Development and improvement of pretreatment technologies to enhance ferrochrome production

Abstract

Ferrochrome (FeCr) is a vital alloy mostly used for the production of stainless steel. It is produced from chromite ore, the only economically exploitable natural chromium (Cr) resource, ttirough carbothermic smelting in submerged arc or direct current furnaces. FeCr production is an energyintensive process. FeCr producers strive towards lower overall energy consumption due to increases in costs, efficiency and environmental pressures. In South Africa, in particular, higher electricity prices have placed pressure on .FeCr producers. Currently, the pelletised chromite prereduction process, also referred to as solid state reduction of chromite, is most likely the FeCr production process with the lowest specific electricity consumption (SEC), i.e. MWh/t FeCr produced. The unique process- considerations of clay binders in the pelletised chromite pre-reduction process were highlighted and demonstrated utilising two case study clays (Chapter 3). It was demonstrated that the clay binder has to impart high compressive and abrasion resistance strengths to the cured pellets in both oxidising and reducing environments (corresponding to the oxidised outer layer and pre-reduced core of industrially produced pellets), while ensuring adequate hot strength of pellets during the curing process. The possible effects of the clay binder se-lection and the amount of binder addition on the degree of chromite pre-reduction achiev_ed were also investigated, since it could have substantial efficiency and economic implications. The case study results presented in this paper indicated that it is unlikely that the performance of a specific clay binder in this relatively complex process can be predicted, based only on .the chemical, surface-chemical and mineralogical characterisation of the clay. The effects of carbonaceous reduotant selection on chromite pre-reduction and cured pellet strength were investigated (Chapter 4 ). Multiple linear regression analysis was employed to evaluate the effect of reductant characteristics on the aforementioned two parameters. This yielded mathematical solutions that can be used by FeCr producers to select reductants mo~e -optimally in future. Additionally, the results indicated that hydrogen (H)- (24% contribution) and volatile content (45.8% contribution) were the most significant contributors to predicting variance in pre-reduction and compressive strength, respectively. The role of H within this context is postulated to be linked to the ability of a reductant to release H that can induce reduction. Therefore, contrary to the current operational selection criteria, the authors believe that thermally untreated reductants (e.g. anthracite, as opposed to coke or char), with volatile contents close to the currently applied specification (to ensure pellet strength), would be optimal, since it would maximise the H content that would enhance pre-reduction.