Postęp w obiegach mielenia z młynami HIG (Wysokiej Intensywności Mielenia) produkcji Outotec

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Recent progress of regrinding circuits with Outotec

HIGmills

Ville Keikkala1), Harri Lehto1), Ilkka Roitto1)

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Outotec (Finland) Oy,Espoo Finland,

ville.keikkala@outotec.com; harri.lehto@outotec.com; ilkka.roitto@outotec.com

Abstract

With better liberation the diminishing ore bodies can be used more efficiently. Regrinding circuit effect is essential, and can boost the performance of the flotation circuits, when performed and controlled properly. While tumbling mills can do the work in some extent, new and more energy efficient ways of grinding needs to be utilized. Outotec has been doing extensive test work with different size HIGmill units, previously used only in the white minerals processing applications. The outcome of the tests is very promising and has gained a broad interest. Especially energy efficiency, space saving and versatile ways of running the new HIGmill regrinding circuit are in common interest. This paper discusses the latest results acquired from the different size mills test work. It also includes latest information of the installed plant size unit with the erection process. Paper also gives outlook for the controlling philosophy of the regrinding circuits with the HIGmill, and discusses the main benefits related to unique technology advantages with a HIGmill.

Key words: grinding, fine grinding, vertical mill, energy efficiency, regrinding

Postęp w obiegach mielenia z młynami HIG

(Wysokiej Intensywności Mielenia) produkcji Outotec

Streszczenie

Lepsze uwolnienie pozwala na efektywniejsze wykorzystywanie malejących zasobów rud i uzyskiwanie coraz wyższych wskaźników w obiegach flotacji, o ile tylko ich ruch prowadzony jest prawidłowo, a obiegi są właściwe kontrolowane. Mimo, że młyny bębnowe w pewnym stopniu wykonują swoją pracę, istnieje silne zapotrzebowanie na nowe i bardziej wydajne sposoby mielenia. Outotec wykonał wiele badań z młynami HIG (Wysokiej Intensywności Mielenia) o różnej wielkości, które poprzednio stosowane były tylko do przeróbki „białych” minerałów. Wyniki tych testów są bardzo zachęcające i wzbudziły szerokie zainteresowanie. Szczególnym zainteresowaniem cieszą się takie kwestie jak efektywność energetyczna, niskie zapotrzebowanie na powierzchnię oraz różne możliwości pracy obiegów domielania, w których wykorzystywane są młyny HIG. W niniejszej pracy omówiono najnowsze wyniki uzyskane w badaniach prowadzonych z wykorzystaniem młynów różnej wielkości. Podano także najnowsze informacje o młynach zainstalowanych w zakładach. Ponadto, w pracy dokonano przeglądu filozofii sterowania obiegami domielania z wykorzystaniem młynów HIG oraz omówiono najważniejsze zalety technologiczne, wynikające z ich zastosowania.

Słowa kluczowe: mielenie, drobne mielenie, młyn pionowy, efektywność energetyczna,

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V.Keikkala, H. Lehto, I. Roitto, Recent progress of regrinding circuits…

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Introduction

Since 2012 Outotec have had a new fine grinding technology, trademark named the “HIGmill”, which comes from High Intensity Grinding mill. This technology has roots in the calcium carbonate industry and is a well proven technology with over 30 years of history. Previously the HIGmill had not been available for hard rock minerals processing duties. Outotec has now made the technology available to this industry, which has included conducting extensive test work in different laboratories worldwide, utilising several different size HIGmills.

The HIGmill is used essentially in regrinding circuits to obtain maximum recovery and improve grade from mineral ore bodies. Increased demand for better mineral liberation is due to diminishing ore bodies. Outotec’s test HIGmills are permanently located in Finland and Austria. In addition to these permanent units, there are several other test HIGmills which can be used for testing. Each country has a different size test mill units, which can be used depending upon the sample size and material available. Test units include one mill installed in a transportable container, which can be delivered to a customer site and connected to the existing process for continuous pilot testing. Multiple other HIGmills (comprising 3 different sizes) are available on request and can be installed in a local laboratory or at a mine site’s own laboratory for longer and thorough testing.

1. HIGmill operational principles

The HIGmill circuit usually consists of a scalping cyclone cluster prior to the mill (Fig. 1). The purpose of this cyclone cluster is to remove fine particles already existing in the feed stream to prevent overgrinding and the creation of unwanted slimes, plus to maintain the ideal milling density. The feed slurry is pumped to the cyclone cluster and the underflow (U/F) is directed to the HIGmill (actual plant data, Fig. 2). The overflow (O/F) of the cyclone cluster is usually combined with HIGmill product and directed to the downstream flotation or leaching stage. Before slurry enters the HIGmill, process make-up water is added to the feed tank to control the density so that the best possible grinding conditions are achieved. The slurry with the correct density (40-60% solids) is pumped to the HIGmill through the bottom feed inlet flange of the mill. Inside the grinding chamber are located the rotating mill shaft with grinding discs attached, stator rings mounted to the shell walls and grinding media beads (typically ceramic, but also steel media can be utilised).

