WO2019227165A1

WO2019227165A1 - Method for the treatment of metal-containing materials

Info

Publication number: WO2019227165A1
Application number: PCT/AU2019/050552
Authority: WO
Inventors: Harry Philip WATTS
Original assignee: The University Of Western Australia
Priority date: 2018-05-31
Filing date: 2019-05-30
Publication date: 2019-12-05

Abstract

In using aqueous solutions to extract metals from ores, it is not uncommon for compounds to precipitate from the solution onto the ore surface resulting in passivation. This passivation mechanism impinges the dissolution of the ore and thus metal extraction. Some examples of ores that exhibit this passivation mechanism include monazite, haematite and calcite. This invention discloses a new method of dissolving metal-containing materials in water, by adding an oxalic acid ester. Hydrolysis of the oxalic acid ester provides an environment that aids dissolution of the metal-containing material and precipitation of the metal oxalates.

Description

“Method for the treatment of metal-containing materials”

FIELD OF THE INVENTION

[0001 ] The present invention relates to methods for the treatment of metal-containing materials.

BACKGROUND ART

[0002] Metals are substances that are typically good conductors, lose electrons to form cations and form oxides and hydroxides. Due to their favourable properties (chemical, physical, electrical etc), they are used for a wide range of purposes ranging from buildings, vehicles to jewels. Most metals found in nature occur in ores and are extracted by various means such as hydrometallurgy, pyrometallurgy and electrometallurgy. These methods can be used individually or complementarily.

[0003] The method used in extracting a metal from its ore depends largely on the properties of the ore. Hydrometallurgy involves the use of aqueous chemistry in extracting metals from ores or minerals. In using aqueous solutions to extract metals from ores, it is not uncommon for compounds to precipitate from the solution onto the ore surface resulting in passivation. This passivation mechanism impinges the dissolution of the ore and thus metal extraction. Some examples of ores that exhibit this passivation mechanism include monazite, haematite and calcite.

[0004] Monazite is an abundant phosphate ore of the lighter rare earths with a pure form of one rare earth for every phosphate. The rare earths are the metals in the lanthanide series on the periodic table, from lanthanum to lutetium, as well as yttrium and scandium. Rare earth metals are needed for the technology of renewable energy, such as the lightweight neodymium magnets in wind turbines and electric cars. Monazite is thought to be refractory - broken down industrially in harsh conditions such as 93 % sulphuric acid at about 180 °C or 70 % sodium hydroxide at higher than 140 °C for over four hours.

[0005] Oxalic acid has been tested as an alternative for dissolving phosphate from monazite. However, it has been reported that recrystallization of rare earth oxalates on the monazite surface slows dissolution. It has also been reported that the oxalic acid will be consumed in side reactions. [0006] Iron oxides can occur as contaminants in clay and silicate minerals. The iron oxide content of these materials generally has to be reduced to less than 0.1 % to achieve an acceptable level of whiteness during the production of high quality ceramics. Typically, the iron content of these minerals is reduced using a costly high-temperature chlorination technique. Oxalic acid has been tested as an alternative but haematite dissolution was reported to be negatively affected by the formation of ferrous oxalate on the haematite surface.

[0007] The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. This discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0008] Each document, reference, patent application or patent cited in this text is expressly incorporated herein in their entirety by reference, which means that it should be read and considered by the reader as part of this text. That the document, reference, patent application or patent cited in this text is not repeated in this text is merely for reasons of conciseness.

[0009] Throughout the specification and claims, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

DISCLOSURE OF THE INVENTION

[0010] In accordance with the present invention, there is provided a method for the treatment of metal-containing material, the method comprising the steps of: combining metal-containing material, water and an oxalic acid ester; at least partially hydrolysing the oxalic acid ester; dissolving the metal-containing material by the hydrolysis products of the ester; and precipitating metal oxalate. [001 1 ] In one form of the invention, the metal-containing material is a rare earth- containing material. In a second form of the invention, the metal-containing material is an iron-containing material.

