BR112020021941B1 - PROCESS FOR RECOVERING RARE EARTHS - Google Patents

Abstract

PROCESS FOR RECOVERING RARE EARTHS. The present invention relates to a method for precipitation of rare earth sulfate, the method including subjecting a crude solution of rare earth sulfate to precipitation in the presence of a water-soluble, volatile organic compound to produce a sulfate precipitate. of rare earth and an acidic supernatant. The organic compound is preferably selected from the group consisting of methanol, ethanol, isopropanol, tert-butanol, acetone and mixtures thereof and is preferably methanol. Preferably, the organic compound is used in precipitation in a weight ratio between 0.25:1 to 1.5:1 and preferably 0.5 to 1.25:1, with the crude sulfate solution.

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 rare earth elements from a rare earth sulfate solution, such as a solution derived from sulfation (acid cooking) and subsequent aqueous leaching of a monazite ore, concentrate or associated residue. pre-leaching.

BACKGROUND OF THE INVENTION

[002] 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. To facilitate discussion and because of their abundance and similar properties, Total Rare Earth Elements (TREE) are often divided into three groups; the Light Rare Earths (LRE) which are lanthanum, cerium, praseodymium and neodymium; the Medium Rare Earths (MRE) which are samarium, europium and gadolinium (promethium does not exist as a stable element); and the Heavy Rare Earths (HRE) which are terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. Yttrium and scandium are often added to this list, although they are not strictly heavy rare earth elements.

[003] During the last 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 memory, DVDs, rechargeable batteries, cell phones , catalytic converters, small electric motors, magnets and fluorescent lighting.

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

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

[006] Rare earths containing naturally occurring 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, Sri Lanka, Thailand, and the United States constitute the second largest segment.

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

[008] As described in Gupta and Krishnamurthy (2005), rare earth elements have typically been extracted from monazite ores and concentrated by sulfation processes. In sulfation (also called "acid cooking"), the ore or concentrate is mixed with concentrated sulfuric acid and cooked at high temperatures (such as 200 to 500°C) to break down the mineral matrix and convert the earth elements. rare in sulfate salts which can then be introduced into solution by aqueous leaching of the cooked solids. Since the rare earth elements are in solution, rare earth sulfate solutions being impure will be referred to herein as "raw rare earth sulfate solutions", the rare earth elements are typically recovered by several methods other than the prior art. .

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

[0010] A prior art process strategy that has been adopted to recover the rare earth elements contained in such crude rare earth sulfate solutions is to neutralize the acid and precipitate problematic impurities, followed by recovery of the rare earth elements using one of the various 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.

[0011] The problem with the first strategy is the need for substantial neutralization before recovery of the rare earth elements. For phosphate-rich systems, magnesium hydroxide or ammonia neutralizing agents, along with a significant addition of iron, are required to prevent the loss of rare earths through rare earth phosphate precipitation. This can be cost prohibitive in cases such as low temperature acid cooking systems where sulfuric and phosphoric acid contents are high.

[0012] The problem with the second strategy, namely the direct recovery of rare earth elements by double sulfate precipitation, is that the resulting mixed acid sterile rare earth solution contains significant sodium sulfate impurities, which can render - it is unsuitable for use. For example, the presence of sodium sulfate is incompatible with the operation of a phosphoric acid pre-leaching system. Furthermore, double precipitation with sulfate results in a precipitate that requires caustic conversion before being processed further. Furthermore, the precipitation process often requires a sufficiently large sodium addition to ensure recovery of the sodium sulfate, which requires neutralization of the sterile mixed acidic rare earth solution.

[0013] It is an object of the present invention to provide a simplified and more economical process for the selective recovery of rare earth elements from crude rare earth sulfate solutions.

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

SUMMARY OF THE INVENTION Rare Earth Sulfate Precipitation

[0015] The present invention provides a method for precipitation of rare earth sulfate, the method including subjecting a crude solution of rare earth sulfate to precipitation in the presence of a water-soluble volatile organic compound to produce a precipitate of rare earth sulfate. rare earth and an acidic supernatant.

[0016] In this method, a water-soluble organic compound is advantageous as it is capable of selectively depressing the solubility of rare earth sulfates over most other components of the crude sulfate solution, resulting in the selective formation of the rare earth sulfate precipitate. rare with insignificant conversion of the organic compound into other different organic compounds. This assists in the recovery and reuse of the organic compound later in the method of the invention.

[0017] The organic compound is preferably selected from the group consisting of methanol, ethanol, isopropanol, tert-butanol, acetone or mixtures thereof, and is preferably methanol. Preferably, the organic compound is used in precipitation in a weight ratio of between 0.25:1 to 1.5:1, and preferably 0.5: to 1.25:1, with the crude sulfate solution.

[0018] Ideally, the raw rare earth sulfate solution is near or above saturation with respect to the contained rare earth sulfate concentration and may contain, although not restricted to, any one or several of the following compounds and/or elements ; sulfuric acid, phosphoric acid, sulfate, phosphate, fluoride, chloride, nitrate, sodium, magnesium, aluminum, silicon, potassium, calcium, titanium, chromium, manganese, iron, nickel, arsenic, selenium, strontium, molybdenum, barium, thorium and uranium.

[0019] Ideally, the rare earth sulfate precipitation step is operated to maximize the extent of rare earth precipitation from solution, minimizing rare earth precipitation in the form of phosphates and fluorides, and minimizing coprecipitation of impurities. As such, in one form, the crude sulfate solution will have a free sulfuric acid concentration above 5% w/w. This applies to raw rare earth sulfate solutions that contain significant phosphate and/or fluoride.

[0020] In a preferred form, precipitation occurs at a temperature in the range between 60 to 65°C with a residence time in the range between 20 to 40 minutes.

[0021] The rare earth sulfate precipitate is expected to contain > 35% TREE, > 12% S, < 2% P, < 0.3% Al and < 0.5% Fe, while all the organics added to the precipitation, except those 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 raw rare earth sulfate solution must be contained in the acidic supernatant.

