WO2023240343A1 - A process and system for extracting rare earth elements using high pulp density cracking - Google Patents

Abstract

The present application provides a process and system for extraction of rare earth elements from a solid feed that employs a high solid to liquid pulp density during cracking. In particular, the present application provides a high pulp density cracking water leaching process for extracting rare earth elements, which comprises: combining a ground solid feed with a strong acid and water to form a slurry or mud having a solid to liquid pulp density of at least 33%; cracking the slurry or mud in a reactor, with or without agitation; and leaching the slurry with an aqueous leaching solution to obtain a leachate comprising the rare earth elements. Also provided is a system for extraction of rare earth elements, which includes a grinding and/or milling device, a cracking reactor and a leaching device configured to perform the high pulp density cracking water leaching process.

Description

A PROCESS AND SYSTEM FOR EXTRACTING RARE EARTH ELEMENTS USING HIGH PULP DENSITY CRACKING

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Patent Application No. 63/351,510, filed on June 13, 2022, the contents of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present application pertains to the field of rare earth recovery. More particularly, the present application relates to a process and system for recovery of rare earth elements from solid feeds using a high pulp density cracking step.

INTRODUCTION

[0003] Rare earth elements (REE) are a group of 17 elements that play an important role in modern society, with many high-tech and clean energy applications, such as in permanent magnets for wind turbines, smart phone components, and rechargeable batteries for electric vehicles (Sadri, Nazari, & Ghahreman, 2017). The group of 17 elements comprises scandium and yttrium in addition to the 15 lanthanides (lanthanum to lutetium).

[0004] There are two main components to the rare earth (RE) production industry: first, the extraction of REE from ore, and, second, the separation of REE mixtures into individual compounds. For extraction of REE from ore the two main commercial methods involve either acid baking or caustic conversion (Demol & Senanayake, 2018). Since REE generally occur together in minerals, the extraction step yields a mixture of rare earths, and purifying these into individual element compounds is a complicated and costly operation, due to the similarity in their chemical properties.

[0005] Currently, industry uses a multi-step extraction process that follows the general formula: decompose

leach

remove impurities convert to useful or marketable products, such as rare earth chlorides or rare earth oxides. Decomposition of the rare earth ore can be achieved using various methods that fall under the two categories mentioned previously (i.e., acid baking or caustic conversion). One such method uses a combination of sulfuric acid baking and water leaching. Sulfuric acid baking is employed commercially by the world's largest producer of rare earths at Bayan Obo in China, where they process mixed bastnaesite/monazite concentrate (Demol & Senanayake, 2018). During baking, the acid reacts with the ore to produce rare earth sulfates and monazite, the two minerals in which the majority of the world's rare earths are found (Qi, 2018, pp. 22, 56). In addition, impurities such as thorium, calcium and iron are also converted to their sulfates.

[0006] Once decomposition by acid baking is complete, the sulfates are dissolved into the leachate and the remaining solid, which contains silica, zircon, and other undigested ore residues, is filtered off. The acid baking process is referred to herein as the "acid baking water leaching" or ABWL process.

[0007] There remains a need for an efficient REE extraction process that reduces capital and operating costs while maintaining or enhancing metallurgical performance.

[0008] The above information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.

SUMMARY OF THE INVENTION

[0009] An object of the present application is to provide a process and system for extracting rare earth elements from a solid feed using high pulp density cracking.

[0010] In accordance with an aspect of the present application there is provided a process for extracting rare earth elements from a solid feed, said process comprising: (a) grinding the solid feed; (b) combining the ground solid feed with a strong acid and water to form a slurry or a mud having a solid to liquid pulp density of at least 33% and cracking the slurry or mud in a reactor at a temperature of from about 30°C to about 300°C for a period of time from about 30 minutes to about 6 hours; and (c) leaching the product of step (b) with an aqueous leaching solution to obtain a leachate comprising the rare earth elements. In some embodiments in step (b) the cracking of the slurry or mud is a static step, and in other embodiments in step (b) during cracking the slurry or mud is agitated in the reactor.

[0011] In accordance with an aspect of the present application, there is provided a process for extracting rare earth elements from a solid feed said process comprising: (a) grinding the solid feed; (b) combining the ground solid feed with a strong acid and water to form a slurry having a solid to liquid pulp density of at least 33% and agitating the slurry in a reactor at a temperature of from about 30°C to about 300°C for a period of time from about 30 minutes to about 6 hours; and (c) leaching the slurry with an aqueous leaching solution to obtain a leachate comprising the rare earth elements.

[0012] In accordance with one embodiment, the strong acid HNO3, HCI or H2SO4.

[0013] In accordance with some embodiments, the cracking step (b) in the process is performed using a solid to liquid pulp density of from about 33% to about 90%, or from about 50% to about 80%, or from about 65% to about 80%, or from about 55% to about 75%, or from about 70% to about 75%, at a temperature (or over range of temperatures) between about 30°C and about 200°C, or between about 30°C and about 100°C.

[0014] In some embodiments the aqueous leaching solution is water. Alternatively, the aqueous leaching solution can be an REE-barren acidic solution, such as may be obtained from other processes. In some embodiments, the leaching step is performed for a period time from about 1 hour to about 10 hours, or from about 1 hour to about 5 hours, or from about 2 hours to about 5 hours, or about 2.5 hours. In some embodiments, leaching is performed at a temperature (or over a range of temperatures) of from about 20°C to about 100°C, from about 20°C to about 90°C, from about 20°C to about 50°C.

[0015] In accordance with another aspect of the present application, there is provided a system for performing the above process for extracting rare earth elements from a solid feed. The system comprises: (i) a grinding and/or milling device for grinding the solid feed to produce a ground solid feed; (ii) a cracking reactor having a ground solid feed ore inlet fluidly connected to the grinding and/or milling device, a strong acid inlet, a water inlet, and a mixed slurry outlet, wherein the ground solid feed is received within the cracking reactor and mixed with a strong acid and water to form a slurry having a solid to liquid pulp density of at least 33% by means of an agitator arranged in the reactor; (iii) means for heating the reactor to a temperature of from about 70°C to about 300°C for a period of time from about 30 minutes to about 6 hours; and (iv) a leaching device comprising a leaching chamber with mixed slurry inlet fluidly connected to the mixed slurry outlet of the reactor, an aqueous leaching solution inlet and a leached slurry outlet.

[0016] In some embodiments, the system additionally comprises means for heating the leaching chamber. Optionally, the system additionally comprises control means for controlling one or both of the means for heating the cracking reactor and the means for heating the leaching chamber.

BRIEF DESCRIPTION OF THE FIGURES

[0017] For a better understanding of the application as described herein, as well as other aspects and further features thereof, reference is made to the following description which is to be used in conjunction with the accompanying drawings, where:

[0018] Figure 1A is a flow chart of a high pulp density cracking water leaching process according to one embodiment of the present application, and Figure IB is a flow chart of a high pulp density cracking water leaching process according to another embodiment of the present application;

[0019] Figure 2 is a schematic of a high pulp density cracking leaching process according to one embodiment of the invention that additionally includes steps downstream of leaching for further purification and treatment of the obtained REE;

[0020] Figure 3 is a graph showing the sample particle size curve for Search Mineral Whole Ore B (SMWB) samples; and

[0021] Figure 4 is a graph showing the grinding curve for ground SMWB samples. DETAILED DESCRIPTION

[0022] Definitions

[0023] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[0024] As used in the specification and claims, the singular forms "a", "an" and "the" include plural references unless the context clearly dictates otherwise.

