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Role of the amorphous phase during sodium aluminosilicate precipitation Hong Peng, Dilini Seneviratne, and James Vaughan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04538 • Publication Date (Web): 07 Jan 2018 Downloaded from http://pubs.acs.org on January 7, 2018
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Industrial & Engineering Chemistry Research
Role of the amorphous phase during sodium aluminosilicate precipitation Hong Peng*, Dilini Seneviratne, James Vaughan School of Chemical Engineering, The University of Queensland, Brisbane, Australia (* corresponding author:
[email protected])
Abstract In the Bayer process, reactive silica associated with bauxite dissolves into alkaline solution and subsequently precipitates as a sodium aluminosilicate “DeSilication Product” (DSP). Multiple DSP phases can be formed, which include an amorphous solid, as well as crystalline zeolite A, sodalite and cancrinite. Even though the ability to control DSP particle size would be of great practical benefit, there remains limited fundamental understanding of the precipitation process, especially during the early stages of the reaction. In this study, DSP is precipitated from synthetic NaOHNaAl(OH)4-Na2SiO3-H2O solution at 75 and 90˚C. Four distinct reaction stages were observed: (1) precipitation of the amorphous solid (2) dissolution of the amorphous solid (3) growth of crystalline phases and (4) maturation of the crystalline phases. In addition to the amorphous phase, the nucleation of both particles and two-dimensional crystal planes appeared in the early stages of the reaction. Subsequently, the amorphous phase dissolves which feeds the early growth of the crystalline DSP phases. After the amorphous phase disappears, the crystal phases continue to grow with reactants from the bulk solution until equilibrium is approached. In the maturation stage, the solution silicate concentration only decreases slightly, however, during this time individual crystal sizes decrease slightly while the overall particle size increases via crystal agglomeration, which is promoted at higher temperature.
Keywords: DSP, Bayer Process, Particle Size Distribution, Amorphous, Sodalite
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1. Introduction Since the Bayer process was disclosed in 1888, this technology has been the main route for producing alumina from bauxite.1, 2 The process begins by dissolving aluminum bearing mineral phases in hot alkaline solution (Bayer liquor) followed by physical separation of the insoluble bauxite residue. Gibbsite is then precipitated from the clarified liquor and calcined to the product alumina. When bauxite contacts Bayer liquor, reactive silica consisting mainly of kaolinite dissolves into the saturated solution with respect to the desilication product (DSP). Once the degree of supersaturation is sufficiently high, silicate precipitates as a sodium aluminosilicate DSP. Desilication represents a loss of both sodium and aluminum as DSP is disposed along with the bauxite residue. The cost of replacing the sodium hydroxide is a major operating expense for the refinery, especially if bauxite high in reactive silica is processed.1, 3 The two main DSP phases formed are sodalite (Na6(AlSiO4)6·2NaCl·2H2O), and cancrinite (Na6(AlSiO4)6·CaCO3·2H2O).1,
4, 5
Although both compounds are tectosilicates with similar
stoichiometry, they differ in terms of crystal structure and particle morphology. The cancrinite phase only readily forms at elevated temperature6. Sodalite is the predominant phase formed in atmospheric pressure reactors at temperatures below 100°C. Sodalite DSP particles are fine (
