THE RELATION OF EXGESS BLEACH, TEMPERATURE, AND CONSISTENCY TO THE LOSS OF STRENGTH AND SHRINKAGE OF WOOD PULP ON BLEACHING SIDNEY PERCY FOSTER THESIS FOR THE DEGREE OF BACHELOR OF SCIENCE CHEMICAL ENGINEERING COLLEGE OF LIBERAL ARTS AND SCIENCES UNIVERSITY OF ILLINOIS 1922 Digitized by the Internet Archive in 2016 https://archive.org/details/relationofexcessOOfost 192 2 FBI UNIVERSITY OF ILLINOIS May ?0 .19^2 THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BY S.ydne_Y_ P . __F os ter ENTITLED delation of Excess Bleach, Temperature, and Consist- ing. encv To The Loss of Strength and Shrinkage of w ood Pulp On Pleach- IS APPROVED BY ME AS FULFILLING THIS PART OF THE REQUIREMENTS FOR THE DEGREE OF __Bac_hejLoj^ _^f _ScJLer^ce in C h e m i c a 1_ _ E n_gi ne er _i n_g_. Instructor in Charge Approved HEAD OF DEPARTMENT OF 500 1 8 ? ACKNOWLEDGEMENT The author desires to express his appreciation to Dr. D. T. Englis, under whose direction this research was carried out, in appreciation of his excellent advice, kindly encouragement, and many favors throughout the course of this work, and also to Lewis Armstrong of the Kimberly Clark Paper Mills Co., Niagara, Wis., for many suggestions, favors, and help. TABLE OF CONTENTS Introduction 1 Object of the Investigation . 1 Nature of Wood Pulp • • 1 Manufacture of Wood Pulp 2 Bleaching of Wood Pulp 5 Experimental Part 7 Procedure . 7 Description of Apparatus 10 Preparation of Materials 10 Results 11 Discussion and Conclusion 15 Summary 17 Bibliography 18 1 INTRODUCTION Object of the Investigation , The object of this investi- gation was to determine the relation of excess bleach, tempera- ture, and consistancy to the loss of strength and shrinkage of wood pulp on bleaching. It is logical to expect that the strength and shrinkage of pulp on bleaching will vary inversely as the temperature and the percent excess bleach. In other words, the higher the temperature, and the greater the excess bleach, the greater will be the loss in the strength and weight of the pulp. However, the problem here was to determine the quanti- tative relation of these factors and, if possible, to get the necessary data required to state exactly what the optimum temp- erature and percent excess bleach would be for a minimum loss in strength and shrinkage of pulp. No doubt much work has been done on this subject, but only in a disjointed way insofar as literature is concerned. Ap- parently there has never been anything published that covers the work completely. Much of the knowledge of this phase of paper manufacture is in the form of plant secrets and, conse- quently, very little is to be found about it in the literature. Nature of Wood Pulp . A brief statement of a few facts about wood pulp and the processes and problems involved in its bleaching may be helpful in giving a clearer conception of what is to follow. Wood pulp is a product coming almost entirely from soft woods. It consists mainly of cellulose associated with a smaller amount of lignin occurring in the raw fibers. This 4 • . . • . - . • • jjj|* iif Q&J . i 1 •' - . . ■ . . . 2 lignin associated with the cellulose carries with it certain colored bodies of complex composition that give the wood pulp a brownish color. Wood pulp, if it is to be used for making any light colored papers, must, therefore, be bleached, as these colored impurities cannot be removed by any amount of washing or other mechanical treatment. It is believed that they are combined in a chemical manner with the fiber or cellulose and, therefore, a chemical process is necessary for their removal. In addition to these colored impurities which are ordinarily present in the pulp, other dark colored substances are produced during the process of manufacturing the pulp. The production of these coloring substances is due to the chemical action of the qcidic or alkaline solutions used in the sulphite and soda processes respectively. Manufacture of Wood Pulp . A brieifi outline of the processes of chemical wood pulp manufacture may help to make this distin- ction clear. Most vegetable fibers are converted into pulp by alkaline processes, that is, by digesting the raw materials with caustic soda and similar alkaline substances. Wood may be treated in one of two ways, one of which