WATER RECOVERY FROM A MINE IN THE ATACAMA DESERT
By: Bob Chambers, Howard Plewes, John Pottie, Len Murray and Alan Burgess
Water in Mining – 2003
October 13-15, 2003
Brisbane, Australia
CONTACT
Bob Chambers
Klohn Crippen Berger Ltd.
500-2955 Virtual Way
Vancouver BC V5M 4X6
CANADA
604.251.8504 t
604.251.8510 f
[email protected]
1
WATER RECOVERY FROM A MINE IN THE ATACAMA DESERT
Bob Chambers1, Howard Plewes1, John Pottie1, Len Murray2 and Alan Burgess3
1. Klohn Crippen Berger Ltd.; formerly: Klohn Crippen Consultants Ltd.
500-2955 Virtual Way, Vancouver BC V5M 4X6 CANADA
2. Klohn Crippen Berger Ltd.; formerly: Klohn Crippen Consultants Ltd.
Level 7-240 Queen Street, Brisbane, QLD, Australia 4000
3. Peak Downs Mine, Australia; formerly: Minera Escondida,
Av. De la Mineria 501-Casilla 690, Antofagasta, Chile
2
WATER RECOVERY FROM A MINE IN THE ATACAMA DESERT
Water recovery from tailings for reuse in mineral processing is essential to operation of mines
in the Atacama Desert of Chile. With annual rainfall of 5 mm, the prudent and efficient use of
available water resources is a major issue for the Escondida Mine and other mines in the area.
The Escondida Mine is located at an altitude of 3100 m, in a very arid, seismically active part
of the Atacama Desert of northern Chile. The site is located about 170 km from the coastal
city of Antofagasta. From 1994 to 2002, tailings were discharged into the Hamburgo Tailings
Facility, which comprises a series of “paddies” located near the open pit and mill. Because
water in the region is only obtainable from groundwater sources, the paddy system
incorporated a number of design features with the aim to maximize water recovery from the
tailings.
As part of the recent mill expansion to 237 500 metric tonnes per day, a new tailings facility,
the Laguna Seca Tailings Facility, projected to be one of the largest tailings facilities in the
world, was commissioned in 2002.
Extensive studies were conducted for the design of the new facility to predict water recovery
from the conventional wet disposal of tailings. The predictions were calibrated against the
performance of the Hamburgo Tailings Facility and subsequently re-calibrated against actual
performance during the first year of operation at the Laguna Seca Tailings Facility.
Initial operation of the Laguna Seca Facility was marked by a coincidental change in tailings
characteristics from design, which highlighted the need for thoroughly understanding tailings
behaviour during deposition, and the parameters and deposition techniques that could
enhance water recovery. Systems have been put in place to manage the tailings deposition
and respond to changes in key parameters such that water recovery targets can be reliably
achieved.
This paper describes the background studies that were conducted to evaluate evaporation and
tailings behaviour, summarizes the water balance and then presents some observations from
the commissioning and start-up of the facility.
3
1. Introduction
The Escondida copper mine, which is owned and operated by Minera Escondida Limitada
(MEL), is located in the Atacama Desert in northern Chile at an elevation of approximately
3100 m (Figure 1). In 1998, MEL upgraded the existing Los Colorados Concentrator
(Phase 3.5) to 127 500 tonnes per day (tpd) and in 2003, commissioned the Laguna Seca
Concentrator (Phase 4) with a capacity of 110 000 tpd. As part of the upgrading, MEL
commissioned a new tailings facility at Laguna Seca to replace the existing Hamburgo
Tailings Facility. A total combined tailings tonnage of 3.3 billion tonnes will be delivered to
the Laguna Seca Tailings Facility over the next 40 years.
Figure 1
Site location plan
4
With annual rainfall of 5 mm, the prudent and efficient use of available water resources is a
major issue for MEL. As a result, water recovery from tailings for reuse in processing is a
key to mine operation. Water is typically released from tailings by:
•
thickening in the concentrator;
•
initial release of transport water as tailings is deposited in the storage facility;
•
long-term release from tailings consolidation; and
•
seepage out the base of the tailings facility or through the containment dam.
