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What Jagersfontein diamond mine staff should have done to avoid the dam wall collapse

03 october 2022
andrew_vietti_xx.pngA mine dam wall at South Africa’s Jagersfontein diamond mine in Free State province recently collapsed twice within two weeks, killing one person and damaging properties.

The dam at the disused mine held liquid waste from a tailings reprocessing operation.

De Beers sold the mine and tailings in 2010 to Superkolong Consortium, which comprised black investors.

However, Stargems bought the mine and the surrounding dry tailings dumps last April and remain the owner and operator through its South African subsidiary Jagersfontein Developments.

Vietti Slurrytec director Andrew Vietti told Rough&Polished’s Mathew Nyaungwa in an exclusive interview that the tailings behaviour can be modified for the clays to settle and the thickener to operate properly.

He said it was unfortunate that the mine staff were unaware that modifying the process water chemistry could have solved the problem.

Below are excerpts from the interview.

In your view, what went wrong at the mothballed Jagersfontein diamond mine?

In my opinion, it was a combination of factors which build up to the failure.

Firstly, the clays in the Jagersfontein fine tailings are colloidally stable which means that they will not settle naturally in a thickener. This property means that any thickener is incapable of thickening the tailings and therefore very watery tailings were pumped out to the tailings facility. It is unfortunate since (i) the thickener at the Jagersfontein processing plant was designed to dewater the tailings to a toothpaste and (ii) the tailings behaviour can be modified so that the clays do settle and so that the thickener can operate properly. Unfortunately, the mine staff were unaware that modifying the process water chemistry would have solved this problem.

Secondly, the Jagersfontein ring dyke impoundment tailings storage facility (TSF) is not suitable for storing watery tailings as the basin of the facility is elevated above the surrounding land and can easily become unstable when filled with large volumes of water.

In my opinion, management/operators at Jagersfontein may be excused for not understanding the colloidal properties of their tailings material, however, they should be held to account for allowing the operation to continue when it was obvious that the dam was overfilled with water.

A second breach of the mine dam wall was recorded at the Jagersfontein diamond mine. What needs to be done to avoid further collapse?

The second breach appears to not be related to the TSF from the latest information.

What is your opinion on the South African Heritage Resources Agency’s recent decision to allow the backfilling of the Jagersfontein diamond mining pit with waste from a second compartment of the tailings dam?

I don’t think this is a problem at all. One has a working example already of where fine kimberlitic tailings were backfilled into an open mine pit. In Kimberley, the so-called De Beers pit was backfilled in 2004 with fine tailings which had been generated by re-mining of the old dumps (the same as what was going on at Jagersfontein). In this case, the clays were naturally settling and could be thickened to a high-density paste before they were pumped into the pit. This was done to stabilise and prevent the national railway line from falling into the pit. It has been a resounding success. (see attached presentation).

Critics have opposed the backfilling arguing that it will destroy the heritage site. Are you persuaded by that argument given the dangers posed to the environment?

No, not at all considering the above example. I would be opposed to filling the “Big Hole” which is a historical and tourist attraction, but I don’t think that the Jagersfontain pit is in the same league.

Which measures do you think need to be enforced by Pretoria to avoid future mine accidents of this nature?

The authorities need to be brought up to speed with the improvements in tailings dewatering technology; these technologies should be encouraged for all new projects; they should strictly enforce the closure of unsafe operations when informed by external consultant reporting.

What is the best way to contain tailings?

For context, most tailings can be classified into either coarse solids or fine solids. The coarse solids are generally stored in free-standing dumps which are dry and self-supporting. The fine solids are generated as a liquid slurry which needs to be dewatered by a thickener before disposal to a TSF.

The best way to store the fine solids is to do so with the least amount of water possible. These days special thickeners can reduce the amount of water so that the tailings look like toothpaste. Even higher amounts of water can be removed if the toothpaste is filtered to a dry cake. However, it is not always easy to get a dry filter cake from all tailings because of the clay content in the tailings.

What is the ideal way of constructing a tailings facility?

I don't think that there is one ideal way to construct a tailings dam as it depends on many factors such as the properties of the tailings, the topography of the land etc. Even if the tailings contain a lot of water and are stored correctly, they could be perfectly safe (look at a regular water dam for example). However, it is clear that the vast majority of tailings need to be dewatered as much as possible before being stored – this means that accepted methods of construction using very liquid tailings (upstream; downstream methods) should be phased out in favour of methods which utilise higher density slurries.

The design philosophy of these facilities relies not only on the geotechnical input but also on an understanding of the clays within the tailings. This aspect has been largely ignored because, in liquid tailings, the clays are too dilute to influence the behaviour of the tailings, however, as the tailings are dewatered further (to reduce the risk of dam failure), the influence of the clays becomes more prominent.

The ideal way to construct a modern tailings facility is to firstly understand the types and properties of the clays within the tailings; then make sure they settle and form a mud bed when thickened; then select the thickener to give the best dewatering to provide the geotechnical engineers with the densest tailings product so that a suitable impoundment can be designed and managed.

Mathew Nyaungwa, Editor in Chief of the African Bureau, in Zimbabwe, Rough&Polished