AU2018293909A1

AU2018293909A1 - Separation of actinium from process liquors

Info

Publication number: AU2018293909A1
Application number: AU2018293909A
Authority: AU
Inventors: Chris Griffith
Original assignee: Australian Nuclear Science and Technology Organization
Current assignee: Australian Nuclear Science and Technology Organization
Priority date: 2017-06-27
Filing date: 2018-06-26
Publication date: 2019-12-19
Also published as: WO2019000014A1; CA3065375A1; ZA201908408B; DK202070008A1; EP3645759A1; DK180858B1; EP3645759A4; BR112019026844A2

Abstract

The present specification relates to a method for removing actinium from process liquors comprising ions of interest (which may be, for example, rare earth ions) and actinium ions. In this method, a low solubility metal sulfate salt is caused to precipitate from a solution comprising ions of interest and actinium ions. Actinium ions become associated with the precipitate causing them to be removed from the solution. The method allows actinium to be removed from the solution selectively i.e. without the concurrent removal of substantial amounts of the ions of interest present in the solution.

Description

SEPARATION OF ACTINIUM FROM PROCESS LIQUORS

Field [0001] The present invention relates to the separation of actinium from process liquors.

Background [0002] The present application claims priority from AU 2017902476, the entire contents of which are incorporated herein by cross-reference.

Background [0003] Actinium-227 is frequently present in mineral processing circuits. Its presence is the result of the decay of uranium-235, which may be associated, for example, with rare earth mineral deposits, either occurring in the rare earth mineral itself or otherwise associated with the process feed.

[0004] Actinium-227 is radioactive, with a half-life of approximately 22 years. It decays through β emission to thorium-227, which gives rise to further decay products.

[0005] Actinium contamination is a problem in mineral processing. While standard processing methods remove the majority of the sources of radioactivity present in mineral process feeds, due to its chemical similarity to, for example, the rare earth elements, actinium is not easily removed. Furthermore, regulatory authorities set restrictive limits on the amount of actinium activity that may be present in materials (for example the International Atomic Energy Agency sets a limit of 0.1 Bq/g (or 1.0 Bq/g), depending on the circumstances) for actinium activity, see e.g. IAEA Radiation Protection and Safety of Radiation Sources: International Basic Safety Standards, Series No. GSR Part 3, 19 July 2014).

[0006] In the context of rare earths processing, rare earth processing feeds may be processed either into individual rare earth products, which typically requires the use of solvent extraction, or into a mixed mineral concentrate, which typically does not require solvent extraction. Solvent extraction can be used to remove actinium from rare earth process streams, however, as it is a costly step and involves extensive additional processing, it is best avoided if possible. Using

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PCT/AU2018/000105 previously available technology, if a mixed concentrate is to be produced from a feedstock that is contaminated with actinium, it must be processed using solvent extraction, which would not otherwise be required for the production of a mixed concentrate.

[0007] Additionally, there is interest in processing high uranium content ores, monazite and xenotime ores, and mineral concentrates to produce mixed concentrates. These ores may be contaminated with actinium.

[0008] Therefore, there is a need for a method of selectively removing actinium from processing streams which does not involve solvent extraction. An object of the present invention is the provision of a method of at least partially removing actinium from a feed solution which does not require solvent extraction.

Summary of Invention [0009] In a first aspect of the invention there is provided a method of removing actinium from a solution comprising actinium ions and ions of interest, the method comprising adding metal ions to said solution, a sulfate salt of which has a solubility in water at 20 °C of less than about 0.3 g/100 mL; wherein if the sulfate concentration of the solution is insufficient to cause the sulfate salt of the metal ions to precipitate, a water soluble source of sulfate ions is added to said solution until the sulfate salt of the metal ions precipitates; whereby a precipitate comprising actinium and the sulfate salt of the metal ions is formed and wherein a sufficient amount of metal ions is added to cause at least 40% of the actinium ions to precipitate.

[00010] The following options may be used in conjunction with the first aspect, either individually or in any suitable combination.

[00011] The metal ions may be added in an amount of 0.01-100 mmol per Bq/L of activity of the solution. This may therefore be considered to be a sufficient amount of the metal ions.

[00012] The ions of interest may be rare earth ions. The concentration of the ions of interest in the solution may be essentially unchanged by the method.

[00013] The actinium ions may be Ac-227 ions. The actinium activity of the total dissolved ions of interest contained in said solution may be greater than about 1.0 Bq/g.

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PCT/AU2018/000105 [00014] The metal ions may be divalent metal ions. The metal may be selected from the group consisting of calcium, strontium, barium or lead. It may be strontium, or it may be lead, or it may be barium. The metal ions may comprise a mixture of different metal ions, such as a mixture of barium ions and strontium ions.

[00015] The solution comprising actinium ions and ions of interest may be produced as a result of the processing of an ore or mineral concentrate containing the ions of interest. The ions of interest may be of one or more rare earth elements selected from the group consisting of Sc, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Y, Tb, Dy, Ho, Er, Tm, Yb, and Lu. In this case, the ore may comprise, or may be, monazite, xenotime, or a mixture thereof. The ore or mineral concentrate may be such that if a mixed rare earth concentrate were to be produced from it without the removal of actinium, the actinium activity of the mixed rare earth concentrate would be greater than 1.0 Bq/g. Alternatively, the ions of interest may not be rare earth ions, in this case, the ore or mineral concentrate may be, for example, an ore containing uranium.