[0012] In accordance with the present invention there is provided a method for the treatment of rare earth-containing material, the method comprising the steps of: combining rare earth-containing material, water and an oxalic acid ester; at least partially hydrolysing the oxalic acid ester; dissolving the rare earth-containing material by the hydrolysis products of the ester; and precipitating rare earth oxalate.

[0013] Preferably, the oxalic acid ester is an alkane ester. More preferably, the alkane ester is a short chain alkane ester of oxalic acid. More preferably, the alkane ester is a dialkane ester. Preferably the alkane is a methyl or ethyl alkane. In preferred forms of the invention, the oxalic acid ester is dimethyl oxalate (DMO) or diethyl oxalate (DEO).

[0014] In one form of the invention, the hydrolysis products of the oxalic acid ester are the only species that dissolve the metal-containing material. Said hydrolysis products will include monoalkyl oxalic acid and oxalic acid.

[0015] Preferably, the rare-earth containing material is a rare earth mineral.

[0016] The rare earth-containing material may be rare earth oxides, carbonates, fluorocarbonates, hydroxylcarbonates, phosphates, arsenates, sulphates, vanadates, halides, uranyl carbonates and borates or combinations thereof. The rare earth containing material may also be non-rare earth minerals which include rare earths, such as laterites rich in iron oxides, lignite or zircon.

[0017] The rare earth-containing material may be selected from the group comprising apatite, monazite, xenotime, bastnaesite, calcian bastnaesite, parisite, synchysite, crandallite, goyazite, gorceixite, florencite, black monazite, phosphorites, columbite, nibates, tanteuxenite, gadolinite, yttrotantalite, fergusonite, samarskite, allanite, stillwellite, cheralite, throite and allanite. [0018] In one form of the invention, the rare earth-containing material is a monazite-in- iron concentrate.

[0019] In one form of the invention, the rare earth-containing material is a mineral sands-monazite.

[0020] The mechanism of hydrolysis of oxalic acid ester is described below with reference to dimethyl oxalate.

[0021 ] The dimethyl oxalate initially hydrolyses into monomethyl oxalate and methanol, as per equation (1 ).

(COOCH₃)2 + H₂0 (COOCH₃)(COOH) + CHsOH ... (1 )

[0022] The monomethyl oxalate hydrolyses to oxalic acid and methanol per equation (2).

(COOCH₃)(COOH) + H2O (COOH)2 + CHsOH ... (2)

[0023] Notably, the pH of a solution with an initial concentration of ester at 1 M will have a lower pH than a solution with oxalic acid at 1 M, even though the oxalic acid concentration is lower in the solution with the hydrolysing ester.

[0024] The rare earth phosphate dissolves in the acidic solution per equation (3).

REPO4 RE³⁺ + PO4³- ... (3)

[0025] Hydrogen ions (H⁺) from the acidic solution react with the free phosphate ions (from the dissolving rare earth phosphate) to form phosphoric acid per equation (4). This is intrinsically assisted by the fact that a solution with the hydrolysing ester has a lower pH.

PO4³- + H⁺ HPO4²- ... (4)

[0026] The oxalic acid precipitates the dissolved rare earths from the bulk solution into rare earth oxalates as shown by equation (5), allowing more rare earth phosphate to dissolve by virtue of Le Chatelier’s principle.

3(COO-)2 + 2RE³⁺ + XH2O RE2(C₂04)₃.H20 ... (5) [0027] Advantageously, the alcohol in the solution from equations (1 ) and (2) lowers the solubility of the rare earth oxalates resulting in more precipitation. The consumption of oxalic acid allows more hydrolysis of the ester (1 ) and (2) to replenish the oxalic acid in the solution.

