The Albion Oxidation Process

The patented Albion Oxidation Process is a processing technology that permits the extraction of precious metals from refractory ore by conventional techniques.

Feed to the Albion Process is a sulphide concentrate containing precious metals. The Albion Process utilises both ultrafine grinding and oxidative leaching at atmospheric pressure to oxidise the concentrate and allow metal extraction utilising cyanide-in-leach (“CIL”) technology.

A cutaway view of the Isamill.

Ultra-fine Grinding

Most sulphide minerals cannot be leached at acceptable rates at atmospheric pressure. The first stage of the Albion Process is ultra-fine grinding of the concentrate using an IsaMill supplied by Glencore Technology, the Albion Process patent holder.

The process of ultra-fine grinding introduces a high degree of strain into the sulphide mineral lattice. This causes mineral grain boundary fractures as well as defects within the mineral lattice. As a result, the activation energy for the oxidation reaction of the precious metal-bearing sulphide minerals is reduced, thereby permitting leaching under atmospheric conditions. The associated increase of the exposed mineral surface also increases the rate of leaching.

Ultra-fine grinding also assists in preventing surface passivation during leaching. Passivation occurs when the leach products, such as iron oxides, precipitate on the surface of the reactant mineral. These precipitates inhibit further oxidation of the underlying mineral.

Once a passivation layer exceeds 2 to 3 microns in thickness, it will typically prevent any further underlying mineral reaction.

Ultra-fine grinding uses a different milling action than a conventional ball mill. Fine ceramic media (~3 mm) is agitated through stirring, causing turbulent mixing and attrition. Abrasion is the principal breakage mechanism for size reduction within an IsaMill.

The Isamill at Las Lagunas.

Albion Leach Reactors

After the concentrate has been appropriately ground in an IsaMill (i.e. P80 of <12 micron), the slurry is then leached in agitated tanks, or Albion Leach Reactors.

These reactors are connected in series, with a launder system that allows gravity flow of the slurry through the continuous leach train. All Albion Leach Reactors are fitted with bypass launders to allow any reactor to be isolated from service for periodic maintenance.

Oxygen gas is injected into the base of the Albion Leach Reactor using supersonic velocity injection lances. Coupled with a high power reactor mixer, this maximises oxygen gas dissolution (oxygen mass transfer rate), plus reactant distribution.

The Albion Leach Reactor is designed to operate at close to the boiling point of the slurry. The temperature of the leach slurry is set by the inherent heat released by the exothermic (heat-generating) chemical reactions. No external heat source is necessary.

Further, no cooling is required. Excess heat generated from the oxidation process is removed by the humidification of the vessel off gases (latent heat losses). The maximum leach temperature is typically 94°C. The design oxygen capture efficiency is 85%.

The Albion Process oxidative leach has three primary control parameters:

Albion Leach pH: Leach pH is continuously regulated using an alkali slurry to neutralise the leach acid products.

Albion Leach Density: Slurry density is controlled through the addition of process water.

Albion Leach Oxygen Gas Addition: Oxygen addition is regulated according to each Reactor’s oxygen consumption (rate of reaction – oxygen demand).

An Oxidative Leach Train.

Oxidation Leach Chemistry

Albion Leach uses near-neutral pH leach conditions to oxidise gold-bearing pyrite and arsenopyrite, plus any tellurides and sellenides contained within the sulphide mineral concentrate. The neutral leach is carried out at a selective pH determined from test work (typically between pH 4 and pH 6).

The leach pH is controlled through the continuous addition of an alkali limestone slurry. This neutralises the acids and the iron sulphates produced during the oxidation of the sulphide minerals.

During the leach, contained sulphur is converted to sulphate, specifically as gypsum mineral (CaSO4.2H2O(s)).

The overall pyrite oxidation leach reaction is:

FeS2(s) + 15/4O2(aq) + 9/2H2O(l) + 2CaCO3(s) = FeO.OH(s) + 2CaSO4.2H2O(s) + 2CO2(g)

The resultant iron product, goethite (FeO.OH(s)), is very stable and has no solubility in cyanide. Consequently, the Albion Leach residues do not generate ferro or ferri cyanide species during subsequent gold extraction (i.e. Carbon-in-Leach).

Arsenopyrite is a common gold carrier in many refractory gold concentrates. Arsenopyrite is typically oxidised under the neutral leaching conditions. The resultant product is ferric arsenate (FeAsO4.2H2O(s)).  Ferric arsenate is a stable and environmentally inert arsenic mineral. The overall arsenopyrite oxidation leach reaction is:

FeAsS(s) + 7/2O2(aq) + 4H2O(l) + CaCO3(s) = FeAsO4.2H2O(s) + CaSO4.2H2O(s) + CO2(g)

The resultant Albion Leach slurry product is suitable for direct feeding to a cyanide process. Given the neutral leach conditions, it requires no solid/liquid separation (i.e. filtration or counter-current decantation), nor neutralisation requirements.