Lamellar hydrates of CAC were designed with the introduction of Mg -Al hydrotalcite (M -A -H), and the effects on the early setting behavior, demolding strength, pore structures, mechanical properties, and fracture behavior of alumina-spinel castables were investigated. The results showed that Mg -Al hydrotalcite stimulated rapidly the hydration of CAC and the formation of lamellar C2AH8 and C4AcH11 when curing at 25 and 40 degrees C. In comparison, CAH10 and C2AH8 were detected without M -A -H and were transformed completely into C3AH6 at 40 degrees C. The formation of lamellar C2AH8 and C4AcH11 would contribute to a more complicated pore structure, especially in the range of 1-10 mu m. Meanwhile, the incorporation of MgO from M -A -H also regulates the distribution of CA6 and spinel (pre -formed and in -situ). Consequently, the optimized microstructure and complicated pore structure can induce the deflection and bridging of cracks, thus facilitating the consumption of fracture energy when testing at 1400 degrees C.
Replacing heavy oil with petrol coke can greatly reduce the cost of glass production,but obviously shorten the service life of refractories used in the regenerator checker body of glass tanks.To prolong the service life of the regenerator checker body,the slag chemical composition and alkali-sulfur ratio of glass tanks after using petrol coke and the damage mechanism of the residual magnesia bricks in the regenerator checker body were studied,as well as the corrosion resistance of three magnesia based bricks(direct bonded magnesia chrome bricks,fused rebonded magnesia chrome bricks,and fused rebonded high-purity magnesium aluminate spinel bricks).On this basis,a series of targeted countermeasures were adopted to optimize the configuration of refractories,significantly improving the service life of checker bricks and meeting the requirements of glass industry development.
To meet the emission standard of nitrogen oxides(NOx)in the flue gas of batch furnaces through dry adsorption,a calcium-silica inorganic adsorbent was prepared with limestone and quartz as raw materials.Sample Cu-BTC 1# was obtained by solvothermal synthesis,drying and purification in vacuum at 120 ℃ using trimesic acid(H3BTC)and copper nitrate trihydrate(Cu(NO3)2·3H20)as raw materials;likewise,sample Cu-BTC 3# was obtained at 200 ℃.Sample Cu-BTC 2# was obtained by hydrothermal synthesis,drying and purification in air(metal-organic frameworks,1,3,5-benzene tricarboxylic acid copper).The two types of materials were tested in terms of the NO2 adsorption,and then the specific surface area,pore volume,NO2 adsorption performance,phase composition,microstructure and thermal stability of the adsorbent materials were explored via N2 physical adsorption-desorption,SEM,XRD and TG characterization.The results show that:(1)the Cu-BTC samples have higher adsorption capacity than the calcium-silica adsorbent,in which sample Cu-BTC 34 has the largest specific surface area and pore volume,thus adsorption capacity for NO2;(2)the calcium-silica adsorbent has better thermal stability and lower total mass loss during the entire process than the Cu-BTC samples;sample Cu-BTC 2# has the best thermal stability among the three Cu-BTC samples,and the metal Cu active sites of the Cu-BTC samples can be exposed at least above 150 ℃.
In view of the service failure and low continuous casting production efficiency due to the corrosion of refractories at the slag-line in submerged entry nozzles,a submerged entry nozzle for low-carbon aluminum killed steel continuous casting in one steel plant was sampled and taken as the research object to analyze the causes and the mechanism of refractory corrosion at the slag-line.The results show that the core issue of the corrosion and necking of the slag-line materials is that the slag-line material are eroded,dissolved,and reacted by the mold flux gradually.Therefore,it is important to suppress or avoid the contact and reaction between the mold flux and slag-line materials.
Cr2O3-bearing castables bonded with reactive MgO(RM)or calcium aluminate cement(CAC)were studied to evaluate the binder effect on their performance in corrosive environments.The properties of the as-prepared castables were compared with respect to the differences in phase composition and microstructure.The corrosion behavior of the as-prepared castables by CaO-Al2O3-Fe2O3-SiO2-based slag was systematically compared via refractory cup testing at 1 600 ℃ with respect to the differences in phase composition and microstructure.The analysis indicates that RM bonded castables show higher apparent porosity,lower bulk density and strengths after drying at 110 ℃ and firing at 1 300 ℃,and higher permanent linear change after firing at 1 300 ℃,but better slag corrosion and infiltration resistance compared with CAC bonded castables.