is the ordinary soda process, and the other an acid treatment requiring the use of sulphurous acid. For preparation of the wood to be used, the logs are cut up and then "flaked" into pieces one inch square, and one-half inch thick by means of chippers. To insure uniformity in size, the chips are sifted on wire cloth. The chips are then put into digesters. The digesters are either spherical, cylindrical, or egg-shaped, being constructed 3 to revolve at a slow rate of speed, or fixed permanently in an upright position. The object of boiling the wood under pressure with chemicals in the digesters is to dissociate the valuable fibrous portion of the plant from the resinous and non-fibrous portion. The contents of the digester are heated by means of high pressure steam, which is blown directly into the mass or passed through a coil lying at the bottom of the vessel. Sulphite Wood Pulp. This name is given to pulp prepared by digesting wood with solutions containing sulphurous acid, or salt of the acid (bisulphate of lime). The digesters are charged with the chipped wood and the re- quisite amount of sulphite liquor is added. Some digesters are capable of holding 20 tons of wood at one charge. The length of time of cooking varies Jh»om 7 hours to 60 hours depending on the kind of wood, size of charge, ultimate product desired, and the yield. No hard and fast rule for cooking can be given and each mill has its own particular method. 80 pounds pressure is usually employed. When the wood has been sufficiently boiled, the spent sulphite liquor is drained from the digester. The wood is then washed well with water to remove all residual liquors. The woody mass is next screened. Here all the knots, large pieces of unsoftened wood, and coarse pieces are removed. From the screens, the mixture of pulp and water is pumped into a concentrator. Here some of the water is removed. The mass of wet pulp is then conveyed into a circular reservoir or “stuff chest" which serves to supply the machines used for converting the pulp into dry sheets. The pulp flows in a continuous ] . ■ . ' ■ . t ’ ... 'V v • . « — . ‘ . ’ . .. ! 4 stream on to a horizontal endless wire, which carries it forward as a thin layer; the water drains through the meshes of the wire, further quantities being removed by "suction boxes". The wet sheet then passes between "couch rolls" which compress the pulp, and then through press rolls which finally give a firm adherent sheet of pulp containing 70 percent of water. The sheet is then dried by passing over a number of steam heated cylinders. Soda Wood Pulp. The chipped wood is boiled in digesters at a pressure of 70--80 lbs. A solution of caustic soda is em- ployed, about 16 — 20 percent of the weight of the wood being added to the contents of the digester. Live steam is blown direct into the mass, and after the operation, the spent liquor is kept for subsequent treatment. After digestion, the pulp is washed in such a manner that the amount of water actually used is kept down to the smallest possible volume consistent with the complete removal of soluble matters. This is done in order that the spent liquors may be treated for the recovery of the soda. From here, the v/ood pulp is screened, rolled, and dried as in the sulphite process. Two other processes for making wood-pulp are the sulphate process and the Mitscherlich process. The former consists of digesting the wood with a mixture of caustic soda, sulphide of soda, and sulphate of soda. The latter process consists of di- gesting the wood with bisulphite of lime at a much lower temper- ature, and for a longer period than the ordinary sulphite pro- cess. It is these acidic and alkaline solutions employed in the different processes that react with some constituents in the wood — . • • . . * . . . . 