Pit Lake Chemistry

Condition Case Minerals allowed to precipitate and dissolve 

Note that reactions that involve the precipitation and dissolution of the minerals, with the exception of gypsum, in Davis and Ashenberg s (1989) simulations consume or produce protons. Therefore, they more or less buffer the pH changes upon the mixing of the pH 11.3 tailings fluid with the pH 2.7 pit lake water. It should also be noted that equilibrium with atmospheric CO2 also constitutes a buffer reaction. 

Case 1 Rapid increase of pH occurs with increasing amounts of alkaline tailings fluids mixed with acidic pit water. This is an unrealistic scenario because Fe and A1 in the acidic pit lake water will precipitate and buffer pH in one solid form or another. 

Case 2 This case should be compared with Case 1 to examine the effects of solid precipitation reaction to buffer the pH of mixed fluids. Slow increase of pH from 2.7 to 3.5 is caused by buffering of Fe(OH)3 (see Table 8.6). Rapid increase of pH from 3.5 to 5.5 is due to lack of buffering reactions. It should be noted that if the authors had allowed Al(OH)3 or kaolinite to precipitate in their simulations, the pH of the mixtures would be buffered at about 4.5 until most A1 is precipitated from the solutions (see Section 6.2). The authors measured A1 concentrations of about 200 mgL-1 in the pit lake. 

Case 3 This case should be compared with Case 2 to see the effect of buffering with an external reservoir of CO2 gas. The CO2 gas dissolution should have a buffering effect on the pH in the range of about pH 6.3. The pH stabilizes at 8.0. 

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Table 9 Example water chemistry observed in existing pit lakes. 

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