[Reprinted from PAPER, October 30, 1918] 
 
 GUIDE TO THE BLEACHING OF PULP AND PAPER 
 
 Processes for the Bleaching of Vegetable Fibers in the 
 Pulp and Paper Mill 
 
 By JAMES BEVERIDGE 
 Author of The Papermakers' Pocket Book 
 
 |HE bleaching of vegetable fibers for 
 papermaking has become in recent 
 years not only an important but a 
 widespread operation on the Amer- 
 ican Continent, and for obvious rea- 
 sons. Pulpmakers especially are 
 adopting bleaching methods in order 
 to enhance the value of their product, 
 thereby reaping an additional profit. 
 It is therefore important that the different methods 
 of bleaching fibers, whether these be derived from the 
 acid or alkaline processes of manufacture, should be 
 studied carefully to assure maximum economy not 
 alone in the bleaching agent used, but, also, in all the 
 factors involved in the operation as a whole. These 
 factors so far as the bleaching operation itself is con- 
 cerned are controlled largely by the character and 
 purity of the fiber treated. Thus in the case of linen 
 and cotton rag stock, the bleaching process is reduced 
 to its simplest terms, in virtue of the comparative 
 freedom of such stock from those foreign vegetable 
 substances, which are acted upon and rendered soluble 
 by the oxidizing or bleaching agent. 
 
 In the case of wood and other pulps such as straw, 
 esparto, bamboo, bagasse, etc., the presence of for- 
 eign vegetable matter other than cellulose in some- 
 what larger quantity necessitates a more drastic treat- 
 ment, resulting in a larger consumption of bleaching 
 agent per unit of pulp treated and a greater propor- 
 tionate loss of weight. In point of fact, these two 
 factors viz. : the consumption of bleaching agent and 
 loss of weight other conditions of the treatment be- 
 ing kept normal or nearly so are a measure of the 
 quality or purity of the pulp itself. This is under- 
 stood by all practical pulpmakers and their constant 
 aim is, or should be, so to adjust their manufacturing 
 conditions that they can obtain a product which will 
 conform to a minimum consumption of bleaching 
 agent and minimum loss of weight. 
 
 445,160 
 
The bleaching properties of a pulp depend upon 
 the process by which it is prepared. As a general rule 
 vegetable fibers prepared by the acid or sulphite pro- 
 cess bleach more readily than those prepared by either 
 of the alkaline methods, that is by either the "caustic 
 soda" or "sulphate" methods . In all cases, no matter 
 what process is in use, substances other than cellulose, 
 as above stated, are left behind with the fiber, and it 
 is these substances that are acted on by the bleaching 
 agent and rendered soluble. It may be stated here 
 that, of the three acid solutions viz. : bisulphite of 
 lime, magnesia and soda, it has been found in actual 
 manufacturing practice, those of magnesia or soda 
 yield a purer cellulose than that obtained with lime. 
 And no doubt, this is due to the fact that the sulphur 
 salts of magnesia and soda are much more soluble 
 than those of lime. 
 
 The quality of the water used for washing the crude 
 pulp has also an influence on the bleaching process, 
 especially if the water contains lime in any form. In 
 the sulphite process, the lime salts precipitate insoluble 
 resin soaps on the surface of the fiber, which absorb 
 chlorine in proportion to^ their presence, and in order 
 to avoid such precipitation calcareous water previously 
 heated to boiling, or otherwise chemically treated for 
 the removal of the lime and filtered, is frequently used. 
 A similar precipitation of lime salts takes place when 
 calcareous water is used for washing the pulp prepared 
 by the alkaline process, the lime itself being precip- 
 itated either in the form of carbonate or sulphate by 
 the alkali present, both of which cling to the fiber and 
 carry down with them coloring organic matter, ren- 
 dering the process of bleaching even more difficult 
 and costly than is the case with sulphite pulps. 
 
