Study On Pressure Oxidation Leaching Of Chromite Roasted By Microwave And Preparation Of Sodium Chromate

Chromium salts and related compounds are common chemical products and important basic materials for China's national economy.However,the current production process of chromium salt in China is mainly calcium-free roasting,and there are still problems such as low chromium yield,high energy consumption and chromium slag accumulation.Therefore,many new chrome salt production processes have been developed.Among them,the sodium-based liquid phase oxidation process has the most promising development with the advantages of almost complete leaching of chromium and low energy consumption.However,in order to achieve efficient leaching of chromium,the process requires a long reaction time under high pressure and high alkali concentration,and the reaction conditions are harsh.In this paper,microwave enhanced roasting chromite pressure oxidation leaching and sodium chromate preparation process were studied.Microwave roasting of chromite was carried out to study the ring-breaking mechanism of microwave on chromite structure;The chromite after microwave roasting was used for subsequent pressure oxidation leaching to achieve efficient leaching of chromium;It is also proposed that the intermediate product of the pressurized liquid phase oxidation process of chromite is oxidized by chromium hydroxide alkaline hydrogen peroxide to achieve the effect that the product is only sodium chromate and the chromium oxidation rate is high.The main conclusions are as follows:(1)The heating characteristics of chromite in the microwave field were studied.It is found that chromite can be rapidly heated in the microwave field.When the microwave heating is carried out at the power of 2.0 kw,the temperature can be rapidly increased to about 1150? within 5 min.Through comparative analysis,it is found that the change of physicochemical properties of chromite by muffle furnace roasting is far less than that by microwave roasting.Under the condition of 2.0 kw power,a large number of Cr2O3 characteristic peaks appear in the XRD pattern of chromite calcined at 800? by microwave.SEM analysis shows that a large number of cracks appear on the surface of chromite,and the structure is no longer dense.The strength and crushing energy of chromite are greatly reduced,and the grindability of chromite is obviously improved.After microwave roasting,the median particle size of chromite can be reduced from 51.156 ?m to 16.486 ?m by hand grinding in a mortar,which greatly reduces the particle size of chromite and is conducive to subsequent pressure oxidation leaching of chromite.(2)The pressure oxidation leaching of chromite ore,muffle furnace roasting and microwave roasting were compared.It was found that the chromium leaching rate was only 56.31%after 240 min of direct reaction with untreated chromite.The roasting of muffle furnace can effectively improve the chromium leaching efficiency.The pressure leaching reaction lasted for 240 min,and the chromium leaching rate increased to 74.68%.Compared with the former two,the pressure oxidation leaching of chromite roasted by microwave can greatly improve the leaching efficiency of chromium.Under the same reaction conditions,the chromite roasted by microwave can react for 150 min to make the leaching rate of chromium reach 96.15%.(3)The particle size of chromite ore powder needs to be below 300 meshes(about 50%of the distribution of raw ore powder),and the coarse ore powder needs further fine grinding.However,after microwave-enhanced roasting of chromite,the chromite can be used for pressure oxidation leaching of chromite after hand grinding in a mortar for 120 min,and the chromite leaching rate can reach 98.41%.The process parameters of pressure oxidation leaching of chromite roasted by microwave were optimized.Under the conditions of initial particle size of chromite(0?130 ?m),stirring speed of 700 rpm,reaction temperature of 240?,oxygen partial pressure of 2.2 MPa,alkali concentration of 60%(?)and alkali-ore ratio of 4:1,the leaching rate of chromium can reach 97.81%after 120 min.(4)The comprehensive analysis of microwave-enhanced pressurized oxidation leaching solution and leaching residue showed that Cr,Al and Si were dissolved into the liquid phase with soluble sodium chromate,sodium metaaluminate and sodium silicate,respectively,during the microwave-enhanced pressurized oxidation leaching process of chromite.Fe and Mg existed in the reaction residue in the solid state form of oxides and hydroxides Fe2O3 and Mg(OH)2.(5)The macroscopic kinetics of pressurized oxidation leaching process of chromite roasted by microwave was studied.It was found that the leaching process of chromium was controlled by interface chemistry.The leaching kinetics equation was' 1-(1-X)^(1/3)=k.The activation energy of the leaching process was 26.73 kJ mol-1,where k=Ae^(-26730/RT)and pre-exponential factor A=2.80 min-1.(6)The preparation of sodium chromate by alkaline oxidation of hydrogen peroxide was studied.It was found that under the conditions of reaction temperature 80?,molar ratio of chromium to alkali 1:2(stoichiometry),liquid-solid ratio 3.5:1,stirring speed 300 rpm and theoretical multiple of hydrogen peroxide 1.5 times,the oxidation rate of chromium could reach 99%after 90 min.The kinetics was also studied.The results showed that the preparation of sodium chromate by alkaline chromium hydroxide with hydrogen peroxide was a first-order reaction,and the activation energy was 17.57 kJmol-1.The kinetic equation was-ln(1-?)=kt,where k=Ae^(-17568.7/RT),A=17.201min-1.