As part of the design of the Laguna Seca Tailings Facility, various thickening technologies
were evaluated, including conventional large diameter thickeners, high capacity and high
density thickeners, and belt and vacuum filters. Optimizing the existing conventional, large
diameter thickeners was the preferred option because of the higher capital and operating
costs, equipment limitations, and limited experience at high tonnages with the other
technologies.
Experience in water recovery from the tailings facilities is discussed in this paper.
5
2. Project description
Laguna Seca Tailings Facility Description
The Laguna Seca Tailings Facility is a horseshoe shaped bowl of approximately 50 km2, with
a dry clay lake bed at the centre at elevation 2876 m (Figure 2). The rim of the bowl rises to
over elevation 3000 m. The facility is contained by an earthfill dam (Starter Dam) at the west
end, on a saddle at approximate elevation of 2900 m.
Tailings Drop Boxes
Ditches
Reclaim Pump Barge
Tailings Pipeline
Figure 2
Laguna Seca Tailings Facility (initial configuration at start-up)
The Starter Dam was built in 2001 and will be raised by the downstream construction method
over the next 40 years with downstream and upstream slopes of 2H:1V and 1.8H:1V,
respectively. Construction materials consist primarily of fine fill (silty sand) and coarse fill
(silty sand with gravel) obtained from borrow pits within the tailings pond area. The Ultimate
Dam will be approximately 3 km long with a maximum height of 80 m.
The Starter Dam is about 20 m high and will provide storage for about 6 years. The Starter
Dam includes a cutoff trench through the overburden to bedrock, and a coarse fill zone at the
base of the dam to collect seepage and to keep the phreatic surface low in the dam.
6
System description
The Laguna Seca Tailings system comprises the tailings pumping system from the Los
Colorados Concentrator, a gravity discharge system for the tailings from the Laguna Seca
Concentrator, a water recovery system returning water to both concentrators and the 20 m
high 1 km long Starter Dam.
The Los Colorados Concentrator has five thickeners (120 m diameter). Each thickener has
variable speed underflow pumps, which discharge tailings into the top of the Cerro
Amelunxen header tank at an elevation of 3080 m. The outlet manifold has the ability to
direct tailings by gravity to the Hamburgo Tailings Facility or into two pipelines that take
tailings slurry to the Laguna Seca tailings pump station at an elevation of 3045 m. The
tailings is then pumped through two independent pumps trains, each with four 20/18 GAH
Warmen pumps and each driven by 810 kW WEG motors. The final pump in each train is
equipped with variable speed. The pump trains convey the tailings 12 km to elevation 3135 m
to the “165” collection box where it joins with the gravity tailings flow from the Laguna Seca
Concentrator. From there, the tailings slurry flows 8 km by gravity through two 48 inch lines
to the “171” and “170” drop boxes where it is diverted to the north or south halves of the
Laguna Seca Tailings Facility.
The Laguna Seca Facility is divided into six sectors each containing four 36 inch spigots.
These spigots discharge into ditches which convey the tailings by gravity flow to the bottom
of the tailings basin (Figure 2). Up to four spigots will be used at any time depending on
tailings throughput. Spigotting has started at the lower elevations, and will progressively
move to higher elevations to efficiently control the tailings distribution and fill the tailings
facility. Tailings deposition will be managed to maximize the runoff of supernatant water to
the reclaim pond and pump barge on the north side of the basin (Figure 3). A critical aspect
of the design is the blanketing of the native soils with tailings to reduce the seepage from the
basin. In addition, a tailings beach on the upstream face of the dam is required to keep the
water reclaim pond away from the dam.
