M. Halmann

Vacuum Carbothermic Reduction of Al2O3, BeO, MgO-CaO, TiO2, ZrO2, HfO2 + ZrO2, SiO2, SiO2 + Fe2O3, and GeO2 to the Metals. A Thermodynamic Study

Thermochemical equilibrium calculations are carried out to elucidate improved conditions for the production of Al, Si, FeSi, Ti, Mg, Hf, Zr, Be, and Ge by the high-temperature carbothermic reduction of their oxides, and for the production of Mg by the silicothermic reduction of MgO–CaO. The onset temperature for the formation of free Al, Be, Si, Ti, Mg, Hf, and Zr in the gas phase is considerably lowered by decreasing the total pressure, enabling their vacuum distillation. An important prediction of vacuum operation is the suppression of undesired by-products, such as Al-carbide, Al4C3, and the Al-oxycarbides Al2OC and Al4O4C. These species considerably interfere in the carbothermic Al production at an ambient pressure, as shown in preliminary experiments using induction furnace irradiation. CO coproduced in these reactions may be water-gas shifted to syngas and further processed to hydrogen and liquid fuels.

Vacuum Carbothermic Reduction of Alumina

The current industrial production of aluminum from alumina is based on the electrochemical Hall-Héroult process, which has the drawbacks of high-greenhouse gas emissions, reaching up to 0.70 kg CO2-equiv/kg Al, and large energy consumption, about 0.055 GJ/kg Al. An alternative process is the carbothermic reduction of alumina. Thermodynamic equilibrium calculations and experiments by induction furnace heating indicated that this reaction could be achieved under atmospheric pressure only above 2200°C. Lower required reaction temperatures can be achieved by alumina reduction under vacuum. This was experimentally demonstrated under simulated concentrated solar illumination and by induction furnace heating. By decreasing the CO partial pressure from 3.5 mbar to 0.2 mbar, the temperature required for almost complete reactant consumption could be decreased from 1800°C to 1550°C. Deposits condensed on the relatively cold reactor walls contained up to 71 wt% of Al. Almost pure aluminum was observed as Al drops, while a gray powder contained 60–80% Al and a yellow-orange powder contained only Al4C3, Al-oxycarbides and Al2O3.

Thermo-neutral production of metals and hydrogen or methanol by the combined reduction of the oxides of zinc or iron with partial oxidation of hydrocarbons

Stoichiometry and temperature requirements are determined for combining the endothermic reduction of metal oxides (ZnO, Fe2O3, and MgO) with the exothermic partial oxidation of hydrocarbons (CH4, n-butane, n-octane, and n-dodecane) in order to co-produce simultaneously metals and syngas in thermo-neutral reactions. Thermogravimetric and GC measurements on the combined reduction of ZnO and Fe2O3 with the partial oxidation of CH4 were conducted at 1400 K to experimentally verify the products predicted by equilibrium computations, and resulted in the complete reduction to Zn and Fe, respectively, while producing high quality syngas. A preliminary economic assessment that assumes a natural gas price of 11.9 US

Vacuum Carbothermic Reduction of Bauxite Components: A Thermodynamic Study

/MWh and credit for zinc sale at 750 US

Carbothermic Reduction of Alumina by Natural Gas to Aluminum and Syngas: A Thermodynamic Study

/metric ton, indicates a competitive cost of hydrogen production at 6.0 US

Reforming of Blast Furnace Gas with Methane, Steam, and Lime for Syngas Production and CO2 Capture: A Thermodynamic Study

/MWh, based on its high heating value. The proposed combined process offers the possibility of co-producing metals and syngas in autothermal non-catalytic reactors, with significant avoidance of CO2 emission.

