Alexander Watt

TREATMENT OF WOOD (continued).

Sulphite Processes.—Francke's Process.—Ekman's Process.—Dr. Mitscherlich's Process.—Ritter and Kellner's Boiler.—Partington's Process.—Blitz's Process.—McDougall's Boiler for Acid Processes.—Graham's Process.—Objections to the Acid or Sulphite Processes.—Sulphite Fibre and Resin.—Adamson's Process.—Sulphide Processes.—Mechanical Processes.—Voelter's Process.—Thune's Process.

Sulphite Processes.—An important and successful method of treating wood has been found in employing sulphurous acid, combined in certain proportions with soda, lime, or magnesia, whereby a bisulphite of the alkaline or earthy base is obtained. One of the principal attributes of these agents is that in boiling wood at high pressures oxidation and consequent browning of the fibres is prevented. Of these sulphite, or more properly bisulphite, processes, several of those referred to below have been very extensively adopted, and vast quantities of so-called "sulphite pulp" are imported into this country from Norway, Germany, Scandinavia, &c., the product from the latter source being considered specially suited for the English market. Some of these processes are also being worked in this country, but more particularly those of Partington, McDougall, and Ekman.

Francke's Process.—In this process, which is known as the "bisulphite process," the active agent employed for the disintegration of wood is an acid sulphite of an alkaline or earthy base, as soda or potassa, lime, &c., but it is scarcely necessary to say that the process has since been modified by others. The invention is applicable to the treatment of wood, esparto, straw, etc., and may be thus briefly described:—A solvent is first prepared, which is an acid sulphite of an alkali or earth, that is, a solution of such sulphite with an excess of sulphurous acid. As the cheapest and most accessible base the inventor prefers lime. It has long been known that a solution of sulphite of lime, combined with free sulphurous acid, would, at a high temperature, dissolve the intercellular portions of vegetable fibres, leaving the fibres in a suitable condition for paper manufacture; but Mr. Francke claims to have determined the conditions under which this can be effected with rapidity, and in such a way as to preserve the strength of the fibres, and to have obtained a practical method of preparing pulp by his process. For his purpose he employs a moderately strong solution of the solvent at a high temperature, with gentle but constant agitation. The acid sulphite is produced by this process at small cost and at a temperature nearly high enough for use in the following way:—A tower or column is charged with fragments of limestone, which are kept wetted with a shower of water; fumes of sulphurous acid, produced by burning sulphur, or by roasting pyrites, etc., are then passed through the tower. The liquid which collects at the bottom of the tower is the desired solvent, which should have a strength of 4° to 5° B. It is not essential that the limestone should be pure, as magnesian limestone, etc., will answer equally well. The soluble alkalies, as soda and potassa, may also be used when their greater cost is not an objection. But for these alkalies the treatment is modified, as follows:—The tower is charged with inert porous material, such as coke, bricks, etc., and these are kept wetted by a shower of caustic alkali at 1° to 2° B., while the sulphurous acid fumes are passed through the tower. In like manner carbonate of soda or potassa may be used, but in this case the solution showered on the porous material should be stronger than that of the caustic alkali, so that it may contain approximately the same amount of real alkali. Whichever alkaline base be employed, the liquid collected at the bottom of the tower should have a strength of 4° to 5° B.; this being the acid sulphite of the base is used as the solvent employed for the manufacture of pulp. When wood is to be treated, it is freed as much as possible from resinous knots by boring and cutting them out, and is then cut—by preference obliquely—into chips of a ¼ to ¾ of an inch thick. Esparto, straw, and analogous fibres are cut into fragments. The fibrous material and solvent are charged into a digester heated by steam at a pressure of four or five atmospheres, and consequently capable of raising the temperature of the contents to about 300° F. As agitation greatly promotes the pulping of the materials, Mr. Francke employs a revolving cylindrical boiler, which is allowed to revolve while the charge is under treatment.