7
Figure 3
Reclaim water pond and starter dam
Key Design Parameters
The following are some of the key parameters that were used in the design of the Laguna
Seca Tailings Facility:
•
Tonnage on average: 240 000 tpd.
•
Tailings slurry density: 50%.
•
Tailings beach angle: 0.3%.
•
Expected annual water recovery after 1 year was 0.40 m3 / tonne of treated ore.
8
3. Climate review
The climate of the Minera Escondida area is extremely arid with annual average precipitation
estimated to be 5 mm/year. Due to the arid climate, most of the surface water in the Laguna
Seca region is ephemeral in nature. There are numerous dry streambeds (quebradas), which
collect runoff during infrequent storm events and facilitate groundwater recharge during and
immediately after storm events. The average monthly evaporation ranges from 4 mm/day to
11 mm/day, with a yearly average of 7 mm/day or over 2.5 m/annum.
9
4. Evaporation studies
A key consideration in the design of the Laguna Seca Tailings Facility was the prediction of
evaporative water losses from the tailings areas including deep and shallow ponded water,
and wetted and dry beach surfaces. Site specific studies were carried out at the Hamburgo
Tailings Facility in February 1997 to quantify these losses. Pond water evaporation was
measured in evaporation pans installed at the edge of the operating tailings pond (Figure 4).
The tops of the pans were set flush with the ground surface to mimic the actual tailings pond.
In order to observe the influence of water depth on evaporation, the base of the pans were
partially filled with tailings to provide water depths between 50 mm and 200 mm.
Figure 4
View of evaporation pan with 50 mm water depth
The average evaporation rate was 9.7 mm/day for the 50 mm water depth and 9.6 mm/day for
200 mm of water. It was found that the evaporation that occurred during the night was of the
same magnitude as the evaporation that took place during the day. The explanation for this
was the noticeably higher wind velocities starting in the late afternoon. This wind velocity
coupled with the highest water temperature led to increased evaporation in the late evening.
The daily evaporation rates from two evaporation test pans were about 1 mm higher than
recorded for the Class A pan evaporation at the closest weather station. Therefore, for design
purposes, the average monthly evaporation values from the Class A pan plus 1 mm/day were
adopted for standing water in the tailings pond. The representative average monthly values
are given in Table 1. The average monthly evaporations range from 3.8 mm/day to
10.7 mm/day, with a yearly average of 7.2 mm/day or about 2600 mm per year.
10
A simple apparatus called a “microlysimeter” was used to measure evaporative water losses
on wetted and dried tailings beaches (Figure 5). The evaporation rate for freshly deposited
tailings was found to be equal to standing water (about 10 mm/day at the time of site
investigation). A thin crust was noted (0.1 mm to 0.2 mm thick) to form on the tailings
surface in a short period of time after deposition. The crust was composed of a combination
of dried tailings, dust carried by wind and precipitated salts from the saline tailings process
water. The formation of this drying crust after about 24 hours reduced evaporation to about
25% of the rate for standing water. As the crust developed over time, the evaporation reduced
to about 10% of the rate for standing water.
Table 1
Representative average monthly evaporation for ponded tailings water
Month
January
February
March
April
May
June
July
August
September
October
November
December
Yearly Average
Figure 5
Average evaporation
(mm/day)
10.5
9.4
8.1
6.1
4.8
3.8
3.9
4.6
6.7
8.4
9.8
10.7
7.2
Microlysimeter installations in fresh tailings and dried tailings beach (with and
without crust on tailings surface)
11
Figure 6 summarizes and compares the evaporation rates measured in February 1997. Studies
to confirm these evaporation rates at the operating Laguna Seca Tailings Facility are
underway.
Figure 6
Evaporation rates measured in February 1997
12
5. Tailings studies
Several tailings investigations and studies were conducted at the Hamburgo Tailings Facility
in 1997 and 1998 for design of the Laguna Seca Tailings Facility. The objectives of the
studies were to characterize the geotechnical properties of the tailings, estimate the initial
settled density, and predict longer term consolidation behaviour. Pertinent aspects are
described below. Table 2 summarizes the basic index parameters for the tailings.