[0028] Overall, the process allows extraction of the rare earth elements from, for example, monazite, followed by the precipitation of the rare earth oxalates without recrystallization of rare earth oxalates on the surface of the monazite particles, thus enabling near complete recovery at a moderate pH and temperature. Additionally, oxalic acid is less available for side reactions. Expressed alternatively, the process regulates the oxalic acid content to dissolve/extract the rare earths which are precipitated as rare earth oxalates. The rare earth oxalates are the lightest component in the system, and can be decanted for further processing. The rates of equations (1 ), (2) and (3) are increased by higher temperature, lower pH, and higher concentration of the reactants. The rate is decreased by a higher concentration of products.

[0029] The step of combining the metal-containing material, water and an oxalic acid ester may comprise combining the metal-containing material and the water prior to adding the oxalic acid ester.

[0030] The step of combining the rare earth-containing material, water and an oxalic acid ester may comprise combining the rare earth-containing material and the water prior to adding the oxalic acid ester.

[0031 ] Preferably, the concentration of ester is at least 0.2 M + 1.5 times the number of moles of rare earths, notwithstanding if some of the ester is initially insoluble. Increasing the ester concentration above 2.5 M will decrease the amount of water available for dissolution.

[0032] The step of combining the metal-containing material and the water may comprise the formation of a slurry.

[0033] Alternatively, the step of combining the metal-containing material, water and an oxalic acid ester; may comprise combining the oxalic acid ester and the water prior to adding the solution to the metal-containing material.

[0034] The step of combining the rare earth-containing material and the water may comprise the formation of a slurry. [0035] Alternatively, the step of combining the rare earth-containing material, water and an oxalic acid ester; may comprise combining the oxalic acid ester and the water prior to adding the solution to the rare earth-containing material.

[0036] The step of combining the metal-containing material, water and an oxalic acid ester may be conducted between 20 °C and below the atmospheric boiling point of water. In one form of the invention, the step of combining the metal-containing material, water and an oxalic acid ester is conducted between 20 °C and 95 °C. In an alternate form of the invention, the step of combining the metal-containing material, water and an oxalic acid ester is conducted between 20 °C and 80 °C. In an alternate form of the invention, the step of combining the metal-containing material, water and an oxalic acid ester is conducted between 20 °C and 70 °C. In an alternate form of the invention, the step of combining the metal-containing material, water and an oxalic acid ester is conducted between 20 °C and 60 °C. In an alternate form of the invention, the step of combining the metal-containing material, water and an oxalic acid ester is conducted between 20 °C and 50 °C. In an alternate form of the invention, the step of combining the metal-containing material, water and an oxalic acid ester is conducted between 20 °C and 40 °C. In an alternate form of the invention, the step of combining the metal- containing material, water and an oxalic acid ester is conducted between 30 °C and 95 °C. In an alternate form of the invention, the step of combining the metal-containing material, water and an oxalic acid ester is conducted between 40 °C and 95 °C. In an alternate form of the invention, the step of combining the metal-containing material, water and an oxalic acid ester is conducted between 50 °C and 95 °C. In an alternate form of the invention, the step of combining the metal-containing material, water and an oxalic acid ester is conducted between 60 °C and 95 °C. In an alternate form of the invention, the step of combining the metal-containing material, water and an oxalic acid ester is conducted between 70 °C and 95 °C. In an alternate form of the invention, the step of combining the metal-containing material, water and an oxalic acid ester is conducted between 80 °C and 95 °C.

