Cerro Verde began operations in April 1977, originally formed and operated by Minero Peru, as a state-run mining company. The electrowinning tankhouse was designed to produce 33,000 metric tonnes per year (mtpy) of copper cathode operating with current density of 184 amps per square meter (A/m2) and 88 percent (%) electrical current efficiency. In 1994 the mine was privatized and purchased by Cyprus Amax, which undertook an expansion project to expand cathode production to 48,000 tonnes per year using a current density of 250 A/m2 and 90% current efficiency. Cathode production surpassed this design by 1997 through innovative process optimization and increasing current density. Phelps Dodge Corporation purchased the property in 1999 and production continued to increase with current densities reaching 368 A/m2 in 2002.
This article discusses modifications to tankhouse electrolyte flows, tenors and temperatures, electrode geometry changes, cell modifications, smoothing agents, cycle times and operating practices which resulted in higher production levels while improving the copper cathode quality
The Cerro Verde mine is located in southwestern Peru, about 20 kilometers south of Arequipa at an elevation of 2700 meters. The mineral resource is a porphyritic sulfide deposit that had a copper oxide capping. Mining commenced under Minero Peru in April 1977 with dump leaching of coarsely crushed oxide ores, principally chrysocolla and bronchantite. Copper was extracted on asphalt-lined leach pads and recovered in the first solvent extraction and electrowinning (SX/EW) plant in Latin America. Production capacity was 33,000 metric tonnes per year (mtpy) of high purity copper cathodes.
At the time, the use of solvent extraction followed by electrowinning for low grade, impure leach solutions was a novel approach. The economic viability of this process had only recently been demonstrated at the Bluebird Mine (Arizona), with larger commercial installations at Bagdad (Arizona) and Chingola (Zambia). The decision to employ this new technology gave added value to the oxide ore overlying the copper porphyry ore body.
In the early 1980’s the oxide ore reserve was exhausted and the mining operation began to exploit secondary sulfide ore, principally chalcocite and covellite. Copper cathode production levels declined to 18,000 mtpy as the existing process was less effective for treating secondary sulfides.
Cyprus Amax acquired the mine in 1994 as part of a national privatization process and undertook a process improvement project with the aim of producing 48,000 mtpy of cathodes. The initial project goals were quickly surpassed and continued improvements over the past seven years enabled production to reach the current level of 90,000 mtpy. In 1999 Phelps Dodge acquired Cyprus along with its participation in Cerro Verde.
Since 2002 the Cerro Verde EW tankhouse has been operating at current density levels of 270 A/m2 while producing good quality cathodes. Optimization programs are underway to utilize the rectifier capacity fully and operate at even higher current densities to develop more copper production from the leaching operation.
ORIGINAL electrowinning DESIGN: The original process design, as developed by Minero Peru, was part of two-staged plan to develop the reserve. The oxide ores would be mined and leached in Phase I exposing the secondary sulfides. Phase II would later mine the uncovered secondary sulfides for processing in a conventional mill along with the primary sulfides. Wright Engineering Limited from Canada designed the leach/SX/EW operation. The original operation is described briefly, much of the early EW flow sheet remains the same today.
A two-stage crushing plant was operated in open-circuit. Ore was truck dumped into an Allis Chalmers 60 X 89 inch gyratory crusher and the product conveyed to a single, seven-foot Symons standard cone crusher. Crushed ore was set to 80% minus 50 mm size. In 1982, due to recovery concerns, a second cone crusher was added to the secondary and the product sized was reduced to 80% passing 25 mm.
The secondary crusher product was conveyed to a load-out bin and transferred to haulage trucks and transported to rubberized-asphalt lined pads for leaching. The ore was dumped in 4.5-meter high lifts on the valley-fill, permanent pads. Strong sulfuric acid was applied as a cure followed by leach solution application over a cycle time that varied between 45 to 120 days depending on the soluble copper content of the ore. The pregnant leach solution (PLS) was pumped from the solution collection ponds to the SX plant.
The SX plant was built with four parallel trains each having three extraction and two stripping mixer-settlers. The total flow capacity was 4,921 liters per minute (l/min) of aqueous leach solution using a 1:1 organic to aqueous ratio. The design was typical first generation with single-stage round mixers and long, rectangular settlers.
