Hydrochemical Method for the Production of Alumina from Nepheline Using Effective Calcium Reagents

Abstract

The use of alumina-containing nepheline raw materials as an alternative source of alumina is relevant in the context of the limited bauxite reserves in Kazakhstan. Nepheline processing can result in products such as alumina, sodium and potassium salts, silicate products and rare metals. In terms of economic value, alumina is the most important. This article considers an advanced technology for nepheline processing for the extraction of alumina that is first purified from potassium. The application of calcium sulphate and calcium oxide as additives to the nepheline raw materials is studied. The optimal conditions for two-stage leaching with calcium additives in the form of calcium sulphate and calcium hydroxide are determined.

1. Introduction

The reserves of Bayer bauxite are limited in Kazakhstan, which affects the industrial production of alumina, but it is possible to use non-bauxite raw materials, such as the nepheline syenites of the Kubasadyr deposit, for the same purpose. The generally accepted processing method for nepheline consists of sintering the ore with limestone. The main disadvantages of the sintering process are high energy consumption, high capital costs and significant atmospheric emissions. Due to technological advancements, it is possible to use a hydrochemical alkaline-sulphate method for the processing of nepheline syenites. The technology includes preliminary chemical enrichment and leaching with a new active calcium additive. The global aluminium industry is growing rapidly, and aluminium is a vital structural material [1].

The traditional source of aluminium is bauxite. However, quality reserves of bauxite are limited, and alternative sources of aluminium are being sought. These include aluminosilicate minerals such as kaolins or nepheline [2,3,4,5,6].

Nepheline is an important aluminium non-bauxite source [7,8]. The transition to alkaline aluminosilicate rocks is significant because they generate other valuable products at the same time as alumina without any waste. The structure and chemical composition of nepheline varies in different deposits [9]. The value of nepheline depends on the content of K₂O, Na₂O and Al₂O₃ and on the presence of rare elements [10,11,12,13]. The excessive amount of silica found in nepheline is a negative aspect of this rock as it is of no value. However, it can be isolated as individual valuable chemicals.

Nephelines are processed only in Russia, Canada, Norway, Turkey, and Brazil [14].

Currently, there are only two nepheline processing facilities in the world, the Achinsk Alumina Refinery and Pikalevsky Alumina Refinery, which are located in Russia. In these plants, the lime sintering process is used to produce soda, potash, and Portland cement in addition to alumina.

Kazakhstan has discovered significant amounts of non-bauxite aluminium raw materials. Alkaline nepheline is present in high-alumina minerals found in ultrabasic and granitoid rocks. There are three primary types of rocks that contain nepheline: nepheline rocks of alkaline or ultrabasic massifs, nepheline–lycite rocks of alkaline–basic massifs and nepheline syenites of alkaline granitoid massifs

Several dozens of nepheline rock massifs have been found in different regions of the Republic of Kazakhstan [15,16]. The processing of nepheline in Kazakhstan requires the development of an effective technology that takes into account the peculiarities of the mineral composition of the raw materials.

With the feasibility of processing substandard alumina-containing raw materials, ways to expand the raw material base of Kazakhstan’s alumina industry must be found, with a focus on the broader use of low-quality but more common natural aluminium raw materials, namely, nepheline ores.

Processing nepheline, a material that is highly siliceous, involves the use of a substantial amount of waste sludge. It is difficult to create a waste-free production in this case. In this regard, it is more advantageous to process rocks of this type with preliminary chemical enrichment, which enables the partial desilicification of ore. When decomposing nepheline using the hydroalkali method, this method is a viable option.

The sintering method is an industrial method for the processing of alkaline aluminosilicate feedstock into alumina [17]. Alumina is the most important economic element, but cement production from belite sludge may not be profitable in some situations. Therefore, a high content of alumina in the raw material charge is of primary importance. The preparation of such a nepheline–limestone–soda charge consists of thorough grinding and mixing of all components in compliance with the necessary stoichiometric proportions. The sintering process is carried out in tubular rotary furnaces, in which the charge is heated to a temperature of 1270 °C. The main reactions during sintering are the formation of sodium aluminate and bicalcium silicate. During the subsequent leaching, bicalcium silicate is converted into belite sludge, from which cement can be produced. It has significant disadvantages, such as high energy consumption, high capital costs, process complexity, high cost of sintering furnaces, the loss of expensive rare earth elements and high atmospheric emissions [13,17,18]. It is estimated that the aluminium industry produces 0.45–0.5 Gt of carbon dioxide (CO₂) equivalent emissions per year and is responsible for 1% of anthropogenic GHG emissions and 2.5% of CO₂ emissions [17].

