The basic principles involved are the reduction of lead compounds by carbon and carbon monoxide to metallic lead so that gold and silver can be recovered. Other important aspect is related to the provision of sufficient heat to allow fusion of the basic oxides with silica to form slag, with some formation of matte (copper sulphide) and speiss.
Approximately, above 910 oC, nearly 100% carbon monoxide is formed, which reduces oxides to metal with the production of carbon dioxide. Below 660 oC, the reaction of carbon and oxygen favors carbon dioxide formation. The temperature in the furnace decreases, falling to 100 oC at the top. Between 105 oC and 390 oC, moisture and water of crystallization are lost with absorption of heat, which helps to keep the top of the furnace at the appropriate temperature. Between 390 oC and 710 oC, dissociation of carbonates and sulphates begins. In the fusion zone, above 1050 oC, matte becomes completely liquid and slag forms from the liberated lime and other basic oxides in combination with silica.
xSiO2 + yFeO + zCaO = xSiO2.yFeO.zCaO
Basically the slag is orthosilicate (2RO.SiO2) with the presence of metasilicate or subsilicate according to the ratio of acids to bases. Theoretical composition of the slag is completed considerably by the presence of alumina, sodium, boric oxide and other compounds. Lead reduced in the upper parts of the furnace trickles through the charge collecting gold and silver. Other bases such as alumina and zinc oxide are absorbed into the slag. If present in sufficient quantity, reduced arsenic and antimony will combine with nickel to form speiss, in which platinum group metals have a high solubility. The slag notch is opened, a clay channel molded over it and two settling ponds arranged in series beneath the slag lip. Slags, matte and speiss run continuously into the first pot, where they are separated according to their specific gravity: 2.4-3.7 slag, 4.3-5.8 matte and 6.8-9.2 speiss.
Pouring lead into the ladle
Speiss collects in a well at the bottom of the pot and is recovered when the pond is changed. Matte is tapped periodically from the pot, while the slag is allowed to run into the second pot. The addition of pyrite and other fluxes form further matte, all of which result in the scavenging of some gold and silver in the slag. The slag is finely granulated in a water trough, flushed to a classifier and dumped. Lead bullion is tapped from the furnace every four hours into moulds. At the end of the campaign, a few blank charges are fed to the furnace followed by a charge of coke. It has been noted that the physical nature of the charge is very important and all components must be of similar size in order to ensure a good mix of the materials and prevents channeling. The chemical composition of the charge is very important.
- Slag must be sufficiently fluid at fusion temperatures (1420-1480 oC) to flow easily over the slag lip and to remain reasonably fluid down 1110 oC at which temperature, it is granulated.
- The lead fall is sufficient to give maximum efficiency in the collection of gold and silver, but not too high so that the cupellation process could be excessively lengthy.
- Sulphur must be present in appropriate proportion to ensure a reasonable matte fall, primarily to collect copper.
- A small quantity of metallic iron is available. This helps the reduction of lead, but in the furnace, replaces lead in the matte.
- Nickel and arsenic are present in small quantities in order to form speiss in which platinum group metals are especially soluble.
Slag, matte, speiss and lead bullion comprise the four products from the furnace. The perfect process would be when all the gold and silver in the charge is collected in the lead. Nevertheless, the distribution is other, next table shows average values,
|
Metal |
Distribution, % | |||
|
Slag |
Matte |
Speiss |
Lead Bullion | |
|
Gold |
0.25-0.35 |
3.5-6.0 |
0.1-0.3 |
92-95 |
|
Silver |
1.5-2.5 |
22-29 |
0.02-0.07 |
69-78 |
The slag assays in the region of 1-2 g/t gold and 85-95 g/t silver present the following composition,
|
Compound |
% |
Compound |
% |
|
SiO2 |
25-30 |
MgO |
1.5-3 |
|
FeO |
34-40 |
ZnO |
1.8-2.5 |
|
CaO |
11-16 |
MnO |
0.5-1.8 |
|
Pb |
0.8-2 |
Cu |
0.2-0.4 |
|
Al2O3 |
6-10 |
S |
0.3-0.6 |
|
B2O3 |
2-5 |
Na |
2-3 |
Matte varies in composition, depending on the copper content. In this way, matte containing more than 30% copper is sold and matte of lower copper content is recycled. Speiss formation is the order of 0.1-0.3% of the charge. Next table show average composition of Matte and speiss.
|
Product |
Au
g/t |
Ag
kg/t |
Pb
% |
S
% |
Cu
% |
Fe
% |
Ni
% |
As
% |
Sb
% |
|
Matte |
190-230 |
22-25 |
9-18 |
17-25 |
12-38 |
22-32 |
0.5-4 |
0.3-1.5 |
0.01-0.08 |
|
Speiss |
850-910 |
4.5-6 |
6-12 |
2-4 |
8-18 |
10-20 |
12-27 |
18-28 |
2-8 |