PASTE- GOLDER_proof 25/05/2016 09:36 Page 1
Paste Supplement
Cemented tailings versus paste
Thomas Skocir and Isaac Ahmed of Golder
Associates in Mississauga, Ontario,
Canada; and Glenn Barr of Twin Metals
Minnesota, outline how continued binder
testing led to process optimisation and
potential savings in backfill costs for the
TMM project
investigating different strategies for using mill tailings as mine backfill
material. One proposed mining method only requires low strength (150
kPa), non-liquefiable backfill material to fill the mined out stopes because
each stope is independent from another, and mining will not take place
beside exposed backfill material.
Binder selection and optimisation
One of the main options investigated as a backfill material was cemented
tailings (CTT). Extensive laboratory testing was performed, investigating
binder types, binder blends, binder addition quantities, and tailings slurry
Cast cylinder before (left) and after (right) UCS testing, CTT with 8% binder at 14 days curing time
win Metals Minnesota LLC (TMM) is currently pursuing the
development of an underground mine in northeast Minnesota.
Situated between the towns of Ely and Babbitt, the proposed project
will produce two concentrate products, containing copper, nickel, gold,
platinum and palladium.
As part of the project development and engineering stage, TMM is
T
Figure 1: Unconfined compressive strengths for two different binders at
varying concentration at a 7 in slump paste blend. Results for the use of FA
are not shown
P8 International Mining | JUNE 2016 Supplement
density. Samples were cast into cylinders and allowed to cure for specific
periods, most typically 7, 14 and 28 days. After being cured to the
prescribed time period, the cylinders were removed from their moulds, and
compressed to failure in a load frame, resulting in an unconfined
compressive strength (UCS) measurement of the fill, which provided a
relative means of evaluating the variables under consideration. Binder
types tested with the TMM tailings sample were:
n 100% Normal Portland Cement (NPC)
n 50% NPC/50% Fly Ash (FA)
n 70% NPC/30% Fly Ash
n 80% Blast Furnace Slag (BFS)/20% NPC
n 90% BFS/10% NPC.
Initial testing was undertaken with 7 in slump tailings (79% solids),
consistent with a typical ‘paste’ material. Figure 1 presents the results of
the 7 in slump paste test work with NPC and BFS binders. A comparison
between the 90% BFS/10% NPC binder (red) and the straight NPC binder
(blue) tests show that significantly higher strength material is achieved
with BFS binder than with NPC binder.
In order for mill tailings to reach a paste consistency, two stages of
dewatering and one stage of repulping are required: mill tailings are
thickened to 73 wt% solids, dewatered (filtered) to 83% solids, and then
repulped to a 7 in slump slurry.
Based on the favourable results obtained using 90% BFS/10% NPC, and
a continued desire to simplify the backfill process, a second round of test
work was undertaken, with the goal of determining the material strength
that could be achieved with addition of BFS binder directly to the thickened
tailings. The results can be seen in Figure 2.
The results were better than expected. Binder contents on tested
samples only overlap at 1 wt%; at this value there is only a marginal
decrease in strength, in the 10-20% range at the 28 day break. Although