SEPARATION OF RADIUM AND RARE EARTH ELEMENTS FROM MONAZITE

Abstract

A method of chemically extracting radium-228, rare earth metals, thorium, the decay products of thorium, and phosphates from thorium-containing ores. The method involves breaking thorium-containing ore into fragments, wetting the fragments with a concentrated strong acid to make a slurry, heating the slurry, passing the heated solution through a first anion exchange column, retaining metals and radium-228 captured on the resin, allowing the radium-228 ions to decay to actinium-228, purifying the actinium-228 fraction, sending the actinium-228 fraction through a capture column, eluting the captured thorium-228 with acid, removing radium from the solution, retaining the radium-228 fraction for isomer in-growth, retaining decay products from the radium-228, separating the REEs from the process stream; and eluting and retaining the REEs.

Description

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to a novel method of chemical extraction fashioned to extract and separate rare earth metals, thorium, its decay products, and phosphates from thorium-containing ores. The novel process efficiently recovers phosphoric acid, radium-228, radium-228's decay sequence, including its isomers, actinium-228 and thorium-228, and the rare earth elements including lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium ytterbium, lutetium, yttrium and scandium from various thorium containing head ores or feed materials including monazite and bastnaesite.

2. Background Art

Radium-228 is present in small quantities, averaging approximately 3.5 grams per thousand tons of refined monazite sand. Radium-228 is a highly valuable material for medical applications as some of its natural or artificial decay products can be used as alpha emitting components in targeted alpha therapy. Monazite is the heavy sand material from which thorium oxide and the rare earth elements are commonly commercially produced. Thorite is thorium silicate. Bastnaesite is a carbonatite source material for rare earth elements (“REE”).

Rare earth elements are in increasing industrial demand. Many electronics related applications need lower cost REE elements. The REE are used in making superior lasers, stronger magnets, special glasses, various semiconductors, light emitting materials, phosphors, and the like. Radioactive decay products from thorium-containing materials, including monazite mining tailings, add cost to monazite mining operations.

Thorium (element 90 in the periodic table) is mildly radioactive. Thorium is widely distributed in nature with an approximate average concentration of 10 ppm in the lithosphere. It is present in the earth's crust in association with many phosphates, silicates, carbonates and oxide minerals. Thorium occurs in association with uranium and the rare earth elements, the lanthanides, many types of rock as veins of thorite, thoranite, uranothorite, and as monazite in granites. Monazite is a mixed phosphate mineral with the general chemical formula (REE/Th/U) POsub.4. Monazite is a major source for REE and a secondary source for phosphate, thorium and uranium. Phosphate mine tailings, uranium and rare earth mine tailings contain radioactive thorium. For the most part this radioactivity is from the first decay product of thorium-232, which is radium-228, and from the decay sequence starting with radium-228.

Because thorium is radioactive, thorium is a regulated material and must be stored and accounted for by mine operator under strict international, national and local standards and set out in a locally issued mining permit. This stewardship cost for the rare earth, thorium, and phosphate extractive industries is a significant commercial expenditure. Each tailing dump, or other location where thorium-containing materials accumulate, must be accounted for as radioactive waste. Because thorium-containing materials are classified as low level radioactive waste, potential third party environmental, personal injury and property claims against the thorium possessor may ripen over time, exposing the possessor to liability and damages. These property and injury claims are typically excluded from insurance coverage under the mining operators' and mine owners' insurance contracts.

DISCLOSURE OF INVENTION

This invention discloses means and methods of a novel chemical separations process that more efficiently recovers and separates the economically valuable elements, phosphoric acid, the REE in chemically purer form, and separates the radium-228, the radioactive “mesothorium,” from the worked material.

The present invention removes radium-228 from the thorium-containing materials produced in the monazite mining waste stream to temporarily reduce the radioactivity of stockpiled thorium-containing materials. The process permits neutralized thorium to be stockpiled so that it can be recovered in the future for its nuclear energy content or later re-milked for its radium-228 content. Because the radioactive emissions from the thorium containing tailings are significantly reduced after the radium-228 is separated and removed, the remaining neutralized thorium in the tailings or stockpile will have significantly lower radioactivity until the natural decay of in the thorium replaces the radium-228 and the physical equilibrium of the decaying isotopes is restored. The process also removes the valuable REE so that these can be efficiently separated into pure metals or pure compounds. Finally, to the extent the ore contains phosphates, the inventive extraction process recovers the phosphates as phosphoric acid.

