Development of a European standard for the determination of acid potential and neutralisation potential of sulfidic waste

Transcription

1 Development of a European standard for the determination of acid potential and neutralisation potential of sulfidic waste Tommi Kaartinen 1 and Margareta Wahlström 1 1 VTT Technical Research Centre of Finland, P.O. Box 1000, FI VTT, Finland tommi.kaartinen@vtt.fi, margareta.wahlstrom@vtt.fi ABSTRACT European Directive (2006/21/EC) on the management of waste from the extractive industries requires guidance on waste characterization as a basis for the development of waste management plans. CEN, the European standardization organization, has commenced work on standards for extractive waste. One of the work tasks is to develop tools for determination of acid generation behavior. Test methods for the determination of acid generation behavior can be divided in static and kinetic tests. A static test is usually relatively fast to perform, but gives only indicative information based on total composition of the waste material. The kinetic test gives more detailed information on behavior based on reaction rates under specified conditions. This paper describes a European static method draft developed for waste characterization of sulfide bearing materials which may generate sulfuric acid when subjected to weathering. In the test draft specified methods for determining both the acid potential (AP) and the neutralization potential (NP) of the material are presented. Here the focus is more on the determination of NP. From these results the net neutralization potential (NNP) and the neutralization potential ratio (NPR) are calculated. Basis for the standardization work were the existing static test methods applied in the extractive industry and research. Background for the choice of the method to be standardized is discussed in the paper. INTRODUCTION Acid drainage is considered as one of the most significant environmental problems associated with extractive industry. Iron sulfide oxidation is responsible for the majority of the acid production by mine wastes. Iron sulfides (e.g. Pyrite) in tailings or waste rock are stable as long as reducing conditions are maintained, but as soon as oxygen and moisture are present iron sulfides begin to oxidize. The resulting acidic, iron and sulfate rich pore water may (i) enhance the weathering process by reacting with sulfide minerals (ii) evaporate to precipitate e.g. hydrated iron-sulfates (iii) react with the waste and, depending on the buffering capacity, leave the mining waste as acid drainage (Lapakko, 2002). Acid drainage from pyrite oxidation may occur if the acid potential exceeds the acid neutralization potential of the minerals in the waste. However, kinetics describe the reaction pathways towards equilibrium. Only if the acid production rate exceeds the rate at which the buffering minerals can neutralize the produced acidity or if the buffering minerals simply become exhausted, acid generation reactions will result in a decrease of ph in the drainage that may lead to e.g. high metal leaching from the waste. The mining waste directive (2006/21/EC) lays down requirements for the safe management of extractive waste including proper characterization of the waste. In 2006 European Commission gave a mandate to CEN, the European standardization organization, for the development of standardized methods relating to the characterization of wastes from the extractive industries. One of the work tasks covered by the mandate includes the 1

2 development of a standardized static test with the status of a European norm (EN) for the determination of net acid production potential in sulfidic waste. 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 sulfur/sulfide content of waste and based on this the calculation of acid potential (AP). Determination of neutralization potential (NP) is usually done by digesting a finely ground sample with acid and measuring the acid consumed by the waste. The ratio of AP and NP or the subtraction NP-AP is then used in the judgment of potential risk for acid drainage. Static tests are designed for the single purpose of measuring extractive waste s capacity for producing acidity and its acid neutralization capacity. Accordingly, static tests are commonly used for screening purposes and to provide an answer to whether the mine waste material has a potential to be an acid producer or an acid neutralizer. These tests do not consider parameters such as the actual availability of acid-producing and acid-neutralizing minerals and differences between the respective dissolution rates of acid-producing and acidneutralizing minerals. Kinetic tests which allow determination of oxidation rates of sulfides and the release rates of acid-producing and neutralizing minerals as well as e.g. metals are needed for deeper characterization of extractive waste. Here we describe the development of a static test to a European Norm (EN). DETERMINATION OF NEUTRALISATION POTENTIAL Background for the choice of method In CEN/TC 292 Working group 8 Wastes from the extractive industry it was agreed from the beginning that in the first place no completely new static test method would be developed. Instead, already existing non standardized static test or tests would primarily form the basis of a test to be standardized. The first step in this work was a review on the current approaches to determine the net acid production in extractive waste (Stenvall 2006). Based on the review as well as discussions among experts in Working group 8 a modified acid-base accounting procedure introduced by Lawrence and Wang (1997) was agreed to be the method for the determination of neutralization potential to form the basis of an EN. Rationale listed below favored the choice of this method to be standardized. The method itself along with development needs is described in detail further in the text. The method has been widely used in industry and research The test is relatively simple to perform Cost of a single test remains reasonably low It 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 2

