Abstract:

Commonly found silicate minerals are often overlooked as effective long-term buffers able to neutralize weak mine effluent acidity. This process can be of importance in rocks containing low sulfide concentrations that generate acid at low rates that do not require soluble carbonates to buffer pH to circumneutral levels. Commonly found silicate minerals such as plagioclase and olivine may possess significant bulk neutralization potential and need to be considered during the determination of Acid Rock Drainage (ARD).

A kinetic study was carried out using a novel flow-through dissolution technique developed at the University of British Columbia: Flow-Through Time-Resolved Analysis (FT-TRA). During FT-TRA, a solution – here, deionized water in equilibrium with atmospheric CO₂ – is continuously pumped using an Ion Chromatography (IC) pump through a small volume reactor in which a mineral sample is gradually dissolved. The effluent is periodically collected using a fraction collector for subsequent compositional analysis. In-line pH sensors monitor both eluent as well as effluent pH.

The pH buffering capacity of two tailings samples as well as end-member mineral samples were studied. The tailings samples were sourced from a carbonate-deficient ultramafic complex. End-member minerals sourced included plagioclase, calcite, quartz and forsterite. A series of flow-through reactor-based dissolution experiments (approximately 140 hours each) were then carried out on small fractions of homogenized tailings samples using deionized water in equilibrium with the atmosphere. Effluent fractions were collected for subsequent compositional analysis using ICP-MS as well as pH.

Based on effluent [Ca, Mg] concentrations, simple pH buffering calculations based on H⁺ consumption by calcite, olivine and plagioclase were used to identify the minerals responsible for eluent pH buffering. The three most soluble (in decreasing order) minerals present in the tailings samples included carbonates (in one tailings sample only), followed by forsterite then plagioclase. In both tailings samples, eventually olivine becomes the dominant mineral consuming H+, providing ~80% of pH buffering.

This study empirically demonstrates the ability of acid-consuming silicate minerals to neutralize weak acidity generated in low-sulfide rocks. These findings further emphasize the need for ARD potential evaluations to adequately consider this process.

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Dr. Bart De Baere