BR112020021369B1 - METHOD FOR PROCESSING A PURIFIED RARE EARTH SULFATE SOLUTION - Google Patents

Abstract

The present invention relates to a method of processing a purified rare earth sulfate solution, wherein the method includes the steps of: placing the purified rare earth sulfate solution in contact with sodium hydroxide to precipitate the rare earths as rare earth hydroxide, including the addition of an oxidant to oxidize the cerium contained in the rare earth hydroxide precipitate; and selectively leaching the rare earth hydroxide precipitate with hydrochloric acid to form a rare earth chloride solution and a residue.

Description

TECHNICAL FIELD

[001] The present invention generally relates to the recovery of rare earth elements from rare earth ores or concentrates containing these elements. More particularly, the invention relates to the recovery of separated cerium and cerium-free rare earth products from a rare earth sulfate solution, such as a purified rare earth sulfate solution.

[002] A purified rare earth sulfate solution can, for example, be formed from a crude rare earth sulfate derived from sulfation (acid cooking) and subsequent water leaching of a monazite ore, a concentrate, or an associated pre-leach residue, wherein the purified solution may, for example, be derived from such a crude solution through the formation of a rare earth sulfate precipitate followed by a water leaching of the precipitate.

BACKGROUND OF THE INVENTION

[003] Rare earth elements include all lanthanide elements (lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium), as well as the rare metals scandium and yttrium. For purposes of facilitating discussion and because of their abundance and similar properties, Total Rare Earth Elements (TREE = Total Rare Earth Elements) are often divided into three groups: Light Rare Earths (LRE = Light Rare Earths) which are lanthanum, cerium, praseodymium and neodynium; the Medium Rare Earths (MRE = Middle Rare Earths) which are samarium, europium and gadolinium (promethium does not exist as a stable element); and the Heavy Rare Earths (HRE = Heavy Rare Earths) which are terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. Yttrium and scandium are frequently added to this list although they are not strictly heavy rare earth elements.

[004] During the past twenty years there has been an explosion in demand for many items that require rare earth elements, which now include many items that people use every day, such as computer memories, DVDs, rechargeable batteries, mobile phones , catalytic converters, small electric motors, magnets and fluorescent lighting.

[005] Rare earth elements also play an essential role in the generation of electricity from wind energy, new generation electric vehicles and military applications. In this regard, military uses include night vision goggles, precision-guided weapons, communications equipment, GPS equipment and batteries.

[006] The increase in the use of rare earth elements in new technology devices has led to an increase in demand, and a need for diversification in a supply chain that has been dominated by China since the early 1990s. In fact, the development of non-Chinese resources for rare earth mining and processing has expanded in recent years, particularly since China announced in 2010 that it will sharply restrict its export of rare earth elements to ensure supply for manufacturing. domestic.

[007] Naturally occurring rare earth-containing minerals that are sufficiently rich in rare earth elements to be of current or potential future commercial interest include fluorocarbonates (such as bastnasite), fluorides, oxides, phosphates (such as monazite , xenotime and apatite), and silicates (such as ionic clays and allanite). The world's resources are mainly contained in bastnasite and monazite. Bastnasite deposits in China and the United States constitute the largest percentage of the world's known economic rare earth resources, while monazite deposits in Australia, Brazil, China, India, Malaysia, South Africa , in Sri Lanka, Thailand and the United States constitute the second largest segment.

[008] Conventional methods for extracting rare earth elements from their ores or concentrates are described in "Extractive Metallurgy of Rare Earths" by C. K. Gupta and N. Krishnamurthy, CRC Press, 2005.

[009] As outlined in Gupta and Krishnamurthy (2005), rare earth elements have typically been extracted from monazite ores and concentrates by sulfation processes. In sulfation (also called "acid baking"), the ore or concentrate is mixed with concentrated sulfuric acid and cooked at high temperatures (such as 200 to 500°C) to break up the mineral matrix and convert the earth elements. rare in sulfate salts, which can then be brought into solution by dissolving in a water leach of the cooked solids. Since the rare earth elements are in the solution, which are impure rare earth sulfate solutions that will be referred to herein as "raw rare earth sulfate solutions", the rare earth elements are typically recovered by a number of different prior art techniques.

[0010] Gupta and Krishnamurthy (2005) also describe processes for extracting rare earth elements from bastnasite ores and concentrates (bastnasite is a rare earth fluorocarbonate). In China, bastnasite concentrates are processed by heating with concentrated sulfuric acid to 500°C in a rotary calcination furnace. The residue is then treated with water to dissolve the soluble rare earth sulfates, again forming a crude rare earth sulfate solution.

[0011] A prior art process strategy that has been adopted to recover the rare earth elements contained in such raw rare earth sulfate solutions is to neutralize the acid and precipitate the problematic impurities, thereby forming rare earth sulfate solutions. "purified", followed by recovery of the rare earth elements using one of several alternative processes such as solvent extraction, rare earth carbonate precipitation, rare earth oxalate precipitation, rare earth hydroxide precipitation, or ion exchange. Another strategy is to directly recover rare earth elements from raw rare earth sulfate solutions, for which a process option is available in the prior art, namely double sulfate precipitation.

[0012] Many of these process strategies can produce a rare earth hydroxide directly, such as in hydroxide precipitation, or indirectly through caustic conversion of intermediates, such as sulfate, carbonate or double oxalate. The cerium contained in the resulting rare earth hydroxides can be oxidized with a strong oxidizing agent, or by drying in the presence of oxygen at 120 to 150°C. The mixed rare earth hydroxides can then be selectively leached in hydrochloric acid at a pH of 3 to 4 to produce a rare earth chloride solution and a residue. The problem with existing processes, as described by Morais et al in "Recovery of Cerium by Oxidation/Hydrolysis With KMNO4- Na2CO3", Hydrometallurgy 2003 - Fifth International Conference - volume 2: Electrometallurgy and Environmental Hydrometallurgy, is that selective dissolution is very slowly and that about 1% of cerium is leached into the rare earth chloride solution.

