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Molybdenum Flotation Circuit and Flotation Cells D Meadows1, D Jensen2, F Traczyk3, S Yu4 and L Riffo5 1. Vice President of Global Process Technology, FLSmidth SLC Inc, Salt Lake City, Utah, USA. Email: [email protected] 2. Director of Global Process Technology, FLSmidth SLC Inc, Salt Lake City, Utah, USA. Email: [email protected] 3. Director of Flotation Products, FLSmidth SLC Inc, Salt Lake City, Utah, USA. Email: [email protected] 4. Senior Metallurgical Engineer, FLSmidth SLC Inc, Salt Lake City, Utah, USA. Email: [email protected] 5. Senior Metallurgical Engineer, FLSmidth Chile, Santiago, Chile. Email: [email protected]

ABSTRACT With the steady increase of molybdenum demand worldwide and the favorable molybdenum market, many technological innovations on both process and equipment have been developed and applied in many modern molybdenum plants around the world. This paper introduces the most commonly used molybdenum flotation flowsheet, together with the introduction of the flotation chemistry. Many different types of flotation cells are used in modern moly plants, and each type of flotation cell has its own unique advantages suited for particular ore and process conditions. This paper tries to provide some insight and guidelines in flowsheet development and flotation cell selection in the molybdenum plants.

INTRODUCTION Molybdenum occurs in approximately 12–14 different recognizable minerals, but only molybdenite (MoS2) is the mineral bearing economic importance. Molybdenite can be recovered from either primary molybdenum deposits or copper-molybdenum deposits. In a primary molybdenum deposit, molybdenite is floated directly from gangue minerals. In byproduct molybdenum deposits—which are typically associated with copper—the molybdenite is separated from the bulk concentrate, with copper minerals depressed by NaHS or similar depressants. Molybdenite typically has a platy particle shape and exhibits intrinsic hydrophobicity from its crystalline structure. Consequently, only fuel oil is required as the molybdenum flotation collector to further boost the recovery. Some other depressants may also be required to depress other impurities in the ore feed. Today, around 88% of the total molybdenum produced in the world is still used as metal alloy in metallurgical applications. Since the early 1960s, the world molybdenum production steadily increased from approximately 40,000 metric tons to 250,000 metric tons (in 2011). It is projected that the molybdenum demand will continue to grow at a rate of around 4.6% per year until 2016, and that the price will remain above the historical levels. With these favorable market conditions, many technological innovations for both process and equipment have been developed. This technology is mainly targeted to increase molybdenum recovery and minimize the production cost. These developments are mainly related to flotation chemistry and mineral characteristics, which will be introduced in the following sections. The flotation circuit design can vary depending on the flotation chemistry and different types of molybdenum properties (either primary molybdenum or byproduct molybdenum). The most typical molybdenum flotation circuits will be introduced in later sections, together with a recommended molybdenum flotation flowsheet. Many different flotation cells are used in modern molybdenum plants. Some of the flotation cells are designed specifically for the unique flotation chemistry and certain molybdenite mineral characteristics. The most commonly used flotation cells will also be introduced in later sections.

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FLOTATION CHEMISTRY Molybdenite typically has a platy particle shape, from its crystalline structure it shows hydrophobic on its planar surfaces but hydrophilic on its edge surface. Because of this, only fuel oil will be required as the collector to further improve the molybdenum recovery in a primary molybdenum plant. Other depressants could be required to depress other easily floatable gangue minerals. The commonly used depressants for primary molybdenite ore include,    

Lime to adjust pH Iron depressant Orform D8 Nokes as main copper and lead depressant Sodium silicate for non-sulphide gangue depression.

The flotation chemistry is more complex for byproduct molybdenite plants. The feed to byproduct molybdenum flotation contains mainly copper minerals, with minor amounts of gangue and certain amounts of molybdenite. A strong depressant, typically sodium hydrosulfide (NaHS), or to a less extent sodium sulfide, will be used to depress the bulk of the copper mineral. Currently, the most commonly used depressant is still NaHS, which is added in the molybdenum rougher conditioner and scavenger and produces an HS- ion that consequently strips the xanthate from the surface of copper minerals and renders them hydrophilic. To maintain a sufficient HS- ion concentration in the pulp phase, the pulp must have the right combination of redox potential (ORP) and pH value, as shown in Figure 1. (The Eh in Figure 1 is based on a standard hydrogen electrode.)

