Titanite Ores of the Khibiny Apatite-Nepheline- Deposits: Selective Mining, Processing and Application for Titanosilicate Synthesis

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Article Titanite Ores of the Khibiny Apatite-Nepheline- Deposits: Selective Mining, Processing and Application for Titanosilicate Synthesis

Lidia G. Gerasimova 1,2, Anatoly I. Nikolaev 1,2,*, Marina V. Maslova 1,2, Ekaterina S. Shchukina 1,2, Gleb O. Samburov 2, Victor N. Yakovenchuk 1 and Gregory Yu. Ivanyuk 1 1 Nanomaterials Research Centre of Kola Science Centre, Russian Academy of Sciences, 14 Fersman Street, Apatity 184209, Russia; [email protected] (L.G.G.); [email protected] (M.V.M.); [email protected] (E.S.S.); [email protected] (V.N.Y.); [email protected] (G.Y.I.) 2 Tananaev Institute of Chemistry of Kola Science Centre, Russian Academy of Sciences, 26a Fersman Street, Apatity 184209, Russia; [email protected] * Correspondence: [email protected]; Tel.: +7-815-557-9231

Received: 4 September 2018; Accepted: 10 October 2018; Published: 12 October 2018

Abstract: Geological setting and mineral composition of (apatite)-nepheline-titanite ore from the Khibiny massif enable selective mining of titanite ore, and its processing with sulfuric-acid method, without preliminary concentration in flotation cells. In this process flow diagram, titanite losses are reduced by an order of magnitude in comparison with a conventional flotation technology. Further, dissolution of titanite in concentrated sulfuric acid produces titanyl sulfate, which, in turn, is a precursor for titanosilicate synthesis. In particular, synthetic analogues of the ivanyukite group minerals, SIV, was synthesized with hydrothermal method from the composition based on titanyl-sulfate, and assayed as a selective cation-exchanger for Cs and Sr.

Keywords: apatite-nepheline-titanite ore; sulfuric-acidic decomposition; titanyl sulfate; hydrothermal synthesis; ivanyukite

1. Introduction The world largest apatite-nepheline deposits of the Khibiny massif (NW Russia) host complex ores containing ﬁve economic minerals are: Fluorapatite, nepheline, titanite, aegirine and titanomagnetite. However, at present, only two of them (ﬂuorapatite and nepheline) are economically attractive, while the rest are accumulated in tailings ponds [1]. Titanite, CaTiSiO5, is the most important mineral among them because titanium and its compounds are widely used in different industrial technologies. In addition to titanite and titanomagnetite, the apatite-nepheline deposits contain numerous pegmatites with rare titanium minerals that have important functional properties, for example, ferroelectric loparite-(Ce) [2], molecular sieve chivruaiite [3,4] (Ca-analog of synthetic microporous titanosilicate ETS-4 [5,6]), cation exchangers kukisvumite and punkaruaivite [7–10] (respectively, Zn and Li analogues of synthetic titanosilicate AM-4 [11,12]), cation exchangers sitinakite [13] (natural analogue of synthetic titanosilicate Ionsiv IE-911 [14–16]), and minerals of the ivanyukite group [17–20]. Besides, these and other synthetic titanium-based compounds (ETS-2, ETS-10, JDF-L1, LHT-9, MIL-125, etc.) are utilized as agents for cation relocation, hydrogen storage and heat transformation, dehydrating agents, drug nanocarriers, adsorbents and membranes [20–25]. In particular, ivanyukite-Na and its synthetic analogue SIV described herein have pharmacosiderite- related crystal structure consisting of Ti4O24-clusters coupled into a framework by single SiO4 tetrahedra

Minerals 2018, 8, 446; doi:10.3390/min8100446 www.mdpi.com/journal/minerals Minerals 2018, 8, 446 2 of 13

Minerals 2018, 8, x FOR PEER REVIEW 2 of 13 (Figure1). In this framework, there is a system of wide channels ( ≈6.1 Å), with extra-framework cations of K andsingle Na SiO and4 tetrahedra water molecules (Figure 1). inside In this that framework, can be easily there exchanged is a system withof wide Rb +channels, Cs+, Tl (+≈,6.1 Ag Å),+, Srwith2+, Ba2+, extra-framework cations of K and Na and water molecules inside that can be easily exchanged with Cu2+, Ni2+, Co2+, La3+, Ce3+, Eu3+, Th4+, etc. [18–20,26]. Moreover, (137Cs, 90Sr, 154Eu)-exchanged forms Rb+, Cs+, Tl+, Ag+, Sr2+, Ba2+, Cu2+, Ni2+, Co2+, La3+, Ce3+, Eu3+, Th4+, etc. [18–20,26]. Moreover, (137Cs, 90Sr, of ivanyukite can be transformed by heating into stable SYNROC-like titanate ceramics for long-time 154Eu)-exchanged forms of ivanyukite can be transformed by heating into stable SYNROC-like immobilizationtitanate ceramics of these for long-time radionuclides immob [20ilization]. of these radionuclides [20].

