Preparation of Titanium Dioxide from the Ammonium Hexafluorotitanate–Silicon Dioxide System

Abstract

Data on the thermodynamic analysis of the chemical affinity values of the oxides of the elements that constitute titanium-containing concentrates with respect to fluorinating reagents (HF, NH₄F, NH₄HF₂) and on the thermochemical reaction processes of silicon dioxide, various dispersions, and ammonium hexafluorotitanate upon steam heating are provided. The predominant formation of ammonium hexafluorosilicate, which binds the products of pyrohydrolysis of titanium salts upon formation, sublimates at a temperature of 250–300°C, is captured outside the pyrohydrolysis zone, and reduces the temperature of pyrohydrolysis, is confirmed and recommended for application in the hydrofluoride technology used in the production of titanium dioxide. Data on the morphology of titanium dioxide powders are obtained.

INTRODUCTION

Before the collapse of the USSR, its reserves of explored titanium-containing raw materials were ranked first in the world. According to the published data [1, 2], more than eighteen deposits of titanium-containing raw materials were known by 2010 in Khabarovskii Krai and Amur Oblast alone; it was planned to develop two of them—Kuranakhskoe and Belyi Seiim. Employees of the Far East Research Institute of Mineral Raw Materials (Khabarovsk, Russia), the All-Russian Institute of Mineral Raw Materials, the Leading Research Institute of Chemical Technology, and OOO ORROS (Moscow, Russia), and members of the German business community were actively involved in a study of the mineral raw materials of this region in 2001–2005.

It is known that German company Sachtleben Chemie GmbH was projecting a plant for the production of 150 000 tons of titanium dioxide and the extraction of 2.2 million tons of ore per year, and the Olekminskii mine would produce, according to the plan, up to 390 000 tons of ilmenite concentrate with a titanium dioxide content of 48.5–49 wt %. There is no information on its current operation, but data on the supply of ilmenite to pigment plants in China and Japan from this field in quantities of up to 160 000 tons in 2015 have been published [3]. The Kedimiiskii massif in Khabarovskii Krai and the Ariadnenskii deposit in Primorskii Krai are mentioned as promising deposits of titanium-containing ores in the Far Eastern region [1, 2].

In addition to the above-mentioned deposits, the main reserves of titanium raw materials in the Russian Federation are concentrated in the following deposits: Chineiskoe, Kruchininskoe, Tuganskoe, Medvedevskoe, Tsentral’noe, Yaregskoe, and Lovozerskoe; there are large deposits in Stavropolskii Krai, such as Beshpagirskoe and others.

The majority of (more than 95%) of the ilmenite and rutile concentrates extracted in the world is used for the production of pigments based on titanium dioxide. Titanium dioxide is classified as a biocompatible material in accordance with its physical and chemical properties and is used in the food and pharmaceutical industries, but the main part (more than 50%) is used in the production of paints and varnishes, plastics, and paper, and in the aerospace, chemical, petroleum, ferrous and nonferrous metallurgy, and other industries.

After the collapse of the USSR, Ukraine inherited the plants for the production of titanium dioxide in Sumy and Armyansk; the existing mining and processing plants (MOPs) for the production of ilmenite concentrates also remained in Ukraine. Since 1991, the Russian Federation has been an importer of pigments, paints, ilmenite concentrates, and titanium slags used in the production of metallic titanium. Over the past years after the collapse of the USSR, no MOPs have been created for the production of ilmenite concentrates (the enterprise VSMPO-AVISMA currently imports ilmenite concentrates for the production of metallic titanium from Senegal).

It should be noted that the consumption of titanium dioxide per capita in the Russian Federation is an order of magnitude less than in the United States, Japan, and many European countries. Over the past twenty years, China has increased pigment production to 1.7 million tons per year. Plants in China for the production of the pigment use mainly the sulfuric acid method (one plant operates using the chloride method).

