Managing Pogo’s tailings: the 1% solution

Nestled in the delicate Alaska wilderness, the Pogo gold mine project posed major challenges in terms of designing a tailings system that would have a small footprint.

Management of any potential impact to water bodies was of extreme importance; Pogo is located next to a key salmon run, and the area has low annual precipitation. The terrain is steep, subject to permafrost, and rattled by frequent earthquakes. Given these considerations, a conventional tailings pond was out.

Enter AMEC, an international project management and engineering company, which proposed a two-fold solution.

Tailings are separated into two streams. Those that could have any potential residual cyanide, about 10% of the total, are thickened into a paste and mixed with cement. The resulting mixture is placed underground with other tailings (to a total of about half of the total tailings) in mined-out areas as low-permeability cemented backfill. This process has the advantage of storing cyanide-bearing tailings in a very secure fashion. The cemented backfill also provides structural support to the mine workings. This technique can be used on the surface, but there are no viable above-ground sites at Pogo.

The remaining tailings (containing no cyanide) are placed on the surface. They are filtered to an unsaturated state, using large-capacity, pressure filter technology. The water removed from the tailings can be recycled, a big advantage in a region of little precipitation and extreme cold. In this dewatered state, tailings can be placed and compacted like other earthfill materials and are seismically stable. They are virtually impermeable due to their fine-grained nature, degree of compactness, and unsaturated makeup.

Sites with terrain and environmental setting challenges like Pogo offer an assured stable tailings mass that can conform to geometries not possible with traditional tailings ponds. This technique, dry stacking, has so far been used in less than 1% of the world’s mines.

Dry stacking is no panacea for tailings management. The process is expensive, possibly costing more than conventional treatments — at the front end and during operations. But a growing number of mines are turning to dry stack and this may lead to lower capital and operating costs as the technology matures. The high up-front costs begin to appear far less onerous at the end of a project life cycle through the realization of lower closure costs and reduced liabilities. Dry stacks can be readily covered with a layer of soil at the end of a project and even progressively reclaimed as the operational constraints allow.

In addition, the process offers other attractive features, including:

* minimizing water demand through recycling of process water;

* conformity to stricter environmental and regulatory requirements; and

* no risk of catastrophic failure, such as slides.

But tailings filtration and dry stack may pose some risks and these must be carefully evaluated before implementation. The risks include:

* potential buildup of deleterious chemicals and dissolved solids in the recycled process water that can affect the efficacy of the process; and

* thorough definition of the tailings filtration characteristics and the variability of filtration rate can be difficult and costly, but also essential, as the process throughput is limited by the filtration rate.

Dry-stack tailings disposal is not for every project, but when conventional tailings management is not possible, and a “regulator-friendly” process is called for, it can be an alternative worth considering.

— The author is a principal geotechnical engineer with AMEC.