3 The method (with limited amount of modifications) has the potential to produce results with good repeatability and reproducibility Modifications to the method Determination of neutralization potential as described in the modified acid-base accounting by Lawrence and Wang (1997) is performed on a pulverized sample by adding water and predetermined amounts of acid to the suspension. The suspension is mixed throughout the test. Acid is added at start and after two hours of mixing. After 22 hours the ph of the suspension is measured and, if needed, adjusted between 2,0 and 2,5, After 24 hours the test is terminated and the suspension is back-titrated with base to ph 8,3. The acid consumed in the test is converted to neutralization potential (NP) in calcium carbonate equivalents (kg CaCO 3 /t of waste). The modified acid-base accounting method by Lawrence and Wang (1997) uses the so called Fizz-test as a preliminary test to determine the amount of acid added to the test suspension in the early stages of the test. In the Fizz-test a few drops of strong hydrochloric acid is added to a small amount of pulverized sample. The rate of bubbling caused by the acid is evaluated visually and the acid amount to be added is chosen from four alternative categories based on the rate of bubbling. The idea behind the preliminary test is to adjust the digestion-ph in the test to be on the same level for samples with different buffering capacities. The type of visual evaluation that the Fizz-test includes was seen impossible to be part of a European Norm. A European Norm has to be such that tests performed in different laboratories by different technicians produce as similar results as possible. It has been shown that different Fizz-ratings with the same sample can cause significant differences in the resulting NP (Stenvall 2006). Another weakness in the modified acid-base accounting method by Lawrence and Wang is that samples with high neutralization potential are digested most of the test time in a relatively high ph. The highest amount of acid (1 M HCl) to be added corresponds to neutralization potential of 125 kg CaCO 3 /t of waste and this amount of acid is soon consumed by samples with high neutralization potential. Particle size of the test sample has also been shown to have great impact on the results (Stenvall, 2006, Tammelin, 2007), The modified acid-base accounting by Lawrence and Wang does not fix the particle size though pulverized is mentioned in the procedure. Table 1 shows the fundamental steps in the modified acid-base accounting procedure. Along with these are the corresponding steps in the modified procedure pren that now has the status of a European standard draft. Explanations to modifications are given further in the text. 3

4 Table 1. Test parameters in the modified acid-base accounting by Lawrence and Wang and pren Test parameter pren Remarks Modified acid-base accounting by Lawrence and Wang (1997) Particle size Pulverized 95 % <0,125 mm Sample amount 2,00 g 2,00 g (dry mass) Amount of water added 90 ml 90 ml Acid used in digestion 1 M HCl 1 M HCl Amount of acid added at start Amount of acid added at time= 2 h Amount of acid added at time = 22 h 1 to 3 ml based on bubbling in the Fizz-test 1 to 2 ml based on bubbling in the Fizz-test So that ph of the suspension is between 2,0-2,5 0,5 to 10 ml based on carbonate content of the sample 0 to 10 ml based on carbonate content of the sample So that ph of the suspension is between 2,0-2,5 In pren not more than 50 % of the total amount of acid is allowed to be added at time = 22 h. If ph of the suspension is below 2 at 22 h the test has to be redone with reduced volume of acid Test duration 24 h 24 h Condition for final ph of the suspension 2,0-2,5 2,0-2,5 If not in the domain test has to be redone Base used in back 0,1 or 0,5 M NaOH 0,1 M NaOH titration End point of back 8,3 8,3 titration Expression of NP kg CaCO 3 /t mol H + /kg In pren conversion factors given to kg CaCO 3 /t It is known that carbonate minerals are the biggest source of neutralization potential in extractive waste. Therefore it was seen feasible to replace the subjective Fizz-test 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 neutralization 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 neutralization potential. However, in two types of cases the acid additions based on carbonate content may be misleading. Firstly, not all carbonates have neutralization potential and the digestion ph might fall two low. Then the test has to be done again with reduced volume of acid. Secondly, extractive waste may contain other neutralizing 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. 4

5 DETERMINATION OF ACID POTENTIAL The potential of extractive waste to produce acid mainly originates from sulfides. However, no standard methods for the determination of sulfur species (sulfate-, sulfide-, disulfidesulfur) are established for extractive waste. Analytical methods that are referred to in a European Norm should also have a status of a standard method. Analysis of total sulfur is a well established method and is regarded as a default method in the standard draft pren Total sulfur is determined either by bomb combustion according EN or by high temperature combustion according to ISO 351 or ISO If substantial part of total sulfur is in sulfate, then the calculation of acid potential based on total sulfur will lead to significant overestimation of AP. pren gives informative guidance on the determination of sulfur species in this type of cases. Calculation of AP as instructed by pren assumes all sulfur to appear as pyrite. 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 sulfur in pyrite creating 2 moles of H +. FUTURE ACTIONS A couple of major steps have to be taken before a standard draft pren can be finalized to a European Norm (EN). These major steps include robustness testing of the method for determination of neutralization potential and a validation study of the method. In the robustness testing some of the most critical test parameters are tested on their sensitivity to affect the results. These could include e.g. the timing of the acid additions in the test and the means of mixing the suspension during the test. The results from the robustness testing are evaluated statistically and the testing conditions can be further adjusted. In the validation study the idea is to see if results of same magnitude can be produced in different laboratories. Extractive waste samples are shipped to participating laboratories and by following the procedure the participating laboratories determine the neutralization potential of the samples. The validation study also includes statistical evaluation of the results. If the NP-values determined in different laboratories are within reasonable range then the reproducibility of the method is adequate to be accepted as an EN. ACKNOWLEDGEMENTS European Commission is funding the development of the static test to a European Norm. The work is planned to be ready and pren finalized to EN in

6 REFERENCES European Committee for standardization pren 15875, Characterization of waste Static test for the determination of acid potential and neutralization potential of sulfidic waste. International Organization for Standardization ISO 351 Solid mineral fuels - Determination of total sulphur - High temperature combustion method International Organization for Standardization ISO Soil quality -- Determination of total sulfur by dry combustion Lapakko, K Metal mine rock and waste characterization tools: an overview, MMSD, IIED, no 67, 30p. Lawrence, R.W. and Wang, Y 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 Stenvall, M Determination of neutralization potential in sulfide bearing mining waste. Master s thesis, Åbo Akademi Finland (In Swedish) Tammelin, M Development of a static test for determination of neutralization potential in mining waste towards a European Norm. Master s thesis, University of Turku Finland (In Finnish) 6