Extraction and yields of rare metals from the rare-earth-containing ores of the carbonatite weathering crust by means of thermochemical preparation

Introduction

The Tomtor deposits occupy a position of prime importance among the rare-earth ores of Russia. Rare-earth-containing ores of the weathering crust are characterised by their complicated and unsTable mineralogical, chemical and granulometric composition. Their minerals occur in intergrown form and exhibit thin layers of iron hydroxides and other minerals on their surface [1]. The great majority of such ores are made up of particles of sizes of less than 10 µm. These ores are fine-particled, soft, and have a proclivity to the formation of extremely small particles. The use of traditional physical preparation methods (flotation, magnetic separation, gravitational preparation) proved not to be suitable for the recovery of any concentrates relevant for the production of rare-earth products. Against this background, a decision was taken to apply the widely used alkaline autoclave process practised for ore preparation, which is effective only for monazite mineral resources and not for the ores of the weathering crust. This method also has its limitations with respect to Fe, Ti, Ca and Si oxide contents. It must be emphasised that these ores are of the complex category and that they also contain Nb, Ti, P, Fe, Th, U and other useable components, in addition to rare-earth elements. This article examines the potentials for the creation of melts, i.e., thermochemical preparation, in the implementation of which two melts differing by composition and properties are formed, namely the silicate-salt- and phosphate-salt-containing melts. This method is founded on the principles and technological solutions of liquation melting of mineral resources. The ore system examined here is complicated and, for this reason, only the process of melt liquation and of the extraction of the rare-metal elements from the phosphate salt melt are directly discussed in this publication.

Ore composition

Two ore types are found in the Tomtor deposits: rich pyrochlore-monazite ores and pyrochlore-monazite-crandallite medium ores, the chemical composition of which is shown in Table 1.

Rich pyrochlore-monazite-crandallite ore has an extremely high rare-earths and niobium content and an extremely low SiO₂ content. Medium pyrochlore-monazite-crandallite ores have a low rare-earths and niobium content and a significantly higher Al₂O₃ and SiO₂ content. A low Na₂O- and K₂O-alkali contents (< 0.15 to 0.5 %), fluorine (< 0.8 %), and not a high Ti, Ca, Mg, Sr and Ba oxides content (up to 10 %), is characteristic of these ores. Iron (7 to 15 %) is present in bivalent and trivalent form. The quantitative mineral composition of the ores differs greatly in the various districts of the ore field. Three minerals make up the definitive part: monazite, pyrochlore and crandallite (Table 2). Some 90 % of the phosphorous are contained in pyrochlore and crandallite. The remaining phosphorous content is fixed to the apatite group. As for niobium, it is contained mainly in pyrochlore. Of the iron minerals, siderite, magnetite and ironhydrooxides are present. Siliceous minerals content is highly variable within the deposits. It is extremely low in the rich ores. Kaolinite-crandallite ores sometimes contain much SiO₂ and Al₂O₃.

The average chemical composition of the ore-forming minerals in the ore seam have been determined using a microprobe and are shown in Table 3.

The principal ore minerals are represented by oxygen salts: pyrochlore by oxide salt of niobic acid, monazite by rare-earth salt of orthophosphoric acid, crandallite by oxide salt of aluminophosphoric acid. Rutile, ilmenorutile and columbite feature among the complex oxides. The same chemical components make up the basic ore minerals. TR₂O₃, for example, is distributed between monazite, pyrochlore and crandallite. Niobium is contained in pyrochlore, rutile and columbite. Al₂O₃ is a constituent of crandallite and kaolinite. Fe is contained in siderite, crandallite and kaolinite. The high value of the coefficient of variation for content, which amounts to around 50 %, is characteristic of all components with the exception of P₂O₅. This means that such ores need to be subjected to thorough homogenisation before they are passed for technological processing. Such homogenisation makes it possible to achieve a composition in the feed which will permit, during liquation melting and in the presence of fluxes, the obtainment of two non-miscible melts, from which useful products can be recovered.

Melting of rare-earth ores

The melting of the ore with the addition of fluxes (NaF, K₂SiF₆, etc.) proceeds with the formation of two non-miscible melts, one of these being the siliceous and the second the phosphate-salt-containing. These melts exhibit contrary compositions and properties. The liquation process is rapid, lasting only 10 to 15 minutes. The main heat input in consumed for heating-up of the ore sample. Figure 1 shows a diagram in principle of the technology of the liquation process for ores.

The results of melting of the rich pyrochlore-monazite and pyrochlore-monazite-crandallite medium ores are shown in Figures 2 and 3. The melts are separated from each other by the flat boundary of the phasen interface (flat phase interface linie).

Yield by weight of the silicate melt was 31.2 % for the pyrochlore-monazite ore (Figure 2) and 68.8 % in the case of the phosphate salt melt. The figures are, correspondingly, 67.5 % and 32.5 % for pyrochlore-monazite-crandallite ore (Figure 3). The empirical phase diagram of the “pyrochlore-monazite ore SiO₂-NaF” system, in which the fields of the liquation melts are shown for five isometric cross-sections (850, 900, 1000, 1100 and 1200 ° C) is shown in Figure 4.