[0022] While the rare earth sulfate precipitate subsequently goes through other process steps that will be described below in the section titled "Rare Earth Processing", the acidic supernatant produced in the above precipitation step will also ideally go through other stages of the process aimed at recovering the organic compound and regenerating phosphoric acid.

[0023] In one form, the method of the invention therefore preferably also includes recovering the organic compound from the acidic supernatant by distillation, resulting in the formation of the recovered organic compound and a dilute mixed acid solution, with the recovered organic compound. preferably being recycled for use in the precipitation step mentioned above, and the diluted mixed acid solution being passed directly and/or after further concentration by evaporation for use in a phosphoric acid regeneration stage which will also be described later.

[0024] In the preferred form, where methanol is the organic compound, distillation is a methanol removal stage and is conducted in a continuous distillation column designed and operated to recover almost all of the feed methanol (>99.5% ), to produce a distillate fraction of nearly pure methanol (>95% methanol), and to produce a bottom fraction containing most of the water and all of the acid and other water-soluble salts contained in the feed solution. In the preferred form, the distilled fraction may be recycled for use as a vapor, condensed for recycling as a solution, or combinations thereof as deemed appropriate to minimize overall energy consumption.

Rare Earth Rich Calcium Phosphate Preprocessing

[0025] The method of the invention preferably also includes some additional steps before the precipitation step, in particular including steps related to producing a suitable crude rare earth sulfate solution from a rare earth rich calcium phosphate concentrate. rare earths, most likely where the rare earth-rich calcium phosphate concentrate is a product of the beneficiation of calcium phosphate-rich ore containing the rare earth by flotation of the entire ore. In a preferred form, the ore contains apatite-hosted monazite and the concentrate contains a higher grade of apatite-hosted monazite relative to the ore.

[0026] 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 to water-soluble rare earth sulfates and finally an aqueous leaching of the heated mixture to place the rare earth sulfates in solution and thus form the crude rare earth sulfate solution suitable for the precipitation step described above.

[0027] Generally speaking, rare earth-containing calcium phosphate concentrate may contain rare earth elements in any rare earth mineral, including, but not limited to, one or more of monazite, xenotime, allanite and bastnesite, while that calcium phosphate may be present as one or several of apatite, hydroxyapatite and brushite. Broadly speaking, crude rare earth sulfate solution is any solution containing rare earth elements, in stoichiometric quantity or greater than sulfate, and sufficient impurities to ensure the application of facilitated precipitation of water-soluble organic rare earth sulfate to recover the rare earth elements contained in the solution relative to alternatives, regardless of the origins of the solution.

[0028] More specifically, with respect to these preferred preprocessing steps, the method of the invention preferably includes removing calcium phosphate from a rare earth-rich calcium phosphate concentrate by pre-leaching with phosphoric acid to form a pre-leach residue enriched in rare earths and a pre-leach solution containing monocalcium phosphate, impurities and small amounts of rare earths.

[0029] Pre-leaching of phosphoric acid is preferably operated to balance the competing criteria of maximizing the dissolution of calcium phosphate while minimizing the dissolution of impurities. The pre-leach is preferably set up in a multi-stage countercurrent configuration, with the leaching operated at a low temperature (typically 30 to 45°C) and with residence times maintained between 30 to 90 minutes for each stage, and the Mass ratio of total acid feed to feed concentrate ideally maintained between 2 and 12 grams of P in the acid per gram of Ca in the concentrate, and ideally between 4 and 8 grams of P in the acid per gram of Ca in the concentrate.

[0030] The pre-leach residue and any rare earth precipitates formed from the subsequent recovery stages (described below), are then preferably combined and cracked using concentrated sulfuric acid in a sulfation stage to convert the minerals that contain rare earths in the mixture in water-soluble rare earth sulfates. Such a sulfation stage is preferably operated so that the downstream dissolution of rare earth elements in a subsequent aqueous leaching is maximized while the dissolution of iron, aluminum, silicon and magnesium impurities is minimized.

[0031] In this form, the pre-leach residue and rare earth precipitates are preferably mixed with sulfuric acid (referred to as an "acid mixture") at or below the acid cooking temperature or for a time of up to approximately 30 minutes to ensure that the resulting mixture is fully homogenized before the next steps. The temperature of the acid mixture is controlled by controlling the sulfuric acid feed temperature. For the case where the acid mixture is operated below the acid cooking temperature, the acid mixture discharge mixture is then heated to an acid cooking temperature of about 250°C and then proceeds to the holding period of acid cooking. For the case where the acid mixture is operated at an acid cooking temperature, the discharge proceeds directly to the acid cooking waiting period. During the acid cooking holding period, the sulfation mixture is preferably held at the acid cooking temperature for a period of time up to about 60 minutes to convert the rare earths into the pre-leach residue and the rare earth phosphates into water-soluble rare earth sulfates.

[0032] The water-soluble rare earth sulfates are then preferably cooled to a temperature below about 50°C for a period of time of up to about 300 minutes to remove maximum heat from the sulfated material that discharges the acid cooking. as practical before use in subsequent aqueous leaching. This helps minimize localized temperature-accelerated leaching of impurities during aqueous leaching.

[0033] The water-soluble rare earth sulfates are then preferably subjected to an aqueous leaching to bring into solution the rare earth sulfate, phosphoric acid and any remaining sulfuric acid and thus form the crude earth sulfate solution. mentioned above and an aqueous leach residue containing insoluble gangue material for disposal.

