Unique challenges of clay binders in a pelletised chromite pre–reduction process : a case study
Kleynhans, Ernst Lodewyk Johannes
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As a result of increasing cost, efficiency and environmental pressures ferrochrome producers strive towards lower overall energy consumption. Increases in local electricity prices have placed particular pressure on South African ferrochrome producers. Pelletised chromite pre–reduction is likely the currently applied ferrochrome production process option with the lowest specific electricity consumption. In this process fine chromite, together with a carbonaceous reductant and a clay binder is milled, pelletised and pre–reduced. In this dissertation it is demonstrated that the functioning of the clay binder in this process is not as straightforward as in conventional metallurgical pelletisation processes, since the cured pre–reduced pellets are characterised by an oxidised outer layer and a pre–reduced core. Conventional performance characteristics of clay binders (e.g. compressive strength and abrasion resistance) therefore have to be evaluated in both oxidative sintering and reducing environments. Two clay samples, i.e. attapulgite and bentonite, were obtained from a local ferrochrome producer and investigated within the context of this study. Results indicated that the compressive and abrasion resistance strengths of oxidative sintered pellets for both clays were substantially better than that of pre–reduced pellets. Thus, although the objective of the chromite pre–reduced process is to achieve maximum pre–reduction, the strength of pre–reduced chromite pellets is significantly enhanced by the thin oxidised outer layer. The strength of the bentonite–containing pellets was found to be superior in both pre–reducing and oxidative sintering environments. This is significant, since the attapulgite clay is currently the preferred option at both South African ferrochrome smelting plants applying the pelletised chromite pre–reduction process. Although not quantitatively investigated, thermo–mechanical analysis indicated that the hot strength of the attapulgite pellets could be weaker than the bentonite–containing pellets. The possible effects of clay binder selection on the level of pre–reduction were also investigated, since it could have substantial efficiency and economic implications. For both case study clays investigated, higher clay contents resulted in lower pre–reduction levels. This has relevance within the industrial process, since higher clay contents are on occasion utilised to achieve improved green strength. The average pre–reduction of the bentonite–containing pellets were also consistently higher than that of the attapulgite–containing pellets. Again, this is significant, since the attapulgite clay is currently the preferred option. In general the case study results presented in this dissertation 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.