HPGR AT TROPICANA GOLD MINE – CASE STUDY

*Andrew Gardula1, Dilip Das1, Johan Viljoen2, Cameron Tickner2, & Steve Piscicelli2
1
Koeppern Machinery Australia Pty Ltd
73 Pavers Circle, Malaga WA 6090
2
AngloGold Ashanti, Tropicana Gold Mine
44 St. Georges Terrace, Perth WA 6000

(*Corresponding author: agardula@koeppern.com.au)

Abstract

At SAG 2015 AngloGold Ashanti Australia Ltd and Koeppern presented a case study on the first year of high-pressure
grinding rolls (HPGR) operation at the Tropicana Joint Venture’s (AngloGold Ashanti Australia 70% and manager,
Independence Group NL 30%) Tropicana Gold Mine (TGM). It is nearly six years since the mine was commissioned.
This case study update describes process optimisation that was undertaken by AngloGold Ashanti with assistance
of Koeppern Machinery Australia (KMA). The result of the optimisation is an increase in mine productivity of over
30%. The presentation describes the challenges in all areas of operation—process control, studded lining
performance and wear rates, mechanical availability, and maintenance. During the six years of operation, the HPGR
demonstrated it was the correct choice, with the project being an unquestionable success story.

Keywords

HPGR, high pressure comminution, process optimisation

Introduction

The Tropicana Joint Venture (JV) in Western Australia was formed in 2002 between AngloGold Ashanti Australia
Ltd (70% and manager) and Independence Group NL (30%). Tropicana was discovered 330 kilometres (km) east-
northeast of Kalgoorlie in 2005. Exploration identified the deposit at the time containing 5.01 million ounces
(Moz) and an ore reserve of 3.3 Moz.

Figure 1 – Tropicana Gold Mine Location

The project was approved by the JV partners in November 2010 following a successful bankable feasibility study
that was based on open cut mining of the Tropicana and Havana deposits and treatment of 5.5 Mt/a of ore to
deliver average gold production of 330,000 to 350,000 ounces per annum (oz/a) over a 10-year mine life. Start-
up was targeted for the December 2013 quarter. Forecast capital costs were between AU$820 million and
AU$845 million.

2 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019

 Figure 2 – 3D Image of Tropicana Gold Mine Deposit
(courtesy of AngloGold Ashanti – Geology Department)
AngloGold Ashanti selected the high-pressure comminution process for its Tropicana Gold Mine in Western Australia.
• The project was developed by AngloGold Ashanti with the support of EPCM contractor Lycopodium
Ltd, Perth, on processing design and construction.
• Maschinenfabrik Köppern – Germany designed, manufactured, and supplied the HPGR. The mine was
commissioned during September 2013 and it reached full operational capacity within six months.
The Tropicana Gold Mine’s original design was based on the following parameters:

• Average annual production – first 3 years ......................... 470,000 to 490,000

• Average life-of-mine annual production ............................ 330,000 to 350,000 (3.6 Moz in total)
• Mine life.............................................................................. +11 years
• Plant throughput ................................................................ 5.8 Mt/a
• Average head grade............................................................ 2.01 g/t
• Gold recovery ..................................................................... 90.4%
Summary of HPGR utilisation and performance from commissioning until April 4, 2019:

• Commissioning completed ................................................. October 10, 2013

• Total “on load” operation hours as at April 4, 2019 ........... 40,373 h
• HPGR utilisation (on load hours / total hours available) .... 84%
• Average lining wear life (on load hours)............................. 7,540 h
• Total net tonnes milled since October 10, 2013 ................ 37.7 million
• Ball mill throughput rate (original design 680 t/h) ............. 950 to 1,050 t/h
• HPGR availability ................................................................ >95%

The HPGR’s high mechanical and process availability and excellent performance of studded wear protection are
the primary contributors to the HPGR success.

