Method for the preparation of anhydrous thorium(IV) chloride

Abstract

The invitation pertains to a process for the manufacturing of anhydrous thorium(IV) chloride comprising the steps of providing a thorium compound comprising a thorium (IV) cation, contacting the compound of (a) with an organic chlorinating agent; and isolating the anhydrous thorium(IV) chloride. The thorium compound may be a thorium oxalate prepared from a soluble thorium salt.

Description

[0001] P37008PC00 / RLA / MOV

[0002] Title: Method for the preparation of anhydrous thorium(IV) chloride

[0003] Field of the invention

[0004] The invention pertains to a method for the preparation of anhydrous thorium(IV) tetrachloride. The invention also pertains to a method for the purification of thorium(IV) tetrachloride. The route is suitable to produce nuclear-grade, anhydrous thorium tetrachloride, of the appropriate purity and quality levels directly, which could be used to make nuclear fuels and blankets. The reaction products obtained can be recycled for re-use.

[0005] Background of the invention

[0006] Controlling the oxidation state and impurity levels of a nuclear fuel is of the utmost importance. With the recent interest in exploring alternative nuclear energy options, such as molten salt nuclear reactors and thorium-based reactors, there is an increased interest in the production of thorium salts, and ThCU is no exception.

[0007] The use of chloride salts as nuclear fuels has been considered previously. For many years, the concept lay dormant, and for the large part of the history of nuclear science, scientific activity around actinide chlorides has not been oriented towards their practical applications as nuclear fuels, but rather towards the chloride state as a practical one for pyrochemical applications. In recent years, there is an increased interest in the use of thorium in the nuclear arena for many reasons.

[0008] Now there is a need for simple, scalable routes to thorium salts in high purity.

[0009] Known methods for making anhydrous thoriumtetrachloride (ThCU, Th(IV)Cl4, thorium(IV) chloride) are for instance described in US2928721A starting from thorium nitrate via thorium carbide and chlorine gas. US2762684A starts from thorium fluorides. Deubner et al. in Journal of Inorganic and General Chemistry, 2017, 643, 2005-2010 described a process starting from ThC>2 and AI2O3. Also GB843261 converts thorium oxide in a melt of sodium and potassium chloride using carbon monoxide and chloride gas. LIS2011282039 describes conversion of thorium nitrate to thorium halides using a halide containing strong acid. Other routes have been described in O. C. Dean & J. M. Chandler (1957), The Preparation of Anhydrous Thorium Tetrachloride, Nuclear Science and Engineering, 2:1 , 57-72. There remains a need for improved methods that yield anhydrous thorium chloride on large scale and with high yields and purity. Summary of the invention

[0010] The present inventors have found a simple, high yield method for the preparation of anhydrous ThCU by the reaction of a thorium compound comprising a thorium (IV) cation with an organic chlorinating agent. The thorium compound itself can for example be made from readily available water-soluble thorium salts and their hydrates by reaction of an aqueous solution of the thorium salt with e.g. a polybasic organic acid. The process allows for the conversion of thorium salts and hydrates thereof into anhydrous thorium chloride. The resultant ThCU is obtained as a white powder in high yield and purity.

[0011] Detailed description of the invention

[0012] The invention pertains to a process for the manufacturing of anhydrous thorium (IV) chloride comprising the steps of: providing a thorium compound comprising a thorium (IV) cation; contacting the compound of (a) with an organic chlorinating agent; isolating the anhydrous thorium(IV) chloride.

[0013] The process of the invention starts by providing a thorium compound comprising a thorium (IV) cation. The thorium compound may further comprise an oxygen atom, a nitrogen atom, a hydroxide group, a nitrate group, a carbonate, a polybasic organic acid or a mixture thereof. Preferably, the thorium compound is selected from the group consisting of thorium oxide, thorium nitrate, thorium oxynitride, thorium hydroxide, thorium carbonate, thorium oxalate, thorium malonate, thorium succinate, thorium glutarate, thorium citrate, thorium isocitrate or thorium aconitate, preferably thorium oxalate, thorium malonate, or thorium citrate, more preferably thorium oxalate. In a preferred embodiment, the thorium compound comprises a thorium (IV) cation and a polybasic organic acid, preferably thorium oxalate.

