PROCESS FOR THE SEPARATION OF RARE EARTH ELEMENTS AND USABLE GYPSUM FROM RADIUM-BEARING PHOSPHOGYPSUM

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser. No. 63/567,064, filed Mar. 19, 2024, the disclosure of which is hereby incorporated by reference in its entirety, including all figures, tables, and drawings.

BACKGROUND

Phosphogypsum is a solid waste byproduct from processing phosphate ore to make phosphoric acid that is later used in fertilizer. Phosphogypsum contains significant quantities of rare earth elements and yttrium that are in high demand in modern technologies. Phosphogypsum also contains radium, which decays to form radon gas, both of which are radioactive and harmful to humans. Because of its radioactivity, phosphogypsum is typically stored in large stacks that pose physical and chemical hazards to the surrounding communities. Radium is in demand for medical isotope production for targeted alpha radiotherapy. Radium has to be concentrated from extremely diluted sources as the raw ingredient for medical isotope production, where phosphogypsum is a potential raw material.

BRIEF SUMMARY

Embodiments of the subject invention provide novel and advantageous systems and methods for separation of rare earth elements and yttrium, radium, and/or gypsum from phosphogypsum based on solubility differences between the phases that host each of these products. Gypsum is more soluble than the hosts of rare earth elements and yttrium, which in turn is more soluble than the hosts of radium. Gypsum can be extracted from the phosphogypsum by dissolution with a pH-controlled aqueous medium that can be buffered and can include salinity, followed by reprecipitation of the calcium sulfate dihydrate (gypsum) and optional recycling of the solvent, leaving an intermediate residue containing the rare earth elements, yttrium, and radium. A further extraction, of rare earth elements, can be performed on the intermediate residue from the gypsum extraction step. The further extraction can be done by using one or more (dilute) acidic, aqueous media on the intermediate residue, leaving a final residue that contains most or all of the radium that was present in the initial phosphogypsum.

In an embodiment, a method for separation of gypsum from phosphogypsum can comprise: i) grinding and/or dissolving the phosphogypsum in a first aqueous medium to give a first solution; ii) precipitating gypsum from the first solution to give extracted gypsum and an intermediate residue; and iii) treating the intermediate residue with a second aqueous medium that is acidic to give a second solution and a final residue. The second solution can comprise the rare earth elements and yttrium. The extracted gypsum can have a concentration of radium that is lower (e.g., at least 50% lower, at least 65% lower, at least 70% lower, at least 75% lower, at least 80% lower, at least 85% lower, at least 90% lower, at least 95% lower, at least 95% lower, or at least 98% lower) than that of the phosphogypsum. The first aqueous medium can have a pH in a range of from 7 to 9 and can be buffered (e.g., using a carbonate buffer) to maintain the pH at such levels. It can have a liquid to solid (L:S) ratio in a range of from 50:1 to 800:1. It can comprise at least one salt, and a concentration of the at least one salt can be in a range of, for example, from 0.01 molar (M) to 6 M. The at least one salt can comprise, for example, a chloride salt, a nitrate salt, a sodium salt, and/or an ammonium salt. The precipitating of the gypsum from the first solution can comprise using evaporation of the first solution. Steps i) and/or ii) can be performed in a first tank (e.g., a first sedimentation tank), which can be optionally stirred. Step iii) can be performed in a second tank (e.g., a second sedimentation tank), which can be optionally stirred. The first tank can be the same as the second tank or different from the second tank. The second aqueous medium can comprise at least one mineral acid and can have a L:S ratio in a range of from 0.5:1 to 100:1. A concentration of the at least one mineral acid can be in a range of, for example, from 0.01 M to 3 M. The at least one mineral acid can comprise, for example, nitric acid, sulfuric acid, and/or hydrochloric acid. Step iii) can be performed at ambient temperature. The second solution can comprise at least 80% by mass (and/or at least 80% by volume) of the rare earth elements and yttrium that were present in the phosphogypsum. The extracted gypsum can have a radium (e.g., radium-226) activity of, for example, 15 picocuries per gram (pCi/g) or less (e.g., 10 pCi/g or less, 5 pCi/g or less, 4 pCi/g or less, 3 pCi/g or less, 2 pCi/g or less, 1 pCi/g or less, or about 1 pCi/g or less).

