Slag and its Role in Blast Furnace Ironmaking Aug07

Slag and its Role in Blast Furnace Ironmaking...

Slag and its Role in Blast Furnace Ironmaking Blast furnace (BF) is the oldest (more than 700 years old) of the various reactors which are being used in the steel plants. It is used for the production of liquid iron (hot metal). The blast furnace is a complex high temperature counter current reactor and is in the shape of a shaft in which iron bearing materials (ore, sinter/pellet) and coke are alternately charged at the top along with flux materials (limestone, dolomite etc.) to create a layered burden in the furnace. Preheated air is blown in from the lower part of the furnace through tuyeres. This hot air reacts with the coke to produce reducing gases. Descending ore burden (iron oxides) is reduced by the ascending reducing gases and is melted to produce hot metal. The gangue materials and coke ash melt to form slag with the fluxing materials. The liquid products (hot metal and slag) are drained out (tapped) from the furnace at certain intervals through the tap hole. The quality of hot metal obtained is dependent on the formation of the slag and its mineralogical transformations. A good quality slag is necessary for a quality hot metal. The slag is a mixture of low melting chemical compounds formed by the chemical reaction of the gangue of the iron bearing burden and coke ash with the flux materials in the charge. All unreduced compounds such as silicates, aluminosilicates, and calcium alumino silicate etc. also join the slag. It is well known that the components of slag namely silica (SiO2) and alumina (Al2O3) increase the viscosity whereas the presence of calcium oxide reduces the viscosity. The melting zone of slag determines the cohesive zone of blast furnace and hence the fluidity and melting characteristics...

Blast furnace slag granulation at cast house May05

Blast furnace slag granulation at cast house...

Blast furnace slag granulation at cast house Blast Furnaces normally generate 250 to 350 Kg of liquid slag for every ton of hot metal produced. This molten slag is at around 1400 – 1550 deg C temperature.  Till seventies blast furnace (BF) slag was considered a waste product and was being dumped at a convenient place away from the blast furnace.  During early seventies granulation of molten slag with high pressure water started near the slag dumps. The processes of granulation of molten slag were developed during late seventies and early eighties. The processes differ in the method of dewatering of the wet granulated slag. Today granulation of molten BF slag is the well accepted technology and is being used in most of the blast furnaces around the world. In India the credit of commissioning the first cast house slag granulation plant goes to Visakhapatnam Steel Plant when the first blast furnace of the plant was commissioned on 28th march 1990. Major slag granulation processes presently under operation are OCP granulation system, Gipromez designed plants, Rasa system and INBA slag granulations plants of Paul Wurth, modified INBA process or IDE process etc. Concept of slag granulation The process of slag granulation involves pouring the molten slag through a high pressure water spray in a granulation head, located in close proximity to the blast furnace. Granulation process is the controlled quenching of the slag in cold water which does not give time for crystalline growth to take place. Large volume of water is required (10 parts of water to 1 part of molten slag being optimum). During this process of quenching, the molten slag undergoes accelerated cooling under controlled water flow condition and gets converted into glassy sand with 97 % of the solid granulated...

Blast Furnace Slag

Blast Furnace Slag  Blast furnace (BF) slag is a nonmetallic by product produced during the process of iron making in a blast furnace. It consists primarily of silicates, aluminosilicates, and calcium-alumina-silicates. The molten slag usually absorbs most of the sulfur from the blast furnace charge. Blast furnace slag is mildly alkaline and exhibits a pH in solution in the range of 8 to 10. Although blast furnace slag contains a small component of elemental sulfur (1 % to 2 %), the leachate tends to be slightly alkaline and does not present a corrosion risk to steels in pilings or to the reinforcement steels embedded in concrete structures made with blast furnace slag cement or aggregates. In certain situations, the leachate from blast furnace slag may be discolored (characteristic yellow/green color) and have a sulfurous odor. These properties appear to be associated with the presence of stagnant or slow moving water that has come in contact with the slag. The stagnant water generally exhibits high concentrations of calcium and sulfide, with a pH as high as 12.5. When this yellow leachate is exposed to oxygen, the sulfides present react with oxygen to precipitate white/yellow elemental sulfur and produce calcium thiosulfate, which is a clear solution. Aging of blast furnace slag can delay the formation of yellow leachate in poor drainage conditions but does not appear to be a preventative measure, since the discolored leachate can still form if stagnant water is left in contact with the slag for an extended period. BF slag is normally produced in two forms (Shown in Fig. 1). These two forms are described below.  Fig 1 Granulated and air cooled BF slag 1.   Granulated blast furnace slag – When liquid slag is cooled and solidified by rapid water quenching to...