PRECIOUS METAL CONCENTRATIONS IN COAL COMBUSTION SLAGS

As coal is combusted in a coal-burning boiler at over 1538^oC (2800^oF), the bituminous portion of coal is gasified, while the mineral portion is separated as a molten slag. The slag will solidify into an iron phase with the bulk of the precious metals and a silicate oxide phase depleted in these metals.

The precious metal concentration in combustion slag is controlled by combustion process as well as their original concentrations in coal before combustion. The higher the combustion temperature the longer the molten slag flows through the combustion process before it quenched and solidifies in a water tank the better the precious metals are partitioned into the iron phase. For this reason, wet-bottom cyclone boiler appears to be most efficient in concentrating precious metals. Precious metals in circulating groundwater or hydrothermal solution could be reduced and deposited in coal bed as a secondary mineralization process. Secondary enrichment of precious metals is seen in coal beds of eastern Siberia, northeastern Wyoming, and Kueichow province of China.

Precious metal concentration in iron phase is roughly 2 to 4 times that of silicate oxide phase. Therefore the bulk of the precious metals could be recovered from slag by liberating the iron phase through stage crushing. The magnetic iron phase could be separated magnetically. Density of pore-free slag can be as high as 2.2 gm/cm³ but density of iron could be as high as 7.9 gm/cm³. So the iron phase could also be separated gravimetrically with various density liquids or simply using coal washing jigs.