Other Pulping Process
( Originally Published 1920 )
THE three chemical processes and the groundwood process ac-count for the great bulk of papermaking fibers, but there are a number of other methods which are of interest for various reasons. Some produce, by chemical treatment, fibers with special characteristics; some employ entirely different chemical reactions for pulping; and some have the chief advantage of much higher yields of fiber from the woods used. Fibers produced by these processes vary in quality all the way from those with many of the properties of high-grade rag fibers, to those containing most of the impurities of the wood.
In attempting to classify and analyze fibers, chemists have divided cellulose into three portions by arbitrarily chosen methods of treatment with caustic soda in a solution of 17 to 18 per cent strength. When treated under very carefully specified and con-trolled conditions, that portion of the fiber remaining undissolved is called alpha-cellulose; the portion dissolving, but which may be reprecipitated by addition of acid, is called beta-cellulose; and the dissolved portion which will not precipitate is known as gamma-cellulose. None of these is a definite chemical entity, and even slight variations in the treatment will cause differences in the proportions of each found in any given fiber. In spite of this, the classification of fibers by their alpha-cellulose content has been widely used, and it is generally considered that a high alpha-cellulose content denotes greater purity, and hence a more permanent fiber.
The preparation of purified fiber depends upon this ability of alkalies to dissolve the less resistant portions of the cellulose, as well as residues of lignin, ash constituents, resins, etc., remaining from the regular pulping processes. The earliest method of making "alpha-fiber," as it was called, involved the treatment of sulfite fiber with calcium hydroxide at high temperature and pressure. This has been superseded by caustic soda, which is used under varying conditions of temperature, fiber concentration and pro-portions of caustic to fiber. After the alkaline treatment the fiber is given a light bleach and then is very thoroughly washed.
Ordinary sulfite fibers generally contain 86 to 88 per cent of alpha-cellulose, and if the purification process raises the alpha-cellulose content from 87 per cent to 96 per cent the yield of fiber obtained will be only about 70 to 75 per cent of the original sulfite fiber. There is no definite alpha-cellulose content which will allow a fiber to be graded as "purified," but anything above 93 per cent generally shows the papermaking properties which are associated with high alpha pulps. These properties are a high degree of whiteness and a softness and resistance to beating which make alpha pulps excellent for papers of high opacity and bulk, and for those which are to be parchmentized, vulcanized or impregnated with resins for lamination. Alpha pulp is not suitable for hard, rattly papers or glassines, even when beaten for very long times. It is even possible to purify a pulp so highly that it will be of no value in papermaking. In one instance a pulp with an alpha-cellulose content of 99.1 per cent failed to develop any beaten characteristics after 90 minutes of beating, and sheets made from it were so weak that they could not be handled.
In general the purified fibers are less extensively used in paper-making than for dissolving purposes in the manufacture of rayon, cellulose acetate, and other cellulose compounds. The requirements for this work are considerably more stringent than for paper production and the limiting amounts of metallic impurities have to be very carefully controlled.
The preparation of purified fiber might not be considered as a pulping process, since it starts with fibers already produced in a previous operation. There are, however, two pulping processes yielding high grade fiber, which are worthy of some attention.
The neutral sulfite process, sometimes called the "Keebra" process, dates back at least as far as 1880, when a method was developed for using alkaline solutions of sulfite in iron digesters. The name "Keebra" was not connected with it until about 1923. The process employs sodium sulfite with enough alkali added to neutralize the acids developed during the cooking of the wood. The alkali maybe caustic soda, soda ash, or sodium bicarbonate. If one of these is not used, corrosion of iron digesters, evaporators, etc., is likely to be severe, but even under the best conditions it is much worse than in the soda or sulfate processes, and lined digesters are recommended.
This method of cooking is said to give appreciably higher yields of fiber than the soda or sulfate process on the same woods. The fiber from softwoods is very similar to sulfate pulp in strength characteristics, though bleaching more readily; the fiber from hardwoods is about the equivalent of soda fiber from such woods.
Although the process gives a high yield of good fiber it has not been widely adopted, probably because of corrosion troubles and the complications involved in efficient recovery of the alkali.
The second process for producing high-grade fiber is that which uses a combination of treatments with alkali and chlorine. Such methods were used for bleaching as early as 1787, and it is claimed that straw pulp was so made in 1830. It was not until considerably later that any real progress was made, but in the early part of the present century two processes were developed commercially. The deVains process employed chlorine water for the chlorination step, while the Cataldi (later known as the Pomilio) process, used gaseous chlorine. Both began as batch processes, but the deVains method proved unsatisfactory, while the Pomilio process later developed into a successful continuous method.
The Pomilio process as applied to straw involves four steps:
(1) an alkaline pretreatment, or digestion, of the raw material,
(2) chlorination of the predigested material, (3) an alkaline wash to dissolve and remove the products of chlorination, and (4) a final slight bleaching with hypochlorite.