[0025] The terms "comprising" and "including" as used herein will be understood to mean that the element or list following is non-exhaustive and may or may not include any other additional suitable items, for example one or more further feature(s), element(s), component(s) and/or ingredient(s) as appropriate.

[0026] Reference throughout this specification to "one embodiment," "an embodiment," "another embodiment," "a particular embodiment," "a related embodiment," "a certain embodiment," "an additional embodiment," or "a further embodiment" or combinations thereof means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the foregoing phrases in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0027] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as temperature and reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about." As used herein, the term "about" refers to variation in the numerical quantity that can occur, for example, through typical measuring and liquid or solid handling procedures; through inadvertent or systemic error in these procedures; through differences in the source, manufacture, or purity of ingredients used to make compositions or carry out the processes; and the like. The term "about" also encompasses amounts that differ due to different equilibrium conditions for a composition resulting from a particular initial mixture or environment.

[0028] The following acronyms are used herein:

REE: Rare earth elements, which include: lanthanides, scandium and yttrium

LREE: Light rare earth elements, which include:

Lanthanum (La)

Cerium (Ce)

Praseodymium (Pr)

Neodymium (Nd)

Promethium (Pm)

Samarium (Sm)

Europium (Eu) and

Gadolinium (Gd)

Scandium (Sc) is generally included in LREE due to his similar chemical behavior

HREE: Heavy rare earth elements, which include:

Terbium (Tb)

Dysprosium (Dy)

Holmium (Ho)

Erbium (Er)

Thulium (Tm)

Ytterbium (Yb)

Lutetium (Lu)

Yttrium (Y) is generally included in LREE due to his similar chemical behavior

TREE Total rare earth elements

[0029] The present application provides a process and system for REE extraction from a solid feed, in which cracking is performed using a high pulp density at elevated temperature. This is referred to herein as the "high pulp density cracking water leaching" or "HPDCWL" process. As used herein, the term "solid feed" refers to an REE-bearing solid, typically a concentrate or an ore. A solid feed can be, for example, an REE-bearing concentrate, ore or ore concentrate, however, it should be recognized that the presently described process can be employed using any REE-bearing solid as the solid feed.

[0030] The HPDCWL process comprises the steps of: grinding the solid feed; mixing the ground solid feed with a strong acid and water to form a slurry or mud having a high solid pulp density; cracking the slurry or mud; and leaching the cracked slurry or mud following dilution with an aqueous leaching solution to obtain a pregnant leachate comprising the rare earth elements. An embodiment of the overall process is shown in Figure 1A, where the cracking can be performed with or without agitation. Each step of the process is described below.

[0031] Step-1: Grinding

[0032] The solid feed is initially processed by crushing and grinding. Given sufficient time, the attacking chemical used for cracking can eventually penetrate many large particles to maximize the recovery of REE. However, for HPDCWL process, the process duration must be within a reasonable range. The reaction rate for REE mineral conversion and REE dissolution is an important factor for the success of HPDCWL. Crushing and grinding large particles into finer ones can accelerate the cracking process.

[0033] A standard industrial practice for leaching of REE-bearing solid feed particles is to reduce the size of particles to a degree that facilitates accessibility of the valuable minerals to the attacking chemicals. The optimal grinding target particle size can be obtained using mineralogical approaches that include measuring the particle size of the valuable minerals present in the solid feed. Alternatively, and more commonly, a set of tests are performed with the particle size as the target variable to identify the optimum particle size, or size range.

[0034] In an embodiment of the present disclosure, the particle size, or particle diameter, of the rare earth solid feed is not particularly limited, and may be selected by those skilled in the art according to actual needs of particular circumstances. An appropriate grinding particle size, or size range, for use in the present process is selected to avoid over grinding. In the high pulp density cracking step of this invention, over-grinding may cause the formation of high-viscosity mud, which will make the cracking reaction less efficient. Milling or grinding to a very small particle size also consumes excessive energy and can also result in an increase in co-dissolved impurities.

[0035] On the other hand, it has been found that if the particle size, or size range, is too large, it is not conducive to reaction between the solid feed and the acid. In particular, insufficient grinding resulting in large particles may save some cost, by reduced energy consumption in the grinding step, but will also slow down the reaction, possibly even to the point that completion of the reaction in the cracking step is prevented.

[0036] A balance needs to be maintained in selecting a particle size that is beneficial to the solid-liquid reaction rate without being so small that the costs are high and/or there are too many co-dissolved impurities. As would be appreciated, the exact particle size range used in a particular HPDWL process can be optimized by the skilled person, based on the requirements of each particular situation.

[0037] In an embodiment of the present HPDCWL process, the typical range of particle sizes is within the range of about 10 microns to about 4000 microns. In some embodiments the range of particle sizes is from about 10 microns to about 1000 microns, or from about 20 microns to about 200 microns. Again, the optimum particle size(s) within this range can be selected for different situations, by considering various criteria, including, but not limited to minerals characteristics, operation cost, metallurgy performance, reaction rates for various species.

[0038] Step-2: Acid Cracking

[0039] The next step of the HPDCWL process comprises mixing the ground solid feed with a strong acid and water to produce a slurry, or mud, which is then heated, with or without agitation, for a period of time. The amount of acid and the amount of water used in this step is kept at a minimum level, but one that can be employed without causing significant material handling or mixing difficulties. [0040] For example, in the processing of a Canadian ore, the crushed and ground ore is mixed with low amount of the strong acid, and the resulting paste is then diluted by adding a small amount of water to form a slurry, or mud, having high pulp density.

[0041] The term "slurry" as used herein refers to a flowable suspension of small particles in liquid. The term "mud" is used herein to refer to a mixture of water, or an aqueous solution, and small or fine particles. A mud will have a higher viscosity than a slurry.

[0042] In the HPDCWL process, the high pulp density mixture formed by combining the ground solid feed with the strong acid and water (or aqueous solution) will initially be either a slurry or a mud, depending on the amount of acid and water added versus the amount of solid. However, it has been observed that the chemical reaction between the added acid and silicate minerals found in the ground feed, at an elevated temperature, can quickly elevate the viscosity of the slurry such that the mixture becomes a mud or behaves similarly to a mud. This can occur during the initial stages of the cracking step. The pulp density is determined by calculation based on the mass of solid and liquid in the mixture. However, because the water and acid is quickly adsorbed and reacted with the solid ore particles upon addition, the calculated pulp density does not reflect the actual state of the mixture.

Consequently, the calculated pulp density can be considered to be a "nominal" pulp density, as referred to herein.

[0043] In some embodiments of the HPDCWL process, the conversion of the slurry to mud is avoided, for example, by diluting the mud with sufficient additional water, or aqueous solution, to allow the mixture to maintain or regain the properties of a slurry. In this embodiment, care is taken to avoid adding too much water, or aqueous solution; if the mixture is diluted too much, then the pulp density and the acidity of the slurry will be reduced to a point that is detrimental to the reaction rate.

