Effect of thiourea on sulphuric acid leaching of bastnaesite

The possibility of leaching bastnaesite pre-concentrate in dilute sulphuric acid solutions at 25 °C was studied. For this purpose, unroasted and roasted bastnaesite pre-concentrates were leached with 100 mL of 1–3 M H2SO4 and H2SO4+thiourea solutions at 25 °C with a liquid/solid ratio of 20 mL/g for 6 h. Roasting affected the dissolution behaviour of rare earths in H2SO4 solution and dissolution of total rare-earth elements (REEs) was increased from 8.5% to 47.4%. The addition of thiourea to the leaching medium further increased the dissolution of the rare-earth elements in the unroasted and especially in the roasted pre-concentrate due to the reduction of cerium (IV) to cerium (III) by thiourea, thus increasing the cerium dissolution value from 22.8% to 93.0%, which in turn increased the total dissolution of REEs from 17.6% to 89.0%. Thus, it was possible to precipitate 93.6% (CeO2=92.4%, La2O3=93.2%, Nd2O3=98.4%) of total REEs in the filtrate and the total REO content of the precipitate was determined as 92.6% (CeO2=58%, La2O3=29%, Nd2O3=5.6%).