SAN FRANCISCO STATE OF CALIFORNIA EARL WARREN. Governor DEPARTMENT OF NATURAL RESOURCES WARREN T. HANNUM, Director DIVISION OF MINES FERRY BUILDING. SAN FRANCISCO 11 OLAF P. JENKINS, Chief SPECIAL REPORT 12 DECEMBER 1951 HYDRAULIC FILLING IN METAL MINES By WILLIAM EWART LIGHTFOOT Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/hydraulicfilling12ligh HYDRAULIC FILLING IN METAL MINES By William Ewart Eightfoot' OUTLINE OF REPORT Page Abstract 3 Introduction 3 Historical review of hydraulic filling 5 Properties of hydraulic fills Applications of hydraulic filling 8 Techniques Preparation of the pulp Transportation of the pulp 12 Underground practice 16 Costs 24 Bibliography 26 Illustrations Figure 1. Photo of thickener-type preparation plant 10 2. Photo of hatch-agitation plant 11 3. Photo of Central Eureka double-tank hatch agitation plant 11 4. Diagram of velocity break for vertical sections 14 5. Horizontal cut-and-fill stoping 15 6. Horizontal cut-and-fill stoping 16 7. Photo of bulkhead construction in a raise 17 S. Photo of the surface of a hydraulic fill 18 0. Photo of crib chutes calked with excelsior 18 10. Diagram of typical open stope 10 11. Diagram of effect of filling a stope without constructing a bulkhead 21 12. Diagram of South African filling practice 21 13. Square-set panel in pillar stope 22 14. Diagram of concrete bulkhead 23 15. Filling against an irregular back 24 ABSTRACT "Hydraulic filling." or "sand tilling," is a system by which the material needed to fill underground openings is supplied as a dense pull). Commonly, mill-tailing is used. The resultant fill, after deposi- tion and draining, is known as a "hydraulic fill." Such fills have been efficiently employed to: (a) provide immediate support of the working-face during stoping, (b) facilitate the recovery of pillars, (c) support access openings, (d) limit surface damage, (e) enable the mining of parallel, hanging-wall orebodies, (f) permit the extraction of ore under water-bearing formations, Ig) stabilize old workings prior to re-entry, (h) reduce rock-burst hazard, (i) con- trol and prevent mine fires, ( j ) seal off ground-water, and (k) al- leviate stream pollution and tailing disposal problems. These various uses of hydraulic-filling have been developed during the last 85 years. I'sed initially in 1K04 to arrest surface subsidence, the system has since been applied successfully to meet specific ground-support problems, and in some mines, has made possible continued operation by contributing essential cost reductions. Methods of handling mine-fill material derived from an external source include transportation of the material by hand, mechanical, pneumatic, and hydraulic means, or a combination of such methods. The hydraulic method has many advantages not found in the others, a significant advantage being the low compressibility of the material as compared to other mine-till material. Slime con tent of the fill material is most important as a factor in the cementa- tion and consolidation of the fill. The techniques of hydraulic rilling, which include preparation and transportation of the pulp, and underground practices in placing fills, vary from mine to mine to meet different C litions. Exper- iments prior to adoption of hydraulic filling at any one mine is advisable. Formerly, the exceptional support inherent in a hydraulic fill was the chief reason for the use of hydraulic filling, but recently, experience proved that it is also an important means of reducing mining cost. This is especially true where labor is a large part of the cost of filling by an alternate method, or where timber require- ments are greatly reduced, which is often the case in "cut-and-fill" or "square-set" mines. * On active duty, U. S. Army. Condensation of a thesis submitted in partial fulfillment of the requirements for the degree of Bachelor of Science in Mining Engineering, University of Washington. Manu- script submitted for publication June 1950. Indirect benefits are probably as important as the direct benefits derived from hydraulic-filling, and few significant disadvan- tages exist. INTRODUCTION Much of the information presented here is from pub- lished sources, although some has been taken from unpub- lished reports. The remaining material represents data gathered during the author's employment at the Central Eureka and the Holden mines, and from observation and consultation during visits to several other mines that are using hydraulic filling. Some discrepancies between data incorporated herein and published data indicate modifi- cations in the methods now being used. Recent informa- tion from some mines, particularly foreign ones, is not available. Most of the significant features of hydraulic filling systems are mentioned, but comprehensive descriptions of particular operations is not attempted, nor are complete details of construction of equipment included. Acknowledgments. Nearly all of the information presented in this report has as its source the men who pioneered hydraulic filling methods. The author deeply appreciates the hospitality, the helpfulness, and the kind personal interest shown by the men of the several mines and plants that were visited. They include : the staff of the Consolidated Mining and Smelting Company of Can- ada, Limited, at the Sullivan Mine and the Chapman Camp Concentrator, Kimberley, British Columbia ; Mr. Paul M. Price, mining engineer, and the staff of the Che- lan Division, Howe Sound Company, Holden, Washing- ton; Mr. E. H. Syms, mine superintendent, and the staff of the Central Eureka Mining Company, Sutter Creek, California; Mr. Richard Krebs, mill superintendent, and Mr. J. C. O'Donnell, mine superintendent, at the New Brunswick Mine of the Idaho-Maryland Mines Corpora- tion, Grass Valley, California ; Mr. C. N. Kravig, assist- ant mine superintendent, Mr. N. Herz, chief metallurgist, Mr. Frank Howell, cyanide plant foreman, and the staff of the Ilomestake Mining Company, Lead, South Dakota; Mr. C. M. Mathews, fire foreman, Mr. Elmer Norris, assist- ant mine foreman, Emma mine; Mr. Wm. H. Trudeau, mine foreman. Tramway mine, and several others on the staff of the Anaconda Copper Mining Company, Butte, Montana; Mr. Rollin Farmin, superintendent, Mr. C. E. Sparks, general mine foreman, and the staff of the Day- rOck mine, Day Mines Inc., Wallace, Idaho. My thanks are extended to Mr. C. W. Plumb, former general manager of the Sliger mine, and Mr. Fred H. Howell, geologist, Page mine, for special counsel and encouragement. This report has been prepared under the direction and guidance of D. A. Pifer, Director of the School of Mineral Engineering, University of Washington. The assistance and encouragement of the California Division of Mines assured the completion of the desired field work. The permission of the various mining companies to publish the data concerning their mines is gratefully acknowledged. (•°. ) Special Rkpokt 12 />> finition of T< mis. Hydraulic filling is used to de- note a system of filling. It includes the flushing of solids underground, the preparation of the pulp, the transpor- tation of the pulp, the preparation of the underground excavation, the dewatering of the pulp, And the entire operation that is involved when a fill is placed. Hydraulic fill is till that has been hydraulically placed. The material of which a hydraulic fill is composed is referred to gener- ally as fill material. Good descriptive terminology for the fill material is lacking, hut the following arbitrary definitions are used in this report: Fines is used to denote material suitable for use in hydraulic filling. It indicates no particular size range, except that the particle size is small enough to be satisfactory. Slimes is used, except where noted expressly to the contrary, to denote all material that will pass a 200-mesh screen regardless of its physical-chemical char- acter. This distinction is convenient, but does not mean that the (piantities and relations of the very minute par- ticles are not significant. They probably are, but data are not complete enough to be conclusive. In keeping with these definitions, desliming is used to mean a reduction in the amount of minus 200-mesh material. 1 A large fill is a hydraulic fill of many hundreds or thousands of tons that is placed continuously or in batches of considerable volume. A small fill may aggregate several thousands of tons, but is placed a few hundred tons at a time so that the depth of fill is never great between periods of thorough dewatering. Tonnage is given in dry tons. Bulkhead is used to indicate any of the various de- vices constructed to confine the fill material. Filter-bulk- head identifies bulkheads that are constructed so as to permit drainage from the fill to pass through the bulkhead. Dewatering is separation of the water from the solids when the fill material is originally deposited and the drainage of the water from the fill. Fill Material. Broken rock, including waste from development, is the material commonly used for mine filling It is not generally suited to hydraulic methods all hough crushed rock has been deposited hydraulically in some European mines. Glacial material, to be satisfac- tory, must not contain large boulders; the ratio of fines must he high. All of the largest and most successful appli- cations of hydraulic filling utilize a fill material of small particle size exclusively, usually below 10-mesh. This pro- vides superior fill which requires less water than fill of larger size, and produces less pipe line wear. Good fill materials are river sand, dune sand, granu- lated slag, or mill tailing. Mill tailing is obtained from operating mills or dumps. In addition clay, culm, finely • rushed nick, ashes, decomposed rock, burnt slate, rock salt, flue dust, boiler ash, and washery waste have been used to a varying extent throughout the world, although not always in metal mines. Methods of Filling. Any material used for stope filling not derived from the stope must be transported and distributed. Several methods are used in handling the material: by hand, mechanically, pneumatically, or hydraulically, or a combination of these. Distribution of the fill material within the stope bv hand is slow, costly, and results in a comparatively poor nil, especially in flat stopes. 1 All screen sizes refer to the Tyler series. Mechanical methods include the use of cars, trucks harrows, scrapers, or conveyor belts to transport the material from the skip or wastepass to the stope or transfer-raise. Interference and low efficiency in the transportation of ore, men, and supplies and cost of labor and maintenance is sometimes considerable. The same kinds of equipment are used for distri- bution of the fill material in the stopes. In Europe, cen- trifugal stowing machines have been used. They are said to be suitable for stowing in flat veins, but require con- siderable space and labor, are costly to maintain, and of small capacity. Two systems of pneumatic transport are adaptable to mine filling: the high-pressure system, usually inter- mittent in operation, and the low-pressure system, usually continuous. The high-pressure system has a limited effec- tive radius and is suited only to the distribution of mate- rial within the stope ; the low pressure system has a greater radius and is suitable for the transportation of material tor longer distances in the mine. In the high-pressure system, a container is alter- nately charged with material and then discharged by the air at a pressure of 70 to 100 pounds per square inch the volume of air required is relatively low, as is the capacity. In the low-pressure system, the fill material is car- ried in a hue in which a volume of air about 300 times the volume of material is flowing. Some rather lono- low- pressure lines have been used. Material as fine as mill tailing has apparently not been handled in low-pressure systems. All pneumatic systems are extravagant in the use of power. In addition, extremely rapid pipe wear is common, and is said to have been excessive in the South African gold mines. In the German and British coal mines, where pneumatic methods are most used at present, replacement ot pipelines due to wear is one of the largest expenses 1 he moisture content of the material supplied to a pneu- matic system must be closely controlled. Pneumatic systems can handle material of larger particle size than hydraulic systems ; they do not intro- duce additional water into the mine, and can be used where it is necessary to deposit the material in locations above the elevation of the end of the pipeline. The dust produced can be overcome by the use of water sprays although such sprays complicate the filling procedure' r-neumatic methods can produce a tighter fill against a nearly horizontal back than can most hydraulic filling methods; however, the advantage is questionable because the walls are not as well supported. A fill that is placed pneumatically is not as well compacted and over a period i lme ,. 