[0044] Whether or not the high pulp density mixture will be a slurry or a mud, or will transition between the two, will also largely depend on the mineralogy response in the cracking conditions.

[0045] In general, during the acid cracking step of the present HPDCWL process: (i) high pulp density is maintained; (ii) the amount of acid and water, or aqueous solution, is selected to optimize general metallurgical performance; and, optionally, (iii) the viscosity of the mixture is maintained in a range to allow agitation of the slurry or mud. Item (iii) is not satisfied when a sticky mud forms that sticks to the reactor and such that it hardens and the becomes static, which is not desirable for an effective or efficient cracking reaction when agitation is used during cracking. However, as shown herein, agitation is not necessary for effective or efficient cracking.

[0046] The acid used in the cracking step is a strong acid, such as, sulfuric acid (H2SO4), nitric acid (HNO3) or hydrochloric acid (HCI). In some embodiments, a mixture of two or more strong acids is used in this step.

[0047] The acid can be added in its most concentrated form, or as a diluted acid. The final concentration of the acid in the cracking step will depend on the acid used, however, in some embodiments the concentration can range from 100% to about 50%. The actual minimum concentration of acid used in this step is dependent on optimized nominal pulp density and optimum acidity in both cracking and leaching steps of HPDCWL. Conventional ABWL requires a very pure and concentrated acid to operate properly. In contrast, in HPDCWL, the presence of water in the acid is not a problem, and has also been found to be useful in providing improved metallurgical performance.

[0048] As used herein, the term "high pulp density" is intended to refer to a solid pulp density in the slurry, or mud, of at least about 33%. In some embodiments, the process is performed using a solid pulp density of from about 33% to about 90%, or from about 50% to about 80%, or from about 65% to about 80% or from about 55% to about 75%, or from about 70% to about 75%. In a conventional leaching operation, the pulp density is at 20% and mostly 10% in REE leaching.

[0049] The high pulp density slurry/mud is heated to a temperature between about 30°C and about 300°C, or between about 30°C and about 200°C, or between about 30°C and about 100°C. In one embodiment, the upper limit of this temperature is the boiling point of the aqueous component of the slurry, or mud, in the reactor. By maintaining the temperature below the boiling point, water loss by evaporation is minimized. In an alternative embodiment, in which a temperature above the boiling point of the aqueous component is employed, the high pulp density slurry is then heated to a temperature between the boiling point of water and 300°C in an autoclave reactor.

[0050] The slurry, or mud, is then allowed to react in the reactor or an autoclave for 30 minutes to 24 hours with or without agitation. For example, the reaction is allowed to proceed for a period from about 30 minutes to about 18 hours, or from about 30 minutes to about 12 hours, or from about 30 minutes to about 6 hours, or from about 30 minutes to about 5 hours, or from about 1 hour to about 4 hours. Agitation is optionally used during this step, to accelerate the reaction. In addition, the use of an agitator during this step helps to maintain homogeneity of the slurry, or mud, and prevent the formation of concentration and temperature gradients. However, agitation during cracking is not necessary.

[0051] During the cracking step, when agitation is used, the ground rare earth solid feed in the slurry and the strong acid can be mixed rapidly and forcibly by agitation such that the rare earth-containing solid feed is sufficiently infiltrated by the strong acid, thus preventing agglomeration from occurring in the subsequent and providing favorable conditions for the cracking reaction.

[0052] In some embodiments, the cracking step is performed without agitation. The static HDPCWL process is referred to herein as an SHDPCWL process, which is a subset of the HDPCWL process. Accordingly, unless otherwise specified, any reference to the HDPCWL process includes the SHDPCWL process. In such static process embodiments, the cracking time is for a period from about 30 minutes to about 18 hours, or from about 30 minutes to about 12 hours, or from about 30 minutes to about 6 hours, or from about 1 hour to about 6 hours, or from about 2 hours to about 6 hours.

[0053] It has now been found that cracking efficiency is improved when the cracked solid residue is drier, rather than wet. This was particularly observed in the static process, where it was found that a longer cracking time resulted in a drier cracked solid residue and an improved REE recovery.

[0054] The process parameters described above and exemplified in the Examples, are designed to maintain the amount of acid in a concentrated form, which enhances the mineral decomposition rate. In some embodiments, increased amounts of acid added are used during cracking, which can increase or maximize the REE value from the subsequent leaching step.

[0055] Step-3: Leaching

[0056] Following the cracking step, the solubilized rare earth elements are removed from the solid feed by leaching, in particular water leaching. Leaching can be performed by, for example, heap leaching, or tank or vat leaching (for example, with stirring). The aqueous leaching solution is water or an REE-barren acidic solution (for example, recycled from other operation steps).

[0057] The leaching step can proceed by:

- adding the aqueous leaching solution to the slurry, or mud, from the cracking step and, optionally, allowing the mixture to cure for a period of time; and

- performing a heap leaching step for a period of time in which the leaching solution is passed through the slurry, or mud, (with or without the above curing step) at a slow flow rate to facilitate long term and slow reactions; or

- performing a stirred leaching step in which the slurry, or mud, from the cracking step (with or without the above curing step) is stirred with the aqueous leaching solution in a tank or vat for a period of time to facilitate fast reactions.

[0058] Initially, water or an aqueous solution, such as an aqueous solution recycled from other operations, is added to the slurry to improve the efficiency of the leach. The slurry, or mud, from Step 2 is slowly diluted to reduce the solid to liquid pulp density to lower than about 50%. A sudden dilution of acidity by adding too much water in a short time period should be avoided to prevent local over-dilution and REE precipitation. In particular, the slurry, or mud, from Step 2 is diluted with a step-wise, or gradual, addition of water to reach the desired pulp density for leaching - rather than immediate, quick dilution by a single, bulk addition of water.

[0059] The solid pulp density in the leaching step, as described herein, is calculated based on the mass of the solid feed introduced in the cracking step and the amount of water or aqueous leaching solution added to the residue from the cracking step. Any change in solid mass that occurs during the cracking step is not accounted for in the calculation of the solid pulp density for the leaching step.

[0060] The higher the solid pulp density, the lower the fresh water needed to be added during the leaching step. Using the present HPDCWL process it is possible to successfully operate at much higher a pulp density during leaching than in a conventional ABWL process. When selecting the appropriate pulp density for leaching, it should be selected to be as high as possible without scarifying the REE recovery.

[0061] In the conventional process, about 10 parts by mass of water (g) is required for every 1 part (g) of solid feed, which corresponds with a solid to liquid pulp density of about 9%. While this pulp density can be used in the present HPDCWL process, reducing the pulp density this low is unnecessary. Rather, the use of a pulp density that is within the above range, but higher than 9%, does not significantly affect the REE recoveries from leaching. The use of higher leaching pulp densities can significantly reduce both the capital and operating costs associated with the overall REE recovery process, in comparison to the conventional process.