The alkaline digestion is much more important than would be inferred from the name "chlorination," which is often applied to the process. It is carried out by boiling with dilute caustic soda at atmospheric pressure, unless the material is highly lignified, in which case a digestion under pressure is desirable. For straws and grasses the time of boiling may be from 1 to 2 hours and the strength of the caustic soda from 5 to 30 grams per liter. The ratio of straw to liquid in the charge may vary from 1 to 3 to as low as 1 to 6. Nine per cent of caustic soda on the weight of straw is about average practice.
In the continuous process the mixture of straw and alkali enters at the top of a sheet iron tower and when part way down comes in contact with steam. The rate of passage through the tower is regulated by extractors at the bottom, which remove the treated material and allow the mass above to pass gradually downward. Because of the dilution of the alkali used in cooking, its recovery is not attempted and the alkaline stock from the digestion tower goes directly through a washing and pressing device. The washed stock is then loosened up to put it into good condition for chlorine absorption in the chlorination tower, to which it next passes. This tower is similar to the digestion tower, except that it must be tile lined because of the acid and corrosive nature of its contents. After chlorination the pulp is given a washing with dilute alkali to remove chlorinated compounds, and is then bleached with hypochlorite. For fully bleached straw pulp about 10.5 per cent chlorine on the weight of straw is needed.
The yield of pulp and its quality vary to some extent with the kind of straw used. Wheat straw yields about 38.5 to 42 per cent of fiber, rice straw 32 to 39 per cent and esparto about the same as wheat straw. Fiber from rye straw is said to produce much stronger paper than that from wheat. The pulps made by this process are said to be very different from those made by cooking with caustic soda; they give softer and more bulky papers with much less of the hard, rattly character usually associated with papers made from straw.
The Pomilio process is not used in America but it is finding rather wide application in other parts of the world, chiefly for straws and grasses. It can be operated advantageously only under certain conditions, since it depends on the electrolysis of salt for its chemicals, and it must be located near a source of cheap salt and power. There are at present plants of this type operating in Italy, Argentina, Chile, Uruguay and South Africa.
A third chemical process, which has attracted considerable attention as a means of utilizing agricultural wastes, is that which employs nitric acid as the pulping agent. This treatment can be used with woods also, and has been applied to beechwood in Germany, but it has not been employed on a commercial scale in America. As applied to bagasse this process includes steeping the material in a 5 per cent solution of nitric acid for 3 hours, draining, heating for 1 hour at 176°F., washing, boiling in a 2 per cent solution of caustic soda for 35 to 45 minutes, and again washing. With other wastes, or with woods, the conditions of treatment would vary in intensity or duration, but the essential steps are the same. Good yields of readily bleachable fiber are said to be obtained. The fibers are said to be more resistant to beating than soda fibers from the same wood, and to have very interesting papermaking characteristics. This process can be carried out in open tanks at atmospheric pressure, but all of the equipment must be constructed of acid-resistant material.
It is difficult to predict the future of this process. The fibers would seem to be useful for both papermaking and dissolving purposes, but whether they can be made cheaply enough to compete with other processes depends on the cost of nitric acid, the expense of installing acid-proof equipment, and the possibilities of re-use of the acid and of recovery of by-products.
Some study has been made of processes depending on the solution of the lignin in the wood by organic reagents, and it has been found that fibers could be prepared by digesting with a mixture of water and some of the higher alcohols. The cost of such reagents is relatively high and very efficient recovery would be necessary if they were to be used. Since the fiber seems to have no advantages over that prepared by customary processes the possibilities of the treatment depend on finding a profitable use for the lignin dissolved during the digestion.
In addition to these processes, which depend almost entirely on chemical reactions for their success, there are others which employ both chemical and mechanical treatments. These semi-chemical processes for the production of fiber are finding a wide use for certain types of paper and boards. The basic principle is that of a slight cooking treatment with chemicals, which is insufficient to pulp the chips fully, followed by a mechanical disintegration and refining to separate the fibers of the softened chips. The chemicals for such processes may be any of those used in the regular cooking procedures, and quite a number of others have been proposed, such as lime, calcium polysulfide, chlorides and nitrates of the alkali and alkaline earth metals, etc. By reason of its more general use, neutral sodium sulfite is usually thought of when semi-chemical pulp is mentioned. This is perhaps unfortunate, since soda, sulfate or acid sulfite cooking liquors can serve equally well.
The cooking conditions vary with the wood used and the type of product desired. As an example, a neutral sulfite cook might be made by impregnating the chips under pressure with a solution containing sodium sulfite and sodium carbonate in the ratio of 4 to 1, and blowing out the excess liquor after the penetration period. The cook would then be completed by steaming at 285° to 340°F., for 1 to 6 hours. The sodium carbonate in the liquor serves to neutralize the acids set free from the wood, and thus maintain neutrality and prevent corrosion of the digesters. The quantity of chemicals used need not exceed 15 per cent of the weight of the wood in the digester charge.
Such cooks do not reduce the chips to pulp, and originally they were made in rotary digesters from which the cooked chips could be dumped readily. It has been found that by proper manipulation they can be discharged from stationary, vertical digesters, and these are used in the newer mills.