It is apparent in the diagrams (Figure 4 and 5) that the melting range encloses a large interval of the component concentration. The field for the two melts coexisting adjacent to one another spreads out toward the side edge, “SiO₂ ore”. The occurrence of the liquid phase at a temperature of 760 ° C is fixed in the SiO₂-NaF system (the lower portion of Figure 4). The Field of the melt liquation itself is at temperatures of between 1005 and 1010 ° C and higher. The S-shaped fusion curve in the SiO₂-NaF system and the decrease in temperature for SiO₂ fusibility can be explained by the reaction occurring between SiO₂ and NaF, in which silicon oxyfluorides and sodium disilicates are formed [5]:

6 SiO₂ + 2 NaF = Si₄O₇F₂ + Na₂Si₂O₅.

The silicate melt becomes alkaline, resulting at a temperature of ~ 1010 ° C in the formation of the non-miscible silicate and alkali fluoride melts. The addition of the ore components to the SiO₂-NaF system results in the formation of the non-miscible melts at as low as 850 ° C (in the upper section of Figure 4). A significant decrease in the temperature for component melts can be explained by reactions which occur between monazite and sodium fluoride in the salt component of the system. Double phosphates of the rare-earth elements and of sodium – Na₂Ce[PO₄]F₂ are formed in this process [6, 7]. The rare-earth phosphates dissolve in the sodium fluoride melt, resulting in the formation of the phosphate salt melt, which separates from the silicate melt (Figures 2 and 3). The phosphate salt melt crystallises out readily upon cooling. Crystals of rare-earth phosphates, double fluorides and small drops of silicate melt (Figure 6) which, due to their small dimensions (2 to 15 µm), have not combined with the silicate melt layer, are formed in this process.

Phosphate minerals are represented by a phase which is similar to the structure of monazite, double phosphates and fluorid phosphates of the lanthanides and of sodium. The latter are formed as a result of the reaction between the native monazite and sodium fluoride, in accordance with the following equation:

2 Ln[PO₄] + 6 NaF = Na₃Ln[PO₄]₂ + Na₂Ln[PO₄]F₂ + NaLnF₄.

A niobium-rich phase has, in addition to phosphates, also been detected in the salt melt and has been identified as Ln[NbO₄]. Fine silicate globules located in the salt melt are similar in their chemical composition to the macrolayer of the silicate melt (Table 4).

A glass is formed during cooling of the silicate melt and contains fine phosphate globules (Figure 7) which are similar in their chemical composition to the macrolayer of the phosphate salt melt (Table 4). Crystals of the rare-earth phosphates and of the niobium-rich phase are visible in the larger salt droplets (diameter: 60 to 100 µm) which are sometimes present.

The chemical composition of the non-miscible melts for rare-earth-element-rich pyrochlore-monazite ores and pyrochlore-monazite-crandallite medium ores, and also the magnitude of the  distribution coefficient Kp = ciLsi : ciLsi (in which ci = component content in %) between the two non-miscible melts are shown in Tables 6 and 7. These tables also include guideline figures for component extraction which have been calculated in accordance with the weight yield of the melts.

The data shown in Tables 6 and 7 indicates major differences in the composition and properties of the salt and silicate melts. This forms a basis for a number of processing methods for these melts. Hydrochemical methods are applicable for phosphate salt melts, for example, and make it possible to obtain rare-earth products. Reduction melting, which permits the production of niobium products (ferroniobium, niobium carbide, etc.), is appropriate for the silicate melt.

Processing of the phosphate salt melt

The simplest method for the salt melt was its dissolution in weakly concentrated nitrous acid. Here, temperature and duration did not exercise any particular influence on the solubility of the phosphate salt melt (Table 5). Some 62 to 67 % of the salts dissolve, while 33 to 38 % remain in the dissolution residues, in which predominantly silicate globules (Figure 6) and small quantities of CeO₂ and CeF₂, which are poorly soluble in weak solutions of nitrous acid, accumulate. The latter compounds form upon digestion of the phosphate salt melt using nitrous acid, as a result of the incongruent decomposition of the double phosphates and fluorid phosphates. The dissolution residue was submitted to dissolution in 10 % sulphuric acid and then in 10 % alkali lye (deutsch hieß “Alkalielauge”). Ultimately, the yield by weight was 13 to 16 % of the initial sample. Here, Nb₂O₅ increased by four- to five-fold and the SiO₂ and Al₂O₃ contents decreased. This intermediate product, the chemical composition of which is shown in Table 8, can be converted to the niobium compounds using thermochemical methods.

Rare earths are extracted from nitric solutions in two stages. The first stage is performed in saturated oxalic acid solution at pH 1 to 2, the second in 5 % ammonia solution at pH 8.0 to 8.5. The major part of the rare earths (87.4 %) is precipitated out in the composition of the oxalates, and the remainder of the rare earths not yielded in the first stage (12.6 %), mixed with hydroxides, in the second stage. The chemical composition of the rare-earth concentrates obtained using this method is shown in Table 8 (the Na₂O content of the products was not quantitated). The concentrates dissolve readily in weak HNO₃ solution and are then subjected to recleaning, in order to remove impurities. The rare-earths content, together with Y₂O₃ and Sc₂O₃, in the oxalate concentrate dried at 105 ° C was 51.43 %. Rare-earth content increases after cleaning in the HNO₃ solution. This product can be used for the production of individual rare-earth elements by means of the known extraction methods.

After cleaning in the nitric acid solution and elimination of the impurities, the dried hydroxide concentrate can be used for the production of individual rare-earth elements.

Conclusion

The thermochemical technology of processing of rare-earth-containing mineral resources can be used for the ochre ore deposits of the weathering crust of carbonatite ore massif outcrops, which cannot be prepared using traditional physical methods. Melting of these ores in the presence of fluxes results in the formation of two non-miscible melts: the phosphate salt melt and the silicate melt. Y₂O₃, Sc₂O₃ and alkali-earth elements are primarily concentrated in the first melt, while Nb₂O₃, SiO₂, Fe₂O₃ and Al₂O₃ accumulate in the second. Each of these melts constitutes a synthetic mineral resource from which corresponding components can be extracted. Unlike the well-known sulphuric acid and alkali processes for the preparation of monazite resources, this article has examined the nitric acid process for extraction of the rare-earth elements from the phosphate salt melt.