Thermodynamic Justification for the Effectiveness of the Oxidation—Soda Conversion of Ilmenite Concentrates

Abstract

This article presents the results of a thermodynamic analysis of the oxidation soda conversion reactions of minerals in ilmenite concentrates in the temperature range of 373–2273 K. The thermodynamic parameters of pseudorutile, pseudobrukite, and the new minerals, zhikinite and spessartine, were calculated for the first time. It has been established that the most important criterion relating to the stability of titanium minerals and related elements, as well as the reaction properties of the structural oxides of metals and silicon, is their degree of oxidation. Oxides of silicon (IV) and manganese have the best reactivity in solid-phase oxidizing alkaline environments (VI). Modeling this process scientifically substantiates the mechanism involved in the destruction of minerals in ilmenite concentrates in the low-temperature region in the presence of atmospheric oxygen and sodium oxide of soda ash, which are decomposed through the absorption of heat and the evaporation of moisture during the dehydration of hydrated minerals of iron and manganese and the dehydration of the soda–ilmenite batch. Tests conducted during pilot metallurgical production at the Institute of Metallurgy and Enrichment (PMP of JSC) confirmed the feasibility of processing high-chromium and siliceous rutile leucoxene ilmenite concentrates, which are unsuitable for traditional pyro- and hydro-metallurgical enrichment methods, through single-stage oxidation soda roasting, followed by the leaching of easily soluble sodium salts of iron and associated impurities with water and a dilute hydrochloric acid solution. The proposed energy-saving method ensures the production of high-purity (>98%) synthetic rutile while eliminating the formation of strong deposits on the lining of roasting units.

1. Introduction

Mineral deposits of titanium are widespread globally; however, reserves of economically valuable rutile sand ores and high-quality ilmenite sands are limited [1]. According to current estimates, the total world reserves of titanium in ilmenite and rutile combined in 2023 were approximately 745 million metric tons. Moreover, ilmenite accounts for 94% of the world’s titanium reserves, with the remainder in rutile [2]. The bulk (up to 95%) of the annually mined ilmenite raw material is used to produce titanium dioxide. The compound annual growth rate of the titanium dioxide market supplied to manufacturers of white titanium pigments is projected to be 8.9% from 2023 to 2032 [3].

The complexity of the material composition of hard-to-enrich ilmenite concentrates limits the effectiveness of commercial methods used to produce synthetic rutile and electrothermal titanium slags. Numerous studies focused on the processing of ilmenite concentrates are mainly devoted to carbothermal smelting with and without flux additives, particularly soda [4].

Titanium slags obtained as a result of carbothermal smelting are heavily contaminated with impurities, and refinement methods involving sulfuric acid result in the formation of a large volume of sulfate iron, leading to the excessively high consumption of acid during chlorination [5]. The number of raw materials suitable for the hydrochloric acid leaching of titanium slags is limited; therefore, they have not found industrial applications [6]. The Benelite process, designed for the refinement of ilmenite sands containing a high content of titanium dioxide and a low iron oxide content, becomes technically unsuitable when the concentration of chromium and magnesium exceeds the permissible concentration, suppressing the slow rate of aeration leaching [6]. The Altair process is low-yielding and unprofitable due to the multi-stage leaching, conversion, and separation of metals. In addition to the technique of processing ilmenite concentrates using carbo-thermal smelting, other known methods include oxidation–reduction roasting [7].

In particular, the sulfate method used for the production of titanium dioxide through rutile and anatase modification is not suitable for processing leucoxene ilmenite concentrates containing rutile that is insoluble in acids. The enrichment process used for ilmenite sand concentrates with a high content of titanium dioxide and a low content of iron oxide is technically unacceptable when the permissible concentrations of magnesium and chromium are exceeded. This is due to the suppression of the slow aeration leaching of metallic iron when using ammonium chloride solution [8]. The ERMS SR technology used for the production of high-purity synthetic rutile (>97%), developed by the Australian company Austpac Resources, includes a two-stage redox roasting of the heavy mineral concentrate with intermediate magnetic separation [9]. The technology for producing UGS (upgraded titania slag) matches the quality (94–95% TiO₂) of synthetic rutile by using the energy-consuming double thermal oxidation–reduction process with intermediate magnetic separation, as developed by the Canadian company QIT-Fer & Titane Inc. (Havre-Saint-Pierre, QC, Canada) [10,11]. These are not applicable for processing rutilized ilmenite concentrates due to the inability to separate rutile from rock-forming vein minerals using magnetic separation.