Figure 1. Crystal structure of ivanyukite-Na-T [17]: cubano-like clusters Ti4O24 consisting of four edge- Figure 1. Crystal structure of ivanyukite-Na-T [17]: cubano-like clusters Ti4O24 consisting of four shared TiO6 octahedra (dark-blue) are connected by vertexes SiO4 tetrahedra (yellow) to form the edge-shared TiO6 octahedra (dark-blue) are connected by vertexes SiO4 tetrahedra (yellow) to form the microporous framework, with Na (cyan), K (pale green), and H2O (magenta) in channels. microporous framework, with Na (cyan), K (pale green), and H2O (magenta) in channels. There are a lot of studies related to titanite concentrate chemical processing [27–29]. Among the There are a lot of studies related to titanite concentrate chemical processing [27–29]. Among the known technologies, sulfuric-acidic ones are the most perspective because they enable to obtain a knownwide technologies, spectrum of sulfuric-acidicmaterials including ones titanium are the mostpigments, perspective fillers, adhesives, because theytanning enable agents, to obtainsorbents a wide spectrumand molecular of materials sieves. including High titanite titanium content pigments,is a typical property fillers, adhesives,of apatite-titanite tanning and nepheline-titanite agents, sorbents and molecularrocks, which sieves. are High currently titanite stockpiled content in is dumps a typical due propertyto difficulties of apatite-titanitewith titanite separation and nepheline-titanite from apatite rocks,and which nepheline are by currently flotation. stockpiledWe would like in to dumps show he duerein tonew difficulties perspectives with for production titanite separation of synthetic from apatiteivanyukite-type and nepheline compounds by flotation. and Weother would materials like me to showntioned herein above newwith perspectives sulfuric-acidic for cleaning production of syntheticfollowed ivanyukite-type by hydrometallurgical compounds processing and of other ground materials apatite-titanite mentioned and abovenepheline-titanite with sulfuric-acidic ores. cleaningThis followedapproach byfeatures hydrometallurgical better productiveness processing in relation of ground to a conven apatite-titanitetional technologies and nepheline-titanite of titanite ores.extraction This approach by flotation features and better titanosilicate productiveness synthesis from in relation compositions to a conventional based on titanium technologies chlorides. of titanite extraction2. Geological by flotation Setting and titanosilicate synthesis from compositions based on titanium chlorides. 2. GeologicalThe world’s Setting largest Khibiny alkaline massif (35 × 45 km) is situated in the West of the Kola Peninsula (Murmansk Region, Russia), at the contact between low-metamorphosed volcano- sedimentaryThe world’s rocks largest of the Khibiny Proterozoic alkaline Imandra-Varzuga massif (35 greenstone× 45 km) belt isand situated the Archaean in the ultra- West of the Kolametamorphic Peninsula rocks (Murmansk of the Kola-Norwegian Region, megablock Russia), at(Figure the contact2). Its age betweenis 380–360 low-metamorphosedmillion years [30]. volcano-sedimentaryThe main rock of the rocks massif of theis coarse-grained Proterozoic Imandra-Varzuganepheline syenite (foyaite) greenstone that forms belt funnel-shaped and the Archaean ultra-metamorphichomogeneous body rocks divided of the into Kola-Norwegian two parts by the Main megablock Ring intrusion (Figure of2 ).foidolite Its age (melteigite–urtite is 380–360 million yearsrock [30 ].series), The with main related rock poikilitic of the massif (kalsilite)-ne is coarse-grainedpheline syenites nepheline (rischorrite syenite and lyavochorrite) (foyaite) that and forms funnel-shapedfine-grained homogeneous alkaline syenite bodywith xenolites divided of into feinitized two partsvolcano-sedimentary by the Main Ring rocksintrusion [31,32]. The of Main foidolite (melteigite–urtiteRing thickness rockon the series), day surface with varies related from poikilitic 0.1 to 10 km, (kalsilite)-nepheline and rapidly decreases syenites with depth. (rischorrite Within and the outer part of the foyaite body, there is another one semi-ring zone of foidolites, alkaline syenite lyavochorrite) and fine-grained alkaline syenite with xenolites of feinitized volcano-sedimentary and fenite. In addition, monchiquite–carbonatite veins and explosion pipes occur in the Eastern part rocks [31,32]. The Main Ring thickness on the day surface varies from 0.1 to 10 km, and rapidly of the Main Ring. decreasesThe with rock-forming depth. Within minerals the outer of foyaite part include of the foyaitenepheline, body, microcline, there is anotherorthoclase, one albite, semi-ring aegirine, zone of foidolites,augite, alkaline Na–Ca- syeniteand Na-amphiboles, and fenite. In addition,aenigmatite, monchiquite–carbonatite titanite, eudialyte, lamprophillite, veins and and explosion annite. pipes occur in the Eastern part of the Main Ring.

The rock-forming minerals of foyaite include nepheline, microcline, orthoclase, albite, aegirine, augite, Na–Ca- and Na-amphiboles, aenigmatite, titanite, eudialyte, lamprophillite, Minerals 2018, 8, 446 3 of 13

Minerals 2018, 8, x FOR PEER REVIEW 3 of 13 and annite. Rischorrite and lyavochorrite contain rock-forming nepheline, sodalite, orthoclase, aegirine,RischorriteMinerals KNa-amphiboles, 2018 ,and 8, x FORlyavochorrite PEER REVIEW annite, contain titanite, rock-forming aenigmatite, nepheline, ilmenite, sodalite, lamprophillite,orthoclase, aegirine, astrophyllite, KNa-3 of 13 amphiboles, annite, titanite, aenigmatite, ilmenite, lamprophillite, astrophyllite, and fluorapatite [31,32]. and ﬂuorapatiteRischorrite and [31 lyavochorrite,32]. The main contain constituents rock-forming of thenepheline, foidolites sodalite, are nepheline, orthoclase, clinopyroxenesaegirine, KNa- of the diopside–aegirine-augiteThe main constituents of the series, foidolites KNaCa-amphiboles are nepheline, clinopyroxenes (potassicrichterite, of the diopside–aegirine-augite potassicferrorichterite, series,amphiboles, KNaCa-amphiboles annite, titanite, (potassicrichterite, aenigmatite, ilmenite, potassicf lamprophillite,errorichterite, astrophyllite, etc.), annite, and titanite, fluorapatite fluorapatite, [31,32]. etc.), annite, titanite, ﬂuorapatite, titanomagnetite (members of the magnetite–ulvöspinel series), titanomagnetiteThe main constituents (members of ofthe the foidolites magnetite–ulvöspinel are nepheline, series), clinopyroxenes ilmenite, and of eudialyte,the diopside–aegirine-augite with wide-ranging ilmenite, and eudialyte, with wide-ranging relations between them (Figure3). relationsseries, KNaCa-amphiboles between them (Figure (potassicrichterite, 3). potassicferrorichterite, etc.), annite, titanite, fluorapatite, titanomagnetite (members of the magnetite–ulvöspinel series), ilmenite, and eudialyte, with wide-ranging relations between them (Figure 3).

