Abstract. The paste pH test (1 part solid: 2 parts water) is one method used to determine the acidic nature of a rock/soil sample. In conjunction with kinetic NAG testing a classification scheme has been developed for the Cypress Prospect within the Stockton coal mining region, West Coast, New Zealand. Samples having a paste pH of < 4.0 are considered potentially acid forming (PAF) and contain significant acidic sulfate salts (up to 30.1 kg H 2 SO 4 /t equivalent) that will immediately produce acid upon exposure to water. Samples with a paste pH of 4.0 -5.0 are considered PAF, but have a lower stored acidic salt content (up to 9.0 kg H 2 SO 4 /t equivalent). In the field the lithologies represented by both these rock types are likely to generate ARD immediately upon exposure to water. Circum-neutral paste pH values (> pH 5.0) for samples classified PAF indicated that they have a short-term acid neutralization capacity (ANC) that is greater than the readily available short-term acid generating capacity of the sample. This resulted in a time lag (2 -356 minutes) prior to decrease to pH 4 in the kinetic NAG test. Samples having a paste pH > 6.0 typically produced a longer lag period than those with a paste pH of 5 -6. As previous researchers have demonstrated this represents a lag period prior to the onset of laboratory acid rock drainage in larger column leach tests. These results have direct application to strategic mine planning at the proposed Cypress mine including separating waste rock into immediate acid generators (high management priority) from acid generators with a lag to acid formation (lower priority) and non-acid forming. Field validation of this classification system is still needed.

During acid mine drainage (AMD) treatment by alkaline reagent neutralisation, Ni and Zn are partially removed via sorption to Fe and Al hydroxide precipitates. This research evaluated the effect of surface area of precipitates, formed by neutralisation of AMD using three alkalinity reagents (NaOH, Ca(OH) 2 , and CaCO 3 ), on the sorption of Ni and Zn. The BET surface area of the precipitates formed by neutralisation of AMD with NaOH (173.7 m 2 g −1 ) and Ca(OH) 2 (168.2 m 2 g −1 ) was an order of magnitude greater than that produced by CaCO 3 neutralisation (16.7 m 2 g −1 ). At pH 6.5, the residual Ni concentration was 0.32 and 0.41 mg L −1 for NaOH and Ca(OH) 2 neutralised AMD, respectively, resulting in up to 60% lower Ni concentrations than achieved by CaCO 3 neutralisation which had no effect on Ni removal. The residual Zn concentration was even more dependent on precipitate surface area for NaOH and Ca(OH) 2 neutralised AMD (0.33 and 1.02 mg L −1 ), which was up to 85% lower than the CaCO 3 neutralised AMD (2.20 mg L −1 ). These results suggest that the surface area of precipitated flocs and the selection of neutralising reagent critically affect the sorption of Ni and Zn during AMD neutralisation.

Predicting the acid and metalliferous drainage (AMD) contribution from waste rock dumps (WRDs) containing potentially acid forming (PAF) material is a key step when planning for closure. For sites already demonstrating impacts from the generation and release of AMD, estimating final water quality and flow rates emanating from WRDs is key to quantifying the level of remediation and/or management required at closure. Predictions of final water quality need to be compared with regulatory limits for closure, stakeholder expectations and any anticipated treatment options (including treatment longevity and costs). In the absence of WRD sample data collected from intrusive investigations, there are often numerous WRD seeps and impacted streams that can be used to determine typical water quality, solubility constraints, flow rates, contaminant loads and thus source terms for PAF WRD drainage. The preceding step critical to the determination of source terms is the development of a conceptual model that incorporates potential/stored acidity components, flow rates and water quality. The developed conceptual model can then be further refined and strengthened with geochemical modelling. The potential acidity component, that is primarily associated with acid generating sulphides, is typically estimated from assay databases and materials placement records. Laboratory derived pyrite oxidation rates can be used to estimate the remaining potential acidity component as well as the formed stored acidity component. The mobilisation of stored acidity and other oxidation products is often constrained by solubility controls, particularly in older WRDs. These solubility controls are often associated with the formation and dissolution of melanterite-type soluble acidity, jarosite-type sparingly soluble acidity and other secondary phases such as gypsum. The determination of these mineral and/or the proportion of which they make up the estimated oxidised sulphur content allows for more accurate determination of the stored acidity component for source term derivation. Geochemical testwork can then confirm the presence of such minerals, which is incorporated into an acid-base accounting modelling process and the determination of three key phases of closure water quality; (1) the draindown water quality phase; (2) the transition water quality phase; and, (3) the long-term water quality phase. During the WRD draindown phase, after cover system installation, the seepage quality can be assumed to be equal to the derived WRD source term with the duration of this phase determined by numerical modelling. Seepage quality for the transition phase is determined from the stored acidity (or metalliferous oxidation products), which also incorporates elemental loading. The long-term water quality can be determined by forward reaction path modelling or by using key mineral dissolution kinetics (first principal approach). Combining these three phases then produces a model for the prediction of long-term water quality after operations, which can be utilised f...

