CN-D™

CN-D™ Cyanide Destruction Technology

The same Aachen HSR technology forms the basis of MMSA’s CN-D™ cyanide destruction process. This proprietary process simply relies on correct combinations of SO2, oxygen and shear to effect catalytic oxidation of weak acid dissociable cyanide. MMSA can perform scouting or pilot testwork to evaluate the technical feasibility for a specific application. The results are used for a techno-commercial assessment of the process application including a comparison to competing processes.

 

Amongst many requirements, the limitation on Tailings Storage Facilities of < 50 ppm CN WAD and on the effluent of < 0.5 ppm CN WAD have seen a rising need for cyanide destruction technology to be deployed in order to comply with these specifications. Often, over and above the International Cyanide Management Institute (ICMA) criteria, local regulatory limitations that are more stringent contribute to the need to lower the cyanide content in gold leach residues. Indications are that the compliance level of < 50 ppm CN WAD could be lowered to 20 ppm or less for some geographic areas.

Commissioning of the CN-D circuit at Avesoro Resources’ New Liberty mine in Liberia was successfully completed in 2016, with Pan African Resources BTRP operation in South Africa commissioned soon after.

CN-D™ METHODOLOGY

Reagent Consumption

The Maelgwyn CN-D™ process depends on catalytical activity (where no consumption of the reagent (granular activated carbon) takes place) rather than reagent addition in its standard layout. Oxygen supplied as compressed gas in moderate excess is the main oxidant, therefore no chemical reagent consumption other than carbon losses associated with normal attrition losses during in-pulp use and elution/regeneration can be associated with the process. As the process is not directly reliant on reagent additions linked to relatively minor cyanide level fluctuations (±30% from average), such changes will as a rule not influence the operational cost.

Site specific circumstances can influence the potential contributors to reagent consumption and hence OPEX:

  • Should the slurry display a buffering behaviour towards lowering of the pH (to achieve optimal conditions for the catalytic environment), the acid consumption will have to be weighed against the cost of generating protons through oxidative dissolution of remaining sulphide minerals. This can lead to additional gold recovery if the material is slightly refractory;
  • CN SAD (largely Fe based cyanide types) would require precipitation steps for complete elimination;
  • The high-shear environment requires a high level of power; making the process sensitive to power cost (but always remains cost competitive);
  • The high amounts of oxygen and the price of oxygen to be introduced into the slurry, could be a major contributing factor of costs, but as for power, will generally leave the process more than competitive and can often be linked to up-stream optimisation on pre-oxidation). The amount of oxygen fed through the Aachen units can be managed within a reasonable range.

CAPEX Related Needs

CAPEX requirements:

  • Adequate tank capacity to allow for the required residence time for the reaction;
  • Oxygen feed systems, DO monitoring instruments;
  • Aachen™ reactors for oxygen dispersing and shear contribution (the units are usually supplied on lease, but can be purchased);
  • Acid dosing installation for pH adjustments;
  • pH/Eh monitoring; and

Cyanide on-line analysis instruments.

Application

This process is extremely well suited for the treatment of slurries. Solutions would require a modified approach of the operation. All ICMI criteria based testwork indicated that the process will be compliant with the use of a 2 – 4 tank system.

In applications where thiocyanate as well as CN WAD must be reduced, the CN-D™ process offers advantages. Testing of post BIOX® leach residues (where residual cyanide levels are much higher than that of ordinary leach tails) and other refractory ore environments indicated considerably reduced thiocyanate levels.

In cases with temporarily or permanent high gold levels, which offers the prospects of additional gold recoveries; the potential benefits of using the CN-D™ process should be evaluated from a gold recovery perspective, rather than savings in OPEX.

Advantages

The Maelgwyn CN-D™ process can be applied in a wide variety of residue environments with comparably little change to the main cost contributing factors. Since the process is not linked to a metered reagent dosage, fluctuations within reasonable levels can be accommodated without changes to the system.  To improve the chemical performance in more challenging environments; dosing of copper could be added. Detoxification costs can be off-set against additional gold loadings onto the activated carbon used in the process. The system units are standard CIL application technology (e.g. carbon, carbon screens, oxygen, measuring instrumentation etc.), with the only new technology being the Aachen™ high-shear reactors used for the gas mass-transfer, although the units have been in trouble free operation at numerous mines for years.

CN-D™ Design Critical Input Data

The hyper-oxygenation is achieved through the use of the Aachen™ high-shear reactors. Refer to Figure One: Reactor Installation for dimensions and physical shape. The units are available in different sizes to accommodate the optimal oxygen/shear requirements for each specific application. The definition of technology deployment is based on the concept of passes.

For a typical layout and installation option for an Aachen™ reactor receiving slurry from a carbon containing tank, refer to Figure 2: Installation Layout. If a carbon free oxygenation is required, no separate carbon screening will be necessary. Slurry feed will be from the lower part of the tank (1.5 m – 2 m) above the bottom, avoiding oversize material intake.

For wide ranged engineering specifications – refer to Table 1: Typical Example of Installation Criteria.

Simultaneous deployment of Aachen™ based hyper-oxygenation in combination with activated carbon based catalysis in several stages, the slurry transfer between stages will have to be through inter-stage screening and the Aachen™ inlet itself would require carbon screening.

Typical hyper-oxygenation level tank example using Aachen REA 450 unit (s)

Tank volume (process dependent)
1500

m3

Minimum residence time during hyper-oxygenation
To be defined

minutes

Acid (HCl) dosing capacity, ratio controlled maximum addition
1000

g/t

REA 450 Feed Pump requirement, flow (per unit)
600

m3/h

REA 450 Feed Pump requirement, head pressure
50

m water head

Oxygen delivery, capacity range (per unit)
30 – 150

kg/h

Oxygen flow, nominal operational (per unit)
60

kg/h

Aachen unit pressure drop
4

bar

Oxygen delivery excess head pressure
2

bar

Interlocks (oxygen pressure and flow) to stop slurry pump
DP to be defined

bar

Physical Address

1331 Staal Street (corner Spokeshave)
Stormill Ext 2
Roodepoort
South Africa

Postal Address

PO Box 2437
Honeydew
2040
South Africa

Contact Details

Tel: +27 11 474 0705
Fax: +27 11 474 5580
Maelgwyn Mineral Services Africa (Pty) Limited | South Africa | Company Reg No: 2003/007416/07
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