Synthesis Of Boride Powders Via Induction Heating And Their Effects On Carbon Composite Refractories

Carbon composite refractories are widely used in Iron and steel industry because the introduction of graphite gives refractories excellent slag resistance properties and thermal shock resistance properties.However,the biggest drawback is their susceptibility to oxidation,which results in loose structure,thus damages the performances of materials.To improve the oxidation resistance of carbon composite refractories,the so-called antioxidants have often been used.Borides are pointed out as a noval additive and receives widespread attention in carbon composite refractories,because it can not only improves oxidation resistance properties and erosion resistance properties by the formation of a surface protective layer,but also reduce carbon loss by reducing CO into C,thus reduces porosity and improves mechanical properties.In addition,the types and features of borides also have a great influence on the properties of refractories.Therefore,the application of borides in carbon composite refractories has attracted extensive attention.At present,the synthesis of borides is mainly based on carbothermal reduction method,but the traditional heating method usually requires high temperature and long holding time,which leads to the waste of energy and high-cost.It greatly limits the application of borides in refractories.Induction heating,as a noval heating method,has received more and more attention due to the advantages of rapid heating rate,easily controlling,clean and environmental protection.In view of the above problems,in this thesis,CaB₆ and boride composite powders were firstly synthesized via induction heating combined with carbothermal reduction by using different raw materials.Then,these borides were introduced into carbon composite refractories?MgO-CaO-C refractory brick and Al₂O₃-SiC-C castables?,and their effects on phase composition,microstructure,mechanical properties,oxidation resistance and slag resistance were investigated.Finally,the oxidation kinetics and mechanism of the carbon-contained refractories with borides addition were revealed.The main conclusions can be made as follows:?1?CaB₆ powders were synthesized by combining induction heating with carbothermal reduction from a CaO-B₂O₃-C ternary system.Cubic single-phase CaB₆powders with an average particle size of 12.3?m were successfully synthesized at1700°C for 10 min in an Ar atmosphere;On this basis,CaB₆ powders were successfully prepared at 1650°C for 20 min by induction heating technology using low-cost natural colemanite and petroleum coke as starting materials.The final products obtained after HCl pickling,washing,and drying have good crystallization with a uniform particle size distribution.With the increase of the mass ratio of petroleum coke and colemanite,the phase composition of powders showed no significant change,but the lattice constant of CaB₆ decreased from 4.147 to 4.145?,and the distribution of particle size became narrower.By the present carbothermal reduction method under induced field,the reaction time can be obviously shortened.Trough the synergistic effects of high temperature and induced field,the liquid phase in the reaction system can be formed rapidly.This can obviously promote the mass migration and the crystallization of B₄C transition phase,leading to the rapid formation of the borides.?2?The composite powders of CaB₆-LaB₆,CaB₆-ZrB₂ and CaB₆-B₄C and ZrB₂-B₄C were synthesized via induction heating.Under the condition of induction heating,pure CaB₆-ZrB₂ and CaB₆-LaB₆ composite powders could be synthesized at1650?for 30 min in Ar;Proper B₂O₃ content and carbon content were beneficial for the synthesis of these composite powders.While CaB₆-B₄C composite powders could only be obtained at 1700?for 30 min.Excessive boron content can promote the synthesis of composite powders,while excessive carbon content were harmful.High-purity ZrB₂-B₄C composite powders with a median particle diameter of 3.07?m could be easily obtained via induction heating at 1400°C for 30 min in an argon atmosphere using ZrO₂,B₄C,and pitch powder as raw materials.?3?The effects of the introduction of CaB₆ and the composite powders containing CaB₆ on the anti-oxidantion,mechanical properties and slag resistance of the MgO-CaO-C refractories were studied.The results showed that the antioxidant property of MgO-CaO-C refractories could be significantly improved by adding a small amount of additives.The addition of composite powders could further improve the antioxidation of MgO-CaO-C refractories.The apparent porosity of the materials could be significantly improved by the introduction of CaB₆ and composite powders after carbonization at 1000?.The cold crushing strength after 1000?carbonization and hot modulus of rupture of the materials were also enhanced.The slag resistance test showed that the slag corrosion resistance and slag penetration resistance properties were enhanced simultaneously when CaB₆-B₄C and CaB₆-ZrB₂ composite powders were added.while the addition of CaB₆-LaB₆ composite powders showed no significant change in slag penetration and slightly improvement in slag corrosion.?4?The oxidation kinetics of MgO-CaO-C refractories containing CaB₆composite powders was investigated.The oxidation kinetics of MgO-CaO-C refractories with CaB₆ and composite powders added in air is basically similar.The oxidation process is controlled by two mechanisms:At the initial oxidation stage??100 min?,the oxidation is controlled by chemical reaction.In the late oxidation period??100 min?,due to the formation of the surface oxidation layer,the oxidation is controlled by diffusion reaction.With the increase of oxidation temperature,the oxidation rate constant shows an increasing trend,and the time of chemical reaction control becomes longer.The oxidation rate constant decreases with the increase of boride content.In the temperature range of 900¹200?,At the chemical reaction control stage,the oxidative activation energy of the refractories containing CaB₆,CaB₆-LaB₆,CaB₆-ZrB₂ and CaB₆-B₄C were 47.1,53.3,53.7 and 62.2 kJ·mol^-1,respectively;At the diffusion reaction control stage,the oxidation activation energy were 141.2,196.5,185.7 and 231.1 kJ·mol^-1,respectively.The results indicated that the introduction of boride composite powders containing CaB₆ further improved the antioxidation of the materials.?5?The application of ZrB₂-B₄C composite powders in Al₂O₃-SiC-C castables was investigated.The results showed that the bulk density,cold modulus of rupture,hot modulus of rupture,and anti-oxidation performance of Al₂O₃-SiC-C castables can be synchronously improved via the introduction of ZrB₂-B₄C composite powders.The improvement of hot modulus of rupture is mainly because ZrB₂-B₄C composite powders promote the development of columnar mullite crystals.The slag corrosion resistance of Al₂O₃-SiC-C castables was slightly impaired,but the slag penetration resistance was obviously enhanced.