Acidic methods of processing aluminosilicate raw materials are known. There are known works on the hydrochloric acid leaching of nepheline at temperatures of 90–150 °C, aluminium extraction from solution by crystallisation of AlCl₃∙H₂O followed by calcination with HCl release [1].

The dissolution of nepheline syenite ore with oxalic acid reduces the Fe₂O₃ content and produces a high value-added concentrate for many industrial purposes. Considering the results of the dissolution tests carried out, a nepheline syenite concentrate was obtained with 0.15% Fe₂O₃ content when 0.4 mol/dm³ oxalic acid and sulfuric acid were used for 2 h leaching [5].

Chemical leaching of nepheline syenite with dilute sulphuric acid after magnetic separation could recover only ~40% of the potassium values [18].

The use of acid methods in industry is limited by several significant drawbacks: the necessity of expensive acid-proof apparatus, difficulties in separating and washing silica residue, difficulties in regeneration of acid solutions and the problem of utilising sludge, which mainly consists of silicic acid gels. In addition, the processing of alkaline aluminosilicates (nepheline) will lose the alkali contained in these rocks, as well as the acids used for the decomposition of silicic acid.

The hydrochemical processing method consists of autoclave leaching at high temperatures and highly concentrated solutions of NaOH in the presence of calcium compounds. Calcium compounds introduced into the aluminosilicate leaching process have a decisive influence on alumina recovery. Many studies have been devoted to the effect of calcium in aluminosilicate systems [19,20,21].

Although nepheline decomposes in solutions without calcium addition, alumina is not extracted from the solution. Since the process of aluminium leaching from aluminosilicates is based on the formation of sodium-calcium hydrosilicate (SCHS) Na₂O × 2CaO × SiO₂∙H₂O, its composition is determined by the dosage of calcium oxide. This compound is an unstable compound in alkaline solutions [22]. After desilicification of the solution, alkali is regenerated from the unstable SCHS compound by hydro-rolysis to yield a precipitate of alkali-free calcium silicate. Sodium and potassium alkalies, which are found in nepheline ore, are also present in the resulting beneficiation solution. The separation of potassium and sodium alkali during desiliconisation results in the formation of SCHS since potassium alkali does not create compounds like SCHS. The presence of potassium alkali should be considered in the hydrochemical processing of nepheline, unlike bauxite.

The physicochemical properties of NaOH and KOH solutions differ significantly. The electrolyte activity coefficient in NaOH solutions is about an order of magnitude smaller than in potassium solutions, and consequently, the chemical activity of sodium solutions is lower [23].

Experiments proved that during the alkaline treatment of nepheline, first, excess silica enters the solution. If the silica content in the rock is greater than that needed for the formation of alkaline hydroaluminosilicate, then the alumina from the rock does not enter the solution at all, and in the alkaline solution, only silica remains in the form of sodium and potassium metasilicate. If there is not enough alumina to completely bind all the silica in the solution, then it will only move into the solution. Nepheline cannot be processed without the use of calcium aluminate additives [24]. In alumina production, calcium oxide plays an important role in binding SiO₂ from aluminosilicate raw materials (bauxite, nepheline, kaolinite, etc.) to form aluminium-free compounds.

At present, calcium oxide is obtained from limestone (calcium carbonate), which requires additional capital investment, energy costs, etc. To produce 1 ton of alumina, it is necessary to roast about 3 tons of limestone. Replacing lime with natural limestone will eliminate the limestone roasting operation and, therefore, improve technical and economic performance. Calcium sulphate is of particular interest as a calcium-containing reagent, significant amounts of which are in the form of the toxic technogenic products of phosphogypsum. The replacement of calcium sulphate with a manmade compound will improve the technological and economic feasibility of nepheline processing and will allow for the use of both of its main components: calcium for binding silicon in the form of calcium silicate and sulphate for obtaining the scarce fertiliser potassium sulphate from the potassium contained in nepheline.