The inventive chemical separation process has the advantage that as radium-228 is collected, phosphoric acid is also collected, and each of the rare earth elements is collected in a highly purified form. The radium-228 is useful for medical isotope applications. When it is separated from thorium, thorium emissions are reduced so that in some jurisdictions it would be classified as the least regulated naturally occurring radioactive materials.

Referring to FIGS. 1-1B and FIG. 2, we can see that the half life of thorium is approximately the age of the universe, 15 billion years or so. The half-life of thorium's first decay product, radium-228 is comparatively short: 5¾ years. The other decay products have shorter half lives. Radium-228 decays to actinium-228. Actinium-228 has a half life of 6.15 hours. Actinium-228 decays to thorium-228 with a half life of 1.91 years. Thorium-228 decays to radium 224 with a 3.66 day half life, which decays to radon-220 with a 55.6 second half life, which decays to polonium-216 with a 0.15 second half life, which decays to lead-212 with a 10.64 hour half life that decays to bismuth-212 with a one hour half life that branches with the ultimate end product being stable lead-208.

It is the radioactivity of thorium's decay daughters and not the parent thorium-232 isotope that imposes the regulatory and stewardship costs on operators having thorium in feed material and tailings. The regulatory costs can be reduced significantly by the present invention because the radioactive disintegrations of concern from the thorium-containing materials are mostly from thorium's comparatively short lived decay products, commencing with radium-228. After radium-228 is removed from the process stream, thorium-containing tailings will include less material ionizing from the decays of the radium-228 in the decay chain. Instead, radium-228, already separated and concentrated, will be available for highly valuable medical isotope applications for the treatment of diseases.

The separation of radium-228 from thorium-232 reduces potential liabilities associated with the possession of thorium for the mine operator for the period of time before sufficient new radium-228 “grows in” the thorium by the inexorable alpha decay. Immediately after the separations the remaining thorium-232 will have lower activity because significant quantities of radium-228 will have been removed. The radioactivity from radium-228 gradually returns in the thorium-232, but a mine operator will have sufficient time to permanently bury or otherwise properly dispose of the thorium-containing materials during the period of time when the rate of treated thorium-232 disintegrations is reduced, perhaps below the applicable regulatory threshold in some jurisdictions.

Other novel features which are characteristic of the invention, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing, in which preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawing is for illustration and description only and is not intended as a definition of the limits of the invention. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. The invention resides not in any one of these features taken alone, but rather in the particular combination of all of its structures for the functions specified.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein:

FIGS. 1-1B comprise a chart 100 showing the short-lived decay chains of thorium-232 and thorium-238, as well as the decay chains of uranium-238, uranium-234, and uranium-235;

FIG. 2 is a table 200 showing the natural decay products of thorium; and

FIG. 3 is a schematic flow chart showing the method steps of the industrial chemical separation process of the present invention.

Various method steps and operational features may change in practice. The details concerning the precise composition of the production resins, columns, filters, solvents and catalysts will vary depending on the size and volume of the production facility, and the degree of purity sought for the rare earth elements, the phosphates, the thorium and the radium-228 recovered. Modifications and changes will be driven by practical and economic concerns and according to the most prudent practices in view of the characteristics of the monazite or other feed material supplied to the plant.

BEST MODE FOR CARRYING OUT THE INVENTION

The chemical separation process summarily described above is now set out fully for monazite. A similar approach works for bastnaesite and other thorium-containing ores. The monazite sand is first conventionally concentrated. This is accomplished by separating the uniform particle size grains based on physical properties, specific gravity, magnetic susceptibility, electrical conductivity, and surface properties. The monazite-containing sand is processed and conductive ilmenite and conductive rutile constituents are removed. Non-conducting monazite, which is heavy and moderately magnetic, is isolated from non-magnetic components and other materials remaining in the feed material. The resulting concentrate having been processed by magnetic and electrical means is generally more than 90% monazite. This concentrated monazite sand is finely ground and is ready for digestion by the inventive process described herein.