An attractive method for synthetic rutile production is a one-stage oxidizing soda process involving the conversion of polymineral ilmenite concentrates and the enrichment of titanium slags. Morsi et al. [12] determined the possibility of completely extracting titanium dioxide by sintering ilmenite ore with a molar ratio of sodium oxide to titanium dioxide of 2.5, followed by roasting at 1000 °C, 1100 °C, or 1200 °C for 120, 60, or 30 min, respectively. They reported that higher sodium titanates of Na₈Ti₅O₁₄ and Na₆Ti₂O₇ formed during the sintering process turn into lower-sodium titanates, which can be easily soluble in dilute sulfuric acid and easily separated from iron. Lasheen [13] determined the possibility of modifying titanium slag smelted from Rosetta ilmenite concentrate by roasting with soda ash at an optimal ratio of Na₂CO₃ and slag equal to 0.55:1 at a temperature of 850 °C for 0.5 h. Nafeaa et al. [14] found that the degree of conversion of granular and powdered Rosetta ilmenite concentrate, determined by analyzing the composition of the resulting sodium titanates, increases with increasing roasting temperature in air and the percentage of soda ash in the concentrate. Abhishek [15] studied the oxidative roasting of Bomar ilmenite with soda ash in the temperature range of 873–1173 K, concluding that the process proceeds in two stages—with the formation of sodium titanate and ferruginous sodium titanate in the first stage, followed by the decomposition of ferruginous sodium titanate with the formation of sodium ferrite and titanium dioxide in the second stage—assuming that divalent iron ions are replaced by sodium soda ions as a result of diffusion from the ilmenite crystal lattice.

The relevance of such research lies in the limited reserves of economically valuable rutile sand ores and high-quality ilmenite sands, increasing consumption, and the need to increase the production capacity of synthetic rutile, necessitating the processing of ilmenite concentrates with complex material compositions.

The results of our fundamental research clarify the real decomposition mechanisms of valuable titanium ore minerals and related elements under the action of oxygen and soda ash, confirming the feasibility of processing high-chromium and -silica rutilized leucoxene ilmenite concentrates—which are unsuitable for processing when using traditional pyro- and hydro-metallurgical methods of enrichment—using oxidative soda conversion. These results are presented below [16].

2. Materials and Methods

2.1. Thermodynamic Analysis

The numerical values relating to the thermal effects of the separate and total reactions of mineral decomposition for a representative batch of ilmenite concentrate from an Obukhov deposit were determined. This calculation considered the heat capacity ∆C°_p of reacting substances and the formed products and involved computing the enthalpy ∆H°_T, entropy ∆S°_T, and Gibbs energy ∆G°_T in each temperature interval from 373 to 2273 K, using reference standard thermodynamic values at 298 K [17]. Due to the absence in the literature of standard thermodynamic values of ∆H°₂₉₈, ∆S°₂₉₈, and ∆C°₂₉₈ for minerals, such as spessartine Mn₃Al₂[SiO₄]₃, pseudorutile Fe₂Ti₃O₉, and pseudobrukite FeTi₂O₅, these values were determined using the incremental Kumok method, the additive Neumann–Kopp rule, and the Mostafa group contribution method [18]. Thermodynamic calculations of chemical reactions were additionally verified using the HSC Chemistry 8.0 (2014) program.

2.2. Physicochemical and Experimental Studies

The phase composition of the Obukhovskoye ilmenite concentrate provided by the Thioline Mining and Processing Company was analyzed through classification on a vibratory shaker with a set of sieve webs with different mesh sizes. The mineral composition of the concentrate was identified using a D8 Advance (BRUKER) X-ray diffractometer with Cu-Ka emission. Thermal analysis was performed using an STA 449 F3 Jupiter synchronous thermal analyzer (Munchen, Germany) with NETZSCH Proteus software v.7.1, applying a linear heating rate of 15 °C/min in a highly purified argon atmosphere with a linear cooling rate of 17 °C/min to 500 °C [19]. Local areas of polished samples, sputtered with a thin layer of graphite for contrast surface display, were analyzed using a JEOLJXA-8230 scanning electron-probe microanalyzer from JEOL (Japan, Tokyo). The chemical composition of the products was analyzed using an Axios X-ray fluorescence wave-dispersive spectrometer, an Optima 2000 DV “Perkin Elmer Inc.” atomic emission spectrometer with inductively coupled plasma (Shelton, CT, USA) and an Agilent 7500 A ICP-MS (Santa Clara, CA, USA) mass spectrometer on the basis of inductively coupled plasma. A “pure analytical reagent” of sodium carbonate was used as a sintering alkaline reagent. The soda–ilmenite charge with the addition of 2% aqueous molasses solution as a binder was briquetted via compaction in a cylindrical steel mold with a diameter of 16 mm on a PSU-10 pressing machine with a pressure force of 50 mPa. The oxidative firing of briquettes of soda–ilmenite charge was carried out in alunda boats placed in a quartz tube with a through hole. Continuous ventilation of the reaction zone was supplied using atmospheric oxygen. A Nabertherm horizontal tubular electric furnace with a B-180 controller for 60–120 min was used when the temperature reached 1100–1200 °C. The soda concentrate charge cake was refined with deionized water, then with 20% hydrochloric acid solution at a solid-to-liquid ratio of 3, a temperature of 90 °C, and a pulp stirring speed of 150 rpm for 1 h in thermostatized beakers heated using a circulating hot water thermostat LT-108 “Loip” (Saint Petersburg, Russia). The alkaline pulp was filtered under a vacuum using a water jet pump through a Buechner funnel with a “blue ribbon” filter, while the acidic pulp was filtered via centrifugation. Washed and dehydrated acid filtrate cakes were calcined at a temperature of 900 °C.