3 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019

Project Design and Optimisation

The original flowsheet was presented at SAG Conference 2015 (see Figure 3):

• Primary crushing – open circuit and stockpile

• Secondary crushing – circuit closed with 42 mm screen
- Screen underflow reports to HPGR
- Screen oversize recycled to cone crushers for re-crush
• HPGR – circuit closed with 4 mm screens
- Screen underflow reports to ball milling
- Screen oversize recycled to HPGR
• Ball mill product reports to carbon-in-leach (CIL) process
In addition to the process flow, TGM operates an emergency stockpile. A portion of HPGR product is diverted to
a 5 mm aperture screen.

• Screen underflow reports to emergency stockpile

• The oversize is returned to the HPGR product conveyor

Figure 3 – Process Flow Overview (courtesy of AngloGold Ashanti)

4 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019

 Figure 4 – Main View of HPGR Installation at Tropicana (courtesy of AngloGold Ashanti)

Process optimisation that was undertaken by AngloGold Ashanti aimed to increase plant productivity. The
following was trialled:

• Feeding the ball mill with a finer ore – this was achieved via reduction of HPGR screen apertures
• Operation of the HPGR at high pressing forces – aimed at improving comminution effect
• Addition of a second ball mill – reduction of milled particle size, improvement throughput, and gold recovery
• All conveyors servicing the HPGR circuit were upgraded to cope with increased throughput rate.

Figure 5 – Current Process Flowsheet at Tropicana Gold Mine (courtesy of AngloGold Ashanti)

5 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019

The design of Tropicana processing plant includes cameras on the main conveyor belts for online PSD as to
prevent the need to stop for sampling of individual streams to assess performance. The plant however does
conduct circuit surveys on a 2 yearly basis to obtain real life particle size distributions by means of crash stopping
conveyor belts. Such an exercise is costly and for this reason, not performed frequently. The analysis of process
performance was, however, necessary in order to evaluate the process, so KMA participated in performance
assessment process that did not require a crash stop.

The HPGR control system (designed and manufactured by KMA) has its own process data logging function. KMA
can remotely log in to the HPGR and observe the process live or download process parameters records for
analysis. KMA developed a statistical process data analysis that allowed to assess process performance and
report it back to Tropicana. A selection of process results is presented in the next section of this document.

Process Improvements

Soon after the plant was fully commissioned and achieved full operation capacity the work commenced aiming
at increasing ball mill throughput.

The authors initially experimented with HPGR screen apertures. The presentation at SAG 2015 mentioned
operation with screen aperture reduced from 4 mm to 3 mm. The result was evident—ball mill throughput
increased from its designed 680 t/h to well in excess of 800 t/h.

A variety of screen apertures were tested including 2.5 mm, although results were not stable. The ball mill
throughput oscillated from 850 t/h to 950 t/h depending on ore properties.

In early 2016 a new operation concept was evaluated. TGM attempted to apply the following assumption for
selection of roller speed and specific pressing force:

If the HPGR is operated at maximum speed and maximum pressing force, this will result in an increase of power
consumption and in effect comminution process performance will improve, thus achieving higher ball mill throughput.

The selection of HPGR process parameters is of paramount importance to achieve a good balance between
production rate of “on specification” product and HPGR operation expense. The HPGR operation costs are
influenced mainly by specific energy consumption and wear rate of the studded lining.

The HPGR was operated at maximum speed, maximum specific pressing force and with feed gates fully open for
several months—process performance was analysed frequently, and variety of interpretation was considered.

At the end of this process the following was accepted as an explanation of process behaviour:

• Comminution effect is controlled by specific energy consumption, not power draw

• HPGR roller speed has a negligible effect on comminution efficiency
• For a given ore the specific energy consumption (kWh/t) is constant, regardless of roller speed
• Operation at maximum speed, maximum specific pressing force and with feed gates fully opened
results in HPGR screen overloading and this leads to the following:
- Reduction of screening efficiency
▪ Fewer near size particles report to underflow
▪ Ball mill receives a finer feed, hence apparent throughput improvement
- HPGR recycle carries increased fines content; this brings excessive water back to the HPGR
▪ Wear rates increase, studded lining must be changed more frequently.

