The safe alternative in mining

1‌ Introduction

As a consequence of devastating dam collapses, tailings management in mining has come into increasing focus in recent years [1]. Worldwide, there are around 3500 dams for retention basins in the mining industry. Following the catastrophic dam bursts at Brumadinho (2019) and Samarco (2015) with almost 300 fatalities, the safety concepts of such structures were investigated in greater detail and scrutinized. Despite this, another eight incidents in China, India, Brazil, Mexico, Peru, and Myanmar have become known, with grave consequences in some cases. Accordingly, safety standards have had to be revised. In August 2020, a new industry standard reference on tailings management was published, unfortunately it contains no reference to alternative processes to retention dams [2].

Leading companies in the mining industry have upgraded tailings management to one of the most important strategic issues in the scope of their sustainability activities. These include Canadian Teck Resources. This mining company has to manage a total of 55 TSF (Tailings Storage Facilities), including 16 active and 39 closed facilities that are not supplied with any new tailings. The facilities also include the now closed largest sand retention dam (Fig. 1) in the Highland Valley Copper Mine, 50 km southwest of Kamloops in British Columbia/Canada. For such TSFs, every five years at the latest, costly dam safety inspections (DSI) have to be conducted to comply with relevant Canadian regulations. The last inspection took place in 2018, during which 29 suggestions were made to improve dam safety, of which two were on the second highest priority level [3].

2 Alternatives to tailings storage facilities

Wet, fine-grained tailings from mineral processing can be avoided completely if the entire ore beneficiation is effected in a dry process [4]. Whether dry processes can be employed often depends on the degree of ore purity. For iron ore, dry processing is possible for an iron content > 60 %. Vale, one of the largest iron ore producers worldwide, already produces 60 % of its iron ore from the Carajas, Serra Leste and S11D mines in a dry process. In five years, 70 % are planned. For this, investments totalling 2.5 bill. US$ have been estimated. Vale will introduce Fines Dry Magnetic Separation (Fig. 2) from the year 2022. A first plant in Minas Gerais is set to have a capacity of 1.5 mill. tonnes per year (MTA).

Parallel to this, Vale, which incurred great financial damage as a result of Brumadinho and Samarco and whose image suffered equally, is planning to invest in mechanical dewatering processes for tailings and dry stack tailings (DST). In the year 2024, of the remaining 30 % wet ore processing, 16 % is to be converted to filter technology or dry tailings. For this, US$ 1.8 bill. are being invested. The technology is to be introduced in the Varcem Grande, Pico, Caue, Conceicao and Brucutu mines. Tests conducted up to now with filter technology at the Brucutu mine have not yet brought the desired results as only 40 % of the capacity of the mine could be utilized. Possibly, not the right filter equipment was used or the preliminary dewatering in thickeners was insufficient.

Worldwide, in mining, around 7 – 9 billion TPA tailings are produced. According to a study by Metso Minerals, in 2018 only 5 % of tailings in the mine industry were dewatered to produce thickened sludge or a sludge paste and less than 1 % of tailings were dewatered in DST plants with filters. According to Metso, the applications with filters are, however, set to grow to 13 % by the year 2025, According to studies, for example, by Anglo American and consulting firms, the DST plants so far installed are almost exclusively designed for relatively low-to-medium throughput rates from 10 000 to 20 000 t/d. Such plant throughput rates are found, for instance, in gold ore processing. For the much larger plant throughput rates for copper or iron ore, no plants have been installed so far.

The few larger DST systems installed so far include the La Coipa gold-silver mine operated by Kinross Gold in the Atacama Desert region. In the mine, up to 2013, around 18 000 t/d tailings were dewatered on vacuum belt filters and the residual moisture reduced to < 20 % for dry stacking (Fig. 3). In the meantime, the mine has been mothballed. Kinross is now planning to resume operation in the year 2020 with deposit 7. Like at La Coipa, DST systems have so far almost only been introduced in arid areas in Chile, Peru, and other parts of South America, in Western Australia, in Southwestern USA and in several Arctic areas. Processes with thickeners that produce a sludge with up to 60 % dry substance are on the other hand installed much more frequently, especially when the sludge is used for backfilling underground mines.

