Alteration of chrome-to-iron ratio in chromite ore by chlorination

Abstract

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 toD90 = 75µm 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 carbo-chlorination. The above mentioned 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 Cr-to-Fe ratios, together with other parameters, were measured. Analyses undertaken included Scanning Electron Microscopy, with Energy-Dispersive X-ray Spectroscopy (SEM-EDS), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and cured breaking strength. Although this investigation did not focus on the cured breaking strength of the pellets, it is a very important industrial parameter and was therefore measured. Results indicated that the addition of NaCI had a definite effect. In both oxidising and partially reducing atmospheres the cured breaking strength of the cured pellets increased up to 800°C exposure, where after it decreased. This was attributed to melting of NaCI at 801 °C. In the oxidising atmosphere, the cured breaking strength increased again at temperatures higher than 1 000°C, due to the formation of a thin oxldised layer on the outside of the pellets, which could be confirmed by SEM analysis. Fine, metallic-like crystals were noticed inside and on the lids of crucibles in which pellets containing NaCI as an additive were cured at temperatures of 900°C or higher. SEM-EDS analysis and weight-ratio calculations revealed that these crystals were pure FeO. This indicated that some iron might have been liberated from the chromite spinel matrix. However, ICP-OES analyses revealed that Cr-to-Fe ratios did not change significantly under any of the experimental conditions (NaCI addition 5wt% to 15wt%, curing between 500'C and 1200'C, and oxidative/partially reducing atmospheres). The observed FeO crystals did not make any meaningful difference to the Crto-Fe ratio of the chromite, but was of great academic interest as iron was extracted from the chromite spinel. This indicated that it is not only the formation of low melting point species, such as those proposed in previous mechanistic studies of carbochlorination of chromite, but that molten NaCI alone could also initiate the extraction of iron out of chromite. According to the knowledge of the author, this is the first report of its nature in open literature. SEM and SEM-EDS analyses also proved that the addition of NaCI to the chromite/carbon/clay mixtures enhanced the rate of chromite pre-reduction. This finding was in agreement with earlier literature reports. In conclusion, it can be stated that the addition of NaCI alone cannot alter the Cr-to-Fe ratio of chromite during high temperature treatment. NaCI addition did, however, have an effect on other important parameters i.e. initiation of iron removal, cured breaking strength and the rate of chromite pre-reduction. From the results and experience gained in this study, certain recommendations with regard to possible future studies could also be made. This included investigating i) other single component additives to possibly alter the Cr-to-Fe ratio during high temperature treatment, ii) the effect of industrially relevant additives such as CaO/CaC03, Mg03 and SiOz on the rate of chromite pre-reduction and iii) the effect of different clays (e.g. attapulgite, bentonite, etc.) on the rate of chromite pre-reduction.