Henan Chiefway’s main products are fused cast AZS series. In order to have the products quality increased constantly, the technological innovations are often implemented. One by one, the new breakthroughs were achieved, the “bottom oxygen blowing” is the one among them.
The fused cast AZS blocks are the key installation materials of all kinds of glass and glass products, due to its characteristics of magnificent resistance to glass liquid and alkali vapor, its high strength, the stability of chemical and mechanical properties in high temperature.
It is well known, that the properties of material do not only depend on the chemical components, but also its inner structure，which depends on the production technology condition and process of the material. There are 3 types of of fused AZS blocks for general use, including AZS33#,AZS36# and AZS41#. their chemical components are concentrated in the eutectic area of Al2O3-ZrO2-SiO2 phase equilibrium diagram.
Theoretically, during the cooling of fused refractories, along with the temperature decreasing and condenser depression decreasing, the primary crystal of zirconia would appear in the liquid phase firstly, followed by the separation of α or β alumina transfered from the rich Al2O3 due to the different chemical components, in the mean time, because of the fluctuation of chemical components, the corundum phase would be attached to the zirconia primary crystal and create the layered eutectic structure. Therefore, the fused AZS block is generally composed by zirconia, corundum, zirconia-corundum eutectic and vitreous phase.
Zirconia, corundum and zirconia-corundum eutectic are characterized by their higher melting point and high temperature chemical stability, which make them the important constituent elements of anti-corrosion against glass liquid. The melting point of vitreous phase is lower, which will be easy to create the liquid state in the block during its application. The micro-pores left after the liquid run over will cause the penetration of glass liquid, then increase the contacting area between glass liquid and refractories, and enhance the corrosion and destruction to the furnace. During the solidification of fused AZS molten materials, the transfer of zircolnia from square to monoclinic crystal, in addition to the transfer of corundum from β to α lattice would be happened, which will lead to a larger volume expansion, but due to the exiting of low melting point vitreous phase, the firstly crystalized zirconia and corundum phase will have more buffer space, which would remove the defects of cracks caused by innerstress created by the valume expansion.
There are three binary systems in the ternary systems of Al2O3-ZrO2-SiO2 system, Al2O3-ZrO2 system is characterized by the comparatively small crystal in the crystalizing temperature zone, the ideal phase structure of fused AZS refractories is that the eutectic crystal of zirconia-corundum could run through the block in the form of interdendritic segregation, and vitreous phase fill in the space of interdendritic acting as binder and buffer agent.
Micro structure of fused AZS refractories
1. ZrO2 primary crystal, 2. Eutectic of Al2O3-ZrO2, 3. Vitreous phase matrix
In order to maintain the designed service life of glass furnace, the engineer will select the variety type of fused AZS products in respect to the different sections of furnace. But in reality, many furnaces can not reach to its expected service life, the problem is caused by the quality of refractories, if exclude the factors of designing and application. Usually, the quality defect of refractory is found on the specific section of the refractory block, so the whole furnace life will be affected by the worst qualified products. The only way to gurantee the service life of furnace is to make sure the refractories selected, especially each AZS block’s quality meet the requirements.
Due to limitation of casting and melting techniques, in practice, the segregation of components and structure was usually found in the fused AZS materials. As per the photos indicating the different micro-structures of fused AZS36# block made by a factory, obviously, the great structure differences were found according to varies sections. These existing phenomenons were caused by the characteristics of the material itself, in addition to the differences of cooling conditions of different sections during the solidification, and the varieties of chemical components.
Generally, the vitreous phase in the fused AZS block would be separated out due to the melting effect, when the temperature inside the furnace is increased to 1400℃, and leave the eutectoid (eutectic) of baddeleyite and corundum phase with porous. The liquid glass will penetrate into the block through the porous and lead to the spalling and corrosion to the fused refractories.
Zirconia is characterised by its high melting point and the chemical stability under the high temperature. If in the residue solidity, there were existing the uniformly distributed fine baddeleyite crystal phase and creating the web shaped eutectic with corundum phase, which would effectively restraint the spalling speed of refractories, so the uniform distribution of zirconia in the fused refractories is a crucial factor to the performance of erosion and spalling resistance of the glass furnace.
The picture indicates the variation of chemical components in different sections in fused AZS41# block: the chemical components in the nearest area to the cast are identical to the theoretical, as far apart from the out surface, the content of zirconia gets increased, but alumina decreased, and vitreous phase ( silica) also decreased slightly; while reaching to a certain distance, the content of zirconia reach to its highest value and more than 10% higher than the theoretical，then following it, as it goes far from the block surface, the content of zirconia gets decreased but alumina increased, till the content of zirconia more than 10% lower than the theoretical and alumina more than 10% higher, the constituents begin to stable. According to the testing, this phenomenon is existing in all the three types of fused AZS blocks.
