Limestone is required in considerable quantity for the preparation of sulphite of lime for the manufacture of wood pulp.
Recovered Ash.—The black liquor obtained during the process of the boiling of straw, esparto, and other paper-making fibres contains a large proportion of non-fibrous organic constituents derived from the fibres, the quantity of which may be gauged from the fact that these fibres generally lose 50 per cent. of their weight when being boiled. The black liquor on evaporation yields a thick resinous mass, which is converted into carbonate of soda when burnt.
Advantage is taken of this fact to carry out a process of incineration on a large scale, so that heat derived from the burning off of the resinous mass is utilised for evaporation of weaker liquors. The ash is drawn from special furnaces, put aside, and allowed to char quietly, so that the carbonaceous matter is more or less completely burnt away. The ash in this form contains about 40 per cent. of soda, its composition being determined by the nature of the fibre which has been treated. In the case of straw, the amount of silicate is considerable, as shown by the following typical analysis:—
| Sodium carbonate | 70·2 |
| Sodium hydrate | 2·3 |
| Sodium sulphate | 4·1 |
| Sodium chloride | 7·5 |
| Silica | 7·5 |
| Oxides of iron and alumina | 0·75 |
| Unburnt carbon, etc. | 7·65 |
| 100·00 | |
At the present time there is no process in general use for the recovery of the liquors used in the treatment of wood by the sulphite process. Many schemes have been proposed, the most promising of which is that of Drewsen.
Sulphur and Sulphites.—The pale yellow brittle substance known as sulphur is too familiar to require any detailed description. It unites with oxygen in various proportions, and these in contact with water form the various sulphur acids known to commerce. Sulphur burned with a limited quantity of air forms sulphurous acid gas, and this substance is the chief product of oxidation, which by further treatment can be converted into sulphites.
In the manufacture of the sulphur compounds required in the preparation of wood pulp, the furnace for burning the sulphur consists of a flat-bottomed cast iron retort which is very shallow, and provided with a curved top, to which a pipe is fixed, so that the sulphurous acid may be conveyed away from the furnace. In the most recent form of sulphur oven a small conical-shaped revolving furnace is employed, which produces a satisfactory gas of constant composition very economically.
Bisulphite of Lime.—This compound is obtained when the sulphurous acid gas is brought into contact with moistened limestone. In the manufacture of bisulphite of lime on a large scale the sulphurous acid gas is drawn or pumped up tall circular towers filled with blocks of limestone, kept moistened by a carefully regulated stream of water flowing from the top of the tower.
In another system known as the acid tank process, the gas is forced into large circular vats containing milk of lime.
In either case a solution is prepared containing bisulphite of lime, together with a certain proportion of free sulphurous acid, the object of the pulp manufacturer being to obtain a solution containing as large a proportion of free sulphurous acid as possible. The composition of a solution will vary on this account, and the following may be quoted as being an example of such a liquor:—