The term SMELTER BYPRODUCT can be summarized as the SLAG produced by smelting. SLAG is a byproduct of the smelting process, which is the process of melting ore to separate out valuable metals. It is a non-metallic material that is composed of the impurities from the ore that are left behind after the smelting process. SLAG is usually discarded, but it can also be used in construction materials and fertilizer.