Distribution of Rare Metals Obtained from the Alkaline Sulfate Processing of Nepheline Syenite

Abstract

This article presents the results of studies on the distribution of rare metals among the products of the alkali sulfate processing of nepheline syenites. In response to the limited reserves of Bayer bauxite in the alumina industrial production region of Kazakhstan, the feasibility of using alternative alumina-containing nonbauxite raw materials was investigated. The most promising nonbauxite raw materials in Kazakhstan are nepheline and kaolinite clays. At present, there is no effective technology for processing nepheline ores. This article describes a proposed complex technology involving nepheline processing with the associated extraction of gallium and vanadium. The technology includes the activation of raw materials, followed by two-stage leaching, where potassium is extracted in the first stage. The sludge and solution obtained from the second stage of the leaching process are utilized for calcium silicate production and two-stage carbonization, respectively. In the first stage, aluminum hydroxide is extracted, and, in the second stage, a concentration of rare metals, such as gallium and vanadium, is obtained. Vanadium is extracted from the solution via crystallization, and gallium is extracted via electrodeposition. Overall, 38.48% of the Ga₂O₃ and 56.12% of the V₂O₅ are recovered from raw nepheline syenite. A technological scheme of the developed technology is presented in this article.

1. Introduction

In response to the limited reserves of Bayer bauxite in the alumina industrial production region of Kazakhstan, it is possible to use nonbauxite raw materials, such as the nepheline syenites of the Kubasadyr deposit [1,2].

The products of nepheline processing include alumina, sodium, potassium salts, silica, and rare metals. Sintering the ore with limestone is the conventional processing method for nepheline. The sintering process has significant disadvantages, such as a high energy consumption, high capital costs, and high atmospheric emissions [3,4]. In the sintering technology, the processing of 2 tonnes of nepheline produces about 3 million tonnes of waste sludge, which is not recycled and, when stored in a landfill, is a source of soil, groundwater, and atmospheric pollution [5]. However, a hydrochemical method for processing nepheline syenites is now feasible due to technological advancements [6].

Impurities in the composition of nepheline are very diverse and include Ca, Mg, Fe, Ti, Ga, Li, Rb, Cs, Ba, Sr, and rare earth elements [7].

This paper presents a hydrochemical alkaline sulfate technology that facilitates the wasteless utilization of nepheline to produce aluminum hydroxide, soda, potash, potassium sulfate, construction materials, gallium, and vanadium without polluting the environment. The main product is aluminum hydroxide. The technology is complex because it also recovers other valuable components that are found in nepheline syenites, including rare metals such as rubidium, gallium, and vanadium.

The developed technology includes a two-stage leaching process with the use of an active calcium additive. The advantage and novelty of this technology lie in the use of an alkaline hydrochemical method of processing, which includes pre-activation followed by a change in phase composition [8] and the separation of potassium alkali at the beginning of the process [9]. It allows for increased Al₂O₃ recovery, as its presence is detrimental to its recovery. The distribution of rare metals in the intermediate products that result from the processing of nepheline has been previously determined, and the methods of their extraction for obtaining V₂O₅ and metallic gallium have been investigated.

Advancements in modern industry are impossible without the use of rare metals, and the demand for rare metals in the long term will remain stable, with the need increasing every year.

The largest consumers of rare metals are developed countries, such as the USA, Western European countries, Japan, and others [10]. Japan, which does not have its own rare metal raw material resources, has the highest rare metal consumption growth rates, with some increases calculated to be in the tens of percentage points per year [11]. At the same time, the list of countries mining rare metals is short. The country that occupies the first place in terms of the production and export of rare metals is China, which accounts for approximately 50% of the world’s reserves. China currently controls the mining and processing of more than 90% of the total volume of key rare earth elements [12].

The application of rubidium in ion engines, photocells, analytical chemistry, etc., demonstrates the unique properties of this valuable element. The annual world production of Rb is limited to 2–4 tonnes per year [13].

Gallium is widely used in various applications, such as semiconductors [14] and monoclinic gallium oxide (β-Ga₂O₃) in optoelectronics [15]. Moreover, it is mainly used for the fabrication of integrated circuits, GaAs for high-frequency communication [16] and for fiber optic communication, and LEDs.

