Alexander Watt

TREATMENT OF WOOD.

Chemical Processes.—Watt and Burgess's Process.—Sinclair's Process.—Keegan's Process.—American Wood Pulp System.—Aussedat's Process.—Acid Treatment of Wood.—Pictet and BrÉlaz's Process.—Barre and Blondel's Process.—Poncharac's Process.—Young and Pettigrew's Process.—Fridet and MatussiÈre's Process.

The advantages of wood fibre as a paper material have been fully recognised in the United States and in many Continental countries, but more especially in Norway, Sweden, and Germany, from whence large quantities of wood pulp are imported into this country. There is no doubt that our home manufacturers have recently paid much attention to this material, and it is highly probable that wood, as an inexhaustible source of useful fibre, will at no distant date hold a foremost rank. Indeed, the very numerous processes which have been patented since the Watt process was first made known, indicate that from this unlimited source of fibre the requirements of the paper-maker may be to a large extent satisfied, provided, of course, that the processes for reducing the various suitable woods to the condition of pulp can be economically and satisfactorily effected. The great attention which this material has received at the hands of the experimentalist and chemist—the terms not being always synonymous—shows that the field is considered an important one, as indeed it is, and if successfully explored will, it is to be hoped, yield commensurate advantages both to inventors and the trade.

The object of the numerous inventors who have devised processes for the disintegration of wood fibre—that is, the separation of cellulose from the intercellular matters in which the fibres are enveloped—has necessarily been to dissolve out the latter without injury to the cellulose itself, but it may be said that as yet the object has not been fully attained by either of the processes which have been introduced. To remove the cellular matter from the true fibre or cellulose, without degrading or sacrificing a portion of the latter, is by no means easy of accomplishment when practised on an extensive scale, and many processes which present apparent advantages in one direction are often found to exhibit contrary results in another. The field, however, is still an open one, and human ingenuity may yet discover methods of separating wood fibre from its surrounding tissues in a still more perfect manner than hitherto.

The various processes for treating wood for the extraction of its fibre have been classified into: (1) chemical processes; and (2) mechanical processes. We will give precedence to the former in describing the various wood pulp processes, since the pulp produced by the latter, although extensively used, is chiefly employed, in combination with other pulps, for common kinds of paper. In reference to this part of our subject Davis says:—"Experience has dictated certain improvements in some of the details of those earlier methods, by which so-called 'chemical wood pulp' is manufactured very largely on the Continent of Europe.... It is possible to obtain a pulp of good quality, suitable for some classes of paper, by boiling the chipped wood in caustic soda, but when it is desired to use the pulp so prepared for papers having a perfectly white colour it has been demonstrated in practice that the action of the caustic soda solution at the high temperature which is required develops results to a certain degree in weakening and browning the fibres, and during the past five years much labour has been expended in the endeavour to overcome the objections named. The outcome of these efforts has been a number of patents, having for their object to prevent oxidation and subsequent weakening of the fibres." In several of these patents, to which we shall refer hereafter, bisulphite of lime is employed as the agent to prevent oxidation and consequent degradation of the fibres, and in other processes bisulphite of magnesia has been used for the same purpose. Davis further remarks: "Although a common principle runs through all these methods of preparing cellulose from wood, they differ in detail, as in the construction of the digesters employed, methods of treating the wood stock before boiling it in the sulphurous acid solution, and also as regards pressure, blowing off the sulphurous acid gas, etc., but all these processes present a striking similarity to the method patented by Tilghmann in 1867." There can be no doubt that the action of caustic soda, under high pressures, is highly injurious both to the colour and strength of the fibres, and any process that will check this destructive action in a thoroughly practical way will effect an important desideratum.