 The loss of weight in bleaching pulp, together with 
 the cost, depend on many factors, the most important 
 of which may be enumerated as follows : 
 
 i The raw fibrous plant from which the pulp is 
 prepared: Spruce, hemlock, balsam, pine and others, 
 among the conifers; poplar, maple, birch, and others 
 of the broadleafed trees; straw, esparto, bamboo, 
 bagasse, etc., among the grasses and canes. 
 
 2 The process employed for manufacturing the 
 cellulose or fiber, whether this be acid or alkaline, i. e., 
 soda or sulphite process. 
 
 3 The purity of the pulp obtained, controlled large^ 
 ly by the conditions under which the fiber is prepared, 
 such as the amount, character and composition of the 
 resolving chemicals used; the temperature employed; 
 
and the time given to complete the process, together 
 with the purity of the water used for washing. 
 
 4 The dilution of the pulp with water, or density 
 of the stock, during the bleaching operation, which in- 
 sures a more intimate and closer contact of the bleach- 
 ing agent with the fiber. 
 
 5 The temperature at which the bleaching is car- 
 ried out, chiefly with a view to accelerate the chemical 
 action between the chlorine and the coloring matter. 
 
 6 The time allowed for bleaching, controlled by the 
 temperature and density of the stock under treatment. 
 
 With regard to these factors, it may be stated in 
 general terms, first that it has been found in manu- 
 facturing practice, the greater the yield of fiber from 
 unit weight of raw fibrous plant, no matter by what 
 process this fiber has been made, the greater is the 
 loss of weight of pulp operated upon, and the amount 
 of bleaching agent required ; and, second, that the dilu- 
 tion of the fiber with water, or density of the stock, 
 and the temperature employed for bleaching are most 
 important since the first manifestly results in great 
 economy of steam and the other in economy of bleach. 
 
 The oxidizing material universally employed in the 
 pulp industry is chlorine in the form of bleaching pow- 
 der (calcium hypochlorite) or an electrolyzed solution 
 of common salt, for chlorine is unquestionably the 
 cheapest bleaching agent known. Ozone has been ap- 
 plied, but failed to compete with chlorine on account 
 of its cost; and although permanganates may in spe- 
 cial cases be used to obtain a greater degree of white- 
 ness than can be obtained with hypochlorites alone, 
 yet, the use of a permanganate is scarcely admissible 
 in the pulp and paper industry on account of the diffi- 
 culty of removing the last traces of manganese from 
 the fiber. Hydrogen peroxide is out of the question at 
 the moment, owing to its cost. 
 
 In the older methods of bleaching, the hollander 
 with a paddle wheel to throw the pulp over the back- 
 fall instead of a roll, and sometimes called a "pocher," 
 was used, the fiber being allowed to circulate either 
 cold or hot until the required degree of whiteness was 
 attained. The stock at first was washed in the hol- 
 lander after bleaching, with drum washers, the wash- 
 ings containing the excess bleach being thrown away. 
 Or, instead of washing, a quantity of sodium hypo- 
 sulphite was added to destroy the excess of hypo- 
 chlorite present. This was obviously a very wasteful 
 method and a distinct improvement was the intro- 
 duction of a special draining tank, usually built of con- 
 
crete and provided with a perforated false bottom of 
 earthenware tiles, into which the pulp was emptied and 
 drained, the liquor being pumped back again to the 
 "pocher" to be mixed with a quantity of fresh un- 
 bleached pulp, so as to exhaust any available chlorine 
 it contained. These draining tanks are in use today 
 in many papermills. 
 
 A still further advance was made when a series of 
 
 open concrete tanks were put down to bleach and store 
 large quantities of pulp, the main principle being to 
 thoroughly mix the bleach liquor and pulp together 
 in a suitable hollander or "pocher," and after steaming 
 to the required temperature, running the whole charge 
 into a tank, there to remain till the fiber came up to 
 the requisite degree of whiteness, a slight excess of 
 bleach liquor being added for this purpose. The liquor 
 was then drained off into a well and from there pump- 
 ed back to the "pocher" to meet fresh unbleached pulp, 
 thus becoming exhausted of its available chlorine. 
 The density of the stock was in all cases, attained 
 with the drum washer. 
 
