Formation And Inhibition Mechanism Of Hexavalent Chromium In Chromia-containing Corundum Castables For Hazardous Waste Incinerator

Chromia-containing corundum castables are widely used in hazardous waste incinerator due to their good construction and mechanical properties and excellent slag corrosion resistance.However,Cr₂O₃ can be easily oxidized into highly toxic and water-soluble Cr(?)compounds Na₂Cr O₄,K₂Cr O₄,Ca Cr O₄ and Ca₄Al₆Cr O₁₆ under oxidizing atmosphere in the presence of alkaline oxides such as Ca O,Na₂O,K₂O,which causes serious environmental pollution.Recent researches have confirmed that calcium aluminate cement(CAC)binder can facilitate Ca Cr O₄ and Ca₄Al₆Cr O₁₆ formation.Moreover,the complex service condition also plays an important role towards Cr(?)formation.Furthermore,the limited availability of literature resources on hexavalent chromium,creates some roadblocks which otherwise would have been essential towards better understanding of the formation and inhibition mechanism of hexavalent chromium in chromia-containing corundum castables.It is therefore of great importance to investigate the formation and inhibition mechanism of Cr(?)during the preparation and service of chromia-containing corundum castables.In this research work,the valence states evolution of chromium in CAC bonded chromia-containing corundum castables was primarily investigated.Thereafter,pre synthesized(Al_1-x,Cr_x)₂O₃ solid solution and zeolite were used to improve the chemical stability of chromium and regulate the ratio of Ca O/Si O₂ in the matrix,respectively,which is potential towards the successful inhibition of Cr(?)formation during preparation of CAC bonded chromia-containing corundum castables.Secondly,in order to avoid the reaction between Cr₂O₃ and CAC to form Cr(?)compounds,hydratable alumina and Siox X-Zero were used as binders to prepare cementless chromia-containing corundum castables.Finally,the evolution of chromium valence states during service of chromia-containing corundum castables was not only evaluated by firing under simulated oxidizing/reducing atmosphere but also through static cup test method.The detailed research work includes:1)The effect of temperature on the valence states evolution of chromium and then the Cr(?)inhibition mechanism of(Al_1-x,Cr_x)₂O₃ solid solution in CAC bonded chromia-containing corundum castables were revealed.2)The effect of zeolite on valence states of chromium in CAC bonded chromia-containing corundum castables was investigated.The adsorption mechanism of zeolite addition on Cr(?)at room temperature and the inhibition mechanism at intermediate and high temperature were revealed.3)The chromium valence states evolution and the inhibition mechanisms of hydratable alumina and Siox X-Zero binders on Cr(?)in cementless chromia-containing corundum castables were investigated.4)The effects of oxidizing/reducing atmosphere and slag composition on chromium valence states were respectively investigated.The above investigation provides theoretical guidance for the harmless application of chromia-containing corundum castables in hazardous waste incinerator.Based on the above research work,the following conclusions can be drawn:1.Heat treatment temperature has a significant effect on the valence states of chromium in CAC bonded chromia-containing corundum castables,which change with an increase in temperatures according to the following order:Cr(III)?Cr(?)?Cr(III).Cr₂O₃preferentially reacts with CA from CAC to form Cr(?)compounds Ca Cr O₄ and Ca₄Al₆Cr O₁₆ at 300 ^oC-1100 ^oC and 900 ^oC-1300 ^oC,respectively,while dissolves in Al₂O₃to form Cr(III)compound(Al_1-x,Cr_x)₂O₃ solid solution(Cr³⁺)at 1500 ^oC.2.(Al_1-x,Cr_x)₂O₃ solid solution is pre-synthesized by Cr₂O₃ dissolving into Al₂O₃,which improves the chemical stability of chromium.The pre-synthesized(Al_1-x,Cr_x)₂O₃solid solution can dramatically inhibit Ca Cr O₄ formation at 300 ^oC-1100 ^oC and partially inhibit Ca₄Al₆Cr O₁₆ formation at 900 ^oC-1300 ^oC.The inhibition performance of(Al_1-x,Cr_x)₂O₃ solid solution is negatively correlated with Cr₂O₃/Al₂O₃ ratio,which shows the highest Cr(?)inhibition rate of 98.1%.Besides,Cr₂O₃ and CAC react with Al₂O₃ to form(Al_1-x,Cr_x)₂O₃ solid solution and CA₆ respectively after pretreatment at 1500 ^oC,which completely inhibits the formation of Ca Cr O₄ and Ca₄Al₆Cr O₁₆ during the secondary heat treatment and shows highest Cr(?)inhibition rate of 99.1%.3.Zeolite can dramatically decrease the Cr(?)concentration in chromia-containing corundum castables with CAC.Cr(?)concentration is lowered by zeolite due to physical adsorption with highest inhibition rate of 25.3%at 110 ^oC,while inhibition of Cr(?)compounds to form phases Ca Si O₃ and Ca₂Al₂Si O₇ at 500 ^oC-1300 ^oC.Ca O-Al₂O₃-Si O₂glass phase leads to the formation of more(Al_1-x,Cr_x)₂O₃ solid solution at 1300 ^oC-1500 ^oC.The inhibition rate of Cr(?)increases with the increase in zeolite content and a remarkable Cr(?)inhibition rate of 96.3%can be achieved.4.A drastic Cr(?)inhibition occur by substituting CAC with hydratable alumina and Siox X-Zero binders in chromia-containing corundum castables.Cr(?)compounds are inhibited at 500 ^oC-1300 ^oC by Al₂O₃ and Si O₂ present in hydratable alumina and Siox X-Zero binders.Both of these binders also promote the formation of(Al_1-x,Cr_x)₂O₃ solid solution at 1500 ^oC,which leads to the inhibition rate of Cr(?)up to 99.9%and a lower Cr(?)concentration than the limits of EPA and China in cementless chromia-containing corundum castables(1100 ^oC-1500 ^oC).5.The valance states of chromium vary with service atmosphere in chromia-containing corundum castables with CAC.Cr₂O₃ is oxidized to Ca Cr O₄ and Ca₄Al₆Cr O₁₆in air at 500 ^oC-900 ^oC and 900 ^oC-1300 ^oC,respectively,while the surface of trace Cr₂O₃particles is reduced to Cr₃C₂ or Cr₄C without any Cr(?)compounds and CAC reacts with Al₂O₃ at 700 ^oC-1300 ^oC to successively form C₁₂A₇ and CA₂ in coke bed.Therefore,the Cr(?)concentration exceeds in air at 500 ^oC-1300 ^oC the limits of EPA and China,while in coke bed is lower than 27.4 mg/kg(below both limits).6.The valence state of chromium is closely related to slag composition in hazardous waste incinerator.Ca O,K₂O and Na₂O in high basicity slag(Base/Acid ratio>1)can promote the formation of Cr(?)compound Ca₄Al₆Cr O₁₆ at high temperature,thereby,increasing Cr(?)concentration.However,Si O₂,Al₂O₃ and Ti O₂ in acid slag(Base/Acid ratio<1)drastically decrease the Cr(?)concentration due to chemical reduction of Ca₄Al₆Cr O₁₆.Besides,Fe Cr₂O₄ and Fe(Al,Cr)₂O₄ spinel are formed by the interaction of Fe O from slag and Cr₂O₃/Al₂O₃ from castables,which improves the stability of chromium.The Cr(?)concentrations in original,slag and penetration layers of hydratable alumina bonded chromia-containing corundum castables are lower than the limits of EPA and China.