I. Chemical Processes: Watt and Burgess's Process.—This process, which, with some modifications, is extensively worked in America, consists in boiling wood shavings, or other similar vegetable matter, in caustic soda ley, and then washing to remove the alkali; the wood is next treated with chlorine gas, or an oxygeneous compound of chlorine, in a suitable vessel, and it is afterwards washed to free it from the hydrochloric acid formed. It is now treated with a small quantity of caustic soda in solution, which instantly converts it into pulp, which only requires to be washed and bleached, and beaten for an hour and a half in the beating engine, when the pulp is ready for the machine. The wood-paper process as carried out in America has been described by Hofmann, from whose work[15] we have abridged the following:—

The wood, mostly poplar, is brought to the works in 5-feet lengths. The bark having been stripped off by hand, it is cut into ½-inch slices by a cutter which consists of four steel knives, from 8 to 10 inches wide by 12 to 15 inches long, which are fastened in a slightly inclined position to a solid cast-iron disc of about 5 to 7 feet diameter, which revolves at a high speed, chopping the wood—which is fed to the blades through a trough—into thin slices across the grain. The trough must be large enough to receive the logs, usually 10 or 12 inches thick, and it is set at such an angle that the logs may slide down towards the revolving cutters; this slanting position only assists the movement of the logs, while a piston, which is propelled by a rack, pushes them steadily forward until they are entirely cut up. The piston, or pusher, then returns to its original position, fresh wood is put into the trough, and the operation repeated. In this way many tons of wood can be chopped up by one of these cutters in a day. The sliced wood is conveyed by trucks to an elevator by which it is hoisted up two storeys to a floor from which the boilers are filled. The boilers are upright cylinders, about 5 feet in diameter and 16 feet high, with semi-spherical ends, provided inside with straight perforated diaphragms, between which the chips from one cord of wood are confined. A solution of caustic soda, at 12° B., is introduced with the chips, and fires are started in a furnace underneath. At other works the boilers are heated by steam circulating through a jacket which covers the bottom and sides of the boiler.

The boiling is continued for about six hours, when the digestion is complete, and the contents of the boilers are emptied with violence, under the pressure of at least 65 lbs. of steam, which had been maintained inside. A large slide valve is attached to the side of each boiler for this purpose close to the perforated diaphragm, and connected by a capacious pipe with a sheet-iron cylinder of about 12 feet diameter and 10 feet high, which receives the contents—pulp, liquor, and steam. The object of these large chambers—one of which serves for two boilers—is to break the force of the discharging mass. The steam passes through a pipe on the top of each, and from thence through a water reservoir, while the liquid containing the pulp flows through a side opening and short pipe into movable boxes, or drainers, mounted on wheels, and each capable of holding the contents of one boiler; these boxes are pushed along a tramway up to the collecting chambers, where the pulp is received. In a building 132 feet long and 75 feet wide, ten digesting boilers are arranged in one straight line, and parallel with the boilers runs the main line of rails, side tracks extending from it to each of the chambers, and a turn-table is supplied at every junction. By this arrangement the drainer waggons can be pushed from the side tracks on to the main line, which leads to the washing-engines in an adjoining room. A system of drainage is established below the tramways, by which all the liquid which drains from the waggons is carried away and collected for treatment by evaporation; these carriers remain on the side tracks until the pulp is ready for the washing-engine.

When the greater portion of the liquor has drained off, warm water is sprinkled over the pulp from a hose for the purpose of extracting all the liquid which is sufficiently concentrated to repay the cost of evaporation—the most advantageous method of recovering the soda. The contents of the waggons—from the same number of boilers—are then placed in two washing-engines, each capable of holding 1,000 lbs. of pulp. After being sufficiently worked in these engines the pulp is transferred to two stuff-chests, and from thence conveyed by pumps to two wet-machines. The screens (strainers) of the wet-machines retain all impurities derived from knots, bark, and other sources, and the pulp, or half-stuff, obtained is perfectly clean and of a light grey colour. The pulp is bleached with solution of bleaching powder like rags, then emptied into drainers and allowed to remain therein with the liquid for twenty-four to forty-eight hours, or long enough to render the use of vitriol in the bleaching unnecessary. The portion of the white pulp which is to be worked up into paper in the adjoining mill is taken from the drainers into boxes running on tramways in the moist state, but all the pulp which has to be shipped to a distance is made into rolls on a large cylinder paper-machine with many dryers. The object being merely to dry the pulp, a very heavy web can be obtained, since the water leaves this pulp very freely. The wood pulp thus obtained is perfectly clean, of a soft, white spongy fibre, and a greater portion of it is mixed with a small proportion of rag pulp and worked into book and fine printing papers. Sometimes the wood pulp is used alone or mixed with white paper shavings for book paper. The fibres are rather deficient in strength, but as a material for blotting paper they are said to be unsurpassed, while the wood paper is much liked by printers.

