use of hydrometallurgy in direct processing of base metal pgm concentrats

USE OF HYDROMETALLURGY IN DIRECT PROCESSING OF BASE

METAL/PGM CONCENTRATES

Joe Milbourne, Marcus Tomlinson and Lynton Gormely1

1AMEC Mining and Metals Consulting

111 Dunsmuir Street, Suite 400 Vancouver, B.C. V6B 5W3

Abstract

The potential for treating copper-nickel-cobalt-platinum group metal concentrates by a fully hydrometallurgical route is discussed. Processes specifically developed for treating these feeds and the potential application of the processes developed for the treatment of chalcopyrite copper concentrates are included in the evaluation. Block flowsheets are provided and the technical characteristics of various processes are compared. The current state of development for each process is summarized and probable economic impacts of the technical features are qualitatively assessed. Perceived risks and rewards associated with each technical concept are provided in the conclusions.

Introduction

The AMEC Mining and Metals consulting group offers considerable expertise in pressure hydrometallurgy, and as such, is interested in finding new applications for the technology. Recently, AMEC has investigated the applicability of a direct hydrometallurgical approach to recovery of platinum, palladium and gold from base metal concentrates. Motivations for this investigation include the cost of treatment for flotation concentrates at existing smelters and refineries, and the length of time required to achieve payment in full. In this paper, we present some of the general conclusions of our work. Our investigations have focused on PGM occurrences with base metal sulfides, which can be concentrated initially by flotation. By PGMs we mean platinum, palladium, and gold (not classed as a platinum group metal) – which are most often sought in these concentrates, and most susceptible to recovery by aqueous leaching. There are significant values in the base metals that can be produced from the same concentrates, and the choice of process route must address their recovery. Historically, primary separation and rejection of iron from these concentrates has taken place in a smelter. Iron is oxidized, combined with silica and lime, and separated as a slag, leaving the base and platinum group metals in a separate matte phase. These operations may have different focuses. They may be: • either a base metal smelter, where PGMs are produced as a by-product, for example, in

anode slimes from a nickel refining process, or • a PGM smelter, where the focus is on PGMs which are first concentrated in a base metal

sulfide matte for subsequent additional concentration by aqueous oxidation. After the smelting step, hydrometallurgy often comes into play to separate and recover the base metals (primarily copper, nickel, cobalt and any residual iron) associated with the PGMs. Further processing of the leach solutions to recover the base metals represents a significant consideration by itself, but it is not the primary focus of this paper. Typically the base metals

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Edited by C.A. Young, A.M. Alfantazi, C.G. Anderson, D.B. Dreisinger, B. Harris and A. JamesTMS (The Minerals, Metals & Materials Society), 2003

Hydrometallurgy 2003 – Fifth International Conference in Honor of Professor Ian Ritchie –Volume 1: Leaching and Solution Purification

are dissolved in a simple sulfate system, in which the PGMs have a very low solubility. This factor is used advantageously in a number of plants processing smelter matte where PGMs are quantitatively concentrated into an insoluble leach residue by selective oxidation of the base metals. This is somewhat analogous to the oxidation stage in refractory gold ore processing, where it is also intended to leave gold quantitatively in the residue. Although hydrometallurgy is already used extensively in PGM processing plants, the objective of this paper is to discuss the possibility of providing a purely hydrometallurgical alternative to the smelt/matte leach process that is less expensive, less complex (fewer and less capital intensive operations), and less subject to losses. As well, by adopting an entirely hydrometallurgical route, environmental/workplace benefits are possible, and smaller mine site plants may be economic.

Hydrometallurgy for PGMS

In PGM processes, hydrometallurgy is not viewed as effective for PGM/iron separation because of the large volume and high surface area of iron precipitates relative to quantities of PGMs typically involved. As well, complete dissolution of PGMs is not attainable in a simple sulfate system (for example, refractory gold processes are intended to leave gold quantitatively in the residue). The same concept is used to recover base metals from smelter matte. Careful control of matte leaching conditions quantitatively concentrates PGMs in an insoluble residue. On the other hand, hydrometallurgy is used in refineries to dissolve the PGMs for final purification. Refinery feeds may come from a variety of sources, e.g.: • anode slimes • matte leach residue • secondary scrap • recycled catalyst but sulfide concentrates are not usually processed directly. Pressure Leach/Oxidation for Sulfide Destruction Given that autoclave technology is now a proven, low risk processing tool, we want to examine how it might be applied to direct treatment of sulfide flotation concentrates for PGM recovery. Its role would be the destruction of the sulfide matrix that holds the base and PG metals. Strategies that could be adopted include using pressure oxidation to: • Dissolve the base metals and PGMs together, with subsequent separation of each metal

