Editing Thorium (section)

====Alkaline digestion====
Alkaline digestion is carried out in 30–45% [[sodium hydroxide]] solution at about 140&nbsp;°C for about three hours. Too high a temperature leads to the formation of poorly soluble thorium oxide and an excess of uranium in the filtrate, and too low a concentration of alkali leads to a very slow reaction. These reaction conditions are rather mild and require monazite sand with a particle size under 45&nbsp;μm. Following filtration, the filter cake includes thorium and the rare earths as their hydroxides, uranium as [[sodium diuranate]], and phosphate as [[trisodium phosphate]]. This crystallises trisodium phosphate decahydrate when cooled below 60&nbsp;°C; uranium impurities in this product increase with the amount of [[silicon dioxide]] in the reaction mixture, necessitating recrystallisation before commercial use. The hydroxides are dissolved at 80&nbsp;°C in 37% hydrochloric acid. Filtration of the remaining precipitates followed by addition of 47% sodium hydroxide results in the precipitation of thorium and uranium at about pH&nbsp;5.8. Complete drying of the precipitate must be avoided, as air may oxidise cerium from the +3 to the +4 oxidation state, and the cerium(IV) formed can liberate free [[chlorine]] from the hydrochloric acid. The rare earths again precipitate out at higher pH. The precipitates are neutralised by the original sodium hydroxide solution, although most of the phosphate must first be removed to avoid precipitating rare-earth phosphates. [[Solvent extraction]] may also be used to separate out the thorium and uranium, by dissolving the resultant filter cake in nitric acid. The presence of [[titanium hydroxide]] is deleterious as it binds thorium and prevents it from dissolving fully.{{sfn|Stoll|2005|p=8}}