[0013] An object of the present invention is therefore to provide an effective and efficient process for the selective recovery of rare earth elements from purified rare earth sulfate solutions, and thereby ultimately a more economical overall process for the production of rare earth products separated from ores containing rare earths.

[0014] The above discussion of the Background is included to explain the context of the present invention. It should not be considered an admission that any material mentioned was published, known, or forms part of common general knowledge (in any country) on the priority date of any of the claims.

SUMMARY OF THE INVENTION

[0015] As mentioned above, in general terms in rare earth processing, sulfation is where an ore or concentrate is mixed with concentrated sulfuric acid and cooked at elevated temperatures to break up the mineral matrix and convert the mineral elements. rare earths into sulfate salts which can then be brought into solution by dissolving in a water leach of the cooked solids. Once the rare earth elements are in the solution, which are so-called "raw" rare earth sulfate solutions, the rare earth elements then require recovery.

[0016] For example, in the applicant's co-pending international patent application filed on the same day as this patent application, a method for precipitation of rare earth sulfate is described, which includes subjecting a solution of rare earth sulfate to crude to precipitation in the presence of a water-soluble volatile organic compound to produce a rare earth sulfate precipitate and an acidic supernatant. The rare earth sulfate precipitate is then washed and dried, and subsequently leached in water to dissolve the soluble rare earth sulfate and form a leaching solution rich in rare earth sulfate and a leaching residue containing impurities in the form of phosphates. insoluble. The rare earth sulfate leaching solution is then subjected to precipitation of impurities to form a "purified rare earth sulfate solution" and a purification residue. It is such "purified" rare earth sulfate solution to which the present invention applies. The present invention can also be applied to a purified rare earth sulfate prepared by neutralizing crude rare earth sulfate as described in the Background.

[0017] The present invention thus provides a method of processing a purified rare earth sulfate solution, wherein the method includes the steps of: a) placing the purified rare earth sulfate solution in contact with sodium hydroxide to precipitating rare earths as rare earth hydroxide, including adding an oxidant to oxidize the cerium contained in the rare earth hydroxide precipitate; and b) selectively leaching the rare earth hydroxide precipitate with hydrochloric acid to form a rare earth chloride solution and a residue.

[0018] In one form, in preparing a purified rare earth sulfate solution, washing a rare earth sulfate precipitate formed from a crude rare earth sulfate solution is carried out with a volatile organic compound soluble in water, such as methanol, in which the sulfate precipitate is then dried (ideally with the volatilized organic compound recovered and reused) before a water leaching of the sulfate precipitate. Such water leaching preferably occurs at about 40°C for up to 180 minutes to dissolve the soluble rare earth sulfate, while leaving most impurities behind as insoluble phosphates (such as thorium) that can be recycled or discarded. . Trace amounts of soluble impurities will also be dissolved in such a process.

[0019] Then, after solid-liquid separation, the leaching residue can be discarded while the resulting leach solution rich in rare earth sulfate is ideally subsequently subjected to a rare earth sulfate purification stage to form the purified rare earth sulfate solution corresponding to the present invention and a purification residue.

[0020] With regard to this purification stage, magnesium is preferably added to the rare earth sulfate leaching solution to purify this solution by precipitation of impurities, leaving the desired purified rare earth sulfate solution and a purification residue. . Purification is ideally operated to maximize precipitation of impurities while minimizing co-precipitation of rare earth elements. In this way, the magnesia is preferably dosed into multiple tanks at a temperature of up to about 55°C, with a residence time of 30 to 120 minutes in each tank, at a target pH of 4.5 to 6.0. extreme point. Then, after solid and liquid separation, the purification precipitate can be recycled or discarded, and the purified rare earth sulfate solution is recovered for further processing.

[0021] Returning to a description of the processing method of the present invention, sodium hydroxide is added to the purified rare earth sulfate solution in order to precipitate the rare earths as rare earth hydroxide, wherein the addition of the oxidant helps to oxidize the cerium contained in the precipitate. Precipitation step a) is operated in this manner ideally to maximize the precipitation of rare earth elements and to maximize the conversion of cerium (III) to cerium (IV), while minimizing the stoichiometric excess of the reactant dosage, and minimizing the concentration of sulfate in the rare earth hydroxide precipitate.

[0022] The precipitation step a) preferably occurs in a two-stage countercurrent process, including a precipitation stage and a refining stage, in which the rare earth sulfate solution is fed into the precipitation stage with the solution used from the refining stage, to precipitate the sulfate containing rare earth hydroxide. The sulfate containing rare earth hydroxide can then be converted into clean rare earth hydroxide in the refining stage with the addition of fresh sodium hydroxide.

[0023] In a preferred form, both the precipitation and refining stages operate at a temperature of 40 to 80°C, and for a time of about 30 to 60 minutes, with the sodium hydroxide addition stoichiometry in the range from 100 to 110%, and with 100 to 130% oxidant stoichiometry dosage.

[0024] Preferably, the oxidant is hydrogen peroxide and/or sodium hypochlorite, which is added to the precipitation and/or refining stages after the addition of sodium hydroxide. For the case where hydrogen peroxide is the oxidant, it is preferable that the operating temperature is in the range of 50 to 60°C at the addition point(s).

[0025] In leaching step b), selective leaching of the rare earth hydroxide precipitate is preferably carried out in two stages, wherein each stage has multiple tanks and each stage uses hydrochloric acid diluted to about 10% by weight /weight when using the leaching solution from the first leaching stage. The leaching solution from the second leaching stage is preferably used to repulp, precipitate the leaching rare earth hydroxide, and precipitate the cerium (IV) that was dissolved in the second stage before the first leaching stage.

