Fundamental Research On The Cleaner Preparation Process Of Basic Chromium Sulfate

Basic chromium sulfate,as a valuable chemical product,is tranditionally prepared by alkaline roasting technology,which generates Cr(?)-bearing substances causing serious environmental pollution in worldwide.Acid leaching of chromite in the presence of oxidant is regarded as a promising option to produce chromic salt in a cleaner way.However,some work still needs to be done on leaching condition optimization,chromium-rich sulfate precipitation control,and deep separation of chromium and iron to accelerate the development of this process.Based on the summary of published work,the investigations focused on the decomposition mechanism of chromite,kinetics model establishment,chromium-rich sulfate precipitation control,effective separation of chromium and iron,and reducing roasting pretreatment were carried out in this study.Furthermore,a Cr(?)-free process for basic chromium salt preparation from chromite was proposed.The main conclusions were obtained as follows.(1)The extraction yield of chromium reached 93%when the leaching process conducted in the optimum leaching conditions obtained in this study:leaching temperature is 160?,duration is 60min,sulfuric acid concentration is 80%,oxidant/chromite ratio is 0.08,agitation speed is 300r · min-1.(2)During the sulfuric acid leaching process,silicate phase in chromite transformed to an amorphous silica phase preventing the internal spinel phase from leaching.As for the spinel phase,corrosion firstly occured at the crystal boundary and developed along with it in a groovy form with leaching duration.It was speculated that the lattice stability of spinel could be reduced by the oxidization of Fe(?)exists in interstitial void.(3)Sulfate could precipitate from solution during the leaching process because of the local supersaturation of metallic ions,resulting in serious chromium loss and pollution issues.By optimizing the leaching conditions,the main phases of residues were controlled to be amorphous silica and unreacted spinel,so this problem could be successfully solved.(4)A kinetics model was established as 1/(1+B)ln(1-B?)/(1-?)=Kt(0??<1),where B=3.48,which was proved to describe experimental finding very well.It was also found that the leaching process was controlled by chemical reaction under the employed conditions and the activation energy of the reaction is 48kJ·mol-1.(5)Aiming to separate iron ions from the leachate,treatments of goethite,oxalic acid,and liquid-liquid extraction were conducted,respectively.Results revealed that iron and magnesium content was reduced to a low level by forming oxalate,and no chromium loss was found in oxalic tests.Liquid-liquid extraction was proved as the most promising approach for iron separation from leachate owing to its excellent extraction efficiency.(6)A new chromic salt production technology was proposed in this work,and some industrial basic sulfuric acid products and valuable by-products were prepared in a Cr(VI)-free way.Moreover,all wastes generated in the whole process met the requirements of discharge standard of China.This work provides some theoretical foundations for the acid leaching process design and industriallation,and promotes the development of cleaner technology for chromium salt preparation.