[0014] The use of a thorium compound, preferably a thorium compound based on a polybasic organic acid, lies in the reduced solubility of such compounds in water and hence make them a suitable starting point for the intended anhydrous product. Typically, the organic chlorinating agent is a di-, tri, poly- or perchlorinated organic compound. Perchlorinated organic compounds have a preference as the number of potential side reactions is less.

[0015] As chlorinating agents there is a preference for di-, tri, poly- or perchlorinated alkanes, alkenes, mono or dicyclic aromatic compounds and mixtures thereof.

[0016] The di-, tri, poly- or perchlorinated alkane, alkene, mono- or dicyclic aromatic compound are preferably selected from the group consisting of tetrachloride, pentachloroethane, methylchloroform (1,1,1 trichloroethane), 1,2,3- trichloropropane, 1 ,1,2-trichloroethane, ethylidene chloride (1 ,1- dichloroethane), propylenechloride (1, 2-dichloro propane), chlorinated C3-C7 alkanes such as polychlorinated propane, butane, pentane, hexane and heptane, 1 ,1,2- trichloroethene, tetrachloroethylene, trichloroacrylylchloride, hexachlorobutadiene, pentachlorobenzene, tetrachlorocumene, polychloronapthalene, chloral (CCh-CHO), hexachloropropene and mixtures thereof.

[0017] Good results have been achieved with perchlorinated alkenes, such as those selected form the group consisting of tetrachloroethylene, hexachloropropene, hexachlorobutadiene and mixtures thereof, preferably hexachloropropene or tetrachloroethylene, most preferably hexachloropropene. The additional advantage also lies in the potential for regeneration of these compounds. Other actinic chlorides have been prepared using hexachloropropene (HOP), (Patel, Dipti, et al. New Journal of Chemistry 39.10 (2015): 7559-7562).

[0018] The thorium compound is contacted with, preferably an excess of, the chlorinating agent. In preferred embodiments, the molar ratio of organic chlorinating agent to thorium compound is more than 2, preferably 5, more preferably 8, most preferably 10, such as more than 15. A solvent or diluent may be used, but is preferred that the chlorinating agent is used as diluent or solvent.

[0019] The thorium compound can be contacted with the organic chlorinating agent under heating to advance the reaction, and optionally under pressure to speed up the reaction, if needed. When the reaction temperature is too low, the chlorinating agent may not be reactive enough and / or the thorium compound or polybasic organic compound may not react and / or decompose or the whole process may be too slow. The reaction mixture may be heated to more than 150, preferably 180, or more preferably 200 degrees Celsius and / or reflux. The temperature may be varied based on, for instance, the boiling point and volatility of the chlorinating agents. The reaction of the thorium compound with the chlorinating agent can be a radical reaction. During the reaction, the thorium compound may (partially) decompose, or the ligand may simply be substituted with chlorine. Depending on the chlorinating agent and the thorium compound (e.g. comprising a polybasic organic acid), other reaction products may form that are driven off (for instance as a gas) or that can be separated from the reaction mixture and may be reused or recycled. The reaction may continue for a prolonged period such as embodiments wherein the process is continued for a period of more than 2, 4, 5, 8, 12, 18, or 24 hours. If the reaction is too slow at ambient pressure, the chlorination may be performed under pressure by heating the reaction mixture in a closed / sealed reaction vessel. With the use of increased pressure the reaction can be completed within an hour. Preferably, the reaction is performed under solvothermal conditions. Using a microwave-assisted solvothermal synthesis may also considerably shorten reaction times. The product resulting from the reaction can be isolated or separated from the reaction mixture by known means such as by filtration or centrifugation. In an alternative embodiment, the reaction mixture is allowed to settle and the liquid fraction collected, for instance by removal of the solvent / diluent or reactants. The isolated product of the method of the invention can be further purified by washing by conventional means and or washing liquids. In preferable embodiments the product is preferably washed with a chlorinated or perchlorinated volatile solvent, preferably perchlorinated or chlorinated methanes, ethanes or ethenes, more preferably selected from the group consisting of dichloromethane, trichloromethane, tetrachloromethane, hexachloroethane, pentachloroethane, 1,1 ,1,2-tetrachloroethane, 1,1,2,2-tetrachloroethane, 1 ,1 ,2-trichloroethane, 1,1 ,1 -trichloroethane, 1,2-dichloroethane, chloroethane, dichloroethylene trichloroethylene, tetrachloroethylene, dichloroethylene, vinyl chloride and mixtures thereof. In view of the possible presence of water, there is a preference for a solvent that is part miscible with water such as dichloromethane. Thus in a preferred embodiment, the product of the invention, anhydrous thorium(IV) chloride, is washed with dichloromethane. As an optional step, organic impurities may be removed through a controlled decomposition step. In some circumstances, organic impurities may remain within the obtained ThCI4powder. The ThCI4powder containing such impurities may be heated up to 550 °C, such as in the range of 500-550 °C, under an inert gas atmosphere. This may lead to decomposition of the organic residues and formation of a mixture of ThCI4and carbon. This mixture can then be dissolved in an organic solvent, such as acetone, ethanol, methanol or a mixture thereof, after which the residual carbon is removed by filtration. The solvent is subsequently evaporated under mild conditions to yield a solvated ThCI4. A further heat treatment, not exceeding 400 °C, may optionally be applied to achieve full desolvation of the product and obtain the desired anhydrous ThCI4.

