The completion of the Nkomati valley TSF is not only the largest nickel TSF in South Africa but  also one of the largest valley-fill TSFs in Africa.

Nkomati Mine is situated in Mpumalanga between the towns of Machadodorp, Waterval Boven and Badplaas, and the miner’s rights stretch over the Slaaihoek and Uitkomst farms. The mine is a joint venture between African Rainbow Mineral and Norilsk Nickel, and produces mainly nickel.

During the early 1990s, African Rainbow Minerals – previously Anglovaal Mining – explored Slaaihoek and in 1993 undertook a major surface-infill and delineation-drilling programme.

From the drilling, it was established that the Uitkomst Complex comprises three disseminated, sulphide-mineralised zones, namely the basal mineralised zone (BMZ), main mineralised zone (MMZ) and chromititic peridotite mineralised zone (PCMZ). At the base of the Uitkomst Complex, a fourth sulphide orebody, the massive sulphide body (MSB), was discovered.

During 1993 and 1994, as part of another feasibility study, a vertical shaft was sunk to a depth of 450 m to obtain a bulk sample for metallurgical test work. A pilot plant was constructed on site and commissioned towards the end of 1995. Results of the study showed that it would be viable to mine the MSB orebody.

Nkomati currently produces approximately 4 500 to 5 000 t of nickel-in-concentrate per annum and when fully expanded, the output will increase fourfold to 20 000 t/a.

Nickel was the hardest hit of all the base metals during the recent economic downturn and during the last quarter of 2008, the chrome market ground to a standstill. This has had a significant effect on the profitability of the operation and its ability to fund expansion. The shareholders, however, remain committed to the completion of the project in anticipation of the next global economic cycle.

The labour-intensive nature of the expansion phase does offer extensive job opportunities and, at the peak, some 1 500 contractor employees were involved in the construction. Unfortunately, these jobs are not sustainable and job losses, on completion of the project, are unavoidable.

Construction

Golder Associates Africa, a ground-engineering and environmental services company, played a significant role in the design and construction of a valley-fill tailings storage facility (TSF) at African Rainbow Minerals (ARM) and Norilsk Nickel’s jointly owned Nkomati Mine. The company’s participation in this project has resulted in not only the largest nickel TSF in South Africa but one of the largest valley-fill TSFs in Africa. 

As part of Nkomati Mine’s phase 2, large-scale mining expansion plan to increase production and extend the life of the mine by 18 years, Golder was appointed to undertake the planning, design and construction supervision of a new valley-fill TSF. Golder was also responsible for the feasibility study, value engineering, detailed design and quality assurance for the new return-water dam (RWD) and related infrastructure.

Golder’s tailings division head, Eddie Cunningham, says that the expansion required that the TSF be able to store approximately 402 000 t per month, which resulted in a site footprint measuring 180 hectares with a perimeter of 5 500 m and a finished wall height of 85 m.

The TSF dimensions brought with it the complexities of the site’s diverse geology and topography, which cover the Highveld, Lowveld and escarpment areas. Cunningham explains that to address these challenges, the TSF had to be located 14 km from the mine with an elevation of 300 m below the processing plant ground level.

In addition, innovative engineering solutions were required to accommodate the changing dynamics of the TSF as it is filled. Cunningham explains that a variety of deposition methods will be employed over its lifetime, including making use of an open-ended dropper pipe, upstream cycloning and spigotted deposition.

Stable TSF

The tailings are delivered to the TSF from the processing plant via a 14 km pipeline. After passing through a choke station to reduce velocity, the tailings are piped to the starter wall of the TSF. (The TSF was constructed so that following deposition, the supernatant water flows to the pool at the head of the valley. This water is then pumped back via the 14 km pipeline to the plant for reuse.)

Cunningham adds that the design and construction of a well-founded and stable TSF wall is integral in the design of the TSF. At 85 m high, the starter wall required extensive earthworks as well as the construction of drainage systems to catch seepage from the TSF.

State-of-the-art pollution control

Cunningham says that state-of-the-art pollution control was used in the planning, design and construction of the TSF to minimise environmental impacts. Extraordinary measures that were taken included:

• a significant design modification to reduce the  impact on the underlying groundwater system greatly
• the addition of emergency outlet pipes in the RWD embankment to enable draining of the dam in 21 days
• using a floating barge and pump system to deliver supernatant water to the RWD
• minimising seepage into the groundwater system by lining the RWD with a geomembrane
• filling and inspecting the RWD for leakages prior to  commissioning
• using a comprehensive underdrainage system to minimise the risks of infiltration into groundwater.

“Collectively, all the components of the design met the needs of the environmental constraints and the challenging size of the required TSF,” says Cunningham.

Moreover, management of the water resources – including both the surface water and groundwater – was the main focus of this project in partnership with the Nkomati Mine. Optimised surface water management was relatively simple using storm water drainage, dewatered tailings disposal, under-drain systems and various diversions for the natural waterways around the site.

Nkomati Mine requested that Golder conduct a phase 1 technical risk assessment of downstream water quality impacts at the Onverwacht site to quantify the risk and provide decision support for mitigating this impact.

Groundwater pumping below the TSF, using three scavenging boreholes, was demonstrated as the optimal and most cost-effective measure to prevent the downstream contamination of water resources during the operational phase.

Cunningham notes that it was recommended that this water abstraction continue for 10 years after decommissioning to intercept the residual leaching from the TSF as it dries out. An engineered cap will then be implemented to optimise the required infiltration and oxygen ingress performance criteria.

Meanwhile, the RWD is located within a main tributary on the higher ground of the TSF’s northern side. It has a capacity of 250 000 m3 and a maximum vertical height of 23 m. Unusually, the RWD is located above the TSF, which required additional engineering solutions to overcome this rare challenge. Acting as a surge dam between the TSF and the plant water supply, the RWD ensured that the water stored on the TSF does not exceed 50 000 m3.

Extensive measures

Although extraordinary measures were undertaken to minimise environmental impacts from the TSF, the project came in on time and well within the budget. “The complex constraints of this project – combined with a capacity of 90 Mt, storing 402 000 tpm for 18 years – makes it a megastructure,” concludes Cunningham.  

Golder has entered the project into the upcoming CESA Awards, saying that the Onverwacht nickel TSF is an extraordinary feat of engineering, overcoming significant environmental sensitivity as well as geological and topographical challenges. Golder successfully saw this project through from start to completion, including the planning, design and construction.

To view additional images related to this article, please go to:

Picture 01 (pump station)

Picture 02 (view of project)

This article was originally published in the Inside Mining magazine.