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AERIALVIEWOF THE SOUTH AFRICAN PULPAND PAPER INDUSTRIES’ FACTORY AT G ~ D U LWHILE D ITWASUNDER CONSTRUCTION
Gas Chlorination in Cellulose
Manufacture UMBERTO POMILIO South African Pulp and Paper Industries, Ltd., P. 0. Geduld Station, Transvaal, South Africa
D
URIKG the past ten years a great deal has been published concerning the manufacture of cellulose by chlorination, and in view of misstatements in the technical press (12) with regard to the nature, scope, results, and merits of the process, it seems desirable to supply further information.
decomposition of brine; therefore it is necessary only to add a cell room to the pulp plant and use the chemicals as obtained from the cells. I n contrast with other industrial cellulose methods, the gas chlorination, or Pomilio, process is continuous throughout its complete cycle. It does not digest under pressure and generally does away with the recovery of black liquor. Water pollution is generally not to be feared. Grasses, straw, agricultural by-products in general, textile residues, and wood (whether resinous, free of resin, hard, or soft) all give satisfactory yields of cellulose with the sodachlorine treatment. The final products may vary within wide limits; that is, the cellulose may be bleached or unbleached, of high purity for chemical uses, or semichemical pulp to be substituted for groundwood. Reference to the classical works of Cross and Revan and to the more recent work of Dor6e ( 2 ) will indicate the superiority of this soda-chlorine process as a n extraction method. Apart from a sound theoretical basis, the process has proved practical on a large scale. Wenzl, who was one of the first cellulose specialists to grasp the importance of the
Gas Chlorination Process The complete cycle consists of ( a ) predigestion with weak alkali solution a t moderate temperature and for a short period, ( b ) exposure of the digested, washed, spongy mass to moist chlorine gas in the cold, (c) treatment of the waterwashed chlorinated mass with a weak alkali solution and washing, and (d) very gentle bleaching with a hypochlorite solution. Each step is comparatively short; the total time required is not more than 6 to 8 hours, and the first three steps may be complited in 3 to 4 hours. The only chemicals needed are weak caustic soda solution for steps a and c, gaseous moist chlorine for step b, and hypochlorite for step d. The ratio of soda to chlorine may vary and may be made the same as that obtained by electrolytic 657
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chloririatinn proi'ess, included in his book ( 1 1 ) a survey of the gas chlorination process, and gave figures on chemicals consumed, yields, and efficiency (in each case wheat Straw of the same oririn was used) : l'i"C*SS
Sodium hydroxide: 1:sed on dry straw Conaumed on dry strew-
Used on cellulose obtained Consumed on cellulase obtained Chioiine: Consumed on atraw Consumed on oeliulose Yields, air-dry Effioienoy
A
B
c PiroToMICRoGRAPEs OF ( A ) CELLULOSE SOUTHAPRICANSTRAW, ( B ) CELLUL~SE FROM Pinus palula, AND ( C ) SEMIPULP FROM Pinus p a t u b
FIGURE 1. FROM
Hypo- Chlorine Chlorine olrlorite G s ~ WBter Alkali
%
%
%
%
7.6
6.9 16.87 15.35
7.0 6.8 14.56 13.95
12.5
22.4
6.9 44.6 24.6
54.6 26.8
18.4 40.8 45.0 65.4
24.8 51.7 48.0 76.5
15.0 53.5 28.0 43.3
41.0
11.0
3:O
58.3
Contrary to misstatements that have appeared in the public press, the raw material for the gas chlorination process is salt, and not soda and chlorine. a The principle of the process is elemental chlorine chlorinating (delignifying) agent, and not a bleaching (oxidizing) agent. If a bleach liquor is used with the same amount of chlorine as in tlie gas chlorination process, the yield is lowered and the quality is spoiled. Fifteen years of experience have shown that high pressure is objectionable. The long time required for cooking and the longer period for bleaching involves a considerably higher cost for installation and operation in the case referred to by Wingfield et al. (12). In the digestion, which is carried out with soda concentrations and temperatures relatively low and of short duration, cellulose material is not attacked. A soda solution containing betvreen 1 and 3 per cent by volnme of sodium hydroxide has no effect on cellulose below 100" C. in the case of grasses, straws, ete., and between 125" and 135" C. in the case of roods, especially if digestion is only partial-that is, if a large amount of noncellnloses is left to act as a shield. In tlie next step the material is submitted to a continuous stream of chlorine gas (continuous process), and therefore tlie concentration of this reagent is constant. KO other pulping process has solved this important point, which accounts for a substantial saving of time. The next favorable condition is t.hat, during thc chlorination by @as, a definite pH is created in the mass. The higher the lignin content, the greater the quantity and proportion of hydroclrloric acid formed and the higher the pH of the mass. This acidity establishes an ideal condition for the prevention