Evaluation of NP determination by static tests for ARD prediction

• Main Author: Wang, Ying
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/7733

In the prediction of acid rock drainage (ARD), static tests play a very important role. The objective of static prediction tests is to determine the balance between the acid producing potential (AP) and the acid neutralization potential (NP) of waste material. The results of static tests are used as initial screening data to characterize waste materials either as "safe" or "unsafe" for disposal. The classification of the waste can be based on either the difference between NP and AP, termed the Net Neutralization Potential (NNP) or the ratio between NP and AP, termed NPR. Errors in measuring either of AP or NP can result in a mis-classification of a material type. In the natural environment, ARD will only occur if there is not sufficient neutralizing alkalinity. Such alkalinity can be provided by many common rockforming minerals. These minerals can neutralize acid in different pH ranges. More importantly, different minerals have different values of reactivity. For low reactivity minerals, their presence in a waste will not necessarily provide protection against ARD if the rate of neutralization is less than the rate of acid generation. Carbonate minerals, especially calcite and dolomite, are the most effective neutralizing minerals because they have relatively high reactivity and neutralize acid in the circum-neutral pH range. Silicate minerals can also be important neutralizers because of their abundance but their reactivities relative to the carbonates range from intermediate (for example, the calcium-rich feldspars) to extremely slow (for example, K-feldspar). This limits the contribution of silicate minerals to the effective field-NP. This thesis focuses on the problems that can arise in the determination of neutralization potential, NP, and in the interpretation of NP data from static methods. In this study the neutralization potentials of 119 samples were determined by three commonly used static tests (the Sobek ABA and variations, the Modified ABA and carbonate NP). Some selected samples were tested using the method proposed by Lapakko in 1994. Several conclusions and recommendations were made from this study as follows: 1. NP values depend on sample mineralogy and test conditions. The NP value of a sample using the Sobek ABA test, the Modified ABA test, C02 analysis and the Lapakko method were found to have a wide variation. 2. In most cases, the Sobek method gives the highest NP value due to its vigorous test conditions. The increased acidity in the Sobek digestion procedure resulted in an increased dissolution of silicate minerals as evidenced by a corresponding increase in silicate mineral cation concentrations. The Sobek method can, therefore, be expected to overestimate the effective field-NP value for many samples. 3. Compared to the Sobek ABA, the Modified ABA is considered to provide more practical NP values by accounting for only most reactive silicate minerals in addition to the carbonate minerals. For the same reason, therefore, the modified ABA NP values are often higher than the Carbonate NP values. 4. The Lapakko method attempts to measure the NP value that is available to neutralize acid before the onset of ARD or in the early stages of acid generation within wastes. It is a useful tool for research, but it is not suitable for the use as a routine procedure in industry and commercial labs since it is very time consuming procedure. 5. In the Sobek ABA method, the fizz test is very important but a subjective measurement. It is used to determine the acid quantity added during the digestion stage, which in turn is critical for the NP value obtained. Misinterpretation or misuse of the fizz test can lead to significant variations in the NP values obtained by the procedure. Generally, when the fizz rating is increased, the NP value of a sample is increased correspondingly. 6. Back titration curves can be obtained for the titration stage in Sobek test. The shape of the curve can indicate if significant silicate mineral dissolution has occurred in the test. The use of such curves might be a helpful supplement in the interpretation of Sobek test results. 7. For the reference standard material NBM-1, the impact of test method on NP and NP:AP ratio was found to be particularly significant. NP:AP ratios of 1.7 to 10.2 were obtained for the same material. 8. Prediction of ARD potential, including the practical NP of a waste, may be obtained by a variety of methods, including detailed mineralogical characterization, comparisons with other sites, drainage monitoring, static laboratory tests, kinetic laboratory test and on-site field trials. A conclusion from this thesis is that no single test alone is conclusive. To get a confident and accurate predictive result, a combination of the above methods and other analyses should be carried out. 9. In practice, only a single static test is likely to be carried out. If this is the case, the modified ABA procedure is recommended since it has the following advantages: It is rapid and easy to perform; It has a low cost and can be used to screen a large number of samples for possible further selective and more detailed evaluation; No special equipment is required; AP value calculated based on the sulfide sulfur is more realistic assessment than the AP value based on the total sulfur; The test determines the NP contributed by the reactive silicate minerals in addition to carbonate minerals.