The wood from poplar, which is generally preferred, furnishes a very white fibre, and is easily digested, but since the fibres are short it is sometimes found advantageous to mix them with longer fibres, as those of the spruce or pine, although the latter wood requires a much more severe treatment in boiling with alkali than the former. In reference to this process the following remarks appeared in The Chemist,[16] 1855:—"The process occupies only a few hours; in fact, a piece of wood may be converted into paper and printed upon within twenty-four hours." An interesting verification of this was published a few years since in an American paper, the Southern Trade Gazette, of Kentucky, which runs as follows:—"At a wood-pulp mill at Augusta, Ga., a tree was cut down in the forest at six o'clock A.M., was made into pulp, and then into paper, at six o'clock in the evening, and distributed amongst the people as a newspaper by six o'clock the next morning. From a tree to a newspaper, being read by thousands, in the brief round of twenty-four hours!" The wood-paper process referred to has given rise to many subsequent modifications, some of which we will briefly describe.

Sinclair's Process.—The wood is first cut into pieces about 1 inch broad, ?th inch thick, and from 2 to 3 inches long. It is then placed in a boiler and a solution of caustic soda, in the proportions of 600 gallons to 10 cwts. of dry wood, is poured over it. The boiler having been securely closed, the heat is raised till a pressure of 180 to 200 lbs. on the square inch is obtained, when the fire is withdrawn and the boiler allowed to cool, after which the ley is blown off, the top door removed, and the contents scalded. The discharge door is now opened and the pulp transferred to a poaching-engine to be washed with pure water, when the resin, &c., are easily removed and the clean fibres obtained, which, it is said, are longer and firmer than those obtained by other methods.

Keegan's Process.—By this method soft deal or pine is sawn up into pieces from 6 to 12 inches long and ½ inch thick, it being preferable that all the pieces should be of an equal size, but the smaller they are the more rapid, of course, will be the operation. The pieces of timber are placed in a cylindrical boiler, turning upon a horizontal axis while the digestion is progressing. In a second boiler is prepared a solution of caustic soda of about 20° B. (specific gravity 1·161), which is introduced through a pipe into the first boiler, this being afterwards hermetically closed, and the soda is forced into the pores of the wood by means of a pump. When the wood is not more than half an inch in thickness a pressure of 50 lbs. on the square inch is sufficient, and the injection of the caustic soda solution is completed in half an hour. The superabundant liquor is pumped back into the second boiler for the next operation. The excess of liquor having been removed from the wood as stated, steam is introduced between the double sides of the first boiler, and the temperature of the wood raised from 150° to 190° C. (334° to 438° F.). The wood is next washed in the usual way until the liquor runs off perfectly limpid, and the half-stuff thus produced may be converted into pulp either before or after bleaching, according to the quality and colour of the paper to be produced.