Figure 2. Geological map of the Khibiny massif (a, after [32]) and vertical section of the Figure 2. Geological map of the Khibiny massif (a, after [32]) and vertical section of the KukisvumchorrKukisvumchorr titanite-apatite-nepheline titanite-apatite-nepheline ore ore body body along along A–B line A–B (b, after line F.M. (b, Onokhin). after F.M. Titanite- Onokhin). apatite-nephelineFigure 2. Geological deposits: map 1—Valepakhk, of the Khibiny 2—Pa massifrtomchorr, (a, after 3—Kuelporr, [32]) and 4—Snezhnyvertical section Circus, of 5—the Titanite-apatite-nepheline deposits: 1—Valepakhk, 2—Partomchorr, 3—Kuelporr, 4—Snezhny Kukisvumchorr,Kukisvumchorr titanite-apatite-nep6—Yuksporr, 7—Apatiteheline ore Circus, body 8—Platoalong A–B Rasv lineumchorr, (b, after F.M.9—Eveslogchorr, Onokhin). Titanite- 10— Circus, 5—Kukisvumchorr, 6—Yuksporr, 7—Apatite Circus, 8—Plato Rasvumchorr, 9—Eveslogchorr, Koashva,apatite-nepheline 11—Niorkpakhk, deposits: 1—Valepakhk,and 12—OleniyRuchei. 2—Partomchorr, Red star 3—Kuelporr, indicates the 4—Snezhny place of titaniteCircus, ore5— 10—Koashva, 11—Niorkpakhk, and 12—OleniyRuchei. Red star indicates the place of titanite sampling.Kukisvumchorr, 6—Yuksporr, 7—Apatite Circus, 8—Plato Rasvumchorr, 9—Eveslogchorr, 10— ore sampling.Koashva, 11—Niorkpakhk, and 12—OleniyRuchei. Red star indicates the place of titanite ore sampling.

Figure 3. Modal composition of alkaline rocks estimated with grain squares of feldspars (A),

feldspathoids (F), and dark colored minerals (M) in polished hand-sized specimens of the Khibiny FiguremassifFigure 3. (after3.Modal Modal [32,33], composition composition with additions). of of alkaline alkaline Ab—alb rocks rocksite, estimated Amp—amphiboles, estimated with withgrain Ano—anorthoclase, grainsquares squares of feldspars of Ap— feldspars (A), (A), feldspathoidsfluorapatite,feldspathoids Bt—biotite, (F), (F), and and dark darkCcn—cancrinite, colored colored minerals minerals Cpx— (M) inclinopyroxenes, (M)polished in polished hand-sized Eud—eudialyte, hand-sized specimens specimensofKls—kalsilite, the Khibiny of the KhibinyMag—magnetite,massif massif (after (after [32,33], Mc—microcline, [32 ,33with], withadditions). additions). Nph—nepheline, Ab—alb Ab—albite,ite, Amp—amphiboles,Ntr—natrolite, Amp—amphiboles, Or—orthoclase, Ano—anorthoclase, Ano—anorthoclase, Sdl—sodalite, Ap— Ap—ﬂuorapatite,andfluorapatite, Ttn—titanite. Bt—biotite, Ccn—cancrinite, Ccn—cancrinite, Cpx— Cpx—clinopyroxenes,clinopyroxenes, Eud—eudialyte, Eud—eudialyte, Kls—kalsilite, Kls—kalsilite, Mag—magnetite, Mc—microcline, Nph—nepheline, Ntr—natrolite, Or—orthoclase, Sdl—sodalite, Mag—magnetite, Mc—microcline, Nph—nepheline, Ntr—natrolite, Or—orthoclase, Sdl—sodalite, and Ttn—titanite. and Ttn—titanite.

At apical parts of the Main Ring, there are fractal stockworks of specific apatite-rich foidolite, apatite-nepheline rock (see Figures2 and3), which form 12 apatite deposits developed by two mining Minerals 2018, 8, 446 4 of 13 companies: JSC “Apatit”, and JSC “North-Western Phosphorus Company” [32]. Major minerals of apatite-nepheline rock are fluorapatite, nepheline, diopside–aegirine-augite, potassicrichterite, potassicferrorichterite, orthoclase, titanite, magnetite, and ilmenite, contents of that vary in a wide range of values (Figure3). Titanite is a typical apatite-nepheline rock mineral of accessory to minor rock-forming types (Table1). It occurs as separate fine-grained lenses (up to 5 cm in diameter) within fluorapatite bands (Figure4a). Content and size of titanite lenses increase towards the upper contact of apatite-nepheline rock covered by rischorrite–lyavochorrite that accommodates layers (up to 50 m thick) of titanite-dominant (up to 80 modal %) apatite-titanite and nepheline-titanite rocks (see Figure2b). In these rocks, the main part of titanite is presented by brown prismatic crystals (up to 1 cm long) with lustrous faces (Figure4b–d). Average chemical composition of such titanite (Table2) corresponds to 3+ the formula: (Ca0.95Na0.04Sr0.01)Σ1.00(Ti0.96Fe 0.02Nb0.01)Σ0.99 (Si0.99Al0.01)Σ1.00O5 [33].

Table 1. Titanite resources of the Khibiny titanite-apatite-nepheline deposits [33].

Measured Ttn Average Ttn Content in Fraction of Ap-Ttn and Deposit Resources, kt (Ttn)-Ap-Nph ore, wt % Nph-Ttn Ores, vol % Partomchorr 59,615 6.9 13.6 Kuelporr 995 5.2 4.3 Kukisvumchorr 17,550 4.2 6.9 Yuksporr 23,340 4.4 8.1 Apatite 3840 3.2 1.9 Circus Rasvumchorr 13,395 4.0 2.1 Eveslogchorr 45 8.9 27.7 Koashva 38,780 4.7 8.4 Niorkpakhk 1595 2.4 0.0 OleniyRuchei 12,075 3.1 0.0

Table 2. Chemical composition of titanite from titanite-apatite-nepheline rock, wt % [33].