[0062] Leaching is performed at a temperature that facilitates or increases leaching efficiency. For example, the water leaching temperature can be an ambient temperature, standard room temperature or a raised temperature. For example, during leaching, the temperature of the slurry can be maintained at a higher level (e.g., from about 70°C to about 100°C), which would require heating. However, such heating is not necessary. Instead, in some embodiments, the temperature is reduced during leaching by simply removing the heating source and allowing the remaining heat from the cracking step to naturally release to the environment, without providing additional heating. In this embodiment, the temperature range during leaching can transition from the temperature used during the cracking step to ambient temperature (which is dependent on environmental conditions). [0063] In other embodiments, the temperature during leaching is maintained in the range of from about 20°C to about 100°C, for example, from about 20°C to about 90°C, from about 20°C to about 50°C, at about 25°C, or at about 50°C.

[0064] In the conventional leaching process, for example as used with ABWL, a higher temperature (e.g., 90°C) is required to be maintained during the leaching process in order to achieve good REE recovery. However, when employing the present HPDCWL process, the leaching temperature can be much lower without negatively impacting REE recovery.

[0065] The conventional process, for example as used with ABWL, also requires a long duration in the leaching step to allow the REE to be fully dissolved (up to 36 hours). In contrast, the leaching step used with the present HPDCWL process can be completed in a much shorter time. Consequently, the leaching step can be performed over a time of from about 1 hour, or from about 2 hours. In some embodiments, the leaching time is from about 1 hour to about 20 hours, or from about 1 hour to about 10 hours, or from about 1 hour to about 5 hours, or from about 2 hours to about 5 hours, or about 2.5 hours or about 3 hours.

[0066] In accordance with other embodiments, the water leaching is performed by a method that comprises washing the slurry, or mud, from the cracking step and collecting the water washes. The temperature of the water used for the water washes is in the range of from about 20°C to about 100°C, for example, from about 20°C to about 90°C, from about 20°C to about 50°C, at about 25°C, or at about 50°C.

[0067] In this embodiment, each wash stage uses water or an REE-barren acidic solution, which can be, for example, a solution recycled from other operation steps. The wash volume can vary largely and depends on the method of washing.

[0068] CCD (Counter Current Decantation) is a classic wash process used in industry for the best wash performance.

[0069] Finally, the extracted REE product from the leaching step can be processed according to standard techniques to purify the REE, as necessary depending on the downstream application. A variety of downstream processing techniques can be applied and are in no way limited by the present HPDWL process. [0070] In a specific, non-limiting example, the REE leaching product is processed using a direct oxalate precipitation, as illustrated in Figure 2. In the direct oxalate precipitation process a precipitate of REE is obtained from the acidic composition produced by the leaching step by adding a reducing agent to the acidic composition, which has a pH of 0.5 to 3 or is adjusted to a pH of 0.5 to 3 using a basic agent, and adding oxalate directly to the composition with the reducing agent. This forms an REE oxalate precipitate in the mixture, which is removed using a solid-liquid separation. The resultant REE oxalate can then be washed and further processed to marketable REE or REE salts. This downstream process is referred to herein as a direct oxalate precipitation process, since the oxalate is added directly to the acidic composition comprising a reducing agent without prior purification or precipitation steps, as required by the conventional REE recovery processes.

[0071] Also provided herein is a system for performing the HPDCWL. The system comprises: a grinding and/or milling device for grinding the rare earth-containing solid feed to produce a ground solid feed (or rare earth ore concentrate); a reactor having a solid feed inlet fluidly connected to the grinding and/or milling device, a strong acid inlet, a water inlet, and a mixed slurry outlet, wherein an agitator is arranged in the reactor; means for heating the reactor; and a leaching device having a leaching chamber with mixed slurry inlet fluidly connected to the mixed slurry outlet of the reactor, an aqueous leaching solution inlet and a leached slurry outlet.

[0072] Optionally, the system additionally includes means for controlling various aspects of the HPDCWL process, such as the heating of the cracking reactor and/or the leaching chamber. In addition, in some embodiments it can be useful to monitor the slurry during the cracking reaction and, depending on the state of the slurry, adding additional water or aqueous solution to the cracking reaction via the water inlet. This can be performed manually or automatically through incorporation of additional control components (e.g., computerized sensor and control components) in the system.

[0073] The components of the system for performing the HPDCWL process can be standard components similar to those used in the conventional ABWL process. Typically, the cracking reactor is an open-air reactor having some form of cap or cover to minimize moisture loss during the cracking reaction. The reactor can include any means for efficient agitation, as are well known in the field. However, it should be appreciated that successful mixing of a sticky, high-temperature, low liquid ratio, high viscosity, high acidity slurry as generated in the HPDCWL process requires reasonably strong agitation to avoid the slurry forming a paste or cement-like mixture before solidifying. Further, the stirring/agitation means should be selected to avoid build-up of any solidified slurry along the reactor wall. One non-limiting example of such a stirred or agitation reactor is a cement truck-type mixing equipment that can resist high temperature and high acidity.

[0074] The agitator in the reactor is configured to be turned on and off as necessary. For example, in the SHDPCWL process the agitator is used only to mix the components of the initial slurry or mud to be cracked, and then turned off to allow the cracking to continue under static conditions.

[0075] An example of a system suitable for performing the present HPDCWL process is schematically depicted in Figure 2. It should be appreciated that components of the system can be varied, for example, based on site requirements, availability and/or cost considerations.

[0076] To gain a better understanding of the invention described herein, the following examples are set forth. It should be understood that these examples are for illustrative purposes only. Therefore, they should not limit the scope of this invention in any way.

EXAMPLES

[0077] EXAMPLE 1: Variation in high pulp density cracking water leaching process conditions, with agitation

[0078] The sample used in this study was a crushed whole ore sample as the solid feed, at a particle size of 6 mesh. The ore had a grade of 1.05% TREE, with the rare earths hosted mostly in allanite and fergusonite.

[0079] Figure IB provides a conceptual flowsheet of a high pulp density cracking water leaching process according to the embodiment studied in the present Example. For this HPDCWL study, all feed samples were ground to a Pso ( i.e., the particle size at which 80% of the material will pass when screened) of 121 microns. The ground solid feed was then mixed in a smaller reactor (ore cracking reactor) with acid and a small amount of water, often resulting in a sticky paste mixture. The agitation of this paste was provided by stirrers designed to maximize the agitation effect. The ore cracking reactor was placed in a heating kettle, which provided a stable temperature to the paste in the reactor. The cracking duration was 4 hours. Afterwards, the "cracked" sample, as a paste, was transferred into a larger reactor (leaching reactor) and deionized (DI) water was added to lower the pulp density (leaching pulp density). The pulp was stirred using a regular stirrer at 250 rpm and leached for 2.5 hours. After leaching, the pulp was filtered and washed to obtain a solid and a liquid sample for chemical analysis.

[0080] The cracking tests were designed to ensure efficient and sufficient agitation was provided to the high pulp density paste. Some standard stirrers for leaching reactions were tested in the first few trials, but without success. A 3-D printed stirred with a diameter close to the reactor size was found to provide a much better agitation to the paste-like pulp. However, other stirrers can be used.

[0081] Results

[0082] During this study certain conditions were varied to maximize the recoveries from the HPDCWL process, i.e., a ground feed; high temperature (90°C), high pulp density, high acidity (but under the same acid dosage) in the cracking process. The findings from these are discussed below.

