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Coarse-particle flotation applied to copper sulfide ores

J. Concha1, E Wasmund2, M. Mankosa3

1. Deputy Managing Director, Eriez Flotation Division Peru, jconcha@eriez.com 2. Global Managing Director, Eriez Flotation Divison Canada, ewasmund@eriez.com

3. Vice-President Operations, Eriez Corp. USA, mmankosa@eriez.com

SUMMARY

In recent years, innovation in the mining industry has become increasingly important since it will be a necessity to develop economically attractive projects. Within innovation, a new type of flotation is gaining more and more importance. We are talking about Coarse-Particle Flotation (CPF), for particles larger than 150 µm and up to 850 µm. It is estimated that for brownfield projects this type of flotation could allow the plant capacity to be expanded by up to 25%, with the single implementation of the CPF system, and without the need to install more mills. For greenfield projects, it is estimated that this type of flotation could help reduce operating costs for the entire project by more than 10%, and also help optimize water recovery and tailings disposal, since a portion of these could be disposed of in dry coarse fractions. However, it is known that, for sulfide minerals, the best recoveries are usually obtained when conventional flotation cells process particle sizes in the ranges of 30 µm to 150 µm. In industrial operations, particles that are outside this range are generally reported in tailings, due to the inherent restrictions associated with the physical interactions that occur in the pulp and froth phases of conventional flotation equipment. To overcome these limitations of conventional cells, a new type of flotation cell has recently been developed that works under the fluidized bed principle. With this new cell type, it has been possible to obtain recoveries up to 90% Cu and 89% Mo when a mineral has been floated in the range of 600 µm x 150 µm.

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1. INTRODUCTION

The largest source of copper supply worldwide comes from copper produced through flotation processes. It is estimated that more than 82% of primary copper production comes from concentration by flotation processes. In Peru, the latest mining megaprojects have been concentrator plants (Cerro Verde, Las Bambas, Constancia, Antapaccay, and so on). Another characteristic of copper projects is that the deposit grade has been decreasing, and as demand for this metal has increased, the size of the concentrator plants has also needed to grow. In Peru, the latest copper concentrator plants were designed to have processing capacities greater than 70 ktpd, and up to 240 ktpd. Projects of these dimensions have also brought to discussion if there are new processes and/or technologies that allow optimizing their profitability. In this regard, one of the processes that has been arousing increasing interest is Coarse-Particle Flotation. Floating coarse particles would bring many advantages to a mining project or operation, such as:

• It is estimated that an increase in the P80 of the flotation feed from 200 to 300 µm, for example, could increase the plant capacity by up to 25% (Mankosa et al, 2016). Studies have shown that the HydroFloat® cell can efficiently float particles larger than 150 µm. For copper minerals, recoveries between 85% - 93% Cu have been reported, when minerals have been floated in fractions of 600 µm x 150 µm.

• Floating coarser means grinding coarser too, and this brings significant energy savings, which helps to reduce OPEX and positively impact the profitability of the operation in the end. Some studies (Jameson et al, 2013) show that coarse-particle flotation could bring savings in operating costs of more than 12% (study conducted for a 50-ktpd open-pit project).

• Other benefits would be associated with tailings disposal, and water recovery. For a greenfield project, it is estimated that around 30% of the total processed solids could be deposited as tailings in coarse fractions (e.g. 600 µm x 150 µm). These coarse tailings have a sedimentation rate greater than ten times as compared to a conventional tailings, allowing more efficient recovery of water, and also reducing the losses due to evaporation that usually occur in tailings. Disposal of these coarse tailings could help increase the life of the tailings facility and make it safer, or in the case of underground mines, they could be used for paste backfill.

The benefits of coarse-particle flotation are diverse, so the question is why is it not applied today? The main reason perhaps is that conventional flotation technologies are not efficient at coarse-particle flotation. Figure 1 shows the relationship between particle size vs. recovery of valuable ore for various operations using conventional cells. In this curve it can be observed that, in all cases, for particles larger than 150 µm, the recovery of the valuable element falls sharply. The low recovery in coarse fractions is usually attributed solely to liberation issues. However, various studies have shown that in certain cases despite having an adequate degree of release in coarse fractions (>150 µm), the minerals of interest they cannot be recovered in conventional cells, where the loss of these particles is due to physical and hydrodynamic phenomena inherent to conventional technologies (Concha and Christodoulou, 2014). Factors such as turbulence inside the cells, or the transport from the pulp phase – froth – -bubble-particle aggregate, among others, are phenomena that affect the recovery of coarse particles. To overcome the limitations of conventional cells for coarse-particle flotation, in 2002, Eriez developed a new type of flotation cell called HydroFloat®. This cell works under a fluidized bed principle, and the description of its operation and characteristics will be presented in the section below.

