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."