[0037] The step of combining the rare earth-containing material, water and an oxalic acid ester may be conducted between 20 °C and below the atmospheric boiling point of water. In one form of the invention, the step of combining the rare earth-containing material, water and an oxalic acid ester is conducted between 20 °C and 95 °C. In an alternate form of the invention, the step of combining the rare earth-containing material, water and an oxalic acid ester is conducted between 20 °C and 80 °C. In an alternate form of the invention, the step of combining the rare earth-containing material, water and an oxalic acid ester is conducted between 20 °C and 70 °C. In an alternate form of the invention, the step of combining the rare earth-containing material, water and an oxalic acid ester is conducted between 20 °C and 60 °C. In an alternate form of the invention, the step of combining the rare earth-containing material, water and an oxalic acid ester is conducted between 20 °C and 50 °C. In an alternate form of the invention, the step of combining the rare earth-containing material, water and an oxalic acid ester is conducted between 20 °C and 40 °C. In an alternate form of the invention, the step of combining the rare earth-containing material, water and an oxalic acid ester is conducted between 30 °C and 95 °C. In an alternate form of the invention, the step of combining the rare earth-containing material, water and an oxalic acid ester is conducted between 40 °C and 95 °C. In an alternate form of the invention, the step of combining the rare earth-containing material, water and an oxalic acid ester is conducted between 50 °C and 95 °C. In an alternate form of the invention, the step of combining the rare earth-containing material, water and an oxalic acid ester is conducted between 60 °C and 95 °C. In an alternate form of the invention, the step of combining the rare earth-containing material, water and an oxalic acid ester is conducted between 70 °C and 95 °C. In an alternate form of the invention, the step of combining the rare earth-containing material, water and an oxalic acid ester is conducted between 80 °C and 95 °C.

[0038] It will be appreciated that dissolution of monazite is facilitated by higher temperatures. However, precipitation of rare earth oxalates is inhibited by higher temperatures and oxalic acid is more easily oxidised at higher temperatures.

[0039] If the feedstock has a lot of iron oxides, then higher temperatures can result in large amounts of oxalic acid being consumed in dissolving the iron particularly at temperatures over 70 °C.

[0040] However, if the feedstock has high concentration of thorium, for example, over 0.5 %, then temperatures over 70 °C are preferred.

[0041 ] Advantageously, the present invention facilitates the precipitation of metal oxalates without the need for acid addition prior to the addition of the oxalic acid ester. Prior art methods require the initial addition of mineral acid (e.g. hydrochloric, nitric, sulfuric, perchloric acids) to dissolve the metal-containing material. The present invention identifies that said dissolution can be facilitated by the hydrolysis products of the oxalic acid ester.

[0042] In one form of the invention, the method comprises the step of combining the metal-containing material, water and oxalic acid ester without a mineral acid.

[0043] In one form of the invention, the oxalic acid ester is added to a mixture of metal- containing material and water. In one form of the invention, the mixture of metal- containing material and water has no mineral acid.

[0044] In one form of the invention, the oxalic acid ester is added directly to a mixture of metal-containing material and water. In the context of the present specification, the term directly will be understood to encompass a mixture to which no mineral acid has been added. In the context of the present invention, the term mixture shall be understood to encompass a slurry containing undissolved metal-containing material.

[0045] Some rare earth minerals are known to contain levels of thorium. Advantageously, the present invention can facilitate the separation of rare earths from thorium by the selective precipitation of rare earth oxalates.

[0046] Where an ore contains high amounts of thorium the combination of rare earth containing-material, water and an oxalic acid ester may be buffered to facilitate the dissolution of thorium. Preferably, the combination is buffered to a pH of between 3 and 3.5. In one form of the invention, the buffer is sodium citrate. In an alternate form of the invention, the buffer is glycine.

[0047] In one form of the invention, the step of combining metal-containing material, water and an oxalic acid ester is conducted with agitation.

[0048] In one form of the invention, the step of combining rare earth-containing material, water and an oxalic acid ester is conducted with agitation.

[0049] Where the method of the present invention is used in processes without agitation and/or heating, such as heap leaching, it can be advantageous to add the oxalic acid ester to a glycine solution to lower the rate of hydrolysis of dimethyl oxalate. Slowing the rate of hydrolysis can allow the rare earths to diffuse longer distances before precipitating. [0050] Advantageously, it is possible to control the rate of ore dissolution and oxalic acid ester hydrolysis by adjusting the pH and/or temperature. A higher concentration of hydrogen ions or temperature can increase the rate of dissolution of the mineral and hydrolysis of the ester.

