Speciation Transformation And Stabilization Of Chromium During Heat Treatment Of Chromium-containing Tannery Sludge

Safe and effective disposal is the key to sustainable development of leather industry due to the fact that tannery sludge is rich in organic matter and heavy metal chromium.Heat treatment possesses the advantages of volume reduction,harmlessness and resource utilization,making it an important method for the disposal of heavy metal-containing organic solid waste.However,there is a risk that Cr(?)will be oxidized to Cr(?)in the incineration process of tannery sludge and an associating technology is need to inhibit the oxidation of Cr(?)to Cr(?).Considering the engineering application of the heat treatment of chromium-containing sludge and the complexity and heterogeneity of sludge,it is very important to identify and clarify the speciation transformation of chromium and the formation mechanism of Cr(?).At present,there are few reports and a lack of systematic research about the speciation transformation and stabilization of chromium during the heat treatment of tannery sludge.In this paper,TG,XRF,SEM,EDS,XRD,XPS,FAAS and TCLP analytical methods and thermodynamic calculation were used to study the effects of different oxygen supply conditions,temperatures and components on the transformation and stabilization of chromium during the heat treatment of tannery sludge.The oxidation mechanism of Cr(?)was obtained,and the effective methods to inhibit Cr(?)oxidation and realize chromium stabilization were proposed.The main results are as following:(1)Chromium-containing tannery sludge(Cr-ITS)used in this paper is rich in organic matter and inorganic salts,among which the contents of the ash and volatile are 49.86%and 43.11%,respectively.The main components of Cr-ITS were Cr2O3,CaO,Al2O3 and Fe2O3,as well as a small amount of SiO2,P2O5,MgO and TiO2,which all affected potentially the oxidation of Cr(?)during thermal treatment.In addition,Six steps sequential extraction procedure showed that Chromium in Cr-ITS existed in stable acid soluble and organically bound states,whose leaching toxicity was lower than the limit of chromium in Hazardous waste identification standard.(2)The microstructure,chemical composition,crystal structure and chromium valence state of Cr-ITS sintered products were systematically studied under the oxic,anoxic and N2 atmospheres.At the range of 300?1200?,a variety of chromium-containing crystalline phases like CaCrO4,MgCrO4,MgCr2O4,Cr5O12 and Cr2O3 were detected in the oxic sintered products of Cr-ITS;At the temperature range of 300?600?,the oxidation of Cr(?)was significantly inhibited under anoxic atmosphere with no chromium-containing crystalline phase being detected in the sintered products;The formation of Cr(?)could be avoided during Cr-ITS heat treatment under N2 atmosphere while Cr3C2 and Cr7C3 crystalline phases were observed above 700?.In summary,oxygen plays an important role in the formation and transformation of chromium-containing microcrystalline phase,and the oxidation of chromium requires the joint action of temperature,alkaline earth metal and external oxygen at a certain temperature.(3)Cr2O3 and simulated chromium-containing sludge(Cr-SS)were taken as the research objects,mixing with different oxides(CaO,SiO2,Al2O3 and Fe2O3),then were heated at 200?1200?.It is found that doping CaO significantly promoted the oxidation of Cr(?),and the oxidation rate of Cr(?)was positively correlated with the mixing amount of CaO.At 900?,the characteristic peak of CaCrO4 was the strongest,and the oxidation rate of Cr(?)reached the maximum level.With the temperature increasing,the characteristic peak of CaCrO4 gradually weakened,which was attributed to the formation of a large number of crystalline phases of Ca3(CrO4)2 and CaCr2O4;Cr(?)oxidation in Cr-SS-CaO/Cr2O3-CaO system was significantly inhibited by doping SiO2 above 600?,and the oxidation rate of Cr(?)decreased most at 900?;Al2O3 and Fe2O3 inhibited the oxidation of Cr(?)above 1100?.(4)With ZnO and TiO2 as dopants,the oxidation behavior of Cr(?),competitive formation of chromium spinel,immobilization and TCLP leaching toxicity of Cr(?)in the sintered products at 800?1200? were investigated.The results proved that doping TiO2 inhibited the oxidation of Cr(?)and promotes the formation of crystalline phase containing Cr(?).Furthermore,the simultaneous doping of ZnO and TiO2 in Cr-ITS could significantly inhibit the oxidation of Cr(?)and accelerate the conversion of Cr(?)to ZnCr2O4 spinel.The higher the temperature,the more conducive to the formation of spinel,so the higher the fixation rate of chromium,the less the amount of leaching of Cr(?).At 1000?,the fixation rate of Cr(?)was 99.85%and the 25 d leaching concentrations of Cr and Zn were 13.32 mg/L and 23.07 mg/L respectively,which were lower than their respective regulatory limits.Continuing to increase the temperature to 1 100?,the leaching toxicity of chromium and zinc continued to be significantly reduced,and the simultaneous stabilization of chromium and zinc was successfully achieved.Based on the above analysis,the influence rule of inorganic components on Cr(?)oxidation was obtained:the order of promoting Cr(?)oxidation is Ca>Mg>Zn,and the order of inhibiting Cr(?)oxidation is Si>Ti>Al/Fe.(5)The power plant fly ash rich in SiO2(54.96%)was mixed with Cr-ITS for synergistic heat treatment.The results showed that the content of Cr(?)in the product decreased with the increase of temperature,in particular,it decreases sharply at 1000?,with a decrease of more than 22%.As a result,the oxidation of Cr(?)was inhibited by the doping of fly ash.In the low temperature range of 600?1000?,the oxidation of Cr(?)was affected by the extension of incineration time and the increase of Cr-ITS doping amount.However,at the high temperature range of 1000?1200?,its effect on Cr(?)oxidation was significantly reduced.Higher incineration temperature gave higher immobilization rate of chromium and lower the leaching toxicity of TCLP.When Cr(?)is replaced by Si(?),the number of holes will be reduced thus the oxidation of Cr(?)could be inhibited.In addition,the anions generated by the dissociation of SiO2 at high temperatures would combine with Ca(?),Mg(?)and other metal ions to form a stable crystalline phases,which significantly weaken the promotive effect of Ca(?)and Mg(?)on Cr(?)oxidation.The co-incineration of fly ash and Cr-ITS formed aluminosilicate,MgCr2O4 spinel and uvarovite(Ca3Cr2(SiO4)3)microcrystals,which chromium to be doped in the crystal lattice or mineral microcrystals,thus realizing the stabilization of chromium in Cr-ITS.