[0026] Selective leaching ideally occurs at a temperature of 60 to 80°C, where the first stage of leaching is being operated to maximize the dissolution of rare earth while minimizing the dissolution of cerium (IV), which is achieved with a pH endpoint of about pH 3 to 4, and the second leaching stage is operated to minimize the concentration of rare earth elements other than cerium in the residue. In this regard, it is preferable that the rare earth chloride solution contains cerium at a negligible level, and that the residue consists mainly of cerium (IV) hydroxide.

[0027] The cerium (IV) hydroxide residue can be compacted as a crude cerium product or processed to produce a higher purity cerium product. Additionally, barium chloride can be subsequently added to the rare earth chloride solution, with sulfuric acid when sulfate levels are low, to remove radium through coprecipitation with the barium sulfate to form a rare earth chloride solution. purified. The purified rare earth chloride solution can then be concentrated through evaporation.

[0028] The diluted and/or concentrated purified rare earth chloride can then be separated into two or more different rare earth products by solvent extraction wherein each rare earth product contains one or more individual rare earth elements.

[0029] The method of the invention may also include some additional steps related to producing a suitable crude rare earth sulfate solution from a rare earth rich calcium phosphate concentrate, most likely where the rich calcium phosphate concentrate in rare earth it is a product of the processing of an ore rich in calcium phosphate containing rare earth by floating the entire ore. In a preferred form, the ore contains apatite-hosted monazite, and the concentrate contains a greater degree of apatite-hosted monazite relative to the ore.

[0030] In general terms, these pre-processing steps preferably include pre-leaching a rare earth-rich calcium phosphate concentrate with phosphoric acid to form a rare earth-enriched pre-leaching residue, the mixture of the pre-leaching residue with sulfuric acid with subsequent heating of the mixture to convert the rare earths in the pre-leaching residue into water-soluble rare earth sulfates, and finally a water leaching of the heated mixture to place the earth sulfates rare earth sulfate solution suitable for the precipitation step described above.

[0031] The raw rare earth sulfate solution can then be subjected to precipitation in the presence of a water-soluble volatile organic compound to produce a rare earth sulfate precipitate and an acidic supernatant.

[0032] Ideally, the rare earth sulfate precipitation step is operated to maximize the extent of rare earth precipitation from solution while minimizing rare earth precipitation in the form of phosphates and fluorides, and while minimizing coprecipitation of impurities. The rare earth sulfate precipitate must contain > 35% TREE, > 12% S, < 2% P, < 0.3% Al and < 0.5% Fe, while all organic compounds added to precipitation, except that which is volatilized, will be contained in the acid supernatant. In fact, virtually all of the water, sulfuric acid, phosphoric acid, Mg, Al, Fe and U contained in the crude rare earth sulfate solution must be contained in the acidic supernatant.

[0033] Although the rare earth sulfate precipitate is subsequently subjected to other process steps to form the purified rare earth sulfate solution of the present invention, the acidic supernatant produced can also be subjected to other process steps aimed at recovering of the organic compound and the regeneration of phosphoric acid. This may include recovering the organic compound from the acidic supernatant by means of distillation, which results in the formation of the recovered organic compound and a dilute mixed acid solution, wherein the recovered organic compound is preferably recycled for use in the precipitation step. and the diluted mixed acid solution is passed directly and/or after further concentration by evaporation to be used in a phosphoric acid regeneration stage which will also be described later.

[0034] After pre-leaching of phosphoric acid from the pre-processing stages described above, heat may be applied to the phosphoric acid leaching solution to precipitate out of solution all minor amounts of rare earths such as rare earth phosphates, remaining a recovery solution. It is these rare earth phosphates that can then be returned to the sulfation and acid cooking steps described above.

[0035] The recovery solution can then be dosed with sulfuric acid and/or the diluted and/or concentrated mixed acid solution from the methanol separation stage to convert the monocalcium phosphate into phosphoric acid and to form a precipitate of monocalcium sulfate. calcium that can be discarded. The objective is a balance between maximizing the reactivity of the phosphoric acid and minimizing the amount of calcium sulfate precipitate formation in the aforementioned pre-leaching.

[0036] The phosphoric acid formed from the recovery solution can then be used as needed by pre-leaching in the pre-processing steps described above, and excess phosphoric acid can be bled out of the system and sent to the phosphoric acid purification.

[0037] In such phosphoric acid purification, the impurities (mainly uranium and thorium) in the dilute phosphoric acid bleed can be removed by ion exchange, solvent extraction or any other commercial process together with some elements of rare Lands. Furthermore, the concentration of calcium and sulfur in the diluted acid can be adjusted by adding sulfuric acid as necessary to meet consumer specifications. The purified and diluted phosphoric acid can then be concentrated to generate a phosphoric acid byproduct, which can be stored before being shipped to customers.

DESCRIPTION OF THE PREFERRED MODE

[0038] Figure 1 is a schematic representation of a flowchart for the precipitation of rare earth sulfates, which also illustrates a preferred preprocessing stage for rare earth rich calcium phosphates, and an acid regeneration stage preferred associated phosphoric acid; It is

[0039] Figure 2 is a schematic representation of a flowchart for a preferred embodiment of a method of processing a purified rare earth sulfate solution in accordance with the present invention.

DETAILED FLOWCHART DESCRIPTION

[0040] Before providing a more detailed description of a preferred embodiment of the present invention with reference to experimental data, it will be useful to provide some explanation of the flowchart of Figures 1 and 2.