Vacuum Carbothermic Production of Aluminum and Al-Si Alloys From Kaolin Clay: A Thermodynamic Study

The possibility of direct vacuum carbothermic reduction of bauxite minerals to metallic aluminum at a temperature of 1400–1500 K and a pressure of 10−7 bar is examined by thermochemical equilibrium calculations on AlO(OH) (boehmite and diaspore) and Al(OH)3 (gibbsite) in the absence and presence of SiO2, TiO2, and FeO(OH). Carbon monoxide and hydrogen coproduced by the reaction may be used as combustion fuel or further processed to liquid hydrocarbons. The vacuum carbothermic reduction of iron-rich calcined bauxite was studied for Al2O3–Fe2O3–C mixtures at 10−4, 10−5, and 10−6 bar, indicating narrow temperature ranges at which the equilibrium for the release of gaseous Al is favored relative to gaseous Fe. Alternatively, for iron-rich bauxite, a preliminary step of iron-removal would be necessary. The proposed process could potentially decrease energy consumption and greenhouse gas emissions, and avoid the production of “red mud.”

Carbothermal reduction of alumina : Thermochemical equilibrium calculations and experimental investigation

The carbothermic reduction of alumina to aluminum by methane is analyzed by thermochemical equilibrium calculations in order to determine its thermodynamic constraints. Calculations predict that in the temperature range 2300–2500°C at 1 bar pressure, the reaction Al2O3 + 3CH4 = 2Al +6H2 + 3CO should occur without significant interference by the formation of unwanted byproducts such as Al2O, Al4C3, and Al-oxycarbides, and with higher yields than by using solid carbonaceous compounds as reducing agent. The reaction was examined for several initial Al2O3/CH4 molar ratios. The proposed process may be carried out in a fluidized bed reactor using concentrated solar energy, induction furnaces, or electric discharges as sources of high-temperature process heat. An important advantage of such a process would be the coproduction of syngas, with the molar ratio H2/CO = 2, suitable for the synthesis of liquid hydrocarbon fuels and polymeric materials.

Solar Aluminum Production by Vacuum Carbothermal Reduction of Alumina—Thermodynamic and Experimental Analyses

The production of iron and steel by the blast furnace process is a major source of CO2 release, with blast furnace gas contributing about 5% to the anthropogenic greenhouse gas emission. Its main components are CO2, CO, N2, and H2. Chemical equilibrium calculations are made to determine the thermodynamic constraints for converting these components into valuable syngas for producing hydrogen, methanol, Fischer–Tropsch hydrocarbons, or ammonia, by either reforming with CH4, water-gas shift reaction, partial oxidation, or CaO carbonation—while achieving partial or complete CO2 capture. By a two-step thermochemical cycle, the CaCO3 formed by lime carbonation could be calcined back to CaO, while releasing relatively pure CO2 for utilization. The implications of such reactions with respect to hydrogen production, CO2 emission avoidance, and process efficiency are examined.

Fuel saving, carbon dioxide emission avoidance, and syngas production by tri-reforming of flue gases from coal- and gas-fired power stations, and by the carbothermic reduction of iron oxide

Examination of the thermodynamic constraints for the carbothermic reduction of iron-free kaolinite, Al2Si2O5(OH)4, or of its calcination product mullite, Al6Si2O13, either at atmospheric pressure or under vacuum of 10−3 to 10−5 bar, indicates the conditions required at equilibrium to produce either elementary Al or Al-Si alloys. At atmospheric pressure, a very high temperature of 3200 K would be required to obtain from Al2Si2O5(OH)4 + 9C an Al-Si alloy with 39 wt.% Si. At 10−4 bar and 1800 K, the predicted Al-Si alloy would contain 2.4 wt.% Si. From mullite, the reaction of Al6Si2O13 + 13C at 10−4 bar and either 1800 K or 2200 K should produce an Al-Si alloy with 0.65 or 24 wt.% Si. The CO produced by the carbothermic reactions may be by water-gas shift converted to syngas, and further either to methanol or by a Fischer–Tropsch reaction to liquid fuels or chemical intermediates. Concentrated solar energy may be used to supply the required process heat of these high-temperature reactions.