Lime addition is a common method for the treatment of acid rock drainage (ARD) whereby neutralization promotes a reduction in acidity and the precipitation of metals as voluminous sludges that may contain gypsum, calcite, Fe-oxides and a spectrum of other phases. Due to the extremely fine-grained and often amorphous (i.e., non-crystalline) character of sludge solids, the composition of these materials has been difficult to elucidate. Traditional methods such as X-Ray Diffraction (XRD) and optical microscopy, for example, have proved largely ineffective. In order to provide further insight into the solid-phase characterization of neutralization sludges, high density sludge (HDS) materials from seven mine sites across Canada were examined by high-resolution microscopy techniques in combination with influent and effluent characterization. The primary objectives of the study were to: 1) define the nature of metal phase associations in sludge materials; 2) define the links between ARD influent/effluent chemistry, treatment process and sludge composition; and 3) provide the basis from which to develop a sludge management framework from the perspective of long-term chemical stability.
High-resolution microscopy methods utilized included Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscopy (STEM), and X-ray Absorption Near Edge Structure (XANES). SEM, and in particular STEM, provided the spatial resolution required to resolve the trace-metal associations in the sludge samples. With respect to XANES, there is a general absence of suitable model compounds analogous to many of the sludge phases examined, and as a result, the applicability of XANES to discern metal-associations in sludge samples is currently limited. However, the XANES data acquired for Zn offer insight into the potential stability of Zn through differentiation of potentially labile and non-labile complexes.
Predictably, SO4 was a dominant component in all ARD influents. However, considerable variability was observed with respect to other major parameters. Dominant influent chemistries on a molar basis included:
SO4>>Mg>Al>Fe>>Ca>Cl>F>Mn>Zn (Equity Mine),
SO4>Fe>>Ca=Mg (Geco Mine),
SO4>Ca>Mg>>Al>Cl>Cu =>Zn (Britannia Mine),
Na> SO4>Mg>Ca=Cl>Zn (Brunswick Mine),
Cl>Na>Ca> SO4>Mg>Zn (Chisel North Mine),
SO4>Mg>Ca>Fe>Zn (Samatosum Mine) and
SO4>Mg>>Ca>Fe>Na=Cl (Sullivan Mine).
Outflow compositions showed uniformly circum-neutral pH and greatly reduced values for Fe, Mn, Al and trace elements (e.g., Zn, Cu). In some cases, values for major species also showed pronounced declines through the HDS process, including SO4 (Equity, Geco, Samatosum) and Mg (Equity, Samatosum and Sullivan).
Treatment sludges show variable elemental compositions, but in all cases, the elemental abundances can be linked to ARD influent chemistry. Crystalline materials identified by XRD include calcite or Mg-calcite (Britannia, Brunswick, Chisel, Samatosum and Sullivan Mines) and gypsum (Equity, Geco, Samatosum and Sullivan). However, none of these phases were shown to be significant repositories for precipitated trace metals.
SEM and STEM data demonstrate that the trace metal-bearing phases in the HDS materials are amorphous or poorly crystalline, and variable in composition (relatively pure to highly heterogeneous). The trace metal host phases are invariably fine-grained, often occurring as oval aggregates ranging in size from <5 to 20 μm, and interspersed with other non-metal-bearing material (e.g., gypsum). Compositional zonation, often in concentric layers, is common, with the zones showing contrasting major ion signatures (e.g., Fe, Mg, Al). Such zonation is predicted to result from the recycling of sludge within the HDS process.
The dominant metal-hosting phases were site-specific, and included relatively pure Fe-oxyhydroxide (Geco Mine), amorphous Mg-Al-(Fe) hydroxysulfate (Equity Mine), Zn-Cu oxyhydroxide (Britannia Mine), Zn-Fe-Mn oxyhydroxide (Brunswick and Chisel North Mines), and Fe-Mg oxyhydroxide (Samatosum and Sullivan Mines). For all HDS samples, selected area electron diffraction patterns revealed broad diffuse rings, consistent with poorly crystalline to amorphous phases. Zn K-edge x-ray absorption near edge structure (XANES) spectra revealed a mixture of labile (outer-sphere complexes) and less labile (structurally incorporated) Zn species.
The data indicate that the nature of the dominant metal-hosting phase in the HDS materials is strongly dependent on ARD influent chemistry, with both the concentrations and relative proportions of Fe, Mg, Mn, Al and SO4 being dominant variables. In this manner, the results of the assessment highlight the potential for the development of a sludge management framework, which may permit prediction of “sludge type” from the ARD composition. In order to develop a defensible framework, further sludge characterization would be required to assess both within-mine and between-mine variability. A further requirement would also be a detailed understanding of the chemical stability of the various sludge types in varying depositional environments. Currently, there is not sufficient information available from which to assess the chemical stability of the various trace metal-bearing phases identified in this study. Given the substantial contrasts in the nature of the various metal-hosting phases, significant differences in chemical stability can be expected. In order for a potential framework to be applied successfully, sludge chemical stability as a function of varying pH and redox conditions must be established. This would be best achieved through the in situ collection of sludge porewaters and laboratory studies designed to assess pH- and pE-dependent solubility controls.
