Under Control

The following interview with SRK Principal Scientist (Hydro) / Partner / Director Peter Shepherd was one of the sources for the Mining Decisions article Under Control

1.    What have you observed as common oversights relating to water usage monitoring in mines? How do they impact on mining operations?

The tailings dams on mines are generally the areas where the greatest water savings can be realised, so good monitoring and management of these facilities is vital. One of the most significant changes in water use on a mine can occur when there is a variation in the density of slurries emanating from the processing plant – leading to an unexpected increase in the volume of water transferred to the tailings dam. With typical platinum tailings, for example, a slurry comprises one tonne of water for every tonne of solid tailings that needs to be pumped to the tailings dam. If not properly monitored, the water-to-tailings ratio could rise and lead to excessive water being disposed of in the tailings dam.

While the change in ratio may be for a variety of reasons, such as the inefficient functioning of the thickeners, there are negative impacts in terms of both wastage and potential environmental harm. Firstly, more water may lead to an increased surface area on the dam, meaning more wastage through evaporation.  Evaporation is the principle way that water is lost on a mine. Secondly, higher levels of water in the dam also raise the danger of seepage of water beyond the collection facilities – posing an environmental hazard and legal compliance risk. Solving this problem is generally not easy, as it may need changes to the plant and changes to the infrastructure.

2.    What should be key priority areas in water usage monitoring in mines to ensure good husbandry of the scarce resource during the current drought conditions, considering that most mines are located in remote areas?

Mines are increasingly prioritising the recycling of water on site, to become as self-sufficient as possible and to reduce the cost of buying in water from outside sources. The recent drought has focused minds even more, and mines have found better ways of utilising water flowing into their operations. It is important for mines to understand the flow of water into mine workings – both through surface water run-off (in the case of open pit mines) and groundwater ingress – so that they can control and harness this resource for operational purposes. Most mines have to pump considerable quantities of water out of workings continuously, and this can be put to valuable use in processing and other activities. Careful monitoring of water flow into an open pit mine also has safety implications, as excess water around the mining area can negatively impact the stability of the pit-walls.

Making the best use of the available water flowing into a mine also requires effective separation of solids from water, such as sediment traps and settlement facilities; alongside this is the suitable means to pump both water and slimes to surface, so that it can be put to appropriate use.

A key strategy in utilising water sustainably on mines is the establishment of a water hierarchy where different qualities of water can be put to different uses; re-circulating and re-using water in this way limits the amount of unnecessary treatment that is required. The amount of potable water required by the mine – for drinking and washing – can therefore be kept to a minimum.

3.    What should be considered in water quality monitoring, considering the high rate of pollution?

Coal and gold mining have brought with them particularly high levels of pollution in the South African context, so water quality is certainly something that needs to be closely monitored in these sectors. This means constant monitoring and management of water levels, especially in mined out areas where mining operations have ceased; mines even have to be aware of when neighbouring mines close down or cease operating in certain areas – as this may also lead to more water ingress in their own mines.

If a mine is close to a river, they need to be particularly vigilant, and should be able to collect and use water so that it does not flow from site into a river system. Also, the plume of untreated water must not extend beyond the mine boundaries where possible, so boreholes and/or other seepage collection facilities necessary around the tailings dams to monitor and collect water.

4.    Any latest standards or innovations in water quality monitoring that you might wish to share with mines?

Despite the high cost of forcing water out of tailings before disposal – using filter presses, for example – this kind of technology is already is wide use in very dry countries like Namibia. The principle is that we will need to remove more and more water from tailings before this material is removed for dumping. The filter press option, which is energy-intensive, also tends to raise the transportation cost of tailings – as it can no longer be pumped in liquid form and has to be transported by conveyor or truck.

There is also scope for preventing pollution by better controlling the plume around tailings dams through interception trenches or boreholes strategically positioned around these facilities. These interventions can reduce environmental and compliance risk, while also providing a source of water for certain suitable applications. The location of water on a mine site may also suggest a valuable use for it; for instance, if it is not close enough to be used in a plant, it may be usable for dust suppression on mine roads.

Mines have become steadily more proactive and innovative in their application of water conservation strategies in the last three to four years, spurred on further by the recent drought. These positive practices, however, are likely to endure – as they have cost-related and environmental benefits that mines now appreciate.