3 Technological aspects and solutions

The particle sizes of the solid matter in the tailings differ widely depending on ore and upstream grinding. For metal ores, for example, particle sizes of a few micrometres to several millimetres are obtained. The dewatering options for the tailings therefore depend largely on the particle sizes, roughness, and other parameters. Fig. 4 shows how the cohesion forces of wet monoparticles increase in relation to gravitational forces. For fine particle fractions, these forces can increase further. It can be seen that the van der Waals forces dominate in the fines range in comparison to the capillary forces (liquid bridges). For the mechanical dewatering of fine-grained sludges, this is of crucial importance.

Fig. 5 shows the basic dewatering processes and the achievable residual moisture content (water content) or dry substance content. In the processing of metal ores (not including iron ore), the suspensions from mineral processing generally have 70 – 80 % water content. That is, for example, from 100 000 t/d tailings, around 75 000 t/d are water and 25 000 t/d solids. In thickeners, the sludges are reduced to residual moisture contents of 40 – 55 %. High-performance thickeners already achieve 25 – 35 %. For further dewatering, filtering equipment is used, which depending on the process and filtration times enables residual moistures of 10 – 20 % in the filter cake. Accordingly, the filter cake contains around 25 000 t/d solids and still up to 5000 t/d water for quasi “dry stacking”. There are also high-performance filters that enable lower residual moisture contents, these are, however, not used for dewatering tailings.

Fig. 6 shows the most commonly used dewatering equipment with their area of application. There are four pressure ranges (gravitation processes, vacuum processes, resp. low pressure as well as medium pressure and high pressure). In vacuum filters, negative pressures to 1 bar are generated, the medium pressure range goes to 25 bar positive pressure. Dewatering screens and spirals are used in the millimetre range, thickeners and sedimentation equipment is used over a wide range, but the finer the particles are and the lower the sedimentation velocity is, the larger and more efficient the thickeners have to be. For further mechanical dewatering with filters, different filter equipment is used. For the dewatering of relatively fine sludges, on account of increasing cohesion forces, higher filter pressures or accordingly longer filter times must be applied.

From a combination of different dewatering stages, a series of different solutions result in comparison with the conventional deposition of the tailings in retention basins (Fig. 7). The goal of all processes is to reduce the quantity of tailings or eliminate them completely. With high-performance thickeners (Fig. 8) or vacuum disc and belt filters, solid content of sludges of 68 – 70 % and higher can already be achieved. Such solid sludges can be stored above ground or, with the addition of binders like cement, used for backfilling underground mines. If, however, higher dry substance contents of sludges or filter cake are required, in the mining industry mainly filter presses are used. In numerous cases, instead of filter presses, decanter centrifuges are suitable [5].

Filter presses have undergone enormous development over the last 2 to 3 years. The conventional membrane filters (Fig. 9) used up to now do not achieve the required throughput rates and residual moistures for large quantities of tailings of  > 120 000 t/d, as required, for example, in the beneficiation of copper ore. For smaller quantities or the dewatering of coal tailings, these filters certainly present a viable option. To achieve larger throughputs for fine tailings, both FLSmidth and Metso Outotec have further developed filter press technology to meet the requirements of the mining industry. With its Colossal filter presses (Fig. 10) with single plates or chambers with dimensions of 2 x 4 m, FLSmidth enables filter areas > 2000 m², volume flows > 40 m³/cycle and filter pressures up to 15 bar. For FLSmidth, in the development of the new filter press generation, the focus was on increasing the specific filter capacities and consequently on lowering specific filtration costs.

Metso Outotec (MO), which only introduced its VPX filter presses (Fig. 11) on the market in June 2019, previously concentrated on membrane presses, too. In its development, MO focussed on a compact filter, fast cycle times and high capacity data. The filter press has a filter volume of 1391 m² at 36 m³/cycle volume flow and filter pressures of maximum 25 bar. The filter cycle times can be adapted relatively easily to the applications for different ores and finenesses of the sludges. This is particularly important as over the course of operation, process parameters can change, and accordingly the filter press can be simply adapted to future requirements. As for the FLS Colossal Filter, easy repair and maintenance facilities have been designed.