Gallium is mainly obtained as a byproduct from the processing of bauxite to produce aluminum and, to a much lesser extent, from the processing of zinc ores. The gallium content in bauxite exceeds 1 million tonnes, and approximately the same amount of Ga is found in zinc deposits [14]. Recycling gallium from industrial waste or recycled electronic components has proven to be a crucial means of expanding the supply of this critical resource, providing a more reliable supply of gallium resources [17].

Most of the gallium in the world is produced in China, and 99.99% pure gallium is produced in Germany, Hungary, Russia, Japan, etc. The largest consumers of 99.99% pure gallium in the world are China, the USA, and Japan [18].

There are several methods of gallium extraction from an aluminate solution: sorption with subsequent cementation using aluminum gallama; extraction of gallium; carbonization followed by cementation with aluminum gallama; mercury cathode electrolysis and cementation with aluminum gallama; flat stationary gallium-covered cathode electrolysis; water-cooled solid cathode electrolysis and cementation with aluminum gallama; and rotating gallium-covered cathode electrolysis [19,20].

Vanadium and its compounds are used in a variety of industrial sectors [18], but the largest volume of vanadium is needed in the metallurgical and chemical industries.

The bulk of the world’s vanadium consumption—approximately 87 percent—comes from the metallurgical industry [21]. Vanadium is mainly used for alloying high-quality structural steels to optimize their performance characteristics. The combination of metamaterials and VO₂ is a promising approach for the development of tunable terahertz devices such as switches, filters, modulators, sensors, and optical storage [22] to improve the efficiency of energy storage in zinc-ion batteries [23,24]. The unique properties of vanadium–carbon compounds are complex. These carbides are characterized by refractoriness, a high hardness, and thermal and electrical conductivity [25,26].

Vanadium is characterized by a high chemical activity, which determines the possibility of its application in various industrial sectors, including the chemical industry. Many compounds of this metal, primarily vanadium acid salts (vanadates), vanadium oxides, and carbides, are used in the chemical industry. Specific examples of vanadium include the production of dyes and the production of catalysts [27,28]. Moreover, vanadium is undeniably advantageous in the production of high-quality tools made of various chrome–vanadium steels.

To obtain vanadium, alkaline methods of decomposition are most often used. In such methods, vanadium passes into a solution, as well as aluminum and silicon, which then must be freed using various methods. Single extraction [29], sequential extraction [30], and other chemical extraction methods are also used for vanadium extraction [31,32].

To date, acceptable, continuous technologies and methods for the integrated processing of nepheline with the associated extraction of rare metals have not been developed. Considering its potential benefits, the extension of the process to the treatment of nepheline for the recovery of values such as Al(OH)₃, K₂SO₄ metallic gallium, V₂O₅, and Rb₂SO₄ was investigated in this study.

2. Materials and Methods

An X-ray fluorescence analysis of chemical composition was performed using a Venus 200 wave dispersion spectrometer (Panalytical B.V., Almelo, The Netherlands). A chemical analysis was performed using an Optima 2000 DV inductively coupled plasma optical emission spectrometer (Optima, Perkin Elmer, Waltham, MA, USA). A semiquantitative X-ray phase analysis was performed using a D8 Advance diffractometer (BRUKER, Billerica, MA, USA) with copper Cu Kα radiation at an accelerating voltage of 36 kV and a current of 25 mA. An X-ray spectral microanalysis with scanning electron microscopy (SEM) was performed using a JEOL JXA-8230 (Tokyo, Japan) instrument.

Activation was carried out in a solution of 120 g/dm³ NaHCO₃ at a liquid-to-solid ratio of 3 and a temperature of 280 °C using an autoclave with a volume of 5 dm³ (Figure 1). The duration of activation was 90 min [9].

After activation, stage I of the leaching process was carried out in a solution containing 240 g/L Na₂O. Then, CaO mixed with CaSO₄ was added for the selective extraction of potassium and part of the SiO₂ into the solution [33]. The temperature was 280 °C, the duration was 120 min in a thermostat unit with 6 autoclaves rotating through the head, and the working volume was 250 cm³ (Figure 2).

