Operating at between 1 100 metres and 2 300 metres below surface, Zondereinde is the deepest platinum mine in the world. This depth and the mine’s specific geology have presented unique operational challenges since Zondereinde’s inception.

The geothermal gradient – the increase in rock temperature with increasing depth – is greater in the Bushveld Igneous Complex, where Zondereinde is situated, than in mines at similar depth. This results in virgin rock temperatures (temperatures of rock before ventilation and cooling is applied) of up to 72°C in the deepest level of the mine.

In order to create a safe and productive work environment for our people, and in compliance with the Mine Health and Safety Act, it is necessary to ventilate and cool the working areas to below 29°C. This level of ventilation and cooling is expensive, highly energy intensive and almost impossible to achieve using conventional methods.

Zondereinde has overcome this challenge by pioneering, developing and establishing two energy-efficient technologies not generally used in combination in mining – hydropower and backfill. In addition to environmental control, these innovations have mitigated some of the harsh heat and dust in underground workings, and following decades of ongoing development and optimisation they have been transformed into reliable, dependable, and indeed essential technologies.

Backfilling

Zondereinde is one of the first mines in the industry to use backill as a support regime in its mined out underground areas.

Backfill is waste which is supplied from the metallurgical plants in slurry form. It is pumped to the shaft and gravity fed to the stope horizon. The classified tailings are pumped tightly into porous ‘backfill bags’ which release the residual water, and leave a restricted compression structure to resist and contain roof lowering. Backfill quality and installation is strictly controlled, with specialised backfill contractors employed to undertake and monitor the backfill placement underground.

By placing backfill concurrently with advancing the mining face underground, the backfill support enhances both local and regional rock stability, and also serves to contain ventilation requirements, given that it reduces the transmission of heat from the surrounding rock.

Backfill has proved to be an industry-leading effective and enduring method of lowering the risk of rockfalls by supporting the roof of mined areas; it reduces energy use by assisting in the control of ventilation and cooling in working areas of the mine; and it significantly reduces the surface environmental impact of tailings storage.

Approximately 60% of the total mined out area at Zondereind has been backfilled in this way.

The health, safety and environmental benefits of backfilling include:

• Reduces hoisting and installation of timber support: energy savings of 10 080 MWh per year (equivalent to 10 890 tonnes of CO₂e avoided);
• Reduces cooling requirements: energy savings of 50 400 MWh per year (equivalent to 54 432 tonnes CO₂e avoided); ƒ Reducing tailings storage on surface by 26%: 14 Mt of tailings have been used as backfill since the initiative started; ƒ Reduced potential for falls of ground; and
• Reduced potential for seismic events.

Hydropower

Zondereinde’s depth below surface offers the advantage of using the passive pressure of water to drive machinery underground. Chilled water is cascaded through high pressure pipes through the underground workings which in turn becomes the source of power for most equipment such as rock drills, water loaders, water jetting guns etc. Exhaust water from hydropowered machinery cools the surrounding air where miners are working. This vastly improves the efficiency of the cooling process given that chilled water absorbs more heat from the hot surrounding rock. Zondereinde also uses water-jetting guns instead of electricity-driven scraper winches at the stope face, which further cools the working area.

he health, safety and environmental benefits of hydropower are:

• Contributes to lowering the temperature in working areas;
• Reduces the need for electricity-driven equipment, thereby avoiding 187 820 MWh of energy use per year (equivalent to 202 850 tonnes of CO₂e);
• Higher efficiencies compared with traditionally powered equipment;
• Enhanced dust suppression; and
• Reduced noise and vibration levels
• Reduced safety risks associated with scrapers.

We estimate that hydropower and backfill technologies have delivered a combined 37% saving in electricity use since Zondereinde went into production in 1992.