OA19464A - Processing of iron-rich rare earth bearing ores. - Google Patents

Abstract

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.

Description

[0014] The accompanying drawing is a flow sheet of steps in a method according to the invention for the extraction of rare earth éléments from a mineralogically complex iron-rich rare earth ore 10. Typically rare earth oxide minerais in this type of ore occur in a complex minerology 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 minerais. A conventional milling and physical séparation 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 éléments.

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

[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 métal and slag phases produced by the furnace, are dépendent on the smelting conditions and particularly on the furnace operating température, 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 réduction to the métal phase, and optimum molten slag properties while the furnace température is selected to effect efficient slag-metal séparation.

[0017] A flux 16 is added (in this example) to the furnace 14 during the smelting process. The nature of the fluxing is such as to modify the slag, to improve the recovery of major métal 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, Na2CO3, 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 ofthe ore 10 the slag 20 may contain appréciable amounts of BaO, ThOsand SrO in addition to rare-earth species and other slagging éléments such as S1O2, AI2O3, CaO and MgO.

[0019] Apart from concentrating the rare earth éléments into the slag phase, the smelting process précipitâtes manganèse and iron into a low-manganese pig iron 22 in the métal phase. The pig iron 22 can be recovered in a downstream 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 érosion 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 séparation step 40 which produces a leach residue 42 which is disposed of by a suitable technique, and a leach solution 46. In a subséquent impurity rejection step 48 lime 50 is added to the leach solution 46. A resulting product 52 is subjected to a solid/liquid séparation 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 éléments 68 which are thereafter recovered by a solid/liquid séparation step 70.

[0022] Sulphuric acid 74 is added in a step 76 to liquid from the séparation step 70 to enable hydrochloric acid (78) in solution to be recovered in a solid / liquid séparation step 80. A CaSO₄ 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 hâve shown that more than 90% of the total rare-earth éléments contained in the iron-rich rare earth bearing ore 10 is 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 dépends 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 réduction 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 are leached and the leach residues 42 are collected, weighed and sampled for Chemical and mineralogical analyses. It is established that the extraction yield of the rare-earth éléments 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 ofthe precipitate 68, is in the range of 80 to 90%.

[0026] The économie viability ofthe process shown in the accompanying flow sheet dépends largely on mining and electricity costs and on the total rare-earth element grade ofthe ore 10. The nature of the furnace crucible which is used during the smelting step 14 can hâve an effect on technical and économie aspects ofthe method ofthe invention. If a graphite crucible is used then the slag 20 need not necessarily be fluxed and direct HCl leaching of the unfluxed slag can be effected. Tests hâve 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 HCl leaching ofthe slag, compared to acid baking and caustic (NaOH) cracking, is préférable. It has also been observed that the extraction efficiency of light rare-earth éléments which include La, Ce, Nd and Pr is loared when the slag is treated with a flux prior to leaching.

[0027] A benefit of the fluxing process is that the température of the smelting can be decreased from about 1700° to 1600°C. Use of a graphite or carbon-based refractory crucible is préférable as it minimizes the contamination ofthe slag product and this results in a higher concentration of the rare-earth éléments in the slag. It has been noted that due to the effect of Chemical érosion the rare-earth oxide grade ofthe 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 5 of goethite (FeO(OH)). This ore was calcined prior to crucible smelting test work as goethite décomposés at about 300 °C to produce Fe₂O3 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 adéquate size for laboratory test work 10 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”

MgO	AI2O3	SiO2	CaO	TiO2	V2O5	Cr2O3	MnO	FeO(OH)	S/A	S/M	P2O5
%	%	%	%	%	%	%	%	%			%
1.13	6.48	6.08	2.06	3.87	0.109	0.073	9.09	46.9	0.94	5.38	1.77
La	Ce	Pr	Nd	Sm	Eu	Gd	Dy	Er	TREE	Th	U
ppm	3pm	ppm	ppm	ppm	PPm	ppm	ppm	ppm	ppm	PPm	ppm
6060	10200	921	3900	478	145	435	166	99.8	23420	221	71.6

Ho,Tm, Lu, Yb ; REE with concentrations iess than IGOppm

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

MgO	Al 203	SiO2	CaO	TiO2	V2O5	Cr2O3	MnO	Fe2O3	S/A	S/M	P2O5
%	%	%	%	%	%	%	%	%			%
1.6	9.29	8.01	3.47	3.97	0.117	0.03	10.8	50.1	0.86	5.01	NA
Ls	Ce	Pr	Nd	Sm	Eu	Gd	Dy	Er	TREE	Th	U
ppm	PPm	ppm	ppm	PPm	ppm	PPm	ppm	PPm	%	PPm	ppm
6756	11233	1222	4133	443	117	355	164	954	2.57	279	NA

Ho,Tm, Lu, Yb ; REE with concentrations Iess than 100ppm; S=SiO2; A=AlaO3; M=MgO 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 100 kVA DC arc smelting tests. The approximate analysis of the anthracite used is given in Table 3.

Table 3: Summary ofthe bulk Chemical composition ofthe anthracite (mass %)

As h	Volatile	Fixed Carbon	Total Sulphur
4.74	6.19	89.1	0.56

Fluxes

High purity laboratory grade Na2CO3, 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 fumaces.

[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 température was reached. Thereafter the crucible was held for specified durations at the target température. The furnace power was then switched off and the crucible was left to cool in the argon gas atmosphère inside the furnace.

Table 4: Conditions for laboratory smelting tests.

Test	Anthracite %	Fluxes, %	Température, °c	Crucible
Na2C 03 Borax	K2CO3	CaO
	variation of anthracite addition and température
1	100					1700	Al
2	90					1700	Al
3	80					1600	Al
4	70					1600	Al
	Variation of crucibles and température
5	100					1700	MgO
6	100					1700	G
		Variation of flux addition and température
7	100	5				1600	G
8	100	10				1600	G
9	100	25				1600	G
10	100	50				1600	G
11	100		5			1600	G
12	100			5		1600	G
13	100			50		1600	G
14	100	50				1700	G
15	100					1800	G
16	100				1	1700	G
17	100				3	1700	G
18	100				7	1700	G
19	100				1	1600	G
bo	100				3	1600	G
21	100				7	1600	G

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 ail at once when the pot is assumed 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.

Test		Ore Condition	Power [kW]	Objective
22	Batch 1	Calcined ore	50	Warm-up & baseline
23	Batch 2	Calcined ore	50	Slag and Métal production
24	Batch 3	Calcined ore	50	Slag and Métal production
25	Batch 4	As is	60	Slag and Métal production
26	Batch 5	As is	60	Slag and Métal production
27	Batch 6	As is	45	Slag and Métal production

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 rare19464 earth éléments in the slag, clean séparation between slag and métal products as well as the amenability to leaching of the slag product were the main parameters for the évaluation of the smelting process.

