Study On Formation And Leachability Of Hexavalent Chromium In Cr-containing Corundum Based Calcium Aluminate Cement Castables

Refractory castables as the formost monolithic materials have an advantage over brick lining due to their easier installation,lower energy requirements and lower production cost.Corumdum based calcium aluminate cement castables is an important refractory materials,and are therefore increasingly being used in high-temperature industries.Cr₂O₃ is added to prolong service life by improving their corrosion resistance.Unfortunately,Cr?III?in chromium-containing refractories can transform into Cr?VI?which is toxic during service process.Cr?VI?can pose a serious threat to the environment and human health causing skin disease,cancer and so on.Cr?VI?in the spend refractory materials can solube in water,and easily contaminates soil and groundwater.For a long time,people have been indiscriminate that all the Cr-containing refractory castables after used will form Cr?VI?,but the formation,existence and control methods of hexavalent chromium in Cr₂O₃-containing castables during the service process.Therefore,the study on the formation and inhibition of Cr?VI?in Cr-containing corundum based castables have important practical guiding significance for promoting the harmless use of Cr-containing castables.Based on the problems mentioned above,the effect of Cr₂O₃ addition on corrosion mechanism of refractory castables was studied in the present work.The formation mechanism and existent form of hexavalent chromium were analyzed by simulating the service temperature of castables.Then,samples from different parts of a spent Cr-containing castables were collected from a local plant.And the presence of Cr?VI?compounds in different parts and effect of different leaching agents on Cr?VI?leachability were studied.Based on clearly understanding of the formation mechanism of hexavalent chromium in Cr-containing corundum based calcium aluminate cement castables,through studying the pure Al₂O₃-CaO-Cr₂O₃ system,the effect of different calcium aluminate cement on the formation and leachability of Cr?VI?were analyzed.And the effect of different existent forms of chromium on the formation and leachability of Cr?VI?were studied.On this basis,an environmentally friendly Cr-containing castables was designed.Based on the above research,the main conclusion can be drawn as follows:?1?The addition of Cr₂O₃ significantly improves the corrosion resistance ofcastables.The slag penetrations depth and corrosion depth decreased with Cr₂O₃addition.By simulating the service temperature,in the low temperature regime favors the formation of CaCrO₄,in the intermediate it forms Ca₄Al₆CrO₁₆.Samples fired at1500°C predominantly froms the?Al,Cr?₂O₃ solid solution and CA₆ phase.Leaching test show that samples treated at high temperatures exhibit very high concentration of Cr?VI?,all of which far exceed the limit values,except the sample treated at 1500°C.And the total Cr?VI?contents of the sample treated at 700°C is highest.?2?The actual Cr-containing castables in different temperature region of existent form of Cr and laboratory conditions are basically the same.Because the chemical composition of refractory castable after used is complex,it will affect the leach result.During the service process,spatter of slag will cause sample to contain reductive ferrous.When the acidic leach agent is used,the Cr?VI?will be reduced,thus affecting the evaluation of hexavalent chromium in sample.Therefore,the actual evaluation of hexavalent chromium in waste should be based on the chemical composition of the sample,so as to avoid misjudgment.?3?Cr?VI?-compoud is formed in the presence of alkali or alkali earth metal oxides under an oxidizing atmosphere.Independent hexavalent chromium or encased in soluble silicates can be leached in water or in an acidic conditions.For the Cr?VI?-containing refractory castables,the leaching kinetic process can be studied using the kinetic model of single block leaching.?4?When MgO was added to the Al₂O₃-CaO-Cr₂O₃ system,it was favored to form Cr?III?-containing spinel phase Mg?Al,Cr?₂O₄.With the increase of MgO content,the spinel phase increased while the Cr?VI?phase Ca₄Al₆CrO₁₆ gradually decreased.The leaching test also confirmed that MgO could significantly inhibit the formation of Cr?VI?.And Cr solid in the spinel phase could not be leached out in acidic or neutral leach agent,while Ca₄Al₆CrO₁₆ was significantly faster in acidic leach agent than in water.At the same time,all Cr?VI?phase in samples can be leached out regardless of the leach agent.?5?By changing the mass percentage of Al₂O₃/CaO in Al₂O₃-CaO-Cr₂O₃ system,it can be seen that it has an important influence on the formation of Cr?VI?.When the mass percentage of Al₂O₃/CaO is 1.5-2.3,Cr?VI?compounds will be formed.However,when it reaches 4.0,Cr?VI?-containing phase disappears completely,and only cement phase CA₂ and CA₆ are present.The chromium is solid in CA₆ in the form of trivalent,and Cr₂O₃ and CA₆ reaction will not form Cr?VI?.?6?The existent form of chromium has an important influence on the formation and the existence form of Cr?VI?.When it exists as free Cr₂O₃,it will react with cement phases to form CaCrO₄ in low temperature regime and Ca₄Al₆CrO₁₆ in middle or high temperature regime.But when it exists as the form of?Al,Cr?₂O₃ or Ca?Al,Cr?₁₂O₁₉solid solution,it can significantly inhibit CaCrO₄ formation,only in the middle temperature area?1100-1300°C?can be formed Cr?VI?-compound Ca₄Al₆CrO₁₆.?7?When Ca?Al,Cr?₁₂O₁₉ replaced Cr₂O₃ to add into castables,the erosion resistance and mechanical properties of refractory castables had little effect,but the formation of Cr?VI?could be significantly inhibited.The samples after sintering at different temperatures and leach according to standard leach test,which were lower than the landfill standards of the general industrial solid waste of category II stipulated by state.