Passive treatment of Acid Mine Drainage with high metal concentrations: Results from experimental treatment tanks in the Iberian Pyrite Belt (SW Spain)

Passive treatment of acid mine drainage (AMD) have been applied successfully at many sites where metal concentrations are relatively low, but experience is scarce for waters with higher metal loads. In this study, we tested the suitability of caustic magnesia (MgO) as an alkaline reagent at field scale, with and without a prior calcite dissolution step. Three pilot systems where tested at the “Monte Romero” mine (Southwest Spain). One treatment tank was filled with a mix of MgO and silica gravel only, while a second one contained two layers of calcite followed by one of MgO. The third tank was similar to a reducing and alkalinity producing system (RAPS). It contained layers of calcite mixed with sheep manure followed by MgO mixed with silica gravel. The RAPS received water with an very high acidity of up to 29000 mg/L as CaCO3 equivalents, while inflow acidity of the other two tanks was around 2380 mg/L as CaCO3. The tank which contained only MgO had severe clogging problems from the beginning, and showed a poor chemical performance. The tank containing calcite and MgO and the RAPS removed high portions of Al and Cu, but only small to insignificant portions of Fe and Zn. Aeration and decantation improved the removal of Fe. Calcite eliminated acidity more effectively than MgO. Nevertheless, the calcite and magnesia tank clogged after seven months of operation due to precipitation of Al-phases. Comparison with published data revealed that even though percentage removals were below our expectations, performance in absolute values was good compared to other systems. The high acidity removal rates are due to the elevated Al concentrations which are easily removed. INTRODUCTION During the last two decades, passive systems have become a widely used option for the treatment acid mine drainage (AMD) in northern America and increasingly in Europe (e.g. Younger et al., 2002, Ziemkiewicz et al., 2003). Nevertheless, these systems have mainly been applied at coal mines and other sites with relatively low metal concentrations, while experience is scarce for waters with higher metal loads. The abandoned mines of Iberian Pyrite Belt (SW Spain) discharge acid waters with high heavy metal concentrations to the Rio Tinto and Rio Odiel river basins (e.g. Olias et al., 2004). Passive treatment systems which once built only require naturally available energy sources and infrequent maintenance may be an economical option to improve water quality in this region. Passive treatment system are usually based on calcite dissolution which achieves an outflow pH up to about 8 (Younger et al., 2002, Cortina et al., 2003). Nevertheless, this pH may be insufficient to precipitate divalent metals (Cortina et al., 2003) such as Zn, Mn, Cu, Pb, Ni, Co, and Cd which are often found in AMD (Younger et al., 2002). Caustic magnesia (MgO) has the attractive property of raising solution pH up to 9-10, where divalent metals precipitate readily as hydroxides. In laboratory column studies (Cortina et al., 2003), it was found that MgO can reduce concentrations of 75 mg/L of Zn, Cu, Pb and Mn in the inflowing water to values below 0.04 mg/L. Nevertheless, MgO has not yet been applied in passive treatment systems at field scale. In this study, we tested the suitability of caustic magnesia (MgO) as an alkaline reagent at field scale in three pilot systems, with and without a prior calcite dissolution step. MATERIALS AND METHODS The present study was carried out at the “Monte Romero” abandoned mine in South-western Spain (Huelva province, Fig. 1). This was a massive pyrite deposit with minor amounts of Zn, Pb and Cu sulphides. The enclosing rocks are siliclastic schists with no carbonate beds present. The AMD emerging from the adit has a mean pH of 3.3, an acidity of over 2300 mg/L as CaCO3 equivalents and contains 400 mg/L Zn, 350 mg/L Fe (83% Fe(II)), 260 mg/L Mg, 210 mg/L Ca, 130 mg/l Al, 18 mg/L Mn, 11 mg/L Cu and 1200 mg/L sulphate. Flow rate is usually around 2 L/s. 9 INTERNATIONAL MINE WATER CONGRESS 642 Figure 1: Schematic map of Monte Romero field site and location of experimental treatment tanks Filling materials • Caustic magnesia (MgO) was provided by Magnesitas de Navarra S.A. In this study, a residue from the magnesite calcination process was used. It contains 60-70% of MgO, together with minor amounts of silicates and carbonates. It is economic, but it has a wide range of particle sizes from powder to pebble size (0-2 cm) • Calcite gravel (99% CaCO3, particle size 0.6-1.2 cm) was purchased at a quarry near Morón de la Frontera. • Silica gravel (0.5-2 cm) was purchased at a local quarry. • Sheep manure was obtained from a local farm. Description of treatment tanks 220 L plastic barrels were used as upflow reactors. Two treatment tanks (“A” and “B”) were installed close to the adit where AMD emerges from Monte Romero Mine (Fig. 1). The input water is taken directly from the adit and feeds the tanks by gravity flow. Tank “C” was installed at the exit of the tunnel that passes underneath the tailings pond. The input water is taken from a V-notch weir located inside the tunnel and feeds the tank by gravity flow. In all tanks AMD enters at the bottom where a layer of quartz gravel distributes flow evenly, flows upwards through the reactive material and exits the barrel though an elbow pipe in the lid. Flow rate was controlled by a compression valve at the inlet. All tanks were equipped with piezometric tubes connected to the inand outlet and to the intermediate sampling points in order to measure hydraulic conductivity. The tanks were set up from September 17th to 19th of 2003 and operated until July of 2004. Tank A (Fig. 2) contains a mixture of 15 vol. % of MgO and 85 vol. % of quartz gravel. It was designed to act in a similar manner as an anoxic limestone drain. Design flow rate was calculated as 220 mL/min (residence time 6 hours with a supposed porosity of 40%). In laboratory trials (Cortina et al., 2003), this residence time had proven sufficient for equilibrium between caustic magnesia and the input solution. Tank B contains (from bottom to top) 25 cm of 50 vol. % calcite gravel and 50 vol. % quartz gravel, 32 cm of pure calcite gravel, and 25 cm of 50 vol. % MgO and 50 vol. % quartz gravel. A sampling port is located at the contact between the upper calcite and the caustic magnesia layer. The calcite layer was introduced to provide a first rise of pH, MgO was meant to boost pH up to higher values (ph 9-10) than those achievable by calcite dissolution (pH 8). Design flow rate was calculated as 55 mL/min as to provide a residence time of 16 hours within the calcite layers and 6 hours within the MgO layer. Tank C contains (from bottom to top), 25 cm of 35 vol. % sheep manure and 65 vol. % quartz gravel, 38 cm of 25 vol. % sheep manure and 75 vol. % calcite gravel, and 16 cm of 50 vol. % caustic magnesia and 50 vol. % quartz gravel. These three reactive zones are separated from each other by layers of 6 cm of quartz gravel equipped with sampling ports (Fig. 2). Tank C was designed to act in a similar manner as a reducing and alkalinity producing system (RAPS), again with MgO as a final pH booster. Design flow rate was calculated as 30 mL/min (residence time of 24 hours within the manure layers and 6 hours within the MgO layer). 500 km