Fabrication and Improving Properties of Lightweight Al_(2)O_(3)-MgO Castables for L adle Working Lining

Three lightweight Al_(2)O_(3)-MgO castables were fabricated with tabular alumina or microporous corundum as the aggregates,reactiveα-Al_(2)O_(3)micropowder,tabular alumina powder,and fused magnesia powder as the matrix,calcium aluminate cement as the binder,and MgO ultrafine powder(d50=5.4μm)and Al(OH)3 ultrafine powder(d50=8.2μm)as additives.The influence of aggregates and ultrafine powders on the properties,including pore size distribution,heat conductivity,thermal shock resistance,and slag resistance of lightweight refractory castables was investigated.The results show that the incorporation of microporous corundum reduces the bulk density of Al_(2)O_(3)-MgO castables,and MgO and Al(OH)3 ultrafine powders further increases the proportion of micropores in castables,which is beneficial to reducing the heat conductivity,and improving the thermal shock resistance and slag resistance of castables.Additionally,MgO ultrafine powder and Al(OH)3 ultrafine powder increase the fluidity and the strength of castables.

Thermal Evolution of Al-containing Magnesia Refractories for RH Snorkels

Resin-bonded, Al-containing magnesia refractories have been an alternative of rebonded fused-grains magchrome bricks to be used as inner lining in the snorkels and lower vessel of RH degassers, having the advantages of being chrome-free and energy-saving(without burning), with a comparable performance. Microstructural investigations indicate that metal Al powder remains independent in the matrix after heating at 700 ℃ and 900 ℃, and Al_4C_3, Al_2OC and Al_4O_4C crystalline whiskers start forming from 1 100 ℃. The whiskers grow and connect into a network matrix with increasing temperatures,remaining up to 1 600 ℃. Comparison of used bricks from different snorkels indicates a strong whisker networks formation in the brick achieving long service life and superior performance, which is attributed to flexible network structure of stable and interwoven whiskers. On contrary, there is a strongly-oxidized layer on the hot face and almost no whisker found anywhere in the brick with the short service life. It is vital to form the bonding phase of whiskers at high temperatures,generating overall properties of high hot strength, high erosion resistance and thermal shock resistance, which are essential in the working conditions of RH snorkels.

Approaches to High-grade Sintered Magnesia from Natural Magnesite

It is hard to manufacture high purity,dense sintered-magnesia from natural magnesite in the prevailing process of calcining and sintering,for two reasons:a)impurity,and b)low sintering activity of coarse-grained magnesite.In this paper,95%MgO magnesite was used as the starting material and finally magnesia of high density(3.47 g/cm^3)and high purity(98%MgO)was obtained after the flotation purification and a three-step sintering process.An industrial scale of the beneficiation system of natural magnesite was introduced for magnesite purification.In lab experimental,the three-step sintering process was implemented by calcining,hydrating,re-calcining and sintering,which has proven the possibility of densified magnesia from natural magnesite.

Indispensability and Vulnerability of Magnesia-carbon Bricks for Steelmaking Process

The manufacturing techniques of magnesia-carbon bricks in China have been documented from raw materials,production(process and facilities)to performance and wear issues in the ladle.Magnesia-carbon bricks made of ordinary fused magnesia is the prevailing material used in the slagline of the ladle,but its service life is substantially lower than the bricks based on large-periclase-crystal fused magnesia.In two types of fused magnesia,the average values of periclase crystal size are in double for their difference.It is suggested that large-periclase-crystal fused magnesia should be used for manufacturing magnesia-carbon bricks for the slagline of the ladle by abandoning ordinary fused magnesia,in order to have a prolonged service life,increase the availability of the ladle and reduce the number of downtimes of the ladle.Free phenol in resin produced in China should be as low as that of resin made in Europe,to improve production environment and reduce smoke emission during the ladle preheating.There are large spaces to promote the productivity of magnesia-carbon bricks in China,with high intensity mixers and hydraulic presses.Expansion controlled magnesia-carbon bricks in the ladle depend on the optimized combination of sintered magnesia,Carbores and antioxidants as the matrix,to minimize the premature wear of vertical cracks and joint opening formed in the ladle lining.

