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ISSUE NO. 26 -- January 2002

Acid Pressure Leaching of Copper Sulfides Part 1

Over the past few months, Pincock Allen & Holt (PAH) has been working on the due diligence of the Las Cruces copper project in Spain, owned by MK Gold of Salt Lake City, Utah. One of the interesting unit operations employed by the project is the use of an autoclave to promote the oxidation of ferrous ions to ferric, which in turn increases the reaction rate and thus the overall recovery. Over the course of our involvement in the project, several of our clients have asked questions about pressure leaching of copper, focusing particularly on the technical and the economic risks. The two articles in Pincock Perspectives issues for January and February 2002, address some of the history and technology examined in the past, and also present a description of new innovations used in the design of the Las Cruces Flow sheet. These articles were prepared for PAH by Dr. Roman Berezowsky of Dynatec, Canada. Dr. Berezowsky has over 25 years of experience in the field of pressure leaching, previously with Sherritt Gordon and now with Dynatec. Dynatec has performed the metallurgical testing and process design for the Las Cruces project. Introduction The proportion of primary copper metal production from oxidic ores and tailings--by leach-solvent extraction-electrowinning processing--has been increasing significantly during the past three decades, and currently accounts for about 20 percent of total world copper production. The major portion of primary copper production, however, is still derived from sulfide ores, and virtually all of this involves pyrometallurgical techniques. The predominant copper sulfide mineral is chalcopyrite. Most copper sulfide minerals are associated with iron sulfides, usually pyrite, and many also contain economically significant concentrations of gold and silver. The grade of copper in a typical sulfide ore deposit is relatively low--often less than 1.0 percent. Prior to treatment at pyrometallurgical facilities, the copper sulfides are upgraded, by froth flotation, to concentrates containing between 20- and 40-percent copper--the grade depending on the sulfide mineralogy. A typical chalcopyrite-pyrite concentrate may contain about 26-percent copper, 31-percent iron, and 36-percent sulphur. Pyrometallurgical practices typically involve smelting, converting, anode-casting, and electro-refining of the anodes to high purity copper metal. The smelting and refining processes use well established technologies, are energy efficient (but weren't always), and have high metals recoveries-- including those of gold and silver. The associated iron and most of the impurities are effectively converted into a stable slag waste. Smelting operations are, however, highly capital intensive--requiring a large scale of operation. For a typical concentrate, such as the one described above, the production of each tonne of copper will result in the production of almost 2.8 tonnes



Gateway to Discoveries Cordilleran Exploration Roundup 2002 January 21­25, 2002 Fairmont Hotel, Vancouver British Columbia, Canada Contact: www.chamberofmines. Investing in African Mining, Indaba 2002 International Investment Conferences February 12­14, 2002 Cape Town, South Africa email: [email protected] Minerals for the Future @ SME 2002 SME Annual Meeting & Exhibit February 25­27, 2002 Phoenix Civic Plaza Phoenix, Arizona email: [email protected] International Convention, Trade Show & Investors Exchange Prospectors and Developers Association of Canada March 10­13, 2002 Metro Toronto Convention Centre Toronto, Canada email: [email protected]

Pincock Perspectives is published as a free information service for friends and clients. Call 303-986-6950 or email [email protected] with address changes.

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BARRICK GOLD CORP. FINALIZES ACQUISITION OF HOMESTAKE The end of an era occurred on December 14, 2001 when Barrick Gold Corporation completed the merger with Homestake Mining Company to create the second largest gold mining company in the world after AngloGold. The merger increases Barrick's annual production to approximately 5.7 million ounces gold, from a revised mineral reserve base for the combined company, of 84.3 million ounces, with cash gold production costs of US $165 per ounce. The merged company is expected to have a market capitalization of approximately US $9.0 billion. The new company will have additional assets in Australia including a portion of the Superpit at Kalgoorlie, in South America where the Veladero project will be combined with the Pascua-Lama project, and in North America where operations in Nevada will be combined. The merger also enhances Barrick's gold sales program, with the inclusion of Homestake's hedge position, and this is expected to generate approximate US $200 million annually in premiums for the merged company. CAMBIOR INC. TO DEVELOP GROSS ROSEBEL IN SURINAME In October 2001, Cambior Inc. announced an agreement to acquire the 50-percent interest in Gross Rosebel owned by Golden Star Resources Ltd. for C$8 million, so that they would control 100 percent of the project. The undeveloped gold property in Suriname in the Guyana Shield has been re-evaluated by Cambior in a recent pre-feasibility study, which examines only the mining and processing of soft rock and transition ore. This new scope of work would provide 3.6 million tonnes per year of ore material for processing from a resource base of 25.2 million tonnes grading 1.7 grams gold per tonne, for an annual production of 170,000 ounces. Mine life is expected to be 7 years for this operation, gold price is calculated at US $300, and production is tentatively scheduled for 2005.