Fig. 1 - Eh-pH Diagram of NaHS System (After Pourbaix)

A sufficient dosage of NaHS has to be added in order to maintain the required ORP. For sufficient copper minerals depression, an ORP between -400 and -600 mV (measured against an Ag/AgCl Pt electrode) is typically required in the molybdenum flotation circuit. To reach such a pulp potential, a plant typically can consume two to ten kilograms or more NaHS per metric ton of bulk concentrate. HS- concentration in the pulp phase is also influenced by slurry pH. Figure 2 shows the relative molar fraction between S2-, HS- and H2S against pH value. At a high pH of 11, the concentration of HSbegins to decrease and the concentration of S2- begins to increase, which will negatively influence the Metallurgical Plant Design and Operating Strategies (MetPlant 2013) 15 - 17 July 2013, Perth WA

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molybdenum separation. The optimum pH value for molybdenum separation should be between 9 and 11. Some plants run pH levels above 11 because of the high pH requirement of the bulk flotation circuit, especially when there is a considerable amount of pyrite to depress. In case the pH value from bulk flotation is too high for optimum molybdenum flotation, some acid agents (CO2 or H2SO4) can be used to reduce the pH in the molybdenum rougher conditioner. Amelunxen (2009) illustrated that CO2 has improved flotation performance compared with H2SO4 at the same pH value, which explains why more plants use CO2 to adjust pH for molybdenum rougher flotation. Another benefit is that CO2 is a mild acid-generating agent and has much slower kinetics in reducing pH value. Consequently, it is less likely to overdose and bringing pH lower than the designed value, which is a potential safety hazard, especially with open tank cells.

1.000 0.900

Molar Fraction (0/1)

0.800

H2S

HS-

0.700 0.600 0.500 0.400 0.300

S-2

0.200 0.100 0.000 1

2

3

4

5

6

7

8

9

10

11

12

13

14

pH (H2S)

(HS-)

(S-2)

Sumatoria

Fig. 2 - Hydrogen Sulphide Species with pH Value (After Juan Aravena, originally from Pacific Chemical Brochure)

NaHS tends to oxidize quickly with the oxygen, either from the air bubbles or the dissolved oxygen in the pulp phase. With fresh air as the flotation gas supply, significant amounts of NaHS will be wasted from this oxidation, leading to a very high NaHS consumption. To reduce NaHS consumption, nitrogen or inert gas is used as the flotation gas supply. The Delany Patent (1972) showed that using inert gas or nitrogen rather than fresh air can cut the NaHS consumption in half. Nitrogen was first tested in enclosed WEMCO® cells for byproduct molybdenum flotation at Anamax Mining Company in 1983. The test resulted in increased rougher molybdenum recovery and reduced NaHS consumption. Inert gas is produced in an enclosed system, and the initial ambient air inside the cell is recycled as the flotation gas supply. The oxygen in the air is gradually depleted from the reaction with NaHS. The inert gas typically contains 2–6% oxygen, minor amounts of H2S depending on pH, and the remaining nitrogen. Current field operations have also indicated that at least 30–50% of the NaHS can be saved by using inert gas or nitrogen. Today, nearly all byproduct molybdenum plants using NaHS use either nitrogen or inert gas in the flotation. There is a recent trend of running a low pH in a byproduct molybdenum flotation circuit, which was proposed by Christensen (2012) and became a popular practice in South America. The pH is typically between six and eight, which is intended to generate large amounts of H2S inside the enclosed cells. With a higher concentration of H2S gas inside the molybdenum cell, the aqueous H2S concentration in the slurry phase will also increase from the chemical equilibrium. This increases HS- concentration in the pulp phase and forces the HS- curve in Figure 2 to shift to the left. This low pH practice has significantly improved the molybdenum flotation kinetics and selectivity. Consequently, less retention time and fewer stages of flotation are required to produce the targeted concentrate grade. This considerably reduces the capital costs and the floor area required in the molybdenum plant. With Metallurgical Plant Design and Operating Strategies (MetPlant 2013) 15 - 17 July 2013, Perth WA

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fewer cleaner stages, the recycle streams are also smaller, which is beneficial for the overall molybdenum recovery. Molybdenum plants running with this scheme also reported an improved molybdenum recovery. H2S gas will be produced by any molybdenum flotation circuit running with pH below 9, and a gas collection and scrubbing system should be designed and incorporated into these plants. The collected H2S is typically treated with caustic solution in the gas scrubber. This low pH practice in the byproduct molybdenum plants requires completely enclosed flotation cells so the generated H2S will not leak out to the environment. In such plants, extra safety measures are required to be taken during operation and maintenance.

