South Africa is listed as one of the top 50 water-stressed countries in the world. The National Water Management Strategy focuses strongly on Water Conservation and Water Demand Management (WCWDM).

The mining industry is a major water consumer in South Africa. As such, it is becoming critical for the industry to reduce its consumption of freshwater, and to improve its internal water reticulation systems.

Watercare Mining has an esteemed reputation in the mining industry, built on our dedicated service delivery and focus on solving customer problems. The ability to understand our customers’ water reticulation systems intimately, and to implement practical solutions, is a core competency of Watercare Mining.

The silting of service water reticulation dams is a major problem faced by customers. Our intimate knowledge of mine service water reticulation systems has helped us to understand the impact and consequences of reduced reticulation capacity, which results from silted dams.

Our dedicated service delivery includes the cleaning of underground and surface mine dams, thereby improving water conservation and water demand management by the mines, to ensure that they are aligned with the National Water Management Strategy.

Poor water management can lead to flooding in mine shafts.

Regular dam cleaning will prevent critical system failures

A typical mine water reticulation system consists of clarification (or settling), pumps, pipes and dams. The bulk of the solids suspended (99%) in mine service water is removed in the underground clarifier.

The settled mud is removed from the circuit and managed via the mud handling infrastructure. The remaining 1% of suspended solids settle in the buffer dams and pipes. One per cent suspended solids, in a flow of 3000 m³/day, will fill a 3 500m³ dam with solids within 10 months, assuming that the specific gravity of the mud is 1,4kg/m³.

Critical system failures occur when underground dams fill with mud and silt.

The threat of internal dam mud-rushes moving the mud towards the dam suction point is heightened when the dam’s capacity is reduced by 30% or more. The solids enter the pump feed manifold, damaging the clear water pumps and, in severe cases, causing flooding of the shaft.

The consequences of such critical system failures include the replacement of multistage pumps, loss of production due to work stoppage and the cost of cleaning the shaft and dam.

Watercare Mining recommends that dams be cleaned every three to six months, depending on the nature of the dam and its water quality. The cleaning procedure depends on the availability of space, the material out of which the dam is constructed, and ventilation.

Underground dam cleaning using ZebraTube® dewatering bags.

Cleaning method depends on the type of mining water dam

The reticulation dams may be underground or on the surface, constructed from materials ranging from the natural rock ore within the mine to concrete or synthetic liner. These conditions vary significantly depending on whether the dam is below or above the surface.

Underground dam cleaning is done with either a vacuum or positive displacement pump – and labour. The mud which is removed is placed into dewatering bags or sent to a filter press, where the moisture content of the mud is reduced, making it easier to handle for disposal.

The risks associated with underground dam cleaning are:

• the threat of mudslides and
• a lack of adequate ventilation

Surface dams constructed of concrete, that have an access ramp available, are cleaned with a front-end loader, or pumped clean, as is done with underground dams. The mud is generally discharged to a drying pad or also placed into dewatering bags.

The primary risk associated with surface dam cleaning is the containment and management of the drained dirty water. Lined dams are more difficult to clean due to the environmental risk associated with the potential for the liner to tear. These dams are generally dredged, with the mud slurry contained in dewatering bags.

Before & after: Dam cleaning technologies with a small eco footprint

Dam cleaning generates large volumes of slurry that requires environmentally acceptable and safe containment and disposal. The geotextile dewatering and containment ZebraTubes® offer a high volume dewatering capacity utilising a small footprint.

The geotextile allows the water content of the slurry to drain completely, drying the solid material sufficiently to be handled with an excavator for disposal or re-use.

The silt particles settle inside the tube, causing the formation of a filter cake on the surface of the geotextile. The filter cake enhances filtration efficiency, allowing clear filtrate to percolate through the geotextile. This filtrate can be captured and re-used.

Watercare Mining completed a dam cleaning project when the mine stopped production between Christmas and New Year. The project scope was to clean a surface hot well dam with a capacity of 3 500m³.

It was estimated that 750m³ could be removed from the dam in the seven days available to us during the mine shut down. However, the commitment and hard work of the team on the project enabled a further 321m³ to be removed during this time.

The mud in the silted dam was watered to a slurry that could be pumped into 42m³ ZebraTube® dewatering bags and packed next to the dam. The filtrate dewatered from the bag drained into the mine’s waste dam.

It took approximately five days for the moisture of the slurry to dewater to a point where the mud could be moved to the mine’s solid waste handling system.

While we were unable to clean all the mud from the dam, the dam’s capacity was cleared for an additional 1 071m³ of water, equivalent to 30% of its capacity.

• Dawie Terblanche is Watercare Mining’s District Manager for the North West.