Constituent n Mean Min Max SD

Na2O 15 0.61 b.d. 1.18 0.30 Al2O3 15 0.24 b.d. 0.66 0.17 SiO2 15 30.31 29.61 32.38 0.62 K2O 15 0.01 b.d. 0.20 0.05 CaO 15 27.16 25.22 28.04 0.95 TiO2 15 38.93 37.28 42.86 1.30 V2O3 15 0.02 b.d. 0.15 0.04 MnO 15 0.01 b.d. 0.06 0.02 FeO 15 0.88 0.33 1.29 0.29 SrO 15 0.32 b.d. 0.53 0.15 ZrO2 15 0.14 b.d. 0.43 0.15 Nb2O5 15 0.38 b.d. 1.35 0.41 La2O3 15 0.05 b.d. 0.17 0.07 Ce2O3 15 0.30 b.d. 0.68 0.16 Nd2O3 15 0.04 b.d. 0.26 0.08

At the apatite deposits, content of (apatite)-nepheline-titanite rock reaches 30 vol % (Table1) with average titanite content about 20 wt %. Taking into account stable position of this rock at the contact between bedding foidolite (including apatite-nepheline rock) and covering rischorrite–lyavochorrite, as well as signiﬁcant thickness of these layers (Figure2b), selective mining of titanite ores gains practical importance. Under the present studies, about 1.5 tons of nepheline-titanite rock were selectively mined from the apical part of the Koashva ore-body, and then processed with the sulfuric-acid method without preliminary concentration. Minerals 2018, 8, 446 5 of 13 Minerals 2018, 8, x FOR PEER REVIEW 5 of 13

Figure 4. TitaniteTitanite segregations segregations in inapatite-nepheline apatite-nepheline (a), ( aapatite-titanite), apatite-titanite (b) and (b) andnepheline-titanite nepheline-titanite (c,d) (oresc,d) of ores the ofKoashva the Koashva apatite deposit, apatite the deposit, Khibiny the massif. Khibiny Photos massif. of polished Photos samples of polished (a–c) and samples thin section (a–c) andof the thin sample section c in oftransmitted the sample light c in(d). transmitted Ap—fluorapatite, light ( dCpx—clinopyroxene,). Ap—ﬂuorapatite, Nph—nepheline, Cpx—clinopyroxene, Ttn— Nph—nepheline,titanite. Ttn—titanite.

3. MaterialsAt the apatite and Methods deposits, content of (apatite)-nepheline-titanite rock reaches 30 vol % (Table 1) with average titanite content about 20 wt %. Taking into account stable position of this rock at the For this study, we used blocks (up to 0.5 m in diameter) of (apatite)-nepheline-titanite ore, mined at contact between bedding foidolite (including apatite-nepheline rock) and covering rischorrite– the Koashva open pit. The reagents used included reagent-grade sulfuric and hydrochloric acids, lyavochorrite, as well as significant thickness of these layers (Figure 2b), selective mining of titanite sodium and potassium hydroxides (H2SO4 93.0%, HCl 37.5%, NaOH 99%, KOH 99%), analytical ores gains practical importance. Under the present studies, about 1.5 tons of nepheline-titanite rock reagent grade pentahydrate sodium metasilicate, Na2SiO3·5H2O, and ammonium sulfate, (NH4)2SO4, were selectively mined from the apical part of the Koashva ore-body, and then processed with the (Reachem, Moscow, Russia), and distilled water (Tananaev Institute of Chemistry, Apatity, Russia). sulfuric-acid method without preliminary concentration. At first, ore blocks were crushed on a jaw crusher, and then in a 1.5 kW AGO-2 planetary mill (NOVIC, Novosibirsk, Russia) for 6 h, with the rock:balls ratio of 1:5 [34]. Then classification (dry sieving) 3. Materials and Methods was carried out (with separation of a powder fraction less than 40 µm). Modal and chemical composition of theFor crushed this study, sample we used was determinedblocks (up to by 0.5 means m in di ofameter) a FT IR of 200(apatite)-nepheline-titanite spectrophotometer (Perkin ore, mined Elmer, Waltham,at the Koashva MA, USA). open pit. The reagents used included reagent-grade sulfuric and hydrochloric acids, sodiumThe and produced potassium powder hydroxides sample was (H2 separatedSO4 93.0%, from HCl apatite 37.5%, and NaOH nepheline 99%, inKOH H2SO 99%),4 or HClanalytical dilute waterreagent solutions grade pentahydrate (50–100 g/L), sodium with solid/liquidmetasilicate, ratioNa2SiO S:L3·5H = 1:3–1:42O, and (Figure ammonium5). The sulfate, suspension (NH4)2 wasSO4, mixed(Reachem, in a Moscow, magnetic Russia), stirrer at and the distilled temperature water of (Tan 50 ±anaev5 ◦C Institute for 2 h. Afterof Chemis dissolutiontry, Apatity, of apatite Russia). and nepheline,At first, a puriﬁedore blocks titanite-aegirine were crushed concentrateon a jaw crusher, was separated and then by in ﬁltration. a 1.5 kW AGO-2 planetary mill (NOVIC, Novosibirsk, Russia) for 6 h, with the rock:balls ratio of 1:5 [34]. Then classification (dry sieving) was carried out (with separation of a powder fraction less than 40 μm). Modal and chemical composition of the crushed sample was determined by means of a FT IR 200 spectrophotometer (Perkin Elmer, Waltham, MA, USA). The produced powder sample was separated from apatite and nepheline in H2SO4 or HCl dilute water solutions (50–100 g/L), with solid/liquid ratio S:L = 1:3–1:4 (Figure 5). The suspension was mixed in a magnetic stirrer at the temperature of 50 ± 5 °C for 2 h. After dissolution of apatite and nepheline, a purified titanite-aegirine concentrate was separated by filtration.

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Figure 5. A principal scheme of apatite-nepheline-titanite ore processing for SIV synthesis. Mineral Figure 5. A principal scheme of apatite-nepheline-titanite ore processing for SIV synthesis. Mineral abbreviations seesee inin FigureFigure3 3..

