| Blast furnace(BF)slag is a by-product of ironmaking process.The main chemical composition of BF slag is CaO,SiO2,MgO and Al2O3.The chemical composition of BF slag is similar to the natural ore and portland cement,and thus a good secondary resource.The tapping temperature of BF slag is 1400-1550?,which contains latent heat of 1260-1880MJ/t,corresponding to 43-64 kgce.Currently,BF slag is recycled in the filed of cement production,environment,agriculture and chemical industry.The latent heat contained in BF slag has not been recycled efficiently.The production of slag fiber using the BF slag is an effective approach to utilize both the waste material and its latent heat.In the process of slag fiber production,the BF slag may crystallize.The crystallization of BF slag results in the fracture of slag fiber and decreaseof strength.To clerify the crystallization behavior of BF slag,FactSage simulation,microstructure analysis and thermal state experiment were conducted.The minerals precipitation and transformation of BF slag during the cooling process were investigated.The initial crystallization temperature and the controlling phase of initial crystallization temperature were determined.In addition,the influence of basicity,MgO content and Al2O3 content on the crystallization behavior of BF slag were investigated,respectively.In order to control the crystallization behavior of BF slag,the BF slag was modified by adding iron ore tailings,fly ash and coal gangue-respectively.The crystallization behavior of the modified BF slags were experimentally investigated.The optimummodifier and its addition quantity were determined.Based on the investigation results,industrial tests were carried using BF slag modified with fly ash.The initial crystallization temperature of BF slag is determined as 1350?.The obtained initial crystallization temperature is lower than that of the FactSage simulation due to kinetic effect.The main mineralogical composition of BF slag includes gehlenite and akermanite,and a small amount of pyroxene,okenite,forsterite and calasite can be crystallized during the cooling process.The initial crystallization temperature is controlled by the crystallization of gehlenite and akermanite.The growth index of melilite and pyroxene are 2.07 and 1.89,respectively.The activation energy for the crystallization of these two phases are 324 and 384 kJ/mol.The crystallization of melilite is bulk nucleation and one-dimensional growth.The crystallization of pyroxene is between surface nucleation and one-dimension growth.The average growth rate of the minerals increased,and then decreased with the decrease of temperature.The highest growth rate can be obtained at the temperature of 1300?.The initial crystallization temperature and quantity of BF slag decreased with the increase of cooling rate.The critical cooling rate of BF slag is 6?/s.In the required viscosity region,the temperature of BF slag is 1337-1371?,which has the temperature interval of 34?.Considering the small temperature gap,the BF slag should be modified before using for slag fiber preparation.The initial crystallization temperature and the critical cooling rate of slag increased with the increase of basicity and MgO content,which is harmful for slag fiber preparation.The increase of Al2O3 content decreased those two parameters,and this is beneficial for slag fiber preparation.Based on these results,the basicity of the modified BF slag should be 0.8.The MgO content of the modified BF slag should be decreased,while the Al2O3 content should be increased.Adding iron ore tailings,fly ash or coal gangue decreased the initial crystallization temperature and increased the temperature interval of BF slag for fibre preparation.The optimum additions of iron ore tailings,fly ash and coal gangue are 20%,15%and 20%,respectively.With these additions,the suitable temperature interval for slag fiber preparation are122,131 and 116? respectively.The initial crystallization temperature of BF slag modified by adding 20%iron ore tailings or 15%fly ash is lower than 1000?.The initial crystallization temperature of the modified BF slag with coal gangue of 20%is lower than 1150?.The BF slags modified by adding iron tailing,fly ash and coal gangue have the properties of low initial crystallization temperature and wide temperature interval for slag fiber preparation.The industrial tests showed that the diameter of the slag fiber produced by the fly-ash modified BF slag increased with the increase of the fly ash content.The diameter of the slag fiber can not meet the national standard when the addition content of fly ash reaches to 20%and 25%.The addition of fly ash improved the forming rate of slag fiber,but increased the power consumption.The modified BF slag with fly ash content of 15%significantly increased the forming rate of the slag fiber,while,the power consumption increased slightly.In addition,the slag fiber produced using the modified BF slag with 15%fly ash can meet the national standard.Therefore,the modified BF slag with fly ash content of 15%is suitable for slag fiber production. |
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