Improved leach recovery

In simple leaching circuits the purpose of grinding is liberate the particle and expose fine grained minerals to leachant (e.g. exposing fine gold to cyanide). In more complex leach environments such as those treating refractory ore bodies the 'refractory' minerals can be passivated by reaction products forming a two to three micron 'rim' on the particle. This rim prevents leaching of material at the centre of the particle.

In both simple and more complex leaching systems the size of the mineral particle is critical. In a simple circuit achieving and maintaining an optimal feed size to the leach circuit ensures surface area is maximised and there is optimum exposure of fine grained minerals to leachant. In more complex circuits the optimal grind size is the size where the values can be transferred before the passivation layer inhibits this transfer.

A rim of passivating reaction products on a nine micron particle may not prevent leaching as the rim does not have time to grow thick enough to prevent molecular transfer. Molecular transfer still occurs through the crystal lattice defects. For an 'oversized' 30 micron particle the rim may have time to grow sufficiently thick preventing molecular transfer and the reaction proceeding deeper into the particle. The valuable mineral in this particle would be lost.

While the IsaMill™ achieves efficient grinding to fine P80 it also has achieves a very sharp size distribution and a finer P98 sizing than alternative grinding technologies.

The rate of the leaching reaction for sulphide minerals is controlled by: 

1. diffusion of ions at the mineral : sulphate layer interface. The sulphur or sulphate phase is formed during the leaching reaction as a rim on the surface of the mineral. For example for acid ferric sulphate leaching of chalcopyrite this is the diffusion of copper and iron ions through the sulphur layer formed during the reaction. 
2. rate of actual chemical reaction
3. diffusion process within the bulk mineral. If a mineral surface has few lattice deformities and defects the diffusion of ions through the mineral : sulphate layer interface is the rate determining step. If the mineral surface is activated, having a high number of crystal lattice deformities and defects, the diffusion process is accelerated and the chemical reaction becomes the rate determining step. Mechanically activated material results in a disordered crystal lattice with many more dislocations than natural, non-activated material. Reaction rates increase with the number of defects.

IsaMilling is a high energy intensive process –introducing up to 10 times the energy introduced by ball or tower mills – around 300kW/m3 of volume in the IsaMill™ compared to regrind or tower mills utilising 20 to 40kW/m3. 

The IsaMill™ efficiently reduces the particle size and also increases the internal and surface energy creating highly stressed surfaces, increasing the number of mineral lattice defects and fractures and reducing the crystalline structure of minerals. This process is known as mechanical or mechanochemical activation.

Mechanical activation of a mineral follows four steps: 

1. Prior to structural disordering a small force applied to the mineral deflects atoms from their normal positions and disorders the crystal lattice. 
2. From the structural disordering a new surface is formed and cracks originate. 
3. Further fine grinding forms new surfaces and accumulates energy in the surface layer – the result is significant changes in structure and material properties. 
4. Further (ultrafine) grinding can result in the mineral losing its original identity – turning into substance with different structure, properties and sometimes different composition – this is known as mechanochemical activation.

The high intensity environment within the IsaMill™ results in the defects on the surface acting as electron transfer sites to 'activate' the mineral. This change in the surface structure allows in minerals to leach under much less aggressive conditions, increasing leach kinetics and reducing the size of the leach circuit required and therefore overall cost – both capital and operating!

Several emerging leaching processes have been based on fine grinding of feed – the Activox process, the UBC/Anglo process, the Phelps Dodge Process, and XT's patented Albion Process™  for atmospheric leaching after fine grinding (Albion Process^TM).

The use of steel media in regrind mills prior to leaching can be detrimental to leach performance.

When leaching circuits are designed to recover precious metals, such as gold or silver, from pyrite concentrates it is common to follow the fine grinding stage with a pre-aeration stage. The preparation stage removes small quantities of active pyrite and pyrrhotite prior to cyanidation.

Pre-aeration increases the Eh and oxidises the active pyrite and pyrrhotite to reduce cyanide consumption in the leach process. This is different from the patented Albion Process, where significant portions of sulphides (over 10%) are oxidised by atmospheric leaching to increase metal recovery.

If steel media is used in the regrind mill a galvanic cell forms between the mineral and media. The formation of a galvanic cell increases the corrosion rate of the media and the deposition of iron ions into solution. The galvanic cell results in a cathodic reaction and the precipitation of metal hydroxides on the mineral surface.

The worn steel media in the ground pyrite concentrate can significantly increase the pre-aeration time required for leaching while the formation of metal hydroxides on the mineral surfaces can retard the leaching reaction.

The use of inert rather than steel media in the grinding circuit has the dual effect of: 

• reducing pre-aeration residence time and cyanide consumption in leaching
• increasing reaction kinetics by reducing the formation of non-reactive layers.

The net result will be a reduction in circuit capital and operating costs.