The optimization of the technological conditions established that the oxidative soda conversion of briquetted ilmenite concentrate charge, with an equivalent mass ratio with alkaline reagent at a temperature of 1200 °C for 2 h and subsequent refining of concentrate cinder with water, followed by 20% hydrochloric acid solution at a solid-to-liquid ratio of 1:3 and a temperature of 90 °C for 1 h, provides high-purity synthetic rutile washed with deionized water and calcined at a temperature of 900 °C.

The oxidizing soda conversion of the briquetted concentrate charge to produce porous specks eliminates the formation of strong deposits on the lining of the roasting furnace (Figure 1 and Figure 2).

3. Results and Discussion

3.1. Characteristics of Ore Raw Materials and Ilmenite Concentrate

The Obukhovskoye deposit of titanium–zircon sands is located in the Taiyshinskiy district of the North Kazakhstan region. Formed under coastal–marine and partly lagoonal conditions, the deposit belongs to gravel–sandy–clay formations of the Chegan Formation and amounts to 6,150,000 cubic meters. Proven reserves of the most seasoned fine-grained ore sands have the following mineral contents (kg/m³): ilmenite—82.09; rutile—21.36; leucoxene—5.74; and zircon—80.41. Approximately 80–85% of the useful components of the granular fraction of sands are concentrated in the size class of −0.2 + 0.04 mm. The clay fraction of the sands is represented by particles smaller than 0.02 mm. Ore and rock-forming minerals, in order of decreasing grain size, are arranged in the following sequence: quartz, tourmaline, sillimanite, staurolite, distene, leucoxene, rutile, ilmenite, chromospinelides, and zircon. The high content of basic minerals, the complete absence of mineral aggregates, and the fine-grained nature of ore sands provide the possibility of obtaining ilmenite, rutile, and zircon concentrates. X-ray phase diagnostics of fractional separation products of ilmenite concentrate with grain sizes ranging from +0.071 to −0 mm established the presence of individual and complex minerals of titanium, silicon, zirconium, chromium, and aluminum (Figure 3,

Table 1. Mineral composition of ilmenite concentrate.

Yield, %	Components	Formula	Content, in wt %
Sample size from −0.071 to +0.063 мм (a)
11.47	Quarts, syn	SiO2	40.8
	Rutil, syn	TiO2	16.9
	Pseudorutil	Fe2(TiO3)3	16.9
	Pseudobrukite	Fe2(TiO5)	11.3
	Spessartine	Mn3AI2[SiO4]3	14.2
Sample size from −0.063 to +0.044 мм (b)
88.53	Rutil, syn	TiO2	27.1
	Pseudorutil	Fe2(TiO3)3	23.5
	Zhiqinite	TiSi2	15.9
	Aluminian Chomit	Fe(Cr,Al)2O4	16.6
	Aluminian Manganese	MnAI6	17.0
Sample size from +0.071 to −0 мм (c)
100	Ilmenit	FeTiO3	20.0
	Rutil	TiO2	20.8
	Pseudorutil	Fe2(TiO3)3	25.9
	Zircon, metamict	ZrSiO4	23.0
	Quarts, syn	SiO2	10.3

).

Thermal analysis showed the presence of hydrated minerals of iron–lepidocrocite and manganese–manganite (Figure 4).

The temperature range for the dehydration and phase transformation of lepidocrocite and manganite was determined via the identification of extremes of endo- and exothermic effects on the thermogram curves of dDTA and DTG.

$$\begin{array}{l}\mathrm{Lepidocrocite}\mathsf{\gamma }·\mathrm{FeOOH}\stackrel{414.3 ÷ 415 °\mathrm{C}}{\to }\mathrm{goethite}\mathsf{\alpha }·\mathrm{FeOOH}\stackrel{457.8 ÷ 479.2 °\mathrm{C}}{\to }\mathrm{maghemite}\mathsf{\gamma }·{\mathrm{Fe}}_2{\mathrm{O}}_3\stackrel{536 ÷ 537.5 °\mathrm{C}}{\to }\\ \mathrm{hematite}\mathsf{\alpha }·{\mathrm{Fe}}_2{\mathrm{O}}_3\end{array}$$

(1)

$$\mathrm{Manganite}\mathrm{MnOOH}\stackrel{479.2 °\mathrm{C}}{\to }\mathrm{bixbyite}\beta \text{-}{\mathrm{Mn}}_2{\mathrm{O}}_3\stackrel{949 ÷ 965.5 °\mathrm{C}}{\to }\mathrm{hausmanite}\beta ·{\mathrm{Mn}}_3{\mathrm{O}}_4\stackrel{1169.8 °\mathrm{C}}{\to }\mathsf{\gamma }·{\mathrm{Mn}}_3{\mathrm{O}}_4$$

(2)

Enantiotropic polymorphic transformation of quartz

$$\alpha \text{-}{\mathrm{SiO}}_2\stackrel{562.2 ÷ 565 °C}{\leftrightarrow }\beta \text{-}{\mathrm{SiO}}_2$$

(3)

Decomposition of ilmenite to form amorphous iron trioxide and titanium dioxide under the action of residual oxygen

$${\mathrm{FeTiO}}_3\stackrel{310.2÷898.9°\mathrm{C}}{\to }{\mathrm{Fe}}_2{\mathrm{O}}_3+{\mathrm{TiO}}_2$$

(4)

Crystallization of rutile and pseudobrukite.

