Introduction
The mining of aggregates or mineral ores and subsequent processing of these materials to extract desirable products typically generates a non-valuable fraction called tailings.
When extracting rich veins of highly concentrated materials, the tailings fraction can be very low, but the long history of humans' mining activities has depleted many of the highly concentrated aggregate and ore deposits.
Today, with high volume mining/processing operations, especially when mining precious and trace minerals, the amount of tailings generated can be greater than 90% of the product being mined.
Old methods for handling tailings
The aggregate and mineral industries have historically used the simplest and cheapest methods to manage waste slurry tailings. The oldest methods include dumping directly into streams, rivers and lakes or using mining surfaces or underground quarries/pits for disposal.
Since the middle of the 20th century, these industries have been pressured to eliminate older ways of waste dumping and look for safer, environmentally friendly and efficient technologies. The most common method of managing tailings today is stockpiling diluted or concentrated tailings slurries in ponds or impoundment dams.
Unlike large hydroelectric water dams, tailings ponds are constructed to permanently store the mining solid waste tailings forever. At the beginning of the 21st century, there were an estimated 3,500 tailings containment ponds globally. Some of the largest containment ponds constructed are over several miles in diameter and hundreds of feet deep.
New technology for managing tailings
Until the past 10 years, little history has been documented on the success and failure rate of these tailings dams. Due to several catastrophic large tailings pond failures that have occurred this century, more attention is being given to improving tailings management practices further.
In addition to the loss of up to several hundred human lives, these major containment failures have destroyed large areas of land and caused major ground and waterway pollution. Furthermore, pressures to improve tailings management are being forced by tighter environmental regulations, better understanding of the high operating cost, the need to reduce waste handling costs and the increasing need to improve water conservation.
These pressures are forcing industry leaders to evaluate and better understand their total life cycle cost of tailings management. Based on this knowledge, pro-active companies are now justifying the upfront investment in improved tailing management systems. Some of the more common improved tailing management systems include:
Underground tailings backfill with:
- Paste thickened tailings pumped underground.
- Thickened tailings mixed with cement placed underground.
Aboveground tailings placement with:
- Paste thickened tailings pumped into a pond or impoundment.
- Thickened tailings mixed with cement and dry stacked.
- Thickened and dewatered tailings dry stacked.
Underflows from a Paste Thickener.
Placing paste thickened tailings in large tailings containment ponds has been the most common practice of improving tailings management over the past 20-plus years. Paste thickening is similar to other gravity settling thickening technologies but can generate higher tailings concentrations, eliminating up to 70-80% of the volume of material being stored in the tailings pond compared to old non-thickening storage methods.
To reduce the tailings volume further, dewatering technologies have to be employed to remove additional water from thickened tailings slurries before placement in the containment pond.
Dewatering technologies can reduce tailings volume an additional 5-10%, but more importantly, sufficient water is removed creating a dry, stackable material. Dewatered tailings can still contain 10- 20% moisture, but the material behaves more like a solid, making it possible to dry stack the tailings in containment areas. After being placed in the containment areas, the low-moisture tailings are very stable, significantly reducing safety and environmental risks.
Common dewatering technologies that have been in existence for decades or even centuries are being evaluated and used for tailings dewatering. Some of these technologies include:
- Plate filter presses
- Vacuum belt filters
- Belt filter presses
- Centrifuges
Recessed/Membrane Plate Filter Press technology has been found to be one of the most efficient methods of dewatering slurry tailings, recovering the maximum amount of water and generating the driest cakes. Although the capital cost of plate Filter Presses is higher compared with the other technologies, the operating costs are much less.
The relative upfront investment and ongoing operating cost of the common dewatering technologies are shown below.
Advantages of dry stack tailings
Industry leaders are finding dry stack tailings can offer many advantages over older, less desirable methods despite the potential for increased upfront and ongoing cost.
Improve structural stability of material storage
Dry stack tailings storage has been found to greatly improve the structural stability of the material storage, especially in high seismic rated zones, near high population centers or near highly sensitive environmental areas.
Maximum recovery of recycled water
In the most arid regions of the world, dry stack tailings permit the recovery of the maximum amount of recycled water. The increased recycled water use can help significantly lower highly regulated and scarce water sourcing costs.
Clear water recovered from a Filter Press.
Recovery of residual process chemicals
Dry stack tailings also promote the recovery of residual process chemicals that can be recovered from the recycled water.
Most effective physical storage
Especially in locations with limited available space, dry stack tailings offer the most effective physical storage. The dry stack tailings minimize the complexity of permitting and minimize the potential of public concerns from nearby communities.
Drip-free, dewatered cakes discharge from a Filter Press into a pile.
Dry stack tailings management is becoming a more accepted practice due to the recurrent disasters seen with older, non-thickened and even more conventional thickened tailings storage facilities. The tragic loss of human life and significant environmental and physical damage to large tracks of land and communities are accelerating the development of more stable dry stack tailings permitting and practices.
Putting economic value to all the potential advantages of dry stack tailings are beginning to justify the higher upfront capital investment cost for efficient dewatering equipment. Industry leaders are beginning to recognize the higher upfront investment and operating costs are offset when considering the full life cycle of the mine, including the ongoing tailings management cost after the processing facilities are closed.
