Study On New Production Process Of Zirconium Oxychloride By Decomposition Of Zircon Sand With CaO-NaOH

Zirconium oxychloride (ZrOCl2·8H2O) is the most important basic chemical products of zirconium salt and the main raw material for zirconium chemicals such as zirconium oxide, zirconium sulfate, and zirconium carbonate. The demand for zirconium chemicals has increased with the rapid development of national economy, especially sanitary ceramics, electronics, nuclear power and automobile manufacturing industry. According to the statistics conducted by China Nonferrous Metals Industry Association, zirconium oxychloride production in China has been over200,000ton by2011, ranking the first in the world. Alkali fusion of zircon sand concentrate, which is the key method to produce zirconium oxychloride, has salient features of large-scale production and high efficiency, in comparison with chloride and lime sintering method. However, alkali fusion method, being used to produce zirconium oxychloride, consumes large amounts of caustic soda, which is0.8t for It of zirconium oxychloride, and producing about6t spend lye and0.5t silica residue containing zirconium. For this reason, United States, Japan and Europe mainly import zirconium chemicals from China while reduce or stop producing zirconium oxychloride.Alkali fusion process of zirconium oxychloride production consumes large amounts of caustic soda and discharges spend lye and silica residue containing zirconium, resulting in serious environmental problems. In view of the problems, a novel decomposition process for zircon using mixed base of NaOH partly replaced by CaO was proposed in the present paper. The aim of this process is to reduce the amount of caustic soda and eliminate the pollution of spend lye and silica residue containing zirconium. The major results and conclusions are as follows:(1) Zircon sand decomposition with CaO-NaOHBy thermodynamic calculation, the possible products and theirs transformation were investigated in the decomposition process of zircon sand with CaO and NaOH, and the equilibrium compositions of the reactions were calculated by HSC thermodynamic calculation software. The primary thermodynamic calculation in ZrSiO4-NaOH system indicated that the rise of temperature is favorable for the reactions of ZrSiO4and NaOH at600-900℃, which equilibrium products are sodium zirconate and sodium silicate. The existing form of sodium silicate is related to the raw materials ratio of zircon sand and sodium hydroxide and reaction temperature. The products of sodium zirconate is really steady during600-900℃in ZrSiO4-NaOH-CaO system. In view of the equilibrium products, substitution effect may exist of zircon decomposition by calcium oxide for that of sodium hydroxide.Effects of reaction temperature, calcium oxide to zircon sand molar ratio, time and of zircon sand on zircon decomposition rate were studied. Approximately97%of zircon sand could be decomposed under the optimal reaction conditions of CaO/NaOH/ZrSiO4molar ratio of0.75:4.5:1,800℃,1h and the particle size in48-58um, and it reduced25%sodium hydroxide consumption in comparison with alkali fusion method. The product was characterized to be Na2ZrO3and Na2CaSiO4by XRD(X-ray diffraction) and SEM (scanning electron microscope), which was loose particle.TG-DTA (thermal gravimetric and differential thermal) analysis for the sample with CaO, NaOH and ZrSiO4molar ratio of1:4:1was recorded on thermal analyzer, with linear heating rate of10℃min-1from room temperature to1000℃, then the products were characterized by XRD. There is a exothermic peak during528-631℃. The reaction equation of NaOH, CaO and ZrSiO4is:ZrSiO4+4NaOH+CaO=Na2ZrO3+Na2CaSiO4+2H2O.According to the results of TG-DTA and XRD, the reaction process of ZrSiO4decomposition with CaO and NaOH was inferred as following:(1) the reaction occured first between NaOH and ZrSiO4. The reaction equations were:ZrSiO4+4NaOH=Na2ZrO3+Na2SiO3+2H2O, ZrSiO4+6NaOH=Na2ZrO3+Na4SiO4+3H2O. The products of Na2ZrO3, Na2SiO3and Na4SiO4swathed on the surface of zircon sand concentrate.(2) Then the reaction occurred between CaO and Na2SiO3and Na4SiO4. The reaction equations were: Na2Si03+Ca0=Na2CaSi04, Na4SiO4+CaO=Na2CaSiO4+Na2O.(3) that Na2O, which was generated by the reaction of CaO and Na4SiO4, increased the decomposition rate of zircon owing to its spread to the surface of zircon and playing in role in decomposition. The overall reaction equation of NaOH, CaO and ZrSiO4is:ZrSiO4+4NaOH+CaO=Na2ZrO3+Na2CaSiO4+2H2O.