Thorium is a metallic element with the symbol Th and the atomic number 90. It is silvery and tarnishes black when it comes into contact with air, forming thorium dioxide; it is moderately soft and malleable and has a high melting point.
It is an electropositive actinide that can be oxidized in the +4 oxidation state, making it a relatively reactive element. It is found in several natural sources and has been extracted as a radioactive waste from nuclear power plants.
The mobility of Th in hydrothermal fluids is a significant source of uncertainty in the development of mineral deposits for use as sources of uranium and other rare earth elements (REE). Many studies employ extrapolations of low-temperature solubility data to high temperature5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30.
In natural hydrothermal systems, sulfate-rich fluid inclusions6,9,20,22,23,24,26,29,30,32,33 are believed to mobilize Th from sulfate-bearing minerals. These fluid inclusions typically contain some sulfuric acid, as well as iron and other REE compounds, as a result of the interaction of Th with these compounds.
One approach to the mobilization of sulfate-rich REE from sulfate-bearing ores is to use hot, aqueous solutions that contain Th as a component. This has been shown to be an effective technique for the recovery of sulfate-rich REE34,35,36 from hydrothermal fluid inclusions, and is also likely to be suitable for mobilizing Th from aqueous-rich rocks.
In a typical process, crude aqueous saline compositions of thorium sulfate are mixed with dissolving water and passed through a clarifier. The resulting clarified filtrate is then heated to a predetermined temperature, such as E-85 C., and crystallized from the mother liquor by addition of sulfuric acid to depress its solubility for subsequent precipitation in the form of a heptahydrate, and in the form of other hydrates when desired.