Patent Description:
Large deposits of iron-rich rare earth-bearing ores is 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.

<CIT> discloses a process to upgrade rare earth elements (REE) in an iron containing ore by carbothermic smelting in the presence of a flux, and subsequent removal of pig iron. The REEs are recovered by acidic leaching of the milled slag.

<CIT> discloses a process to upgrade REEs in an iron containing ore. The process makes use of HCl for washing purposes.

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

The invention provides a method of processing an iron-rich rare earth bearing ore according to claim <NUM>.

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

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.

Without being restrictive the flux may be lime, Na<NUM>CO<NUM>, K<NUM>CO<NUM> and other suitable fluxing agents.

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.

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

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 <NUM>. Typically rare earth oxide minerals in this type of ore occur in a complex minerology of grains, and crystal clusters of less than <NUM> micron in size is 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.

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.

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

A flux <NUM> is added (in this example) to the furnace <NUM> during 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 <NUM> may be lime, Na<NUM>CO<NUM>, K<NUM>CO<NUM> or borax (these flux types is exemplary only and is not limiting). The optimum flux addition may be adjusted according to the type of ore which is being processed.

A slag <NUM> is tapped from the furnace <NUM>. Depending on the composition of the ore <NUM> the slag <NUM> may contain appreciable amounts of BaO, ThO<NUM> and SrO in addition to rare-earth species and other slagging elements such as SiO<NUM>, Al<NUM>O<NUM>, CaO and MgO.

Apart from concentrating the rare earth elements into the slag phase, the smelting process precipitates manganese and iron into a low-manganese pig iron <NUM> in the metal phase. The pig iron <NUM> can be recovered in a downstream process <NUM> using suitable techniques.

As an alternative to adding the flux <NUM> to the smelt in the furnace <NUM> 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 <NUM>.

The slag <NUM>, once solidified, is milled in a step <NUM> to produce a milled product <NUM> of suitable size, e.g. of the order of -<NUM> micron. The product <NUM> is then directly leached or upgraded before leaching (step <NUM>). Hydrochloric acid <NUM> is used to leach the slag. The product <NUM> produced by the leaching step <NUM> is subjected to a solid/liquid separation step <NUM> which produces a leach residue <NUM> which is disposed of by a suitable technique, and a leach solution <NUM>. In a subsequent impurity rejection step <NUM> lime <NUM> is added to the leach solution <NUM>. A resulting product <NUM> is subjected to a solid/liquid separation step <NUM> to remove impurities <NUM> such as Al, Fe and Th which is precipitated. Lime <NUM> is added to liquid <NUM> coming from the step <NUM> to precipitate (<NUM>) the rare earth elements <NUM> which is thereafter recovered by a solid/liquid separation step <NUM>.

Sulphuric acid <NUM> is added in a step <NUM> to liquid from the separation step <NUM> to enable hydrochloric acid (<NUM>) in solution to be recovered in a solid / liquid separation step <NUM>. A CaSO<NUM> precipitate <NUM> produced by the step <NUM> is disposed of in an appropriate way, while the recovered hydrochloric acid <NUM> is recycled to the direct leaching step <NUM>.

Laboratory and pilot scale tests undertaken to demonstrate the efficiency of the smelting step <NUM> and the recovery of the rare-earth oxides into the slag <NUM> have shown that more than <NUM>% of the total rare-earth elements contained in the iron-rich rare earth bearing ore <NUM> is recovered into the slag phase <NUM>. A concentration ratio of from <NUM> to <NUM> times the feed head rate is achieved. A pull mass of from <NUM>% to <NUM>%, and a total rare-earth element recovery from the slag <NUM> of more than <NUM>%, is 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 <NUM>.

For each unit of the ore <NUM> which is processed about <NUM>,<NUM> to <NUM>,<NUM> units of pig iron <NUM> is produced. The pig iron composition varies with the extent of reduction and the nature of the ore <NUM>. Alloys containing from <NUM> to <NUM>% Fe, and from <NUM> to <NUM>% Mn, with the balance being mainly Si and C, is produced.

The slags from the laboratory and pilot tests is leached and the leach residues <NUM> is collected, weighed and sampled for chemical and mineralogical analyses. It is established that the extraction yield of the rare-earth elements is over <NUM>%. The mass of the residue <NUM> is from <NUM> to <NUM>% of the initial mass of the slag <NUM>. In general the overall recovery rate of the rare-earth element concentration in the slag <NUM> to the production of the precipitate <NUM>, is in the range of <NUM> to <NUM>%.

