Alteration of chrome-to-iron ratio in chromite ore by chlorination / Bryson Wade Neizel

The industrial importance of the chromium industry to South Africa is emphasised by the fact that it is considered the largest chromite (chromium ore) and ferrochrome (chrome-iron alloy) producing country in the world. Although South Africa holds three quarters of the world's chromite ore reserves, the chrome-to-iron (Cr-to-Fe) ratio of the local chromite ores is relatively low (1.47 to 1.55), compared to other deposits in the world (2.6 to 3.5). Additionally, iron is more readily reduced than chromium. The combination of these two factors implies that ferrochrome produced from South African chromite ore contains 47-53% chromium. Current pricing practises in the world ferrochrome industry dictate that ferrochrome producers are only paid for the chromium content in the ferrochrome, which implies that South African ferrochrome producers export a large percentage of their product without any financial benefit. Research to improve the Cr-to-Fe ratio is therefore essential to support sustainability of the local ferrochrome industry. Conventional beneficiation methods such as gravity concentration, magnetic separation and floatation are unlikely to increase the Cr-to-Fe ratio, since both iron and chromium are part of the same mineral phase, i.e. the spinel, which requires structural dissociation. It has been proven on laboratory scale that high temperature carbochlorination (CO and Clz atmosphere) can be used to selectively remove iron from chromite. However, such methods are unlikely to be implemented on an industrial scale due to health, environmental and cost considerations. In light of this, an alternative approach to chromite chlorination, avoiding the use of chlorine and other toxic gasses, was investigated during this study. Since it was found that NaCI addition significantly improved the effectiveness of carbochlorination of chromite, the effect of adding only NaCI during high temperature treatment of chromite was investigated. The material utilised during this investigation consisted of local chromite, anthracite (source of carbon) and attapulgite clay (serving as a binder). These materials were mixed in a ratio and subsequently milled to 0 90 = 75fJm to represent materials and specifications similar to those used during pelletisation of the chromite in the pre-reduction ferrochrome production process. This mixture could also be used to generate a partially reducing atmosphere (CO rich) during high temperature treatment, which was similar to the reaction conditions utilised during carbochlorination. The abovementioned milled mixture was pelletised into cylindrical pellets with a die set and a hydraulic press. This experimental investigation was based on a mono-variance procedure, in that the four different variables investigated, i.e. maximum pellet treatment temperature, exposure time, wt% NaCI addition to the pellets and the atmosphere the pellets were exposed to, were varied one at a time during experimentation. After each alteration of the afore-mentioned variables, the…