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Figure 1. Conventional flotation data obtained in industrial sulfide flotation circuits (Mankosa et al, 2016)

2. HYDROFLOAT® - COARSE-PARTICLE FLOTATION TECHNOLOGY

The HydroFloat® cell has a tank with three sections: Freeboard, fluidized-aerated area, and dewatering cone.

During operation, the deslimed pulp is fed to the top of the cell tank. At the same time, a controlled flow of process water is added exactly above the conical section of the tank bottom, and this water is added through a network of distribution pipes. The upward water flow creates a fluidized bed of suspended particles, which are primarily hydrophilic gangue (tailings). Finely dispersed air bubbles generated by an injector system are transported by the water to the fluidized bed, where these bubbles are forced to come into contact with the dense bed of solid particles. After the bubble adhesion on the surface of the hydrophobic particles, the resulting particle-bubble aggregate is transported by the upward flow of fluidizing water, until it is collected in the cell launder. While the hydrophilic solid particles continue to pass through the fluidized bed and accumulate in the dewatering cone, located below the ring of fluidization water distribution pipes, to later be discharged.

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Figure 2. Schematic Diagram of the HydroFloat® Cell. The HydroFloat® cell has the following characteristics:

• Processability: >15 t/h-m2. • Flotation Air Consumption: It is about 10% of the air consumed by conventional cells of

similar flotation area. • Reagent Consumption: In some applications, after HydroFloat installation, a 10%

reduction of reagent consumption (collector) has been possible. • Spare Parts: The HydroFloat® cell, since there are no internal moving parts, does not

have the problem of premature abrasive wear. • Feed Particle Size: This cell is being used industrially to float particles up to 6,000 µm

(Phosphates). In the case of Cu sulfides, the ranges usually vary from 600 µm x 150 µm to 850 µm x 150 µm in some cases.

• To optimize performance, it is recommended to work within a particle size distribution of 1:6.

• Solids concentration: The HydroFloat® cell can work with pulps with solid percentage of 40% - 80%. The tailings produced in this cell come out with a solid percentage similar to feed because of the existing dewatering area.

• Release: 3D Tomography and MLA studies report that this cell would require very little exposed hydrophobic surface to recover the mineral of interest. According to these studies, a range from >1% to 10% of the exposed hydrophobic surface would suffice for a particle to be recovered by the HydroFloat® cell. (Miller et al, 2016, Mehrfert, 2017)

• The HydroFloat® cell works as a single flotation stage, that is, it does not need cells in series.

The HydroFloat® cell has been used industrially since 2004. To date, there are more than 50 units in operation, in phosphate, potassium, coal, and diamond plants. The first industrial facility to process copper sulfide ores will be built in 2018.

3. POTENTIAL USES OF COARSE PARTICLE FLOTATION

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As mentioned above, coarse particle flotation has been gaining increasing interest within the mining industry, due to all the potential advantages that can help develop more profitable projects. Some of the possible applications that have been studied include the use Coarse-Particle Flotation as a pre-concentration stage (coarse gangue rejection). A second option is the use of this cell to recover the valuable elements contained in the coarse fractions of the rougher tailings of the plants in operation.