OverView of test work development - Thermodynamic évaluation

Smelting operation

[0036] The liquidus température of the fluxless smelting test work slag was determined. The unfluxed slag composition was estimated to be 44% A12O3 -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 - SiO2 phase diagram, are expected to hâve effects on the liquidus température of the slag. FactSage thermodynamic package was used to investigate and predict the effects of these other slag components on the slag liquidus température and viscosity. Table 6 shows different possible slag compositions and their relative melting points as predicted by FactSage. The liquidus température prédictions are done assuming an oxygen partial pressure of 1 atm and also at a typical iron making oxygen partial pressure of T¹⁰ atm.

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

Compo -sition	Slag composition, %	PO2 = 1 atm	PO2 = log10 atm
AI2O 3	SiO 2	Ca 0	Mg 0	TiO 2	Mn O	FeO	Ce2 03	T°C	Liqui d %	T°C	Liqui d %
1	44	42	14						1620	100	1620	100
2	33	31	10.5	5.75	19.7				1470	99.5	1468	99.8
Alumina refractory compositions, with	high AI2O3 in s	ag
3	52.7	16.3	5.6	6.9	9.4	6,48	2.51		1678	99.9	1668	99.5
4	49.7	15.4	5.3	6.51	8.9	10.7	3.55		1653	100	1646	99.7
5	47.5	14.7	5.1	6.21	8.5	13.6	4.52		1631	100	1625	99.8
6	46.4	14.4	5	6.08	8.3	13.3	6.63		1615	100	1610	99.8
7	49.3	15.3	5.3	6.46	8.8	6.06	2.35	6.46	1710	91.6	1696	91.1
MgO refractory compositions, with high MgO in slag
8	24.5	16	6.7	28.8	9.8	11.7	2.45		1690	99.9	1705	99.8
9	23.1	15	6.4	27.2	9.2	11	2.31	5.78	1725	92.4	1715	91.6
Graphite refractory test with relatively lower AI2O3 and MgO in slag
10	38	20.9	13.2	6.98	13.2	5.43	2.33		1567	100	1551	99.8
11	32.9	18.1	11.4	6.04	11.4	4.7	2.01	13.4	1549	82.4	1537	82.3

Table 7: FactSage data used to predict the operating températures for the fluxed smelting 5 tests.

Compo sition	Slag composition, %	Flux, %	P 02 = 1 atm	PO2 = log-10 atm
AI2 03	Si 02	Ca O	M go	Ti 02	M nO	Fe O	Ce2 03	Bor ax	Na2 CO3	K2C 03	T °c	Liq uid %	T °c	Liq uid %
12	31. 3	17. 3	10 .9	5. 75	10 .9	4. 47	1. 92	12.8	4.8			14 24	83. 3	14 19	83. 2
13	26. 3	14. 5	9. 1	4. 83	9. 1	3. 76	1. 61	10.7		20		14 78	86	13 96	77. 6
14	26. 3	14. 5	9. 1	4. 83	9. 1	3. 76	1. 61	10.7			20	16 23	86	-

[0038] Overall, the data generated from Factsage gave an indication that a portion ofthe rareearth oxides in the slag would be in the form of a solid solution of AlCeOa which may affect the viscosity of the slag, in spite of relatively lower slag liquidus températures 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 prédictions; 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 température and fluidity (to improve metal-slag séparation).

•Tests conducted at 1600°C; decreasing anthracite additions will increase residual FeO in the slag, and thus lower the operating températures. Solid AlCeOs 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 séparation.

•Tests conducted at 1700°C in ail crucible types. Besides the effect of température, 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 séparation; 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 resuit in a relatively lower slag liquidus température, however because of the possible presence of solid perovskite phase and that the slag may be acidic, a higher température 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 séparation by decreasing viscosity.

(E) A Na₂CO₃ flux test at 1700°C as compared to 1600°C to evaluate the effects of a higher température on the viscosity and séparation of métal and slag.

(F) An unfluxed test in a graphite crucible at 1800°C, also to investigate the effects of higher température on metal-slag séparation. 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 séparation.

Distribution of rare-earth éléments

[0039] Pyrosim and FactSage thermodynamic packages were used to estimate the distribution of rare-earth éléments 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 is added for the réduction of Fe₂O3, MnO and P₂Os.

(C) Operating température of 1700°C Ce/Ce₂O3 was used to represent the total rare-earth elements/oxides in the FactSage while yttrium was used in the Pyrosim model. The results of the Pyrosim prédictions are given in APPENDIX A. Only the métal and slag Chemical analyses and recoveries for selected éléments predicted by the models are summarised in Table 8 and Table 9.

Table 8: Chemical analyses/slag quality in mass %

Slag	Al2o3	SiO2	CaO	MgO	TiO2	MnO	FeO	Y2O3
Pyrosim	25.0	23.2	7.95	4.36	13.4	16.1	2.10	7.24
FactSage	21.3	22.3	7.54	4.13	13.7	28.7	2.33	2.93
Métal	Fe	Mn	p	C	Re/Y
Pyrosim	87.0	10.2	1.59	0.312	-
FactSage	90.1	2.21	1.48	5.78	-

Table 9: Recovery of essential éléments in mass %

Slag	AI2O3	SiO2	CaO	MgO	TiO2	MnO	FeO	Y2O3
Pyrosim	99.9	99.1	99.9	99.8	99.0	42.4	1.20	100
FactSage	85.4	95.4	95.2	94.9	91.9	76.0	1.19
Métal	Fe	Vin	P	C	Re/Y
Pyrosim	98.5	57.6	100	9.99	1.00-5 6 * * * 10 * * * * 15
FactSage	>99.0	11.1	75.6

[0040] The theoretical prédictions indicate that ail 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 réduction as compared to that of the Pyrosim model. The calculated content of Ce in the FactSage métal is 0.000001% at 1700°C, which also indicates that ail the rare earth oxides report to the slag phase. In practice, the presence of solid AICeO₃ phase in the slag will not hâve an overall effect on the slag final grade while a more efficient réduction of MnO is possible. The actual concentrations of rare earths in the slag may be heigher than predicted levels. The métal 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 métal will be produced from these recipes. Minimising the slag tonnage and optimising its grade in

REE are 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.

