0001] This invention relates to the extracting of rare earth minerals from an iron-rich rare earth-bearing ore.

[0002] Large deposits of iron-rich rare earth-bearing ores are found worldwide. These ore deposits carry significant reserves of rare earths but, nonetheless, some of these deposits have not been exploited because milling of the ore and physical separation processes to produce a concentrate from which rare earth elements could be extracted by hydrometallurgical means have been found to be challenging, inefficient and uneconomical.

[0003] An object of the present invention is to provide a method for extracting rare earth elements from an iron-rich rare earth-bearing ore.

SUMMARY OF THE INVENTION

[0004] The invention provides a method of processing an iron-rich rare earth bearing ore which includes the steps of smelting the ore to concentrate rare earth oxide minerals in the ore into a slag phase and extracting the rare earth oxide minerals from the slag.

[0005] In the smelting step iron and manganese oxides in the ore may be reduced to a low manganese pig iron in a metal phase.

[0006] Smelting of the ore can be effected through the use of a suitable furnace.

[0007] The extraction of the rare earth oxide minerals may be carried out in any suitable way. Preferably though the slag is conditioned through controlled cooling and, after solidification, is milled and leached directly or upgraded further by flotation/magnetic separation before leaching.

[0008] The slag may be milled to a suitable size, eg. of the order of -35 micron.

[0009] The milled slag may be directly leached in hydrochloric acid or any other suitable lixiviant.

[0010] Prior to the extraction step the slag may be treated to enhance the leaching process. For example at least one suitable flux may be added to the melt and conditioning of the slag through controlled cooling may be undertaken. The fluxing may take place in the furnace or the flux may be added to the slag when it is tapped from the furnace, for example into a conditioning casting ladle or into a separate reactor.

[0011] Without being restrictive the flux may be lime, Na2C03, K2CO3 and other suitable fluxing agents.

[0012] A function of the fluxing is to facilitate the breaking of bonds between spinel phases, rare earth bearing phases and other phases in the slag, with the aim of improving the downstream upgrading and leaching of the slag.

BRIEF DESCRIPTION OF THE DRAWING

[0013] The invention is further described by way of example with reference to the accompanying drawing which depicts steps in the method of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

[0014] The accompanying drawing is a flow sheet of steps in a method according to the invention for the extraction of rare earth elements from a mineralogically complex iron-rich rare earth ore 10. Typically rare earth oxide minerals in this type of ore occur in a complex mineralogy of grains, and crystal clusters of less than 20 micron in size are disseminated through an iron oxide matrix or as coatings on the iron oxide minerals. A conventional milling and physical separation process is generally technically and economically not viable to yield an ore concentrate which can be further processed by hydrometallurgical techniques to obtain the rare earth elements.

[0015] The method of the invention uses a selective carbothermic smelting step for concentrating the rare earth oxide species into a slag phase and for precipitating iron and manganese in the ore, as low manganese pig iron, in a metal phase. Thereafter the slag is processed by hydrometallurgical techniques to extract and then to separate the rare earth elements.

[0016] Referring to the flow sheet the ore 10 and a suitable reductant 12, e.g. anthracite, are fed in appropriate quantities to a furnace 14. The process energy requirement of the furnace, and the quality and mass of the metal and slag phases produced by the furnace, are dependent on the smelting conditions and particularly on the furnace operating temperature, the composition of the ore 10 and the quantity and quality of the reductant 12. The reductant input is regulated to achieve at least 98% iron reduction to the metal phase, and optimum molten slag properties while the furnace temperature is selected to effect efficient slag-metal separation.

[0017] A flux 16 is added (in this example) to the furnace 14 d uring the smelting process. The nature of the fluxing is such as to modify the slag, to improve the recovery of major metal values, to improve furnace operation, as well as improve downstream upgrading and leaching of valuable rare earth species in the slag. The flux 16 may be lime, Na2C03, K2CO3 or borax (these flux types are exemplary only and are not limiting). The optimum flux addition may be adjusted according to the type of ore which is being processed.