After the annexation of Crimea by the Russian Federation in 2014, the pigment production plant in Armyansk ended up in Russia, but its share of the supply of titanium dioxide to the Russian Federation does not exceed 20–30% of the country’s annual consumption, which is about 80 000 tons per year; the Russian Federation imports the rest of the pigment volumes from other countries (U.S.A., China, Germany, Finland, Great Britain, Belgium, etc.). According to experts, the demand for titanium dioxide in the Russian Federation will be at least 250 000 tons per year by 2030 if industrial demand rises.

The industrial production of titanium dioxide of pigment quality from its anatase modification by the sulfuric-acid method began in the 1930s; the production of titanium dioxide from the rutile modification, as the most resistant to exposure to ultraviolet irradiation, began in the 1940s, but this still requires additional systematic research. Worldwide, the amount of pigment production by the sulfuric-acid method exceeds 3.5 million tons per year, but this method is associated with large volumes of waste per ton of pigment, large volumes of water consumption, and large environmental loads on the territories adjacent to the plants.

In the 1950s, a new stage began in the production of titanium pigments. DuPont researchers developed and proposed for industrial implementation a new chloride method for the production of titanium dioxide. To implement it, a titanium slag enriched in titanium dioxide (more than 70%) or a rutile concentrate, reserves of which are insignificant over the world, is used, and an ilmenite concentrate subjected to electroslag remelting with the separation of cast iron is mainly used to obtain the titanium slag. After briquetting, the titanium slag is subjected to high-temperature chlorination, and the resulting titanium tetrachloride purified from contaminants is hydrolyzed or burned to obtain titanium dioxide and a gaseous product in the form of chlorine or hydrogen chloride (depending on the method of combustion), which are used in the technological cycle. The quality of the pigment obtained by this method is higher than by the sulfuric-acid method, but it has a number of drawbacks, namely

— an increased hazard associated with the nature of the initial reactants and reaction products;

— the presence of chlorine or hydrogen chloride (phosgene formation is possible at the chlorination stage);

— increased requirements for the composition of the raw materials used;

— the presence of three high-temperature technological rearrangements.

In the early 2000s, Altair Nanomaterials Inc. proposed a method for leaching an ilmenite concentrate with highly concentrated hydrochloric acid with subsequent extraction of titanium from the solution, but the trivalent iron is preliminarily reduced to a divalent iron by treatment with an iron powder. The entire technological process corresponding to the proposed technology is quite complex and characterized by low recovery rates in an autoclave under a pressure of 0.5 MPa for 3–6 h. Moreover, the extraction of titanium from ilmenite does not exceed 85% and the productivity of titanium dioxide from 1 m³ of autoclave is low [4]. There are no published data on the large-scale industrial implementation of the technology using concentrated hydrochloric acid.

Fluoride Methods for Processing Titanium-Containing Concentrates

With the advent of sulfuric-acid production of titanium dioxide, researchers have been actively trying since the mid-1940s to scientifically substantiate new methods for pigment production. Over the same period, studies of the possible use of fluoroammonium salts as reagents for recovery of titanium-containing raw materials have been undertaken [5, 6]. To recover mineral raw materials, including ilmenite concentrates and other minerals, researchers have tried, in addition to fluoroammonium salts, hydrogen fluoride [7], iron tetrafluoride [8], potassium hexafluorosilicate [9], uranium tetrafluoride [10], etc. With the implementation of atomic research by the leading world powers, i.e., the United States and the USSR, information on fluoride technologies was generally classified (protected) until the beginning of the 2000s.

In recent years, the studies related to the processing of titanium-containing raw materials using ammonium fluoride NH₄F and/or ammonium hydrodiforide NH₄HF₂ as fluoridated reagents are of particular interest due to the possibility of isolation of the use of a recovery reagent in the technological cycle. In the studies devoted to the recovery of titanium-containing raw materials with fluoride reagents, researchers have used both thermal methods based on different evaporation and sublimation temperatures of fluoride compounds and methods associated with their different solubility in fluoride salt solutions to separate the obtained compounds of the main elements of the ilmenite titanium concentrate from iron and some other concomitant impurity elements.