A Retrospective Review of Aluminamagnesia-carbon Refractories

The residual expansion of in-situ spinel formation in using of alumina-magnesia-carbon(AMC)bricks monolizes the lining of steel-making ladles with the closure of their joints,which has been an effective solution avoiding washing out of the joints in ladle lining by the reduction of the penetration of liquid slag and molten steel.Alumina-magnesiacarbon refractories are overall reviewed,in terms of major raw materials,thermal evolution,corrosion and oxidation,and thermomechanical behavior,as well as type,addition and fraction of magnesia used.General commercial products contain 5%-10%MgO and 5%-10%C with a certain amount of metallic aluminum powder,which is believed to facilitate spinel formation at early stage of heating-up,although high magnesia containing AMC bricks are studied and used sometimes.With low ratio of Al_(2)O_(3)/C=12.9 and the carbon content of 6.4%C,AMC brick exhibits the highest corrosion resistance.It is important to determine the type,addition and fraction of magnesia used in AMC refractories for demonstrating high corrosion resistance and superior thermomechanical behavior.

Multifunctional Refractory-lined Vessel: Ladle Aggregate

A ladle primarily is a container used to transport molten steel from the steelmaking unit to the casting facility.The essential requirements are heat resistant,insulative and strong enough to hold molten steel up to 300 t,with a structure of multilayered refractory lining in a steel shell vessel.When a ladle is assembled with three-phase graphite electrodes,a ladle furnace forms,starting steel refining process with/without the vacuum tank degasser and RH circulating degasser,in order to meet the growing demand on high purity and higher quality steel.The working lining is under aggressive conditions that comprise the chemical reaction with molten steel,the severe corrosion of liquid slag,the disaggregation through oxidation,and the strong stress due to the impacting of the melts and gases and the effects of thermomechanical behaviors.Magnesiacarbon brick is one of the major materials,being indispensable for the slagline of a ladle furnace.Alumina-magnesiacarbon brick has played an important role to substantially increase the service life in the metal zone,from the early lining materials of high alumina and doloma bricks.The permanent lining must assume sufficient responsibility to allow finishing the process of the engaged charge in case of a failure in the working lining.The insulation layer must be as thin as possible in order to maximize the ladle’s capacity,and reduce the shell temperature for saving energy.In this issue,several integrant refractories are reviewed or investigated in order to compile a lining overview,and to contribute a prolonged service life under aggressive working conditions of the ladle furnace.

Refractories Utilizability for Slagging Gasifiers

Slagging coal gasification process became a highlight of coal chemical industry in China during the last decade. Refractory lining＇ s life of slagging gasifiers is one of the most critical factors for a cost - effective operation. The paper introduces current status of coal gasification in China, lining structure of slagging gasifiers and performance of refractory lining. It also summarizes the major factors impacting on refractory wear in slagging coal gasifiers in four Chinese chemical plants, based on ten years of industrial experience. The utilizability is discussed in terms of cost -effectiveness of high chromia refractories and possibility of the alternatives.

Insulating Permanent Lining of Calcium Hexaluminate Based Castable for 300t Ladle in Baosteel

Minimizing of heat loss of liquid steel through the ladle lining led to the integration of increasing insulation function of the permanent lining and high-quality insulating layer between the permanent lining and the ladle steel shell.In this paper,a study was made on physical and thermomechanical properties and corrosion resistance of calcium hexaluminate based castables.The chemical and physical properties of CA6 based castables fabricated in this work can fulfill general requirements of the permanent lining in ladle.The permanent linings of CA6 based castables offer superb thermal insulation and corrosion resistance,compared to conventional refractory systems of 300t ladle in Baosteel.Both functions of insulation and safety work effectively in whole process of ladle operation.