[0051 ] Iron-containing materials include but are not limited to haematite, goethite, amorphous iron oxide and combinations thereof.

[0052] The iron oxide dissolves in the oxalic acid by reduction of the iron and consumption of acid. For example, the dissolution of haematite proceeds as follows:

H⁺ + Fe203 + 5HC2O4 - > 2Fe(C204)2^2_ + 3H2O + 2CO2 ... (6)

[0053] The iron enters solution as a soluble iron oxalate complex and the solution can be decanted, filtered or washed from the solids. If the decanted solution is exposed to air for a period of days, or to an oxidant, the oxalate will decompose to carbon dioxide and the iron will precipitate as an iron oxalate as follows:

2Fe(C₂04)2²- + 0.502 + 2H⁺ - > H2O + 2CO2 + 2Fe(C₂04) ... (7)

[0054] The preferred technique is to add 2.5 mole of ester for 1 mole of iron in an agitated suspension of iron oxide and water at 90 °C.

BRIEF DESCRIPTION OF DRAWINGS

[0055] The present invention will now be described, by way of example only, with reference to the following Figures in which:

Figure 1 is a plot demonstrating the effect of varying dosages oxalic acid on monazite dissolution;

Figure 2 is a back-scattered electron image of monazite particles prior to contact with oxalic acid;

Figure 3 is a back-scattered electron image of monazite particles after 8 hr contact with oxalic acid; Figure 4 is a plot demonstrating the effect of varying dosages of dimethyl oxalate on monazite dissolution;

Figure 5 is a back-scattered electron image of rare earth oxalates after addition of dimethyl oxalate;

Figure 6 is a back-scattered electron image of monazite particles after contact with dimethyl oxalate;

Figure 7 is a plot demonstrating the effect of varying temperature on monazite dissolution;

Figure 8 is a plot demonstrating the effect of varying reagent on monazite dissolution;

Figure 9 is a plot demonstrating the effect of varying temperature on monazite dissolution; and

Figure 10 is a plot demonstrating the changes in dissolution of various components in monazite.

DESCRIPTION OF EMBODIMENTS

Example 1

[0056] A flotation concentrate of an iron-rich monazite ore (containing 3 g of monazite) was wet screened to a size range of 36 to 44 pm and agitated in oxalic acid (1 L) at different concentrations (0.1 M, 0.2 M, 0.4 M, 0.6 M and 0.8 M) at a temperature of 25 °C, stirring speed of 510 rpm without any pH adjustment. Figure 1 shows that the dissolution rate decreased rapidly within half an hour, even with a vast excess of oxalic acid.

[0057] Figure 2 and Figure 3 show the monazite particles before and after dissolution in oxalic acid. After dissolution, the monazite particles in the lighter shade were covered in clusters of rare earth oxalates. These clusters are not only likely to impede transport of fluid, but are also attaching the oxalates to the monazite, making decantation of oxalates impossible. [0058] Figure 4 demonstrates that at higher temperatures, the use of dimethyl oxalate provides significantly better results than oxalic acid. At 70 °C, more phosphorus dissolves in water with dimethyl oxalate added than in concentrated oxalic acid. It is known within the art that the amount of phosphorous dissolved from rare earth phosphates is an indication of the efficacy of the dissolution.

[0059] Figure 5 shows the smaller rare earth oxalate crystals as independent crystals from the larger crystals of monazite greater than 30 pm across. Figure 6 shows the residual monazite crystals without attachment of rare earth oxalates, from which the rare earth oxalates were decanted.

Example 2

[0060] A monazite concentrate rich in goethite and amorphous iron oxides had the assay in Table 1. One hundred grams were agitated at 512 rpm in one kilogram of solution with an initial composition of DMO (1 M) and water at various temperatures in a baffled container (Duran GLS 80). The initial rate of phosphorus dissolution increased with temperature, but the most phosphorus dissolved over 12 hours at 70° C as shown in Figure 7. The effects of DMO, DEO and oxalic acid at 70 ° C were compared in Figure 8. More phosphorus dissolved in dimethyl oxalate than the others. Flowever, the rate of dissolution in diethyl oxalate was similar after two hours. This indicates an induction period for diethyl oxalate, which can be shortened by higher temperature as shown in Figure 9.

