Savelsberg explains the reaction of this process as follows:
“1. The particles of limestone act mechanically, gliding in between the particles of lead ore and separating them from one another. In this way a premature sintering is prevented, and the whole mass is rendered loose and porous.
“2. The limestone moderates the reaction temperature produced in the combustion of the sulphur, so that the fusion of the galena, the formation of dust and the separation of metallic lead are avoided, or at least kept within the limits permissible. The lowering of the temperature of reaction is due partly to the decomposition of the limestone into caustic lime and carbon dioxide, in which heat is absorbed, and partly to the consumption of the quantity of heat which is necessary in the further progress of the operation for the formation of a slag from the gangue of the ore and the lead oxide produced.
“3. The limestone gives rise to chemical reactions. By its decomposition it produces lime, which, at the moment of its formation, is converted into calcium sulphate at the expense of the sulphur in the ore. The calcium sulphate at the time of slag formation is converted into silicate by the silica present, sulphuric acid being evolved. The limestone therefore assists directly and forcibly in the desulphurization of the ore, causing the formation of sulphuric acid at the expense of the sulphur in the ore, the sulphuric acid then acting as a strong oxidizing agent toward the sulphur in the ore.”
The most conclusive proof for the correctness of the opinion which I expressed above, that it is very important to create at the beginning of the operation the conditions for the formation of as much sulphate as possible, has been furnished by Carmichael and Bradford. They recommend that gypsum be added to the charge in place of limestone. At one of the works of the Broken Hill Proprietary Company (where their process has been carried on successfully, and where lead ores very rich in zinc had to be worked up) the dehydrated gypsum was mixed with an equal quantity of concentrate and three times the quantity of slime from the lead ore-dressing plant, as in the table given herewith:
| Contents | Of the Slime | Of the Concentrate | Of the Calcium Sulphate | Sulphate Whole Charge |
|---|---|---|---|---|
| Galena | 24 | 70 | — | 29 |
| Zinc blende | 30 | 15 | — | 21 |
| Pyrites | 3 | — | — | 2 |
| Ferric oxide | 4 | — | — | 2.5 |
| Ferrous oxide | 1 | — | — | 1 |
| Manganous oxide | 6.5 | — | — | 5 |
| Alumina | 5.5 | — | — | 3 |
| Lime | 3.5 | — | 4.1 | 10 |
| Silica | 23 | — | — | 14 |
| Sulphur trioxide | — | — | 59 | 12 |
The charge is mixed, with addition of water, in a suitable pug-mill. The mass is then, while still wet, broken up into pieces 50 mm. (2 in.) in diameter, which are then allowed to dry on a floor in contact with air; in doing so they set hard, owing to the rehydration of the gypsum.
As in the case of the Savelsberg process, the converters are heated with a small quantity of coal, are filled with the material prepared in the manner above described, and the charge is blown, regulating the blast in such manner that, after the moisture present has been dissipated, a gas of about 10 per cent. SO2 content is produced, which is worked up for sulphuric acid in a system of lead chambers.
The reactions are in this case the same as in the Savelsberg process, for here also calcium sulphate is formed transitorily, which, like other sulphates, reacts partly with sulphides, partly with silica.