Ekman's Process.—In this process, which in some respects bears a resemblance to the preceding, native carbonate of magnesia (magnesite) is first calcined to convert it into magnesia; it is then placed in towers lined with lead, and sulphurous acid gas, obtained by the burning of sulphur in suitable furnaces, is passed through the mass, a stream of water being allowed to trickle down from the top of the towers. The supply of gas is so regulated that a continual formation of a solution of bisulphite of magnesium, of an uniform strength, is obtained; great care, however, is necessary to avoid excess and consequent loss of sulphurous acid by its conversion into sulphuric acid. In boiling, the fragments of wood, previously crushed by heavy rollers, are placed in a jacketed, lead-lined, cylindrical boiler, suspended on trunnions, so that it can be inverted to remove the charge. The pressure in the outer jacket is 70 lbs. per square inch, and that within the boiler is 90 lbs. per square inch. The boiling occupies twelve hours. This process has been extensively worked by the Bergvik and Ala Company, of Sweden, for many years with great success, and we understand that the company has been turned over to an English company—the Bergvik Company, Limited. The Ilford Mill and Northfleet Works have been largely supplied with sulphite pulp from the Swedish works.

One great drawback to the bisulphite processes is that the boiling cannot be effected in iron boilers unless these be lined with some material which will protect the iron from the destructive action of the bisulphite, which, being an acid salt, would exert more action upon the iron than upon the fibre itself, and the solution of iron thus formed would inevitably prove injurious to the colour of the fibre. In several of the systems adopted iron boilers lined with lead have been used, but the heavy cost of this material and its liability to expand unequally with the iron, especially at the high temperatures which the solvent necessarily attains under pressure, causes the lead to separate from the iron, while it is apt to bulge out in places, and thus becomes liable to crack and allow the acid liquor to find its way to the interior of the iron boiler which it was destined to protect. To overcome this objection to the simple lead lining, Dr. Mitscherlich patented a process which has been extensively adopted in Germany, and is now being carried out by several companies in different parts of America. This process is briefly described below.

Dr. Mitscherlich's Process.—The digester employed in this process is lined with thin sheet lead, which is cemented to the inner surface of the boiler by a cement composed of common tar and pitch, and the lead lining is then faced with glazed porcelain bricks. In this process a weaker bisulphite of lime is used than in Francke's, and the time of boiling is consequently considerably prolonged.

Ritter and Kellner have proposed to unite the inner surface of the boiler to its lead lining by interposing a soft metal alloy, fusible at a temperature lower than that of either metal, and it is claimed that the iron and lead are thus securely united, while the alloy being fusible under the normal working temperature of the digester, the lead lining can slide freely on a boiler shell.

Partington's Process.—This process, which has been for some time at work at Barrow, and for the further development of which a private company, entitled the Hull Chemical Wood Pulp Company, Limited, has been formed, consists in the employment of sulphite of lime as the disintegrating agent. The process consists in passing gaseous sulphurous acid—formed by burning sulphur in a retort, into which is forced a current of air at a pressure of 5 lbs. to the square inch—through a series of three vessels, connected by pipes, the vessels being charged with milk of lime. The first two of these vessels are closed air-tight, and the gas is then introduced, while the third vessel remains open; from this latter a continuous stream of nitrogen escapes, due to the removal of the oxygen by the burning sulphur from the air passed into the retort. This process is said to be a very economical one, so far as relates to the cost of materials used.

Blitz's Process.—This process consists of employing a mixture composed of bisulphite of soda 2 parts, caustic soda 1 part; and vanadate of ammonia 1 gramme, in hydrochloric acid 4 grammes to every 6 kilogrammes of the bisulphite. The wood, after being cut up in the ordinary way, is submitted to the action of the above mixture, under a pressure of three or four atmospheres, for from four to eight hours, and the pulp is then ground; it is said to possess some of the qualities of rag pulp and to look much like it.