in a sequence of hydrometallurgical operations. • Dissolve the base metals and leave the PGMs in a highly concentrated residue suitable

for refining. • Dissolve the base metals and leave the PGMs in an intermediate residue for further

concentration and then refining. The conditions commonly employed for sulfide oxidation are: • High temperature (above 200oC)to give complete sulfur oxidation to sulfate. • Intermediate temperatures (150-160oC) to give oxidation primarily to elemental sulfur,

require finer feed.

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• Low temperatures (below 120oC) to give oxidation primarily to solid elemental sulfur, require fine grinding and/or alternate chemistry. Atmospheric leaching processes fall into this category.

Other than sulfate systems, nitrate with oxygen, chloride/bromide with chlorine/bromine as oxidant, and iodide with iodate as an oxidant have been developed for sulfide oxidation, either at ambient conditions or under pressure. Chemistry for PGM Dissolution While oxidation in the sulfate system works for gold (the values largely report to the oxidation residue), matte leaching experience suggests that PGMs may partially leach, increasing the metal recovery effort required, and also the potential for losses. Obviously it is desirable to have the PGMs either fully leach or remain fully inert. The most desirable process for PGM concentrate processing would dissolve all the values sought (both base metals and PGMs), eliminate the residue, and then separate each with high efficiency and low losses (the first strategy outlined above). To achieve this, we first need to look for a suitable chelating agent to hold the metals in solution, which might include, for example: • chloride • other halides • cyanide • thiocyanate • thiosulfate • thiourea • ammonia Of these, cyanide, thiocyanate, thiosulfate, and thiourea would not likely survive the pressure oxidation conditions. Undesirably, they might survive long enough to provide some initial mobility to the PGMs, with possible redeposition in a more refractory form.

At the same time, the selected chemistry and conditions should produce a residue that does not contain substances reactive towards dissolved PGMs. Key products to avoid would be carbonaceous materials, elemental sulfur, or hydrated ferric iron precipitates.

PGMs in Leach Residues

If the second or third strategies are pursued, then chelating ions should be avoided, and the constraints on the chemical nature of the residue are set mostly by the nature of further processing for PGM recovery. It will be desirable to produce a low mass, highly concentrated PGM residue as a product.

Developed Process Technology

Over the last 20 years or so a number of processes have been developed for dissolving base metal concentrates and these provide a number of potentially applicable hydrometallurgical processes for processing PGM concentrates. In order to avoid undertaking a major research and development program, before pursuing this list of chemical environments, we should survey the technology already available.

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For this discussion, the processes will be grouped according to whether or not the PGMs are leached into solution or remain in the insoluble residue. Obviously, testing of the various options would be needed to confirm PGM deportment.

Insoluble PGM Processes

When treating primary PGM concentrates there are no processes that will generate a PGM residue directly suitable for refining. As mentioned above, this is the aim of the matte leach plants, but these reject iron via a smelting step ahead of leaching.

Most of the processes developed for the treatment of copper concentrates should not dissolve significant quantities of PGMs. This is due to either the lack of a suitable chelating agent or the presence of significant quantities of elemental sulfur, which should reduce any soluble PGM in the system.

Total Pressure Oxidation [1]

This process, developed for copper by Placer Dome and UBC, essentially uses refractory gold conditions to achieve total oxidation in a sulfate system, i.e., with no suitable chelating agent present. A conceptual flow schematic is shown in Figure 1.