[0020] In a step preceding or separate from the reaction with the chlorinating agent, the thorium compound can be prepared from a water-soluble thorium salt. Thorium salts are commercially available. Suitable salts are thorium chloride, thorium sulphate, thorium fluoride or thorium nitrate, preferably thorium nitrate or their hydrates thereof.

[0021] In the particular embodiment of the use of thorium chloride, the process of the invention not only provides anhydrous thorium chloride, but further serves as a purification method to remove other, possibly insoluble, components present in the starting material (oxides, contaminants etc.). As the intended use is in molten salt nuclear reactors, purity is of importance.

[0022] In an embodiment of the invention, the thorium compound is prepared by contacting the water-soluble thorium salt in an aqueous solution with the polybasic organic acid. The polybasic organic acid can be a dibasic or tribasic organic acid and may be selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, citric acid, iso-citric acid or aconitic acid and mixtures thereof, preferably oxalic acid or citric acid. The molar ratio of the polybasic organic acid to thorium cation can be more than 2, 3, 4, 5, 6, 7, 8, or even 10, i.e. an excess of polybasic organic acid to ascertain that the maximum yield of thorium compound is formed. By using a polybasic organic acid an insoluble thorium compound is formed or at least a compound with a reduced solubility in water. This drives the reaction to completion instead of forming an equilibrium. Following the reaction, the thorium compound comprising a thorium (IV) cation and dibasic organic acid can be isolated, for instance through filtration or centrifugation.

[0023] In the case of the most preferred organic acid, oxalic acid, thorium oxalate is formed. Thorium oxalate, Th(C2O4)2, is formed when oxalic acid is added to a thorium salt solution. Thorium oxalate is a white insoluble solid prepared by the reaction of thorium(IV) salts with an oxalic acid. The material is a coordination polymer. Each Th(IV) center is bound to 10 oxygen centers: eight provided by the bridging oxalates and two by a pair of aquo ligands. Two additional water of hydration are observed in the lattice which can be removed by careful dehydration. The composition of the air- dried salt is expressed by the formula Th(C2C>4)2.6H2O; when heated to 100 °C or dried over sulphuric acid the dihydrate Th(C2C>4)2.2H2O can be formed, and at 160 °C the hydrate 4Th(C2C>4)2.3H2O can be obtained. Thus in preferred embodiments, the isolation of the thorium compound is achieved by separating solids from liquids such as by filtration or centrifugation or by tank settling and extracting liquid from the top. This can also be done in a continuous process of blending the two solutions and separating off the solids from liquids using conventional means. The thorium compound may be dried to preferably remove any remaining free or crystal water. To that end, the thorium compound may preferably be dried at elevated temperatures and / or under reduced pressure. The drying may be for a first period of more than 2, 4, 5, 8, 12, or 24 hours. Preferably, the drying is a temperature of more than 100, 150, or 200 °C. In preferred embodiment the drying is additionally for a second period of more than 2, 4 ,5, or 8 hours. Preferably at more than 200, 220, 240, 260, 280, 300 °C which also may be under reduced pressure. The two-step drying process allows both the free and the water molecules in the crystal lattice to be expelled from the solid lattice without decomposing the thorium oxalate. The process of drying thorium oxalate under heat and / or reduced pressure can be cumbersome due to the risk of partial decomposition of the oxalate and potential formation of thorium oxy chlorides. The method of the present invention allows for the combination of a more gentle drying process of the formed thorium oxalate with an additional water removal step in the chlorination phase by using a washing solvent that is partly miscible with water and can be used to remove any of the remaining water to achieve an anhydrous thorium chloride product with a water content of less than 1000 ppm, preferably less than 100 ppm. In preferred embodiments, the steps of the process of the invention (oxalate formation and chlorination) are performed under an inert atmosphere, preferably the drying steps are performed under dry air or inert (nitrogen, argon) atmosphere, more preferably under dry air. The product of the process of the invention is an anhydrous thorium (IV) chloride, wherein the anhydrous thorium chloride contains less than 1000 ppm water, more preferably less than 100ppm.