Phosphogypsum is a multitonne waste product from phosphate fertiliser production, consisting mainly of gypsum CaSO₄ at 94–95%, undecomposed apatite Ca₅(PO₄)₃ at 1.77% and monocalcium phosphate Ca(H₂PO₄)₂ at 0.18% [25].

According to expert estimates, the accumulated phosphogypsum reserves in the dumps of industries amount to approximately 140 million tonnes, with an annual increase of 14 million tonnes [26].

There are 14 million tonnes of phosphogypsum in the dumps of Kazphosphate LLP, and the company gives it away at a price of 1 tenge per tonne [27].

The purpose of this article is to develop an effective technology for processing nepheline syenites to study the influence of the forms of calcium additive in the leaching process. The possibility of partial or complete replacement of CaO by CaSO₄ at complex technology of nepheline processing is considered. The use of CaSO₄ will reduce the energy costs associated with limestone firing.

A hydrometallurgical alkali-sulphate technology for processing nepheline ore was developed, including preliminary chemical activation and leaching in HMAS with the use of CaSO₄ as a calcium additive in the first stage, which provides the amount of constituents necessary for the formation of potassium sulphate. As a result, we extract potassium and purify the aluminate solution from potassium. Potassium has a negative effect on aluminium recovery. In the second stage, leaching should be carried out in recycled aluminous-alkali solution with CaO added in the amount necessary for the complete binding of silica in CaO∙SiO₂. The addition of a calcium additive in the second stage to a ratio of CaO:SiO₂ = 1.6 is optimal, as above such a ratio, SiO₂ does enter the solution, the presence of which negatively affects the regeneration of the solution and the quality of the obtained aluminium hydroxide. The recovery of Al₂O₃ remained at approximately 85%.

2. Materials and Methods

X-ray fluorescence analysis to determine chemical composition was performed on a Venus 200 wave dispersion spectrometer (Panalyical B.V., Almelo, The Netherlands). The sample, ground to a particle size of 0.074 mm, \was applied to a boric acid substrate under a force of 5 tonnes. The sample was evenly distributed throughout the cuvette, compressed and placed in the apparatus. Chemical analysis was performed using an Optima 2000 DV (Perkin Elmer, Waltham, MA, USA) inductively coupled plasma optical emission spectrometer (Optima, Perkin Elmer). X-ray experimental data were obtained using a BRUKERD8 ADVANCE (Karlsruhe, Germany) device using copper radiation at an accelerating voltage of 36 kW and a current of 25 mA. The sample, ground to a particle size of 0.056 mm, was placed in a cuvette pretreated with alcohol. The sample was evenly distributed throughout the cuvette, compressed and placed in the apparatus. International Centre for Diffraction Data ICDD PDF-2 2023 database was used to confirm the accuracy of phase identification. Electron probe microanalysis (EPMA) was performed on a JEOL JXA-8230 (Tokyo, Japan) and scanning electron microscopy was used to clarify the elemental composition of the particles in different areas of the cake after leaching as a function of duration.

An average representative sample of nepheline syenites from the Kubasadyr deposit was used in this work, which was obtained by sampling and quarting. Leaching in alkaline solution at temperatures of 200–300 °C was carried out in an autoclave with an overpressure of up to 100 bar, which allowed the temperature to increase to 300 °C with the help of an electric heater with a power of 5 kW. The accuracy of temperature maintenance in the autoclave was ±2 °C. The design of the autoclave enabled sampling with quenching throughout the experiment. The design of the autoclave also allows the introduction of the solid phase at a given temperature and stirring by a falling stirrer with a frequency of 0.1–1 Hz.

3. Results and Discussion

To develop an effective technology for the direct extraction of alumina from nepheline syenites, the influence of the form of calcium added during leaching was investigated. CaO and CaSO₄ were used as the calcium additives.

