Low-Cost Electrowinning
Copper processors using solvent extraction/electrowinning (SX/EW) have known for years that eliminating the electrolyte bleed could benefit process economics dramatically. Until now, there was no practical solution to eliminate the bleed. Available processes either could not reduce iron contaminants sufficiently, or they pulled too much valuable copper and cobalt out of the electrolyte along with the iron. Pilot tests of a new ion exchange separation system on representative electrolytes at three mine sites have resulted in an alternative to bleeding electrolytes. Processors can now shelve the wasteful electrolyte bleeding practice. A major copper producer in Mexico is presently installing a full-scale system.
Two new technologies have made this possible. The first is an innovative complexing ion exchange resin called Diphonix Resin from Eichrom Industries Inc. of Darien, Illinois. It is the first resin that binds the iron while leaving valuable copper and cobalt in the electrolyte circuit, where it belongs. Second, is a continuous ion exchange separator which makes it economically feasible to sustainably reduce iron to at least 2.0 grams per liter—levels necessary for efficiency in copper electrowinning. Called ISEP® by its developer, Calgon Carbon Corporation, the continuous separator dramatically increases productivity of the Diphonix Resin.
Though a newcomer to copper mining and minerals processing, the ISEP continuous separation system has proven itself in more than 250 chemical, food, pharmaceutical, wastewater, and water processing operations since 1990. The ISEP has improved efficiencies and process economics in these more familiar ion exchange applications.
Compared with intermittent fixed-bed separation, the ISEP equipment improves process optimization and resin productivity in this copper application. In addition, it reduces water usage and waste discharge.
Mexicana de Cananea of Sonora, Mexico, commissioned its first full-scale solvent extraction/electrowinning (SX/EW) plant in 1980. It will be the first company to implement this innovative iron control process. Cananea's copper mine hosted one of the pilot studies.
"Economic analyses of the ISEP/Diphonix pilot studies indicate dramatic savings in electricity and recovered copper and cobalt," says Gordon Rossiter, Calgon Carbon Corporation's Director of New Development Projects. Projections are that a typical 50,000 ton-per-year copper electrowinning plant would see a payback in 18 months.
One of the persistent and costly challenges associated with the SX/EW process for copper recovery is the build up of iron (Fe+3). Iron is leached along with copper from the ore and accumulates in the electrolyte circuit as the copper is removed in electrowinning. Soluble cobalt is added to the copper electrolyte to control corrosion of the lead anode and to prevent spalling and possible lead contamination of the copper cathode. Until now, processors have bled a portion of the electrolyte. In doing so, the loss of copper and cobalt contained in the bled electrolyte has increased their operating costs.
Cananea's operation provides a typical example of iron accumulation in a solvent extraction and electrowinning circuit. Leaching of copper ore contributes about 900 kilograms of iron per day to the electrolyte circuit. Factor in the iron inventory which has built up over years of partial bleeding, and the total iron concentration is a significant 9-10 grams per liter. The high iron levels reduce the efficiency of the electrical current. Electricity that could be spent electrowinning copper is diverted to convert iron from Fe+3 to Fe+2. By reducing iron concentrations in the electrolyte, a copper processor can improve process efficiencies and reduce electricity costs per ton of electrowon copper.
Eichrom Industries had determined in the laboratory that their innovative resin would remove the iron and that the company would benefit from a more efficient method of resin contacting.
Calgon Carbon Corporation and Eichrom determined that the ISEP ion exchange separator loaded with Diphonix Resin had the potential to remove iron from the copper electrolyte and allow recycle of incremental copper and cobalt. This innovative process would eliminate the electrolyte bleeding practice.
The ISEP system in tandem with the innovative Diphonix Resin completed three rounds of pilot tests amounting to greater than 600 cycles over a period of six months. The conditions and results of these tests follow.
The first pilot test was performed in San Manuel, Arizona. The ISEP handled 230 sequential loading and stripping cycles. During the testing, iron concentration at the San Manuel mine was 1.5 grams per liter. The purpose of the test was to confirm the economic viability of the ISEP/Diphonix commercial plant and to optimize the ISEP ion exchange parameters for copper SX/EW plants. Areas of interest included iron removal capability, resin stability, resin capacity and hardware reliability, effects of acid and copper concentrations, and optimum iron stripping time and temperature.
