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...

Magnesia

Magnesia Magnesia or magnesium oxide (MgO) is a white hygroscopic solid mineral that occurs naturally as periclase. It forms magnesium hydroxide in the presence of water [MgO + H2O = Mg(OH)2], but this reaction can be reversed by heating magnesium hydroxide to separate moisture. Magnesium (Mg) is the eighth most abundant element and constitutes about 2 percent of the crust of the earth. It is the third most plentiful element dissolved in seawater, with a concentration averaging 0.13 %. Although magnesium is found in over 60 minerals, only dolomite, magnesite, brucite, carnallite, and olivine are of commercial importance. Magnesium and magnesium compounds are produced from seawater, well and lake brines and bitterns, as well as from the above mentioned minerals. Magnesite (MgCO3), the naturally occurring carbonate of magnesium (Mg) is one of the key natural sources for the production of magnesia (MgO) and subsequently fused magnesia. It is the world’s  largest source of magnesia. It contains a theoretical maximum magnesia content of 47.6 %. It occurs in two distinct physical forms namely (i)  macro-crystalline and (ii) crypto-crystalline. Crypto-crystalline magnesite is generally of a higher purity than macro-crystalline ore, but tends to occur in smaller deposits than the macro-crystalline form. The word magnesite literally refers only to the natural mineral, but common usage applies this name to three other types of materials, dead burned magnesia (DBM), electro fused magnesia and calcined magnesia also called caustic calcined magnesia. Often magnesia word is replaced by magnesite in these products. These products of magnesite often differ mainly in density and crystal development that results from different levels of heat application. The three products of magnesite are shown in Fig 1. Fig 1 Products of magnesite  Magnesia is an alkaline earth metal oxide. Magnesium oxide is normally produced by the calcinations of...

High Alumina Slag and Blast Furnace Operation May27

High Alumina Slag and Blast Furnace Operation...

High Alumina Slag and Blast Furnace Operation Blast furnace (BF) process of iron making is a process where liquid iron (hot metal) and liquid slag are produced by the reduction of iron bearing materials (sinter and/or pellet and lump ore) with coke and by fluxing of the gangue material of the feed materials. The process is the result of a series of chemical reactions which takes place in the blast furnace. The separation of slag from the hot metal takes place in liquid state. Slag has a lower melting point and is lighter than hot metal. In the blast furnace it is at a higher temperature than the hot metal. Blast furnace slag contain predominantly silica (SiO2), alumina (Al2O3), lime (CaO) and magnesia (MgO) along with smaller amounts of FeO, MnO, TiO2, Na2O, K2O and S. Blast furnace (BF) slag composition has very important bearing on its physicochemical characteristics which influences to a great degree the smooth operation of the blast furnace, slag handling, coke consumption, blast furnace productivity and the quality of the hot metal. Low alumina slag normally has low viscosity, high sulphide capacity and low liquidus temperature as compared to high alumina slag. Blast furnace slag alumina (Al2O3) is mainly dependent on the alumina content of the input materials mainly iron ore. In those cases where iron ore alumina is less than 1 % the alumina content in the slag hardly goes above 10 %. But in some iron ores (normally found in India) alumina content is 2 % and higher. Such ores raise the alumina levels in blast furnace slag to 20 % and higher. To operate a blast furnace with such high alumina slag is quite difficult and need a different type of skill from the blast furnace operators...