Generation Transport and Uses of Mill Scale Sep23

Generation Transport and Uses of Mill Scale...

Generation Transport and Uses of Mill Scale  Mill scale is the flaky surface of hot worked steel and is formed by the oxidation of the steel surface during reheating, conditioning, hot rolling, and hot forming operations. It is one of the wastes generated in steel plants and represents around 2 % of the produced steel. It is a hard brittle coating of several distinct layers of iron oxides formed during the processing of steel and composed mainly of iron oxides and may contain varying amounts of other oxides and spinels, elements and trace compounds. It flakes off the steel easily. Characteristics of mill scale Mill scale is normally present on rolled steel and is frequently mistaken for a blue coloured primer. The very high surface temperature combined with high rolling pressures result in a smooth, bluish grey surface. Under visual inspection, mill scale appears as a black metal powder made up of small particles and chips. Its physical state is solid and powdered. The specific gravity of mill scale is in the range of 5.7 to 6.2. Mill scale’s melting point is around 1370 deg C and boiling point is around 2760 deg C. It has a stable state and is insoluble in water and alkalis but soluble in most of strong acids. It is normally classified as non dangerous waste material. Fig 1 shows a small pile of mill scale. Fig 1 Small pile of mill scale  The size of the mill scale normally varies from dust size in microns up to usually 6 mm. The average mill scale obtained during the hot working of steels has iron content ranging from 68 % to 72 %. The iron in the mill scale is present in different chemical forms as given below. Magnetite, Fe3O4,...

Steel making slag

                          Steel making slag Steel making slag is defined as the solid material resulting from the interaction of flux and impurities in the smelting and refining of steels. It is a by product of steel making. It is produced during the separation of the molten steel from impurities in steel making furnaces. The slag occurs during steel making in liquid state. It is a complex solution of silicates and oxides that solidifies during cooling. Steel making slag is mainly calcium alumino silicate. This compound contains elements like sulfur, selenium, carbon, cadmium, lead, copper, and mercury. Many of the residuals are encased within a glassy matrix. The matrix is soluble and releases calcium and manganese oxides, which can increase the pH of the dissolving fluid to 10 or 11. As slag is a coarse glass, it maintains high permeability (~4.5 x 10-2 cm/sec) regardless of how much water has passed through it. The permeability of the slag is reduced when it is compacted or grounded into smaller particles. Unlike lime, steel making slag does not absorb CO2 from the air and convert back to relatively insoluble limestone. This is an important property. Because of this property steel making slag, when left outside exposed to the atmosphere for years, achieves high levels of alkalinity upon dissolution. The neutralization potential (NP) of steel making slag is in the range of 45 % to 78 %. Most of the residuals are in the form of alumino silicates and iron oxides Steel making slag aggregates usually exhibit a tendency to expand. This is due to the presence of free lime and magnesium oxides which have not reacted with the silicate structures and which can hydrate and expand in humid environments. This potentially expansive nature (volume changes of up to...

Pig iron

Pig iron  Pig iron is an intrmediate product of a steel plant produced during smelting of iron ore in a blast furnace. Iron ore is reduced by coke using limestone and dolomite as flux for removal of impurities. The resulting liquid iron (hot metal), when cast in the pig casting machine, produces pig iron. Fig 1 shows pig iron.                        Fig 1 Pig iron The term ‘pig iron’ has arisen from the old method of casting liquid iron from a blast furnace into moulds arranged in sand beds. The traditional shape of the molds used for these ingots was a branching structure formed in sand. It had many individual ingots at right angles to a central channel or runner. Such a configuration is similar in appearance to a litter of piglets suckling on a sow. Name of these ingots was given from the word “piglets”. When the metal had cooled and hardened, the smaller ingots (the pigs) were simply broken from the much thinner runner (the sow). Since pig iron was intended for remelting, the uneven size of the ingots and inclusion of small amounts of sand were insignificant when compared with its ease of casting and of handling. Presently most of the liquid iron is produced and consumed within integrated steel plants for steel making. In this context the term ‘pig iron’ is something of a misnomer within integrated steel plants since blast furnace iron (hot metal) is transferred directly to the steel melting shop in liquid form. The liquid iron from a blast furnace which is not sent for steel making is cast into pigs in pig casting machine for use in steel making later as cold charge or is sold to foundries or to mini steel plants having induction furnaces as merchant...