6 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019

 Figure 6 – Main Motor Power Draw as a Function of Roller Speed

Figure 7 – Specific Energy Consumption as a Function of Roller Speed

The operation returned to normal in the beginning of 2018. The HPGR continues operating at near maximum
specific pressing force and roller speed is controlled to maintain total discharge rate at the correct level.

To utilise the extra process capability of the HPGR, TGM installed a second ball mill. This in effect resulted in an
increase of plant throughput to well above 1,000 t/h.

Additional improvements included:

• Redesign of the HPGR feed bin conveyor head chute to produce a more homogeneous particle size
distribution in the HPGR feed.
- Prior to this design change, the HPGR feed in the bin was segregated. This resulted in fluctuations
in the particle size distribution of the ore being drawn from the bin and into the HPGR. This, in
turn, led to variation in the HPGR performance. The new design resolved segregation issues and
the HPGR performance improved.

7 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019

 • Installation of automated swing gates.
- The installation of automated swing gates has improved operations ability to position feed
directly over the centre of the HPGR gap. Prior to the upgrade, the position of the gates was
largely unknown due to lack of proximity sensors. This meant that the feed to the rolls was quite
often biased over one roll resulting in skewing of the rolls. With the installation of the automated
gates containing proximity sensors the exact position of the gates was known, ensuring a
consistent central feed to the HPGR.
• Installation of white iron billets on the face of the gates.
- The white iron billets have extended the wear life of the swing gates, thus allowing for the feed to
be positioned over the centre of the HPGR gap for longer.

Process Analysis – March 2019

KMA analyses the process on a very frequent basis. Presented in this section is process analysis for the month
of March 2019. This is the most recent month at the time of writing, and it is representative of typical HPGR
operation at Tropicana during a month without a planned shutdown.

The analysed data is a spot value recorded every five seconds of operation.

Table 1 – Summary of HPGR Average Operating Parameters during March 2019

Parameter Unit Value

Total on-load operating hours h 707
HPGR utilisation % 95
Total fresh feed processed t 820,517
HPGR total throughput t 1,565,151
Total ball mill feed t 751,365
Average ball mill feed rate t/h 1,009
Total to emergency stockpile t 75,042
Roller speed rpm 17.4
Operating gap mm 62
Roller skew mm 1.4
2
Specific pressing force kN/m 3,379
Specific energy consumption kWh/t 1.32
3
Specific throughput (m-dot) ts/hm 318
Comminution efficiency % 57

March 2019 was a steady production month at Tropicana Gold Mine. Only very few interruptions of ball mill
feed took place. The trend below presents the ball mill feed rate for the months.

8 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019

 Figure 8 – Ball Mill Feed Rate for March 2019

The HPGR process was very well controlled and the average process parameters are presented in the Table 1.
Histograms below provide an indication of process stability.

Figure 9 – Average Roller Gap and Roller Skew for March 2019

The roller gap was a very steady 60 to 68 mm. The HPGR operated with the automatic roller skew controller
engaged, keeping the roller skew safely away from the 16 mm threshold. The automatic skew controller function
was described in our presentation at SAG 2015. This control loop automatically adjusts pressing forces at drive
and non-drive roller sides to ensure as close as possible parallel position of the floating roller.

Our presentation at SAG 2015 compared HPGR parameters and process performance for oxide, intermediate and
fresh ore. These days Tropicana processes fresh ore only and for this reason process specific pressing force is
maintained at near maximum. This is reflected in higher energy consumption.

9 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019

 Figure 10 – Process Specific Pressing Force and Specific Energy Consumption for March 2019

Assessment of comminution effect is not an easy task. As mentioned, Tropicana operates an emergency stockpile
(ESP). A proportion of HPGR total product is diverted to a 5 mm dry screen. Screen underflow reports to ESP
and oversize returns to the HPGR product conveyor. This oversize report back to the HPGR via main HPGR
screens. When estimating the comminution effect, we consider only operation when no divert to ESP is in
operation, this eliminates a potential error.

