Gold miner Gold Fields is currently using four of engineering company M-Tech Industrial’s Mk II modular underground air-cooling units (ACU) at its Beatrix No 4 gold mine, instead of using its static surface refrigeration plant during the past winter months.

Beatrix mine is located 240 km south-west of Johannesburg and has 17.1-million ounces in mineral resources and 5.7-million ounces in mineral reserves. The gold mine operates at depths of between 700 m and 2 200 m below surface.

M-Tech Industrial programme manager Dr Martin van Eldik says that the four ACUs are collectively providing about 1 MW of cooling, with two more units to be commissioned by the company as the mine expands.

The ACUs are majority manufactured by M-Tech Industrial, with the marketing infrastructure and after-sales service coming from manufacturer of underground support systems Mine Support Products (MSP).

Van Eldik explains that development of its latest ACU Mk II began 2009 and the first prototype was tested and used for three years at global gold mining company AngloGold Ashanti’s Mponeng gold mine, located 65 km south-west of Johannesburg. Gold is mined at an average depth of 2 800 m to 3 400 m below surface and is one of the world’s deepest and richest gold mines. The Ventersdorp Contact Reef extension from 109 levels to 120 levels is expected to increase the mine’s life by eight years to 2024. This project is expected to add 2.5-million ounces to production.

The nominal 250 kW Mk II was launched about two years ago and Van Eldik explains that the modular underground ACU can use water at temperatures of up to 35 ºC as heat sink, adding that it is based on a “vapour compression heat-pump cycle with efficiencies comparable to that of surface chillers”.

The ACUs are designed to create a com- fortable working environment with temperatures ranging between 26 ºC and 28 ºC in areas where cooling capacity is either inadequate or inefficient, or where additional cooling is required as a result of mining regulations, says Van Eldik.

He adds that it is particularly well suited for remote underground areas where temperatures approach the critical 32 ºC wet-bulb limit.

ACU Concept

Heat is extracted locally from ventilation air through a finned-tube evaporator coil. The heat is transferred through a vapour compression cycle to water in a specially designed condenser unit that operates efficiently with a wide range of inlet water temperatures as well as the capability of dealing with a large water temperature rise.

The ACUs can operate with a greater range of water temperatures compared to conventional chilled water cooling cars, as the water does not have to be chilled. The water temperature that rises through the condenser is also much larger, subsequently reducing the amount of water used. The reduction in flow rate results in less water to be pumped from the mine, leading to increased savings on pumping power.

Van Eldik explains that the ACU is mobile and comes standard fitted onto rolling stock to suit any mine’s infrastructure, noting that the ACUs are extremely positional efficient, as cooling can be provided exactly when and where it is required.

Challenges

MSP GM Conrad Engelbrecht says that major issues affecting operations are the quality and supply of both water and electricity. The challenges associated with the robustness of the condenser, were overcome by using high-grade stainless steel piping in the Mk II condenser, compared with the copper piping used in the nominal 100 kW ACU Mk.

Engelbrecht explains that from a marketing perspective, customers are sometimes sceptical of the new technology used in the cooling system, as many companies have been using the same system for decades and are not prepared to change their ways.

Further, the cooling system is not a consumable product and, therefore, requires the mine to invest in the product. However, Van Eldik adds notes that the ACU system typically uses 20% to 50% less energy than conventional cooling systems, owing to the reduced pumping power requirement.

Exporting

Van Eldik states that the ACU has been exported to Peru, South America, with interest also being shown by Canada and Australia.

Engelbrecht explains that MSP does not have a big enough footprint overseas yet to provide the after-sales service that the company usually provides locally.

However, he states that high volume orders from abroad will enable the com- pany to develop infrastructure and, in turn, a bigger footprint.