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 <NUM>. The nature of the furnace crucible which is used during the smelting step <NUM> can have an effect on technical and economic aspects of the method of the invention. If a graphite crucible is used then the slag <NUM> need not necessarily be fluxed and direct HCl leaching of the unfluxed slag can be effected. Tests have shown that total rare-earth element leaching efficiencies ranging between <NUM>% and <NUM>%, at different acid dosages, is achieved. Additionally it has been demonstrated that direct HCl 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 loared when the slag is treated with a flux prior to leaching.

A benefit of the fluxing process is that the temperature of the smelting can be decreased from about <NUM>° to <NUM>. 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 as compared to that of the slag produced in a graphite crucible. Virtually no slag contamination took place through the use of a graphite crucible.

Zandkopsdrift (ZKD) iron-rich rare-earth bearing is used. Iron in the ore is in the form of goethite (FeO(OH)). This ore is calcined prior to crucible smelting test work as goethite decomposes at about <NUM> to produce Fe <NUM> and H2O. A summary of the chemical composition of the ore before and after calcining is given in Table <NUM> and Table <NUM>.

The granulometry of the ore supplied is <NUM>% passing to <NUM> sieve. The ore is milled to <NUM>% passing to <NUM> micron sieve, adequate size for laboratory test work while - <NUM> passing is used for the <NUM> kVA DC arc smelting test work.

The particle size distribution of the as is anthracite is <NUM>% passing to a sieve of <NUM> size. It is milled to <NUM>% passing to a <NUM> micron sieve for the crucible tests and used as is in the <NUM> kVA DC arc smelting tests. The proximate analysis of the anthracite used is given in Table <NUM>.

High purity laboratory grade Na2CO3, K2CO3, Borax and CaO is used as fluxing agents.

Laboratory tests is conducted in the <NUM> kW and <NUM> kW induction furnaces.

The raw material components at specified composition according to the test recipe in Table <NUM> is blended and packed in either an alumina, magnesite or graphite crucible. Power is increased at a rate of <NUM> per minute until the target temperature is reached. Thereafter the crucible is held for specified durations at the target temperature. The furnace power is then switched off and the crucible is left to cool down in the Argon gas atmosphere inside the furnace.

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 is employed.

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

The main objective of all the smelting test work is to investigate the smelting conditions that would yield to the production of optimal grade of the rare-earth bearing slag. The test work is 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 is the main parameters for the evaluation of the smelting process.

The liquidus temperature of the fluxless smelting test work slags is determined. The unfluxed slag composition is estimated to be <NUM>% A12O3 - <NUM>% CaO - <NUM>% SiO2 when FeO is fully reduced and MgO is assumed to be negligible. The melting point of this slag is thus estimated to be between <NUM> and <NUM> using an Al2O3 - CaO - SiO2 phase diagram.

The other components not accounted for in the Al2O3 - CaO - SiO2 phase diagram, is expected to have effects on the liquidus temperature of the slag. FactSage thermodynamic package is used to investigate and predict the effects of these other slag components on the slag liquidus temperature and viscosity. Table <NUM> shows different possible slag compositions and their relative melting points as predicted by FactSage. The liquidus temperature predictions is done assuming an oxygen partial pressure of <NUM> atm and also at a typical iron making oxygen partial pressure of <NUM>-<NUM> atm. The planned test work conditions include variation of anthracite additions, use of different crucible refractories as well as use of different fluxes.

Overall, the data generated from Factsage give an indication that a portion of the rare-earth oxides in the slag would be in the form of solid solution of AlCeO3 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 either 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 is developed as follows.

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 is conducted at <NUM>; decreasing anthracite additions will increase residual FeO in the slag, and thus lower the operating temperatures. Solid AlCeO3 may still exist in the slag.

Fluxless smelting at <NUM>% 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 is conducted at <NUM> 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.

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 is conducted at <NUM>. 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 is used because these fluxes is aggressive to refractories.

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

A Na2CO3 flux test at <NUM> as compared to that at <NUM> to evaluate the effects of a higher temperature on the viscosity and separation of metal and slag.

An unfluxed test in a graphite crucible at <NUM>, also to investigate the effects of higher temperature on metal-slag separation.

Additional tests at relatively lower CaO flux additions at <NUM> to <NUM>% relative to ore input. These tests is conducted at <NUM> and <NUM> to investigate the effect of slag basicity on metal-slag separation.