 An arrangement of the kind described above is 
 shown in Fig. I. The fiber direct from the screens 
 flows into the pocher a, and after the desired density 
 has been obtained with the drum washer, the bleach 
 liquor is added and the whole circulated for a couple 
 of hours or so, steam being injected meanwhile till 
 the temperature reaches 100 to no to 120 Fahr. 
 The charge is then run off through the chute b, into 
 any one of the series of tanks c, where it remains at 
 rest for twelve or sixteen hours. By this time the 
 pulp will have reached a good color. The liquor is 
 then drained off through the plug hole d, into the pipe 
 
e, which conveys it to the well f, from whence it is 
 pumped back to the pocher again. The drained and 
 bleached fiber is afterward conveyed to the beater 
 floor in trucks, or thrown on the traveling belt g, and 
 conveyed to the stuff chest h, mixed with water and 
 pumped partly to a wet machine on the beater floor, 
 to be made into laps, and partly direct into the beating 
 engines. Such a system manifestly involves much 
 labor, plant and floor space, not to mention a somewhat 
 large expenditure of steam for heating, when hot 
 bleaching is carried on. As a rule not more than 2.5 
 
 to 3 percent density of stock is obtained from the 
 pocher. 
 
 Bleaching apparatus were then designed fulfilling 
 more perfectly the conditions for economy . The ves- 
 sels or pochers were built of meunier work, or rein- 
 forced concrete, and the mixture of pulp, bleach liquor 
 and water, kept in continued motion by means of an 
 Archimedean screw or propeller, until the process of 
 bleaching was practically completed. Such a system 
 is illustrated in Fig. 2. 
 
 The propeller is placed at one end, and in this par- 
 ticular case causes the stock to travel in the direction 
 
 5 
 
of the arrows. These bleaching engines are built to 
 hold from one to ten tons of air-dry pulp per charge, 
 and are operated with from four to six percent stock. 
 Such a degree of concentration insures fair economy 
 in bleach and steam, but they are intermittent in their 
 action, and, in consequence, there is considerable loss 
 of time in filling and emptying. The stock, after it 
 has acquired the right degree of whiteness, is emptied 
 
 into the chest beneath, from whence it is pumped to 
 the washing and drying machines. The names of 
 Kellner, Partington, Bdmer, Hromadnik, and others 
 are identified with bleaching engines and systems of 
 this kind. 
 
 It has always been the aim of pulp manufacturers 
 to invent a continuous system, or one that is nearly 
 so, and quite a number of such have been constructed 
 and operated for many years in Europe and America. 
 Such plants are known in Great Britain as the "tower 
 system" and in America as the "continuous tank sys- 
 tem." Neither of these fulfills the most perfect^ con- 
 ditions for bleaching, although the tower system is the 
 better of the two. The towers are usually concrete 
 tanks, with or without conical bottoms, connected to- 
 gether by channels or passages as shown in ^ Fig. 3, 
 which illustrates the "continuous tank system" in use 
 in many mills in America. This consists of six or 
 more circular concrete tanks 12 feet in diameter by 
 about 20 feet deep, connected together with passages 
 or pipes, at top and bottom alternately, as shown in 
 the section through A B. These tanks have fiat bot- 
 toms and agitators driven by spur gearing at the top, 
 which keeps the pulp in continuous motion. The pipes 
 or passages connecting the tanks at the bottom, are all 
 
 6 
 
on the same level, but those for the overflow from 2 
 to 3, 4 to 5, 6 to 7, and so on through the series, are 
 all on different descending levels, in order to permit 
 the pulp to flow by gravity from the first to the last 
 tank in the series in the direction as shown by the 
 arrows. 
 
