PRESSURE LEACH PLANT SHOWS POTENTIAL
Carter, Russell A
PD’s high-temperature pressure-leach demo plant effectively processes primary sulfide concentrates
With the startup of the Bagdad pressure-leach demonstration plant earlier this year, copper producer Phelps Dodge (PD) threw its hat back into the ring as a serious contender in the pursuit of lower-cost, non-pyrometallurgical methods for treating concentrates from primary sulfide ores.
The $40-million plant began processing concentrate in March and achieved design capacity within two weeks, according to PD executives. Startup of the facility represents a major milestone in the company’s renewed focus on finding alternatives to conventional methods for processing concentrates.
The Phoenix-based producer had been active in concentrate-leach R&D during the ’70s and early ’80s, but devoted most of its metallurgical efforts throughout the ’90s to fine-tuning its conventional heap-leaching, beneficiation and smelting practices. Starting in 1998, however, the company began looking closely at various technologies for concentrate processing, including sulfate- and halide-based leaching, biological oxidation, and ammonia-based chemistry. The work was driven by concerns over rising capital and operating costs for smelting and refining, the need to lower its production costs and develop safe, environmentally sound alternatives for processing concentrates.
Although all the various third-party processes reviewed offered some degree of commercial viability, none were deemed universally appropriate for solving the company’s long-term concentrate processing objectives. Instead, PD began work on developing a proprietary method for handling gold-bearing copper concentrates from the Candelaria mine in Chile, owned 80% by PD and 20% by Sumitomo Metal Mining/Sumitomo Corp. According to John O. Marsden, PD’s vice-president of technology and development, batch-scale laboratory testing of various systems identified two promising technologies: high- and medium-temperature pressure leaching.
Both approaches offered certain advantages, based on a producer’s specific needs-or lack thereof-for the sulfur extracted from the concentrates. A high-temperature pressure leach system yielded almost all of the sulfur in acid form, while the medium-temperature approach produced some acid and a large amount of elemental sulfur. Based on batch testing conducted by several metallurgical labs, and more than 30 long-term pilot-plant runs performed by Hazen Research, PD decided to initially install the high-temperature process, which had provided copper recovery rates of 98% or more over a number of different operating conditions.
In August 2001, PD entered into a technology partnership arrangement with Placer Dome Inc., in which the two companies agreed to further develop and commercialize technology for extraction and recovery of copper, gold, silver and other co-products from base metal and precious metal concentrates. Just a few months later, in October, PD decided to proceed with construction of a high-temperature concentrate-leach demonstration plant at Bagdad.
For Phelps Dodge, which produced 1,275,600 st of copper in 2002, lower-cost processing of primary sulfide ores is a high-priority objective. These ore types account for a large majority of the world’s known copper reserves. Chalcopyrite, one of the most prevalent sulfide ores, is difficult to dissolve in traditional heap and stockpile leaching. Depending on the ore size and the specific leaching conditions, between 50% and 75% of contained copper can be leached from oxide ore and secondary sulfide ores such as chalcocite, but traditional leaching of chalcopyrite typically recovers only about 10% to 15% copper even over an extended treatment period.
Consequently, the choice of Bagdad as site for the demonstration plant was fairly easy, according to Marsden. “We wanted to target a concentrate that was not amenable to heap leaching and a site for which the economics favored concentrate leaching. Bagdad’s product was ideally suited for this.” Approximately 95% of the concentrate produced by the 82,000-st/d Bagdad mill is chalcopyrite.
“We are able to use the sulfuric acid generated by the high-pressure leach process in our stockpile leaching operations at Bagdad,” Marsden explained. “Formerly, concentrated acid for this purpose had to be shipped to Bagdad from our smelters.
“Freight costs are a major factor. By installing the pressure leaching process at Bagdad, we can reduce or eliminate the cost of shipping concentrate to the smelters. And, since the mining operation has to pay the smelters for the cost of sulfuric acid itself as well as shipping charges, we can eliminate those costs by using the acid from the pressure leach operations.
“Also,” Marsden noted, “Bagdad is 100% owned by Phelps Dodge. That’s an important factor when you’re trying out new, proprietary technology. Plus, we have a well-trained and adaptable workforce there. Over the years, they’ve proved to be very amenable to new technology.”
The demonstration plant can handle annual throughput of 63,000 st of concentrate (depending on the concentrate grade), producing 35 million lb of copper at a recovery rate of 98%. Sulfuric acid production is 140 st/d. In brief, Bagdad’s chalcopyrite ores are mined and milled to produce copper concentrate. Part of the concentrate is sent to PD’s facility in Miami, Arizona, for smelting and refining. The remainder of the concentrate is sent in slurry form to the demonstration plant. Pregnant leach solution (PLS) from the concentrate leaching process is added to PLS from traditional leaching at the site. The PLS goes to Bagdad’s electrowinning tank house to produce copper cathode. Sulfuric acid produced through concentrate leaching is recovered and added to the heap leach stockpiles, replacing some of the acid that is supplied by the Miami smelter. In order to accommodate the additional input, Bagdad’s electrowinning capacity was expanded from 13,600 tpy to 26,000 tpy and a number of modifications were made to the mine’s solution extraction facilities.
The pressure-leach setup at Bagdad consists of a concentrate repulping system, pressure leach vessel, flash letdown and gas scrubbing, four stages of countercurrent decantation, a four-stage solid residue neutralization system, and a copper-bearing solution storage pond. The centerpiece of the plant is the 53-ft-long, 11.5-ft-diameter pressure leach vessel (PLV). The interior walls of the steel PLV are protected by a chemical-resistant lining and three courses of refractory brick. The PLV is designed to operate at a nominal pressure of 475 psi and temperature of 225° C (440° F), and is divided into five compartments; oxygen and water can be injected into all of these compartments for oxidation and slurry cooling, respectively. Average slurry retention time is 70 minutes.