3.1 Pre-Concentration

This application is believed to be particularly important for marginal deposits. Figure 3 shows a simplified flowsheet of the application as pre-concentration. This application will allow to have a coarser grinding. For example, the Overflow of the first cyclone could have a P80 from 400 to 500 µm. Then in the second stage of cycloning, the classification would be made to divide the load that will go to conventional flotation (particles below 150 µm), and the coarse fraction will go to the HydroFloat® cell (particles above 150 µm). However, as the classification process in a cyclone is not efficient, and there is a by-pass of fine particles in the U/F, then an additional classification stage is required, which could be carried out in a hydraulic classifier (e.g. CrossFlow). This is more efficient than a hydrocyclone, in such a way as to ensure that the load that is fed to the HydroFloat® contains a minimum amount of fine particles (<150um). Depending on the P80 used, it is estimated that approximately 35% - 50% of the total processed tonnage can be treated with the Coarse-Particle Flotation technology. This type of application seeks that the tailings generated in the HydroFloat® cell be a final tailings product. For this reason, it aims at recoveries usually higher than 85% Cu at this stage. The HydroFloat® cell concentrate will be a pre-concentrate. On average, the enrichment ratio for copper sulfide ores is around 4. For example, if you had a marginal mineral with a head grade in the coarse fraction of 0.25% Cu, the coarse pre-concentrate would have a grade of around 1% Cu. This pre-concentrate will need to be dewatered, and then go to coarse regrinding for improving the liberation, then join with the conventional rougher concentrate and then be sent to regrinding to later be cleaned in the cleaner stages of the conventional plant. With this type of application, it is estimated that around 30% of the processed solids could be disposed of as tailings in a coarse fraction, facilitating the recovery of water, and optimizing its final disposal in the tailings facility.

Figure 3. Flowsheet for coarse-particle flotation as pre-concentration

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3.2 Rougher tailings recovery

As shown in Figure 1, a large part of the losses occurring in a concentrator plant is related to coarse particles (>150 µm). Depending on the P80 at which the plant operates, it is estimated that around 30% - 50% of the Cu that is lost in the rougher tailings would be contained in the coarse fractions. So, recovering the valuable elements contained in these fractions can represent a good opportunity for mining companies to optimize the performance of their plants, and therefore their profitability. Laboratory and pilot scale studies have shown that through Coarse-Particle Flotation it is possible to recover between 60% - 90% of the Cu contained in the coarse fractions of rougher tailings. This would allow to increase the global recovery of the plant by 2 to 5% of Cu. This type of arrangement has proven attractive to plants that are in operation since it does not require changes to the existing flowsheet. The construction, commissioning, and operation of the new plant for the recovery of coarse particles from the tailings could be carried out without interfering with the normal operation of the existing plant. As shown in Figure 4, the rougher tailings will need to be classified to remove fine particles (<150 µm) and this fine fraction will be disposed of as usual (final tailings), while the coarse fraction (>150um) will be fed to the HydroFloat® cell. The coarse concentrate produced in this application usually have grades similar to the grade of fresh head. As the HydroFloat concentrate contains a large amount of coarse mixed particles, it will need grinding to improve the liberation, and then it will be fed to the conventional rougher cells, or a new “rougher” flotation circuit can be implemented just to clean this concentrate.

Figure 4. Flowsheet for coarse-particle flotation for rougher tailings recovery

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4. EXPERIMENTAL RESULTS

Currently Coarse-Particle Flotation is being studied by the main copper mining companies, as well as by the mineral processing departments of the main universities. Eriez has been studying this process for more than 18 years, and in the case of sulfide copper minerals, studies have been carried out since 2013. Currently, studies have been carried out in more than 9 countries, and more than 20 different types of minerals have been studied. A large number of these studies have been developed in Peru, and the summary of these is presented in Figure 5. This Figure shows that mostly for enrichment ratios of 4, the copper recovery ranged from 85% to above 90%. Higher enrichment ratios have been obtained; however, these have been specific cases, and above all it has been due to the presence of native copper, or other types of copper-rich minerals. This work will discuss tests carried out with fresh ore where Coarse-Particle Flotation has been used as a Pre-Concentration stage, and also tests conducted to recover valuable elements from the rougher tailings.

Figure 5. Enrichment ratio vs. recovery. Data obtained in laboratory scale tests carried out with Cu minerals from deposits located in Peru

4.1 Equipment

The laboratory tests performed in Peru were conducted on a HydroFloat® HF-150 cell. It is an automated cell, where bed control is carried out through a loop that has a pressure transmitter, a local controller, and a pinch valve. The control of the flotation air and fluidizing water is done manually. This cell allows continuous testing, that is, feeding, obtaining concentrate and tailings is performed continuously. This type of equipment can simulate most of the parameters expected to be used on an industrial scale. In addition, it allows obtaining results similar to those expected in an industrial facility.

4.2 Pre-concentration Tests

This study was condcuted for a greenfield project located in the South of Peru. The sample received went through the preparation stages that are described in Figure 6.