Estimation of viscosity of rare-earth oxide-containing melt

[0041] The slag produced is largely constituted of FeO, MnO, SiO2, AI2O3, CaO and MgO with a small portion of up to 13% RE2O3. Rough analysis of the slag viscosity is 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 is assumed to fully reduce; which would be an idéal 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 is used to calculate the viscosity of the liquid at the liquidus température. For the calculations at 1600 °C, the viscosity of the liquid portion of the melt is 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 is présent in the melt (spherical particles is assumed).

[0042] As a resuit of refractory érosion when operated in alumina and magnesia crucibles, viscous slags of higher liquidus température 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 température will increase the fluidity of these slags.

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

Table 10: Normalisée! compositions, liquidus températures and viscosity calculation results

Test	MgO	AI2O3	SiO2	CaO	Liquidus	Viscosity at Liquidus	% Liquid at 1600C	Viscosity of liquid 1600C	Apparent viscosity 1600C	Solids
	%	°C	Poise	mass % Poise
1	4.28	69.3	19.6	6.82	1843	1.24	56.4	11.9	109	AI2O3
2	3.71	72.6	17.9	5.79	1872	1.03	50.1	13.1	216	AI2O3
3	4.56	51.7	32.7	11.1	1678	6.81	87.8	17.8	27.9	AI2O3
4	4.75	48.7	34.6	11.9	1644	10.2	93.3	17.8	22.6	AI2O3
5	38.6	21.2	30.2	10.1	1989	0.3	81.1	1.59	3.33	MgAI2O4+ MgO
6	8.12	37.8	38.6	15.5	1488	32.6	100	12.6	12.6	-
7	7.51	39.8	39.4	13.2	1527	30.4	100	16.2	16.2	-
8	6.99	39.8	40.2	13	1530	34	100	18.4	18.4	-
9	7.77	35.8	42.9	13.6	1469	64.7	100	19.2	19.2	-
10	7.81	35.2	42.9	14	1458	69.3	100	18.6	18.6	*
11	7.2	44.4	29.8	18.6	1578	8.73	100	8.81	8.81	-
12	7.46	40	38.2	14.3	1525	27	100	14.3	14.3	•
13	10.7	37.8	36.4	15.2	1547	12.3	100	8.32	8.32	-
14	10.8	41.1	29.5	18.7	1646	3.77	94.4	5.56	6.78	MgAI2O4
15	11.2	42.8	26.4	19.6	1695	2.39	88.4	4.94	7.56	MgAI2O4

3. EXPERIMENTAL RESULTS

Mass balance and test work overview

[0045] The overall mass balance of the laboratory smelting test work is given in Table 11 and

Table 12. These tables include the masses of the raw materials (ore, flux and reductant), slag and métal products for the various conditions investigated. The tests are grouped below according to particular objectives investigated.

[0046] Tests 1 to 4 investigated the effect of anthracite addition on the slag quality and melting température for tests carried out in alumina crucibles. Tests 1 to 4 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 are contaminated by eroded refractory material; REO contents in the slag are diluted.

[0047] Tests 1, 5 and 6 investigated the effect on the slag chemistry and final slag REO content of using different crucibles/refractories (as a resuit of crucible erosion). Tests 1,5 and 6 were carried out in alumina, magnesite and graphite crucibles, respectively. The metal-slag séparation in these tests seemed good. The best refractory is the one thaï has minimal erosion (or contaminâtes 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 6 gave the best results and subséquent tests is ail carried out in graphite crucibles.

[0048] Tests 7 to 13 investigated the effects of different flux additions on the slag phases produced for leaching purposes. These tests are ail conducted in graphite crucibles because of the corrosive nature of the fluxes used towards alumina and magnesite refractories. The métal and slag masses for Tests 7 to 13 conducted at 1600°C are not recorded and only the combined masses of slag and métal are presented. Ail the tests led to virtually no metal-slag séparation. Smelting of the ore is effective at 1600°C as is observed from Visual inspection of the crucible products. The séparation is most probably affected by the high slag viscosity which could be a resuit of low basicity index and the presence of solids. Whilst the addition of slag modifiers lowers the liquidus température of the slag, a portion of rare-earth oxides in the slag may exist as a high melting point solid.

[0049] Tests 14 to 21 are conducted to investigate conditions leading to improved metal-slag séparation. Test 14 is conducted at 1700°C to investigate the effect of température increase on the metal-slag séparation of tests fluxed with Na₂CO3, specifically Test 10. Test 15 is conducted at 1800°C to investigate the effect of température increase on the metal-slag séparation ofTest6 which is unfluxed. The slag-metal séparation of Tests 14 and 15 appeared better than that of Test 10 and Test 6, respectively.

[0050] Tests 16 to 21 are fluxed with varying levels of CaO to evaluate the effect of increasing slag basicity index on metal-slag séparation as well as on the réduction of MnO. These are conducted in graphite crucibles to evaluate them against fluxless Test 6. Tests 16 to 18 are conducted at 1700°C and Tests 19 to 21 are conducted at 1600°C to evaluate the effect of basicity index on the liquidus température. As indicated in the mass balance results in Table 12, these fluxed tests resulted in much better metal-slag séparation. The Chemical analyses indicated increased basicity index resulted in increased réduction of MnO. Tests 19 to 21 demonstrated that the addition of CaO also lowered the liquidus température of the slag; more efficient smelting is carried at 1600 and 1700°C as compared to Test 6 with no flux addition. The Chemical analyses of ail the tests follows.

Table 11: Mass balance

Test	Ore (g)	Anthracite (g)	Fluxes	Total mass In (g)	Products (g)	Total mass out (g)	ToC Crucible
N82CO3	Borax	K2CO3	CaO	Alloy+slag	Alloy	Slag	Gas/LOI
variation of anthracite addition and température
1	100	16.0					116	76.2	40.8	35.4	39.8	114	Al 1700
2	100	14.0					114	57.4	34.5	22.9	56.6	114	Al 1700
3	100	12.5					113	81.3	36.9	44.4	31.2	114	Al 1600
4	100	11.0					111	78.0	36.5	41.5	33.0	107	Al 1600
Variation of crucibles and température
5	100	16.0					116	79.1	42.9	36.2	34.3	113	Mg1700
S	400	50.0					460	265	158	107	192	457	G 1700
Variation of slag modi	fying flux addition and température
7	200	30.0	2.67				233	146	NA	NA	87	233	G 1600
8	200	30.0	5.33				235	125	NA	NA	111	235	G 1600
9	200	30.0	13.3				243	138	NA	NA	105	243	G 1600
10	200	30.0	26.7				257	128	NA	NA	129	257	G 1600
11	200	30.0		2.67			233	123	NA	NA	109	233	G 1600

