Aachen Reactors™ for Mass Transfer
Several mineral and metallurgical processes require gas-liquid or gas-slurry mass transfer for chemical reactions to occur.
- Gold leaching using aerated cyanide solutions
- Oxidation of sulphide minerals
- Ferric generation
- Chemical decontamination and soil remediation
- Alkaline sulphidisation
- Waste water and effluent oxidation
In general, mass transfer at the gas-liquid interface is rate controlling. Since 1997 Maelgwyn Mineral Services have been improving the design and operation of the Aachen Reactor and today many of the reactors are installed world-wide in a range of applications. MMS lease the Aachen Reactors to clients on maintenance and support service agreement.
The concept for mineral slurry reactors originated in Germany, and was developed in the 1970’s for flotation. The Aachen Reactor is designed to improve gas-liquid mass transfer using energy provided in pipeline flow. The fundamental principle is the use either of a slot aerator or a micro-fine gas diffuser made from high-tech, non-blinding materials in a high velocity flowstream. Additionally, a secondary chamber provides regeneration of bubble surfaces using various hydrodynamic effects.
The objective is to increase utilisation of the gas phase, thus reducing overall energy and reagent costs. The reactor contains no moving parts, and is designed to withstand erosive effects of mineral slurries. Materials of construction can be selected according to application.
The Aachen Reactor is designed to facilitate mass transfer by increasing the dispersion of gas in a process fluid or slurry. This results in improved gas utilisation and efficiencies. This is particularly appropriate, for example, in the leaching of gold ores with cyanide and in chemical and waste water treatment processes where large amounts of gases are required to be dissolved in fluids. The reactor is especially efficient for high rate oxidation of sulphides. The reactor accelerates the slurry or solution stream at the gas addition point and increases shear rates in the subsequent flow mixing zone. The reactor has been designed to maximise the phase interface surface area at this point in the mixing zone by means of a proprietary gas diffusion system which generates extremely fine gas bubbles. The total pressure, under which the unit operates, can be selected according to the requirements of the process.
The Aachen Reactors consist of two sections:
- A novel bubble generation system made from advanced materials
- The pressure/ after mixing/ cavitation chambers
The pulp enters the unit on the reactor’s side and exits through the pressure chambers. gas is injected into the reactor section perpendicular to the pulp flow. The reactor, which is manufactured from advanced materials, is designed to generate very fine bubbles. These fine bubbles then pass through the pressure chambers where pressure, excellent mixing characteristics and cavitation are employed to ensure that the bubbles do not coalesce or flash off, so forcing the micro bubbles into solution.
The Aachen Technology sets itself apart from spargers and other in-line reactors in the following ways:
- It has a unique mode of injection into the pulp.
- The novel ceramic reactor generates micro bubbles that do not coalesce or flash off.
- Moderate back-pressures of around 2.5 bar are employed which minimises energy costs and pump maintenance.
- Custom units for specific applications can be built.
- Pulp flow rates of up to 1000m3/h can be accommodated.
- Maintenance requirements are minimal.
Aachen Reactors – Application 1: Cyanide Leaching of Gold
Almost universally the chemical recovery of gold from milled slurries uses the cyanide leach process, usually followed by counter current adsorption using in-pulp activated carbon. The process requires the alkaline slurry to be treated with dilute solutions of cyanide ions to enable aurocyanide complex formation. The process kinetics are generally considered to be diffusion dependent (less than 1st order) with a key factor being the provision of oxygen at the reaction site.
Cyanide leaching of gold encompasses a variety of oxygen assisted reactions involving dissolution in dilute cyanide solutions. A generally accepted mechanism is described by the Elsner equation.
4Au + 8NaCN + O2 + 2H2O = 4NaAu (CN)2 + 4NaOH
Elsner’s Equation shows oxygen as a requirement for the gold leach reaction – how much is still under debate. The above equation shows a cyanide to oxygen ratio of 8:1. Habashi et al propose a CN:O2 ratio of 6:1, eg. If the cyanide concentration is 150ppm, then the corresponding DO should be 25ppm.