1S i > , 1 ? served to settle an d compress more than a hydraulic fill. Hydraulic systems are continuous from the surface to the stope. No rehandling of material is required. Pilling by hydraulic methods is rapid and simple, requires little labor involves a minimum of maintenance of equipment, permits filling , n inaccessible parts of a mine, introduces no dust hazard, and does not interfere with underground traffic. A hydraulic-filling system requires only the in- stallation of a preparation plant on the surface, apparatus tor conveying the material underground, and the prepa- ration ol each stope for containing and dewatering the material. b Hydraulic Filling in Metal Mines Fills placed hydraulically are claimed to be the tight- est possible kind of fill. Many laboratory experiments and observations on full-scale operations have demon- strated that hydraulic fills are the most resistant of any continuous support used in mining. This results in cheaper mining methods, reduced timber consumption, greater recovery of ore, and less dilution, as well as decreased development per ton of ore mined and less mine maintenance. Material deposited hydraulically flows into the small fissures and cracks in the adjoining rock, and penetrates areas of broken rock or of matted timber and waste. Ex- cept where the upper limit of the stope is nearly horizon- tal or is very irregular, the material is tight against all of the confining walls. Low dips require more care during filling than do steep dips to assure tight filling and good drainage. Among the problems imposed by the use of hydraulic methods are : pumping of the excess water to the surface, pressure at depth on the pipeline, wear on the pipeline, possible necessity of desliming the material, effects on ventilation, construction of suitable bulkheads, and pro- vision for adequate drainage. HISTORICAL REVIEW OF HYDRAULIC FILLING Hydraulic filling is thought to have been first used in the anthracite region of Pennsylvania. It was origi- nated by a Catholic priest in the city of Shenandoah in 1864, who, in the hope of saving his church from destruc- tion by subsidence, prevailed upon the president of the Philadelphia and Reading Coal and Iron Company to slush the breaker waste and culm into the old mine work- ings beneath his church. 2 In 1884 culm was flushed underground at a mine near Schuykill, Pennsylvania, to extinguish a fire after attempts at drowning it with large volumes of water had failed. In the following two or three years hydraulic filling was used in neighboring regions for sealing off or extinguishing mine fires, arresting squeezes and sup- porting the surface. By the 1890 's hydraulic filling had become well established. From these early applications the process of "silt- ing," "flushing," or "slushing," as hydraulic filling is called in the coal mines, became an important part of coal mining methods, especially in the Pennsylvania anthracite fields, where it has been employed continu- ously. It is now used as an important aid in the conserva- tion of coal reserves. The technique of hydraulic filling in coal mines dif- fers in many respects from the technique employed in metal mines ; it is characterized by rather shallow depths, large range of particle size, intermittent filling with small quantities, and relatively low pulp densities. Glacial material, silt, or culm, the finely divided waste product from coal preparation plants, is commonly used for fill material, although many other materials are satisfactory. After visiting the Pennsylvania anthracite region in 1893, a party of German engineers introduced hydraulic filling in Europe where it rapidly became important. The Myslowitz colliery first used hydraulic filling extensively, and many other mines in Silesia soon followed. It was called "sand replacement" and was employed as an 2 Ash, S. H., and Westfield, James, Backfilling problems in the anthracite region as it relates to conservation of anthracite and pre- vention of subsidence: U. S. Bur. Mines, Info. Cir. 7843, 18 pp., 1946. integral part of the coal-getting operation. This was in contrast to the applications in Pennsylvania where hy- draulic filling had been principally used as a secondary, temporary, or stop-gap measure and was seldom em- ployed for immediate support during extraction of coal from the face. Hydraulic filling soon became standard practice not only in Silesia, but in Westphalia, France, and to some extent in Bulgaria, Servia, Poland, Spain, Austria, Hun- gary, Belgium, Italy, Britain, Russia, and Manchuria. The European technical literature of the period 1900 to 1915 contains many descriptions and discussions of these early operations. Although a wide variety of materials was used for hydraulic filling in the European mines, glacial or fluvial sand and gravel, or crushed rock were chiefly employed. Because these unsized aggregates usually contain very little slimes and require large volumes of water, high velocities in the pipeline were so common that wear on the pipe became a principal source of trouble. Various kinds of pipe, including wood, terracotta, cast iron, wrought iron, mild steel, glass-lined, porcelain-lined, wood-lined, and pipe of oval shape with renewable iron liners were investigated and compared. The most difficult problems confronting the engineer were wear on the pipe, lack of sufficient fill material, and the difficulty of clarify- ing and lifting the stowing water. 3 Hydraulic filling is still used extensively in Silesia 4 where sufficient surface material is available, but in other parts of Europe it has been curtailed or abandoned in favor of pneumatic and mechanical methods. In recent years hydraulic filling has been widely practiced in the Jharia and Raninganj coal fields in India. 5 The government and the coal mining industry have cooperated in developing economical methods of using river sands for filling. The program is aimed pri- marily at the conservation of coal reserves by leaving a mined area in good enough condition that it can be mined at a later date either for recovery of the pillars or for overlying narrower seams. Although hydraulic filling was used at an early date in the lead mines of Germany, the gold mines of Australia, and at Cripple Creek, Colorado ° — operations that have not been well documented — it was first applied in large- scale metal mining on the Witwatersrand in South Africa. The Village Main gold mine began placing stamp sands underground hydraulically in 1909. Other hydraulic fill- ing operations were soon begun at the Ferreira, Gelden- huis Deep, Robinson, Simmer and Jack, and Cinderella Deep, and have since been employed at most of the mines on the Rand for such varied purposes as support for cur- rent stoping, reclaiming pillars, mining parallel reefs, protecting drives, and controlling rock bursts. Millions of tons of tailing have been placed under- ground at the South African mines; several mines have placed much more than a million tons each. The material used for filling at first was exclusively stamp sands, and was of such size that most of it would be retained on a 8 Paton, J. Drummond, Modern developments in hydraulic stow- ing, and suggestions for its application in British collieries : Inst. Min. Eng. Trans., vol. 47, 31 pp. 1913-1914. ♦Whetton, J. T., Gob stowage: Colliery Eng., vol. 25, no. 292, p. 188, no. 293, p. 235, and no. 294, p. 255, 1948. 5 Turton, A. C, Sandfilling at Mufulira : Inst. Min. Met. Bull. 478, 24 pp., 1946; Discussion and reply Bull. 480, pp. 1-10, 1946. "Origin of sand filling: Min. Sci. Press, vol. 109, p. 791, 1914. Spe< iai, Report 12 100-mesh screen. Modifications in the milling practice have lowered the average grain size of the sands, but the South African sands are still coarser than most of the taijings available from modern dotation mills. Hydraulic- lillum practice on the Witwatersrand, where the dip of the ore bodies is rarely greater than :{."> degrees, is charac- terized by this particle size and by the use of sand cones, boreholes, underground distribution launders, and filter- bulkheads. However, as in all hydraulic filling applica- tions, the details of practice vary greatly from mine to nunc. Hydraulic filling was used at Butte prior to 1920 for combating mine fires. Except for the small hydraulic- filling operation at Cripple Creek, this application ap- pears to be the first use of hydraulic filling in the metal mines of the United States. The use of hydraulic filling tor fighting mine fires has become an accepted effective technique. It has been successfully employed against fires al the United Verde, the Tintic Standard, and at other metal mines and coal mines throughout the world. At the Matahambre copper mine, Cuba, in 1927, hydraulic filling was adapted to the cut-and-fill method of mining a steeply dipping vein. The fill material, mill tailing, was reduced to a very low slime content before being sent underground. The installation of rubber-lined pipe was found to be necessary, as it has in many hy- draulic-filling systems since then. Regardless of the cost Of the rubber-lined pipe, however, hydraulic filling re- duced the total cost of mining and was beneficial in sev- eral other respects. There soon followed several more applications of hydraulic filling in metal mines throughout the world- these applications at more mines and to different filling problems continue to -row in number. About the time that large hydraulic-filling operations were begun in the cop- per mines at Mount Lyell, Tasmania, and at Mufulira Northern Rhodesia, 1929-32, the Homestake mine applied hydraulic filling to the control of surface subsidence sin- tailing from the cyanide sand-leaching operation hydraulic filling soon became a part of the regular mining method - T,H ' " es *ake Mining Company has developed tli«' techniques of hydraulic filling to a high degree- the < ompany ran completely recover an irregular ore body by a selective method of mining at a minimum cost. In 1940 hydraulic filling was introduced at the Sliger mine, Georgetown, California. Large savings in labor and timber were realized. Since World War II hydraulic filling has been intro- '''"•;■:' ", I 1 " 1 1;, ," V n s - including the South mine, Frood- Stobie Ilohlen, Greater Butte Project, New Brunswick, , n " ,,;ii Eureka, and Dayrock. The last three mines are ■''" rl " and-fil] operations; small fills are deposited The Practice oi hydraulic filling as employed at these mines appears to have evolved into a rather distinct system which is proving very effective. PROPERTIES OF HYDRAULIC FILLS , ''""' "'•!'"'"■ Although little cementation of hy- draulic fills ,s observed, even after several years, the lack ••I -men ;„,,,„ ,,„.. not significantly impair the effective- ness oi the fill. Experiments on fill material at the Sulli- a »'">^haye shown that the time required for cementa- 1 s fonwderablj longer to,- fine material than for •oars,, sands or waste. Thus mill tailings- especially those with a high proportion of slimes — may be expected to require many years for complete cementation. Most fills apparently are compressed to the maximum before suf- ficient time has elapsed for much cementing to take place. Because the chemical composition and environment of any fill determines the effectiveness of cementation, the results obtained are seldom identical. Although the fills at both Butte and Mufulira are in sulfide areas, the percolating sulfate waters have caused no extensive cementing during the period of observation, which has exceeded ten years at both mines. Few fills consisting of tailing contain much sulfide. In South Africa some of the fills have a pyrite content as great as 4 percent, but even these show no cementation. The tailing used at Homestake, which contains as much as 2 percent sulfides, usually cements to a shallow depth around the edges of the fill. The cemented material re- sembles a moderately indurated, ferruginous sandstone. Larger amounts of sulfides have been introduced into granulated slag and into sink-float tails in order to pro- duce a thoroughly cemented rocklike fill. These cement well in a relatively short time, but develop high tempera- tures during oxidation. Calcareous material may also produce a cemented fill. Only 7 months after deposition, the 55-percent lime- stone fill at Cerro de Pasco required blasting to drive through it. The 85-percent dolomite material used in a southeast Missouri hydraulic grouting operation cemented sufficiently to recover drill cores 6 inches in length. The methods of chemical soil solidification that have been developed recently may eventually prove to be a valuable adjunct to hydraulic filling, especially as an emergency measure. 7 Consolidation. Consolidation is used to mean the compaction or "setting up" of the material within a fill, exclusive of cementation. It depends chiefly upon the retained moisture and the slime content. The amount of moisture retained in a fully drained fill may be as much as 15 percent of the weight of the fill. Moisture content of the Mufulira fill, for example, was 11 percent 2 years after deposition. Except for coarse sands, the material of most fills is damp to the touch even after many years. The slime assists the consolidation of the fill in two ways : it is important in retaining water, and it provides closer packing of the particles, the finer ones filling inter- sticies. If the amount of slimes is too great, however, too much water may be retained and poor consolidation 'will result. Examination of fills several months or more after deposition shows that the degree of consolidation varies considerably from mine to mine. Some fills react as crumbly aggregates and can be easily broken up ■ others are more^blocky, a.s the material will hold together in chunks. Most fills, however, especially if there is sufficient slime, are firmly compacted, and will retain a sharp im- print of any tool thrust into them. A shovel, for example can be e asily driven into the material, but this does not f^^flr' «^: £a,^vW Z tfft w., t ,:::;;T-:;:,;' U( Vi:-^ w ^jf i f^'}^f-e., sand , a „ d W e ak r o Ck: i-..iv.r.Vfei^iSubl£^g l §!S?