[0034] Aqueous leaching is preferably operated to minimize the leaching of impurities from the gangue material (iron, aluminum, magnesium and silicon), while also maximizing the concentration of rare earth elements in the raw sulfate solution. Ideally, aqueous leaching will operate near but below the rare earth solubility limit and will be conducted at a temperature less than about 30°C for a period of time less than about 20 minutes. Phosphoric Acid Regeneration

[0035] After pre-leaching the phosphoric acid from the pre-processing stages described above, heating is preferably applied to the phosphoric acid pre-leaching solution formed in the pre-processing, to precipitate from the solution any minor amounts of rare earths. such as rare earth phosphates, leaving a recovery solution. It is these rare earth phosphates that will preferably return to the sulfation and acid cooking steps described above.

[0036] In a preferred form, heating of the phosphoric acid pre-leaching solution occurs in several stages of increasing temperature, all temperatures being in the range between 60°C to 110°C, with residence times in the stage between 60 and 180 minutes.

[0037] The recovery solution is then preferably dosed with sulfuric acid and/or the diluted and/or concentrated mixed acid solution from the methanol separation stage described above, to convert the monocalcium phosphate into phosphoric acid and form a sulfate precipitate. of calcium that can be discarded. This acid addition is preferably carried out in stages at a temperature of about 40°C and a residence time of between 30 and 90 minutes per stage. Ideally, sulfuric and/or mixed acids are added to produce a phosphoric acid solution that contains about 6 g/L Ca and 1.6 g/L S. In this situation, about 2 of the 6 g/L Ca are associated with soluble calcium sulfate and the remaining 4 of the 6 g/L of Ca are associated with monocalcium phosphate. This objective is a balance between maximizing the reactivity of the phosphoric acid and minimizing the amount of calcium sulfate precipitate that forms in the pre-leaching mentioned above.

[0038] The phosphoric acid formed from the recovery solution is used as required by pre-leaching in the pre-processing steps described above, excess phosphoric acid can be purged out of the system and sent for phosphoric acid purification .

[0039] In such purification of phosphoric acid, the impurities (mainly uranium and thorium) in the dilute phosphoric acid purge can be removed by ion exchange, solvent extraction or any other commercial process along with some rare earth elements. Additionally, the concentration of calcium and sulfur in the diluted acid can be adjusted by adding sulfuric acid as needed 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 consumers.

Rare Earth Processing

[0040] The method of the present invention preferably also includes subsequent steps for processing the rare earth sulfate precipitate produced by the above precipitation step.

[0041] Generally speaking, this subsequent processing includes washing and drying the rare earth sulfate precipitate, leaching the washed and dried rare earth sulfate precipitate in water to dissolve soluble rare earth sulfate and form a rich leaching solution. into rare earth sulfate and a leach residue containing impurities in the form of insoluble phosphates, the precipitation of impurities from the rare earth sulfate leach solution to form a purified rare earth sulfate solution and a purification residue, the precipitation of rare earths in the purified rare earth sulfate solution as rare earth hydroxide and selectively leaching the rare earth hydroxide precipitate with hydrochloric acid to form a rare earth chloride solution and a residue.

[0042] In a preferred form, washing of the rare earth sulfate precipitate is conducted with a water-soluble, volatile organic compound, such as methanol, with the sulfate precipitate then dried (with the volatilized organic recovered and reused) before from aqueous leaching of the sulfate precipitate. The rare earth sulfate precipitate is then preferably leached in water at about 40°C for up to 180 minutes to dissolve the soluble rare earth sulfate while the majority of impurities remain as insoluble phosphates (such as thorium) which can be recycled or discarded. Residual amounts of soluble impurities will also be dissolved in this process. Then, after solid-liquid separation, the leaching residue can be discarded while the resulting rare earth sulfate-rich leach solution subsequently passes through a rare earth sulfate purification stage to form a rare earth sulfate solution. purified and a purification residue.

[0043] Regarding the purification step, magnesium is preferably added to the rare earth sulfate leaching solution to purify this solution by precipitation of impurities, leaving the purified rare earth sulfate solution and the purification residue. Purification is operated to maximize the precipitation of impurities while minimizing the co-precipitation of rare earth elements. As such, the magnesium is preferably added in several tanks at a temperature of up to about 55°C, with a residence time of 30 to 120 minutes in each tank, for a target end point of pH 4.5 to 6.0. .

[0044] Subsequently, sodium hydroxide can then be added to the purified rare earth sulfate solution to precipitate the rare earths as rare earth hydroxide, with the addition of an oxidant such as hydrogen peroxide to oxidize the cerium contained in the precipitate. . This precipitation is preferably operated to maximize the precipitation of rare earth elements and maximize the conversion of cerium (III) to cerium (IV), while minimizing the stoichiometric excess of reactant dosage and minimizing the sulfate concentration in the earth hydroxide precipitate. rare. As such, the production of rare earth hydroxide preferably occurs in a two-stage countercurrent process, with the purified rare earth sulfate solution fed to a precipitation stage (stage one) to precipitate the rare earth hydroxide containing some sulfate. using spent solution from one refining stage (stage two).

[0045] The sulfate containing rare earth hydroxide is subsequently converted into clean rare earth hydroxide in the second stage with the preferred addition of fresh sodium hydroxide. Furthermore, hydrogen peroxide is preferably added during the precipitation and/or refining stage in the presence of sodium hydroxide at a temperature of about 55°C and each stage in the production of rare earth hydroxide preferably operates at a temperature of 40 to 80°C and for a time of about 30 to 60 minutes, with the stoichiometry of addition of sodium hydroxide ideally in the range of 100 to 110%, and ideally with 100 to 130% of stoichiometric dosage of hydrogen peroxide.

[0046] The purification precipitate can be recycled or discarded, while the rare earth hydroxide precipitate can then undergo selective leaching with hydrochloric acid to form the rare earth chloride solution and the residue, with the rare earth solution containing negligible cerium and the residue consisting mainly of cerium(IV) hydroxide.