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Table 1. Chemical assays of the sample studied

Figure 6. Scheme of the sample preparation procedure to perform coarse-particle flotation tests as a pre-concentration

The bulk sample as well as the fine and coarse fractions generated during sample preparation were assayed, and the results are shown in Table 1. As can be seen, it is a sample with a low head grade, 0.255% Cu, which could be considered “marginal”. In this regard, the objective of the tests was to determine if the Coarse-Particle Flotation could be used as a pre-concentration stage, in such a way that it allows obtaining a head grade for conventional flotation, with a grade greater than 0.5% Cu. Sample preparation consisted of classification into two fractions:

• Coarse fraction (600 µm x 150 µm) to perform Coarse-Particle Flotation tests using the HydroFloat® cell

• Fine fraction (<150 µm), sample stored for conventional flotation tests using the Denver cell.

Cu(%)

Au(g/t)

Fe(g/t)

Fraction 600 um x 150 um (HF Feed) 60% 0.202 0.011 5.345Fraction <150 um (Storage for Conventional Flotation) 40% 0.336 0.019 6.220

TOTAL (Bulk Sample) 100% 0.255 0.014 5.693

Size Fraction DescripctionGrades

Weight (%)

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The assays showed that in the fine fraction, a natural pre-concentration occurs during the classification, going from a head grade of 0.255% Cu to a grade of 0.336% Cu, and the grade in the coarse fraction was 0.202% Cu. It was also observed that approximately 60% (w/w) of the mass was reported to the coarse fraction. The results of the CPF test works are presented in Figure 7.

Figure 7. Coarse-Particle Flotation test results – Pre-concentration

The results achieved for this type of mineral were encouraging. The recovery of Cu was 93%, and an enrichment ratio of 13.5 was obtained. In addition, the mass pull was 6.8%, being below the average obtained in studies with other ores. That means, from early stages, and in coarse fractions, approximately 55% (w/w) of the total feed to the plant could be disposed of as final coarse tailings, which would mean significant savings in both the CAPEX and OPEX for the project.

Figure 8. Simplified scheme of rougher grinding and flotation stage WITHOUT pre-concentration

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Finally, Figure 9 shows the results of the evaluation in terms of grades and flows, which would mean to implement a of a Coarse-Particle Flotation stage as Pre-Concentration in a 70,000 tpd Cu project.

Figure 9. Simplified scheme of rougher grinding and flotation stage WITH pre-concentration

Figure 8 shows the implementation of a project without pre-concentration, vs. Figure 9 that shows the same project including a pre-concentration stage. One of the main differences is the feed grade and the tonnage that would enter the conventional rougher flotation. Without pre-concentration, 70,000 tpd would be fed with a grade of 0.255% Cu, while in a plant with pre-concentration only 30,880 tpd would be fed with a grade of 0.561% Cu. In addition, a plant with pre-concentration would allow the disposal of around 39,120 tpd of coarse tailings at early stages, which could be easily dewatered and the possibility of disposing them in dry form could even be evaluated.

4.3 Rougher Tailings Recovery Testing

One of the recurring problems in most concentrator plants is the significant loss of valuable elements in particle size fractions coarser than 150 µm. As mentioned at the beginning of this document, these losses occurred because of issues related to the mineral liberation needed to be recovered in conventional cells as well as the principle of operation of conventional cells. Conventional cells, due to the turbulence generated by rotors promote loss of adherence of the coarse particles that were collected in the air bubbles. Despite this, copper concentrating plants have been increasing the P80 at which they perform flotation, and currently in Peru most of them have been working with P80 (in rougher stages) greater than 150 µm, even up to 250 µm.

This issue was observed in one of the large mining Cu/Mo concentrator plants in southern Peru. A sampling program of the plant allowed identifying that approximately 47% of copper and 43% of moly that is lost in the rougher tailings is contained in particle sizes coarser than 150 µm. Knowing this, it was decided to take a sample from the rougher tailings, and to carry out Coarse-

Tonelaje (tpd) 70,000Ley (% Cu) 0.255PS (um) - P80 400 - 500

O/F Ciclon 1Tonelaje (tpd) 28,000Ley (% Cu) 0.336PS (um) < 150

Fracción Fina

Tonelaje (tpd) 42,000Ley (% Cu) 0.202PS (um) >150

Fracción Gruesa

Tonelaje (tpd) 2,880Ley (% Cu) 2.728PS (um) >150

Concentrado HFTonelaje (tpd) 39,120Ley (% Cu) 0.016PS (um) >150

Relave Final HF

Tonelaje (tpd) 2,880Ley (% Cu) 2.728PS (um) <150

Concentrado HF Molido

Tonelaje (tpd) 30,880Ley (% Cu) 0.561PS (um) <150

Alimento Rougher

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Particle Flotation tests to try to recover the valuable minerals contained in this fraction of the tailings. Figure 10 shows the scheme of the procedure as the tests were conducted, and Table 2 shows the grades of the rougher tailings, and of each of its fractions, coarse (>150 µm) and fine (<150 µm).

