Teemu Karlsson, Geological Survey of Finland, P.O. BOX 1237, FI-70211 Kuopio, FINLAND, e-mail: teemu.karlsson(at)gtk.fi
The net acid generation (NAG) test is a static test developed to predict the generation of acid rock drainage (ARD) during and after the mining operation. It is suitable to use as a stand-alone prediction tool (Miller et al. 1997) and as a supplement to other static tests such as the acid-base accounting (ABA) test
(Räisänen et al. 2010, Morin & Hutt 1999).
The NAG test is based on the reaction of a sample with hydrogen peroxide, which accelerates the oxidation of sulphide minerals in the sample. During the test, acid generation and acid neutralization reactions can occur simultaneously, end-result representing a direct measurement of the net amount of acid generated by the sample. The test does not estimate neutralisation potential, therefore e.g. the AMIRA guidebook (2002) recommends to use the net acid production potential (NAPP) together with NAG for more detailed classification of acid generation. (Räisänen et al. 2010, Morin & Hutt 1999)
Description of the method
Details of the different NAG methods have been obtained from the AMIRA guidebook (2002), Stewart et al. (2006), and Miller et al. (1997).
Single addition NAG test
The most basic NAG test involves a mixture of a 250 ml of 15% (m/V) H2O2 and 2.5 g of pulverised (less than 75 µm) sample, which is allowed to react overnight (for about 12 hours). The sample is boiled until the visible reaction has ceased, i.e. the excess H2O2 is removed (approximately 1-2 hours). After cooling to room temperature, pH (“NAGpH”) and electric conductivity (EC) are measured from the NAG solution, after which the suspension is titrated with NaOH (0,1 mol/L) to pH 4.5 and 7.0. The NAG value is calculated from the consumption of NaOH and expressed as kg H2SO4/t. A NAGpH less than 4.5 indicates that the sample is acid producing. This test is suitable for samples that contain less than about 1.5% of total sulphides and have low metal concentrations. To use as a stand-alone test, the single addition NAG test should be calibrated for a particular site with NAPP and sequential NAG tests.
Sequential NAG test
In the single addition NAG test the peroxide may be depleted before all the reactive sulphides have oxidised, thus the basic test may not account for the total acid generation potential of the sample. To overcome peroxide decomposition effects, successive additions of peroxide can be used (sequential NAG test) to provide a better estimate of the total acid generation capacity. The sequential NAG test involves a series of single addition NAG tests on one sample. After each single NAG test, the sample is filtered and the NAGpH and titrated NAG acidity of the solution are measured. The test is repeated on the solid residue, until the NAGpH is > 4.5. The individual NAG acidities are summed to calculate the total sequential NAG acidity. The filtered solutions are topped up to the original 250 ml with deionised water.
The acid production potential of the sample can be obtained from the NAGpH and NAG value as presented in the table 1.
Table 1. Interpretation of the NAG test results (AMIRA 2002)
||NAG (kg H2SO4/t)
||Acid Production Potential
||Non-acid forming (NAF)
||Potentially acid forming – lower capacity (PAF-LC)
||Potentially acid forming (PAF)
Kinetic NAG test
In the kinetic NAG test the pH and temperature are monitored during a single addition NAG test, resulting in pH and temperature profiles providing information about the reaction kinetics of sulphide oxidation and acid generation during the NAG test.
NAG test and the net acid production potential (NAPP)
The net acid production potential (NAPP) can be used together with NAG for more detailed classification of acid generation and to highlight potential issues in ABA results. The NAPP can be calculated as the difference between the maximum potential acidity (MPA) and the acid neutralizing capacity (ANC). According to AMIRA (2002), the ANC can be converted from the NP value of the modified NP test (ABA, NP/50*49) and the MPA from the total sulphur content in units of kg H2SO4/t (S% *30.6). The NAPP values can then be plotted against the NAGpH, as presented in an imaginary example diagram in Figure 1.