American Wood-Pulp System.—Another method of carrying out the wood-pulp process has recently been described by Mr. E. A. Congdon, Ph.B.,[17] from which we extract the following:—"Poplar, pine, spruce, and occasionally birch, are used in the manufacture of chemical fibre. Pine and spruce give a longer and tougher fibre than poplar and birch, but are somewhat harder to treat, requiring more soda and bleach. Sticks of poplar, freed from bark, and cleansed from incrusting matter and dirt, are reduced to chips by a special machine having a heavy iron revolving disc set with knives, and are then blown by means of a Sturtevant blower into large stove chambers after passing over a set of sieves having 1¼-inch for the coarse and 1?-inch mesh for the fine sieves, from whence they pass to the digesters, which are upright boilers 7 by 27 feet, with a manhole at the top for charging the chips and liquor, and a blow-valve at the bottom for the exit of the boiled wood. A steam-pipe enters at the bottom, beneath a perforated diaphragm, and keeps the liquor in perfect circulation during the boiling of the wood by means of a steam-ejector of special construction."

Boiling.—The average charge of wood for each digester is 4·33 cords,[18] giving an average yield of 4,140 lbs. of finished fibre per digester. A charge of 3,400 gallons of caustic soda solution of 11° B. is given to each digester charged with chips, and the manhead is then placed in position and steam turned on. Charging the digester occupies from thirty to forty-five minutes, and steam is introduced until the gauge indicates a pressure of 110 lbs., which occupies about three hours. This pressure is kept up for seven hours, when it is reduced by allowing the steam to escape into a large iron tank which acts as a separating chamber for the spent liquor it carries, the steam entering in at one end and passing out at the other through a large pipe, the liquor remaining in the tank. The steam is allowed to escape until the pressure is reduced to 45 lbs., when the digester is blown. The blow-cap being removed, the blow-valve is raised and the contents of the digester are discharged into a pan of iron covered with a suitable hood. The contents strike against a dash-plate placed midway in the pan, which thoroughly separates the fibres of the wood. The time occupied in the foregoing operations is from eleven to eleven and a half hours. It takes from nine to ten hours to free the pans from alkali, when they are removed to washing-tanks with perforated metal bottoms, where the material receives a final washing before being bleached.

Washing.—Each of the three digesters has a pan into which its contents are discharged, and there are also four iron tanks used for holding the liquors of various strengths obtained from the cleansing of the pulp and a fifth tank is kept as the separating-tank before mentioned. When the digester is blown, the pulp is levelled down with a shovel, and the liquor from the separating-tank is allowed to flow into it. The contents of the next strongest pan are pumped upon it, while at the same time the strongest store tank flows into this pan. This flowing from the tank to the pan, pumping from here to the pan just blown, and from there to the evaporators, is kept up until the liquor is not weaker than 6° B. hot (130° F.). The second pan is now down to 4° B. hot, and the process of "pumping back" is commenced. The two weakest tanks are put upon this pan and pumped out of the bottom of it into the two tanks in which are kept the strongest liquors. The two weak tanks have been filled in the process of completing the cleansing of the third pan (the weakest) on which water was pumped until the last weak tank stood at only ½° B. This pan, now cleaned, is hosed and pumped over to the washing tanks. A fresh blow is now made in this pan, and the same treatment kept up as with the first pan.

The foregoing system is thus illustrated by Mr. Congdon:—

A is levelled down; contents of separating-tank allowed to flow upon it; B is pumped on to A; at the same time liquor from the two strong store tanks is put on it (B), and this continued to be sent from A to the evaporator until it is now weaker than 4° B. hot; the process of "pumping back" is then commenced. The two weakest are allowed in succession to flow on to it, and the liquor purified from the bottom of B into the two strong tanks, filling No. 1, the stronger, before No. 2. The weakest are filled in the process of completing the cleansing of C, on which water is pumped until the last tank from it tests only ½° B. C is now hosed and pumped over to the washing tanks. A fresh digester is blown in C, and the process repeated as with A.

The above system has been modified by having an extra pan into which the liquor from the last pan blown (after sending to the evaporators until down to 6° hot, and bringing down to 4° hot, by the stored liquor) is pumped. When the strength is reduced to 4° the pumping is stopped. The liquor from this pan is put in the next pan blown, after the liquor from the separating-tank has been put upon it, whereby an economy in time is effected.

