| The stainless steel pickling sludge(SSPS)produced from pickling wastewater contains a variety of heavy metals e.g.Cr,Ni et al.during the surface treatment by lime neutralization precipitation technology,which affects the surrounding soil and surface water.It has been regarded as a toxic waste in the Chinese National Hazardous Waste List,and thus SSPS should be seriously treated.However,to recycle SSPS is very difficult because it has complicated components,high water content and the contents of each component cover large fluctuation ranges.Without any treatment,these components could turn into toxic substances in air or in soil,which can pollute the environment.On the other hand,SSPS is a valuable renewable resource because it contains many useful metals such as Fe,Cr,and Ni.Therefore,how to effectively realize the comprehensive utilization of SSPS to alleviate environmental pollution caused by SSPS and reduce the external dependence of Cr and Ni strategic resources is an important issue to be solved urgently in China.In view of this,this paper proposes a microwave radiation deep dehydration and coal-based smelting reduction process to achieve resourceful disposal of the SSPS.The conclusions have been obtained as follows:(1)Study on the electromagnetic parameters and temperature rising characteristics of SSPS in microwave fieldUsing agilent vector network analyzer,the electromagnetic parameters of SSPS were measured.The results showed that the wet pickling sludge belong to microwave absorber materials and the microwave absorbing capacity of the wet pickling sludge is stronger than that of the dry pickling sludge in a microwave frequency of 2450 MHz at room temperature.The temperature rising characteristics of SSPS in the microwave field were studied systematically through the reformed microwave equipment in laboratory.The temperature rising characteristics of dry pickling are as follows:first,temperature rises slowly to a turning point temperature,then rises rapidly.That is to say,the dry pickling sludge has good absorbing ability at high temperatures.The temperature rising characteristics of wet pickling sludge are as follows:first,temperature rises rapidly to about 110?,then rises slowly until reaches a turning point temperature after the water evaporation.The temperature rising characteristics of SSPS in microwave field show that the microwave heating technology can be used for a deep dehydration treatment of SSPS.(2)Deep dehydration of SSPS by microwave radiationThe effects of different microwave output power,sludge loading,sludge thickness and sludge size on microwave radiation dehydration rate of pickling sludge were investigated by the reformed microwave equipment in laboratory.The results show that with the prolongation of microwave radiation time,the dehydration rate of pickling sludge increases gradually.With the increase of microwave output power,the time required for pickling sludge to reach 100%dehydration rate decreases gradually.With the increase of sludge loading,the time required for pickling sludge to reach 100%dehydration rate increases gradually.When the thickness of sludge is less than 20 mm,the time needed to reach the 100%dehydration rate of pickling sludge is gradually shortened with the increase of the thickness.When the thickness exceeds 20 mm,the thickness has little effect on the dehydration rate.It takes the longest time for the sludge with a particle size of 3?6 mm to reach the 100%dehydration rate,and the shortest time for the sludge with a particle size of 8?10 mm to reach the 100%dehydration rate.Therefore,when using the microwave heating technology to dehydrate the pickling sludge in depth,the sludge size from 3?6 mm should be avoided,and the sludge size from 8?10 mm should be preponderant.The dehydration rate of pickling sludge experienced the process of accelerated dehydration,constant dehydration and decelerated dehydration.Response surface methodology was used to optimize the deep dehydration process of pickling sludge under microwave irradiation.The optimal process parameters were obtained as follows:the microwave irradiation time 350 s,microwave output power 616 W and sludge loading capacity 65 g.Under these optimum conditions,the dehydration rate of the pickling sludge can reach 95.95%after microwave irradiation,and the water content in the sludge is 1.76%,which meets the requirement of raw materials for blast furnace.The energy consumption for a ton of pickling sludge with microwave irradiation deep dehydration is 3.3170 kJ/g.Established by response surface methodology,the optimization of the microwave radiation surface pickling sludge deep dehydration process model is that:Y=93.75+8.84X,+20.42X2-5.55X3-5.88X1X2+1.28X1X3+4.39X2X3-3.05X12-11.78X22+0.37X32In the formula,X1 is the microwave radiation time,X2 is the microwave output power,X3 is the sludge loading,Y is the dehydration rate of pickling sludge.The deep dehydration kinetics of SSPS under microwave irradiation was studied.The R2 value of the Wang and Singh model is between 0.9755 and 0.9934,which is closer to 1 and its x2 is less than or equal to 0.0458,and RMSE is less than or equal to 0.00352.The microwave radiation dehydration process of SSPS can be explained by the Wang and Singh model.The relationship between the water ratio MR and time t of different power density m/P during the microwave radiation deep dehydration of SSPS was fitted.The activation energy of microwave radiation deep dehydration Ea was 28.4234 W/g,the pre-exponential factor A0 was 1.3611×10-4.(3)Experimental study on coal-based smelting reduction of SSPSIn the process of coal-based smelting reduction of SSPS and oolitic hematite,the thermodynamic conditions of carbon thermal reduction of Fe2O3,Fe3O4,FeO,NiO,Cr2O3 and complex metal oxides FeO·SiO2,2FeO·SiO2,2CaO·Fe2O3,CaO·Fe2O3,FeOCr2O3,NiO·Fe2O3 and NiO·Cr2O3 are good in the range of 1673.15-1873.15 K.It is possible to produce 2FeO·SiO2,2CaO·Fe2O3,CaO·Fe2O3,FeO·Cr2O3,NiO·Fe2O3 and NiO·Cr2O3 in complex metal oxides and it is impossible to produce FeO·SiO2.CaSO4 in stainless steel pickling sludge may be reduced to SO2 or CaS.Apatite 3CaO·P2O5 in oolitic hematite can not only be reduced to P2 or[P]by C,but also react with CaF2 in pickling sludge to form stable 3 CaO·P2O5·CaF2.However,apatite 3CaO·P2O5 in oolitic hematite is easier to react with CaF2 to form 3CaO·P2O5·CaF2.The "deformation temperature" of SSPS is 1154?,the "melting temperature" is 1207?,the "flowing temperature" is 1225? the "deformation temperature" of oolitic hematite is 1044?,the "melting temperature" is 1095?,and the "flowing temperature" is 1126?.With the increase of slag basicity,the "flow temperature" of the mixed samples of SSPS and oolitic hematite increases gradually,ranging from 1148? to 1216?.In order to maximize the recovery of valuable metals such as Fe,Cr and Ni from SSPS,to reduce the content of harmful elements such as S and P in recovered ferroalloys,and to reduce the cost comprehensively,the optimum technological parameters for coal-based smelting reduction of SSPS and oolitic hematite are as follows:the slag basicity is 1.5,the reduction temperature is 1550?,the reduction time is 90 min,the C/O amount of substance ratio is 2.0.Under the optimum reduction process parameters,the recovery rates of Fe,Cr and Niare 98.91%,98.46%and 99.44%respectively;the total Fe content in ferroalloys is 74.85%,the Cr content is 13.61%,the Ni content is 2.13%,the S content is 0.032%,and the P content is 0.3%;the ferroalloys can be used as raw materials for wear resistant white cast iron;the slags can be used as raw materials for cement production.The smelting reduction kinetics of apatite shows that the smelting reduction reaction of apatite is a first order reaction,the apparent activation energy Ea is 69.39 kJ/mol and the pre-exponential factor A0 is 1.3951. |
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