| The iron ore oxide pellets have the advantages of high grade of TFe,uniform particle size,physical properties and good metallurgical properties,which are considered as high quality furnace materials.Binder is the necessary material for preparing iron ore pellets,and its physical and chemical properties directly affect the quality of pellets.The general application of bentonite binder will reduce the iron grade of pellet,reduce blast furnace yield and increase the energy consumption of blast furnace.However,the preheated and roasted pellets prepared by the widely studied organic binder have the problems of insufficient strengths and large quantity of dusts.Therefore,it is of great significance to develop a new type of high efficient binder.In this paper,based on the binding properties of different kinds of organic binders and inorganic additives,it is preferable to prepare nano-calcium carbonate based binders with carboxymethyl starch,carboxymethyl cellulose,guar gum and nano-CaCO3 as the functional components.FTIR,zeta potential measurement,contact angle measurement,SEM,AFM,adsorption capacity determination,viscosity measurements,TG-DSC,XRD and porosity determination were applied in this paper.DLVO theory calculation,adsorption kinetics,balling growth kinetics and oxidation kinetics were also conducted.Firstly,the interaction mechanisms of nano-CaCO3 and organic polymer with the surface of iron ore concentrate were studied in detail.Secondly,the contributions of organic polymer and nano-CaCO3 to the strength of the pellets were investigated.Thirdly,the effects of nano-CaCO3 based binder on snowballing of magnetite concentrate were ascertained.Fourthly,the effect of nano-CaCO3 on the oxidation of magnetite concentrate and the high temperature consolidation mechanism of magnetite concentrate in presence of nano-CaCO3 based binder were investigated.Lastly,based on all above investigations,the preparation technology of nano-CaCO3 based binder was optimized and the application of nano-CaCO3 based binder in binding two magnetite concentrates was examined.(1)With carboxyl or hydroxyl as the function group,carboxymethyl starch(CMS),carboxymethyl cellulose(CMC)and guar gum(Guar)were optimized as organic components,while nano-CaCO3 was optimized as inorganic components.The thermal weight loss of organic components was 90%,and 200-600 ? was the main weight loss temperature range.Nano-CaCO3 was characterized by calcite crystal shape,particle size of 60-80 nm,thermal decomposition temperature of 650-750?.The residue is CaO,which is benefit for blast furnace ironmaking and sintering of pellets.Nano-CaCO3 based binder is characterized by a node-type cross-linking structure.(2)CMS and CMC interacted with magnetite concentrate surface mainly through chemical interaction,electrostatic interaction and hydrogen bond by-COO-and-OH groups.They increase the hydrophilic ability of magnetite concentrate surface.The electrically neutral Guar adsorbed onto magnetite concentrate mainly through the hydrogen bond of-OH group.In addition to the van der waals and electrostatic interactions,there are positively charged active sites(-Ca(OH)+)on the surface of nano-CaCO3,which can interacted with negatively charged Fe-O-site of magnetite concentrate.This specific interaction can leads to a reduction of electrostatic repulsion force and an increase in adsorption of organic constituents.The adsorption capacity of CMS on magnetite concentrate is the largest,followed by CMC and Guar.The equilibrium adsorption data of organic components can be described by the Freundlich isotherm and the pseudo-second-order kinetic model.The addition of nano-CaCO3 does not change the isothermal adsorption model and adsorption kinetics model of organic components,but it can improve the adsorption capacity and adsorption rate of organic components.(3)The drop strength is mainly formed by viscous force,chemical interaction force and capillary attraction force.The order of contribution of the three forces to the drop strength is:viscous force>chemical force>capillary force.Nano-CaCO3 can promote the chemical interaction force and viscous force between organic components and magnetite particles,which ultimately improve the drop strength.The compressive strength of dry pellet is mainly formed by the adhesive force at the interface and binder cohesive force.Nano-CaCO3 enhances the compressive strength of dry pellet by promoting the adhesive force and binder cohesive force.(4)Increasing the dosage of binder will reduce the nucleation rate of iron ore concentrate,while increasing the moisture content of iron ore concentrate will increase the nucleation rate and the mass percentage of magnetite concentrate nucleated.Compared with bentonite,nano-CaCO3 based binder can reduce the nucleation rate.When the nano-CaCO3 based binder is used for snowballing of magnetite concentrate,as the water content increases,the diameter of the growing pellet gradually decreases,and the mass percentage of magnetite concentrate layered(MPL)first increases and then decreases.With the increase of the dosage of binder,the diameter of the growing pellet increases firstly and then decreases,while the MPL decreases gradually.Compared with bentonite,the diameter of growing pellet and the MPL are larger when using nano-CaCO3 based binder.The particle size of green pellet produced with nano-CaCO3 based binder is more uniform.The layering process of magnetite concentrate is well described by pseudo-first-rate equation.The rate constant in above equations can represent the layering rate of magnetite concentrate.(5)Compare with organic binder,the compression strength of preheated pellets and heated pellets increase by 100-200 N/p and 1100-1300 N/p respectively when using nano-CaCO3 based binders.Nano-CaCO3 is the main functional constituent to improve the strengths of preheated and heated pellets.Nano-CaCO3 does not change the mechanism function of the magnetite oxidation reaction,but increases the number of active sites participating in the oxidation reaction and increases the oxidation rate of magnetite.Nano-CaCO3 promotes the formation of recrystallization and solid bridge between the hematite particles,and the formation of the liquid phase and the slag bonds,which ultimately reduce the porosity of preheating and heating pellets.(6)Fine grinding process is used to produce the nano-CaCO3 based binder.The dosage of CMS is optimized as 0.10%of dry magnetite,while the sum of the dosage of CMC and Guar is above 0.05%.The minimum amount of nano-CaCO3 is 0.50%and the dispersant is preferably sodium hexametaphosphate.Compared with 2.0%bentonite,TFe grade of fired pellets produced with 0.70%nano-CaCO3 based binder is higher,with the increase of 1.04%of Nanfen magnetite concentrate and the increase of 1.01%of Dagushan magnetite concentrate.Meanwhile,FeO contents of two kinds of fired pellets are slightly lower than using bentonite.The metallurgical property of fired pellets produced with nano-CaCO3 based binder is quite the same as using bentonite. |
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