It’s in the melt

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

Summary: The problem of processing of the rare-earth metal ores of the weathering crust of the Tomtor deposits (in Yakutia) has been discussed in this article. The ores of this deposit, due to their fluctuating composition, are extremely complicated. They are characterised by fine mineral particles of sizes of less than 10 µm. These ores are composed of monazite, pyrochlore, crandallite, iron and titanium oxides and kaolinite, among other minerals. The same chemical components form part of the chemistry of industrial minerals. They are not susceptible to physical preparation methods (flotation, magnetic separation, etc.). Hydrochemical alkali processes have proven suiTable only for the preparation of ores with limited iron, calcium and silicates contents. Liquation melting technology with the addition of fluxes (thermochemical method) has been presented in this context. Two non-miscible melts, silicate and phosphate-salt melt, are formed at temperatures of 1000 to 1200 ° C. Silicate melt is a niobium concentrator, and the phosphate salt melt a concentrator of the rare-earth elements. Treatment of the phosphate salt melt with a nitric acid solution renders rare earths in the form of oxalates or oxides. The silicate melt is processed using thermal methods to ferroniobium or to niobium carbide.

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.

Literature:

[1] Lazareva E. V., Zwodik S. M., Dobrezov N. L., Tolstov A. W. et al.: Principal ore-forming minerals of the anomalous rich ores of the Tomtor deposits (Arctic Siberia). Geologia I Geophisika.  2015. Volume 56, No. 6, pp. 1080 – 1115

[2] Paschkov G. L., Kuzmin W. I. et al.: The technology of the winning and extraction of usable components from the rare-metal ores of the Tomtor deposits. Topics for papers at the international conference on “Rare metals: Resource processing, their use for production of compounds and materials”. Krasnoyarsk (Russia): 1995, pp. 71 – 74

[3] Lichnikeviz E. G., Ozogina E. G., Astachova Ju. M., Fatov A. S. The influence of the mineralogical composition of the pyrochlore-monazite-crandallite ores on technological characteristics. Zoloto I Technologii. 2016. No. 4 (34), pp 68 – 71

[4] Sarytschev G. A., Tarasov A. W., Kosynkin W. D. et al.: The development of industrial technology for the processing of the ores of the Tomtor deposits. Trudy mezdunarodnoj nautschno- praktitscheskoj konferenzii “Aktualjnye voprosy polutschenia I primenenia RZM I RM”. 2017, 21 – 22 June 2017. Moscow. AG “Institut Ginzwetmet”

[5] Delitzyn L. M. “Liquation effects in magmatic systems”. Moscow. “Geos” publishers. 2010. 222 pp.

[6] Delitzyn L. M., Sineljtschikov W. A., Batenin W. M. et al.: Experimental investigation of the formation of double phosphates in the LaPO4 - NaF system. Doklady Akademii nauk. Volume 465, No. 4, pp. 451 – 455

[7] Delitzyn L. M.: The distribution of niobium and lanthanum in non-miscible melts in the LaPO₄ - SiO₂ - NaF - Nb₂O₅ - Fe₂O₃ system. Doklady akademii nauk. 2017. Volume 477. No. 3, pp 307 – 312

Authors

Dr. habil. Leonid M. Delitzyn, Chief research scientist Joint Institute for High Temperatures RAS (JIHT)

Nach seinem Studium am Moskauer Institut für Buntmetalle und Gold, wo er das Diplom des Ingenieurgeologen bekam, hat er sich mit Reaktionsverhältnissen zwischen Silikaten und Haloiden beschäftigt. Das Ergebnis dieses Studiums war seine Promotionsarbeit. Im Jahre 1986 verteidigte er erfolgreich seine Habilitationsarbeit, die der Rolle der Liquationsvorgänge bei der Entstehung der Apatit-Nephelin-Lagerstätte des Hibinski Gebirgsmassives gewidmet wurde. Zur Zeit liegen seine wissenschaftlichen Interessen auf dem Gebiet der Aufbereitung und Verarbeitung der Seltenerderzen, der aschehaltigen Abgänge der Kraftwerke, der Reinigung der Betriebsabwässer.

Dr. Gelij B. Melentjev, Senior scientific researcher, Joint Institute for High Temperatures RAS

Er hat das Moskauer politechnische Institut absolviert und den Grad des Ingenieurgeologen erhalten. Er ist einer der führenden Fachleute in Russland auf dem Gebiet der Forschung und der komplexen Wertschätzung der Seltenmetallrohstoffe und technogener Ressourcen mit dem Zweck ihrer industriellen Ausnutzung.

Dr.-Ing. Jury V. Ryabov, Senior scientific researcher Joint Institute for High Temperatures RAS

Nach dem Studium an der Bergakademie Freiberg hat er am Forschungsinstitut “Uralmechanobr“ gearbeitet, wo er sich mit der Aufbereitung von Buntmetallerzen beschäftigte. Von 1961 bis 2014 arbeitete er am Forschungsinstitut für bergbauchemische Rohstoffe. Seine Promationsarbeit hat er der Flotation von Silvingrobkörnern aus der Wirbelschicht gewidmet. Er hat neue Verfahren der Flotation von Kali, Schwefel und Phosphaten in Russland und im Ausland erprobt und praktisch umgesetzt. Seit 2014 nimmt er an den Untersuchungen der Aufbereitung von Seltenen Erden und aschehaltigen Abgängen an der JIHT der russischen Akademie der Wissenschaften teil.

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