of hypochlorous acid formation-that is, of oxidation, which is a very rapid reaction. The combined action of hydrochloric acid and heat tends to hydrolyze cellulose, but this is a much slower reaction, and only strong acidity and temperatures never reached during chlorination accelerate it. The alkali wash is a rapid operation which requires only 8 few minutes. Therefore the process is characterized by rapidity, efficiency, and safety. Since it is a multistage process, the quality of production may be raised before the final stage is reached; this is of paramount importance (7). That delignification and defibering of cellulosic fibers by the chlorination process are complete is shown by photomicrographs (Figure 1 ) of straw cellulose, A , and of pine wood, B. Yet t,lie gields arc high, and generally over 95 pcr cent of the theoretical cellulose content of the raw material is extracted in a largeacale operation. However, mnichemical pulps, with higher yields and lower costs, may be obtained if desired. Figure 1C is a yhotomicrograplr of seiuipulp ohtaincd from tlie samo South African Pinus palula rrom m+icli pure cellulose (Figure 1B) was obtained. C h l o r i n a t i o n Process in the United Stales The possibility of producing cheap seniipulps, from both woods and straws or grasses, at a prier comparable in some cases to that. of mechanical pulp and of a quality suitable for
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PLANTOF CELULOSA AT ARGENTINA, ROSARIO, ARGENTINA
'HE
newsprint paper, should attract attention. There has recently been considerable agitation in the United States for making newsprint from cereal straws as was done in Italy more than ten years ago (1). The problem of cellulose production by chlorination has probably attracted less attention in the United States than in any other country, despite the fact that it would be very suitable for American requirements. The process is rapid, and the total time required is about half that necessary for sulfite or sulfate cooking. A total of 4 hours (2 for digestion, 1.5 for chlorination, and 0.5 for alkali wash) is sufficient, compared with an average of 8 hours or more for sulfite or sulfate cooking, from the introduction of chips into the digester to their discharge.
Since the gas chlorination process is continuous throughout, utilization of machinery is 100 per cent as against a n average of 50 to 60 per cent for the batch process, so that the cost of the plant per ton output is reduced to about one fourth. In addition, the chlorination process does away with equipment for the recovery of waste liquor. The process applies industrially to any raw material; grass, cereal straws, hemp stalk, flax straw, rice straw, esparto, bagasse, alfa grass, poplar wood, and pine wood are materials from which cellulose has been or is to be produced on a large scale. It is therefore possible to select any fibrous raw material; in addition, salt and power, both steam and electrical, are needed. The difficulties in straw collection were covered in a recent lecture (IO).
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THE FAcToaY O F CELLULOSA CLoEo-SoDA AT NAPLES
The combined use of wood and straw is possible in a gas chlorination plant, since the digestion black liquors from wood treatment are directly used for cooking straw. Woods, especially if resinous and hard, necessitate a higher soda concentration and temperature during preliminary digestion and result in a higher soda residuein the black liquor. It has been proved ( S , 6 ) that residual black liquors, such as those from sodadigested pine, are a n excellent cooking liquor for cereal straws. This combined use of pine wood and straw suggest the feasibitity of the process for the southern part of the United States.
Treatment of Wastes Waste waters from a soda-chlorine pulp plant may be the following: Black liquor (8 nt lye alter digestion) is the result of s mild Gestion carried out with soda a t a concentration less than that required to combine with the materials treated. The resultina black liauor is nearly neutral (2 to 3 crams It absorbs oxygen proportionally more, generally, than ordinary sewage waters. Wash waters from digested materials are similar to but much more dilute than t h o black liquor. Acid wash waters after chlorination contain Eree hydrochloric acid (generallyless than 1 gram per liter) and traces of dissolved chlorolignin (one of the chloroligninsformed appean to be slightly water soluble). Spent lye siter alkalization may be considered as a black liquor, but it is more nearly neutral and contains the alkaline derivatives of the chlorolignins,which are oxygen absorbing. The waste from the washing of pulp alter alkalieation is a much more dilute solution than the spent 1 e The wasg &atem alter bleaching may contain, in addition to traces of calcium chloride, the last residues of active chlorine, which will he immediately destroyed by the oxygen-absorbing capacity of some a1 the other waters. The aoidity 01 the wash water after chlorination ia obviously neutralizcd by the spent lye alter digestion. suspegsion.
