Editing Thorium (section)

==Production==
{{See also|List of countries by thorium resources}}
<div style="float: right; margin: 2px; font-size:85%; margin-left:18px; margin-bottom:18px>
{| class="wikitable sortable collapsible" cellpadding="3" rules="all" style="background:#f9f9f9; border:1px #aaa solid"
|+'''Lower-bound estimates of thorium reserves in thousand [[tonne]]s, 2014'''<ref name="World Nuclear Association Thorium" />
! Country !! data-sort-type="number"|Reserves
|-
| [[India]] || align="right"|846
|-
| [[Brazil]] || align="right"|632
|-
| [[Australia]] || align="right"|595
|-
| [[United States]] || align="right"|595
|-
| [[Egypt]] || align="right"|380
|-
| [[Turkey]] || align="right"|374
|-
| [[Venezuela]] || align="right"|300
|-
| [[Canada]] || align="right"|172
|-
| [[Russia]] || align="right"|155
|-
| [[South Africa]] || align="right"|148
|-
| [[China]] || align="right"|100
|-
| [[Norway]] || align="right"|87
|-
| [[Greenland]] || align="right"|86
|-
| [[Finland]] || align="right"|60
|-
| [[Sweden]] || align="right"|50
|-
| [[Kazakhstan]] || align="right"|50
|-
| Other countries || align="right" |1725
|-
| align="center" |''World total'' || align="right" |6355
|}
</div>

The low demand makes working mines for extraction of thorium alone not profitable, and it is almost always extracted with the rare earths, which themselves may be by-products of production of other minerals.{{sfn|Stoll|2005|p=7}} The current reliance on monazite for production is due to thorium being largely produced as a by-product; other sources such as thorite contain more thorium and could easily be used for production if demand rose.<ref>{{cite web |url=https://minerals.usgs.gov/minerals/pubs/commodity/thorium/mcs-2012-thori.pdf |title=Thorium |author=United States Geological Survey |date=2012 |access-date=12 May 2017 |archive-date=29 April 2017 |archive-url=https://web.archive.org/web/20170429174611/https://minerals.usgs.gov/minerals/pubs/commodity/thorium/mcs-2012-thori.pdf |url-status=live }}</ref> Present knowledge of the distribution of thorium resources is poor, as low demand has led to exploration efforts being relatively minor.<ref>{{cite report|url=http://www.iaea.org/inisnkm/nkm/aws/fnss/fulltext/0412_1.pdf |title=An Overview of World Thorium Resources, Incentives for Further Exploration and Forecast for Thorium Requirements in the Near Future |last=Jayaram |first=K. M. V. |year=1987 |archive-url=https://web.archive.org/web/20110628234922/http://www.iaea.org/inisnkm/nkm/aws/fnss/fulltext/0412_1.pdf |publisher=[[Department of Atomic Energy]]|archive-date=28 June 2011}}</ref> In 2014, world production of the monazite concentrate, from which thorium would be extracted, was 2,700 tonnes.<ref name="USGS">{{Cite report|url=https://minerals.usgs.gov/minerals/pubs/commodity/thorium/index.html#mcs|title=Thorium. Statistics and Information|year=2017|publisher=[[United States Geological Survey]]|language=en|access-date=6 January 2018|archive-date=10 January 2019|archive-url=https://web.archive.org/web/20190110140206/https://minerals.usgs.gov/minerals/pubs/commodity/thorium/index.html#mcs|url-status=live}}</ref>

The common production route of thorium constitutes concentration of thorium minerals; extraction of thorium from the concentrate; purification of thorium; and (optionally) conversion to compounds, such as thorium dioxide.{{sfn|Stoll|2005|p=8}}

===Concentration===
There are two categories of thorium minerals for thorium extraction: primary and secondary. Primary deposits occur in acidic granitic magmas and pegmatites. They are concentrated, but of small size. Secondary deposits occur at the mouths of rivers in granitic mountain regions. In these deposits, thorium is enriched along with other heavy minerals.{{sfn|Stoll|2005|p=6}} Initial concentration varies with the type of deposit.{{sfn|Stoll|2005|p=8}}

For the primary deposits, the source pegmatites, which are usually obtained by mining, are divided into small parts and then undergo [[froth flotation|flotation]]. Alkaline earth metal carbonates may be removed after reaction with [[hydrogen chloride]]; then follow [[thickening]], filtration, and calcination. The result is a concentrate with rare-earth content of up to 90%.{{sfn|Stoll|2005|p=8}} Secondary materials (such as coastal sands) undergo gravity separation. Magnetic separation follows, with a series of magnets of increasing strength. Monazite obtained by this method can be as pure as 98%.{{sfn|Stoll|2005|p=8}}

Industrial production in the 20th century relied on treatment with hot, concentrated sulfuric acid in cast iron vessels, followed by selective precipitation by dilution with water, as on the subsequent steps. This method relied on the specifics of the technique and the concentrate grain size; many alternatives have been proposed, but only one has proven effective economically: alkaline digestion with hot sodium hydroxide solution. This is more expensive than the original method but yields a higher purity of thorium; in particular, it removes phosphates from the concentrate.{{sfn|Stoll|2005|p=8}}

====Acid digestion====
Acid digestion is a two-stage process, involving the use of up to 93% [[sulfuric acid]] at 210–230&nbsp;°C. First, sulfuric acid in excess of 60% of the sand mass is added, thickening the reaction mixture as products are formed. Then, fuming sulfuric acid is added and the mixture is kept at the same temperature for another five hours to reduce the volume of solution remaining after dilution. The concentration of the sulfuric acid is selected based on reaction rate and viscosity, which both increase with concentration, albeit with viscosity retarding the reaction. Increasing the temperature also speeds up the reaction, but temperatures of 300&nbsp;°C and above must be avoided, because they cause insoluble thorium pyrophosphate to form. Since dissolution is very exothermic, the monazite sand cannot be added to the acid too quickly. Conversely, at temperatures below 200&nbsp;°C the reaction does not go fast enough for the process to be practical. To ensure that no precipitates form to block the reactive monazite surface, the mass of acid used must be twice that of the sand, instead of the 60% that would be expected from stoichiometry. The mixture is then cooled to 70&nbsp;°C and diluted with ten times its volume of cold water, so that any remaining monazite sinks to the bottom while the rare earths and thorium remain in solution. Thorium may then be separated by precipitating it as the phosphate at pH&nbsp;1.3, since the rare earths do not precipitate until pH&nbsp;2.{{sfn|Stoll|2005|p=8}}

====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}}

===Purification===
High thorium concentrations are needed in nuclear applications. In particular, concentrations of atoms with high neutron capture [[cross-section (physics)|cross-sections]] must be very low (for example, [[gadolinium]] concentrations must be lower than one part per million by weight). Previously, repeated dissolution and recrystallisation was used to achieve high purity. Today, liquid solvent extraction procedures involving selective [[complexation]] of {{chem2|Th(4+)}} are used. For example, following alkaline digestion and the removal of phosphate, the resulting nitrato complexes of thorium, uranium, and the rare earths can be separated by extraction with [[tributyl phosphate]] in [[kerosene]].{{sfn|Stoll|2005|p=8}}