[00016] The method may comprise a step of processing an ore or mineral concentrate using an acid cracking process so as to produce the solution comprising actinium ions and ions of interest. In this event, the metal ions may be lead(II). The lead(II) may be in the form of lead(II) chloride or lead(II) nitrate.

[00017] The method may alternatively comprise a step of processing an ore or mineral concentrate using an alkaline cracking process so as to produce the solution comprising actinium ions and ions of interest. In this event, the metal ions may be strontium or barium, or a mixture thereof, which may be in the form of barium chloride and/or strontium chloride. A water soluble source of sulfate ions may be added to said solution until the sulfate salt of the metal ions precipitates. The water soluble source of sulfate ions may be selected from the group consisting of sulfuric acid, sodium sulfate, magnesium sulfate, or a combination of any two or all of these. The molar ratio of sulfate ions to metal ions may be in the range of about 1:1 to about 1000:1.

[00018] In the situation where the method comprises processing an ore or mineral concentrate using an alkaline cracking process, the solution may further comprise radium ions. In this case, the metal ions are barium ions, or a mixture of strontium and barium ions. Strontium ions and barium ions may also be added sequentially, in either order. The metal ions may be in the form of barium chloride and/or strontium chloride, and a precipitate comprising actinium ions, radium

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PCT/AU2018/000105 ions, and barium sulfate and/or strontium sulfate, may be formed. A sufficient amount of metal ions may be added to cause at least 40% of the actinium ions to precipitate and at least 99% of the radium ions to precipitate.

[00019] The metal ions may be added to the solution in the form of a solid. They may be added in the form of an aqueous solution. The water soluble source of sulfate ions may be added to the solution in the form of a solid. It may be added in the form of an aqueous solution.

[00020] The method may further comprise separating the precipitate from the solution. Following this, the solution may have a reduced concentration of actinium or its actinium activity may be reduced or both.

[00021] Where the ions of interest are rare earth ions, the method may further comprise obtaining a solid mixed rare earth concentrate from the solution. The solid rare earth concentrate may have an actinium activity of less than or equal to 1.0 Bq/g.

[00022] In one embodiment there is provided a method of removing actinium from a solution comprising actinium-227 ions and rare earth ions, the method comprising adding to the solution lead chloride or strontium chloride; wherein if the sulfate concentration of the solution is insufficient to cause the sulfate salt of the lead or strontium to precipitate, a water soluble source of sulfate ions such as sodium sulfate, sulfuric acid, magnesium sulfate, or a combination of any two or all of these, is added to the solution until the sulfate salt of the lead or strontium precipitates; whereby a precipitate comprising actinium and the sulfate salt of the metal ions is formed. In this embodiment a sufficient amount of metal ions is added to cause at least 40% of the actinium ions to precipitate.

[00023] In another embodiment there is provided a method of removing actinium from a solution comprising actinium-227 ions and rare earth ions, the method comprising processing an ore or mineral concentrate using an acid cracking process so as to provide the solution comprising actinium ions and rare earth ions and adding to the solution lead(II) chloride, whereby a precipitate comprising lead(II) sulfate and actinium is formed. In this embodiment a sufficient amount of metal ions is added to cause at least 40% of the actinium ions to precipitate. The method may further comprise separating the precipitate from the solution. Following said separation, the solution may have a reduced actinium activity and the concentration of rare earth ions in said solution may be essentially unchanged.

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PCT/AU2018/000105 [00024] In another embodiment there is provided a method of removing actinium from a solution comprising actinium-227 ions and rare earth ions, the method comprising processing an ore or mineral concentrate using an alkaline cracking process so as to provide the solution comprising actinium ions and rare earth ions; adding to the solution strontium chloride and, a water soluble source of sulfate ions such as sodium sulfate, sulfuric acid or magnesium sulfate; whereby a precipitate comprising strontium sulfate and actinium is formed. In this embodiment a sufficient amount of metal ions is added to cause at least 40% of the actinium ions to precipitate. The method may further comprise separating the precipitate from the solution. Following said separation, the solution may have a reduced actinium activity and the concentration of rare earth ions in said solution may be essentially unchanged.

[00025] In another embodiment there is provided a method of removing actinium and radium from a solution comprising actinium-227 ions, radium ions and rare earth ions, the method comprising processing an ore or mineral concentrate using an alkaline cracking process so as to provide the solution comprising actinium ions, radium ions and rare earth ions; adding to the solution barium chloride and a water soluble source of sulfate ions, such as sulfuric acid, sodium sulfate, magnesium sulfate, or a combination of any two or all of these; whereby a precipitate comprising barium sulfate, radium and actinium is formed, and wherein a sufficient amount of metal ions is added to cause at least 40% of the actinium ions to precipitate and at least 99% of the radium ions to precipitate. In this embodiment, the method may further comprise separating the precipitate from the solution. Following said separation, the solution may have a reduced actinium activity, a reduced radium activity, and the concentration of rare earth ions in said solution may be essentially unchanged.

[00026] In a second aspect of the invention there is provided a solid mixed rare earth concentrate obtained by the method of the first aspect of the invention wherein the ions of interest are rare earth ions. The solid mixed rare earth concentrate may have an actinium activity of less than 1.0 Bq/g.

[00027] In a third aspect of the invention there is provided the use of a metal ion, the sulfate salt of which has a solubility in water at 20 °C of less than about 0.3 g/100 mL, for removing actinium from a solution comprising actinium ions and ions of interest.

