Phosphoric Slag Leaching with Mineral Acids Process
The thermodynamic calculation was performed for the probability of the main slag components leaching reaction ∆G°T with mineral sulfuric (H2SO4), phosphoric (H3PO4), hydrochloric (HCl) and nitric (HNO3) acids, taking into account a phase and chemical compositions of the initial product. Gibbs energies were calculated for the reactions in the temperature range from 20 to 100 °C. It was found that the reactions were thermodynamically favorable and were almost not-reversible. The interaction of Fe2O3 with hydrochloric acid was an exception. With the temperature increase from 60 to 100 °C, hydrolysis went due to the increase in the dissociation degree of water molecules affecting the results of hydrochloric acid treatment. Experiments on the phosphorus slag dissolution were performed with H3PO4, HCl and HNO3 solutions in the concentration of 2 mol/dm3. The distribution of calcium, aluminum, and iron were analyzed over the slag leaching products. The choice of nitric acid as a leaching reagent was justified. A thermodynamic analysis of the reactions of the interaction of the slag rare-earth metals components with nitric acid was performed. The use of nitric acid in the analyzed temperature range from 50 to 100 °C was acceptable for all rare-earth components. For thermodynamic purposes reactions of rare-earth metals and their ions interaction with nitric acid were the most favorable for lanthanum sulfide, yttrium phosphate, lanthanum and neodymium phosphate, as well as for rare-earth metals carbonate ions. The Gibbs energy for these compounds was within from –424.93 to –36.16 kJ/mol. Rare-earth metals (REMs) remained in the cake by ~ 85% when phosphoric slag was leached with a HNO3 solution in the concentration of 2 mol/dm3. It was found that the use of nitric acid as an opening reagent was favorable for the decomposition of the main components of phosphorus slag.