In [11], data on the fluoridation of many minerals (including ilmenite, sphene, and rutile) with ammonium hydrodifluoride are published. In the early 2000s, experiments on the production of titanium dioxide pigment by sublimation (evaporation) in an inert medium of fluoridation products of ilmenite concentrate were conducted in the Institute of Chemistry, Far East Branch, Russian Academy of Sciences. As was noted in [11], the titanium dioxide prepared by this method was contaminated with sublimation products, in particular, iron compounds, due to which the color of the titanium dioxide became a yellowish gray. Such contaminants are unacceptable for pigment-quality titanium dioxide, and the use of the sublimation method in large-scale pigment production does not seem appropriate. Similar studies related to the production of pigments using pyrometallurgical processes of separation of iron and titanium salts after fluorination of titanium-containing raw materials were carried out by researchers of the Tomsk Polytechnic Institute in [12].

The Institute of Chemistry, Far East Branch, Russian Academy of Sciences has since the 1990s been conducting systematic research related to the development of physicochemical foundations and technological regimes for fluoride technologies, including hydrofluoride technology for the complex processing of ilmenite concentrates with the production of pigment titanium dioxide and iron oxides, and to the study of their composition, structure, and functional properties. In [13], the results of studying the phase composition and structure of ammonium fluorotitanates obtained by the interaction of ilmenite concentrate with ammonium hydrodifluoride are presented, and their hydrolysis in aqueous alkaline media and behavior in water vapor and in a dried medium during heating are described.

In [14], the regimes of recovery of titanium-containing raw materials (ilmenite concentrate) with fluoroammonium salts at temperatures of 170–200°C for 2–2.5 h without adding water are described. This is the so-called “dry” recovery, but water is released in the reaction of ilmenite with NH₄HF₂ by the scheme

$$\begin{gathered} 6{\text{N}}{{{\text{H}}}_{4}}{\text{H}}{{{\text{F}}}_{2}} + {\text{FeTi}}{{{\text{O}}}_{3}} + {1 \mathord{\left/ {\vphantom {1 {4{{{\text{O}}}_{2}}}}} \right. \kern-0em} {4{{{\text{O}}}_{2}}}} \\ = {{\left( {{\text{N}}{{{\text{H}}}_{4}}} \right)}_{2}}{\text{Ti}}{{{\text{F}}}_{6}} + {{\left( {{\text{N}}{{{\text{H}}}_{{\text{4}}}}} \right)}_{3}}{\text{Fe}}{{{\text{F}}}_{6}} + {\text{N}}{{{\text{H}}}_{3}} + 3.5{{{\text{H}}}_{{\text{2}}}}{\text{O}}{\text{,}} \\ \end{gathered} $$

and this process can be called dry only conditionally. Subsequent technological operations are associated with the leaching of hot sinter with an aqueous solution of ammonium fluoride, in which titanium salts are not crystallized. Unreacted ilmenite is separated from an aqueous suspension containing iron salts and titanium salts and returned for subsequent recovery, and a solution of titanium salts is separated from the suspension by decantation and filtration after settling. As a result of the change in the S : L ratio upon leaching of the sinter with water, the ratio of the main elements, i.e, iron and titanium, in the solution also changes, as was shown in [15], but it is essential that the fluoroammonium titanium salts subjected to pyrohydrolysis contain no more than a few thousandths of the mass fraction of the impurities of the coloring elements (primarily, iron) in order to obtain high-quality pigments based on titanium dioxide [16]. To obtain pure (for pigment production) titanium salts from hypothetically ideal ilmenite as regards the elemental composition, it is necessary to take into account the effects of iron and titanium, the pH of the solution, the content of fluorine ions, the temperature, and the concentrations of salts that are background with respect to NH₄F (all of these parameters affect the value of the Ti/Fe concentration ratio in the filtrate) on the solubility of the fluororammonium salts.

