Open Cut » Environment
A technique was developed that could be used on mine sites to quantify salt uptake and release from coal during processing. Tests were conducted on coals from 14 sites from the Bowen Basin, Hunter Valley and Illawarra. Results showed that in general salt uptake and release was a linear function of the salt concentration of the initial solution. Salt was released from all coals in deionised water and taken up by the coal as the solution salt concentration increased. In general, Na content of the coals during these experiments increased on the order of ~10 % depending on the conductivity of the water used during processing.
This work has shown that in most cases the compromise to product quality is likely to be relatively small. Consequently, concerns around product quality should present less of a constraint to using salty water in the CHPP. It is recommended that water re-use be actively promoted as part of the water management strategy for coal preparation on sites. Clearly for sites were the coal has a high Na content before processing, even a small increase may represent a threat to product quality. This threat could be assessed routinely on site using the method described in this work.
The project also tested a passive salt concentration control mechanism (called Passive Salt Governor, PSG) to maintain maximum salt concentration levels for various water uses across a mine site. Results of the PSG trials showed that while the coal could be used to capture salts from solution the amount of freshwater required to flush the salt from the coal to regenerate the PSG was greater than the volume of salty water used to load the PSG. In addition there was a small, but significant amount of salt that became tightly bound to the coal and could not be released during the flush cycles. Thus, the PSG system would not be an effective means by which to remediate saline water on sites as the amount of freshwater required to regenerate the column would be greater than the volume of desalinated water produced by the system.
A mechanism was proposed to explain the cation uptake/release characteristics observed in this work. Further research is required to determine the coal characteristics that affect uptake and release of cations and the implications for coking quality.