Stage II of the leaching process was carried out in a recycled high-modulus alkaline solution (HMAS) with a caustic modulus, α_k, of 30.0, which was determined from the Na₂O to Al₂O₃ ratio, with a Na₂O concentration of 240 g/dm³ at 280 °C. Calcium oxide addition was necessary for calcium silicate formation to obtain an aluminous alkaline solution with α_k = 10.0.

The synthesis of tricalcium hydroaluminate (TCHA) was carried out at 100 °C for 3–4 h [33], with the addition of CaO from a stoichiometric amount of 100–120%.

TCHA decomposition was carried out in a solution containing NaO₂ at 140–160 g/dm³, a temperature of 180 °C, a leaching time of 90 min in an autoclave, and a liquid-to-solid ratio of 4. The carbonization of the solutions was carried out by supplying CO₂ gas from a cylinder.

Stage I of the carbonization process of the TCHA decomposition solution was carried out up to a NaO₂ concentration of 10–20 g/dm³ for 8–10 h.

Stage II of the carbonization process was conducted at 70 °C for 300 min [34]. After each stage, the pulp was incubated for 1 h without a carbon dioxide supply.

Leaching of the gallium and vanadium concentrates was carried out in a solution containing 280 g/dm³ of Na₂O at 90 °C for 60 min with a liquid-to-solid ratio of 2.

Electrodeposition was carried out according to a previously described method [9] under the following conditions: a temperature of 50 °C, a Dk of 50 mA/cm², and a rotation speed of the gallinated cathode of 2.0 m/s. An electrolyzer with a rotating gallinated surface was used for electrodeposition (Figure 3).

Complex hydrochemical technology was employed to process the nepheline syenites (Figure 4).

3. Results and Discussion

Nepheline syenites of the Kubasadyr deposit in the Republic of Kazakhstan served as the initial raw material for this study. The chemical composition in wt % was as follows: Al₂O₃—17.77; SiO₂—48.37; Fe₂O₃—3.18; CaO—2.89; Na₂O—4.24; K₂O—7.7; Ga₂O₃—0.021; and V₂O₅—0.034.

The phase composition of the nepheline syenites is shown in Figure 5.

The nepheline syenite was subjected to thermochemical activation in a sodium hydrogen carbonate solution. The activation operation facilitates the dispersion of the source material and modifies its mineral composition, which prepares the raw aluminosilicate material for leaching [8]. As a result of activation, we obtained the following ore compositions, shown in mass percentages: Al₂O₃—17.88; SiO₂—47.8; Fe₂O₃—2.99; CaO—2.76; Na₂O—4.16; K₂O—7.5; Ga₂O₃—0.004; and V₂O₅—0.031.

The phase composition of the nepheline after activation is shown in Figure 6.

After stage I of the leaching process, a cake was obtained with the following composition: Al₂O₃—11.86%; SiO₂—21.9%; Fe₂O₃—1.4%; CaO—17.7%; Na₂O—11.73%; K₂O—0.89%; Ga₂O₃—0.167%; and V₂O₅—0.022%.

The distributions of rare metals obtained by the stage I leaching process are presented in

Table 1. The distribution of rare metals obtained from stage I of the leaching process.

Products	Mass		Ga2O3			V2O5			Rb2O
	m3	kg	Content, %	kg	Distribution, %	Content, %	kg	Distribution, %	Content, %	kg	Distribution, %
Stage I Leaching
Loaded
Nepheline Syenite		10.00	0.021	0.002	100.0	0.034	0.0034	100.0	0.103	0.01	100.0
Ca2SO4		7.840				0.0004	0.00004	100.0
Recycled alkaline solution	89.20		0.004	0.0004
Total				0.003			0.003			0.01
Received
Stage I leaching precipitate		14.00	0.017	0.002	92.06	0.022	0.003	88.74	0.008	0.011	10.87
Alkaline solution	65.00		0.002	0.0002	7.940	0.006	0.0004	11.26	0.14	0.01	89.13
Total				0.002	100.0		0.003	100.0		0.01	100.0

.

