Anode Coke from Coal - A Low Cost Approach

The decline in quality and availability of petroleum cokes that have traditionally been used for the production of carbon anodes is obliging aluminum producers to use cokes that would previously have been regarded as unsuitable[1]. Domestic reserves of coal represent an alternative carbon resource for anode production, provided coke specifications can be met at an economically viable cost. Previous studies[2] have shown that carbons suitable for aluminum production could be produced by the solvent refining of coal. To conform to coke specifications (eg the metals content), it was necessary to remove mineral matter in the coal by inclusion of a filtration step. This adds significantly to the cost of the process, making it uncompetitive with coke from other sources. However, cokes with properties very similar to the required specifications have been produced from very low ash coals by solvent extraction, but without the uneconomic solids removal step[3]. Extraction in a suitable solvent is used to produce a coal digest, which when injected at high temperature into a coking vessel generates the anisotropic needle structure characteristic of high quality cokes. It is proposed to investigate whether low-ash coals from the eastern Kentucky coalfield can be used to produce suitable cokes using these principles. The high cost filtration step would be omitted. High volatile ‘A’ bituminous coals of eastern Kentucky produce anisotropic cokes and many have been used as blend constituents for production of metallurgical coke and are prime candidates for this application. It should be possible to reduce the ash content of the feed coal yet further through coal cleaning, e.g. selective oil agglomeration applied to finely divided coals. Hence, it is proposed to convert a range of low-ash coals into cokes via solvent extraction and coking with nitrogen sparging to promote solvent stripping and simultaneously applying shear to the polymerizing residue. The resultant cokes will be calcined and their properties compared to that of typical petroleum anode cokes. The effect of process variables including coal type and ash content, solvent type and process conditions would be investigated. This proposal represents a proof of concept study to examine the feasibility of producing anode-grade coke by the solvent extraction of coal, but without a costly solids separation step. It is intended to select suitable high volatile ‘A’ coals from the Eastern Kentucky coalfield (eg Knox and Whitley counties) and determine whether the process can be adapted to provide an economically viable alternative for producing anode-grade coke. Western Kentucky primary aluminum producers (RioTinto Alcan and Century Aluminum located in Henderson and Hancock counties) have expressed interest in the project and offered their backing.