THE RECOVERY OF RARE-EARTH-OXIDES FROM A PHOSPHORIC-ACID BY-PRODUCT .1. LEACHING OF RARE-EARTH VALUES AND RECOVERY OF A MIXED RARE-EARTH-OXIDE BY SOLVENT-EXTRACTION

The development of a process for the recovery of a mixed rare earth ox ide from calcium sulphate sludges obtained in the manufacture of phosp horic acid from a South African apatite ore is described. The leaching of the rare earth values from the sludges by dilute nitric acid is co nsiderably enhanced by the addition of calcium nitrate to the lixivian t, enabling recoveries of up to 85% to be achieved. The rare earth val ues can be recovered from a leach liquor containing 1.0 M nitric acid and 0.5 M calcium nitrate by the addition of 2.5 M ammonium nitrate an d extraction into 33 vol% dibutyl butylphosphonate in Shellsol 2325. T he organic phase can be stripped with water, preferably at temperature s above ambient, to yield a solution of rare earth nitrates from which the mixed oxide can be recovered by the addition of oxalic acid, and calcination of the precipitate. In a continuous counter-current labora tory trial of the process, a total of 140 kg of sludge was processed t o produce 2651 of leach liquor, which was treated in five extraction a nd five stripping stages to give a strip liquor from which 4 kg of mix ed rare earth oxide of 98% purity was recovered. In a pilot-plant eval uation, the organic phase was changed to 40 vol% tributyl phosphate in order to enhance the stripping characteristics (as well as for econom y), and the leach liquor was changed to 1.0 M nitric acid plus 3.0 M c alcium nitrate, the addition of ammonium nitrate then being unnecessar y. Some 3200 kg of mixed rare earth oxalates were prepared and calcine d in a rotary kiln to give 1600 kg of mixed oxide of 89-94% purity. In all of the continuous trials the raffinate from the solvent-extractio n process was recycled to the leaching stage, without any deleterious effect on the leaching efficiency being apparent.