 Instead of the stock flowing by gravity,- it is some- 
 times pumped from one tower to the next in series. 
 This obviously is forced circulation, and has certain 
 advantages over circulation by gravity, but can scarce- 
 ly be called a continuous system. It permits o*f great- 
 er concentration of stock, a stronger bleaching fluid 
 in intimate contact with the fiber, and economy of 
 steam for heating when hot bleaching is employed, 
 but under the best conditions, seldom more than 4 
 to 4.5 percent stock can be handled, which in the 
 opinion of many is too dilute to yield the most econ- 
 omical results in any continuous system. 
 
 Skjold (Svensk Pappers-Tidning, 1905, No. 15, 
 pp. 85-86) shows a method of continuous bleaching 
 in which he employs a series of flat-bottomed upright 
 circular tanks, built of concrete and containing agita- 
 tors, four or more tanks being employed in the series, 
 connected by an arrangement of pipes, so that the 
 pulp can be pumped from one to the other continu- 
 ously, or circulated at will from the bottom to the top 
 of each tank; a mixing tank is provided between the 
 wet machine and the first bleaching tank, for mixing 
 the sheet of wet pulp with water of 50 'Cent. (122 
 Fahr.) and bleaching powder solution of 3.5 Be. 
 The bleached pulp leaving the system is washed on 
 wet machines before it is dried. The tanks in this 
 system are each 4.9 meters in diameter by 8 meters 
 deep, giving a total capacity for the four, of about 
 600 cubic meters, and it is stated, that from forty to 
 forty-five tons of air-dry pulp can be bleached in 
 twenty-four hours, equivalent to nearly 500 cubic feet 
 of tank space a ton a day. This is a large output, 
 which is made possible, perhaps, by the high tempera- 
 ture employed viz. : f rom 130 to 140 Fahr. The 
 operations of this system are somewhat broken, and 
 it is doubtful if these conditions as to output could be 
 maintained in constant practice, unless the stock treat- 
 ed were of the easiest bleaching character. 
 
 J. E. Heiskanen, in his apparatus (U. S. Pat. 
 1277926), has overcome certain difficulties and has 
 greatly simplified the continuous system. His appa- 
 ratus is illustrated in Fig. 4, 5 and 6. All stock pipes, 
 centrifugal pumps and agitators, are eliminated, the 
 
 7 
 
inventor substituting for these propellers for mixing, 
 agitating and circulating the stock through the whole 
 system. These propellers are driven by small motors 
 fixed on the upright shafts, or by overhead gearing, 
 and as he attains a density of 6 to 8 percent stock he 
 fulfills 4:he best conditions for economy of bleach 
 liquor and economy of steam for heating. The floor 
 space occupied by the plant is about half of that re- 
 quired for the "continuous tank system" mentioned 
 above. 
 
 The unbleached pulp falls from the pulp thickener 
 into the screw conveyor where it is mixed with the 
 necessary quantity of bleach liquor and hot water, the 
 mixed stock being conveyed automatically into the 
 first bleaching tank la. The fiber in this tank is forced 
 upwards by the propeller to the top of Ib, where the 
 stream is divided by the regulating gates into two 
 {Jarts, one part giving back into la while the other 
 part flows into I la. The proportion going forward 
 into Ha varies from one-tenth to one-fifth of the total 
 volume passing the propeller, so that the stock in la is 
 kept circulating vigorously and is in continual motion. 
 These regulating gates are placed at the top of Ib, 
 lib, I lib, and so on through the whole series, the flow 
 forward from one tank to another being adjusted by 
 them in accordance with the amount required, and 
 kind of pulp to be bleached. Steam is introduced at 
 the -bottom of each tank immediately below the pro- 
 peller as shown in Fig. 5, to maintain a uniform tem- 
 perature throughout the apparatus. After the pulp 
 Has traversed through the series of tanks it is dis- 
 charged into the chest at the end, from whence it 
 flows by gravity, or it is pumped, to washers and 
 finally to the drying machine. The return pump shown 
 in Fig. 4 is not essential and is seldom or never used, 
 but is added to enable the operator to pump the stock 
 a number of tanks back, if by accident insufficient 
 Bleach liquor has been added. 
 