of sulfur dioxide or, potentially, about 4.2 tonnes of sulfuric acid. Traditionally, most of the sulfur dioxide--along with various other airborne pollutants--was discharged to the atmosphere. Growing concerns regarding the deleterious impact on the environment brought about increasingly stringent restrictions on gaseous emissions, such that the sulfur dioxide became either a minor credit (where the gasses are used to make sulfuric acid and there is a market for sulfuric acid) or a serious liability, depending on the location of the pyrometallurgical plant and the nature of the sulfur dioxide bearing gases. It also caused many companies to consider hydrometallurgy as an alternative to pyrometallurgy for treating copper sulfides. Commercial Introduction of Pressure Hydrometallurgy for Base Metal Sulfides The first commercial applications of pressure hydrometallurgy for the treatment of base metal sulfides came onstream in the 1950s. Sherritt's nickel refinery at Fort Saskatchewan, Canada, was commissioned in 1954, and employed ammoniacal pressure oxidative leaching for the extraction of nickel, cobalt and copper from a pentlandite-chalcopyrite concentrate. The refinery is still operating today, at a much higher throughput than the original design, on relatively high-grade nickel-cobalt sulfides. Nickel and cobalt are both recovered as high purity metals, copper as a sulfide by-product, and the sulfur is converted to fertilizer-grade ammonium sulfate. The 1950s also saw the first two sulfuric acid pressure oxidative leach-based plants come onstream, both in the United States, for the treatment of cobalt-containing sulfide concentrates. The Garfield Cobalt Refinery (Utah) of the Calera Mining Co. came onstream in 1953, for the treatment of a cobaltite (CoAsS)- chalcopyrite-pyrite concentrate, and the Fredericktown Metals Refinery


(Missouri) of National Lead Co. commenced operations in 1954, for the treatment of a siegenite ([Co,Ni]3S4)-- chalcopyrite-pyrite concentrate. Both used an elevated temperature (190 to 245°C), elevated pressure-leach process with compressed air as the oxidant, developed by the Chemical Construction Co. (Chemico), primarily for the extraction and recovery of the cobalt. Under the aggressive leach conditions, all of the contained sulfide sulfur was fully oxidized to sulfate. Metals recovery from solution was by hydrogen reduction, developed by Sherritt and Chemico. These plants were partly dependent on various economic concessions, and ceased operation in the late 1950s, due both to economic considerations and shortage of feed supply. Early Process Development on Copper Sulfides Dynatec (and formerly, as Sherritt) has conducted numerous research programs-- on laboratory, miniplant and pilot-plant scale--seeking to develop economically competitive and environmentally acceptable acid oxidative pressure leach processes for the treatment of copper-iron sulfide ores and concentrates. The major objectives included high recoveries of both copper and precious metals, and conversion of sulfur to the elemental state or, alternatively, its fixation as gypsum. In the 1960s The earliest work, in the early 1960s, was on a chalcocite-pyrite concentrate, containing 23-percent copper, 28-percent iron and 39-percent sulfur. Mineralogically, the concentrate comprised about 29percent chalcocite and 60-percent pyrite, with the pyrite accounting for about 90 percent of the total sulfur. Pyrometallurgical processing would have resulted in the generation of about 3.4 tonnes of sulfur dioxide for each tonne of copper. The hydrometallurgical process involved a mild oxidative pressure leach in dilute sulfuric acid (at between 80 and 105°C under 140 to 350 kPa oxygen overpressure). This enabled selective

TVX GOLD INC. TO RESUME OPERATIONS IN GREECE TVX Gold Inc.'s Greek subsidiary TVX Hellas has been granted permission to resume mining and milling operations at its Stratoni base metal operation. Since 1995, when TVX acquired the Kassandra mines, it has performed all tasks and acquired all permits in accordance with the stated regulations. However, the company has been prevented from mining in the last 3 weeks by the imposition of a suspension order from the mining inspector, because of lack of permits for the extension of operations at Stratoni, which should have been approved by November 26, 2001. The threatened "force majeure" has been averted.