MOLYBDENUM FLOTATION FLOWSHEET Primary molybdenum circuit The processing of primary molybdenum ore typically includes crushing, grinding, multi-stage flotation and multi-stage regrind. Due to the lower head grade in the flotation feed, primary molybdenum plants typically have more stages of flotation and regrind, as indicated in Figure 3. Besides rougher tailings and the first cleaner scavenger tailings, tailings from all other cleaner stages typically have a counter current recycle in order to maximize the overall molybdenum recovery.

Fig. 3 - Flotation Section of a Typical Primary Molybdenum Plant

By-product molybdenum flowsheet The current molybdenum production in North America and South America comes mainly as a byproduct of copper concentrators. The flotation chemistry is more complex due to the need to depress the majority of copper minerals in the bulk concentrate feed. This paper will mainly focus on the byproduct molybdenum circuit. In a typical byproduct molybdenum plant, the feed is the bulk concentrate generated from the copper flotation circuits. Because the majority of the feed is copper sulphide mineral, a strong copper mineral depressant is necessary to dissolve the collector absorbed Metallurgical Plant Design and Operating Strategies (MetPlant 2013) 15 - 17 July 2013, Perth WA

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The main benefits of this flowsheet developed by Thompson are slurry density control and flexibility. An intermediate thickener can be used after the first molybdenum cleaner to remove the excessive water from later cleaner stages. The water quality of the bulk thickener overflow is difficult to control because it is typically not desirable to add flocculant in the bulk concentrate thickener, considering its possible effect on the downstream copper molybdenum separation. A clarifier is designed to take the bulk concentrate thickener overflow in case the water from the bulk thickener overflow does not meet quality standards. Depending on the molybdenum content of the clarifier underflow, it can be sent to the copper concentrate thickener or recycled back to bulk thickener for further recovery. Depending on the solids concentration of the first molybdenum cleaner concentrate, the concentrate can be sent to the second cleaner or to the intermediate thickener for water control. Because it is typically difficult to obtain sufficient and representative test samples for any Greenfield byproduct molybdenum project, some assumptions have to be made for the circuit design. This makes circuit flexibility even more important.

TYPES OF MOLYBDENUM FLOTATION CELLS & THEIR SELECTION In current molybdenum plants, the trend has been to move away from column cells and use a combination of mechanical cells in the rougher and cleaner stages, with column cells as the final cleaner. The most commonly used FLSmidth molybdenum flotation cells in the roughers are WEMCO® inert gas cells. The cleaners can be WEMCO® inert gas, or forced air cells such as DorrOliver® or XCELL® flotation cells. The cleaners can be forced with nitrogen to save NaHS costs or operated with air.

Flotation cell type in use in molybdenum plants Table 1 summarizes the information related to the flotation cell type, reagent information, process gas supply and pH value used in some molybdenum plants. The major mechanism of each type of flotation cell and its advantages will be introduced in the following sections. Table 1 - Cell type, gas supply, reagents and ph for some molybdenum plants Main Reagents

Process gas supply

Cell type

Cells enclosed

PH

Andina

NaHS + H2SO4

Inert gas

WEMCO®

yes

7–8

Bagdad

NaHS

N2 + air

Denver

no

11.5

NaHS + CO2

Inert gas

WEMCO®

yes

10–11 7–8

Plant

Cerro Verde Chuqui

NaHS + H2SO4

N2

Denver

no

El Salvador

NaHS + As2O3

N2

Agitar

no

Gibraltar 84

NaHS + COOH

N2

Outotec

no

7–8

Las Tortolas, original

NaHS

Inert gas

WEMCO®

yes

9

Pelambres, original

NaHS

N2

WEMCO®

no

9.0–9.5

Sierrita

NaHS

N2

XCELL®

no

11–12

Teniente

NaHS + Nokes

N2

Metso

no

9

Plant A (North America)

NaHS

N2

Denver

no

9.5–10.5

Plant B (North America)

NaHS + CO2

N2

Outotec

yes

8.5–9.0

Plant C (North America)

NaHS

N2

XCELL®

yes

8.5

Plant D (North America)

NaHS

N2

Outotec

no

11.5

Plant E (South America)