Unburned magnesium aluminate spinel refractories were prepared using sintered magnesium aluminate spinel as the main raw material,phenolic resin as the binder,aluminum powder(2％,4％,and 6％ by mass)and silicon powder(when Al powder addition is 4％,Si powder addition varies:1％and 2％,by mass)as additives.The effects of the Al powder and Si powder additions on the properties and microstructure of the refractories heat treated at different temperatures(1 000,1 400,and 1 600 ℃ for 3 h)were studied.The results show that the Al powder addition can greatly enhance the cold modulus of rupture of the samples fired at 1 000 or 1 400 ℃,and meanwhile AIN reinforcement phase forms in the matrix,which greatly improves the hot modulus of rupture of the samples at 1 400 ℃;however,the heat treatment at 1 600 ℃ has little influence on the strength;the addition of Al powder and Si powder results in the formation of low melting point phases,greatly reducing the hot modulus of rupture.However,the low melting point phases promote sintering,which enhances the density and the cold modulus of rupture,and decreases the volume change during heating.The samples added with Al and Si all have higher cold modulus of rupture than those added with Al powder only.
Ultra-high temperature ceramic coatings have ultra-high melting points,excellent mechanical properties and high temperature ablation resistance.These unique performance combinations turn it into a promising material for use in extreme environment structures in rockets and hypersonic vehicles,particularly nozzles,leading edges and engine components.In this paper,various preparation methods of ultra-high temperature ceramic coatings were reviewed,including plasma spraying,chemical vapor deposition,pack cementation,slurry sintering,hot pressing and their research progress.Meanwhile,some new preparation methods of high temperature coatings,such as ion beam deposition,ultrasonic spraying,metal organic frame work coating,and magnetron sputtering,were introduced.The development trend of ultra-high temperature coatings was prospected as well.
Microporous mullite aggregates were prepared from bauxite by dry pressing and firing at 1 700 ℃ for 3 h with corn starch,PMMA microspheres or rice husk powder as the porogenic agent.The effects of some porogenic agents and their additions(mass fraction of 0,5％,10％,15％and 20％,respectively)on the properties of the microporous mullite aggregates were analyzed by XRD,SEM and Image-Pro Plus software.The results show that the comprehensive performance of the sample with 10％rice husk powder is the best:the closed porosity is 7.1％;the bulk density is 2.63 g·cm-3;the compressive strength is 122 MPa;the median pore size is 15.16 μm;aandtthe thermal conductivity at 500 ℃ is 2.30 W·m-1·K-1.
In order to improve the performance of Al2O3-SiC-C castables for iron trough,samples were prepared using brown corundum,dense corundum,activated alumina micropowder,ball pitch,and silicon as main raw materials,calcium aluminate cement as the binder.Several kinds of silica fumes(93SiO2,96SiO2,and 99SiO2)with different particle size distributions and chemical composition were added to research the effects on the properties of castables.The results show that the sample with 99SiO2 fume has high water requirement for molding,but the bulk density is the lowest,the apparent porosity is the highest,the oxidation resistance is the worst,and the permanent linear change on heating is low after firing at 1 450 ℃ for 3 h.The bulk density and the apparent porosity of the sample with 93SiO2 fume or 96SiO2 fume are equivalent,the hot modulus of rupture of the sample with 93SiO2 fume is the lowest,and the slag resistance of the sample with 96SiO2 fume is the best.The comprehensive performance of the sample with 96SiO2 fume is the best.
The in-situ formation mechanism of MgAl2O4 was introduced,focusing on the formation process by solid phase reaction and gas phase reaction as well as the phenomenon of secondary spinelization.The influencing factors of the in-situ MgAl2O4 formation and its effect on the microstructure and the properties of materials were systematically summarized for the Al2O3-MgO-MgAl2O4 system and the carbon-containing refractories systems.It was pointed out that the in-situ formation of MgAl2O4,including secondary spinelization,can regulate the microstructure and the service performance of materials.Its expansion effect can not only offset the shrinkage caused by sintering to improve the corrosion resistance of refractories,but also seriously restrict the reliability of functional refractories.The composition,the particle size,the atmosphere,and the temperature are important factors affecting the in-situ formation of MgAl2O4.In the carbon-containing materials systems,the solid-solid reaction and the gas-solid reaction coexist to produce MgAl2O4,which provides an effective way to further regulate the microstructure and the properties of materials through the reaction process.
In order to investigate the effect of zirconia corundum on the preparation of chrome corundum castables,samples were prepared using tabular corundum,white fused corundum,active alumina micropowder,chromium oxide micropowder and fused ziconia corundum particles as the main raw materials,and pure calcium aluminate cement as the binder.The effects of the fused zirconia corundum particles addition(1-0.5 mm,0,4.5％,9％,and 18％,by mass)on the linear change rate,the apparent porosity,the cold modulus of rupture,and the thermal shock resistance of chrome corundum castables were studied,and the microstructure of the samples was analyzed.The results show that:(1)with the zirconia corundum addition increasing,the permanent linear change on heating of the samples gradually increases,the apparent porosity gradually increases,the cold modulus of rupture gradually decreases,and the thermal shock resistance gradually increases;(2)according to SEM,it can be found that the phase transformation of ZrO2 after heat treatment leads to a large number of microcracks in the material.On one hand,the cold modulus of rupture of the material is reduced;on the other hand,the thermal shock resistance of the material is improved.
