A quantitative approach for the estimation of the “fizz rating” parameter in the acid-base accounting tests: A new adaptations of the Sobek test

Acid-base accounting (ABA) is a widely used technique for the determination of the acid generating potential of geological material. It consists of a screening method where the neutralization potential (NP) and the acid-generating potential (AP) of rock sample are determined. Among the first ABA test is the one proposed by Sobek et al. (1978) and it is the most used test in North America. AP is calculated based on the sulfide sulfur content in the sample. However, NP is assessed by using acid-base titration with some limits. In fact, discrepancies in results obtained for a same sample by Sobek test were pointed out by several authors depending on fizz rating, on sample mineralogy (iron-bearing carbonates and Fe–Al-silicates), on the titration method, or the laboratory practitioner. Thus, some authors have proposed modifications of the Sobek test to improve the test reliability and the resulting waste classification. Nevertheless, no modified method has been able alone to overcome all error sources. The subjectivity of fizz rating, which is performed to determine the concentration and volume of hydrochloric acid to be added to a sample for neutralizing minerals digestion and subsequent NP determination, remains the main limit of the test. So, the aim of this study is to develop a quantitative approach, replacing the subjective visual fizz, to calculate the exact volume of hydrochloric acid (HCl) needed to digest the entirety of Fe–Mn–Mg-carbonates that give to a sample their main neutralizing potential. Then, a new adaptation of the Sobek test, based on the quantitative Fizz, is proposed in this paper. Testings were conducted on samples prepared by mixing pure minerals in well known quantity mimicking concentrator tailings (here named synthetic tailings). The obtained NPs by the quantitative fizz based method are well correlated with the theoretical NP obtained by the mineralogical Paktunc method. This new Sobek test adaptation requires knowledge of detailed sample mineralogy or at least the inorganic carbon content. If such information is not available, alternative digestion methods are proposed giving results which are also well correlated with the Paktunc method. The synthetic tailings were subjected to a kinetic test to evaluate their acid-generating potential. The NP values obtained by the quantitative fizz based methods, and resulting sample classifications, are consistent with kinetic test predictions demonstrating the effectiveness of the Sobek test refinement suggested by the authors to enhance the confidence of acidity potential classification.