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Characterization and Settling of Solid Contaminants in Coal Mine Effluent

Oil Sands and Hydrocarbon Recovery Dep., Alberta Research Council, P.O. Box 8330, Edmonton, AB, Canada T6H 5X2;
Fuel Supply Dep., TransAlta Utilities Corp., 110-12th Avenue S.W., Calgary, AB, Canada T2P 2M1.

^* Corresponding author (zhou{at}arc.ab.ca).

ABSTRACT

Groundwater and runoff water at surface coal mines are often contaminated with suspended solids and should be treated before it is discharged to surface water bodies. The suspended solids in the waste effluent of the Highvale mine, Alberta, Canada, are dominantly micron and submicron sized, dioctahedral smectite. They form stable colloidal suspensions in sodium bicarbonate solutions. Theoretical consideration suggests that the suspended smectite can be flocculated by divalent cations such as Ca²⁺. Experimental results show that even a small amount (2.7 mM) of gypsum can flocculate the system and cause clays to settle. Depending on the solution composition, up to 85% of the effluent volume can be reduced after 24 h settling. The clay settling rate is affected by both NaHCO₃ and gypsum concentrations. Without gypsum addition, no settling occurs regardless of NaHCO₃ concentrations (2 to 30 mM). At a fixed NaHCO₃ concentration, clay settling rate increases with an increase in gypsum concentration. At a fixed gypsum concentration, the rate of clay settling decreases with an increase in NaHCO₃ concentration. The initial settling rate (ISR) can be correlated to the equivalent fraction of Na⁺ on the clay surface (E_Na) or the exchangeable sodium percentage (ESP) on the smectite surface. Linear regression of the experimental data gives an empirical equation ISR = –0.170 – 0.459 log(E_Na) with a regression coefficient of 0.917, where E_Na = ESP/100. The functional relation between solid settling rate and ESP can be useful in the design of sediment ponds for treatment of coal mine effluent.