Table 2
1997 tailings index properties
Property
Slurry Density
Specific Gravity
Gradation
•
Fines (<74 µm)
•
Clay (<2 µm)
In-place Density
•
Initial
•
Consolidated
•
Average
Value
50%
2.7 to 2.8
Comment
W solids/W total
55% to 70%
5% to 15%
1.1 tonnes/m3
1.6 tonnes/m3
1.3 tonnes/m3
55% water content
24% water content
The raw tailings consisted of a clayey sandy silt mixture with typically 70% passing the
No. 200 sieve (74 micron) and 10% clay-sized particles less than 2 microns. The specific
gravity of the tailings solids was between 2.7 and 2.8. The tailings were pumped to the
tailings facilities at nominally 50% solids density. Upon discharge into the tailings facilities,
the tailings segregated to deposit clayey silt (slimes) and silty sand layers in the first 100 m
from the discharge point. Beyond 100 m, the tailings were predominantly clayey silt (slimes)
which became slightly finer with distance from the discharge point.
The water content of freshly deposited tailings slimes was 55% (0.55 m3/tonne of tailings).
With 1 month to 2 months of consolidation, the water content of the tailings slimes reduced
to 35% to 45% water content in the upper 450 mm below the tailings surface. Sampling to
determine water contents at deeper depths was not possible.
The undrained shear strength of the tailings below the surface crust was low (2 kPa to 9 kPa)
even after 3 months of inactivity and “drying”. The low strength was attributed to the surface
crust that inhibits evaporation and drying of the tailings. The effect of this crust was
demonstrated by a test where the crust was removed and shear strengths in the upper 7.5 cm
increased 100% in only four days.
13
Specialized slurry consolidation testing of the tailings was conducted on samples to
determine representative relationships of compressibility and permeability as a function of
effective confining stress (Figure 7). These relationships were incorporated into a “large
strain” consolidation numerical model for the tailings. These model results were used to
estimate the water entrained in the tailings voids, and the seepage expelled to the surface of
the tailings beach and into the underlying foundation soils. Typical output is presented in
Figure 8.
Figure 7
Consolidation relationships for 1997 Escondida tailings
14
Figure 8
Example results from large strain consolidation model
The “large strain” consolidation analyses for the deposited tailings predicted faster
consolidation than previously predicted by the “small strain” consolidation models. The
average water content of the consolidated tailings deposit was estimated to be 24%. The
relatively low water content was influenced by the underdrainage provided by the pervious
foundation soils.
The average seepage rate into the underlying foundation soils was predicted to be about
0.3 mm/day due to the low permeability of the consolidated tailings. During periods of active
tailings deposition, the water expelled by consolidation to the beach surface was predicted to
be about 12 mm/day. This expulsion rate agreed with field observations that the expulsion
rates in recently inactive paddies at the Hamburgo Facility exceeded the daily evaporation
rate of 10 mm/day in summer.
After tailings deposition is halted, the rate of water expelled to the tailings beach surface
progressively decreases to less than 0.5 mm/day after 11 months. Representative average
monthly expulsion rates are summarized in Table 3.
15
Table 3
Rate of water expulsion to top of tailings deposits
Month after cessation of tailings deposition
Average water expulsion rate (note 1)
(mm/day)
12
0 (active deposition)
8.5
1
4.0
2
2.0
3
1.3
4
1.0
5
0.8
6
0.8
7
0.5
8
0.5
9
0.5
10
0.5
11
Note 1. Following continuous tailings deposition for 15 to 30 days.
A runoff ratio was developed (Figure 9), which expresses the ratio of the total annual water
expelled to the top of the tailings beach in excess of the annual evaporation rate of 7 mm/day,
and the total annual amount of water expelled to the beach surface during that year. The
runoff ratio increases with the rate of tailings deposition. The maximum ratio is 0.30 at
tailings deposition rates greater than 4 m per year. Below 1 m per year, the runoff ratio
approaches zero. Maintaining high tailings deposition rates is therefore key to maximizing
water recovery from the consolidating tailings.
