Study On Preparation Of Chromium Salt Via Liquid-phase Oxidation Leaching Of Chromite

As one of the inorganic chemical products,chromium salt is an important pillar of the national economy.The traditional calcium-free roasting process has the problems of low chromium extraction rate and chromium residue pollution,which poses a huge threat to the environment and human health.The development of cleaner production processes for chromium salts is the key to solve the current problems.The liquid phase oxidation process of chromite is considered to be a promising chrome salt clean production process,which has the advantages of low energy consumption and high leaching efficiency.In order to improve the solid-liquid separation efficiency and alkali reuse rate of the leaching system and expand the production process of chromium salt products,this paper proposes a process for the preparation of chromium salt by chromite liquid-phase oxidation method.The multi-phase mass transfer process explored its flow field regulation mechanism and realized the efficient leaching of chromium;according to the properties of the mixed slurry after leaching,a method of thermal insulation settlement was developed to achieve the efficient separation of the solid-liquid system and improve the alkali Recycling efficiency.Aiming at the intermediate product of this process,barium chromate,a solution of hydrochloric acid to reduce the conversion of barium chromate to a trivalent chromium salt was proposed to realize the multi-directional product development of chromium salt platform products.In conjunction with the previous steps,a new process for the preparation of chromium salts by liquid phase oxidation of chromite was formed.This article has achieved the following results:?1?According to the needs of strengthening the pressure leaching process of chromite,a new dynamic and static combined stirring system was proposed.Under the same reaction conditions,the leaching rate of chromium is about 10%-50%higher than that of the traditional dynamic stirring blade.Systematic experiments were performed on the chromite oxidative liquid leaching process under a combined dynamic and static mixing system.The results showed that the reaction time was 240 minutes,the chromite particle size was less than 58?m,the speed was 800 rpm,the oxygen partial pressure was 2.4 MPa,the reaction temperature was 240°C,and the alkali concentration Under the conditions of 60%?wt%?And alkali-mine ratio?mass?of 4:1,the leaching rates of chromium and aluminum are more than 97%,and the leaching efficiency is extremely high.Kinetic studies show that the leaching process of chromium and aluminum is controlled by the step of the interface chemical reaction rate.The kinetic equation is 1-?1-X?¹3=kt.The apparent activation energies are 34.36 kJ·mol^-1 and 30.69 kJ·mol^-1,respectively,and the pre-finger factor are 7.1 min^-1and 3.41 min^-1.?2?The mechanism of the enhanced leaching process of the new dynamic and static combined stirring system was simulated and analyzed.The numerical simulation results of the flow field show that the asymmetry of the flow field distribution of the combined dynamic and static mixing system is enhanced,the turbulent kinetic energy is smaller,and the blade vortex is less,which effectively disrupts the orderly flow of the fluid and reduces"Spin"phenomenon,while reducing the energy dissipation in the reaction process,is conducive to the energy transfer in the stirred tank,thereby enhancing gas-liquid-solid three-phase mass transfer.?3?Aiming at the problem of solid-liquid separation in the chromite oxidative liquid leaching system,a solid-liquid separation method of thermal insulation settlement was proposed.The differences of three solid-liquid separation methods including dilution filtration,centrifugal separation,and thermal sedimentation in separation time,liquid alkali concentration,and dry-wet ratio of reaction slag were compared and studied.The results show that,compared with dilution filtration and centrifugation,the thermal sedimentation can increase the separation efficiency by 54%and 73%respectively,while ensuring that the alkali concentration after separation can reach the standard of direct circulating leaching,and it is found that chromium,aluminum,and alkali during the sedimentation process The change in the distribution in the liquid phase is conducive to the subsequent separation of chromium and alkali.The sedimentation speed is inversely proportional to the viscosity of the liquid phase.The sedimentation is complete at 90 min at 150°C with the shortest separation time.The particle sedimentation speed is 0.48 m/h and the liquid viscosity is 2.5Pa·s.?4?The response conditions were used to optimize the reaction conditions for the conversion of barium chromate intermediates by the hydrochloric acid dissolution reduction method.The results showed that the effects were respectively the ratio of the hydrochloric acid/barium chromate material>reaction temperature>liquid-solid ratio>reaction time.Taking the temperature,liquid-solid ratio,and hydrochloric acid/barium chromate material ratio as independent variables,the response rate model of chromium conversion rate was used to establish a response surface model.The obtained linear regression equation is:Y=84.92+5.48X₁-2.22X₂+21.23X₃+2.45X₁X₂-3.90X₁X₃+1.15X₂X₃+0.76X₁²+1.65X₂²-12.75X₃².Residual,variance,and response surface analysis all show that this response surface model predicts the chromium conversion rate of barium chromate dissolution reduction experiments more accurately.Under the conditions of a reaction time of 30 minutes,a reaction temperature of 95°C,and a hydrochloric acid/barium chromate material ratio of3.73:1 and a liquid-solid ratio of 8:1,the chromium conversion rate reached more than 99%.?5?Through cyclic experiments on the leaching process of chromite,it is shown that the aluminum content in the circulating leaching solution is always about 1.65 g,and the leaching rate of chromium in multiple cycles is about 96%,which has no obvious inhibitory effect on the leaching of chromium;Sodium silicate removes 96%of aluminum from the slag washing solution after heat preservation and sedimentation,and the generated sodium aluminosilicate has a higher utilization value.Compared with the existing calcium oxide and calcium hydroxide dealuminum methods,the efficiency is higher,better and more environmentally friendly;when the amount of ethanol is 3 times the theoretical amount,the reduction rate of Cr?VI?is 98%,the pH of the solution after the reaction is 0.75,and the pH of the solution is adjusted to 8.5⁹ with barium hydroxide to make Cr?III?Completely converted to chromium hydroxide precipitate.