Desilication of concentrated alkali solution by novel desilication reagent calcium hydroferrocarbonate: Part III. Standard thermodynamics investigation of desilication reaction using hydroferrite desilication reagents

Highlights

• The hydroandradite and aluminum-hydroandradite is the main compositions in DSPs in desilication reaction using hydroferrite agents

• That CHFC has the better desilication performance in the thermodynamic point

• The free energy of desilication reaction using hydroandradite is affected by the initial silica coefficient, that of DSPs and temperature

• The substitution reaction between aluminum to iron mainly occurs when the higher initial silica coefficient and alumina coefficient of product

Abstract

The property of lower soda and alumina contents of iron hydrogarnet can reduce the loss of soda and alumina in Bayer and Sub-molten salt process. Thus, the economic efficiency of alumina production could be improved. How to solve the effective formation of iron hydrogarnet in concentrated caustic aluminate solution and its cycle in the digestion process has become an important research topic. The base of hydroferrire desilication agents (calcium hydroferrite CHF, calcium hydroferrocarbonate CHFC, calcium hydroferrosulfate CHFS) were synthesized, the desilication experiments and desilication products (DSPs) were characterized, the results show that the hydroandradite and aluminum-hydroandradite are the main components in DSPs. The free energy of synthesis reaction of desilication agents and desilication reaction in concentrated caustic solution was investigated using the complex silicate theory. The investigation shows that CHFC has the better desilication performance for hydroferrite desilication agents. The free energy of desilication reaction in concentrated caustic solution using hydroandradite is affected by the initial silica coefficient, the reacted silica coefficient of DSPs, and the reaction temperature. From the point of view of thermodynamics, the substitution reaction between aluminum to iron that occurred in the concentrated caustic aluminosilicate solution main occurs when the higher initial silica coefficient of hydroandradite and the higher alumina coefficient of reaction product.

Introduction

The Bayer process is the principal method for the production of alumina from bauxite worldwide. The modern version of the process maintains the key steps of dissolution of alumina-rich minerals into hot caustic solution, separation of the insoluble phases, followed by gibbsite precipitation and calcination of the gibbsite to alumina (Smith, 2009; Smith, 2017).

The nature of crystal structure i.e. the gibbsitic and boehmitic bauxites are easier to digest than diasporic minerals by Bayer process leads to variations in processing conditions, such as less severe caustic concentrations, temperature and holding time. For the diasporic bauxite, especially Chinese and Eastern-Europe minerals, classical Bayer process at elevated temperatures of 220–300 °C and high pressures of 3–10 MPa(Chen and Peng, 1997; Li, 2010), only an incomplete extraction of alumina (<80%) can be conducted in practice.

Based on the enhanced oxidation property of concentrated alkali solutions, a new decomposition/separation technology for chromite, diaspore, iserine, and uhligite was introduced and developed (Zhou et al., 2004; Sun et al., 2007; Cao et al., 2009; Ma et al., 2009; Zhang et al., 2010; Chen et al., 2013; Chen et al., 2014; Liu et al., 2017). Due to the similar Lewis acidity and oxidation characteristics of concentrated alkali and molten salt (Angell, 1978), the new technology was termed the “sub-molten salt” method (Zhang et al., 1999; Wang et al., 2010; Liu et al., 2017). It has been demonstrated that >95% alumina in Chinese diaspore can be dissolved in a concentrated alkali solution using the sub-molten salt process at low temperatures of 140–200 °C and pressures of 0.3–1.2 MPa.

In the Bayer and Sub-molten salt process, the reactive silica (kaolinite, illite) is dissolved into the caustic aluminate solution (Whittington, 1996; McCormick et al., 2002; Smith, 2009; Yuan and Zhang, 2009; Smith, 2017) and the inert silica in Bayer method (pyrauxite, quartz) is dissolved in the concentrated alkali solution of Sub-molten salt process. A slurry storage or predesilication stage in which slurry which is kept at high volume-fraction at higher temperature for several hours to encourage the re-precipitation of the solubilized silica to sodalite, i.e. desilication products (DSPs) (Whittington and Cardile, 1996; Whittington and Fallows, 1997; Whittington et al., 1998; Smith, 2009; Yuan and Zhang, 2009; Xu and Smith, 2012).

In order to decrease the loss of soda and alumina of alumina production, the DSPs produced within the process should be calcium silicates and iron-hydrogarnet (Jozsef et al., 1980; Zoldi et al., 1987; Li et al., 2010; Lu et al., 2017; Wang et al., 2018); but the calcium silicate can be occurred in hydrothermal lime process (McCormick et al., 2002; Medvedev et al., 2003; Xu and Smith, 2012). Ni et al. and Zoldi et al. promoted the iron-hydrogarnet additives in high temperature digestions for the reduction of soda losses (Zoldi et al., 1987), Solymar et al. developed the technology that based on the production of iron-hydrogarnet as a residue digestion to offset the alumina losses (Zoldi et al., 1987). Medvedev et al. put forward the sodium ferrite method which was used into digestion process for iron-hydrogarnet production, and it was used in five refineries of Eastern Europe(Medvedev et al., 2003). Although iron hydrogarnet technology is very attractive, it needs to solve the problems that effect the formation of iron hydrogarnet in high concentrated caustic aluminate solution and cycle in digestion process.

In previous work (Hong et al., 2018a; Hong et al., 2018b), investigated the multiphase synthesis of calcium hydroferrocarbonate (CHFC), the desilication reaction factor and reaction mechanism of CHFC in concentrated caustic aluminosilicate solution. The investigation shown that CHFC had good desilication activity in concentrated alkali solution, the silica reacted with CHFC on the surface to form iron hydrogarnet (3CaO Fe₂O₃·6H₂O) and hydroandradite (3CaO Fe₂O₃·xSiO₂·(6–2×)H₂O). The crystallinity of hydroandradite improved with an increase in the reaction time and temperature. Silica and alumina exchanged with OH^– ions and Fe₂O₃ respectively in the hydroandradite structure by isomorphic substituting reaction; calcium aluminum iron hydroxide (CAIH, 3CaO·yAl₂O₃·(1–y)Fe₂O₃·6H₂O) was formed when the reaction continued. With an increase in Na₂O concentration and temperature, hydroandradite and CAIH dehydrated to calcium aluminum iron silicate (CAIS, 3CaO·yAl₂O₃·(1–y)Fe₂O₃·kSiO₂) and Fe(OH)₃.

Although there are a number of studies regarding the use of iron hydrogarnet in digestion and deep desilication at atmospheric pressure, the formation conditions of iron hydrogarnet, the thermodynamics of iron hydrogarnet and desilication are still unclear. In this paper, we report our recently study of standard thermodynamics investigation of reaction process using hydroferrite desilication reagents at atmospheric pressure. The hydroferrite desilication agents, including calcium hydroferrite (CHF), calcium hydroferrocarbonate (CHFC) and calcium hydroferrosulfate (CHFS) were synthesized and reacted with concentrated caustic aluminosilicate solution, then the DSPs were characterized. On the base of experiments, the standard Gibbs energy, the Entropy or the total differential functions of free energy for hydroferrite agents and DSPs were calculated; the standard free energy and equilibrium constant that the synthesis reaction of hydroferrite, desilication reaction using hydroferrite were explored.