In a third step, the second residue can be treated with concentrated nitric acid (e.g., 1 M to 16 M) to remove more than half of the radium from the second residue. The radium can then be extracted from the resulting solution (of the concentrated nitric acid after treating the second residue).

In another embodiment, a system for separation of gypsum from phosphogypsum can comprise: at least one sedimentation tank configured to perform steps i), ii), and iii) as disclosed herein.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a bar chart of percentage of gypsum extracted (in %) in each chemical step for phosphogypsum composites from three different gypstacks in separate locations in Central Florida. The (light gray) bars are mostly made of portions represented by Step 1. The small bars at the top of some of the stacks or locations represent step 2 (dark gray), and step 3 (black). FIG. 1 shows that greater than 98% of gypsum was extracted in Step 1 for all stacks and locations within each stack.

FIG. 2 shows a bar chart of percentage of total rare earths and yttrium (SREY) sequentially extracted in Steps 1-3 for the samples of phosphogypsum shown above. No REY were extracted in Step 1, so there is no light gray bar. FIG. 2 shows that greater than 95% of REY were extracted in Step 2a, except for Gypstack 1. A second application of Step 2 (Step 2b) yielded only a few percent extra REY. Except for two samples from Gypstack 1, complete extraction of REY was achieved by Step 2.

FIG. 3 shows a bar chart of radium remaining in clean gypsum for the same samples of phosphogypsum from the three different stacks shown above. The vertical axis shows radium-226 (²²⁶Ra) activity (in picocuries per gram (pCi/g)). Radium was determined by gamma-ray spectrometry using a high-purity germanium well detector (Mirion, SAGe™).

DETAILED DESCRIPTION

Embodiments of the subject invention provide novel and advantageous systems and methods for separation of gypsum, rare earth elements and yttrium, and radium from phosphogypsum. Gypsum can be extracted from the phosphogypsum by dissolution with a basic (e.g., a pH in a range of from 7 to 9) aqueous medium, with a liquid to solid (L:S) ratio in a range of from 50:1 to 800:1, that can optionally include salinity (e.g., at a concentration in a range of from 0.001 molar (M) to 6 M), followed by reprecipitation of the calcium sulfate dihydrate (gypsum) and optional recycling of the solvent. A further extraction, of rare earth elements and yttrium, can be performed on the intermediate residue from the gypsum extraction step. The further extraction can be done by using one or more acidic, aqueous media on the intermediate residue, leaving a final residue that contains most or all of the radium that was present in the initial phosphogypsum.

The extraction of the gypsum can be done by dissolving the phosphogypsum in a basic aqueous medium (e.g., pH in a range of from 7 to 9) that can include at least one buffering agent (e.g., calcium carbonate). The aqueous medium can include salinity (e.g., at a concentration in a range of from 0.001 M to 6 M). The salinity can be provided by salts that include, for example, chlorides or nitrates (e.g., NaCl, NH₄Cl, etc.). Clean gypsum can be extracted from the solution. The clean gypsum is gypsum having a much lower concentration of radium than the original phosphogypsum (e.g., at least 75% less, at least 80% less, at least 85% less, at least 90% less, at least 95% less, at least 97% less, or at least 98% less radium activity than the original phosphogypsum). For example, the clean gypsum can be extracted by evaporation of the gypsum solution created in Step 1. The extraction of the gypsum can be performed, for example, in an evaporation pan that can optionally be heated. The extracted gypsum can be low enough in radium to meet requirements of the United States Environmental Protection Agency (EPA) for all applications (i.e., less than 5 picocuries per gram (pCi/g)). The residue remaining after the gypsum extraction can retain the majority of the radium from the original phosphogypsum. This residue can either be disposed (e.g., as technologically enhanced naturally occurring radioactive materials (TENORM) waste) or can be used in a further extraction process (e.g., to extract rare earth elements).

