Mechanochemical decomposition of mixed rare earth concentrate in the NaOH-CaO-H2O system

This paper describes mechanochemical decomposition of the mixed rare earth concentrate (mixed concentrate) in the NaOH-CaO-H2O system. Phase transformation and effects of NaOH, CaO and time on mechanochemical decomposition of mixed concentrate were investigated. We also studied strengthening mechanism of mechanochemical decomposition by comparing its results with the results obtained by pressurized decomposition. Mixed concentrate first decomposed into RE(OH)(3), CaF2 and Ca2P2O7. Then, Rare Earth Elements (REEs) and phosphorus were extracted by HCl, while CaF2 remained in the leach residue. After mechanochemical decomposition, leaching efficiency of REEs, which represented decomposition efficiency of the mixed concentrate, reached 91.8%. At the same time, 93.8% of fluoride and over 99% of phosphorus remained in the decomposition residue when 35% of NaOH and 20% of CaO (relative to mixed concentrate weight) were added and decomposition time was 4 h. Mechanochemical treatment improved concentrate decomposition efficiency from 58.9% of the pressurized decomposition to 91.8%. Such an improvement was attributed to the beneficial effect of the mechanical force, which loosened the surface structure as well as decreased particle size in the decomposition residue.