5 pulp to produce colored complexities that have to be bleached out. Bleaching of Wood Pulp . The object of all successful bleach- ing is to thoroughly bleach the pulp ao as to produce a product of maximum whiteness and purity, which will remain white inde- finitely, and at the samfe time not impair the strength and natural properties of the fiber, and not to cause too great a shrinkage in weight and volume. The process of bleaching is essentially one of oxidation and the success attained depends on the fact that the impurities are attacked and resolved into soluble products much more easily than the comparitively inert cellulose of which the impure fiber is largely composed. Many different oxidizing agents can be utilized in the bleaching process and a number of them have been applied with more or less success. Practically all commercial work is performed by chlorine, or some of its compounds* Some of the bleaching agents used are chlorine gas, hypochlo- rites, bleaching powder, and electrolytic bleach. There are many factors entering into the bleaching of wood pulp. Those upon which the shrinkage and loss in strength depend are as follows: 1. The raw material from which the pulp is made. 2. The process employed in the manufacture. 3. The purity of the pulp obtained. 4. The density of the stock during bleaching. 5. The temperature at which the bleaching was carried out, 6. The time allowed for bleaching. 7. The percent excess bleach used. . .. 6 It has been found that the greater the yield of fiber from unit weight of raw material, no matter by what process the pulp has been made, the greater is the loss of weight of pulp bleached and the amount of bleaching agent required. The quality of water used for cleaning the pulp also has an influence on the bleaching factors, especially if the water con- tains any lime. in the sulphite process, the lime salts pre- cipitate insoluble resin soaps on the surface of the fibers. These resin soaps absorb chlorine. A similar precipitation takes place when water containing lime is used for washing soda or sulphate pulp. In both of these cases, the resin soaps formed cling to the fibers, and carry down with them the organic coloring matter in the fibers, thus rendering the bleaching process more difficult and costly. . % -• s 7 EXPERIMENTAL PART Procedure . The procedure followed in the determination of the relation between the temperature, excess bleach, consistency, and the loss of strength and weight on bleaching, was a series of tests in which the conditions of bleaching were varied. In one test, the temperature, excess bleach, and consistency were all kept constant. In the next test, the excess bleach and consistency remained the same as in the preceding one while the temperature was raised 10 degrees. In the next test, the temperature was again raised 10 degrees. The kind of pulp used was called No. 2 Balsam Dryer.. For each test, a 700 gm. sample (air dry) was used. This amount was taken so as to be able to run strength tests on each sample. The beaters and strength testers to be used could not handle an amount smaller than 700 grams. The bleaching agent used was commercial bleaching powder. Following is the procedure in detail. The pulp after first being weighed, was torn into small pieces / and placed in the container in which it was to be bleached and about three-fourths of the total volume of distilled water was added. This was stirred mechanically until the pulp was tho- roughly broken up. In the meantime, the container and contents were brought up to temperature by means of a heating coil and maintained at that temperature by a thermostat. Then the balance of the water and the required amount of bleach liquor was added. At this point after the solution had been stirred for about five minutes, a sample of the liquor was taken and 8 titrated with 1.41 h/lO areenite to determine the amount of chlo- rine present in grams per liter. This was repeated every 15 minutes to get the rate of chlorine absorption. From time to time during the bleaching, the whiteness of the pulp was com- pared with standard color plates so as not to overbleach, which would cause a greater loss in shrinkage and strength. Vihen the pulp had been bleached white, as indicated by the color plates, check tests were made on the chlorine present in the solution. Then 5% sodium bisulphite or sodium thiosulphate was added to "kill" the remaining bldach. The pulp was then removed to a large Buchner funnel where it was thoroughly dried by suction. A sample was taken from the pad of pulp and the percent of moisture in it was determined. The total weight of the pad was multiplied by this number to get the bone-dry weight of the pad. To get the air-dry weight, the bone-dry