SkimAir flash flotation improves gold recovery for Polyus' Olimpiada in Russia

Polyus

Polyus is the largest gold producer in Russia and one of the top ten gold miners globally with the lowest cost position. Based on its 2018 Ore reserves and Mineral Resources, Polyus group ranks the third by attributable gold reserves among the world’s largest gold mining companies.

The Polyus group’s principal operations are located in Krasnoyarsk, Irkutsk and Magadan regions and the Sakha Republic (Yakutia) and include 5 operating mines, alluvial operations and several advanced development projects. One development project is Sukhoi Log, in the Irkutsk region, which is one of the world’s largest gold greenfield projects.

Polyus operates a portfolio of well-established mines characterised by efficient use of modern equipment, low-cost, large-scale processes and minimum reliance
on third-party infrastructure. Focusing on maximisation of the potential of its existing producing assets, the company is well-positioned to capitalise on low-risk cost-efficient expansion opportunities provided by its operating mines.

Olimpiada site

Located in one of Russia’s most prolific gold mining provinces, Olimpiada is Polyus’ largest operation. The mine began production in 1996 and currently accounts for over a half of the company’s total gold output. The ore mined at the site is processed at three plants with a combined capacity of 13 million tonnes of ore annually.

Long relationship with Outotec

Polyus has a long relationship with Outotec. To date Outotec has modernized seven existing thickeners and supplied a 30-meter thickener and automation equipment to their concentrator plants. Outotec has also supplied leading-edge automation and analysis technologies for Polyus to help optimize their flotation circuit further, including PSI 300i online analyzer, FrothSense froth imaging analysis camera, LevelSense slurry/froth interface tool and an ACT advanced control system. 

SkimAir technology – a profit maker in the grinding circuit

SkimAir is designed for flotation in the grinding circuit to process cyclone underflow where heavy minerals concentrate. The SkimAir unit reduces overgrinding of valuable mineral particles, provides a more stable feed to the conventional flotation circuit and improves dewatering with coarser final concentrate. Benefits include minimized loss of fines due to overgrinding, improved overall recovery, increased mill throughput and improved dewatering. SkimAir technology also has a small footprint and can deliver benefits such as lower energy consumption in the downstream conventional flotation circuit.

The SkimAir is suitable for all kinds of grinding circuits, and is applicable for heavy and soft minerals, such as lead, zinc, nickel, silver, gold, copper and platinum bearing minerals. The unit recovers a considerable quantity of valuable minerals from the grinding circuit, resulting in a more stable feed to the conventional flotation circuit, which in turn leads to improved control. In addition, grinding mill throughput can be increased as a consequence of the controlled density of the SkimAir tailings.The dewatering performance is improved, as the final concentrate is coarser. 

Typically, the SkimAir flash flotation cell is operated as a standalone single-stage unit or as a two-stage process with a dedicated mechanical cleaning flotation stage. The single-stage process can be recovery- or gradedriven. In the two-stage process, the SkimAir is used as a rougher unit to produce a lower grade concentrate at high recovery, and an Outotec TankCell® is used as a cleaner to achieve target concentrate quality. 

SkimAir pilot test program at Olimpiada 

The SkimAir flash flotation project commenced at Olimpiada in Autumn 2014 with pilot and laboratory scale testwork. The first trial was conducted at the Olimpiada ZIF-3 concentrator (ZIF is a Russian acronym for gold recovery concentrator). This concentrator has a 3mm gravity circuit pre-screen underflow. In the trial a side cut from this gravity circuit pre-screen underflow was directed to a SkimAir 3 test unit.

During the pilot test program the following conditions were investigated:

• Reagent dosing
• Feed % solids
• SkimAir operating parameters

Pilot test results

The pilot test program proved that the Olimpiada gold floats very fast. The measured gold stage recovery for the test unit varied from 50 to 75 % and concentrate gold grade from 68 to 187g/t. The average gold recovery and grade were 62.4% and 111.5g/t, respectively. During the trial the estimated feed rate of the unit varied from 3.8 to 5.8 t/h and feed solids content from 54.6 to 69.7% solids. 

