Study On The Release Mechanism And Extraction Mechanism Of Hexavalent Chromium In Ferrous Sulfate-reduced Chromite Ore Processing Residue (rCOPR)

Chromium salt,an important industrial raw material,plays a vital role in the national economic construction,but a large amount of chromite ore processing residue（COPR）is discharged during the production of chromium salt.However,COPR contains a large amount of Cr（Ⅵ）which is highly toxic and highly mobile.The treatment of COPR is a tough task of environmental protection in the world.Various approaches have been proposed to detoxify and immobilize COPR,including stabilization/solidifcation（S/S）,biologically mediated reduction,dry reduction and wet reduction.Among them,wet reduction is generally considered a suitable method and has been widely used to remediate COPR in the past few decades due to its simple process and easy equipment selection,especially the use of ferrous sulfate to detoxify COPR.However,the reduced COPR（rCOPR）will slowly release Cr（Ⅵ）during the deposition process.Many studies have shown that this is due to mass transfer limitations that the structurally incorporated Cr（Ⅵ）is slowly released from the mineral phases during the chemical reduction process,resulting in incomplete Cr（Ⅵ）reduction and residual Cr（Ⅵ）in the rCOPR.Moreover,rCOPR will slowly release Cr（Ⅵ）when COPR undergoes weathering reactions during the deposition process,with the risk of Cr（Ⅵ）transfer to the surrounding soil and water.Nevertheless,previous studies only evaluated the long-term stability of rCOPR.The chemical speciation and leaching behavior of Cr（Ⅵ）in rCOPR have rarely been studied.In addition,rCOPR need further detoxification in view of its dangers.In this paper,the host phases of Cr（Ⅵ）in rCOPR,the microscopic interfacial mechanism between Cr（Ⅵ）and its host phases and the leaching behavior of Cr（Ⅵ）were studied.The residual Cr（Ⅵ）in rCOPR was further extracted by NaHCO₃ under hydrothermal conditions,and the extraction mechanism in the process was studied.The main research contents and innovation points are as follows:（1）X-ray diffraction（XRD）,scanning electron microscope integrated with energy dispersive spectrometer（SEM-EDS）,spherical aberration corrected scanning transmission electron microscopy integrated with X-ray energy-dispersive spectroscopy（Cs-STEM-XEDS）and micro-X-ray fluorescence（μ-XRF）were used to study the phase composition of rCOPR and the host phases of Cr（Ⅵ）.XRD results indicated that there were various mineral phases in rCOPR,including brownmillerite,brucite,calcite,portlandite,hydrotalcite and ettringite.The results of SEM-EDS,Cs-STEM-EDS andμ-XRF indicated that ettringite was the host phase of Cr（Ⅵ）in rCOPR,and quantities of Cr may exist in brownmillerite,chromite and chromium hydroxide in the form of Cr（III）.The chromium hydroxide was a reduction product of Cr（Ⅵ）in the COPR reduced by FeSO₄·7H₂O,which was usually an amorphous substance and cannot be observed in the XRD pattern.The ability of ettringite to incorporate Cr（Ⅵ）is attributed to its structural properties.The channel structure enabled the relatively easy replacement of sulfate with chromate possessing similar structure and thermochemical radius to form a chromium-containing ettringite solid solution.（2）The release characteristics of Cr（Ⅵ）in rCOPR when rCOPR was eroded by Na₂CO₃,Na₂SO₄ and HCl were investigated by leaching experiments.The release mechanism of Cr（Ⅵ）was studied by XRD and SEM.The results revealed that Na₂CO₃,Na₂SO₄ and HCl promoted the release of Cr（Ⅵ）in rCOPR when rCOPR was eroded.That phenomenon was caused by the following three reasons:1)Na₂CO₃ can react with ettringite;2)HCl can destroy the stability of ettringite;3)the chromate in the ettringite structure can be replaced by the sulfate of Na₂SO₄ due to the smaller thermochemical radius,resulting in the release of Cr（Ⅵ）from the ettringite structure.The erosion of rCOPR by Na₂CO₃ and Na₂SO₄ not only promoted the release of Cr（Ⅵ）in rCOPR,but also facilitated the migration of Cr（Ⅵ）.In addition,due to the strong pH buffering of rCOPR,the erosion effect of HCl on rCOPR was significantly lower than that of Na₂CO₃ and Na₂SO₄.（3）The Cr（Ⅵ）in rCOPR was extracted by NaHCO₃ under hydrothermal conditions to detoxify rCOPR,and the potential extraction mechanism was further studied by means of XRD,X-ray absorption fine structure（XAFS）,Cs-STEM andμ-XRF.The results showed that NaHCO₃ can promoted the release of Cr（Ⅵ）from ettringite at room temperature,but Cr（Ⅵ）can not be completely extracted because partially released Cr（Ⅵ）will be incorporated in the crystal lattice of the formed CaCO₃.Subsequently,Cr（Ⅵ）co-precipitated with CaCO₃ to form a CaCrO₄ surface complex,which inhibited the growth of CaCO₃ crystals and formed nano-scale CaCO₃.NaHCO₃ can achieve complete extraction of Cr（Ⅵ）in rCOPR under hydrothermal conditions.This was ascribed to the fact that the introduction of hydrothermal conditions can reduce the supersaturation of CaCO₃ in the reaction system and effectively inhibited the incorporating of Cr（Ⅵ）into the crystal lattice of CaCO₃,thereby achieving the complete extraction of Cr（Ⅵ）in rCOPR and promoting the crystal growth of CaCO₃.In addition,the increase of hydrothermal temperature and hydrothermal time was beneficial to the crystal growth of CaCO₃,and promote the extraction of Cr（Ⅵ）.