The Basic And Applied Research Of Ga Electrowinning From High Aluminium Fly Ash

With the great progress of science and technology, the usages of Ga in thin film solar cells, magnetostrictive materials, liquid metal materials and hydrogen storage materials become increasingly mature. Ga and its compounds, especially the GaAs semiconductor, have been widely used in the photoelectric devices and integrated circuits. Ga mainly distributes in the bauxite resources, vanadium titanium magnetite, sphalerite and coal fly ash. At present, about 90% of Ga production is recycled from mother liquor of alumina production in the world. While high aluminium fly ash is most likely to compete with bauxite for raw material of Ga as its high content of aluminum and Ga. As the key subprocess, during the electrolytic recovery of Ga from high aluminium fly ash, there are serious electrochemical polarization and concentration polarization phenomenon due to the low concentration of Ga, which causes extremely low current efficiency. Meanwhile, high content impurity in the mother liquor further affects the quality of Ga products.Based on the process of Al-Si-Ga poly-metallic synergistic extraction, the paper researched the bottleneck problems of Ga electrowinning from high aluminium fly ash. With the study of electrochemical kinetics, efficient Ga electrodeposition was aimed by the means of process intensification and product decoupling control. The original results are obtained as following:(1) The kinetics of Ga electrodeposition with hydrogen evolution was analyzed, and the effect rules of electrochemical reaction and mass transfer were revealed. The influence mechanism of solution composition, electrolytic technology and outfield intensification on diffusion kinetic parameters were researched. It was found that, the values of apparent coefficients and exchange current densities of Ga electrodeposition were both less than those of hydrogen evolution. Under the control of mass transfer, the increase of NaOH concentration decreased the electric repulsion of cathode to GaO2- ions while also lessened the diffusion coefficient due to the viscosity increase. Ga concentration, pulse current and stirring could hardly affect the thickness of diffusion layer. The cavitation effect of ultrasonic produced the mix action on the solution next to the cathode, and consequently increased the reaction rate coefficient k for Ga electrowinning by decreasing the thickness of diffusion layer.(2) The competing reaction of hydrogen evolution and Ga electrodeposition at cathode were regulated by interfacial modification including the electrode activation and adding surfactant. Electrode activation improved the wettability and surface roughness of the electrode, which provided more active sites for Ga electrodeposition. From nucleation kinetic calculation, the value of pre-exponential factor (K) increased to 4.1 and 20.5 times that of none after alkaline activation and acid activation, respectively. The addition of perfluorinated surfactant promoted the formation of clustered particles on cathode surface, with the increasing of the specific surface area of cathode. From the kinetic calculation, it was found that the exchange current density of Ga electrodeposition increased from 0.99 mA cm-2 to 1.57 mA cm-2 by the addition of the surfactant. That increased the electrodeposition rate of Ga, consequently.(3) Diaphragm electrolysis of carbonated spent liquor was carried out. The NaOH and NaHCO3 by-products could be obtained while direct Ga electrodepostion through diaphragm electrolysis. The current efficiency of NaOH and NaHC03 decreased with time due to the increase of molar ratio of Na+ion between cathode and anode. There were three competing reactions of H+which formed during oxygen evolution, with the formation of HCO3-, CO2 and H2O respectively. The anolyte composition, current density, anode material and stirring changed the distribution of H+ of the three reactions, and consequently affected current efficiency of the products. The expanding scale experimental verification was conducted.(4) The influence mechanism of Fe (Ⅲ) and V (Ⅴ) on Ga electrowinning were studied. The deposition of iron oxide enhanced the wettability of Ga with electrode surface, and caused the development of aggregated Ga particles, which might be ascribing to the difference of surface properties between Ga and impurities. The deposition of vanadium oxide catalyzed the hydrogen evolution and caused the formation of spherical Ga particles, which depressed Ga electrodeposition. The experimental exploration of the co-electrodeposition of CIGS solar film and Ga electrorefining were implemented.(5) The interactions of anode corrosion behavior and Ga electrodeposition were conducted in alkaline sulfide solutions with low Ga concentration. It was found that Ga was hardly electrodeposited using stainless steel anode due to anode corrosion. Anode corrosion introduced iron ions into Ga solutions, and Ga electrodeposition is displaced by the deposition or absorption of iron oxide. The corrosion rate of nickel is much lower than that of stainless steel due to the formation of stable passive films which consists of Ni oxide and hydroxide. Bright metal gallium with high purity is obtained by using nickel anode at high S2- concentration from online pilot plant test.