| PbO = | 78.11 per cent. |
| Sb2O4 = | 8.75 ” |
| As2O3 = | 2.18 ” |
| CuO = | 0.36 ” |
| CaO = | 1.10 ” |
| Fe2O3 = | 0.42 ” |
| Al2O3 = | 0.87 ” |
| Insol. = | 4.10 ” |
Antimony dross is usually kept separate and worked up from time to time, yielding hard antimonial lead, used for type metal, Britannia metal, etc.
Desilverization.—The softening being completed the charge is tapped and run to a kettle or pan of cast iron or steel, holding, when conveniently full, some 12 or 13 tons. The lead falling into the kettle forms a considerable amount of dross, which is skimmed off and returned to the softening furnace. By cooling down the charge until it nearly “freezes” an additional copper skimming is obtained, which also is returned to the softener. The kettle is now heated up to the melting point of zinc, and the zinc charge, determined by the gold and silver contents of the kettle, is added and melted. The charge is stirred, either by hand or steam, for about an hour, after which the kettle is allowed to cool down for some three hours and the first zinc crust taken off. When the charge is skimmed clean a sample of the bullion is taken for assay, and while this is being done the kettle is heated again for the second zinc charge, which is worked in the same way as the first; sometimes a third addition of zinc is necessary. The resulting crusts are kept separate, the second and third being added to the next charge as “returns,” allowing 3 lb. of zinc in returns as equal to 1 lb. of fresh zinc. An alternative method is to take out gold and silver in separate crusts, in which case the quantity of zinc first added is calculated on the gold contents of the kettle only. The method of working is the same, though subsequent treatment may differ in that the gold crusts are cupeled direct.
As to the quantity of zinc required:
1. Extracting the gold with as little silver as possible, the following figures were obtained:
| Total Gold in Kettle, oz. | Amount taken out by 1 lb. zinc, oz. |
|---|---|
| 300 | 1.00 |
| 200 | 1.00 |
| 150 | 0.79 |
| 100 | 0.59 |
| 60 | 0.45 |
2. Silver zinking gave the following general results with 11-ton charges:
| Total Silver in Kettle, oz. | Amount taken out by 1 lb. zic, oz. |
|---|---|
| 1,200 | 4.1 |
| 930 | 3.8 |
| 755 | 3.5 |
| 616 | 3.4 |
| 460 | 2.6 |
3. Extracting gold and silver together: