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This case study is copyright (©) 1997 by Kevin A. Morin and Nora M. Hutt, and is adapted from Chapter 5 of Environmental Geochemistry of Minesite Drainage: Practical Theory and Case Studies.
One of the most misunderstood concepts in the prediction of drainage chemistry is "neutralization potential" (NP). For acidic-drainage predictions, NP basically represents the amount of acidity that can be neutralized by a sample of rock, tailings, or soil. For alkaline-drainage predictions, NP represents the amount of base that can be released into drainage waters.
NP has historically been defined as the result of a laboratory analysis. However, as the number of methods for NP has increased (Table 1), an interesting problem has arisen: for a particular sample, each method gives a different value of NP (Table 2)! Long ignored has been the question of which is correct. The answer is not so obvious because, to come full circle, NP is simply the result of a laboratory analysis. So all are right!
ABA Methods Used by Lapakko (1993)
|Standard Sobek ABA||Sobek et al., 1978||Total S: LECO; SO4: not explained||Crushed to <0.25 mm||Determine fizz rate, then add specific volume and normality of HCl to 2 g sample; heat to nearly boiling until fizzing stops; add distilled water to obtain 125 mL; cool; titrate to pH 7 with NaOH|
|Modified ABA||Lawrence, 1990||Total S: not explained Sulfide: LECO after non- oxidative digestion with HCl||Like Standard ABA||Determine fizz rate, then add specific volume and normality of HCl to 2 g sample; let stand 24 hours at room temperature; if pH not between 1.5 and 2.0, start again with adjusted acid addition; when pH between 1.5 and 2.0, titrate to pH 8.3 with NaOH until pH constant for 30 seconds|
|BC Research||Duncan and Bruynesteyn, 1979||Total S: Leybold- Heraeus carbon- sulphur analyzer||Crushed to 70% <0.044 mm||Titrate 10 g of sample and 100 mL distilled water with 1.0 N H2SO4 until pH 3.5 reached and less than 0.1 mL H2SO4 added over 4 hours; NOTE: Lapakko also performed same technique lowering pH only to 6.0 and 5.0|
|Net Acid Production||Lawrence et al., 1988||Does not provide sulfur levels||Crushed to 80% <0.074 mm||Does not provide NP; incrementally add 100 mL of >30% H2O2 to 1 g sample; heat to near boiling for 1 hour; if H2O2 depleted, add 50 mL of H2O2 and heat for 30 minutes; cool for 10-15 minutes, add 0.0157 M Cu solution, boil for 10 minutes; filter out solids, wash with 1 M CaCl2; titrate to pH 7.|
Comparison of Five NP Techniques for Acid-Base Accounting and Mineralogy (adapted from Lapakko, 1993 and 1994, and Lapakko et al., 1995)
Sample (RK = waste rock, TL = tailings)
|Sobek NP (pH<2)||12||35||15||28||27||18||46||3.8||7.5||99|
|Modified Sobek NP (pH 1.5-2.0)||9.6||33||14||28||27||20||61||2.9||3.2||72|
|BCR NP (pH 3.5)||7.7||11||25||33||30||25||82||15||20||58|
|Lapakko pH 5 NP||4||4||5||29||25||19||34||6||16||57|
|Lapakko pH 6 NP||3.0||2.8||3.3||28||24||16||30||3.8||15||55|
|1 As t CaCO3 equivalent/1000 t; CaNP calculated from CO2 measurements.|
|2 As weight percent.|
To resolve this confusion, a concept known as Empirical, or Effective, Neutralization Potential (ENP) has appeared. ENP is defined as the actual, real NP that exists in a sample under the environmental conditions that it will reside in. While this gives a better definition, the issue is to determine which NP method gives ENP. Unfortunately, none do!
Due to site-specific variations in factors like mineralogy and climate, no laboratory-based NP method gives ENP. Instead, all NP values must be converted to ENP values by adding and/or subtracting amounts. The only NP method with large database to demonstrate how the adjustments should be made is the Standard Sobek Method (also known as EPA 600 Method). These database include the International Static and Kinetic Databases (ISD and IKD).
Some publications and websites say that the Standard Sobek NP is often the highest laboratory-based value because of the aggressive test conditions. That is wrong, as demonstrated in reports by the international NP expert, Kim Lapakko of the Minnesota's (USA) Department of Natural Resources. His work shows that other methods can yield higher values, particularly the B.C. Research Method. Other studies show that all NP methods can greatly overestimate or underestimate ENP, so all methods require adjustments.
© 1997 Kevin A. Morin and Nora M. Hutt
Lapakko, K. 1994. Evaluation of neutralization potential determinations for metal mine waste and a proposed alternative. IN: International Land Reclamation and Mine Drainage Conference and Third International Conference on the Abatement of Acidic Drainage, Pittsburgh, PA, USA, April 24-29, Volume 1, p. 129-137. U.S. Bureau of Mines Special Publication SP 06A-94.
Lapakko, K. 1993. Evaluation of Tests for Predicting Mine Waste Drainage pH. Report to the Western Governors' Association by the Minnesota Department of Natural Resources. 76 p. plus appendices.
Lapakko, K, J. Wessels, and D. Antonson. 1995. Long Term Dissolution Testing of Mine Waste. Report to the U.S. EPA, Grant X-8200322-01-0, 85 p. plus appendices.
For more details and case studies on NP and ENP, see Environmental Geochemistry of Minesite Drainage: Practical Theory and Case Studies.
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Created by K.Morin