Refractories and Classification of Refractories...

Refractories and Classification of Refractories Refractories are inorganic, nonmetallic, porous and heterogeneous materials composed of thermally stable mineral aggregates, a binder phase and additives. The principal raw materials used in the production of refractories are normally the oxides of silicon, aluminum, magnesium, calcium and zirconium. There are some non-oxide refractories like carbides, nitrides, borides, silicates and graphite. Refractories are chosen according to the conditions they face during their use. Some applications require special refractory materials. Zirconia is used when the material is required to withstand extremely high temperatures. Silicon carbide and carbon are two other refractory materials used in some very severe temperature conditions, but they cannot be used in contact with oxygen, since they oxidize and burn in atmospheres containing oxygen. Refractories are the materials which are resistant to heat and exposure to different degrees of mechanical stress and strain, thermal stress and strain, corrosion/erosion from solids, liquids and gases, gas diffusion, and mechanical abrasion at various temperatures. In simplified language, they are considered to be materials of construction which are able to withstand high temperatures. Refractories are usually inorganic non-metallic materials with refractoriness greater than 1500 deg C. They belong to coarse-grained ceramics having microstructure which is composed of large grains. The basis of body is coarse-grained grog joined by fine materials. Refractory products are a specific sort of ceramics that differs from any ‘normal’ ceramics mainly with their coarse-grained structure being formed by larger grog particles joined by finer intermediate materials (bonding). ASTM C71 defines refractories as ‘non-metallic materials having those chemical and physical properties that make them applicable for structures or as components of systems that are exposed to environments above 538 deg C’. Refractories are to be chemically and physically stable at high temperatures. Depending on the operating environment, they...

Slag and its Role in Blast Furnace Ironmaking Aug07

Slag and its Role in Blast Furnace Ironmaking...

Slag and its Role in Blast Furnace Ironmaking Blast furnace (BF) is the oldest (more than 700 years old) of the various reactors which are being used in the steel plants. It is used for the production of liquid iron (hot metal). The blast furnace is a complex high temperature counter current reactor and is in the shape of a shaft in which iron bearing materials (ore, sinter/pellet) and coke are alternately charged at the top along with flux materials (limestone, dolomite etc.) to create a layered burden in the furnace. Preheated air is blown in from the lower part of the furnace through tuyeres. This hot air reacts with the coke to produce reducing gases. Descending ore burden (iron oxides) is reduced by the ascending reducing gases and is melted to produce hot metal. The gangue materials and coke ash melt to form slag with the fluxing materials. The liquid products (hot metal and slag) are drained out (tapped) from the furnace at certain intervals through the tap hole. The quality of hot metal obtained is dependent on the formation of the slag and its mineralogical transformations. A good quality slag is necessary for a quality hot metal. The slag is a mixture of low melting chemical compounds formed by the chemical reaction of the gangue of the iron bearing burden and coke ash with the flux materials in the charge. All unreduced compounds such as silicates, aluminosilicates, and calcium alumino silicate etc. also join the slag. It is well known that the components of slag namely silica (SiO2) and alumina (Al2O3) increase the viscosity whereas the presence of calcium oxide reduces the viscosity. The melting zone of slag determines the cohesive zone of blast furnace and hence the fluidity and melting characteristics...

Alumina and Alumina Refractories...