McDougall's Boiler for Acid Processes.—This invention is intended to obviate the difficulties which arise in using lead-lined boilers, owing to the unequal expansion and contraction of the lead and the iron on their being alternately heated by steam and cooled, on the discharge of each successive batch of pulp. This invention consists in constructing the boilers with an intermediate packing of felt, or other compressible and elastic material, so that when the interior leaden vessel is heated, and thereby enlarged and pressed outwards by the steam, the compressible and elastic packing yields to the pressure and expansion. Also in the cooling of the vessels the packing responds to the contraction, and approximates to its original bulk and pressure between the two vessels, and so prevents the rupture or tearing of the lead and consequent leakage and other inconveniences. Another part of this invention consists in the construction of the outer iron or steel vessel in flanged sections, which are fitted to incase the interior leaden vessel with a space between the two vessels, into which the compressible and elastic materials are packed. In the construction of these vessels the iron or steel flanged sections are placed on to the leaden vessel and packed with the compressible and elastic lining in succession. As each section is packed it is screwed close up to the adjoining section by the screw bolts, fitted into corresponding holes in the flanges of the contiguous section until completed. This method of construction secures economy by the retention of the heat, which is effected by the packing between the two vessels. The materials used for the packing are caoutchouc, felt, flocks, asbestos, etc., and a space of about two inches between the vessels is preferred, into which the packing is filled.

Graham's Process.—This process consists in boiling fibrous substances in a solution of sulphurous acid, or a sulphite or bisulphite of soda, potash, magnesia, or lime, or other suitable base and water. The boiling is preferable conducted in a closed boiler, lined with lead, to protect it from the action of the chemical substances used, and is fitted with a valve which can be opened to allow the gases and volatile hydrocarbons contained in and around the fibres to escape. The method of carrying out the process has been thus described:—"In carrying out the process there is a constant loss of sulphurous acid gas going on, and consequently a continual weakening of the solution employed, to avoid which it is preferable to employ monosulphite of potash, soda, magnesia, lime, or other suitable base, and water. Either of these substances, or a suitable combination of them, and water are placed in the boiler with the fibrous substances to be treated, and the temperature raised to the boiling point. After the hydrocarbons, air, and gases natural to the fibrous substances have been driven out by the heat and allowed to escape, sulphurous acid, in its gaseous or liquid state, or in combination with either of the bases referred to, is pumped or injected into the boiler. There is thus forming in the closed boiler a solution containing an excess of sulphurous acid above that required to form, in combination with the base, a monosulphite. The operation of injecting sulphurous acids, or the sulphites, may be repeated from time to time during the boiling, so as to fully maintain, and if necessary increase, the strength and efficiency of the chemical solution. It is said that by this process a saving of the chemicals employed is effected, as little or no sulphurous acid gas is lost during the time the gaseous hydrocarbons, air, and other gaseous matters are being expelled from the fibrous materials. If an open vessel is used instead of a closed boiler, it will be necessary to keep the solution at a fairly uniform strength, and if necessary to increase the strength, but the result will be substantially the same; but as it is evident that, when using an open boiler, the excess of sulphurous acid supplied during the boiling will be constantly driven off as gas, it must be replaced by further injections, while the acid fumes may be conveyed away and condensed, so as to be available for further use. When the fibrous substances are boiled as above, with the addition of potash, soda, etc., during the boiling, the result will be equally beneficial. The inventor prefers to inject the sulphurous acid or its combinations into the boiler at the bottom, and to cause it to come in contact with the solution therein before reaching the fibrous materials. For this purpose there is formed a kind of chamber beneath the boiler, but separated from it by a perforated disc or diaphragm of lead or other suitable material not acted upon by the solution, so as to allow the latter to fill the chamber, to which is connected a pipe, through which the sulphurous acid or solutions of the sulphites is forced by any suitable apparatus.