As was shown in [17, 18], a change in the concentration of NH₄F in a solution from 0 to 25 wt % changes the titanium-to-iron ratio in the leached solution. Based on the data obtained, the researcher can select the optimal concentration of the background saline solution from which it is necessary to isolate titanium salts suitable for the production of pigments or for other purposes, while a change in the pH of the aqueous solution of the titanium and iron salts obtained after recovering the ilmenite ore to 7.5–8 by adding NH₄OH and subsequent elimination of the precipitate allows one to obtain an iron concentration in the solution on the order of a few hundredths of a milligram per 1 mL. Purification of the solution from iron salt occurs owing to partial hydrolysis of the initial solution containing ammonium hexafluorotitanate, which is associated with the formation of ammonium oxofluorotitanate (NH₄)₃TiOF₅, which co-precipitates together with ammonium hexafluoroferrate, since the compounds are isostructural [19]. After the separation of the precipitate, a fluoroammonium salt of titanium suitable for the production of pigments precipitates from a solution with an iron content of up to 0.03 mg/mL when it is evaporated. In addition to obtaining titanium salt of the (NH₄)₂TiF₆–(NH₄)₃TiF₇ type upon evaporation of the solution, titanium oxyfluoride compounds can also be obtained during ammonia hydrolysis.

Hydrofluoride Method of Integrated Processing of Ilmenite Concentrates

As follows from the analysis of the physicochemical processes that accompany the reaction of fluorommonium salts with ilmenite concentrates and the subsequent processes of isolation of titanium salts with sufficient purity necessary for the preparation of high-quality titanium dioxide, the hydrofluoride (hydrometallurgical) method of integrated processing of ilmenite concentrates is the most promising one. This name given to the method corresponds to the main technological rearrangement in this technology, the purification and separation of pure titanium salts from solutions, but there are also pyrometallurgical rearrangements in this technological process. It is possible to use both the dry method and aqueous solutions of fluoroammonium salts in the first technological stage of ilmenite-concentrate recovery, and pyrohydrolysis is the final stage in the preparation of pigments from titanium salts, including those obtained during the ammonia hydrolysis of complex fluoroammonium salts of titanium.

The presence of fluorine ions and ammonium groups in the initial fluorommonium salts of titanium during steam heating leads to the release of ammonia and hydrogen fluoride. The processes of thermal destruction of the complex fluoroammonium salt and pyrohydrolysis products create a chemically aggressive environment in pyrohydrolysis apparatuses, and increased requirements for chemical resistance are imposed on the structural materials used in such equipment [20]. In addition to the chemical resistance of the materials used in pyrohydrolysis apparatuses, special attention should be paid to the fact that the fluorination products of the structural materials used in pyrohydrolysis apparatuses do not impair the functional properties of the obtained pigments; primarily, this applies to titanium dioxide, in particular, to its whiteness.

Devices made of materials (platinum, carbon glass, etc.), the use of which in large items for industrial applications does not seem appropriate both for economic reasons and due to the technological parameters of their production, are often used in the laboratory when studying fluorination processes that involve small volumes and weights of the studied concentrates or individual compounds subjected to fluorination in the amounts of a few tens to hundreds of grams.

The search and physicochemical substantiation of materials that operate reliably in pyrohydrolysis apparatuses during the production of pigments with use of the hydrofluoride technology is an urgent problem, on the solution of which the implementation of the developed technology on industrial scales depends.

The aims of this study were as follows: preparing titanium dioxide by pyrohydrolysis of the (NH₄)₂TiF₆–SiO₂ batch; calculating the values of the chemical affinity of the oxides included in the composition of the ilmenite concentrate with respect to the fluorinating reagents (HF, NH₄F, and NH₄HF₂) and of the silicon dioxide with respect to the hexafluoroammonium salt of titanium under consideration of the possible final reaction products (NH₄)₂SiF₆, TiO₂, 2NH₄F, TiO₂, SiF₄, 2NH₃, TiO₂, and 2HF, in accordance with the reaction equations given below; and obtaining data on the thermal behavior of the mixtures of ammonium hexafluorootitanate and nanodisperse silicon oxide with different morphologies upon their heating to 800°C in a steam-containing atmosphere.