Innovative Lining of Unburnt Al_(2)O_(3)-MgO Brick for 300t Steel Ladle in Baosteel

The manufacture of clean steel needs high performance carbon-free bricks for ladle lining.Based on long term application experiences of the prefab Al_(2)O_(3)-MgO blocks,unburnt Al_(2)O_(3)-MgO brick has been developed by pressing a mix of alumina,spinel and magnesia as major raw materials and Al_(2)O_(3)-MgO gel powder as binder.In addition to low open porosity and high strength,the unburnt Al_(2)O_(3)-MgO brick shows superior corrosion resistance and thermal shock resistance to the prefab block.Field trials in a 300t steel ladle have indicated the residual lining of unburnt Al_(2)O_(3)-MgO bricks showed smooth surface without large spalling and seldom steel infiltration,consequently longer service life than the prefab blocks.The reliable,unburnt Al_(2)O_(3)-MgO bricks have more advantages of high efficiency,energy-saving and eco-friendly production.It will be one of the best options for ladle lining in the manufacture of high-purity steel.

Bonding Mechanisms of Basic Refractories for RH Snorkels

Magchrome bricks,as the inner lining of RH snorkels,have played a vital role in the operation of RH degassers for a long term.In chrome-free campaigns,resin-bonded,Al-containing magnesia bricks have been an alternative of magchrome bricks with a comparable performance in the last decade.It is important to have found whisker formation in the matrix of Alcontaining magnesia bricks above 1100°C and in the correlation to their high performance of RH snorkels.In this paper,the bonding mechanisms of both refractories are investigated to differentiate from other refractories.In magchrome bricks,the bonding modes of fused magchrome grains are characterized by the reactions between magnesia and chrome ore at different burning temperatures.At 1500°C,liquid forms around chromite grains.It is sucked into surrounding magnesia and a gap forms around chromite grains at 1600°C.Plenty of Fe2O3,Cr2O3 and Al2O3 have diffused from chrome ore into magnesia at 1670°C.A complete dissolution of the chrome ore takes place at 1750°C,with chromite precipitating entirely.In unburnt,Al-containing magnesia bricks,a dense network of whiskers is formed during heating,which is a prevailing bonding feature,instead of traditional particle growing and merging.It is believed that the whiskers are formed by vapoursolid mechanism since there is no liquid droplet observed at the tip of whiskers.In most stringent working conditions of RH snorkels,the bonding mechanisms are emphasized for their application,instead of chromia component.

Sintering Complexity of Magnesia-chrome Refractories

Magnesia-chrome refractories are appropriate to work under cyclic temperatures or atmospheric conditions because some oxides of chrome ore readily release oxygen(are reduced)upon heating and pick up oxygen(are oxidized)upon cooling or upon changing the atmosphere.They have been ideal lining materials for RH degassers for many years,despite challenging of Cr^(6+) disposal issue after using.The varieties of magnesia-chrome refractories depend on their raw materials and burning temperatures,which could be well demonstrated by their microstructures.But Indian chrome concentrate cannot be directly used in the production of magnesia-chrome bricks because of reducing sintering density.Silicate bonded magnesia-chrome bricks are produced with low purity magnesia after burning at relatively lower temperatures,resulting in liquid film forming as silicate bond around chrome ore particles.Direct bonded and rebonded fused-grains magnesia-chrome refractories are made of high purity magnesia or fused magnesia-chrome grains,forming euhedral and intergranular secondary chromite spinels as the main feature.The amount of secondary spinel increases with the rising burning temperature,leading to increasing hot modulus of rupture and overall hot properties as well.It is important to burn magnesia-chrome bricks under weakly reducing atmosphere.The oxygen content of burning atmosphere from 800℃ to 1650℃ would better controlled below 0.5%to increase the burnt strength and to avoid inner cracks and loose bonding of magnesia-chrome bricks,while the atmosphere contains 3%-6%O_(2) under most economical firing condition.