Under total oxidation conditions (225°C, 700 kPa O2, 3400 kPa total pressure), the sulfide content of the ore is completely oxidised to sulfate and it is expected that the PGMs would follow iron, and report to the hematite residue as an oxide or in elemental form. Some iron would remain dissolved, but the PGM concentration ratio from concentrate to residue is unlikely to be more than 2 to 3. It might be possible to manipulate conditions to keep more iron in solution and obtain a higher concentration ratio. A secondary leach is required to recover the PGMs. Any leach performed would ideally be selective against iron; analogous to refractory gold processing, a chelating leach may be an option, followed by ion exchange. The hematite residue is relatively dense and likely would be slow to dissolve. This process has strong precedents in refractory gold processing, making it a relatively low risk option. Copper and nickel can be recovered from the leach solution by SXEW and/or precipitation routes. Complete sulfide oxidation with resultant high oxygen consumption, high maintenance costs and disposal of sulfate are among the operating cost concerns. Almost all the other hydrometallurgical copper process fall into the partial sulfide oxidation category. The temperature, pressure and chloride content of the leaches are variable but the outcome is the formation of elemental sulfur. In all of these processes (discussed below) it is anticipated that the presence of elemental sulfur will prevent the dissolution of the PGMs, even in the presence of low concentrations of chloride, a suitable chelating agent. Copper and nickel in the concentrate will be leached during the oxidation process and can be recovered from the leach solution using a number of different processing options.

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Figure 1: Conceptual block diagram for PGM and base metal Recovery by total pressure oxidation.

Figure 2: Conceptual block diagram for PGM and Base metal recovery by the CESL process.

PGM concentrate

U'flow

Neutralization Gypsum Residue

Fe, Cu, Co, etcPrecipitation ResiduesReagents

Limestone, lime

Nickel Releach

Ni CathodeGypsum tailing

PGM leach

PGM recovery

PGM concentrate

Nickel Precipitation and S/L Separation

Liquid/Solid Separation

Nickel Electrowinning

Pressure Leach

Residue to tails

Solution Purification: Pptn/SX

PGM concentrate Evaporator

Atmospheric Leach

U'flow

Cu CathodeGypsum residue Neutralization


Ni Cathode

Pressure Leach

PGM concentrate

Primary SX


Copper Electrowinning

Sulfur Removal

Tertiary Cu SX

Residue to tails

PGM recovery

PGM leach

Sulfur Product

Ni Precipitation/Releach Nickel Electrowinning

Neutral Effluent to Tails



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The leach residue from all these processes could be subjected to a flotation step to recover the elemental sulfur and unleached sulfide minerals. The elemental sulfur could then be removed by either hot filtration of chemical dissolution in, for example, perchloroethylene. Ideally, the PGMs would remain in the residue from this process, which could then be the feed to a secondary process in which the PGMs would be dissolved and recovered. Obviously, the deportment of the PGMs through such a circuit requires investigation but it appears that their concentration into a flotation concentrate essentially free of base metals may open up the possibility of an economic processing route, provided they are not removed with the elemental sulfur. CESL Process [2, 3] This process is based on chloride-enhanced pressure leaching. A conceptual block diagram is shown in Figure 2. In spite of the chloride environment, typical CESL conditions (150 oC, 700 kPa O2, 1100 kPa total pressure, 12 g/L Cl-) would not likely dissolve (or only partially dissolve) the PGMs, which would need to be at least partly recovered from a residue containing high levels of elemental sulfur. CESL has patented a process for the recovery of gold from the leach residue, which entails the following main steps: • removal of elemental sulfur using a hot perchloroethylene (PCE) leach, • total oxidation of the remaining sulfides to release refractory gold, • neutralisation, and, • cyanide leaching of the solids for gold recovery.

As published, PGMs are excluded from consideration.

This process has been extensively tested for copper at demonstration plant scale, but not for copper-nickel. No publications indicate that the precious metal recovery circuit has been run at the same scale.

Dynatec [4]

A conceptual flow schematic for the Dynatec process if applied to PGM recovery is shown in Figure 3. This is a mid-range (150 oC) autoclave process which produces elemental sulfur in a sulfate medium and is therefore likely to dissolve less PGMs than the chloride enhanced CESL process. Coal is used as a source of surfactant for elemental sulfur dispersion. PGM recovery from the residue would be complicated by the presence of coal and elemental sulfur, both of which would tend to pick up PGM values from a leach solution. This process has been piloted but not demonstrated; its operating conditions have a good pedigree in zinc leaching.

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Figure 3: Conceptual block diagram for PGM and Base metal recovery by the Dynatec Process.

