Acid-base accounting (ABA)

Henna Punkkinen, Markku Juvankoski, Tommi Kaartinen, Jutta Laine-Ylijoki, Elina Merta, Ulla-Maija Mroueh, Jarno Mäkinen, Emma Niemeläinen & Margareta Wahlström, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland

A variety of static tests methods exists for rough estimation of whether a waste material will potentially generate acid mine drainage. Acid-base accounting (ABA) is the most commonly used static test method for characterizing mine wastes. It is a screening method for determining the acid generation potential and acid neutralisation potential of the mine waste material tested, and does not give information on the acid production or neutralisation rates (Technical Committee CEN/TC 292 2012). Thus, the test implications need further verification (Lapakko 2002), for example with kinetic test methods.

ABA tests include the determination of sulphur/sulphide content of waste to calculate acid potential (AP) of the waste. The neutralisation potential (NP) depends on the amount of carbonates and other alkaline material. The most important acid producing minerals in this respect are iron containing sulphides, especially pyrite (FeS₂) and pyrrhotite (Fe_(1-x)S), while the carbonates, especially calcite (CaCO₃) are the main rapid neutralisers (Technical Committee CEN/TC 292 2012).

The ratio of NP and AP (neutralisation potential ratio, NPR) or the subtraction NP-AP (net neutralisation potential NNP) is then used for estimating a potential risk for acid mine drainage. The NPR and NNP can be evaluated by the following criteria (Price 2009):

However, these interpretation criteria are only simplifications. There are several different variations of ABA test, including standard ABA (“Sobek method”), modified ABA (“modified Sobek”), and the upgrade of the most used methods “acid addition on the basis of the carbonate content” (EN 15875), which is a recommended method within Europe. Although these tests have common features, different results may occur when using alternative methods for testing same samples. This should be kept in mind especially when interpreting the test results so that incorrect conclusions would be avoided. Also, it is critical to understand that the test interpretation may be misleading when (Technical Committee CEN/TC 292 2012):

• the test material is not homogenous. In this case the result of the test will depend upon the sampling procedure. Acid production may occur in the parts of the waste material where no neutralising material is available, even if the test result does not indicate any acid generation potential.
• the sulphides oxide but there are neutralising materials available. In these cases, some hazardous elements that are mobile in neutral conditions may release and cause neutral rock drainage (NRD). (Technical Committee CEN/TC 292 2012)

And:

• AP may be overestimated if there is a remarkable amount of other sulphide minerals than (rapid acid producer) pyrite
• AP may be underestimated if waste contains much easily dissolvable and acid generating iron sulphate minerals, or if siderite (FeCO₃) (that acts as an acid generator above pH 3.5 – 4 and below that as an acid neutraliser) is present in a remarkable amount.
• NP may be underestimated if silicate minerals have a long reaction time, as many of them have a potential to neutralise acid if the time period is long enough. (Technical Committee CEN/TC 292 2012)

References

Lapakko, K. 2002. Metal Mine Rock and Waste Characterization Tools: An Overview. Minnesota Department of Natural Resources, US. April 2002 No. 67.

Price, W.A. 2009. Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials. MEND Report 1.20.1. 579 p.

Technical Committee CEN/TC 292 2012. Characterization of waste – Overall guidance document for characterization of wastes from extractive industries. CEN/TR 16376:2012.