Calculation of the Values of Chemical Affinity of Oxides to Fluorinating Reagents

The free Gibbs energies of the reactions and the values of the chemical affinity of the oxides contained in the ilmenite concentrates to the recovery reagents HF, NH₄F, and NH₄HF₂, and of the titanium dioxide to ammonium hexafluorotitanate. are calculated from the published thermodynamic data [20, 21] for the chemical compounds involved in the reactions, which are given in Tables 1–3. The Gibbs energies of the reactions were calculated taking into account the temperature coefficient M_o, according to the following equation [22]:

$$\Delta G_{{{\text{reaction}}}}^{^\circ } = \Delta H_{{298}}^{^\circ }{\text{ }} - T\Delta S_{{298}}^{^\circ } - \Delta {{c}_{{p298}}}{{M}_{{\text{o}}}},$$

where Т is the reaction temperature in K and М_о is a coefficient depending on the temperature (Table 1).

Table 1.   Dependence of M_o from equation (1) on the temperature [22]

In Tables 1–3, the compounds that are obtained by the reaction with fluorine-containing recovery reagents in the presence of steam or from aqueous solutions are given as products of the reactions of oxides. The reaction equations with the calculated values of the chemical affinity of oxides to the corresponding reactants are arranged in a decreasing sequence (with respect to their absolute values). As follows from the analysis of the values of the chemical affinity of oxides to HF (see Table 2A), titanium dioxide shows the lowest chemical activity.

Table 2.   Gibbs energy of reactions and the chemical affinities of oxides in standard states:

If ammonium fluoride is used as a recovery reagent (see Table 2B), then the arrangement of oxides according to the chemical affinity energies alters: reactions under normal conditions are impossible for the oxides of divalent iron, silicon, cobalt, and nickel.

Data on the chemical affinity of oxides to fluoroammonium salt NH₄HF₂, which is most commonly used for fluorination of titanium-containing concentrates, are given in Table 2B.

It is of interest not only to calculate the values of the chemical affinity of oxides to fluorinating reagents, but also to compare them with each other and with respect to the selected oxide (in our case, titanium dioxide), and to use these data for substantiation of technological operations in the preparation of titanium dioxide from fluoroammonium salts of titanium.

It is known that the selected reagent acts, in a certain temperature range, as a reducing agent (for oxidative processes) in relation to the rest of the chemical compounds in the reagent series of elements, with higher (in absolute value) chemical affinities for the same type of reactions [23]. By analogy with this idea, the oxides of Fe(III), Ca, Mg, Al, Mn, Fe(II), and Si in this case should be considered as reductants that reduce titanium dioxide from its fluoroammonium salt in the reaction with ammonium hexafluorotitanate (see Table 2B), but the complex fluoroammonium compound of silicon is the most interesting reagent for hydrofluoride technology, being formed during the annealing of a charge composed of silicon oxides and ammonium hexafluorotitanate taken in a stoichiometric ratio according to equations (1)–(3) in Table 3. It was assumed when calculating the free Gibbs energy and chemical affinity values of silicon dioxide to a specific titanium salt that various reaction products are possible, but only one reaction is thermodynamically viable according to the thermodynamic calculations (see Table 3, reaction 1). Silicon hexafluorosilicate and titanium dioxide are formed when such a charge is heated in water vapor. The resulting silicon hexafluorosilicate compound is resistant to water vapor (the Gibbs energy of its reaction with water at 25°C is 353.1 kJ/mol), easily sublimated at temperatures of 250–300°C, and removed from the reaction zone; it condenses in adsorbers at temperatures below 250°C. During the ammonia hydrolysis of ammonium hexafluorosilicate salts from the adsorber and subsequent filtration, silicon dioxide is separated from the ammonium fluoride solution; both hydrolysis products are used in the technological cycle or for other purposes.