^ &r ans of chemical injection: Hydraulic Pillino in Metal Minks cause the material to loosen or run. Most hydraulic fills will stand up well in free vertical faces. None is known to behave as a clean dry sand with uninhibited running ten- dencies, although the coarser the material, the more a fill might be expected to approach such a condition. The fills in South Africa are an example. The effectiveness of a hydraulic fill for support is evi- dently unchanged by the manner of consolidation. Routine mining methods are employed at many mines in which free vertical faces of hydraulic fill are exposed. In cut-and-fill mining, heights of two floors are commonly exposed, and greater heights are known. The South Afri- can practice of mining parallel reefs by using the hydrau- lic fill of the underlying stope for the footwall of the overlying stope also demonstrates the satisfactory consol- idation of hydraulic fills. Whenever the free side of a high fill is to be later exposed, safe practice requires that a bulkhead be con- structed so that the material will be contained at the time of exposure. Maximum height to which the material will stand sufficiently well without a bulkhead depends upon the manner in which the fill consolidates, and must be determined by experience ; both the Homestake mine and the South Mines have constructed bulkheads. Compressibility. Many observers agree that hydrau- lically deposited sands or other fine material are the least compressible materials used for mine filling. 8 Rarely does hydraulic fill compress more than 20 percent ; measure- ments and estimates generally indicate from 5 to 10 per- cent. 9 Hydraulic fills are more resistant than other fills because water-deposited particles are very compact and because the innumerable small particles provide a large bearing surface. The strength of a hydraulic fill is not developed until sufficient dewatering has changed the material from the quick condition to the consolidated condition. Pressure on a poorly consolidated fill causes flowage and is some- times transmitted through the fill, as though it were a fluid, to the bulkheads. Permeability. The permeability of a fill is import- ant. It is more important in large fills than in small ones. Large fills must be either impermeable, a rare condition, or they must be sufficiently permeable to permit the quick dissipation of any amount of water that may find its way into the fill so that the water will not form ponds or other- wise be retained. Enough water in the fill either from poor original dewatering or from subsequent influx, to cause a mushy or quick condition may cause immense pressures on the bulkheads which may not become evi- dent before the bulkheads burst. Suitable permeability will permit any water that might enter to drain off through fissures in the adjoining rock or through the drains pro- vided in the bulkheads. 8 Griffith, Wm., and Conner, E. T., Mining conditions under the city of Scranton, Pennsylvania: U. S. Bur. Mines Bull. 25, 1912. Jeppe, C. B., Gold mining on the Witwatersrand : Transvaal Chamber of Mines, vol. 1, pp. 814-826, 1946. Watermayer, G. A., and Hoffenherg, S. N., Witwatersrand min- ing practice: Gold Producers' Committee, Transvaal Chamber Mines, Johannesburg, South Africa, pp. 459-471, 1932. * Jones, A. A., Sand filling methods at Hodbarrow mines, South Cumberland: Inst. Min. Met. Bull. 229, pp. 1-19, 1932; Discussion: Bull. 330, pp. 1-13 ; Reply : Bull. 332, pp. 51-53. Eaton, Lucien, Sand filling through pipes and boreholes : Am. Inst. Min. Met. Eng. Trans., vol. 102, pp. 33-41, 1932. Richert, George L., Mining methods at Mines De Matahambre, Matahambre, Pinar Del Rio, Cuba: IT. S. Bur. Mines Info. Circ. 6145, pp. 8-9, 1929. The permeability of small fills is probably less im- portant because entrapped water may drain through less fill material, as the proportion of wall rock surface and bulkhead surface exposed to the fill is much larger. Also, the condition of the bulkheads is more easily observed and dangerous conditions can be detected. In addition to required minimum permeability of the emplaced fill, the permeability of the fill at the time of deposition is important in determining the methods of dewatering to be used. The permeability of a fill, and consequently the rate of percolation, depend chiefly on the slime content of the fill material, although the effect of other factors, such as the range of sizes and shape of particles, are important. Slime Content. Control of the amount of slimes in the fill material is very important. The slime content is a factor in the cementation and consolidation of fills, and is important for its effect on viscosity and abrasion dur- ing the transportation of the pulp. However, the amount of slimes is probably most important in its effect on the permeability of the fill and the methods used for dewater- ing. There is a maximum limit to the amount of slimes that is satisfactory. The effect of slimes is easily demonstrated in the cor- relation between the types of bulkheads and the slime content of the fill material with which they are used. Filter-bulkheads are ordinarily used with fills that are comparatively coarse and contain few slimes. Most of the dewatering is accomplished by percolation with final egress of the water through the bulkheads. The fills at Hodbarrow, the Emma and Travonna mines in Butte, the Matahambre mine, and on the Witwatersrand 10 are ex- amples (see table 2). Pills consisting of finer material require at least partial dewatering by decantation or by continual drain- ing of water from the surface of the fill. The fills at Central Eureka, Dayrock, Great Boulder, New Brunswick, Sliger, and the South Mine are examples. After these fills have become consolidated only part of any water that is run in on top of the fill will seep slowly into the fill ; most of it will flow over the surface and seek an outlet elsewhere. The effect of slimes on permeability and percolation rate has been studied at a few of the mines. The work is difficult to correlate because the tests are conducted for different purposes, the methods employed in testing are not the same, and the results are recorded in different units. The distinction, for example, between fines, slimes, and true colloidal phases is not always clear. Probably there is a lower limit of particle size below which the suitability of any material rapidly diminishes, especially if such sizes are present in large quantities. This limit is undoubtedly not the same with different materials and therefore no generally applicable figure is to be expected. An arbitrary limit on the amount of slimes allowable in fill material is set at some mines. At the Sullivan mines, for example, where glacial drift is used, 15 percent is the maximum amount of minus 200-mesh material permitted. Large stopes are filled hydraulically, but the methods of hydraulic filling as defined herein are not used. '" Regardless of the apparently satisfactory permeability of the coarse sands commonly used on the Rand — drainage is almost always provided for by percolation methods — occasionally destructive wash- outs have occurred due to the accumulation of water within the fill. Special Report 12 At Homestake, percolation tests on fill material ob- tained from the sand-leaching plant and deslimed at a separation near 325 mesh, have shown that an increase oi several percent in the minus 200-mesh portion, which ordinarily runs about 4") percent, reduced the percolation rate only slightly. However, it was also shown that an increase m the mums 325-mesh portion of only 1 percent reduced the percolation rate by 50 percent. The material is derived from a schistosic ore. At Mulfulira" a change in mill practice increased tln> pulp density of the feed to the desliming cones con- sequently, the amount of minus .S25-mesh material going under-round increased from 13 to 22 percent. The cumu- lative minus 200-mesh material increased onlv 1 percent As a result of the increased slimes a major bulkhead tailed causing a bad spill, an d a thin pillar of rock (the mud-seam") between the parallel stopes also failed. 1 his was corrected by diluting the feed to the cones until the quantity of minus 325-mesh in the fill material was reduced to its ori-i„ a l value. Fifty percent of the minus 325-mesh is said to be colloidal. The cumulative, minus 200-mesh fraction equals 32 percent. Prior to the introduction of hydraulic filling at the Great Boulder mine l2 several test runs were made in a stope on the 300-level. The stope was 100 feet long 4 feet wide a„d was sealed with water-tight bulkheads at the ends. Before the first fill was deposited, used filter cloths were placed on top of the waste fill already in the stope. Thereafter, drainage was chiefly through a per- forated vertical pipe in which the holes were plugged as the level of the fill rose, thus decanting the excess^water inThlT tk ' £, am ° UntS UP t0 85 tons ' are reeor d^ in table 1. These fills required from 3 to 14 days to dry at he surface and remained puggy several more days They required up to 6 weeks to dry. Thereafter, the mate- rial used m fills was classified before being sent under- ground and the amount of slimes greatly reduced (table 1). According to the report, "the resultant surfaces were a 1 excellent and the drying time was less than 24 houTs " Since these experiments were performed, the results of actual operations have shown that the slime content can be increased to 50 percent and still be within satisfactory The results so far obtained indicate that the effect s. „ es^m' 'a\ draUli ° ^ * ^ Predsel - V what ^ •i u • 'v ' t° &n l mUie C u ann0t aS - vet be Prated witn accuracy; it is known, however, that too hkdi n percentage of slimes may be damaging. g & APPLICATIONS OF HYDRAULIC FILLING . Hydraulic filling can be used in any application for nc convent,ona. nils are used and JjSbg'Stad ;.;;<<;;<»>> uses. I„ m08 t operations it serves more than one Control of Subsidence. The effectiveness of hydrau- rength and lou compressibility of such fills. Hydraulic 0IJta« with (Mcken? :, : s^l uTno^U^ ,V^ ^ E ^ Stope Mm M.-t IT..,, vol I87,pp "fii, i945 ,0n t&M ^s : Australasian Inst. Mm.^\!!;;?^,;!.K,^:; , :: 1: ;'' i ^;;' i ;-;;';;' j K. j, and n >, E . w., stope Mm Ifel Proc.VOl 137. pp l-li.iaV!-, ta,Un S": Australasian [nit Table t. Pulp densities nnd .screen .size of material used in experi- mental hydraulic filling at the Great Boulder mine' 3 Test Pulp density percent solids 65-mesh percent —65 + 200-mesh percent — 200-m. sh percent 1 65 65 66 63 65 68 67 A 2.5 2.2 0.3 1.4 B 24.1 23.0 21.3 22.8 2._. 73.4 3 74.8 4___ 78.4 1--. 75.8 2 36.8 3 27. 1 1 35.0 fills have been employed in order to minimize surface damage, 14 to facilitate the mining of parallel hanging- wall stringers, 1 ' to permit mining below water-bearing forma- tions 1G and to protect underground workings. 17 Table 2. Average screen analyses of material used for hydraulic fills at various mines Locality — 65-mesh percent —200-mesh „ percent Remarks Butte, Emma and Travonna Decomposed surface rock. Butte, (fire fighting) 52 30 45 55 35 20 48 Butte, (greater Butte project) . ^ nil 1 Central Eureka Cerro de Pasco Dayrock 4 10 2 Frood-Stobie Great Boulder Hodbarrow Dune sand, analysis unknown. 24% in plus 100-mesh. From sand-leaching plant. Very little minus 325-mesh. Note lack of slimes. 17% is minus 325-mesh. 13% is minus 325-mesh of which about 50% is colloidal. Ho'den... 35 44 3 39 24 30 25 30 Homestake. 