Objections to the Acid or Bisulphite Processes.—While the various methods of boiling wood in caustic soda at high temperatures are well known to be open to serious objections, the acid treatment of wood also presents many disadvantages, which it is to be hoped may be yet overcome. In reference to this, Davis makes the following observations:—"In the acid treatment of wood for the purpose of converting the fibres into pulp for use in paper manufacture, the general practice has been to use alkaline solutions of soda, combined in various proportions with certain acids, such, for instance, as sulphurous acid, hydrochloric acid, etc. These solutions have been heated in digesting vessels, and the high temperature resulting from this process of heating developing a pressure of from six to seven atmospheres, the wood being disintegrated by the action of the boiling solutions, the gum, resinous constituents, and other incrustating or cementing substances that bind the fibres together are decomposed, destroyed, or dissolved, while pure cellulose, which constitutes the essential element of the ligneous fibres, is separated therefrom. To this end high temperatures had to be employed, otherwise the disintegration was found to be only partial, the wood remaining in a condition unfit for further treatment. The high temperature not unfrequently converts a large proportion of the resinous and gummy constituents of the wood into tar and pitch—that is to say, carbonaceous bodies that penetrate into the fibre and render its bleaching difficult, laborious, and costly, while the frequent washing and lixiviation necessary to bleach such products seriously affect the strength of the fibre and its whiteness, and also materially reduce the percentage of the product, in some instances to the extent of 18 per cent. These difficulties and detrimental results materially enhance the cost of production, while the fibre itself suffers considerably in strength from the repeated action of the chloride of lime.... The difficulties are chiefly due to the carbonisation of certain constituent parts of the fibres under temperatures exceeding 212° F., such carbonised matters being insoluble and incapable of being bleached, and as they permeate the fibre, cannot be entirely removed.

"To overcome these difficulties, the wood should be chemically treated at a temperature sufficiently low to ensure that the decomposition of the connecting substances of the fibres will remain chemically combined with the other elements, such as hydrogen, oxygen, and nitrogen, in order to obtain an increased product of superior quality and render the process more economical."

Sulphite Fibre and Resin.—A German manufacturer sent the following communication to the Papier Zeitung, which may be interesting to the users of sulphite pulp:—"In making [disintegrating] cellulose by the soda or sulphite process, the object in boiling is to loosen the incrusting particles in the wood, resin included, and to liberate the fibres. The resin is dissolved both in the soda and sulphite processes, but in the former it is at the same time saponified, and is consequently very easily washed out. In the case of sulphite fibre, however, the resin attaches itself by its own adhesiveness to the fibres, but can also be removed by as hot washing as possible, and adding a little hydrochloric acid, which produces a very great effect. At the same time, however, sulphite fibre loses in whiteness by thorough washing, and assumes a reddish-grey shade. As the paper manufacturer insists upon white fibre, the manufacturer of sulphite fibre not only often omits washing, but adds some sulphite solution (bisulphite of lime). This not only enables him to give his customers white fibre, but he also sells a quantity of the incrusting particles and sulphite residuum as cellulose.

"So long as the manufacturer looks more to white than to well-washed cellulose, or does not wash it well before working up the fibre, these annoyances cannot be avoided. Not only this, but other disadvantages will be added in the course of time, as the action of the sulphurous acid in the pulp will have very injurious consequences on metals—[and on the fibre itself?] especially iron—coming in contact with it. This should be the more avoided, as the whiteness of the unwashed cellulose is of very short duration. The paper made from it soon turns yellow and becomes brittle. Well-washed sulphite fibre, on the other hand—provided no mistakes have been made in the boiling process—makes a strong, grippy paper, which can withstand both air and sunlight. I have made no special studies as to resin, but believe that pine and fir act differently, especially with solvents."

Adamson's Process.—Mr. W. Adamson, of Philadelphia, obtained a patent in 1871 for the use of hydrocarbons in the treatment of wood. His process consisted in treating the wood with benzine in closed vessels, under a pressure of 5 to 10 lbs., according to the nature of the wood. His digester consisted of an upright cylinder, in which the wood-shavings were placed between two perforated diaphragms. The mass was heated beneath the lower diaphragm by a coil through which steam was passed. The vapours which were given off were allowed to escape through a pipe on the top of the digester, to which was connected a coil immersed in a vessel of cold water, and the condensed liquid then returned to the lower part of the digester. The remaining portion of the benzine in the digester, which was still liquid but saturated with the extracted matters, was drawn off through a faucet at the bottom. Benzine being a very cheap article in America, a similar process was recommended in another patent by the same author for extraction of pitch and tar from rags [tarpaulin, ropes, &c.?], and for removing oil from rags and cotton waste.

