Research On The Thermodynamics And Sulfur Removal During The Desulfurization Slag Hydrothermal Leaching Process

The reutilization of desulfurization slag is rare since the high content of sulfur in the slag.However,the hydrothermal leaching method can effectively reduce the sulfur content in desulfurization slag.Therefore,it is essential to study on the thermodynamics of leaching slag and the dissolution behavior of components and sulfur during the leaching process,which has a great value on the recycling of desulfurization slag.Aiming at the mechanisms for hydrothermal leaching of sulfur from slag,the dissolution equilibria of multicomponent and influences of sulfur leaching,the changes of physical and chemical properties of the leaching solution and slag during hydrothermal leaching process.In this paper,changes of Eh-pH diagrams during hot water leaching process and dissolution equilibria of components in the leaching system were studied.Meanwhile,experimental research on desulfurization slag hydrothermal leaching was carried out.The research on the the Eh-pH diagram for Ca-S-H₂O system shows that the diagram is divided into 10 stable regions,the leaching reactions could occur spontaneously under normal conditions and it was advantage for the sulfur leaching into leachate in forms of S^2-,HS^-,H₂S,S,SO₄^2-and so on.With an increasing of temperature,HS^- will convert into S^2-or H₂S.Also,H₂ S,HS^-,S^2-will be oxidized to elemental sulfur or SO₄^2-directly.When the oxygen partial pressure increased,S^2-tends to be oxidized into SO₄^2-.With the decreasing of the activity of dissolved species,the elemental sulfur tends to be converted into other ions through redox reactions.At the same time,Ca?OH?₂ might be dissolved into Ca²⁺ easily.The lg[Me]T-pH diagrams for dissolved components in the desulfurization slag hydrothermal leaching system show that CaS?s?is easily soluble in water.When pH>11.4,CaS?s?dissolved to form Ca?OH?₂?s?.CaSO₄?s?is difficult to dissolve and its solubility is always lower than CaS?s?.So the oxidation of the slag has negative effects on sulfur removal.However,when the pH of leaching environment is controlled exceed 12.3,CaSO₄?s?is transformed into Ca?OH?₂?s?with pH increased.Al2?SO₄?3?s?is easily soluble and Al³⁺ is difficult to form a precipitate with sulfur in the solution.The experimental results show that,the leaching solution pH increased with time and remained at about 12.The calcium concentration of leaching solution is high.However,the aluminum in the slag is rarely dissolved and had little effects on the leaching of sulfur.The sulfur concentration in the solution is increasing with the leaching of slag,which indicated that hydrothermal leaching has a good effect on sulfur separation from slag.The effect of hydrothermal treatment for the composition of spent slag was tiny and the main constituents of the slag still were 3CaO×Al₂O₃,2CaO×SiO₂,12 CaO×7Al₂O₃ and CaO after hydrothermal leaching.But there were corresponding hydrates formed during the hydrothermal leaching process,and the lumps of slag were decomposed into small granular or flake particles.The size of particles decreased with increasing hydrothermal leaching temperature.