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PCT/AU2018/000105 [00028] In a fourth aspect of the invention there is provided a precipitate comprising a sulfate salt of a metal ion and further comprising actinium-227, said precipitate being obtained by the method of the first aspect of the invention.

[00029] In a fifth aspect of the invention there is provided a method of preparing a radioisotope, the method comprising: obtaining a precipitate according to the fourth aspect of the invention; separating the actinium-227 from the precipitate; purifying the actinium-227; allowing a portion of the actinium-227 to decay to a decay product, which is the radioisotope, and purifying the radioisotope.

[00030] The radioisotope may be thorium-227 or radium-223.

[00031] In one embodiment there is provided a method of preparing thorium-227 or radium223, the method comprising: obtaining a precipitate according to the fourth aspect of the invention; separating the actinium-227 from the precipitate; purifying the actinium-227; allowing a portion of the actinium-227 to decay to thorium-227 or radium-223 and purifying thorium-227 or radium-223 from the resulting mixture of radioisotopes.

[00032] In another embodiment there is provided a method of preparing thorium-227 or radium223, the method comprising: obtaining a precipitate comprising a sulfate salt of metal ions and actinium-227 by providing a solution comprising actinium-227 ions and ions of interest; adding to the solution metal ions, a sulfate salt of which has a solubility in water at 20 °C of less than about 0.3 g/100 mL; wherein if the sulfate concentration of the solution is insufficient to cause the sulfate salt of the metal ions to precipitate, a water soluble source of sulfate ions is added to said solution until the sulfate salt of the metal ions precipitates; whereby a precipitate comprising the sulfate salt of the metal ions and actinium-227 is formed and wherein a sufficient amount of metal ions is added to cause at least 40% of the actinium ions to precipitate; separating the actinium-227 from the precipitate; purifying the actinium-227; allowing a portion of the actinium-227 to decay to thorium-227 or radium-223 and purifying thorium-227 or radium-223 from the resulting mixture of radioisotopes.

[00033] In a sixth aspect of the invention there is provided use of the precipitate of the fourth aspect of the invention to produce a radioisotope, such as a thorium or radium radioisotope.

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PCT/AU2018/000105

Description of Embodiments [00034] The present specification relates to a method for removing actinium from process liquors comprising ions of interest (which may be, for example, rare earth ions) and actinium ions. In this method, a low solubility metal sulfate salt is caused to precipitate from a solution comprising ions of interest and actinium ions. Actinium ions become associated with the precipitate causing them to be removed from the solution. The method allows actinium to be removed from the solution selectively i.e. without the concurrent removal of substantial amounts of the ions of interest present in the solution.

[00035] Surprisingly, the inventors have found that when the low solubility metal sulfate salt precipitates in the presence of actinium ions, the actinium ions become selectively associated with the precipitate, causing them to be removed from the solution. Without wishing to be bound by any particular theory, it is suggested that this association may occur through any one of the following mechanisms: co-precipitation of an actinium salt with the metal sulfate salt, adsorption of actinium into defects in the crystal lattice of the metal sulfate salt, incorporation of actinium ions into the metal sulfate salt crystal or occlusion of actinium into the growing metal sulfate salt crystal. Removal of actinium from the solution may occur through a combination of the above mechanisms or through a mechanism not listed here.

[00036] Advantageously, the inventors have discovered that if the process liquors derive from an alkaline cracking process and contain radium and/or lead ions as well as actinium ions, the method of the invention may result in actinium, radium and lead ions becoming associated with the precipitate of the metal sulfate salt. In this way, it is possible to remove a proportion of actinium, radium and lead from process liquors in a single process step. If the removal of radium and/or lead, as well as actinium, is desired, the amount of metal ions required (and corresponding amount of sulfate ions) is greater than that required for the removal of radium alone. An advantage of removing actinium and radium simultaneously is that an additional process step is not required for actinium removal because there is already a radium removal step in the process.

[00037] The solution which is used as a feed to the method of the invention comprises water, however it may in some instances also comprise one or more non-aqueous co-solvents, or other organic or inorganic additives. In some instances, there may be no co-solvents or additives. In

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PCT/AU2018/000105 the event that co-solvents or additives, e.g. organic solvents, are present, they may be in a proportion of less than about 20% by volume of the solution, or less than about 15%, 10%, or 5%, or in a proportion of about 0-10, 10-20, 5-15, 0-5, 5-10, 10-15, 15-20% by volume of the solution. The co-solvents or additives may be present in an amount of about 5, 10, 15 or 20% by volume. The solution may be provided as a result of the processing of an actinium-containing material, which may be an ore or mineral concentrate containing ions of interest, such as rare earth elements. Two methods which may be used in the processing of an ore or mineral concentrates are acid cracking and alkaline cracking.

[00038] In the context of this specification the term “about” is taken to mean ±10% of the stated value, unless signified otherwise by the context.

[00039] In the context of this specification, the term “comprising” is taken to require the presence of the recited integer(s) but does not preclude the presence of others and does not imply any particular concentration or proportion of the recited integer(s).

[00040] The process of acid cracking comprises a step of treating the ore or mineral concentrate with concentrated sulfuric acid at elevated temperature. The resulting mixture may be leached with water. Thus sulfate ions, which are subsequently precipitated to remove actinium, are present in the process liquor from the sulfuric acid.

