Technical Market Support » Metallurgical Coal
During the past 30 years there has been a major trend in the metallurgical industry towards the use of high intensity bath smelting reactors for large-scale production of metals. Coal is commonly used as a fuel and/or reductant in these reactors.
GK Williams Cooperative Research Centre for Extractive Metallurgy undertook this project to improve understanding of the behaviour of coal and the importance of coal properties in the coal injection process and subsequent bath smelting reactions.
The sponsors were ACARP, Ausmelt, Pasminco, WMC and Rio Tinto. The project commenced on 1 July 1995.
The project aims were to predict and interpret the performance of slag bath reactors using coal as the reductant and to determine the relative importance of coal properties to the performance of slag bath reactors. A combination of experimental work and mathematical modelling of molten bath reactors was used to meet the project aims.
A generic PC-based model for slag bath reactors was developed which can be tuned to particular reactors using plant trial data. The slag fuming version of the model was tuned to the operation of the Pasminco, Port. Pirie, No.1 slag fumer.
Experimental investigations were carried out on coal devolatilisation at bath smelting temperatures, the interfacial reaction rates between fumer slags and gases derived from the submerged combustion of coal with slags, and the reduction rates of slags by coal chars and other carbons.
The key experimental findings are:
Impact of slag viscosity: The particle velocity required for capture of injected coal increases with increasing slag viscosity and decreases with increasing particle size and density. It can be predicted using a dimensionless plot based on a dynamic force balance model developed in the project.
Volatiles yield: The actual volatiles yield for short heating times is determined by the devolatilisation rate and may be significantly less than the proximate volatiles.
Mass transfer limitation: The gases in zinc slag fumer plumes during normal operation are oxidising relative to the slag bath. The interfacial oxidation reaction is likely to replace gas phase mass transfer as the rate limiting step for oxidation of the bath by the plume bubble (for bubble sizes less than about 0.01 m).
Liquid phase mass transfer: The reduction rate of FeO by solid carbonaceous reductants in iron making slags containing less than 20% FeO is controlled by liquid phase mass transfer between 1300 -1375?C.
Ferric oxide reduction: The ferric oxide reduction rate of a typical non-ferrous (high iron content) smelting slag by coal added to the bath is directly proportional to the char yield of the coal measured by heating to 1400?C but is apparently independent of the proximate properties of the coal.
Carbon content: The fixed carbon content of the coal is the most important factor affecting reduction efficiency.
Reduction efficiencies: High fixed carbon coals are predicted to give the highest reduction efficiencies. For these coals, a high entrainment rate is also desirable.
Entrainment: High entrainment will increase efficiency and reduce the excess heating effect that occurs due to the higher available heat from the coal. However, this does not take into account the lower reactivity of the high carbon coals which may increase the amount of coal bypassing the furnace.
Lower fixed carbon content: For coals with a lower fixed carbon content, the PC-based modelling predicts an optimum entrainment rate which is related to the heating value of the coal.
Moisture and ash: Reduction efficiency is improved by the reduction of both the moisture and ash contents of the coal. This is primarily due to the corresponding increase in the fixed carbon content that this represents.