A study has been carried out to evaluate static test methods for determining the neutralization potential, NP, of mining waste as part of prediction testing for acid rock drainage. In particular, results of the widely used Sobek method under standard and altered conditions have been assessed and compared with those obtained from the Modified procedure of Lawrence and with those based on the inorganic carbon content (Carbonate NP). The method of Lapakko has also been assessed for selected samples. Special attention has been given to the effect of mineralogical composition in interpreting NP results. A supplemental method to determine NP based on a calculated mineralogical composition and mineral reactivity has been proposed and results compared with the empirical methods of determination. 120 samples of waste rock or tailings from 12 mines were used in the study.

The principal findings and conclusions were as follows:

• The Sobek method provides NP values which are significantly higher than those obtained by the Modified or Carbonate NP methods.

• NP values obtained by the Modified procedure were nominally higher than those obtained by the Carbonate NP method.

• A limited number of tests using the Lapakko procedure gave NP values similar to the Carbonate NP method. The method is time consuming and is not recommended for routine assessments.

• Misinterpretation or misuse of the fizz test, used to determine the quantity of acid added in the Sobek procedure, can lead to significant variations in the NP values obtained by the procedure. The degree of excess acidity at the end of the digestion stage was found to be greater as the fizz rating is increased. NP values increased correspondingly.

• 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. Therefore, the higher values of NP obtained by the Sobek method relative to the Modified and Carbonate procedures are overestimates of the neutralization capacity that will be available under field conditions.

• The Modified procedure is considered to provide practical NP values by accounting for only the most reactive of the silicate minerals in addition to the carbonate minerals.

• Variations in NP values for the same sample as determined by different methods or by using procedural variations for the same method can result in wide variations of Net NP or, more importantly, the NP:AP ratio used to classify wastes as ‘safe’ or ‘requiring further testing or requiring ARD control’. For a significant number of samples, both classifications were shown to be possible, depending on the NP values used.

• Plotting the back titration curve in the determination of the remaining acidity at the end of the Sobek digestion procedure has been shown to be a reliable indicator of NP overestimation for the majority of samples. The shape of the curve indicates if significant silicate mineral dissolution has occurred in the test.

• The impact of test method on NP and NP:AP ratio was found to be particularly significant for the reference standard material NBM-1. NP:AP ratios of 1.7 to 10.2 were obtained for this material.

• A method of calculating NP based on whole rock chemistry, a CIPW normative calculation, and relative reactivities of component minerals gave NP values which correlated well with values from the Modified procedure. The method is considered to have a good potential as a routine method of NP determination following calibration with empirically determined values for specific lithological units.

• Reliable and confident determination of the practical NP value of a waste cannot be achieved by a single test. In all cases, mineralogy is the key parameter which must be evaluated and considered in a waste characterization program. The Modified procedure is considered to provided the most reliable static test for providing a practical NP value. Other analyses and supplemental tests are recommended, some or all of which might be applicable for a specific program.

• Static test procedures, with or without other analyses and tests, must, however, be used as a preliminary method of classification. For many applications, kinetic test methods will be required to establish the rate and extent of the depletion of neutralization potential over a wide range of conditions which might be encountered in the waste disposal facility.