2 4 Then thethe concentrateconcentrate was was mixed mixed with with sulfuric sulfuric acid acid (500 (500± 100 ± 100 g/L g/L H2SO H4SO) at) the at ratiothe ratio of S:L of = S:L 1:3.5, = and1:3.5, the and mixture the mixture was poured was poured into a ﬂask into equipped a flask equipped with a stirrer with and a stirrer a condenser and a [condenser35]. The concentrate [35]. The concentrate interacted with H2SO4 at the boiling point of the reaction mass for 9–10 h. The produced interacted with H2SO4 at the boiling point of the reaction mass for 9–10 h. The produced Ti-bearingTi-bearing sulfuricsulfuric acidacid liquidliquid phasephase waswas separated fromfrom insolubleinsoluble aegirine byby ﬁltration,filtration, and used for crystallization of titanyl sulfates TiOSO4·H2O (STM) and (NH4)2TiO(SO4)2·H2O (STA) [28,36]. The last crystallization of titanyl sulfates TiOSO4·H2O (STM) and (NH4)2TiO(SO4)2·H2O (STA) [28,36]. The last compound waswas obtainedobtained byby meansmeans ofof reactionreaction ofof STMSTM withwith ammoniumammonium sulfate.sulfate. Hydrothermal synthesis of microporousmicroporous titanosilicates (ETS-4, SIV, AM-4,AM-4, etc.)etc.) waswas performedperformed by analogy with ETS-10 [[37],37], with STM as Ti source. To prepare aa titaniumtitanium precursor, wewe usedused STMSTM solution with concentrations of 82 ± 2 g/L TiO2 and 9 ± 1 g/L H2SO4. Four-valent titanium contained solution with concentrations of 82 ± 2 g/L TiO2 and 9 ± 1 g/L H2SO4. Four-valent titanium contained inin thethe solutionsolution was was partially partially reduced reduced by by addition addition of zincof zinc powder powder (4.4 ±(4.40.1 ± g/L).0.1 g/L). Zinc Zinc consumption consumption was was assumed on the basis of Ti2O3 concentration of 5–10 g/L. Solution of sodium silicate assumed on the basis of Ti2O3 concentration of 5–10 g/L. Solution of sodium silicate Na2SiO3·7H2O Na2SiO3·7H2O containing 140 ± 10 g/L of SiO2 was also used. The reduced titanium solution was containing 140 ± 10 g/L of SiO2 was also used. The reduced titanium solution was gradually dosed intogradually the sodium dosed silicateinto the solution; sodium silicate after that, solution; sodium after and that, potassium sodium alkaliand potassium solutions alkali were addedsolutions to adjustwere added pH. Total to adjust time forpH. the Total mixture time preparationfor the mixtur wase preparation 3–3.5 h. The was resulting 3–3.5 h. gel-like The resulting suspension gel-like was placedsuspension in the was Parr placed 4666-FH-SS in the Parr autoclave 4666-FH-SS (Parr autocl Instrument,ave (Parr Moline, Instrument, IL, USA) Moline, and held IL, USA) at 195 and± 5held◦C forat 195 3–5 ± days. 5 °C Thefor 3–5 produced days. The precipitate produced was precipitate separated was and, separated after washing, and, placedafter washing, in a drying placed cabinet. in a Dryingdrying cabinet. time at theDrying temperature time at the of 70temperature± 5 ◦C was of 7–10 70 ± h.5 °C was 7–10 h. Modal composition of the synthesizedsynthesized samplessamples was determined by means of aa XRD-6000XRD-6000 diffractometer (Shimadzu, (Shimadzu, Kyoto, Kyoto, Japan), Japan), with with a Cu a Cu X-ray X-ray tube tube operated operated at 60 at kV 60 and kV 55mA. and 55mA. Bulk chemical compositions of the powder samples were determined using an X-ray fluorescence spectrometer Spectroscan MAKS-GV (Spectron, St. Petersburg, Russia), operating in WDS mode at 40 kV. Morphology of the produced particles was analyzed with an optical microscope DM-2500P

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Bulk chemical compositions of the powder samples were determined using an X-ray ﬂuorescence spectrometer Spectroscan MAKS-GV (Spectron, St. Petersburg, Russia), operating in WDS mode at 40 kV. Morphology of the produced particles was analyzed with an optical microscope DM-2500P (Leica, Wetzlar, Germany) and a scanning electron microscope XL 30 (Philips, Amsterdam, The Netherlands) equipped with an Oxford INCA EDS analyzer (Oxford Instruments, Oxford, UK). The BET surface properties and the porosity of the ﬁnal products were characterized by nitrogen and adsorption/desorption method at 77 K using a surface area analyzer Micromeritics TriStar 3020. The average pore diameter (Dav) was calculated as 4 V/S. Prior to adsorption/desorption measurements the sample was degased at 393 K for about 24 h. Low degassing temperature was chosen to avoid any structural changes of the material.

4. Results

4.1. STA and STM Production Acidic cleaning of (apatite)-nepheline-titanite ore occurs due to consecutive ﬂuorapatite and nepheline dissolution under the inﬂuence of diluted sulfuric or hydrochloric acids:

Ca5(PO4)3F + H2SO4 → 5CaSO4↓ + 3H3PO4 + HF (1)

Ca5(PO4)3F + 10HCl → 5CaCl2+3H3PO4 + HF (2)

(NaK)2Al2O3SiO2 + 4H2SO4 = (NaK)2SO4 + Al2(SO4)3 + SiO2·H2O + 2H2O (3)

(NaK)2Al2O3SiO2 + 8HCl = 2NaCl + 2KCl + 2AlCl3·2H2O + 2SiO2·2H2O↓ (4) Because of close titanite intergrowth with nepheline and especially fluorapatite, it is necessary to ensure fine grinding of ore (<63 µm). It is obvious that at this stage we should rather use hydrochloric acid, which does not cause formation of silica and insoluble calcium compounds that can inhibit titanite dissolution at the next stage. We found out that cold acidic cleaning of homogeneous finely ground ore was not very effective. Temperature increase up to 50 ◦C activates apatite dissolution with the corresponding phosphorus transfer into the liquid phase (Table3); however, nepheline remains undissolved. Two-stage ore cleaning with cold and then heated sulfuric acid gives satisfactory results for both apatite (P) and nepheline (Al), without Ti losses. Addition of hydrochloric acid is the most effective for apatite separation; however, absence of insoluble secondary products compensates quite well slow dissolution of nepheline.