3 49 7 6 18 nil 17 Matahambre Mount Lyell Mufulira New Brunswick Sliger. . South Mine Approximate conversion from IMM screen series. Formerly mostly plus 100-mesh. Now may be somewhat finer. Witwatersrand (South Africa) . Stope Skpport During Mining. Hydraulic filling has always proved advantageous where it is used as an integral part of the stoping operation. The quick effec- tive support of the walls reduces sloughing and slabbing from the back, allows safer mining, and helps substan- tnillyjoward minimizing dilution. Commonly it has been Tech.. , V?l° 8 3'no'4 J 'i5 M Dn S t??Q fl i" n8 , at ft? Homestake mine: Min. 1075. ' PP - 1939 - Am - Inst - Min - Met. Eng., Tech. Paper mining ^r£™e? Gold ' p'roduo^^^f ' S ^ N " Witwatersrand Mines^ohannetbuVt^South^^Tca, pp.T^ "?! , f^T^' ^"^ Cumberland 8 ' fts^Mm^il'"^!?, 6 ^^ at "°? barrow mi "es, South Bull. 330 pp. iSfrgfc ^u.K, P p 9 51 P - P 53 ^ 1M » ! Discussio " ' 478 2< dd °194S'- nVJ?. anjlfimn 5 a * Mufulira': Inst. Min. Met. Bull ' ,. PI - 1!H6 ' Dls cussion and reply Bull. 480, pp. 1-10, 1946 Jep D a e te c le B er 'p^iH A ' and Hoffe " b ^g, S. N., op. cit.. 1932. Chamber &„£,' v^'l, p p n, 8T4-8 , 26, t, } e 94? ritWaterSrand ! Transvaal Hydraulic Filling in Metal Mines 9 possible to modify the method of mining to permit savings in labor, timber, and development work. Hydraulic filling permits maximum selectivity, flexibility, and rapidity in stoping operations. The fills ordinarily consolidate suffi- ciently to permit resumption of mining within 24 hours. Some of the achievements in stoping credited to hydraulic filling are remarkable. The Central Eureka and New Brunswick mines, for example, were able to convert square-set stopes to cut-and-fill methods. At the Sliger mine the layer of serpentine gouge 100 feet thick along the hanging wall was so effectively held as to reduce the timber required by more than 50 percent. At the Dayrock and Matahambre mines modified stope layout decreased the amount of development work. The necessity of leav- ing crown pillars at the Homestake mine was entirely eliminated. Pillar Recovery. On the Witwatersrand the value of hydraulic filling for the recovery of pillars is undis- puted, except in the deep mines. As in most other appli- cations of hydraulic fills, it is the strong compressive strength and rapid succession of mining, filling, and tak- ing weight that makes hydraulic filling suitable for use in pillar recovery. When the concentration of stresses in pillars is lessened by quick and effective support within the adjacent excavations the mining of the pillars pro- ceeds rapidly, reducing cost and increasing safety and the maintenance of access through pillars is facilitated. lx Mine Stabilization. Hydraulic filling has been effective in improving the general stability of ground in mines. In mines containing large open stopes it has helped to minimize ground movement and diminish the possi- bility of rock-bursts. Hydraulic filling is being used to stabilize previously mined ground in preparation for large-scale block caving at Butte. Control and Prevention of Mine Fires. Hydraulic filling has controlled many sulfide and timber fires in metal mines, even after other methods have failed. Ac- cording to Rahilly, 19 it is particularly effective with inac- cessible fires of large extent and high temperature. Usu- ally hydraulic filling is employed for fire fighting by inun- dating with fill material the entire area in which the fire is burning. 20 Drainage Control. In the southeast Missouri lead district hydraulic filling has been employed to control the flow of underground water that was entering the mines through solution channels in dolomite. The water w Ross, A. J. M., Sand filling at the Homestake mine : Min. Tech., vol. 3, no. 4, 15 pp., 1939. Am. Inst. Min. Met. Eng., Tech. Paper 1075. Plumb, C. W., Filling mine stopes with mill tailings: Min. Cong. Jour. vol. 28, no. 1, pp. 12-14, 1942. Assoc. Mine Managers Transvaal, Some aspects of deep level mining on the Witwatersrand gold mines with special reference to rock bursts: Transvaal Chamber Mines, Johannesburg, South Africa, 1933. 10 Rahilly, H. J., Mine fires and hvdiaulic filling: Am. Inst. Min. Met. Eng. Trans., vol. 68, 12 pp., 1923. » Alenius, E. M. J., Methods and costs of stripping and mining at the United Verde open-pit mine, Jerome, Arizona : L\ S. Bur. Mines Info. Circ. 6248. p. .34, 1930. Brusset, J., Battling a mine fire in Algeria : Explosives Eng., vol. 8, no. 3, 4 pp., 1930. Hanson, Earl F., Control of underground mine fires at Tintic Standard mine: Am. Inst. Min. Met. Eng. Trans., 126, 12 pp., 1937. McCutchen, V. L., Mining methods at the Cerro de Pasco proper- ties : Mining and Metallurgy, vol. 26, no. 467, pp. 521-523, 1945. Rahilly, H. J., op. cit., 1923. was effectively dammed by the introduction of mill tail- ing through boreholes into areas adjoining the mines.- 1 Tailing Disposal. Although hydraulic filling has never been used in metal mining exclusively for the pur- pose of disposing of the tailing, such disposal might be economical where land is valuable, streams are seriously polluted, or it is expensive to prepare for tailing storage. Effects on Mine Ventilation. Ventilation of mines is commonly improved when hydraulic filling is substi- tuted for filling with waste. The air does not short-circuit through the fill and the maintenance of openings required for ventilation is facilitated. Where the fill material is tailing from a cyanide plant, precautions may be necessary to prevent the genera- tion of hydrogen cyanide. The Mine Regulations in South Africa impose a limit of 0.005 percent potassium cyanide in the moisture of the fill and in the water that drains from the fill. In order to keep within this limit an oxidiz- ing reagent is added to the alkalized pulp that goes under- ground to oxidize the cyanide to the innocuous cyanate. 22 At the Homestake mine the material from the sand- leaching plant requires no special treatment. The water draining from the fill is neither harmful to the skin nor dangerous if taken internally. The material used at Holden, which also contains some cyanide plant tailing, also requires no special treatment and produces no harm- ful effects. Hydraulic filling increases the humidity of a mine and therefore may add to the ventilation problem. Also, openings into a hydraulic fill are sometimes deficient in oxygen and need special ventilation. The heat of oxida- tion from a hydraulic fill probably never is great enough to be troublesome, since the oxidation proceeds too slowly. Techniques Preparation of the Pulp Before the fill material is sent underground it must first be mixed with water at the preparation plant, which also controls somewhat the quality of the pulp. The chief functions of a preparation plant are regu- lation of the volume or velocity, control of the pulp den- sity, provision of homogeneity in the pulp, and regulation of the slime content. Precise control of these factors is seldom required and therefore elaborate equipment is rarely warranted. Some preparation plants provide a continuous flow of pulp, whereas others prepare the pulp in batches for intermittent delivery. A continuous type plant is limited in the rate of output to the rate of feed, and fluctuations in the feed or discharge usually affect the efficiency of the unit. Where the pulp is sent underground in a closed pipe, 23 gravity system, the rate of discharge will vary with the depth and horizontal distance to which the mate- rial is being conveyed. Because of these limitations a con- tinuous-discharge system is practically restricted to use -i Weigel, W. W., Mine drainage, southeast Missouri lead dis- trict : Am. Inst. Min. Met. Eng., Trans., vol. 153, 8 pp., 1943. -"Jeppe, C. B., Gold mining on the Witwatersrand: Transvaal Chamber of Mines, vol. 1, pp. 814-826. 1946. 33 Closed pipe is used to indicate a continuous, unbroken line, from the tank of the preparation plant to the point of discharge in the stope. The term open pipe means that somewhere the line is open to the air. in Special Report 12 with large fills where conditions are constanl for consid- erable periods of time and where the pulp is either pumped from the plant or transported in an open pipe, gravity system. SimpU Sluicing Plants. The simplest type of plant provided only for sluicing the material from dumps, stor- age bins, or leaching tanks into pipelines, boreholes, or launders. This system obviously has merit for simplicity and economy of investment. It has been particularly suc- cessful in shallow mines where the pulp could be intro- duced directly into the stope through a borehole. It is not useful in small, widely scattered fills because it cannot be controlled through long pipelines. Unnecessarily large amounts of water are often introduced into the mine. Sluicing systems were extensively used in South Africa where rather low pulp densities were used in the underground launder system. Other sluicing systems were used at Ilodbarrow and at the United Verde sliming operation. Thickeners, Classifiers, and Sand Cones. These de- vices are employed for better control of the pulp than is possible in sluicing plants. They are well adapted to handling large tonnages and are easily arranged to per- mit continuous operation. Sand cones have been used extensively on the Rand, at the Mufulira mine, and at Homestake during the early years of hydraulic-filling. They are erected directly over boreholes. At .Mufulira. sand cones were used after it was found that they recovered more of the fines than a classifier and yet made a suitable separation of the true slimes Bat- teries of four locally constructed wooden cones are used They give a recovery of 58 percent of the solids at a feed density oi less than :{() percent solids, and have a capacitv of 1,400 to 1, •>()() tons per day per battery. Sand cones on the Rand are used chiefly for dewater- Ulg the sand after pumping if H long distance at low dens- ity. At the Government Gold .Mining Areas, for example batteries ot l(j cones are used at each borehole They are capable of handling 2000 tons of sand in 8 hours with BIOURB l. ThI.kener-type preparation plant. discharge at 70 percent solids. The sand cones at Home- stake were likewise used principally for dewaterin- after pumping. * Classifiers were used at the Matahambre mine to re- duce the minus 200-mesh fraction in the mill tailing from 4!) percent to only 3 percent. At the Ilolden mine a thickener is used for pulp density control and for mingling the two different tailing products that are used as sources for the fill material" I he discharge from the thickener, about 50 percent solids is ted to pumps and must be uniform (fig. 1). Agitators. The third and largest group of prepara- I tion plants is that m which agitators are employed The agitator is a means of smoothing out surges in the feed to the pipeline and is employed primarily as a device to provide a supply of homogeneous pulp. It is most satis- factory where large volumes are handled and desliming is not important. Equipment ranges from small tanks with compressed air agitation to large installations with agita- tion by a combination of methods. Where the control of density or slimes is important, thickeners or classifiers are sometimes used m conjunction with agitators. In general an agitator plant requires close surveillance during oper- ation. b H At the Great Boulder mine and at Mount Lyell the agitators are preceded by combination thickener-deslim- ers. This arrangement permits continuous desliming of the null discharge with intermittent discharge of the pulp to the stopes, and provides excellent control of slimes and pulp density The control of pulp density is especially necessary at Mount Lyell where all the fill material is pumped upward into the mine and the point of discharge rS pJai^ " el6Vati ° n ""* distance f ™ the ^repf- The Homestake agitator plant consists of a 12-foot diameter Dorr dewaterer with four rakes revolving at 5 revolutions per minute. The sand from the leaching tanks s sluiced either directly into this agitator or into a set- ting pond behind a small dam for temporary storage. The sand in the pond is then sluiced to the agitator when- 1 TVl 1S "L eded under g rou « d - The pulp density is regu- althoni nS S Water i° r by dewateri »" at the agitator, although offer! no regulation is necessary. Seven hundred tons of sand from one leaching tank is handled per shift If such a plant could be laid out more compactly only one operator would be required. , • ♦ f* B " tte , the mil1 filing' is sluiced by high-pressure jets directly from railroad cars into a 25-foot diame er agitator beneath the tracks. Agitation is provided™ an impeller turning at 25 revolutions per minute and driven by a 50-horsepower motor. A similar plant of large capac- iL USed at ^/"^"Stobie mine, but here the dry in n?l9 dm ! lped + fi ft int o a storage bin and then sluiced into a 12-foot agitator equipped with a rake mechanism Another arrangement was used at the Sliger mine. Tailing from the mill, in the form of dilute pulp, was collected in a storage tank and allowed to settlelsome deshmmg was thus effected. It was then sluiced by vari- ous means from this tank through a small agitator into the underground pipeline. This system did not provide easy control of the pulp density and required constant attention during operation. Hydraulic Filling in Metal Mines 11 Several features of the Slider operation should be noted: storage of the mill tailing was necessary in order to convert the continuous discharge of the mill to an inter- mittent discharge for filling ; some desliming was desir- able ; some thickening was necessary ; a homogeneous pulp was desirable because it would eliminate much of the pipeline trouble ; the volume of material to be handled was comparatively small. This is a combination of circum- stances that may be expected to exist at many mines of moderate size. It was to satisfy conditions such as these that the following type of plant was developed. Strict Batch Agitation. This kind of plant is now used at the New Brunswick, the Central Eureka, and the Day rock mines. 24 One unit of equipment is employed, a combination deslimer, thickener, and agitator. Details of design and operation can be varied to effect almost any desired con- trol over the pulp. Tailing is run into the tank from the mill until the depth of settled pulp indicates that a suffi- cient quantity is available to provide the desired pulp density when the water level is at a certain height. De- sliming is controlled by the length of overflow weir and by the arrangements for introducing the tailing into the tank. After the tank is charged with tailing and water in correct ratio, feed from the mill is by-passed to the tailing pond or to another similar tank. The tank of tailing is then mixed by a mechanical impeller assisted by air jets located directly below the impeller and around the peri- phery of the tank. Only after the pulp has become thor- oughly homogeneous is it turned into the pipeline leading to the mine. One man is required for operation of the tank. This kind of plant has been used exclusively with closed-pipe systems. Owing to the complete control of pulp density and the reliable operation resulting there- from, this strict batch agitation type of plant has been quite successful. It is particularly good where small fills are placed (see figs. 2 and 3). Figure 2. Batch-agitation plant. This is the New Brunswick mines tank which has a capacity of 240 tons per batch. 