JOHANNESBURG (miningweekly.com) – A cool breeze of change has swept over South Africa’s oldest listed gold mine – the Barberton gold mine, in Mpumalanga – where a refrige- rated cooling system has been installed to improve underground conditions as the mine reaches greater depths, where ambient air temperatures become hotter.
The mine is a subsidiary of precious metals producer Pan African Resources and the miner’s flagship project. It has a yearly production target of about 125 000 oz of gold. The mine also pioneered biological oxidation – an environment-friendly process of releasing the gold from the sulphide that surrounds it using organisms that perform this process naturally.
Prior to the installation of the refrigerated cooling system, the mine relied solely on a fresh-air ventilation system to provide fresh ambient-temperature air for the underground areas, but mining at greater depths further away from the surface results in increasing ambient air temperatures.
The cooling system was commissioned through Pan African Resources by Bryanston-based mine ventilation and cooling system consultancy BBE Group, which comprises consultancy BBE Consulting and project implementation house BBE Projects. BBE Consulting provided the technical drawings and specifications for the cooling system, while BBE Projects undertook the project implementation.
BBE Projects was given access to site in November 2016 and the plant was operational in July 2017 – on time and within budget.
Barberton is synonymous with the early gold prospecting days of South Africa, with some of the first major gold nuggets found there, resulting in a gold rush in the 1880s and the rise of gold-mining boom towns.
The gold rush started in Barberton after significant finds by Tom McLachlan and Auguste Roberts from 1881 to 1883, after which prospectors flocked to the area. The Witwatersrand gold rush occurred later, in 1886, subsequently resulting in the founding of Johannesburg as a mining-boom town.
According to geologists and scientists, some of the oldest exposed rocks on earth – which date back about 3.5-billion years – are found in Barberton. These rock formations are located in the Barberton greenstone belt and are historically renowned for their gold mineralisation.
For more than 100 years since the mine started operating, cooling was not required; however, at its current depth and expansion into the surrounding mountainside, the Barberton mine has had to implement a cooling system to ensure suitable and safe underground conditions to comply with health and safety protocols.
The mine consists of the Sheba, New Consort and the Fairview shaft operations, with Fairview – also the deepest operation – receiving the new cooling system.
BBE Projects MD Richard Gundersen says, in terms of mining health and safety regulations for underground conditions, the ambient air temperature needs to be below a certain threshold and the Fairview shaft was nearing that threshold prior to the installation of the cooling system.
“The mine reached a point in its life in which the existing fresh-air ventilation system could no longer provide enough cooling for the high temperatures experienced in some of the deeper points of the mine,” he explains.
Therefore, the Fairview shaft’s ventilation system had to be augmented using a mechanical cooling system to make the underground environment more tolerable in terms of temperature, essentially enabling the mine to continue operating safely for personnel and machinery and ensuring that workers are comfortable.
“The simplest description of what we did was to ‘create winter in summer’, as the mine is cool enough to operate without a cooling system during winter, but not in summer,” highlights Gundersen.
In addition, the existing ventilation system was nearing the threshold of its mine cooling capability, because, when ambient air is drawn deeper into a mine, the energy from the compression of the air produces a heating effect, thereby countering any cooling effect the fresh air might have had.
The 3 MWr refrigeration plant – comprising two York R134a refrigeration machines, condenser cooling towers, pumps and electrical systems – is located on a rehabilitated corner of the main terrace of the mine.
Gundersen says the refrigeration plant employs tried-and-tested technology, which is highly reliable, having been used in similar applications worldwide.
While the plant is located outside the mine, the bulk air cooler (BAC) is located deep in the mountain, but is technically at the same elevation. At this location, the BAC cools air in the vicinity of the working places of the mine, where it is ultimately required. The BAC comprises an underground horizontal spray chamber located in an intake airway more than 1 km into the mountain.
Because many of the Barberton gold mine’s adits extend horizontally into the mountain (as opposed to vertical shafts) the BAC can operate under low pressure, as the water for the cooling system does not need to be pumped from great depths. Therefore, although on the same level as the outside refrigeration plant, the BAC is about 250 m below the surrounding hillside.
Accessing the terrace where the outside refrigeration plant is located requires navigating through a tortuous road with a novel “α-shaped” hairpin corner that presented challenges for the transport of the long heat exchanger shells of the refrigeration machines and the large cooling tower components, says Gundersen.
“The size of the plant enabled us to transport the bulk of the components in a knocked-down format – they were assembled on site quickly . . . reducing the amount of construction required.”
It is preferable to undertake as much fabrication and assembly work as possible at a factory, thereby reducing the amount of work required on site. For example, the refrigeration machines (manufactured in the US) were shipped fully assembled, complete with compressors and electric motors.
Similarly, all electrical equipment, comprising the motor control centre and programmable logic controller panel, could be packaged into a container, facilitating transport to the mine.
However, the two condenser cooling tower cells had to be supplied in four halves to keep the module sizes manageable for transport.
The operating capacities of the refrigeration machines were selected by BBE Consulting to provide the best balance between economies of scale and modularity for load matching and operational redundancy. “There is capacity to operate the system at a higher load. This enables the mine to increase the duty of the system as the mine progresses further,” says Gundersen.
Warm water from the bulk air cooler is pumped to the refrigeration machines and back through a 3 200-m-long pipeline. The low-pressure application, predominantly in quiet intake airways, made it possible to use high-density polyethylene piping, which has the added benefit of providing sufficient insulation properties for this particular situation. The returning cold water is then pumped through two stages of sprays to increase the efficiency of the BAC.
As part of the complete turnkey project, BBE Projects went to great lengths to use local contractors as far as possible for all civil construction and mechanical and electrical installation works.
Gundersen tells Mining Weekly that the mine was consulted to find appropriate contractors. “We contacted a few and enquired about their competence to undertake various work packages . . . and informed them of the requirements of this project.”
He points out that BBE personnel managed to find a civil construction contractor simply by driving around Barberton. “We noticed a local contractor performing some civil works for a newly built petrol station . . . and subcontracted the contractor to do the civil works on site for the project.” The contractor was required to do the same scope of work at the mine, such as steel columns and concrete decking.
As a consultancy and project house, Gundersen says, BBE Group’s role is to design the systems, produce all the drawings across all disciplines, source all the components and find suitable contractors to do all the civil construction and mechanical, electrical and control installation work. Of the on-site works, over 90% went to local contractors.
Since the commissioning of the refrigeration plant, the environmental conditions in the mine have improved drastically, enabling the mine to continue operations in line with health and safety protocols and regulations, as well as contributing to the mine’s overall sustainability.