 Heiskanen represents that his plant occupies half 
 the floor space of the ordinary "continuous tank sys- 
 tem" ; that he can attain a density of 7 percent stock 
 on an average; and that by so doing the consumption 
 of steam ancTBleach is reduced to a minimum. The 
 consumption of power and labor is extremely low; 
 for in the first case there is very little weight to be 
 moved by the propellers, the columns of stock in la 
 and Ib equalizing themselves, while in the second one 
 man is sufficient to run the tanks from the first tank 
 to the pulp chest. He also allows eight hours or so 
 
II 
 
for bleaching and completes this with a total tank 
 capacity of about 200 cubic feet a ton of pulp a day. 
 
 The plant as shown is capable of handling 100 tons 
 of pulp a day, is well designed and as it is constructed 
 of concrete, lined internally with glazed tiles if so de- 
 sired, and fitted with bronze working parts in con- 
 tact with the fiber, the risk of iron spots appearing in 
 the dried pulp is avoided. 
 
 From the foregoing it is apparent that any econ- 
 omical system of bleaching must involve a high den- 
 sity of stock, primarily to economize steam for heat- 
 ing, and as this is a very important factor, I append 
 my formula for calculating the amount required, 
 which is applicable to every case for hot bleaching: 
 (W S -\- w' s' -f w" s" + ) (Pf fa') _ 
 
 ~T~ T-tf 
 
 in which 
 
 S = It) of steam required. 
 
 W = Wt. of air-dry pulp in the charge, in lb. 
 
 s=Sp. heat of air-dry pulp (0.65). 
 
 w' Wt. of water associated with the pulp, in lb 
 
 s' = Sp. heat of water (i.oo). 
 
 w" Wt. of vessel in which pulp is bleached, in ft>. 
 
 s"=Sp. heat of material of which w" is constructed. 
 
 ti = Initial temperature of stock in degrees Fahr. 
 
 tf = Final temperature of stock in degrees Fahr. 
 
 T = Total B. thermal units in lit) of steam used for heating. 
 
 From this formula the following quantities of steam 
 required for different densities of stock bleached at 
 different temperatures have been calculated, taking 
 the initial temperature t, as 60 Fahr. ; the final tem- 
 perature tf, as 90, 100, no and 120 Fahr., and the 
 total British thermal units in one pound of steam T as 
 1190, i. e., steam at notb pressure above atmosphere. 
 
 Density of 
 Stock 
 Pulp Water 
 
 Water 
 associated 
 with 20Oott> 
 air-dry 
 pulp 
 
 Lbs. of steam required 
 the mixture containing 
 air-dry pulp to 
 
 90 F. 100 F. 110 F. 
 
 to heat 
 
 2OOOft) 
 120 F. 
 
 37 
 .! ' 
 5* 
 
 D ' 
 
 7] 
 
 8 ' 
 
 o 97% 
 96" 
 95" 
 94" 
 93" 
 92" 
 
 646661b 
 48000 " 
 38000" 
 31333 " 
 26571 " 
 23000" 
 
 1881 
 1427 
 
 H54 
 972 
 
 842 
 745 
 
 2421 
 1809 
 1442 
 1197 
 1023 
 892 
 
 3053 
 2282 
 1819 
 1510 
 1290 
 1125 
 
 3699 
 2764 
 2203 
 1830 
 1562 
 1269 
 
 NOTE. This table is based on the above formula but the 
 weight of the apparatus w" has been eliminated. Three per- 
 cent should be added to "the above quantities of steam in cols. 
 4, 5, 6 and 7 to allow for loss of heat by radiation. 
 
 12 
 
5160 
 
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