24 Krebs, Richard, and O'Donnell, J. C, Sand-slime stope filling proves satisfactory: Eng. and Min. Jour., vol. 150, no. 1, pp. 54-60, 1 049. Sand filling pays off: Mining World, vol. 11, no. 11, pp. 34-36, 1949. Figure 3. Central Eureka double-tank batch agitation plant. Underground Plants. Dry material is mixed into a pulp underground in two localities. This method requires provision for getting the dry material underground, may necessitate excavation for the mixing apparatus, and limits the radius in which gravity hydraulic-filling opera- tions are possible. At the Emma and Travonna mines in Butte the de- composed granitic material is dumped down waste-passes and drawn off one level above that on which it is to be used. Air-powered, screw-feeders carry the sand into launders from which it is sluiced into the pipeline and conveyed to the level below. In certain square-set stopes where only small volumes of fill are required at a time, the dry sand is dumped down the service raises and drawn off in the stopes by bazookas, which are simple water-jet devices that mix the sand in the chute with enough water to cause it to flow freely in a launder to all parts of the stope. Several underground mixing stations are used at the South Mine, Broken Hill, and are located throughout the mine to provide hydraulic filling to stopes in all areas of the mine below the stations. The apparatus consists essentially of conical-shaped, wooden receivers, equipped with suitable screens and water jets that draw the dry tailing from the sand passes and mix it with appropriate amounts of water for it to flow through the pipelines. One man can operate two stations. The mixing is done under- ground chiefly because of the storage capacity provided in the sand-passes ; large volumes of fill material are re- quired at irregular intervals inconsistent with the output of the mill. Construction of Slime Ponds. The ponding of slimes may be required at a mine where the mill tailing must be deslimed before being sent underground. It was reported from the Sliger operation "that by careful work, a very excellent pond can be made from the slimes themselves." 25 Experience at the Central Eureka has also shown that a good pond can be constructed. The dis- charge of slimes into the pond is shifted regularly to per- mit localized drying so that the slimes can be shoveled into dykes where they make a tough, leathery bank. 2S Plumb, C. W., Filling mine stopes with mill tailings: Min. Cong. Jour. vol. 28, no. 1. pp. 12-14, 1942. Special Report 12 '"''''' , { Vetaih of some closed-pipe, gravity transportation systems '■ WM5K3B4r ,r inc,l " ed sllaf, - totaI Ungth of line 8 ' 200 feet - Transportation of the Pulp The transportation of the pulp from the preparation Plant to the workings may be accomplished by several methods. These include gravity systems, employing pipe- lines, launders, or boreholes, and pumping systems or a combination of methods. The most suitable system is de- termined largely by the layout of the mine and plant the tonnage requirements, and the physical character of the Pulp, especially the settling rate and the abrasiveness In all systems the pulp density should be as high as possible to minimize the amount of water introduced into the mine; it is usually limited to the density at which Plugs > or spills occur. The velocity should be as low as mn Cbsed Pipe, Gravity Systems. A common means of conveying the pulp underground is through a closed pine in which then- is no break nor opportunity for air To enter the pulp flows at constant velocity throughout the ength of the line if the pipe size remains the same T his system has been used chiefly in connection with the stric ut ( :r^ o ed n n sm ' fa< ' ( ; pIants - for whieh * «■*££ suited. Closed-pipe systems may not be well suited to appheations m which tonnage requirements necessitate arge size lines, but where it is applicable, it provSes good control oyer the velocity and thereby may sometimes allow substitution of steel for rubber-lined pipe SuS and ^heavy vibration do not occur in a closed^ system Details of tour of these systems are given in table 3 Once a line of this type has been installed it is obyi o us that the .interrelations of density, viscosity and f ric "" V fr mporUnl ; they control the tonnage translrted ;;■; the velocity. Variations in the location o tn IZl? n,, ( l,scl,aroc necessitate some control of the syste ni and this m usually obtained by regulation of the pulp pui-t ant tZv! "V "' "" Visc ° sity and w * » ■8 nnpo.tant. I he viscosity ...creases with increased nnln density and may vary proportionally oyer a condderablp .1.-.-.-.,-.^ tl... v..|.., it .v. Likewise, wilJla constant deosi fflg ,, ' , • friCti0D h6ad ~ ^ Snte "^W^W^»« fe at M t »Kia CO a n vSlf~ inStaIled ''"" stMl pi <* — «*&■ at th^Ce^ralFnrif ° n ^ e °T picU0Usl y demonstrated at tne Central Eureka mine where pulp of 70 percent heaf is 3^ i*f let G 1 P r SeC ° nd alt W» ^ "CS nead is dbOO feet and the ratio of horizontal to vertical pipeline length is only 1.2 to 1. Small variations in Zshv i\Z\™Z nmch variation in velocity in th »«- d - . The depth to which a closed-pipe system can be used lZerZn n Z r0hah K° m C ? uld be -iied consider! y ZIF k! ! P eSent maximum of 3,600 feet There is undoubtedly some depth below which extra heavy pipe AltL r T re " releaSe - ValveS WOuld have to be insta ed Although pressure in the line while the pulp is flowing s not great, a pug may cause high static heads The longest steep section in the ordinary steel pipeline used at the Central Eureka is 3,000 feet dowi 7 70 Se r ee incline; this section has withheld the static pressures exerted when plugs occurred in the lower parts of line In determining the size of pipe for a closed nine system the tonnage requirements are important en cial y when steel pipe is used and the velocity musX kept at a minimum. From observation of the New Br ms wick line it was concluded that a 3-inch line would have been better than the 2-inch line." In order to e uffi cient tonnage in the 2-inch line the density hfd to be lowered so as to gain higher velocity, which', in turn in creased abrasion. If the density of a pulp rema n the same a smal increase in the pipe sizeVoula provide a considerable increase in the tonnage without a Tare fin crease in velocity, because the only factor Increasing velocity would be decreased friction, which varies aTthf square of he diameter. Any increase due to friction mie t be checked by further increasing the velocity These X unships, however, cannot be expected to exit t in very arge o r very S mall pipes. Another consideration in fil u ? mg the size of pipe is that the larger the pipe the " reader" will be the total tonnage passed before ^ replacement elTln'le^raVTl ^ ^T^ ^-dTould suit n less wear 1 he pipe size, however, cannot be so partoj the friction head, because too great a velocitv an, The 3-inch line at the Dayroek mine has not been ^"dedfar enough nor been used long enough to de ter nillle lts characteristics and effectiveness. Hydraulic Filling in Metal Mines 13 Tnhle '/■ Details of sonic open-pipe gravity transportation systems ("mainline" or shaft piping only ) Location Kind of pipe Pipe size (in inches) Maximum elevation head (in feet) Horizontal to vertical ratio Percent solids a Dry tons per hour (average) Screen analyses percent Tonnage passed Resultant wear + 65 —200 steel rubber-lined rubber-lined rubber-lined cast-iron' . _ 4 6 6 2H 6 870 1200 2300 d 1700 3400 i 7.4:1 1' 2.5'' 3.0:l(g) 2.7:l(k) 70-75 60 65 50 50-55 160 125 100 35 17 10 3 49 nil 30 20 44 e 3 30 500,000>\ ■ 3,000.000 500,000 little to date „ Home-stake. - f h Gt. Butte Project " a Measurements taken at agitator or mixer — density along lines varies owing to entrained air. b Maximum tonnage to date. ' From four different "mixing stations" — no pipe replacement yet necessary. d Maximum uninterrupted vertical drop of 1500 feet. <• Deslimed to almost no minus 325-mesh. ' In upper part of vertical sections. Open-Pipe, Gravity Systems. An open-pipe system is so constructed that air is admitted to the pipeline, with the result that velocity probably varies along: the length of the line, and in the upper parts of vertical sections the pulp probably drops freely. In some of the systems special provision is made to allow air to enter ; in others the air is drawn in at the head of the line through the agitator of the slucing apparatus. This system has been used generally where greater tonnages are handled than by the closed-pipe systems, and where larger size pipes are used. Details of some of these systems are presented in table 4. The exact conditions under which the pulp flows in open-pipe systems is not known. The intake end of the Ilomestake line, for example, draws in air and the dis- charge is violent with sprayed pulp and intermingled air under pressure. The velocity at the discharge appears to he considerably greater than the calculated velocity for pulp alone would indicate. This effect is noticeably different from those in the closed-pipe systems where the pulp issues in a smooth, quiet stream. At Butte the volume of air taken in through the air-intake valve at the collar of the shaft was found to be 6 times the volume of pulp. The diverse conditions within the line are further evi- dent by the non-uniform wear along the line. The upper parts of vertical drops commonly show the greatest wear. 28 Irregularities in the alignment of the pipe in the sections where free falling takes place cause extreme wear. Ver- tical sections require strong support and careful blocking to check vibration. The ratio of horizontal length to elevation head is important in all gravity systems. The conditions that exist below the area of "free fall" in the open-pipe systems are probably somewhat like those in the closed pipe sys- tems, and are governed by the same relationships, i.e., density, viscosity, and friction. An outstanding example is reported from the South Mine, Broken Hill in which a pulp at 72 percent solids was run through a line in which the ratio of horizontal to vertical length was 7.4 to 1 (total length 1305 feet). The layout of lines for the (Jreater Butte project is based on the rule of allowing a ratio of 2.7 to 1. This contrasts remarkably with the former practice during the nre-nghtin»' operations when pulp - s Watermeyer, G. A., and Hoffenberg, S. N., Witwatersrand mining practice: Cold Producers' Committee, Transvaal Chamber .Mines, Johannesburg, South Africa, pp. 459-471, 1932. s Average. h No replacement necessary. 1 Experimenting with rubber lining in upper (100 feet. 1 1000 foot maximum vertical interval between "break," and vent. k Maximum allowable. n Data not available. densities of 30 percent and a ratio of 8 to 1 were used. The lines at most mines are within a ratio of 3 horizontal to 1 vertical. Open hoppers inserted in vertical lines at intervals of 300 feet have been used in South Africa and Europe in order to diminish ' ' surging, hammering, velocity, wear, and pressure." They were originally used in Butte at intervals of 1000 feet. They are a disadvantage in that they limit the head that can be developed for propelling the pulp through subsequent horizontal lengths of pipe, and they are subject to spills. In favor of the use of hoppers, the breaks in the lines at Butte are now made by the introduction of two long-shanked tees with a short horizontal piece between them, thus keeping the line closed (fig. 4). From each break a 2-inch vent-pipe, equipped with a valve, extends upward about 40 feet. Whenever the pulp backs up to one of these vents the valve is closed, otherwise it remains open and either admits or ejects air, depending probably on the relative position of the bottom of the free fall area in the pipe. Pumping Plants. The pumping of pulp for hydrau- lic filling is not unlike other pumping problems involving suspensions, except that high pulp densities are com- monly employed. Table 5 gives details of some of the pumping plants that have been used in connection with hydraulic filling. Except where raises are driven to provide all-gravity flow, some pumping, either of the mill tailing or of the prepared pulp, is required at most mines. Mount Lyell is apparently the only application to date of hydraulic filling at a mine in which all of the fills are deposited at an elevation above the level of the mill. Hydraulic filling here has proven successful, however, and costs are not excessive. Six pumps are spaced along a pipeline that delivers pulp to stopes that are as much as 10,700 feet in distance and 819 feet in elevation from the point of preparation of the pulp. All stations along the line are connected by telephone. Pumping is started with the fill material 50 percent solids and increased until any one pump is fully loaded. The pulp density varies consider- ably depending on the location of the stope and the amount of sealwater introduced by the pumps. 29 20 Horsefall, R. A., Flotation tailings for mine filling: Chem. Eng. and Min. Rev., Oct. 14, pp. 5-7, 1937. Hudspeth, G. F., Pumping tailing for mine filling at Mount I.yell : Chem. Eng. and Min. Rev., vol. 31, pp. 414-417, 1939. 