12.8 Tailings impoundments

There are two basic types of structures used to retain tailings in impoundments, the raised embankment and the retention dam. The four main types of impoundments include the Ring-Dike, In-Pit, Specially Dug Pit, and variations of the Valley design. The design choice is primarily dependent upon natural topography, site conditions, and economic factors. Most tailings dams in operation today are a form of the Valley design. Because cost is oftenly directly related to the amount of fill material used in the dam or embankment (i.e., its size), major savings can be realized by minimizing the size of the dam and by maximizing the use of local materials, particularly the tailings themselves. Retention dams are constructed at full height at the beginning of the disposal whereas raised embankments are constructed in phases as the need for additional disposal capacity arises. Raised embankments begin with a starter dike with more height added to the embankment as the volume of tailings increases in the impoundment.

 

Tailings dams are built to allow the remains from processing ores to settle out from the water that is used in the processing. The construction of tailings dams can vary significantly and is mainly dependent on the type of tailings being stored and the topography of the storage facility.  There are four main types of tailings storage: water retention Dam, up stream dam, down stream dam and centre line dam. Table show the comparison between different types of water retention impoundments.

 

Comparison of Surface impoundment Embankment Types

 

Water Retention

Upstream

Downstream

Centerline

Mill Tailings Requirements

Suitable for any type of tailings

At least 40-60% sand in whole tailings. Low pulp density desirable to promote grain-size segregation

Suitable for any type of tailings

Sands or low-plasticity slimes

Discharge Requirements

Any discharge procedure suitable

Peripheral discharge and well-controlled beach necessary

Varies according to design details

Peripheral discharge of at least nominal beach necessary

Water Storage Suitability

Good

Not suitable for significant water storage

Good

Not recommended for permanent storage. Temporary flood storage acceptable with proper design

Seismic Resistance

Good

Poor in high seismic areas

Good

Acceptable

Raising Rate Restrictions

Entire embankment constructed initially

Less than 4.5-9 m/yr most desirable. Greater than 15 m/yr can be hazardous

None

Height restrictions for individual raises many apply

Embankment Fill Requirements

Natural soil borrow

Natural soil, sand tailings, or mine waste

Sand tailings or mine waste if production rates are sufficient, or natural soil

Sand tailings or mine waste if production rates are sufficient, or natural soil

Relative Embankment Cost

High

Low

High

Moderate