Monazite is a phosphate ore. It occurs in three varieties: (Ce La Nd Th Y) POsub.4, (La Ce Nd) POsub.4 and Nd, La Ce)POsub.4. Its general formula is (REE)Th U POsub.4. Its economically significant minerals include the REE group metals, phosphate, thorium and radium-228. All of these are extracted by the novel method disclosed herein.

Referring now to FIG. 3, the first step 300 in processing monazite under the present invention is to pulverize concentrated monazite by passing it through a 200 mesh screen (0.074 millimeters) or through larger dimensions of up to 10 mesh (2.000 millimeters). Pulverized monazite sand or other thorium-containing ore is then first wetted with 8M nitric acid 310 to make a slurry, which is then heated. This solution is filtered and passed over an anion exchange resin 320. The solution passing through the first column contains (non-retained) REE ions, actinium ions and radium ions 330. Several metals are retained 340 on the resin in the first column, including Th, Fe, Co Ni, Cu Ag Sn Zn Ce A. Sc Te Zr Hf Cr Mo Mn and U 350. The metals retained on this column can be removed and recovered 360 using a 90% methanol-10% nitric acid solution. Thorium may be eluted from the first resin column's resin using 1 M HNOsub.3.

After passing through the first column, the liquid solution contains REE, actinium and radium ions in the nitrate solution. The radium ions are allowed to decay 370 to Actinium-228, as actinium-228 is the first decay product of radium-228,which has a 100 hour maximum life (this is the time needed for all the separated actinium-228 to entirely decay). The purified actinium-228 fraction is sent through a final thorium-228 capture column 380. Thorium-228 390 is eluted with 1M nitric acid and can be further purified as needed by passing the solution over additional anion exchange resin columns (not shown in the flow diagram).

Next, radium is removed from the solution via co-precipitation with barium nitrate 400. Secondary and tertiary co-precipitations of barium nitrate ensure complete separation of the radium fraction. This fraction 410 is withheld for “isomer in-growth.” The decay products from the radium-228 are retained for use as the alpha-emitting material that ultimately finds use in medical isotope “generators”.

The solution 420 containing the REE, after the radium-228, thorium and other metals are separated, is ideal for the production of highly pure forms of the rare earth elements. In a preferred method, the REE are separated 430 according to the size of the rare earth metal ions, with the smallest being separated first and the largest last. The detailed steps for separating the rare earth lanthanides of the preferred technique include procedures using reversed-phased partition chromatograph. This technique for separating the rare earths has gained increasing importance in recent years. This is because the separation factors between adjacent rare earths elements are in several cases better when extracting these elements with organic phosphorous compounds than when eluting from cation exchange resins in the presence of organic complexing agents.

The liquid is then passed through a stationary phase 440. The organic phosphorous compounds that are most frequently used as stationary phases are bis(2-ethylhexyl)-o-phosphoric acid (HDEHP) and tri-n-butylphosphate (TBP) Also, bis(di-n-hexyl-phosphinyl)methane (HDPM) and di-n-butylphosphate have been recommended for this purpose. Also, long-chain amines, e.g. trioctylamine and dinonylnaphalenesulphonic acid in heptane, have been employed as stationary phases.

The following substances can be used as supports for these stationary phases: Corvic (poly(vinyl chloride-vinyl acetate) co-polymers), siliconized kieselguhy or silica gel, Kel-F (polychlorotrifluoroethane), or filter paper. The mobile phases are mostly pure aqueous solutions containing acids, such as nitric, hydrochloric or perchloric acids.

The best separations factors (about 2 to 5) between adjacent rare earths are obtained with HDEHP as the stationary phase. These are in most cases, better than in cation exchange systems when using α-hydroxyisobutyrate or lactate as eluting agents for the rare earths. TBP is less suitable, although the separation factors (about 2) that are obtained with this extraction are higher than in adsorption on cation exchange resins from citrate, glycollate or lactate solutions.