(2) Separation process of Zr with Na, Ca and SiThe separation process of Zr with Na, Ca and Si were studied. Sodium and calcium of Na2CaSiO4and Na2ZrO3were gradually leached by hydrogen ion with the pH value lower in aqueous solution. Sodium ion was entirely leached when the pH value was less than5. The leaching rate of calcium ion was above95%when the pH value was2. The separation process of Zr with Na and Ca can be carried out by controlling the pH value in the leaching process. Effects of liquid-solid ratio, acid leaching temperature, time and stirring intensity on leaching rates of sodium ion and calcium ion were investigated. Leaching rates of sodium ion and calcium ion were84%and78%respectively under the optimal leaching conditions of6ml·g-1(liquid-solid ratio),60℃,200r·min-1and40min.Effects of the initial concentration of hydrochloric acid, stirring intensity, leaching temperature on zirconium ion leaching rate were studied. The results show that the initial concentration of hydrochloric acid and leaching temperature have significant influence on zirconium ion leaching rate, whereas the stirring intensity has little effect. The leaching kinetics of fusion products with hydrochloric acid indicates that the shrinking core model with chemical reaction controlled process is most applicable for the leaching fusion products, with the apparent activation energy of64.52kJ·mol-1Zirconium oxychloride (ZrOCl2·8H2O), which ZrO2content was more than36wt.%, was prepared by liquid-solid separation, concentration and crystallization. The product conforms to Chinese standard of zirconium oxychloride for industrial use (HG/T2772-2004).(3) Recovery and utilization of Spent lyeEffects of liquid-solid ratio,water washing temperature, time and stirring intensity on Na2O recovery rate of fusion products were investigated. Na2O recovery rate was above50%under the optimal washing conditions of5ml·g-1, washing temperature90℃,30min and200r·min-1. The process of NaOH recovered from spend lye with SiO2removed by CaO precipitant was introduced, and the factors affecting SiO2removal efficiency were studied. Under the optimal removing SiO2conditions of2.5molar ratio of CaO and SiO2,90℃,30min and200r·min-1, the removal rate of SiO2was over95%. The product of NaOH solution can be used in alkaline fusion process of zircon sand by concentrating.According to the reseach results, the processing line of200,000tons spend lye has been built for NaOH recovery and utilization, which can produce about35,000tons of42wt.%NaOH solution. The product of NaOH solution can be used in alkaline fusion process of zircon sand. The environmental pollution of spend lye was eliminated by the production line, also which made an economic and social benefit.(4) Recovery and utilization of silica residue containing zirconiumThe comprehensive utilization process of silica residue containing zirconium was studied. Effects of the hydrochloric acid concentration, liquid-solid ratio, leaching temperature, time and stirring intensity on the recovery rate of soluble zirconium were investigated. The result shows that the recovery rate is above60%under the optimal conditions of60℃,0.5mol·L-1,4ml·g-1,30min and200r·min-1. Soluble zirconium solustion instead of hydrochloric acid can be used in the process of washing products to leaching sodium ion and calcium ion. The silica residue containing zirconium can be disassembled rapidly in alkaline solution, thus the zircon sand in silica residue would be recovered. Effects of the pH value, alkali dissolution temperature, stirring intensity and time on the recovery rate of zircon sand were investigated. The recovery rate of zircon sand was over98%under the optimal conditions of pH9,90℃,30min and200r·min-1. According to the chemical composition and the morphology of the recovered zircon sand, it can be used to produce zirconium oxychloride in fusion process.The main solid phase composition of silica residue containing zirconium is silica slag, which has large specific surface area and activity. And it can convert to sodium silicate in sodium hydroxide solution. Hydrated silicon dioxide precipitation can be produced by hydrochloric acid adding in the sodium silicate solution. Then the sediment would be converted into silica hydrated by dehydration.The optimum conditions of silica residue converted into sodium silicate in sodium hydroxide solution were ensured by the influence of temperature, sodium hydroxide mass and time investigated. The leaching rate of silica residue containing zirconium was more than73%under the optimal conditions. It analyzes the influence of the pH value on silica hydrated quality during sodium silicate solution decomposed by hydrochloric acid. Based on the exploratory experiment, the preparation process of silica hydrated with silica residue containing zirconium as raw material is feasible.