[00041] The process of alkaline cracking comprises a step of treating the ore or mineral concentrate with a concentrated solution of a base such as sodium hydroxide at elevated temperature. This is followed by solubilisation of the caustic residue in an acid, such as hydrochloric acid, adjusting the pH of the solution to 3-4. As no sulfate is provided during this process, it is commonly necessary to add a source of sulfate ions in order to form a precipitate so as to remove actinium.

[00042] Actinium is present in the feed solution. The actinium may be present as a result of the decay of uranium-235 or it may occur as a result of any natural or artificial process. The actinium may be actinium-227. Other isotopes of actinium may also be present. The method of the invention may be used to successfully remove all isotopes of actinium present in the solution. As used in this specification, “actinium” includes elemental actinium as well as actinium ions in any oxidation state. Commonly the actinium will be present as Ac³⁺ ions.

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PCT/AU2018/000105 [00043] Radium may also be present in the feed solution. The radium may be present as a result of any natural or artificial process. The radium may be radium-226 or radium-228. Other isotopes of radium may also be present. The method of the invention may be used to successfully remove all isotopes of radium present in the solution.

[00044] The feed solution will typically contain ions of interest. The ions of interest are nonactinium ions present in the solution which the operator of the method of the invention may wish to separate from the actinium ions. A goal of the method may be to obtain the ions of interest free from undesirable levels of actinium activity or with reduced actinium activity. However, a goal of the method may equally be to obtain actinium ions and, optionally, discard the ions of interest. The ions of interest may be rare earth ions. Rare earth ions present in the solution may be ions (of any oxidation state) of one or more of the following elements: Sc, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Y, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Alternatively, the solution may contain no rare earth ions. Another example of an ion of interest may be a uranium ion. In some instances, the ions of interest may comprise both rare earth ions and non-rare earth ions. Other organic or inorganic components may also be present in the solution. The concentration of the ions of interest (and their counter ions) may be about 1 to about 200 g/L, or about 1-100, 100-200, 50-150, 1-25, 25-75, 75-100, 100-125, 125-150, 150-175 or 175-200 g/L.

[00045] In the method of the invention, metal ions are added to the feed solution. The metal ions may be of any metallic element in any oxidation state, provided that the solubility of a sulfate salt of the metal ions in water at 20 °C is less than about 0.3 g/100 mL. The solubility of the sulfate salt of the metal ions in water at 20 °C may be less than about 0.25, 0.2, 0.15, 0.1, 0.05, 0.02, 0.005, 0.002, 0.001, 0.0005 or 0.0001 g/100 mL. It may be in the range of about 0.00010.3, 0.0001-0.25, 0.0001-0.2, 0.0001-0.15, 0.0001-0.1, 0.005-0.05, 0.0001-0.005, 0.005-0.05 or 0.05-0.3 g/100 mL. It may be about 0.00024, 0.0044, 0.013 or 0.25 g/100 mL. Typically, the metal ions are divalent metal ions such as lead, barium or strontium, or combinations thereof. Where a combination of different divalent metal ions is added, the metal ions may be added simultaneously or sequentially.

[00046] The feed solution may comprise sulfate ions. For example, if an ore or mineral concentrate is processed using acid cracking the solution will generally comprise sulfuric acid. The sulfate ions may be present in a sufficient concentration to cause at least some of a sulfate salt of the metal ions to precipitate on addition of the metal ions. In general, a sufficient

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PCT/AU2018/000105 concentration of sulfate ions is such that, for the given metal ion concentration, the solubility product of the metal sulfate salt is exceeded by the product of the metal ion and sulfate concentrations. At a sufficient concentration of sulfate in the solution, addition of the metal ions may result in formation of at least some precipitate which comprises actinium and the sulfate salt of the metal ions.

[00047] Alternatively, the concentration of sulfate ions in the solution may be insufficient to cause at least some of the sulfate salt of the metal ions to precipitate. This is commonly the case if the processing of the ore or mineral concentrate comprises an alkaline cracking step. In general, an insufficient concentration of sulfate ions is one at which the solubility product of the sulfate salt of the metal ions is not exceeded by the product of the metal ions and sulfate concentrations. In this case, it may be necessary to increase the concentration of sulfate ions in the solution in order to precipitate at least some of the sulfate salt of the metal ions so as to remove actinium. This is achieved by adding a water soluble source of sulfate ions. Any water soluble source of sulfate ions may be used, such as sodium sulfate, magnesium sulfate, potassium sulfate, sulfuric acid, or any mixture of these. Sulfuric acid, magnesium sulfate, or a combination thereof, are preferred. The water soluble source of sulfate ions may be added, continuously, intermittently or as a single bolus, until at least some of a precipitate comprising the sulfate salt of the metal ions forms. Once a sufficient concentration of sulfate ions is achieved, at least some of a precipitate comprising the sulfate salt of the metal ions and further comprising actinium forms.

[00048] The method of the invention is at least partially selective, and in some circumstances, substantially or essentially completely, for the removal of actinium over ions of interest from the feed solution. The concentration of ions of interest in the solution may be essentially the same before and after applying the method of the invention. In the context of the specification “essentially the same” means the concentration of ions of interest is not reduced by more than about 1%, or not reduced by more than about 2, 5, 10 or 15%. The concentration may be reduced by about 0-15%, or about 0-10, 0-5, 5-10, 5-15 or 10-15%. The term “essentially unchanged” has a corresponding meaning. In the context of the specification the term “the concentration of ions of interest” may refer either to the total concentration of ions of interest or it may refer to the concentration of any or each ion of interest individually.