Multi-phase nitrides bonded silicon carbide lintel blocks were prepared using industrial SiC(SiC ≥ 98 mass％,3-0.5,≤0.5 and ≤ 0.044 mm),Si powder(Si ≥98 mass％,≤ 0.044 mm),and SiO2 micropowder(SiO2 ≥ 96 mass％,d50=0.15 pm)as raw materials,and calcium lignosulfonate as the additive,batching,mixing,and molding on a vibration pressure molding machine,drying and then firing at 1 420 ℃ for 10 h in high-purity N2.The apparent porosity,the bulk density,the cold modulus of rupture,the hot modulus of rupture,and the linear expansion coefficient of the samples were tested.The phase composition and the microstructure of the samples at different nitriding depths(50,100,and 150 mm)were analyzed by XRD and SEM.The field application effects of the blocks were studied.The results show that:(1)the multi-phase nitrides bonded silicon carbide refractories can dynamically adjust their own phase composition and minimize structural and thermal stresses,improving the service life of key parts of dry quenching furnaces;(2)calcium lignosulfonate can improve the nitriding micro-environment of multi-phase nitrides bonded silicon carbide lintel blocks,successfully increasing the effective nitriding thickness of the blocks to 300 mm;(3)Sinosteel LI RR provides a unique concept in the design of materials and block types as well as the stable and scientific overall structure,promoting the industrialization process of dry quenching furnaces with long service life in China.
SiC-MgAl2O4 composites with SiAlON polytypoids(12H and 21R)as the bonding phases were prepared using SiC fine powder,MgAl2O4 fine powder and Al powder as raw materials.The effects of the firing temperature and the mass ratio of SiC to MgAl2O4 on the formation of SiAlON polytypoids as well as on the properties of samples were studied.The formation mechanism of SiAlON polytypoids was discussed.The results show that firing at 1 650 ℃ in the nitrogen atmosphere,SiC-MgAl2O4 composites with SiAlON polytypoids can be obtained.With the gradual increase of the mass ratio of SiC to MgAl2O4 in the raw materials,12H forms first,and then coexists with 21H,finally,all 12H converts into 21R in samples.Both phases are plate crystals with flat and smooth edges.Compared with 21R,12H is conducive to improving the density and the cold mechanical properties of materials.After firing at 1 650 ℃ in the nitrogen atmosphere,with the mass ratio of SiC to MgAl2O4 of 6:4,the sample has the cold modulus of rupture of 27 MPa,showing the optimal performance.
To prevent dense chromium oxide covers from cracking during firing,such as internal cracking,surface cracking and fracture,the firing process of dense chromium oxide products was simulated,and the performance parameters at several specific temperatures before and after densification of chromium oxide green bodies were tested.The temperature difference and the stress difference of chromium oxide green bodies were calculated by Finite Element Software at heating and cooling rates of 10,20,30 and 40 ℃·h-1,respectively.The results show that large temperature difference and thermal stress difference are the main causes of internal cracking,surface cracking and fracture of the brick.Suitable heating rates reduce or avoid internal cracking of the brick.When the heating rate is 30 ℃·h-1 below 1450 ℃ and 10 ℃·h-1 above 1 450 ℃,the temperature difference and the thermal stress difference in the brick are below 4 ℃ and 4 MPa,respectively,and there is no internal crack in the brick after firing.The initial cooling stage is the key stage that causes cracking or fracture of bricks.When the cooling rate is 20 ℃·h-1 above 1 450 ℃,the thermal stress difference in the brick is less than 4 MPa,which can significantly reduce the surface cracks and fracture of the brick.
Commercial high purity corundum brick,corundum-mullite brick,mullite brick,calcium hexaaluminate brick and calcium hexaaluminate-corundum brick were studied to investigate their reaction state and alkali vapor corrosion mechanism at 1 370 and 1 500 ℃.The results show that:(1)served in hot alkali vapor,the high-purity corundum brick has no obvious slag layer but expands dramatically;(2)the corundum-mullite brick shows obvious reactive expansion at 1 370 ℃ and melting corrosion happens at 1 500 ℃;(3)at 1 370 ℃ alkali vapor and mullite brick react and form a slag layer without volume effect;when the temperature increases to 1 500 ℃,the reaction melting corrosion intensifies;(4)the calcium hexaaluminate brick and the calcium hexaaluminate-corundum brick form a thin slag layer in alkali vapor,and the sample surface absorbs Na+to form a dense layer with small volume effect.