[0041] Figure 1 shows the precipitation of rare earth sulfates (section A), together with the pre-processing of rare earth-rich calcium phosphates (section B) and an associated phosphoric acid regeneration stage (section C) . In Figure 2, a series of rare earth processing stages (section D) are shown. It should be appreciated that the focus of the present invention is on the rare earth processing stages in section D.

[0042] With respect primarily to Figure 1, and in the order of the process flow beginning with section B, several steps can be seen relating to the production of a suitable crude rare earth sulfate solution 10 from a phosphate concentrate of rare earth-rich calcium 12, which is a product of the beneficiation of a calcium phosphate-rich ore containing rare earth by flotation of the entire ore (not shown).

[0043] These preprocessing steps include pre-leaching the concentrate with phosphoric acid in a multi-stage countercurrent configuration 14, 16 to remove calcium phosphate and form a pre-leach residue 18 enriched in rare earths and a pre-leaching solution 19 containing monocalcium phosphate, impurities and minor amounts of rare earths. In this embodiment, the leach 14, 16 is operated at a low temperature (typically 30 to 45°C), with residence times maintained between 30 to 90 minutes for each stage, and with a mass ratio of total feed acid and the feed concentrate maintained between 2 and 12 grams of P in the acid per gram of Ca in the concentrate.

[0044] The pre-leaching residue 18 is then combined and cracked using concentrated sulfuric acid in a sulfation stage, being mixed 20 with sulfuric acid 22 at or below an acid cooking temperature for a time of up to about 30 minutes to ensure that the resulting mixture is completely homogenized before the following steps. Subsequent heating 24 of the mixture converts the rare earths in the pre-leaching residue 18 to water-soluble rare earth sulfates 26.

[0045] The water-soluble rare earth sulfates 26 are then cooled to a temperature less than about 50°C for a period of time of up to about 300 minutes to remove so much heat out of the sulfated material 26 that it discharges the cooking of acid 24 as practical before its use in a subsequent water leaching 28.

[0046] The cooled water-soluble rare earth sulfates 27 are then subjected to a water leaching 28 to bring into solution the rare earth sulfate, phosphoric acid and any remaining sulfuric acid, and thereby form the rare earth sulfate solution. aforementioned "raw" rare earth 10 and a water leaching residue 30 containing insoluble gangue material for disposal.

[0047] Turning now to the next section, section A, which is the precipitation of rare earth sulfates, Figure 1 shows the sulfate precipitation stage 32 where the raw rare earth sulfate solution 10 is subjected to precipitation in the presence of a water-soluble volatile organic compound (such as methanol) 34 to produce a rare earth sulfate precipitate 36 and an acidic supernatant 38. Sulfate precipitation 32 ideally occurs at a temperature in the range of 60 to 65°C with a residence time in the range of 20 to 40 minutes.

[0048] Although the rare earth sulfate precipitate 36 is subsequently subjected to other process steps in accordance with the present invention, which will be described below in relation to Figure 2, the acidic supernatant 38 produced in the sulfate precipitation step 32 it is also subjected to other process steps (section C) whose objective is the recovery of the organic compound and the regeneration of phosphoric acid.

[0049] Section C of Figure 1 thus shows the recovery of the organic compound from the acidic supernatant 38 by means of distillation 40, which results in the formation of the recovered organic compound 34 and a dilute mixed acid solution 42, in which the compound recovered organic 34 is recycled to be used in the sulfate precipitation step 32 as methanol 34, and the dilute mixed acid solution 42 is subjected to further concentration 44 by means of evaporation to form a concentrated mixed acid solution 45 to be used in a phosphoric acid regeneration stage 46.

[0050] After the aforementioned phosphoric acid pre-leaching 14, 16, heat 48 is applied to the phosphoric acid pre-leaching solution 19 to precipitate out of the solution any minor amounts of rare earths such as rare earth phosphates 50, leaving a recovery solution 52. As can be seen, the rare earth phosphates 50 are then returned to the acid mixing 20 and acid cooking 24 steps described above. Heating 48 of the phosphoric acid pre-leaching solution 19 occurs in several stages of increasing temperatures, with all temperatures in the range of 60°C to 110°C, with stage residence times between 60 and 180 minutes. .

[0051] The recovery solution 52 is then dosed with sulfuric acid 54 in the acid regeneration stage 46 and, in this embodiment, also with the concentrated mixed acid solution 45, to convert the monocalcium phosphate into recoverable phosphoric acid 56 and form a precipitate of calcium sulfate 57 that can be discarded. This addition of acid 46 is carried out in stages at a temperature of about 40°C and a residence time of between 30 and 60 minutes per stage. A portion of the phosphoric acid formed from the recovery solution 58 is also used by pre-leaching 14, 16, while excess phosphoric acid 59 is bled out of the system.

[0052] Turning now to Figure 2, and the final section of the invention (section D) of the flowchart, which shows the rare earth processing stages, this subsequent processing includes washing and drying the rare earth sulfate precipitate (shown indeed in Figure 1 as stage 60), and subsequently leaching 62 in water of the washed and dried rare earth sulfate precipitate 36 to dissolve the soluble rare earth sulfate and form a leaching solution 54 rich in rare earth sulfate and in a leaching residue 66 containing impurities in the form of insoluble phosphates.

[0053] Next, the impurities are precipitated 68, with the addition of magnesia 63, from the rare earth sulfate leach solution 64 to form a "purified" rare earth sulfate solution 70 and a purification residue 72, followed by by precipitation 74 of rare earths in purified rare earth sulfate solution 70 as rare earth hydroxide precipitate 76.