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PCT/AU2018/000105 [00049] The selectivity of the method is sensitive to the amount of metal ions added. If an excessive amount of metal sulfate is caused to precipitate, the concentration of ions of interest may be unacceptably reduced. If an insufficient amount of the metal ions is added, it may not be effective in removing the desired quantity of actinium. In the method of the invention, the metal ion is added in a sufficient amount so as to remove actinium to an acceptable level, and no more. Alternatively, if the removal of radium and/or lead, as well as actinium, is desired, the amount of metal ions added will be greater than that required for the removal of radium alone. A sufficient amount of the metal ions is in the range of about 0.01-100 mmol per Bq/L of actinium activity of the feed solution, or about 0.01-0.1, 0.01-1, 0.01-10, 0.01-50, 1-10, 1-50, 50-100, 0.11, 1-10, 10-25, 25-50, 50-75, 75-100 mmol per Bq/L of actinium activity of the feed solution. The amount of metal ions which is sufficient to remove actinium without unacceptably reducing the concentration of ions of interest will depend on the identity of the metal ions. If the metal ions are Pb²⁺ ions, a sufficient amount of the metal ions is about 0.01-5 mmol per Bq/L, or about 0.01-0.5, 0.01-1, 0.01-2, 1-5, 2-5, 0.01-0.05, 0.05-0.1, 0.1-0.5, 0.5-1, 1-2, 2-3, 3-4, or 4-5 mmol per Bq/L. If the metal ion are Sr²⁺ions, a sufficient amount of the metal ions is about 0.1-15 mmol per Bq/L, or about 0.1-1, 0.1-5, 0.1-10, 0.1-0.2, 0.2-0.5, 0.5-1, 1-5, 1-10, 1-15, 5-10, or 515 mmol per Bq/L. If the metal ions are Ca²⁺ions, a sufficient amount of the metal ions is about 10-100 mmol per Bq/L, or about 10-50, 50-100, 10-20, 20-30, 30-40, 40-50, 50-60, 60-70, 7080, 80-90, or 90-100 mol per Bq/L. If the metal ions are Ba²⁺ ions, a sufficient amount of the metal ions is about 0.1-15 mmol per Bq/L, or about 0.1-1, 0.1-5, 0.1-10, 0.1-0.2, 0.2-0.5, 0.5-1, 1-5, 1-10, 1-15, 5-10, or 5-15 mmol per Bq/L.

[00050] In the context of this specification, “activity” refers to radioactivity. “Actinium activity” refers to the radioactivity of actinium-227,. Activity is measured in Bq/g or Bq/L, depending on the context. The activity of Ac-227 is typically determined by gamma spectroscopy of in-growth ofTh-227.

[00051] In order to remove a desired amount of actinium, a sufficient amount of the metal ion that is added to the solution must be caused to precipitate as a sulfate salt. It may be titrated into the solution to a desired actinium activity endpoint. This may be achieved by adjusting the relative amounts of metal ions and sulfate ions such that the stoichiometry allows a sufficient amount of the metal ions to form a sulfate salt. For example, if the processing of the ore or mineral concentrate comprises an alkaline cracking step and thus the metal ions are strontium ions and a water-soluble source of sulfate ions is required, typically an at least equimolar

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PCT/AU2018/000105 amount of sulfate ions, relative to the amount of strontium or barium ions, will be added. Preferably, an excess of sulfate ions is added relative to the strontium or barium ions. The molar ratio of sulfate ions to the total amount of metal ions (such as strontium ions, or barium ions, or a combination thereof) may be in the range of about 1:1 to 1000:1, or about 1:1 to 2:1, 5:1, 10:1, 100:1, 200:1, 300:1; 400:1, 500:1, 600:1, 700:1, 800:1 or 900:1, or about 10:1 to 100:1, 200:1, 300:1; 400:1, 500:1,600:1, 700:1, 800:1, 900:1 or 1000:1, or about 100:1 to 200:1, 300:1; 400:1, 500:1, 600:1, 700:1, 800:1, 900:1 or 1000:1, or about 200:1 to , 300:1; 400:1, 500:1, 600:1, 700:1, 800:1, 900:1 or 1000:1. The ratio of sulfate ions to metal ions may be about 1:1, 1.5:1, 2:1, 5:1, 10:1, 50:1, 100:1, 200:1, 300:1; 400:1, 500:1, 600:1, 700:1, 800:1, 900:1 or 1000:1. The skilled person will be able to determine, by routine trial and error guided by the information disclosed herein, a ratio of sulfate to metal ions which is appropriate to remove actinium from particular process liquors.

[00052] The total dissolved ions of interest, for example total dissolved rare earths, contained in the solution may originate from an ore or from a mineral concentrate. The ore or mineral concentrate is processed, resulting in the formation of the solution used as a feed to the method of the invention. The dissolved ions of interest may derive from various salts such as carbonates or oxides. The total dissolved ions of interest contained in the solution may be radioactive. The radioactivity may originate from actinium-227. Other radioactive elements may be present. The actinium activity of the total dissolved ions of interest may be greater than about 0.1 Bq/g, or about 1, 10, 100 or 500 Bq/g. The actinium activity may be in the range of about 0.1-500 Bq/g, or about 0.1-100, 0.1-10, 0.1-1, 1-10, 10-100 or 100-500 Bq/g. The actinium activity ofthe total dissolved ions of interest may be about 0.1 Bq/g, or about 1, 10, 100 or 500 Bq/g. Alternatively, the actinium activity may be expressed per unit volume of the feed solution. The actinium activity may be in the range of about 1-500 Bq/L, or about 1-100, 1-250, 100-250, 250-500, 1100, 1-200, 1-300, 1-400 Bq/L. The actinium activity may be about 1 Bq/L or about 2, 5, 10, 20, 50, 100, 150, 200, 250, 300, 350, 400, 450 or 500 Bq/L.