Table 3.   Chemical affinity of ammonium hexafluorotitanate to SiO₂

In [17], it was proposed to use the process of binding chemically active products of pyrohydrolysis of ammonium hexafluorotitanate with silicon dioxide for the selection of refractory lining materials of pyrohydrolysis devices used in the production of titanium dioxide by hydrofluoride technology.

EXPERIMENTAL

To study the thermal behavior of the silicon dioxide–ammonium hexafluorotitanate charge, the following reagents were used: ammonium hexafluorotitanate (NH₄)₂TiF₆ of reagent grade, ammonium hydrogen fluoride NH₄HF₂ of reagent grade, an aqueous 25% solution of ammonia NH₄OH of analytical purity grade, and ammonium fluoride NH₄F of analytical purity grade. Samples of silicon dioxide nanotextured with a specific surface area of S_sp = 57 m²/g were synthesized from ammonium hexafluorosilicate by ammonium hydrolysis at a concentration ratio of Si : OH = 1 : 8 between the salt and hydroxide groups with a slow addition of the salt solution to the alkali solution (sample A); sample B, with a specific surface area of 120 m²/g, was obtained when adding an aqueous ammonia solution to a solution of ammonium hexafluorosilicate salt at a ratio of Si : OH = 1 : 8. The ammonium hexafluorosilicate from which the samples of silica (A and B) were prepared was synthesized using silicon(IV) oxide (aqueous oxide of pure grade, GOST 4214–78) and ammonium hydrogen fluoride.

The X-ray diffractograms of the starting materials and pyrohydrolysis products were recorded in a D8 Advance diffractometer (Germany) with rotation of the samples in CuKα radiation. A phase analysis of the XRD patterns was performed using the EVA searching tool and the Powder Diffraction FileTM database (Soorya N Kabekkodu, 2007). The specific surface area of the samples was determined by low-temperature adsorption of nitrogen with use of a SORBTOMETER-M device (Russia). Thermogravimetric analyses of ammonium hexafluorosilicate, ammonium hexafluorotitanate, and the charge were carried out using a Q-1000 derivatograph of the Paulik–Paulik system (MOM, Hungary) in an open platinum crucible with heating of the weighed samples at a rate of 5°C/min. Calcined Al₂O₃ was used as a standard. Quantitative determination of the elemental composition of the samples was performed using the energy-dispersive X-ray fluorescence method in a Shimadsu EDX-800HS spectrometer. The morphology of the original samples and pyrohydrolysis products was analyzed using a Hitachi S 5500 scanning electron microscope (SEM) with a Thermo Scientific energy dispersive spectrometer (EDS).

RESULTS AND DISCUSSION

Figure 1 shows the SEM images of nanotextured X-ray amorphous silicon dioxide powders which were obtained on a Hitachi S 5500 scanning electron microscope equipped with a Thermo Scientific energy dispersive spectrometer. The mean size of the SiO₂ particles was about 200 nm in sample A and a few nanometers in sample B.

Fig. 1.

Scanning electron microscope images of silicon-dioxide powders.

Given that the most likely product in the process of pyrohydrolysis of the silicon-dioxide–ammonium-hexafluortitate charge should be ammonium hexafluorosilicate, a thermogram was obtained to clarify the temperature range of sublimation of ammonium hexafluorosilicate with water vapor (Fig. 2), from which it follows that the sublimation begins at a temperature of 225°C and terminates at 350°C.

Fig. 2.

Thermogram of nanodisperse ammonium hexafluorosilicate.

A thermogram reflecting the processes of pyrohydrolysis of the ammonium hexafluorootitanate component of the charge is shown in Fig. 3.

Fig. 3.

Thermogram of pyrohydrolysis of (NH₄)₂TiF₆.

