The first patented process claiming the use of sulphurous acid, in the form of its aqueous solution, for the purpose of disintegrating wood, was that of Tilghmann (1866). So far as is known, however, the results obtained by the inventor with the acid alone were such as to lead him to abandon it for solutions containing a certain proportion of base. The reasons, doubtless, were the greater facility of preparation and simplicity of application of such solutions.
More recently Professor Pictet, of Geneva, on the basis of the experience acquired by him in the preparation and employment of sulphurous acid in his patent freezing processes, has revived and perfected a process which in other hands had proved impracticable.
The method and apparatus employed will be found fully described in the author’s specification. The process consists in digesting the wood in closed vessels with an aqueous solution of the acid (10–12 per cent. SO2) at 176°–194° F. (80°–90° C.), and under the corresponding pressure (60–75 lb. per square inch). A complete disintegration of pine wood is effected in about twenty-four hours’ treatment. The yield of pulp is, we are informed, from 40–50 per cent. of the wood (Pinus sylvestris); it is also stated to be readily bleached {71} with a moderate amount of bleaching powder, and to yield a pulp of the highest quality. Further, the recovery of the sulphurous acid, for which special apparatus is provided, is practically complete (95–98 per cent.). In view of the probable utilisation of the bye-products, it is interesting to observe that they are obtained in a form in which they can very readily be treated.
The process being as yet undeveloped on a large scale, we cannot do more than recommend it to the consideration of paper-makers. At the same time we would draw attention to the features of interest which it possesses in regard to the general aspect of our subject. The resolution which it effects is obviously one of simple hydrolysis, and it affords an indirect proof of the chemical complications which attend all other processes for disintegrating wood which employ temperature much above the boiling-point of water. The soluble bye-products are obtained in the simplest form, and may be isolated by mere evaporation. As regards the chemical features of the pulp, they will presumably be different from those of any other, whether on the side of advantage or not it remains to decide. Provided the process fulfils the condition of economical working, we have no hesitation in predicting for it a considerable development.
The difficulties attending the employment of a gaseous acid have doubtless operated in deterring many, in addition to the originator, from a complete investigation of its action, and have led them to adopt in preference one of its several compounds with bases, the employment of which defines a group of processes to which we now desire to call attention. There is a variety of processes agreeing in the essential feature of employing a bisulphite as the disintegrating reagent, and differing from one another only in the less essential details of method.
The general principle of these processes will be readily understood from what has gone before. The chief agency is the hydrolytic action of sulphurous acid, aided by the conditions of high temperature and pressure: and the subsidiary {72} agencies are—(1) The prevention of oxidation; (2) The removal from the sphere of action of the soluble products of resolution in combination with the sulphite as a true double compound. This mode of union is well known to be peculiarly characteristic of all aldehydes, to which class we have shown that the non-cellulose constituents of wood belong. The combination is expressed by the general equation—
M.CHO
Aldehyde.
+
SO3HNa