Refrigeration Projects at African Mines Keep Miners Cooler, Energy Costs Lower
BBE recently announced completion of innovative refrigeration projects at deep mines in South Africa and Tanzania, applying technologies ranging from ice and water thermal storage to bulk air cooling.
It has commissioned an innovative ice thermal storage scheme for a refrigeration facility at AngloGold Ashanti’s Moab Khotsong mine near Klerksdorp, North West province, South Africa, which provides both chilled service water and air cooling for the mine.
The refrigeration machine for the ice maker operates outside of peak powertariff periods, when availability of electricity is more reliable and cheaper— and at night when outside temperatures are lower and the refrigeration machines can operate more efficiently. Ice is formed on the outside of submerged coils in an existing surface water dam and stored until required. In peak tariff periods, all of the refrigeration machines—providing some 40 MWR (megawatts of refrigeration)—can be turned off and cooling is then provided by melting ice in the dam. Overall, in a 24-hour cycle, the ice thermal storage system uses less power than a conventional water cooling system and the system operates outside of national power utility Eskom’s peak tariff periods.
This turnkey project, awarded to BBE in 2008, included engineering design, procurement, construction, management and commissioning. It is the second installation of its kind for AngloGold Ashanti, the first being an 800-ton ice mass thermal storage system at the Mponeng mine near Carletonville. The Moab Khotsong installation is a third-generation design in which the ice is formed directly on refrigerant carrying coils.
According to BBE, ice thermal storage schemes are an ideal response to the critical need for power and energy management in mine ventilation, refrigeration and cooling systems. These schemes have become an integral part of BBE’s expertise. BBE is a registered energy services company (ESCO) with Eskom, a classification that recognizes the company’s technical competency gained from its work in the field of energy saving and load-shift management for mine cooling and ventilation systems. As an ESCO, BBE says it is able to initiate these projects in conjunction with an interested mine customer who provides a letter of intent with which BBE can then apply to Eskom for funding through the DSM (demand side management) program. BBE guarantees the performance of the system, while the mine contracts to operate the system as required.
BBE also conceptualized and designed a unique water thermal storage scheme for Impala Platinum in the North West province which promises to achieve significant power savings. “The challenge has been to introduce water thermal storage into the cooling systems on seven shafts using existing water storage dams for both hot and cold water storage at different times during the day,” said BBE Managing Director Richard Gundersen.
“Each dam has been fitted with five plasticized canvas bladders, with a combined capacity of 3 megaliters per dam, the largest measuring 30 m x 7 m wide by 6 m high when full. The bladders have been secured to the base of the dam and a piping system feeds and extracts the cold water in and out of the bladder while the hot water is fed and extracted from the surrounding space,” said Gundersen.
Capital costs for the simultaneous storage of hot and cold water in the same reservoir are a fraction of the cost of a new dam. “The electrical operating costs will be reduced by producing more cold water than is required during off-peak tariff periods, then switching the refrigeration machines off during the peak tariff periods of the day and using the stored cold water. The saving is achieved through the tariff differential,” said Gundersen.
In July, BBE reported it was on schedule for commissioning of a new 7- MW bulk air cooler and two ammonia surface refrigeration machines at African Barrick Gold’s Bulyanhulu mine in northwest Tanzania, to counter the increased heat load arising from the mine’s expansion and deepening.
This turnkey project represents the first phase of a proposed 14-MW system and is believed to be the first bulk air cooling system to be installed on a Tanzanian mine. The project comprises a four-cell vertical forced-draft concrete bulk air cooling tower, two axial flow fans, a plant room housing two ammonia refrigeration machines with plate heat exchangers, a four-cell condenser cooling tower, and associated pumps, pipework, electrical, instrumentation and control system.
The $13-million contract was awarded in 2009. Bulyanhulu is an underground trackless operation that uses long-hole and drift-and-fill as its principal stoping methods. Ore reserves are accessed via a surface shaft and an internal ramp system.