Acid-base accounting (ABA) and a suite of quick geochemical tests have become mining’s early warning system, separating benign rock from material that can leach acid and metals for decades. Field data show the approach can predict drainage pH with up to 96% accuracy — and drive designs that prevent costly cleanup later.
Acid mine drainage (AMD) isn’t subtle. When sulfide minerals — mainly pyrite (FeS₂) — in coal waste oxidize, they generate sulfuric acid that can push water to pH values often below 3 and mobilize metals like Fe, Al, Zn, Cu, Pb, and As (link.springer.com) (researchgate.net). A coal-overburden study in Indonesia found that oxidation of sulfide‑bound sulfur produced highly acidic waters rich in dissolved iron and manganese (researchgate.net).
Because AMD can persist for decades and is very costly to clean up, regulators push geochemical characterization to the start of the mine life. In Indonesia, about 95% of surface coal/gold/copper mines have AMD potential, prompting laws and guidelines that require environmental management plans — including AMD control — for permitting and closure (researchgate.net) (researchgate.net). Static tests of waste rock and tailings are the first line of defense.
Acid‑base accounting fundamentals
Acid-base accounting (ABA) quantifies a sample’s acid‑producing potential (AP) against its neutralization potential (NP). The core pieces are:
Maximum Potential Acidity (MPA): Calculated from sulfur content, assuming all sulfur oxidizes to H₂SO₄. For pyritic sulfur, MPA = 30.6 kg H₂SO₄ per tonne per 1% S; 1% pyritic S produces ~30.6 kg H₂SO₄ per tonne (≈0.3 mmol H₂SO₄/g sample) (researchgate.net).
Acid Neutralization Capacity (ANC or NP): Measured by wet titration: treat the sample with excess acid (e.g., HCl), then back‑titrate with base; the consumed acid gives ANC in kg H₂SO₄ neutralized per tonne (researchgate.net).
Net Acid-Producing Potential (NAPP): NAPP = MPA – ANC; a positive value signals potentially acid‑generating waste (PAG). It’s often reported as Net Neutralization Potential, NNP = –NAPP (researchgate.net).
NP:AP ratio (NPR): The NP/MPA ratio frames risk. In a study of 56 West Virginia coal mine sites, an NP:MPA ratio <1 generally yielded acid drainage, 1–2 was mixed/uncertain, and >2 generally yielded neutral/alkaline drainage; interpretation agreed with observed drainage pH in 50/52 cases (96% accuracy) when anomalies were excluded (researchgate.net) (researchgate.net). Indonesian studies likewise classify waste as “Potential Acid Forming (PAF)” if NAPP > 0 or NP:MPA < 1, and “Non‑Acid Forming (NAF)” if NAPP < 0 or NP:MPA ≫ 1 (researchgate.net) (researchgate.net).
Quick static tests: paste‑pH and NAG
The paste‑pH test mixes pulverized rock (typically <250 μm) with deionized water at about a 1:1 ratio and measures equilibrium pH; a significantly sub‑neutral value (often <5) signals stored acidity and acid salts (mineclosure.gtk.fi) (mineclosure.gtk.fi). Weber et al. (2006) suggest paste pH <5 warrants aggressive AMD control measures (mineclosure.gtk.fi). It’s quick and cheap, but it only detects existing stored acidity — not total acid potential — so it doesn’t replace ABA/NAPP for prediction (mineclosure.gtk.fi) (mineclosure.gtk.fi).
The Net Acid Generation (NAG) test oxidizes a sample — commonly by adding hydrogen peroxide — and measures leachate pH or direct acidity; low NAG‑pH (e.g., <4.5) correlates with acid‑generating potential (researchgate.net). In one Kalimantan coal study, a sample with NAG‑pH 1.69 and NAPP = 262.7 kg H₂SO₄/t was classed as PAF, whereas a sample with pH 2.28 and NAPP = 27.6 kg/t was “uncertain” (researchgate.net). These empirical thresholds (NAG pH ~4–6) align with international practice.
Sulfur speciation and leachable metals
ABA often uses total sulfur to compute MPA, but only part of sulfur sits in acid‑producing minerals (primarily pyrite). A study of South African coal processing wastes found acid‑generating sulfur was only 53–64% of total S, meaning MPA from total S would significantly overestimate acid potential (mdpi.com). Advanced protocols measure sulfur speciation (e.g., distinguishing pyritic from organic S) or carbonate content to refine NP.
Heavy‑metal content analysis (XRF or ICP) flags elements like As, Pb, and Ni that may leach under acidic conditions. Simple extraction tests — such as USA EPA TCLP or aqua regia digest — screen metals. In Finland, aqua regia extraction tended to over‑predict some elements in neutral drainage, while the NAG leachate test correlated well for Al and Cr under strongly acidic conditions (link.springer.com). These geochemical screens help refine risk, for example when a sample has low NAPP but high leachable metals.
Kinetic tests and reactive transport modeling
Static tests don’t capture long‑term release rates. Kinetic tests — humidity cells (ASTM static humidity cell) and column tests — cycle crushed waste through wetting and drying or repeated leaching for weeks to months, tracking pH, conductivity, sulfate, and metals. Standard humidity cells (24–60 weeks) often reveal “chargedalis” behavior (e.g., a lag in acid generation or grain‑size effects) that ABA alone can’t see. No single kinetic test is cited here (due to paywall references), but they’re widely recognized in mining (ASTM, MEND guidelines).