The results proved that the Olimpiada type ore was suitable for the flash flotation technology. The customer decided to proceed with front-end-engineering of a full-scale SkimAir installation. Following the completion of this front-end-engineering and based on the promising pilot test results, Polyus ordered one SkimAir 500 flash flotation cell in 2016. The cell was installed in the Olimpiada ZIF-4 concentrator on one of the parallel processing lines, enabling comparison of lines with and without SkimAir.

Support services

As part of the SkimAir scope, Outotec provided a range of services to help Polyus achieve target results. The services included full on-site cover during installation for commissioning phase, as well as ramp-up of the SkimAir unit to operating mode and optimization of process parameters.

Olimpiada ZIF-4 process

Olimpiada ZIF-4 concentrator has two parallel process lines which process slightly over 1000 t/h of crushed ore. Prior to the implementation of the SkimAir flash flotation cell, both processing lines consisted of a SAG-ball mill grinding circuit. This SAG mill discharge is screened with a 12mm screen. The screen overflow is crushed and recirculated back to the SAG mill.

The screen underflow is fed to a gravity separation stage which is designed to recover the gravity recoverable gold. The tails of the gravity separation are pumped to a secondary hydrocyclone, and these tails are then fed to the ball mill. 

The ball mill discharge is pumped with the SAG mill discharge to the primary hydrocyclone. Overflows of the primary and secondary hydrocyclones are fed to two parallel conventional flotation circuits. The flotation concentrates are further treated in a leaching circuit.

SkimAir in the process

The SkimAir was installed in processing line two of the grinding circuit. The SkimAir’s duty was to process the gravity separation prescreen underflow. The tails of the SkimAir were pumped to the gravity separation, whilst the concentrate went to a separate gravity separation stage. As SkimAir was installed in just one of the two parallel processing lines, it was easy to compare performance between the lines with and without SkimAir technology.

Results

After completion of commissioning and optimisation of the SkimAir operating parameters, several sampling campaigns were carried out. These campaigns were designed to compare the metallurgical performances of the parallel process lines. From the sampling campaigns, the following conclusions were drawn:

• Significant reduction in gold recirculation load: Samples were collected from the grinding and flotation circuits to assess the performances of the process lines. The samples collected from the primary cyclone underflow showed that the line with SkimAir had over 60% less gold in the cyclone underflow compared to the parallel line without SkimAir. This shows that SkimAir very effectively recovers gold which the gravity circuit was unable to recover.
• Reduction in flotation feed head grade: SkimAir was able to recover a significant proportion of the gold which the gravity separation was unable to recover. This was proven by 40% lower gold head grade to the conventional flotation circuit compared to the flotation head grade of the parallel line without SkimAir.
  • Lower gold content in the final tails: The average gold grades of the lines with and without SkimAir were 0.17 and 0.19 g/t, respectively. This shows a fall of 11% in the tails gold content from the line where SkimAir was implemented.
  • Higher overall gold recovery: With less overgrinding and less gold recirculation, the line with SkimAir had close to 2% higher overall recovery compared to the line without SkimAir.

The 2% recovery improvement is significant, resulting in a payback time of just four months for the investment. Polyus decided to invest in five more SkimAir units for all the remaining processing lines at the Olimpiada concentrator plants.

Q1/2019

Over the course of the first quarter of 2019, Polyus continued the roll-out of the flash flotation project at Olimpiada. Currently, the company is conducting a ramp-up of two flash flotation units at ZIF-3 concentrator and proceeding with construction and installation works at ZIF-1 and ZIF-2 concentrators.

*Metso Outotec was formed July 1, 2020 when Metso and Outotec merged into one company. This case study has been written prior to the merger under the old company name.