Table 12: Additional tests - mass balance

Anthracite (g)	Fluxe s	Total mass In (g)	Products (g)	Total mass out (g)	ToC Drucible
Na2CO3	Borax	K2CO 3	CaO	Alloy+sla g	Alloy	Slag	Gas / LOI
Varying separatio	emperature and add Ί	ing CaO flux to improve viscosity/ slag and meta
30	26.7				257	127	63.4	63.6	124	251	G 1700
60					460	249	164	85.0	211	457	G 1800
16				3.5	116.5	66.3	44.3	22.0	50.2	116.5	G 1700
16				1.4	117.4	67.6	43.4	24.2	49.8	117.4	G 1700
16				3.2	119.2	70.8	42.9	27.9	48.4	119.2	G 1700
16				0.5	116.5	70.8	42.1	28.7	45.7	116.5	G 1600
16				1.4	117.4	71.5	40.6	30.9	45.8	117.4	G 1600
16				3.2	119.3	74.5	42.5	32.0	44.8	119.3	G 1600

Chemical analyses of slag

[0051] The Chemical analyses of the slag are given in Table 13 and Table 14.

[0052] Tests 1 to 4: The metal-slag séparation is good. The slags contained a relatively lower concentration of REO as a resuit 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 are lower than 0.2. The slag RE2O3 concentrations ranged from 4.09 to 6.36 %

[0053] Test 5: The metal-slag séparation is also good. The slag contained lower RE2O3 concentration at 5.36% due to contamination of the slag by MgO eroded from the MgO crucible. The slag had a relatively higher slag basicity index at 0.93 and this had a positive effect on MnO réduction. The concentration of MnO in the slag is lower than that for slags from Tests 1 -4 conducted in alumina crucibles.

[0054] Test 6: The metal-slag séparation is good. The RE2O3 grade of the slags produced in the graphite crucible at 11.6% is higher than that in alumina and MgO crucibles. Virtually no slag contamination took place in the graphite crucible as is observed in the alumina and MgO crucibles. Graphite crucible érosion contributes to provide an excessive reducing environment, which resulted in relatively higher réduction of MnO than in the alumina and MgO crucibles. However a relatively higher FeO content in the slag at 3.16% is observed. Iron spéciation analyses on the slag revealed that FeO (reported as Fe2+) in the slag is in fact 2.2%. The slag contained 4.2% entrained Fe. The entrainment of submicron metallic prills to the slag could be attributed to a relatively higher slag viscosity / higher liquidas température as is predicted in the FactSage model for high REO contents in the slag. Higher rare earth concentrations in the slag may resuit in a higher liquidus température and a significant amount of solid perovskite phase (AlCeOa). Between the unfluxed different crucible tests (1,5 and 6), fluxless smelting in a graphite crucible is more préférable.

[0055] Tests 7 to 13: Metal-slag séparation is poor. Clean slag pièces were collected and analysed. The REO concentration is relatively higher in the range of 7.13 - 11.9%. Because these tests are carried out in graphite crucibles and thus in excessively reducing environment, relatively higher réductions of iron and manganèse were observed. The FeO levels ranged between 0.19 and 4.98 %. The entrainment of Fe métal prills in the slag ranged from 2.8 to

32.4%. Poor metal-slag séparation could be attributed to high slag viscosity, which would be a resuit of low basicity index and possibly high liquidus température (as a resuit of high REO content).

[0056] Tests 14 and 15: These tests were carried out to investigate means to improve the metal-slag séparation. Increasing température appeared to hâve a positive effect on the slag viscosity and réduction of reducible oxides. Based on the Fe analyses in the slag, Test 14 metal-slag séparation is better than in Tests 7 to 13 séparations, and Test 15 séparation is better than that achieved in Test 6. Fe content in the slag is relatively low.

[0057] Tests 16 to 21: Fluxing the smelting recipe with lime was investigated to improve the metal-slag séparation. Good metal-slag séparation is achieved at ail CaO levels and operating températures. This is attributed to increased slag basicity index as a resuit of lime addition as a fluxing agent to the smelting recipe. The grade of REO in the slag is in the range of 10.913.8% for Test 16-18 and 8.39- 8.87% for tests 19 to 21. The slag REO grades of Tests 1921 conducted at 1600°C are relatively lower than those of Tests 16 to 18, due to higher réduction of MnO at 1700°C than at 1600°C. At 1600°C, anthracite addition may be increased to improve the réduction of MnO and subsequently the content of REO in the slag.

[0058] Compared to the optimal unfluxed condition in Test 6, the addition of CaO is found to be optimal in the tests that resulted in good metal-slag séparation with REO grade at least equal to that in Test 6 slag. Tests 16 and 17 met these requirements. The slag REO grades are 13.6 % and 12.5 %, respectively as reported in Table 13. Improved réduction of MnO is achieved in these tests as compared to Test 6 as a resuit of increased slag basicity and operating température (1700°C).

[0059] In larger commercial operations, CaO additions of up to 3% may be carried out as these will resuit in higher REO, lower FeO, lower MnO in the slag as well as better furnace operation, better metal-slag séparation, and virtually no métal 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.