[0061 ]

Table 1 - Elemental assay of monazite concentrate

Example 3 [0062] Two grams of relatively pure monazite from a mineral sand deposit were agitated at 512 rpm in one kilogram of solution at 90 ° C with an initial composition of DMO (1 M) and water in a baffled container (Duran GLS 80). The particles of monazite in this experiment had a median diameter of 134 pm, significantly larger and with a lower surface area to mass ratio compared to the monazite concentrate particles in Example 2 above. The specific surface area of these monazite particles was 0.0423 m²/g compared to 1.39 m²/g for the monazite concentrate particles in Example 2 above - a factor of 33 times lower. In that context, the dissolution of between 6 to 7% of phosphorus and thorium in Figure 10 was comparable to the dissolution achieved in the monazite concentrate in Example 2 above. Most notably, the phosphorus and thorium continued to dissolve throughout the twelve hours, whereas the rare earths precipitated at around two hours. This enables a separation of radioactive thorium from rare earths at the same time as breaking down the monazite.

Example 4

[0063] One kilogram of a solution of goethite and amorphous iron oxide-rich limonitic nickel laterite ore was agitated at 512 rpm at 80 °C with an initial composition of DMO (1 M) and water in a one litre baffled Duran GLS 80 leaching vessel. The liquids to solids weight ratio was 50:1. After 6 hours, 83% of the iron oxide content was dissolved as shown in Table 2.

[0064]

Table 2 - Fraction of iron dissolved at the end of the time period for a variety of iron oxide samples. [0065] In another example, one kilogram of a solution of haematite ore was agitated at 512 rpm at 80 °C with an initial composition of DMO (1 M) and water in a one litre Duran GLS 80 leaching vessel. The liquids to solids weight ratio was 20:1. 30% of the iron oxide content was dissolved after 3.7 hours as shown in Table 2.

[0066] In another example, one kilogram of a solution of monazite rich in goethite and amorphous iron oxides was agitated at 512 rpm at 90 °C with an initial composition of DMO (1 M) and water in a one litre Duran GLS 80 leaching vessel. The liquids to solids weight ratio was 10:1. 85% of the iron oxide content was dissolved after 12 hours as shown in Table 2.

[0067] In yet another example, one kilogram of a solution of monazite rich in goethite and amorphous iron oxides was agitated at 512 rpm at 90 °C with an initial composition of DEO (1 M) and water in a one litre Duran GLS 80 leaching vessel. The liquids to solids weight ratio was 10:1. 63% of the iron oxide content was dissolved after 12 hours as shown in Table 2.

[0068] It is known that the use of aqueous oxalic acid to dissolve iron oxides, especially haematite, results in the formation of a passivation layer of iron oxalate on the surface of the haematite, severely limiting the dissolution process. The present invention overcomes this limitation by keeping the oxalate concentration at a minimum within the bulk solution.

[0069] The present invention provides a number of commercial advantages over the use of oxalic acid.

[0070] It is possible to dissolve virtually all of the metal-containing material in one pass, meaning that recirculation of the metal-containing material through the process is not needed.

[0071 ] In the case of a relatively pure monazite feed, the rare earth oxalates can be decanted from the sunken monazite residue.

[0072] The speed of the dissolution of the metal-containing material and hydrolysis the ester can be controlled by adjusting pH and temperature. The conditions of dissolution can be controlled by addition of a buffer and temperature. [0073] A minimal and controlled concentration of precipitant (oxalate) means that they are less likely to precipitate on the surface of the metal-containing material and block further dissolution.