Water chemistry was monitored monthly for ten months from an acid mine drainage (AMD) seep emanating at Stockton Coal Mine within the Mangatini watershed in New Zealand. Metal concentrations of the seep water were Fe (4.31-146 mg/L), Al (7.43-76.7 mg/L), Cu (0.0201-0.0669 mg/L), Ni (0.0629-0.261 mg/L), Zn (0.380-1.39 mg/L), Cd (0.000540-0.00134 mg/L) and Pb (0.0049-0.0056 mg/L), pH was 2.49-3.34 and total acidity (pH 8.3) was 78.5-626 mg/L as CaCO 3. Water chemistry signature prompted laboratory mesocosm studies measuring the effectiveness of sulfate-reducing bioreactors (SRBRs) for generating alkalinity and sequestering metals. Alkaline materials utilized in the SRBRs included industrial waste products such as mussel shells, nodulated stack dust (NSD) derived from the cement industry, and limestone. Organic substrate materials included post peel, a by-product from fence post manufacture, Pinus radiata bark and compost. Seven SRBRs comprised of varying substrate mixes received aerated AMD for nearly four months. AMD was sourced from the pond that collected the seep water. The SRBR containing NSD successfully removed all metals, but effluent was caustic with pH>9. Bioreactors consisting of 20-30% mussel shells were most successful at immobilizing metals and generating circumneutral effluent. Systems containing mussel shells sequestered more than 0.8 moles of metals/m 3 of substrate/day at stable operating conditions and yielded effluent concentrations (removal efficiencies) of 0.120-3.46 mg/L Fe (96.5-99.8%), 0.0170-0.277 mg/L Al (99.5-99.9%), <0.0005-<0.001 mg/L Cu (>99.7->99.9%), <0.0005-0.0020 mg/L Ni (99.3->99.7%), <0.001-0.005 mg/L Zn (99.7-99.9%), < 0.00005 Cd (>98.3->98.9%) and <0.0001-0.0001 Pb (99.5-<99.7%). The system consisting of limestone as the only alkalinity generating material was less effective (15.4-64.3 mg/L Fe). Results from duplicate systems but different reactor shapes indicated reactor dimensions influence flow characteristics and therefore treatment efficacy.

Acid and metalliferous drainage (AMD) management plans are generally developed as part of a site's closure plan to inhibit or mitigate the generation and release of AMD for sites with problematic materials. They are typically constructed around a body of knowledge involving multiple geoscience and environmental disciplines. However, despite the volume and degree of scientific investigations completed, if the waste rock classification system and therefore AMD management plan developed is not practical and does not take into consideration other site drivers such as production, its successful implementation and adoption is unlikely. A common weakness of AMD plans developed based on industry best practice is that they often fail the practicality test, as the characterisation process produces ambiguous outcomes such as the classification of material as uncertain with respect to acid generating potential. At the Escarpment Coal Mine, West Coast, New Zealand, a new process flow method for geochemical classification is being trialled. Results indicate that classification by a process flow method results in far fewer samples being classified as uncertain compared to the current resource consent matrix-style classification. Results presented in this paper indicate that ABA data and field column leach trials validate this approach. https://papers.acg.uwa.edu.au/p/1608_48_Pearce/ A risk-based approach using process flow diagrams for operational waste rock S Pearce et al. classificationcase studies 650 Mine Closure 2016, Perth, Australia