[0047] Regarding selective leaching, selective leaching of the rare earth hydroxide precipitate can be conducted in a multi-stage configuration, with hydrochloric acid diluted to about 10% w/w using polished leaching solution in stage one before of its addition to stage one and stage two leaching tanks, preferably in multiple tanks in each stage and the stage two solution is used as slurry again, leaching of the rare earth hydroxide residue and precipitation of the cerium(IV) dissolved in stage two, before stage one leaching. Furthermore, selective leaching preferably occurs at a temperature of 60 to 80°C, stage one leaching is ideally operated to maximize the dissolution of rare earths while minimizing the dissolution of cerium (IV) which is achieved with a final pH of about pH 3 to 4 and stage two leaching is ideally operated to minimize the concentration of rare earth elements other than cerium in the cerium (IV) hydroxide residue.

[0048] The cerium (IV) hydroxide residue can be packaged as a raw cerium product or further processed to produce a higher purity cerium product. Rare earth chloride solution can be dosed with barium chloride, along with sulfuric acid for situations where sulfate levels are low, to remove radium via coprecipitation with barium sulfate and form a purified rare earth chloride solution . The purified rare earth chloride solution can then be concentrated by evaporation if necessary.

[0049] The diluted and/or concentrated purified rare earth chloride can then be separated into two or more different rare earth products by solvent extraction with each rare earth product containing one or more individual rare earth elements.

DESCRIPTION OF THE PREFERRED MODE

[0050] Figure 1 is a schematic representation of a flow diagram for the precipitation of rare earth sulfates according to a first preferred embodiment of the method of the present invention, which also illustrates a preferred preprocessing step for rare earth phosphates. rare earth-rich calcium and a preferred associated phosphoric acid regeneration stage; It is

[0051] Figure 2 is a schematic representation of a flow diagram for preferred subsequent rare earth processing stages.

DETAILED DESCRIPTION OF THE FLOW DIAGRAM

[0052] 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 flow diagram of Figures 1 and 2.

[0053] 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 subsequent rare earth processing stages (section D) are shown. It will be appreciated that the focus of the present invention is on the precipitation of rare earth sulfates in section A.

[0054] With reference first to Figure 1, and in the order of the process flow beginning with section B, several steps can be seen in relation to the production of a crude solution of rare earth sulfate 10 from a concentrate of calcium phosphate rich in rare earths 12, being a product of the processing of an ore rich in calcium phosphate that contains a rare earth by flotation of the entire ore (not shown).

[0055] 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 small amounts of rare earths. In this embodiment, the leaching 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 the total feed acid to the acid concentrate. Feed maintained between 2 and 12 grams of P in the acid per gram of Ca in the concentrate.

[0056] The pre-leaching residue 18 is then combined and broken down 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 the resulting mixture is fully homogenized before the next steps. Subsequent heating 24 of the mixture converts the rare earths in the pre-leaching residue 18 to water-soluble rare earth sulfates 26.

[0057] The water-soluble rare earth sulfates 26 are then cooled to a temperature lower than about 50°C over a period of time of up to about 300 minutes to remove maximum heat from the sulfated material 26 that discharges acid cooking 24 as practical before use in a subsequent aqueous leaching 28.

[0058] The cooled water-soluble rare earth sulfates 27 are then subjected to an aqueous leaching 28 to bring into solution the rare earth sulfate, phosphoric acid and any remaining sulfuric acid and thus form the crude earth sulfate solution rare 10 mentioned above and an aqueous leaching of residue 30 containing insoluble gangue material for disposal.

[0059] Turning now to section A, being the inventive 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 an organic compound water-soluble volatile substance (such as methanol) 34 to produce a rare earth sulfate precipitate 36 and an acidic supernatant 38. Precipitation of sulfate 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.

[0060] While the rare earth sulfate precipitate 36 subsequently goes through other process steps that will be described below in relation to Figure 2, the acidic supernatant 38 produced in the sulfate precipitation step 32 also goes through other process steps. (section C) aiming at the recovery of the compound and the regeneration of phosphoric acid.

[0061] Section C of Figure 1 thus shows the recovery of the organic compound from the acid supernatant 38 by distillation 40, resulting in the formation of the recovered organic compound 34 and a dilute mixed acid solution 42, with the recovered organic compound 34 being recycled for use in the sulfate precipitation step 32 as methanol 34 and the dilute mixed acid solution 42 being subjected to further concentration 44 by evaporation to form a concentrated mixed acid solution 45 for use in a phosphoric acid regeneration step 46.

[0062] Following the phosphoric acid pre-leaching 14, 16 mentioned above, heat is applied 48 to the phosphoric acid pre-leaching solution 19 to precipitate from 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 being in the range of 60°C to 110°C, with stage residence times between 60 and 180 minutes.

[0063] 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 eliminated. 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. Some of the phosphoric acid formed from the recovery solution 58 is also used by pre-leaching 14,16, while excess phosphoric acid 59 is purged out of the system.

[0064] Turning now to Figure 2 and the final section (section D) of the flow diagram, which shows the subsequent stages of rare earth processing, this subsequent processing includes washing and drying the rare earth sulfate precipitate (in true shown in Figure 1 as stage 60), and subsequent 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 64 rich in rare earth sulfate and a leach residue 66 which contains impurities in the form of insoluble phosphates.

[0065] Next, impurities 68 are precipitated, with the addition of magnesium 63, from the rare earth sulfate leaching solution 64 to form a purified rare earth sulfate solution 70 and a purification residue 72, followed by by precipitation 74 of the rare earths in the purified solution of rare earth sulfate 70 as a precipitate of rare earth hydroxide 76.

[0066] 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 containing some sulfate using a spent solution 71 from the refining stage (stage two) 73, with the conversion of rare earth sulfate compounds to rare earth hydroxide compounds occurring with the addition of fresh sodium hydroxide 77 in sodium hydroxide refining. rare earths (stage two) 73.

[0067] The rare earth hydroxide precipitate 76 then undergoes 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 negligible cerium and the residue 88 consisting mainly of cerium(IV) hydroxide.