A further extraction can be performed on the residue remaining after the gypsum extraction (which can be referred to herein as “the intermediate residue”). The intermediate residue can be treated with an acidic aqueous solution. For example, one or mineral acids (e.g., nitric, hydrochloric, sulfuric, etc.) can be used. The concentration of each mineral acid and/or the concentration of all acids together (there may be only one acid present) can be in a range of from 0.01 M to 3 M (e.g., 0.1 M to 3 M). The intermediate residue can be treated with the solution at ambient temperature. The L:S ratio of the acidic aqueous solution to the intermediate solid can be in a range of from 0.5:1 to 100:1. This further extraction can result in dissolving rare earth elements and yttrium (e.g., at least 80% (by volume or by mass)) of the rare earth elements and yttrium while radium (e.g., radium-226) and other elements remain in the least soluble phase (i.e., the final residue). After the treatment the final residue can contain most or all of the radium that was initially present in the original phosphogypsum before any extraction(s). The final residue can be disposed (e.g., as TENORM waste). The further extraction can be performed, for example, in a tank, such as a sedimentation tank that can optionally be stirred. The tank can be the same as that used for the extraction of gypsum (if present).

A further extraction can be performed on the residue of the second step to extract radium from sand and other insoluble minerals. This residue can be treated with nitric acid at a concentration in a range of from 0.1 M to 16 M to extract greater than 50% of the radium from the second residue. The radium can then be extracted from the resulting solution. The residue remaining can be comprised of sand.

Embodiments of the subject invention extract clean gypsum from phosphogypsum, the clean gypsum having significantly reduced levels of radium (e.g., radium-226) relative to the initial phosphogypsum. The intermediate residue after the gypsum extraction can then be treated with an acid (such as a dilute nitric acid solution) that selectively dissolves rare earth elements (e.g., at least 80% of the rare earth elements) in the intermediate residue while retaining radium (and other elements) in the least soluble phase, which is the final residue. The final residue comprises a multitude of low-solubility minerals (e.g., sand and/or clay that was incorporated into the stack of phosphogypsum during outdoor exposure), and can have half or more of the radium (e.g., radium-226) from the initial phosphogypsum. The extraction of embodiments of the subject invention can advantageously separate phosphogypsum into separate phases of gypsum, rare earth elements and yttrium, radium, and a final residue with some or all of the radium from the initial phosphogypsum.

Embodiments of the subject invention have significant utility, including but not limited to: the creation of raw materials for a rare earth element supply chain from an existing waste stream; and the valorization of the gypsum currently contained in about a billion tons of phosphogypsum that cannot be utilized due to the presence of low-level radioactivity (e.g., no more than 35 pCi/g) within it. The phosphogypsum is typically stored in large stacks that pose physical and chemical hazards to the surrounding communities. Embodiments of the subject invention enable the consumption of the phosphogypsum and extraction of usable gypsum and/or valuable rare earth elements and yttrium. Radium (i.e., the final residue) can subsequently be disposed of as low-level radioactive waste, occupying considerably less volume than the initial phosphogypsum stack. Radium could also be chemically extracted from the sand to produce an ingredient for medical isotopes.

Embodiments of the subject invention take advantage of the differential solubility of gypsum, rare earth phases, and radium hosts, in aqueous pH-controlled, and/or saline, media. A two-step dissolution process can be used to: effectively separate the gypsum first from the other components (e.g., using a basic solution that may contain salinity); and extract the rare earth elements and yttrium into a dilute acid medium. The final residue remaining after these two steps holds a variety of mineral material, much of it sand and/or clay, as well as the majority of the radium (e.g., radium-226) (from the original phosphogypsum), which can be coprecipitated as insoluble barium sulfate (barite) grains.