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

Ore analyses is based on the ZKD ore given in Table <NUM> and Table <NUM> of the raw material analyses.

<NUM>% stoichiometric carbon is added for the reduction of Fe2O3, MnO and P2O5.

Operating temperature of <NUM> Ce/Ce2O3 is used to represent the total rare-earth elements/oxides in the FactSage while yttrium is used in the Pyrosim model. The results of the Pyrosim predictions is given in APPENDIX A. Only the metal and slag chemical analyses and recoveries for selected elements predicted by the models is summarareed in Table <NUM> and Table <NUM>.

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 <NUM> times that in the ore while the FactSage model predicts a relatively lower REE concentration in the slag at <NUM> 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 <NUM>% at <NUM>, which also indicates that all the rare earth oxides report to the slag phase. In practice, the presence of solid AlCeO3 phase in the slag will not on overall affect the slag final grade in rare earths 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 <NUM> while that predicted by FactSage is <NUM>; 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 is able to minimise impurities to the hydrometallurgical plant, reduce consumptions of consumables in the extraction process as well as minimise plant size and its capital cost. These thermodynamic models will be refined when comprehensive rare-earth data (compounds and solutions) become available in the FactSage and Pyrosim packages.

The slag produced is largely constituted of FeO, MnO, SiO2, Al2O3, CaO and MgO with a small portion of up to <NUM>% RE2O3. Rough analysis of the slag viscosity is done by ignoring the RE2O3 portion, although thermodynamically not correct. FactSage® <NUM> is used to estimate the viscosity of the portion of the melts composed of SiO2, Al2O3, CaO and MgO by normalising the slag composition to four components, i.e., SiO2, Al2O3, CaO and MgO. FeO and MnO is assumed to fully reduce; which would be an ideal situation. The FTOxid database is used to calculate the liquidus of the melt as well as the phase composition of the melt at <NUM>. The viscosity module from FactSage is used to calculate the viscosity of the liquid at the liquidus temperature. For the calculations at <NUM>, the viscosity of the liquid portion of the melt is calculated using the viscosity module in FactSage and then adjusted to an "appisnt" viscosity of the overall melt, using the Roscoe relationship to account for solids that is present in the melt (spherical particles is assumed).

As a result of refractory erosion when operated in alumina and magnesia crucibles, viscous slags of higher liquidus temperature would be produced in Tests <NUM> to <NUM> shown in Table <NUM>. In these slags, alumina solid solutions is precipitated. However the presence of FeO and increased temperature will increase the fluidity of these slags. Lower liquidus slags below <NUM> (in the absence of rare-earth oxides) would be produced in the graphite crucible; the viscosity of these slags is relatively high. Good separation of metal and slag is achieved in Tests <NUM>, <NUM> and <NUM>; these slags have lower viscosity and slightly higher basicity index. Increasing the slag basicity index by adding lime is employed to improve the slag-metal separation.

The overall mass balance of the laboratory smelting test work is given in Table <NUM> and Table <NUM>. These tables include the masses of the raw materials (ore, flux and reductant), slag and metal products for the various conditions investigated. The tests are grouped below according to particular objectives investigated.

Tests <NUM> to <NUM> investigated the effect of anthracite addition on the slag quality and melting temperature for tests carried out in alumina crucibles. Tests <NUM> to <NUM> demonstrated (validated) that the melting point of the slag decreases with decreasing anthracite addition as predicted by FactSage. The optimal operating condition could not be assessed as the resulting slags is contaminated by eroded refractory material; REO contents in the slag is diluted.

Tests <NUM>, <NUM> and <NUM> investigated the effect on the slag chemistry and final slag REO content of using different crucibles/refractories (as a result of crucible erosion). Tests <NUM>, <NUM> and <NUM> is carried out in alumina, magnesite and graphite crucibles, respectively. The metal-slag separation in these tests seemed good. The best refractory is the one that has minimal erosion (or contaminate least) by the primary slag generated by the ore (and will subsequently provide optimal REO concentration in the slag). In addition, the slag thus produced should also be leachable. Test <NUM> gave the best results and subsequent tests is all carried out in graphite crucibles.

