Notice: This page was translated automatically. If anything seems unclear, please refer to the German version.

Tailings dams: Sealing systems through the ages

Introduction

Tailings dams are mining waste dumps where the products of mineral extraction are deposited. These residues consist of fine-grained material and are usually deposited hydraulically. In principle, these processing residues can be generated in the energy sector (e.g. ash) or in the mineral extraction industry (gold).

Neglected for decades, the industry's attitude towards tailings dams has now changed, away from viewing them as "rubbish dumps" at the end of the value chain that only incur costs, towards recognising that tailings dams are an essential and fundamental part of mining. Without the possibility of safe storage, no production can take place. At the same time, processes have improved in areas such as gold mining to such an extent that the dams are often reworked to extract minerals that were not leached out in earlier processes (remining). Recent catastrophic incidents, such as those in Brazil in 2015 and 2019, have also contributed to these structures receiving more attention.

The size of the dams is often unfamiliar, which is why some key figures – typical for gold mining in South Africa – are listed below:

• Production: 100,000 tpm – 500,000 tpm (tonnes per month)
• Rate of rise (RoR): 1-4 m/year
• Surface area: 50-250 ha
• Height: 50-150 m
• Operating life: decades
• 200 active dams

Using South Africa as an example, the following article explains how changes in environmental protection guidelines are affecting the planning and construction of dams (base sealing), as well as the entire industry. Gold mining is used as the primary example.

Sealing systems through the ages

Gold mining in South Africa has a tradition dating back over 130 years, especially in the Johannesburg area. However, production volumes have declined in recent decades for various reasons. At the same time, the average gold content has also decreased. This trend is illustrated in Fig. 2.1. It is obvious that the combination shown leads to economic challenges for mine operators.

Fig. 2.1: Development of gold production and average gold content between 1880 and 2016

The situation described above is exacerbated by the regulations introduced in 2010 regarding the base sealing of dams (National Environment Management: Waste Act, Notice R 636). Whereas the legal requirements up to that point could be described as rather lax, the latest regulations are groundbreaking. Until 2010, there were basically no binding regulations regarding base sealing. Base sealing was generally only provided if it was advantageous from a production point of view, for example to save water in arid areas. Fig. 2.2 shows and explains schematically the three key development phases that have been observed in gold mining since around 1880.

Fig. 2.2: Key development phases of base sealing between 1880 and 2016

Basically, base sealing must now be provided for in the new planning of dams. The prescribed requirement for gold tailings dams is shown schematically and explained in Fig. 2.3.

Fig. 2.3: Key sealing combination for the new planning of gold tailings dams

Challenges

Overview

The new legal regulations currently pose several challenges in South Africa. In general, these can be divided into the following main groups:

• Drainage
• Expertise
• Consideration of operational processes
• Theft and damage

The above challenges are explained in more detail below.

Drainage

Due to hydraulic deposition processes, tailings material is generally highly anisotropic, see Fig. 3.1. This leads to difficulties, particularly when designing drainage systems and, in the worst case, to stability problems if the design is inadequate. This situation is exacerbated by the fact that designs are often based on gut feeling. There is clearly a need for research in this area. The same applies to the long-term stability and functionality of geotextiles.

Fig. 3.1: Deposition-related anisotropy

Expertise

The difficulties in dimensioning drainage systems already described in Section 3.2 are often exacerbated by the fact that dimensioning is carried out based on gut feeling. The compatibility of different liners, friction angles between the individual components, etc. are often not determined in the laboratory, but rather estimated, if they are taken into account at all.

Consideration of operational processes

Due to the nature of dams, which are essentially under constant construction, base waterproofing must either be completed a priori or permanently supplemented. Both approaches have an impact on the integrity of the waterproofing. If waterproofing is supplemented during ongoing operations, this can take years. This requires a continuous supply of materials of consistent quality. If a complete base waterproofing is produced a priori, the liner is often exposed to the weather conditions for years without protection. Erosion of the filter materials/protective layer in particular can be observed here.

Theft and damage

In practice, it can be observed time and again that geosynthetics are generally stolen in whole or in part (misuse as roofing). At the same time, damage to the liner is often not reported by the dam operator, but rather concealed. Both of these factors naturally increase leakage rates and are certainly not in line with legislation.

Summary

The legal requirements to only construct tailings dams with base sealing are certainly a step in the right direction, especially when considering issues such as acid mine drainage in gold mining. In the short term, however, this development may lead to capacity bottlenecks, as the construction of new dams is often postponed for cost reasons and the service life of older dams that are protected by grandfathering is extended. This approach is therefore not in line with the objectives of the current legislation.

It should also be noted that there is still a considerable need for research into the base sealing of tailings dams. Unlike conventional landfills, tailings dams are considerably larger and higher structures. This raises questions that can currently only be answered to a limited extent (clogging of filter materials caused by chemical process residues, effects of high overburden stresses, etc.).

It is to be expected that the legislation introduced by South Africa will soon be adopted in a similar form in other African countries. Despite all the challenges, these developments can therefore be seen as an opportunity to increase the level of knowledge regarding the application limits of geosynthetics and thus the scope of application itself.