The use of hydrogen as a reductant in the smelting industry

MSc (Chemistry), North-West University, Potchefstroom Campus

Both the smelting of chromite and manganese (Mn) ores consume a significant amount of carbon (C) and electrical energy. The smelting processes contribute to the elevation of carbon dioxide (CO2) emissions in the atmosphere as stable metal oxides are reduced to their metallic states via the removal of oxide atoms via a carbothermic reaction. The incorporation of hydrogen as a reductant during the smelting process will not only reduce the C footprint but may also contribute to lowering material and energy consumption during smelting procedures. This dissertation presents the use of hydrogen as a reductant during the reduction of chromite and Mn-ore. The effect of hydrogen as a reductant on the reducibility of chromite was investigated in Chapter 3. Also, thermodynamic modelling, the influence of time, and particle size on the reduction of chromite by hydrogen were studied. The chromite was reduced with pure hydrogen at 750 mL/min flowrate at 1100 °C for 30 min. It was determined that at the experimental conditions the iron (Fe)-oxide constituency in chromite could be metalized and removed with hot-acid leaching, while the chrome (Cr)-oxide constituency did not undergo metallization. However, the removal of Fe leads to the formation of eskolaite-type phase ((Cr1.4Al10.6)O3) which may be unfavorable for the reducibility of chromite. As the reduction time increased the %Fe metallization increased near-linearly. Also, the particle size had a noticeable effect on the %Fe and Cr metallization. Chapter 3 showed that the Fe-oxide constituency of chromite could be metalized by hydrogen with relative ease. In Chapter 4, several techniques were employed to optimize the %Fe metallization during chromite reduction with hydrogen at 1100 °C. The effect of hydrogen availability was explored. The results revealed that an increase in hydrogen flowrate significantly improved the %Fe metallization, while Cr metallization remained relatively unchanged (and close to 0%). Furthermore, chromite was pre-oxidized, which altered the structure of the chromite spinel, before reduction using hydrogen. This alteration improved the reactivity of chromite with hydrogen. Also, the effects of pre-oxidation and reduction time, and chromite particle size were investigated. By pre-oxidizing the chromite before being subjected to hydrogen, the reduction time could be reduced from 180 to 60 min, while maintaining similar levels of Fe-metallization. For instance, unoxidized ore reduced for 180 min achieved a Fe metallization of 92.1%, whereas chromite pre-oxidized for 60 min and reduced for 60 min reached 77.5% Fe metallization. An increase in pre-oxidation time from 60 to 240 min had no significant impact on the reducibility of chromite using hydrogen. However, an increase in Fe metallization was observed with a decrease in particle size. A similar observation was made for Cr metallization; however, Cr metallization did not exceed 2.0 % (which was significantly lower than Fe metallization). The influence of hydrogen on the degree and rate of pre-reduction in Mn-ore was investigated in Chapter 5. The Mn-ore was treated using three atmospheres, i.e., 100% H2, 70% H2 30% H2O, and 70% CO 30% CO2 at 4 L/min during the isothermal pre-reduction process performed at 700, 800, and 900 °C. A thermogravimetric (TG) furnace (furnace coupled with a TG analyzer) was employed for the pre-reduction of the ore. The weight loss and off-gas composition were measured and quantified. The results revealed that the utilization of hydrogen significantly increased the degree and rate of pre-reduction and carbonate decomposition. Furthermore, the utilization of 100% H2 allowed the Fe-oxides to completely reduce to metallic Fe. The metallization of the Fe-oxide was shown to be thermodynamically and kinetically possible. The study also showed that the utilization of hydrogen as a reductant did not affect the decrepitation and porosity of United Manganese of Kalahari (UMK) ore after the pre-reduction process.

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