Research On Mechanical Properties Of Carbon-bearing Pellets And Modification Of Its Thermal Strength

Nowadays,to develop the direct reduction ironmaking method is one of the most significant countermeasures to solve resource short of the high quality ore and coke,and to maintain the sustainable development of the China steel.The carbon-bearing pellet is a new type ironmaking material with the development of coal-based direct reduction.It provides an important menthod to produce of metallized materials and to utilize the complex iron ore and secondary resources containing iron.However,the raw materials of the carbon-bearing pellet is remarkably different from that of the synthetic high-grade ore,as a result,the strength of carbon-bearing pellet is difficult to meet the produce requirement of high-performance reactor.Consequently,the mechanical properties of carbon-bearing pellets had been analyzed systematically in the stages at cold and high-temp using iron concentrate powder and solid carbonaceous reducing agent(coal and coke)as main objects.And the thermal strength of carbon-bearing pellets was improved by regulation of gangue component and development of bi-component binding agent.The current study is expected to exhibit some theoretical and practical reference on the reduction of the carbon-bearing pellet.The interaction force between raw material particles in the carbon-bearing pellet was calculated by employing the equation of classical micromechanics.The effects of solid carbonaceous reducing agent and inorganic binder on the cold mechanical performance of the pellet were systematic studied.The results showed that the main cohesive forces between particles in the carbon-bearing pellet without binder are F_cap(capillary force),F_van(Van der Waals'force)and F_mes(meshing force),and the non-cohesive forces are F_gra(gravity)and F_ele(electrostatic force).The order of magnitude of the F_capand F_mesis about 10¹N,which is much larger than the others.The cold mechanical performance of the pellets is mainly based on the meshing force and capillary force.The weak of the cold mechanical performance of the carbon-bearing pellet is caused mainly by the introduced solid reducing agent,which reduces the meshing force between particles that is in the pellet.The cold mechanical properties of carbon-bearing pellet were improved by cementation of the binder,and the effects are related on the physical properties of the binder and its hardening mechanism.The thermal mechanical properties and changing mechanism of the carbon-bearing pellet at high temperature were analyzed with the help of TG/DTG-DTA,SEM-EDS and XRD.The results showed that the solid carbonaceous reducing agent addition has a significant effect on the thermal strength of carbon-bearing pellets.The trend of the strength of the carbon-bearing pellets was change from inverted V-type(reducing agent:0?8%)to N-type(reducing agent:18%)as the roasting temperature increases.When the C/O ratio equal to 1 is unchanging,the thermal strength trend of the carbon-bearing pellets was slightly influenced by the type of the reducing agent and heating rate,while remarkable differences can be obtained for the maximum value,the corresponding calcined temperature and the change rate of stage intensity.The study showed that the maximum thermal strength of carbon-bearing pellets generally occurs at 600??800?,while the thermal strength is the lowest at 1000??1100?.The carrying capacity of carbon-bearing pellet at high temperature is depended on the interaction between particles of the pellet and its inner structure.The changing of the strength can be divided into 3 stages during roasting.(1)Strength increasing slowly stage.The pellet strength is depended on the meshing interaction between the particles of pellet and binding force of the binder.The enhanced strength is relied on the cementing performance of the binder.(2)Strength decreasing rapidly stage.The pellet carrying capacity at high temperature was decreased rapidly because the meshing interaction between the particles of pellet and binding force of the binder were weaker.The forces that maintain the pellet strength are still the meshing interaction between the particles of pellet and binding force of the binder.(3)Strength recovery stage.The forces that maintain the pellet strength are the formation and aggregation of new formed phase which possess metallic bond and ionic bond.The bi-component binder was designed based on the action mechanism of the binder,the strength trend of the pellet and phase diagram theory.The study on the carbon-bearing pellet with bi-component binder showed that the strength of green ball is matched with the pellet containing bentonite when the proportion of binder is 2.5%,the maximum crushing force of dry pellet and roasting pellet are 35.49 N/Pellet and145.3 N/pellet,respectively,these values are much more than 16.98 N/pellet and 49.4N/pellet of the pellet with bentonite.The strength of pellet at room temperature was improved through the organic components and hydration reaction of the aluminate in the bi-component binder.In roasting process,the pellet hardening and consolidation were formed under the condition of persistence hydration of inorganic component,simultaneously,some CA and CA₂were formed by other components added into the binder in the solid reaction at high temperature.The formed phases intensify the formation of A₃S_2,decrease the destructive effects of the reduction reaction on the pellet structure,and improve the thermal strength of the carbon-bearing pellet.