Claims

1. A method for the treatment of metal-containing material, the method comprising the steps of: combining metal-containing material, water and an oxalic acid ester; at least partially hydrolysing the oxalic acid ester; dissolving the metal-containing material by the hydrolysis products of the ester; and precipitating metal oxalate.

2. A method according to claim 1 , wherein the step of combining the metal- containing material, water and an oxalic acid ester comprises combining the metal-containing material and water prior to adding the oxalic acid ester.

3. A method according to claim 1 , wherein the step of combining the metal- containing material, water and an oxalic ester comprises combining the oxalic acid ester and the water prior to adding the solution to the metal-containing material.

4. A method according to any one of the preceding claims, wherein the step of combining the metal-containing material, water and an oxalic acid ester is conducted between 20 °C and below the atmospheric boiling point of water.

5. A method according to any one of the preceding claims, wherein the step of combining metal-containing material, water and an oxalic acid ester is conducted with agitation.

6. A method according to claim 2, wherein the step of combining the metal- containing material and water includes the formation of a slurry.

7. A method according to any one of the preceding claims, wherein the said metal- containing material is an iron-containing material, the method comprising the steps of: combining iron-containing material, water and an oxalic acid ester; at least partially hydrolysing the oxalic acid ester; dissolving the iron-containing material by the hydrolysis products of the ester; and precipitating iron oxalate.

8. A method according to claim 7, wherein the concentration of the said oxalic acid ester is 2.5 mole per mole of iron.

9. A method according to claim 7 or 8, wherein the step of combining the iron- containing material, water and an oxalic acid ester is conducted with agitation at 90 °C.

10. A method according to any one of claims 7 to 9, wherein the said oxalic acid ester is dimethyl oxalate (DMO) and comprising the steps of: combining iron-containing material, water and DMO; at least partially hydrolysing the DMO into monomethyl oxalate (MMO) and methanol; at least partially hydrolysing the MMO into oxalic acid and methanol; dissolving the iron-containing material in the resulting acidic solution; reacting the dissolved iron material with the oxalic acid to form iron oxalates; and precipitating the iron oxalates by exposure to air or an oxidizing agent.

1 1. A method according to any one of claims 7 to 10, wherein the iron-containing material is any of the following: haematite, goethite, amorphous iron oxide, limonitic nickel laterite and combinations thereof.

12. A method according to any one of claims 7 to 9, wherein the said oxalic acid ester is diethyl oxalate (DEO) and comprising the steps of: combining iron-containing material, water and DEO; at least partially hydrolysing the DEO into monomethyl oxalate (MEO) and ethanol; at least partially hydrolysing the MEO into oxalic acid and ethanol; dissolving the iron-containing material in the resulting acidic solution; reacting the dissolved iron material with the oxalic acid to form iron oxalates; and precipitating the iron oxalates by exposure to air or an oxidizing agent.

13. A method according to claim 12, wherein the said iron-containing material is any of the following: haematite, goethite, amorphous iron oxide, limonitic nickel laterite and combinations thereof.

14. A method for the treatment of rare earth-containing material, the method comprising the steps of: combining rare earth-containing material, water and an oxalic acid ester; at least partially hydrolysing the oxalic acid ester; dissolving the rare earth-containing material by the hydrolysis products of the ester; and precipitating rare earth oxalate.

15. A method according to claim 14, wherein the step of combining the rare earth- containing material, water and an oxalic acid ester comprises combining the rare earth-containing material and water prior to adding the oxalic acid ester.

16. A method according to claim 15, wherein the step of combining the rare earth- containing material and water includes the formation of a slurry.

17. A method according to claim 14, wherein the step of combining the rare earth- containing material, water and an oxalic ester comprises combining the oxalic acid ester and the water prior to adding the solution to the rare earth-containing material.

18. A method according to any one of claims 14 to 17, wherein the step of combining the rare earth-containing material, water and an oxalic acid ester is conducted between 20 °C and below the atmospheric boiling point of water.

