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.
[CHAPTER VII.]
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.