Mine tailings and their removal

While the metallurgist has accomplished technical marvels — recovering upward of 85-90% of the valuable minerals from low-grade ores — he has also created a mountain of a problem; namely, millions of tons of tailings for which there is no commercial use.

Even under the most favorable circumstances, little more than 50% of a mill’s tailings can be sent back underground. Crushed rock occupies more space than the rock from which it originally came. There are myriad open spaces between the particles of tailings and it is not possible to fill them. Only by melting the tailings can the open spaces be eliminated and the rock returned to its original volume.

The majority of Canadian mines return only 20-35% of their tailings underground. The extremely fine sands are first removed. If they are not, the fill will not drain properly after it has been placed underground and quicksand will result. Cement and water are added to the deslimed tailings and the slurry piped to wherever it is needed. Water content is 30-40%. (This is conventional hydraulic fill.)

The slurry is fluid and is transported long distances without difficulty. On the other hand, dilution permits a degree of separation of the cement and of the sands into their various sizes, and this results in a structurally imperfect, though adequate, support medium.

The problems of not being able to use extremely fine tailings and the low strength of the resulting fill have been resolved to a large extent by “paste fill.”

Much of the technology and skill required for the underground application of paste fill has been developed by Inco (TSE) during the past 12 years. Major tonnages have been placed underground on an experimental basis in Sudbury, Ont. Inco expects 20-30% of its total fill requirements to be routinely met with paste in the near future.

This particular type of fill is composed of unsized mill tailings, cement and only sufficient water (20-22%) to keep the mixture lubricated. It has the consistency of toothpaste and, at Sudbury, it is delivered in that form by the same pipe system used to handle hydraulic fill.

Because so little water is present, the extremely fine sand is not flushed away nor are the extremely finely divided cement particles. In addition, the less the amount of water in a concrete mix of any kind, the stronger it is. Consequently, paste fill makes a more effective support than conventional hydraulic fill. It is billed as the fill of the future.

The phenomenon of extremely fine sands holding on to their moisture (as in “quicksands” noted earlier) can have serious consequences where tailings dams are in earthquake country. A tailings dam may long since be filled, the lagoon atop evaporated and the tailings mass apparently dry. The dryness is an illusion. Repeated seismic vibrations will cause the water surrounding each particle of the finest tailings to coalesce with that of its neighbor. There is a chain reaction. The mass liquefies. If the earthquake has weakened the retaining wall as well, instead of dry sand spilling through the rupture, there is a mud rush.

The La Coipa mine, owned 50% by TVX Gold (TSE) and 50% by Placer Dome (TSE), is in such a seismically active area of northern Chile. There, one of the world’s largest filtration plants is capable of handling up to 20,000 tonnes of tailings daily.

A considerable proportion of the mill’s water is recovered. And most importantly, the tailing is sufficiently dry (15% moisture) to ensure a stable product, safe for impoundment.