The data showed that the ISEP/Diphonix separation performed above expectations on all counts. Consistent iron loading of 8 to 12 kilograms per cubic meter of resin was demonstrated for the duration of the test. The tests determined the ideal strip solution temperature to be 70°C, 15°C lower than determined in laboratory tests. Furthermore, with the revised operating parameters, the tests demonstrated 10-20% less Diphonix Resin than originally projected would be required in commercial operation.
"The word about the pilot tests quickly traveled across the border to Mexico. Cananea heard about the breakthrough and asked to observe the system in action," says Rossiter. "Once they witnessed the capabilities of continuous separation with Diphonix Resin, they asked for a pilot test of their own."
Cananea's particular hydrometallurgy was more demanding than that experienced in the first pilot test in Arizona. Iron concentration at 905 grams per liter is unusually high, so it provided a challenge for the ISEP/Diphonix Resin tests.
The pilot test lasted for a two-month period with more than 250 loading and stripping cycles. The purpose of this test was to confirm how much iron Diphonix Resin would adsorb and to continue the evaluation of the system parameters for commercial operation in the Mexicana de Cananea environment.
Cananea's specific leaching environment created a more stringent requirement that iron control be demonstrated while not significantly increasing the acid balance associated with the current bleed operation.
The Cananea pilot studies involved two electrolyte recycle tests to stimulate long-term operation of the ISEP control unit on the iron levels in the total tankhouse electrolyte. The lean copper electrolyte used as the feed solution had the following approximate composition:
Iron (g/L)
Average: 9.5
Range: 8.0-11.0
Copper (g/L)
Average: 35.7
Range: 28.0-41.1
Treated electrolyte was collected in a 55-gallon drum after processing through the resin columns. This collected electrolyte was then recirculated through the unit repeatedly. The objective was to demonstrate that ferric iron could be removed from the electrolyte. Electrolyte also contains 15-20% ferrous ion (Fe+2), or about 2.0 grams per liter, which would not be removed by the resin, but does not impair the electrowinning process.
The objective of lowering the acid concentration in the process effluent was met during a final phase of the study. The low pH and difficulties with acid balance in the past were a direct result of electrolyte bleeding and the characteristics of the Cananea hydrometallurgy. By adjusting the acid concentration in the rinse solution and the flow of operating parameters, a total reduction in acid effluent of about 40-50% was demonstrated. Iron control was not affected by these changes. The ability to control acid content of plant effluent while controlling iron will be of interest to sites mining acid-generating ore bodies.
A third pilot test was performed in Arizona. The system processed more than 100 loading and stripping cycles, with a copper electrolyte solution with an iron concentration of 3 grams per liter. The purpose was to prove the efficiency of the system in removing iron impurities, thereby improving electricity utilization.
The ISEP/Diphonix Resin system removed at least 50% of the electrolytic iron, without removing any of the valuable copper and cobalt, demonstrating Diphonix Resin's selectivity for Fe+3 over copper and cobalt. These results improved progressively in subsequent studies, culminating with processing of iron concentrations as high as 11.5 grams per liter.
ECONOMIC IMPROVEMENTS
With the three pilot tests, iron removal, resin stability and low copper, cobalt and acid losses were demonstrated with ISEP/Diphonix continuous separation. The predicted effects on copper processing economics are as follows:
Diphonix Resin, a styreno-divinylbenzene resin, was recently developed and patented at Argonne National Laboratory and the University of Tennessee and exclusively licensed to Eichrom Industries, Inc. Unlike a conventional ion exchange resin, Diphonix material contains additional complexing ligands that select certain classes of elements, including Fe+3. This resin is able to keep the copper and cobalt in the electrolyte, while removing and controlling iron.
The ISEP continuous ion exchange separation system was developed by Calgon Carbon Corporation. In a typical ISEP, a carousel arrangement of 20 moving columns of small ion exchange beds slowly rotates between 20 stationary ports. The carousel, rotating under the distribution ports, moves the ion exchange beds through normal ion exchange sequence: adsorption, wash, strip, and rinse. This results in lower resin inventory compared to conventional ion exchange equipment. The countercurrent stripping and rinse flow combine to reduce the amount of strip chemical required to regenerate the resin.
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