Table 13: REE Chemical composition in the Slag

Test	La	Ce	Pr	Nd	Sm	Eu	Gd	□y	Ho	Er	îm	Yb	Lu	Y	Tb	Th	J	RE E	RE2O3 (REO)
	ppm	Ppm	ppm	Ppm	Ppm	jpm	Ppm	Ppm	Ppm	Ppm	Ppm	Dpm	ppm	Ppm	Ppm	Ppm	ppm	PP m	ppm
1	11550	20050	1790	8199	1128	279	799	389	60.3	175	17.9	118	15.4	1770	113			4.65	5.44
2	8570	14700	1360	5525	905	227	552	314	48.4	140	14.3	96.3	124	1320	91.5			3.49	4.09
3	14900	26500	2430	8960	1131	333	997	441	73.0	168	20.0	164	21.0	2162	101	487	201	5.84	6.84
4	13800	24400	2260	8400	1096	325	923	426	70.0	171	19.0	152	20.0	2110	100	455	184	5.43	S.36
5	12835	21064	2302	7986	1198	317	1088	472	78	205	26.3	163	22.6	5731	124	401	112	5.36	6.28
6	21601	42805	4328	19338	2327	829	1416	972	170	380	50.4	257	45.3	3904	139			9.86	11.60
7	24114	48756	4935	14227	2225	447	1572	570	112	260	38.3	208	32.2	3645	191	780	270	10.1	11.9
8	23142	46677	4742	13568	2131	431	1507	636	107	227	366	182	29.9	3568	181	886	289	9.72	11.4
9	20092	31385	3576	9863	1656	740	434	576	92.2	304	29.0	359	34.7	4154	160	863	277	7.35	8.61
10	17673	27398	3226	9082	1560	702	387	508	51.8	271	26.0	318	31.0	4258	146	792	250	6.57	7.69
11	22994	41985	4175	15364	2339	521	3978	653	136	287	48.3	345	52.6	4245	457			9.77	11.5
12	20453	37770	3724	13659	2059	531	3408	566	117	246	40.8	291	44 0	3731	391	-		8.70	10.2
13	22659	34902	3898	15130	1768	p17	482	321	101	338	33	404	40	4552	185	963	309	8.59	10.1
14	28023	47152	5161	18198	2560	689	2240	961	159	418	53.7	334	46.2	4145	261	787	189	11.0	12.9
15	29459	49500	5444	19144	2685	727	2420	1048	173	456	58.3	362	49.9	4547	275	863	210	11.6	13.6
16	[28970	57197	5232	15077	2335	503	1816	785	153	364	53.6	321	44.6	5015	210	986	256	11.8	13.8
17	26989	49852	4891	14067	2213	575	1726	740	146	343	50.8	305	42.0	4786	199	987	249	10.7	12.5
18	23589	43524	3593	12309	1936	500	1505	642	127	297	44.1	264	36.4	4090	172	859	245	9.26	10.9
19	19429	32083	3569	12516	1756	550	1654	598	113	292	38.8	234	32.5	2536	195	862	452	7.57	8.87
20	18381	30337	3385	11851	1670	521	1564	660	106	276	36.4	220	30.4	2420	185	894	436	7.16	8,39
21	18348	30478	3395	11848	1661	526	1595	663	106	277	36.6	221	30.8	2399	187	941	433	7.18	8.41

Table 14: Chemical composition ofthe other métal oxides in slag, with total REO

Test	MgO	AI2O3	SiO2	CaO	TiO2	V2O5	Cr2O3	MnO	FeO	Fe entrained	Bl	S/A	S/M	REE	RE2O3 (REO)
	%	%	%	%	%	%	%	%	%					%	%
1	2.86	46.3	13.1	4.56	7.47	0.09	0.07	10.3	2.45	-	0.12	0.28	4.58	4.65	5.44
2	2.49	48.7	12	3.88	6.47	0.14	0.1	13.2	3.1	-	0.1	0.25	4.82	3.49	4.09
3	2.4	27.2	17.2	5.82	8.81	0.23	0.07	19.1	4.09	-	0.19	0.63	7.17	5.84	6.84
4	1.92	19.7	14	4.81	7.49	0.21	0.08	17.4	11	-	0.2	0.71	7.29	5.43	6.36
5	24.8	13.6	19.4	6.51	9.52	0.09	0.07	7.37	0.98	«	0.95	1.43	0.78	5.36	6.3
6	5.24	24.4	24.9	9.97	10.5	0.1	0.08	5.81	3.16	-	0.31	1.02	4.75	9.86	11.6
7	3.94	20.9	20.7	6.95	6.85	0.3	0.11	19.5	1.75	-	0.26	0.99	5.25	10.1	11.9
8	3.53	20.1	20.3	6.59	6.13	0.23	0.1	16.5	2.08	-	0.25	1.01	5.75	9.72	11.4
9	4.84	22.3	26.7	8.45	9.05	0.11	0.09	9.07	1.7	9.28	0.27	1.2	5.52	7.35	8.61
10	4.48	20.2	24.6	8.05	5.62	0.16	0.12	15.6	4.98	32.4	0.28	1.22	5.49	6.57	7.69
11	4.23	26.1	17.5	10.9	10.3	0.13	0.18	8.12	0.19	5.82	0.35	0.67	4.14	9.77	11.4
12	4.88	26.2	25	9.36	6.8	0.1	0.08	7.01	1.21	17.8	0.28	0.95	5.12	8.7	10.2
13	7.56	26.8	25.8	10.8	3.47	0.11	0.09	3.49	2.96	2.8	0.35	0.96	3.41	8.59	10.1
14	7.27	27.7	19.9	12.6	7.84	0.13	0.106	7.72	3.08	-	0.42	0.72	2.74	11	12.9
15	7.74	29.6	18.3	13.6	4.4	0.108	0.089	4.69	1.39	-	0.45	0.62	2.36	11.6	13.6
16	6.82	25.8	13.2	14.1	5.1	0.089	0.073	1.67	3.04	-	0.54	0.51	1.94	11.8	13.8
17	6.47	24.8	13.4	16.7	4	0.089	0.073	1.82	5.31	-	0.61	0.54	2.07	10.7	12.5
18	5.94	21.4	16	19.6	5.6	0.089	0.073	2.43	4.43	-	0.68	0.75	2.69	9.26	10.9
19	5.74	19.9	22.8	10.1	11.4	0.089	0.073	5.24	4.64	-	0.37	1.15	3.97	7.57	8.87
20	4.18	17	21.6	11.9	9.3	0.089	0.073	5.62	9.31	-	0.42	1.27	5.17	7.16	8.39
21	5.46	19.1	22.7	16.7	8.5	0.089	0.073	3.41	4.52	-	0.53	1.19	4.16	7.18	8.41

The effect of refractory on REO grade and slag quality.

[0059] As indicated in Table 13 and Table 14, the concentration of REO in the slag phase varied from 4.09 to 13.80 %, dépendent on the smelting conditions. At its highest, the total rare-earth element in the slag is up to about 5 times its concentration in the ore, a significant upgrade. The Chemical érosion is acute in the alumina crucible while it is still significant in the MgO crucibles. Consequently, slags of relatively lower REO concentrations are produced in the test work conducted in alumina and magnesite crucibles while higher REO concentrations are obtained in the tests conducted in graphite crucibles (Tests 6-21).

[0060] Based on the above évaluations of effects of crucible érosion 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.

[0061] 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 économies

Alloy quality

[0062] The compositions of the iron alloy produced is presented in Table 15. A carbonsaturated iron-manganese alloy is produced from these tests. In the smelting process, iron is preferentially reduced over manganèse. The réduction of iron is almost complété in ail the various conditions investigated. The composition of the alloy appeared to be strongly related to the extent of manganèse réduction. For instance, increase of manganèse réduction increases the alloy manganèse content while it decreases its iron concentration by dilution.