ActiveOx [5]

The generic flow schematic is shown in Figure 4. The Activox process is a mild pressure leaching process employing fine grinding (p80 5-15 micron, 100-110 oC, 1000 kPa oxygen) in a sulfate system. This process has been demonstrated at the continuous pilot plant level.

Albion [6]

The Albion Process is another sulfate-based process employing fine grinding (10-15 micron), to be successful at mild conditions (85-90 oC atmospheric leach, 24 hours residence time). Oxygen and air sparging are used for oxidation. The process has been demonstrated at the continuous pilot plant level. Mount Isa Mines, the process owners, have said they wish to keep the technology internal for use in their own projects.

AAC/UBC [7]

This process combines features of the Albion, Activox, and Dynatec processes. It is an autoclave sulfate leach at 150 oC, 700 kPa oxygen that is able to achieve high copper recoveries from chalcopyrite by completing a fine grind of the feed (10-20 micron). As with zinc concentrate leaching, a surfactant is required for satisfactory performance to control sulfide wetting by molten elemental sulfur. The AAC/UBC process is currently being piloted for copper.

Unleached sulfides

U'flowFloat tails

NeutralizationGypsum Residue


Limestone, lime

Nickel Releach

Residue to tails


Nickel Cathode

Concentrate Fine Grinding

Flotation, Sulfur Melting & Filtration

Sulfur Product

PGM recovery

PGM concentrate

PGM leach


Flotation Concentrate

Gypsum tailing



Oxidation Leach

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U'flow

Neutralization Gypsum Residue


Limestone, lime

Nickel Releach

Flotation Concentrate

Gypsum tailing



Oxidation Leach


Residue to tails


Nickel Cathode

Concentrate Fine Grinding

Sulfur Product

Sulfur Removal

PGM recovery

PGM concentrate

PGM leach

Figure 4: Conceptual block diagram for PGM and base metal recovery By the Activox, Albion, AAC/UBC or Nitrogen Species Catalyzed processes.

Nitrogen Species Catalyzed (NSC) Process [8]

In this process a sulfate leach system is augmented with 2 g/L sodium nitrite. Both total and partial oxidation processes have been proposed. It operates with mild conditions (125 oC, 400 kPa oxygen). The partial oxidation process was commercialized as a batch operation at the Sunshine Mine in Idaho on chalcocite-tetrahedrite minerals. The silver, rhodium and palladium can either be leached or retained in the solids by controlling the chosen process conditions. Ammonia Leach Processes [9, 10, 11]

Flow schematics are not provided for these processes. The Arbiter and Coloso processes were commercialized for copper, the Sherritt process for nickel. All three would likely produce the PGMs in a voluminous iron hydroxide leach residue, and produce ammonium sulfate that would need to be recovered and sold. A low PGM concentration in the residue and poor liquid/solid separation properties would make subsequent recovery of the PGMs expensive. The undesirable features of this technology led Sherritt to develop the acid leach process for Impala Platinum matte leaching many years ago.

Soluble PGM Processes

The following processes have the potential to dissolve significant quantities of PGMs, due to the presence of a suitable chelating agent.

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PGM concentrate

U'flow to tailing

Reductant or IX Resin PGM Concentrate

Lime/LimestoneNeutralization Gypsum Residue


Limestone, lime

Nickel Releach

Ni Cathode

Pressure Leach

PGM Ppptn or Ion Exchange


Gypsum tailing




Figure 5: Conceptual flowsheet for PGM and base metal recovery by the Platsol process.

Platsol [12]

This process was developed specifically to treat PGM sulfide flotation concentrates. It leaches base and precious metals in a single step with oxygen in sulfate media under total oxidation conditions (220°C, 700 kPa O2, 3200 kPa total pressure) using chloride (or other halide) as a complexing agent. The high temperature used in the leach implies a high level of sulfur oxidation, which may be necessary to keep PGMs in solution. In addition, the majority of the iron present in the concentrate could be rejected into the residue, thereby effecting a separation. The presence of the chloride (20 g/L) is also believed to assist in achieving high recoveries of the base metals.

Total oxidation of sulfide in the primary concentrate leads to high operating costs due to high oxygen consumption and sulfate neutralisation. To reduce overall processing costs the Platsol process could be used to treat the PGM bearing residues (after sulfur removal) from the partial sulfide oxidation processes discussed above. This would allow the generation of higher grade solutions than if the whole concentrate was processed and may open up a number of options for PGM recovery, while keeping equipment size down.