The copper-loaded electrolyte from the SX flowed by gravity to one of three electrolyte tanks at the EW tankhouse. The rich electrolyte with a high copper tenor was pumped from the starter recirculation tank to 18 starter sheet cells each with a flow rate of 378 l/min. The starter cell discharge reported by gravity to the second, commercial recirculation tank from which the electrolyte was pumped to the 208 commercial cells each with a flow rate of 189 l/min. The commercial cell discharged to the weak electrolyte tank where most, approximately 80%, overflowed a weir back to the commercial recirculation tank and the rest was returned to the SX as lean electrolyte. Make-up water, acid and cobalt additions were done in the weak electrolyte tank. The system operated at ambient temperature without the use of heat exchangers.
The EW tankhouse was built with three separate cell sections; one starter sheet section and two commercial sections, Block A and Block B. Within each section the cells were connected in parallel hydraulically and in series electrically. The general layout of the original tankhouse is shown in Figure 1.
The individual cells in each section were fed from branch lines coming off a main manifold. The electrolyte entered the cells through a vertical bored pipe with a butterfly valve at the top for flow control. The cells were constructed with reinforced concrete and lined with paraliner lining and acrylic sheeting as bumper boards. The cells were placed on glass insulators for electrical insulation and supported on columns 1.3 meters above the tankhouse floor.
The three cell sections each represented a separate electrical power block equipped with a transformer and rectifier for converting alternating current to direct current. The starter sheet section was arranged in two rows of 9 cells each, a total of 18 cells, with a 20,000-amperes transformer/rectifier. The commercial sections each had two rows with 52 cells per row for a total of 104 cells per power block. Each commercial block was supplied with a 20,000-amperes transformer/rectifier.
The electrode spacing in the cells was set at 101.6 mm with 46 cathodes and 47 anodes. The starter sheet blanks were of stainless steel design, 316L, and harvested daily to produce 1.0-meter by 1.0-meter size sheets. The commercial section cells operating design was for 182 A/m2 with a seven-day plating cycle.
EXPANSION PROJECT: In 1994 an expansion was undertaken with the objective of increasing cathode production 45% to 48,000 mtpy. At the time cathode production had fallen to a level of 18,000 mtpy about half the nominal EW design capacity. Since initiating operations the cathode quality had been reasonably good and the facilities were well maintained. The general opinion was the plant was capable of producing high quality cathodes at 250 A/m2 with some basic process improvements. Project design and engineering was contracted to Fluor Daniel Wright of Canada.
The evaluation of the existing tankhouse design identified areas for incorporating EW process improvements that had become accepted practices in the industry in the twenty years since the Cerro Verde start-up.
A trade-off study was performed between continued use of starter sheets and conversion to permanent cathode blanks. The experience of the newer operations coming into operation in the mid-1990’s using both Mount Isa and Kidd Creek technologies pointed to the advantages in higher current densities, operating flexibilities and reduced manpower. However, results of the study indicated manpower reductions did not justify the capital required for such a conversion and the decision was made to continue with the existing copper starter sheets using 7 to 8 day harvest cycles.
Included in the expansion project was the construction of a fifth SX train to accommodate increased PLS volume from the leaching operation and nearly doubling the SX/EW flow capacity. The electrolyte flow between the SX and EW incorporated a system of dual-media filtration to reduce organic entrainments and solids. The system included seven Di-Sep filters with anthracite/garnet beds operating in parallel.
The operating parameters for rich electrolyte copper tenor were increased to 47 grams per liter (gpl) and 35 gpl for the lean electrolyte to ensure sufficient copper concentration in the cells to maintain starter sheet and cathode quality. The electrolyte bleed system capacity was increased to assure the 1.5 gpl Fe parameter. In addition, a new water purification plant was installed for make-up water supply.
An electrolyte heating system was installed consisting of a new fuel oil boiler and four plate and frame heat exchangers. Rich electrolyte from the SX first passes through two heat exchangers in series and countercurrent to the lean electrolyte. The other two exchangers operating in parallel using hot water from the boiler heat the slightly warmed electrolyte to the final temperature. The temperature exiting the commercial cells was controlled to 42°C.
The capacity of the commercial Power Block A and Power Block B needed to increase to deliver the higher amperage required. A new 30,000 amperes transformer rectifier was installed on Block B. The 20,000 amperes units removed from Block B were installed in parallel with the existing units on Block A to give that side an installed capacity of 40,000 amperes. Some minor changes were required in the main bus bars to the power blocks but the inter-cell bus bars and cathode hanger bars were properly sized for the increased current.
