(1) The Chemicals employed.—In both cases sulphur is used as the source of the sulphurous acid. The Ekman process starts from magnesite, a native carbonate of magnesia. This is converted by calcination into magnesia, and in this state is introduced into lead-lined towers, through which is passed sulphurous acid gas, obtained by burning sulphur in suitable furnaces, while a stream of water trickles down from the top. The supply of the gas being properly adjusted, a continuous production of a solution of magnesium bisulphite of uniform strength is obtained. To secure this uniformity, constant attention is necessary, and further, the loss of sulphurous acid by oxidation to sulphuric acid, and by diffusion, is, in consequence of its distribution over so large a surface, {73} and with free exposure to the air, necessarily very large. It is unnecessary, however, to point out that the preparation of the magnesium bisulphite is capable of very considerable modification; in fact, we understand that numerous improvements in this direction have already been introduced by the proprietors of the process.
Partington’s method, in addition to the advantage of starting from lime, the cheapest of all basic substances, is free from the disadvantages referred to as attending the use of a tower. It consists in passing the sulphurous acid gas through a series of three vessels connected together with pipes. Into these vessels a charge of milk of lime is introduced, and the two first being closed air-tight, the gas is passed in. The third remains open to the air, and from it escapes a continuous stream of nitrogen, resulting from the removal of the oxygen by the burning sulphur, from the stream of air which is blown through the sulphur retort at a pressure of 5 lb. to the square inch. The expenditure for chemicals is under 1l. per ton, whereas, by the Ekman process, it has been variously estimated at from 2l. to 3l. 10s. per ton. Provided therefore, that as good pulp can be obtained by Partington as by Ekman, the former offers considerable advantages both from point of view of simplicity and economy. Doubtless the cost of chemicals could be somewhat reduced in both systems by the adoption of a cheaper form of sulphur, such as pyrites.
Preparation of the Wood.—Although the acid sulphite processes can, by suitable modifications, be made available for the preparation of cellulose from almost all kinds of wood, yet it is necessary, in order to insure uniformity of result, not only to take care that the wood is carefully selected, but that it is freed from bark and roots, which, if allowed to remain, would be imperfectly pulped, and would be exceedingly difficult to bleach. The most suitable kind of wood is Pinus sylvestris.
After the bark has been carefully removed the logs are sawn into boards of ordinary sizes. This treatment {74} exposes the knots, which are then removed either by boring or cutting out with an axe. The pieces are then passed through a machine, which cuts them or rather breaks them into fragments about an inch long. In order that the acid sulphite may as quickly as possible enter into the wood, these fragments must be crushed and opened out by passing through heavy rollers: moreover, the wood is cut across the grain, by which means it is more or less split up into laminæ. A machine for this purpose has been patented by Fry, Ransome, and Wilkie (No. 11,389, 1884).
The Boiling Process.—Ekman uses a lead-lined, jacketed cylindrical boiler, suspended on trunnions, so that it can be turned upside down for discharging. The pressure in the outer jacket is 70 lb. per square inch, while inside the boiler it is 90 lb., the difference being due to the tension of the sulphurous acid gas. The duration of boiling is from ten to twelve hours. Partington, on the other hand, boils with live steam, and therefore is enabled to use a boiler of simpler construction. His boilers are spherical, lead-lined and rotary. The pressure employed does not exceed 60 lb. per square inch, the time of boiling being about sixteen hours.
Various other sulphite processes have been from time to time devised, which differ but little either in principle or in working detail from those described, and therefore a brief mention of them will suffice. Mitscherlich employs a bisulphite of lime in somewhat weaker solution than that employed by Francke or Partington, and in order to produce the same result he is obliged to extend the time of boiling to from 40 to 60 hours.
Graham, after treating the wood with a mono-sulphite, injects into the boiler either a solution of a bisulphite or gaseous or liquid sulphurous acid.
Flodgvist uses a solution prepared by dissolving bones in sulphurous acid solution.
The cellulosic products of the various processes present certain slight differences, but so far as has been investigated, not such as to constitute a distinct advantage for one or {75} another. One feature of these processes, to which but little attention has hitherto been paid, is that of the utilisation of their bye-products. These bodies are, as already indicated, obtained in a condition not materially different from that of the parent substance, the non-cellulose constituents of the wood. One important property of these bodies, and one capable of considerable application, has been made the basis of a patent by the authors. When a solution of gelatin is added to the liquors in which wood has been treated by any of the bisulphite or sulphurous acid processes, a body is precipitated in the form of a caoutchouc-like mass which can be dissolved in neutral sodium sulphite solution, or in weakly alkaline solutions, and can be reprecipitated on addition of alum to the solution. The substance consists of a compound of the gelatin with the non-cellulose constituents resembling, but not identical with, as has been supposed, the precipitate obtained by adding tannic acid to a solution of gelatin. The solution of this substance in sodium sulphite can be applied to many of the purposes to which gelatin itself is ordinarily applied.
A serious objection to all processes involving the use of sulphurous acid or acid sulphite, and one which has to some extent deterred paper-makers from adopting them, is the necessity of using a boiler lined with lead. In addition to the fact that such boilers are heavy and costly, they are objectionable on account of the difficulty and expense of keeping them in thorough repair. This difficulty arises from the fact that, owing to the different co-efficients of expansion of lead and iron, the lead lining originally placed in contact with the iron or steel shell of the boiler, becoming detached on heating, swells out in various places, which are thus rendered weaker and liable to fracture. This is especially the case if, in the course of lining the boiler, any air has been allowed to remain between the two linings. Various expedients have been proposed to obviate these defects but with only partial success. Graham (Eng. Pat. No. 5367, 1883) coats the plate of his boiler by pouring molten lead upon the perfectly clean surface which has been {76} previously prepared with zinc chloride. Sometimes minute holes are drilled in the iron plate to allow the escape of air.