Paste pH

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.

Introduction

pH is a measure of the hydrogen ion activity in an aqueous solution. pH is the main factor controlling reaction rates of the primary minerals and solubilities of the secondary minerals. As pH value of the water in contact with sample reflects on the drainage chemistry behaviour of the material, pH measurements are commonly used as a part of determination of acid production potential of mine wastes (e.g. in ABA tests). (Price 1997) Paste pH is a simple static test for assessing the presence of soluble acid salts (Lapakko 2002, European Commission 2009) and reactive minerals, or in other words, the presence of stored acidity that is readily available in mine waste samples (Weber et al. 2006).

Paste pH can be used to forecast whether there is likely to be acid or circumneutral drainage as a result of weathering. Paste pH reflects the immediate pH characteristics of the sample at issue when subjected to dissolving conditions (that is, mixed with water), but it must be kept in mind that it is not possible to analyse the total acid generation capacity of the waste sample using paste pH test (such as the relation between acidifying and neutralizing minerals in the sample). In addition, paste pH does not give information of the metal release from the waste material connected to acid generation. (Weber et al. 2006)

Paste pH is a so called sum parameter for minerals. In general, it is presumed that acid generating sulphate salts (e.g. melanterite), reactive sulphides (e.g. greigite), together with high surface area carbonate and pyrite represent the group of minerals, which pH characteristics can be analysed by using paste pH. However, at least Weber et al. (2006) suggest that also other minerals can affect to the paste pH.

Description of the test method

There are numerous different procedures available for performing the test. The paste pH procedures introduced by Sobek et al. (1978) and Page et al. (1982) and their modifications are probably amongst the most cited methods. US EPA has developed a standard method to measure paste pH (Method 9045C: “Soil and Waste pH”). In Europe, no standard method for paste pH determination exist; however, first eluate collected in standard leaching test can be used to measure paste pH.

In the test setup air dried waste material and deionized water are mixed together. After allowing minerals and water to react with each other for a certain time period, pH is determined at the solution. Sample masses and reaction times before pH determination vary among different methods. Waste material used is typically crushed <250 µm particle size; Weber et al. (2006) even used the particle size as fine as < 75 μm. Different methods use different water to solid ratios. For example, Sobek et al. (1978) used 1:2 water to solid weight ratio (or alternatively “near saturation conditions” were created by adding more waste material or water depending on case) and Weber et al. (2006) 2:1 ratio, however, most methods use 1:1 weight ratio (Lapakko 2002, European Commision 2009).

pH results below neutral indicate acid formation. According to Weber et al. (2006), strategic acid rock drainage management is suggested for the mine sites if paste pH values go below 5.

Appropriate applications

Method can be used on both waste rocks and tailings (European Commission 2009). Paste pH is not suitable for measuring acidification of unweathered or fresh waste rocks or tailings, but it can be useful when the ongoing acidification of weathered mine waste is of interest (Technical Committee CEN/TC 292 2012).

According to Price (2009), the most important advantage of the traditional paste pH determination created by Sobek et al. (1978) is that the water to solid ratio of pore water in wastes can be more closely observed than it could be done with other procedures. However, Price (1997) noted that sample crushing may expose minerals occluded from weathering processes, which may alter pore water pH conditions.

The use of inappropriate test procedure, misinterpretation of results, and misleading or inaccurate descriptive terminology may lead to false results and cause wrong conclusions. These disadvantages are in fact quite commonly related to pH measurements, despite the simplicity of the tests. (Price 1997) Inaccurate test results may occur if the paste is not mixed well enough, as the results may not reflect the pH situation of the whole waste sample. Rough particles may also scratch the electrode. (Price 1997; 2009)

The usefulness of paste pH has been criticised, as the test provides only little information on the sample’s ability to generate acid drainage (e.g. because the oxidation of sulphide bearing minerals is time dependent). In fact, the paste pH of well weathered waste sample containing only low amount of sulphur, but including small amounts of products formed in consequence of residual weathering, may have significantly low paste pH. (Kania 1998) However, according to Weber et al. (2006) the comparison of the paste pH test results to other standard geochemical test results (e.g. to kinetic NAG test) shows that the potential of paste pH determination is indicating the immediate acid-base reactivity of mine waste sample.

Although the paste pH has limited usefulness in predicting the mine drainage chemistry, it can be used as a site specific screening tool to characterise mine wastes with different acid drainage implications during operational management of the mine (Pope et al. 2010). However, paste pH determination in itself is inadmissible for predicting acid generation potential of mine waste material and other methods should be used in conjunction with it.

Performance and design requirements

Paste pH measurements can be performed either in the laboratory or in the field (Technical Committee CEN/TC 292 2012). The method is non vigorous as only deionized water is used (Weber et al. 2006). No expensive equipment is needed to perform the measurement. Paste pH determination is simple and relatively rapid to perform. Several samples can be measured one after another.

The calibration of pH electrode is highly important for achieving reliable results. The solid:water ratio together with the relative magnitude of the unweathered interior and weathered surfaces of crushed particles are the main variables affecting the pH measurement (Price 2009). Also drying the sample, content of soluble salts in sample, amount of crushing, electrode junction potential, and seasonally influenced content of CO₂ can be added to the list (Jackson 1958 and Peech 1965, cited by Sobek et al. 1978)

References

European Commission 2009. Reference Document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities. January 2009.

Kania, T. 1998. Laboratory methods for acid-base accounting: an update. In Brady, K. B. C., Smith, M. W. & Schueck, J. (Eds.) Coal mine drainage prediction and pollution prevention in Pennsylvania. Dept. of Env. Protection. Harrisburg. p. 6-1 to 6-9.

Lapakko, K. 2002. Metal Mine Rock and Waste Characterization Tools: An Overview. Minnesota Department of Natural Resources, US. April 2002 No. 67. Page, A. L., Miller, R. H. & Keeney, D. R. 1982. Methods of Soil Analysis. Part 2 – Chemical and Microbiological Properties. American Society of Agronomy Inc., Soil Science Society of America Inc. Second Edition, pp. 199-209.

Pope, J., Weber, P., Mackenzie, A., Newman, N. & Rait, R. 2010. Correlation of acid base accounting characteristics with the Geology of commonly mined coal measures, West Coast and Southland, New Zealand, New Zealand Journal of Geology and Geophysics, 53:2-3, 153-166.

Price, W.A. 1997. DRAFT Guidelines and Recommended Methods for the Prediction of Metal Leaching and Acid Rock Drainage at Minesites in British Columbia. British Columbia Ministry of Employment and Investment, 142 p.

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

Sobek, A.A., Schuller, W.A., Freeman, J.R. & Smith, R.M. 1978. Field and Laboratory Methods Applicable to Overburdens and Minesoils. U.S. Environmental Protection Agency EPA 600/278054, 204 p.

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

Weber, P. A., Hughes, J. B., Conner, L. B., Lindsay, P. & Smart, R. S. C. 2006. Short-term Acid Rock Drainage Characteristics Determined by Paste pH and Kinetic NAG Testing: Cypress Prospect, New Zealand. 7th International Conference on Acid Rock Drainage (ICARD), March 26-30, 2006, St. Louis MO.