Reaction mechanism of alumina, sulfur and gallium in desulfurization concentrate from diasporic bauxite during high-temperature digestion

Abstract

To mitigate the detrimental effects of sulfur and enhance the enrichment efficiency of valuable elements in desulfurized diasporic bauxite, the effects of CaO dosage, caustic alkali concentration, reaction temperature and time on the digestion behavior of alumina, sulfur and gallium were illustrated, and the digestion thermodynamics and mechanism were also revealed. During the high-temperature Bayer process, alumina and gallium were digested synergistically, while pyrite was digested to S2− and SO42−. Appropriate CaO dosage promotes the digestion of alumina and gallium, and facilitates the precipitation of sulfur as calcium sulfoaluminate hydrate, effectively removing sulfur from the solution. Excess CaO leads to the formation of hydrogarnet, wherein Ga3+ incorporates into the crystal lattice by substituting for Al3+, reducing the digestion efficiency of gallium. Under the optimum conditions (CaO dosage of 3%, reaction temperature of 260 ℃, reaction time of 60 min, caustic alkali concentration of 260 g/L), the corresponding alumina and gallium digestion efficiencies reach 90.82% and 77.58%, respectively, with a significantly reduced sulfur concentration of 1.32 g/L in the solution. This work provides theoretical guidance for the efficient co-extraction of alumina and gallium from high-sulfur bauxite via the Bayer process.

1. Introduction

With the depletion of high-grade bauxite, the development of alumina industry has been seriously restricted in China [1]. High-sulfur bauxite reserves are abundant and partly rich in rare metal gallium, which can be used as an effective supplement to available bauxite resources [2]. The sulfur content in high-sulfur bauxite is usually more than 0.7%, even reaching over 2%, and sulfur mainly exists in the form of pyrite and its isomers as well as sulfates such as CaSO4 [3 −5]. The sulfur-bearing minerals entering the alumina production process can corrode the equipment, increase alkali consumption, decrease alumina extraction, and reduce product purity [6, 7]. To realize the efficient utilization of high-sulfur bauxite resources, it is essential to desulfurize to reduce the influence of sulfur [8, 9].

At present, the primary desulfurization methods include roasting [10 −12], flotation [13 −15], electrochemical [16] and biological methods [17, 18]. The roasting method focuses on converting the sulfur-bearing minerals into SO2 gas, which is not conducive to saving energy consumption and production costs. Electrochemical and microbial desulfurization have low production costs and energy consumption, but remain at the experimental stage [8]. Flotation is one of the most economical and effective methods to utilize high-sulfur bauxite. Nevertheless, the efficient separation of sulfur-containing minerals through flotation remains challenges due to the fine dissemination of pyrite and its polymorphs, as well as their close combination with diaspore. Consequently, the sulfur content in the concentrate remains as high as 0.50% [19, 20], and is converted into sulfide and sulfate under high temperature, resulting in substantial alkali consumption and reduction in alumina digestion efficiency [14]. The wet oxidation and precipitation technologies have been proposed as effective methods for desulfurization [21]. ZHOU et al [22] adopted the seed precipitation to remove sulfur from sodium aluminate solution, and the removal efficiency can reach 76% at the condition of temperature 60 ℃, agitation speed 200 r/min with 2 mL/L seed. LIU et al [23] used a method of adding ZnO to remove sulfur during the digestion process and the optimum digestion conditions in this experiment are as follows: 260 ℃, 60 min, lime dosage of 13%, finding the sulfur entering into the red mud in the form of ZnS. WANG et al [24] studied the effect of reducing oxidant and reductant on the reaction of pyrite in the digestion process. The results showed that the presence of oxidant retards the pyrite reaction with sodium aluminate solution, but the presence of reductant promotes at elevated temperatures. However, the cost of wet oxidation and precipitation technology is high, and lacks efficient desulfurization reagents for high-sulfur bauxite [25]. Many studies focused on the desulfurization behavior for high-sulfur bauxite, but little attention has been paid to the reaction mechanism of alumina, sulfur and gallium during high-temperature digestion.