Supercritical fluid extraction of minerals in Martian soils

Abstract

The objective of this dissertation was to develop a supercritical extraction process for extracting useful materials from in-situ resources on Mars. Carbon dioxide, composing approximating 95% of the Martian atmosphere, provides an in-situ source of carbon dioxide as a supercritical solvent. We explored two potential processes. The first was direct extraction with supercritical carbon dioxide. The second was extraction using supercritical carbon dioxide combined with small quantities of a chelator and water. To explore the types of minerals in Martian soil that could be extracted in supercritical carbon dioxide, we performed an initial screening to determine soluble species of inorganic minerals that may be present in the Martian regolith. The results showed that simulated Martian soil (JSC-Mars1) and hydrated inorganic compounds have significant weight change during extraction. We also confirmed that it would be possible to recover water from hydrated species using supercritical carbon dioxide. We ran further tests on ferrous sulfate and calcium sulfate to determine the temperature at which the water of hydration is removed and the enthalpy values for various dehydration steps. We compared the dehydration behavior of these two compounds for two processes, a thermal heating process to recover water, and CO2 supercritical extraction process to recover water. To enhance the solubility of metals in supercritical carbon dioxide, we used a small amount of water and a chelate. The chelate was a high performance perfluoropolyether. We studied the solubility of copper (II) nitrate trihydrate (Cu(NO3)2·3H2O) and magnesium(II) chloride hexahydrate(MgCl2·6H2O). The influence of parameters including extraction pressure, extraction temperature, concentration of chelate, concentration of water and molar ratio of chelate to metals was examined. Finally, we used a thermodynamic model incorporating conventional mixing rules and the Peng-Robinson equation of state (EOS) to model the solubility oh the chelate complexes in supercritical carbon dioxide. We obtained the model parameters, Tc, Pc, vs, Psat and k12, for Cu-(PFPECOO)2 and Mg-(PFPECOO)2 by fitting experimental data using the Marquardt-Levenberg least squares (MLLS) method. Good agreement between experimental and calculated results was achieved.