Figure 11 – Comminution Effect Estimate (% passing 4 mm)

10 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019

Studded Lining Wear Performance

Studded lining wear performance at Tropicana Gold Mine was the most discussed issue during project design.
Wear tests were performed and provided adequate information to estimate expected lining service life. Based
on Köppern calculations expected wear life was 8,000 on load hours. The wear warranty agreed, and the contract
was settled at 6,000 on load hours.

Currently Lining #6 is in operation, Table 2 presents wear life of each lining since plant commissioning in October 2013.

Table 2 – Tropicana Gold Mine Studded Lining Wear Record

Wear Lining Set 1 2 3 4 5 6
Date put into service 8/10/2013 16/06/2015 5/07/2016 20/03/2017 29/11/2017 3/12/2018

Date taken out from service 16/06/2015 5/07/2016 20/03/2017 29/11/2017 3/12/2018 in service

Total days in service 616 385 258 254 369 122

Service time 11,689 7,471 5,386 5,444 7,704 2,679

(on-load hours)
Main reason for removal Worn out Tramp Metal Damage Ball Mill Reline Bearing Failure Bearing Failure

Premature removal by ~2,000 h ~2,000 h ~1,500 h

The wear performance of studded linings at TGM shows very large fluctuations. The initial lining lasted twice as
long when compared with linings # 3 and #4.

Replacement of the lining at Tropicana is often an economic decision. The highest cost at Tropicana is revenue
loss resulting from plant shutdown. Comments below provide additional information for lining replacement:

• Wear Lining #1:

- The lining was kept in service until it was fully worn

Figure 12 – Studded lining set #1 at the time of removal from service, having experienced advanced wear and
large sections on the rollers show complete removal of the tungsten carbide studs.

11 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019

 • Wear Lining # 2:
- In November 2015 (when the lining was less than 3,000 h old) TGM suffered from suspected
tramp metal passing through the HPGR.
- In an instant hundreds of studs were broken. TGM and KMA evaluated potential repair.
Replacement of such a large number of studs would be very time consuming and require a long
shut down.
- The decision was made to continue operation. The lining was finally removed from service seven
months later – the lining did not reach its originally expected wear life.

Figure 13 – Studded lining shortly after tramp metal damage. The majority of studs marked with the red line
were broken. Stud length was approximately 6 mm shorter than those on the remaining wear surface.

• Wear Lining # 3:
- High wear rates resulting from selection of operating parameters
- Lining #3 was removed early during plant shut down for ball mill reline
• Wear Linings # 4 and 5:
- High wear rates resulting from selection of operating parameters
- Premature removal from service due to bearing failures.
The current lining has been in service for nearly 2,700 h and it performs well. Process parameter selection is
adequate; the lining is expected to last in excess of 10,000 on load hours.

Stud breakage

Except for Lining #2 when major stud breakage occurred due to tramp metal, stud breakage is minimal.

To-date, stud breakage has not been a reason for stopping the HPGR for repairs. Damaged studs are replaced
during planned plant shutdowns every 12 weeks. During these shutdowns, KMA carries out wear measurement
and inspection of studded lining. Based on inspection results and time available, damaged studs are marked for
replacement:

12 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019

 • Studs that shatter (broken off stud tip and shattered stud stem) as well as all broken (chipped) edge
protection carbides are nominated as a priority or “must do”
• Broken groups of studs (more than three studs side by side) are nominated as second priority for
replacement—replace at least some of these
• Single studs broken clean without shattering—if time allows.
• Small chips are left in—no replacement.

Chipped Snapped tip Shattered

Figure 14 – Examples of Broken Centre Studs

As mentioned previously, a shutdown for TGM is a significant cost (revenue loss). For this reason, stud lining
repair must be completed as quickly as possible. If this means that some broken studs are left in the lining, it is
a well justified risk.

In general, at Tropicana stud breakage is not an issue. Typically, the inspection will reveal 0 to 10 studs that
require attention. Worst case to date (excluding the major failure in November 2015) was approximately 30
broken studs out of which ~10 needed replacement.

