Aachen Shear Reactors for Increased Gold and Silver Recovery

Aachen reactors can be used to increase the leach kinetics in gold and silver cyanidation resulting in significantly higher gold recoveries. The Aachen rectors can be used on most gold ore types including

  1. Oxides
  2. Transitional material
  3. Refractory ores
  4. Gold tailings retreatment

Background

The Aachen reactor is basically a highly efficient mass transfer device developed out of the experience with Maelgwyn’ s Imhoflot flotation technology. The unit can be used for any application where it is required to intimately mix a gas with a liquid but has been found to be particularly suitable for gold leaching applications where it is required to boost the dissolved oxygen levels of slurry either prior to, or during the leach reaction. Historically in many operations this has been done by various types of lance arrangements and their derivatives. However, lances by their very nature tend to be very inefficient resulting in large oxygen bubbles and high oxygen consumptions. A second drawback of lance based systems is that they fail to address the problem of surface passivation from oxidised species which can retard the leach dissolution process .The Aachen shear reactor in contrast is able to produce significantly higher dissolved oxygen levels whilst also introducing an element of shear to clean up mineral surfaces. The net benefits of this include: –

  • Higher gold recoveries/lower residue grades
  • Accelerated leach kinetics
  • Reduced cyanide consumption
  • Improved oxygen utilisation
  • Increased tonnages without compromising recovery

The reactors are particularly suitable for the treatment of transitional material consisting of a mixture of oxide and sulphides where sulphide grades are too low and erratic for flotation but nevertheless compromise leach efficiency and cyanide consumption

As mentioned above the Aachen reactors can be used on a variety of ore types to increase gold and silver recoveries but the way that the Aachen reactor is used will vary according to the ore type .These can be summarised as

Currently there are over 60 Aachen reactors installed with the majority of these used for pre-oxygenation or Aachen assisted leaching
Fig 1.0 Top view of Aachen reactor showing oxygen addition to cartridge

Fig 1.0 An Aachen reactor installation

How Aachen reactors work

The current Aachen Reactor design consists of a cartridge insert which is used to introduce gas under a pressure of approximately 8 bar into the slurry via a slot contained in silicon carbide tubes contained within the cartridge. Slurry is accelerated through the tubes to a speed of approximately 12-13m/s at the gas addition point resulting in the generation of extremely fine gas bubbles. The cartridge is protected by sacrificial wear plates and inserted within a stainless-steel housing. This cartridge is illustrated in Fig 2.0 below.
Figure 2.0–Aachen Reactor inlet cartridge with slot aerator
Additional turbulence and shear is created within the subsequent four chambers which again contain nozzles through which the slurry is forced. The complete reactor as illustrated below in Fig 3.0. is mounted vertically on a leach or pre-leach tank. Feed to the Reactor is pumped via a variable speed pump into Reactor with the oxygenated slurry returning to the leach tank.
Fig 3.0 Schematic diagram of Aachen reactor illustrating the operating principles

Why Aachen Reactors work

The Aachen reactor is specifically designed to meet all the requirements for gold cyanidation.

In order to be able to determine the potential suitability of an oxygen addition device to improve cyanidation it is necessary to have a basic understanding of the cyanidation reaction itself and the factors driving it and importantly the role of oxygen. Whilst cyanide is one of the major drivers if not the major driver of gold dissolution it cannot be viewed in isolation particularly in respect of the relationship between cyanide and oxygen derived from the well-known Elsner’s equation.From the above it is apparent that both cyanide and oxygen are required in an aqueous solution to leach gold. For the leaching of pure gold, the stoichiometry of Elsner’s reaction gives the required molar ratio of free cyanide to oxygen as 8: 1 (or 6½: 1 when expressed as a mass ratio). As cyanide is the more expensive reagent, it is desirable from a financial point of view to ensure that cyanide is the rate-limiting reagent. It is important to note that the rate of dissolution of gold in alkaline cyanide solutions is controlled by the rate of dissolution of oxygen from the bulk solution to the metal surface.

This explains why the Aachen Reactors are so successful in boosting leach kinetics and reducing reagent consumption as the Reactors are specifically designed to maximise the phase interface surface area.

For a gold ore with a 10 ppm head grade, 0.4 ppm of oxygen in 50% solids by weight slurry should theoretically be sufficient for gold dissolution alone. However, in practice significantly more oxygen is required to maintain gold dissolution due to numerous competing side reactions that also take place consuming oxygen and so potentially “starving” the leach of oxygen. The net effect of this can be to slow down leach kinetics or even, potentially bring the leach to a halt.

Many of these undesirable side reactions have kinetics faster than the gold dissolution reaction and so unfortunately need to be satisfied ahead of the gold dissolution; there is little other choice.

In certain processes it may also be beneficial for some side reactions to be encouraged e.g. partial sulphide oxidation of gold bearing minerals such as in the Leachox™ process, depending on the gold deportment within the mineralogical phases.

The reactions consuming oxygen (and often cyanide as well) can be categorised in terms of increasing oxygen demand into two basic groupings namely solution and solid species.