Figure 10. Scheme of the coarse-particle flotation test procedure for tailings recovery

Table 2. Characterization of the rougher tailings studied

The rougher tailings grade was 0.105% Cu. However, upon classification, the grade in the coarse fraction (>150 µm) of this tailings was 0.162% Cu. Using this coarse fraction, the Coarse-Particle Flotation tests were conducted. The results of the CPF test are presented in Figure 11.

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Figure 11. Coarse-Particle Flotation Test Results for Tailings Recovery

The recovery achieved in the Coarse-Particle Flotation stage was 90% Cu, 65% Mo and 74% Ag. The enrichment ratio was 3.3 Cu, 2.4 Mo, and 2.7 Ag. The mass pull was 27%. The grade of the concentrate produced was 0.532% Cu, which is similar to the grade of the fresh ore of this plant. These results are encouraging and have led to additional studies to evaluate the different mineralogies of this plant, and based on this, develop a business case that allows evaluating the feasibility and profitability of applying this type of process. In order to determine what would be the effect on the global recovery of a Coarse-Particle Flotation stage, a preliminary balance of the tailings flotation was carried out, and is presented in Figure 12.

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Figure 12. Global Balance of tailings recovery through a Coarse-Particle Flotation process

Figure 12 shows that Coarse Particle Flotation could likely recover 33% of the copper that is currently being lost in the rougher tailings at this plant. In addition to recovering 16% of Molybdenum and 14% of Silver. The Coarse-Particle Flotation would allow the generation of a new rougher tailings whose Cu grade would be 0.075% Cu, as compared to the current condition that produces tailings with grades of 0.105% Cu. In terms of global recovery of the plant, this would mean increasing the total copper recovery by more than 3 percentage points, which for a plant of this magnitude, would mean additional sales of approximately US$ 37 million per year, considering current copper prices.

5. CONCLUSIONS

Coarse-Particle Flotation presents a series of potential benefits to optimize and make mining projects and operations more profitable. Although it is a new technology for flotation of copper sulfide ores, it has already been applied successfully for more than 14 years in other industries (phosphates, potassium, etc.), and its mechanical, structural, and control designs have been already validated.

As has been seen, the HydroFloat® Cell can be used in a pre-concentration stage to obtain recoveries of up to 90% Cu by floating fractions of 600 µm x 150 µm for the evaluated case. For this same application, enrichment ratios greater than 10 were obtained. The application of this technology for the studied project would allow increasing the head grade that would be fed to

Grade Cu (%) Grade Mo (ppm) Grade Ag (g/t)Distribution Cu (%) Distribution Mo (%) Distribution Ag (%)

Weight (kg) Weight (%) Size Fraction (µm)

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the conventional rougher flotation from 0.255% Cu (without pre-concentration), to a grade after the pre-concentration of 0.561% Cu, and reducing tonnage fed to conventional flotation from 70,000 tpd (without pre-concentration) to 30,880 tpd (with pre-concentration). In addition, approximately 39,000 tpd of tailings could be disposed of at early stages of the process, and in a coarse fraction (600 µm x 150 µm) that will improve the water recovery process and safer disposal of said tailings.

As regards of the tailings recovery application, it is seen that it is possible to recover 33% of the copper that is being lost in the rougher tailings at the studied plant. After classifying the tailings, the coarse fraction (>150 µm) had a grade of 0.162% Cu, and from this fraction a concentrate of 0.532% Cu was produced. It is estimated that an application of HydroFloat® technology would allow the plant's global copper recovery to be increased by more than 3 percentage points, allowing additional sales for the company of approximately USD 37 million a year, considering current copper prices.

The application of this technology should be evaluated on a case-by-case basis, since its application will depend on the liberation degree that each type of mineral has, mineralogy, etc.

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