[0054] Sodium hydroxide 77 is added to the purified rare earth sulfate solution 70 to precipitate the rare earths as rare earth hydroxide 76, with the addition of hydrogen peroxide 78 to oxidize the cerium contained in the precipitate. Production of rare earth hydroxide 76 occurs in a two-stage countercurrent process, with the purified rare earth sulfate solution 70 fed to the precipitation stage (stage one) 74 to precipitate a crude rare earth hydroxide 75 that contains a little sulfate when using the spent solution 71 from the refining stage (stage two) 73, in which the conversion of rare earth sulfate compounds to rare earth hydroxide compounds occurs with the addition of fresh sodium hydroxide 77 in refining the rare earth hydroxide (stage two) 73.

[0055] The rare earth hydroxide precipitate 76 is then subjected to selective leaching 79, 80, 82 with hydrochloric acid to form the rare earth chloride solution 86 and the residue 88 with the rare earth solution 86 containing an insignificant amount of cerium, and residue 88 consists mainly of cerium(IV) hydroxide.

[0056] Selective leaching 79, 80, 82 of the rare earth hydroxide precipitate 76 is carried out in a multi-stage configuration, with diluted hydrochloric acid up to 10% w/w when using stage one polishing 80 of the leaching prior to its addition to the stage one 80 and stage two 82 leach tanks, in multiple tanks at each stage, and the stage two 82 solution is used for pulp reforming and leaching the rare earth hydroxide cake before from stage one 80 leaching.

[0057] The cerium(IV) hydroxide residue 88 is then compacted as a crude cerium product, while the rare earth chloride solution 86 is dosed with barium chloride 90 to remove radium through coprecipitation with barium sulfate and forming a purified rare earth chloride solution 92. The purified rare earth chloride solution 92 is then concentrated by evaporation 94.

DESCRIPTION OF EXPERIMENTAL DATA

[0058] Attention will now be directed to a description of the experimental data throughout the spreadsheet, developed to illustrate a preferred embodiment of the present invention.

Rare Earth Sulfate Precipitation

[0059] Rare earth sulfate precipitation tests were carried out by placing a measured amount of preheated water leaching solution in contact with a measured amount of methanol at room temperature in a suitable well-stirred baffled vessel equipped with a condenser reflux to minimize evaporative loss. The resulting mixture was maintained at a set point temperature for a specific duration, and then vacuum filtered. The filter cake was then washed thoroughly with methanol to remove entrained solution before drying.

The influence of temperature (Tests 1 and 2)

[0060] Two rare earth sulfate precipitation tests were carried out to evaluate the influence of reaction temperature on performance. One test was performed at 60 to 65°C (Test 1), while the other was performed at 40 to 45°C. Both tests were performed in the same water leach solution (Table 1) and were contacted with 1 gram of methanol per gram of water leach solution for 30 minutes. The results are summarized in Tables 2 through 4. The results indicate that operation at lower temperatures results in reduced coprecipitation of Al, P, Fe, Th, and U with rare earth sulfate. Table 1: Feed solution composition Table 2: Composition of the final solution Table 3: Composition of rare earth sulfate precipitate (% w/w or g/t) Table 4: Extent of precipitation (%)

The influence of contact ratio and residence time (Tests 3 to 9)

[0061] Two sets of rare earth sulfate precipitation tests were carried out to evaluate the influence of the contact ratio between the organic and aqueous phases and residence time on performance. Four contact ratios (0.25, 0.5, 0.75, and 1) were tested in the first set of tests (Tests 3 to 6, respectively). Subsamples were collected at 30, 60, and 120 minutes (subsamples A, B, and C) in each of Tests 3 through 6. The second set of tests (Tests 7 through 9) evaluated shorter residence times (10, 20 and 30 minutes, respectively) with a contact ratio of 1 when using a different feed solution for the first set (see Table 5). All tests (Tests 3 to 9) used methanol as the organic phase, and were performed with a target temperature of 60 to 65°C, while the temperatures experienced varied between 55 to 70°C. The results are summarized in Tables 6 to 8.

[0062] The results indicate that if the operation occurs at contact ratios between the organic and aqueous phases, it reduces the extent of precipitation of the rare earth element. The results indicate that precipitation of rare earth sulfate is effectively complete within the shortest residence time tested (10 minutes in Test 7), whereas precipitation of impurities such as thorium is effectively complete within 20 minutes, with a Negligible additional precipitation observed with prolonged contact durations. Table 5: Feed solution composition Table 6: Composition of the final solution Table 7: Composition of rare earth sulfate precipitate (% w/w or g/t) Table 8: Extent of precipitation (%)

The influence of feed composition (Tests 10 to 24)

[0063] Fifteen rare earth sulfate precipitation tests were carried out to evaluate the influence of variation in feed composition on precipitation performance. Each test used a different raw rare earth sulfate feed solution (Table 9), was contacted with methanol at a contact ratio of 1 to 1 by weight for 30 minutes, with a target temperature of 60 to 65°C, while whereas the temperatures experienced varied between 55 and 70°C. The results are summarized in Tables 10 to 12, from which it can be seen that most tests resulted in a relatively rare earth sulfate precipitate with some variation in the coprecipitation of aluminum, iron and phosphate.

[0064] To understand this coprecipitation of impurities, the results were condensed and arranged in sequence according to the free sulfuric acid content of the raw rare earth sulfate feed solution for each test (Table 13). This free acid value is a calculated value for convenience purposes and may not reflect the actual speciation occurring in the feed solution. The free acid content was determined by summing all cations and anions measured and inferred from the solution test result with the proton concentration calculated to balance the cations and anions. It was then assumed that all of the phosphate is present as phosphoric acid with the remaining protons designated as sulfuric acid.