Figure 9
Runoff and evaporation for tailings beaches
16
A significant change in the tailings behaviour was noted by MEL starting in early 2000.
Higher water retention by the tailings reduced water reclaim from the Hamburgo Tailings
Facility by up to 50%. Additional geotechnical studies were conducted in 2001 to investigate
the apparent change in tailings behaviour. The key findings from this work were:
•
The tailings produced in 2001 are slightly finer overall than the tailings produced
in 1997. The most important change is the increase in clay content to 14% versus
10% in 1997. The higher clay content in the tailings was probably related to a
change in the composition of the ore.
•
Field sampling and laboratory flask settling tests (Figure 10) showed that the 2001
tailings initially settle to a lower density after 24 hours (water content = 75%)
than the 1997 tailings (water content = 55%). Figure 11 compares all settling tests
performed to date on the 1997 and 2001 tailings. The settling behaviour of the
2001 tailings is clearly distinct from the 1997 tailings. One plausible explanation
for this phenomenon is the development of an apparent cohesion possibly caused
by geo-chemical interactions between the saline slurry water and the colloidal
clay particles.
•
Additional consolidation testing and large-strain consolidation analyses show that
the water initially entrapped in the 2001 tailings is released when the apparent
cohesion is overcome by stresses induced during further tailings deposition. The
water expulsion under loading will make up for some of the water losses due to
the initial water retention. The amount of water release is dependent upon the rate
of tailings loading or stacking, with more water proportionally released at higher
deposition rates and longer periods of tailings deposition.
17
6. Summary of water balance
A water balance model was developed for the Laguna Seca Tailings Facility to predict the
water available for recovery from the facility. The model considers the effect of the initial
solids content and variable deposition (loading) rates likely for the tailings (25 mm/day to
130 mm/day). The water balance model also incorporates the initial 24-hour water release
and the long-term consolidation water that is generated due to subsequent consolidation of
the tailings and uses the evaporation from the open water of the reclaim pond, active beach
areas and inactive beach areas.
Initial Height 167mm
0.0
24 hours of settling
Change in Sediment Height (mm)
-10.0
-20.0
w/c = 81%
w/c = 77%
-30.0
1997 U of A sample (50% solid content)
-40.0
2001 sample (50% solid content)
-50.0
w/c = 56%
w/c = 53%
-60.0
0
1000 1440 2000
3000
4000
5000
6000
7000
8000
Settling Time (min)
Figure 10
Comparison of settling behaviour of 1997 and 2001 tailings
18
70
60
U of A - 1997 Tailings
U of A - 2001 Tailings
50
Change in Height (%)
KC - 1997 Tailings (tested in 2002)
KC - 2001 Tailings
40
30
20
10
0
0
10
20
30
40
50
60
70
Solids Density (%)
68%
Figure 11
Normalized tailings settlement versus solids density
The key elements of the water balance are shown schematically on Figure 12 and described
as follows:
Inflows
•
Slurry Inflow: Mill ore production and tailings slurry solids density reported by
MEL.
•
Bleed from active beaches: Water produced due to consolidation of the tailings
during active deposition periods. The flow in excess of evaporation will report to
the water pond for reclaim. Bleed rates were determined based on large strain
consolidation modelling.
•
Bleed from inactive beaches: Water produced due to consolidation of the tailings
during inactive deposition periods and field observations.
•
Precipitation: Actual precipitation values measured at the Laguna Seca Weather
Station.
19
Storage
•
Water lost to tailings voids: The average amount of water permanently retained in
the tailings deposit depends on the consolidation of the tailings over time. Based
on the slurry consolidation testing and large strain consolidation modelling, an
average water content of 24% over the depth was assumed.
