Alumina and its Role in Iron and Steelmaking...

Alumina and its Role in Iron and Steelmaking Alumina is a chemical compound of aluminum (Al) and oxygen (O2) with the chemical formula aluminum oxide (Al2O3). It is the most commonly occurring of several aluminum oxides. It is significant in its use to produce aluminum metal. It is being used as an abrasive material because of its hardness. It is also being used as a refractory material owing to its high melting point. Aluminum oxide is an amphoteric substance. It can react with both acids and bases, acting as an acid with a base and a base with an acid, neutralizing the other and producing a salt.  It is insoluble in water. Aluminum oxide has a white solid appearance and is odorless. The molar mass of aluminum oxide is 101.96 grams per mole. Specific gravity of alumina is 3.986. It is insoluble in water. Melting point of aluminum oxide is 2072 deg C while the boiling point is 2977 deg C. Alumina affects the processes of producing iron and steel during the production of iron and steel. Besides alumina is a very important refractory material for the lining of furnaces and vessels in iron and steel plants. Role of alumina in ironmaking Alumina during ironmaking enters the process through impurities in the input materials mainly iron ore. Alumina affects the sintering of iron ore. The most harmful effect of alumina is to worsen the RDI (reduction degradation index) value of sinter. RDI value increases as the alumina content rises. It is seen that within a 10 % to 10.5 % CaO content range, an increase of 0.1 % in the alumina content raises the RDI by 2 points. The strength and quality of sinter deteriorate as the alumina content rises. Alumina promotes the formation of SFCA (silico ferrite of calcium and aluminum), which is beneficial for sinter strength, but the strength of the ore components is lower, since a...

Limestone and Lime

Limestone and Lime Limestone is an odorless white, grayish-white or tan material that ranges from sized stone to a granular powder. It is often described as the most versatile mineral. Limestone is the name given to any rock formed which consists mostly of calcium carbonate (CaCO3), but to geologists, limestone is only one of several types of carbonate rocks. These rocks are composed of more than 50 % carbonate minerals, generally containing the mineral calcite (pure CaCO3). Limestone is a sedimentary rock composed mainly of CaCO3. It is formed by the deposition either of the skeletons of small creatures and/or plants (organic limestones), or by chemical precipitation, or by deposition of fragments of limestone rock, on the beds of seas and lakes. Limestones are contaminated to a greater or lesser extent by the deposition of sand or clay which is the source of the impurities usually found in the limestone. Generally there is a difference in quality in a deposit from one layer to the next. The purest carbonates and the most suitable from the production point of view tend to be the thick bedded type. Carbonate deposits may be found in horizontal layers as deposited, or at an angle from the horizontal due to earth movements. They will vary in density, hardness and chemical purity. Limestone rocks are extremely common and make up a significant portion of the crust of the Earth. They serve as one of the largest carbon repositories on our planet. The properties of limestone make it one of the most widely used minerals. Some limestones may contain small percentage of magnesium carbonate (MgCO3). These limestones are known as dolomitic limestones. Impurities (such as clay, sand, organic remains, iron oxide, and other materials) cause limestones to show different colours, especially with weathered surfaces. Limestone may be crystalline, clastic, granular,...

Dolomite – A Useful Mineral...

Dolomite – A Useful Mineral Dolomite is also known as dolostone and dolomite rock.  It is a sedimentary rock which primarily consists of the mineral dolomite. It is found in sedimentary basins worldwide. Dolomite rock is similar to limestone rock. Both dolomite and limestone rocks share the same colour ranges of white-to-gray and white-to-light brown (although other colours such as red, green, and black are also possible). Both the rocks have approximately the same hardness, and they are both soluble in dilute hydrochloric (HCl) acid. The original mineral name ‘dolomie’ was given by NT Saussare, in 1792, in honor of the French geologist Deodat Guy de Dolomieu (1750–1801). Dolomite, the rock, contains a large proportion of dolomite the mineral. Ideal dolomite has a crystal lattice consisting of alternating layers of Ca and Mg, separated by layers of CO3 and is typically represented by a stoichiometric chemical composition of CaMg(CO3)2, where calcium and magnesium are present in equal proportions. Dolomite originates in the same sedimentary environments as limestone i.e. in warm, shallow, marine environments where calcium carbonate (CaCO3) mud accumulates in the form of shell debris, fecal material, coral fragments, and carbonate precipitates. Dolomite is thought to form when the calcite in carbonate mud or limestone is modified by magnesium-rich groundwater. The available magnesium facilitates the conversion of calcite into dolomite. This chemical change is known as dolomitization. Dolomitization can completely alter a limestone into a dolomite, or it can partially alter the rock to form a dolomitic limestone. Dolomite is a complex mineral. It is relatively a soft mineral which can be easily crushed to a soft powder. The mineral is an anhydrous carbonate mineral consisting of a double carbonate of calcium (Ca) and magnesium (Mg). It is chemically represented by CaMg(CO3)2 or CaCO3.MgCO3. It theoretically contains...