[00053] The precipitate may be separated from the solution by any method known to the skilled person. Methods of separation include, but are not limited to, fdtration, gravitational settling, centrifugation or flotation and skimming, or any combination of these. The precipitate may comprise a single chemical species, or may comprise multiple species. The word ‘precipitate’ is intended to refer to the totality of species caused to precipitate out of solution as a result of carrying out the method of the invention.

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PCT/AU2018/000105 [00054] Following the precipitation, the solution may have a reduced concentration of actinium when compared to the concentration of actinium prior to carrying out the method of the invention. The actinium concentration may be reduced by up to 100%, or up to about 95, 90, 85, 80, 70, 60, 50,40,30 or 20%. The reduction may be in the range of about 100-20%, or about 100-60, 100-85, 85-20, 60-20, 20-40, 40-60, 60-85, 85-90, 90-95 or 95-100%. The concentration may be reduced by about 100%, or about 95, 90, 85, 80, 70, 60, 50, 40, 30 or 20%. Equivalently, up to 100%, or up to about 95, 90, 85, 80, 70, 60, 50, 40, 30 or 20% of the actinium ions may be caused to precipitate. The proportion of actinium ions caused to precipitate may be in the range of about 100-20%, or about 100-60, 100-85, 85-20, 60-20, 2040, 40-60, 60-85, 85-90, 90-95 or 95-100%. The amount of actinium ions caused to precipitate may be about 100%, or about 95, 90, 85, 80, 70, 60, 50, 40, 30 or 20%. The degree of reduction (amount of precipitation) may be controlled in part by the quantity of the metal ion, which may be determined with reference to the measured actinium activity of the solution.

[00055] The method may therefore further comprise measuring the actinium activity of the solution and determining from the measured actinium activity the required quantity of the metal ions to be added. The method may also comprise adding to the solution metal ions, a sulfate salt of which has a solubility in water at 20 °C is less than about 0.3 g/100 mL and, if required, a source of water soluble sulfate, whereby a precipitate to form comprising actinium and the sulfate salt of the metal ions is formed; measuring the actinium activity of the solution, and determining from the measured actinium activity if further addition of the metal ions (and water soluble source of sulfate, if required) is needed. This step may be repeated as often as required until an acceptable level of actinium activity is reached. The actinium activity may be measured by gamma spectroscopy of in-growth of thorium-227.

[00056] Following separation of the precipitate, the actinium activity of the solution is reduced when compared to its actinium activity prior to carrying out the method of the invention. The actinium activity may be reduced by up to or about 100%, or about 95, 90, 85, 60, 40 or 20%. The reduction may be in the range of about 100-20%, or about 100-60, 100-85, 85-20, 60-20, 20-40, 40-60, 60-85, 85-90, 90-95 or 95-100%. The actinium activity may be reduced by about 100%, or about 95, 90, 85, 60, 40 or 20%. The same proportional reductions apply, independently, to radium and/or lead.

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PCT/AU2018/000105 [00057] Following separation of the precipitate, where the ions of interest are rare earth ions the resulting solution may be processed further to produce a solid mixed rare earth concentrate. A mixed rare earth concentrate is a solid comprising a mixture of rare earth elements intended for further processing. A mixed rare earth concentrate produced by the method of the invention may have an actinium activity of less than or equal to about 1.0, 0.75, 0.5, 0.25, 0.1, or 0.05 Bq/g. The actinium activity may be in the range of about 0-0.05, 0-0.1, 0-0.5, 0-1.0, 0.05-0.1, 0.1-0.25, 0.25-0.5, or 0.5-1 Bq/g. The actinium activity may be about 0.05, 0.1, 0.25, 0.5, 0.75, or 1.0 Bq/g.

[00058] The invention also encompasses a method of preparing a radioisotope from the precipitate comprising the sulfate salt of the metal ion and further comprising actinium-227, obtained by the method of the invention. Actinium-227 may be separated from the precipitate by any method known to the skilled person. A suitable method involves treating the precipitate with hydrochloric acid. The liberated actinium-227 may then be purified by any suitable method, e.g. ion exchange or solvent extraction. The resulting purified actinium compound may then be allowed to decay, resulting in the formation of decay products which may be the radioisotope. For example, the decay product may be thorium-227 or radium-223. Since actinium-227 decays to thorium-227, which decays to radium-223, selection of the desired radioisotope may be accomplished by selecting a suitable time for which the actinium-227 is allowed to decay. Thorium-227 or radium-223 produced by the above method may be used in medical imaging or radiotherapy.

Examples [00059] Example 1: To sulfate containing process liquor containing about 8 Bq/L Ac-227 and rare earths, mainly as yttrium, was added strontium. Strontium chloride was dissolved in a minimal amount of water, added to the actinium containing sulfate liquor at room temperature, and agitated for 20 hours. The sulfate precipitate was filtered off and the Ac-227 removal determined by gamma spectrometry analysis of the liquor.

