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...

Importance of Hearth, Dead man and Tapping in Blast Furnace Operation Apr13

Importance of Hearth, Dead man and Tapping in Blast Furnace Operation...

Importance of Hearth, Dead man and Tapping in Blast Furnace Operation  A trend of deterioration in ore quality is seen these days with the increasing demand for iron ore. The deterioration in ore quality is accompanied with higher quantities of slag which in turn affects burden descent and liquid flow through the hearth. These conditions provide a catalyst for lining wear mechanism with bosh, stack and hearth linings coming under additional stress. Tapping in the blast furnace is adversely affected and trough and runners in the cast house get under strain due to higher slag volume. All these put increased pressure on blast furnace operations. The poor quality of iron ore affects the operation of the blast furnace in the following way. Slag volume – Poor quality of iron ores bring into the furnace higher quantities of impurities resulting into increase in the slag volumes. Heat load – The furnace thermal condition undergoes changes since a large quantity of heat is required to melt the additional slag as well as to keep it in proper fluid state for its drainage. This introduces higher heat loads inside the blast furnace. Coke rate and productivity – Increasing slag volumes needs a higher fuel input into the furnace, and where pulverized coal injection rates are already running at optimum, this results into a higher coke rate. Higher coke means introduction of higher amount of ash in the furnace resulting into further increase in the slag volume. This has got a deteriorating effect on the productivity of the furnace. Process stability – The deterioration in the ore quality affects the process stability adversely and has an unfavourable effect on the smooth running of the blast furnace. Due to the above factors, the production process in the blast furnace...

Improved Designs  and Campaign Life of a Blast Furnace May23

Improved Designs and Campaign Life of a Blast Furnace...

Improved Designs  and Campaign Life of a Blast Furnace The cost of rebuilding or relining a blast furnace (BF) is very high. Hence techniques to extend BF campaign lives are important and need to be pursued very actively. Large BFs usually have a slightly higher campaign output per unit volume. This difference is because larger BFs generally are of more modern design and are well automated.  Since the viability of an integrated steel plant depends on a continuous supply of hot metal (HM), which, in a plant with a small number of large BFs, puts great importance on long campaign life. The techniques for prolongation of BF campaign life falls under the following three categories. Operational practices – The control of the BF process has a major effect on the campaign life. BF is to be operated not only for meeting the production needs but also to maximize its life. Hence it is necessary to modify operating practices as the campaign progresses and in response to the problem areas for the maximization of campaign life. Remedial measures – Once wear or damage that affects the life of the BF becomes evident, engineering repair techniques are to be used or developed to maximize campaign life. Improved designs – As improved materials and equipment are developed, these are to be incorporated into future rebuilds to extend the life of critical areas of the BF, where it is cost effective to do so. Improved designs of the BF for improving the campaign life are discussed in this article. The correct design of the furnace proper is fundamental to reliable operation, metallurgical performance, sustained high productivity, long campaign life and an availability of more than 98 %. BF design has had many improvements in recent decades and campaigns...

Fireclay Refractory Bricks...

Fireclay Refractory Bricks  Fireclay refractory bricks are manufactured from unfired refractory bond clay and fireclays (chamotte), fired refractory clay or similar grog materials . Fireclay refractory bricks have two main components namely 18 % to 44 % of alumina (Al2O3) and  50 % to 80 % of silica (SiO2). The variety of clays and manufacturing techniques allows the production of numerous brick types appropriate to particular applications. The usefulness of fireclay refractory bricks are largely due to the presence of mineral mullite, which forms during firing and is characterized by high refractoriness and low thermal expansion. Raw materials for fireclay refractory bricks Refractory fireclay essentially consists of hydrated aluminum silicates with minor proportion of other minerals. The general formula for these aluminum silicates is Al2O3.2SiO2.2H2O, corresponding to 39.5 % alumina, 46.5 % silica, and 14 % water (H2O). Kaolinite is the most common member of this group. At high temperature, the combined water is driven off, and the residue theoretically consists of 45.9 % alumina and 54.1 % silica. However even the purest clays contain small amounts of other constituents , such as compounds of iron, calcium, magnesium, titanium, sodium, potassium, lithium, and usually some free silica. The total quantity of these fluxing agents, which lower the melting point, should be at a level of 5 % to 6 % maximum. TiO2 is not regarded as fluxing agent and was previously counted together with alumina. The name fireclay is given to a group of refractory clays which can generally withstand temperatures above pyrometric cone equivalent (PCE) value of 19. Refractoriness and plasticity are the two main properties needed in fireclay for its suitability in the manufacture of refractory bricks. A good fireclay should have a high fusion point (greater than 1580 deg C) and...

Silica Refractories

Silica Refractories Silica refractories were first produced in United Kingdom in 1822 from Ganister (caboniferous sandstone) or from so called Dinas sand. Silica occurs in a variety of crystalline modifications, e.g. quartz, tridymite, and cristobalite and also as an under-cooled melt called quartz glass. The crystalline modifications each have a high and low temperature forms which can transform reversibly. The crystal structure of the individual SiO2 modifications can differ widely, so that distinct density changes occur during transformation. This is of great importance during heating and cooling because of the change in the volume. Quartz requires the smallest volume and the quartz glass the largest. During firing above approximately 900 deg C, quartz transforms into the other modifications and melt completely at 1725 deg C. During slow cooling , reversible volume decreases take place  which are a result of the spontaneous transformation of the crystal structure from the high to the low temperature modification (Fig 1). The reversible and irreversible volume effects can cause considerable stress within the refractory brick structure. Fig 1 Calculated volume and density changes Production of silica refractories The silica refractories are manufactured as multiple asymmetric shapes, which are normally keyed or interlocked with each other by means of tongues and grooves. It is the objective of the manufacturer of silica refractory bricks to select the raw materials and the firing process in such a manner that the degree of quartz transformation is suitable for the intended application of the brick. The raw material for silica brick is naturally occurring quartzite which must meet certain requirements in order to achieve optimum brick properties. If refractoriness or thermal expansion under load (creep) are the main requirements, a quartzite of high chemical purity must be selected. Raw materials for volume stable products...