A cathode press was installed to ensure the starter sheets hang down straight and parallel to the anode. This proved to be a critical process improvement considering the conversion to rigid permanent blanks was not deemed economical. The starter sheets were removed and pressed 36 hours after initial placement in the cells. The pressing operation has an important impact on current density and quality in a starter sheet operation.
A new Wenmec starter sheet assembler was installed to replace the manual loop punch. The machine rigidized the sheets and attaches them to the hanger bars with copper loops. This increased the safety, speed and productivity of the starter sheet operation and also improved sheet performance in the cell until presses by the cathode press.
Operating at the higher current densities required changing the crane pull rack to avoid subjecting cathodes in the cell to peak densities during pressing or harvesting. The original rack pulled half a cell, 23 cathodes, at a time. Operating at 250 A/m2 would subject the cathodes in the cell to peaks of over 400 A/m2 for periods of 10-12 minutes. A new pull scheme to pull the cell by thirds was adopted with a new rack design.
CURRENT SX-EW OPERATION: By the mid-1996, modifications to the tankhouse as part of the expansion had been completed. The production goals were quickly met and quality remained 100% Grade A cathode. Process changes in the crusher and leaching areas resulted in higher than expected metallurgical performance and Cerro Verde sought to increase plating capacity even more. Further changes were pursued through process optimizations to fully utilize the installed rectification capacity.
Operating at a current density of 255 A/m2 with an 8-day harvest cycle was considered to be reasonable parameters for a starter sheet operation. Even so, it was felt the cathodes were relatively heavy at 110-120 kilograms each. To reduce the cathode weight the surface area was increased. The number of electrodes per cell was increased to 48 cathodes and anodes to 49. This increased the cathode surface area from 92 to 96 m2. The starter sheet area was increased 2 centimeters (cm) in length by raising the solution level in the cells and 0.5 cm in width with new mother blanks. Overall the cathode surface area was enlarged to 98.4 m2 per cell.
Copper production from the leach pads had reached 150 metric tonnes per day (mtpd) by the end of 1996, about 14% above the expansion design of 132 mtpd. To meet the higher demand on cathode output the rectifier amperage was raised to 27,000, corresponding to current density levels of 270-280 A/m2. The higher amperage caused the inter-cell bus bars to overheat. The original design capacity with a cross sectional area of 750 square millimeters (mm2) was exceeded. All the bus bars were replaced with a new triangular bar of 1,048 mm2. In addition, the bus bars were water-cooled using drip emitters placed on the electrodes and directly over the bus bars.
In 1998, an evaluation of the operation was performed to increase current densities to 300 A/m2 to process additional copper resulting from improved leach recoveries. A time study of the tankhouse crane operation indicated the feasibility of reducing harvest cycles from 8 days to 7 days. This would allow an increase in amperage without increasing the cathode weight. An additional cathode was placed in the cells by modifying the feed pipe to provide more space. Additional starter sheets were required for the larger cell harvest and the starter sheet production was modified.
The starter sheet loops were reduced in length from 60 cm to 50 cm allowing a more efficient use of the sheets. Each sheet could then produce 20 loops instead of 14, reducing the scrap production by half. The mother blanks were replaced with a larger design to provide greater surface area. Together with the additional cathode in the cells these modifications resulted in a cathode surface area of 102.4 m2. A diagram illustrating the change in starter sheet utilization is given in Figure 2.
The production gradually reached a level of 170 mtpd operating at a current density of 300 A/m2. The current capacity rating of Power Block B was been reached at 30,000 amperes. The rectifier refrigeration system was enlarged to allow the unit to safely operate at 35,000 amperes. The flow to the commercial cells was also increased 20% from 189 l/m to 227 l/m. This ensured continued Grade A quality at the higher operating levels.
During this same period an important improvement was made to the anode insulators. The new “Victoria” design essentially consisted of a clip placed on the lower corners of the anode to maintain electrode separation and reduce short circuits.
In 1999, a series of tests were performed to determine the impact of higher current densities on cathode quality. Current densities ranging from 335 A/m2 to 390 A/m2 were achieved in certain cells by removing cathodes. The cycle time was evaluated at these current density levels for 6, 7 and 8 days. In general, the results indicated quality could be achieved at all levels of current density. The physical appearance of the higher current density and longer cycle cathodes was poorer than those at lower current density and shorter cycle times. This was not unexpected and because the tests were performed over a short period of several months there was some question regarding long-term deterioration of the anodes.