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Test ID SrCE. 6H2O addition (mmol per Bq/L activity)	A-2-13 0.7	A-2-11 4.7	A-2-12 9.5
Sr	59	Precipitation (%) 95	98
Y	<5	<5	<5
TRE+Y	<5	<5	<5
Ac-227 (% precipitation)	74	97	>99

RE = total rare earths [00060] Example 2: To sulfate-containing process liquor containing ~ 8 Bq/L Ac-227 and rare earths, mainly as yttrium, was added lead(II) chloride. Lead chloride was dissolved in a minimal amount of water, added to the actinium containing sulfate liquor at room temperature, and agitated for 20 hours. The sulfate precipitate was filtered off and the Ac-227 removal determined by gamma spectrometry analysis of the liquor. The Ac-227 removal was approximately 70-80%.

Test ID PbCL addition (mmol per Bq/L activity)	A-2-7 0.24	A-2-9 0.59
Pb	Precipitation (%) >92 >96
Y	<5	<5
TRE+Y	<5	<5
Ac-227	-68	-79

[00061] Example 3: To a chloride containing process liquor containing ~70 Bq/L Ac-227 and minimal radium was added strontium. The radium had been previously removed by coprecipitation with barium sulfate by addition of barium chloride and sulfuric acid. Strontium chloride solid was added to the process liquor and dissolved in it. Magnesium sulfate was added as a 250 g/L solution to induce the precipitation of strontium sulfate at room temperature. The mixture was agitated for 20 hours at room temperature. The sulfate precipitate was filtered off and the Ac-227 removal determined by gamma spectrometry analysis of the liquor. There was approximately 50% removal of Ac-227 with 2% precipitation of rare earths.

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Test ID SrCL. 6H2O addition (mmol per Bq/L activity) SO+'Sr molar ratio	61-2 1.3 1.5
Sr	% Precipitation 60
LRE (La+Ce+Pr+Nd)	2
TRE+Y	2
Ac-227 (% precipitation)	47

LRE = light rare earths [00062] Example 4: To a chloride containing process liquor containing ~90 Bq/L Ac-227 and minimal radium was added lead. The radium had been previously removed by co-precipitation with barium sulfate by addition of barium chloride and sodium sulfate. Lead chloride solid was added to the process liquor and lead chloride that had not dissolved was filtered off (0.98 g PbCB/L feed dissolved). Sodium sulfate was added as a 150 g/L solution and the mixture agitated for 20 hours at room temperature. The precipitate was filtered off There was 41% removal of Ac-227. The relatively high precipitation of rare earths is attributed to sodium sulfate addition and precipitation of a sodium sulfate rare earth double sulfate salt.

Test ID	AMI
PbC’L addition (mmol per Bq/L activity)	0.041
SO i/Pb molar ratio	42
	% Precipitation
Pb	23
Nd	13
TRE+Y	13
Ac-227	41

[00063] Example 5: To a chloride containing process liquor containing >100 Bq/L Ac-227 and >10,000 Bq/L Ra-228 was added barium. Barium chloride solid was added to the process liquor and dissolved. Sulfuric acid solution was added as the sulfate source to induce precipitation of barium sulfate. The mixture was agitated for 20 hours at room temperature. The precipitate was filtered off. Results are presented in the table below.

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Test ID	56-1	56-2	56-3
BaCf.ZtLO addition (mmol per Bq/L Ac-227 activity)	0.4	0.7	1.4
SO i:Ba molar ratio	1.1	1.1	1.1
Dilution of feed volume	0.80	0.80	0.80
		% Precipitation
Ba	>99	>99	>99
LRE	0.4	0.8	2
TRE+Y	0.4	0.8	2
Ac-227 (% precipitation)	27	43	66
Ra-228 (% precipitation)	>99.9	>99.9	>99.9

[00064] Example 6: To a chloride containing process liquor containing >100 Bq/L Ac-227 and >10,000 Bq/L Ra-228 was added barium and strontium, as their chloride salts. In one test (test ID = 59-3), precipitation of barium sulfate and strontium sulfate was carried out simultaneously. Both barium chloride and strontium chloride salts were added to the process liquor and dissolved followed by addition of sulfuric acid solution to initiate barium sulfate and strontium sulfate precipitation. In another test (test ID = 59-4), precipitation of barium sulfate and strontium sulfate was carried out sequentially in the same vessel. Barium chloride was dissolved in the process liquor and sulfuric acid added to initiate barium sulfate precipitation. Without filtration of the barium sulfate, strontium chloride was dissolved in the slurry and more sulfuric acid was added to initiate strontium sulfate precipitation. The Ac-227 removal was slightly higher when precipitation of barium sulfate and strontium sulfate occurred sequentially rather than simultaneously. Radium removal was >99.9% in both cases.

Test ID	59-3	59-4
BaC12.2H2O addition (mmol per Bq/L Ac-227 activity)	0.3	0.3
SrC12.6H2O addition (mmol per Bq/L Ac-227 activity)	0.3	0.3
SOafBa+Sr) molar ratio	1.6	1.6
SO i:Ba molar ratio		1.1
SO₄: Sr molar ratio		2.1
	% Precipitation
Ba	>99	>99
Sr	85	83
TRE+Y	1.7	1.0
Ac-227 (% precipitation)	42	55
Ra-228 (% precipitation)	>99.9	>99.9

WO 2019/000014

Claims

1. A method of removing actinium ions from a solution comprising actinium ions and ions of interest, the method comprising adding to said solution metal ions, a sulfate salt of which has a solubility in water at 20 °C of less than about 0.3 g/100 mL, wherein if the sulfate concentration of the solution is insufficient to cause the sulfate salt of the metal ions to precipitate, a water soluble source of sulfate ions is added to said solution until the sulfate salt of the metal ions precipitates, whereby a precipitate comprising actinium and the sulfate salt of the metal ions is formed, wherein a sufficient amount of metal ions is added to cause at least 40% of the actinium ions to precipitate.