Irrespective of the kind of stationary phase that is employed, the rare earth elements are eluted in the order of increasing atomic number, that is, lanthanum first and lutetium last. This is the reverse order of elution as observed when using cation exchange chromatography. It is also the order of increasing partition coefficient for the lanthanides, when portioning between aqueous mineral acid solutions and a solution of HDEHP, TBP, etc. in an inert solvent at constant acidity and concentration of extractant.

Besides the higher separation factors there are other advantages of these reverse-phase techniques over the conventional methods using cation exchange in the presence of complexing agents. One advantage is that the elution curves of the rare earths obtainable by reversed-phase partition chromatography are narrow, quite symmetrical and show no tailing. Furthermore, the effluent from the column does not contain any salts or complexing agents but only mineral acids which can be readily removed by evaporation. Also, the procedure is not time consuming and it may be carried out satisfactorily at room temperature.

The recovered products from the process stream include thorium-232, rare earth elements, scandium, yttrium, radium-228, actinium-228, thorium-228 (by decay) 450, and phosphoric acid 460. A distillation of nitric acid 470 can be effected to introduce the distillate back in the process stream for use at steps 310 and 430.

Thorium-232, now depleted of radium-228, can be reserved and used for nuclear fuel or other applications. When stripped of its decay products thorium-232 can be of high commercial value as a catalyst or as an alloying metal. The Rare Earth Elements can be marketed and sold in highly pure form. Value is added by the ability of this chemical separations process to produce pure rare earth elements. Other forms of chemical separation methods for the rare earth elements are more expensive because all of the rare earth elements have similar or identical chemical properties. High purity rare earth elements are industrial commodity materials in demand that have growing applications to enhance magnetic performance, to enhance semi-conductor performance, to enhance phosphor performance, to enhance diode performance and to provide for photon to copper and back again transducers. Rare earth elements can be more economically produced by this disclosure from monazite and other REE ores because the process contemplates the segregation of high purity rare earth metals.

Importantly, radium-228, actinium-228, and their decay product thorium-228, are concentrated and recovered by the techniques disclosed. These materials can be used as generators of valuable medical isotope materials. The three isomers, radium-228, actinium-228, and thorium-228, can be irradiated either with high energy neutrons for n, 2n reactions or with high energy gamma photons for gamma, n reactions or simply reserved for harvest of useful decay products. For example radium-227 can be produced by gamma, n on radium-228. Radium-227 has a 42.2 minute half life and its immediate decay product is actinium-227 that has a half life of 21.773 years. Actinium-227 can be used to supply the highly sought medical isotope thorium-227 having a half life of 18.72 days that decays to radium-223 useful for the treatment of bone cancers and for pain palliation. This disclosure provides means to collect valuable precursor isotopes with uses in the treatment of cancer along with valuable rare earth metals and commercial phosphate containing materials.

Alpha particles from short-lived alpha emitters in the natural thorium decay chain provides superior treatment options for many diseases. High z alpha emitters from thorium-containing ores can be linked with molecules that seek and bind to abnormal cells. For many cancer treatments targeted alpha therapy provides better outcomes than chemotherapy, gamma radiation therapy, x-ray therapy or beta radiation therapy, as are presently practiced.

The inventive method disclosed herein further teaches the important steps and novel techniques to increase the purity of rare earth metals produced from monazite and other ores, it provides means to reduce the radioactivity of thorium produced from monazite and other ores, and it provides the means to recover radium-228 to provide a reliable source of isotopes useful in the treatment several types of cancer and other infectious diseases.

The foregoing disclosure is sufficient to enable those with skill in the relevant art to practice the invention without undue experimentation. The disclosure further provides the best mode of practicing the invention now contemplated by the inventor. While the particular chemical separation method herein shown and disclosed in detail is fully capable of attaining the objects and providing the advantages stated herein, it is to be understood that it is merely illustrative of the presently preferred embodiment of the invention and that no limitations are intended concerning the detail of process steps other than as defined in the appended claims. Accordingly, the proper scope of the present invention should be determined only by the broadest interpretation of the appended claims so as to encompass obvious modifications as well as all relationships equivalent to those illustrated in the drawings and described in the specification.

