Mineral Evolution And Precipitation Activity Of Desilication Products(DSPs) In Bayer Process

With the increasingly fierce competition in alumina enterprises and the increasing dependence on foreign bauxite,the hydrolysis problem in the Bayer process has been becoming more and more prominent.How to inhibit the hydrolysis loss of the digested solution during the dilution settling,that is,to ensure the stability of sodium aluminate solution,is one of the major scientific and technical problems which need to be solved urgently in Bayer enterprises.On the basis of calculating the thermodynamics of different types of complex desilication products（DSPs）in the Bayer process and simulating the desilication reaction system of different types of bauxite during the digestion process,the mineral evolution,microstructure and the structure characteristics of sodium aluminate solution under different conditions were systematically investigated in Na₂O-Al₂O₃-SiO₂-H₂O system,CaO-Na₂O-Al₂O₃-SiO₂-H₂O system and the actual bauxite digestion system using XRF,XRD,IR,Raman,SEM-EDS combined with the particle size,viscosity,conductivity analyses.Finally,the theoretical basis and practical guidance of precipitation control of DSPs and solution conditions for reducing hydrolysis loss of the Bayer process were established.The main conclusions of this research are as follows:The Gibbs free energies of variously complex DSPs were calculated according to Wen’s theory,and the mineral evolution behavior and precipitation activity of hydrated sodium aluminosilicates in Na₂O-Al₂O₃-SiO₂-H₂O system with different solution concentration,reaction temperature,time and predesilication were systematically studied.All the Gibbs free energies of zeolite,sodalite,hydroxyl-sodalite and hydrogarnet are below zero,which decrease with the increase of reaction temperature from 0 to 300℃.The Gibbs free energies of zeolite and hydrogarnet are larger than sodalite and hydroxyl-sodalite.The hydrated sodium aluminosilicates in Na₂O-Al₂O₃-SiO₂-H₂O system are comprised of amorphous zeolite,zeolite A,sodalite,hydroxyl-sodalite and zeolite,which contain cancrinite when carbonate is present at high precipitation temperature.Zeolite A is easy to generate with octahedral morphology at a large driving force which disappears gradually with the increase of reaction temperature and time;the zeolite and sodalite（hydroxyl-sodalite）are the stable phases in hydrated sodium aluminosilicates with disc-crossing spherical and cubic morphologies respectively.The crystallinity,mineral composition and microstructure of DSPs can affect the precipitation activity,while the stability of sodium aluminosilicate hydrates in sodium aluminate solution is:zeolite>sodalite>zeolite A>amorphous zeolite.Increasing the desilication temperature and time can significantly increase the crystallinity of hydrated sodium aluminosilicates,and decrease their aggregation degree,which are beneficial to the stability of sodium aluminate solution.The effects of different anion impurities and their concentration on the precipitation behavior and activity of hydrated sodium aluminosilicates at 145℃ were investigated.Cl^-,SO₄^2-,CO₃^2-and CH₃COO^-are involved in the desilication reaction to form the corresponding Cl^-sodalite,noselite,CO3-sodalite and hydroxymethyl sodalite,while C₂O₄^2-is not involved in the desilication reaction.Anion impurities have little effect on the morphology of hydrated sodium aluminosilicates,but change the thickness of disc structure of zeolite.With the increase of anion impurity concentration,the crystallization degrees of hydrated sodium aluminosilicates decrease and their precipitation activities increase,and the sodalite content increases and the zeolite content decreases.The ability of inorganic anions to participate in desilication is CO₃^2->SO₄^2->Cl^-.The addition of hydrated sodium aluminosilicate seed into the desilication process can improve the desilication depth of sodium aluminate solution,and increase the crystallinity of hydrated sodium aluminosilicates,which is beneficial to improve the stability of sodium aluminate solution.The mineral evolution behavior and precipitation activity of hydrated sodium aluminosilicates in CaO-Na₂O-Al₂O₃-SiO₂-H₂O system with different reaction temperature,time,CaO addition,carbonate concentration and predesilication were systematically studied.The DSPs in CaO-Na₂O-Al₂O₃-SiO₂-H₂O system are mainly comprised of hydrogarnet,tricalcium aluminate and zeolite;the amorphous sodalite content is very little,and zeolite A only exists at low temperature desilication.When sodium carbonate exists in sodium aluminate solution with CaO addition,the DSPs in 145℃ are comprised of hydrogarnet,tricalcium aluminate and two zeolites with different morphology;the DSPs in 245℃ are comprised of hydrogarnet,tricalcium aluminate and Ca-cancrinite.With the increase of carbonate concentration,the crystallinity of DSPs decreases,the content of hydrogarnet decreases and the content of zeolite or Ca-cancrinite increases gradually.The two-step desilication improves the crystallinity of DSPs,which decreases the content of amorphous phase and increases the content of zeolite in hydrated sodium aluminosilicates without CaO addition,but increases the content of hydrogarnet and decreases the content of zeolite when CaO is added.The addition of CaO in the predesilication obviously increases the content of hydrogarnet and greatly reduces the precipitation activity of DSPs.The stability of DSPs in sodium aluminate solution is:hydrogarnet>zeolite>Ca-cancrinite.Increasing the desilication temperature and time can increase the crystallinity of DSPs,promote the transformation of tricalcium aluminate to hydrogarnet,increase the SiO₂ saturation coefficient of hydrogarnet and decrease the content of zeolite.The ionic structures of sodium aluminate solution were systematically studied by the Raman spectra as well as the physical properties of viscosity and conductivity,and the mechanism of the ionic structure change of sodium aluminate solution affecting its stability was established.With the increase of temperature,caustic ratio and SiO₂ concentration of sodium aluminate solution,Al（OH）₄^-ions transform into Al₂O（OH）₆^2-ions and[（HO）₃Al-O-Al（OH）₃]^2-ions respectively,meanwhile the Al-O-Al bond of Al₂O（OH）₆^2-ions gradually transforms into more stable Al-O-Si bond with the increase of SiO₂ concentration,which decrease the number of Al（OH）₄^-and improve the stability of sodium aluminate solution.CO₃^2-and SO₄^2-promote the transformation of Al（OH）₄^-ions to Al₂O（OH）₆^2-ions,which improve the stability of sodium aluminate solution;C₂O₄^2-and CH₃COO^-increase the content of Al（OH）₄^-ions and Al₂O（OH）₆^2-ions,and promote the transformation of Al₂O（OH）₆^2-to Al（OH）₄^-,which deteriorate the hydrolysis of sodium aluminate solution.Finally,the precipitation behavior of DSPs and the stability of digested liquor during the digestion process of bauxites were studied.Predesilication and addition of lime can improve the desilication performance of the bauxite,while increasing the content of hydrogarnet and decreasing the content of zeolite are helpful to reduce the precipitation activity of DSPs.Predesilication and lime addition can greatly improve the stability of the digested solution,especially when the two are combined,for which the hydrolysis hardly occurs.