Study On Cr Speciation In Chromite Ore Processing Residue And Selective Intensified Leaching Of Cr(Ⅵ)

Chromium is widely used in industrial manufacturing,and it is an important strategic resource.Chromite ore processing residue（COPR）is a hazardous waste produced in the production of chromate,which contains highly toxic Cr（VI）.Due to the difficulty in detoxification and large yield,the discharged COPR has caused serious environmental pollution.Pollution caused by COPR has long been a widespread concern worldwide,and it has become one of the focuses of pollution control in China.The main bottleneck of COPR treatment is that the encapsulated Cr（VI）is not easy to leach or be reduced,which makes the wet detoxification process generally less efficient.Relatively,there are problems such as high energy consumption in the dry detoxification process.There are two key issues in the study of pollution control of COPR.One is the material properties of COPR,especially the speciation of Cr（VI）,which is the basis of understanding the law of pollution and provides guidance for developing utility model treatment technology.The second is how to regulate COPR to achieve the selective separation of Cr（VI）,which is an ideal way to eliminate pollution and recover resources concurrently.Based on the above issues,this paper focuses on COPR from a typical lime-less calcination process,the material properties of COPR and Cr speciation in COPR were studied.On this basis,selective intensified leaching of Cr（VI）by using inorganic salts and CO₂ as regulators was studied.Then,achieve the detoxification of COPR with the aid of reductant.The main conclusions of this study are as follows:（1）The material properties of COPR were studied systematically,and the Cr speciation in COPR was investigated by means of spectroscopy,microscopy and chemistry.The results indicate that COPR is a hazardous waste with corrosivity and toxicity.COPR mainly consists of ten elements including O,Ca,Fe,Mg and Cr.Cr makes up 4.8%of COPR,of which 35.0%is Cr（VI）and the rest is Cr（III）.There are mainly ten minerals in COPR,including residual raw ore（spinel）,high-temperature phase（brownmillerite,periclase,spinel）,hydrated and weathered phase（hydrocalumite,stichtite,hydrogarnet,portlandite,brucite,calcite,dolomite）.In particular,hydrogarnet exists in two conformations.The crystallinity of COPR is not high,of which 56.7%is amorphous.For Cr speciation,24.2%,19.5%and 1.4%of Cr（VI）exist in brownmillerite,hydrogarnet and hydrocalumite,respectively.Nearly 50%of Cr（VI）is soluble in water and exists in amorphous phase,part of which is physically trapped in the solid phase.71.9%,25.4%and 2.7%of Cr（III）exist in amorphous phase,spinel and stichtite,respectively.The findings provide a basis for subsequent research.（2）Selective intensified leaching of Cr（VI）by using inorganic salts and CO₂ as regulators was studied.The results indicate that F^-,CO₃^2-,PO₄^3-,SO₄^2-and H₂CO₃ have intensified leaching effects on Cr（VI）.The mechanism of the four ions is metathesis,that is,they react with material in COPR to release the encapsulated Cr（VI）.For H₂CO₃ and its derived HCO₃^-,the mechanism includes acidolysis and metathesis.Extending reaction time,increasing salt dosage or CO₂ pressure,increasing reaction temperature and decreasing sample granularity are all beneficial to intensified leaching of Cr（VI）when use Na F,Na₂CO₃,Na₃PO₄,pressurized and supercritical CO₂ as regulators.For Na₂SO₄,excessive dosage and excessively low granularity are sometimes not conducive to leaching of Cr（VI）.The intensified leaching effect is ranked according to Na₂SO₄<Na F<Na₃PO₄<Na₂CO₃,while pressurized CO₂<supercritical CO₂.After salt leaching or carbonating,many minerals in COPR reduce or even disappear.Fluorite,calcite,hydroxyapatite and hydrated calcium sulfate are the reaction products of different salt leaching,respectively.Aragonite,calcite and dolomite are the reaction products of carbonating.Under the given reaction conditions of salt leaching（over 100 mesh,5 mol/kg,90℃,24 h）and carbonating（over 200 mesh,0.4/16 MPa,80/110℃,24 h）,the leaching amounts of Cr（VI）reach 9-12 g/kg and 11-12 g/kg,respectively.At this moment,all the neogenic minerals are free of Cr.（3）The detoxification process of COPR by leaching-reduction treatment was studied.The results indicate that it is difficult to make COPR reach the discharge standard by salt leaching,carbonating or reduction,but it is not difficult to make it via salt leaching or carbonating followed by reduction.Using ferrous sulfate as reductant,the leaching toxicity of treated COPR is in the range of 1-2 mg/L under the optimized reaction conditions.The better the effect of salt leaching or carbonating,basically the more favorable the subsequent reduction.The weight gain rate of treated COPR is basically less than 20%,and the minimum is only 10%.Cr makes up nearly 3%of treated COPR,of which Cr（VI）makes up 7-21%.The optimal number of reuse times of leachate from salt leaching is in the range of 5-9.The leaching toxicity of treated COPR increases but is still below the standard when reuse leachate.Pure hydrated magnesium carbonate（Ca makes up 0.55%）can be precipitated from the leachate from carbonating,and the recovery rate of Mg can reach 31%.All the leachates can eventually be reused in the water leaching process of chromate production.