The diagram of zirconia segregation of the melting in EAF
The process of the formation of zirconium segregation in the melt in the electric furnace
The components segregation of fused refractories could be caused during the melting process, in addition to the casting process. We have experienced the testing to the chemical components of the first and the last block in the same melting circle, the result indicated the difference between them could reach to 8-11%, and the content of zirconia in the furnace bottom material could reach to more than 60%.
The research indicates that the components segregation occured during solidification or melting has a relation with the specific gravity of oxidizes in the material itself. At the moment when the melting contact the mold cavity, a hard shell will be created on the surface at first, due to the sharp cooling, because the materials have no enough to move, so the components on the surface will be distributed uniformly and are identical to the theoretic value. The hard shell created by the cooling of molten material has a certain of insulating function to the non-solified material due to its poor thermal conductivity. At the primary stage of solidifying, the molten liquid is characterized by it high temperature and low viscocity; because the higher density of zirconia more than silica and alumina, the primary crystal of zirconia will be sediment and enrichment, which will cause the ingredients segregation among different sections of one same block; during the melting process, if the liquid molten was not stired insufficiently, there would be a temperature difference between the liquid molten near the electric arc and the far below it at the furnace bottom,which would cause the liquid molten layered inevitably during the melting process. The more densified melting zirconia will be deposited under the alumina and silica, leading to the chemical components segregation among different blocks; accordingly, there would be great quality difference among different blocks. In this way, during the application, the inadequate zirconia section in one block, or the inadequate zirconia blocks in one circle casting will be eroded by liquid glass earlier, which will lead to the scrap of furnace due to the partial decrease of service life.
With the develpent of melting, the reduction method has been replaced by the oxidation method,whose character is the option of oxidizing atmosphere in the melting process. The products are characterized by its lower carbon content, the high valence of variable valence oxides (Fe2O3, TiO2), which decreases the low melting impurities in the vitreous phase and increases the exudation temperature of vitreous phase. The baddeleyite’s and vitreous’ corrosion resistance of liquid glass get released abundantly, and the campaign of furnace get increased obviously. The main function of oxidation method is to achieve the de-carbon treatment to the melting materials in the last melting process. The usual operation is to input oxygen to the furnace by using lance through the hole on the furnace cover, when the temperature inside the furnace reach to 2000℃, so that the oxidation reaction on the surface of molten material could be increased to gurantee the quality of product. According to the discovery, the oxidation method with long arc heating can only transfer the electric energy to heat and have the solid materials melted, which has no performance to have the molten material inside the furnace flowed and stirred. The molten materials inside the furnace almost remain the static state, or only a slight stirring occured on the molten materials surface and around the electric arc. This shortcoming is not benifited to the floating and clearing of the low density impurities involved, in addition that the zirconia enriched components would be prone deposited and layered during the melting process, then cause the segregation of components and structure among blocks.
In order to overcome the above mentioned shortcoming, we invented the bubbling production technique of fused refractories and bottom oxygen blowing technology one after another, and achieved the national invention patents of China-- during the process of temperature increasing inside the furnace, the oxygen and compressed air separated from oil and water are put into the furnace slowly, through the valve which is located on the bottom shell of furnace and penetrated the shell and furnace lining. The advantages lay as the followings: 1. No need of cutting off the electric power while blowing oxygen to the furnace, which could solve the temperature decrease of materials inside the furnace defined by the power cut off in current oxygen blowing technique, thus, reduce the consumption of electricity needed in fused refractories production, then the production cost also get reduced; 2. Because of the continuing blow of oxygen from the furnace bottom, the flowing speed and stirring to the molten liquid get increased and homogeneoused, then not only the reaction opportunity of carbon and low-valent oxides with oxygen will be increased, but also the efficiency of de-carbon and the proportion of high-valent oxides, on the other hand, the weight segregation caused by the higher proportion of zirconia than alumina and silica will be reduced, then the components of molten liquid will get even. In the mean time, because the time of oxygen blowing is prolonged without the temperature reducing and the stop of oxygen blowing, the low melting point impurities and micro-bubble dissolved in the molten liquid will be gathered sufficiently and floated, thus the porosity of the products will be reduced, on the other hand, the density will be increased and the service life of products will be prolonged. 3. because of the simplified production technique, the automatic online control will be more easy to be realized.
The application of Bottom Oxygen Blowing Process increased the purity of the molten liquid greatly, in the mean time, the layer segregation of different components, cause by its different proportion get decreased, then the chemical constituents in each section get evened, by this way, the segregation of zirconia is controlled effectively, the corrosion and erosion resistance of fused refractories against liquid glass is improved. Take the AZS-33#Y-PT as an example, the testing results before and after the technique adopted shows the variations of zirconia content in one furnace heat can be reduced from 4.6% to 0.3%, and the vitreous phase from 2.5% to 0.4%, in the mean time, the volume weight and corrosion resistance to liquid glass get increased and developed.