2. The method of claim 1, wherein the metal ions are added in an amount of 0.01 -100 mmol per Bq/L of actinium activity of the solution.

3. The method of claim 1 or claim 2, wherein the concentration of the ions of interest in said solution is essentially unchanged.

4. The method of any one of claims 1 to 3 wherein the actinium ions are Ac-227 ions.

5. The method of any one of claims 1 to 4 wherein the actinium activity of the total dissolved ions of interest contained in said solution is greater than about 1.0 Bq/g.

6. The method of any one of claims 1 to 5, wherein the metal ions are divalent metal ions.

7. The method of any one of claims 1 to 6 wherein the metal ions are selected from the group consisting of calcium, strontium, barium and lead ions.

8. The method of any one of claim 1 to 7 wherein the metal ions are strontium or lead or barium ions.

9. The method any one of claims 1 to 8, wherein the ions of interest are rare earth ions.

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10. The method of any one of claims 1 to 8 wherein the solution comprising actinium ions and ions of interest is produced as a result of the processing of an ore or mineral concentrate containing the ions of interest.

11. The method of claim 10 wherein the ions of interest are of one or more rare earth elements selected from the group consisting of Sc, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Y, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

12. The method of claim 10 or claim 11 wherein the ore comprises monazite, xenotime, or a mixture thereof.

13. The method of claim 10 wherein the ore contains uranium and the ions of interest are uranium ions.

14. The method of claim 11 or claim 12 wherein the ore or mineral concentrate is such that a mixed rare earth concentrate produced from said ore or mineral concentrate without the removal of actinium has an actinium activity of greater than about 1.0 Bq/g.

15. The method of any one of claims 1 to 14 comprising the step of processing an ore or mineral concentrate using an acid cracking process so as to produce the solution comprising actinium ions and ions of interest.

16. The method of claim 15 wherein the metal ions are lead(II) ions.

17. The method of claim 16 wherein the lead (II) ions are in the form of lead(II) chloride.

18. The method of any one of claims 1 to 14 comprising the step of processing an ore or mineral concentrate using an alkaline cracking process so as to produce the solution comprising actinium ions and ions of interest.

19. The method of claim 18 wherein the metal ions are strontium or barium ions or a mixture thereof.

20. The method of claim 19 wherein the strontium ion is in the form of strontium chloride.

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21. The method of claim 19 or claim 20 wherein the barium is in the form of barium chloride.

22. The method of any one of claims 18 to 21 wherein a water soluble source of sulfate ions is added to said solution until the sulfate salt of the metal ion precipitates.

23. The method of claim 22 wherein the water soluble source of sulfate ions is selected from the group consisting of sulfuric acid, sodium sulfate, magnesium sulfate, or a combination of any two or all thereof.

24. The method of any one of claims 18 to 23, wherein the solution further comprises radium ions, wherein the metal ions are barium ions, strontium ions, or a mixture thereof, and whereby a precipitate comprising actinium ions, radium ions and barium sulfate is formed, and wherein a sufficient amount of metal ions is added to cause at least 40% of the actinium ions to precipitate and at least 99% of the radium ions to precipitate.

25. The method of any one of claims 18 to 24, wherein the molar ratio of sulfate ions to metal ions is in the range of about 1:1 to about 1000:1.

26. The method of any one of claims 1 to 25 wherein the metal ions are added to said solution in the form of a solid or in the form of an aqueous solution.

27. The method of any one of claims 1 to 26 wherein the water soluble source of sulfate ions is added to said solution in the form of a solid or in the form of an aqueous solution.

28. The method of any one of claims 1 to 27 further comprising separating said precipitate from said solution.

29. The method of claim 28 wherein the actinium activity of said solution is reduced.

30. The method of any one of claims 8 to 12 or 14 to 29 further comprising obtaining a solid mixed rare earth concentrate from said solution, the rare earth concentrate having an actinium activity of less than 1.0 Bq/g.

31. A solid mixed rare earth concentrate having an actinium activity of less than 1.0 Bq/g obtained by the method of claim 30.

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32. Use of a metal ion, the sulfate salt of which has a solubility in water at 20 °C of less than about 0.3 g/100 mL, for removing actinium from a solution comprising actinium ions and ions of interest.

33. A precipitate comprising the sulfate salt of a metal ion and further comprising actinium-227, said precipitate being obtained by the method of any one of claims 1 to 30.

34. A method of preparing a radioisotope, the method comprising:

- Obtaining a precipitate according to claim 33

- Separating the actinium-227 from said precipitate

- Purifying said actinium-227

- Allowing a portion of said actinium-227 to decay to a decay product, which is the radioisotope

- Purifying the radioisotope.

35. The method of claim 34 wherein the at least one decay product is thorium-227 or radium223.

36. Use of the precipitate of claim 33 to produce a radioisotope.

37. The use of claim 36 wherein the radioisotope is thorium-227 or radium-223.