The pulp, after being partially cleaned in the pans, still contains an appreciable quantity of soda. It is hosed over to the washing-tanks and receives a final washing with hot water. When the pulp is thoroughly free from alkali, and the water flowing from under the tank is colourless, the contents are hosed down by hot water into the bleaching-tanks. The superfluous water is removed by revolving washers, and about 1,000 gallons of a solution of chloride of lime at 4° B. are then introduced, and the contents agitated as usual. The bleaching occupies about six or seven hours, when the pulp is pumped into draining tanks, where it is left to drain down hard, the spent bleach flowing away. The stock is then hosed and pumped into a washing-tank, where it acquires the proper consistency for the machine. From here it is pumped into the stuff chest, whence it goes over a set of screens and on to the machine, from which the finished fibre is run off on spindles. The rolls are made of a convenient size to handle, averaging about 100 lbs. each. The fibre is dried on the machine by passing over a series of iron cylinders heated by steam. The finished product is a heavy white sheet, somewhat resembling blotting paper. The whole of the foregoing operations are stated to occupy forty-five hours.

Aussedat's Process.—By this method the wood is disintegrated by the action of jets of vapour. In one end of a cylindrical high-pressure boiler, about 4½ feet in diameter and 10 feet high, is fixed a false bottom, whereby the wood placed upon it may be removed from the liquor resulting from steam condensed in the chamber, the whole being mounted on lateral bearings which serve for the introduction of the vapour, and the wood is fed through a manhole at the upper end of the boiler. Taps are fixed at the upper and lower ends for the liquid and uncondensed vapour. The wood having been placed in the boiler, the jet is gradually turned on in such a way that at the end of three or four hours the temperature becomes about 150° C., the pressure being about five atmospheres, which point is maintained for an hour. As the slightest contact between the wood and the condensed water would at once discolour the former, it is essential that the liquid be removed from time to time by one of the outlets provided for the purpose.

The treatment above described is said to be suitable for all kinds of wood, and although it is the usual practice to introduce it in logs about a yard long, any waste wood, as chips, shavings, etc., may be used. It is preferable, though not necessary, to remove the bark, but all rotten wood may be left, as it becomes removed in the condensed water. The logs, after the above treatment, by which the fibre is disintegrated and the sap and all matters of a gummy or resinous nature are removed, are afterwards cut up by any suitable means into discs of about an inch, according to the nature of the fibre required. These are then introduced into a breaker, in which they become converted into half-stuff, which, after being mixed with a suitable quantity of water is passed through mills provided with conical stones, in which it becomes reduced to whole-stuff. The pulp thus prepared is principally used in the manufacture of the best kinds of cardboard, but more particularly such as is used by artists, since its light brownish shade is said to improve the tone of the colours. Bourdillat says that in the above process the vapour has a chemical as well as a mechanical action, for in addition to the vapour traversing the cellular tissues of the wood and dissolving a considerable portion of the cell-constituents, acetic acid is liberated by the heat, which assists the vapour in its action on the internal substance of the wood.

Acid Treatment of Wood.—A series of processes have been introduced from time to time, the object of which is to effect the disintegration of wood fibre by the action of acids. The first of these "acid processes" was devised by Tilghmann in 1866, in which he employed a solution of sulphurous acid; the process does not appear to have been successful, however, and was subsequently abandoned, the same inventor having found that certain acid sulphites could be used more advantageously. Other processes have since been introduced, in which wood is treated in a direct way by the action of strong oxidising acids, as nitric and nitro-hydrochloric acids, by which the intercellular matters of the wood become dissolved and the cellulose left in a fibrous condition.