Sulphide Processes.—Many attempts were made about thirty years ago, and in subsequent years, to employ the soluble sulphides as a substitute for caustic soda in boiling wood and other fibres, but these processes do not appear to have been very successful. Later improvements in the construction of boilers or digesters, however, seem to have induced further experiments in this direction, and we understand that several sulphide processes are being worked on the Continent, the processes of MM. Dahl and Blitz being amongst them. One of the supposed advantages of these sulphides over caustic soda is that by evaporation and calcination of the liquors, or leys, by which the organic matters become destroyed, the original product would be recovered, which merely requires to be dissolved out for further use. There are, however, several important objections to the use of sulphides in this way, amongst which may be mentioned the deleterious vapours which they emit; and this alone would doubtless prevent their employment—at all events in this country.

II. Mechanical Processes.—Besides the various chemical methods of separating cellulose from woody fibres, before described, certain processes have been devised for reducing wood to the condition of pulp directly by mechanical means without the aid of any chemical substance whatsoever. In this direction Heinrich Voelter, of Wurtemburg, appears to have been the first to introduce a really practical process for the conversion of wood into pulp for paper-making, although, as far back as 1756, Dr. Schaeffer, of Bavaria, proposed to make paper from sawdust and shavings mechanically formed into pulp: the process was not successful, however, with the machinery then at his command.

Voelter's Process for Preparing Mechanical Wood Pulp.—In 1860-65 and 1873 Voelter obtained patents in this country for his methods of treating wood mechanically, and the process may be thus briefly described:—Blocks of wood, after the knots have been cut out by suitable tools, are pressed against a revolving grindstone, which reduces the material to a more or less fine condition, but not in a powdery form, and the disintegrated fibre is caused to press against a wire screen, which allows the finer particles to pass through, retaining the coarser particles for further treatment.

The apparatus employed, which is shown in Fig. 17A, consists of a pulping apparatus A, with vat K, in which the revolving stone S is placed; the blocks of wood are held against the stone at p p, and water is introduced at G, and the revolving stone carries the pulp against the screen E, which admits the passage of the finer particles of the wood, while the coarser particles are led by the trough F to the first refining cylinder B, after passing through an oscillating basket, which retains the coarser particles. From thence it is led through a distributing apparatus and hopper C, to be uniformly supplied to the refining cylinder D, these cylinders being of the ordinary construction, and, as usual, covered with fine gauze wire sieves. The ground material which fails to pass through the sieves is transferred by an elevator to the millstones E, which are of ordinary construction, and after leaving these unites with the finer fibres which pass through E, the whole now entering a mixing reservoir F, whence it is thrown on to the cylinder G, and the pulp which passes into this is distributed on to a similar cylinder H, the contents of which then passes through the last cylinder I, which is differently constructed to the others, inasmuch as its lower part is surrounded by an impervious leather jacket, so that the pulp ascends in order to enter it. The disintegrated fibres that are retained by the wires of the cylinders pass into the refiners, which consist of a pair of horizontal cylinders of sandstone, one of which (the upper one) only revolves, and by the action of these the coarser fibres become further reduced, the finer particles, as before, passing through the wire gauze of the cylinders, the operation being repeated in the same order until the whole of the fibres have passed through the sieves.

Thune's Process.—Mr. A. L. Thune, of Christiana, U.S.A., has recently patented an apparatus for disintegrating wood, which consists of a grinding apparatus connected to a turbine. In this arrangement the grindstone, fixed on a shaft, is worked by a turbine, and the wood, which is used in small blocks, is pressed against the stone by means of a series of hydraulic presses. The fine pulp is afterwards made into thick sheets by means of a board-machine, the pulp, mixed with water, passing down a shoot into a vat beneath, in which is a revolving cylinder covered with wire-cloth, which in its revolution carries with it a certain quantity of pulp in a continuous sheet; this is taken on to an endless travelling belt by means of a small couch-roll, and passes on to a pair of rolls, round the upper one of which the sheet becomes wound, and is removed when sufficiently thick.