Underground » Environment - Subsidence and Mine Water
Mines prioritise their water use strategies not just because it is a limited shared resource, but also to achieve environmentally safe discharge after use. Water treatment is already adopted in various coal mines and a greater emphasis is placed by the Government for more water treatment and for mines to have positive water balance. Demand for water is also increasing for their use in coal handling and preparation plants and other mine operations.
Generally, the direct reuse or disposal of the mine impacted water is not possible, due to the presence of salt and other ions, without some form of treatment and/or amendment. Reverse osmosis has been one of the leading desalination treatment technologies, which can typically achieve 50-60% water recovery, depending on the feed mine water characteristics. Some mine impacted waters require various pre-treatment such as neutralisation, pre-filtration etc. to make it suitable to treat through the reverse osmosis process. Yet, management of the brine produced as a by-product of RO treatment remains a challenge as a proportion of available storage capacity must be dedicated to brine. Minimisation of brine volume reduces one of the key constrains of brine management for the industry.
So far, evaporation ponds have been used to concentrate the brines, but these are only practical where the right climatic conditions are available and require large surface area and careful management. Besides, with the increasing value of freshwater, water evaporated from the evaporation ponds cannot be collected and reused, thereby making no contribution to improving water usage efficiency. Further to loss of water, additional expense is incurred for the mine to replace this lost water. Most coal mine operations in Australia are located inland and do not have the option of ocean disposal and are looking for alternate methods to handle brine. A portion of the brine generated from the treatment process could be diluted with mine water and used for dust suppression. Runoff from dust suppression would be recaptured, with the increasing salt concentration within the salt management system. Remaining brine is stored on-site, reticulated to mining or waste emplacement areas. Minimising the salt brine volume will significantly minimise the salt disposal cost. It was estimated that in some areas of inland desalination treatment, every 1% reduction of their brine volume offered a savings of about $1M in disposal costs.[1] Hence, developing and adopting an effective brine management option would lower the brine waste footprint for the coal industry.
In this project, a proof-of-concept of a low thermal based membrane distillation process coupled with a crystalliser (MDC) was demonstrated using actual brine and mine water samples. Two coal mine reverse osmosis brines and one mine impacted water were used as test samples in this project to further concentrate and simultaneously recover reusable clean water using the lab scale MDC process operated in a batch mode. The experimental results obtained show that the MDC process was able to concentrate and further reduce the volume of brine produced from the reverse osmosis process and also the acidic mine impacted water, operating at 55°C feed water temperature. This process also recovered a high-quality reusable permeate water both with hypersaline brine and acidic mine water. Under batch mode of operation, water recovery of about 60-90% was obtained with a flux rate of about 15-20 LMH. The solute rejection of the membrane was found to be over 99.5%, recovering clean permeate from the vapour permeating through the membrane. Integrating with a crystalliser, resulted in achieving crystallisation for all three samples tested through the MDC process.