(5)

The temperature range and character of phase transformations of ilmenite concentrate minerals under an argon atmosphere are presented in

Table 2. Temperature range and character of phase transformations of ilmenite concentrate minerals under an argon atmosphere.

Mineral	Formula	Temperature, °C			Processes
		Samples
		a	b	c
Goethite	FeO(OH)	479.2 *	459.2 *	457.8 *	Dehydration
Lepidocrocite	γ-FeO(OH)	414.3 *	414.3 *	415 *	Dehydration and formation of γ-Fe2O3
		537.5 **	537.4 **	536 **	Transition to α-Fe2O3
Manganite	γ-MnO(OH)	479.2 *	−	−	Dehydration and formation of β-Mn2O3
		965.5–949 *			Decomposition with oxygen scavenging and formation of β-Mn3O4
		1169.8 *			Enantiotropic polymorphic transformation to γ-Mn3O4
Ilmenite		887.9 **	853.9 **	310.2 **–898.9 **	Oxidation of Fe+2 and formation of amorphous Fe2O3 and TiO2
Rutile	TiO2	1005.3 **	1010.1 **	1005.6 **	Crystallization
Pseudobrookite
Quartz	SiO2	562.2 *	562.2 *	563.4 *	Enantiotropic polymorphic transformation
Clay mineral		517.4 *	508.1 *	504.8 *	Oxidation

Temperature range: *—endothermic effect; **—exothermic effect.

.

When heating up to a temperature of 1205.4 ÷ 1205.6 °C under an inert gas atmosphere, it was found that mass loss of Obukhov ilmenite concentrate samples does not depend on their grain size (

Table 3. Weight loss of Obukhovskoye ilmenite concentrate samples in argon atmosphere.

Samples	Coarseness, mm	Initial Mass, g	Residual Mass, %	Weight Loss, %
a	−0.071 +0.063	0.301	98.62	1.38
b	−0.063 +0.044	0.306	98.77	1.23
c	+0.071 −0	0.304	98.65	1.35

).

The manifested thermal endothermic effects indicated extremes at temperatures of 479.2 and 517.4 °C, 451.7 and 508.1 °C, and 461.6 and 504.8 °C in fractional samples of concentrate with sizes of −0.071 +0.063 (a), −0.063 + 0.044 (b), and −0.044 + 0 (c), respectively, demonstrating that the decomposition of the minerals present in the concentrate and the formation of new varieties of minerals are accompanied by mass losses of suspensions of the analyzed products.

The chemical composition of the concentrate, in accordance with the mineral composition, is characterized by an abnormally high content of chromium trioxide (

Table 4. Chemical compositions of different types of ilmenite concentrates.

Components		Content, in wt %
	Kazakhstan	Ukraine	Australia [9]	South Africa [20]	Egypt [21]	India [15]
	Obukhovskyi	Volnogorskyi	Capel	Hillendale	Abu Ghalaga	Bomar
TiO2	51.16	66.8	55.43	48.40	36.78	69.25
FeO	3.15	20.5	22.51	36.15	25.85	15.2
Fe2O3	28.78	n/a	18.16	12.29	29.86	6.1
SiO2	1.8	0.82	1.41	0.44	4.46	3.8
Al2O3	2.0	1.8	0.25	0.27	0.72	2.4
Cr2O3	7.49	1.5	0.03	0.09	0.21	0.2
CaO	0.1	0.57	n/a	0.02	0.15	0.10
MgO	0.71	0.89	n/a	0.50	0.81	0.9
MnO	1.61	0.85	1.44	1.06	0.36	0.37
V2O5	0.18	0.38	0.13	0.25	0.38	-
P2O5	0.18	0.25	0.15	0.01	0.03	0.3
ZrO2	0.41	0.12	0.08	0.08	LOI 0.17	LOI 0.8
	Other 2.43	Other 5.52	Other 0.41	Other 0.44

).

Electron probe microscopy of concentrate cinders revealed that, during air firing, the crystal lattice of iron-oxide mineral chromite—which has a dense structure and approximately the same hardness (5.5 on the Mohs scale) as ilmenite—is deformed, and as a result of oxidation, results in the loosening of divalent iron and the formation of reactive iron trioxide Fe₂O₃ (Figure 5 and Figure 6).