[0024] The product of the process of the invention is an anhydrous thorium (IV) chloride that contains less than 0.1 wt.% of contaminants or impurities.

[0025] The solvents, chlorinating agents and the reduction products other than thorium(IV) chloride can be recycled, for instance using conventional distillation.

[0026] The resulting reactants from the formation of thorium chloride, such as HCI, can be recycled or reused, for instance using a molecular sieve.

[0027] The recycling of HCP is partly described in Patel, 2015 as cited herein elsewhere. After reaction, the HCP mother liquor is filtered from the ThCU precipitate and collected. The distillation of the HCP mother liquor gave 3 fractions. The first two fractions were acid chlorides but the characterization was not made further by Patel. The third faction was essentially pure HCP.

[0028] The leftover compounds collected after filtration of ThCU can be separated by chromatography for example with flash columns with stationary phases including silica or cellulose and with or without addition of a solvent phase.

[0029] One of the fractions which can be collected by the aforementioned methods can be of the acid chlorides family: R-COCI, R being any carbon chain. Acid chlorides can be recycled into HCP with a series of reaction including shortening or extending of the carbon chain, chlorination of the carbon chain and dehydrochlorination of the acid chloride.

[0030] If the compound isolated / separated is an acid chloride of the form: R’R-COCI, R and R’ being unchlorinated or partially chlorinated carbons, chlorination is ensured with Cl2and a radical initiator, which can be UV or peroxide:

[0031] RR’-COCI + Cl2CI3C-C(CI)2COCI

[0032] For example, with 1,1,2-trichloropropanoyl chloride Cl2C=C(CI)COCI: CI2C=C(CI)COCI + Cl2CI3C-C(CI)2COCI

[0033] HCP is then obtained by dehydrochlorination reaction to remove hydrogen chloride (HCI) and form the double bond needed. A strong base, such as KOH or NaOH in ethanol, can be used.

[0034] CI3C-C(CI)2COCI + base CCI2=CCI-CCI3+ HCI

[0035] Another possible fraction that can be collected by the aforementioned methods can be of partially or fully chlorinated short alkane compounds (<=3 carbon length chain). In that case, the chloroalkanes can be dechlorinated with hydrogenolysis or by reaction with an electropositive metal followed by an hydrolysis. The short alkane or alkene obtained can then be used to synthesize HCP following the method below.

[0036] HCP can be synthetized starting from Cl2(chlorine gas) and simple organic compounds.

[0037] For example, carbon tetrachloride can be produced by reaction between methane and Cl2: CH4+ 4 Cl2CCI4+ 4 HCI

[0038] Dichloroethane can be produced by the reaction of ethylene and Cl2. The use of a ferric chloride catalyst is preferred:

[0039] CH2=CH2+ Cl2CICH2-CH2CI

[0040] Dichloroethane can be heated to around 400 °C with additional chlorine and a catalyst like a mixture of potassium chloride and aluminum chloride or porous carbon to produce trichloroethylene:

[0041] CICH2-CH2CI + 2 Cl2CICH=CCI2+ 3 HCI

[0042] This reaction produces tetrachloroethylene CCI2=CCI2as a byproduct and depending on the amount of chlorine fed to the reaction, tetrachloroethylene can even be the major product. Typically, trichloroethylene and tetrachloroethylene are collected together and then separated by distillation.

[0043] Heptachloropropane is then prepared from trichloroethylene and carbon tetrachloride through electrophilic addition reaction in the presence of Lewis acid serving as a catalyst, wherein the lewis acid can be aluminum trichloride, zinc chloride or stannic chloride.

[0044] CICH=CCI2+ CCI4CCI3-CHCI-CCI3

[0045] Dehydrochlorination of heptachloropropane with a strong base such as KOH or NaOH in an ethanol solution is then used to remove hydrogen chloride (HCI) and form the double bond.

