Metso Outotec HIGmill increases gold recovery by approximately 6% for Sunrise Dam Gold Mine

As the amount of sulphide material treated increased, gold recovery decreased in response to the refractory nature of the ore. Testwork indicated that a 6% increase in recoverable gold could be obtained by grinding the sulphide material to 80% passing 10μm and leaching the mill product combined with the flotation tails in the existing carbon-in leach (CIL) circuit. This is where Metso Outotec comes in.

World’s largest stirred mill

In 2017, AngloGold Ashanti (AGA) embarked on a Recovery Enhancement Project to increase gold recovery. The project required the existing screened cyclone overflow to be redirected into a new flotation and ultra-fine grinding circuit, producing concentrate up to a rate of 400,000 t/a.

Metso Outotec supplied six TC200 TankCell® flotation units, one 14m High Rate Thickener and one HIGmill® HIG3500/23000. At this time (June 2021), the HIGmill is the world’s largest stirred mill supplied into a pyrite concentrate regrind duty. The HIGmill grinds pyrite/arsenopyrite flotation concentrate to 10μm in order to expose refractory gold for leaching. Despite its size and significance, only a single operator is required for the flotation, thickener and HIGmill area.

Process and mechanical design

Based on the vigorous testwork and strict design criteria, a HIGmill size of 3500kW with a 23,000L body was selected. To match the finer grind size, media of a 2 to 3mm size range and media specific gravity (SG) range of 3.8 to 4.1 was required. Media is retained in the mill due to media fluidisation conditions facilitated by settling velocity and flowrate.

Media is also held within the grinding chambers (stator compartments) by the centrifugal force generated by the rotors. The design inlet temperature of 30°C, 60kWh/t, 45%w/w yielded an exit temperature of 66°C. The HIGmill was designed with a high alarm temperature limit of 70°C and a trip limit of 85°C. An energy balance also confirmed that the exit temperature was suitable for the materials of construction.

Delivery and installation

Due to a successful collaboration with AngloGold Ashanti, the Recovery Enhancement Project was delivered very quickly. Installation time for the complete upgrade project was approximately six months. Prior to these works, the HIGmill delivery time was 30 weeks ex-works.

Once the civil foundations were ready, the HIGmill installation took four weeks, with cold and wet commissioning taking just one week. Both Metso Outotec and the site are happy with how the Recovery Enhancement Project was completed two weeks earlier than expected - and under budget.

Commissioning

Commissioned in May 2018, the mill has been operating since June 2018. Survey data was collected during ore commissioning to track performance as well as to further optimise the mill. It is important to measure the speed/power/slurry density/ media filling percentage flowrate relationship during commissioning for both water and slurry, as the relationship is different for each application.

The water calibration curve conducted during wet commissioning was performed with 19 tonnes of media, a media size range of 2-4mm and a media specific gravity (SG) of 3.9 t/m3. These values are used to assess the mill’s capability and for future operational guidance. These results can be put into power models for prediction of the operation and assist with future optimisation of the rotor design.

During ore commissioning, the slurry density was optimised to account for the slurry viscosity effect on energy efficiency. The mill was filled with 2-4mm ceramic media with an SG of 3.9 t/m3. The operating mill with greater than 50%v/v media filling, combined with longer run time for media wear in, resulted in improved energy efficiency.

Successfully meeting expectations

After successful commissioning, optimizing the life of wear component was crucial. AGA expected the HIGmill to meet a minimum plant shutdown interval of 17 weeks.

Within 18 months, the HIGmill service interval between shutdowns gradually increased to 23 weeks, thus effectively meeting AGA’s central target.

By matching the HIGmill shutdown timing to normal plant shutdowns, the annual HIGmill availability is im-proved, in addition to greater machine utilisation and increased gold recovery.

Excellent component wear life

The component wear life currently stands at 26 weeks for certain mill shell liner components, and up to 45 weeks for rotors.

Close customer collaboration

Sunrise Dam’s optimisation journey continues with a close collaborative AGA and Metso Outotec effort, intent on reducing the total cost of ownership whilst maintaining safety, machine availability and key performance parameters.

Castellated rotor development

In December 2018, Metso Outotec agreed to implement rubber castellated rotors in the Sunrise Dam HIGmill. As this was a staggered approach, the initial quantity of rotors was four units mounted from the bottom of the shaft, as this location is exposed to the highest pressure and wear.

The improvement in wear life seen with the four rubber rotors led to the decision to increase the installed quantity to eight units in the next cycle, and eight to eleven in the next cycle.

In addition to previous developments, a recent modernization is a moulded cranked design. These three units have been used in two operating cycles over 50 weeks.

High plant and machine availability

HIGmill’s overall utilisation is 97%, with an availability of 98.6%. The availability is governed by the shell liner wear life interval of approximately 26 weeks, harmonizing with other general plant shutdowns. This allows for a further two inspections per year.

The reline periods run for 36 hours each, excluding operator shutdown and start up time. The main plant shutdown period is generally between 36-48 hours, governed by the primary milling circuit relines.

Scaleup testwork

In 2020, Metso Outotec conducted onsite scaleup pilot testwork to study the effect of residence time and operational density on the energy efficiency. It was shown that residence time of 2 and 3 mins in the HIG5 test unit didn’t significantly affect the energy efficiency.

These results indicate that the full-scale operation is slightly more energy efficient than the HIG5 pilot testwork. The measurements allow for the gearbox efficiency, motor under partial load and motor cable transmission.

Proven high performance

In the first 18 months of operation, the HIGmill obtained an average product size P80 of 9.4μm, with a corresponding gold recovery rate of 81.5%. For the twelve months after that, the HIGmill grind size was 12.5μm, with a gold recovery rate of 82.6%.

This increased grind size is attributed to several factors including highly variable mineralogy of the feed ore, increase in ore hardness and high talc concentrations. Overall gold recovery has increased by approximately 6% as a result of the installation of the fine grind circuit, with continuous further optimisation of the circuit.

Easy operation

According to operators, the HIGmill has been and is continuing to run smoothly. It is very easy to change the mill speed, adjust the energy input and change the P80 during operation.

As the thickener has been installed prior to the HIGmill, with large ~45 min mass inventory, a steady state process is easily achieved that delivers consistent throughput, flowrate and density.

The rotor life can also be maximised by increasing the media load in the mill and slowing the rotor speed down. Once the maximum media fill level or an upper torque limit is reached, the mill speed can be increased as the rotors wear to maintain the required power draw and product particle size.

Conclusion

The regrind project at Sunrise Dam has so far achieved an estimated average increase in gold recovery of 6%, and provides consistent and steady operation. The mill is achieving the design product size of 80% passing 10μm and continues to be operated with minimal operator input. Since the wear life of rotor is up to 45 weeks and a shell liner component minimum 26 weeks, the mill does not need to be stopped for regular wear inspections, resulting in higher availability.

With an overall utilisation of 97%, an availability of 98.6%, and an outstanding component wear life, Metso Outotec was able to provide the right solution. There’s strength in partnerships, and due to the close collaboration with AGA, the HIGmill optimisation journey continues at Sunrise Dam.

That’s why, together, we are the partner for positive change.

This is an abbreviation of the paper “The Application of Ultra Fine Grinding for Sunrise Dam Gold Mine” (Paz et al), from AusIMM’s Mill Operators conference (2021), Brisbane, Australia.