[0063] As manganèse oxide is an undesirable impurity in the leaching process contributing to increased acid consumption, its réduction tothe alloy in the smelting step should be optimised. The réduction of manganèse oxide is affected by the reductant addition, température and slag basicity index. MnO réduction in the graphite crucible tests fluxed with CaO is even better due to higher slag basicity index. There is a noticeable différence in the réduction of FeO and MnO for Test 5 carried out in a magnesite crucible as compared to the results achieved in the graphite crucibles with CaO fluxing.

Table 15: Alloy analyses
Test	Si	Ti	V	Mn	Cr	Ou	Ni	Ca	Fe	Mg	Al	P	C
	%	%	%	%	%	%	%	%	%	%	%	%	%
1	0.75	0.25	0.15	7.1	0.08	0.03	0.16	0.18	88.5	0.04	0.6	0.72	1.55
2	0.03	0.05	0.04	1.06	0.04	0.02	0.1	0.11	96.8	0.02	0.39	0.71	0.6
3	0.61	0.14	0.03	0.86	0.04	0.02	0.04	0.07	94	0.02	0.43	3.42	0.35
4	0.47	0.03	0.01	0.14	0.02	0.02	0.06	0.01	96.1	0.01	0.08	3.01	0.08
5	2.61	0.71	0.12	11.9	0.05	0.02	0.04	0.05	79.5	0.05	0.14	0.72	4.08
6	3.3	0.35	0.1	10.1	0.05	0.05	0.05	0.05	81.2	0.05	0.14	0.83	3.77
7	-	-	-	-	-	-	-	-	-	-	-	-	-
8	-	-	-	-	-	-	-	-	-	-	-	-	-
9	0.84	0.85	0.12	11.8	0.08	0.06	0.03	0.36	79.7	0.17	0.69	0.72	4.63
10	4.12	1.31	0.12	12.5	0.08	0.07	0.04	0.08	75.5	0.05	0.45	0.73	4.98
11	-	-	-	-	-	-	-	-		-	-	-	-
12	-	-	-	-	-	-	-	-	-	-	-	-	-
13	-	-	-	-	-	-	-	-	-	-	-	-	-
14	2.52	0.96	0.13	13.7	0.08	0.01	0.04	0.13	75.8	0.08	0.24	0.72	5.63
15	0.78	0.67	0.11	12	0.08	0.03	0.04	0.19	83.9	0.09	0.34	0.73	4.09
16	4.56	0.37	0.1	12.4	0.05	0.05	0.05	0.1	79.2	0.05	0.26	1.26	1.57
17	5.74	0.71	0.12	11.2	0.07	0.05	0.05	0.13	76.9	0.05	0.29	1.25	3.48
18	4.96	0.7	0.12	11.6	0.05	0.05	0.05	0.13	74.9	0.05	0.2	1.29	4.9
19	1.72	0.39	0.13	11.4	0.04	0.05	0.04	0.09	81.5	0.04	0.26	0.69	3.68
20	1.72	0.38	0.12	11.4	0.04	0.04	0.04	0.07	81.6	0.02	0.26	0.72	3.55
21	1.47	0.49	0.12	11.9	0.03	0.1	0.03	0.05	81.1	0.02	0.11	0.72	3.85

Carbon and Phosphorus in the métal

[0065] Saturated-carbon iron-manganese alloys are produced in the crucible smelting tests. The highest levels of P are from Tests 3 and 4. These tests are conducted at relatively lower température and anthracite addition in the recipe is Iess than the stoichiometric amount. As a conséquence, a lower amount of métal is produced while P2O5 is almost fully reduced to the alloy.

[0066] The métal composition corresponding to the optimal slag production is considered as being the optimal métal composition. Optimal metals are produced in Test 6, and Tests 16 and 17. Based on these recipes, the optimal alloy composition produced from this particular Zandkopsdrift ore sample would be: 75-79 % Fe, 10-12.5 % Mn, 2-4 % C, 3-6 % Si and 0.71.3 % P. This alloy composition falls within the commercial manganèse steel composition range which consists of 11-13% Mn.

Métal to slag ratio

[0067] The metals to slag ratios reported in Table 16, are calculated only for the tests which resulted in good slag-metal séparation. These results are compared to the theoretical values of the métal to slag ratio calculated using Pyrosim and FactSage (in section 3.1.1.2). These ratios can be used to assess the extent of contamination of the slag by crucible érosion, extent of réduction relative to the prédictions and the mass pull of the REE containing slag relative to the ore.

Table 16: Métal to slag ratio

Test	Anthracite(%)	Alloy(g)	Slag(g)	Metal/slag	Crucible	ToC
Pyrosim	100	49.0	30.0	1.56		1700
FactSage	100	45.1	30.8	1.46		1700
1	100	40.8	34.0	1.20	Al	1700
2	90	34.5	22.9	1.51	Al	1700
3	80	36.8	40.5	0.91	Al	1600
4	70	36.5	41.5	0.88	Al	1600
5	100	42.9	35.2	1.22	Mg	1700
6	100	158	107	1.48	G	1700
7-13	100				G	1700
14	100	63.4	63.6	0.997	G	1700
15	100	164	85.0	1.94	G	1800
16	100	44.3	22.0	2.01	G	1700
17	100	43.4	24.2	1.79	G	1700
18	100	42.9	27.9	1.54	G	1700
19	100	42.1	28.7	1,47	G	1600
20	100	40.6	30.9	1.31	G	1600
21	100	42.5	32.0	1.33	G	1600

[0067] The metal-to-slag ratio of the fluxless smelting conditions for Tests 3 and 4 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 érosion that increases the slag volume.

[0068] 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 érosion, and increased MnO réduction to the alloy.

[0069] A metal-to-slag ratio of 1.48 is measured in Test 6; which is doser to the values predicted using Pyrosim and FactSage models. Test 15 which is a repeat of Test 6 at a higher température resulted in a ratio of 1.94. The Test 15 ratio is the highest as a resuit of better metal-slag séparation as well as higher MnO réduction.

[0070] Compared to Test 6, Tests 16 to 21 resulted in relatively higher metal-to-slag ratios which decreased with increasing CaO addition. As indicated in section 3.1.2.2, lime addition promoted the réduction of MnO, improved the metal-slag séparation, and also diluted the slag. The metal-to-slag ratio results indicative that, 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 érosion 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.

Recoveries to slag and métal

[0071] The recoveries of REE and métal oxides to the slag phase are given in Table 18 and Table 20, respectively. Recoveries to the alloy are given in Table 20. Recoveries are only calculated for tests yielding good metal-slag séparation.