Conventionally this has been effected by compressed air sparging of agitated pulp. While this is quite straightforward there may be interferences to contend with including:
- cyanide consumers
- oxygen consumers
- unintended adsorption media or preg-robbers
A typical gold leach may require at least two oxidation steps:
- pre-aeration to overcome the chemical oxygen and cyanide demand of mineral components such as pyrrhotite or other sulphides
- aeration to provide oxygen for the cyanidation leach reactions
Efficient aeration is required for both steps to optimise energy and reagents. Additionally the use of gaseous oxygen from on-site generators or bulk liquid provides distinct advantages, not least of which is an increase in the driving force for oxygen dissolution. In this case poor efficiency of mass transfer has direct consequences in losses of gas to atmosphere. The oxygen utilisation provided by tank sparge systems using air may be as low as 2-5%, by comparison with in-pipe oxygenation which can achieve 30-80%. Additionally the availability of oxygen in solution reduces any tendency to form ferrocyanide and thiocyanate. Benefits include reduced leach time, increased recovery, and reduced cyanide consumption. From an economic perspective air is obviously less costly than oxygen, but in view of the above factors the capital and operating costs of compressors outstrip those of using on-site oxygen generators and high efficiency reactors.
Aachen Reactors – Application 2: Cyanide Leaching of Gold where Sulphides are present
A process for the treatment of refractory mineral ores has also been developed with the first successful commercial application already in operation.
Oxygen is essential for the gold leach reaction, as illustrated in Elsner’s Equation previous. With no pre-oxidation iron-sulphide minerals will react with cyanide to produce ferrocyanide and thiocyanate, and so consume cyanide that should be leaching gold. The following equation describes this interaction:
FeS + 1/2O2 + 7CN- + H20 = Fe(CN)64– + SCN- + 2OH-
In the case of ore containing sulphides pre-oxidation of the pulp before cyanide addition results in the production of a surface film of ferric hydroxide which prevents further reaction of the sulphides and so ensures that the cyanide added is consumed for gold leaching purposes only. There are various other species that also act as cyanicides, but often they also consume oxygen, so it’s logical to satisfy these consumers with oxygen first, bearing in mind that oxygen is a lot cheaper than cyanide. If the ore treated results in low dissolved oxygen levels then the use of an Aachen Reactor can reduce O2 consumption, reduce cyanide consumption and increase leach kinetics with an associated increase in gold recovery.
Aachen Reactors – Application 3: Ferric Generation
Acid ferric sulphate solutions used to leach copper minerals such as chalcocite can be regenerated from spent ferrous solutions. Ferric to copper molar ratios of more than 2.5 are required for effective sulphide copper leach conditions.
Reactors to be used in highly oxidising and scaling conditions would generally require wetted parts to utilise materials combining chemical resistance with suitable solid/liquid interface characteristics. The diffuser required for this application has to resist effects of crystallisation. The extensive use of PTFE and similar thermosetting polymers has been employed. Structural components of the reactors likewise have been found to require high grade austenitic stainless steels, typically ASTM grade 316 or equivalents.
Aachen Reactors – Capacities and Configurations for Aachen Reactor Applications
|Model||Slurry Capacity m3 /h||Nominal Oxygen Capacity Nm3 /h||Nominal Pipe Diameter mm||Approx. Weight kg||Length mm|
|1.6 – 2.2
8 – 12
50 – 70
150 – 210
250 – 350
350 – 480
650 – 920
|0.3 – 0.5
2 – 5
7 – 20
28 – 70
50 – 180
100 – 280
200 – 520
Aachen Reactors – Installation and Operation of an Aachen Reactor System
This six minute video (home made by the MMS commissioning engineer!) shows the installation, commissioning and operation of an Aachen Reactor system at a large West African Gold operation. It demonstrates the extreme action of the Aachen Reactors taking the dissolved oxygen levels from 1.2 mg/L before start up of the Aachen’s to 28.7 mg/L immediately after start up. The mine indicated that after start up of the Aachen Reactors, gold recovery measurably increased whilst cyanide consumption dropped appreciably.