[0083] Effect of grinding

[0084] In the first set of studies, the effect of a 4-minute grinding (Pso 121 micron) was compared to a test without grinding. As shown in Table 1, the 4-hour cracking and 2.5-hour leaching test without grinding (baseline) recovered 64.6% TREE. The same test on the ground sample successfully elevated the TREE recovery to 72.1%. The LREE, HREE, Dy, Nd recoveries in the 4-minute grind test were all higher than the baseline test. Table 1: Effect of grinding on high pulp density cracking water leaching process

Percentages are by weight unless otherwise notec .

[0085] Effect of leaching temperature

[0086] A comparison was performed between two tests using different leaching temperatures (25°C and 90°C). As shown in Table 2, the final TREE recovery remained unchanged, indicating that an elevated leaching temperature is not necessary for the high pulp density cracking water leaching process.

Table 2: Effect of leaching temperature on high pulp density cracking water leaching process

[0087] Effect of cracking temperature

[0088] When a lower temperature was tested in the cracking stage (rather than in the leaching stage), the metal recovery obtained was found to be very different. As shown in Table 3, the cracking process was very effective at 90°C, resulting in 72.7% TREE recovery.

However, at the lower temperature, 50°C, the cracking leaching only recovered 51% of TREE.

Table 3: Effect ofcracking temperature on high pulp density cracking water leaching

[0089] Effect of foreign ion addition

[0090] The additions of two foreign ions at the beginning of the cracking stage were examined. As shown in Table 4, the addition of Mg and Mn ions did not provide any significant improvement on the TREE, Dy and Nd recoveries.

Table 4: Effect of two foreign ion additions in cracking stage on high pulp density cracking water leaching

* based on the sulphate salts [0091] Effect of cracking pulp density

[0092] To investigate the effect of cracking pulp density, three larger batch tests were all conducted using samples of 300 grams. The nominal pulp density in the cracking stage does not appear to have a significant effect on the TREE recovery (Table 5). The TREE recoveries were 72.3%, 72.4% and 70.1% when the pulp density was set at 58%, 71%, and 75% respectively.

Table 5: Effect of nominal pulp density in cracking stage on high pulp density cracking water leaching

[0093] Effect of acid type

[0094] Finally, the use of different types of acid in the high pulp density cracking water leaching process was compared in three 300 g batch tests. Using the same H⁺ concentration provided from each acid dosage, the use of HNO3 and HCI appeared to recover more TREE than H2SO4 (Table 6). Using H2SO4, the process leached 72.4% TREE. Using HCI, 75.5% TREE was recovered. Using HNO3 pushed the recovery of TREE up to 79.9%. Table 6: Effect of acid type on high pulp density cracking water leaching

[0095] Effect of leaching pulp density

[0096] The 100 g batch size tests (Tables 1 - 4) conducted in this study were leached at a pulp density of 9.1%; the 300 g batch size tests (Tables 5 and 6) were leached at 20% pulp density. Due to the difference in the batch size of the tests, a direct comparison of the results is inappropriate. However, given that the REE recoveries were not obviously affected at the higher leaching pulp density in the 300 g tests, the results demonstrate the feasibility of increasing the leaching pulp density in the HPDCWL process without incurring negative effects. A higher leaching pulp density can significantly reduce both the Capex and Opex of the leaching process.

[0097] The present study used a Canadian REE whole ore as the target solid feed sample, which contains Nd (in allanite) and Dy (in fergusonite) as the main values. Without grinding and at the same acid dosage (150 kg/t), a conventional ABWL process recovered 71% TREE. As shown above, the use of the HPDCWL process achieved a recovery of 72.4% TREE using sulphuric acid. When using HCI or HNO3, the recovery was elevated to 75.5% and 79.9% respectively.

[0098] To accelerate the duration of the cracking reactions from weeks to hours, the HPDCWL process relies on the use of ground samples as the feed material. The rotary kiln operation in ABWL is avoided in this process, thereby reducing the Opex and Capex significantly. The HPDCWL process is conducted using a high pulp density and at a moderate temperature. The duration of the leach and the leaching temperature are both significantly reduced in comparison to ABWL, which results in a significant reduction in the overall processing cost. The leaching cost can be further reduced by using less water (20% pulp density vs. 9.1%), as the TREE recovery was found not affected with the higher leaching pulp density. The HPDCWL process can be further optimized in many aspects. Both engineering design and metallurgical control can be adapted to optimize the efficiency and economics of the HPDCWL process for individual situations.

[0099] For comparison to the results obtained using the above variations in the HPDCWL process, a conventional ABWL process was conducted on the same whole solid feed sample, with a recovery of 71% TREE achieved.

[00100] Overall, these results demonstrate that the high pulp density cracking water leaching process is an effective and economically attractive alternative to the standard acid baking process.

[00101] EXAMPLE 2: Variation in high pulp density cracking water leaching process conditions, without agitation

[00102] An objective of this Example was to demonstrate the effect of agitation in the cracking step and to provide cracking conditions optimization in static (i.e., without agitation) high pulp density cracking water leaching (SHPDACWL).

[00103] The studies were performed using a batch of crushed whole ore samples (Search Mineral Whole Ore B, SMWB). The sample was crushed to 10 mesh and divided into 1 kg lots as received. The size analysis of the crushed ore is shown in Figure 3. Although most of the work was done on the raw sample as received, some tests were performed using ground samples of SMWB. The grinding curve for the ground samples is shown in Figure 4.

[00104] Table 7 provides a summary of mineralogy analysis for typical Search Minerals Whole Ore. The major value of this ore are the elements, neodymium (Nd) and dysprosium (Dy). Nd is mostly hosted in allanite (92.82% of Nd) and Dy is mostly hosted in fergusonite (88.96% of Dy). SWMB elemental analysis shows that the ore consists of 0.340% cerium (Ce), 0.020% Dy, 0.143% Nd, and 7.42% iron (Fe).

Table 7: Mineralogy analysis of Search Minerals Whole Ore

Mineral Mass (%) Mineral Mass (%)

Orthoclase 19.2 Thorite 0.09

Plagioclase 12.7 Synchysite 0.01

Mica 3.37 Other REE 0.01

Amphibole 5.81 Titanite 1.17

Epidote/Zoisite 0.25 Mn silicate 0.16

Chlorite 0.54 Zn Silicate 0.03

Calcite 3.34 Zircon 3.24

Quartz 36.5 Ca K Silicate 0.14

Apatite 0.16 Fe-Oxides 8.52

Allanite 4.51 Other 0.09

Fergusonite 0.2 Total 100

[00105] Figure 1A schematically depicts the conceptual flowsheet of the high pulp density acid cracking water leaching process used in this example, where the cracking step is performed under static conditions (i.e., without agitation). For the HPDACWL test work, all feed samples were ground to a Pso of 121 microns. The ground feed solid was then mixed in a smaller reactor (ore cracking reactor) with acid and a small amount of water, often resulting in a sticky paste mixture. The ore cracking reactor was placed in a heating kettle which provided a stable temperature to the paste in the reactor. The cracking duration was 4 hours. Following cracking, the "cracked" sample, as a wet or dry paste, was entirely transferred into a larger reactor (leaching reactor) and deionized (DI) water was added to lower the pulp density (leaching pulp density). The pulp was stirred using a regular stirrer at 250 rpm and leached for 2.5 hours. After leaching, the pulp was filtered, washed, and dried to obtain a solid and a liquid sample for chemical analysis.

