Applied Fundamental Research On Precipitation And Separation Of Vanadate In The Process Of Extracting Vanadium

Vanadium is an important scarce resource and strategic metal with excellent physical properties and is widely used in metallurgy,chemical industry,aerospace and other fields.Vanadium is mainly derived from vanadium-titanium magnetite.The vanadium slag obtained after smelting of vanadium-titanium magnetite is the main material for extracting vanadium.The main technology for vanadium extraction from vanadium slag is high temperature roasting-water leaching-ammonium salt precipitation-vanadium pentoxide calcination technology,in which the vanadium precipitation is the key to the production and purification of vanadium products.Acidic vanadium precipitation technology has the advantages of fast vanadium precipitation rate and low dosage of vanadium agent,which is the mainstream vanadium precipitation method.The main process of acidic ammonium salt precipitation is to add ammonium salts such as ammonium sulfate under acidic conditions to combine the polyvanadate and ammonium ions to precipitate insoluble polyvanadate,thereby realizing the seapartion of vanadium with the impurities such as sodium,calcium,manganese,sulfate,and chloride ions etc.,further taking advantage of the characteristics that ammonium salts can be easily decomposed at high temperatures with no ammonium residued.Basic products such as vanadium pentoxide and vanadium trioxide can be obtained by calcining ammonium polyvanadate at high temperatures.However,this method has the following shortcomings,such as the lack of precipitation mechanism,intermittent operation,which leads to difficult control of the precipitation process,high content of impurities,unstable product quality,and high risk of ammonia nitrogen pollution in wastewater,etc.Aiming at solving the above-mentioned problems,this paper systematically studies the crystallization kinetics of acid ammonium salt vanadium product ammonium polyvanadate and the effect of impurities on the crystallization process,explores new vanadium precipitation methods,and starts the design of continuous vanadium precipitation processes.The main conclusions are as follows:(1)The solubility and crystallization kinetics of ammonium polyvanadate crystals were investigated.The results shows that the solubility in the sulfuric acid medium decreases first,then remain stable and then rises rapidly with increasing acidity.The solubility of ammonium polyvanadate in water with increasing temperature.Based on the classic nucleation theory,a series of primary nucleation parameters were obtained.The crystal growth mode of ammonium polyvanadate crystal was two-dimensional,and the crystallization rate equation was obtained by moment transformation method.(2)The effects of impurity elements such as sodium,potassium,chromium,and phosphorus on the crystallization induction period and product morphology of ammonium polyvanadate were investigated.The results show that sodium can inhibit nucleation,but has little effect on the morphology of the product;potassium can extend the induction period,and when its concentration exceeds 12g/L,a new phase KV₃O₈appears;chromium has almost no effect on the crystallization process.Phosphorus significantly affects the induction period and the morphology of the product.It not only adsorbs on the surface and inhibits crystal growth,but can also enter the product to form heteropoly acids.(3)A new vanadium precipitation process by using ethylene diamine as precipitation agent was proposed for the first time,which can operate at normal temperature and neutral condition,and eliminate the generation of acidic ammonia nitrogen wastewater at the source.The precipitation agent does not participate in the precipitation reaction can be recycled.Through orthogonal and conditional experiments,optimized technological conditions of vanadium precipitation were obtained.By analyzing the composition and structure of the precipitated product,the structural formula and thermal decomposition product of the product were inferred,and the reaction equilibrium constant was measured.(4)A crystallization model and reactor operation optimization parameters for a continuous precipitation process were proposed.A continuous mixed-flow reactor was designed,and it was found that different feeding positions had a greater impact on the residence time distribution of a single tank and a smaller effect on a two-tank reactor in series.The residence time distribution density function was calculated by fitting the1stopt software,and the distribution density function of multiple tanks in series was predicted accordingly,and the equation for predicting the reaction conversion rate using the residence time distribution was derived.At the same time,the process of continuous vanadium precipitation in a two-tank reactor in series was optimized.