Alumina and Alumina Refractories Alumina (Al2O3) refractories are the part of alumina- silica (SiO2) group of refractories and belongs to the SiO2 -Al2O3 phase equilibrium system as shown in diagram at Fig 1. They differs from fire clay refractories in term of Al2O3 content and normally have Al2O3 content of more than 45 %. The raw material base for these refractories are different than the fire clay bricks. Fig 1 SiO2 – Al2O3 phase diagram As seen in the diagram, refractoriness increases with the increase in the Al2O3 content. The eutectic at 1595 deg C has a composition of 94.5 % SiO2 and 5.5 % Al2O3. As the Al2O3 content is increased, the melting point of the refractory increases to a maximum of 2054 deg C which is the melting point of pure corundum. The only stable compound in the system is mullite, which has a defective space lattice and decomposes into corundum and liquid phase at around 1840 deg C. The classification of Al2O3-SiO2 refractories as per the Al2O3–SiO2 phase equilibrium diagram is given in Tab 1. Tab 1 Classification of Al2O3-SiO2 refractories as per the Al2O3–SiO2 phase equilibrium diagram Range of Al2O3  Phases as per common terminology General performance of refractories in conditions of the absence of slag corrosion or alkali attack Al2O3 less than 50 % Fireclay (Chamotte); Phases on phase diagram are mullite and glass; can contain free SiO2 Normally made from 100 % fireclay, Highest quality grades (super duty bricks) usable to about 1600 deg C,  Usually contain 38 % to 42 % Al2O3 and are based on fireclay minerals Al2O3 50 % or 60 % Sillimanite, andalusite, or kyanite; Phases on phase diagram are mullite as major phase and glass as minor phase; can contain free SiO2 These...

Refractory lining of blast furnace Aug15

Refractory lining of blast furnace...

Refractory lining of blast furnace  A modern blast furnace (BF) is refractory lined to protect the furnace shell from the high temperatures and abrasive materials inside the furnace. The refractory lining is cooled to further enhance the protection against the dispatch of excess heat that can destroy the refractory lining. BF has a complex refractory system to provide a long, safe life that is necessary for the blast furnace availability and for permitting nearly continuous furnace operation and casting. Conditions within the blast furnace vary widely by region and the refractories are subjected to a variety of wear mechanisms. Details are given in Tab 1. The application condition of different regions of a blast furnace is not the same due to the very nature of its geometry and also due to the pyrometallurgical process occurring at different stages. There are diverse physical and chemical wear mechanisms in the different regions of the blast furnace and they are complex in nature. For example mechanical wear or abrasion occurs mainly in the upper stack region and is caused by the decent of the charge materials and by the dust laden gases. High thermal loads are a major factor in the lower stack and the belly regions. In the hearth region, horizontal and vertical flow of hot metal combined with thermal stresses often form undesirable elephant foot shaped cavitation. The refractory materials in these regions are to take care of these wear mechanisms to avoid damage due to them. Therefore, the BF stack (upper middle and lower), belly, bosh, raceway and tuyere region, hearth, and taphole all require different quality of refractories depending on the respective application conditions. Tab 1 Attack mechanisms in different regions of blast furnace       Region Attack mechanism Resulting damage       Upper stack Abrasion...

CAS-OB Process of Secondary Steelmaking Oct03

CAS-OB Process of Secondary Steelmaking...

CAS-OB Process of Secondary Steelmaking The CAS-OB process consists of Composition Adjustment by Sealed argon bubbling with Oxygen Blowing. It was developed by Nippon Steel Corporation. Typical schematic diagram of a CAS-OB installation is shown in Fig 1. Fig 1 Schematic diagram of a CAS-OB installation The process allows alloy additions to be made under an inert argon environment. It allows simultaneous addition of Al and O2 gas blown through a top lance. These react to form Al2O3 and generate a considerable amount of heat due to exothermic nature of the reaction. The CAS-OB process, therefore results into chemical heating of the liquid steel. The Equipment Liquid steel processing is carried out in ladles, equipped with slide gates and a porous plug for blowing argon. Equipment for the process consists of a snorkel or bell fixed to the movable bracket. To the top of the bell, a port is provided, which serves the purpose of feeding of ferro alloys into the bell and for removal of gases to the gas cleaning system. The design of the bell has provision for lowering of oxygen lance and process and instrument (PI) lance for sampling, measuring of the temperature and for measuring of the dissolved oxygen as well as a lance for injecting a metal powder, desulphurizing compound and CaSi wire. Bell consists of two parts. The upper part is lined only from the inside, while bottom is lined both inside and outside. Lining of the bell is usually done with of high-alumina castables reinforced with 2 % stainless steel needles. These castables are also used for the lining of the oxygen lance and lance for blowing argon into the liquid steel, which is used when argon cannot be supplied to the liquid steel through the bottom porous plug. Chrome magnesite bricks...