| Basic pellets,which characterized with lower pollutant emission,less solid fuel consumption and less return rate compared with production of that of sinter,always demonsrtae excellent high temperature metallurgical properties compared with that of acid pellets,and slag rate,fuel consumption ratio and pollutant discharge level will get declined when used as raw material for blast furnace production practice.Strait grate process,which roasted basic pellets with gas as fuel,is the dominating rote for foreign enterprises.Cosidering coal as primary energy for china,domestic enterprises,especially independent enterprises,always adopt coal-fired grate-kiln machine to produce basic pellets.So it is necessary to carry out research for production of basic pellets and utilization in blast furnace by experiments and industry trials combined with fundamental physical chemistry principles of metallury.Based on TISCO's future blast furnace production planning and burden structure readjustment,this paper present the research on production and application of basic pellets by using self-produced iron concentrate,including roasting properties,firing parameters by grate-kiln process,swelling mechanism,effect of high proportion basic pellets on smelting thermodynamics by following means,such as theoretical analysis,model calculation,experiments simulation and industrial trials.The research results will provide theoretical foundation for promotion of grate-kiln process basic pellets and application of higher proportion basic pellets as blast furnace burden.Based on the establishment of thermal model by applying molecule theories,the research was carried out focusing on effect of basicity on types and content of complicated molecule during roasting process of basic pellet.After preparing basic pellets samples with wider range bascity with self-produced iron concentrate as raw material,the detection of types and content of complicated molecule and verification of calculation accuracy for thermal model were performed by applying XRD?SEM?EDS and Image-Pro Plus.The thermal model,which developed by using molecule theories,will provide pioneering means for further research of basic pellets roasting parameters and the effect of raw materials composition and roasting temperature on roasting process will also be predicted correctly.The research,which combined with thermal model and other experiments,was performed concentrating on effect of bascity on roasting mechanism of basic pellets.The research indicates that Fe2O3 micro-grains will recrystallized and form recrystallization bonds during roasting process for acid pellets,however for basic pellets,calcium ferrites and calcium silicate,which act as low melting temperature agent,will replace Fe2O3 micro-grains and become main bonding phase,the types of bonding phase depend on basicity of basic pellets.Calcium fayalite will presents in pellets with basicity less than 1.0,compound need-like calcium fayalite is spotted in pellets and calcium ferrites will replace calcium fayalite and become main bonding phase with basicity over than 1.0.Effect of basicity on swelling properties of basic pellets was carried out based on research of roasting and solidifying mechanism.The results indicate that independent Fe2O3 micro-grains and Fe2O3 micro-grains coated by calcium fayalite will exhibit different reduction rate,cracks will present at Fe2O3 particles while internal stress concentrate on certain point for basic pellets with basicity less than 1.0.For basic pellets with basicity over than 1.0,calcium ferrites,which acted as the main bonding phase with lower melting temperature,will exhibit higher reduction rate and form liquid phase and porosity,so internal stress was alleviated and swelling index is decreased remarkably during reduction process in blast furnace.Based on the results of lab-scale balling and roasting experiments,grate-kiln simulation test and industrial trials,the comprehensive experiments were caried out on the selsction of reasonable preheating and roasting parameters for production of basic pellets with TISCO's self-produced iron concentrate as raw materials.The results indicate that compression strength,total iron content,binary basicity will reach 3500N/pellet,62.3%and 1.0 respectively,while swelling index will get less than 15%with parameters as follows,updraft blast drying temperature being 230?,downdraft blast drying temperature being 420?,preheating temperature being 1160-1180?,roasting temperature being 1165-1175?.The technical indices of basic pellet could meet demands of large volume blast furnace.The investions were also finished on effect of proportion of basic pellets on indirect reduction for lump area burden in blast furnace by applying gas-solid thermal model established with principle of minmium free energy.It is found that ruduction index of lump area burden will decrease with increase of proportion for basic pellets,the mecanism lies in the changes of composition of iron oxides,gas-solid reducing conditions and equilibrium composition for lump area burden.The thermal model,which used for calculation of desilication reaction between hot metal and slag in blast furnace and silicon distribution coefficient,is also established based on principle of coexistence theory of ions and molcules.Effect of components composition in slag on silicon distribution coefficient was investigated and calculation results demonstrate desilication ratio precisely for each components and complicated molcules.The results indicate that silicon distribution coefficient is affected signiificantly by large and complicated molcules such as CaO·SiO2?2CaO·SiO2?CaO·MgO·2SiO2 and simple molcules such as CaO,MgO.Desilication reaction will progress smoothly as CaO content for basic pellets being higher than that of acid pellets when basic pellets replace acid pellet in blast furnace. |
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