14 Special Report 12 6" Pipe FlGURK 4. Velocity break for vertical sections in an open-pipe transportation system. Pipe and Fittings. The kind of pipe used at any installation is governed chiefly by the cost, total tonnage to be passed, and the abrasive resistance that the pipe exhibits toward the particular pulp. High slime content, high pulp density, and low velocity tend to minimize abrasion. Whether rubber-lined pipe should be used at a given nunc is important because of the cost of the pipe. From the analyses presented in tables 3 and 4, it is evident that most of the installations of rubber-lined pipe have been m the open-pipe systems. In addition, rubber-lined pipe lias been used at Mufulira where it was employed for distribution of the pulp from boreholes, and rubber hose was used at the Sliger mine. In all of the applications where rnbber-lined pipe is used, except perhaps at the Frood, various other kinds of pipe, including all grades of iron and steel were first tried, but were found to wear rapidly. In contrast to these results, the satisfactory applica- tion of steel pipe at certain mines is conspicuous', although it should be noted that at no plant using steel pipe has the total tonnage passed been great when compared to some of the others. Possibly for long life and great ton- nage, a rubber-lined pipe should be installed in gravity operations regardless of whether an open-pipe or 'closed- pipe type of transportation system is used. The choice of pipe to use in surface pumping plants where the velocity is adequately controlled, or in under- ground distribution laterals, which will not carry so great a total tonnage, is not difficult. Table 5 indicates the kinds of pipe that are commonly used in pumping plants. Any kind of used pipe or other inexpensive, readily available pipe is generally used for underground laterals. The laterals from the cast iron shaft-piping at Butte are wrought iron— a combination that has also been used extensively in the Pennsylvania coal fields and in Europe. Flange connections are generally used with the larger pipe sizes and with the rubber-lined pipe. Smaller lines usually are joined by some type of butt connector; threaded couplings causes turbulence and undue abrasion! Victaulic couplings are preferred at many mines, because less wear results and they are easy to break for inspection to connect to different laterals, or to disconnect to unpluj? the line. Valves are used sparingly, if at all. They wear rapidly and are always a possible cause of plugging in the line. 1 he flow of pulp is usually directed from one line to an- other by reconnecting the lines. Where the pipeline is too heavy or rigidly held, disks are inserted in the line (as at Homestake) or valves may be necessary. Cone or plug valves are best; gate valves are nearly worthless Any device for prohibiting the flow of pulp through a lateral must be located near the main line and not at the discharge end of the lateral. Reductions in size .along the line are common in the open-pipe systems. They are not common— indeed they defeat the purpose— in closed-pipe systems. Any but slight reduction or restriction may cause plugging in a finely adjusted, high pulp density, closed-pipe system. Ninety-degree elbows are not used in pipelines for hydraulic filling; abrasion would be excessive. Curves of long radius are preferred, but a series of small-angle elbows is commonly used. Where space is limited, 90- degree turns have been made by using long-shanked, Location II. .1,1.,, Mt Lyell Table 5. Details of some pumping plants Kind of pipe wood, wood. wood. Size of pipe (in inches) concrete Bteel . I il 8 8 8 Elevation head (in feet) 125 b b 819* 45 Horizontal distance (in feet) 3,300 2,000 variable 1 0,700 e 11,000 ^nteair before entry into the gravity system.,. 1 M 'Milium Velocity (feet per second) 6-6.5 .7-8.3 Percent solids 45-50 60 50-55 47-55 •> 50 Dry tons per hours (average) 50 125 28 500 Screen analysis —200 (percent passing) 35 20 25 39 Gallons per minute (average) 420 310 1,900 Connected horse- power .SO 50 460 Tonnage passed 325,000 little 500,000 Wear slight 1/16" maximum 932,000 negligible w;it.i^iddiUoTT~ VarieS W " h l0Ca,i ° n ° f St °" e; " ls0 VarieS along line becaust ' of " s<;al - " Smith Africa. " Data not available. Hydraulic Filling in Metal Mines 15 rubber-lined tees. Tbe sand in the stopped shank acts as a cushion for tbe impinging pulp. Other devices some- times used at curves include rubber hose, lengths of curved, rubber-lined pipe, or renewable wear plates in- side of tbe pipe. Location and Layout of the Pipeline. Tbe pipeline is ideally located where spills resulting from a broken line will not restrict the use of a shaft or drift, yet where it is easily accessible for repairs and unplugging. Prac- tically, these conditions are rare. Good alignment of the pipe with a minimum of irregularities promotes long life. This is increasingly im- portant with higher velocities. A constant grade of the laterals is not necessary to How of the pulp, but has occasionally proven helpful in reopening plugged lines. No difficulty has been experienced in extending the discharge end of the line upward to reach stopes above the level of the lateral. Heights of more than 75 feet are common. Operation of the Pipeline. Reliable operation of a pipeline used for hydraulic filling requires standardized procedures and constant vigilance against plugging and leakage. Other than by improper preparation of the pulp, plugging of the line may occasionally be caused by such things as the inadvertent operation of valves or blank disks, by allowing the discharge end of the line to become buried in the fill, or by improper flushing of the line. At all hydraulic-filling operations the line is flushed by clear water after running the pulp, and the admission of pulp is usually preceded by clear water. Plugging of the line is rare after the best operating conditions of a new system have once been determined and put into practice. The location of plugs in a line can usually be found by tapping with a hammer. Unplugging necessitates opening the line at intervals and allowing the head of material behind the plug to force it out. The application of water or of water and compressed air is sometimes required. Air alone dries out the material and is not helpful. Wooden pipes are easily unplugged by boring boles and injecting water. The shorter the time a line is plugged, the easier it is to unplug. A good communication system is an indispensible aid to smooth operation. Telephones are sometimes placed in the stopes to provide the quick communication neces- sary to minimize the etfect of spills and to synchronize operations along the line. Compressed air can be introduced into a pipeline as a means of increasing the effective radius of a system in which the head is limited, without diluting the pulp. This has been done at the Emma and Travonna sand-filling projects in Butte, at Hodbarrow, and at Mufulira. The air should be so injected that a stream of pulp is not directed against the opposite sidewall of the pipe. At Mufulira 72 cubic feet of air per ton of solids in a 4-inch line will extend the effective radius of the system 1200 feet. XwvV| V,AVX// /^v^-// an, \//^\\/f /^W /^\ 7 aW// A.V \//A. v nX//A^V-V^ V/.\\V/A\V /AV/ANV/^N^-^j; ^/AvvM a. v yy a v y//A \>/a \ V/Vn n.v/ as \//a s v/ a\v/a. \ v/av v/avv/axv/a v vwy /An y/Avy/Av y/aaw^ v/ a, WA\yAsyy a w/ana View in plane of vein Stopes A and B: Prepared for filling as one unit Stope C -. Cut being mined by "breasting down" Stope D: Filled FlOUliK 5. Horizontal cut-aml-fill .storing. Filling to back. Hi Special Report 12 ---~-^y-^^A^A^^^ Section Z-Z /Mwav^ V /^WMV^vN/^ V ^y /M v^v M , V/ , N V/ ^ N N , K V/KV/Kv/Kxv ^ v/ ^ v/< ^ A\N View in plane of vein Stope A: Lower cut prepared for filling Stope B: Lower cut filled. Next cut being mined by "blasting the back" Stopes C and D : Next cut being mined by "breasting down " Figure C. Horizontal cut-arid-fill stoping. Two cuts open. Boreholes. Boreholes have been used mostly at the mines on the Witwatersrand where they convey the pulp to the underground launder systems, and by suitable location eliminate the necessity of underground pumping In other localities boreholes have found some use in con- nection with regular underground pipeline systems, or to introduce material into stopes. Starting diameters of 4 to 12 inches are recorded and depths are as great as 3600 feet. The boreholes in South Africa do not require lining. In other places, where the rock is friable, lining has been necessarv. Reinforced concrete was used at Mufulira for lining after steel proved yi'vy short-lived. In South Africa boreholes have passed large tonnages with no maintenance being required. They have the ad- vantage that they do not disturb the shaft with breaks However unless they are large enough and in suitable ground, they are subject to plugging and mav be expensive to reopen. Launders. As an adjunct to hydraulic filling, laun- ders have been used extensively only on the Witwaters- '•;"" • ilH'.y are particularly adapted to certain mines on * "' Rand because of the moderate dip of the reef which allows the use of open fluming for underground distri- """;«•' the pulp. Launder installations on grades of more than degrees are not uncommon, and on grades as high as 11 degrees have been satisfactory. At the out- crop mines launders have been used for transportation "' eneral methods. Where the fill is dewatered chiefly by percolation, filter-bulkheads must be constructed and the workings beneath the fill may be- come very wet. Where other methods of dewatering can be used, these disadvantages are eliminated/but prepara- tions for draining are more elaborate. Perforated drain pipes or special perforated closed launders are used in dewatering by the run-off method. To dewater by decantation it is necessary to provide only some means of raising the level of the overflow as the level of the fill and the surface of the pond rise. The method chosen to accomplish this is commonly the simplest or most convenient, e.g., providing slots or chopping holes in the bulkheads and later closing them over, con- structing the bulkhead as the level of the fill rises, plug- ging cracks between the planks in the bulkhead, adding the upward extension of a drainpipe, or plugging holes in a vertical drainpipe (see fig. 9) . The pulp is distributed in a stope from pipes, hoses, or launders. In general, it should be introduced as far from the drainage outlet as possible. Long stopes mav necessitate occasional shifting of the pulp stream to pro- hibit segregation of the slimes. Where the stope is to be filled to the back, a light trestle must be erected to carry the pipeline to the far end of the stope. At times during the filling operations some of the fill material may escape through fissures in the rock or through poorly calked bulkheads. This sometimes can be stopped by depositing the incoming pulp directly over Hydraulic Filling- in Metal Mines 19 "___ Slush er drift fS3 Ore-pass Raise Haulage level 100 FEET Bulkhead Figure 10. Typical open stope. the point of leakage and providing special drainage away from the particular locality. The sands so deposited will usually form a filter and quickly stop further loss. Ac- tually, the filling of fissures in the adjacent rock accounts for part of the effectiveness of hydraulic fills, and, if the fissures are not a part of an open system through which the material is conveyed to other workings, such filling is desirable. Where the hanging wall is very badly broken an ef- fective seal over a drift may necessitate leaving a small pillar. Recovery of such pillars was facilitated at the Sliger mine by the addition of Portland cement to the pulp that formed the first layer of fill placed above the pillar. Hydraulic filling as applied in cut-and-fill stoping permits flexible mining operations. The fill may be raised to the back and the ore broken by breasting down or, if the ore is strong enough and the vein not too wide, the fill may be placed within 7 or 8 feet of the back and the whole cut mined by stoper drilling. Both of these methods have been used with outstanding results at mines in which square-setting had been required prior to the introduc- tion of hydraulic tilling. The ready availability of fill material in a mine equipped with a hydraulic filling sys- tem permits rapid and effective measures to be taken when changing conditions within the stope necessitate departures from routine stoping. Where hydraulic filling is used, the rate of mining is increased because of the rapidity with which fill material is deposited and because of the reduced amount of timber to be handled. However, filling usually interrupts the mining operation ; ordinarily the mining is resumed the day following filling. Working on the fill during the same shift in which it is poured, however, is practiced at the Dayrock mine. Scrapers are commonly used directly on a hydraulic fill without laying floors. Filling Large, Open Stopes. In some mines it is necessary to fill large, open stopes. Although the rock strength permits mining by open-stoping or by shrinking at these mines, the stope must finally be filled in order to assist the recovery of pillars, in the stabilization of the mine, or in the control of surface subsidence. Hydraulic