Surface refrigeration plant commissioned at Mali’s Loulo Mine

Technical information supplied by Avinash Andhee, senior project leader and mechanical engineer at BBE, edited by Eamonn Ryan

The Loulo Mine in Mali recently underwent a significant HVAC cooling project, aimed at enhancing the refrigeration and ventilation systems within the facility.

Bluhm Burton Engineering (BBE) Projects has commissioned another turnkey surface refrigeration plant at Société des Mines de Loulo (SOMILO) SA gold mine in Mali, West Africa. It was a full turnkey project commencing with a hole in the ground and ending with the handover of a fully commissioned HVAC cooling plant.

Defining the character of the project was its remote location: the Loulo gold field, which includes the Gara, Yalea and Gounkoto deposits, is less than 2km east of the border with Senegal (marked by the Falé river) on the West Mali gold belt. The refrigeration plant as well as other infrastructure will enable the mine to expand its operations and maintain its gold production output.

THE CHALLENGE

BAC overview.Image supplied by Bluhm Burton Engineering

After commencing with open pit mining in 2005, the mine took the opportunity to start looking at mining underground. Underground mining at Loulo mine is relatively shallow, but being at low altitude in an equatorial region, the mine experiences a challenging climate of both humid and hot, through the various seasons, particularly when hot and humid occur at the same time, which is most of the year.

These challenging climate conditions necessitated the turnkey construction and commissioning of a refrigeration plant at both Gara and Yalea dedicated downcast ventilation holes, in 2016, to cool the underground workings. Since 2016, both the Gara and Yalea underground mines have expanded production towards the south requiring further underground cooling.

The Loulo Mine HVAC cooling project involved the multidisciplinary BBE team, each specialising in their respective areas. The design phase took place in South Africa, with a dedicated division handling electrical control and instrumentation design. Local sub contractors were also engaged to assist with the civil and structural design aspects. The mechanical design and process were managed by Andhee, who oversaw the specifications, procurement, and shipment of necessary equipment and materials to the site.

THE SOLUTION

Avinash Andhee, senior project leader and mechanical engineer at BBE.Image supplied by Bluhm Burton Engineering

In 2015. an independent mine ventilation consultant prepared and issued an enquiry on behalf of SOMILO for a turnkey contract to build a 14MWR refrigeration plant at both Gara and Yalea dedicated downcast ventilation holes. BBE Projects was awarded the turnkey contract by means of a competitive tender, after a few rounds of technical and commercial queries. The BBE tender covered all technical, commercial, and statutory requirements laid out in the enquiry documents, and during the execution of the project, BBE Projects carried out detailed design, procurement, manufacturing, quality control, shipping, delivery, construction, erection, installation, supervision, project management, commissioning, testing, certification and handover of the refrigeration plants.

In a similar manner, SOMILO awarded BBE Projects a second turnkey contract to build a refrigeration plant at the Yalea South dedicated downcast ventilation hole in 2021. This time SOMILO requested that all equipment supplied by BBE Projects must be compatible and interchangeable with the existing refrigeration plants to minimise spares holding and to allow the mine to implement the same corrective and preventative maintenance schedules used at the existing plants.

“Standardisation of equipment was another crucial requirement, given the remote location of the mine. Working in Africa presents unique challenges due to remote locations and the need for self-sufficiency. By utilising standardised components and maintenance procedures, the mine could streamline operations, minimise spare parts inventory, and ensure ease of future maintenance,” says Andhee.

The new refrigeration plant capacity is 8MWR and is smaller than the existing refrigeration plants, but with space reserved and design provisions made to expand the refrigeration system in future if required. The new refrigeration plant also has many improvements compared to the existing plant. These innovations were brought about during meetings and consultation with mine employees that maintain and operate the refrigeration plants. The SOMILO ventilation team completed an extensive ventilation study with aid of mine ventilation simulation software.

The most notable innovation is the five-metre diameter steel inlet duct that connects the bulk air cooler to the dedicated downcast ventilation hole complete with self-closing dampers positioned directly above the shaft that vent shockwaves that occur during certain times of day due to blasting underground. The damper blades are of a relatively light construction allowing the blades to open with little pressure and close using adjustable counterweights.

The refrigeration machine is an 8MWR dual-compressor York YD with R134a refrigerant, housed in a plant room erected from steelwork that was designed, shop detailed and fabricated in South Africa. Dual compressors were selected to halve the maximum instantaneous start-up current of the refrigeration machines thereby mitigating the possibility of de-stabilising the mine’s power grid. The mine generates its own power by means of diesel generators and solar plants on site for mining operations.

The Bulk Air Cooler (BAC) is adjacent to the plant room, minimising interconnecting piping and thermal losses. The BAC basin is constructed from reinforced concrete with insulated panel side walls and roof to speed-up construction time, reduce time spent working at heights and increase thermal efficiency of the spray chamber. Similarly, the Condenser Cooling Tower (CCT) used for heat rejection is constructed from lightweight Fibreglass-Reinforced Plastic (FRP) components with similar benefits in cost, time and safety. Furthermore, FRP is a good alternative to concrete because of its high strength, light weight and stability.

All civil, structural, mechanical and piping works were carried out by local contractors in line with the mine’s commitment to drive socio-economic development and training. Site construction, erection and assembly was supervised by BBE Projects with regular inspections by discipline specific engineers from BBE and OEMs.

To manage the challenges of Andhee remotely overseeing the project site, a site manager (a South African, Humphrey Maluleke, a civil engineer) was seconded to Mali to provide a constant presence at Loulo Mine supported by a junior civil engineer recruited locally in Mali. Andhee supplemented this with regular meetings and periodic in-person visits, while inspections were conducted by the site manager and the project team, ensuring effective communication and quality control throughout the project.

“During the period of sanctions, efforts were made to limit visits to the site, relying instead on the site manager’s daily reports and photographs for quality control, as well as bi-weekly online meetings.”

Equipment for the refrigeration plant was sourced and shipped from all over the world. CCT fans and electrical motors from Brazil, FRP CCT structure and header piping from India, FRP CCT fan stacks from Poland, CCT gearboxes from Belgium, FRP CCT Cladding from Turkey, refrigerant from Dubai and the refrigeration machines from USA. The balance of the mechanical equipment (pumps, plant room crane, valves, piping, fasteners, etc.), EC&I equipment (transformers, MV Switchgear, MCC, cables, instrumentation) and civil material and structural steelwork (rebar, plant room steelwork, shaft inlet ducting, pipe supports) were consolidated and shipped from South Africa.

CHALLENGES ON SITE

Mine refrigeration is the area of specialisation of BBE Projects, so while there were the usual challenges, says Avinash Andhee, senior project leader and mechanical engineer at BBE, they were all in a day’s work for them.

He adds: “This was our second such project in Mali and we had already learned a great deal from that. Undertaking the HVAC cooling project at Loulo Mine presented several unique challenges for the team. At the outset, the project coincided with a period of political instability as a coup and subsequent sanctions in the ECOWAS (Economic Community of West African States) region disrupted the logistics chain. The construction of the refrigeration plant also came with a unique set of challenges such as changing the shipping route from Dakar port to Guinea port midway through the project and sourcing new quotations became necessary. A major supplier also went into liquidation (fortunately only near the end of the manufacturing process). The project team also collaborated with the mine to secure alternative clearing agents and transportation routes from Guinea to Mali. Despite the unforeseen challenges, the team persevered to ensure the project’s success.”

Andhee points out that logistics played a crucial role in the project’s execution. The team encountered obstacles not only in the region but also faced delays at the port in Durban following the historic floods in KwaZulu Natal. However, by adapting to the circumstances and exploring alternative solutions such as changing shipping ports, the team managed to overcome these logistical hurdles.

“Health and safety considerations were paramount throughout the project. While Mali is known as a malaria region, the team took appropriate precautions to minimise health risks, and the quality of food was to a high standard,” he adds.

BBE’s expertise in multiple disciplines enabled them to adopt a holistic design approach, covering civil, structural, mechanical, piping, and electrical control and instrumentation aspects. The project’s design phase took into account lessons learned from previous projects, addressing past challenges and incorporating innovative solutions. For instance, previous issues with blasting necessitated a shift from insulated panel structures to robust steel ducting, improving ventilation effectiveness and worker safety. Additionally, prefabricated components, such as an operator cabin with excellent thermal properties, were utilised to streamline on-site construction and enhance energy efficiency.

When it came to the design specifications and client brief for the project, the main emphasis was on incorporating lessons learned from previous endeavours. The team addressed issues encountered in the past, such as blasting problems, by opting for a more robust solution. The use of steel ducting, resembling a hammerhead, was a significant departure from the previous insulated panel structure. The steel ducting provided enhanced durability and improved performance.

“To optimise construction efficiency, the components were meticulously designed and fabricated in South Africa. The steel sections were cut into sizes that could be easily transported in containers, while the panels were assembled on-site based on detailed drawings provided to the contractor.”

Methodologies used to implement energy reductions included:

• A horizontal spray chamber BAC design was selected for minimum air resistance
• High thermal performance cooling towers with low internal resistance
• Plant room adjacent to the BAC for minimum hydraulic and thermal losses
• High efficiency single-stage centrifugal refrigerant compressors (with efficient part load control)
• Process system design to maximise plant coefficient of performance (COP)

Andhee explains that considering the extreme temperatures in the plant room, which often reached 40-43°C, special measures were taken to maintain optimal working conditions. An air-conditioned booth was constructed to house the York Optiview Panel, which
controls the HVAC system. This step ensured that the delicate network cards and equipment inside the booth remained cool and prevented potential heat-related issues.

“Energy efficiency was a key consideration throughout the project. The team implemented various strategies to minimise energy consumption and associated costs. For example, the design included a horizontal spray chamber air cooler, which reduces air
resistance and eliminates the need for large fans. Additionally, a high thermal performance cooling tower with low internal resistance was selected to efficiently dissipate heat. By positioning the refrigeration machine adjacent to the bulk air cooler, the team minimised thermal losses and reduced energy wastage during longdistance cooling operations,” he adds.

BBE Projects is currently working on the Gara South Refrigeration plant that will be commissioned later this year, and the project is on track for completion in the third quarter of 2023. Once complete, the refrigeration system will enable mining in the largely untapped southern area of the orebody, extending the life of the mine significantly.