A flow sheet of the caustic soda process described by Bearse et al. [B2] is shown in Fig. 6.4. The sand is ground with water in a ball mill until 96.5 percent passes 325 mesh. A wet classifier recycles coarse particles and delivers a slurry of fine particles to a stainless steel reactor. A liquid caustic solution containing 73% NaOH is fed to the reactor. At the beginning of the reaction the slurry contains 1.5 kg of NaOH and 1.7 kg of water per kilogram of sand. The mixture is heated to 140°C, and after 3 h at this temperature the sand is completely reacted. The mixture is then diluted with the wash solution of caustic and trisodium phosphate from a later step and digested at 105°C for 1 h to facilitate later filtration. The resulting hot slurry contains practically all the original phosphorus in solution as trisodium phosphate, and thorium, cerium, and rare earths are present as solid hydrous metal oxides. The trisodium phosphate and unreacted caustic are removed by filtering the slurry through Monel wire cloth, and the metal oxide cake is washed with water. The filtration is carried out at 80°C to keep caustic and trisodium phosphate in solution. The filtrate, which contains about two-thirds of the original caustic soda charged, is evaporated in an open steel kettle until the NaOH

concentration is 47%, corresponding to a boiling point of 137°C. The concentrated solution is cooled to room temperature. More than 95 percent of the sodium phosphate crystallizes out of solution and is removed by filtration. The caustic soda liquor is recycled for sand digestion and for later neutralization steps.

The hydrous oxide cake is brought into solution by dissolving in 37% hydrochloric acid (1.5 kg acid/kg sand) at 80° C for 1 h in a glass-lined vessel. Hydrochloric rather than sulfuric acid is recommended because of more selective precipitation from chloride solutions in the later step. About 2 percent of the weight of the original sand is left as residue, which contains undissolved monazite and rutile (Ti02), an impurity in the sand.

The acid solution and undissolved material are transferred to a neutralizer vessel and diluted with water.

Thorium is separated from the rare earths by selective precipitation of thorium hydroxide at a pH of 5.8. This is effected by neutralizing the diluted chloride solution with caustic recovered from the evaporator. The wet cake is reslurried in water solution, filtered, and again reslurried and filtered to effect a high degree of separation of the thorium precipitate from any occluded rare earth solution.

The percentage of thorium and other constituents of the monazite sand recovered in the precipitate is given in the first column of Table 6.17. The composition of the precipitate is given in the second column.

Rare earths are recovered from the combined decantates and filtrates by further neutraliza­tion with NaOH. The hydroxide precipitate is removed by filtration.

The process used in Brazil [B6] is generally similar.