The mechanical defibering of the softened chips can be carried out by any of the various "refiners" which are now available. The principle on which many of these operate is that of tearing the chips to pieces between two discs with roughened surfaces, one or both of which revolve. The chips enter the center of the discs and pass outward between them to be discharged at the periphery. It is not necessary to employ this type of refiner, as any mechanical treatment which breaks the chips up sufficiently will do the work —rod mills, for example, have been found entirely satisfactory. It has even been proposed to treat the chips in a rod mill with 7 to 10 per cent of caustic soda for 10 to 30 minutes to defiberize them.
The advantage of the semi-chemical processes is largely in the high yield of fiber, which is intermediate between the 90 per cent for groundwood and the 45 to 50 per cent for the chemical pulps. Most investigators set 75 to 78 per cent as the maximum practical yield, though a few make higher claims. The strength of such pulps is usually high, but depends somewhat on the degree of cooking; it increases quite rapidly as the yield drops. Some pulps at 70 to 80 per cent yield have strength characteristics approaching those of chemical pulps. The fibers are well separated and only slightly fibrillated. They are intermediate in appearance between groundwood and chemical fibers and on beating they develop strength more nearly like the chemical pulps. Semi-chemical pulps from hardwoods develop more strength than chemical pulps from the same woods, but for softwoods the relative strengths are reversed.
The color of semi-chemical pulps varies with that of the wood from which they are made and with the degree of cooking. The process was developed as a means of utilizing chestnut chips from which the tannin had been extracted, and as the pulp was used in corrugated boards the dark color was not a serious problem. Pulps made by all semi-chemical processes are still employed chiefly for making box boards, but they are also used to some extent in wrap-ping papers, insulating boards, roofing felts, etc. If they are refined sufficiently to eliminate all shives they tend to make hard, brittle, and translucent sheets, and the tearing strength and freeness of the stock are reduced more than is desirable.
Neutral sulfite semi-chemical pulp can be bleached to a fairly good whiteness—about 75 brightness—and this bleached product may be used with groundwood in making body stock for papers that are coated on the paper machine.
Much of this description applies to neutral sulfite as the reagent used, but the general principles are the same when other chemicals are employed. By cooking with calcium bisulfite a yield of 63 to 75 per cent of fiber can be obtained from spruce, and such fiber can be substituted for the sulfite fiber used in newsprint. The calcium polysulfide process was designed to yield bleachable fiber, but the dark-colored, intermediate product proved usable for wrapping papers.
A step below the semi-chemical processes in the grade of fiber produced are those which might be termed "chemi-groundwood" processes. Steaming of wood prior to grinding might be considered such a process, and as early as 1916 a process was developed for treating the logs under pressure with sulfur dioxide and then cooking with water, salt solution, or bisulfite liquor before grinding. This is said to give stronger fiber of better color, which is suitable for newsprint without the addition of sulfite fibers.
More recently such a process has been developed for treating aspen, maple, birch and beech logs and producing mechanical pulp which can be used in place of spruce groundwood in many grades of paper. The wood in log form is placed in a vessel from which the air is then exhausted for a short time. The cooking iquor, of any desired composition, is then admitted, the temperature raised by steaming, and a pressure pump used to force more liquor into the digester until the pressure reaches about 200 pounds per square inch. After about six hours of this treatment the liquor is blown from the digester, and the logs removed and ground in the usual way for preparing groundwood. The yield is said to be 85 to 90 per cent of the original wood, and the pulp can be bleached with peroxide to a color about equal to that of ground-wood made from spruce wood.
Another variation of the semi-chemical process is the Asplund process, which depends on the fact that when wood chips are heated to 212° to 390°F. in the presence of moisture the lignin is so softened that the chips can be reduced to fibers by a mechanical grinding or pulping operation. A yield of 92 to 96 per cent on the moisture free basis is obtained and the fibers are well separated and largely unbroken, differing in this respect from ground-wood, though containing essentially the same chemical impurities. This type of fiber has found considerable use in making insulating wallboard, as well as producing dense, hard boards.
The apparatus which produces this fiber operates continuously. The chips containing about 50 per cent of moisture are forced into the heating chamber by a plunger which presses the chips into such a solid mass that they withstand the pressure of 90 to 160 pounds applied during the steaming operation. After less than 2 minutes of steaming the chips reach the defibering discs, which operate at the same temperature and pressure as in the heating chamber. The discs are two in number, one stationary and the other revolving at 500 to 600 revolutions per minute. The heated chips are fed to the center of the stationary disc and the fibers leave at the periphery, passing through a valve to a discharge chamber from which they are discharged to atmospheric pressure through another valve. These two valves, operating alternately, serve to maintain pressure in the heating and defibering sections.
An improvement in this apparatus enables the chips to be held for a longer time in the heating chamber, and if suitable chemicals are sprayed in with the steam, a semi-chemical pulp can be produced in a continuous operation. The yields are lower in pro-portion to the degree of the chemical treatment, but the fiber finds wider use in grades of paper and boards for which the regular Asplund fiber is not suited.