In order to improve the high-temperature performance of mullite ceramic materials,mullite ceramic bodies were placed in closed containers with AlF3·3H2O powder and kept at 1 600 ℃ for 6 h.AlF3·3H2O reacts with O2 to produce gaseous compounds AlOF and F,which penetrate into the bodies,promote Al2O3 and SiO2 to form mullite whiskers,and strengthen the mullite ceramic materials.The results show that the mullite ceramics have enhanced hot strength,increased bulk density and declined apparent porosity by adding a certain amount of AlF3·3H2O in a closed container.When the addition of AlF3·3H2O is 6％,the bulk density of the ceramic material reaches the maximum and the apparent porosity is the lowest;and when the addition of AlF3·3H2O is 8％,the hot strength of the material is the highest.
Lightweight alumina-silica castables were prepared using closed-cell perlite(2-4 mm),open-cell perlite(4-6 mm)and coal gangue ceramsites(2-5 mm)as aggregates,floating beads(0.3-0.5 mm),sinking beads(0.6-0.8 mm),silica micropowder,a-Al2O3 micropowder,zirconia and zircon micropowder as fines,and Secar 71 cement(calcium aluminate cement)as the binder.The effects of the coal gangue ceramsites addition(0,6％,12％,18％and 24％,by mass)on the properties of the as-prepared lightweight alumina-silica castables were investigated.The results show that:(1)the addition of coal gangue ceramsites can reduce the sintering shrinkage of the specimens and help to improve the strength and thermal shock resistance;(2)the samples with the addition of coal gangue ceramsites can produce pores in the matrix of the sintered samples,which provides enough space for the growth of CA6 complex solid solution and expands the irregular lamellar structure;(3)with the addition of coal gangue ceramsites increasing,the linear shrinkage of the samples heat treated at 1 000 or 1 200 ℃ firstly reduces and then increases,the bulk density increases and the apparent porosity decreases;the cold compression strength and the thermal shock resistance of the specimens heat treated at 1 200 ℃ firstly increase and then decrease.Thus,the optimal addition of coal gangue ceramsites is 18％.
To extend the service life of the upper checker bricks in the regenerator of glass furnaces with petroleum coke as fuel,the corrosion resistance of magnesia-chrome and alumina-chrome bricks with the similar apparent porosity was systematically researched.The results show that the Cr203 content and the microstructure present significant effects on the corrosion resistance.The molten corrosion reagent forms silicate and vanadate phases with low melting points between MgO crystals in magnesia-chrome bricks,and the volume strain generated by the melting and solidification process leads to the cracking and spalling of refractories,which intensifies the corrosion process.The encapsulation of alumina particles by alumina-chrome solid solutions in alumina-chrome bricks avoids contact with the low melting point liquid phases and improves the corrosion resistance of refractories.
The dissolution of alumina-based refractory ceramics in CaO-Al2O3-SiO2 slag melts was performed based on the in-situ observation system of an ultra-high-temperature laser confocal microscope,and the effect of the CaO/SiO2 slag mass ratio(C/S ratio)on the dissolution rate of alumina-based refractory ceramics was investigated.The results indicate that the dissolution rate increases with an increase of the C/S ratio and is mainly controlled by diffusion.During the early stage of dissolution,for all C/S ratios,the dissolution process conforms to the classical invariant interface approximation model.During the later stage of dissolution,when the C/S ratio is≥6,the dissolution process is significantly different from the model above because of the formation of a thick interfacial laver,which can be explained by dissolution kinetics.
To reduce production costs and make full and reasonable use of raw materials,high alumina bricks were prepared using tabular corundum and mullite as aggregates,sillimanite as intermediate particles,and white fused corundum powder,α-alumina micropowder,and Suzhou soil as the matrix,firing at different temperatures(1 420,1 440,1 460,1 480,1 500 and 1 520℃)for 4 h.The apparent porosity(AP),the bulk density(BD),the cold crushing strength(CCS),the thermal shock resistance(TSR),the refractoriness under load(RUL)and the creep rate of the samples were tested.The effects of the firing temperature on the creep rate(1 450℃×50 h,under a load of 0.2 MPa)of the samples were studied.The results show that with the sillimanite addition of 22.5 mass％,the sample fired at 1 460℃for 4 h performs the best comprehensive properties:the AP of 17.5％,the BD of 2.75 g·cm-3,the CCS of 100.5 MPa,the TSR number of 35 cycles,the RUL of 1 682℃,and the creep rate of-0.428％,which can prolong the service life of furnaces.
