Refractory Gold Ore Treatment: Options and Trade-offs

Key takeaways

• A refractory gold ore gives poor recovery by direct cyanidation, usually because the gold is locked inside sulphide minerals such as pyrite or arsenopyrite.
• The main treatment routes break down or open up those sulphides before cyanide leaching: roasting, pressure oxidation (POX), bacterial oxidation, ultrafine grinding, and ultrafine grinding combined with atmospheric oxidation (the Albion Process™).
• At the GPM Gold Project in Armenia, adding the Albion Process™ lifted recovery from the sulphide concentrate from 20% to over 98%, after the existing CIL circuit had recovered only around 20 to 30% of the gold.
• The right route depends on how the gold is locked, so treatment starts with grind and leach testwork and mineralogy on representative samples.

A regrind mill is where many concentrators decide their final recovery. Grinding is one of the most energy-demanding stages in mineral processing, and depending on the ore and circuit design, it can account for up to 50% of a plant's total processing costs. Yet many valuable minerals are not fully liberated at the primary grind. 

They report to the rougher concentrate as composite particles, which require further grinding to liberate valuable minerals from gangue and improve both concentrate grade and overall recovery

A well-designed regrind circuit does more than reduce size. It allows the primary grind to stay coarser, applies energy only to the smaller tonnage that needs it, and produces clean mineral surfaces that float well. A poorly designed one adds circulating loads, harms flotation chemistry, and consumes power without a matching gain in recovery. This article sets out the grinding circuit design principles Glencore Technology developed at Mount Isa, how inert stirred milling changed regrinding, and how the approach now extends into coarser grinding duties.

What makes a gold ore refractory?

A gold ore is refractory when cyanide cannot dissolve enough of the gold to make recovery economic, most often because fine gold is locked inside sulphide minerals. Conventional grinding can liberate the host sulphide, usually pyrite, from the gangue, but it cannot expose gold that is finely disseminated within the pyrite or held in solid solution. Direct leaching of that material gives poor gold extraction, because the cyanide never contacts the locked gold.

Pyrite is the principal gold-bearing mineral in most refractory gold concentrates, and arsenopyrite is also common. The type of host matters. In testwork published by Kalgoorlie Consolidated Gold Mines (KCGM), gold locked in arsenopyrite did not reach the same recovery as gold disseminated in pyrite after fine grinding, because the locked gold particles in arsenopyrite were smaller.

Refractory concentrates can also contain other phases that complicate treatment. Telluride minerals can hold high levels of gold and silver. Carbonaceous material can adsorb dissolved gold, which is known as preg-robbing. Each of these affects which treatment route will work.

Why do refractory gold ores give low recovery?

Refractory gold ores give low recovery because conventional cyanide leaching only dissolves gold it can physically reach. If gold stays encapsulated in sulphide, cyanide cannot dissolve it, and that gold is not recovered.

The GPM Gold Project in Armenia shows the scale of the problem. GeoProMining (GPM) needed to treat sulphide material beneath the oxide ores at its Zod mine. Through its existing conventional carbon-in-leach (CIL) flowsheet, that sulphide material gave only around 20 to 30% gold recovery. The gold was there, but most of it could not be recovered without first breaking down the sulphides.

What are the options to treat refractory gold ores?

The options to treat refractory gold ores all work by breaking down or opening up the host sulphide so cyanide can reach the gold. The established methods are thermal (roasting), pressure-based (pressure oxidation, or POX), biological (bacterial oxidation), mechanical (ultrafine grinding) and atmospheric oxidation combined with ultrafine grinding.

Sources: Ellis, KCGM; Glencore Technology, The Albion Process for Refractory Gold; Glencore Technology Albion Process™ pages.

The oxidative routes are metallurgically sound and achieve high recoveries. As the KCGM paper notes, though, treating their reaction products can change the economics. Capturing sulphur dioxide from roasting, or neutralising acidic liquors from pressure oxidation, can add high cost, and in some cases that makes another route more attractive.

Ultrafine grinding alone can be a practical alternative in particular cases. At KCGM, testwork showed that grinding the refractory concentrate to 10 microns could lift gold recovery from 75% to 92%, without destroying the host mineral.

How does atmospheric oxidation treat refractory gold ore?

Atmospheric oxidation treats refractory gold ore by ultrafine grinding the sulphide concentrate and then oxidising it with oxygen at atmospheric pressure, so the locked gold is exposed for cyanide leaching. The Albion Process™ applies this in three steps.

1. Concentrate. A bulk sulphide concentrate is produced by flotation. A high-grade concentrate is not required, and rougher flotation alone is recommended to produce it.
2. Ultrafine grinding. An IsaMill™ grinds the concentrate to 80% passing 10 to 12 microns. This strains the sulphide lattice so it oxidises at atmospheric pressure, and prevents leach products from passivating the particles.
3. Neutral oxidative leach. Oxygen is injected at supersonic velocity into the base of Albion Leach Reactors. For refractory gold, the pH is held at 5.5 with limestone slurry, which fully oxidises the sulphur to sulphate. The leach runs autothermally, typically at 90 to 98°C, with no autoclave.

The near-neutral pH changes what happens downstream. Acid and iron released from the pyrite are neutralised inside the leach, so the oxidised residue goes straight to the carbon-in-leach circuit without a separate solid-liquid separation or neutralisation stage. That leaves a single tailings stream. Because no elemental sulphur forms, the residue has very low cyanide consumption relative to bacterial or pressure leach circuits, and full dissolution of the gold in the cyanide leach is expected within 8 to 12 hours.