[0018] A slag 20 is tapped from the furnace 14. Depending on the composition of the ore 10 the slag 20 may contain appreciable amounts of BaO, Th02 and SrO in addition to rare-earth species and other slagging elements such as S1O2, AI2O3, CaO and MgO.

[0019] Apart from concentrating the rare earth elements into the slag phase, the smelting process precipitates manganese and iron into a low-manganese pig iron 22 in the metal phase. The pig iron 22 can be recovered in a dov/nstream process 24 using suitable techniques.

[0020] As an alternative to adding the flux 16 to the smelt in the furnace 14 it is possible to add the flux to the slag as it is tapped from the furnace into a separate reactor or into a casting ladle (not shown). Inter alia the fluxing technique is designed to facilitate the breaking of bonds between spinel phases, rare earth bearing-phases and other phases in the slag, with the aim of improving the downstream upgrading and leaching of the slag. It is known that the spinel phases cover the rare-earth oxide grains and prevent or hinder their efficient leaching. Additionally, the fluxing technique which is adopted should be selected to minimise effects such as refractory erosion and off-gas blockages which can disrupt operation of the furnace 14.

[0021] The slag 20, once solidified, is milled in a step 30 to produce a milled product 32 of suitable size, e.g. of the order of -35 micron. The product 32 is then directly leached or upgraded before leaching (step 34). Hydrochloric acid 36 is used to leach the slag. The product 38 produced by the leaching step 34 is subjected to a solid/liquid separation step 40 which produces a leach residue 42 which is disposed of by a suitable technique, and a leach solution 46. In a subsequent impurity rejection step 48 lime 50 is added to the leach solution 46. A resulting product 52 is subjected to a solid/liquid separation step 54 to remove impurities 56 such as Al, Fe and Th which are precipitated. Lime 62 is added to liquid 64 coming from the step 54 to precipitate (66) the rare earth elements 68 which are thereafter recovered by a solid/liquid separation step 70.

[0022] Sulphuric acid 74 is added in a step 76 to liquid from the separation step 70 to enable hydrochloric acid (78) in solution to be recovered in a solid / liquid separation step 80. A CaS04 precipitate 82 produced by the step 80 is disposed of in an appropriate way, while the recovered hydrochloric acid 78 is recycled to the direct leaching step 34.

[0023] Laboratory and pilot scale tests undertaken to demonstrate the efficiency of the smelting step 14 and the recovery of the rare-earth oxides into the slag 20 have shown that more than 90% of the total rare-earth elements contained in the iron-rich rare earth bearing ore 10 are recovered into the slag phase 20. A concentration ratio of from 4 to 7 times the feed head rate is achieved. A pull mass of from 15% to 25%, and a total rare-earth element recovery from the slag 20 of

more than 90%, are measured. The total rare-earth element content in the slag depends on the pull mass and the total rare-earth element grade of the ore 10.

[0024] For each unit of the ore 10 which is processed about 0,4 to 0,6 units of pig iron 22 are produced. The pig iron composition varies with the extent of reduction and the nature of the ore 10. Alloys containing from 75 to 97% Fe, and from 1 to 14% Mn, with the balance being mainly Si and C, are produced.

[0025] The slags from the laboratory and pilot tests were leached and the leach residues 42 were collected, weighed and sampled for chemical and mineralogical analyses. It is established that the extraction yield of the rare-earth elements is over 90%. The mass of the residue 42 is from 30 to 35% of the initial mass of the slag 20. In general the overall recovery rate of the rare-earth element concentration in the slag 20 to the production of the precipitate 68, is in the range of 80 to 90%.

[0026] The economic viability of the process shown in the accompanying flow sheet depends largely on mining and electricity costs and on the total rare-earth element grade of the ore 10. The nature of the furnace crucible which is used during the smelting step 14 can have an effect on technical and economic aspects of the method of the invention. If a graphite crucible is used then the slag 20 need not necessarily be fluxed and direct HCI leaching of the unfluxed slag can be effected. Tests have shown that total rare-earth element leaching efficiencies ranging between 93% and 96%, at different acid dosages, are achieved. Additionally it has been demonstrated that direct HCI leaching of the slag, compared to acid baking and caustic (NaOH) cracking, is preferable. It has also been observed that the

extraction efficiency of light rare-earth elements which include La, Ce, Nd and Pr is lowered when the slag is treated with a flux prior to leaching.