Table 3. Chemical composition of ground ore cleaned under different conditions.

Ore Composition, wt % Experiment Conditions TiO2 Al2O3 P2O5 ◦ 1 H2SO4, 80 g/L, S:L = 1:3, 4 h, 18 C 30.5 0.97 4.23 ◦ 2 H2SO4, 80 g/L, S:L = 1:4, 2h, 50 C 28.0 2.69 1.00 ◦ 1 stage: H2SO4, 80 g/L, S:L = 1:3, 4 h, 18 C; 3 ◦ 32.0 0.27 1.15 2 stage: H2SO4, 100 g/L, S:L = 1:4, 2 h, 50 C 4 HCl, 50 g/L, S:L = 1:4, 2 h, 50 ◦C 31.5 1.62 0.21

Electron-microscope analyses showed that titanite grains cleaned with sulfuric acid were covered with the smallest particles of Ca sulfate and aluminum alum (Figure6a). After cleaning with hydrochloric acid, they became free from any secondary phases, as we predicted (Figure6b). Minerals 2018, 8, 446 8 of 13 Minerals 2018, 8, x FOR PEER REVIEW 8 of 13

Minerals 2018, 8, x FOR PEER REVIEW 8 of 13

FigureFigure 6. 6.SE-images SE-imagesof oftitanite titanitegrains grains cleaned cleaned by by sulfuric sulfuric acid acid ( a(a)) and and hydrochloric hydrochloric acid acid ( b(b).). Figure 6. SE-images of titanite grains cleaned by sulfuric acid (a) and hydrochloric acid (b). TheThe titanite-aegirinetitanite-aegirine concentrateconcentrate processingprocessing stagestage includedincluded dissolutiondissolution ofof titanitetitanite inin heatedheated concentratedconcentrated sulfuric sulfuric acid acid with with formation formation of of insoluble insoluble calcium calcium sulfate sulfate and and amorphous amorphous silica: silica: The titanite-aegirine concentrate processing stage included dissolution of titanite in heated concentrated sulfuric acidCaSiTiO with 5formation + 2H2SO4 =of TiOSO insoluble4 + SiO calcium2·H2O ↓sulfate +CaSO and4↓ + amorphous H2O. silica: CaSiTiO5 + 2H2SO4 = TiOSO4 + SiO2·H2O↓ +CaSO4↓ + H2O. This reaction canCaSiTiO be divided5 + 2H into2SO two4 = TiOSO consecutiv4 + SiOe 2stages:·H2O↓ +CaSOkinetic4 ↓stage + H2 O.and diffusion stage. The firstThis stage reaction gradually can transforms be divided into into the two second consecutive one due to stages: accumulation kinetic stage of solid and reaction diffusion products. stage. This reaction can be divided into two consecutive stages: kinetic stage and diffusion stage. The TheBoth ﬁrst stages, stage especially gradually the transforms second one, into are the seconddiffusio onen limited. due to The accumulation mechanism of of solid titanite reaction dissolution products. is first stage gradually transforms into the second one due to accumulation of solid reaction products. Boththe same stages, for especially both stages, the secondbut the one,dissolution are diffusion rate is limited. different The (Figure mechanism 7). In practice, of titanite proportion dissolution of Both stages, especially the second one, are diffusion limited. The mechanism of titanite dissolution is isleached the same titanium for both (IV) stages, in the butliquid the phase dissolution (i.e., prop rateortion is different of dissolved (Figure titanite)7). In ranges practice, from proportion 35 to 90% the same for both stages, but the dissolution rate is different (Figure 7). In practice, proportion of of[28]. leached Optimal titanium sulfuric (IV) acid in theconcentration liquid phase of 600 (i.e., g/L proportion prevents offrom dissolved precipitation titanite) of rangestitanyl fromsulfate 35 that to leached titanium (IV) in the liquid phase (i.e., proportion of dissolved titanite) ranges from 35 to 90% 90%exists [28 in]. such Optimal solution sulfuric only acid in a concentration stable molecular-dispersive of 600 g/L prevents form. from precipitation of titanyl sulfate [28]. Optimal sulfuric acid concentration of 600 g/L prevents from precipitation of titanyl sulfate that that exists in such solution only in a stable molecular-dispersive form. exists in such solution only in a stable molecular-dispersive form.