weight was multiplied by 1.1. The loss of weight of the pulp is due to the decomposition of the cellulose and lignin by chlorine, and is called shrinkage. Following is a complete plan of the work contemplated, all of which could not, however, be completed. The second test, i.e. Series I, Test 2, was the same process repeated with the same conditions, except that a 4% consistency was used. Series I. Test 3. Temperature and excess bleach same. Consistency 6 %. Series I. Test 4. Temperature and excess bleach same. Consistency Qfi. . . v- . . . Series II. Test 1. Same tests made as in Series I, except that the temperature was kept at 35° instead of 25°. Consistency 2%. Series II. Test 2. Consistency 4%. Series II. Test 3. Consistency 6%. Series II. Test 4. Consistency 8%. Series III. Test 1. Same tests made as in Series I, except that the temperature was kept at 40°, instead of 25° • Series III. Test 2. Consistency 4%. Series III. Test 3. Consistency 6%. Series III. Test 4. Consistency 8%. Series IV. Test 1. Same tests hut with a temperature of 45° C. Consistency 2%. Series IV. Test 2. Consistency 4%. Series IV. Test 3. Consistency 6%, Series IV. Test 4. Consistency 8%. Series V. Test 1. Same tests but with a temperature of 50° C. Consistency 2 %, Series V. Test 2. Consistency 4%. Series V. Test 3. Consistency 6%. Series V. Test 4. Consistency 8%. Then Series I, II, III, IV, and V, and Tests 1, 2, 3, and 4, were repeated making the same tests and observations, but raising the excess bleach to 25%. These series were numbered VI, VII, VIII, IX, and X. Series I, II, III, IV, V, Tests 1, 2, 3, 4, were again re- peated using an excess bleach of 50%. This series was called . • . . . .... 1 . . ■ . . 10 XI, XII, XIII, XIV, and XV. Description of Apparatus . The apparatus consisted of a 20 gallon jar placed inside of a large barrel, the latter being filled with water and acting as a thermostat. The temperature was maintained by means of a heating coil controlled by a mercury regulator and relay as shown in the diagram. The pulp was agitated in the jar by means of a heavy glass rod with paddles attached, driven by a one-sixteenth horse power motor. Two agitators placed on opposite sides of the barrel and dri- ven by a small motor kept the temperature of the apparatus uniform. Preparation of Materials . For the chemical work, the solu- tions used were N/lO iodine, N/lO sodium thiosulphate, 1.41 N/lO arsenite, and bleach liquor. The first two were prepared and standardized in the usual way. The arsenious oxide required purification. This purifica- tion was carried out as recommended by Treadwell and Hall (1). The arsenious oxide was dissolved in hot HC1 (1:3) and the in- soluble sulphide was filtered off. The solution was cooled and the arsenic trioxide allowed to crystallize. The mother liquor was poured off and the remaining crystals were washed with water and then dried. The pure arsenite was obtained by sublimation of these crystals. For preparation of the 1.41 N/lO solution, 7.06 grams of the arsenite were dissolved in 200 cc. distilled water containing 28.49 grams of Na^CO^. The solution was diluted to 1 liter. . . . . -«•. . 11 1.41 N/lO arsenite is used for the reason that when 5 cc. of the bleach liquor is Rested, then 1 cc. arsenite is equal to 1 gm. of chlorine per liter. Bleach liquor is spoken of and expressed in terms of grams per liter. The arsenite was standardized against N/lO iodine which had previously been standardized against sodium thiosulphate. The latter was standardized with copper foil. The bleach liquor was prepared according to the method of Cary and Niuspratl (2) from commercial bleaching powder. The solution was made at a temperature of 70 — 80° F., the ratio of bleach powder to the water used being .5 lbs. per gallon. This mixture was agitated in a jar for one hour and then allowed to stand until most of the suspended material had settled. The clear liquid was then drawn off into a 5 gal. carboy and stored in the dark since light hastens the loss of strength of a bleach- ing solution. Results . Normality of I 2 solution .07351 N Normality of arsenite solution 1.4323 N/lO 1 cc. 1.41 N/lO arsenite * 1 gm. Cl per liter. Then 1.4323 N/lO arsenite ■ 1.015 gm. Cl. per liter. The bleach liquor contained 