The dissolution of the rare earths depends on both
sulfuric acid concentration and the reaction temperature.
After digesting 100 g of ore in 1.0 L of 18 M acid at
190 °C less than 40% of total rare earth into the sulfate
leach liquor, while using 9 M acid at 100 °C resulted in
97.8% dissolution after 2 h. It was mentioned that in strongly acidic solution some rare earth elements (e.g.
Ce, Eu) tend to form stable sulfate salts which are
insoluble due to hydrolysis (Molycorp, 1997).