Bf Slag 5a3t61

BLAST FURNACE SLAG

Slag formation in Blast furnace. Slag is a fused product formed by the action of a flux (Limestone/Dolomite) upon gangue of ore or fuel or upon the oxidized impurities of metal. The BF slag is defined as “ the non metallic product essentially consisting of silicates and alumino-Silicates of lime and other bases which are developed simultaneously with iron in a BF. Slag is an essential and important output of BF process. Slag is a medium which absorbs unwanted or unavoidable impurities of iron bearing burden and coke ash. The course of slag formation can be divided into 3 zones. 1. Primary Slag- Mainly consists of FeO 2. Bosh slag – Basicity is twice the final slag basicity 3. Final slag.

The formation of the first slag occurs through fluxing of the alumino-silicates by unreduced iron oxide. The molten slag then flows over lumps of limestone and incandescent coke and in the process picks up lime, forming what is known as primary slag which loses its ferrous oxide (FeO) rapidly by reaction with coke at the prevailing high temperatures. These processes occur simultaneously in the belly and the bosh where most of the Silica and Alumina from coke ash have not been released yet. The slag formed in this region above the tuyere is termed as secondary slag or bosh slag. The final slag or hearth slag is formed when the coke ash released at the tuyeres and the still un dissolved lime is incorporated in the descending bosh slag which flows down to the hearth.

Chemical Composition of Slag CaO- 32%, SiO2- 33%, Al2O3- 20%, MgO- 10%., S- 1.1%, MnO- 0.54%, TiO21.01%, FeO- 0.54%, K2O, Na2O- 1%, traces of Oxides, Total- 100%. Basicity = CaO/SiO2. Total Basicity = (CaO+ MgO)/SiO2

Basic requirement of slag for normal operation of a BF 1.

The slag volume should be kept as low as possible, keeping in view alkali removal and desulfurization problems.

2.

Slag formation should be confined to a limited height of BF.

3.

The slag must be enough fluid to get sufficiently heated abd should provide good permeability in the zone of slag formation.

4.

The melting temperature (liquidus temperature) of the slag should neither too high or too low.

Function of Slag Since slag has a low thermal conductivity, it acts as a barrier for heat to go out of the bath, thus maintaining the temperature of the metal bath. It also acts a physical barrier to the age into the metal ot the gaseous species such a H2, N2, S.

Slag Regime: Slag is formed by oxides of Si, Ca, Mg, Al and small quantities of ferrous & Magnesium oxides not reduced during the heat. Fluxes like limestone & Dolomite are added to fuse the high M.P gangue material like SiO2, Al2O3, which are present with the ore. In BF slag the basic constituents are CaO, MgO, MnO, FeO, Na2O and the acidic constituents are SiO2, P2O5. Slags with Al2O3 below 10-15% is considered as intermediate oxides. Al2O3 doesn’t form a sulfide, it is not a desulfuriser

Al2O3 is retained or dissolved in slag in presence of SiO2. So more Al2O3 in slag will need more SiO2 i.e Quartzite. To neutralize this siliceous gangue (Al2O3+ SiO2), more limestone will required. CaCo3. If the slag is low melting, such as acid slag, it melts at higher zone in the shaft and after melting, it trickles down fast (due to high fluidity) to absorb enough heat of the hearth. If the slag melting temperature is high, such as basic slag, it melts at lower level in the shaft and reaches to the tuyere level heated to a high temperature. Thus the basic slag i.e high melting slag's heat the hearth and low melting slag cool the hearth.

Liquidus Temperature of slag The liquidus temperature should be such that enough degree of superheat is available to maintain adequate fluidity of slag, viscosity of slag should be around 5 poise. Liquidus temperature of the slag should neither too high or too low. Alkali removal capacity of slag Alkali removal capacity of slag = % Alkali in slag x Slag Vol. Since Alkali content of slag increases with decreasing basicity and temperature, lower slag basicity, lower slag temperature and higher slag volume are desirable for better Alkali removal. Viscosity of Slag The higher the basicity ratio the lower the viscosity. Viscosity increases with decreasing temperature.

PROCESSING OF SLAG Molten slag has a temperature of 1450-1500 deg.C. Granulated Slag- The molten slag is chilled. This is done to prevent crystallization. The granulated slag which is in the form of glassy beads to light –weight froth. Glass wool- Slag wool is manufactured by re-melting the solidified slag in a cupola furnace and tapping the molten slagin a thin steam through nozzle where it is suddenly chilled to vitreous threads by jets of air or steam. Utilization of Slag Granulated slag is a Raw Material for cement industries. Slag wool is used as a insulating material.

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