[00106] Both the ASWL and ABWL processes were conducted on this same whole ore sample, with recoveries of 81% total rare earth elements (TREE).

[00107] Using HPDCWL, for example as described in Example 1, the slurry or paste employed in the cracking step has high viscosity and high acidity, and is maintained at an elevated temperature during cracking. The combination of the high viscosity, acidity and temperature in the reactor means that the stirrer used in the cracking step may be subject to engineering challenges, especially material corrosion. However, corrosion will be drastically reduced if the cracking is conducted without significant agitation.

[00108] In this Example, the HPDCWL process was performed without agitation during the cracking step and was, instead performed under static conditions. Additional conditions were evaluated using static cracking conditions, as summarized below.

[00109] Results

[00110] Effect of pulp density during cracking

[00111] Table 8 summarizes the effect of water addition when using the SHPDACWL process. It was found that with a lower amount of water was added, the overall metal recoveries are the lowest in this group (e.g., 77.3% TREE recovery). When the water addition was doubled or tripled, the metallurgical recoveries were improved for both Nd and Dy and resulted in a higher TREE recovery (e.g., >=81.9% TREE). However, when the water addition reached 5 mL, where the nominal pulp density dropped to 61%, there was no increase in the beneficial effect of adding water with the recoveries falling from their maximum. Here the concept of nominal pulp density was used to reflect the fact that the mixture of the ground ore, acid and water appeared as a wet paste initially, and later turned into a completely dried solid cake at the end of the cracking process. The dried cake, however, was easily softened and transferred into a slurry when water was added before the water leaching step. Using the concept of nominal pulp density means that the mixture in the cracking process is a wet paste or a "dry" cake. There was no real pulp observed, even when water was added in the cracking process.

Table 8: Effect of water addition in SHPDCWL

[00112] Effect of acid addition during cracking

[00113] As illustrated in Table 9, the addition of more acid (300 kg sulfuric acid per ton of ore) in the cracking step of SHPDCWL resulted in a significant elevation of metal recoveries. TREE recovery and Nd recovery increased by almost 10%. Dy recovery increased from 64.7% to 79.4%. The significant correlation between Nd/Dy recovery versus the total acid dosage indicate that the lack of sufficient acid in the cracking and/or leaching steps is a constraining factor in the recovery of REEs. Table 9: Effect of acid dosage in SHPDCWL

[00114] Effect of cracking duration

The data provided in Table 10 illustrates the effect of longer cracking duration time (18 hours) in the SHPDCWL. The results indicated that an extended cracking step of 18 hours instead of 4 hours did not provide any improvement in the metallurgical recoveries. Considering the higher cost associated with extended hours of cracking at an elevated temperature, the use of an extended cracking duration was not necessary to obtain improved metallurgical results. This result could be readily explained by the observation that all the wet pastes tested became a dry cake in a duration of less than 4 hours. The dry state of the mixture leads to very slow, if not completely ceased, reaction.

Table 10: Effect of cracking duration in SHPDCWL

[00115] Effect of Leaching Duration

[00116] The data provided in Table 11 shows a comparison of results for REE recoveries using an SHPDCWL process with different leaching duration times. The leaching duration of 1.5 and 2.5 hours both extracted similar amounts of total rare earths. However, the result of Dy recovery indicated a significant increase when the duration was extended from 1.5 hours to 2.5 hours. Doubling the leaching hours further to 5 hours failed to show any benefit on REE recoveries.

Table 11: Effect of leaching duration

[00117] Effect of Leaching Temperature

[00118] The effect of leaching temperature in an SHPDCWL process is shown in the data provided in Table 12, which indicates that higher temperatures are not a significant factor for Nd recovery, but can be beneficial to the dissolution of Dy. At 90°C, 64.7% Dy was leached, while only 57.4% Dy recovery was obtained when the leach was conducted at ambient temperature of 25°C.

Table 12: Effect of leach temperature

[00119] The results of the above studies, as summarized in Tables 7 to Table 12 have demonstrated that for the target ore sample used, agitation in the cracking step of HPDCWL process is not required. The cracking of REE bearing minerals can be realized in a comparatively short time (4 hours) at moderately elevated temperature (50~90°C).

[00120] To further confirm these results, similar studies were performed using larger ore samples. In the following studies, tests were performed on a 100g or 1kg scale to help identify any factors hidden in the test scale. [00121] Effect of leaching pulp density (large scale)

[00122] A major cost of REE cracking and leaching is the high consumption of water and its associated pollution potential. In the conventional ABWL process, the water to ore ratio is kept at 10:1 to allow maximized dissolution of REEs. However, the target ore used as the feed in the present study is a low-grade whole ore. Therefore, it is hypothesized that the full recovery of cracked REE minerals does not require as much water. In this group of tests (Table 13), the leaching pulp density was increased from 16.7% to 20% and 25%, representing a reduction of freshwater dosage of 5.0 ton per ton of ore, to 4.0 and 3.0 ton per ton of ore. The metal recovery result shows that a cut of 25% of water addition in the leaching step does not reduce the metal recoveries. In fact, higher recoveries were obtained for both Dy and Nd. For the same amount of acid residue in the cracked solid, lower water addition means higher pH, which could provide slightly better thermodynamic conditions for REE dissolution. In this case, we saved 25% of fresh water without sacrificing REE recoveries. When the leaching pulp density went up to 25% by cutting the water addition by 40%, the recoveries of both Nd and Dy were significantly reduced, indicating that the solubility of REE in this acidity was becoming the limiting factor for REE dissolution. At 14.3% leaching pulp density, the maximum REE recovery was observed indicating that higher water addition in the leach is beneficial to the dissolution of REE. However, from 20% to 14.3% The elevation of REE recovery is 3% only which required 50% extra freshwater addition.

Accordingly, use of the higher pulp density may be sufficient depending on circumstances; as would be readily understood by the skilled person, an economical calculation can be performed on a case-by-case basis to determine the optimal pulp density in the leach step depending on the situation.

Table 13: Effect of leaching pulp density (at 100 g ore)

[00123] Effect of acid dosage

[00124] As shown in Table 14, the effect of acid dosage in the REE recovery was verified under larger scale (i.e., 100 g). By increasing the acid dosage from 150 kg per ton to 400 kg per ton, the TREE recovery increased by nearly 10%. Both Nd and Dy recoveries were significantly increased.

Table 14: Effect of acid dosage (at 100 g ore)

[00125] Effect of cracking pulp density

[00126] As illustrated in Table 15, the effect of water addition in static high pulp density cracking differed when a larger amount of ore was employed. In the smaller scale tests (10 g), adding water showed significant enhancement to the recovery of REE at the end of the leach (see above). However, in the 100 g scale tests, the TREE recovery was reduced when the added water amount increased. At zero water addition (where only the addition of the acid affected the pulp density), 86.1% TREE was dissolved. When 400 kg of water was added per ton of ore, corresponding to a nominal pulp density of 59%, the recovery reduced to 57.7%.

