The ability to leach primary copper sulphides has, on many occasions, proved too great a hurdle, with laboratory or pilot-scale design work falling on sub-economic or volatile recovery rates when working on field.

This problem tends to result in one of two things: new capital-intensive concentrators are set up to process these sulphides, or brownfield oxide operations are developed that prolong leaching operations. existing for a few more years when – hopefully – copper prices are higher.

Antofagasta proposed an alternative option that takes advantage of chloride-based reagents and 20 years of knowledge of secondary sulphide leaching.

Called Cuprochlor®-T, the proprietary process has undergone five years of intense development, to the point that the company is now ready to talk about its potential.

It builds on the first iteration of Antofagasta’s leaching technology designed for secondary sulphides – Cuprochlor.

Cuprochlor, now working at the Michilla mine in Chile (which Antofagasta sold in 2016), effectively binds ore particles – particularly fine ones in Michilla’s case – into a porous but manageable material that can then be leached into pile. Agglomeration is achieved by mixing the mineral and leaching solution with chloride salts and sulfuric acid, which react to form a plaster-like paste.

Over the years the process has been refined, continuing to consistently deliver recovery rates of around 90%.

Building on a large resource base of primary sulphides – mainly chalcopyrite – and the success of Cuprochlor, Antofagasta, about five years ago, began a series of tests, adjusting variables such as temperature, concentrations of reagents and particle size to see if the chloride leaching process could be suitable for the treatment of primary sulphides.

Temperature proved to be one of the keys, with tests showing that by raising the temperature of the heap to around 30°C, Cuprochlor-T was able to stimulate the chemical reaction needed to recover copper from primary sulphides such as chalcopyrite.

Another key differentiator between the two chloride leaching technologies is “reagent recipe” and particle size distribution (PSD), said Alan Muchnik, vice president of strategy and innovation for Antofagasta. I AM.

“Providing a consistent temperature throughout the process is very important, but the real innovation is the approach we used,” he said. “It involves a combination of factors including but not limited to the recipe of reagent concentrations and the required PSD.”

While not wanting to reveal the “secret sauce,” Muchnik said the PSD consideration goes beyond the usual P80 industry benchmark.

It is this balance that has earned the company recoveries of over 70% after approximately 200 days of heap leaching in test work.

Muchnik developed this: “The Cuprochlor-T process, in simple terms, involves leaching in a chloride environment – ​​where there are no passivation layer bonds. This allows copper, iron and chloride ions to react which, under controlled temperature, results in economical copper production.

It goes through three steps:

  • First, an agglomeration step where the necessary reactants are added and left to stand at constant aeration and temperature;
  • Second, the ore is intermittently irrigated with continuous aeration, also maintained at a constant temperature; and
  • Finally, after 200 days, the ore completes the leaching cycle and allows the company to obtain recoveries of 70% copper or more.

What started with laboratory tests and evolved into pilot tests and a “semi-industrial” test on several different waste dumps at Centinela has recently concluded with an industrial test of more than 40,000 t of primary sulphide containing in average 0.4% Cu – containing over 90% chalcopyrite – which, using the same process described by Muchnik, has shown consistent recoveries of over 70%, he said.

Alan Muchnik, Vice President of Strategy and Innovation for Antofagasta

Asked if the company was considering even higher recoveries that could rival the levels achieved by Cuprochlor on secondary sulphides, Muchnik said it was an economic compromise.

“It may be possible to achieve such a percentage [as Cuprochlor], but that’s not the goal or the expectation with the kinetics we’re currently seeing in Cuprochlor-T,” he said. “There is always a trade-off between irrigation duration, PSD, and recoveries, all of which are related to capital costs, operating costs, and the return on investment associated with the process.”

Antofagasta’s planning and operations teams have now secured the Cuprochlor-T ‘license’ and will be busy outlining potential deployments for consideration in the company’s annual planning cycles.

There are obvious starting points.

The open pit and heap leach copper mine at Zaldívar, 175 km southeast of Antofagasta, is currently in the process of transitioning to chloride leach operations with cuprochlor.

The project, which includes an upgrade to the SX plant and the construction of new reagent facilities and additional wash basins to control chlorine levels, was completed in January 2022 and is now being commissioned . It is expected to increase copper recoveries by around 10 percentage points, increasing production at Zaldívar by around 10,000 to 15,000 tpa over the remaining mine life.

“In addition to moving to chloride leaching with cuprochlor for secondary sulphides, we are currently progressing studies on the primary sulphide deposit which currently lies under the Zaldívar reserves to prove if cuprochlor-T leaching can work. “, said Muchnik. “In our resource base, there are approximately 460 Mt of declared primary sulphide resources here.”

Both Centinela and Antucoya have primary sulphide resources and existing heap leach and SX-EW facilities that would also fit the Cuprochlor-T plan.

Muchnik said, “One of the technological attractions of Cuprochlor-T is the ability to use an otherwise inactive leaching buffer and the SX-EW capability. This is the scale limit of our current operations, but the technology can be gradually rolled out into a plant already suitable for chloride leaching, phasing it in over the life of mine to meet needs.

“It provides a continuous adoption process option rather than an immediate infrastructure project that sees an operation switch from oxide leaching to another type of heap leach all at once.”

The advent of Cuprochlor-T does not mean the company will abandon potential concentrator projects altogether, Muchnik said, pointing to the second concentrator project currently undergoing a feasibility study at Centinela.

In addition to the capital and operating cost benefits that would come with Cuprochlor-T down the concentration path, there is likely to be a sustainability benefit.

“This is only a rough benchmark as each case is different, but you would expect the energy consumption associated with cuprochlor-T leaching and SX-EW treatment to be less than half the way normal copper concentration and SX-EW,” Muchnik said. .

In this respect, it is a favorable consideration for Antofagasta’s long-term carbon neutrality objectives.

While every potential Cuprochlor-T implementation will have to go through corresponding project studies, Muchnik was confident in the prediction that new copper from Cuprochlor-T would be produced this decade.

With five years of substantial testing under its belt, few metallurgists would bet against it.