Physical Chemistry of the Reduction of Calcium Oxide with Aluminum in Vacuum

The physical chemistry of "aluminothermic" reduction of calcium oxide in vacuum is analyzed.Basic thermodynamic data required for the analysis have been generated by a variety of experiments. These include activity measu-rements in liquid Al-Ca alloys and determination of the Gibbs energies of formation of calcium alumirtates. These data have been correlated with phase relations in the Ca-Al-0 system at 1373 IC The various stages of reduction,
the end prr ducts and the corresponding equilibrium
partial pressures of calcium have been established
from thermodynamic conside rations. In principle, the
recovery of calcium can he improved by reducing the
pressure in the reactor. Honvever, the cost of a high
vacuum system and the enhanced time for reduction needed to achieve higher yields makes such a practice uneconomic, Aluminum contamination of calcium also increases at low pressures. The best compromise is to carry the reduction up to the stage where 3CaO'Al_O3 is formed as the product. This corresponds to an equilibrium calcium partial
ptcssure of 31.3 Pa at 1373 K and 91.6 Pa. at 1460 K.
Calcium can be extracted at this pressure using mech-anical pumps in approximately 8 to 15 hr,depending on the site and the fill ratio of the retort and porosity of the charge briquettes.