Study On Preparation Of Zr Starting From ZrO₂ By Aluminothermic Reduction

Zirconium is widely used in industrial production and manufacturing industry because of its excellent machinability,corrosion resistance and nuclear performance,so it is known as the "first metal in the atomic age".The main way of industrial production of zirconium metal is still the traditional Kroll process,which has incomparable advantages:the process is very mature,but because of the relatively small demand for Nuclear-Grade zirconium and high-purity zirconium and the high requirements for product quality,Due to its production scale cannot achieve magnesium chloride cycle by Kroll-process,resulting in low economy,and the process of producing zirconium tetrachloride by chlorination at high temperature involves the use of dangerous gases,which is not completely applicable to the preparation of zirconium with special needs such as nuclear grade zirconium or high purity zirconium.Therefore,it is of great significance to develop a process of preparing metal zirconium by direct reduction of zirconia.Considering comprehensively,the metal reduction method is still used as the basic technological idea in this paper.Because the general metal reduction is carried out at the melting point of zirconium,the products are all powder metals.The flammability of metal powder will cause certain difficulty and danger to the next smelting and purification process.Therefore,the strong exothermic reaction between potassium chlorate and reductant as auxiliary heating agent is proposed to realize the self-heating of the reaction system,so that the system can complete the process of reduction,synthesis,melting and separation at the same time,and obtain bulk metal ingots.Reducing agent is aluminum with low cost and good reducibility.Because aluminum is milder than calcium and magnesium,the reaction process is not easy to gasify,which is a suitable reducing agent.By adding calcium oxide and calcium fluoride to make slag,the melting point and fluidity of slag were improved,good separation of slag and gold was achieved,and Al-Zr master alloy with less impurities were obtained.Based on the difference of saturated vapor pressure between aluminum and zirconium,electron beam melting technology was used to separate them at high temperature and high vacuum,and finally high purity zirconium metal was obtained.The mechanism of reduction reaction was studied by DSC,X-ray diffraction,Scanning Electron Microscopy(SEM)and Chemical Composition Analysis.The optimum technological conditions were screened through factor experiments.The phase,microstructure and composition of Al-Zr master alloy and refined product Zirconium metal were characterized.The following conclusions are drawn:(1)DSC differential thermal analysis showed that the intermetallic compounds mainly consisted of Al2Zr and Al3Zr were obtained when the reduction reaction took place at 700 ℃.The addition of slagging agent CaO not only has slagging effect,but also combines the composite oxides formed by aluminum dioxide in the slagging process,which improves the barrier effect of poor wettability of Al2O3 on the diffusion of Al,and reduces the activation energy from 4084.04 kJ/mol to 1572.3 kJ/mol,which greatly promotes the reduction reaction.(2)Al-Zr master alloy can be successfully prepared by thermite direct reduction.The yield of the alloy can reach 73.4%by smelting with Al2O3(66 wt.%)-CaO(34 wt.%)-CaF2(8 wt.%)slag and 3000 kJ/kg of burden unit heat effect.XRD phase shows that the main phases of the alloy products are Al3Zr and Al.The optimum oxygen content of the alloy product is 0.044%.(3)Al3Zr and Al-rich alloy alternately grow into lamellar alloys due to the huge temperature difference of primary crystallization.Al3Zr growth process makes the Al-rich liquid with low primary crystallization temperature sealed and the Al-rich region in the lamellae formed after cooling.As the temperature decreases,the solid solubility of zirconium in aluminum decreases sharply,resulting in the precipitation of zirconium from the interface in the aluminum-rich region.(4)During the refining process,a little splashing of molten pool is caused by the gasification of aluminum.The internal structure of the refined product is obviously different from that of the original alloy ingot.There are no voids and bright silver metallic luster in the product,and the density increases obviously.XRD phase validation shows that metal Zr is successfully prepared by electron beam smelting furnace de-aluminizing refining,and good de-aluminizing effect is achieved.The chemical composition analysis showed that the content of Zr was 97.44 wt.%,the content of O was 0.23 wt.%,and the content of N was 0.14 wt.%.From the point of view of demand and quality requirement of nuclear grade zirconium and high purity zirconium,the aluminum-zirconium alloy products with good separation of slag and gold are obtained by adding potassium chlorate as heating agent,calcium oxide as slagging agent and calcium fluoride as flux.The products with high zirconium content are obtained by electron beam smelting furnace,which is of great significance to the production of nuclear grade zirconium and high purity zirconium.