[0068] Selective leaching 79, 80, 82 of the rare earth hydroxide precipitate 76 is conducted in a multi-stage configuration, with hydrochloric acid diluted to 10% w/w using a leaching solution 80 prior to its addition to the stage one 80 and to the leaching tanks 82 in stage two, in several tanks in each stage, and the solution from stage two 82 is used for repulping and leaching the rare earth hydroxide cake before the leaching stage one 80.

[0069] The cerium(IV) hydroxide residue 88 is then packaged as a raw cerium product, while the rare earth chloride solution 86 is treated with barium chloride 90 to remove the radium through coprecipitation with cerium sulfate. barium and form a purified rare earth chloride solution 92. The purified rare earth chloride solution 92 is then concentrated by evaporation 94.

DESCRIPTION OF EXPERIMENTAL DATA

[0070] Attention will now be directed to a description of the experimental data developed to illustrate a preferred embodiment of the present invention.

Rare Earth Sulfate Precipitation

[0071] Rare earth sulfate precipitation tests were conducted by contacting a measured amount of preheated aqueous leach solution with a measured amount of methanol at room temperature in a baffled vessel under suitable agitation equipped with a reflux condenser to minimize evaporation loss. The resulting mixture was maintained at a set point temperature for a specified period, then vacuum filtered. The filter cake was then washed extensively with methanol to remove entrained solution before drying.

The influence of temperature (Tests 1 and 2)

[0072] Two rare earth sulfate precipitation tests were conducted to evaluate the influence of reaction temperature on performance. One test was conducted at 60 to 65°C (Test 1) while the other was conducted at 40 to 45°C. Both tests were conducted in the same aqueous leach solution (Table 1) and were contacted with 1 gram of methanol per gram of aqueous leach solution for 30 minutes. The results are summarized in Tables 2 to 4. The results indicate that operating 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 proportion and residence time (Tests 3 to 9)

[0073] Two sets of rare earth sulfate precipitation tests were conducted to evaluate the influence of the proportion of aqueous organic contact 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 to 6. The second set of tests (Tests 7 to 9) evaluated shorter residence times (10, 20 and 30 minutes, respectively) with a contact ratio of 1 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 conducted with a target temperature of 60 to 65°C, while the experimented temperatures ranged between 55 and 70°C. The results are summarized in Tables 6 to 8.

[0074] The results indicate that operating at lower organic to aqueous contact ratios reduces the extent of rare earth element precipitation. The results indicate that precipitation of rare earth sulfate is effectively complete within the shortest residence time tested (10 minutes in Test 7), while precipitation of impurities such as thorium is effectively complete within 20 minutes, with negligible additional precipitation. observed with extended 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)

[0075] Fifteen rare earth sulfate precipitation tests were conducted 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 with a contact ratio of 1 to 1 w/w for 30 minutes, with a target temperature of 60 to 65 C. , while 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 clean rare earth sulfate precipitate with some variation in the coprecipitation of iron, aluminum and phosphate.

[0076] To understand this coprecipitation of impurities, the results were condensed and sequenced 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 value calculated for convenience and may not reflect the actual speciation occurring in the feed solution. Free acid content was determined by summing all cations and anions measured and inferred from the solution assay result with the proton concentration calculated to balance 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.

[0077] For Test 13, this resulted in a negative sulfuric acid content, which suggests that the phosphate is not fully protonated or that the uncertainty of the test was biased towards cations over anions. In any case, Test 13 has a low free acid content and a significant fraction of the precipitating 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% w/w or more free H2SO4 as defined here.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: Composition of rare earth sulfate and extent of precipitation as a function of free acid

Phosphoric Acid Pre-leaching

[0078] Pre-leaching tests were conducted by contacting a measured amount of a temperature-controlled phosphoric acid solution with a measured amount of a concentrate in a suitable container with a well-shaken baffle. The resulting mixture was maintained at a set point temperature for a specified period, then vacuum filtered. The filter cake was then washed carefully with DI water to remove the entrained solution before drying.

The influence of feed concentrate variability

[0079] Five pre-leaching tests were conducted to evaluate the influence of variability in feed concentrate composition (Table 14) on pre-leaching performance. Each test was conducted using the same feed acid (Table 15), with a ratio of acid to contact concentrate 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: Free acid composition (g/L or mg/L) Table 16: Extent of dissolution (%)

The influence of acidic feed composition

[0080] Five pre-leaching tests were conducted to evaluate the influence of variability in the composition of the acid feed (Table 17) on pre-leaching performance. Each test was conducted using the same feed concentrate (Table 18), with an acid to contact concentrate 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: Composition of acid feed (%w/w or g/t) Table 18: Feed concentrate composition (%w/w or g/t) Table 19: Extent of dissolution (%)

Two-stage continuous countercurrent leaching

[0081] A test on the two-stage continuous pre-leaching circuit was conducted (Test 34). For this test, a thickener was used for the first stage of solid-liquid separation, while thickening with underflow filtration of the thickener was used for the solid-liquid separation in the second stage. The first stage featured a single tank with a 30-minute residence time operated at 40 to 45°C, while the second stage contained two tanks with a 30-minute residence time operated at 30°C. Underflow of stage one thickener fed into the two first stage leach tanks along with phosphoric acid (Table 20). The thickener overflow from stage two was combined with primary filtrate and spent wash solution, then fed into the stage one leach tank, along with a wet concentrate (9% moisture) (Table 21). Thickener overflow from stage one was continuously removed from the system as the leach residue cake was washed in stage two. For each kilogram of concentrate (dry basis) fed into the first stage leaching tank, 10.6 kg of phosphoric acid was fed into the two first stage leaching tanks. System performance is summarized in Table 22.Table 20: Free 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

[0082] Rare earth recovery tests were conducted by heating a measured amount of pre-leaching solution (rare earth recovery feed solution) to boiling in a suitable well-stirred baffle vessel equipped with a condenser. reflux. The resulting mixture was maintained under a continuous boiling state for 120 minutes, then vacuum filtered. The filter cake was then washed carefully with DI water to remove the entrained solution before drying.