Tests <NUM> to <NUM> investigated the effects of different flux additions on the slag phases produced for leaching purposes. These tests are all conducted in graphite crucibles because of the corrosive nature of the fluxes used towards alumina and magnesite refractories. The metal and slag masses for Tests <NUM> to <NUM> conducted at <NUM> is not recorded and only the combined masses of slag and metal is presented. All the tests led to virtually no metal-slag separation. Smelting of the ore is effective at <NUM> as is observed from visual inspection of the crucible products. The separation is most probably affected by the high slag viscosity which could be a result of low basicity index and the presence of solids. As indicated in the results of FactSage in Table <NUM>. Whilst the addition of slag modifiers lowers the liquidus temperature of the slag, a portion of rare-earth oxides in the slag may exist as a high melting point solid.

Tests <NUM> to <NUM> are conducted to investigate conditions leading to improved metal-slag separation. Test <NUM> is conducted at <NUM> to investigate the effect of temperature increase on the metal-slag separation of tests fluxed with Na2CO3, specifically Test <NUM>. Test <NUM> is conducted at <NUM> to investigate the effect of temperature increase on the metal-slag separation of Test <NUM> which is unfluxed. The slag-metal separation of Tests <NUM> and <NUM> appeared better than that of Test <NUM> and Test <NUM>, respectively.

Tests <NUM> to <NUM> is fluxed with varying levels of CaO to evaluate the effect of increasing slag basicity index on metal-slag separation as well as on the reduction of MnO. These are conducted in graphite crucibles to evaluate them against fluxless Test <NUM>. Tests <NUM> to <NUM> is conducted at <NUM> and Tests <NUM> to <NUM> is conducted at <NUM> to evaluate the effect of basicity index on the liquidus temperature. As indicated in the mass balance results in Table <NUM>, these fluxed tests resulted in much better metal-slag separation. The chemical analyses indicated increased basicity index resulted in increased reduction of MnO. Tests <NUM> to <NUM> demonstrated that addition of CaO also lowered the liquidus temperature of the slag; more efficient smelting is carried at <NUM> and <NUM> as compared to Test <NUM> with no flux addition. The chemical analyses of all the tests conducted is discussed in the next sections.

The chemical analyses of the slag is given in Table <NUM> and Table <NUM>.

Tests <NUM> to <NUM>: The metal-slag separation is good. The slags contained a relatively lower concentration of REO as a result of contamination by alumina eroded from the crucible refractory as well as relatively higher FeO contents in tests conducted with relatively lower than the stoichiometric amount of anthracite additions. The basicity indexes is lower than <NUM>. The slag RE2O3 concentrations ranged from <NUM> to <NUM> %.

Test <NUM>: The metal-slag separation is also good. The slag contained lower RE2O3 concentration at <NUM>% due to contamination of the slag by MgO eroded from the MgO crucible. The slag had a relatively higher slag basicity index at <NUM> and this had a positive effect on MnO reduction. The concentration of MnO in the slag is lower than that for slags from Tests <NUM> - <NUM> conducted in alumin a crucibles.

Test <NUM>: The metal-slag separation is good. The RE2O3 grade of the slags produced in the graphite crucible at <NUM>% is higher than that in alumina and MgO crucibles. No slag contamination virtually took place in the graphite crucible as is observed in the alumina and MgO crucibles. Graphite crucible erosion contributes to provide an excessive reducing environment, which resulted in relatively higher reduction of MnO than in the alumina and MgO crucibles. However a relatively higher FeO content in the slag at <NUM>% is observed. Iron speciation analyses on the slag revealed that FeO (reported as Fe2+) in the slag is in fact <NUM>%. The slag contained <NUM>% entrained Fe. The entrainment of submicron metallic prills to the slag could be attributed to a relatively higher slag viscosity / higher liquidus temperature as is predicted in the FactSage model for high REO contents in the slag. Higher rare earth concentrations in the slag may result in a higher liquidus temperature and a significant amount of solid perovskite phase (AlCeO3). Between the unfluxed different crucible tests (<NUM>, <NUM> and <NUM>), fluxless smelting in a graphite crucible is more preferable.

Tests <NUM> to <NUM>: Metal-slag separation is poor. Clean slag pieces is collected and analysed. The REO concentration is relatively higher in the range of <NUM> - <NUM>%. Because these tests is carried out in graphite crucibles and thus in excessively reducing environment, relatively higher reductions of iron and manganese is observed. The FeO levels ranged between <NUM> and <NUM> %. The entrainment of Fe metal prills in the slag ranged from <NUM> to <NUM>%. Poor metal-slag separation could be attributed to high slag viscosity, which would be a result of low basicity index and possibly high liquidus temperature (as a result of high REO content).