To completely utilize the advantages of DST systems, there are plans to enable stack heights up to 100 m, depending on the condition of the tailings. This would provide for large space savings compared to conventional tailing systems. This requires, however, special material handling equipment and stackers. Corresponding technologies (Fig. 12) are available in the mining industry. The special challenge is that the stackers with crawler chassis need to be driven on the tailings, and over the operating time the conveying height must be increased step by step. In comparison to such special requirements, conventional and less complex belt conveyors and stackers will be used. In the simplest cases, the dry tailings can be spread by means of mine trucks.

4 Cost analyses, key suppliers and reference projects

So far, in the mining industry, the broadly held view is that DST systems are much more expensive than conventional tailings retention basins. Especially with regard to operating costs, it is assumed that DST systems incur four- or five- or even ten-fold higher running costs.

In view of the quantum leaps in the development of new filter presses, FLSmidth and Metso, however, see the cost level being almost on a par in the future (Fig. 13), especially when the potential for water saving is taken into account. When this is factored in, the operative costs of DST systems average only insignificantly above conventional tailings management systems with retention basins, although here the cost for security surveillance is not yet taken into account. Moreover, higher OPEX is compensated by low CAPEX. It is therefore worth comparing the options.

At present, the most important suppliers of integrated DST systems are FLSmidth, Metso Outotec and TAKRAF Tenova. In addition, there are numerous suppliers that do not yet offer the complete technology from thickener, through filter presses to belt conveyors and stackers. These include, for example, McLanahan, thyssenkrupp Industrial Solutions and WesTech. Going by the number of recently received orders in the sector, FLSmidth and Metso Outotec are ahead of the field here. Metso Outotec evolved from the merger of Metso Minerals and Outotec and has been listed on the Nasdaq in Helsinki since 5 August 2020. The company employs 15 000 people and generated a turnover of around € 4.2 billion in 2019. With these figures, the company ranks as the largest OEM for the mining industry worldwide.

In recent years, Metso Outotec has received several orders for the backfilling of sludge pastes. One of these orders was placed by OZ Minerals for its Prominent Hill gold-copper mine in South Australia (Fig. 14). Supplied was a CHF plant (CHF = Cemented Hydraulic Backfill). For the CHF, a maximum of 2 % free water was allowed to be contained in the sludge paste. The latest order was placed by Salares Norte in Chile for a new greenfield mine. As part of the order, amongst other things, three VPX filter presses were supplied for a DST system. The mine is located at an altitude of 4500 m in an Andes region where water is scarce, making every precious drop count. The commissioning of the plant is scheduled for 2023, and will certainly be a benchmark for the entire industry.

FLSmidth has also been very successful in recent years with regard to projects for DST systems or mechanical dewatering of tailing sludges. Most plants were supplied for capacities of < 20 000 t/d, for example the systems for sludge pastes at Hindustan Zinc and at Lundin Gold. For the Goldcorp EcoTails project in the Atacama region in Chile, a first demonstration plant for a DST system with Colossal filter (Fig. 15) and a capacity of 10 000 t/d has been built and successfully commissioned. Another project with the customer is planned in Penasquito/Mexico. FLSmidth landed the biggest order with the Karara iron ore project in West Australia. There the world’s largest DST system is being installed, as an ecological and economic alternative to a tailings landscape with an area of 8 km² for a mine lifetime of 30 years.

TAKRAF Tenova also supplies complete DST systems, the thickeners and filters being sourced from Tenova Delkor. For a gold mine operator, a plant was supplied to Uzbekistan for a throughput rate of 120 t/h tailings. For the Société Nationale Industrielle et Minière de Mauritanie (SNIM), an integrated system (Fig. 16) was supplied for an iron ore project. The throughput rate there reached 880 t/h or around 490 m³/h. The maximum height for the bridge stacker is 7 m.

5 Outlook

In the new processing lines, according to information from leading suppliers, DST systems are now optionally requested and offered. On account of the currently relatively low number of new projects, the greatest demand is for upgrades of existing tailing systems. This has to do with several reasons: On the one hand, many mines have reached their end with regard to the expansion of the tailings areas and capacities, on the other hand, in many mines there is a mix of different retention basins of which some no longer meet the safety requirements and are set to be replaced, for example, with DST systems. In the course of recent months, therefore confidence is growing among the mining companies that conventional retention basin systems can also be complemented with safe DST systems. The increasing shortage of water is making the option of DST systems additionally attractive.