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