Tests <NUM> and <NUM>: These tests is carried out to investigate means to improve the metal-slag separation. Increasing temperature appeared to have a positive effect on the slag viscosity and reduction of reducible oxides. Based on the Fe analyses in the slag, Test <NUM> metal-slag separation is better than Tests <NUM> to <NUM> separations and Test <NUM> separation is better than that achieved in Test <NUM>. Fe content in the slag is relatively low.

Tests <NUM> to <NUM>: Fluxing the smelting recipe with lime is investigated to improve the metal-slag separation. Good metal-slag separation is achieved at all CaO levels and operating temperatures. This is attributed to increased slag basicity index as a result of lime addition as a fluxing agent to the smelting recipe. The grade of REO in the slag is in the range of <NUM>-<NUM>% for Test <NUM>-<NUM> and <NUM>- <NUM>% for tests <NUM> to <NUM>. The slag REO grades of Tests <NUM>-<NUM> conducted at <NUM> is relatively lower than those of Tests <NUM> to <NUM>, due to higher reduction of MnO at <NUM> than at <NUM>. At <NUM>, anthracite addition may be increased to improve the reduction of MnO and subsequently the content of REO in the slag.

Compared to the optimal unfluxed condition in Test <NUM>, the addition of CaO is found to be optimal in the tests that resulted in good metal-slag separation with REO grade at least equal to that in Test <NUM> slag. Tests <NUM> and <NUM> met these requirements. The slag REO grades is <NUM> % and <NUM> %, respectively as reported in Table <NUM>. Improved reduction of MnO is achieved in these tests as compared to Test <NUM> as a result of increased slag basicity and operating temperature (<NUM>).

In larger commercial operations, CaO additions of up to <NUM>% may be carried out as these will result in higher REO, lower FeO, lower MnO in the slag as well as better furnace operation, better metal-slag separation, and virtually no metal entrainment in the slag. However, the most important parameter for the optimal recipe, either unfluxed or fluxed, will be the amenability of the slags to be leached efficiently.

As indicated in Table <NUM> and Table <NUM>, the concentration of REO in the slag phase varied from <NUM> to <NUM> %, dependent on the smelting conditions. At its highest, the total rare-earth element in the slag is up to about <NUM> times its concentration in the ore, a significant upgrade. The chemical erosion is acute in the alumina crucible while it is still significant in the MgO crucibles. Consequently, slags of relatively lower REO concentrations is produced in the test work conducted in alumina and magnesite crucibles while higher REO concentrations is obtained in the tests conducted in graphite crucibles (Tests <NUM>-<NUM>). The extent of crucible erosion is also evaluated and confirmed by comparing the SiO2/Al2O3 and SiO2/MgO ratios in the slag to their original ratios in the ore as presented in Table <NUM> and Table <NUM>, respectively.

Due to the extent of chemical erosion, the REO grade of the slag produced in the alumina and MgO crucibles is about half of that of the slag produced in the graphite crucible. Figure <NUM> shows the associated Al2O3 and MgO levels in the slag for Tests <NUM>, <NUM> and <NUM>; or in magnesite, alumina and graphite crucibles.

Based on the above evaluations of effects of crucible erosion on the slag quality, a carbon based refractory would be recommended in order to minimise slag contamination and thus maximise slag REO grade. Operating the furnace with a freeze line can also achieve similar results as those for the smelting in the carbon crucible; this option is highly recommended and is standard practice on large scale furnaces.

Producing a higher slag REO grade and lowering the level of deleterious impurities in the slag is very important as it will decrease the consumption of reagents in the hydrometallurgical circuits and ultimately lower the plant size and cost, which will impact positively on the process economics.

The compositions of the iron alloy produced is presented in Table <NUM>. A carbon-saturated iron-manganese alloy is produced from these tests. In the smelting process, iron is preferentially reduced over manganese. The reduction of iron is almost complete in all the various conditions investigated. The composition of the alloy appeared to be strongly related to the extent of manganese reduction. For instance, increase of manganese reduction increases the alloy manganese content while it decreases its iron concentration by dilution.

As manganese oxide is an undesirable impurity in the leaching process contributing to increased acid consumption, its reduction to the alloy in the smelting step should be optimised. The reduction of manganese oxide is affected by the reductant addition, temperature and slag basicity index. MnO reduction in the graphite crucible tests fluxed with CaO is even better due to higher slag basicity index. There is a noticeable difference in the reduction of FeO and MnO for Test <NUM> carried out in a magnesite crucible as compared to the results achieved in the graphite crucibles with CaO fluxing.