The intensive pyrohydrolysis process begins at a temperature of 252°C and is accompanied by the removal of hydrogen fluoride and ammonia to form (NH₄)₃TiOF₃ with a mass loss within Δm = 29.85% by the following equation:

$$\begin{gathered} {{\left( {{\text{N}}{{{\text{H}}}_{4}}} \right)}_{2}}{\text{Ti}}{{{\text{F}}}_{6}} + {{{\text{H}}}_{{\text{2}}}}{\text{O}} \\ \xrightarrow{{250 - 370^\circ {\text{C}}}}{\text{N}}{{{\text{H}}}_{4}}{\text{TiO}}{{{\text{F}}}_{3}} + 3{\text{HF}} + {\text{N}}{{{\text{H}}}_{3}}. \\ \end{gathered} $$

(1)

With an increase in the temperature from 370 to 440°C, pyrohydrolysis of NH₄TiOF₃ with the formation of TiO₂ and titanium oxyfluoride occurs with a mass loss of up to 18.7%, as follows:

$$\begin{gathered} {\text{2N}}{{{\text{H}}}_{{\text{4}}}}{\text{TiO}}{{{\text{F}}}_{3}} + {{{\text{H}}}_{{\text{2}}}}{\text{O}} \\ \xrightarrow{{370 - 440^\circ {\text{C}}}}{\text{ Ti}}{{{\text{O}}}_{{\text{2}}}} + {\text{TiO}}{{{\text{F}}}_{2}} + 2{\text{N}}{{{\text{H}}}_{3}} + 4{\text{HF}}. \\ \end{gathered} $$

(2)

An increase in the temperature from 440 to 550°C is accompanied by a decrease in the mass of the sample by 11.1%, which is associated with the removal of fluorine from the compound by the following reaction:

$${\text{TiO}}{{{\text{F}}}_{2}} + {{{\text{H}}}_{{\text{2}}}}{\text{O}}\xrightarrow{{440 - 550^\circ {\text{C}}}}{\text{ Ti}}{{{\text{O}}}_{2}} + 2{\text{HF}}.$$

(3)

Pyrohydrolysis in the temperature range of 550–750°C is associated with the removal of residual fluorine and mass loss in the range of 2.5%.

The final product of pyrohydrolysis of titanium fluoroammonium salt at temperatures of up to 800°C is titanium dioxide of anatase modification with tetragonal syngony of crystals (Fig. 4).

Fig. 4.

X-ray diffractogram of the titanium-dioxide sample of anatase modification obtained by pyrohydrolysis of ammonium hexafluorotitanate at temperatures up to 800°C.

Figure 5 shows a thermogram of the process of pyrohydrolysis of the silicon-dioxide–ammonium-hexafluorotinate charge with the components taken in a stoichiometric ratio according to equation (1) of Table 1.

Fig. 5.

Thermogram of pyrohydrolysis of the (NH₄)₂TiF₆–SiO₂ charge.

The reaction between ammonium hexafluorotitanate and silicon dioxide begins, according to the thermogram, at temperatures slightly above room temperature (30°C) with the removal of water molecules adsorbed on the silicon dioxide, while the reaction of the formation of ammonium hexafluorosilicate is possible even at room temperature in accordance with the thermodynamic calculation results (see Table 3). With a further increase in the temperature to 230°C, pyrohydrolysis of ammonium hexafluorotitanate and the reaction of silicon dioxide with the released ammonia and hydrogen fluoride, which is accompanied by the formation of ammonium hexafluoroosilicate that intensively sublimates at a temperature of 220°C, are observed. The loss of mass in this case is Δm = 67.8%. In the pyrohydrolysis process (actually, a complex exchange reaction takes place in such a charge), silicon dioxide acts as a reagent that reduces titanium dioxide from its complex compound by the following equation:

$$\begin{gathered} {{\left( {{\text{N}}{{{\text{H}}}_{4}}} \right)}_{2}}{\text{Ti}}{{{\text{F}}}_{6}} + {\text{Si}}{{{\text{O}}}_{2}}{\kern 1pt} \cdot {\kern 1pt} n{{{\text{H}}}_{{\text{2}}}}{\text{O}} \\ \to {\text{Ti}}{{{\text{O}}}_{2}} + {{\left( {{\text{N}}{{{\text{H}}}_{4}}} \right)}_{2}}{\text{Si}}{{{\text{F}}}_{6}}. \\ \end{gathered} $$

