Crystallization Regulation Of Zirconium Oxychloride In Highly Concentrated Hydrochloric Acid And Enrichment Of Rare Earth In Mother Liquor

Zirconium oxychloride is an important basic raw material of zirconium.At present,it is mainly prepared by "one acid and one base" method,including alkali melting,water transfer,acidification,crystallization and other processes.The process has the characteristics of high energy consumption and heavy pollution.To increase the environmental impact,it is very important to achieve a one-step crystallization process of precise regulation.In addition,the mother liquor waste acid after crystallization contains a large number of zirconium,rare earth and other valuable elements,in the process of neutralizing the waste acid to form a large number of neutralizing waste residue,according to environmental protection requirements,need to be stored and disposed of,not only environmental protection disposal pressure,but also caused a waste of resources,to carry out waste residue source reduction is an urgent need.In view of the above problems,this paper mainly carried out research on the crystallization process regulation of zirconium oxychloride and the utilization of waste acid in mother liquor,studied the crystallization mechanism and regulation law of zirconium oxychloride,and provided theoretical and technological reference for the precise regulation of crystallization process and the preparation of high-quality zirconium oxychloride products.Meanwhile,for the crystallization mother liquor,the rare earth enrichment method in waste acid was developed to reduce the amount of neutralizing waste residue at the source.The main progress achieved is as follows:First,the main existing forms of zirconium oxychloride in hydrochloric acid solution were researched,which can provide a theoretical foundation for the nucleation mechanism of zirconium oxychloride.The existence forms of zirconium in solution were studied under the conditions of different zirconium concentration,hydrochloric acid concentration,heating treatment and standing time.The small angle X-ray scattering method was used to test the zirconium solution,the results indicated that the zirconium was mainly existed in polymer species in the solution.The degree of polymerization decreased with the increasing of hydrochloric acid concentration.When the concentration of hydrochloric acid is higher than 4 mol/L,zirconium exists as[Zr（H₂O）₈]₄⁺+monomer.The standing time affects the number of chloride ions in the polymer.Then,the solubility of zirconium oxychloride was measured and the crystallization mechanism of zirconium oxychloride was studied.The solubility and super solubility of zirconium oxychloride in water and acid solution was measured under the conditions of hydrochloric acid concentration of 5-9 mol/L and temperature of 20-60℃,and the solubility data and metastable zone width were obtained.Then,the process of nucleation and growth was observed by using probe type online imaging particle size analyzer and focused beam emission measuring instruments.The nucleation of crystals is explosive.With the decrease of temperature,acicular crystals of zirconium oxychloride crystals（with large length-diameter ratio）are formed continuously.These acicular crystals are then arranged and adsorbed which can form larger particles of acicular or columnar crystals.The crystallization process can be described as follows:zirconium monomers in solution are hydrolyzed and polymerized continuously,which can form nanocrystalline nuclei,the nuclei are continuously attached and connected to form the smaller size of acicular crystals,then the larger size of columnar crystals are further formed.The crystallization conforms to the non-classical crystallization mechanism.Finally,the process of zirconium oxychloride crystallization was investigated.The effects of hydrochloric acid concentration,cooling rate,stirring rate and impurity type on crystals were researched.The results showed that the concentration of hydrochloric acid mainly affects the number of crystal water,and zirconium oxychloride octahydrate is more easily formed when the concentration of hydrochloric acid is 6 mol/L.The cooling rate has little effect.The stirring rate is mainly influencing the crystal particles’size,and the larger particles of crystal was obtained at 50 rpm/min.About the impurities effects,Na+does not affect the crystallization of zirconium oxide chloride,Fe²⁺ and Al³⁺ will probably decrease the bonding ability between zirconium and water or chloride ions,Y³⁺will probably inhibit crystal growth,which resulting in the formation of tiny crystals.In addition,the method of selective precipitation and enrichment of rare earth elements from the crystallization mother liquor was proposed.After washing and drying,the content of rare earth in dry enrichment can reach more than 70%,and the recovery rate of rare earth could reach more than 90%.Through the above research,the crystallization mechanism of zirconium oxychloride was clarified,and the process regulation law of forming robust zirconium oxychloride crystals was obtained,which provided theoretical and technical reference for the crystallization process regulation of zirconium oxychloride in actual industry,and provided reference for reducing the acid consumption and environmental impact of acid fog emission during crystallization.The method of crystalline mother liquor of rare earth is beneficial to reduce the amount of neutralizing waste residue from the source and realize the comprehensive utilization of zirconium mother liquor.