Pre-reduction of United Manganese of Kalahari Ore in CO/CO2, H2/H2O, and H2 Atmospheres

Abstract

The incorporation of hydrogen, which is a relatively unexplored reductant used during ferromanganese (FeMn) production, is an attractive approach to lessen atmospheric gaseous carbon release. The influence of hydrogen on the pre-reduction of carbonate-rich United Manganese of Kalahari (UMK) ore from South Africa was investigated. Experiments were performed in 70 pct CO 30 pct CO2 (reference), 70 pct H2 30 pct H2O, and 100 pct H2 gas atmospheres at 700 °C, 800 °C, and 900 °C. Calculated phase stability diagrams and experimental results showed good correlation. The pre-reduction process involved two reactions proceeding in parallel, i.e., the pre-reduction of higher oxides and the decomposition of carbonates present in the ore. A thermogravimetric (TG) furnace was employed for the pre-reduction of the ore in various atmospheres. The calculated weight loss percentage was used to determine the degree and rate of pre-reduction. It was found that the oxidation state of higher Fe- and Mn-oxides was lowered when treated in 70 pct H2 30 pct H2O and 70 pct CO 30 pct CO2, whereas FeO was metalized when using 100 pct H2. As for the intrinsic carbonates, the majority thereof were decomposed in the CO/CO2 atmosphere at 900 °C, and ≥ 700 °C in the H2/H2O and H2 atmospheres. Additionally, the degree and rate of reduction were accelerated by increasing the pre-reduction temperature and by employing a hydrogen-containing gas atmosphere (70 pct H2 30 pct H2O, and 100 pct H2). Scanning electron microscopy and electron microprobe analysis revealed the presence of three phases in the pre-reduced ore: (i) Mn- and Fe-rich, (ii) Mg- and Ca-rich, and (iii) Mg-, Si-, K-, and Na-rich. It was also found that there were no appreciable differences in porosity and decrepitation of the ores treated in the CO/CO2 and hydrogen-containing atmospheres. The use of a hydrogen atmosphere showed potential for the pre-reduction of carbonate-containing manganese ores as it accelerated the decomposition of the carbonates as well as facilitated the metallization of Fe-oxides present in the ore.