Thermodynamic Analysis of Recovering Chrome from Cr2O3

A thermodynamic analysis is carried out for reducing chromium from chromium contacting carbon in gas phase Н2–Н2О–СО–СО2. Considering the normalization condition $${{x}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}} + {{x}_{{{{{\text{H}}}_{{\text{2}}}}}}} + {{x}_{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}} + {{x}_{{{\text{CO}}}}} = 1$$ , the oxidizing potential $$\left( {{{p}_{{{{{\text{O}}}_{{\text{2}}}}}}}} \right)$$ of the gas phase is determined using two nomograms in coordinates $$\log \left( {\frac{{{{x}_{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}}}}{{{{x}_{{{\text{CO}}}}}}}} \right)$$ –T and $$\log \left( {\frac{{{{x}_{{{{{\text{H}}}_{2}}{\text{O}}}}}}}{{{{x}_{{{{{\text{H}}}_{2}}}}}}}} \right)$$ –T. The potential parameters of chromium reduction from Cr2O3 are determined through calculating the ratio of oxide dissociation pressure to the gas phase oxidation potential. In the CO–CO2–C system, chromium is reduced at a temperature of 1505 K if xCO > 0.9995. At this temperature, compound Cr2O3 is reduced in water-gas having composition $${{x}_{{{{{\text{H}}}_{{\text{2}}}}}}}$$ = 0.0186, $${{x}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}$$ = 0.28 × 10–4, xCO = 0.9809, $${{x}_{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}}$$ = 4.86 × 10–4; this compound features the oxidation potential equal to the oxide dissociation pressure: $$\log {{\left( {{{p}_{{{{{\text{O}}}_{{\text{2}}}}}}}} \right)}_{{{\text{C}}{{{\text{r}}}_{{\text{2}}}}{{{\text{O}}}_{{\text{3}}}}}}}$$ = –17.082. When the hydrogen concentration increases from 0.0186 to 0.9900, the oxidizing potential of water-gas contacting carbon drops by four orders of magnitude: down to $${{\left( {\log {{p}_{{{{{\text{O}}}_{{\text{2}}}}}}}} \right)}_{{{\text{gas}}}}}$$ = –21.09. This should result in a significant increase in the recovery rate. Chromium can be reduced in this gaseous atmosphere at a temperature of 1230 K. There is a simple and cheap process to obtain reducing water-gas, for example, by heating water vapor contacting carbon. It is shown that at a temperature of 1500 K, Н2О and СО2 traces can be found in water-gas having parameters $${{x}_{{{{{\text{H}}}_{{\text{2}}}}}}}$$ = 0.4999, xCO = 0.4996, $$\log \left( {\frac{{{{x}_{{{{{\text{H}}}_{2}}{\text{O}}}}}}}{{{{x}_{{{{{\text{H}}}_{2}}}}}}}} \right)$$ = –3.12, $$\log \left( {\frac{{{{x}_{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}}}}{{{{x}_{{{\text{CO}}}}}}}} \right)$$ = –3.59. The oxidizing potential in this gas is lower than that of chromium oxide with this difference increasing significantly when the temperature grows.