[00127] It was observed that during the smaller scale tests (10 g), all the solids appeared to be dried at the end of the cracking step. In the 100 g scale tests, however, all the cracked residues remained as a wet paste. Such a difference on the nature of the cracked solid residue indicated that due to the increased thickness of the solid paste in the cracking reactor, a portion of water in the 100 g tests remained in the solid residue as free water.

Table 15: Effect ofcracking pulp density (at 100 g ore)

[00128] Effect of cracking duration

[00129] As illustrated by the data provided in Table 16, the effect of cracking duration when performed at larger scape had a similar trend to that observed in the smaller scale studies. By extending the cracking time from 2 hours to 6 hours, the recovery of TREE, Dy and Nd were increased by a significant percentage. The observations obtained using 100 g of ore show that at 2 and 4 hours, the cracked solids were still in a wet paste form.

However, with 6 hours of cracking, the solid residue appeared to be much drier.

Table 16: Effect ofcracking time (at 100 g ore)

[00130] Effect of leaching temperature

[00131] The results provided in Table 17 summarize the effect of different leaching temperatures at the larger, 100 g ore, scale. The results suggest that at lower leaching temperature higher recovery of both Dy and Nd becomes possible. At 25°C, 89.7% of TREE was recovered. Elevating the temperature to 75°C reduced the metal recovery by about 4%. This is significantly different from what has been observed using current industrial conditions for this ore, where 90°C was assumed to be necessary to maximize the TREE recovery after ABWL. Without wishing to be bound by theory, the fact that an elevated temperature becomes unnecessary during SHPDCWL suggests a higher degree of cracking is happening in the SHPDCWL process than that in the convention ABWL process.

Consequently, in the leaching step, there is no need to continue cracking the minerals that locks REE value. Consequently, use of a higher temperature to speed up reactions in the leaching step becomes unnecessary.

Table 17: Effect of leaching temperature (at 100 g ore)

[00132] Following the above studies, the SHDPCWL process was further scaled-up from 100 g to 1 kg in the cracking step. Again, when scaled up, due to the experimental design and equipment limitations, the cracked solid appeared very different. At 10 g scale, the process produced completely dry solids after 4 hours of cracking at 95°C whereas at 100 g and 1 kg scales the solid residue after cracking had a much wetter appearance. This degree of drying during cracking was identified as a hidden factor in the process. The result of this group of tests shows that at smaller scale, with dried cracked solid, a much higher TREE/Nd/Dy recovery was reached. When scaled up, the cracked solid remains wet, which changed the reaction mechanism, and resulted in reduced REE recovery. This conclusion was further supported by the result in the study of the cracking time, where a longer duration of the cracking step resulted in a solid that was much drier than the ones obtained using a shorter cracking step and also resulted in more REE being leached at the end of the leaching step.

[00133] By identifying the improvement associated with a drier residue from cracking in the SHPDCWL process, it becomes possible to maximize TREE recovery using this process in a full-scale operation; in particular, through control of the degree of drying during cracking. Such control can include one or more of the following:

- Monitor the dryness of the cracked solid quantitatively in the flowsheet.

- Extend the duration of the cracking step to allow for a dried solid residue at the end of cracking.

- Adjust water addition amount in the cracking step to avoid producing a wet residue.

- Provide an open reactor for the SHPDACWL process to allow the moisture to escape.

[00134] Conclusions

[00135] High pulp density cracking leaching is a process developed from the acid soaking water leaching process for cracking low-grade ores or concentrates. As the need for expensive rotary kilns are removed from the conventional acid baking water leaching process, HPDCWL becomes a promising alternative method for cracking and leaching REE values. Despite the success of the HPDCWL process with agitation during cracking, it has been found that the process reactor consisting of an agitating tank will face very harsh conditions, such as high acidity, elevated temperature, and high slurry viscosity. These factors may contribute to rapid material corrosion and eventually results in troubled performance and higher maintenance costs. Accordingly, the present study was performed to demonstrate the effect of modifying the HPDCWL process such that there is no agitation used during the cracking step.

[00136] This study confirmed that the agitation in the cracking step of the process is not necessary. A better recovery of metals was observed in this static process where no agitation was required. By adding more acid, the process has potential to produce over 90% of TREE recovery. The study suggested the following conclusions on the process variables:

- By calculating the economic performance overall, the acid addition can be reasonably increased in the cracking step as a factor to maximize the REE recoveries;

- A hidden factor in the process control was identified in this study; namely the cracking process performs better when the cracked solid residue is dried rather than wet;

- For this feed sample, the cracking time was controlled between about 4 to about 6 hours, whereby the duration was selected to be sufficiently long to allow a dry solid residue to be produced by the cracking step;

- The leaching performance was affected by the degree of success of the cracking step, however, in general, the leaching requires short duration (1.5 hours), and ambient temperature if the cracking is successful, where the water dosage during leaching was effective at least in the range of from about 20% to about 14% pulp density.

[00137] References

Bauer, D., Diamond, D., Li, J., Sandalow, D., Telleen, P., & Wanner, B. (2011). Critical Materials Strategy. US Department of Energy. Demol, J., & Senanayake, G. (2018). Sulfuric acid baking and leaching of rare earth elements, thorium and phosphate from a monazite concentrate: Effect of bake temperature from 200 to 800 °C. Hydrometallurgy, 179, 254-267.

- Qi, D. (2018). Hydrometallurgy of rare earths: Extraction and separation. Cambridge: Elsevier.

- Sadri, F., Nazari, A., & Ghahreman, A. (2017). A review on the cracking, baking and leaching processes of rare earth element concentrates. Journal of Rare Earths, 35(8), 739-752.

- Kumari, A., Panda, R., Jha, M.K., Jumar, J.R., and Lee, J.Y., 2015. Process development to recover rare earth metals from monazite mineral: A review. Minerals Engineering, 70: 102-115.

- McGill, I., 1997. "Part Ten: Rare Earth Metals", In: Habashi, F. (ed.) Handbook of Extractive Metallurgy Volume III. Federal Republic of Germany: Wiley-VCH, 1693- 1741pp.

- Verbaan, N., Bradley, K., Brown, J., and Mackie, S., 2015. A review of hydrometallurgical flowsheets considered in current REE projects. In: Simandl, GJ. and Neetz, M., (Eds.), Symposium on strategic and critical materials proceedings, November 13-14, 2015, Victoria British Columbia Ministry of Energy and Mines, British Columbia Geological Survey Paper 2015-3. 147-162.