[0065] For Test 13 this resulted in a negative sulfuric acid content, which suggests that the phosphate is not completely protonated or that the uncertainty of the test drove the cations over the anions. Either way, Test 13 is low in free acid, and a significant fraction of the precipitated rare earth elements precipitated as phosphates. Overall, the results suggest that in order to minimize precipitation of rare earth elements such as phosphates, the raw rare earth sulfate solution containing phosphate should contain 5% wt/wt or more free H2SO4 as defined herein. . Table 9: Feed solution composition Table 10: Composition of the final solution Table 11: Composition of rare earth sulfate precipitate (% w/w or g/t) Table 12: Extent of precipitation (%) Table 13: Rare earth sulfate composition and extent of precipitation as a function of free acid

Phosphoric Acid Pre-Leach

[0066] Pre-leaching tests were carried out by placing a measured amount of the temperature-controlled phosphoric acid solution in contact with a measured amount of the concentrate in a suitable well-stirred baffled vessel. The resulting mixture was maintained at a set point temperature for a specific duration, and then vacuum filtered. The filter cake was then washed thoroughly with deionized water to remove entrained solution before drying.

The influence of concentrate feed variability

[0067] Five pre-leaching tests were carried out to evaluate the influence of variability in the composition of the feed concentrate (Table 14) on pre-leaching performance. Each test was performed using the same feed acid (Table 15), with an acid-to-concentrate contact ratio of 8.4 grams of P in the acid per gram of Ca in the concentrate, at 30°C for two hours. . The results are summarized in Table 16. Table 14: Feed concentrate composition (% w/w or g/t) Table 15: Feed acid composition (g/L or mg/L) Table 16: Extent of dissolution (%)

The influence of feed acid composition

[0068] Five pre-leaching tests were performed to evaluate the influence of variability in feed acid composition (Table 17) on pre-leaching performance. Each test was performed using the same feed concentrate (Table 18), with an acid-to-concentrate contact ratio of 13 grams of acid per gram of feed concentrate, at 30°C for two hours. The results are summarized in Table 19. Table 17: Free acid composition (% w/w or g/t) Table 18: Feed concentrate composition (% w/w or g/t) Table 19: Extent of dissolution (%)

Continuous two-stage countercurrent leaching

[0069] A continuous two-stage pre-leaching circuit test was performed (Test 34). For this test, a thickener was used for the solid and liquid separation of the first stage, while a thickener with filtration of the thickener bottom stream was used for the solid and liquid separation in the second stage. The first stage contained a single tank with a 30-minute residence time operated at 40 to 45°C, whereas the second stage contained two tanks with a 30-minute residence time operated at 30°C. The bottom thickener stream from stage one was fed into two first stage two leach tanks along with the phosphoric acid (Table 20). The thickener overhead stream from stage two was combined with the primary filtrate and spent wash solution, then fed into the stage one leach tank along with the wet concentrate (9% moisture) (Table 21). The thickener overhead stream from stage one was continuously removed from the system while the leach residue cake from stage two was washed. For every kilogram of concentrate (dry basis) fed into the stage one leach tank, 10.6 kg of phosphoric acid was fed into the two first stage leach tanks. Circuit performance is summarized in Table 22. Table 20: Feed acid composition (g/L or mg/L) Table 21: Feed concentrate composition (% w/w or g/t) Table 22: Composition of pre-leaching residue (% w/w or g/t)

Rare earth recovery

[0070] Rare earth recovery tests were carried out by heating a measured amount of the pre-leaching solution (rare earth recovery feed solution) to boiling in a suitable well-stirred baffled vessel equipped with a reflux condenser. The resulting mixture was maintained under a continuous state of boiling for 120 minutes, and then vacuum filtered. The filter cake was then washed thoroughly with deionized water to remove entrained solution before drying.

[0071] Five rare earth recovery tests were performed to evaluate the influence of variability in the composition of the feed solution (Table 23) on rare earth recovery performance. The results are summarized in Tables 24 and 25. Table 23: Feed solution composition (% w/w or g/t) Table 24: Composition of the precipitate (% w/w or g/t) Table 25: Extent of precipitate (%)

Phosphoric Acid Regeneration

[0072] A continuous phosphoric acid regeneration circuit test was performed (Test 40). The circuit contained a single precipitation tank with a 55-minute residence time (Tank 1) operated at 44°C, followed by a 78-minute stabilization tank (Tank 2) and batch vacuum filtration, which was supported by the feed tanks in standby mode and operated with countercurrent washing. The recovery solution was fed into the precipitation tank along with mixed acid (Table 26). Regeneration performance is summarized in Tables 27 and 29. Table 26: Feed solution composition (g/L or mg/L) Table 27: Composition of the precipitate (% w/w or g/t) Table 28: Composition of the regenerated solution (g/L or mg/L) Table 29: Extent of total precipitation (%)

Acid Cooking and Water Leaching

[0073] Acid cooking and water leaching tests were carried out by placing a measured amount of sulfuric acid in contact with a measured amount of pre-leaching residue with thorough mixing in a suitable dish. The resulting mixture was placed in an oven and heated to 250°C for a period of up to 50 minutes, and then held at 250°C for a period of 30 minutes, removed from the oven and allowed to cool. The fresh sulfated material was then added to a measured amount of deionized water at 5°C, stirred for 10 minutes, and then vacuum filtered. The filter cake was then washed thoroughly with deionized water to remove entrained solution before drying.

[0074] Six acid cooking and water leaching tests (Tests 41 to 46) were performed to evaluate the influence of variability in feed pre-leaching residue composition (Table 30) on acid cooking and water leaching performance. water. Each test was performed using an acid-to-residue contact ratio of 1,600 kg of H2SO4 per ton of leach residue, and 2.5 g of deionized water per gram of pre-leach residue. The results are summarized in Tables 31 to 33. Table 30: Feed pre-leach residue composition (% w/w or g/t) Table 31: Composition of the water leaching solution (g/L or mg/L) Table 32: Composition of water leaching residue (% w/w or g/t) Table 33: Extent of dissolution (%) [1] behavior of sulfuric acid in relation to water leaching residue for S

Rare Earth Sulfate Dissolution

[0075] Rare earth sulfate dissolution tests are typically performed by placing a measured amount of dry rare earth sulfate precipitate in contact with a measured amount of thoroughly mixed deionized water in a suitable well-stirred baffled vessel at 40 °C for a period of 120 minutes, and then vacuum filtered. The filter cake was then washed thoroughly with deionized water to remove entrained solution before drying.