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Fig. 1. HIGmill circuit [1]

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___________________________________________________________________________ When the slurry enters the HIGmill via the bottom feed inlet flange it is immediately brought into contact with the grinding media beads. The larger mineral particles are directed to the high media concentration zone at the shell periphery, where they work their way upwards and are progressively reduced in size by the multiple grinding stages between the discs and stator rings (Fig. 3). During time in the high media concentration zones, the distance travelled by the particles is greatest where also the most severe attrition forces exist for efficient grinding. Once the particles are ground to the final product size, they travel upwards near the shaft where there is no media through specifically designed openings in the grinding discs. This minimises particle overgrinding and results in a steep particle size distribution and more desirable conditions for downstream flotation or leaching processes.

Fig. 3. HIGmill grinding principle [1]

The process through the mill is a single pass, commonly referred to as open circuit. Open circuit configuration is simple to control because there are no high flow recirculating loads that need to be handled. After time in the mill the grinding beads become seasoned with a full distribution of media sizes. This ensures constant contact between the particles and media beads remain at all times, which improves the grinding efficiency. Gravity keeps the media and slurry particles compacted together, ensuring the whole mill volume is efficiently used for grinding. The grinding media beads are automatically added to the HIGmill feed tank, along with the feed slurry, and are controlled by the media consumption g/kWh.

Media beads cannot exit the mill until they are the same size as the final product, because at the top of the rotating shaft is located a hydro-classifier that directs coarse media and particles back into the grinding chamber of the mill. During start-up, shutdown and normal operation detailed checks have been performed on all mill sizes ranging from laboratory to production size units and there has been no recorded media bead loss due to the high efficiency of the hydro-classifier. Pumped slurry under pressure prevents media beads flowing backwards into the feed

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return valve, which prevents the media beads flowing backwards when the feed flow and mill have been stopped.

2. HIGmill Control Philosophy

Ore changes constantly during normal plant operation, which causes fluctuations to the feed flow and demands more flexibility from the regrinding circuit. All HIGmills are supplied standard with a variable speed drive (VSD) to alter the mill shaft speed and grinding disc tip velocity. By continuously controlling the grind size and quality of the grind, upstream ore body variations do not negatively affect downstream flotation or leaching performance.

A distinct advantage of the HIGmill is that there is not simply a single method of control philosophy available. Different control strategies can be implemented depending upon the requirements of a specific operation. The main control philosophy of the HIGmill utilising the advantage of the variable speed drive for mill speed control can be selected from the following:

 Specific grinding energy (SGE)  Particle size distribution  Main motor power

SGE based control is obtained from laboratory tests or actual plant data. A predetermined SGE setpoint value in kWh/t (per dry tons) is used to vary the mill speed.

The second option for controlling the mill is from an online particle size, P80 measurement. When Outotec’s PSI500 (particle size analyzer) is installed in the HIGmill circuit, the operation obtains continuous online feed and product particle size distributions. If the product becomes too coarse the mill speed will be increased to control to the P80 setpoint and vice versa if the product becomes too fine.

The third method of mill speed control is by the main motor power draw measurement. In this case, the mill keeps the power in kW constant, no matter what else occurs within the circuit.

There are several other parameters which can be altered in order to reach the optimal grinding results. Comparing similar ores and concentrates already examined as part of the extensive Outotec testing database can also be used to select initial operating parameters. Thorough testing of the following parameters is used for further operation optimisation.

 Filling level of media beads (normally between 60-70%)  Grinding media bead size

 Size distribution of media beads

 Quality and material of beads (typically ceramic or steel)  Slurry density by weight and volume

 Slurry viscosity  Feed flow rate

 Mill motor power draw  Cyclone pressure.

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3. Benefits 3.1. Available sizes

Over 200 HIGmills have been installed to date, with orders now received for 9 HIGmills in hard rock, mining applications, including the large 5 MW units. The largest mills installed have power up to 5.5 MW, making it the largest fine grinding technology on the market. There is a range of mills from 400 to 27,500 litres net volume with corresponding motor drives from 132 to 5.5 MW. Outotec’s HIGmill is the only stirred bead mill on the worldwide market today which can use ceramic grinding media in units above 3 MW in motor size.

3.2. Small footprint

As can be seen from the first commercial installation in a hard rock mining duty, the vertical configuration footprint is compact, making it easy to retrofit into the tight confines of a brownfield plants (Fig. 4). The mill shell shown in Fig. 4 is flanged and hinged vertically for easy access to the normal wear components and ensuring maintenance activities do not require much additional space around the mill. On large mills the shell halves are split vertically using hydraulics. The mill is supported from the top structure, allowing feed pipework to enter the bottom inlet flange and providing good access for media collection prior to a mill inspection. In addition, height above the mill is not required, because the mill shaft with grinding discs attached can be lowered to the ground with permanent electric winches incorporated with the mill design.

