Jan Philip Plog, Thermo Fisher Scientific, Material Characterization, Karlsruhe, Germany
Key words
Rheology, Yield Stress, Mining, Tailings
Introduction
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 [1].
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 [2]. Mine
tailings are usually produced from the mill in slurry form
(a mixture of fine mineral particles and water) [2].
The composition of tailings is directly dependent on the
composition of the ore and the process of mineral extraction used on the ore [1].
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. In 2000 it was estimated that about
3,500 active tailing ponds existed worldwide [3].
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 [1]. 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 τ0. The yield stress
describes the amount of energy needed to overcome elastic behaviour for a given fluid and enter sustainable flow.
1
Fig. 1: Thermo ScientificTM HAAKETM ViscotesterTM iQ rheometer
with Lab stand for holding large containers.
As can be seen later this yield stress value is strongly dependent on solid content of the slurry. The yield stress
value can be extracted easily from a simple measurement
on the HAAKE Viscotester iQ rheometer.
Application Notes V-277
Correlating Yield Stress with Pumpability of
Mining Tailings
2
As soft solids and slurries like tailings are often difficult to
work with when using conventional parallel plate or coaxial
cylinder geometries on rotational rheometers due to possible
wall slip and excessive sample disruption during sample
loading into narrow gaps, vane geometries are recommended here.
When a vane rotor is fully immersed in the sample, the yield
stress itself can then be calculated according to Boger [4]:
[a]
With M being the Torque and K the vane parameter that
depends on the height (H) and the diameter (D) of the
paddle according to:
[b]
Thus, in order to determine the yield stress the Torque
(or corresponding shear stress) needs to be tracked as a
function of time. The maximum value can then be calculated into the yield stress.
Experimental Results and Discussion
We recommend to rheologically test tailings with vane rotors
to prevent wall slip. Fig. 1 shows the HAAKE Viscotester
iQ rheometer for vane configuration as well as an open stand
to allow testing samples in large containers.
For this study, sand mine tailings have been tested at different solid mass fractions so that a yield stress vs. mass
fraction map could be derived later.
Fig. 2 shows as an example the yield stress determination
according to the Boger model. The test was conducted
with a 0.4 solid mass fraction tailing.
With this easy and fast method a range of sand mine
tailings with different solids mass fractions ranging from
0.2 to 0.5 have been tested. The result can be seen in Fig. 3
as a plot of the respective yield stress values versus the
solid content.
With the data derived from a short series of tests shown
in Fig. 3, the pumpability of a specific tailing can then be
easily predicted by calculating the needed pumping
pressure in bar via the cross-section of the pipe and the
yield stress.
Conclusion
The vane rotor method on the HAAKE Viscotester iQ
rheometer is a quick, simple and accurate approach to
measure the yield stress of mining tailings. Those values
can then be easily correlated with the pumpability of a
specific tailing formulation with given solids mass fraction.
Literature
[1] See Wikipedia, Tailings, http://en. wikipedia/wiki/Tailings
(as of July 13, 2014, 16:50 CET)
[2] US EPA. (1994). Technical Report: Design and Evaluation of Tailinfs Dams.
[3] Martin T.E., Davies M.P. (2000). Trends in the
stewardship of tailings dams.
1996, Essen
[4] Dzuy N.Q., Boger D.V. (1985). Direct yield stress measurement with the vane method. J Rheol 29:335-47.
2
Fig. 2: Flow Curve of a tailing with 0.4 solids mass fraction at room temperature (RT) with automatic yield stress extraction
according to Boger.
Application Notes V-277
Fig. 3: Yield Stress as a function of solids mass fraction for sand mine tailings at RT.
thermoscientific.com/mc
© 2014/11 Thermo Fisher Scientific Inc.⋅Copyrights in and to all photographs of instruments are owned by Thermo Fisher Scientific. All trademarks are the property
of Thermo Fisher Scientific Inc. and its subsidiaries. This document is for informational purposes only. Specifications, terms and pricing are subject to change. Not all
products are available in every country. Please consult your local sales representative for details.
Material Characterization
Benelux
Tel. +31 (0) 76 579 55 55
[email protected]
China
Tel. +86 (21) 68 65 45 88
[email protected]
3
France
Tel. +33 (0) 1 60 92 48 00
[email protected]
United Kingdom
Tel. +44 (0) 1785 82 52 00
[email protected]
India
Tel. +91 (22) 27 78 11 01
[email protected]
USA
Tel. +1 866 537 0811
[email protected]
Japan
Tel. +81 (45) 453-9167
[email protected]
International/Germany
Dieselstr.4
76227 Karlsruhe
Tel. +49 (0) 721 4 09 44 44
[email protected]