[0076] Five rare earth sulfate dissolution tests (Tests 47 to 51) were performed to evaluate the influence of variability in feed composition (Table 34) on dissolution performance. Each test was performed using a water-to-feed solids contact ratio of 13 grams of deionized water per gram of feed solids. Test 47 was operated with a 60-minute dissolution at 22°C, while Tests 48 to 51 were operated with a 120-minute dissolution at 40°C. The results are summarized in Tables 35 to 37. Table 34: Feed rare earth sulfate composition (% w/w or g/t) Table 35: Dissolving solution composition (g/L or mg/L) Table 36: Dissolution residue composition (% w/w or g/t) Table 37: Extent of dissolution (%)

Rare Earth Purification

[0077] Rare earth purification tests (Tests 52 and 53) were carried out by dosing magnesia, up to a target endpoint pH 5, up to a measured amount of impure rare earth sulfate solution at 40°C in a vessel well-shaken deflected suitable with in-line pH measurement. The resulting mixture was mixed for a period of 30 minutes after adding magnesia to allow it to stabilize, and then vacuum filtered. The filter cake was then washed thoroughly with deionized water to remove entrained solution before drying.

[0078] Two rare earth purification tests were performed to evaluate the influence of variability in source magnesia (Table 38) on rare earth purification performance from a common feed solution (Table 39). The results are summarized in Tables 40 to 42. In both tests (52 and 53), 0.16 kg of MgO was dosed per ton of feed solution. There was a use of 97.3% of magnesia in Test 52, and a use of 91.1% of magnesia in Test 53. Table 38: Magnesia composition (% w/w or g/t) Table 39: Feed solution composition (g/L or mg/L) Table 40: Precipitate composition (% w/w or g/t) Table 41: Composition of the purified rare earth sulfate solution (g/L or mg/L) Table 42: Extent of precipitation (%)

Rare Earth Hydroxide Precipitation The influence of the Reagent

[0079] The rare earth hydroxide precipitation tests (Tests 54 to 56) were carried out by placing a measured amount of magnesia or sodium hydroxide in contact with a measured amount of purified rare earth sulfate solution at 55°C in a suitable well-shaken deflected vessel. The resulting mixture was mixed for a period of 30 minutes after the addition of each dose of reagent to allow it to stabilize, then the subsample was collected, and vacuum filtered. The subsample filter cake was then washed thoroughly with deionized water to remove entrained solution before drying. For each test, a range of subsamples was collected to cover a range of reagent doses. After subsampling the final precipitation, a measured amount of hydrogen peroxide was added to the remaining slurry. The resulting mixture was mixed for a period of 30 minutes to allow it to stabilize, and then vacuum filtered.

[0080] Two of the rare earth hydroxide precipitation tests (Tests 54 and 55) were performed to evaluate the influence of variability in the magnesia source (Table 43) on rare earth hydroxide precipitation performance, while the third test (Test 56) evaluated the use of sodium hydroxide. All three tests were based on a common feed solution (Table 44). The reagent dosage is summarized in Table 45. The results are summarized in Tables 46 and 47. From the results it can be seen that the use of magnesia results in significantly increased impurities in the resulting precipitate relative to the precipitation when using the hydroxide of sodium. Furthermore, increased dose rates result in reduced total behavior of the sulfate relative to the rare earth hydroxide precipitate. Table 43: Magnesia composition (% w/w or g/t) Table 44: Composition of the feed solution (g/L or mg/L) Table 45: Addition of reagent (1)

[0081] [1] Subsamples A to D correspond to the periods of addition of magnesia (Tests 54 and 55) or sodium hydroxide (Test 56), where the dosage is in units of kg of magnesia or NaOH per ton of load. Subsample E corresponds to a period of hydrogen peroxide dosing with reagent addition expressed as a percentage of stoichiometry. Table 46: Precipitate composition (% w/w or g/t) Table 47: Extent of precipitation (%)

Two-Stage Rare Earth Hydroxide Production

[0082] A rare earth hydroxide precipitation test (Test 57) was performed by placing a measured amount of sodium hydroxide (97% stoichiometry) in contact with a measured amount of purified rare earth sulfate solution (Table 48 ) at 55°C in a suitable well-shaken baffled vessel. The resulting mixture was mixed for a period of 30 minutes to allow it to stabilize, and then a measured amount of hydrogen peroxide (117% stoichiometry) was added. The resulting mixture was mixed for a period of 30 minutes to allow it to stabilize, and then vacuum filtered.

[0083] For the second stage, the unwashed cake from the first stage was remade in deionized water along with a measured amount of sodium hydroxide (same mass as the first stage). The mixture was stirred for 60 minutes at 55°C and then vacuum filtered. The resulting cake was washed.

[0084] The results are summarized in Tables 49 to 50. Table 48: Composition of the feed solution (g/L or mg/L) Table 49: Composition of the precipitate (%, w/w or g/t) Table 50: Extent of precipitation (Stage 1) or dissolution (Stage 2) (%)

Selective Dissolution The influence of rare earth concentration

[0085] A series of four two-stage rare earth hydroxide dissolution tests (Tests 58 to 61) were carried out at 70 ° C in an appropriate well-stirred baffled vessel with in-line pH measurement. Each test begins with simulating the second stage by dosing a measured portion of 10% w/w hydrochloric acid into a measured portion of the rare earth hydroxide cake (Table 51) that has been remade in deionized water (Test 58 ) or a solution of rare earth chloride (Tests 59 to 61), followed by a 30-minute stabilization period. Typically, this results in a slurry pH of about 1.2 to 2.2. Each test is then completed with the second stage by adding a measured amount of rare earth hydroxide cake (typically 1.5 times was used to start the test), observing a 30 minute stabilization period, and then dosing with a measured portion of 10% wt/wt hydrochloric acid, followed by a 30 minute stabilization period. Typically, this results in a slurry pH of about 3 to 4. The test is then completed with vacuum filtration, followed by washing the cake.

