Introduction to Refractories...

Introduction to Refractories Refractories are defined in ASTM C71 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 inorganic, nonmetallic, porous and heterogeneous materials composed of thermally stable mineral aggregates, a binder phase and additives. These materials have ability to retain its physical shape and chemical identity when subjected to high temperatures. Refractories perform four basic functions namely (i) act as a thermal barrier between a hot medium and the wall of the containing vessel, (ii) represent a chemical protective barrier against corrosion, (iii) ensure a physical protection, preventing the erosion of walls by the circulating hot medium and (iv) act as thermal insulation for heat retention. Refractories are classified in the different following ways. Classification based on chemical composition –  Refractories are classified on the basis of their chemical behaviour into following three classes. Acid refractories – These are those refractories which are attacked by alkalis or basic slags. These are used in acidic atmosphere or where slags are acidic. Example of these refractories are silica and zirconia. Basic refractories – These refractories are attacked by acid slags but stable to alkaline slag, dust and fumes at the elevated temperatures. These refractories are used in alkaline atmospheres. Example of these refractories are magnesia, dolomite and chromite. Neutral refractories – These refractories are chemically stable to both acids and bases and used in the areas where slag and environment are either acidic or basic. Examples are carbon graphite, chromites and alumina. Grphite is the least reactive and is extensively used in the furnaces where the process of oxidation can be controlled. Classification based on physical form – Refractories are classified according...

Limestone and dolomite flux and their use in iron and steel plant...

Limestone and dolomite flux and their use in iron and steel plant Limestone is a naturally occurring mineral. The term limestone is applied to any calcareous sedimentary rock consisting essentially of carbonates.  The ore is widely available geographically all over the world. Earth’s crust contains more than 4 % of calcium carbonate. Limestone is basically calcite which is theoretically composed of exclusively calcium carbonate (CaCO3). When limestone contains a certain portion of magnesium, it is called dolomite or dolomitic limestone (CaCO3.MgCO3). Dolomite theoretically contains CaCO3 54.35 % and MgCO3 45.65 % or CaO 30.4 %, MgO 21.9 % and CO2 47.7 %. However, in nature, dolomite is not available in this exact proportion. Hence generally the rock containing 40-45 % MgCO3 is usually called dolomite. When MgCO3 is less than 40 % but more than 20 % then the limestone is called dolomitic limestone. The chemical composition of limestone and dolomite varies greatly from region to region as well as between different deposits in the same region. Therefore, the end product from each natural deposit is different.  Typically limestone and dolomite are composed of calcium carbonate (CaCO3), magnesium carbonate (MgCO3), silica (SiO2), alumina (Al2O3), iron (Fe), sulphur (S) and other trace elements. These minerals are shown in Fig 1 Fig 1 Limestone and dolomite The limestone from the various deposits differs in physical chemical properties and can be classified according to their chemical composition, texture and geological formation. Limestones from different sources differ considerably in chemical compositions and physical structures. The chemical reactivity of various limestones also shows a large variation due to the difference in crystalline structure and the nature of impurities such as silica, alumina and iron etc. The varying properties of the limestone have a big influence on the processing method....