The Effect of Carbonaceous Reductant Selection on Chromite Pre-reduction
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ODUCTION
THE importance of stainless steels to society is obvious, considering the multitude of unique applications thereof and the significant growth in its demand.[1–3] Ferrochrome (FeCr), a crude chromium (Cr)-iron(Fe) alloy, is the only source of new Cr units during stainless steel production.[4,5] According to 2012 data from the International Chromium Development Association (ICDA), 94.53 pct of chromite (the only economically viable Cr-containing ore) was consumed by the metallurgical industry for the production of various FeCr grades, including the most common, i.e., high-carbon (HC) and charge grade (ChG) FeCr. The stainless steel industry consumes the vast majority of HC and ChG FeCr (~70 pct).[6,7] FeCr is mostly produced by means of pyrometallurgical carbothermic reduction utilizing submerged-arc furnaces (SAFs) and direct current arc furnaces (DCFs).[8–10] FeCr manufacturing is an energy-intensive process consuming not only high quantities of electricity, but also large amounts of carbon-based reductants.[11–13] Typical operational costs of FeCr producers, E.L.J. KLEYNHANS, J.P. BEUKES, P.G. VAN ZYL, J.R. BUNT and M. VENTER are with the Chemical Resource Beneficiation, North-West University, Potchefstroom Campus, Private Bag X6001, Potchefstroom, 2520, South Africa. Contact e-mail: paul. [email protected] N.S.B. NKOSI is with the Pyrometallurgy Division, Mintek, 200 Malibongwe Drive, Private Bag X3015, Randburg, 2125, South Africa. Manuscript submitted June 22, 2016. Article published online December 27, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
as presented by Daavittila et al.,[14] can be divided into four cost factors, i.e., chromite ore (30 pct), electricity (30 pct), reductant (20 pct), and other production costs (20 pct). A more recent paper focusing on the South African FeCr smelters indicated that the reductant and electricity cost each account for 30 pct of the overall production costs.[15] Comparing the specific electricity consumption (SEC), i.e., MWh/t FeCr produced, of existing operational FeCr production processes, the pelletised chromite pre-reduction process (also referred to as solid-state reduction of chromite, SRC) has the lowest SEC (~2.4 MWh/t FeCr achieved with approximately 45 pct pre-reduced hot feed). In this process, a carbonaceous reducing agent, chromite ore, and a clay binder are mixed, dry milled, agglomerated by disk pelletisation, and heated in a grate prior to being fed into a counter current rotary kiln, where pre-reduction of the chromite spinel takes place. The pre-reduced pellets are then charged hot into a closed SAF.[10] There have been a number of commercial facilities where the pelletized chromite pre-reduction process has been successfully applied, i.e., in Japan at the Showa Denko K. K. operations at Shunan and Toyama that has since closed down mainly due to the reliance of imported ore, as well as in South Africa at Glencore Alloys’ Lydenburg (previously known as CMI smelter) and Lion smelters. The combined production of the latter two operations was recent
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