Solar carbothermic reduction offers a promising approach for the clean production of several metals from their oxides with significantly lower CO2 emission compared to common pyrometallurgical processes. This process can be used for clean production of magnesium and calcium from dolomite (MgCO3·CaCO3) mineral as the raw material. The carbothermic approach consists of two separate steps: calcination of dolomite to dolime (MgO·CaO) followed by carbothermic reduction (preferably under vacuum) of the dolime to magnesium and calcium. In the present study, the possibility of combining these two steps into a single process step was thermogravimetrically investigated. The direct carbothermic reduction of dolomite not only reduces the process complexities but also decreases the energy consumption and CO2 emissions. Thermogravimetric results indicate that the conversion yield of carbothermic reduction of dolomite is slightly lower than that of the dolime due to the consumption of a portion of the available carbon reacting with CO2 that is released during the calcination of dolomite. Therefore, surplus carbon should be used in the single-step conversion process. The product gases, after the magnesium is condensed, contain mostly CO which can be directly combusted or converted to hydrogen by the water-gas shift reaction. The results also show that CaO is reduced only after a complete reduction of MgO in dolomite occurs, which is quite similar to the results obtained from the carbothermic reduction of dolime. The effect of three carbon sources, including carbon black (CB), activated carbon (AC), and wood charcoal (CC), on the performance of the carbothermic reduction process was studied. AC had a lower conversion yield compared to the CB. CC achieved the highest conversion yield with better kinetics. This can be explained by CC's porous structure that provides a better surface contact of dolomite powder with carbon. Considering the lower adverse environmental impacts and lower cost of CC in comparison to the other carbon sources studied, it can be considered as the best option for use in the solar carbothermic reduction of dolomite on a future commercial scale.
Bibliographical noteFunding Information:
This research has been funded by the University of Minnesota Duluth. N.O. appreciates the diligent effort and support of Professor Aydin Durgunoglu, Professor Rudy Perrault, Ms. Kathleen Adee, and Mr. Michael Swanson in the materialization of the High Flux Gas Dynamics Laboratory.