[0083] Five rare earth recovery tests were conducted to evaluate the influence of variability in feed solution composition (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 precipitation (%)

Phosphoric Acid Regeneration

[0084] A test was carried out on the continuous phosphoric acid regeneration circuit (Test 40). The circuit featured a single 55-minute residence time precipitation tank (Tank 1) operated at 44°C, followed by a 78-minute stabilization tank (Tank 2) and batch vacuum filtration, which was supported by tanks supply with backup service filter and was operated with countercurrent washing. The recovery solution was fed into the precipitation tank along with the acid mixture (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 global precipitation

Acid Cooking and Aqueous Leaching

[0085] Aqueous leaching and acid cooking tests were conducted by contacting a measured amount of sulfuric acid with a measured amount of pre-leaching residue with intense mixing on a suitable plate. The resulting mixture was placed in an oven and heated to 250°C for a period of up to 50 minutes, then held at 250°C for a period of 30 minutes, removed from the oven and allowed to cool. The cold sulfated material was then added to a measured amount of 5°C DI water, stirred for 10 minutes, then vacuum filtered. The filter cake was then washed carefully with DI water to remove the entrained solution before drying.

[0086] Six aqueous leaching and acid cooking tests (Tests 41 to 46) were carried out to evaluate the influence of variability in the composition of the feed pre-leaching residue (Table 30) on the performance of aqueous leaching and acid cooking. Each test was conducted using an acid-to-residue contact ratio of 1600 kg of H2SO4 per ton of leach residue and 2.5 g of Dl per gram of pre-leach residue. The results are summarized in Tables 31 to 33. Table 30: Composition of pre-leaching feed residue (% w/w or g/t) Table 31: Composition of the aqueous leaching solution (g/L or mg/L) Table 32: Composition of aqueous leaching residue (% w/w) Table 33: Extent of dissolution (%) [1] behavior of sulfuric acid for aqueous leaching residue for S

Dissolving rare earth sulfate

[0087] Rare earth sulfate dissolution tests are typically conducted by contacting a measured amount of dry rare earth sulfate precipitate with a measured amount of Dl water with thorough mixing in a suitable container with a well-shaken baffle, at 40°C for a period of 120 minutes, then vacuum filtered. The filter cake was then washed carefully with DI water to remove the entrained solution before drying.

[0088] Five rare earth sulfate dissolution tests (Tests 47 to 51) were conducted to evaluate the influence of variability in feed composition (Table 34) on dissolution performance. Each test was conducted using a water to feed solids contact ratio of 13 grams of Dl 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: 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

[0089] The rare earth purification tests (Tests 52 and 53) were conducted by treating with magnesium, to an endpoint of pH 5, a measured amount of impure rare earth sulfate solution at 40°C in a container suitable with well-shaken baffle with online pH measurement. The resulting mixture was mixed for a period of 30 minutes after adding magnesium to allow it to stabilize, then vacuum filtered. The filter cake was then washed carefully with DI water to remove the entrained solution before drying.

[0090] Two rare earth purification tests were conducted to evaluate the influence of variability in the magnesium source (Table 38) on the performance of rare earth purification 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 added per ton of feed solution. There was 97.3% magnesium utilization in Test 52 and 91.1% magnesium utilization in Test 53. Table 38: Magnesium 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 purified rare earth sulfate solution (g/L or mg/L) Table 42: Extent of precipitation (%)

RE Hydroxide Precipitation The influence of the reagent

[0091] The rare earth hydroxide precipitation tests (Tests 54 to 56) were conducted by contacting a measured amount of magnesium or sodium hydroxide with a measured amount of purified rare earth sulfate solution at 55°C in a suitable container with well-shaken baffle. The resulting mixture was mixed for a period of 30 minutes after each dose of reagent addition to allow it to stabilize, then the subsample was collected and vacuum filtered. The filter cake subsample was then washed thoroughly with DI water to remove entrained solution before drying. For each test, a range of subsamples was collected to cover a range of reagent doses. After the final precipitation subsample, a measured amount of hydrogen peroxide was added to the remaining emulsion. The resulting mixture was mixed for a period of 30 minutes to allow it to settle and then vacuum filtered.

[0092] Two of the rare earth hydroxide precipitation tests (Test 54 and 55) were performed to evaluate the influence of variability in the magnesium 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 magnesium results in significantly increased impurities in the resulting precipitate relative to precipitation using sodium hydroxide. Additionally, increased dose rates result in reduced overall sulfate transport to the rare earth hydroxide precipitate.Table 43: Magnesium composition (% w/w or g/t) Table 44: Feed solution composition (g/L or mg/L) Table 45: Reagent Addition[1] [1] Subsamples A to D correspond to periods of addition of magnesium (Tests 54 and 55) or sodium hydroxide, where the dosage is in units per kg of magnesium or NaOH per ton. feed. Subsample E corresponds to a period of hydrogen peroxide addition 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 production of rare earth hydroxide

[0093] A rare earth hydroxide precipitation test (Test 57) was conducted by contacting a measured amount of sodium hydroxide (97% of stoichiometry) with a measured amount of purified rare earth sulfate solution (Table 48) at 55°C in a suitable container with a well-shaken baffle. The resulting mixture was mixed for a period of 30 minutes to allow it to stabilize, 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 settle and then vacuum filtered.

[0094] For the second stage, the unwashed cake from the first stage was repulped in DI water along with a measured amount of sodium hydroxide (the 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.