Leaching conditions are similar to those used in refractory gold plants and are thus within the range of current industrial practice. The addition of chloride to the leach solutions means that care must be taken when selecting materials of construction for the plant.

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Figure 6: Conceptual block diagram for PGM and Base metal recovery by the Intec process.

Intec [13, 14]

The novel chloride/bromide chemistry featured in this process (developed for copper) provides a strong oxidant at nearly ambient (85°C, atmospheric pressure) conditions. The use of a high concentration of chloride/bromide (greater than 5M) is believed to be sufficient to allow dissolution of the PGMs even in the presence of elemental sulphur. Dissolution of gold is an integral feature of the Intec copper process.

As with all of the processes discussed, test work is required to determine the deportment of the PGMs and the method of recovery. PGM recovery in the Intec process could use ion exchange, or reduction using recycled copper dendrites.

It is suspected that problems may be encountered with redox and pH control to achieve low sulfur oxidation with high PGM dissolution.

The Intec process for copper has been run at demonstration plant scale. Treatment of primary PGM concentrates has been limited to batch laboratory work. Materials of construction, impurity deportment, and product quality all need to be proven before development of a commercial plant.

The mixed chloride-bromide chemistry in this process represents a significant departure from current sulfide processing technology. There are potential benefits to the process, but the future plant owner must be aware of the risks in implementing this new technology.

Other Chloride-Based Copper Processes

CLEAR [15] and CUPREX [16] are chloride processes that were commercialized or at least extensively piloted. Neither focused on PGM recovery. Without focus on these constituents, the likelihood is that they would report partially to both solution and residue, an unsatisfactory state of affairs.

Copper dendrites Cathode Copper

Catholyte

Anolyte

Reagents

Cu, Fe, Co, etc. products

Flotation concentrate Nickel Cathode

O'Flow

Sulfur PGM Product Concentrate

Anolyte

O'Flow

Third Stage Leach

Impurity Removal


Leach Residue to Tails

First stage leach Copper Electrowinning

Sulfur Removal


Second Stage Leach



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PGM Recovery from Concentrate Leach Solutions

If the PGMs are dissolved, there is extensive chemistry available for metal recovery and purification from current refinery practice [17]. However, a departure is the lower strength solutions resulting from a concentrate leach, when compared to refinery chemistry. This can place a significant constraint on the use of high strength reagents to achieve particular chemical results. PGM mineral processors usually leave separating the various PGMs to a refinery, and economics may continue to dictate that these rather sophisticated separations be done in only a few centralized locations, with special equipment and expertise. At the mine site, we may be content to produce a high-value small-volume product, which will return a high proportion of the value of the constituent metals, in a reasonable time.

A significant chemical characteristic of dissolved PGMs is that the ions are strong oxidants, easily reduced to metal, and easily hydrolyzed. Thus, they tend to precipitate and deposit in undesirable locations, and this can make development of a robust, controllable process difficult. They are capable of a wide range of reactions, however, and so we can think of various chemical concepts for winning them from solution including:

• Dilution. Reducing the strength of the complexing agent can cause metals to hydrolyze and precipitate. However, the volume of precipitate is small, and by the dilution operation, we have made its recovery even more difficult. Losses are likely to be high in these circumstances, and the dilution makes economic use of reagents through solution recycle impractical.

• Salts Precipitation. There are numerous insoluble compounds of these metals that are used in refining processes. For example, platinum and palladium can be precipitated in appropriate circumstances with ammonia or sulfide. Once again, with relatively low concentrations produced from concentrate leaching, high recovery of the precipitated solids requires great care, reagent dosages may be uneconomic, and the large volume of barren solution may be an environmental liability.

• Reduction. The complex ions are relatively strong oxidants, easily reduced by a range of compounds including:

- sulfide - qsulfite - ferrous iron - zinc metal - metallic iron - elemental sulfur - activated carbon - alkenes, e.g., ethylene, propylene - oxalic acid The concentration of reduced product produced from typical concentrate leach solutions will be low. Thus, it may be useful to employ a carrier, for example, a base metal sulfide, or excess unreacted cementation agent, to facilitate recovery from the solution, even though a less desirable product may result.