As a result of the stage I leaching process, the recovered components were as follows: K₂O—92.06%; Al₂O₃—6.57%; SiO₂—22.84%; Rb₂O—89.13%; Ga₂O₃—7.94%; and V₂O₅—11.26%.

Approximately 90% of the Rb₂O was extracted into the solution as a result of the stage I leaching process. The stage I leaching solution was fed to the leach rack for the extraction of K₂SO₄. Rb₂SO₄ was obtained by evaporating the alkaline solution from the stage I leaching process to obtain K₂SO₄ via crystallization.

Consequently, gallium and vanadium were concentrated in the residue.

The phase composition of the cake from stage I of the leaching process is shown in Figure 7. According to the X-ray phase analysis, the cake from stage I of the leaching process consisted of cancrinite, sodium calcium hydrogen silicate mayenite, pyrite, ferro-ferri-taramite, aluminum hydroxide, and others.

The distribution of rare metals obtained by the stage II leaching process is presented in

Table 2. The distribution of rare metals obtained from stage II of the leaching process.

Products	Mass		Ga2O3			V2O5
	m3	kg	Content, %	kg	Distribution, %	Content, %	kg	Distribution, %
Stage II Leaching
Loaded
Stage I leaching precipitate		14.000	0.017	0.0023	92.061	0.022	0.003	88.74
Recycled HMS	70.000		0.001	0.0001	4.264	0.001	0.0001	3.91
Ca(OH)2		2.910				0.000	0.000
Total				0.002	96.33		0.003	92.65
Received
Stage II leaching precipitate		11.480	0.001	0.0001	4.509	0.002	0.000	6.753
Aluminous alkaline solution	80.000		0.023	0.002	91.82	0.030	0.003	85.90
Total				0.002	96.33		0.003	92.65

.

As a result of the stage II leaching process, the mass percentages were as follows: Al₂O₃—2.065%; SiO₂—22.87%; Ga₂O₃—0.002%; V₂O₅—0.006%; and Rb₂O—0.0017%. The Al₂O₃ yield was 85.72%.

The chemical composition of the stage II leaching solution was as follows (g/dm³): Na₂O—260.0; Al₂O₃—38.87; SiO₂—0.1; Ga₂O₃—0.017; and V₂O₅—0.0214 (α_k = 10.5).

Gallium was extracted into more than 90% of the aluminous alkaline solution, and more than 90% of the gallium was extracted as a result of the stage II leaching process.

The stage II leaching precipitate consists of 8.18% Ga₂O₃ and is used for construction material production. During the sintering process, 30% of the Ga₂O₃ is transferred to the sludge and lost [35].

The stage II leaching solution was a medium-modulus solution processed using tricalcium hydroaluminate (TCHA) synthesis.

The aluminous alkaline solution was converted via the synthesis of TCHA to bind the aluminum extracted as a result of the stage I leaching process. Recycled HMS with α_k = 30.0 was obtained using the following reaction:

Na₂O·AI₂O₃ +3Ca(OH)₂ + 4H₂O = 3CaO·AI₂O₃·6H₂O + 2NaOH

(1)

The caustic modulus of alkaline solutions (αk) was determined from the ratio αk = Na₂O/Al₂O₃ × 1.645.

The chemical composition of TCHA was as follows (wt.%): Al₂O₃—23.4; CaO—46.6; Ga₂O₃—0.0011; and V₂O₅—0.0059. The recovery of AI₂O₃ in TCHA was 70.4%. The αk of the obtained HMAS was 30.0. After adjustment for caustic alkali content, the obtained HMAS was recycled as a solution for leaching a new portion of nepheline ore.

As a result of the synthesis of TCHA, 87.56% of the tricalcium hydroaluminate gallium and 81.30% of the vanadium were extracted (

Table 3. The distribution of the rare metals obtained from the synthesis of tricalcium hydroaluminate.