Pictet and BrÉlaz's Process.—By this process wood is subjected to the action of a vacuum, and also to that of a supersaturated solution of sulphurous acid at a temperature not exceeding 212° F. In carrying out the process a solution of sulphurous acid is used, consisting of, say from ? to ? lb. avoirdupois of sulphurous acid to each quart of water, and employed under a pressure of from three to six atmospheres at 212° F. Under these conditions the cementing substances of the wood "retain their chemical character without a trace of decomposition of a nature to show carbonisation, while the liquor completely permeates the wood and dissolves out all the cementing constituents that envelop the fibres." In carrying out the process practically, the wood is first cut into small pieces as usual and charged into a digester of such strength as will resist the necessary pressure, the interior of which must be lined with lead. Water is then admitted into the vessel and afterwards sulphurous acid, from a suitable receiver in which it is stored in a liquid form until the proportion of acid has reached that before named, that is, from 100 to 150 quarts of the acid to 1,000 quarts of water. The volume of the bath will be determined by the absorbing capacity of the wood, and is preferably so regulated as not to materially exceed that capacity. In practice it is preferable to form a partial vacuum in the digester, by which the pores of the wood are opened, when it will be in a condition to more readily absorb the solution and thereby accelerate the process of disintegration. When disintegration is effected, which generally occurs in from twelve to twenty-four hours, according to the nature of the wood under treatment, the liquor, which is usually not quite spent in one operation, is transferred to another digester, a sufficient quantity of water and acid being added to complete the charge. In order to remove the liquor absorbed by the wood, the latter is compressed, the digester being connected with a gas-receiver, into which the free gas escapes and in which it is collected for use again in subsequent operations. The bath is heated and kept at a temperature of from 177° to 194° F. by means of a coil in the digester supplied with steam from a suitable generator. The wood, after disintegration, undergoes the usual treatment to convert it into paper pulp, and may thus be readily bleached by means of chloride of lime. The unaltered by-products contained in the bath may be recovered and treated for use in the arts by well-known methods.

Barre and Blondel's Process consists in digesting the wood for twenty-four hours in 50 per cent. nitric acid, used cold, by which it is converted into a soft fibrous mass. This is next boiled for some hours in water and afterwards in a solution of carbonate of soda; it is then bleached in the usual way.

Poncharac's Process.—In this process cold nitro-hydrochloric acid (aqua regia) is employed for disintegrating wood in the proportions of 94 parts of the latter to 6 parts of nitric acid, the mixture being made in earthen vessels capable of holding 175 gallons. The wood is allowed to soak in the acid mixture for six to twelve hours. 132 lbs. of aqua regia are required for 220 lbs. of wood. When it is desired to operate with a hot liquid, 6 parts of hydrochloric acid, 4 parts of nitric acid, and 240 parts of water are used in granite tubs provided with a double bottom, and it is heated by steam for twelve hours and then washed and crushed.

Young and Pettigrew's Process.—These inventors use either nitric or nitrous acids, and the acid fumes which are liberated are condensed and reconverted into nitric acid.

Fridet and MatussiÈre's Process.—This process, which was patented in France in 1865, consists in treating wood with nitro-hydrochloric acid, for which purpose a mixture of 5 to 40 per cent. of nitric acid and 60 to 95 per cent. of hydrochloric acid is used, which destroys all the ligneous or intercellular matter without attacking the cellulose. After the wood (or straw) has been steeped in the acid mixture, the superfluity is drawn off, and the remaining solid portion is ground under vertically revolving millstones. The brownish-coloured pulp thus obtained is afterwards washed and bleached in the usual way.

It is quite true that cellulose can be obtained from wood and other vegetable substances by treatment with nitric acid alone, or with a mixture of nitric and hydrochloric acids, but it will be readily seen that the employment of such large quantities of these acids as would be required to effect the object in view on a practical scale, would be fraught with incalculable difficulties, amongst which may be mentioned the insuperable difficulty of obtaining vessels that would resist the powerful corrosive action of the acids. Moreover, since nitric acid forms with cellulose an explosive substance (xyloidin) of the gun cotton series, the risk involved in the drying of the cellulose obtained would be quite sufficient to forbid the use of processes of this nature.