3.2. Mechanism of Oxidizing-Soda Conversion of Ilmenite Concentrate

3.2.1. Thermodynamics of Ilmenite Decomposition Reactions

In chemical processes, two opposing factors—entropic (TΔS) and enthalpic (ΔH)—are known to act simultaneously. The total effect of these opposing factors in processes proceeding at constant pressure and temperature determines the change in Gibbs energy (ΔG). The nature of the change in the latter allows us to judge the fundamental possibility of the process.

Thermodynamic analysis of reactions of oxidative soda conversion of ilmenite concentrate minerals revealed that the process of ilmenite decomposition by soda ash can proceed at temperatures above 773 K with the formation of ferruginous sodium titanates NaFeTiO₄ and NaFeTi₃O₈, iron monoxide, sodium trititanate sodium ferrite Na₂Ti₃O₇, and the release of carbon dioxide due to the entropic factor of irreversible endothermic Reactions (6)–(8) and (10) with the following numerical values: ΔH > 0, ΔS > 0, ΔG < 0, and T > ΔH/ΔS (Figure 7). The formation of ferruginous sodium titanates of the indicated composition during the intermediate stages of ilmenite roasting was found by Abhishek et al. [15].

FeTiO₃ + 0.5Na₂CO₃ + 0.25O₂ → NaFeTiO₄ + 0.5CO₂ ↑

(6)

3FeTiO₃ + 0.5Na₂CO₃ + 0.25O₂ → NaFeTi₃O₈ + 2FeO + 0.5CO₂ ↑

(7)

6FeTiO₃ + 3Na₂CO₃ + 1.5O₂ → 4NaFeO₂ + 2NaFeTi₃O₈ + 3CO₂ ↑

(8)

3FeTiO₃ + 1.5Na₂CO₃ + 0.75O₂ → 3NaFeO₂ + 3TiO₂ + 1.5CO₂ ↑

(9)

3TiO₂ + Na₂CO₃ → Na₂Ti₃O₇ + CO₂ ↑

(10)

The endothermic Reaction (9) of ilmenite decomposition with the formation of sodium ferrite and the release of titanium dioxide from the crystal lattice of the mineral under the numerical values ΔH > 0, ΔS > 0, and ΔG < 0 reveals that the initial stage of the sodium trititanate formation process can proceed spontaneously at any temperature, with subsequent heat release. The decomposition of ilmenite, which begins in the low-temperature region, is attributed to the adsorption of oxygen on the mineral surface, leading to the oxidation and transition of divalent iron to the reactive trivalent oxide state, similar to natural conditions. As a result of the interaction of amorphous salt-forming iron trioxide with strongly basic sodium oxide of soda ash, sodium ferrite is formed. The titanium dioxide released in this process, reacting with sodium oxide, forms sodium trititanate.

It should be explained that sodium oxide is a binary thermally stable, refractory (T_sinter 1405 K under high pressure) substance with rigid donor–acceptor bonding of the alkali metal ion with oxygen. Calcined soda is a hygroscopic substance that decomposes upon heating through the absorption of evaporating moisture into sodium oxide and carbon dioxide. The release of carbon dioxide indicates the decomposition of soda ash and the interaction of the mineral and intermediate products of its decomposition with sodium oxide alkaline reagent.

The ilmenite mineral is primarily a metatitanate of iron. The formation of ferric sodium titanates in the low-temperature region is the initial incomplete stage of the gradual decomposition of ilmenite. The possible decomposition of ilmenite using sodium oxide in soda ash to form ferruginous sodium titanate NaFeTiO₄ at temperatures ranging from 373 to 773 K, as well as sodium ferrite and ferruginous sodium trititanate NaFeTi₃O₈ at temperatures ranging from 373 to 573 K, can proceed due to the enthalpy factor of exothermic Reactions (11) and (12). These reactions are characterized by the following numerical values: ΔH < 0, ΔS < 0, ΔG < 0, and T < ΔH/ΔS. At temperatures above 773 K and 573 K, the decomposition process of ilmenite can proceed spontaneously due to the dominant (ΔS > 0) entropic factor (Reactions (11) and (12), Figure 8).

2FeTiO₃ + Na₂O + 0.5O₂ → 2NaFeTiO₄

(11)

3FeTiO₃ + 1.5Na₂O + 0.75O₂ → 2NaFeO₂ + NaFeTi₃O₈

(12)

2FeTiO₃ + 4Na₂O + 0.5O₂ → 2NaFeO₂ + Na₂TiO₃ + Na₄TiO₄

(13)

3FeTiO₃ + 2.5Na₂O + 0.75O₂ → 3NaFeO₂ + Na₂Ti₃O₇

(14)

2FeTiO₃ + Na₂O + 0.5O₂ → 2NaFeO₂ + 2TiO₂

(15)

3FeTiO₃ + 1.5Na₂O + 0.75O₂ → 3NaFeO₂ + 3TiO₂

(16)

2TiO₂ + 3Na₂O → Na₂TiO₃ + Na₄TiO₄

(17)

3TiO₂ + Na₂O → Na₂Ti₃O₇

(18)

