Molten slag

Another type of combustion unit operates at about 1600°C to produce a molten slag which forms a granular frit on quenching rather than the usual ash. The higher operating temperature is obtained by preheating the combustion air or by burning auxiUary fuel. 

Normally ca 50% of the coal ash is removed from the bottom of the gasifier as a quenched slag. The balance is carried overhead in the gas as droplets which are solidified when the gas is cooled with a water spray. A fluxing agent is added, if required, to the coal to lower the ash fusion temperature and increase the molten slag viscosity. 

The molten slag and the molten Hon, called hot metal or pig Hon, ate tapped from the hearth of the blast furnace. A modem blast furnace yields 5000—9000 t/d of Hon. The compositions of the pig Hon and the slag are determined by the furnace temperature, the composition of the ore, and the added flux. Pig Hon always contains 3.5—4.5 wt % carbon, variable amounts of siHcon, manganese, sulfur, and phosphoms. 

Still another process, called BI-GAS, was developed by Bituminous Coal Research in a 73 t/d pilot plant in Homer City, Peimsylvania. In this entrained-bed process, pulverized coal slurry was dried and blown into the second stage of the gasifier to contact 1205°C gases at ca 6.9 MPa (1000 psi) for a few seconds residence time. Unreacted char is separated and recycled to the first stage to react with oxygen and steam at ca 1650°C to produce hot gas and molten slag that is tapped. 

The carbides are generally not resistant to molten slags and fused salts. Their resistance to molten metal is usually poor. For instance, TiC is attacked by nickel, cobalt, chromium, and silicon. SiC is attacked by aluminum. 

Mills has concluded in his review article on molten slags that (1) most viscosity measurements were subject to experimental imcertainties of 25% (2) in some cases experimental uncertainties could be > 50% and (3) experimental uncertainties as low as 10% could be achieved by careful calibration of viscometers with high and low temperature reference materials. 

In most unit operations it is of considerable importance that material is transferred from one phase to another across a boundary. The transfer of material from a solid phase to a liquid phase (as typically in leaching), or the transfer of material between one liquid phase to another liquid phase (as typically in molten metal and molten slag phases), extraction or between liquid and vapor phases (as typically in distillation) are well-known examples encountered in practice. 

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