[0095] The results are summarized in Tables 49 to 50. Table 48: Composition of the feed solution (g/L or mg/L) Table 49: Precipitate composition (% w/w or g/t) Table 50: Extent of precipitation (Stage 1) or dissolution (Stage 2) (%)

Selective Dissolution The influence of rare earth concentration

[0096] A series of four two-stage rare earth hydroxide dissolution tests (Tests 58 to 61) were conducted at 70°C in a suitable container with a well-shaken baffle with online pH measurement. Each test begins by simulating the second stage by dosing a measured portion of 10% w/w hydrochloric acid to a measured portion of rare earth hydroxide cake (Table 51) that has been repulped in DI water (Test 58) or solution. of rare earth chloride (tests 59 to 61), followed by a 30 minute stabilization period. Typically this results in an emulsion pH of between about 1.2 to 2.2. Each test is completed with the second stage, adding a measured amount of rare earth hydroxide cake (typically 1.5 times that used to start the test), observing a 30 minute stabilization period, and then adding a portion measure of 10% w/w hydrochloric acid, followed by a 30 minute stabilization period. Typically this results in an emulsion pH of between about 3 to 4. The test is then completed with vacuum filtration, followed by washing the cake.

[0097] Tests 59 to 61 were initiated using a rare earth chloride solution from test 58, with 2.8, 5.3 and 8.4 g of 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 global dissolution

Two-stage selective dissolution

[0098] A two-stage rare earth hydroxide dissolution test (Test 62) was conducted at 70°C in a suitable container with a well-shaken baffle with online pH measurement. The test was initiated by repulping 60% of the rare earth hydroxide cake (Table 54) in deionized water followed by the controlled addition of 10% w/w hydrochloric acid to online pH targets of pH 2 (sample 1), then pH 1 (sample 2). At pH 1, all rare earth hydroxide precipitate was dissolved. The remaining 40% of the rare earth hydroxide cake was then added and the mixture allowed to stabilize under stirring for 30 minutes (sample 3). Controlled acid addition was then resumed 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 emulsion was left under stirring for another 50 minutes before collecting the final sample (sample 7). After each period of adding acid and obtaining an online target pH, the resulting mixture was left under mixing for at least 15 minutes before sample collection. The subsamples were vacuum filtered, followed by washing the cake.

[0099] The results are summarized in Tables 55 to 57. From the results, despite completely dissolving all of the cerium in rare earth hydroxide by sample 2, the addition of additional rare earth hydroxide led to re-precipitation so that, for sample 4 (which aims at pH 3, obtained pH 3.2 after stabilization), the cerium concentration in the solution was below the detection limit. This suggests that the 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 solids composition (% w/w or g/t) Table 57: Extent of global dissolution (%)

[00100] The person skilled in the art will understand that there may be variations and modifications beyond 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 referred to or indicated in this application, individually or collectively, and any and all combinations of any two or more of the steps or features.

Claims (38)

