In a previous article, Determining the Rheological Properties of Mine Tailings,we discussed the fact that the composition of tailings depends on the composition of the ore and the extraction process used to obtain the ore. Tailings are the materials left over after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are distinct from overburden, which is the waste rock or materials overlying an ore or mineral body that are displaced during mining without being processed.
The extraction of minerals from ore can be done two ways: placer mining, which uses water and gravity to extract the valuable minerals, or hard rock mining, which uses pulverization of rock, then chemicals. In the latter, the extraction of minerals from ore requires that the ore be ground into fine particles, so tailings are typically small and range from the size of a grain of sand to a few micrometres. Mine tailings are usually produced from the mill in slurry form (a mixture of fine mineral particles and water). The composition of tailings is directly dependent on the composition of the ore and the process of mineral extraction used on the ore. Usually, the major quantity of a tailings product consists of rock, mainly ground to a fine size ranging from coarse sands down to powder consistency. Many tailings may also contain small quantities of different metals that are found in the host ore as well as added components used in the extraction process.*
Usually tailings are stored in tailing ponds. Tailing ponds are areas where the waterborne refuse material is pumped into a pond to allow the sedimentation of solid particles from the water. Tailings ponds are instrumental in storing the waste coming from separating minerals from rocks, or the slurry produced from tar sands mining. Often, tailings are mixed with other materials like bentonite to slow down the release of impacted water to the environment. However, no matter which composition the tailing has, it needs to be pumped from the mine to the pond. In order to understand if a (modified) tailing can be pumped with the existing pumping equipment, not just the viscosity needs to be determined but also the yield stress. The yield stress describes the amount of energy needed to overcome elastic behaviour for a given fluid and enter sustainable flow.
This yield stress value is strongly dependent on solid content of the slurry. Rheological measurements such as viscosity, elasticity, processability, temperature-related mechanical change, and the yield stress value of mining tailings can be extracted easily from a simple measurement using a rheometer. Those values can then be easily correlated with the pumpability of a specific tailing formulation with given solids mass fraction.
Our next article will summarize the procedure and results of rheological tests conducted to evaluate the pumpability of specific tailings.
* Note: References used:
- Wikipedia, Tailings, http://en. wikipedia/wiki/Tailings (as of July 13, 2014, 16:50 CET)
- US EPA. (1994). Technical Report: Design and Eva- luation of Tailinfs Dams.
- Martin T.E., Davies M.P. (2000). Trends in the stewardship of tailings dams. 1996, Essen
- Dzuy N.Q., Boger D.V. (1985). Direct yield stress measu- rement with the vane method. J Rheol 29:335-47.
For specifics, see Correlating Yield Stress with Pumpability of Mining Tailings