16.90 gm. Cl per liter. The grams per liter of bleach liquor necessary for 700 grams of pulp were determined by the following method. First of all, the "bleachability" of the pulp must be known. Bleachability is the number of grams of dry bleach (35 % avail- able Cl) necessary for bleaching 100 grams of air-dry pulp. An average bleachability is figured at about 16 %. That means that . . . " : 1 i. , ; 12 16 pounds of commercial calcium hypochlorite is necessary for every hundred pounds of pulp. Bleaching powder is figured on a 35% basis. Then using grams instead of pounds, 16 gms. bleaching powder contain .35 x 16, or 5.6 gms. Cl, the amount of Cl. necessary for every 100 gms. of pulp. So for 700 gms. of pulp, 7 x 5.6 or 39.2 gms. of Cl would be required. To determine the liters of bleach liquor necessary then: 39^2 gras, chlorine required „ 2.320 liters (- % excess) 1679 gms. Cl per liter in liquor All of these calculations may be reduced to the following formula : gl«ac. h ?billty x 700 x .85 x (100 .ft excess) , liters re q u i re c 100 x gm. 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The rate of chlorine absorbed was the same in each run in spite of the fact that the rate of absorption of the chlorine and the loss of pulp weight was dif- ferent • Insufficient time prevented more tests from being made and, consequently, not enough data was obtained to state exactly what the optimum temperature and percent excess bleach would be for a minimum loss in strength and shrinkage of pulp on bleaching. Some work, however, was done with varying consistances, the temperature and percent excess bleach remaining constant. The apparatus used was inadequate and the pulp mixture could not be stirred sufficiently at the higher consistancies, so the work along this direction was abandoned for the time being. Strength tests were not run on any of the series so nothing can be definitely said about them here, but it is believed that the strength decreases rapidly as the temperature and percent excess bleach are increased. However, the samples are to be sent to the Kimber ly-Clark Paper Mills, Niagara, Wis., to have the strength tests run on them and the data will be obtained later. The three factors: (1) The raw material. (2) The process employed in the manufacture of the pulp. (3) The purity of the pulp. . ' . • •: . . . . 16 were not dealt with as it would have been impossible to effect any control on the second and third factors. Just one kind of pulp was used and not enough time was available to complete all the tests on this particular pulp. It would have been very interesting to compare the results on “No. 2 Balsam Dryer" with spruce and hemlock. . . 17 SUMMARY From this investigation of the study of bleaching wood pulp, it was found that the higher the temperature and the greater the percent excess bleach, the more rapid will be the rate of bleach- ing and the greater will be the loss in weight. The loss in weight varied all the way for 5% at 25°C. to 20.5$ at 50°C. In the rate of chlorine absorption, the quantity of chlorine left in the solutionwhen the pulp had been bleached to a standard color was the same in almost every test. It maintained almost a constant in spite of a variation in the weight losses. The amount of chlorine in grams per liter of a 12.5$ excess and at a 2% consistency, was 1.22 grams per liter. At the end of each run, the amount of chlorine left was approximately .29 grams per liter. An attempt was made to study the effect of consistancy variation on the bleaching, but the laboratory teehnique required was too involved so it was temporarily abandoned. The other factor, namely, percent excess bleach, was not varied. A 12.5% excess was maintained throughout the investi- gation and the effect of different percent excesses was not studied due to lack of time. . * . . . . 18 BIBLIOGRAPHY (1) Treadwe 11-Hall, Vol. II, 651 (2) J. Soc. Chem. Ind. (1903) 674 I Chemistry of Pulp and -Paper Making. E. Sutermeister (1920) II. The Manufacture of Paper. R. V/. Sindall (1908) III An Elementary Manual of Paper Technology. R. W. Sindall (1910) IV The Chemistry of Paper Making. R. B. Griffin and A. D. Little (1894) V Chapters on Paper Making. C. Beadle (1907) VI Paper Making. A. F. Cross and E. J. Bevan (1888)