Saturated-carbon iron-manganese alloys is produced in the crucible smelting tests. The highest levels of P is from Tests <NUM> and <NUM>. These tests is conducted at relatively lower temperature and anthracite addition in the recipe is less than the stoichiometric amount. As a consequence, a lower amount of metal is produced while P2O5 is almost fully reduced to the alloy.

The metal composition corresponding to the optimal slag production is considered as being the optimal metal composition. Optimal metals is produced in Test <NUM>, and Tests <NUM> and <NUM>. Based on these recipes, the optimal alloy composition produced from this particular Zandkopsdrift ore sample would be: <NUM>-<NUM> % Fe, <NUM>-<NUM> % Mn, <NUM>-<NUM> % C, <NUM>-<NUM> % Si and <NUM>-<NUM> % P. This alloy composition falls within the commercial manganese steel composition range which consists of <NUM>-<NUM>% Mn.

The metals to slag ratios reported in Table <NUM>, is calculated only for the tests which resulted in good slag-metal separation. These results is compared to the theoretical values of the metal to slag ratio calculated using Pyrosim and FactSage (in section <NUM>. These ratios can be used to assess the extent of contamination of the slag by crucible erosion, extent of reduction relative to the predictions and the mass pull of the REE containing slag relative to the ore.

The metal-to-slag ratio of the fluxless smelting conditions for Tests <NUM> and <NUM> conducted with anthracite additions below the stoichiometric amount is relatively lower due to the presence of unreduced FeO and MnO in the slag and also due to significant crucible erosion that increases the slag volume.

Higher ratios is achieved in the graphite crucible unfluxed tests. The high ratios is attributed to the following factors: minimal flux addition, absence of crucible erosion, and increased MnO reduction to the alloy.

A metal-to-slag ratio of <NUM> is measured in Test <NUM>; which is closer to the values predicted using Pyrosim and FactSage models. Test <NUM> which is a repeat of Test <NUM> at a higher temperature resulted in a ratio of <NUM>. The Test <NUM> ratio is the highest as a result of better metal-slag separation as well as higher MnO reduction.

As compared to Test <NUM>, Tests <NUM> to <NUM> resulted in relatively higher metal-to-slag ratios which decreased with increasing CaO addition. As indicated in section <NUM>. <NUM>, lime addition promoted the reduction of MnO, improved the metal-slag separation, and also diluted the slag. The metal-to-slag ratio results further support the following recommendations:.

For the purpose of producing a leachable slag feed of higher REO concentration, a higher metal-to-slag ratio must be targeted by minimising crucible erosion or the contamination of the slag with crucible material. This can be done either by using a carbon based refractory or by developing a crucible freeze line during operation.

The recoveries of REE and metal oxides to the slag phase is given in Table <NUM> and Table <NUM>, respectively. Recoveries to the alloy is given in Table <NUM>. Recoveries is only calculated for tests yielding good metal-slag separation.

Rare earth oxides is stable at the conditions of the reduction of iron oxides. Tests <NUM> to <NUM> carried out in graphite crucibles resulted in REE recoveries ranging from <NUM> to <NUM>%. These tests and particularly those yielding a clean metal-slag separation demonstrated that all the rare earth oxides would report to the slag phase at the smelting conditions investigated.

The distribution of rare earths in the product streams is calculated based on the REE analyses and masses of the slag and metal produced. The concentration of TREEs in selected alloys is very low as indicated in Table <NUM>,.

It is also worth noting that FeO and MnO is significantly reduced at higher slag basicity index. This appeared specifically in Tests <NUM> to <NUM> fluxed with CaO. The recoveries of FeO to the slag in all these tests is below <NUM>%, indicating that FeO is effectively reduced in all the test work in spite poor metal-slag separation in some tests. However closer to about <NUM>% of MnO stayed unreduced in the slag.

Recoveries of Fe, P and Mn to the metal phase.

As indicated in Table <NUM>, the recovery of Fe to the alloy calculated on the basis of the content of this element in the feed ranged between <NUM>% and <NUM>%. This further validates that the reduction of FeO in the tests conducted is effective in spite of poor metal and slag separation in some tests.

The recoveries of P to the metal is highest at low anthracite additions and lowest at high anthracite additions and temperatures.