Outflows
•
Evaporation: Measured evaporation from field tests was assumed representative
of rates from active and inactive tailings beaches, and from the reclaim water
pond. The evaporation was varied monthly according to the selected design
evaporation values.
•
Seepage from tailings: The rate of seepage from the base of the tailings was taken
to be 20% of the water expelled to the tailings surface based on large strain
consolidation modelling.
•
Groundwater losses: Groundwater losses occur where the tailings water contacts
the native ground around the perimeter of the developing tailings deposit and at
the edge of the pond and the ground beneath the tailings.
•
Reclaim Water: Water available to be reclaimed to the concentrator. Reclaim
Water = Inflows - Storage Water – Outflows
20
Figure 12
Schematic of water balance formulation
The water balance was initially calibrated using performance data measured for the
Hamburgo Tailings Facility. The predicted water recoveries were compared to actual water
recoveries reported by MEL to assess the accuracy of the selected tailings parameters and
operating assumptions. The parameters and assumptions made for Hamburgo were then
extended to the Laguna Seca Tailings Facility to predict water recovery from the Laguna
Seca Facility. The actual and predicted values for the Hamburgo and Laguna Seca Tailings
Facilities are shown in Table 4 and on Figure 13. The water balance model is accurate to
within + 10%. The model is currently being used to forecast water recovery for a 12 month
period. The model input parameters are also updated regularly to reflect actual field
performance.
21
Table 4
Actual and predicted values of water recovery
Month
Predicted
(m3/tt)
0.27
0.22
0.24
0.27
0.39
0.43
0.48
0.31
0.43
0.38
0.43
0.33
0.38
0.34
0.35
January 2002
February 2002
March 2002
April 2002
May 2002
June 2002
July 2002
August 2002
September 2002
October 2002
November 2002
December 2002
January 2003
February 2003
Average
Actual reported
(m3/tt)
0.27
0.27
0.22
0.28
0.42
0.42
0.32
0.40
0.29
0.44
0.40
0.35
0.36
0.34
0.34
Difference
(m3/tt)
0.00
-0.05
0.01
-0.01
-0.04
0.01
0.16
-0.08
0.14
-0.06
0.03
-0.03
0.02
0.00
0.01
tt – tonnes of ore treated in mill
Figure 13
Actual and predicted values of water recovery
22
7.
Observations during commissioning and start-up
Chronology
The following is a brief chronology of the start up and key events at the Laguna Seca Tailings
Facility:
•
Design commenced for the Phase 4 project at Escondida Mine in 1996.
•
The final engineering for the civil, electrical and mechanical components of
Phase 4 undertaken between 1999 and 2001.
•
Phase 4 project approved for construction in November 2000.
•
1st Pump train from Los Colorados Concentrator ran for short time December 19,
2001 and started to run continuously February 3, 2002
•
2nd Pump train commissioned and operating April 1, 2002
•
Water reaches 4.5 m depth beneath reclaim barge on March 22, 2002.
•
Water pump system commissioned May 7, 2003.
•
Tailings distribution to Laguna Seca Tailings Facility halted due to failure of
Cerro Amelunxen Tank on May 28, 2002.
•
Tailings distribution to Laguna Seca Tailings Facility restarted July 17, 2002.
•
Laguna Seca Concentrator starts commissioning tests August 2002 and full
production tests September 9, 2002 with the tonnage ramp up commencing on
October 13, 2002.
Observations on start up of Laguna Seca Tailings Facility
In general, the performance of the Laguna Seca Tailings Facility following start-up was as
predicted during the design with several key exceptions. The tonnage of tailings delivered to
the Laguna Seca Tailings Facility was less than assumed during the design. This was
primarily due to problems with commissioning the pump trains, a 7 week period during
which no tailings could be sent the Laguna Seca Facility, and production restrictions due to
the world copper market.
23
Actual water recovery was hindered by operational issues including:
•
Higher water contents (lower solids density) in the tailings slurry, which
decreased the beach angle to around 0.2% (0.3% design).
