Mill Scale

Mill Scale Mill scale is the product of oxidation which takes place during hot rolling. The oxidation and scale formation of steel is an unavoidable phenomenon during the process of hot rolling which involve reheating of steel in a reheating furnace, multi-pass hot rolling and air-cooling in the inter-pass delay times and after rolling.  Mill scale is usually removed by process water used for descaling, roll and material cooling, and by other methods. It is subsequently separated by gravity separation techniques. The formation of oxide scale not only results in a significant loss of yield of steel, but also deteriorates the surface quality of the steel product caused by rolled-in scale defects or roughened surface. In addition, the presence of a hard scale layer on the steel can have an adverse effect on roll wear and working life. The amount of mill scale generated in a rolling mill depends on the type of the reheating furnace and on the practice of rolling adopted in the mill. It is generally in the range of 1 % to 3 % of the weight of the steel rolled. Mill scale mill scale is a layered and brittle material, composed of iron oxides with wustite as a predominant phase. It is normally considered as waste material. From the chemical and physical analysis performed on the mill scale, and with respect to the environmental concerns, mill scale is considered to be non-dangerous waste and normally considered as a green waste. Scale formed during the heating of steel to rolling temperatures in the reheating furnace is known as primary scale. This primary scale is removed generally by hydraulic descaling before hot rolling. The removal of the primary scale formed during the reheating operation before hot rolling is usually done for...

Solid Waste Management in a Steel Plant Sep29

Solid Waste Management in a Steel Plant...

Solid Waste Management in a Steel Plant  Steel industry in general, produces large amounts of solid wastes while processing materials through its various processes. These solid wastes have many valuable products, which can be reused if recovered economically. Steel industry throughout the world has already taken up a number of innovative measures and continues to take further for 100 % utilization of these wastes with the ultimate objective of improving the operational efficiency and economics of steel industry. These measures not only reduce the cost of waste disposal and environmental pollution but also provide substantial amount of iron ore and flux materials as well as fuel rate benefits to the existing process, thereby conserving matching amounts of raw materials. Steel industry is both capital and energy intensive and its production volumes are very high. Process chains within the industry are long. Many different technologies are applied and the industry has a significant impact on the environment. One of the major concerns of world steel industry is the disposal of wastes generated at various stages of processing. Because of natural drive to be cost-effective, there is a growing trend of adopting such waste recovery technologies which convert wastes into wealth, thereby treating wastes as by-products. This has led to aiming at development of zero-waste technologies. The technologies developed to economically convert wastes of steel plants into wealth also provide new business opportunities for prospective entrepreneurs. As per World Steel Association (WSA), the world steel industry applies the principles of reduction, reuse and recycling (3 ‘R’s) in many ways, in order to improve the sustainability of the industry. Due to this the industry has dramatically reduced need for raw materials. In the 1970s and 1980s, modern steel plants needed an average of 1.44 tons of raw...

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,...