Crushed minerals are extracted with hydrochloric acid or sulfuric acid, which converts the insoluble oxides into soluble chlorides or sulfates. The acidic filtrates are partially neutralized with caustic soda to pH 3–4. Thorium precipitates as its hydroxide, and is then removed. The remaining solution is treated with ammonium oxalate to convert rare earths into their insoluble oxalates. The oxalates are converted to oxides by heating. The oxides are dissolved in nitric acid that excludes one of the main components, cerium, whose oxide is insoluble in HNO3. The solution is treated with magnesium nitrate to produce a crystallized mixture of double salts of gadolinium, samarium and europium. The salts are separated by ion exchange chromatography. The rare-earth ions are then selectively washed out by a suitable complexing agent.[