A Chilean copper mine devises an action plan to combat unexpected levels of impurities in its ore.

Bolidens Lomas Bayas copper mine in Chile is implementing an action plan to combat unexpected levels of impurities in its ore, which has cut planned production by 30 percent.

Following the start-up at Bolidens Lomas Bayas copper mine, production of copper cathode at the mine averaged only 70 percent of design capacity, hampered by impurities in the ore.

Lomas Bayas uses solvent extraction-electrowinning (SX EW) technology to produce copper cathode. Both crushed and lower-grade run-of-mine (uncrushed) ore is spread on large leach pads. A mild acidic solution is irrigated over the pads. Solution containing dissolved copper passes through the solvent extraction plant and is transferred to the electrowinning tank house where an electrolysis process produces 99.99 percent pure copper cathode.

"Reaching full production levels has been made more difficult by higher than expected levels of chlorides and nitrates in the ore," says mine manager Bob Bennett.

How the Impurities Impact Production

The elevated level of chlorides has made it difficult to strip copper cathodes from the stainless steel cathode blanks used in the EW tankhouse.

The level of nitrates has degraded the organic reagent used in the SX plant, thereby reducing amount of copper transferred from the solvent extraction plant to the eletrowinning circuit and ultimately copper cathode production. The increased nitrates are also responsible for creating unacceptable roughness on the surface of the cathodes.

The metallurgical testing carried out in connection with the feasibility study did not identify the elevated levels. "The combination of impurities in such high levels is unusual," Bennett says.

Solving the Problem, Step-by-Step

"We sought opinions from a wide range of experts, including the international engineering firms who worked on the original design and construction of Lomas Bayas," Bennett says. "We have had our own company experts on SX EW technology research the problem and we have discussed potential solutions with experts in SX EW technology, both within Chile, as well as internationally."

Bennett relates, "We have solved the chloride problem by relatively simple changes in the SX circuit, which have decreased the content in EW chloride to acceptable levels. We are moving on several fronts to reduce the nitrate levels." Bennett explains the operation is now periodically removing and replacing the degraded reagent and rerouting the leach system piping to allow recirculation of nitrates.

"We are evaluating the feasibility of a pre-wash stage before leaching and an evaporation pond to bleed off nitrates," says Bennett. The action plan includes studies on reclamation of degraded reagent and research on new reagents more tolerant to nitrates.

In addition, the mine is examining information from previous drilling and new drilling of the orebody to determine extent of nitrate distribution. Preliminary indications show the elevated nitrate levels are concentrated near the surface of the orebody, and at depth, only in certain parts.

Reprinted with permission from Minett Media.