Recoveries to slag

[0072] Rare earth oxides are stable at the conditions of the réduction of iron oxides. Tests 6 to 21 carried out in graphite crucibles resulted in REE recoveries ranging from 80 to 100%. These tests and particularly those yielding a clean metal-slag séparation demonstrated that ail the rare earth 5 oxides would report to the slag phase at the smelting conditions investigated.

[0073] The distribution of rare earths in the product streams is calculated based on the REE analyses and masses of the slag and métal produced. The concentration of TREEs in selected alloys is very low as indicated in Table 17,

[0074] FeO and MnO are significantly reduced at higher slag basicity index. This appeared specifically in Tests 16 to 21 fluxed with CaO. The recoveries of FeO to the slag in ail these tests are below 3%, indicating that FeO is effectively reduced in ail the test work in spite poor metal-slag séparation in some tests. However doser to about 40% of MnO stayed unreduced in the slag.

Table 17: REE Analyses of selected alloy

	Ce	Dy	Er	Gd	La	Nd	Pr	Sc	Sm	Tm	r	Yb	Th	U
Test	ppm	Ppm	apm	Ppm	ppm	spm	ppm	ppm	ppm	ppm	dpm	opm	opm	ppm
6	250	5.26	1.96	15.9	143	123	31.3	17.8	1.32	10.7	1.18		5.95	50.10
16	155	2.80	1.49	7.00	98.9	59.2	18.3	56.4	8.22	1	15.4	1	B.65	31.5
17	49.8	1	1	2.16	25.1	18.1	5.50	41.3	2.63	1	5.68	1	5.10	30.4
18	48.7	1.49	9.75	11.2	27.3	14.6	5.80	1	1.96	2.50	11.6	6.98	6.35	24.9

Test	_a	Ce	Dr	Nd	Sm	Eu	3d	Dy	Ho	Er	rm	rb	-U)	r	Fb	-tEO/RezOj
	%	%	% %	%	%	%	7o	%	7o	%
1	57.4	60.2	49.6	57.2	B6.3	B0.7	76.2	30.6	55.0	51.7	17.8	53.4	50.9	52.6	95.0	52.7
2	41.2	42.7	36.4	51.8	67.0	63.4	51.0	52.8	42.7	47.6	56.9	42.1	59.8	15.2	74.7	45.5
3	58.1	94.7	80.1	87.4	103	115	113	107	79.2	70.4	83.5	38.3	52.8	91.1	101	93.8
4	33.6	89.3	76.3	34.0	102	116	107	107	77.8	73.4	61.8	83.8	80.8	91.0	103	89.3
5	56.1	65.4	66,1	□7.7	94.8	94.9	107	101	73.4	74.8	72.7	76.4	77.5	210	109	74.8
6	B4.3	101	94.1	125	140	188	106	158	122	105	106	91.1	118	108	92.2	105
7-13 [ [			1-	l·	Ir
14	130	132	134	140	183	186	200	186	135	138	134	141	143	137	206	139
15	91.5	92.8	94.2	98.1	128	131	144	136	98.3	100	97.2	102	103	101	145	98.1
16	93.1	111	93.7	BO.O	116	113	112	105	90.4	82.8	92.4	94.0	95.7	115	114	103
17	95.5	106	96.4	B2.1	121	118	117	109	94.5	B6.0	96.5	98.1	99.1	121	119	103
18	96.2	107	B1.6	B2.8	122	119	118	109	94.9	B6.0	96.5	98.0	99.1	119	119	103
19	81.5	B1.3	I33.4	86.6	113	134	133	122	B6.6	86.9	B7.4	89.4	91.0	75.8	139	86.2
20	83.0	82.7	B5.2	B8.3	116	137	136	124	88.0	88.4	B8.3	90.4	91.5	77.8	142	37.8
21	B5.8	B6.1	B8.5	91.4	120	143	143	129	91.3	91.7	91.9	94.2	95.9	79.9	149	91.1

Table 19: Recoveries of Oxides

Test	VgO		SiO2	3aO	no2	ΆΟ5	Dr2O3	vlnO	FeO
	%	%	%	%	%	%	’/o	%	%
1	50.9	196	49.3	45.2	64.2	26.1	B1.3	32.3	1.64
2	51.3	200	43.5	37.1	53.8	38.4	107	39.9	2.01
3	60.9	137	77.1	68.6	90.1	80.8	96.8	71.2	3.26
4	49.9	101	64.1	58.1	78.5	76.4	103	66.5	9.01
5	546	59.6	75.5	66.7	84.7	27.0	84.2	23.9	0.68
6	87.6	81.1	73.4	77.6	70.8	21.9	38.2	14.3	1.66
7-13			-		-
14	145	110	70.0	117	63.1	35.4	110	22.6	1.93
15	103	78.3	43.1	84.0	23.6	19.8	61.8	9.17	0.58
16	94.0	70.6	32.0	91.0	28.5	16.9	52.6	3.37	1.32
17	98.0	74.6	35.7	117	24.3	18.6	57.9	4.05	2.53
18	104	74.2	49.3	159	39.2	21.4	66.7	6.23	2.43
19	103	71.2	72.2	84.0	82.7	22.0	68.7	13.8	2.62
20	80.9	65.4	73.8	107	73.0	23.7	73.9	16.0	5.70
21	109	76.0	80.2	155	68.4	24.6	76.6	10.0	2.84

Τοκιθ 20 Recoveries to motsl

Test	Si (%)	Ti(%)	V(%)	Vln(%)	3r(%)	Cu(%)	Mi(%)	Ca(%)	Fe(%)	vig(%)	M(%)	3(%)
	%	%	%	%	%	%	%	%	%	%	%	%
1	7.20	4.22	91	34.4	149.3	24.5	126	2.99	101	1.84	5.71	22.5
,2	0.204	0.713	21.1	4.34	71.0	16.6	69.0	1.55	94.0	3.752	3.16	18.7
3	5.02	2.06	16.0	3.56	59.9	11.8	30.6	1.02	92.0	0.795	3.52	91.2
4	4.05	0.416	5.60	0.61	36.7	14.6	42.3	3.07	98.7	0.190	3.687	84.1
5	26.5	12.9	79.1	80.5	102.7	19.7	30.0	0.87	91.2	2.23	1.41	23.6
6	30.8	5.84	58.9	47.5	94.6	39.6	39.6	3.81	86.8	2.05	1.30	25.1
7-13
14	18.9	12.8	53.2	51.6	121	3.17	21.5	1.68	67.5	2.63	1.79	17.6
15	7.60	11.6	59.3	58.3	157	21.3	31.2	3.18	93.4	3.83	3.28	22.9
16	47.7	6.90	55.3	65.3	112	44.3	44.3	1.75	98.7	2.30	2.66	42.8
17	58.8	13.0	79.9	57.7	152	43.4	43.4	2.30	93.8	2.25	2.96	41.6
18	50.3	12.7	79.0	59.1	103	42.9	42.9	2.27	91.6	2.23	2.02	42.5
19	17.1	6.90	B0.8	57.0	72.5	38.3	30.7	1.57	96.5	1.51	2.53	22.3
20	16.5	6.50	74.8	55.0	85.5	33.3	29.2	1.21	93.2	0.97	2.48 —	22.4
21	14.8	8.80	78.3	60.1	61.0	85.0	27.2	0.83	97.0	075_______________	]1.1O	23.3