Figure 1. An example of ARD classification in NAGpH / NAPP diagram with imaginary samples (Teemu Karlsson, GTK)
In the NAGpH/NAPP diagram (Fig. 1) the samples 1-3 are classified as non acid forming (NAF), i.e. they have a negative NAPP and NAGpH > 4.5. The samples 6-8 are classified as potentially acid forming (PAF), i.e. they have a positive NAPP and NAGpH < 4.5. The samples 4 and 5 have an uncertain (UC) classification. Sample 4 has high positive NAPP (high total sulphur and low ANC), but according to NAGpH it is not acid producing. This could be e.g. due to the fact that most of the sulphur is present as gypsum. Sample 5 has a negative NAPP value but a NAGpH below 4.5. This could be e.g. due to iron carbonates, which result in high ANC, but not all of the measured ANC is effective.
The test was developed in the late 70’s, originally to measure the sulphide content (Sobek et al. 1978), but later its interpretation changed to estimating net acid generation (Morin & Hutt 1999). The extract contents produced by the NAG test (see NAG test with leachate analysis
) could be useful in assessing contaminant mobility during long-term acid generating reactions (e.g. Räisänen et al. 2010).
Advantages (Lapakko 2002, Stewart et al. 2006)
- Does not require separate sulphur determinations; more readily conducted in a field laboratory than other static tests
- When used together, ABA /NAPP and NAG provide a more powerful routine screening technique than either test alone.
Disadvantages (Morin & Hutt 1999, Charles et al. 2015)
- Does not give a value for neutralisation potential
- Additional NP measurements required
- Some uncertainties when compared to ABA results, an “uncertain” range should be incorporated.
- More understanding needed about the nature of NAG tests.
Popular in Australasia and in some countries in South America (Morin & Hutt 1999) and in Europe (Räisänen et al. 2010).
AMIRA 2002. ARD Test Handbook. Project P387A Prediction & Kinetic Control of Acid Mine Drainage. AMIRA international May 2002.
Charles, J., Barnes, A., Declercq, J., Warrender, R., Brough, C. & Bowell, R. 2015. Difficulties of Interpretation of NAG Test Results on Net Neutralizing Mine Wastes: Initial Observations of Elevated pH Conditions and Theory of CO2 Disequilibrium. In Proceedings of the 10th International Conference on Acid Rock Drainage & IMWA Annual Conference, April 21-24, Santiago, Chile. Gecamin.
Lapakko, K. 2002. Metal mine rock and waste characterization tools: an overview. Minin, Minerals and Sustainable Development (MMSD) Working Paper No. 67. http://pubs.iied.org/pdfs/G00559.pdf
Miller, S., Robertson, A. & Donahue, T., 1997. Advances in acid drainage prediction using the net acid generating (NAG) test. Proceedings Fourth International Conference on acid rock drainage, Vancouver, B. C. Canada May 31 – June 6, 1997, volume II, p. 533–547.
Morin, K.A. & Hutt, N.M. 1999. Internet Case Study #10: Comparison of NAG Results to ABA Results for the Prediction of Acidic Drainage. Minesite Drainage Assessment Group (MDAG). http://www.mdag.com/case_studies/cs1-99.html
Räisänen, M.L., Kauppila, P.M. & Myöhänen, T. 2010. Suitability of static tests for acid rock drainage assessment of mine waste rock. Bulletin of the Geological Society of Finland, Vol. 82, 2010, p. 101-111.
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, Cincinnati, Ohio, 45268. EPA-600/2-78-054, 47-50.
Stewart, W.A., Miller, S.D. & Smart, R. 2006. Advances in acid rock drainage (ARD) characterization of mine wastes. Proceedings of the 7th International Conference in Acid Rock Drainage, p. 2098-2119. http://www.imwa.info/docs/imwa_2006/2098-Stewart-AU.pdf