NaHS

Inert gas

WEMCO®

yes

Plant F (South America)

NaHS

Inert gas

WEMCO®

yes

Plant G (South America)

NaHS

N2

Outotec

yes

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WEMCO® INERT GAS CELL The WEMCO® cell is the most common cell used in the roughing application. It is also the market’s only flotation cell with a self-aspiration mechanism that does not require nitrogen. The rotor and aeration are in the upper section of the cell, which allows a shorter transport distance for bubble particles to agglomerate to the froth phase. This feature is especially beneficial for coarse particles or particles with a high detachment rate. The rotor-disperser in the cell delivers intense mixing and aeration, which uniformly distributes the air throughput the pulp and provides optimum air-particle contact and improved recovery. The high energy helps to increase moly recovery. Because of the selfaspiration mechanism, the ambient air is induced throughout the pulp phase by the turning rotor. This feature makes the WEMCO® cell the best flotation cell for inert gas applications. In the totally enclosed and tightly sealed system, the oxygen in the ambient air is depleted by the reaction with NaHS, and the oxygen-depleted gas is re-circulated as the gas supply for the air bubbles inside the cell. Once the operation reaches a steady state, the inert gas that’s generated typically contains 2–8% oxygen, various percentages of H2S depending on slurry pH, and the remaining nitrogen. Figure 6 below shows a WEMCO® inert gas cell installation in a South American byproduct molybdenum plant, alone with its internal gas recirculation mechanism. Once started, the WEMCO® cell does not need much makeup gas due to the internal gas recirculation inside the cell. A window port is installed in each WEMCO® inert gas cell in order to observe the froth condition in the cell for better monitoring and operation. This is shown in Figure 6.

Fig. 6 - Molybdenum Plant with WEMCO® Inert Gas Flotation Cell

The concentrate froth in molybdenum flotation is typically dry and sticky, and sometimes difficult to recover from the cell to the froth launder. If this poses a problem in the plant, the rectangular shape of a WEMCO® 1 + 1® cell has an advantage due to the convenience of installing a froth paddle to push the concentrate froth to the froth launder. Due to the tightly sealed system of the WEMCO® inert gas cell, sufficient H2S gas can be accumulated in the inert gas inside the cell when flotation is running with low pH. This helps to produce a higher hydrosulfide ion (HS-) concentration in the pulp phase. It also increases the flotation kinetics and selectivity. Currently, all byproduct molybdenum flotation with low pH practice uses WEMCO® inert gas cells. This low pH molybdenum flotation with WEMCO® inert gas cell considerably reduces the number of cleaner stages, maximizes the molybdenum recovery and reduces required floor area.

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To Gas Scrubber

Fig. 7 - Inert Gas Collection System

Considering the safety issues related to H2S in the inert gas, the whole system (including the concentrate sump) is enclosed and the inert gas is collected and treated in the gas scrubber as shown in Figure 7. This inert gas collection system also equilibrates the gas pressure in the whole flotation system. To maximize the hydrosulfide ion concentration, the pressure of the inert gas inside the system is typically maintained around four inches of water. This also helps convey the inert gas to the gas scrubbing system.

XCELL® flotation cell The flotation cells with forced air mechanisms are often used in high pH or medium pH operation, or in the cleaner applications. Compared to most of the other forced air mechanism cells, XCELL® flotation cells have unique characteristics, mainly from their good performance in recovery, especially for minerals in the fine particle size range. The unique design of XCELL® flotation cells creates higher energy intensity in the bubble particle contact zone, though the overall power consumption from the cells is still low. This high energy intensity in the “effective” region of the bubble particle collision zone allows fine particles easier to penetrate the stream line of an air bubble and collide with the bubble surface. This improves the fine particle recovery. For the molybdenite particle, the planar surface is hydrophobic and the edge surface is hydrophilic. Consequently, the smaller particles with a lower aspect ratio also have lower hydrophobicity. Besides the lower collision probability of fine particles, this further reduces hydrophobicity of fine particles and leads to lower probability of bubble particle attachment. One major molybdenum plant in North America has bi-modal size distribution in the rougher feed, and the particle size is concentrated around 130 µm and 25 µm size fractions, respectively. In the tailings stream, there is a considerable amount of molybdenum lost around 25 µm size fraction. XCELL® flotation cells—due to their Metallurgical Plant Design and Operating Strategies (MetPlant 2013) 15 - 17 July 2013, Perth WA