However in Test <NUM> conducted in a graphite crucible at <NUM>, with <NUM>% CaO flux addition in the smelting recipe, the highest proportion of REEs is present in the Ca-Silicate phase, lower amounts is detected in the CaAl silicate and the Ba-rich Ca-silicate phases as can be seen in Figure <NUM>. This distribution is similar to that of Test <NUM> slag.

Laboratory smelting test work demonstrated that the smelting of the ZKD ore can be conducted without flux addition at a temperature of about <NUM>. However the temperature of the smelting can be decreased to about <NUM> with the addition of fluxes.

Fluxless smelting in various crucible types demonstrated that a graphite or carbon-based refractory should be used as it minimises the contamination (dilution) of the slag product and thus results in higher concentration of REE in the slag. Operating the furnace with an efficient freeze line is however highly recommended to prevent crucible erosion.

Fluxing with a minimal lime addition of <NUM> to <NUM>% relative to the ore is investigated. This provided a clean metal-slag separation as well as promoted the MnO reduction. Fluxing with minimal CaO (<NUM>-<NUM>%) may be recommended in order to minimise acid consumption in the leaching step; it will improve the reduction of MnO whilst producing a high REO grade slag.

As the effects of RE2O3 on the slag chemistry is not well known, especially the effects of the presence of solid perovskite phase in the molten slag, more work will be required in this field.

Various slag samples produced in the smelting tests is subjected to leaching in order to determine the amenability of the rare-earth elements to leaching. Three leaching methods as listed below is explored to determine the most economical route to be used:.

The slag used in the acid baking leaching tests is produced in the <NUM> kVA furnace in Test <NUM> in an alumina crucible furnace. It is saturated with Al2O3 (due to crucible erosion), has low concentration of TREE and high MnO content. The slag composition in REEs and other metal elements is shown in Table <NUM> and Table <NUM>, respectively. La and Ce is the major REE elements present in the feed solids, constituting almost <NUM>% of the total rare earth elements (TREE) content of <NUM>%. The major impurities in the sample is Fe, Mn, Si, Mg, Ca and Al.

The slag is contacted with pre-determined amount of concentrated H2SO4 (<NUM>% (m/m). The mixture of the acid and slag is weighed and transferred into a baking tray. The acid contacted slag is baked in an oven at specified test temperature. At the end of the baking period, the samples is weighted prior to subjecting them to water leach.

The baked samples is subjected to water leach to solubilise the rare-earth sulphates; deionised water is used as the lixiviant. Water leaching is conducted for <NUM> hour residence time, at a pulp density of <NUM>% (m/m). At the end of the test, the entire reactor content is filtered. The filtrate volume is measured and wet un-washed cake weighted. The weighted cake is washed three times initially with pH adjusted water and thereafter with deionised water at a ratio of <NUM>:<NUM> (i.e. for <NUM> of sample <NUM> of deionised water will be used).

The slag sample is subjected to cracking with <NUM>% sodium hydroxide (NaOH), for a period of <NUM> hours, at a temperature of <NUM> and in initial pulp density of <NUM> % (m/m). At the end of the test the entire slurry is diluted with deionised water then filtered. The filtered wet cake is then re-pulped once with deionised water to remove entrained Na and dried in an oven overnight prior to water leaching. The filtrate and residue is measured and analysed for REE and base metals.

The residues from the caustic cracking tests is used as feed for the water leach. The water leach test is conducted in order to wash entrained Na in the sample. The washed residue from the water leach test is then subjected to HCl leach. The HCl leach is conducted in order to dissolve the REE hydroxides and recover them in the chloride form. One test is conducted using glucose as a reductant and the other test is conducted without a reductant. The addition of glucose into the slurry is aimed at reducing the Ce (IV) in order to improve the leaching of other REE in the sample. A stoichiometric amount of glucose is added upfront targeting <NUM>% stoichiometry based on total Ce in feed. Both tests is conducted at <NUM>, targeting a pH value of <NUM>, for <NUM> hours.

The slag is milled and then slurried in HCl solution (<NUM>% (m/m) or <NUM>% (m/m), targeting the required target pulp density (<NUM>% and <NUM>%) and agitated. The temperature is then increased to <NUM>. After <NUM> hours of reaction, the mixture is filtered and the mass of the wet unwashed residues is recorded. The filtered cake is weighed, subsequently re-slurried and washed three times, initially with acidified deionised water (deionised water acidified to the pH of slurry) and thereafter with deionised water. The cake is initially washed at a ratio of <NUM> time the mass of wash liquor to the wet cake mass and the second and third washes at a ratio of <NUM> times the mass of the wash liquor to the wet cake mass.