5.    What are the ways mines are constructing water controls to capture water to accommodate climate variability?

Mines have for many years been increasing the amount of run-off that they capture on mine sites, in a trend that was initially driven by legislation like the Water Act (Act 36 of 1998) ad specifically Regulation 704 which requires mines to separate clean and dirty water. Clean water run-off from areas upstream of the mine needs to be diverted away from mining infrastructure, while dirty water emanating from the mine site needs to be collected and reused on the site.

Climate variability – where heavier rainfall is being experienced in a shorter space of time alongside longer dry spells – therefore creates a more demanding situation in which mines’ water storage facilities must accommodate larger quantities at a time. This year has already seen an example of this; February was a wet month and March was one of the driest.

6.    How much buffer storage is needed to ensure consistency across wet and dry periods?

For a large mine with concentrators, shafts and pits, the volume stored could vary from 500,000 m3 to 1,5 million m3.  Sizing of dams is therefore very important, and makes the field of hydrology exciting.

You don’t want a dam that is mostly empty and only fills up every 10 years, but you do want to capture, reuse and recycle as much water as possible to minimise the water you need to buy from water service providers.

Also, the less that mines need to consume treated water from these providers, the more of it is available to communities and other industries that perhaps don’t have the potential to recycle their water.

7.    Which mines and/or mine companies have successfully achieved or are striving to achieve the correct amount?

The platinum mining sector has made considerable progress in reusing and recycling water, with 50-60% of water pumped to tailing dams being returned to the plant for re-use. Ten years ago, this figure would have been closer to 30%.  The aim is to further increase the water returned to above the 60%.

There has been extensive work undertaken on this front, as tailings dams tend to be where mines lose most of their water – through seepage, evaporation or water locked up in the dam itself.

8.    Please detail the full scope of SRK’s work in this matter.

SRK has been at the forefront of developing ways to maximise the re-use of water; even before the introduction of legislative requirements, we recommended and installed features that would collect water. These included groundwater barriers and collection facilities which allowed mines to minimise the areas on tailings storage facilities (so that there was less evaporation), compartmentalise return water dams (to minimise exposed areas) and create thickened tailings.

Creating compartments within dams allows water of different qualities to be stored within the same facility – providing the versatility necessary to implement a ‘water hierarchy’ on the mine. [see more on ‘hierarchy’ below]

More advanced thickener technology is becoming more popular, although it has traditionally come with a high price tag. This allows less water to be lost to the tailings dam, by focusing on the aim of drier tailings leaving the plant.

9.    Please detail the extensive technologies that could be implemented by mines to reduce evaporation.

It is important to reduce the area exposed to the sunlight; this can be achieved by creating deeper dams or by covering dams with plastic covers. An innovative solution applied abroad has been the floating of solar panels on effluent dams; unfortunately this particular innovation would be fairly costly, and in South Africa may not be feasible due to the likelihood of theft.

10.    How do these technologies work? What are the benefits?

The benefits of deeper or covered dams could save a mine 1-2 million m3 of water each year.

11.    How are mines opting to recycle/reuse their water?

An important strategy for mines is creating a ‘hierarchy’ of water use, so that water-consuming activities are only allocated the minimum quality of water that they require. This allows the consumption of treated, expensive water to be limited to potable purposes while other applications like irrigation and plant processes can use water that is less treated.

12.    How much does this affect the quality of the water being re-circulated?

An inevitable outcome of re-cycling mine water is that the salt levels rise steadily, as there is no dilution of fresh water from an external source. While this ‘brine’ is kept safely on site, it still needs to be treated or disposed of at some stage.

13.    How does this allow for managing of shallow groundwater, especially over conventional technologies?

Directional drilling is a valuable strategy for reducing seepage from tailings dams and controlling pollution. By drilling underground holes in a ‘U’ shape instead of just vertically, you can cover a greater area along the whole front of a tailings dam – to pick up more of a polluted plume, for instance, and treating it for re-use. It allows any seepage from a dam to be more effectively treated.

14.    Other comment:

A significant area of mine water conservation has been the wide use of liners under tailings storage facilities; this has reduced the level of seepage from dams, leaving more water on surface to be re-used.
 

Mining_Decisions_-_Under_control.pdf

Acknowledgement: Mining Decisions_Silke Colquhoun

 

 

 

SRK Africa