Although the principal constituents of monazite are rare earth and thorium phosphates, its composition varies widely within a given deposit and from place to place. Table 6.16 gives the composition of monazite concentrates from different locations.

1.13 Processes for Opening Up Monazite

Monazite is chemically very inert. Two general methods that are used for opening up monazite and making its constituents sufficiently reactive to permit extraction and separation of thorium, uranium, and rare earths are

1. Reaction with hot, concentrated caustic soda solution

2. Dissolution in hot, concentrated sulfuric acid

The caustic soda process has been used on a large scale in Brazil [B6] and India [Dl] and has been investigated on a pilot-plant scale at Battelle Memorial Institute [B2, Cl] in the United States. A brief description is given in Sec. 8.4.

Sulfuric acid has been used to dissolve monazite in Europe, Australia, and the United States. The numerous processes used to separate thorium from the acid leach liquors are listed

in Sec. 8.5, with a more detailed description of one process, developed at Iowa State College [В1].

Solvent extraction processes for recovering thorium from monazite sulfuric acid leach liquor are described in Sec. 8.6.