VIEWS IN THE
NAPLESPLANT: (top) IdohDING %PART0 Ganss DIGESTING TOWERS; (bouom) SILOS
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The admixture of the six effluents is practically neutral, nonfermentable, considerably diluted by wash waters, but oxygen absorbing a t an extremely slow rate. In contrast to sulfide liquors, the effluent from the chlorination pulp mill does not contain sulfur compounds; its principal mineral constituent is sodium chloride, regenerated from soda and chlorine compounds, but sodium chloride is present to a less extent than in sewage waters. The organic materials are the encrustants from which the cellulose was freed, but because of the high yield no degradation products of cellulose complicate or increase the black liquor problem. The combination of the six effluents is thoroughly mixed and discharged as sewage. ‘The plant a t Rosario, Argentina, now discharges into the I’aran& River, the Santiago, Chile, factory into a small river, and the plant a t Foggia, Italy, into marsh lands since no outlet to sea or river is available; the Naples factory has for some years discharged all waste water into the near-by harbor. In no case have difficulties with effluents been experienced. The plant at Springs, South Africa, will discharge black liquors into the gold-mine dumps. Should special conditions arise by which the disposal of the effluents, especially black liquor and spent lye, must be handled otherwise, the problem will become economic rather than technical. Black liquor may have sodium hydroxide added to it and be re-used for digestion before it is evaporated and burned, and its volume may be thus reduced; or it can be used for the alkali wash. Its small amount of free residual alkali is thus utilized and its concentration of organic materials increased. Both of these treatments have been carried on a t Rosario for years, not to decrease the volume of the waste water but t o prepare a strong liquor of small volume. After the addi-
tion of other chemicals, this liquor is used as a weed killer which is sold in considerable quantities by Celulosa Argentina throughout the Pampa under the trade mark of “Celarite.” Re-use of black liquor for redigestion or for alkali wash is feasible and requires only a slight increase in the consumption of active chlorine for bleaching. The two black liquors sufficiently concentrated by re-use may then be evaporated. B u t evaporation is not the only way to dispose of such black liquors; it is possible to precipitate organic material and recover salt. The gas chlorination process is being developed in countries where no previous pulp industry existed-for example, Argentina, Chile, Uruguay, Brazil, the Transvaal, the Philippines, Italy, etc. (4). Recent Developments
In 1937 a short account of initial operations in the Foggia plant was presented (9). After two years the results have proved so satisfactory that production of wheat straw pulp by the chlorination process has more than doubled. Another large wheat straw pulp factory is now being erected b y Cellulosa d’Italia a t Chieti, Italy, with a capacity of 60 tons per day of bleached high-grade straw pulp, and operations are expected to commence in October, 1939. During 1939 the plants a t Montevideo, Uruguay, erected by Fabrica Nacional de Papel, with a capacity of 18 tons per day of bleached straw pulp and at Central Bais, Philippine Islands, built by Compania General de Tabacos, with a capacity of 12 tons per day of rayon from bagasse, will start production. Another plant which utilizes esparto and broom is being erected at Castelraimondo, near the famous Fabriano paper mills in central Italy. Celulosa Argentina began a t Rosario with an output of 12 tons of pulp per day in 1930, increased it to 40 tons by. 1937,and in August, 1938, successfully started a second wheat, straw pulp plant with a production of 40 tons per day. A gas chlorination unit of latest design was used; it is continuaus throughout and includes digestion. Celulosa Argentina began in 1929 with a capital of 1,500,000pesos and one paper machine, and now has a capital of 18,000,000 pesos and twelve paper machines in three factories. The production of chemicals connected with brine electrolysis has been so developed that at present about 50 per cent of the profits come
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from the sale of such chemicals as caustic soda, hydrochloric acid, sodium hypochlorite, bleaching powder, chlorides, liquid chlorine, and synthetic ammonia, both anhydrous and in solution, made from by-product hydrogen gas. This shows the elasticity of the gas chlorination process; the pulp and paper factory may or may not enter the chemical market and, if it chooses, can do so on a very profitable basis. The plant of the South African Pulp and Paper Industries, Ltd., a t Springs, pear Johannesburg, Transvaal, began operations in December, 1938 (6, 8). There are both straw pulp and pine pulp units; both are producing bleached or unbleached chemical pulp as well as semichemical pulp. The caustic soda, in concentration of 3 to 4 per cent of sodium hydroxide, is used to cook pine wood under slight pressure; and the black liquor obtained, containing about 1.5 per cent of sodium hydroxide, is utilized for cooking straw in continuous digestion towers under atmospheric pressure. The plant, located near three large gold mines, will supply the water pumped from their shafts, which sink as low as 6000 feet. (The Johannesburg Rand is on a high plateau about 6000 feet above sea level.) The black liquors and some waste are discharged into the “dumps” (milled and exhausted aurific rocks on the surface). The black liquors are mixed with the cake of exhausted ore coming from huge Oliver filters in order to dilute them, and the mud is pumped to the top of the mine dumps. Solar evaporation quickly dries the mud. Thousands of tons of exhausted ore are dumped every day by each of these three mines.
