The refining slags used are inert stable silicates and are based upon CaO-FeO-SiO2 or CaO-Al2O3-SiO2 systems. If the latter is employed, higher operating temperatures are necessary and has the advantage that alumina can be slagged at 1200 oC over wide compositional ranges. This temperature isotherm covers a large area of the slag system. The second system has the disadvantage that they degrade easily and can hold substantial values, particularly in the presence of halides. By increasing the operating temperature at which materials are smelted, it became possible to employ the CaO-Al2O3-SiO2 slag system. The effect of calcium rather than sodium in the slag matrix is to help reduce precious metals solubility in the slag. The slags generated from the system have shown themselves to be stables and inert.
The increase in operating temperatures can be achieved by selecting an appropriate burner. For example an oxy-fuel burner has advantages over aspirated burners such as increased smelted temperature; increased capacity due to increased smelting rate, decreased extraction requirements because nitrogen is not present in the combustion air, decreased dust to the bag house because of decreased extraction requirement, and better energy utilization of the fuel oil. Offset against these benefits are the extra cost of oxygen and increased refractory wear. Next figure shown phase diagram for CaO-Al2O3-SiO2 slag system, black area indicates slag composition.
Phase Diagram for CaO-Al2O3-SiO2 slag system