Waste Heat Recovery Devices...

Waste Heat Recovery Devices  Industrial furnaces are used for carrying out certain processes which requires heat. Heat in the furnace is provided by (i) fuel energy, (ii) chemical energy, (iii) electrical energy or (iv) a combination of these energies. Gases which are generated during the process leaves the furnace at a temperature which is the inside temperature of the furnace and hence have a high sensible heat content. Sometimes the exhaust gases carries some chemical energy, which raises the temperature of exhaust gases further due to post combustion because of this chemical energy. The heat energy contained in the exhaust gases is the waste energy since it gets dumped in the environment. However, it is possible to recover some part of this energy if investments are made in waste heat recovery devices (WHRDs). Methods for waste heat recovery include (i) transferring heat between exhaust gases and combustion air for its preheating, (ii) transferring heat to the load entering furnaces, (iii) generation of steam and electrical power, or (iv) using waste heat with a heat pump for heating or cooling facilities. WHRDs work on the principle of heat exchange. During heat exchange the heat energy of the exhaust gases gets transferred to some other fluid medium. This exchange of heat reduces the temperature of the exhaust gases and simultaneously increases the temperature of the fluid medium. The heated fluid medium is either recycled back to the process or utilized in the production of some utilities such as steam or power etc. The benefits of WHRDs devices are multiple namely (i) economic, (ii) resource (fuel) saving, and (iii) environmental. The benefits of these devices include (i) saving of fuel, (ii) generation of electricity and mechanical work, (iii) reducing cooling needs, (iv) reducing capital investment costs in...

Air Pollution Control Devices...

Air Pollution Control Devices Air pollution control devices (APCD) are a series of devices which are used to prevent a variety of different pollutants, both gaseous and solid, from entering the atmosphere mainly out of the industrial stacks. These control devices can be separated into two broad categories namely (i) devices which control the amount of particulate matter escaping into the environment, and (ii) devices which controls the acidic gas emissions into the atmosphere. By and large the air pollutants are generated due to the combustion of fuels in the furnaces. The major combustion-generated pollutants are the oxides of nitrogen (NOx), sulphur dioxide (SO2), carbon monoxide (CO), unburned hydrocarbons, and particulate matter. The generated pollutants are carried by the exhaust gases produced during the combustion of the fuel. These exhaust gases are then normally passed through the APCDs before releasing them to the atmosphere.  The pollutants are removed, destroyed, or transformed in the control devices before the discharge of the exhaust gas into the atmospheric air. Common methods for removing the pollutants from the exhaust gases work on the following principles. Destroying pollutants by thermal or catalytic combustion, such as by use of a flare stack, a high temperature incinerator, or a catalytic combustion reactor. This technique is used when the pollutants are in the form of organic gases or vapours. During flame combustion or catalytic process, these organic pollutants are converted into water vapour and relatively less harmful products, such as carbon dioxide (CO2). Changing pollutants to less harmful forms through chemical reactions, such as converting nitrogen oxides (NOx) to nitrogen and water through the addition of ammonia to the exhaust gas in front of a selective catalytic reactor. In the technique known as ‘absorption’, the gaseous effluents are passed through scrubbers or absorbers. These contain a suitable liquid absorbent, which removes or modifies one or more...

Refractories for a Reheating Furnace...

Refractories for a Reheating Furnace Refractories are inorganic, nonmetallic, porous and heterogeneous materials composed of thermally stable mineral aggregates, a binder phase and additives. They 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, refractories are considered to be materials of construction which are able to withstand high temperatures. The general requirements from the refractories for are (i) ability to withstand high temperatures and trap heat within a limited area such as a reheating furnace, (ii) ability to withstand sudden changes of temperature, (iii) ability to withstand load at service conditions, (iv) ability to withstand chemical and abrasive action of the materials such as liquid metal, liquid slag, and hot gases etc. coming in contact with the refractories, (v) ability to resist contamination of the material such as scale etc. with which it comes into contact, (vi) ability to maintain sufficient dimensional stability at high temperatures and after/during repeated thermal cycling, (vii) ability to conserve heat, (viii) ability to withstand load and abrasive forces, and (ix) to have low coefficient of thermal expansion. Properties of the refractories can be classified to resist four types of service stresses namely (i) chemical, (ii) mechanical, (iii) thermal, and (iv) thermo-technical. A suitable selection of the refractories for the lining of the reheating furnace can only be made with an accurate knowledge of the refractory properties and the stresses on the refractories during service. The relationship between service stresses and important properties of the refractories are at Tab 1.  Tab 1 Relationship between type of stress and refractory property Sl.No. Type of stress Important refractory property 1 Chemical...