ARTICLE
Simplifying arsenic sludge management with passive Geotube® dewatering
The extraction of metals and minerals can generate large volumes of wastewater and sludge that must be effectively managed and treated. A gold mine in Atlantic Canada was producing between 1.5 and 2.5 million cubic metres of sludge annually at 0.13% average solids, and was considering storing the arsenic-laden sludge in onsite ponds. However, the lagoons would eventually require the accumulated sludge to be removed, creating an ongoing management challenge. Over time, the sludge would have to be dredged, dewatered and relocated, requiring additional handling and maintenance.
Passive dewatering with Geotubes
In the search for a scalable and long-term sludge management and dewatering solution, the gold mine determined that a “clean-as-you-go” approach using the Bishop Solids Management Solution would provide a more effective way to manage its mining sludge.
The semi-passive process allows sludge to be contained directly within Geotube® units, where it continues to consolidate and dewater over time. The dewatered material can remain contained onsite for extended periods or, where appropriate, provide permanent onsite containment. The large geotextile bags are made from woven polypropylene fibres and serve as large containment vessels, while allowing water to drain from the sludge.
A unique benefit of Geotube units is the performance enhancement provided by natural freezing and thawing cycles that come from outdoor exposure. As water freezes, it expands, compressing the solids and creating microchannels through which water can drain when the sludge thaws. As water is released, the material consolidates and the height of the Geotube containers can decrease by as much as half, creating additional capacity for incoming sludge. Operators can then resume filling, or top up the Geotubes to achieve maximum fill height.
Results
Bishop Water conducted lab testing to determine the optimal polymer application for the gold mine’s arsenic-laden sludge, seeking to determine dosage rate, filtrate quality from the Geotube units, speed of dewatering, and polymer type.
The full-scale system receives polymer-conditioned arsenic sludge at up to 7,000 L/min and up to 1,700 mg/L TSS. The Geotubes provide both filtration and containment, producing filtrate with TSS of approximately 200 mg/L and arsenic levels of 0.013 mg/L. The filtrate is then directed to a pond, where gravity settling reduces the TSS to less than 15 mg/L before discharge.