Maintenance

All ongoing on-site and off-site maintenance of the HPGR and its components is performed by Koeppern
Machinery Australia. The KMA facility in Perth is fully equipped to carry out change of wear lining on TGM rollers
and refurbish (repair) bearing housings and other parts. KMA also manufactures cheek plate wear parts for TGM.
The KMA facility is well equipped for removal of worn tyres as well as installation of new wear lining. KMA uses
an induction heating process developed by Köppern for tyre pre-heat.

13 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019

 Figure 15 – Roller Maintenance for Tropicana

Theoretically, maintenance of a HPGR is a relatively straightforward process. The maintenance is limited to:

• Preventative maintenance visit (every 3 to 4 weeks) no HPGR shutdown needed

• 12 weekly shutdowns (48 h) repair and/or replacement of faulty parts
• The major shutdown is the roller change-out.

At Tropicana, as at many other mining operations in Western Australia’s Goldfields there is a battle with corrosion
due to the quality of the ground water. The availability of fresh water on site is very limited. Tropicana has no
other option but to use hyper-saline ground water for all dust suppression, equipment washdowns, etc. This
leads to severe corrosion issues. The battle with corrosion and HPGR component redesign is an ongoing
challenge. Corrosion protection measures are applied to every exposed bolt, hinge, or crevice. When
preparations are made for a roller change-out, Oxy-lance is always at the ready and often used. Some
components of the HPGR have already been replaced with stainless steel to counteract corrosion.

14 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019

 Figure 16 – Bearing slide rails (carbon steel) after ~11,000 h in service.
The slide rails were replaced with a duplex stainless steel

Figure 17 – Recrystalized salt on an accumulator bank and corrosion of bearing sliding pads

The combination of hypersaline water and high ambient temperatures leads to ever-increasing salt present on
machine components. This leads to crevice corrosion of the machine frame, bearing housings and many other
components of the HPGR and surrounding structures. When compared with other operations, TGM’s HPGR
requires much more maintenance and more frequent replacement of machine components.

15 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019

Crevice corrosion at the joints of main bearing housings, hydraulic system and in other locations is a major issue
at Tropicana.

As noted in Table 2, during operation of Linings #4 and #5, TGM experienced main bearing failures that led to
premature removal of the rollers from service. Extensive investigations were conducted into the reasons for the
failures. The main conclusion pointed to salt water ingress into the bearings, which leads to corrosion, loss of
lubrication, and subsequent catastrophic bearing failures.

Project Summary

Every project has its own challenges. At Tropicana, the battle with corrosion is the main issue. The HPGR suffers
from corrosion damage in the same way as many other pieces of equipment on site. Redesign of individual HPGR
components is ongoing. Anticorrosion measures are a standard task undertaken by KMA during every
preventative or shutdown visit to the site.

In summary, the performance of the HPGR at Tropicana is regarded as excellent:

• Mechanical availability is well above 95%

• Stud breakage is minimal
• Wear life of studded lining is as expected; some linings were removed from service prematurely for
various reasons
• Process control is simple – automatic control loops (roller skew and motor torque) function very well
• The throughput measured by the on-specification product rate is as predicted by KMA during the
project design stages.
The selection of Köppern HPGR for the Tropicana processing plant was a correct choice. The HPGR contributed
well to the project’s success.

References

Broeckmann C., & Gardula A. (2005). Developments in High Pressure Grinding Technology for Base and
Precious Metal Minerals Processing. 37th Canadian Mineral Processors Conference, Ottawa, 18-
20.01.2005, p. 285-300.

Gardula, A., Das, D., DiTrento, M., & Joubert, S. First year of operation of HPGR at Tropicana Gold Mine – Case
Study. SAG Conference, Vancouver 2015

Günter, H. (1996). The Application of Roller Presses for High Pressure Comminution. Symposium on Grinding
Processes, Toulouse, France, 1996

Morley C. (2003). HPGR in hard rock applications. Mining Magazine, 9, p. 118-127.

Kirkup L. (1994). Experimental Methods.

16 | SAG CONFERENCE 2019 VANCOUVER | SEPTEMBER 22–26, 2019