filling has been beneficial in such mines. The bulkheads to contain the fill material are constructed in the draw- points and in the sub-level entrances. Adequate dewater- ing of the fill to prevent excessive pressures is the most important consideration. no Special Repokt 12 Sand filling was adapted at the Mufulira mine for support of the hanging wail in order to prevent flooding of the mine by caving into the overlying water-bearing formation, to prevent air blasts, and to attain predictable rates of ore extraction. The orebody consists f three superimposed, ore-bearing strata that range from 10 t.» • hydraulic fill was deposited. " A "'" il; " ''' employing burlap was unsuccessful at Home- The filling operations at Ilolden and Ilomestake also are of this type. At both of these mines fills of large vol- umes are deposited in a more or less continuous operation In contrast to Mufulira. dewatering is done chiefly by decantation and secondarily by percolation ; therefore, the bulkheads are constructed watertight, although they usu- ally contain a drain pipe with a valve. The drain pipe provides for some dewatering, is helpful in estimating the condition of the fill, and provides a means of final drain- age or drainage for stray water. Most bulkheads at Ilolden are constructed of con- crete, usually about 3 feet thick, are well hitched into the surrounding rock, and are reinforced with old drill steel The pressure for which to design such bulkheads is one ot the most enigmatic of hydraulic-filling problems A liberal safety factor is always used. Shrinkage of the con- crete sometimes causes leakage around the bulkheads but this is stopped by gunniting. The pulp is introduced into the stope from points near the back. Unless the discharge is shifted occasionally . cones of sand will build up and segregation of slimes re- sults. Much of the water is drained off through fissures and cracks in the footwall, either from the pond on top of the fill or after some downward percolation through the fill. Wherever possible, water is decanted through the sub-levels. If the pool of water becomes too deep, a sink- ing-pump is lowered into the stope and is employed in decanting some of the water. A vertical drainage launder has been tried, but the long line and manv curves caused the hue to plug easily. Pour months after completion of filling, only slight drainage was noticeable. The fissures surrounding a fill serve effectively as a means of dewatering. When the fill material first 'enters a fissure in the country-rock, all sizes of particles <>o through. However, as the height of the fill is raised above the entrance to the fissure, the velocity of the stream is checked, which causes the sands to settle and act as a filter for the slimes. Thus the fissures are filled, in part at least, so that they contribute to the support of the mine and assist materially in dewatering the fill or in removing other water. At the Ilomestake mine the methods of mining in- volve two kinds of stopes that need different fillin* methods. The ore is localized along the minor folds that parallel the axis of the plunging anticline along which the folds occur. The orebody is mined by a series of vertical shrinkage stopes, 60 feet wide, alternating with pillars 40 feet wide. The long axis of the stopes and pillars is trans- verse to the axis of the anticline. The average elevation of each stope is less than the previous one as mining pro- gresses downward along the length of the anticline It is the shrinkage stopes that are considered here under the heading of filling large open stopes. The vertical height of the ore usually exceeds 150 leet, which is the level-interval. To eliminate the neces- sity ot a level-pillar, each stope is mined from the lower >m.t of the ore upward to a height 25 feet above the next level. After the ore is drawn, fill is deposited to the height of the level only, and as soon as it is sufficientlv consoli- dated (about two weeks at least), a line of timber is con- structed on the fill. Then the fill is carried to the back some of the fill is drawn out to form hoppers above the Hydraulic Filling" in Metal Mines 21 Figure 11. Effect of filling a stupe without construction of bulkhead in adjoining drift. chutes in the line of timber, and the stope is ready to shrink to the level above. Thus the shrinkage stopes, for the purposes of hydraulic-filling, are 60 feet wide, some- what less than 150 feet high, and as much as 300 feet in length. These stopes have sub-level access from the pillars at vertical intervals of 20 feet. These entrances, as well as the openings at the bottom, are closed by wooden bulk- heads. A typical sub-level bulkhead is constructed of 12- by 12-inch timbers laid together horizontally and braced with a single vertical 12-by 12-inch timber at their cen- ters. Each bulkhead is hitched in cement and covered with a layer of burlap to retain the fill material in case the timbers do not fit together well. A drain pipe and valve are usually provided. Wooden bulkheads are preferred to concrete because they are not as easily broken by moving ground. Dewatering is accomplished chiefly by decantation at the sub-levels. Several irregularities in past filling practice serve to illustrate the flexibility of these filling methods as em- ployed at the Homestake mine. A fill has been deposited, owing to lack of sub-level openings, for a full height of 150 feet in a single operation without special provision for dewatering except by decantation on the top level. The fill apparently set as compactly as in better drained stopes. In dead-end stopes with no access above the level on which the ore was drawn, decantation pipes extending upward, one for half the height of the stope and the other for the full height, have been successfully used for de- watering. The more commonly used method, however, is to construct a timbered chimney which is an extension of a line of square-sets one above the other, and the whole enclosed with suitable lacing. The chimney is raised three sets at a time as the fill level rises. Continuous decanta- tion is accomplished through slots in the lacing. This method provides good drainage and better observation of the fill, but may be dangerous unless the chimney is strongly constructed. Another fill was deposited in a certain stope that would ordinarily require one bulkhead. However, no bulk- head at all was constructed (fig. 11). The fill material, introduced through the back of the stope, built up in the drift at an angle of repose of about 10 or 12 degrees until it blocked the drift. Thereafter, that block of fill material in the drift exerted the only constraining influence on the remainder of the fill, which was carried to a height of nearly 100 feet. Water filtered slowly through the fill into the drift. Complete Filling of Parts of a Mine. Hydraulic rilling has been used in some mines to fill completely cer- tain parts of the mine. The pulp is introduced into the area at the bottom of the fill; or, as the fill progresses upward, at the lowest level at which the pulp will flow. Thus, the pulp, after being injected behind the bulkheads, Hows upward through material already in place. This method of filling has been used chiefly in fight- ing fires. It was used at Butte, Tintic, and Cerro de Pasco ; it is now being used at Butte to prepare parts of the mines for mining by caving. To prepare an area for filling, all entrances to the area are closed by suitable bulkheads. Figure 14 shows the kind of bulkheads constructed for this purpose at Butte and Cerro de Pasco Delivery launder^ y/A\V/AxV/AVv/^\V/ANV/A%V/XxV/^\V//^\V/^-,V/ A\V//.sV/A\V/A\V/AW/A\V/ Bulkhead Drive A. Stope mined out, then filled Delivery launder v/a\V//,w/a^«\V/a\V;a \ViaW;a\V/a\V// ,W/,<\y/AV)-,-,ck and ,n previously placed waste fill are completely "U"'l ; an effeel that is very significant in fire fighting \ Becond important characteristic of this system of filling is that the fill is placed tightly against the back regardfess of large differences in elevation along the back. Figure 15 illustrates the principle involved. If the pulp is intro- duced from above, it will not rise and fill cavities beyond a downward projecting obstruction, as in A. Where the pulp is introduced from below, as in B, the fill material carries upward directly to the back, and voids are left only where there is no escape for entrapped air. 33 Other Examples of Hydraulic Filling Practices. The methods of hydraulic filling practiced on the Wit- watersrand are unique among metal-mines. This is be- cause of the low dip of the reefs and the relatively coarse nature of the fill material. Details of the many methods and modifications that are employed in hydraulic filling m South Africa are found in abundance in the technical literature. Only enough data are presented here to afford general comparisons with the methods that are more familiar in this country. Q t .,„,i " ^ anson ' ? arl F -' Con trol of underground mine fires at Tintic .•standard mine : Am. Inst. Mln. Met. Eng. Trans., vol. 126, 12 pp., 1937. ti^o . M? Cutchen . Y; L., Mining methods at the Cerro de Pasco proper- ties . Mining and Metallurgy, vol. 26, no. 4(i7, pp. 521-523. 1SI45. \i at ^ M ^ y ' H ' J -, Min e fires and hydraulic filling: Am. Inst. Min. Met. Eng. Trans., vol. 68, 12 pp., 1923. Hydraulic Filling in Metal Mines 23 Plan View Elevation View Figure 14. Concrete bulkhead. Sand filling has long: been used in South Africa for many purposes, including the recovery of pillars, the sup- port of heavy ground during stoping, the support of over- lying reefs, and the protection of shafts and drives. This discussion is confined to the filling methods used within the stopes. The common method of mining is by advancing a long-wall face that is continuous between the drives. Figure 12 shows two such stopes. Hydraulic fills are em- ployed either to fill mined-out stopes, as in A, or to provide support during current stoping, as in B. The predominant type of bulkheading consists of a row of vertical posts to which split lagging arc attached horizontally. In addi- tion, wire rope or a large-mesh wire screen is used to support a layer of coconut matting. 34 Other devices have been used for filtering and many variations have been described. The pulp is distributed to the stopes by launders. Periods of filling in any particular stope are alternated with periods of draining. Dewatering is accomplished principally by percolation through the fill. Where the fill is bounded more by rock than by filter-bulkheads, drainage launders are sometimes used. These are simply ordinary launders covered with slats and coconut matting. As in other large fills, the location of the incoming pulp must be changed occasionally to prevent undue segre- gation. Hydraulic filling has not been used on the Rand as much in the last couple of decades as it had previously. The average depth of mining is now great and not only the strength of the final support but also the time relations in placing it in position are important factors in deter- mining the best kind of support to employ. The greatest disadvantage of hydraulic filling is that it is too cumber- some to handle close to the advancing, working face. The fill cannot be placed quickly enough nor cheaply enough. Other disadvantages that have been mentioned include the difficulty of maintaining supply lines of such great length, heavy pressures collapse bulkheads before fill can be placed, pumping costs increase with depth, amount of fines have increased in recent years, and the difficulty of transporting the large tonnage. According to Jeppe 3r> the economic limit for hydraulic filling is 5000 feet of depth. It is generally agreed that the chief value of hydraulic filling on the Witwatersrand is in stoped-out areas in the shallower workings preparatory to mining of pillars or of hanging- wall reefs, and also for the protection of shafts and drives. Hydraulic filling is presently being used mostly on the Far East Rand (where the dip averages only 8 degrees), but there has been some revived interest throughout the Rand since the War. As a final example of the variety and flexibility of hydraulic-filling methods, the system employed in min- ing the pillars at Homestake is illustrative. The pillars arc vertical, tabular bodies, sometimes several hundred feet in length, about 300 feet in height, and 40 feet wide. They are bounded on both sides by the hydraulic fill pre- viously placed in the former shrinkage stopes. The pillars are mined from the top down in successively lower lifts of 75 feet, by overhand, square-set stoping. The mining is accomplished by stoping out a series of panels, and filling each panel before mining the next. A panel is seven sets wide between the hydraulic fill on the sides, and only three sets wide along the long axis of the pillar. A panel therefore, is 75 feet high and only 7-by-3 sets in hori- zontal area. As a consequence of this method of mining, each panel of square-sets is mined upward to the full ■" The bulkheads used at Hodbarrow and at the Emma and Tra- vonna mines are similar, being wide-spaced lagging covered with burlap. :t "'.Ieppe. ('. B., Cold mining on the Witwatersrand: Transvaal Chamber of Mines, vol. 1, pp. 814-82(1, l'J4C. ( 24 Special Rkport 12 Figure 1 5. Filling against an irregular back. height of 75 feet with hydraulic fill on three sides Oce-, Before filling a square-set panel the side that faces the unbroken ore is laced off with 4-inch ro e an , or access durfn"fi J r ng "'* °^ Tllis forms a — « ' 101 access during filling operations and subsequent mi.i porting the fill material is earned to the top of the panel Also on one side of the manway, the slots that are used .