In operation, filtered concentrate from the mill is fed by wheel loader into a hopper, where it is conveyed to an agitated repulping tank and mixed with water to produce a slurry containing 65% solids. The slurry then reports to an agitated batch transfer tank and from there to another storage tank where additional water is added to adjust its composition to 40% solids. Two Toyo pumps handle the transfer of slurry from storage tank to the PLV at 80 gal/min.
The steam-heated slurry is agitated by Lightnin units in each compartment to maintain slurry suspension and disperse oxygen. Slurry discharged from the PLV reports to a single-stage, 14-ft-diam by 28-ft-high flash vessel which reduces the slurry pressure to about 3 psi. Gases and steam from the flash vessel are scrubbed in two stages, while the still-hot slurry is delivered to a first-stage CCD thickener feed tank where it is mixed with overflow solution from a second-stage CCD thickener.
The plant’s four-stage CCD section comprises four 30-ft-diam by 8-ft-high Westech Hi-Flo thickeners.
Flocculant is added at each thickener to promote solids settling and facilitate solid-liquid separation. The circuit is designed to recover more than 99% of the copper contained in solution in the CCD feed.
Overflow from the first CCD thickener is piped to the product solution tank and then to the strong PLS storage pond, where retention time is seven days. Underflow from the fourth CCD thickener passes through a four-stage neutralization system prior to disposal in the Bagdad flotation tailings facility.
According to Marsden, the plant’s commissioning went quite smoothly, with few problems encountered-and most of those in the nature of minor snags typical of similar processing facilities. In fact, he noted, the plant’s efficiency has surpassed company estimates. “We assumed we’d have to run the plant for at least a year to reach a steady-state operation where we could accurately assess availability and costs,” he said, “but with the smooth startup and operation, I think we’ll be comfortable determining those figures much more quickly than expected.”
What are the potential savings offered by the technology? “Pressure leaching is a very promising area, but it’s not a panacea that’s going to eliminate copper smelting for every producer,” Marsden cautioned. “In some respects, you’re comparing apples with oranges if you have a brown field situation in which conventional smelting capacity is readily available or expandable, as opposed to a green field situation with no existing conventional facilities. In our case, we’re looking at a target operating savings of about 30% over traditional smelting.
“Pressure-leach technology is very site-specific in its suitability and benefits, and its cost-effectiveness will be determined by the same factors we considered when selecting Bagdad as the demonstration site. These factors include the availability of existing smelting and refining capacity, the cost of shipping concentrates to a smelter, acid purchase and shipping costs, and the availability of adequate SX capacity to treat pressure-leach production.”
Concentrate shipping and acid costs are important in determining the suitability of the new technology, Marsden explained, as concentrate transport charges generally are a major component in the cost of production. For example, a $10/ton variance in freight rates for a chalcopyrite concentrate containing 30% copper results in a $0.017/lb change in unit production cost. Plus, acid prices vary greatly throughout the industry and can range anywhere from $10/ton to more than $50/ton.
Scale-up from a commercial demonstration plant to a facility designed to produce 160,000 mt/y would involve a scale-up factor of 10:1. Although the original demo plant cost $40 million, the capital cost of scaling-up to a commercial plant of that size would not be directly linear with the higher production capacity and would actually be quite a bit less per unit of production, according to the company.
In addition to being set-up to handle “outside” concentrates (i.e., non-Bagdad concentrates), the demo plant also has been designed to allow a changeover to medium-temperature pressure leaching with some modifications. These two factors provide PD with a wide degree of flexibility in adapting the technology to its other copper properties.
Medium-temperature pressure leaching “produces a lot less acid and consumes less oxygen,” according to Marsden. “So, it’s a matter of tailoring the process to optimize the economics. The key to suitability for one approach over the other is the acid balance-if we want to apply pressure leaching technology at a location where the acid requirement is far less than at Bagdad, then medium temperature leaching would be appropriate.
“For example, processing requirements at our Cerro Verde property in Peru will be much different than at El Abra in Chile, although both are massive sulfide deposits. We’re currently looking closely at the possibility of applying some form of pressure leaching at either or both of those properties, but a final decision hasn’t yet been made.”
PD also is studying the possibility of adding direct electrowinning-in which solvent extraction is replaced by a solids/liquid separation phase to remove residue from the copper-bearing solution before it goes to EW-to the pressure-leach plant’s product stream. “We’re making good progress [on direct electrowinning] but still have additional development work to do,” said Marsden. “It’s a very exciting prospect, with potential for both capital and operating costs savings.”
In summary, Marsden reports that PD “sees a number of applications coming down the road for the application of concentrate leaching. With about seventy percent of the world’s remaining copper reserves being primary sulfide ore bodies, clearly there will be number of locations that will be well-suited to pressure leaching technology. It certainly isn’t going to shut down every copper smelter overnight, but we see concentrate leaching as an important, emerging alternative.”
Editor’s note: For additional information regarding this technology, readers are invited to refer to the paper “Hydrometallurgical Processing of Copper Concentrate by Phelps Dodge at the Bagdad Mine in Arizona, authored by John O. Marsden and Robert E. Brewer, presented at ALTA Copper-8, May 21-22, 2003, Perth, Western Australia.
By Russell A. Carter, Managing Editor
Copyright Mining Media Sep 2003
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