Research On Process And Mechanism Of Synergetic Preparation Of Zinc-Bearing Dust Metallic Pellets And Activated Carbon

The zinc-bearing dusts from steel plants which accounted for3%～4%of crude steel output were classified as hazardous waste and they are hard to be recycled due to their complex composition and high contents of heavy metals such as zinc and lead,as a result,large amounts of zinc-bearing dusts are stockpiled in steel plants and put huge pressure on them.In addition,the coal-based activated carbon adsorption method has been widely adopted by steel plants to purify flue gases generated in the process of iron and steel making.However,the production and application of coal-based activated carbon was limited by the long process,low capacity of the production process.Therefore,it is significant to develop an efficient and economical process to synergistically dispose of the zinc-bearing dusts and prepare activated carbon.Three kinds of zinc-bearing dusts from steel plant were used as raw material in this paper.The research was carried out to identify mineralogy and physicochemical characteristics of the zinc-bearing dusts.The pre-wetting and pelletizing process was optimized.The thermodynamic analysis of coupling process of direct reduction of carbon-containing zinc-bearing dust pellets and activation of carbon was conducted.The kinetic behaviors of reduction of iron oxides and removal of zinc,lead,potassium and sodium from the pellets were revealed.The process of coal-based direct reduction of carbon-containing zinc-bearing dust pellets was optimized.The phases and microstructure transformation of the pellets were clarified.On this basis,a process of synergetic preparation of zinc-bearing dust metallic pellets and activated carbon was developed.The mechanism of coupling effects between direct reduction of zinc-bearing dust and carbonization and activation of low rank coal was clarified.The catalytic effect of potassium and sodium on the activation process of carbon.The metallized pellets which can be used as a burden for improving blast furnace operation and the high-quality activated carbon which can be used as adsorbent for purification of SO₂ and NO-bearing flue gases were prepared by a one-step process.The high value utilization of solid waste resources and efficient production of activated carbon were realized.The study of pelletizing of zinc-bearing dusts indicated that the contact angle of zinc-bearing dusts would decrease while the hydrophilia would increase after the wetting process.When the zinc-bearing dusts were wetting for 10min with moisture of 10%and then balling for 16min with13.5%moisture,1%bentonite and 10%dust of coke dry quenching,the green balls with drop number of 19.1 times/0.5m,compressive strength of78 Newton per pellets and thermal shock temperature of 518°C,and dry pellets with drop number of 19.6 times/1m,compressive strength of 155Newton per pellets could be obtained.The thermodynamic analysis of coupling process of direct reduction of carbon-containing zinc-bearing dust pellets and activation of activated carbon demonstrated that there is coupling relationship between the gas-solid reaction of metal oxides and activation of carbon.Therefore,the gas-solid reaction of a metal oxide and activation of carbon possess the same initial reaction temperature.The isothermal kinetics study of carbon-containing zinc-bearing dust pellets in coal-based direct reduction process indicated that the reduction of iron oxides and removal of potassium and sodium were controlled by the chemical reaction while the removal of zinc and lead were controlled by the three-dimensional diffusion.The investigation on reaction model of direct reduction of carbon-containing zinc-bearing dust pellets indicated that the reduction of pellets followed the volume reaction model and the pellets possessed high reaction rate.When the zinc-bearing dusts pellets were reduced at 1200°C for50min with lignite as reductant and C/Fe mass ratio of 0.25,the mass contents of Fe,Zn,Pb,K and Na in reduced pellets were found as 73.6%,0.07%,0.02%,0.06%and 0.11%,respectively（removal rates of Zn、Pb、K and Na were 99.06%、97.88%、96.84%and 88.02%,respectively）.The reduced pellets with iron metallization degree of 90.7%and compressive strength of 1164 Newton per pellet can be directly used as a burden for enhancing blast furnace operation.A new process of synergetic preparation of zinc-bearing dust metallic pellets and activated carbon was proposed.The activated carbon with surface area of 300.65m²/g,iodine absorption value of 655.23mg/g,compressive strength of 255 Newton per granule,abrasive resistance of96.2%and the metallized pellets with compressive strength of 905 Newton per pellet,iron grade of 73.59%,iron metallization degree of 97.03%and low content of hazardous metals（0.015%Zn,0.008%Pb,0.017%K and0.039%Na）could be simultaneously prepared under the optimal conditions.The coupling reaction mechanism in the system was clarified.The carbon monoxide in the reduction process of metal oxides can be acted as activator in the preparation process of activated carbon.At the same time,the metal oxides could be reduced by the carbon monoxide generated in the activation reaction of carbon.The activation of the coal and reduction of zinc-bearing dusts were proceeded simultaneously through the coupling relationship.The mechanism of autocatalytic activation was revealed.9.21%of potassium and 32.88%of sodium from the zinc-bearing dust pellets could be adsorbed by the activated carbon.The potassium and sodium with strong reducing property could easily react with carbon dioxide and produce Na₂O,Na₂CO₃,K₂O and Na₂CO₃,which could further react with the carbon to produce carbon monoxide,potassium and sodium.The potassium and sodium played a catalytic role in the activation process.The sinter,oxidized pellets,lump ores and metallized pellets produced can form quaternary blast furnace burden to enhance blast furnace operation.When mass ratio of metallized pellets in quaternary blast furnace burden was set as 18%,the RI and RDI_+6.3mm of comprehensive burden could increase by3.79%and 6.88%,respectively,the initial softening and dripping temperatures could increase by 51°C and 16°C,respectively,while the melting drop range decreased by 46°C.When the activated carbon was used for separate desulfurization and separate denitration,the removal rate of SO₂ and NO could reach 97.5%and 75.7%,respectively,compared with the commercial activated carbon,the maximum removal rates of SO₂ and NO could increase by 13.5%and 14.3%,respectively.Figures 107,Tables 74,References 237...