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The development of a process for the recovery of a mixed rare earth oxide from calcium sulphate sludges obtained in the manufacture of phosphoric acid from a South African apatite ore is described. The leaching of the rare earth values from the sludges by dilute nitric acid is considerably enhanced by the addition of calcium nitrate to the lixiviant, enabling recoveries of up to 85% to be achieved. The rare earth values 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 and extraction into 33 vol%v dibutyl butylphosphonate in Shellsol 2325. The organic phase can be stripped with water, preferably at temperatures 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 laboratory trial of the process, a total of 140 kg of sludge was processed to produce 265 1 of leach liquor, which was treated in five extraction and five stripping stages to give a strip liquor from which 4 kg of mixed rare earth oxide of 98% purity was recovered. In a pilot-plant evaluation, the organic phase was changed to 40 vol% tributyl phosphate in order to enhance the stripping characteristics (as well as for economy), and the leach liquor was changed to 1.0 M nitric acid plus 3.0 M calcium nitrate, the addition of ammonium nitrate then being unnecessary. Some 3200 kg of mixed rare earth oxalates were prepared and calcined 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-extraction process was recycled to the leaching stage, without any deleterious effect on the leaching efficiency being apparent. |
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