Products	Mass		Ga2O3			V2O5
	m3	kg	Content, %	kg	Distribution, %	Content, %	kg	Distribution, %
The Synthesis of Tricalcium Hydroaluminate
Loaded
Aluminous alkaline solution		80.000	0.023	0.002	91.82	0.030	0.003	85.90
CaO		0.840					0.000
Total				0.002	91.82		0.002	85.90
Received
TCHA		6.520	0.034	0.002	87.56	0.036	0.002	81.30
Na2CO3 solution	70.000		0.001	0.0001	4.26	0.001	0.0001	4.61
Total				0.002	91.82		0.002	85.90

).

The obtained TCHA was subjected to decomposition in a soda solution for the regeneration of calcium oxide in the form of CaCO₃ or Ca(OH)₂, as well as for the extraction of Al₂O₃, Ga₂O₃, and V₂O₅ into the aluminous alkaline solution.

TCHA was decomposed with the soda solution to obtain an aluminate solution for carbonization using the following reaction:

3CaO∙Al₂O₃∙6H₂O + Na₂CO₃ = 3CaCO₃ + 2NaAl(OH)₄

(2)

As a result of TCHA decomposition under the above conditions, the percentage of Al₂O₃ extracted into the solution was 95.8%.

The chemical composition of the TCHA decomposition solution was as follows (g/dm³): Na₂O—140.0; and Al₂O₃—25.41 (α_k = 6.5).

As a result of the decomposition of TCHA, 74.13% of the gallium and 78.103% of the vanadium were extracted into the TCHA decomposition solution (

Table 4. The distribution of the rare metals obtained from the decomposition of TCHA.

Products	Mass		Ga2O3			V2O5
	m3	kg	Content, %	Kg	Distribution, %	Content, %	kg	Distribution, %
Decomposition of TCHA
Loaded
TCHA		6.520	0.034	0.002	87.56	0.036	0.002	81.30
Na2CO3 solution	26.080
Total			0.028	0.002			0.002
Received
Precipitate		4.500	0.008	0.000	13.43	0.002	0.000	3.195
TCHA decomposition solution	20.500		0.019	0.002	74.13	0.002	0.002	78.103
Total			0.100	0.002	87.56		0.002	81.30

).

To extract aluminum from the TCHA decomposition solution and obtain rare metal concentrates (gallium and vanadium), carbonization was carried out in two stages.

The aluminate solution was carbonized with CO₂-containing gas to obtain Al(OH)₃, and the Na₂CO₃ solution was recycled using the following reaction:

2NaAl(OH)₄ + CO₂ = 2Al(OH)₃ + Na₂CO₃

(3)

After the first carbonization step, 90.0% of the AI₂O₃ precipitated as hydrates. Together, 15.0% Ga₂O₃ and 9.0% V₂O₅ were precipitated via coprecipitation.

After the second carbonization stage, a precipitate was obtained with the following composition (wt.%): 27.1% Al₂O₃, 0.48% Ga₂O₃, and 15.6% V₂O₅. This precipitate was a concentrate of gallium and vanadium.

As a result of the second stage of carbonization, 54.894% of the gallium and 63.132% of the vanadium were extracted into the cake (

Table 5. The distribution of the rare metals obtained from the carbonization I process.

Products	Mass		Ga2O3			V2O5
	m3	kg	Content, %	kg	Distribution, %	Content, %	kg	Distribution, %
Carbonization I
Loaded
TCHA decomposition solution		20.500		0.019	0.002	74.13	0.002	0.002
CO2
Total				0.002			0.002
Received
Al(OH)3		1.480	0.020	0.000	11.634	0.022	0.000	11.277
Carbonization I solution	18.000		0.015	0.002	62.496	0.002	0.002	66.818
Total				0.002	74.129		0.002	78.10

and

Table 6. The distribution of the rare metals obtained from the carbonization II process.

Carbonization II
Loaded
Carbonization I solution	18.000		0.015	0.001	55.649	0.002	0.002	63.126
CO2
Total				0.001			0.0019
Received
Gallium- and vanadium-containing cake		0.120	1.071	0.001	54.894	1.608	0.002	63.132
Solution	17.600		0.001	0.000	0.752
Total				0.001	55.645		0.0019	63.132

).