[0027] A benefit of the fluxing process is that the temperature of the smelting can be decreased from about 1700° to 1600°C. Use of a graphite or carbon-based refractory crucible is preferable as it minimizes the contamination of the slag product and this results in a higher concentration of the rare-earth elements in the slag. It has been noted that due to the effect of chemical erosion the rare-earth oxide grade of the slag produced in an alumina crucible or in an MgO crucible is relatively lower compared to that of the slag produced in a graphite crucible. Virtually no slag contamination took place through the use of a graphite crucible.

Experimental Procedure for the smelting tests

1.1 RAW MATERIALS

Ore

[0028] Zandkopsdrift (ZKD) iron-rich rare-earth bearing was used. Iron in the ore is in the form of goethite (FeO(OH)). This ore was calcined prior to crucible smelting test work as goethite decomposes at about 300 °C to produce Fe203 and H2O. A summary of the chemical composition of the ore before and after calcining is given in Table 1 and Table 2.

[0029] The granulometry of the ore supplied was 100% passing to 5mm sieve. The ore was milled to 100% passing to 75 micron sieve, which is an adequate size for laboratory test work while -1 mm passing was used for the 100 kVA DC arc smelting test work.

Table 1: Summary of the bulk chemical composition of the ZKD ore "as is"

Table 2: Summary of the bulk chemical composition of the calcined ZKD ore

Ho.Tm, Lu, Yb . REE with concentrations less than 100ppm; S=Si02, A=AI203; = gO NA

not analysed

Anthracite

[0030] The particle size distribution of the as-is anthracite was 100% passing

to a sieve of 5mm size. It was milled to 100% passing to a 75 micron sieve for the

crucible tests and used as-is in the 00 kVA DC arc smelting tests. The approximate

analysis of the anthracite used is given in Table 3.

Table 3: Summary of the bulk chemical composition of the anthracite (mass %)

Ash Volatile Fixed Carbon Total Sulphur

• 4.74 • 6.19 . 89.1 0.56

Fluxes

High purity laboratory grade Na2C03, K2CO3, borax and CaO are used as fluxing agents.

1.2 LABORATORY SMELTING TEST WORK

[0031] Laboratory tests were conducted in 60 kW and 30 kW induction furnaces.

[0032] The raw material components at specified composition according to the test recipe in Table 4 were blended and packed in either an alumina, magnesite or graphite crucible. Power was increased at a rate of 20°C per minute until the target temperature was reached. Thereafter the crucible was held for specified durations at the target temperature. The furnace power was then switched off and the crucible was left to cool in an argon gas atmosphere inside the furnace.

Table 4: Conditions for laboratory smelting tests

1.3 DC ARC FURNACE TEST WORK

Facility description

[0033] The facility used in the preliminary investigation of the smelting of ZKD ore consisted of a DC power supply, a furnace and an off-gas handling system. Manual feeding was employed.

Testing conditions

[0034] A blend of ore and reductant was fed to the DC arc furnace. In total, six batches were processed. Two batches contained calcined ore. In the five first batches, the blend was manually fed into the pot through a roof feed port of the furnace. The sixth batch (Batch 6) was fed all at once when the pot was hot enough. The test work was conducted according to the conditions (feed and energy supply) given in Table 5.

Table 5: 100 kVA DC smelting test work conditions.

2. RESULTS AND DISCUSSION

2.1 SMELTING TESTS

[0035] The main objective of all the smelting test work was to investigate the smelting conditions that would yield an optimal grade of the rare-earth bearing slag. The test work was conducted with the aim of providing the optimal smelting recipe(s), operating temperature(s) as well as the characteristics of the products that would be generated. Concentration of rare-earth elements in the slag, clean separation between slag and metal products as well as the amenability to leaching of the slag product were the main parameters for the evaluation of the smelting process.

