The cleaning of an Australian coking coal by mild acid leaching to improve coke properties is investigated. Anglo Coal supplied a sample of coking coal, from a coal mine located in the Bowen Basin in Central Queensland, to be studied. Coking coal, which is used to produce metallurgical coke, is mainly used in steel blast furnaces as a reducing agent and to provide energy and physical support to the burden. It has been shown that the coke is subjected to chemical attack by oxidising gases (eg CO, CO2) and alkalies (eg Na, K) targeting the structure of coke, and physical abrasion and compression as it moves through the blast furnace. These two factors contribute to coke size and strength reduction resulting in a loss of ability to support this burden.
A measure of coke reactivity with carbon dioxide is determined by the Coke Reactivity Index (CRI) test and a measure of the hot strength is the Coke Strength after Reaction (CSR) test. It has been shown that the ash chemistry is an important factor relating to coke reactivity and strength. It is theorised that basic minerals catalyse coke reaction with carbon dioxide.
Therefore, some coking coals may benefit by altering the ash chemistry to lower the basicity index. Conventional washing cannot selectively remove basic minerals, though this may be achieved by acid leaching with hydrochloric or citric acid. Thus, the following hypothesis was formed:
Leaching a selected size fraction of coking coal with hydrochloric or citric acid can reduce the basic ash forming mineral matter content, thereby significantly improving the coke properties of the bulk coal.
To determine the most appropriate coal particle size for leaching a size and ash analysis was performed on three size fractions: -50 +12mm, -12 +0.5mm w/w, -0.5mm w/w. They were chosen to represent different processes routes in conventional wash plants. The results showed that the basicity index was 0.94, 0.97 and 0.145, and the mass proportions were 17.7%, 55.8% and 26.5% respectively. It was decided that the –0.5mm size fraction was the most appropriate for acid leaching.