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extraction of up to 97 percent of the copper, with less than 10-percent reaction of the pyrite. After partial neutralization of the product liquor for the removal of iron, the solution was deemed suitable for recovery of the copper by direct electrowinning, with recycle of the spent electrolyte to the pressure leach. Although not commercialized at the time, this served as the basis of the process flowsheet ultimately developed for the Las Cruces ores. Dynatec's initial process for the direct leaching of chalcopyrite-containing concentrates was developed in the late 1960s. It proposed fine grinding of the feed to 98 percent passing 44 µm and leaching at about 110°C, with an excess of concentrate-- relative to the provided acid--to produce a high copper, low-iron solution suitable for direct electrowinning. Prior to liquid-solids separation, the oxidized slurry was heated to above the melting point of sulfur (~117°C), then cooled, and the unreacted sulfides, which were agglomerated with/by the elemental sulfur, were recovered and separated from the iron oxides and gangue minerals by screening and flotation. After removal of the elemental sulfur, the unreacted sulfides were recycled to the pressure leach. As in the preceding case for chalcocite, the spent electrolyte was recycled to provide acid for the pressure leach. The flotation tailings were subjected to cyanidation for recovery of gold and silver. This process, although effective for predominantly chalcopyrite concentrates, had difficulty in treating feeds with appreciable pyrite content because of the tendency for extensive oxidation of pyrite ­ to iron sulfate and sulfuric acid. In addition, recovery of the precious metals was incomplete. In the 1970s In the 1970s, the increase in energy prices and the increasingly stringent restrictions on sulfur-dioxide emissions mentioned earlier, initiated an intense interest and activity in the development of hydrometallurgical processes by many others. This included companies such as Anaconda Copper, Cominco, Cyprus, du Pont, Duval, Freeport, Inco, Kennecott, Lurgi, Noranda, Sherritt/Dynatec, Sunshine, amongst others, as well as numerous government and university research organizations. Most processes tested targeted chalcopyrite, with the objective of converting the major portion of the sulfide sulfur to the elemental form, with recovery of the copper by electrowinning. Iron was rejected as a hydrated iron oxide (goethite, hematite) or as a jarosite. The leach systems included atmospheric and pressure leaching in acid chloride or acid sulfate, as well as ammoniacal sulfate media, the latter with oxidation of sulfur to soluble sulfate. Because of the refractory nature of chalcopyrite, many of the processes involved a pretreatment to convert the chalcopyrite to a more reactive form. Some of these included a thermal pretreatment, with metallic copper, iron or with elemental sulfur, to convert it to chalcocite, covellite or idaite, while others involved a hydrometallurgical conversion, using metallic copper or iron at atmo-spheric conditions, or copper sulfate at elevated temperatures. The 1970s also marked the introduction of ultrafine grinding (to less than 10 µm) to "activate" the sulfide minerals. Other developments included the addition of various reagents to the leach system, such as silver, chloride ions, or nitrogen oxide species to enhance the leaching of the chalcopyrite. Several of these were advanced to pilot plant and demonstration plant scales. Although these processes, in general, proved technically viable and avoided the problem of air pollution by sulfur dioxide, most of them involved complex flowsheets, often involving highly corrosive conditions for some of the unit operations. There were concerns regarding the purity of the copper product, as well as that of the sulfur by-product, and concerns regarding the environmental stability or acceptability of some of the solid wastes. Recovery of the precious metals was complex, and less efficient than under pyrometallurgical operations. During the same period, the concerns that had spurred the research activities on hydrometallurgical alternatives also

3 3


Will the British Columbia Government Eliminate the B.C. Geological Survey Branch?

Since its election in mid 2001, the new British Columbia (BC)

government has been undertaking a Core Services Review to refocus government functions and eliminate non-core activities, so that taxpayer dollars are directed to expanding investment in BC. Within this sphere in the last few weeks, the possibility of the elimination of the British Columbia Geological Survey Branch (BCGSB) over the next 3-year period has been rumored. The BC mining community believes that the BCGSB is not an optional activity, but a very necessary core service to provide geoscience data for the mineral industry in BC. Investors must be able to have access to new mapping and surveys, and to information that will lead to new mineral discoveries, or re-evaluation of mines that were previously closed. Under the previous government, the rich mineral endowment of BC was ignored as many investors took their search overseas to countries where large amounts of money were being expended on rapidly upgrading mineraldatabase inventories to attract foreign investors to their resources (e.g.: Argentina, Chile and Peru). The new BC government has implemented flow-through share tax credits, eliminated provincial sales tax on production machinery, and reduced corporate income and capital taxes for mining in an effort to revitalize the mineral and mining exploration community. However, the new government is also indicating that they will focus on a public-private partnership model. Budget targets have not been released but funding for the BCGSB will no doubt be reduced.

