PASTE SUPPLEMENT 2019
Paste and filter cake for in-pit tailings disposal
Ibrahim Karajeh of Golder Associates Ltd in Mississauga,
Ontario explains how in the right situation, paste tailings or
filter cake can be safely and usefully placed in an inactive
open pit
any mines start as an open pit
operation and advance into
underground developments. In some
cases, paste is selected as the backfill method
of choice. For these operations, the potential to
place tailings in the existing inactive open pit
can be beneficial from an environmental and
cost perspective. The operation can delay or
eliminate the need to permit a new tailings
storage facility. One of the key challenges with
in-pit deposition is the management of water. A
M
Pinos Altos gold/silver mine Oberon de Weber
open pit in Mexico prior to start of fill
Placement of roller compacted concrete layer
P10 International Mining | APRIL 2019 Supplement
highly dewatered tailings such as paste or
filter cake can be the solution, and to those
operations that have a paste backfill plant, the
advantages are apparent.
Key challenges
Water management
Open pits are practically sinks where water
flows into from either the surface or ground.
Part of that water either evaporates or
continues to flow to other underground voids
and cavities. The balance of the water in the
open pit is pumped out, if access to the pit is
required. Typically, the mine water management
plan involves procedures to monitor, test and
manage this water. However, depending on the
chemistry of the paste or filtered tailings, the
contact water may require further treatment.
The extent of this risk may be complicated by
site specific climatic, topographic and
hydrogeological conditions. There are several
ways to counter this challenge including:
n Create ditches to divert surface water away
from the open pit
n Selective placement of engineered products
As an example of selective placement, a
method known as the Pervious Surround
Method is used in uranium mines. Here, a
layer of a highly permeable material is
installed around the tailings to allow for
groundwater circulation around in-pit
deposited tailings. Paste and filter cake can
be highly impermeable and hence nearly no
exchange of contaminants between tailings
and groundwater takes place
n Adjusting the water management plan by
including additional measures for water
monitoring, testing, pumping and treatment.
Drainage layers at the
bottom of the open pit can
be used to collect any
infiltrated or seepage water
and pumped back to the
surface
n Use cementitious binder to
blend with the paste in
order to minimise the
potential for metals to
leach into the contact water
Risk to underground
Liquefaction and sudden rush into the
underground workings is a risk that poses a
deterrent to putting tailings in an open pit
where an active underground mining operation
is in proximity. In addition, to the water
management steps above, these risks can be
mitigated by:
n Installing structural barriers that prevent
entry of material into underground working
in the case of liquefaction. Barricades can be
built to seal adits and other openings where
underground workings intersect open pit
walls. This requires proper mapping of
underground workings which can be a
challenge for inactive sites. A plug can also
be poured at the bottom of the pit, at the
interface between the open pit and
underground. This could serve to further
stabilise the crown pillar
n Placement of paste or filtered cake in thin
layers and allowing it to dry out and
desiccate hence reducing the impact of
liquefaction. The advantage of a more
heavily dewatered tailings is an obvious
Installation of the first section of pit dewatering
pump well
Placement of a rock fill drainage layer