1. Introduction
Nowadays, the Bayer process is the predominant method of alumina production in the world. This method is based on the leaching of pre-crushed bauxite in alkaline solutions. After leaching, aluminum hydroxide separates from the aluminate solution and is then calcinated to produce alumina. High-quality bauxites with an Al
2O
3 to SiO
2 mass ratio of more than 8 are required as a raw material for alumina production by the Bayer process [
1]. The world’s reserves of high-quality bauxite reduce every year [
2,
3,
4]. In this regard, a lot of intensive research focused on the expanding of the raw material base with the involvement of the low-quality aluminum raw materials (high-silica bauxite, kaolin clay, nepheline, ash) has been carried out [
5,
6,
7,
8].
The development of highly-efficient processing methods of low-quality aluminum raw materials with high silica content is an urgent task. One of the possible ways of high-silica raw materials processing is the sintering method. However, this method is multi-stage and characterized by high complexity as well as high fuel and reagents consumption.
One of the most promising methods of obtaining alumina from high-silica aluminum ores is a hydrochloric acid technology, followed by the selective separation of aluminum compounds from acidic solutions [
9,
10,
11]. The main advantage of this technology is the removal of silicon oxide at the first stage of the process. Silica separation helps to simplify the process and prevent the formation of harmful alumina production waste [
12,
13,
14]. The advantage of hydrochloric acid as a leaching agent is the possibility of aluminum chloride hexahydrate (ACH) selective precipitation from leaching solutions. In addition, the hydrochloric acid method is the easiest in terms of acid regeneration compared to sulfuric and nitric acids. [
15,
16]. After the thermohydrolysis of aluminum chloride and iron crystals, gaseous HCl is captured by water and returned to leaching.
The purpose of this research is the study of the physical and chemical properties of alumina obtained by the hydrochloric acid technology using kaolin clays from Eastern Siberia, as well as the assessment of the obtained alumina suitability for aluminum electrolysis.
3. Results and Discussion
The main reactions of kaolin clays during hydrochloric acid leaching were Equations (1) and (2):
where Me is Fe, Ca, K, Na, Mg.
As shown in Equations (1) and (2), aluminum dissolves in hydrochloric acid to form ACH. The amount of aluminum recovered into the hydrochloric acid solution was 95%. In this case, silica (the main component of kaolin clays) was not dissolved in hydrochloric acid and was separated from the solution by filtration. Other impurities also transferred into the leaching solution with a different extraction value. The composition of the aluminum chloride solution is presented in
Table 2.
After separation from the solid residue, the aluminum chloride solution was subjected to a two-stage crystallization of AlCl
3·6H
2O. Selective deposition of AlCl
3·6H
2O crystals occurred during crystallization and most of the metal chloride impurities remained in the solution. Precipitated crystals of AlCl
3·6H
2O were particles with a hexagonal form combined into agglomerates of different sizes (
Figure 3) with a grain size of up to 550 microns.
The main impurity in the ACH crystals was iron. The main part of the iron was concentrated on the surface of the crystals and was represented by the residual master solution (see
Figure 3). At the same time, part of the iron impurities was distributed point-wise throughout ACH crystals volume (see
Figure 4 and
Table 3).
To determine the temperatures of dehydration, water and chlorine removal, as well as phase transitions, the obtained samples of ACH were subjected to thermal analysis (see
Figure 5 and
Figure 6). The measurements were performed on the initial ACH under dynamic heating of the sample up to 1200 °C at a rate of 10 degrees per minute in an argon atmosphere.
The first stage of mass loss occurred between 105 °C and 156 °C and was associated with the removal of adsorbed water. Further heating of the sample (up to 1200 °С) removed water and chlorine from AlCl
3·6H
2O, with a mass loss of 76.66%. The main release of water and chlorine occurred up to 400 °C. At high temperatures, the DSC curve showed an exothermic peak associated with the beginning of γ-Al
2O
3 to α-Al
2O
3 transition. The thermal decomposition reaction of ACH crystals can be represented by the following Equation (3):
To determine the residual chlorine content in the products of ACH calcination, ACH thermal decomposition in a stationary mode at 450–1250 °C and a duration of 30–90 min was made. Calcination (in air) was carried out in the RT 50-250/13 tube furnace (Nabertherm GmbH, Lilienthal, Germany). The weight of the ACH sample was 10 g. The results of the study are presented in
Table 4. The calcination temperature had the most influence on the residual chlorine content in rough alumina. The chlorine content decreased from 7.20% to 0.12% with increasing temperature from 450 °C to 1250 °C during the 30 min duration of the calcination process. With increasing duration from 30 to 90 min, the chlorine content reduced from 7.20% to 4.26% at the temperature of 450 °C. The minimum chlorine content in alumina (0.05 wt %) was obtained at 1250 °C and 90 min of calcination. The alumina obtained at 1250 °C was characterized by an increase in α-Al
2O
3 content, which adversely affected the subsequent electrolysis of aluminum. The calcination of ACH below 900 °C contributed to an increase of chlorine content in the final product.