No related art methods exist for valorizing phosphogypsum with the effectiveness of embodiments of the subject invention. The Merseburg Process (reaction of sodium or ammonium carbonate and gypsum) has been proposed to utilize phosphogypsum for production of ammonium sulfate (as a fertilizer) and calcium carbonate, including for carbon dioxide sequestration, but this does not address the fate of the radioactivity contained within the phosphogypsum stacks (see, e.g., Avsar et al., A Review on Ammono-Carbonation Reactions: Focusing on the Merseburg Process, (5) 83-91, 2022, 10.22034/crl.2022.329067.1154; which is hereby incorporated by reference herein in its entirety). Some methods have been proposed in Russia to attempt to extract rare earth elements from phosphogypsum, but these methods are not concerned with the removal of radium from the phosphogypsum or the valorization of gypsum, and they return the impure gypsum residue to the stacks after extraction of rare earth elements. One process uses a saline solution to selectively dissolve gypsum and separate rare earth elements from the resulting intermediate phase (see WO 2024072201A1). However, this method does not control for a fraction of the radium that is co-extracted from phosphogypsum via this process and integrated into the gypsum separate, causing it to be radioactive and thus unfit for industrial purposes that require gypsum with a radium activity of less than 5 pCi/g.

Embodiments of the subject invention have several commercial applications. The extractions give rare earth elements and yttrium, clean gypsum, a radium separate, and a final residue containing radium (e.g., radium-226). The rare earth elements can form the input for a rare earth element separation supply chain, yielding materials for, e.g., permanent magnets developed for use in electric vehicle (EV) traction motors, wind turbine generators, and other applications, including in military applications, particularly drones and the F-35 fighter program. Depending on the amount of radium remaining in the purified (clean) gypsum, applications for gypsum include use as a soil amendment (if it has no more than 10 pCi/g of radium activity), as aggregate in road construction (if it has no more than 10 pCi/g), and/or for drywall and other building materials (if it has no more than 5 pCi/g).

When ranges are used herein, combinations and subcombinations of ranges (e.g., any subrange within the disclosed range) and specific embodiments therein are intended to be explicitly included. When the term “about” is used herein, in conjunction with a numerical value, it is understood that the value can be in a range of 95% of the value to 105% of the value, i.e. the value can be +/−5% of the stated value. For example, “about 1 kg” means from 0.95 kg to 1.05 kg.

A greater understanding of the embodiments of the subject invention and of their many advantages may be had from the following examples, given by way of illustration. The following examples are illustrative of some of the methods, applications, embodiments, and variants of the present invention. They are, of course, not to be considered as limiting the invention. Numerous changes and modifications can be made with respect to embodiments of the invention.

Example 1

The extraction methods disclosed herein were performed on three different phosphogypsum stacks-referred to as “Gypstack 1”, “Gypstack 2”, and “Gypstack 3”. FIG. 1 shows the percentage of the gypsum extracted in the first extraction (dissolving the phosphogypsum in a basic aqueous medium that can be buffered and can include salinity; labeled “Step 1” in FIG. 1), as well as that for the second extraction (treating the intermediate residue with an acidic aqueous solution; labeled “Step 2a” in FIG. 1). As seen in FIG. 1, a vast majority (greater than 98%) of the gypsum was extracted in the first extraction.

FIG. 2 shows the percentage of the total rare earth and yttrium (EREY) extracted in the second extraction (dissolving the phosphogypsum in a dilute acidic medium; labeled “Step 2a” and “Step 2b” in FIG. 2), as well as that for the third extraction (treating the intermediate residue with a concentrated acidic solution; labeled “Step 3” in FIG. 2). No rare earths or yttrium were extracted in the first step. As seen in FIG. 2, a vast majority (greater than 95%) of the total rare earth and yttrium content was extracted in the second extraction, except for one sample from Gypstack 1 that yielded about 85% of rare earth and yttrium content.

FIG. 3 shows the radium-226 activity (in pCi/g) for the clean gypsum from the same set of phosphogypsum samples shown in FIGS. 1 and 2. Referring to FIG. 3, for all stacks depicted the clean gypsum has a much lower radium-226 activity than the initial phosphogypsum stack, and in most cases the radium-226 activity is about 1 pCi/g or less, reaching the limit found in natural, mined gypsum (dashed line).

It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application.

All patents, patent applications, provisional applications, and publications referred to or cited herein are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification.