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Impala Platinum Mines Refinery Pond


Aurecon, SA's largest consulting engineering company were awarded the R1.5million contract for the review of the conceptual, detail design and site supervision of the new 20 000m Base Metal Refinery Pond complete with associated capital equipment for Impala Platinum Limited.

The total capital value of the project estimated at R15 million, forms part of Impala Platinum's plans to increase its production of nickel, platinum and other precious metals.

Impala Platinum Phase II

The BMR Phase II Expansion has necessitated the installation of a new double lined leak detection pond and effluent pump at its Base Refinery located in the Enstra industrial area of Springs in Gauteng.

Aurecon's project manager on this project, Viren Gajathar says, "The Base Metal Refinery has to install an additional pond to increase its water storage capacity, therefore ensuring that the plant will be able to collect the run-off generated from this new area. Further a new pond is required to allow the BMR to replace the single-liner in the existing BMR pond."

Impala Platinum Mines
Part of the Refinery Pond design [enlarge]

The conceptual design of the new pond was undertaken in a prior contract between Impala Platinum and Africon before their coming together with Connell Wagner and Ninham Shand to form Aurecon.

Aurecon provides engineering, management and specialist technical services for government and private sector clients in the following key sectors: Community Development & Infrastructure, Mining & Industrial, Transport, Property, Energy and Water.

Design & Geology

Viren, who has led the design on the Impala Platinum project, noted in his 2009 Design Report that the size of the proposed new dam would be limited due to the land available for development and the topography of the area.

The area selected for the new effluent pond site lies east of the existing one on a location previously used by Afrox. Following the demolition and removal of the old foundations, Aurecon carried out a geotechnical investigation, the results of which showed the presence of dolomites.

As part of further investigations, five exploratory percussion boreholes were drilled to determine the subsurface conditions. These were evenly distributed across the site and were deemed to be representative of the site conditions.

The boreholes/test pits excavated during the investigation indicated fairly good material to a depth of 5 metres below ground level. Although the council for Geoscience and Aurecon's Geotechnical team suggested that the excavations should be limited to about 3 metres, Viren says this was not an option because of the storage capacity requirements of the new pond.

"Two alternatives were considered. These were the dynamic compaction of the base of the dam material or over excavation. A decision was taken to over excavate the poor material between the 5m depth and solid bedrock and compact suitable material available between the ground level and 3 metres deep in 300mm layers to the final design level."

According to Viren, the soil mattress would prevent any settlement that may compromise the liner integrity and provide an impervious horizon that will prevent water ingress into the bedrock in the event of any spillage or leakage from the pond.
Impala Platinum Mines
Existing Inlet Canal to Base Metal Refinery Pond

Civil Engineering Software

Viren, who used the software package Civil Designer to calculate all the earthworks, is an avid user of Knowledge Base software. He says, "I started off with Stardust from the beginning and was really pleased when Knowledge Base took over the development of AllyCAD and improved it further."

Viren has used AllyCAD, the 2D/3D CAD package and Civil Designer on the design and feasibility studies for water and sewer pump stations, earth dam embankments and bulk water pipelines throughout his eleven-year engineering career.

Constraints & Calculations

Talking about the design for the Impala Platinum project, Viren says, "The pond was designed with 1:2 side slopes and 1:100 base slope."

Because the size of the pond would be limited to the old Afrox site boundaries, a trapezoidal shaped gravity fed earth embankment design was agreed upon.

"The base slope will slant from the base edge towards the middle of the pond and the flow will be forced to the pump sump of the pond. The lining components will follow the slope of the pond."

The lining, designed according to the minimum requirements for Waste Disposal by Landfill published by the Department of Water Affairs and Forestry, is composed of several layers with leak detection lining layers that all come together into a leak detection sump. This is then transferred to a leak detection manhole which will be regularly inspected for the presence of leaks.

Flow Control

The inlet canal to the new effluent pond will connect to the existing canal at the point where the existing canal bends at a 90 angle to enter the existing BMR pond. Furthermore a channel gate will be installed in the new canal and existing canal immediately downstream of the y-section where the new canal splits off.

"A flow control structure will be constructed along the east side of the existing canal, beginning approximately 10m downstream of the manhole."

According to Viren, the purpose of this structure is to bring the velocity of the flow between 0.4m/s to allow grit to settle out and oil and grease to rise to the surface so they can be removed from the flow control structure before it enters the ponds.

"The flow control structure will be an 8m x 8m x 3m concrete basin with a horizontal floor and a dividing wall to force the flow in a U-turn. Similar to the canal, this structure will be set in the ground and the top of the walls will be at ground level."

The depth of the excavation will therefore be 3m, which is within the limit advised by the dolomitic surface assessment.

Construction on the project is scheduled for February or March 2010 and is expected to be complete by September 2010.

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