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In January, 1938, La Cellulose du Pin started producing kraft pulp from p i n des Landes a t a plant near Bordeaux, France, and has erected a chlorination section in order to delignify and bleach about 24 tons of pulp per day with chlorine gas; a cheap and strong bleached pine cellulose is thus obtained which brings a good price. In October, 1938, the Cellulosa Cloro-Soda a t Naples started operations in its esparto pulp plant with a capacity of 24 tons per day. The chlorine process now contributes largely to the Italian Government’s policy of self-sufficiency. Up to a few years ago all the cellulose used was imported into Italy, but in a few years none will have to be brought in.
Literature Cited (1) Chimie & Industrie, 18 (Oct., 1927). (2) DorBe, Charles, “Methods of Cellulose Chemistrv”. 19.73. ---(3) Dupont, G., and Fayard, J. de, Chimie & Indust&, 31,No. 4 bis, 784-7 (1934); BUZZ.SOC. chim., 28,175 (1877). (4)Electrochem. Soc., miscellaneous notes. ( 5 ) Fremy and Urbain, BUZZ. SOC. chim., 37,409 (1882). (6) Grove, J. E., South A f r i c a n Printer & Stationer, 18, 227 (1938). (7) Hall, GGsta, “Some Aspects of Modern Pulp Manufacture”, Paper Makers Association of Great Britain and Ireland, 1937. (8) Paper Trade Rev., Technical Convention No., March, 1938. (9) Pomilio, Umberto, IND.ENG.CHEM.,News Ed., 15,73 (1937). (10) Pomilio, Umberto, “Use of Chlorine Gas in Industrial Pulp Manufacture”, lecture, London, 1938. (11) Wenzl, Hermann, “Zellstofferzeugund mit Hilfe von Chlor”. P . 73, 1927. (12) Wingfield, Baker, Whittemore, E. R., Overman, C. B., Sweeney, 0. R., and Acree, S. F., Natl. Bur. Standards, Misc. Pub. M124 (1936). r
Limitations of Tin as a Packing Material
ALLOTROPIC TRANSFORMATION
A. C. HANSON AND G. 0. INMAN Rock Island Arsenal Laboratory, Rock Island, Ill.
T
IN has been found very satisfactory as a packing material for use in ordnance mat6riel. In certain instances rubber cannot be employed, and a material must be utilized which at least simulates some of its desirable properties. Tin possesses softness and plasticity so that it does not scratch steel but deforms readily under pressure to make a tight seal against the cylinder wall. In addition, it will not gall and is easily formed. Tin exists in its ordinary or white form above 18” 0. (64” I?.). Below this temperature the gray variety is stable. The transformation, however, takes place with considerable slowness, except at very low temperatures. The rate increases as the temperature decreases until a maximum rate is reached a t -50” C. The change, although slow in starting at 18”C., may be facilitated by contact with the stable form. Gray tin is a friable substance; when the change to this form has once started, as indicated by a number of warty masses on the bright surface of the white tin, their number and size continue to grow until the whole of the white tin has passed into a gray powder. This transformation has been called the “tin plague.”
That the conversion of white tin to the gray modification cannot be accomplished a t will merely by cooling the specimen below the transformation point, is a known fact. Mason and Forgeng (2) found a temperature of -40” C., as recommended by Cohen and Van Eijk (I), to be ineffective over a period of 6 months on single crystals of tin. They found that tin from the same source as that used for the single crystals when cast in a cold mold transformed readily in less than 24 hours. Further investigation lead them to the conclusion that the presence of as little as 0.0035 per cent bismuth in tin, in an annealed and homogeneous solid solution, can prevent the transformation; but if the same piece is chill-cast, the transformation can proceed uninhibited along the practically pure tin in the interior of the cored crystals which exist in the chill-cast specimen. Tammann and Dreyer (3) consider bismuth the most effective of common metals in inhibiting the transformation. They found 0.1 per cent bismuth necessary for more or less complete protection against transformation to gray tin. In an effort to determine the amount of time necessary t o induce the transformation of white tin to gray tin, without initial inoculation, the writers made several attempts which are summarized in the table which follows.
