Acid addition on the basis of the carbonate content

Introduction

CEN/TC 292 has developed a European Standard (EN 15875, “Characterization of waste – Static test for determination of acid potential and neutralisation potential of sulfidic waste”) to determine the potential of sulphide containing waste materials to form acidic drainage (Technical Committee CEN/TC 292 2011). A variety of non-standardized static tests methods exists for rough estimation of whether a waste material will potentially generate acid drainage. Methods usually include the determination of sulphur/sulphide content of waste and based on this calculation of acid potential (AP). Determination of neutralisation potential (NP) is usually done by digesting a finely ground sample with acid and measuring the acid consumed by the waste. The method EN 15875 is an upgrade of the commonly used ABA procedures, as it defines the amount of acid to be added on the basis of the carbonate content of the sample. The method is based on so called “modified ABA” method by Lawrence & Wang (1997) that has been widely used in industry and research.

Description of the test method

The method EN 15875 consists of the following steps:

• Determination of acid potential (AP). Analysis of total sulphur by bomb combustion or by high temperature combustion methods, and calculation of AP. Calculation of AP presumes all sulphur to be in pyritic form (worst case scenario). It is done my multiplying the total-S in mass fraction (%) by 0.625 to yield acid potential (AP) in mol H⁺/kg based on 1 mole of sulphur in pyrite creating 2 moles of H⁺. Note: If substantial part of total sulphur is in sulphate form, the calculation of acid potential based on total sulphur will result in notable overestimation of AP. However, it is also possible to determine sulphur species in these cases.
• Determination of carbonate content of the sample by dry combustion method gives the carbonate rating (CR).
• Determination of neutralisation potential (NP). 2 grams of finely ground sample is digested for 24 hours in 90 ml of hydrochloric acid solution. The test aims at final digestion pH of 2.0-2.5, and the amount of 1 M HCl added to the solution in the early stages of the test is determined on the basis of the carbonate content (i.e. carbonate rating, CR). After digestion the solution is back titrated with sodium hydroxide to pH 8.3 to measure the amount of acid left in the solution. The acid consumption in the test is converted to the neutralisation potential (NP) of a sample.
• Calculation of neutralisation potential ratio (NPR) and net neutralisation potential (NNP). (Technical Committee CEN/TC 292 2011)

It is known that carbonate minerals are the biggest source of neutralisation potential in extractive waste. Therefore it was seen feasible to replace the subjective fizz test used in some previous ABA methods with the determination of the carbonate content of the sample as a preliminary test to decide on the amounts of acid added at start and after two hours. Now the amounts of acid added correspond to stoichiometric neutralisation potential in the carbonates assuming all carbonates to appear as calcium carbonate. This will give more equal conditions in the test in terms of digestion pH for samples with different neutralisation potential.

Appropriate applications

The method is suitable to all waste materials from the extractive industries that contain sulphides, excluding wastes that have pH <2 at the beginning of the test (Technical Committee CEN/TC 292 2011). The method may not alone be adequate to determine the actual acid generation potential in the field. As site specific conditions may affect the behaviour, a more detailed assessment may be needed. Kinetic tests which allow determination of oxidation rates of sulphides and the release rates of acid-producing and neutralising minerals as well as e.g. metals are needed for deeper characterization of extractive waste.

The addition of acid based on the total carbonate content instead of a fizz test as in previous ABA test protocols, is in most cases an improvement of the consistency of the result between the laboratories (Technical Committee CEN/TC 292 2012), because it has been shown that different fizz test ratings with the same sample can cause significant differences in the resulting NP (Stenvall 2007).

Other improvements compared to the previous ABA test protocols include:

• Lower digestion pH. In the previous methods samples with high neutralisation potential were digested most of the test time in a relatively high pH. The highest amount of acid (1 M HCl) added corresponds to neutralisation potential of 125 kg CaCO₃/t of waste and this amount of acid is soon consumed by samples with high neutralisation potential.
• The determination of the test samples particle size. Particle size of the test sample has also been shown to have great impact on the results (Stenvall 2007, Tammelin 2008); earlier test methods did not fix the particle size.

However, in two types of cases the acid additions based on carbonate content may be misleading. Firstly, not all carbonates have neutralisation potential or their reaction time may be slow (for example dolomite and siderite). This will cause an overly high acid addition at the beginning of the test and as a result the digestion pH might fall too low. Then new test has to be performed with reduced volume of acid. Secondly, extractive waste may contain other neutralising minerals than carbonates e.g. silicates some of which are reactive at the pH-domain of the test. In this case the pH of the suspension can be found quite high at 22 hours. Then possibly quite a lot of acid is needed to drop the pH back to the desired level. To have the testing conditions (digestion-pH) as equal as possible throughout the test for samples with NP from different minerals an upper limit for the volume of acid added at 22 hours is set to be 50 % of the total volume. If exceeded, the test has to be redone with higher volume of acid. (Technical Committee CEN/TC 292 2011, 2012)

Performance

• The test is relatively simple to perform: Cost of a single test remains reasonably low. The method also allows multiple samples to be tested simultaneously.
• No expensive equipment is needed.
• Test is performed in ambient temperature: No overestimation of NP due to high temperature.
• Test aims at final digestion-pH of 2.0-2.5: No overestimation of NP due to extremely low digestion-pH.
• The method has the potential to produce results with good repeatability and reproducibility.

Method maturity

During the development process of the method EN 15875 three Master’s thesis studies were performed to generate consistent and repeatable method for European use (Stenvall 2007, Tammelin 2008, Punkkinen 2009). Also an inter-laboratory comparison was performed in order to demonstrate the methods’ reproducibility. The idea of the inter-laboratory comparison was to see if results of same magnitude could be produced in different laboratories. The results showed the method to be reproducible. (Kaartinen & Wahlström 2010)

References

Kaartinen, T. & Wahlström, M. 2010. Evaluation of the inter-laboratory comparison on the determination of neutralisation potential according to prEN 15875 organized by CEN/TC 292/WG 8. Validation study on prEN 15875 (Unpublished).

Lawrence, R.W. & Wang, Y. 1997. Determination of neutralization potential in the prediction of acid rock drainage. Proceedings of Fourth International Conference on Acid  Rock Drainage. Vancouver B.C. Canada. Volume I, pp. 451-464.

Punkkinen, H. 2009. Kaivannaisjätteiden haponmuodostus- ja neutraloitumispotentiaalin arviointi – Tutkimusmenetelmien kehittäminen. An assessment of acid generation and neutralisation potential of wastes from extractive industry  – Test method development. Master’s thesis, University of Helsinki, Finland (In Finnish).

Stenvall, M. 2007. Bestämning av neutraliseringsförmågan i sulfidhaltiga gruvavfall. Determination of neutralisation potential in sulfide bearing mining waste. Master’s thesis, Åbo Akademi, Finland (In Swedish).

Tammelin, M. 2008. Staattisen neutraloimispotentiaalin määritysmenetelmän kehittäminen Euroopan Unionin standardimenetelmäksi kaivosjätteiden happaman valuman ennakoinnissa. Development of a static test for determination of neutralisation potential in mining waste towards a European norm. Master’s thesis, University of Turku, Finland (In Finnish).

Technical Committee CEN/TC 292 2011. Characterization of waste – Static test for determination of acid potential and neutralisation potential of sulfidic waste. EN 15875.

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