Design and Fabrication of Pellets for Magnesium Production by Carbothermal Reduction
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For carbothermal reduction (CTR) to be an economic and clean process for magnesium metal production, operational challenges must be overcome. Strong and reactive precursor pellets are necessary to effectively and selectively produce Mg(g) from any feedstock. In this study, the effects of ore (magnesia and dolime), carbon (petroleum coke, charcoal, algal char, and carbon black), and binder (organic and inorganic) on pellet strength and reactivity, product yield and purity, and reduction selectivity were analyzed. Theoretically and experimentally, the CTR of dolime (MgO·CaO) favored MgO reduction over CaO reduction; however, with enough carbon and heat, both oxides could be reduced. CaO carbothermal reduction produced CaC2 and Ca(g). The selectivity to CaC2 remained constant (7 ± 4 pct) for all C/MgO·CaO ratios analyzed, while the selectivity to Ca(g) increased (5 pct → 40 pct) when C/MgO·CaO was increased from 0.5 to 2.0. As the overall metal yield decreased (77.6 pct → 59.7 pct) with increasing CaO reduction (38.2 pct → 78.1 pct), Ca(g) reverted faster than Mg(g). Heavy metal impurities primarily remained in the residue (< 30 pct volatilized) and, when volatilized, condensed at high temperatures (700 °C to 1450 °C), relative to light metal impurities (350 °C to 1000 °C, > 78 pct volatilized). Organic binders added reducing power to the pellets but produced frail pellets (radial crush strength = 9.1 ± 0.7 N) after pyrolysis, relative to pellets with inorganic binders (15.1 ± 3.2 N). Kinetic parameters were determined for extruded pellets to predict the reaction rate as a continuous function of pressure and temperature.
The authors acknowledge the financial support from the National Science Foundation: Award 1622824, and from the Advanced Research Projects Agency-Energy (ARPA-E) of the US Department of Energy (DOE): Award AR0000404. Dragan Mejic provided machining, welding, and design services for all custom vacuum hardware. Dave Sorenson at Dover Resources provided carbon consulting services.
Conflict of interest
Boris Chubukov, Scott Rowe, and Aaron Palumbo are co-founders of Big Blue Technologies and are working to commercialize magnesium production by carbothermal reduction.
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