Geochemical treatment of coal mining waste waters at Highvale Mine, Alberta, Canada

The Highvale Mine is Canada's largest surface coal mine with annual production of 12 million tons. One and half billion gallons of mine water effluent is produced annually from surface runoff and groundwater that enters four different mine pits. This water is contaminated by suspended solids which have to be removed before the water can be discharged downstream. A number of settling ponds were built to settle the suspended solids. However, the settling rate of the solids is so slow that the ponds cannot achieve their purpose. Currently, the effluent is discharged into a cooling pond which serves an on-site power plant. A long-term solution is being sought to treat the effluent water. The suspended solids in the Highvale mine effluent are sub-micron smectite clays. The water is essentially a sodium bicarbonate solution with a pH ranging from 8 to 9. The concentration of calcium in the solution is usually less than 1 mM. Under these conditions the smectite surface is expected to be Na-saturated and an expanded electric double layer is formed at the clay/solution interface. This leads to a strong repulsion force between the clay particles which hinders the clays from settling. Laboratory study shows that the claymore » settling rate is increased significantly when gypsum is added into the mine effluent. The settling rate is influenced most by (Ca[sup 2+])/(Na[sup +]) ratio of the effluent, where (Ca[sup 2+]) and (Na[sup +]) are the activities of Ca[sup 2+] and Na[sup +] respectively. Based on the solution chemistry and solid concentration of the effluent, the optimum gypsum addition can be calculated. The supernatant derived from this process is practically free of suspended solids. A pilot facility is being considered for the water treatment based on this process.« less