Chromium(Ⅵ) Extraction From Calcium Carbonate Chromium Slag By Carbonation Treatment

Chromium is an important industrial raw material,widely used in chemical industry,electroplating,metallurgy,dyestuff and other industries,which also brings a large amount of chromium containing waste discharge.The annual emission amount of chromium residue in China has reached millions of tons.Hexavalent chromium[Cr（VI）]contained in chromium residue has the characteristics of high water solubility,mobility and carcinogenic mutation,which is easy to cause serious pollution of soil and groundwater.Calcium carbonate（Ca CO₃）is a common problem in chromium containing waste treatment.At present,the main treatment method of chromium residue is reduction.Although this method can effectively reduce the toxicity of the residue,there are also risk of secondary pollution.Chromium extraction technology can achieve detoxicfication and recover chromium resources from chromium residue,is an ideal treatment method.But the extraction methods at present can not achieve detoxicfication yet.Our team’s previous research shows that understanding the relationship between heavy metals and mineral phases from a micro perspective,and realize the efficient extraction of heavy metals by regulating the phase transformation of minerals.This article focuses on the key mineral Ca CO₃ in chromium containing waste,developed the method of carbonation phase transformation to extract Cr（VI）,and the chemical mechanism of treatment process was analyzed.The technology of extracting Cr（VI）by carbonation had been formed and applied to the treatment of chromate ore process residue（COPR）,a chromium containing waste with complex components.Finally,the engineering application of carbonation technology was studied.The main research contents and innovations are as follows:（1）The samples of chromium containing Ca CO₃ were synthesized according to actual production process.Based on density functional theory（DFT）calculation,combined with means of XRD,SEM,Raman spectrum,TEM-mapping,the exist state of Cr（VI）in calcium carbonate was analyzed.It was found that Cr（VI）was incorporated in crystal structure of Ca CO₃ in the synthesized sample,existing as state of structure doping.The method of carbonation（pressurized carbon dioxide）treatment can effectively separate Cr（VI）and reduce the leaching toxicity of the residue.In the process of phase transformation reaction,carbon dioxide can dissolve the large-size Ca CO₃and reprecipitate them into small size particles.The Cr（VI）which is doped in the crystal structure of Ca CO₃ was released into the liquid phase.Under the weak acid condition provided by carbon dioxide and water,the ion form of Cr（VI）changed and no longer doping into the structure of Ca CO₃.The long-term stability of raw and treated sample was evaluated by column leaching experiment and heavy metal release model fitting.It was found that the stability of Cr（VI）in treated sample was significantly higher than that of raw.（2）Carbonation method was applied to the treatment of COPR,an actual chromium slag containing kinds of magnesium and calcium minerals,for example,Ca CO₃,Ca（OH）₂,Mg（OH）₂,Ca₆Al₂（SO₄）₃（OH）₁₂?26H₂O,etc.Carbonation treatment can effectively separate Cr（VI）from COPR and achieve the goal of detoxification.In the reaction process,Ca CO₃,Ca（OH）₂,Mg（OH）₂,and Ca₆Al₂（SO₄）₃（OH）₁₂?26H₂O reacted with carbon dioxide and transformed into carbonate phase,forming a large number of small particles of Ca CO₃ in the treated residue.Cr（VI）was released into the liquid phase and separated from minerals.（3）The industrial application of carbonation method to the treatment of chlorate Ca CO₃slag was studied.The main components of the chlorate Ca CO₃slag were Ca CO₃ and calcium sulfate（Ca SO₄）,which was belong to hazardous waste according to the leaching toxicity.The results of Kilogram experiment showed that the Cr（VI）in Ca CO₃slag can be separated effectively by hydrothermal carbonation treatment,using sodium carbonate solution as mineralizer,and the leaching toxicity of the residue can meet the treatment standard.During the treatment process,Ca SO₄ phase disappeared,and the crystallinity of Ca CO₃ increased,Cr（VI）was released into the supernatant.The results of ton-scale experiment showed that the toxicity of three batches of treated Ca CO₃slag can all meet the treatment standard,and the average chromium extraction rate reached 90%,it can be used in engineering application.