The decomposition of ilmenite using sodium oxide from soda ash to form sodium ferrite, meta-, ortho-, and trititanates is reflected in exothermic Reactions (13)–(18). Overall, Reactions (13) (formation of sodium ferrite, meta- and ortho-titanates) and 14 (sodium ferrite and trititanate) and intermediate Reactions (15) and (16) (formation of sodium ferrite and titanium dioxide) proceed due to the enthalpic factor, with numerical values of ΔH < 0, over the entire temperature range from 373 to 2273 K. The reactions have ΔS < 0 from 373 to 573 K and from 373 to 473 K, with ΔG < 0 and T < ΔH/ΔS, and proceed spontaneously at temperatures above 573 and 473 K. The numerical values are ΔH < 0, ΔS < 0, and ΔG < 0 in the whole temperature range and T < ΔH/ΔS in Reaction (17), which completes the general reaction of ilmenite decomposition, allowing us to determine that the formation of sodium meta- and orthotitanates in the interaction of the two released moles of titanium dioxide with three moles of sodium oxide proceeds due to the enthalpic factor. The numerical values of ΔH < 0, ΔS > 0, and ΔG < 0 throughout the entire temperature range of Reaction (18), which completes the general Reaction (14) of ilmenite decomposition, indicate that the formation of sodium trititanate in the interaction of three moles of titanium dioxide with one mole of sodium oxide proceeds spontaneously at any temperature.

Thus, via the thermodynamic analysis of chemical reactions, we have determined the temperature range relating to the formation of ferruginous sodium titanates at the decomposition of ilmenite. However, the formation of ferruginous sodium titanates at the initial stage of ilmenite decomposition is possible only in the presence of sodium ions in the reaction mixture of initial substances. Sodium oxide, formed during the decomposition of soda, as mentioned above, is a binary thermally stable, refractory (T_sinter 1405 K under high pressure) substance possessing a rigid donor–acceptor bond of the alkali metal ion, i.e., Na+, with oxygen. Through the use of thermodynamic calculations, we have established that sodium oxide derived from soda does not decompose into sodium ions across the wide temperature range of 373 to 2273 K (Appendix B). This is convincing evidence in terms of refuting the existing idea that ilmenite decomposes as a result of the diffusion of iron ions from the crystal lattice of the mineral and the diffusion of sodium ions into the crystal lattice of the mineral. The true mechanism of ilmenite decomposition is logically explained by the oxidation of divalent iron by oxygen and its transition to the trivalent reactive oxide state, which forms water-soluble sodium ferrite when interacting with strongly basic soda sodium oxide, which is reflected in the course of thermodynamic Reactions (13)–(16) shown in Figure 8.

3.2.2. Thermodynamics of Pseudorutile Decomposition Reactions

Pseudorutile Fe₂Ti₃O₉, having in its composition trivalent iron oxide in contrast to ilmenite, can be catalytically decomposed without oxygen under the action of sodium oxide with the formation of sodium ferrite and metatitanate in the whole temperature range from 373 to 2273 K due to the enthalpic factor (Figure 9, general exothermic Reaction (19)).

Fe₂Ti₃O₉ + 4Na₂O → 2NaFeO₂ + 3Na₂TiO₃

(19)

Fe₂Ti₃O₉ + Na₂O → 2NaFeO₂ + 3TiO₂

(20)

3TiO₂ + 3Na₂O → 3Na₂TiO₃

(21)

Fe₂Ti₃O₉ + 4Na₂CO₃ = 2NaFeO₂ + 3Na₂TiO₃ + 4CO₂

(22)

3TiO₂ + 3Na₂CO₃ = 3Na₂TiO₃ +3CO₂

(23)

The decomposition of this mineral to form sodium ferrite and titanium dioxide directly can proceed spontaneously at any temperature (initial Reaction (20)). The released titanium dioxide, reacting with sodium oxide, forms sodium metatitanate due to the enthalpic factor (Reaction (21), completing the general Reaction (19)). The destruction of the mineral by soda ash with the formation of sodium ferrite and sodium metatitanate, beginning at 873 K, can occur in the high-temperature region during thermal decomposition of the alkaline reagent, accompanied by the release of carbon dioxide, due to the entropic factor (endothermic Reactions (22) and (23)).

3.2.3. Thermodynamics of Pseudobrookite Decomposition Reactions

The constituent component of pseudobrukite Fe₂(TiO₅) is reactive trivalent iron oxide; therefore, this allotropic ilmenite mineral, like pseudorutile, can be catalytically decomposed without oxygen at low temperatures by sodium soda oxide, leading to the formation of ferrite and then sodium metatitanate, due to the enthalpy factor of general exothermic Reaction (24), initial spontaneous Reaction (25), and final enthalpy Reaction (26) (Figure 10).

Fe₂(TiO₅) + 2Na₂O → 2NaFeO₂ + Na₂TiO₃

(24)

Fe₂(TiO₅) + Na₂O → 2NaFeO₂ + TiO₂

(25)

TiO₂ + Na₂O → Na₂TiO₃

(26)

The hypothetically possible destruction of pseudobrookite by soda ash, starting at 973 K, can proceed in the high-temperature region due to the entropic thermal decomposition of the alkaline reagent accompanied by the release of carbon dioxide.

