Roasting Chemicals | Manganese Dioxide Smelting
The amount of silica, sodium borate flux added with the charge depends on the quantity of base metals and other impurities in the charge. As smelting proceeds the acidity of the slag is gradually reduced as the base metals react with flux materials. Once the reaction is complete the slag should be neutral, or slightly acidic, to protect the furnace lining or crucible from basic corrosion.
Other chemicals may be added to the flux for various reasons, the most important being:
- Calcium fluoride which reduce slag viscosity by the substitution of fluoride ions into the silica lattice.
- Sodium carbonate which improves slag clarity and decrease viscosity, thereby reducing precious metals entrainment.
- Oxidizing agents such as sodium nitrate and manganese dioxide which assist in the oxidation of unoxidized materials (base metals)
The addition of calcium fluoride and sodium carbonate may cause forming and/or increased precious metals volatilization losses, depending on the charge composition and smelting temperature. Consequently, care must be taken when applying such modified fluxes in smelting systems.
Although smelting is most efficient when treating thoroughly oxidized materials, there are occasions when direct smelting of unoxidized products is the most cost effective method. In these cases, oxidation must be achieved during the smelting stage itself. Manganese dioxide can be used when only a small proportion of the feed must be oxidized. Sodium nitrate is preferred when more severe oxidation is required. The use of oxidizing agents during smelting requires care since silver can be oxidized and lost into the slag, and gold losses can be increased.
Flux compositions are selected to optimize slag quality and to maximize crucible or furnace liner life. Some slags compositions are more corrosive than others, e.g. strongly oxidizing fluxes, or flux that react violently with the material to be smelted.
Any sulphides that have not previously oxidized by roasting will form a matte layer between the precious metals and the slag phases during smelting. This may content significant quantities of gold and base metals as well as selenium, tellurium, arsenic and antimony. The matte can be treated to recover the precious metals by:
- Smelting with sodium borate and a cyanide salt at white heat for 2-3 hours.
- Smelting with fluxes, sodium nitrate and finely divided scrap iron.
Once smelting is complete the slag is poured off and the precious metals removed from furnace. The metals are allowed to cool in bar or button mould. Ideally the slag should be clear and uniform, with a grey-greenish coloration.
A typical recipe can be constituted with borax 5 parts, silica sand 40 parts, sodium carbonate 10 parts, sodium nitrate 10 parts. All the components must be mixed in order to get a homogeneous mass that is the flux. For each part of concentrate is possible add 1-2 parts of flux. It is important mention the known fact that there can be 4 - 10 times more Gold in your concentrates than the visible Gold that you can see and recover. There are several ways to recover this material including leaching, smelting and others. But one of the most practicable and proven methods is to smelt the concentrates. Remember one thing, when smelting, you are now a chemist as well as a Gold Prospector. And careful attention must be paid to what you are doing at all times. A mistake can mean serious consequences too and include even death.