Claims

1. A chemical separation process for removing radium-228, from materials containing thorium-232 to make “neutralized thorium”, and to separate rare earth elements from common thorium ores or mining waste streams, comprising the steps of:

(a) breaking concentrated monazite or other thorium-containing ore into small fragments;

(b) wetting the fragments with a concentrated strong acid to make a slurry, wherein the slurry contains, among other things, rare earth elements (REE), actinium and radium ions;

(c) heating the slurry made in step (b);

(d) filtering and passing the heated solution through a first anion exchange column having an anion exchange resin;

(e) retaining metals on the resin the first anion exchange column;

(f) allowing the radium ions to decay to actinium-228;

(g) purifying the actinium-228 fraction;

(h) sending the actinium-228 fraction through a final thorium-228 capture column;

(i) eluting the captured thorium-228 with 1M HNOsub.3;

(j) removing radium from the solution;

(k) retaining the radium-228 fraction for isotope in-growth;

(l) retaining decay products from the radium-228 fraction for use as alpha-emitter used in medical isotope generators;

(m) separating the each individual REEs from the process stream; and

(n) eluting and retaining the separated REEs.

2. The process of claim 1, wherein step (a) comprises pulverizing or comminuting.

3. The process of claim 2, wherein step (a) comprises passing the concentrated monazite or other thorium-containing ore through a mesh screen.

4. The process of claim 4, wherein the mesh screen used in step (a) is between 10 and 200 mesh.

5. The process of claim 1, wherein the strong acid used in step (b) is 8M nitric acid.

6. The process of claim 1, further including the step of removing and recovering the metals retained in step (e) by using a 90% methanol-10% nitric acid solution.

7. The process of claim 6, wherein the metals retained include, Th, Fe, Co Ni, Cu Ag Sn Zn Ce A. Sc Te Zr Hf Cr Mo Mn and U.

8. The process of claim 1, further including the step of eluting thorium from the resin in the first anion exchange column using 1 M HNOsub.3.

9. The process of claim 8, further including the step of further purifying the captured thorium-228.

10. The process of claim 1, further including the step of passing the solution over at least one additional anion exchange resin column.

11. The process of claim 1, wherein step (j) involves removing radium from the solution via co-precipitation with barium nitrate

12. The process of claim 11, further including the step of secondarily and tertiary co-precipitating barium nitrate to ensure complete separation of the radium-228 fraction.

13. The process of claim 1, wherein step (m) involves separating each REE in order of the size of the rare earth metal ions, with the smallest separated first and the largest last.

14. The process of claim 1, wherein step (m) involves separating each REE lanthanides using reversed-phased partition chromatograph.

15. The process of claim 14, wherein the stationary phase of the reversed-phased partition chromatograph employs an organic phosphorous compound.

16. The process of claim 15, wherein the organic phosphorous compound of the stationary phase is selected from the group consisting of bis-(2-ethylhexyl)-o-phosphoric acid (HDEHP), and tri-n-butylphosphate (TBP), bis(di-n-hexyl-phosphinyl)methane (HDPM), and di-n-butylphosphate.

17. The process of claim 14, wherein the stationary phase uses a long-chain amine.

18. The process of claim 17, wherein the long-chain amine is selected from the group consisting of tri-octyl-amine and di-nonyl-naphalene-sulphonic acid in heptane.

19. The process of claim 14, further including the step of using a support for the stationary phase.

20. The process of claim 19, wherein the support is selected from the group consisting of Corvic (poly(vinyl chloride-vinyl acetate) co-polymers), siliconized kieselguhy or silica gel, Kel-F, (polychlorotrifluoroethane), and filter paper.

21. The process of claim 14, wherein the mobile phases are substantially pure aqueous solutions containing strong acids.

22. The process of claim 21, wherein the strong acids are selected from the group consisting of nitric acid, hydrochloric acid, and perchloric acid.

23. The process of claim 1, wherein step (n) involves eluting each REE in order of increasing atomic number.

24. The process of claim 1, further including the step of recovering phosphates as phosphoric acid.

25. The process of claim 1, further including the step of distilling nitric acid from the process stream and placing the distillate back in the process stream for use in an earlier method step.

26. The process of claim 1, further including the step of reserving thorium-232 depleted of radium-228.