•
The lower solids density appeared to affect the stability of the subaqueous
beaches. Beaches form but collapse on a frequent and random basis causing
tailings to flow towards the pump system. In response, the operators change the
tailings discharge point, creating greater wetted areas and higher evaporation
losses.
•
Based on evaporation studies currently underway, the actual evaporation at the
Laguna Seca Tailings Facility may be slightly higher than design.
•
Higher clay contents (different ore mineralogy), which is evidenced by changed
settling characteristics and a higher yield stress in the deposited tailings. More
water is retained within the tailings mass but will be released later as the yield
stress is overcome.
In general, the facility designs considered the average conditions; for example the design is
based on an average flow and average density as supplied by the Owner. The actual
conditions of an operating concentrator have such variants as pebbles and balls (from poorly
operating sizing cyclones), large variation in flows (real condition of an operating
concentrator), surges from dumping during upset conditions (power cuts, mill trips etc.) all of
which create perturbations in the tailings facility. As an example, upset conditions during
commissioning of the new concentrator caused reduced water recovery due to ingress of fine
tailings into the reclaim pond and barge pumps. These conditions have made the start-up of
the facility challenging.
24
8. Future plans
In 2002, MEL initiated several studies to assess ways to further improve water recovery from
the Laguna Seca Tailings Facility. The investigations, which have not been concluded, are
concentrating on three specific components:
i) Equipment improvements
•
Improve performance of existing thickening equipment with small capital
cost;
•
Improved thickening with existing equipment with major capital cost; and
•
Supplement or replace existing thickeners with high capacity or high density
thickeners.
ii) Maximize solids density for pumping from the mill thickeners to the tailings area;
and
iii) Optimize density for maximum water recovery from the tailings facility.
Initial results from the above work show that the best water recovery occurs with maximized
water recovery from the thickeners which has several benefits:
•
Water is recovered closer to where it is needed (at the mill);
•
Water recovery is more reliable and will not be impacted as greatly by
evaporation in the reclaim pond; and
•
Water is not “recirculated” around the tailings distribution system and the water
recovery system.
However, the cost of recovered water from the upgraded thickeners has yet to be established
and any overall system will likely depend on the combined performance of the mill
equipment and the tailings impoundment. High capital cost thickening equipment cannot be
easily replaced or modified. Variations due to tailings properties also can have serious
consequences on water recovery. These variations however can, to some extent, be smoothed
out by water recovery from the tailings disposal facility.
25
9. Acknowledgements
We wish to express our appreciation to Minera Escondida Limitada for the opportunity to
participate in this very interesting and challenging project as well as for the co-operation that
we received throughout the project.
26
10. Disclaimer
The opinions expressed in this paper are those of the authors and not necessarily those of
Minera Escondida Ltds. or BHP Billiton.
27
List of Figures
Figure 1 – Site location plan
Figure 2 – Laguna Seca Tailings Facility (initial configuration at start-up)
Figure 3 – Reclaim water pond and starter dam
Figure 4 – View of evaporation pan with 50 mm water depth
Figure 5 – Microlysimeter installations in fresh tailings and dried tailings beach (with and
without crust on tailings surface)
Figure 6 – Evaporation rates measured in February 1997
Figure 7 – Consolidation relationships for 1997 Escondida tailings
Figure 8 – Example results from large strain consolidation model
Figure 9 – Runoff and evaporation for tailings beaches
Figure 10 – Comparison of settling behaviour of 1997 and 2001 tailings
Figure 11 – Normalized tailings settlement versus solids density
Figure 12 – Schematic of water balance formulation
Figure 13 – Actual and predicted values of water recovery
List of Tables
Table 1 – Representative average monthly evaporation for ponded tailings water
Table 2 – 1997 tailings index properties
Table 3 – Rate of water expulsion to top of tailings deposits
Table 4 – Actual and predicted values of water recovery