-n a -on during filling are exposed. The "v h-lim ■ ; n it i, ,1S I"'V af " ty t0 the °P erator s d^ing intn^^tt^^t^i^T 'T " ' 0Cated S0 as to slots as po b i,r S far h '° ,U the Recantation are sealer! t,v .. v 1 " ' ' a,ul t,1( lower ones "« ''""" **■> -^wiSi.'STrr water introduced i, ti 1,,( ' amount of «««« n V-^^a!JlS^S Cr the system U '""" 1 " < " 1(l tlu ' amount of slimes in the fill material. By the time the water reaches the sump it ma* or may not sti 1 carry the same amount of olYd's Gen erally, enlarged settling chambers at the sump are con structed .here hydraulic filling is employe d and the additional wear on pumps is not noticeable It is no known that special solids handling pumps have eer been required because of hydraulic filling ™ The water that drains from a fill commonly runs to -«. sump through the regular mine ditching system Ordinardy the ditches must be of somewhat larVTcr OSS decantation into this lower section of line C, * " m keeping the mine clean arTotm^t ^topTS be conveniently located for this system to be used with™ tain nnT°-' W M D ^ culties - Hydraulic filling has cer- tain undesirable features, and some inconveniences ami delays will probably always attend its US(? meniences and Besides pipeline troubles and the necessity for ade quate preparation of stopes, occasional spill" whi^h are" evidently almost unavoidable, and increased slonnines* '» openings below filled areas are the moTtrouble one consequences of hydraulic filling. The ^ ™£*Z bio.ken lines, broken bulkheads, and runs of material ri'e ( b SSUr T- iU ,! he r ° Ck - S P ills can --tl b mucked by machine, but often hand muckin- is reauirerl vhjch is a difficult job with the semi-draLKe^maS 1 ml Some : operators suggest that a spare stope be Lntin readiness below the stope being filled so that Sie pulpmav be directed into it if trouble develops P ' irl t f Un f '■ m Pipes ° r ditches ^ best wm r most of the dewatenng is accomplished by percola dam the d.tch causing overflow into the level, depositing Costs Cost data for hydraulic filling are difficult to obtain and to correlate. Most published data are old and most y;P;'bl..shed data of present operations are rest it e d further the figures that are available are seldom com parable because they differ in what they measure "n< are based on different units. The cost figure ^4 wHhin wan.llteT "* °°* ^ ""* ^^ «»S5o^S smal| T flfl! < ? t ! )e V° n f fi " deposited at mines that place small fills ls hlgher than at . j «*< wh »»' «* P^ced. The cost is lowest per L "where laS Hydraulic Filling in* Metal Mixes 27) Table 6. Costs of hydraulic filling* Small fills Large fills Date Cost in dollars per ton Date Cost in dollars per ton 1938 -- 0.89 0.71 0.42 0.33 0.32 0.29 0.26 0.25 0.24 0.20 1938 1944 0.27 1943 0. 11 1944 . . _ 1949 ._ ... 1948 ... 1934 1945 1930 1937 1946 • These costs are each from a different mine. The year is given to establish the general level of economic conditions at the time. The costs are per ton of dry fill deposited. The figures from foreign operations have heen recomputed at the official exchange rate for the country and date. volumes are handled, a minimum of bulkheading is neces- sary, gravity handling is employed, the system is run as nearly continuously as possible, current mill tailing is used, and the depth is not excessive. Direct Costs. The costs of hydraulic filling include labor, materials and supplies, capital investment and maintenance, and power or fuel. The labor involved in hydraulic filling accounted for more than 40 percent of the total cost at each of the mines investigated ; it accounted for more than 50 percent at a majority of them. Labor charges chiefly originate from surface plant oper- tion, pipeline patrol, bulkhead construction, stope filling attention, and ditch maintenance. The labor required at the surface depends on the type of equipment that must be operated. All plants must have a man readily available during the filling period in case of emergency shutdown, and most plants require the attention of one or two men continually for regular operation. The New Brunswick strict-batch agitation plant requires attention only toward the end of the period during which it is being filled and during the mixing period prior to discharge of the tank. Most other agitator plants require some attention during the complete cycle. A pipeline patrol is maintained at two mines where abrasion has caused frequent leaks, but wherever pipe- line leakage is less common and where a suitable com- munication system is in use such a patrol is not used. This cost, therefore, is applicable only in certain mines. Stope preparation, including bulkhead construction, is often the largest single labor expense, being frequently over half of the total cost for labor. Customarily, special crews perform all the work of preparing the stopes. The smaller type of fills require one or two men in the stope to control dewatering, guard against leaks, shift the discharge occasionally, and watch the general progress of the fill. Larger fills do not require constant attention in the stope, but do require constant patrol of the area surrounding the fill and of bulkheads for the discovery of spills, leaks, or weak structures. Increased mucking of ditches and spills was shown to be one of the consequences of hydraulic filling. The charges involved in this are sometimes a substantial part of the labor cost. The second largest portion of the cost of hydraulic filling is usually for the materials and supplies required. Most of this is for material used in bulkhead construction and in sealing the stopes or constructing filtering devices, e.g., matting, burlap, wire mesh, excelsior, cement, etc.; other minor costs are for miscellaneous items such as flocculating agents and lubricants. Because of the cost of burlap and some of the other filtering media, methods of dewatering in which they are not used are sometimes preferable, especially in this country. For example, the Homestake mine has reduced the cost of filling the square- set, pillar stopes by substituting tongue-and-groove lacing for flat-edged planking and burlap. The cost of power that may be charged to hydraulic filling is, in none of the mines for which data are available, more than 25 percent of the total cost ; it is usually less than 15 percent. This includes the power required in returning the excess water to the surface. The costs for equipment, installation, and mainte- nance, or the capital charges for these, have been re- ported, if at all, in such widely differing units that correlation is impossible. The two principal items of capital investment are the preparation plant and the equipment needed for transporting the pulp. The prepa- ration plant, depending mostly on the type of plant and whether it can be constructed locally, may constitute any- thing from a very small part to a very large part of the capital investment. The cost of the transportation system can be calculated for any given property quite accurately. The greatest saving to be investigated is in the use of ordinary steel pipe in lieu of rubber-lined pipe. The cost of installing the pipeline may be considerable, especially where large pipe is used and where elaborate precautions must be taken against vibration of the line. The 300-ton plant at the New Brunswick mine is- a notable example of low capital investment for an effective plant. The con- struction and installation of both the preparation plant ami the pipeline is reported to have cost only $7000.00. 36 The cost of hydraulic filling at several different mines, together with the year in which these costs were incurred, are given in table (J. Costs at some of these mines, as at the Homestake, have undoubtedly been somewhat reduced since the original figures were obtained. Comparative Costs. A comparison of the cost of hy- draulic filling with the cost of another method of filling at the same mine could be an illustration of the economy of hydraulic filling; however, only meager data are avail- able. Prior to the use of hydraulic filling at the Sligar mine, the cost of filling equalled the cost of mining the ore. Specific costs of the hydraulic filling are not disclosed, but listed among the economies claimed are a 50 percent reduc- tion in timber consumption and a reduction in the number of men used in filling from more than 20 to three. At the Matahambre mine the direct cost of filling was reduced from 55 cents to 2!) cents per ton of fill placed. Total savings were about one dollar per ton of ore mined. At the South Mine, Broken Hill, costs of filling were reduced during the war years and the following period of rising wages by the introduction of hydraulic filling in place of mechanically-handled sand or of waste fill. »' Krebs, Richard, and O'Donnrll, .1. C. proves satisfactory: I~"ng. and Min. Jour., \ 1949. Sand-slime stope filling >I. 150, no. l, pp. r.4-G0, 26 Special Report 12 The cost of prewar filling methods was prohibitive at the New Brunswick mine in the period following the war but the introduction of hydraulic filling permitted the resumption of mining operations. At the Central Eureka mine, mining would not have been possible following the war without the economy of hydraulic filling, which has kept the costs of mining at nearly the pre-war level. The cost of labor of filling in 1939, when waste rock was used, was nearly twice the total cost of filling in 1949. Hydraulic filling has been employed in some mines regardless of cost because of development, general mine maintenance, and tailing disposal, sometimes indicates that hydraulic filling is the most economical method although the direct filling costs per ton of fill may show no increase. Hydraulic filling, speaking broadly, is different in every application. Basic principles are the same, but modifications of the methods employed are common and consequently, details of practice vary considerably. The physical layout of the mine and particularly the character of the material utilized for filling are of paramount im- portance in determining the best practice. Some prelimi- nary experiments on the consolidation properties of the material and on the other aspects of the system prior to introduction of large-scale hydraulic filling has proved advisable. The variations and modifications possible, both in the technique of filling and in other phases of mining, are innumerable. Table 7. Metal mines employing hydraulic filling. Butte, Montana, Anaconda Copper Company : for fire fighting and fire prevention; Emma and Travonna mines; Greater Butte project. Central Eureka mine, Central Eureka Mining Company, Sutter Creek, California. Cerro de Pasco, Peru, Cerro de Pasco Copper Corporation. Dayroek mine, Dayroek Mines Incorporated, Wallace, Idaho Frood-Stobie mine; International Nickel Company of Canada Lim- ited, Copper Cliff, Ontario. Great Boulder mine, Great Boulder Proprietary Gold Mines, Limited Finiston, AVest Australia. Hod harrow mines, Hodharrow Mining Company, Limited, Millon, Great Britain. Not operating at present. Holden mine. Hose Sound Company, Chelan Division, Holden, Wash- ington. Homestake mine, Homestake Mining Company, Lead, South Dakota Matahambre mine, Minas de Matahamhre, Matahambre, Pinar del Rio, Cuba. Mount Lyell, Tasmania, Mount Lyell Mining and Railway Company Limited: Royal Tharsia mine; North Lyell mine; Crown Lveli mine. Mufulira mine, Mufulira Copper Mines, Limited, Mufulira, Northern Rodesia. New Brunswick .e, Idaho-Maryland Mines Corporation Crass > alley, California. Philippeville iron mines, Philippeville, Algeria Sliger mine. Middle Fork Gold Mining Company, Georgetown, Cali- fornia. Mining has not been resumed since the war South mine, Broken Hill South Limited, Broken Hill New South Wales. Triton mine, Reedy, West Australia. Not operating nited Verde mine, Phelps Dodge Corporation, Jerome, Arizona W ilwatersrand, South Africa, many mines. Although some other method of filling may be pref- erable at some mines, hydraulic filling has wide applica- tion for many kinds of mining and for many different purposes. It has been used in coal mining, ferrous and non-ferrous metal mining, and undoubtedly could be em ployed in the mining of nonmetallics. It has been used ii steep veins, narrow seams, wide lodes, massive ore bodies and in bedded deposits. 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Paper 1978 1946 " " Am - Inst Min " - *»*Wrsft£cw3-,a - ....aJSTS" " x '""' " " '■"■"■ »" s Mae., v'«i. 37i .Srs;-^,™'3 1 i ' * rt "- ,!B *» t , Ta | U i n |A f " r ^ tope filIing at the Frood-Stobie mine- Enir Min Jour., vol. 150, no. 5, pp. 88-89, 1949. s ' ln Thoenen, J. R., Sand and gravel excavation • II S r„p vr Inf. Circ. 6875, pp. 211-224 1936 "" at,on • U. b. Bur. Mines, o. ,JSr„t ,t,,^;;; i , ,:*°;;i,, , ' I , u ' « j '-^. w. L , F ,„. 022-629, lira. ' ' ■ '"'• '•'"••• a ""- »»'• 31, no. 5, p„. sa u sri £ k s- - v rs m r sr'S lines. SJr Tecl"''" ^^ a » s P-ation of suspended solids in pi pe Eng., Tech. Paper 1785, 1945: "' ' P " 194 °' Am ' InSt Min ' Mrt - Jou,,^ S 143,uI™ P !i 9 S hen deSignInS laUDderS: Eng - Mta. 4(i::07 4-51 2M prill/ej ,,, CALItORN IA STATE PRINtlNI. OFFICE