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- Extractive Metallurgy of Rare Earths | The Nile | TheMarket NZ
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- Extractive Metallurgy of Rare Earths
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Books by C. Rare earths are extracted from the leachate and transformed into pure rare earth oxides, chlorides, or halides by hydrometallurgical means. Pure rare earth metals and rare-earth-metal alloys are produced by direct reduction of rare earth oxides, reduction of anhydrous chlorides and fluorides, and fused salt electrolysis of rare earth chlorides or oxide-fluoride mixtures. Pure rare earth oxides may be prepared by selective oxidation, selective reduction, fractional crystallization, ion exchange, and solvent extraction.
Chlorides and fluorides are produced by transformation of rare earth oxides Gupta and Krishnamurthy, ; Mishra and Anderson, A rare earth deposit is valuable only when the metal values can be extracted in a cost-effective, efficient, sustainable, and environmentally responsible manner. Although a large number of deposits have been explored worldwide, only a few are mined for rare earths.
Bayan Obo is the largest exploited iron-rich rare earth deposit. The Bayan Obo deposit contains bastnaesite and monazite, with magnetite and haematite as the dominant iron ore minerals. This is followed by selective flotation to separate bastnaesite and monazite. The Mount Weld mine, which was opened in , exploits one of the world's highest grade rare earth deposits, with an average REO grade of Mount Weld has been described as a secondary rare-earth phosphate deposit, but the phosphates most likely monazite are encapsulated in iron oxide minerals Haque et al.
Mount Weld produces a rare earth concentrate and an iron ore concentrate for further beneficiation. Upgrading of some of the rare earth ores is challenging, due to the complexity of the mineralogy. This was the case for two specific iron-rich rare earth deposits in the Southern African region. Milling was not able to liberate the rare earth minerals from the iron mineral matrix. In this paper we present the results of an investigation into the processing of iron-rich rare-earth-bearing ores from the Southern African region.
Process overview - iron-rich rare-earth-bearing ores from Southern Africa. Brief mineralogy and implications. Two samples of iron-rich rare-earth-bearing ore from two deposits in Southern Africa were evaluated. The ores also contain bastnaesite, synchysite, ancylite, pyrochlore, REE-apatite, and rhabdophane. Quartz, jarosite, svanbergite, apatite, barite, mica, clays, crandallite, gorceixite, carbonates, and nordite are present as gangue minerals.
Monazite and ancylite often occur as submicroscopic inhomogeneous grains in association with Fe-oxyhydroxides. Ancylite is associated with calcite and apatite. Most of the conventional physical separation techniques were found to be inefficient. Bulk leaching of these ores to extract rare earths inevitably solubilizes iron minerals and increases acid consumption. Removal of the iron from the leachate requires excessive amounts of reagents such as lime.
This, in addition to the discarding of the iron precipitate, increases the cost of the process, usually beyond economic viability.
Extractive Metallurgy of Rare Earths | The Nile | TheMarket NZ
Methods for extracting rare earths without affecting the value of the iron minerals are required to improve the economics of processing these ores. High-temperature processing of iron-rich rare earth ores. Separation of iron and rare earths is theoretically possible by carbothermic reduction and could be an alternative to physical upgrading of the ore. As shown in the Ellingham diagram for pure oxides Figure 1 , iron oxide can be selectively reduced over rare earth oxides.
However, in the present case, smelting would be required to enable efficient separation of iron metal from the rare-earth oxide bearing slag. Rare earth oxides are stable under the iron smelting conditions generally described by the temperature and oxygen partial pressure, therefore they will report to the slag phase while the iron will report to the metal phase.
However, the most important step is the extraction of the rare earths from the slag. Hydrometallurgical methods were investigated for this purpose. In practice, smelting of the ore and solubilization of the rare earth oxides through acid leaching is affected by the ore mineralogy and impurities.
The mineralogy of the ore affects the kinetics of both smelting and leaching. Principle of the PyEarth process. The 'PyEarth' process flow sheet, as presented in Figure 2 , incorporates the smelting of ore, leaching of rare earths from the resulting slag, and recovery of the rare earths from the leach liquor. Smelting of the ore to a slag in which the REE are concentrated, as well as a metal product, followed by leaching of the slag, constitute the main critical steps of this process.
However, for simplicity, the flow sheet considers the use of a DC open-arc furnace to smelt the ore, leaching in a hydrochloric medium, precipitation of impurities, precipitation of the rare earths, and recovery of HCl. As opposed to other smelting reactors such as blast furnaces and submerged-arc electric furnaces, the DC open-arc furnace is able to process ore fines without prior preparation and the complex nature of the mineralogy is easily managed by way of temperature control. This reactor would be an appropriate option for the smelting of the mineralogically complex iron-rich, rare-earth-bearing ores.
Upgrading of the slag prior to leaching to remove constituents other than REOs is not included in the basic flow sheet presented in this paper. However, this option may improve the efficiency of the subsequent hydrometallurgical operations if physical upgrading such as electrostatic and magnetic concentration, froth flotation, and gravity concentration is introduced post-taphole. The results of preliminary smelting and leaching tests are presented and discussed in the following sections. A sample of iron-rich rare-earth-bearing ore from a Southern African deposit was subjected to series of laboratory tests.
The chemical composition of the ore is given in Table I , and the proximate analysis of the anthracite used as reductant is given in Table II. High-purity CaO was used as a fluxing agent to improve the smelting process, particularly to decrease the slag liquidus temperature and viscosity, constraints that are exacerbated at small scale. Experimental procedure. Laboratory smelting tests were conducted in 30 kW and 60 kW induction furnaces. Figure 3 shows a schematic of the induction furnace set-up. The raw material components for the test conditions specified in Table III were blended and packed in a graphite crucible.
Tests 1 and 2 were unfluxed tests conducted at stoichiometric anthracite addition and different temperatures. In this particular case, the anthracite amount was calculated so as to fully reduce the iron and manganese oxides. Tests 3 and 4 were fluxed with 0. The packed crucible was placed at the centre of the graphite susceptor in the induction furnace. A B-type thermocouple was secured next to the crucible, and argon gas was purged into the furnace to create an inert environment.
Extractive Metallurgy of Rare Earths
The crucible was held for about 30 minutes at the target temperature. The furnace power was then switched off and the crucible was left to cool in the Ar gas atmosphere inside the furnace. The cold crucible was removed, weighed, and then broken down to separate the metal and slag phases. The metal and slag samples were subjected to chemical and mineralogical analysis. Smelting results and discussion.