Date of Award

Spring 5-17-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Alan W. Weimer

Second Advisor

David E. Clough

Third Advisor

John L. Falconer

Fourth Advisor

Hans Funke

Fifth Advisor

Ronggui Yang

Abstract

Magnesium metal production by carbothermal reduction (CTR) offers a clean and economic alternative to silicothermic and electrolytic processes. At the reduction temperature, magnesium is liberated as a gas. Magnesium gas and by-product carbon monoxide tend to revert unproductively to the starting materials. This reversion has prevented CTR from being industrially successful. Here, the generation of Mg(g) by CTR and the effective condensation and separation of Mg(g) were investigated experimentally and theoretically. Near isothermal and isobaric batch experiments revealed the reaction kinetics based on product gas evolution. Parallel reaction pathways, solid-solid and gas-solid, propagated the overall reduction. A kinetic model based on a macroscopic species balance predicted the reaction rate continuously with pressure and temperature. Milling, catalysis, vacuum operations, and their interactions were studied as methods to increase the reaction rate. Milling increased the reaction rate by mixing and crystallite attrition. The impregnation of transition metal salts to the C/MgO powder mixture catalyzed the gas-solid reaction pathway, but inhibited the solid-solid reaction pathway. CTR of calcined dolomite (MgO•CaO) favored the reduction of MgO over CaO, but reduction at 1550°C resulted in nearly complete MgO and CaO reduction. Ca(g) reverted more readily than Mg(g) as the overall yield decreased with increasing CaO reduction.

Product CO and Mg(g) did not revert at temperatures near 1000°C even though reversion is thermodynamically favored at these conditions. Condensation of Mg(g) in a CO atmosphere at reduced pressures (pCO < 0.3 kPa) resulted in high Mg(s) yields (Y > 90%), while condensation in a CO2 atmosphere resulted in nearly complete reversion (Y < 10%). The feasibility of continuous Mg(g) condensation onto a moving bed of solid particles was demonstrated using an artificial mixture of MgO CTR product gases. This moving bed condenser was scaled and integrated with a high-temperature furnace where gasification of equimolar C/MgO pellets produced Mg(g) and CO. Continuous reduction was achieved for 3.5 h; however, the yield of Mg(s) from Mg(g) decreased sharply (80%→30%) during this period. Overall ore-to-ingot yield was 29.1±0.5%.

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