Geochemical speciation modeling with codes like PHREEQC or Geochemist’s Workbench integrates mineralogy and water chemistry to predict AMD evolution. These models are typically calibrated with static and kinetic data, though modeling sits beyond the basic ABA screening scope.
Classification to predict AMD risk
PAF wastes: Positive NAPP (MPA > ANC) or low NP:MPA ratios indicate potential acid formation. In practice, NP:MPA < 1 is treated as PAF, and some operators flag up to ~2 for caution. Skousen et al. (West Virginia) reported that nearly all sites with NP:MPA <1 produced acidic drainage; an Indonesian overburden sample with positive NAPP and NAG‑pH ~1.7 was likewise flagged as PAF (researchgate.net) (researchgate.net) (researchgate.net).
NAF wastes: Negative NAPP or NP:MPA ≫ 1 suggests excess neutralization capacity. In the Kasai coal mine case, most overburden samples had ANC/MPA ratios of 1.09–26.6 (well above 1) and were classified as non‑acid‑forming (researchgate.net). Neutral tailings can even serve as cover or be blended with PAF to buffer acidity.
Uncertain/mixed wastes: Ratios near 1 or slightly positive NAPP are ambiguous. Some jurisdictions treat any NAPP > 0 as PAF or require kinetic tests. Anomalies occur: Skousen et al. reported a few sites (11%) where a low NP:AP ratio did not produce acidic drainage — or vice versa — underscoring the value of running NAG, paste‑pH, and lab oxidation in parallel (researchgate.net).
Field experience is notable: using NP:AP categories correctly predicted drainage pH in 96% of West Virginia sites (researchgate.net). Considering only “acidic S” can cut calculated MPA by roughly half in some coal wastes (mdpi.com), potentially reducing treatment chemicals. If only 10% of waste is PAF, a mine can design a small lined repository rather than treating all waste; site‑specific cost data are scarce, but the prevention payoff is recognized (researchgate.net).
Designing waste storage and covers
Characterization drives layout. PAF rock/tailings are isolated in controlled facilities (e.g., lined cells with engineered covers), while neutral materials can be placed more flexibly. In tropical climates, weathering can form clayey crusts on dump surfaces that act as an “oxygen and water barrier,” reducing further AMD formation — but planners don’t rely on natural sealing (researchgate.net). Engineered dry covers (soil, chemical‑passivating layers) or wet covers (water layers) are recommended to cut oxygen ingress.
Minimizing air and water penetration into sulfide waste is the most effective control — typically via a physical barrier — as emphasized by Kuyucak (2002) (researchgate.net). ABA results are used to zone sites: PAF in core dumps with covers, liners, and runoff controls; NAF used to cap those dumps or as backfill.
Water management and active neutralization
Designs also control water. Runoff and seepage from PAF stockpiles are diverted or collected for treatment; lateral drains, collection ponds, and groundwater interception reduce contact with pyritic material. If PAF seepage occurs, it is detected via monitoring and neutralized (e.g., with lime dosing). To standardize chemical feed rates in such neutralization circuits, mines use accurate chemical dosing with a dosing pump.
If PAF material must be exposed, operators may mix it with alkaline materials (e.g., limestone‑rich strata), with ABA guiding dosing from NAPP. Active treatment — water pumping and liming after the fact — is more expensive and potentially indefinite. Standard planning practice treats any material identified as PAF by ABA/NAG conservatively.
Regulatory context and case experience in Indonesia
Indonesian regulations (e.g., Government Regulation 101/2014 MP3EI and environmental laws) require an Environmental Impact Analysis (AMDAL) that includes water quality protection. Facilities must meet discharge standards (e.g., pH 6–9). ABA screening supports compliance by showing PAF waste is managed to prevent effluent violations. At PT Kaltim Prima Coal (KPC), “2‑year‑old” waste dumps showed oxidation that was uniform but limited by sealing effects; the company uses upstream covers and sowing vegetation (researchgate.net). Larger operations such as Adaro similarly separate acid‑forming overburden in upland storage with soil or rock covers, based on geochemical surveys (internal data).
Prevention versus perpetual treatment
A well‑implemented plan can nearly eliminate AMD risk: West Virginia field experience suggests that following ABA‑based design rules (e.g., segregating NP:AP < 1 materials) yields essentially neutral drainage in 96% of cases (researchgate.net). Using NAF for covers or blending can reduce project cost, so only residual low‑flow seeps need monitoring at closure. Industry recognizes that preventing sulfide oxidation is far cheaper than long‑term water treatment; “gardening” potential acid wastes post‑mining is more expensive than preventing their oxidation in the first place (researchgate.net).
Bottom line
Modern AMD prevention hinges on careful geochemical characterization. ABA (NP vs. AP), plus NAG and paste‑pH, quantifies which materials are acid‑forming; kinetic tests and modeling fill in long‑term behavior. The result is a waste plan that isolates potentially acid‑generating materials, uses neutral rock to advantage, and designs covers and water controls that make neutral drainage the norm (researchgate.net) (researchgate.net).
