Rich slag is an item added in Thermal Expansion v2.1.0. It is a by-product of smelting ores (not dust) in an Induction Smelter.
Rich Slag can be used to increase the ingot yield from dusts, when used in an Induction Smelter. Using Rich Slag with an ore block in the induction smelter will yield 3 ingots. This can be better as an endgame system as it is better than using a macerator or a pulverizer as it can produce 3 ingots instead of 2.