[0086] Tests 59 to 61 were initiated by using the rare earth chloride solution from Test 58, with 2.8, 5.3 and 8.4 g of the rare earth chloride solution added to Tests 59 to 61, respectively, per gram of rare earth hydroxide consumed by each test. The results are summarized in Tables 52 to 53. Table 51: Solid feed composition (% w/w or g/t) Table 52: Composition of the final dissolution solution (g/L or mg/L) Table 53: Extent of total dissolution (%) Test La Ce Pr Nd MRE HRE Y Mg Al Ca Th

Two-stage selective dissolution

[0087] A two-stage rare earth hydroxide dissolution test (Test 62) was carried out at 70 ° C in an appropriate well-shaken baffled vessel with in-line pH measurement. The test was initiated by remaking 60% of the rare earth hydroxide cake (Table 54) in deionized water followed by controlled dosing of 10% w/w hydrochloric acid at pH targets in line with pH 2 (sample 1), and after pH 1 (sample 2). At pH 1, all rare earth hydroxide precipitate had dissolved. The remaining 40% of the rare earth hydroxide cake was then added and the mixture was allowed to stabilize under stirring for 30 minutes (sample 3). Controlled acid addition was then restarted at pH 3 (sample 4), pH 2 (sample 5), and pH 1 (sample 6). This time the rare earth hydroxide cake did not dissolve completely. The final paste was stirred for another 50 minutes before collecting the final sample (sample 7). After each period of acid dosing and achieving a target in-line pH, the resulting mixture was allowed to mix for at least 15 minutes before sample collection. The subsamples were vacuum filtered, followed by washing the cake.

[0088] The results are summarized in Tables 55 to 57. From the results, despite the complete dissolution of all cerium in the rare earth hydroxide by sample 2, the addition of additional rare earth hydroxide resulted in reprecipitation in such a way that in sample 4 (target pH 3, reaching pH 3.2 after stabilization) the cerium concentration in the solution was below the detection limit. This suggests that cerium dissolved as cerium IV in this test. Table 54: Solid feed composition (% w/w or g/t) Table 55: Subsample dissolution solution composition (g/L or mg/L) Table 56: Subsample solid composition (% w/w or g/t) Table 57: Extent of total dissolution (%)

[0089] One skilled in the art will understand that there may be variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all steps, features, compositions and compounds mentioned or indicated in this specification, individually or collectively, and any and all combinations of any two or more of the steps or features. The claims defining the invention are as follows.

Claims (10)

1. Method for processing a purified rare earth sulfate solution, characterized in that it includes the steps of: a) contacting the purified rare earth sulfate solution with sodium hydroxide to precipitate the rare earths as rare earth hydroxide , including the addition of an oxidant to oxidize the cerium contained in the rare earth hydroxide precipitate; and b) selectively leaching the rare earth hydroxide precipitate with hydrochloric acid to form a rare earth chloride solution containing an insignificant amount of cerium and a residue consisting primarily of cerium (IV) hydroxide. 2. Method for processing according to claim 1, characterized in that precipitation occurs in a two-stage countercurrent process, including a precipitation stage and a refining stage, in which the purified rare earth sulfate solution is fed to the precipitation stage with the solution used from the refining stage, to precipitate the sulfate containing rare earth hydroxide. 3. Method for processing according to claim 2, characterized in that the sulfate containing rare earth hydroxide is subsequently converted into clean rare earth hydroxide in the refining stage with the addition of fresh sodium hydroxide. 4. Method for processing according to claim 2 or 3, characterized in that the oxidant is hydrogen peroxide and/or sodium hypochlorite, which is added to the precipitation and/or refining stages after addition of sodium hydroxide. 5. Method for processing according to any one of claims 2 to 4, characterized in that both the precipitation and refining stages operate at a temperature of 40 to 80 ° C, and for a time of about 30 to 60 minutes, with sodium hydroxide addition stoichiometry in the range of 100 to 110%, and with 100 to 130% oxidant stoichiometry dosage. 6. Method for processing according to any one of claims 1 to 5, characterized in that the selective leaching of the rare earth hydroxide precipitate is carried out in two stages, wherein each stage has multiple tanks and each stage uses the acid hydrochloric acid diluted to 5 to 15% weight/weight with the leaching solution from the first leaching stage. 7. Method for processing according to claim 6, characterized in that the leaching solution of the second leaching stage is used to repulp, leach the rare earth hydroxide precipitate and precipitate the cerium (IV) dissolved in the second leaching stage, before the first leaching stage. 8. Method for processing according to claim 7 or 8, characterized in that the selective leaching occurs at a temperature of 60 to 80 ° C, wherein the first leaching stage is operated to maximize the dissolution of rare earth while minimizes the dissolution of cerium (IV), which is achieved with a pH endpoint of about 3 to 4, and the second leaching stage is operated to minimize the concentration of rare earth elements other than cerium in the residue. 9. Method according to any one of claims 1 to 8, characterized by the fact that barium chloride is added to the rare earth chloride solution, with sulfuric acid when sulfate levels are low, to remove radium through coprecipitation with barium sulfate to form a purified rare earth chloride solution. 10. Method according to claim 9, characterized by the fact that the purified rare earth chloride solution is concentrated by means of evaporation.