• Adsorption By Ion Exchange Or Activated Carbon. Adsorption systems are well adapted for concentrating dilute values from solution. What is needed is an adsorbent that is easily contacted with and separated from the solution, and which has a selectively high affinity for the desired ions.

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• Chelating Resins. These are ion exchange resins with functional groups that hold the desired metals by mechanisms similar to those which bind ligands into complexes. Some are further characterized as “molecular recognition” resins [18], exhibiting a high degree of selectivity. They tend to be expensive, and therefore are not a good choice for processes where the most practical recovery method might involve incinerating the resin.

• Strong Base Resins. These resins adsorb anions, including the anionic complexes that may be formed when PGMs are dissolved in the systems described above. Essentially, they form a salt with the anions. They are not as selective as the chelating resins, but are much less expensive. These resins have a high selectivity for chloroanionic PGM complexes over base metals found in some of the aforementioned processes [19]. If stripping and subsequent chemical recovery are deemed undesirable or uneconomic, loadings will likely be sufficiently high that the resins can be destroyed as part of the metal recovery process.

• Reducing Resins [20]. These resins have a borohydride functionality, and were designed especially for recovering precious and platinum group metals. The ions are reduced to metals within the resin matrix, and are not easily strippable. Thus, high loadings are essential to make metal recovery by subsequent resin destruction economically feasible.

• Activated Carbon. Activated carbon is an adsorbent well-proven in gold recovery from cyanide leach systems. There, the carbon adsorbent is readily strippable, and can be recycled to the adsorption process many times. There is an indication that PGMs adsorbed on carbon from a chloride solution matrix reduce to the metallic state and may not be as easily recovered in a stripping process.

• Solvent Extraction. Solvent extraction reagents mentioned in connection with PGM refineries include alkyl sulfides, phosphine sulfide, hydroxyoximes, amines, TBP, ketones, and ethers (dibutyl carbitol). For treating dilute solutions such as are produced by direct concentrate leaches, solvent extraction may not be a desirable approach. This is because solvent losses to raffinate are inevitable, and they are directly proportional to the volume of solution treated. As the concentration of PGMs in the pregnant solution decreases, the combined cost of reagent and metal losses in the raffinate soon matches the value of metals in the pregnant solution, defeating the process economically.

Conclusions

A direct aqueous treatment route from flotation concentrate to refinery feed is desired by the industry to provide the benefits of reduced treatment charges, reduced working capital, and better metal winning economy at mine-site scale, as well as greater marketing flexibility for the product. This can be achieved with minimal additional development effort by starting from the extensive range of existing process technology.

The best options currently appear to be those that do not produce elemental sulfur in residue, where it would compete for PGMs that are intended to be leached. Concepts providing a suitable basis for future process development include:

For PGM/base Metals Dissolution

• Total pressure oxidation, PGM leach of residue with an acidic chelating ligand.

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- Industrially proven oxidation conditions and chemistry for sulfide destruction. - Oxidation process does not provide a high concentration factor for PGMs

because of iron precipitation. - Deportment of PGMs between pressure oxidation residue and solution uncertain.

• Total pressure oxidation with a chelating ligand to dissolve all values in one operation. - Oxidation leach conditions and chemistry with chelating ligand not as proven

from engineering standpoint. - Materials of construction need careful consideration.

For PGM Recovery

• Precipitation to a high value concentrate for sale to refinery. - Low solution strength from concentrate leach makes high recoveries difficult. • Concentration by adsorption (ion exchange), sell loaded adsorbent for sale to a refinery. - Technology generally suitable for low strength solutions. - Not yet proven for low strength PGM leach solutions.

- Resin stripping and subsequent precipitation is another option.

