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 ⅕ to ⅓ 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.
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