[0046] CCI3-CHCI-CCI3 + base CI2C=CCI- CCI3

[0047] Examples Example 1

[0048] Preparation of Thorium-oxalate

[0049] Thorium oxalate is prepared from a 0.5M thorium nitrate solution, obtained by dissolving thorium nitrate in distilled water. To this solution a 0.5 M solution of oxalic acid is added to the thorium nitrate solution under stirring. The amount of 0.5M oxalic acid is in excess of 100 mL per 16.7 mL of 0.5 M thorium nitrate solution (about 6-fold excess). A white suspension forms. After addition of all oxalic acid, the suspension is stirred for an additional period of time. When about 5 minutes have passed, stirring is stopped and the suspension allowed to settle to the bottom of the reaction vessel. The white solid precipitate is isolated by filtration, for instance with filter paper, and washed with three portions of 20mL 0.1 M oxalic acid. The resulting white solid is dried in an oven at 220°C for at least 12h, preferably under inert atmosphere, and then under an inert atmosphere at 270°C for 4h without decomposing it. Thorium oxalate was obtained in near quantitative yield.

[0050] Reaction with malonic acid gives similar results and yields.

[0051] Example 2 Conversion of Thorium oxalate to Thorium chloride

[0052] The dried thorium oxalate of example 1 is reacted with hexachloropropene (CaCk) in a molar ratio of 10 moles of C3CI6 per mole of thorium oxalate in a vessel with a magnetic stirrer, under an inert atmosphere. The solution is slowly heated to 180-190 degrees Celsius under reflux and in an inert atmosphere. The reaction is allowed to proceed for 18h. Using a microwave-assisted solvothermal synthesis allows for considerably shorter reaction times. After cooling down to room temperature and draining any excess hexachloropropene, a white product is obtained that is washed three times with dichloromethane and dried in vacuo. A white powder is obtained in a near quantitative yield. The white powder identifies as anhydrous Th(IV)Cl4. The product is stored under dry and inert conditions.

[0053] The product (ThCU) is analyzed and its structure and purity confirmed by XRD, I R, solid phase NMR and ICP-MS.

[0054] Reaction with tetrachloroethylene, hexachlorobutadiene, 1 ,2,3,- trichloropropane or tetrachlorocumene gave reasonable to good (60%-90%) yields and purities.

[0055] Example 3

[0056] Recycling of chlorinating agent

[0057] The collected filtrate of example 2 and washing solutions are collected, stripped from volatiles and distilled in vacuo (10 mbar) to recycle the chlorinating agent, following the method of Patel et al. , cited herein elsewhere. In the case of hexachloropropene, a colourless liquid fraction was collected at 70 degrees Celsius.13C-NMR identified the liquid as hexachloropropene.

[0058] Example 4

[0059] Drying the oxalate under air

[0060] The precipitated thorium oxalate of example 1 is dried under dry air in an oven at 220°C for at least 10h, and then at 310°C for 4h without decomposing it. Thorium oxalate was obtained in near quantitative yield.

[0061] Example 5

[0062] Conversion of thorium oxalate to thorium chloride under solvothermal conditions

[0063] The dried thorium oxalate of example 4 is reacted with hexachloropropene (C3CI6) in a molar ratio of 10 moles of C3CI6 per mole of thorium oxalate in a teflon lined autoclave, under an inert atmosphere. The autoclave is transferred to a preheated oven at 190°C. The reaction is allowed to proceed for 4 days. After cooling down to room temperature and draining any excess hexachloropropene, a white product is obtained that is washed three times with dichloromethane and dried in vacuo. A white powder is obtained in a near quantitative yield. The white powder identifies as anhydrous Th(IV)Cl4. The product is stored under dry and inert conditions.

[0064] Example 6

[0065] Conversion of thorium oxalate to thorium chloride with removal of organic solvent Thorium oxalate, either in a hydrated or anhydrous form, is reacted with hexachloropropene (CaCle) in a molar ratio of 10 moles of C3CI6 per mole of thorium oxalate in a teflon lined autoclave, under an inert atmosphere. The autoclave is transferred to a preheated oven at 190 degrees Celsius. The reaction is allowed to proceed for 4 days. After cooling down to room temperature, any excess hexachloropropene is drained. The product is dispersed in dichloromethane and centrifuged for 5 minutes at 5000 RPM. The resulting dichloromethane liquor is drained and the powder is redispersed in fresh dichloromethane. This process is repeated at least 5 times. After draining the dichloromethane for the last time, the product is left to dry under inert atmosphere in a glovebox. A white powder is obtained in a near quantitative yield. The white powder identifies as anhydrous Th(IV)Cl4. The product is stored under dry and inert conditions.