References

1. J.A. King and D.B. Dreisinger, “Autoclaving of Copper Concentrates,” Proceedings of Copper 95 – Cobre 95 International Conference, ed. W.C. Cooper et al. (Montreal: The Metallurgical Society of CIM, 1995), 511-532. 2. D.L. Jones and J. Hestrin, “CESL Process for Copper Sulphides: Operation of the Demonstration Plant,” paper presented at the Alta 1998 Copper Sulphides Symposium, Brisbane, Australia, October 19, 1998. 3. D.L. Jones “Process for the recovery of Nickel and/or Cobalt from a Concentrate,” United States Patent No. 6,383,460, May 7, 2002. 4. L.A. Barta, K.R. Buban, J. Stiksma, and M.J. Collins, “Pressure Leaching of Chalcopyrite Concentrates by Dynatec,” Proceedings of Copper 99 – Cobre 99 International Conference, ed. S.K. Young et al. (Warrendale, PA: The Minerals, Metals, & Materials Society, 1999), Vol IV, 167-180. 5. G. Johnson, “The Activox Process for Treatment of Copper Concentrates,” (paper presented at the Copper Hydrometallurgy Forum, Brisbane Australia, September 18-19, 1995). 6. M. Hourn and D. Halbe, “The Nena Tech Process: Results on Frieda River Copper Gold Concentrates,” Randol Copper Hydromet Roundtable ’99 (Golden, CO: Randol International Ltd., 1999) , 97-102. 7. D. Dreisinger, J. Marsh, and P. Dempsey, “The Anglo American Corporation/University of British Columbia (AAC/UBC) Chalcopyrite Hydrometallurgy Process,” Alta 2002 Copper – 7 Technical Proceedings, (Lower Templestowe, Australia: Alta Metallurgical Services, 2002). 8. C.G. Anderson, “The Treatment of Chalcopyrite Concentrates with Nitrogen Species Catalyzed Oxidative Pressure Leaching,” Proceedings of Copper 99 – Cobre 99 International Conference, ed. S.K. Young et al. (Warrendale, PA: The Minerals, Metals, & Materials Society, 1999), Vol IV, 139-149.

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9. M.C. Kuhn, N. Arbiter, and H. Kling, “Anaconda’s Arbiter Process for Copper,” CIM Bulletin, 67(752) (1974), 62-73. 10. W. Duyvesteyn and B. Sabacky, “The Escondida Process for Copper Concentrates,” The Paul E. Queneau International Symposium Extractive Metallurgy of Copper, Nickel, and Cobalt,” ed. R.G. Reddy and R.N. Weizenbach, (Warrendale, PA: The Minerals, Metals, & Materials Society, 1993), Vol I, 881-910. 11. J.R. Boldt, Jr., The Winning of Nickel, (Toronto: Longmans Canada Limited, 1967), 299-315. 12. C.A. Fleming, C.J. Ferron, D.B. Dreisinger, and P.T. O’Kane, “A Process for the Simultaneous Leaching and Recovery of Gold, Platinum Group Metals and Base Metals From Ores and Concentrates,” EPD Congress 2000, the Minerals, Metals and Materials Society (TMS), 2000 TMS Annual Meeting, March 12-16, 2000, ed. P.R. Taylor, (Warrendale, PA: The Minerals, Metals, & Materials Society, 1993), 419-431. 13. J. Moyes, D. Sammut, and F. Houllis, “The Intec Copper Process: Superior and Sustainable Metals Production,” Alta 2002 Copper – 7 Technical Proceedings, (Lower Templestowe, Australia: Alta Metallurgical Services, 2002). 14. F. Houllis, Personal Communication, January 2003. 15. G.E. Atwood and R.W. Livingston, “The CLEAR Process, a Duval Corporation Development,” Erzmetall, 33 (1980), 251-255. 16. R.F. Dalton, G. Diaz, R. Price, and A.D. Zunkel, “The Cuprex Metal Extraction Process: Recovering Copper from Sulphide Ores,” Journal of Metals, 43(8) (1991), 51-56. 17. G.P. Demopoulos, “Refining of Platinum-Group Metals,” CIM Bulletin, 82(923) (1989), 165-171. 18. J.B. Dale, N.E. Izatt, R.L. Bruening, A. Reghezza I., J. Vergara Ch., and J.A. Matta V., “Recent Advances in the Application of Molecular Recognition Technology (MRT) in the Copper Industry,” Proceedings of Copper 99 – Cobre 99 International Conference, ed. S.K. Young et al. (Warrendale, PA: The Minerals, Metals, & Materials Society, 1999), Vol IV, 329-343. 19. K. A. Krause and F. Nelson, Proc. Intern. Conf. Peaceful Uses of Atomic Energy, Geneva, 7, 113 (1956) 20. L. Manziek, “Amborane Resins: A New Approach to the Recovery of Precious Metals,” paper presented at the 6th International Precious Metals Conference, Marriott Hotel, Newport Beach, CA, June 7-11, 1982.

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