Leaching Copper Without Burning the Pyrite How Oxygen Pressure Becomes a Selectivity Dial

This application covers a way of dissolving the valuable metals out of a mixed sulphide concentrate while deliberately leaving most of the worthless iron sulphide alone. The claimed method runs a continuous oxidative leach and uses one control handle, the partial pressure of oxygen in the vessel, to hold the slurry at an electrochemical potential where copper, nickel, cobalt and zinc dissolve but pyrite and the precious metals largely do not. It was filed jointly by two Australian process consultancies, Malachite Process Consulting Pty Ltd and Hydromet WA Pty Limited, and names Grenvil Marquis Dunn and Bruce James Wedderburn as inventors. The application is a divisional of Australian application 2023382017.

The Problem

Polymetallic sulphide concentrates are awkward feedstocks. A typical example in this specification assays under three per cent copper, a few tenths of a per cent each of cobalt, nickel and zinc, and then roughly 31 per cent iron and 40 per cent sulphur. The metals worth money are a thin seam running through a large mass of pyrite, and every process that has to oxidise the whole concentrate in order to reach them pays for that mass twice, once in reagent and once in waste treatment.

The specification walks through the conventional options and what is wrong with each. Roasting) burns the sulphur off and produces a calcine that leaches readily, but it generates large volumes of sulphur dioxide and sulphur trioxide off gas that has to be captured, usually as sulphuric acid whose disposal the specification describes as expensive, and it can release mercury and thallium to the environment.

Total oxidative pressure leaching is the main competing route. For a high pyrite concentrate the specification calls it expensive and possibly uneconomic, because of the quantity of oxygen needed to oxidise gangue sulphides and the volumes of sulphuric acid generated afterwards that must be neutralised. Biological leaching, in tanks or on heaps, is described as slow and prone to poor recovery, partly because chalcopyrite passivates in a low intensity leach and stops reacting.

There is a subtler problem underneath, and it is the one the invention actually attacks. Multi stage leach circuits are usually run to a terminal electrochemical potential in the final reactor, and for copper that target is often around 550 mV against a silver chloride reference. The specification points out what else happens at that potential. Pyrite and other iron sulphides oxidise excessively, loading the solution with iron and acid, and precious metals get co extracted with the copper and end up in the anode slimes of the electrowinning circuit rather than being recovered properly. The process hits its copper number and destroys value on the way.

What This Invention Does

Claim 1 is a hydrometallurgical method for treating a sulphide concentrate in a continuously operating oxidative leach reactor, where the concentrate is a blend of electronegative and electropositive elements. The method has two steps. A slurry of the concentrate in an aqueous stream containing at least water is submitted to an oxidative leach that targets the recovery of the more electronegative elements over the more electropositive ones. Then the electrochemical potential is adjusted by controlling the partial pressure of oxygen within the leach circuit, so as to preferentially leach the electronegative elements over the electropositive ones. The claim closes by defining the electronegative elements as including at least one of copper, nickel, cobalt or zinc.

The whole invention sits in that second step. Electrochemical potential determines which sulphide minerals will oxidise and how fast, and the specification treats oxygen partial pressure as the throttle that sets it. Dependent claims put numbers on the window: oxygen partial pressure in the range 20 to 500 kPa, and potential controlled between 280 and 600 mV against a silver silver chloride reference. Compare that with the roughly 550 mV terminal potential the background describes as normal practice, and the point of the invention becomes clear. It is a decision to stop short.

The other structural choice in the specification is the vessel. Most pressure leaches use a multi compartment autoclave, and the specification explains why that defeats potential control: the compartments share a single vapour headspace, so the oxygen partial pressure cannot be set independently in any one of them. Remove the dividing walls, run a single compartment, and controlling the headspace oxygen pressure now controls the slurry potential everywhere in the vessel. Claim 15 covers the single compartment case and claim 17 covers the pressure leach autoclave generally.

The results in the worked examples are what make the argument. In Example 1 a concentrate at 2.8 per cent copper was ground to 50 microns and leached continuously at 180 to 240 kPa oxygen and 150 degrees C with sulphuric acid and 5 g/l chloride, holding the potential between 400 and 480 mV. Copper extraction was 95 per cent, zinc 95 per cent, cobalt 82 and nickel 78, while pyrite oxidation was held below 25 per cent. Example 4 is the same story on an as received 95 micron feed with no grinding at all.

Example 5 is the one that proves the mechanism rather than the outcome. A nickel concentrate leached at 110 to 130 kPa oxygen, holding 460 to 470 mV, recovered more than 95 per cent of the nickel. The identical test at 600 kPa oxygen, which lifted the potential to about 515 to 520 mV, recovered only 87 to 88 per cent. More oxygen produced less nickel, because the higher potential passivated the millerite. That is a direct demonstration that the potential window is a real optimum and not simply a cost saving.