Efficient smelting process yielded clean slag and metal; thus provide a clean separation of the two products. The extent of this separation is visually evaluated on solidified crucible products. This observation, although it did not take into account the reaction yields and kinetics as well as the amount of entrained metallic prills to the slag, is very important in the evaluation of a novel smelting process. Clean separation is achieved at <NUM> and above in the fluxless smelting tests of the as received Zandkopsdrift ore. However, clean separation is observed at <NUM> in the fluxed smelting tests.

Stoichiometric anthracite addition is carried out to reduce both FeO and MnO. A residual slag FeO below <NUM>% could be achieved in most of the smelting tests while the levels of MnO could only be significantly reduced in relatively higher basic slags (fluxed with lime).

The melting temperature decreases by decreasing the reductant addition (increasing slag FeO content). However this produce diluted REE slags.

Levels of about <NUM>% total rare-earth elements in the slag is reached in the smelting tests conducted in graphite crucibles. These is the tests where virtually no contamination of the slag by the refractory occurred. A concentration ratio of <NUM> (from the ore grade to slag grade) with a pull mass of <NUM> - <NUM>% is achieved in these cases.

Acute erosion of alumina and magnesite crucibles is observed which had a significant impact on the REE grade of the final slag products. The slag REE grade is almost half of that achieved in the graphite crucibles.

Based on the distribution of REE in the products streams and the results of the mass balance of the crucible smelting tests, more than <NUM>% of rare-earths is recovered to the slag.

Iron-manganese alloys containing about <NUM>% Fe, <NUM>%Mn, <NUM>%C, <NUM>% Si and <NUM>% P is produced. Specifically because of its high content in P, refining of this alloy or its upgrading may be required for its use in the steel industry. Removal of P should be investigated as well as the use of this alloy for other applications such as in the manufacturing of dense separation media.

More than <NUM>% Fe is recovered to the metal in most of the conditions investigated. MnO reduction appeared to be affected mainly by the slag basicity and temperature. About <NUM> to <NUM>% of Mn is reduced to the metal in the acidic slag conditions while with the addition of lime as flux (increasing the basicity index), up to about <NUM>% is reduced to the metal. As expected, the addition of lime appeared to significantly increase the reduction of MnO to the alloy.

The modelling work conducted by considering Ce/ Ce2O3, found that addition of rare-earth oxides to the molten typical slag in the particular conditions investigated would increase the liquidus temperature and viscosity. FactSage predicted the formation of a perovskite phase which occurrence would increase with increased slag rare-earth grade. Perovskite phase is a solid aluminate-silicate rare-earth oxide that is stable up to a temperature of <NUM>.

Within the scope of this smelting process, slag chemistry and temperature contributed significantly to the metal-slag separation. Better separations is achieved at either higher basicity index or higher temperatures of <NUM> and above. These conditions impacted positively on the manganese reduction.

Leaching efficiency levels of about <NUM>% is attained in the direct hydrochloric acid leaching; other leaching methods all resulted in leaching efficiencies iof less than <NUM>%.

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 <NUM>. The nature of the furnace crucible which is used during the smelting step <NUM> can have an effect on technical and economic aspects of the method of the invention. If a graphite crucible is used then the slag <NUM> need not necessarily be fluxed and direct HCl leaching of the unfluxed slag can be effected. Tests have shown that total rare-earth element leaching efficiencies ranging between <NUM>% and <NUM>%, at different acid dosages, were achieved. Additionally it has been demonstrated that direct HCl 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.

Claim 1:
A method of processing an iron-rich rare earth-bearing ore which includes the step of carbothermic smelting of the ore to produce a slag, the smelting step being regulated to control a level of residual FeO in the slag and to achieve at least <NUM>% iron reduction by weight, and by adding at least one suitable flux and conditioning of the slag through controlled cooling to concentrate rare earth oxide minerals in a leachable species in the slag wherein the molten slag, after solidification, is milled to a size of the order of <NUM> micrometers, and the milled slag is leached in hydrochloric acid to extract the rare earth oxide minerals from the slag, and wherein, during the smelting step, iron and manganese oxides in the ore are reduced to a low manganese pig iron in a metal phase.