Recoveries of Fe, P and Mn to the métal phase

[0075] As indicated in Table 20, the recovery of Fe to the alloy calculated on the basis of the content of this element in the feed ranged between 86% and 98%. This further validâtes that the réduction of FeO in the tests conducted is effective in spite of poor métal and slag séparation in some tests.

[0076] The recoveries of P to the métal are highest at low anthracite additions and lowest at high anthracite additions and températures.

[0077] However in Test 16 conducted in a graphite crucible at 1700°C, with 1% CaO flux addition in the smelting recipe, the highest proportion of REEs is présent in the Ca-Silicate phase, lower amounts are detected in the CaAl silicate and the Ba-rich Ca-silicate phases as can be seen in Figure 12. This distribution is similar to that of Test 6 slag.

Conclusion of the smelting tests

[0078] Laboratory smelting test work demonstrated that the smelting of the ZKD ore can be conducted without flux addition at a température of about 1700°C. However the température of the smelting can be decreased to about 1600°C with the addition of fluxes.

[0079] 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 érosion.

[0080] Fluxing with a minimal lime addition of 1 to 7% relative to the ore is investigated. This provided a clean metal-slag séparation as well as promoted the MnO réduction. Fluxing with minimal CaO (13%) may be recommended in order to minimise acid consumption in the leaching step; it will improve the réduction of MnO whilst producing a high REO grade slag.

4. LEACHING

[0081] Various slag samples produced in the smelting tests are subjected to leaching in order to détermine the amenability of the rare-earth éléments to leaching. Three leaching methods as listed below are explored to détermine the most economical route to be used:

1. Acid baking,

2. Sodium hydroxide cracking followed by HCl leach,

3. Direct HCl leach

Acid baking and water leaching

[0082] The slag used in the acid baking leaching tests is produced in the 100 kVA furnace in an alumina crucible furnace. It is saturated with AI2O3 (due to crucible érosion), has low concentration of TREE and high MnO content. The slag composition in REEs and other métal éléments is shown in Table 25 and Table 26, respectively. La and Ce are the major REE éléments présent in the feed solids, constituting almost 70% ofthe total rare earth éléments (TREE) content of 3.76%. The major impurities in the sample are Fe, Mn, Si, Mg, Ca and Al.

Acid baking procedure

[0083] The slag is contacted with pre-determined amount of concentrated H2SO4 (98% (m/m). The mixture ofthe acid and slag is weighed and transferred into a baking tray. The acid contacted slag is baked in an oven at specified test température. At the end of the baking period, the samples are weighed prior to subjecting them to water leach.

[0084] The baked samples are subjected to water leach to solubilise the rare-earth sulphates; deionised water is used as the lixiviant. Water leaching is conducted for 2 hour résidence time, at a pulp density of 20% (m/m). At the end of the test, the entire reactor content is filtered. The filtrate volume is measured and wet un-washed cake weighed. The weighed cake is washed three times initially with pH adjusted water and thereafter with deionised water at a ratio of 1:5 (i.e. for 1 kg of sample 5 L of deionised water will be used).

NaOH cracking and water leaching

Caustic cracking procedure

[0085] The slag sample is subjected to cracking with 50% sodium hydroxide (NaOH), for a period of 4 hours, at a température of 140°C and in initial pulp density of 20 % (m/m). At the end of the test the entire slurry is diluted with deionised water then fïltered. The fïltered 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 are measured and analysed for REE and base metals.

[0086] The residues from the caustic cracking tests are 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 120% stoichiometry based on total Ce in feed. Both tests is conducted at 40°C, targeting a pH value of 1.5, for 4 hours.

Direct HCl leach

[0087] The slag is milled and then slurried in HCl solution (16% (m/m) or 32% (m/m), targeting the required target pulp density (10% and 20%) and agitated. The température is then increased to 60°C. After 3 hours of reaction, the mixture is fïltered and the mass of the wet unwashed residues is recorded. The fïltered 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 1 time the mass of wash liquor to the wet cake mass and the second and third washes at a ratio of 5 times the mass of the wash liquor to the wet cake mass.

[0088] Leaching efficiency levels of about 95% are attained in the direct hydrochloric acid leaching; other leaching methods ail resulted in leaching efficiencies of less than 60%.

[0089] The économie viability of the process shown in the accompanying flow sheet dépends 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 hâve an effect on technical and économie aspects of the method of the invention. If a graphite crucible is used then the slag 20 need not necessarily be fluxed and direct HCl leaching of the unfluxed slag can be effected. Tests hâve shown that total rare-earth element leaching efficiencies ranging between 93% and 96%, 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 préférable. It has also been observed that the extraction efficiency of light rare-earth éléments which include La, Ce, Nd and Pr is lowered when the slag is treated with a flux prior to leaching.

[0090] A benefit of the fluxing process is that the température of the smelting can be decreased from about 1700° to 1600°C. Use of a graphite or carbon-based refractory crucible is préférable as it minimizes the contamination of the slag product and this results in a higher concentration of the rareearth éléments in the slag. It has been noted that due to the effect of Chemical érosion 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.

Claims (8)

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 minerais in the ore into a slag phase and extracting the rare earth oxide minerais from the slag.

2. A method according to claim 1 wherein, in the smelting step, iron and manganèse oxides in the ore are reduced to a low manganèse pig iron in a métal 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 éléments 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, Na₂CO3, K2CO3 and borax.

8. A method of Processing an iron-rich rare earth-bearing ore which includes the steps of using a sélective carbothermic smelting step for concentrating the rare earth oxide species into a slag phase and for precipitating iron and manganèse in the ore, as low manganèse pig iron, in a métal phase and then processing the slag using hydrometallurgical techniques to extract and then to separate the rare earth éléments.