Overview of test work development - Thermodynamic evaluation

Smelting operation

[0036] The liquidus temperature of the fluxless smelting test work slag was determined. The unfluxed slag composition was estimated to be 44% AI 2O3 - 14% CaO - 42% S1O2 when FeO was fully reduced and MgO was assumed to be negligible. The melting point of this slag was thus estimated to be between 1600 and 1700°C using an AI2O3 - CaO - S1O2 phase diagram.

[0037] The other components not accounted for in the AI2O3 - CaO - S1O2 phase diagram, are expected to have effects on the liquidus temperature of the slag. FactSage thermodynamic package was used to investigate and predict the effects of these other slag components on the slag liquidus temperature and viscosity. Table 6 shows different possible slag compositions and their relative melting points as predicted by FactSage. The liquidus temperature predictions are done assuming an oxygen partial pressure of 1 atm and also at a typical iron making oxygen partial pressure of 1 "10 atm.

Table 6: FactSage data used to predict the operating temperatures of the different conditions

Table 7: FactSage data used to predict the operating temperatures for the fluxed smelting tests.

[0038] Overall, the data generated from Factsage gave an indication that a

portion of the rare-earth oxides in the slag would be in the form of a solid solution of

AICeO3 which may affect the viscosity of the slag, in spite of relatively lower slag

liquidus temperatures of the different planned smelting conditions. The viscosity can be decreased by addition of fluxes such as CaO. However these effects will be weighed against the recovery of REE to the slag; the highest REE concentration in the slag is the primary objective. Based on the ternary phase diagram and FactSage thermodynamic predictions, the test programme was developed as follows.

(A) Fluxless smelting at different anthracite additions to investigate the effects of residual FeO in the slag on the slag smelting temperature and fluidity (to improve metal-slag separation).

• Tests conducted at 1600°C; decreasing anthracite additions will increase residual FeO in the slag, and thus lower the operating temperatures. Solid AICeO3 may still exist in the slag.

(B) Fluxless smelting at 100% anthracite addition in different crucibles, with the objective of optimising the grade (concentration) of REE in the resulting slag and the quality of metal-slag separation

• Tests conducted at 1700°C in all crucible types. Besides the effect of temperature, the presence of perovskite solid phase as well as the basicity index may be the main parameters affecting the viscosity of the liquid slag and thus the quality of metal-slag separation; however the experimental tests would validate this.

(C) Tests to investigate the effects of different slag modifying fluxes (Na2CO3, K2CO3 and borax) on the smelting and extraction of REE in the leaching step.

• These tests were conducted at 1600°C. According to the FactSage simulations, they would result in a relatively lower slag liquidus

temperature, however because of the possible presence of solid perovskite phase and that the slag may be acidic, a higher temperature may be required to decrease the slag viscosity, and also to keep molten the pig iron produced. Graphite crucibles are used because these fluxes are aggressive to refractories.

(D) Additional tests to improve the metal-slag separation by decreasing viscosity.

(E) A Na2C03 flux test at 1700°C compared to 1600°C to evaluate the effects of a higher temperature on the viscosity and separation of metal and slag.

(F) An unfluxed test in a graphite crucible at 1800°C, also to investigate the effects of higher temperature on metal-slag separation. Additional tests at relatively lower CaO flux additions at 1 to 7% relative to ore input.

• These tests were conducted at 1600 and 1700°C to investigate the effect of slag basicity on metal-slag separation.

Distribution of rare-earth elements

[0039] Pyrosim and FactSage thermodynamic packages were used to estimate the distribution of rare-earth elements to the products of the smelting process. The following conditions were considered:

(A) Ore analyses based on the ZKD ore given in Table 1 and Table 2 of the raw material analyses.

(B) 100% stoichiometric carbon added for the reduction of Fe2O3, MnO and P2O5

(C) Operating temperature of 1700°C Ce/Ce2O3 was used to represent the total rare-earth elements/oxides in the FactSage while yttrium was used in a Pyrosim model. Only the metal and slag chemical analyses and recoveries for selected elements predicted by the models are summarised in Table 8 and Table 9.