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3.3. Maintenance

When the mill has been stopped and the media beads drained from the grinding chamber, the mill wear (and condition) can be checked from the bottom of the mill through an inspection hatch. If normal wear component replacement is required the mill shell is opened via the vertical flanges. If only the bottom grinding discs need replacement, the work can be conducted with the main shaft remaining in place. If more grinding discs require replacement, the shaft can be lowered to the ground and new discs fitted, or alternatively an entire spare shaft with new discs can be fitted in place of the old shaft which would then be taken to the workshop for repairs at a convenient time. For this general maintenance work only two skilled personnel are needed.

3.4. Operational Flexibility

The HIGmill can be used in many applications in the mining industry, including regrinding of concentrates, tertiary grinding, precious metal fine grinding for hydrometallurgical processes and ultra-fine grinding. The flexibility of the HIGmill to adapt to process fluctuations in these applications is excellent.

Both ceramic and steel media grinding beads can be used in the HIGmill. Ceramic media is used when iron contamination is an issue for the downstream metallurgical chemistry.

Depending upon the application, different size media beads are available. That is, 0.5-1.5 mm diameter beads for ultra-fine grinding, 1-3 mm diameter beads for fine grinding and 3-6 mm diameter beads for coarser grinding, where the grind size is defined as follows:

 Coarse range, F80 100-300 µm, P80 50-100 µm.  Fine range, F80 50-100 µm, P80 20-60 µm.  Ultra-fine range F80 <70 µm, P80 <20 µm. Typical process parameters for industrial operations are:

 Feed solids 20-30% by volume (40-60% by weight depending solids SG)  60-70% of mill free volume filled with media beads

 Typical media beads are ceramic (zirconia-alumina-silicate, SG ~4 kg/dm³)  Media bead size 0.5-6 mm depending upon the duty and F80 / P80

 Tip speed 8-14 m/s

 Typical retention time 1-3 minutes

 Specific grinding energies from 5 up to > 100 kWh/t (no temperature limitations due to hardened steel lining, compared to other rubber or polyurethane lined stirred mills on the market)

 Power intensity 100-200 kW/m³.

4. First installed HIGmill

The first full-scale, plant size, HIGmill with 700 kW motor was recently installed, commissioned and is operating as part of a brownfield upgrade project at a copper/nickel mine to regrind copper rougher/scavenger concentrate (Fig. 5). The total installation and commissioning lasted only 10 days after which the mill was ready for full operation and producing design product quality (P80 = ~20 micron).

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___________________________________________________________________________ These 10 days were made up of 6 days for assembly and erection, plus only 4 days for commissioning by a single Process Engineer.

The vertical HIGmill design meant it was relatively straightforward to select a suitable location within the already constricted confines of the existing concentrator. The mill was commissioned and brought into the production line while the plant was in full operation. There was no requirement to stop the upstream or downstream processes. Furthermore, the HIGmill can be bypassed easily and taken offline separate to the rest of the process, allowing inspection and maintenance work to be conducted as needed. The first opening of the mill will take place around 3 months after the initial start-up in February/March of 2015.

Fig. 5. HIGmill installation location

All the operational sequences and controlling parameters, including start-up, shutdown, media bead addition, mill speed, product sizing and main motor power control are automated. The control room operator is not required to adjust the HIGmill manually at all, which is a significant operating advantage for a newly introduced technology.

Right after commissioning the new HIGmill regrind circuit was achieving the designed product sizing, P80 of ~20 µm at a specific grinding energy (SGE) in the order of 12-15 kWh/t. A further reduction in SGE can be expected after additional circuit optimization and once the ceramic media bead charge becomes fully seasoned.

A seasoned media charge with a full size range of beads will allow energy efficient grinding of the full size distribution of minerals particles present in the feed stream.

The initial metallurgical results were encouraging with the cleaner circuit recoveries improving and the copper grade increasing by up to 2% soon after the HIGmill was operational. Ongoing work to optimise the process parameters will likely improve the metallurgical results even further. In addition, due to these regrind circuit improvements, the primary mills throughput could be increased with the HIGmill now improving mineral liberation.

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Summary

The first commercial Outotec HIGmill unit in a hard rock mining duty is now up and running at a large copper/nickel concentrator. According to the data from the Cu/Ni plant the HIGmill stirred bead mill technology give very promising results. The versatile process is simple to operate, control and maintain. There have been no plant stops or problems due to the HIGmill since it commenced operation in February/March of 2015. The HIGmill was installed, commissioned and operational, producing designed product size, within a total of only 10 days. Overall, the HIGmill performance was in line with expectations and further circuit optimisation work will be ongoing to still improve the results.

References

[1] Outotec (2012), Internal reports and presentations, 2012. [2] Outotec (2013), Internal reports and presentations, 2013. [3] Outotec (2015), Internal reports and presentations, 2015. [4] Actual plant data from the 1st installed unit to Cu/Ni plant, 2015.

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