Acid generation: Recognition and adjustment for major differences in acid production between different sulfide minerals is essential for correct classification. Standard acid-base-accounting (ABA) analysis assumes all sulfur is pyrite. Pyrrhotite produces less than 20% acid (>80% sulfide reacts to form elemental S) as compared to pyrite; chalcopyrite also produces elemental S; sphalerite and galena do not produces acid on dissolution.

Neutralisation: Recognition and estimation of neutralisation from reactive silicate minerals in addition to carbonates (not siderite) is also important in understanding the long-term acid generation profile.

A comprehensive set of standard ABA experimental methods for assessing the geochemical reaction characteristics of different waste rock samples and their acid forming, or non-acid forming, behaviour is available in the AMIRA ARD Test Handbook (www.amira.com.au/documents/downloads/P387AProtocolBooklet.pdf) developed in our AMIRA project with EGi. Further corrections, developed subsequently, can also be applied by Blue Minerals Consultancy.

MLA (and QEMSCAN) provides an alternative approach to quantitative XRD for mineralogical analysis. MLA is based on stoichiometric definition of phases in contrast to the structural analysis provided by XRD. It is preferable to carry out both analyses due to the contrasting strengths and weaknesses.

LA-ICP-MS can be undertaken subsequent to MLA analyses where a map of mineral particles is obtained. LA-ICP-MS enables the mineralogical location of trace elements to be examined. These elements may be a cause of concern on leaching, e.g. Se and Cd, or may be of value but in solid solution within a range of minerals, e.g. Ni, Ag and Au. Definition of their mineralogical location enables tailored strategies for improved immobilisation or extraction as desired.

The ANC measured in short-term Sobek tests as part of the definition of net acid production potential (NAPP) provides an estimate of the neutralisation available at rates comparable or matched to the initial rates of acid production in an ARD waste. This ANC is primarily derived from carbonates but can include significant contributions from more reactive silicate minerals (e.g. chlorites, anorthite, orthosilicates) in the waste that continue after the carbonates are exhausted.

A methodology for measurement of non-carbonate, long-term, steady state, stoichiometric  acid neutralisation rates (ANRnc) from these mineral sources has been developed. This can be used to evaluate the neutralisation rate requited to match the reduced acid generation rate. The availability of suitable reactive silicate and other minerals in waste rock or adjacent rock can be assessed by QXRD and tested for application in dumping strategies or as covers on existing dumps (e.g. Savage River Project).

(ANC).

Improved assessment of MPA using % reactive sulfide only (CRS test not total S) and adjustment for pyrrhotite and other non-pyrite content

Improved ANC estimate using corrected procedure for siderite (non-acid-forming) and inclusion of acid neutralising reactive silicate minerals.

The full set of corrections can result in wastes previously classified as potentially acid forming (low capacity) being reclassified with EPA acceptance as non-acid forming with resultant major cost savings in handling and storage (https://www.nature.com/articles/s41598-019-40357-4).