Applied Fundamental Research On The Product Engineering Of The Cleaner Chromate Production Process By Sub-molten Salt

Chromium is the most scarce critical metal in China,and the comprehensive utilization and clean production of valuable components in chromite have become strategic needs of the nation.To solve the existing problems of the conventional process,such as low recovery of chromium,high energy consumption,a great deal of Cr-containing residue and serious environmental pollution,a cleaner production process of chromate based on potassium hydroxide sub-molten solution has been proposed by the Institute of Process Engineering,Chinese Academy of Sciences.Clean and efficient utilization of chromium resource and zero emission of Cr-containing residue have been highly ffulfilled during this processHowever,the products structure of the process needs further improvements due to the lack of chromic anhydride which is the main chromic salt product and the low-value added byproducts of aluminium and magnesium.This thesis focused on the production of chromic anhydride by acid decomposition of potassium dichromate,and the preparation of nano materials using the byproducts of alumumum and magnesium as well as their application in the adsorption of heavy metals in wastwater.The main innovative achievements were summarized as follows:?1?The decomposition of potassium dichromate using sulfuric acid and nitrate acid was both analyzed theoretically by investigating the phase equilibrium of the quaternary system KHSO₄-CrO₃-H₂SO₄-H₂O and analyzing the phase diagram of the quinary system K₂Cr₂O₇-KNO3-CrO₃-HNO₃-H₂O.The results showed that high-purity chromic anhydr:ide product was difficult to obtain because potassium hydrogen sulfate and chromic anhydride coexisted in the range of high H₂SO₄ concentration;while chromic anhydride had a separate phase region in the range of high HNO₃ concentration.The following experiments confirmed that higher-purity CrO₃ product could be obtained by HNO₃ method compared to H₂SO₄ method.The process route for the CrO₃ production by decomposing potassium dichromate using nitrate acid was designed based on the experimental results.?2?The effects of reactants concentration,stirring rate,acid feeding rate and cooling rate during the process were explored,and the process parameters were optimized.High-purity and granular CrO₃ crystals?CrO₃ content is more than 99.8%?were prepared after recrystallization at the original K₂Cr₂O₇ concentration of 800 g/L 65%HNO₃,?95%HNO₃:65%HNO₃?volum ratio of 2,stirring rate of 300 rpm,acid feeding rate of 1 mL·min^-1 and cooling rate of 0.1? min^-1,which is superior to the product standards.Additionally,the recovery of Cr could achieve to more than 95%by adjusting the acid concentration of the crystallization process using 95%HNO₃.?3?The Cr0₃ cryatallization process by HNO₃ decomposition of K₂Cr₂O₇ included reaction cryatallization period and cooling crystallization period.The kinetics of the reaction crystallization was investigated and the nucleation rate and growth rate equations were calculated as B=5.928x10¹¹ M_T^0.53N^1.96 Fm^0.76 and G=4.218×1O^-6m^0.74,respectively.High nucleation rate and low growth rate conformed to the characteristics of reaction crystallization.Complete Cr0₃ crystals with smooth surface formed by the nucleus aggregation during the cooling crystallization.?4?Nano y-AlOOH was synthesized with Al?OH?₃ precursor by hydrothermal method.Effects of concentration of Al?OH?₃ precursor,pH values,temperature and time were investigated,indicating that ?-AlOOH nano particles highly depended on these experimental conditions.The growth mechanism of nano ?-AlOOH was thus explicited as the dissolution of Al?OH?₃ precursor,the appearance of transformation phase a-Al?OH?₃,the redissolution of transformation phase,and the formation of?-AlOOH.The as-prepared nano y-AlOOH was used as adsorbent for tha adsorption of Cr???in caustic solution,and the adsorption isotherms and kinetics could be fitted by Langmuir model and pseudo-second order kinetics model,respectively.The maximum adsorption capacity of Cr???on nano y-AlOOH could achieve at 19.85 mg g"1.The adsorption mechanism was expressed as the exchange of-OH and Cr?OH?₄-and formation of complex bonds.?5?Nano MgO was synthesized by ultrasonic exfoliation method.Nano MgO with a BET surface area of 166.44 cm².g^-1 was obtained at ultrasonic frequency of 40 kHz,ultrasonic power of 150 W,DMF:MgO ratio of 600:1 and ultrasonic time of 2 h.The as-prepared nano MgO was used as adsorbent for tha adsorption of Se,and the adsorption isotherms and kinetics could be fitted by Langmuir model and pseudo-second order kinetics model,respectively.The maximum adsorption capacity of Se???and Se???on nano MgO could achieve at 103.52 mg ·g^-1 and 10.28 mg g-1,respectively.The adsorption mechanism was expressed as inner-sphere surface complexes(SeO₃^2-and SeO₄^2-)formation on the surface of the nano MgO.Further studies showed that high adsorption capacity for Se??/??in the presence of coexistent anions and efficient regeneratability of adsorbent by NaOH solution at certain temperature were observed.