Nepheline syenites of the Kubasadyr deposit, Republic of Kazakhstan, were used as a raw material. Chemical composition in wt% was as follows: Al₂O₃ 21.2; SiO₂ 55.86; Fe₂O₃ 3.18; CaO 3.64; Na₂O 5.09; K₂O 5.52; others 5.14.

The use of CaSO₄ as a calcium additive instead of CaO is justified by the fact that it will reduce the energy costs associated with limestone firing.

Leaching was carried out in a high-modulus aluminate solution (HMAS) with caustic modulus α_k = 30.0 (the caustic modulus of alkaline solutions (αk) was determined from the ratio αk = Na₂O/Al₂O₃ × 1.645) at a temperature of 280 °C, L:S ratio of 5 and duration of 90 min (

Table 1. Leach cake of nepheline concentrate with different forms of calcium added.

Additive	Content, wt%								Extraction, Al2O3, %
	CaO	Al2O3	SiO2	Na2O	K2O	Fe2O3	MgO	Others
CaO	15.63	3.93	14.67	24.55	0.53	0.96	0.25	39.48	94.86
CaSO4	13.29	3.07	12.16	22.2	0.5	0.88	0.14	47.76	95.25

), and calcium was added to achieve a CaO:SiO₂ ratio of 1.0 [24].

The chemical composition of HMAS, g/dm³ was Na₂O 235.0; Al₂O₃ 13.7. Ga 0.28; α_k = 30.0.

Leaching resulted in a medium-modulus aluminate solution of composition g/dm³: Na₂O 161.0; Al₂O₃ 38.87; SiO₂ 0.1; α_k = 10.5.

The X-ray phase analyses of the leach cake are summarised in

Table 2. X-ray phase composition of leach cakes.

Name	Formula	Type of Additive
		CaSO4	CaO
Portlandite, syn	Ca(OH)2	53.2%	49.6%
Sodium calcium hydrogen silicate	NaCaHSiO4	17.3%
Tricalcium silicide oxide	Ca3SiO	14.5%
Krotite, syn	CaAl2O4	10.1%
Mullite, syn	Al(Al1.25Si0.75)O4.875	4.9%
Natrite	Na2CO3		33.6%
Cancrinite	Na7.14Al6Si7.08O26.73(H2O)4.87		16.8%

and in Figure 1 and Figure 2.

Analysis of the results showed that it is possible to use CaSO₄ as a calcium additive in the leaching of nepheline ore instead of CaO.

The mechanism of ore leaching using CaO or CaSO₄ as a calcium additive can be represented by Reactions (1)–(3),

2ROH + SiO₂ → R₂SiO₃ + H₂O

(1)

R₂SiO₃ + CaO + H₂O → 2ROH + CaSiO₃

(2)

R₂SiO₃ + CaSO₄ + 2H₂O → R₂SO₄ + CaSiO₃ · 2H₂O,

(3)

where R is Na or K.

Thus, the developed hydrometallurgical alkaline-sulphate technology for nepheline ore processing, including preliminary chemical activation and leaching in HMAS using CaSO₄ as a calcium additive, allows for Al₂O₃ extraction in solution at a recovery rate of 94–95%. As a result of leaching, a medium-modulus aluminate solution with α_k = 10.5 was obtained.

However, the replacement of CaO with CaSO₄ leads to the formation of sodium and/or potassium sulphates in solution and, consequently, to the consumption of caustic alkali (Reactions (1)–(3)), so it is logical to use CaSO₄ only in the amount necessary for the formation of potassium sulphate; this can be realised by dividing the leaching process into two stages. The first stage includes leaching in an alkaline solution with the addition of a part of the calcium in the form of CaSO₄, which is necessary for the binding of potassium during leaching into a sulphate form. In the second stage, leaching is carried out in a recycled aluminous-alkaline solution with the addition of CaO in the amount necessary for the complete binding of silica in the form of CaO∙SiO₂.

From the alkaline solution in the first leaching stage, using the difference in solubility, potassium sulphate can be selectively extracted by crystallisation.