According to the results of the thermal analysis of AlCl
3·6H
2O (see
Figure 5), studies on the calcination of aluminum chloride hexahydrate crystals were carried out. The studies were conducted at three different calcination temperatures for 90 min each. The chemical compositions of alumina obtained by calcination are presented in
Table 5.
Various modifications of alumina were formed depending on the calcination temperature (see
Figure 7). Alumina obtained at 800 °C is mainly represented by γ-Al
2O
3 with a small fraction of δ-Al
2O
3. Identical peaks were received at 900 °C. At 1000 °C there was a transition to θ-Al
2O
3 and α-Al
2O
3. These results were comparable to previous studies [
28,
29]. As can be seen in
Table 5 and
Figure 7, an increase in the calcination temperature above 900 °C led to a decrease in the chlorine content of the final product, but at the same time promoted the transition of γ-Al
2O
3 to α-Al
2O
3, which negatively affected the dissolution of the resulting alumina during electrolysis of aluminum.
To study the effect of temperature conditions on the particle size of alumina, experiments on calcination of ACH were carried out in two technological modes:
With smooth heating of the material to the operating temperature for 2 h, followed by 2 h of exposure at operating temperature;
Thermal shock-placing of the material into a hot furnace with a 4 hr exposure at operating temperature.
The operating temperature of the calcination was 900 °C. The ACH crystal size subjected to calcination was more than 450 microns. The size distribution of alumina obtained by calcination is shown in
Figure 8.
The average size of alumina particles obtained at both temperature modes of calcination was 150 microns. Thus, the particle size decreased by more than three times during ACH calcination, which can be explained both by the destruction of agglomerates of the initial ACH and primary crystals. According to
Figure 8, when the ACH was placed in a hot furnace, there was a slight over-grinding of the material compared to the smooth heating. This was due to the more intense release of the gas phase from the ACH.
Figure 9 shows SEM images of alumina obtained by hydrochloric acid technology. The samples were obtained by the calcination of aluminum chloride hexahydrate at 900 °C. As can be seen on the figure, the surface of alumina obtained from ACH was covered by the net of thin cracks emerging from the grain body. These cracks promote the removal process of hydrogen chloride and water vapor. At the same time, crack development was observed in certain directions according to the structural anisotropy of the AlCl
3·6H
2O crystal, without significant destruction of the ACH particle’s original shape. The destruction of ACH particles during calcination occurred mainly due to the destruction of agglomerates consisting of a large number of fine fractions. In addition, the developed porous surface of alumina obtained from ACH had a high active surface and promoted adsorption of fluorine-containing waste gases of aluminum electrolysis and increased the rate of alumina dissolution in the electrolyte.
Table 6 shows the compliance of the impurity content in the test samples with the requirements for metallurgical alumina. The content of controlled impurities in test samples of alumina obtained by hydrochloric acid technology met the requirements, which demonstrated the possibility of aluminum electrolysis.
Alumina was produced according to the technological scheme (
Figure 1) of kaolin clay processing by the hydrochloric acid technology. The following parameters were used to obtain alumina: leaching temperature was 160 °C with 3 h duration. Then, two-stage crystallization of ACH was carried out: at a temperature of 20 and 80 °C. The duration of both stages was 45 min. In accordance with the studies of ACH heat treatment, the optimal calcination mode was at 900 °C and 90 min duration. The alumina produced satisfied the following properties: Al
2O
3 content was 99.7 wt %, the content of impurities didn’t exceed maximum allowed values. Alumina was represented by γ-modification, the average particle size was 150 microns.
To assess the applicability of alumina obtained by the hydrochloric acid method, studies of its solubility in comparison with alumina produced by the classical Bayer process were made. The results of experimental studies are presented in
Figure 10.
The achievement of a constant voltage on the electrochemical cell using alumina produced by acid technology occurred on average 4 s faster, which indicates a better solubility of the alumina in cryolite.
The electrochemical voltage consists of the voltage drop in the electrolyte (U
1), the voltage drop in the contacts and conductors (U
2), decomposition voltage (U
3), and polarization voltage (U
4) (4):
Taking into account the small contact area of the current-carrying elements and small amperage, the voltage drop in the contacts and conductors (U
2) was assumed to be zero. When alumina was added to the cryolite, the electrolysis reaction took the form (5):
Decomposition voltage (U
3) in the temperature range of 1000–1100 °C was 1.15 V [
30]. According to the results [
31], the polarization voltage in the system С–СO
2|Na
3AlF
6 (2.7–3.1 cryolite ratio) + Al
2O
3|Al-C at the current density of up to 1 A/cm
2 is in the range of 0.03–0.06 V, with accepted 0.04 V.
The voltage drop in the electrolyte depends on the current density, conductivity of the electrolyte, and interelectrode distance, according to Formula (6):
where ρ is the resistivity of the electrolyte, Om·cm; i is the current density, A/cm
2; and l is the interelectrode distance, cm.
The resistivity of the electrolyte at the points of voltage stabilization on the electrochemical cell was determined (
Table 7).
According to
Table 7, the resistivity of the used electrolyte, measured using the developed design of the electrochemical cell, was in the range of 0.623–0.637 Om·cm. This value was consistent with the literature data [
32,
33], which indicates the adequacy of the used method.