Smelting reduction processes
In general, the pyrometallurgical smelting reduction of metals from oxidic residues requires a reduction of the relevant oxides (normally using carbon as a reductant) and a subsequent melting to separate the metals from the other slag-forming components. There are two different types of processes (the Primus® technique and the Redsmelt® process) which are described below. The Primus® technique using a two-step process, consists of the combination of a multiple hearth furnace (MHF) (the Primus® Pre-Reduction Unit) aiming to dry, heat up and initiate reduction followed by an electric arc furnace (EAF) (the Primus® Melting Unit) leading to complete iron reduction and providing hot metal, slag formation and the finalisation of zinc reduction. This technique permits the treatment of all typical iron and steelmaking residues which cannot normally be recycled within the existing plant, such as EAF dust, BF sludge, steelworks sludge and oily mill scale sludge. In the MHF, the mix of residues (pre-pelletised if very fine), and coal are charged from the top. Rotating arms stir and transport the charge, which is dropped from one hearth to the next while being dried, preheated and pre-reduced by a countercurrent flowing stream of gas. The carbon for reduction and the energy for the process are entirely supplied by the volatile coal. The subsequent melting unit (a specially-designed EAF) is continuously hot-charged through a chute between the electrodes. Complete reduction, iron and slag melting and full de-zincing are achieved in this electric smelter. Zinc and lead are transferred to the off-gas system and recovered as a marketable zinc oxide concentrate. The iron content of the feed material is recovered as liquid pig iron or tapped in a pig-caster. The resulting slag, similar to blast furnace slag, can be used for road construction. In the Redsmelt® process, the residues are pelletised together...
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