Figure 7. Ratio of Ti(IV) leaching from titanite (Rtot, line 1) and titanium recovery into the liquid phase (L, line 2) vs. sulfuric acid concentration at S:L = 1:3.5. FigureFigure 7. 7. RatioRatio of of Ti(IV) Ti(IV) leaching leaching from from titanite (R tot, ,line line 1) 1) and and titanium titanium recovery recovery into into the the liquid liquid phase phase (L,(L,During line line 2) 2) vs. vs.the sulfuric sulfuricthird stageacid acid concentration concentration of the experiment, at at S:L S:L = = 1:3.5. 1:3.5. the produced titanyl-sulfate solution was used to crystallize STA and STM salts. Their bulk crystallization from the solutions can be initiated by DuringDuring the the third third stage of the experiment, the produced titanyl-sulfate solution solution was was used used to to addition of salting-out reagents, in particular, ammonium sulfate [38]: crystallizecrystallize STASTA andand STM STM salts. salts. Their Their bulk bulk crystallization crystallization from from the solutions the solutions can be can initiated be initiated by addition by additionof salting-out of salting-out reagents,TiOSO reagents, in particular,4(L) in+ (NHparticular, ammonium4)2SO4(S) ammonium + sulfateH2O = [(NH38 sulfate]: 4)2TiO(SO [38]: 4)2·H2O(S). This composition becomes supersaturated when total concentration of ammonium sulfate and TiOSOTiOSO(L)4(L) + + (NH (NH)4)SO2SO4(S)(S) ++ HH2O = (NH 4))2TiO(SOTiO(SO4)2)·H·H2O(S).O(S). sulfuric acid in the solution4 reaches 45502 g/L,4 with2 the ammonium4 2 sulfate:acid4 2 2 ratio to be 1.2:1. If this This composition becomes supersaturated when total concentration of ammonium sulfate and ratioThis decreases composition to the value becomes of 1:2 supersaturated due to growth whenof sulfuric total acid concentration concentration of ammonium (400 g/L H2 sulfateSO4 and and 80– sulfuric acid in the solution reaches 550 g/L, with the ammonium sulfate:acid⋅ ratio to be 1.2:1. If this sulfuric85 g/L acidTiO2), in another the solution solid reaches “acidic” 550 compound g/L, with the(NH ammonium4)2TiO(SO4)2·0.15H sulfate:acid2SO4 0.89H ratio2O to bewill 1.2:1. precipitate If this ratio decreases to the value of 1:2 due to growth of sulfuric acid concentration (400 g/L H2SO4 and 80– ratioinstead decreases of STA. to the value of 1:2 due to growth of sulfuric acid concentration (400 g/L H SO 85 g/L TiO2), another solid “acidic” compound (NH4)2TiO(SO4)2·0.15H2SO4⋅0.89H2O will precipitate2 4 To produce STM, we pre-evaporated or diluted titanyl-sulfate solution with sulfuric acid up to instead of STA. 850–900 g/L H2SO4. Crystalline STM precipitates during boiling of this solution for 5–7 h. To produce STM, we pre-evaporated or diluted titanyl-sulfate solution with sulfuric acid up to 850–900 g/L H2SO4. Crystalline STM precipitates during boiling of this solution for 5–7 h.

Minerals 2018, 8, 446 9 of 13

and 80–85 g/L TiO2), another solid “acidic” compound (NH4)2TiO(SO4)2·0.15H2SO4·0.89H2O will precipitate instead of STA. To produce STM, we pre-evaporated or diluted titanyl-sulfate solution with sulfuric acid up to 850–900 g/L H2SO4. Crystalline STM precipitates during boiling of this solution for 5–7 h. Minerals 2018, 8, x FOR PEER REVIEW 9 of 13 4.2. Synthetic Ivanyukite-Na (SIV) Synthesis 4.2. Synthetic Ivanyukite-Na (SIV) Synthesis For hydrothermal synthesis of ivanyukite-Na (SIV), we used STM water solutions pre-reacted with metallicFor hydrothermal zinc to transform synthesis Ti4+ ofto ivanyukite-Na Ti3+ [39]. The (SIV), experimental we used conditionsSTM water aresolutions shown pre-reacted in Table4. with metallic zinc to transform Ti4+ to Ti3+ [39]. The experimental conditions are shown in Table 4. Table 4. Experimental conditions of SIV synthesis. Table 4. Experimental conditions of SIV synthesis. ◦ Experiment Pre-Reaction with Zn TiO2:SiO2, Moles pH T C Time, Day Experiment Pre-Reaction with Zn TiO2:SiO2, Moles рН Т °С Time, Day 1 + 1:4 11.5 200 3 21 ++ 1:5 1:4 12.511.5 200 190 3 3 32 ++ 1:4 1:5 12.512.5 190 200 3 5 43 –+ 1:4 1:4 11.512.5 200 200 5 5 54 –– 1:51:4 12.511.5 200 200 5 3 5 – 1:5 12.5 200 3

Long-term crystallization of SIV occurs in the basic (pH up to 12.5) system Na2O-K2O-TiO2-H2SO4- Long-term crystallization of SIV occurs in the basic (pH up to 12.5) system Na2O-K2O-TiO2- SiO2-H2O supersaturated with Na and Si [39,40]. After a three-day synthesis (experiments 1 and H2SO4-SiO2-H2O supersaturated with Na and Si [39,40]. After a three-day synthesis (experiments 1 2), the precipitates consist of two SIV polymorphs: rhombohedral SIV-T, Na4(TiO)4(SiO4)3·nH2O, and 2), the precipitates consist of two SIV polymorphs: rhombohedral SIV-T, Na4(TiO)4(SiO4)3·nH2O, and cubic SIV-C, Na3H(TiO)4(SiO4)3·nH2O (Table5). Longer crystallization (experiment 3) causes and cubic SIV-C, Na3H(TiO)4(SiO4)3·nH2O (Table 5). Longer crystallization (experiment 3) causes protonation of SIV-T on the schema Na + TiO6 + H2O + TiO5(OH) + Na(OH), with the protonation of SIV-T on the schema Na + TiO6 + H2O ⇆ + TiO5(OH) + Na(OH), with the corresponding transformation of SIV-T into SIV-C (Figure8) by analogy with natural ivanyukites [ 19]. corresponding4+ transformation of 3+SIV-T into SIV-C (Figure 8) by analogy with natural ivanyukites When Ti predominates over Ti (experiments 4 and 5), ETS-4, Na6Ti5Si12O34(O,OH)5·11H2O, [19]. When Ti4+ predominates over Ti3+ (experiments 4 and 5), ETS-4, Na6Ti5Si12O34(O,OH)5·11H2O, crystallizes instead of SIV. crystallizes instead of SIV.

