The Decolorization Of Industrial By-product Gypsum Based On Crystal Regulation And The Related Mechanism

In recent years,pollution source control and solid waste recycling have become the major strategic needs of the country.Industrial by-product gypsum as a kind of common industrial solid waste was generated by many industrial fields with a huge output.The ultimate treatment of IBPG was often direct landfill disposal with less than 50%comprehensive utilization rate,which not only causes the waste of resources but also results in secondary pollution.Therefore,the secondary utilization of industrial by-product gypsum is of great significance to the secondary utilization of solid resources and the reduction of environmental pollution.Red gypsum and phosphogypsum are two typical industrial by-products of gypsum.At present,the most common comprehensive utilization method is to replace natural gypsum as building material after pretreatment.The iron in red gypsum and organic matter of phosphogypsum will affect the whiteness of gypsum,further affecting its application in construction.However,there is no ideal decolorization method for industrial by-product gypsum,and there are still some problems of the previous method such as low iron removal efficiency,inability to remove organic matter,soluble phosphorus and fluorine at the same time.Previous studies have shown that heavy metals can be removed deeply from industrial solid waste via regulation of crystal phase transformation and growth under hydrothermal treatment.At the same time,it is the premise of effective separation of impurities to identify the occurrence state of impurities in solids,but the occurrent state of impurities in by-product gypsum is a lack of microscopic understanding.In this paper,the occurrence state of impurities in red gypsum and phosphogypsum and the interfacial interaction between impurities with mineral phases have been studied.Furthermore,different mineralizers were used to remove the impurities through hydrothermal control of crystal phase change,and the removal mechanism of the process was studied.The main research contents are as follows:（1）The phase composition of red gypsum and the occurrence state of iron were studied by XRD,SEM-EDS and other characterization.The effective removal of iron（≥99%）was realized and the red gypsum transformed into white anhydrous calcium sulfate powder through hydrothermal treatment with HCl as mineralizer and the whiteness change from 19.3 to 86.8.Meanwhile,the microscopic mechanism of iron removal is studied.The results show that the main phases in red gypsum are Ca SO₄·0.5H₂O and Ca CO₃,and iron is adsorbed on the surface or encapsulated in the crystal of calcium sulfate as the form of ferric hydroxide or ferric oxide.Furthermore,iron removal efficiency with HCl as mineralizer under the hydrothermal treatment with or without acid and room-temperature acid leaching was conducted,which showed that hydrothermal conditions of hydrochloric acid were essential for iron extraction.On the one hand,HCl accelerated the phase transformation from DH to AH,which promoted the release of adsorbed iron from the solid residue.On the other hand,it provided acidic conditions（p H<1）to transform the iron species from insoluble oxide or hydroxide into Fe³⁺,which release into the solution and easy to be removed by solid-liquid separation.（2）The main phase,phosphorus occurrence state and organic matter species of phosphogypsum were studied by infrared spectroscopy,Raman spectroscopy,XRD,SEM-EDS and GC-MS.It was shown that Ca SO₄·2H₂O is the main phase of phosphogypsum,phosphorus is incorporated in the gypsum residue,and organic matter is mainly alkanes and amides.Furthermore,K₂S₂O₈ was used to remove phosphorus and organic matter from phosphogypsum under hydrothermal conditions,and the phosphogypsum was transformed into white anhydrous calcium sulfate powder（the whiteness from 40.9 to 97）.The main mechanism of impurity removal via hydrothermal treatment with K₂S₂O₈ was described as followed:firstly,incorporated phosphorus was released in the dissolution–recrystallization process of the transformation of calcium sulfate dihydrate into anhydrous calcium sulfate controlled by H⁺;meanwhile,K₂S₂O₈ is activated by the hydrothermal condition to produce free radicals which oxidized the organic matter in gypsum,promoting the increase of whiteness of gypsum.