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PROPERTIES OF TAILINGS DAMS

(last updated 30 Sep 2003)

Contents

> See also Tailings Dam Safety


INTRODUCTION


What are tailings?

Tailings are the residue of the milling process that is used to extract metals of interest from mined ores. During this process, ores are first milled and finely ground, and then treated in a hydrometallurgical plant. Since the extracted metal represents only a small percentage of the whole ore mass, the vast majority of the material mined ends up as a fine slurry.
The tailings contain all other constituents of the ore but the extracted metal, among them heavy metals and other toxic substances. Moreover, the tailings also contain the chemicals added during the milling process. In addition, as a result of the milling process, all these contaminants now are much better available for dispersion into the environment than in the original ore. And, the mechanical stability of the tailings mass is very poor, due to its small grain size and the usually high water contents.

Tailings impoundments

Most of the mill tailings mass produced worldwide is dumped in large surface impoundments ("tailings dams"). The embankments forming these impoundments are earthfill dams. Although water-retention type dams would be very suitable for tailings dams, they are not used for their high cost.

Water-retention type dam for tailings storage

Other than water-retention type dams, tailings dams usually are not constructed initially to completion, but raised sequentially as the impoundment fills.

Types of sequentially raised tailings dams


UPSTREAM TAILINGS DAMS


Upstream-type embankments are the most popular embankments for tailings dams; new parts of the embankment are built on top of the slurries impounded during the previous stage - the dam crest thus moving "upstream".

Upstream tailings dam (click image to view animation of sequential raising) new window

(Click image to view animation of sequential raising of upstream tailings dam new window)

For its low cost, the upstream embankment type is used with most tailings dams worldwide, but it bears the highest risk. Dam stability is of particular concern with this type of tailings embankments:

(see also Tailings Dam Safety)


COMPARISON OF TAILINGS DAM TYPES


Comparison of Surface Impoundment Embankment Types
Water RetentionUpstreamDownstreamCenterline
Mill Tailings RequirementsSuitable for any type of tailingsAt least 40-60% sand in whole tailings. Low pulp density desirable to promote grain-size segregationSuitable for any type of tailingsSands or low-plasticity slimes
Discharge RequirementsAny discharge procedure suitablePeripheral discharge and well-controlled beach necessaryVaries according to design detailsPeripheral discharge of at least nominal beach necessary
Water Storage SuitabilityGoodNot suitable for significant water storageGoodNot recommended for permanent storage. Temporary flood storage acceptable with proper design
Seismic ResistanceGoodPoor in high seismic areasGoodAcceptable
Raising Rate RestrictionsEntire embankment constructed initiallyLess than 4.5 - 9 m/yr most desirable. Greater than 15 m/yr can be hazardousNoneHeight restrictions for individual raises may apply
Embankment Fill RequirementsNatural soil borrowNatural soil, sand tailings, or mine wasteSand tailings or mine waste if production rates are sufficient, or natural soilSand tailings or mine waste if production rates are sufficient, or natural soil
Relative Embankment CostHighLowHighModerate
Source: [Vick1983]


BIBLIOGRAPHY


> see also bibliography on Decommissioning & Tailings Management !


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