3.2.4. Thermodynamics of Titanium Disilicide Decomposition Reactions

Titanium disilicide TiSi₂—zhiqinite can decompose into constituent oxides spontaneously in the whole studied temperature range (Figure 11, exothermic initial Reaction (27)).

TiSi₂ + 2O₂ → 2SiO + TiO₂

(27)

2SiO + O₂ → 2SiO₂

(28)

2SiO₂ + 2Na₂O → 2Na₂SiO₃

(29)

TiSi₂ + 3O₂ + 2Na₂O → 2Na₂SiO₃ + TiO₂

(30)

TiO₂ + Na₂O → Na₂O∙TiO₂

(31)

TiSi₂ + 3O₂ + 3Na₂O → 2Na₂SiO₃ + Na₂O∙TiO₂

(32)

The destruction of titanium disilicide at low temperatures is promoted via the oxidation and transition of divalent silicon into a reactive strongly acidic dioxide state, actively reacting with strongly basic sodium oxide, proceeding with the formation of sodium metasilicate in the temperature range from 337 to 2273 K due to the enthalpic factor (intermediate exothermic Reactions (28) and (29)). At the same time, the interaction of releasing titanium dioxide with sodium oxide to form sodium metatitanate also occurs in the temperature range from 337 to 2273 K due to the enthalpic factor (subsequent exothermic Reactions (30) and (31), completing the overall Reaction (32)). Probable reactions of metasilicate and sodium metatitanate formation collectively reflect the enthalpic character of the process of oxidative conversion of titanium disilicide by sodium oxide of soda ash.

3.2.5. Thermodynamics of Chromite Decomposition Reactions

The mineral chromite FeCr₂O₄, the constituent component of which is divalent iron oxide—which, in contrast to stable reactive trivalent iron oxide, has a smaller charge and a larger ionic radius [22]—can be decomposed by sodium soda ash oxide with the formation of sodium ferrite and sodium metachromite at temperatures ranging from 373 to 573 K due to the enthalpic factor, and spontaneously above 573 K (Figure 12, exothermic general Reaction (33)).

2FeCr₂O₄ + 0.5O₂ + 3Na₂O → 2NaFeO₂ + 4NaCrO₂

(33)

2FeCr₂O₄ + 0.5O₂ + Na₂O → 2NaFeO₂ + 2Cr₂O₃

(34)

2Cr₂O₃ + 2Na₂O → 4NaCrO₂

(35)

The enthalpic nature of the process of chromite decomposition in the low-temperature region is driven by the initial stage of mineral destruction, which is stimulated by the oxidation and transition of divalent iron into the reactive trioxide state. This reactive iron then reacts strongly with basic sodium oxide to form sodium ferrite and release chromium trioxide (initial exothermic Reaction (34)). The process of oxidative conversion of chromite acquires a spontaneous character as a result of the subsequent spontaneous interaction of chromium trioxide, which is resistant to oxidation, with sodium oxide of soda ash, proceeding with the formation of sodium metachromite (exothermic Reaction (35)), completing general Reaction (33).

3.2.6. Thermodynamics of Spessartine Decomposition Reactions

Spessartine Mn₃Al₂[SiO₄]₃—non-silicate manganese-aluminum garnet—is one of many rock-forming vein minerals found as phenocrysts in ilmenite concentrate. The destructive effect on spessartine at low temperatures is exerted by the oxygen and sodium oxide of soda ash. Strongly basic divalent manganese, which is present in the mineral composition, shows great stability, and is easily oxidized by oxygen, acquires an acid trioxide state, forming water-soluble sodium manganate in the process of firing with strongly basic sodium oxide [23]. It is quite possible that the degradation of spessartine is accompanied by the simultaneous interaction of reactive strongly acidic silicon dioxide with strongly basic sodium oxide to form water-soluble sodium metasilicate. The calculation of the Gibbs energy change in the reaction of oxidative-soda conversion of spessartine is presented in Appendix A. The decomposition of spessartine, which proceeds with the formation of sodium manganate and metasilicate and the release of amorphous aluminum oxide, is driven by the enthalpic factor and is represented by exothermic Reaction (37) (Figure 13).

Mn₃Al₂[SiO₄]₃ + 3O₂ + 7Na₂O → 3Na₂MnO₄ + 3Na₂SiO₃ + 2NaAlO₂

(36)

Mn₃Al₂[SiO₄]₃ + 3O₂ + 6Na₂O → 3Na₂MnO₄ + 3Na₂SiO₃ + Al₂O₃

(37)

Al₂O₃ + Na₂O → 2NaAlO₂

(38)

Amorphous aluminum oxide can interact with strongly basic sodium oxide spontaneously over the entire temperature range to form water-soluble sodium aluminate after sodium oxide binds reactive strongly acidic manganese and silicon oxides (exothermic Reaction (38), completing the general Reaction (36) of spessartine decomposition).