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capability to recover fine particles—were an ideal choice to recover this fine size fraction of molybdenum. Due to the platy particle shape of molybdenite, it is more difficult for the molybdenite particles to be entrained up the slurry flow once they settle at the bottom. Consequently, molybdenite particles sand easily especially when the particle is coarse and slurry viscosity is high. XCELL® has a specially designed “Velocity Cone” that constricts the cross-section area of the slurry flow. This produces a higher flow velocity to prevent particles from sanding. If the molybdenum particles happen to have slow floating kinetics, either from low froth recovery or high detachment probability, this unique design of the XCELL® flotation cell is particularly beneficial in order to improve the molybdenum recovery. Figure 8 shows an XCELL® installation for molybdenum flotation and its internal structure.

Fig. 8 - Molybdenum Plant with EXCEL® Flotation Cell

Dorr-Oliver® cell and other types of forced air cells The mechanism in Dorr-Oliver® cells is very similar to most other forced air mechanism cells, including Outotec and Denver cells. A Dorr-Oliver® cell has a simple mechanical design and fewer components, which makes it easy to operate and maintain. It also has relatively lower capital cost and greater availability than other similar cells. Due to its simplicity, the Dorr-Oliver® cell is popular in molybdenum plants and is especially used in the cleaner stage. Figure 9 shows a Dorr-Oliver® installation and an illustration of its internal structure. For all forced air cells — including XCELL® and Dorr-Oliver® cells—when the cells run with medium pH, they should still be covered in order to minimize the escape of H2S to the environment. Also, a gas collection and scrubbing system still needs to be incorporated. Currently, the nitrogen is typically not recycled in most molybdenum plants using nitrogen as gas supply. Consequently, the nitrogen consumption can be high — especially if all the flotation cells are required to use nitrogen. Depending on the addition points of NaHS in the molybdenum flotation circuit, some plants keep the molybdenum rougher covered while others leave some of the cells in the cleaner stages open.

Fig. 9 - Molybdenum plant with DORR-OLIVER® Flotation Cell

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Economics comparison – mechanical cells Depending on the pH running in the flotation circuit, the flotation cells in molybdenum plants can be either completely open, covered or completely enclosed and sealed. The WEMCO® inert gas cell is the only totally enclosed and sealed cell, and it needs an additional gas collection and scrubbing system for H2S safety. However, the WEMCO® cells do not need additional nitrogen plants or air blowers because of the self-aspiration mechanism and the internal recirculation of inert gas. The cells with a forced air mechanism—either open tank cells or covered cells—will need an additional nitrogen plant if no existing nitrogen source is available. Covered cells still need a gas collection and scrubbing system even if pH is relatively high. This major ancillary equipment, including the nitrogen plant and gas scrubbing system, will have an economic impact on the project. Figure 10 shows an economic trade-off study on the major equipment costs in the molybdenum flotation circuit between the WEMCO® Inertgas systems (Option 1), open tank cell systems (Option 2) and covered cell systems (Option 3) based on a 20 mtph byproduct molybdenite plant.

Fig. 10 - Economic Tradeoff (accuracy of +/- 20%) Between Three Types of Molybdenum Flotation Systems

Comparing these three options, the WEMCO® flotation Inertgas systems have the lowest operational cost since no operational cost associated with the nitrogen plant will be incurred. Though the equipment cost of the WEMCO® system is slightly higher than the open tank cells system, it is offset by savings from its lower operational cost. However, if the plant does have an existing nitrogen source, the most economic option would be the open tank cells system, as the WEMCO® flotation cell itself has a higher capital cost than that of the forced air cell. Overall, the cell selection and criteria are often influenced by regional experience, degree of operational skill sets and “past experience on what works.” At the early stages of the process design, a full analysis of the different cell options and configurations is recommended.