[0066] Example 7 Conversion of thorium nitrate to thorium chloride under solvothermal conditions The same procedure as in example 6 is followed, with thorium nitrate (Th(NOs)4) in hydrated or anhydrous form taking the place of thorium oxalate.

[0067] Example 8

[0068] Conversion of thorium hydroxynitrate nitrate to thorium chloride

[0069] Thorium nitrate pentahydrate (Th(NC>3)4-5H2O) is dehydrated at 150 °C for 15 h under Ar, yielding Th(OH)2(NC>3)2. This material is then used as thorium compound for a process as in Example 6.

[0070] Example 9

[0071] Conversion of thorium oxide to thorium chloride

[0072] Thorium oxide is procured or prepared, preferably prepared with a large surface area, as nanoparticles for instance. This material is then used as thorium compound for a process as in Example 6.

[0073] Example 10

[0074] Removal of carbon impurities

[0075] In some circumstances, organic impurities may remain in the ThCU powder. These impurities are decomposed at 550 °C under inert gas, leaving a mixture of ThCI4 and carbon. This mixture is dissolved with an organic solvent, such as a mixture of acetone, ethanol, and methanol, and the carbon can then be filtered from this solution. The solvent is evaporated under mild conditions, yielding solvated ThCk Further heat treatment, which should not surpass 400 °C to achieve full desolvation of the product, leads to anhydrous ThCk

Claims

CLAIMS1. Process for the manufacturing of anhydrous thorium(IV) chloride comprising the steps of: a. providing a thorium compound comprising a thorium (IV) cation; b. contacting the compound of (a) with an organic chlorinating agent; c. isolating the anhydrous thorium(IV) chloride.

2. Process according to claim 1 wherein the thorium compound further comprises an oxygen atom, a nitrogen atom, a hydroxide group, a nitrate group, a carbonate, a polybasic organic acid or a mixture thereof.

3. Process according to claim 1 or 2, wherein the thorium compound is selected from the group consisting of thorium oxide, thorium nitrate, thorium oxynitride, thorium hydroxide, thorium carbonate, thorium oxalate, thorium malonate, thorium succinate, thorium glutarate, thorium citrate, thorium isocitrate or thorium aconitate, preferably thorium oxalate, thorium malonate, or thorium citrate, more preferably thorium oxalate.

4. Process according to claims 1-3, wherein the organic chlorinating agent is a di-, tri, poly- or perchlorinated organic compound.

5. Process according to claims 1-4, wherein di-, tri, poly- or perchlorinated organic compound is a perchlorinated alkene, selected from the group consisting of tetrachloroethylene, hexachloropropene, hexachlorobutadiene and mixtures thereof, preferably tetrachloroethylene or hexachloropropene.

6. Process according to claims 1-5, wherein the molar ratio of organic chlorinating agent to thorium(IV)cation is more than 2, preferably more than 5.

7. Process according to claims 1-6, wherein the thorium compound is manufactured from a water-soluble thorium salt.

8. Process according to claims 2-7, wherein the thorium compound is prepared by contacting the water-soluble thorium salt in an aqueous solution with a polybasic organic acid.

9. Process according to claims 2-8, wherein the polybasic organic acid is a dibasic or tribasic organic acid, preferably selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, citric acid, iso-citric acid or aconitic acid and mixtures thereof, preferably oxalic acid or citric acid10. Process according to claims 2-9, wherein the molar ratio of the polybasic organic acid to thorium cation is more than 2.

11. Process according to claims 1-10, wherein the thorium compound is dried.

12. Process according to claims 1-11 , wherein the thorium compound is dried at elevated temperatures.

13. Process according to any of the previous claims, wherein the anhydrous thorium chloride contains less than 1000 ppm water.

14. Anhydrous Thorium (IV) chloride obtainable by the process of any of claims 1-13.

15. Use of anhydrous thorium (IV) chloride of claim 14, in a molten salt nuclear reactor.