The method of gallium- and vanadium-containing cake processing with the addition of water in an autoclave at 245 °C for 2 h was obtained from [35]. The Ga₂O₃ recovery rate was 80%. Processing the cake in an alkaline solution at 90 °C resulted in a recovery rate of 88%.

As a result of the leaching of gallium and vanadium, 48.482% of the gallium-containing cake and 61.66% of the vanadium-containing cake were extracted into tricalcium hydroaluminate (

Table 7. The distribution of the rare metals obtained from the leaching of gallium- and vanadium-containing cakes.

Products	Mass		Ga2O3			V2O5
	m3	kg	Content, %	kg	Distribution, %	Content, %	kg	Distribution, %
Leaching
Loaded
Gallium- and vanadium-containing cake		0.120	1.071	0.001	54.894	1.608	0.0019	63.13
Na2O solution	0.480
Total				0.001			0.0019
Received
Precipitate		0.050	0.300	0.000	6.406	0.090	0.000	1.472
Solution	0.800		0.011	0.001	48.482	0.019	0.0019	61.66
Total				0.001	54.888		0.0019	63.13

).

The solution from the second carbonization stage after evaporation was used for the preparation of recycled HMAS.

After the concentrate was leached, the obtained solution was cooled to room temperature, and the vanadium cake was isolated via the crystallization method. The composition of the obtained vanadium cake is as follows (wt.%): 8.7 Al₂O₃, 0.04 Ga₂O₃, and 22.1 V₂O₅. After the extraction of the vanadium cake, a gallium-containing solution was obtained with the following composition (g/dm³): Na₂O 169.2; AI₂O₃ 113.5; Ga₂O₃ 2.1; and V₂O₅ 0.5 (α_k = 2.3). The solution served as an initial intermediate for the extraction of metallic gallium.As a result of the crystallization process, 9.737% of the gallium and 56.124% of the vanadium were extracted from the vanadium cake (

Table 8. The distribution of rare metals obtained from the crystallization process.

Products	Mass		Ga2O3			V2O5
	m3	kg	Content, %	kg	Distribution, %	Content, %	kg	Distribution, %
Crystallization
Loaded
Solution	0.800		0.011	0.001	48.482	0.019	0.0019	61.66
Total				0.001			0.0019
Received
Vanadium cake		0.120	0.190	0.000	9.737	1.480	0.0018	56.124
Solution for electrolysis	0.600		0.009	0.001	38.437		0.0001	5.500
Total				0.001	48.175		0.0019	61.660

).

The cake was used to obtain vanadium pentoxide.

A total of 38.487% of the Ga₂O₃ was extracted from nepheline syenite. In the concentrated leaching solution, the gallium content was 2.1 g/dm³. This content meets the requirements for the efficient electrodeposition of metallic gallium.

In summary, the extraction of 97.8% gallium was obtained in this study, with a yield of 9.22%. The power consumption was 87.7 kWh/kg of gallium.

Thus, the advantages of the proposed hydrochemical alkaline sulfate technology in comparison with the conventional sintering method are as follows:

• An increase in gallium recovery by 30%;
• Exclusion of the energy-intensive sintering process;
• A reduction in waste sludge by 22%.

4. Conclusions

In this study, a complex hydrochemical alkaline sulfate technology involving nepheline processing with the associated extraction of gallium and vanadium was developed. This technology allows for the wasteless utilization of nepheline to produce aluminum hydroxide, soda, potash, potassium sulfate, construction materials, gallium, and vanadium without polluting the environment. The proposed technology includes the activation of raw materials, followed by a two-stage leaching process using an active calcium reagent, where potassium and rubidium are extracted in the first stage. Approximately 90% of Rb₂O is extracted into the solution as a result of the first stage of the leaching process. The sludge and solution obtained from the second stage of the leaching process are utilized for calcium silicate production and two-stage carbonization, respectively. Aluminum hydroxide is extracted during the first stage, and a concentration of rare metals such as gallium and vanadium is obtained at the second stage. Vanadium is extracted from the solution via crystallization, and gallium is extracted via electrodeposition. In total, 38.487% of the Ga₂O₃ and 56.124% of the V₂O₅ are extracted from nepheline syenite.