3.2.7. Thermodynamics of Zircon Decomposition Reactions

Metamict zircon ZrSiO₄, found in ilmenite concentrate, is a very hard (7.5 Mohs slag) mineral of the subclass of island silicates, consisting of reactive strongly acidic silicon dioxide and extremely refractory zirconium dioxide (T_sinter. 2715 °C), which is almost 1.5 times stronger than titanium dioxide (T_sinter. 1843 °C).

Due to the active reaction properties of silicon dioxide and sodium oxide of soda ash, zirconium orthosilicate can spontaneously decompose across the entire temperature range, forming sodium metasilicate and releasing zirconium dioxide mineral from the crystal lattice (Figure 14, initial exothermic Reactions (40) and (41)).

ZrSiO₄ + 3Na₂O → Na₄SiO₄ + Na₂ZrO₃

(39)

ZrSiO₄ + 2Na₂O → Na₄SiO₄ + ZrO₂

(40)

SiO₂ + 2Na₂O → Na₄SiO₄

(41)

ZrO₂ + Na₂O → Na₂ZrO₃

(42)

The interaction of amorphous zirconium dioxide with sodium oxide of soda ash, in accordance with its stability, can proceed with the formation of sodium zirconate due to the entropic factor of exothermic Reaction (42), which completes the general Reaction (39), reflecting the spontaneous character of the exothermic decomposition of zircon.

3.3. Evaluation of the Efficiency of Oxidizing Soda Conversion of Polymineral Ilmenite Concentrate

As a result of the thermodynamic analysis of the chemical reactions, a one-stage oxidizing soda conversion of ilmenite concentrate was developed. As a result of testing technological conditions, samples of synthetic rutile were obtained. The composition is presented in Figure 15 and

Table 5. Quality of a representative sample of synthetic rutile, according to the results of X-ray phase analysis.

Name	Formula	%
Rutile, syn	Ti0.912O2	98.2
Silicon Oxide	SiO2	1.8

. The quality of the synthetic rutile samples was estimated at the Central Laboratory of Kazakhstani Ust-Kamenogorsk titanium-magnesium combine (JSC “UK TMC”), certified by the PRI (USA) in terms of conformity to international standards “Nadcap”.

The specified technological mode of the roasting process ensures the completeness of decomposition of all minerals, and the refining of concentrate cinders achieves efficient purification of titanium dioxide from easily soluble sodium salts of accompanying elements.

The chemical composition of a representative sample of synthetic rutile (in wt%) was as follows: TiO₂—96.516; Fe₂O₃—0.185; Cr₂O₃—0.249; SiO₂—1.237; Al₂O₃—0.839; ZrO₂—0.343; MgO—0.017; P₂O₅—0.201; Nb₂O₅—0.081; Y₂O₃—0.005; and CaO—0.033.

Further studies determined the feasibility of regenerating soda and carbonizing alkaline filtrate through water-refining the concentrate cinders; this process yields amorphous silica and sodium bichromate. Additionally, hydrochloric acid evaporation of the acidic filtrate of the pulp in the evaporator columns with the condensation of hydrogen chloride results in the production of an iron oxide pigment.

4. Conclusions

• The thermodynamic parameters of simple and complex minerals, which are absent in the reference literature, can be determined using the incremental Kumok method, the additive Neumann–Kopp rule, and the Mostafa group contribution method;
• The stability of ilmenite concentrate minerals depends on the oxidation degree and the reaction properties of structural metal and silicon oxides. The oxides of iron (III), silicon (IV), and manganese (VI) have the best reactivity in solid-phase oxidizing–alkaline medium;
• The destructive effect on the minerals of ilmenite concentrate is exerted by air oxygen and sodium oxide of soda ash, decomposing in the low-temperature region by absorbing heat and evaporating moisture in the process of dehydrating goethite, lepidocrocite, manganite, and clay minerals and dehydration via heating briquetted soda charge concentrate with an aqueous binder solution;
• The oxidative soda conversion mechanism of ilmenite concentrate is explained by the decomposition of ilmenite, zhikinite, chromite, and spessartine by air oxygen and the transition of iron, silicon, and manganese from the lowest divalent to the reactive oxide state reacting with strongly basic sodium soda oxide to form ferrite, inert sodium titanates, metasilicate, manganate, metachromite, and sodium aluminate. For pseudorutile and pseudobrukite, due to the presence of reactive iron trioxide and zircon in the structure, strongly acid-reactive silicon dioxide catalytically decomposes strongly basic sodium soda oxide without the participation of oxygen to form first ferrite and metasilicate and then meta-titanate and sodium zirconate;
• The one-stage soda conversion of high-chromium, silica, rutilized and leucoxene ilmenite concentrates via air oxygen with the subsequent refining of concentrate cinder with water, followed by diluted hydrochloric acid, simplifies the process of obtaining high-purity synthetic rutile and reduces the cost of producing metallic titanium and titanium pigments.