The technological scheme for processing nepheline ore is shown in Figure 3. In the two-stage leaching process, Stage I includes leaching with the recycled alkaline solution with CaSO₄ added in the amount necessary for the formation of potassium sulphate, and Stage II of leaching contains the recycled HMAS with the rest of the calcium introduced in the form of CaO to obtain CaO:SiO₂ = 1.

Stage I of leaching was carried out in a solution with a Na₂O content of 240 g/L, temperature of 280 °C and duration of 120 min.

Thus, samples of nepheline purified from potassium were prepared in the optimal solution with a Na₂O concentration of 240 g/dm³, 280 °C and duration of 1200 min with 70% CaO added at the rate of CaO:SiO₂ = 1. Under these conditions, the maximum recovery of K₂O in solution was 92.13%, that of Al₂O₃ was 6.57 and that of SiO₂ was 22.84.

The chemical composition of the spent cake from leaching stage I was Al₂O₃ 11.86%; SiO₂ 21.9%; Fe₂O₃ 1.4%; CaO 17.7%; Na₂O 11.73%; K₂O 0.89% and MgO 0.52%.

The optimal ratio of CaO:SiO₂ = 1.6 was chosen because, at this ratio, SiO₂ did not enter the solution, the presence of which negatively affects the regeneration of the solution and the quality of the obtained aluminium hydroxide.

The aim of leaching Stage II was to release aluminium into solution and to bind the remaining silicon to calcium to produce calcium silicate. For this purpose, the effects of the temperature, duration and CaO:SiO₂ ratio on the recovery of Al₂O₃ and SiO₂ were investigated.

The study of the influence of CaO:SiO₂ on the degree of Al₂O₃ extraction at Stage II of leaching was carried out with the addition of Ca(OH)₂, and it was quenched at a temperature of 130 °C to obtain the molar ratio CaO:SiO₂ = 1:2. The influence of temperature in Stage II of the leaching process on the degree of recovery of elements is presented in

Table 3. Effect of the Stage II leaching temperature on the element recovery rate.

Leaching Temperature, °C	Extraction into Solution, %
	Al2O3	SiO2
220	54.27	2.99
240	73.05	19.9
260	85.07	25.94
280	91.37	23.31

and Figure 3.

According to the results, as the temperature of leaching in Stage II increases, the recovery rate of Al₂O₃ in solution increases and reaches 91.37% at 280 °C.

The influence of the amount of calcium added in the stoichiometric ratio of CaO:SiO₂ in Stage II of leaching on the degree of elemental recovery was investigated (

Table 4. Effect of the amount of calcium added in the stoichiometric CaO:SiO₂ ratio in Stage II of leaching on the recovery of elements.

CaO:SiO2	Extraction into Solution, %
	Al2O3	SiO2
1:1	62.37	0
1:1.2	65.71	0
1:1.4	85.13	0
1:1.6	85.72	0
1:1.7	86.43	15.85
1:1.8	88.87	24.07
1:1.9	91.37	23.31
1:2	77.06	21.95

, Figure 4).

According to the results obtained, when calcium oxide is added at a molar ratio of CaO:SiO₂ greater than 1.6, SiO₂ starts to be released into solution, and the recovery of Al₂O₃ remains at approximately 85%.

Thus, the technological parameters for leaching, including two-stage leaching with the use of HMAS and calcium sulphate, were determined (Figure 5).

4. Conclusions

A hydrometallurgical alkali-sulphate technology for processing nepheline ore was developed, including preliminary chemical activation and leaching in HMAS with the use of CaSO₄ as a calcium additive in the first stage, which provides the amount of constituents necessary for the formation of potassium sulphate. In the second stage, leaching should be carried out in recycled aluminous-alkali solution with CaO added in the amount necessary for the complete binding of silica in CaO∙SiO₂. The addition of a calcium additive in the second stage to a ratio of CaO:SiO₂ = 1.6 is optimal, as above such a ratio, SiO₂ does enter the solution, the presence of which negatively affects the regeneration of the solution and the quality of the obtained aluminium hydroxide. The recovery of Al₂O₃ remained at approximately 85%.