The plant operated for the next two years at levels of 320 A/m2 to 330 A/m2 producing between 190 mtpd to 195 mtpd. The Power Block B had nearly reached its rated capacity of 35,000 amperes. In late 2000 two additional starter sheet cells were added to reduce harvest cycle to 6 days and to continue raising the current density on Power Block A, rated at 40,000 amperes.
In early 2001, due to improved recoveries, unleached ore representing over two months production had accumulated on the pads. To bring this material under leach the decision to expand the tankhouse was made.
The aim of the expansion project was to increase capacity in EW to produce 230 mtpd of cathode with an operating current density of 365 A/m2. The rectification in Power Block B was augmented with the addition of a 10,000-ampere transformer rectifier installed in parallel to the existing 35,000-amperes unit. Power Block A was expanded by 14 cells to a total of 118 cells. Two more cells were added to the starter sheet section for a total of 22 cells. The main bus bar system and inter-cell bus bars were replaced with larger bars to operate at the higher amperage. The electrolyte flow in the SX/EW loop was increased for greater copper transfer.
Shortly after completing the 230 mtpd expansion, in 2002, an additional 8 cells were added to Block B for a total of 112 cells. To improve crane time a second cathode press was installed on the end opposite the existing press. Finally, the cathode length was increased once more by 1.5 cm to provide a cathode surface area of 104 m2. This small increase was accomplished by raising the solution level in the starter sheet cells. The tankhouse layout after the most recent changes is shown in Figure 3.
The Cerro Verde EW tankhouse is currently operating with a current density of 370 A/m2 and producing 250 mtpd of Grade A cathode. A historical summary of the EW cathode production since start-up and the respective operating current density is given in Figure 4.
METALLURGICAL PARAMETERS & OPERATING PRACTICES
The operating practices and metallurgical parameters developed to maintain the cathode quality while constantly increasing the production with current density are listed below. Table 1 compares the changes in the tankhouse since the operation began in 1977.
- Electrolyte Cu concentration exiting the commercial cells is maintained at 38-39 gpl. The ratio of current density to the outgoing copper tenor is around 10. Increasing the flow of lean and rich electrolyte to and from the SX reduces the volume recirculated in the tankhouse thereby increasing the tenor of the feed to the cells.
- The lean electrolyte acid content is maintained below 170 gpl to reduce acid corrosion and acid mist propagation. The SX reagent is LIX 984, which performs at these acid levels. Acid mist is suppressed with the use of FC 1100 surfactant and three layers of plastic balls in the commercial cells.
- Increased electrolyte flow reduces electrolyte recirculation in the SX stripping settlers and increases acid availability. As a maximum, the rich electrolyte is kept below 52 gpl copper to avoid crystallization. The operating philosophy is to maintain maximum flow to the SX while adjusting the tenor with changes in the rectifier amperage.
- Electrolyte flow to the cells is kept between 2.7 to 3.0 l/m/m2 of the cathode surface area. This represents a flow of 17.5 m3/h per cell. This flow rate ensures a high concentration of copper at the boundary layer. Cell manifolds along the bottom and length of the cells are critical for distributing the electrolyte. The manifold holes and their diameters must be carefully designed to suit the dimensions of the cell and the electrodes.
- Electrolyte temperature is maintained at 48-49° This increases the mass diffusion rate of the copper ions to the cathode and prevents crystallization of the electrolyte at the operating concentrations..
- Electrolyte cobalt concentration maintained at 150 ppm. This reduces the lead anode polarization rate and corrosion at the high current density.
- The guar smoothing agent concentration has been found to be optimal between 400-500 grams per tonne of cathode at the present electrolyte temperature. Guar degradation rate is influenced by temperature and the appropriate level must be found as temperatures are increased.
- Meticulous monitoring and control of iron, chloride, manganese, etc. content in the electrolyte is important. The correct level varies from plant to plant. Their impact on quality and cost to control through the use of an electrolyte bleed must be determined.
- Cell cleaning and anode replacement programs must be strictly followed. Tankhouse housekeeping becomes more critical as the current densities are increased. The program calls for each cell to be cleaned three times per year. The anode sludge removed during cleaning allows monitoring of the anode corrosion. The electrolyte feed pipes and manifolds are inspected and maintained during the cell cleaning operation.
CONCLUSIONS: The Cerro Verde EW tankhouse operation has demonstrated over the years the feasibility of producing copper cathode of excellent quality at high current densities. Since the beginning it has strived to continually improve the process through innovation, ingenuity and attention to detail. The prospect of still further improvements is likely as new opportunities arise.