Improved acid neutralisation capacity assessment of iron carbonates by titration and theoretical calculation

The acid neutralisation capacity (ANC) of a rock sample containing significant amounts of Fe carbonates, as conducted to determine net acid production potential (NAPP), can be a difficult parameter to determine. Various ANC tests are available to determine the ANC of carbonates. This work does not attempt to create another ANC test protocol; rather, it provides a refinement for existing tests. Results showed that a significant lag period may be needed (up to 432 h) after standard Sobek-type ANC tests for the complete hydrolysis of Fe associated with the ANC testing of siderite. This lag occurred even with standard industry modifications that include the addition of 2 drops of H2O2 at pH 4.5 during the back-titration. In this work the authors used a modification to the Sobek ANC test (the Modified Sobek ANC test) that included filtering and the addition of H2O2 at pH 4.5. This test was further modified by the continuance of this H2O2 addition (the H2O2 ANC test) until there was no subsequent pH drop (which is due to Fe hydrolysis reactions), thereupon the back titration was continued to pH 7.0. Results indicated that the ANC for siderite (after 0 h) using the H2O2 ANC test was similar to the ANC determined after 432 h lag by the Modified Sobek ANC test. This modification reduces the uncertainty related to static-test results for samples containing Fe carbonates. The test is simple to use, has industry application, and provides a better indication of the NAPP. The Modified Sobek ANC results for calcite and dolomite and the H2O2 ANC test for siderite were in good agreement with the mineralogical carbonate ANC (ANCcarb). ANCcarb was determined by calculation based on electron probe micro-analysis. Although lower than both the ANCcarb and the ANC determined by titration, the chemical ANC calculated from the ions present in the ANC digestion liquor also provided a good indication of the overall acid neutralisation capacity of the sample.