1. Method for precipitation of rare earth sulfate, characterized in that it includes subjecting a crude solution of rare earth sulfate to precipitation in the presence of a water-soluble volatile organic compound to produce a precipitate of rare earth sulfate and an acidic supernatant. 2. Precipitation method according to claim 1, characterized by the fact that the volatile, water-soluble organic compound is selected from the group consisting of methanol, ethanol, isopropanol, tert-butanol, acetone or their mixtures. 3. Precipitation method according to claim 2, characterized by the fact that the water-soluble, volatile organic compound is methanol. 4. Precipitation method according to claim 3, characterized in that methanol is used in precipitation in a weight ratio between 0.25:1 and 1.5:1 and preferably between 0.5:1 and 1 .25:1 with the sulfate solution. 5. Precipitation method according to any one of claims 1 to 4, characterized by the fact that before precipitation, the sulfate solution has a free sulfuric acid concentration above 5% w/w. 6. Precipitation method according to any one of claims 1 to 5, characterized in that precipitation occurs at a temperature in the range of 30 to 85°C, or in the range of 40 to 70°C, or in the range of 60 to 65°C with residence time in the range of 5 to 120 minutes or in the range of 10 to 40 minutes. 7. Precipitation method according to any one of claims 1 to 6, characterized in that the organic compound is recovered from the acidic supernatant by distillation, resulting in a recovered organic compound and a diluted mixed acid solution, with the organic compound recovered being recycled for use in precipitation. 8. Precipitation method according to any one of claims 1 to 7, characterized in that the dilute mixed acid solution is concentrated by evaporation to produce a concentrated mixed acid solution. 9. Precipitation method according to any one of claims 1 to 8, characterized by the fact that it includes producing the crude rare earth sulfate solution by pre-leaching a rare earth rich calcium phosphate concentrate with phosphoric acid to form a pre-leach residue enriched in rare earths, mixing the pre-leach residue with sulfuric acid and subsequently heating the mixture to convert the rare earths in the pre-leach residue into water-soluble rare earth sulfates and subjecting to heat treated mixture to an aqueous leach to bring into solution the rare earth sulfates to form the crude rare earth sulfate solution. 10. Precipitation method according to claim 9, characterized by the fact that it includes removing calcium phosphate from the rare earth-rich calcium phosphate concentrate by pre-leaching with phosphoric acid to form the enriched pre-leaching residue in rare earths and a pre-leaching solution containing monocalcium phosphate, impurities and small amounts of rare earths. 11. Precipitation method according to claim 10, characterized by the fact that the leaching is operated in a two-stage countercurrent configuration at a temperature between 30 and 45 ° C, with residence times between 30 and 90 minutes for each stage and with a mass ratio of acid feed to feed concentrate maintained between 2 and 12 grams of P in the acid per gram of Ca in the concentrate and ideally between 4 and 8 grams of P in the acid per gram of Ca in the concentrate . 12. The precipitation method of claim 10 or 11, wherein heat is applied to the pre-leaching solution to precipitate small amounts of rare earths such as rare earth phosphates from the solution, leaving a recovery solution. 13. Precipitation method according to claim 12, characterized by the fact that heating occurs in several stages of increasing temperatures, with all temperatures being in the range of 60°C to 110°C, with stage residence times between 60 and 180 minutes. 14. Precipitation method according to any one of claims 9 to 13, when appended to claim 7 and/or claim 8, characterized by the fact that sulfuric acid and the diluted and/or concentrated mixed acid solution are added to the recovery solution to convert the monocalcium phosphate to phosphoric acid and form a calcium sulfate precipitate. 15. Precipitation method according to claim 14, characterized in that the acid addition is carried out in stages at a temperature between 30 and 50 ° C and a residence time between 30 and 90 minutes per stage. 16. Precipitation method according to claim 14 or 15, characterized in that the phosphoric acid formed from the recovery solution is used as necessary by the pre-leaching as defined in claim 10. 17. Precipitation method according to any one of claims 9 to 16, characterized by the fact that it includes mixing the pre-leaching residue and, if present, the rare earth phosphates as defined in claim 12, with sulfuric acid, heating the mixture to a temperature in the range of 210 to 270°C, then holding the material at this temperature for a period of time of up to about 60 minutes to convert the rare earths into the pre-leach residue and the rare earth phosphates in water-soluble rare earth sulfates. 18. The precipitation method of claim 17, wherein mixing occurs for a time of up to about 30 minutes, either where there is no temperature control in the mixing stage, with the mixed material heated between mixing and sulfation retention periods, or where the sulfuric acid is preheated prior to addition so that the acid mixture operates within the temperature range as defined in claim 17. 19. The precipitation method of claim 17 or 18, wherein the water-soluble rare earth sulfates are cooled to a lower temperature of about 50°C over a period of time of up to about 300 minutes . 20. Precipitation method according to any one of claims 17 to 19, characterized in that the water-soluble rare earth sulfates are subjected to an aqueous leaching to bring into solution the rare earth sulfate, phosphoric acid and the acid remaining sulfuric acid and thus to form the crude rare earth sulfate solution and an aqueous leaching residue containing the insoluble gangue material for disposal. 21. The precipitation method of claim 20, wherein the aqueous leaching is conducted so as to minimize the leaching of impurities from the gangue material and to maximize the concentration of rare earths in the raw rare earth sulfate solution . 22. The precipitation method of claim 21, wherein the aqueous leaching is conducted at a temperature of less than about 35°C and for a period of time of less than about 20 minutes. 23. Method of processing rare earth sulfate precipitate produced by the method as defined in any one of claims 1 to 22, characterized in that it includes washing and drying the rare earth sulfate precipitate, aqueous leaching of the sulfate precipitate washed and dried rare earth sulfate to dissolve the soluble rare earth sulfate and form a leaching solution rich in rare earth sulfate and a leaching residue containing the impurities in the form of insoluble phosphates, precipitation of impurities from the sulfate leaching solution of rare earths to form a purified rare earth sulfate solution and a purification residue, precipitation of the rare earths in the purified rare earth sulfate solution as rare earth hydroxide and selective leaching of the rare earth hydroxide precipitate to form a solution of rare earth chloride and a residue. 24. The processing method of claim 23, wherein washing of the rare earth sulfate precipitate is conducted using a volatile, water-soluble organic compound. 25. Processing method according to claim 23 or 24, characterized in that the dry rare earth sulfate precipitate is leached with water at about 40 ° C for a period of 60 to 180 minutes, leaving a solution of rare earth sulfate mixed with impurities and a leaching residue containing impurities in the form of insoluble phosphates. 26. Processing method according to any one of claims 23 to 25, characterized in that magnesium is added to the rare earth sulfate leaching solution to purify this solution by precipitation of impurities, leaving the rare earth sulfate solution rare purified and purification residue. 27. Processing method according to claim 26, characterized in that the magnesium is added in several stages, each at a temperature of up to about 55 ° C and for a time of about 30 to 120 minutes, with the pH of the purified rare earth sulfate solution being about 4.5 to 6.0. 28. Processing method according to any one of claims 23 to 27, characterized in that the precipitation of the rare earths in the purified rare earth sulfate solution as rare earth hydroxide and the subsequent selective leaching occurs in the steps of: a ) contacting the rare earth sulfate solution with sodium hydroxide to precipitate the 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. 29. Processing method according to claim 28, characterized in that precipitation occurs in a two-stage countercurrent process, including a precipitation stage and a refining stage, in which the rare earth sulfate solution is fed to the precipitation stage with a spent solution from the refining stage, to precipitate sulfate containing rare earth hydroxide. 30. Processing method according to claim 29, 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. 31. Processing method according to claim 29 or 30, characterized by the fact that the oxidant is hydrogen peroxide and/or sodium hypochlorite, which is added to the precipitation and/or refining steps after the addition of sodium hydroxide . 32. Processing method according to any one of claims 29 to 31, characterized in that both the precipitation and refining phases operate at a temperature of about 40 to 80 ° C and for a time of about 30 to 60 minutes, with the stoichiometry of the addition of sodium hydroxide being in the range between 100 to 110% and with 100 to 130% of the stoichiometric dosage of oxidant. 33. Processing method according to any one of claims 28 to 32, characterized in that the rare earth chloride solution contains negligible cerium and the residue consists mainly of cerium (IV) hydroxide. 34. Processing method according to any one of claims 28 to 33, characterized in that the selective leaching of the rare earth hydroxide precipitate is conducted in two stages, each stage having multiple tanks and each stage using hydrochloric acid diluted between 5 to 15% w/w with the leaching solution from the first leaching stage. 35. Processing method according to claim 34, characterized in that the leaching solution from the second leaching stage is used to repulp, leach the rare earth hydroxide precipitate and precipitate the cerium (IV) dissolved in the second stage of leaching, before the first stage of leaching. 36. Processing method according to claim 34 or 35, characterized by the fact that selective leaching occurs at a temperature between 60 to 80 ° C, the first leaching stage is operated to maximize the dissolution of rare earths while minimizes 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 non-cerium rare earth elements in the residue. 37. Method according to any one of claims 28 to 36, 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 co - precipitation with barium sulfate to form a purified rare earth chloride solution. 38. The method of claim 37, wherein the purified rare earth chloride solution is concentrated by evaporation.