Key Features

• Oxygen partial pressure as the control variable. Rather than dosing reagents or chasing a terminal redox reading, the method sets the slurry electrochemical potential by adjusting the oxygen pressure in the autoclave headspace, in a stated range of 20 to 500 kPa.
• A deliberately narrow potential window. Dependent claim 4 holds the leach between 280 and 600 mV against a silver silver chloride reference, well below the roughly 550 mV terminal potential the background describes as conventional for copper.
• Single compartment autoclave. Because compartments in a conventional autoclave share one vapour space, the specification removes the dividing walls so that headspace pressure controls the potential throughout the whole slurry volume.
• Continuous rather than batch operation. The specification states plainly that steady, stable operation of the leach is not possible in a batch process, and that continuous running allows small adjustments to solids density, flowrate, retention time, free acid and oxygen pressure as feed chemistry changes.
• Precious metals left in the residue. Keeping the potential low means gold, silver and the platinum group metals are not co extracted with the copper, so they stay in the leach residue for separate recovery instead of reporting to anode slimes.
• Chloride addition to break passivation. Claims 18 and 19 add 5 to 10 g/l of chloride to the autoclave feed slurry, which the body describes as helping to strip the passivation layer from chalcopyrite and millerite surfaces.

Who Is Behind It

The two applicants are small Australian consultancies rather than mining companies. Hydromet WA Pty Limited is the vehicle of Grenvil Dunn, a hydrometallurgist with a long public record. His profile at Orway Mineral Consultants, where he is Principal Engineer, describes more than fifty years in the industry including twenty six years in hydrometallurgy operations before he started his own consultancy, work spanning nickel, cobalt, uranium, gold, lithium and platinum group metals, and a Fellowship of the Institution of Chemical Engineers. He joined Orway in 2008 as a sub consultant through Hydromet.

Dunn’s earlier patents map neatly onto this one. He is the named inventor on US 7,635,407, an exothermic pressure leach autoclave circuit built around flashing and thickener recycle, which is precisely the flash thicken recycle step used in Example 6 of this specification. He is also lead inventor on US 9,587,290, assigned to Orway Mineral Consultants, covering an electrochemically controlled leach for removing radionuclides from copper concentrates. That patent appears in this specification’s own background section as prior art, credited to Orway. Writing your own earlier work into the background of a later application is unusual enough to be worth noting.

The second applicant, Malachite Process Consulting Pty Ltd, trades as Malachite Consulting and is led by Bruce Wedderburn, a chemical engineer whose stated experience covers nickel sulphides and laterites, copper, cobalt, uranium, manganese, mercury, gold and the platinum group metals, with named project work including Murrin Murrin, Ambatovy and KCGM. The practice lists patent preparation and opposition among its services, which fits a filing of this shape.

On priority, the Australian specification is silent. There is no cross reference paragraph at the head of the body text, and the only relationship the document itself establishes is the title page entry recording it as a divisional of Australian application 2023382017. From the published family record, rather than from this specification, 2023382017 is the Australian entry of PCT/AU2023/051149 filed 13 November 2023, which claims priority from Australian provisional application 2022903424 filed 14 November 2022. That makes the priority country Australia.

Why It Matters

The economics of base metal hydrometallurgy have been shifting for two decades toward concentrates that smelters do not want: too much pyrite, too little copper, awkward penalty elements, and a shrinking global smelting network willing to take them. Every hydrometallurgical answer to that problem runs into the same wall, which is that oxidising a concentrate that is one third iron and two fifths sulphur costs a great deal in oxygen and then costs again in limestone to neutralise the acid produced. Selectivity is not a refinement here, it is the whole business case.

What is interesting about this claim is how little apparatus it asks for. The Albion process, cited in the background, depends on grinding to 20 microns or finer. Other cited routes depend on chloride chemistry, or on feeding pyrite in a controlled ratio, or on a purification mill immediately before the leach. This method asks for a single compartment vessel and a pressure controller, and Example 4 makes the point sharply by achieving 95 per cent copper on a 95 micron feed that was not ground at all.

There is also a metallurgical argument buried in the precious metals clause. In a conventional high potential leach, gold and platinum group metals dissolved along with the copper end up in electrowinning anode slimes, from which recovery is possible but costly and slow. Holding the potential down keeps them in the solid residue, where conventional recovery routes apply. For a polymetallic orebody where the precious metal credits carry a meaningful share of the revenue, that is not a side effect, it is arguably the reason to run the process this way.

Related Concepts

• Pyrite – the iron sulphide gangue mineral whose oxidation this method is designed to suppress.
• Pressure leaching – the autoclave based leaching family the claimed method belongs to.
• Passivation) – the surface layer effect that stalls chalcopyrite and millerite leaching and that the chloride addition targets.
• Platinum group metals – the precious metals the low potential window is meant to keep in the leach residue.
• Electrowinning – the downstream recovery step whose anode slimes collect co extracted precious metals in conventional circuits.

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AU 2026202139 was published in the Australian Official Journal of Patents on 9 April 2026 and is open for public inspection. Patent applications represent inventions that are sought to be protected and do not necessarily reflect commercially available products.