(4)

The exchange processes with the production of titanium dioxide are completed at temperatures of 400–460°C in accordance with the thermogram (see Fig. 5), which is 100–150°C lower compared to the processes of titanium dioxide formation during pyrohydrolysis of pure salt (NH₄)₂TiF₆. It stands to reason that this will lead to a change in both the morphology of the resulting titanium-dioxide product and its properties. In addition, the optical properties and stability of the titanium dioxide under exposure to radiation will depend on the residual content of silicon dioxide, the concentration of which is easy to control. Such studies are planned for the future.

It is follows from the X-ray diffractograms of the titanium-dioxide samples obtained by pyrohydrolysis of ammonium hexafluorotitanate (Fig. 6, sample C) and by pyrohydrolysis of its mixture (charge) with silicon dioxide at maximum pyroidrolysis temperature of 800°C (Fig. 6, samples A and B) that the obtained finely dispersed samples belong to the anatase modification of titanium dioxide with tetragonal syngony of crystals and the following lattice-cell parameters: a = 3.78520 Å, b = 3.78520 Å, and c = 9.51390 Å; however, the shapes of the X-ray reflection peaks in the range of angles of 25.2°–25.7° (see Fig. 6) differ in the peak width at half height values, which seems to be associated with both the size of the crystals and their defectiveness. Establishing the relationship between the dispersity of silicon dioxide and the morphology and properties of the resulting titanium-dioxide powders is a subject for further investigations.

Fig. 6.

Shapes of X-ray diffraction reflection peaks of titanium dioxide in (a) sample A, (b) sample B, and (c) sample С.

According to the electron microscopy data, a considerable number of titanium dioxide crystals obtained by pyrohydrolysis of a titanium salt without silicon dioxide (sample C) have a size on the order of one micrometer, and the crystals of samples A and B have a size on the order of a few tenths of a micrometer (Fig. 7). In the composition of titanium dioxide samples A and B, silicon dioxide is present in the range of 1–1.5% (the amount can be preset), and such a modification affects the functional properties of the titanium dioxide, which requires additional systematic studies.

Fig. 7.

Scanning electron microscope images of titanium-dioxide samples.

CONCLUSIONS

The theoretical calculation and comparative thermodynamic analysis of the chemical affinities of the oxides of the elements contained in titanium-containing concentrates with respect to fluorinating reagents such as HF, NH₄F, and NH₄HF₂, has been performed.

Experimental data on the thermochemical process of interaction of silicon dioxide and ammonium hexafluorotinate during heating in steam have been obtained, which confirm the thermodynamic calculation results predicting the predominant formation of ammonium hexafluorosilicate during the pyrohydrolysis of the charge. In the system under study, silicon dioxide has a high value of chemical affinity to the pyrohydrolysis products, i.e., hydrogen fluoride and ammonia. The reaction proceeds with the formation of salt, which is easily sublimated at temperatures of 230–300°C and removed from the pyrohydrolysis zone. This fact is recommended to be used when choosing lining materials of pyrohydrolysis apparatuses for the hydrofluoride technology of production of titanium dioxide.

Data on the morphology of the titanium dioxide formed during pyrohydrolysis of titanium salts and silicon oxide of different degrees of dispersity indicate that titanium dioxide powders obtained under the same conditions have different dispersity, and the effect of this factor on the functional properties, especially on the optical and radiation resistance of the titanium dioxide obtained in such a system, requires further studies.

It has been established that the process of pyrohydrolysis of the studied charge starts at a temperature that is 100–150°C below the temperature of pyrohydrolysis of pure ammonium hexafluorotitanate salt, which should affect the temperature regimes of the formation of the anatase and rutile modifications of titanium dioxide and its properties.