19. A method according to any one of claims 14 to 18, wherein the concentration of the said oxalic acid ester is at least 0.2 M + 1.5 times the number of moles of rare earths but not higher than 2.5 M.

20. A method according to any one of claims 14 to 19, further comprising the steps of: adding a buffer to the combination of rare earth-containing material, water and an oxalic acid ester; to attain a pH of between 3 and 3.5.

21. A method according to claim 20, wherein the said oxalic acid ester is added to the buffer prior to combination with the rare earth-containing material and water.

22. A method according to claim 20 or 21 , wherein the said buffer is selected from the group consisting of sodium citrate and glycine.

23. A method according to any one of claims 14 to 22, wherein the step of combining rare earth-containing material, water and an oxalic acid ester is conducted with agitation.

24. A method according to any one of claims 14 to 23, wherein the said oxalic acid ester is dimethyl oxalate (DMO) and comprising the steps of: combining rare earth-containing material, water and DMO; at least partially hydrolysing the DMO into monomethyl oxalate (MMO) and methanol; at least partially hydrolysing the MMO into oxalic acid and methanol; dissolving the rare earth-containing material in the resulting acidic solution; reacting the dissolved rare earth materials with the oxalic acid to form rare earth oxalates; and precipitating the rare earth oxalates.

25. A method according to any one of claims 14 to 24, wherein the said rare earth- containing material is in any of the following chemical forms: rare earth oxides, carbonates, fluorocarbonates, hydroxylcarbonates, phosphates, arsenates, sulphates, vanadates, halides, uranyl carbonates and borates or combinations thereof.

26. A method according to any one of claims 14 to 24, wherein the said rare earth- containing material is a concentrate of monazite rich in iron ore.

27. A method according to any one of claims 14 to 24, wherein the said rare earth- containing material is pure monazite from mineral sands.

28. A method according to any one of claims 14 to 24, wherein the said rare earth- containing material is any of the following: apatite, xenotime, bastnaesite, calcian bastnaesite, parisite, synchysite, crandallite, goyazite, gorceixite, florencite, black monazite, phosphorites, columbite, nibates, tanteuxenite, gadolinite, yttrotantalite, fergusonite, samarskite, allanite, stillwellite, cheralite, throite and allanite.

29. A method according to any one of claims 14 to 24, wherein the said rare earth- containing material is a non-rare earth mineral which contains rare earths, such as laterites in iron oxides, lignite or zircon.

30. A method according to any one of claims 14 to 23, wherein the said oxalic acid ester is diethyl oxalate (DEO) and comprising the steps of: combining rare earth-containing material, water and DEO; at least partially hydrolysing the DEO into monoethyl oxalate (MEO) and ethanol; at least partially hydrolysing the MEO into oxalic acid and ethanol; dissolving the rare earth-containing material in the resulting acidic solution; reacting the dissolved rare earth materials with the oxalic acid to form rare earth oxalates; and precipitating the rare earth oxalates.

31. A method according to claim 30, where the rare earth-containing material is in any of the following chemical forms: rare earth oxides, carbonates, fluorocarbonates, hydroxylcarbonates, phosphates, arsenates, sulphates, vanadates, halides, uranyl carbonates and borates or combinations thereof.

32. A method according to claim 30, where the rare earth-containing material is a concentrate of monazite rich in iron ore.

33. A method according to claim 30, where the rare earth-containing material is pure monazite from mineral sands.

34. A method according to claim 30, where the rare earth-containing material is any of the following: apatite, xenotime, bastnaesite, calcian bastnaesite, parisite, synchysite, crandallite, goyazite, gorceixite, florencite, black monazite, phosphorites, columbite, nibates, tanteuxenite, gadolinite, yttrotantalite, fergusonite, samarskite, allanite, stillwellite, cheralite, throite and allanite.

35. A method according to claim 30, wherein the rare earth-containing material is a non-rare earth mineral which contains rare earths, such as laterites in iron oxides, lignite or zircon.