For more information go to:

Pincock Perspectives

accelerated the development of more efficient, high intensity pyrometallurgical processes. Developments such as flash smelting and continuous converting enabled a much higher and more efficient capture of sulfur than from the older smelters using reverberatory furnaces, such that many of the environmental concerns and the emission guidelines were readily addressed. Early Commercial Applications of Pressure Hydrometallurgy for Copper Sulfides Only two hydrometallurgical plants reached the commercial, or semicommercial, scale for direct treatment of copper sulfide flotation concentrates in the 1970s, both in the United States. One was Anaconda's Arbiter Plant, commissioned in 1974, with a design capacity of 36,000 tonnes/year of cathode. The process was based on the ammoniacal oxidative leaching of a concentrate comprising a mixture of chalcocite, bornite and chalcopyrite, with a significant portion of pyrite. Copper recovery from the ammonia leach liquor was by solvent extraction and electrowinning. The ammonia leach was designed to extract the easily leached copper of chalcocite and bornite (70 to 80 percent of the copper), recover the chalcopyrite (and most of the gold and silver) from the leach residue by flotation for treatment in a smelter, while rejecting pyrite, which accounted for almost 70 percent of the sulfur of the original concentrate. The plant was shut down in 1977, as a result of high maintenance costs, due partly to changes in the mineralogy and also to complications associated with sulfate disposal. The second plant was the chloride based CLEAR® Process of Duval, developed for the treatment of chalcopyrite concentrates, which came onstream in 1978. It reached its design capacity of 32,000 tonnes/year of copper and was considered a technical success, although modifications were required to achieve economic viability. The plant, considered by Duval as a demonstration or prototype plant, was also shut down after several years. Although the hydrometallurgical processes developed for the copper-iron-sulfides did not live up to expectations, there were a number of acid pressure leach processes--for simple copper sulfides--which did achieve commercial success. One of these is at Inco's CRED (Copper Refinery Electrowinning Department) plant at Sudbury, Ontario, where the feed material is a precious metals (PM) concentrate containing copper sulfide, resembling chalcocite in composition. The plant, commissioned in 1973, was designed to leach the solids in spent electrolyte at 105°C with oxygen, to generate a copper sulfate solution for electrowinning, and a residue containing the PM and elemental sulfur, for further upgrading by organic extraction of the sulfur. Difficulties with the latter operation precipitated a change in the leach conditions, to a sulfuric acid deficient system, for oxidation of the sulfide sulfur to sulfate, and conversion of the copper to a basic copper sulfate salt. The basic salt was then dissolved in spent electrolyte, leaving a high grade PM residue. Dynatec/Sherritt developed acid pressure leach processes for the treatment of

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platinum group metals (PGM) containing nickel-copper mattes which have been adopted by five PGM producers, including Impala (1968), Rustenburg (1981), Lonmin (1985) and Northam (1992) - all in South Africa, and Stillwater (1996) in the United States. The second stage of oxidative leaching treats a predominantly chalcocitic residue, at between 135 and 160°C, with the objective of dissolving the copper and oxidizing the sulfur to sulfate, leaving a residue highly enriched in PGM content for final refining. In 1984, Sunshine Mining commissioned a novel acid oxidative pressure leach of an argentiferous chalcocite intermediate product, derived from an alkaline preleach of a tetrahedrite concentrate for the prior removal of antimony. This was a batch, nitric acid-promoted sulfuric acid leach operation, initiated at about 90°C and completed by about 150°C, which dissolved the silver, as well as copper and iron, with conversion of some of the sulfur to the elemental form. Silver was recovered by precipitation, and the copper was recovered by solvent extraction and electrowinning. Additional innovations, introduced several years later, included the fine grinding of the concentrate with 80 percent passing 10 µm, and replacement of the nitric acid with sodium nitrite, which proved to be a more effective catalyst. This plant operated for about 16 years, before closing due to severely depressed silver and antimony markets. As noted earlier, these examples of commercial operations are rather specialized cases, for treating a chalcocitic type feed, with total oxidation of the sulfur--in most cases--to sulfate. As such, they did not lend themselves to the direct treatment of chalcopyritic feeds.


· Acid Pressure Leaching of Copper Sulfides--Part 2


Pincock, Allen & Holt is a consulting and engineering firm serving the international mineral resource industry. Your comments and suggestions are always welcome. Contact Pincock, Allen & Holt · 274 Union Blvd., Suite. 200, Lakewood, Colorado 80228 · TEL 303.986.6950 · FAX 303.987.8907 ·




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