Table 5. Modal composition and surface properties of titanosilicate precipitates. Table 5. Modal composition and surface properties of titanosilicate precipitates. Surface Properties Experiment Modal Composition Surface Properties Experiment Modal Composition 2 3 S, mS, m/g2/g V,V cm, cm/g3/g DDavav,, nmnm 11 SIV-SIV-TT+ + SIV- SIV-CC 148.0 148.0± 0.9 ± 0.9 0.68 0.68 18.4 18.4 ±± 0.2 (3.4–68.0) 22 SIV-SIV-TT+ + SIV- SIV-CC 158 158± 1 ± 1 0.73 0.73 18.6 18.6 ±± 0.1 (2.9–65.4) 33 SIV-SIV-CC 143.3 143.3± 0.6 ± 0.6 0.75 0.75 20.9 20.9 ±± 0.1 (4.3–75.0) ± ± 44 70% 70% ETS-4ETS-4 + + 30%-SIV- 30%-SIV-T T 81.0 81.00.4 ± 0.4 0.18 0.18 8.9 8.9 ± 0.1 (2.1–54.3) 5 65% ETS-4 + 30% SIV-T + 5% TiO2 85.1 ± 0.5 0.22 10.3 ± 0.1 (1.8–58.2) 5 65% ETS-4 + 30% SIV-T + 5% TiO2 85.1 ± 0.5 0.22 10.3 ± 0.1 (1.8–58.2) S—speciﬁc surface area, V—speciﬁc volume of pores, Dav—average diameter of pores, mean ± SD (min–max). S—specific surface area, V—specific volume of pores, Dav—average diameter of pores, mean ± SD (min–max).

FigureFigure 8. 8.SE-image SE-image of of SIV- SIV-CCaggregates aggregates (experiment(experiment 3) 3) ( (aa)) and and powder powder X-Ray X-Ray diffraction diffraction patterns patterns of of productsproducts obtained obtained in in experiments experiments 1–51–5 ((bb).).

Size of SIV particles varies from 2 to 15 μm. High specific surface area (143–158 m2/g) and general pore volume (0.68–0.75 cm3/g) of SIV aggregates as well as presence of extra-framework cations and water molecules in wide channels of SIV crystal structure (see Figure 1) result in impressive SIV cation-exchange properties [20,26].

Minerals 2018, 8, 446 10 of 13

Size of SIV particles varies from 2 to 15 µm. High speciﬁc surface area (143–158 m2/g) and general pore volume (0.68–0.75 cm3/g) of SIV aggregates as well as presence of extra-framework cations and water molecules in wide channels of SIV crystal structure (see Figure1) result in impressive SIV cation-exchange properties [20,26].

5. Discussion Environmental technologies play an increasingly important role in various industrial applications, including production of new materials. Being the main world suppliers of new zircon-, niobo- and titanosilicates, massifs of the Kola Alkaline Province are important sources of information on conditions of mineral formation and synthesis of the corresponding mineral-like functional materials. Again, we should like to emphasize that titanosilicate materials of practical importance, such as ETS-4, IE-911, SL3 and SIV, are complete analogues of zorite, sitinakite, punkaruaivite and ivanyukite-Na discovered in Lovozero and Khibiny massifs. On the other hand, synthesis of the natural minerals analogues allows us to study conditions of natural minerals formation for effective exploration of the minerals—prototypes of functional materials. The Kola Science Center of the Russian Academy of Sciences has developed technologies for recovery of all listed mineral-like materials from the products of chlorine processing of the Lovozero’s loparite, and sulfuric acid processing of the Khibiny’s titanite (its resources is about 170 Mt [33]). The latest technology presented in this paper seems to be safer and more profitable, and enables to produce about 1 t of titanium tanning agent/precursor of sorbents, and then about 1 t of SIV from 1 t of titanite concentrate (1.5–2 t of apatite-nepheline-titanite ore), with simultaneous recycling of sulfuric acid produced by the Monchegorsk Copper-Nickel Combine. The main consumer of SIV is the nuclear power industry (direct processing of liquid radioactive waste into titanate ceramics with reduction in waste volume by 2–3 orders). Precursor of STA/STM is used in the leather industry (due to higher safety of titanium tannic in comparison with chromium ones). Other potential applications of the developed materials are selective or collective extraction of non-ferrous and, especially, precious metals from process solutions and effluents, (photo)catalysis and medicine. In addition, during decomposition of titanite with sulfuric acid, a moist precipitate is produced. It contains calcium, silicon and titanium compounds (wt %): CaSO4 40–45, SiO2 15–20, TiO2 5–10, and sulfuric acid. Neutralization of sediment with lime milk followed by annealing produces a titanium pigment filler, which is widely used in paint-and-varnish industry.

6. Conclusions (1) Large lenses and layers of (apatite)-nepheline-titanite ore, up to 50 m thick, are widespread in apical parts of the most apatite deposits in the Khibiny massif. This ore can be selectively mined and processed as a new kind of titanium raw materials; (2) This ore can be processed by acidic cleaning only, without a conventional flotation stage. With a new approach, the yield of Ti-salts becomes 5–6 times higher than in the known process flow diagram of titanite recovery from apatite-nepheline ore with flotation followed by sulfuric-acidic processing of titanite concentrate; (3) Sulfuric-acidic processing of titanite ore enables to recover up to 90% Ti into the liquid phase that becomes a common precursor for hydrothermal synthesis of functional titanosilicates, in particular, synthetic analogues of the ivanyukite-group minerals. To prevent from ETS-4 crystallization, the rate of titanium cations hydrolysis should be reduced by partial transformation of Ti4+ into Ti3+; (4) Perspectives of SIV application include recovering of liquid radioactive wastes into Synroc-type titanate ceramics, selective or collective extraction of non-ferrous and, especially, precious metals from process solutions and effluents. Minerals 2018, 8, 446 11 of 13

Author Contributions: A.I.N., L.G.G. and M.V.M. designed the experiments, performed ore processing and SIV synthesis, wrote and reviewed the manuscript. E.S.S. and G.O.S. participated in the experiments. G.Y.I. and V.N.Y. took and analyzed ore samples and wrote the manuscript. All authors discussed the manuscript. Funding: The research is supported by the Russian Foundation for Fundamental Researches, Grant 18-29-12039, and the Program 35 of the Presidium of the Russian Academy of Sciences. Acknowledgments: We are grateful to JSC “Apatite” for help with titanite ore sampling. We would like to thank the anonymous reviewers for their suggestions and comments. Conﬂicts of Interest: The authors declare no conﬂict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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