Underground transfer dams form an integral part of the water management systems of a mine. The purpose of the transfer dams in underground mine water management is to receive and transfer dirty water from the stopes to the mine surface.
The dirty water is clarified and treated if necessary (e.g. pH adjustment) before it is stored in surface water dams.
From a water conservation perspective, this retained water is incorporated into the site water balance by returning it to the process water systems. The water is utilised where appropriate. For example, as cooling water makeup, wash water or for solids transport.
The underground transfer dams need to be operated at a level to accommodate dirty water inflow, less the water outflow, water losses and required freeboard.
Preventing mine dam failure
The dams contain buffer capacity for surges and other impacts within the process water circuit. The correct operation of these dams is critical in preventing dam failure resulting in overflow and flooding.
The reasons for dam failure include:
- Insufficient storage capacity due to mud and silt sedimentation
- Poor level control because of the failure to remove sediment
- Control and instrumentation errors and failure
- Pump failures due to “slugs” of solids breaking off and entering the pump inlet
The correct operation of transfer dams is also important for the preservation of the water transfer pumps that pump the water from the dams.
When sediment builds up in the dams, mud slugs seed through the pumps causing untimely pump wear and tear, and unbudgeted maintenance costs.
Figure 1: Diagrammatic representation of the dam agitation system installed on the base of an underground dirty water catchment dam.
Using agitation to control mud settling
Watercare Mining has extensive experience in the management of dam level control and dam cleaning. This has been a manual, labour-intensive function, logistically challenging and risky in terms of health and safety.
The dams are notoriously difficult to get to, the risk of mudslides is high and ventilation is poor. It generally requires 15 days to manually clean a transfer dam. The dams are required to be cleaned three times during the year.
Watercare Mining realised the opportunity to better manage the mud levels and to minimise the manual cleaning of the dams by designing, building and commissioning a unique dam agitation system.
The system is installed at the base of the transfer dam. Air originating from two, power-efficient blowers connected in parallel, is forced through a manifold containing downward-facing nozzles.
As the upward moving air exits the manifold it unsettles the sediment causing it to mix and suspend in the water (Figure 1 above), keeping the solids suspended as a homogenous slurry that is consistently and safely pumped through the transfer pumps. The design may also be used to blow water to disturb the settled solids.
Sedimentation of mud and silt in underground process water dams
Preventing sedimentation in underground transfer dams
The system has reliably prevented mud and silt sedimentation in the underground transfer dams, conserving the integrity of the water reticulation systems, and improving the working conditions associated with dam management.
Improved dam level control has reduced the risk of dam flooding, ensuring consistent availability of buffer storage capacity for unforeseen water and mud surges.
This has all been done cost-effectively, with the typical return on the investment of the agitation equipment being between 18 months and 36 months.
The calculated ROI is based on the elimination of labour-intensive manual cleaning cost and has not considered the reduced transfer pump maintenance costs.
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