Solution species oxidation

Some of the mineralogical phases might dissolve and react in the comminution circuit ahead of the main leach circuit and thus lead to solution species prone to oxidation. The circuit process water might also contribute to these species. The more important of these include: –

Ferrous iron:  the oxidation of Ferrous to Ferric (Fe2+ to Fe3+) requires ¼ O2 for each iron in solution to be oxidised. Hence, without the further addition of oxygen, a 50 ppm Fe2+ in solution could consume the entire mass of oxygen contained at usual equilibrium values. Therefore, at Fe2+ levels exceeding 10-20 ppm, it is advisable to apply pre-oxidation to reduce the oxygen consumption during the leach (and also prevent ferro-cyanide formation at the same time).

The mass of oxygen required is relatively low and addition could be achieved using lances or other low efficiency mass transfer devices.

Sulphur species: unstable products of sulphide dissolution may also consume oxygen to form thiosulphate or poly-thionates. The oxygen consumption for these species is higher than the iron oxidation, but the kinetics tend to be slower.

Where fresh meta-stable sulphur compounds are present in solution, more aggressive pre-oxidation might be advisable to try and convert them away from thiocyanate forming species. The oxygen mass requirements are still relatively moderate.

Fig 4.0 Completed Aachen reactors in the workshop

Solid, mineral phase components to be oxidised

Whilst solution species oxidation largely prevents the slowdown of gold leach kinetics due to inadequate oxygen levels as well as limiting cyanide consumption due to iron or sulphur, the solid phase oxidation may be unavoidable, desired and targeted, or best restricted. Minerals such as pyrrhotite (Fe7S8) are very easily oxidised and would consume nearly the same mass of oxygen to be transformed to sulphate and ferric hydroxide. Hence for each 1 kilogram of sulphide oxidised in the leach feed approximately 1 kilogram of oxygen would be required if the oxidation reaction were to advance to completion. It will be appreciated that even at modest sulphur dissolution levels the oxygen requirement will be expressed in tons per day rather than kilograms.

Similarly, arsenopyrite (FeAsS) would consume roughly an equivalent of oxygen equating to half the mass of arsenopyrite to be oxidised. The oxidation kinetics of this reaction are reasonably fast. Lastly, pyrite (FeS2) would consume the same mass of oxygen for each mass equivalent FeS2 transformation to sulphate and ferric hydroxide. However, the kinetics of this reaction are significantly slower than the two aforementioned ones.

Pyrrhotite may fall into the category of unavoidable oxidations and must be accommodated to a degree whether the mineral is gold barren or not. Arsenopyrite and pyrite are often associated with gold and hence the dissolution of these matrix minerals might be deliberately targeted through oxidation in order to release encapsulated gold.

Film boundary layer effects

Whether it is the dissolution of relatively coarse free gold, the oxidation of gold containing sulphides or the diffusion of either oxygen or cyanide to the solid interface (or the diffusion of Au(CN)2from the solid interface into the bulk solution phase) – the reaction kinetics are driven by the film boundary layers. With less agitation or shear, the layer will be thicker than under conditions of raised shear.

A reduction in film boundary layers can increase the gold dissolution reaction rates significantly.

Net benefits can include: –

  • reduced residence time (allowing for higher throughput) if coarse gold is present
  • increased gold recovery if the partial dissolution of sulphides renders some gold leach amenable (either matrix mineral dissolution or opening up of lixiviant access areas)

 

The side effects to be assessed (depending on ore) are:-

  • unintended liberations (arsenic etc)
  • unwanted oxidation reactions (barren sulphides with no direct benefit from Au)
  • where a high oxygen level induced surface coating with iron oxide may support passivation and thus slow down reagent consuming side reactions

 

The constant polishing of particle surfaces at high shear in the Aachen Reactor prevents surface oxide film formation. Whilst this can be a desirable outcome where a sulphide matrix has to be oxidised; it would be detrimental in the case of barren mineral phases where the surface film inhibits unwarranted sulphide dissolution. The degree of shear required therefore requires careful assessment during laboratory trials.

In certain instances, where preg-robbing is more a function of AuCN precipitation due to limiting diffusion of CN to the surface, this effect can be reduced significantly.

Figure 5.0. Mass control across the film boundary layer; -solid to solution (Marsden+House, 1992)

Installation

The Aachen reactors would normally be installed vertically on the top of the pre-oxygenation/leach tank and are fed by a dedicated variable speed centrifugal pump. Oxygen is required to be added to the Aachen reactor. This oxygen would be sourced either from a liquid supply or generated on site for more remote sites using a PSA/VSA unit.

The Aachen reactor should ideally be installed with the aerator cartridge at approximately chest height with a straight length of pipe prior to the Aachen of at least 2m.

Figure 6.0. Top view of Aachen Reactor showing gas injection into cartridge

Sizing

Whilst the Aachen reactors are designed to handle a range of throughputs the most popular units are summarized below

Table of throughputs with O2 addition

The slurry is accelerated through the Aachen reactor where it is intimately contacted with the oxygen under a slight pressure generated by the pump thus meeting all the conditions required for optimum gold cyanidation. The oxygenated slurry is returned to the same tank. The acceleration of the slurry and mixing within the aeration cartridge and subsequent mixing chambers results in a high degree of shear which removes passivating films as they form and thins the boundary layer thus turbo boosting gold dissolution.

The Aachen reactor requires a screened feed to prevent tramp material damaging the Aachen internals. As most gold dissolution plants use carbon-based technologies (CIL/CIP) this screening is an inherent part of the process and satisfactory providing it is maintained and operated correctly.