Table 8: Chemical analyses/slag quality in mass %

Table 9: Recovery of essential elements in mass %

[0040] The theoretical predictions indicate that all the rare earths report to the slag phase as rare-earth oxides. The Pyrosim model gives a slag phase with a REE concentration 4 times that in the ore while the FactSage model predicts a relatively lower REE concentration in the slag at 2.93 times. The lower REE concentration predicted by Factsage is mainly attributed to a relatively lower MnO reduction as compared to that of the Pyrosim model. The calculated content of Ce in the FactSage metal is 0.000001 % at 1700°C, which also indicates that all the rare earth oxides report to the slag phase. In practice, the presence of the solid AICeO3 phase in the slag will not have an overall effect on the slag final grade while a more efficient reduction of MnO is possible. The actual concentrations of rare earths in the slag may be higher than predicted levels. The metal to slag ratio predicted by Pyrosim is 1 .56 while that predicted by FactSage is 1 .46; meaning a relatively lower slag tonnage as compared to that of the metal will be produced from these recipes. Minimising the slag tonnage and optimising its grade in REE minimise impurities to the hydrometallurgical plant, reduce consumption of consumables in the extraction process and as minimise plant size and its capital cost.

Estimation of viscosity of rare-earth oxide-containing melt

[0041] The slag produced is largely constituted of FeO, MnO, S1O2, AI2O3, CaO and MgO with a portion of up to 13% RE2O3. Rough analysis of the slag viscosity was done by ignoring the RE2O3 portion, although thermodynamically not correct. FactSage® 7.0 was used to estimate the viscosity of the portion of the melts composed of S1O2, AI2O3, CaO and MgO by normalising the slag composition to four components, i.e., S1O2, AI2O3, CaO and MgO. FeO and MnO were assumed to fully reduce; which would be an ideal situation. The FTOxid database was used to calculate the liquidus of the melt as well as the phase composition of the melt at 1600 °C. The viscosity module from FactSage was used to calculate the viscosity of the liquid at the liquidus temperature. For the calculations at 600 °C, the viscosity of the liquid portion of the melt was calculated using the viscosity module in FactSage and then adjusted to an "apparent" viscosity of the overall melt, using the Roscoe relationship to account for solids that are present in the melt (spherical particles were assumed).

[0042] As a result of refractory erosion when operated in alumina and magnesia crucibles, viscous slags of higher liquidus temperature would be produced in Tests 1 to 6 shown in Table 10. In these slags, alumina solid solutions are precipitated. However the presence of FeO and increased temperature will increase the fluidity of these slags.

[0043] Lower liquidus slags below 1600°C (in the absence of rare-earth oxides) are produced in the graphite crucible; the viscosity of these slags is relatively high. Good separation of metal and slag is achieved in Tests 5, 14 and 15; these slags have lower viscosity and a slightly higher basicity index. Increasing the slag basicity index by adding lime was employed to improve the slag-metal separation.

CLAIMS

1 . A method of processing an iron-rich rare earth-bearing ore which includes the steps of smelting the ore to concentrate rare earth oxide minerals in the ore into a slag phase and extracting the rare earth oxide minerals from the slag.

2. A method according to claim 1 wherein, in the smelting step, iron and manganese oxides in the ore are reduced to a low manganese pig iron in a metal phase.

3. A method according to claim 1 or 2 wherein smelting of the ore is achieved through the use of a graphite or carbon-based refractory crucible.

4. A method according to claim 1 , 2 or 3 wherein the molten slag is conditioned and after solidification, is milled and upgraded while the milled and upgraded slag is leached.

5. A method according to claim 4 wherein the slag is milled to a size of the order of -35 micron and the milled slag is leached in hydrochloric acid.

6. A method according to any one of claims 1 to 5 wherein, prior to the extraction step, a flux is added to the melt to facilitate the breaking of bonds between spinel phases and the various oxide elements which can occur in the slag while the resulting slag is conditioned during the solidification process.

7. A method according to claim 6 wherein the flux is selected from lime, Na2C03, K2CO3 and borax.

8. A method of processing an iron-rich rare earth-bearing ore which includes the steps of using a selective carbothermic smelting step for concentrating the rare earth oxide species into a slag phase and for precipitating iron and manganese in the ore, as low manganese pig iron,

in a metal phase and then processing the slag using hydrometallurgical techniques to extract and then to separate the rare earth elements.