The same conditions help with other refractory features:

• Arsenic. Arsenic released from arsenopyrite forms a crystalline ferric arsenate that is environmentally stable and has low solubility.
• Tellurides. Ultrafine grinding speeds up telluride oxidation, which is faster at neutral to alkaline pH. The final stage of the leach can be raised to pH 9 with hydrated lime if needed.
• Preg-robbing carbon. The neutral leach does not activate carbonaceous preg-robbing phases in the ore.
• Base metals. Copper, nickel and zinc sulphides do not oxidise under the neutral conditions, so they do not consume reagents.

The flowsheet options page shows how the gold and base metal configurations differ.

What results has atmospheric oxidation achieved on refractory gold?

Glencore Technology reports that the Albion Process™ achieves recoveries of up to 97% from refractory gold concentrates. Published projects and testwork show results across different deposits.

Sources: Glencore Technology GPM case study, installation list and media releases.

For a broader comparison with pressure oxidation, Glencore Technology has published the technical paper "Case Study Reviews of the Albion Process™ as an Effective Alternative to Pressure Oxidation", presented at CMP 2024.

Which leaching process is best for refractory gold ore?

No single leaching process is best for every refractory gold ore. The right choice depends on how the gold is locked, the minerals in the concentrate, project scale and site constraints. Cyanide leaching remains the step that recovers the gold. The decision is which pre-treatment exposes the gold most economically for that deposit.

A few questions narrow the choice:

• Where is the gold? Gold in pyrite responds well to fine grinding and oxidation. Very fine gold in arsenopyrite may need full oxidation of the host.
• What else is in the concentrate? Arsenic, tellurides, carbonaceous material and base metal sulphides each affect reagent use, recovery and residue stability.

• What are the site constraints? Sulphur dioxide handling, acid neutralisation, pressure equipment, capital budget and time to production all shape the economics.

   

How do you choose a treatment route for a refractory gold ore?

Choosing a treatment route for a refractory gold ore starts with testwork on representative samples to measure how the gold responds to each option. Preliminary testwork grinds samples to several target sizes and leaches each product. This shows the potential recovery and gives a first economic view of the main drivers: power consumption, reagent costs and gold recovery.

Mineralogy adds detail. Scans such as QEMSCAN and MLA can measure mineral liberation and gold particle size down to about 5 microns. Below that, methods such as secondary ion mass spectrometry (SIMS) are used.

For the Albion Process™, project development begins with a testwork program to confirm the concentrate responds to the process and to gather design data. Glencore Technology can then provide a +35% Class 4 study of capital and operating costs. A pilot plant stage is not needed, and a process guarantee is provided.

Frequently Asked Questions

What is a refractory gold ore?

A refractory gold ore is an ore that gives low gold recovery through conventional grinding and direct cyanide leaching. Most often, fine gold is locked inside sulphide minerals such as pyrite or arsenopyrite, so cyanide cannot reach it. The ore needs a pre-treatment that breaks down or opens up the sulphide before cyanidation.

Can refractory gold be recovered without roasting or pressure oxidation?

Yes. Atmospheric oxidation with the Albion Process™ combines ultrafine grinding with oxidative leaching at atmospheric pressure, with no roaster or autoclave. For some ores, ultrafine grinding alone is enough: at KCGM, testwork showed that grinding refractory concentrate to 10 microns lifted gold recovery from 75% to 92%.

What happens to arsenic when refractory gold concentrate is oxidised?

In the Albion Process™ neutral leach, arsenic released from arsenopyrite forms a crystalline ferric arsenate. This compound is environmentally stable and has low solubility. It reports to the leach residue, which is treated in the cyanide leach and then sent to a single tailings stream.

Does atmospheric oxidation increase cyanide consumption?

No. In  the Albion Process™, the refractory gold leach runs at near-neutral pH, so no elemental sulphur forms. The oxidised residue therefore has very low cyanide consumption relative to bacterial or pressure leach circuits. Gold dissolution in the cyanide leach is expected to be complete within 8 to 12 hours.

What recovery can the Albion Process™ achieve on refractory gold?

Glencore Technology reports recoveries of up to 97% from refractory gold concentrates. At the GPM Gold Project in Armenia, recovery from the sulphide concentrate rose from 20% to over 98%. Freegold's metallurgical testwork in Alaska achieved over 93% recovery using Albion Process™ oxidation followed by CIL.

Planning treatment for a refractory gold ore

Refractory gold ores are becoming more important as free-milling deposits decline, and many known refractory deposits have not been mined because the gold is locked in sulphides. Treating them is a liberation problem, and there is more than one way to solve it.

Roasting, pressure oxidation and bacterial oxidation all break down the sulphides, but each carries costs in off-gas handling, acid neutralisation, pressure equipment or downstream separation. Ultrafine grinding alone can work where the gold is disseminated in pyrite. Atmospheric oxidation with the Albion Process™ combines ultrafine grinding with oxidation in atmospheric reactors, neutralises acid inside the leach, and sends the residue directly to cyanidation. At GPM, it turned a sulphide resource recovering 20 to 30% of its gold into a plant that exceeded its design recovery.

The best route for any refractory gold ore depends on its mineralogy, so the starting point is always testwork. Grind and leach tests, supported by mineralogy, show where the gold is and which pre-treatment exposes it most economically.

To assess atmospheric oxidation for your refractory gold ore, use the Albion Process™ Calculator for an order-of-magnitude cost estimate, review how the Albion Process™ works, or contact the Albion Process™ team for a free desktop capital and operating cost estimate based on your concentrate composition and throughput.

Technical references

• Ellis, S. Ultra Fine Grinding: A Practical Alternative to Oxidative Treatment of Refractory Gold Ores. Kalgoorlie Consolidated Gold Mines.
• Glencore Technology. The Albion Process for Refractory Gold.
• TOMS Institute. Technology for Processing of Refractory Gold-Containing Concentrates (IMPC 2016).
• Glencore Technology. Albion Process™ Installation List.