[00138] Exemplary Embodiments

[00139] The following are non-limiting exemplary embodiments of the present invention:

1. A process for extracting rare earth elements from a solid feed, said process comprising: a. grinding the solid feed; b. combining the ground solid feed with a strong acid and water to form a slurry or a mud having a solid to liquid pulp density of at least 33% and cracking the slurry or mud in a reactor at a temperature of from about 30°C to about 300°C for a period of time from about 30 minutes to about 6 hours; and c. leaching the product of step (b) with an aqueous leaching solution to obtain a leachate comprising the rare earth elements. A process for extracting rare earth elements from a solid feed, said process comprising: a. grinding the solid feed; b. combining the ground solid feed with a strong acid and water to form a slurry having a solid to liquid pulp density of at least 33% and agitating the slurry in a reactor at a temperature of from about 30°C to about 300°C for a period of time from about 30 minutes to about 6 hours; and c. leaching the slurry with an aqueous leaching solution to obtain a leachate comprising the rare earth elements. The process according to claim 1, wherein in step (b) the cracking of the slurry or mud is a static step in which the slurry or mud is not stirred or agitated following combining. The process according to any one of embodiments 1 to 3, wherein the strong acid is HNO₃, HCI or H2SO4. The process according to embodiment 4, wherein the strong acid is HNO3 or H2SO4. The process according to any one of embodiments 1 to 5, wherein the solid to liquid pulp density in step (b) is from about 33% to about 90%, or from about 50% to about 80%, or from about 55% to about 75%, or from about 65% to about 80%, or from about 70% to about 75%. The process according to any one of embodiments 1 to 6, wherein the temperature for step (b) is between about 30°C and about 200°C, or between about 30°C and about 100°C. The process according to embodiment 7, wherein the temperature for step (b) is between about 85°C and about 100°C. The process according to any one of embodiments 1 to 8, wherein the aqueous leaching solution is water or an REE-barren acidic solution. The process according to any one of embodiments 1 to 9, wherein the leaching is performed for a period time from about 1 hour to about 10 hours, or from about 1 hour to about 5 hours, or from about 2 hours to about 5 hours, or about 2.5 hours. The process according to any one of embodiments 1 to 10, wherein the leaching is performed at a temperature, or over a range of temperatures, of from about 20°C to about 100°C, for example, from about 20°C to about 90°C, from about 20°C to about 50°C. A system for extracting rare earth elements from a solid feed of rare earthcontaining solid feed comprising: a. a grinding and/or milling device for grinding the solid feed to produce a ground solid feed; b. a cracking reactor having a ground solid feed inlet fluidly connected to the grinding and/or milling device, a strong acid inlet, a water inlet, and a mixed slurry outlet, wherein the ground solid feed is received within the cracking reactor for mixing with a strong acid and water to form a slurry having a solid to liquid pulp density of at least 33% by means of an agitator arranged in the reactor; c. means for heating the reactor to a temperature of from about 30°C to about 300°C for a period of time from about 30 minutes to about 6 hours; and d. a leaching device comprising a leaching chamber with mixed slurry inlet fluidly connected to the mixed slurry outlet of the reactor, an aqueous leaching solution inlet and a leached slurry outlet. The system according to embodiment 12, wherein the strong acid is HNO3, HCI or H2SO4. 14. The system according to embodiment 13, wherein the strong acid is HNO3 or H2SO4.

15. The system according to any one of embodiments 12 to 14, wherein the solid to liquid pulp density in of the slurry in the cracking reactor is from about 33% to about 90%, or from about 50% to about 80%, or from about 55% to about 75%, or from about 70% to about 85%, or from about 70% to about 75%.

16. The system according to any one of embodiments 12 to 15, wherein the cracking reactor is heated to a temperature between about 30°C and about 200°C, or between about 30°C and about 100°C.

17. The system according to any one of embodiments 12 to 16, wherein the aqueous leaching solution inlet provides water or an REE-barren acidic solution into the leaching chamber.

18. The system according to any one of embodiments 12 to 17, additionally comprising means for heating the leaching chamber to from about 20°C to about 100°C, from about 20°C to about 90°C, or from about 20°C to about 50°C.

19. The system according to any one of embodiments 12 to 18, additionally comprising control means for controlling the means for heating the cracking reactor.

20. The system according to any one of embodiments 12 to 19, additionally comprising monitoring means for monitoring the slurry properties during a cracking reaction in the cracking reactor.

[00140] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

[00141] All publications, patents and patent applications mentioned in this Specification are indicative of the level of skill of those skilled in the art to which this invention pertains and are herein incorporated by reference to the same extent as if each individual publication, patent, or patent applications was specifically and individually indicated to be incorporated by reference.

Claims

WE CLAIM:

1. A process for extracting rare earth elements from a solid feed, said process comprising: a) grinding the solid feed; b) combining the ground solid feed with a strong acid and water to form a slurry or a mud having a solid to liquid pulp density of at least 33% and cracking the slurry or mud in a reactor at a temperature of from about 30°C to about 300°C for a period of time from about 30 minutes to about 6 hours; and c) leaching the product of step (b) with an aqueous leaching solution to obtain a leachate comprising the rare earth elements.

2. The process according to claim 1, wherein in step (b) the cracking of the slurry or mud is a static step or wherein in step (b) during cracking the slurry or mud is agitated in the reactor.

3. The process according to claim 1, wherein the strong acid is HNO3, HCI or H2SO4.

4. The process according to claim 2, wherein the strong acid is HNO3 or H2SO4.

5. The process according to claim 1, wherein the solid to liquid pulp density in step (b) is from about 33% to about 90%, or from about 50% to about 80%, or from about about 65% to about 80%, or from about 55% to about 75%, or from about 70% to about 75%.

6. The process according to claim 1, wherein the temperature for step (b) is between about 30°C and about 200°C, or between about 30°C and about 100°C.

7. The process according to claim 6, wherein the temperature for step (b) is between about 85°C and about 100°C.

8. The process according to claim 1, wherein the aqueous leaching solution is water or an REE-barren acidic solution.

9. The process according to claim 1, wherein the leaching is performed for a period time from about 1 hour to about 10 hours, or from about 1 hour to about 5 hours, or from about 2 hours to about 5 hours, or about 2.5 hours. The process according to claim 1, wherein the leaching is performed at a temperature, or over a range of temperatures, of from about 20°C to about 100°C, from about 20°C to about 90°C, from about 20°C to about 50°C. A system for extracting rare earth elements from a rare earth-containing solid feed comprising: a) a grinding and/or milling device for grinding the solid feed to produce a ground solid feed; b) a cracking reactor having ground solid feed inlet fluidly connected to the grinding and/or milling device, a strong acid inlet, a water inlet, and a mixed slurry outlet, wherein the ground solid feed is received within the cracking reactor for mixing with a strong acid and water to form a slurry having a solid to liquid pulp density of at least 33% by means of an agitator arranged in the reactor; c) means for heating the reactor to a temperature of from about 30°C to about 300°C for a period of time from about 30 minutes to about 6 hours; and d) a leaching device comprising a leaching chamber with mixed slurry inlet fluidly connected to the mixed slurry outlet of the reactor, an aqueous leaching solution inlet and a leached slurry outlet. The system according to claim 11, wherein the strong acid is HNO3, HCI or H2SO4. The system according to claim 12, wherein the strong acid is HNO3 or H2SO4. The system according to claim 11, wherein the solid to liquid pulp density in of the slurry in the cracking reactor is from about 33% to about 90%, or from about 50% to about 80%, or from about 55% to about 75%, or from about 70% to about 85%, or from about 70% to about 75%. The system according to claim 11, wherein the cracking reactor is heated to a temperature between about 30°C and about 200°C, or between about 30°C and about 100°C. The system according to claim 11, wherein the aqueous leaching solution inlet provides water or an REE-barren acidic solution into the leaching chamber. The process according to claim 11, wherein the system additionally comprises means for heating the leaching chamber to from about 20°C to about 100°C, from about 20°C to about 90°C, or from about 20°C to about 50°C.