Flotation column The advantage of a flotation column is that it can apply froth cleaning water to wash the fine entrainment from the froth layer. This is especially important because molybdenum flotation tends to have sticky and dry froth, which typically leads to higher fine gangue mineral entrainment. To ensure the final molybdenum concentrate grade, a flotation column is typically used as the last one or two stages of cleaning in molybdenum flotation circuits. However, a flotation column is typically poor in recovery due to its lower energy intensity and lack of internal circulation. In addition, this low recovery from the column is typically associated with a high circulating load in the subsequent cleaner circuit, which has a detrimental influence on the overall recovery and poses challenges for process Metallurgical Plant Design and Operating Strategies (MetPlant 2013) 15 - 17 July 2013, Perth WA

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control. Due to the low recovery associated with a flotation column, its tailings are often further treated in mechanical scavenger cells to improve the molybdenum recovery. One reason for low recovery from the flotation column is the uneven gas distribution around the cross-sectional area of the column. Therefore, in selecting a flotation column, special attention should be paid to its sparger design, which is closely related to the quality of gas dispersion inside the column. The froth wash water is an important feature of the flotation column. Some plants place the froth wash water mechanism in the middle of the froth layer, though this approach has the potential to plug the orifices of the spray bar and make the wash water distribution uneven. In most flotation column applications, the froth wash water is held in a pan, and the wash water passes through the numerous orifices at the bottom of the pan and sprinkles to the top of froth layer. The second approach is the favored froth cleaning mechanism by most column suppliers. The optimum design for a molybdenum plant should use mechanical cells as the rougher and most of the cleaner stages, with the last one or two stages of cleaning in flotation columns.

CONCLUSIONS Molybdenum flotation is a complex operation compared with that required for most other minerals. This is because of the required high concentration ratio, the unique particle shape and the complex flotation chemistry. The difficulty in obtaining sufficient and truly representative test samples for testwork poses a challenge for an optimum flowsheet design. Therefore the keys to a successful molybdenum flotation circuit design are allowances in process design for circuit flexibility and slurry density control. The most commonly used flotation cells in molybdenum flotation are WEMCO® inert gas cells, XCELL® flotation cells, Dorr-Oliver cells, Outotec cells, Denver cells and flotation columns. FLSmidth has extensive experience in molybdenum flotation and has virtually all types of cell mechanisms. WEMCO® inert gas cells not only eliminate the nitrogen plant that is required for most other forced air cells, but also provide the opportunity of low pH operation in molybdenum flotation from safety and operational viewpoints. This corresponds to fewer stages of cleaner, improved selectivity and higher recovery. XCELL® is superior in recovery when the feed consists of predominantly fine particle sizes or the slurry is more viscous. Dorr-Oliver® and some other forced air cells have simple mechanical designs, which correspond to lower capital cost and easier maintenance. Flotation columns are still preferred in the last one or two stages of cleaner in molybdenum circuits due to their froth cleaning feature. Each type of cell has distinct advantages, and clients are encouraged to consider all the flotation cell options and size the cell based on the ore characteristics, process chemistry and the client’s own operational experience.

ACKNOWLEDGMENTS The authors would like to express their great appreciation to the valuable input from Phil Thompson, Carlos Letelier, Lorin Redden, Wolfgang Baum and Sean Armstrong.

REFERENCES Amelunxen, P. and Amelunxen, R., February, 2009, “Moly Plant Design Considerations”, SME Annual Meeting, Denver, CO. Christiansen, A., May 8th, 2012, Molybdenum Plant Design Informative Presentation, Santiago, Chile. Dabrowski, B., Lelinski, D., Redden, L., February, 2006, “Final Report: Depleted Oxygen Flotation Project, La Caridad CuMo Rougher and 1st Cleaner Feeds”, Internal Report. Delaney, J.F., 1972, “Separation of Molybdenum sulphide from copper sulphide with depressant”, U.S. Patent 3,655,044.

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Meadows, D., Jensen, D., Traczyk, F., Yu, S., Riffo, R., February 2013, “Molybdenum Flotation Practices – Cell Type Selection and Design Considerations”, 2013 SME annual Meeting, Denver, Colorado. Onstott, Y. K., Person L. P., February 26th- March 1st, 1984, “By-product molybdenum flotation from copper sulfide concentrate with nitrogen gas in enclosed WEMCO® nitrogen flotation machines”., SME-AIME annual meeting, Los Angeles, CA. Redfearn, M., March 6-10, 1983, “The role of nitrogen in the flotation of by-product molybdenite at Gibraltar mines”, SMEMIME Annual Meeting. Shirley, F, J., 1979, “New Concepts in Byproduct Molybdenite Plant Design”, SME-AIME fall meeting. Thompson, P., October, 2012, Molybdenum Flotation Flowsheet, FLSmidth Internal Communication. Weiss, N. L., 1985, “SME Mineral Processing Handbook, Volume 2”, Society of Mining Engineers. WEMCO® Inert Gas Flotation Machine Bulletin FS-B58 (9/83/3.5M), Sacramento, California.

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