Cyanide Destruction Technology

CN-D™ Cyanide Destruction Technology

The same principle of a high shear / high oxygen environment based on Aachen™ units is applied for Maelgwyn’s patented cyanide destruction process.

In addition to the Aachen™ based oxidation technology, standard activated carbon used for the Carbon in Leach (CIL) process on a gold plant forms the basis of the catalytical reactions of the process used to destroy cyanide (CN free, CN Weak Acid Dissociable (WAD) and potentially large portions of thiocyanate) to compliance levels.

The process is not dependant on reagent additions to stabilize rising or falling cyanide levels, therefore OPEX costs tend to be a fraction of competing technology modules (such as peroxide or SMBS based methodologies).

Apart from the Aachen™ reactors, the main equipment required are centrifugal pumps, carbon screens and oxygen supply.

The process has been commissioned at several mines.

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 to be commissioned soon.

Amongst many requirements, the limitation on Tailings Storage Facilities of < 50 ppm CN WAD and on 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.

The need for gold mines to comply with the pressure of authorities requiring lower cyanide levels at reduced operating cost, initiated the development of the Maelgwyn CN-D™ process.

This was developed during the last few years as a response to calls for a low operational cost process able to achieve compliance with ICMI criteria around gold mining circuits. The core elements of the process are:

  • High-shear re-oxidation of the residue after completion of the conventional gold adsorption;
  • pH modification (optionally through in-situ oxidation of contained sulphide minerals or HCl addition) though the use of Aachen™ high-shear reactors;
  • First stage cyanide destruction based on the catalytic properties of standard gold circuit CIL activated carbon to oxidise cyanide to cyanate (at 50 g/L);
  • Simultaneous adsorption of additional trace amounts of liberated gold;
  • Second re-oxidation step prior to second stage catalytic and / or chemical cyanide detox polishing step; and
  • Second phase cyanide destruction using granular activated carbon (50 g/L) and optional reagents (depending on discharge criteria and species to be reduced); this second phase is not always required.

The process can be modified to have successive modules of limited high shear oxidation with simultaneous oxidation on activated carbon (50 g/L), where each stage can be optimised with respect to the pH / Eh environment (the oxygenation remains fixed throughout for operational reasons).

During laboratory and pilot level work, very low levels (< 0.5ppm) of free CN and WAD have been achieved. The process can be engineered to address the thiocyanate and the CN SAD (ferro- and ferri-cyanide) criteria as well.

 

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.

Pass

The Aachen™ high shear reactors circulate slurry in a cross–stream method to the main flow from tank to tank. Should the leach slurry flow rate be 1000 m3/h and the Aachen™ high shear reactor throughput 5000 m3/h, this would represent 5 passes. Depending on the design requirements, the number of passes will be adjusted to achieve design requirements, to ensure optimum benefits. The resulting benefits must offset OPEX cost associated with these measures.

The critical wear components are made from silicon carbide to ensure a low maintenance environment for working with abrasive slurries.

 Aachen™ receiving slurry from a carbon containing tank

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.

Leachox - CN-D layout

Table 1  Typical example of installation criteria

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
Catalytic oxidation tank example
Tank volume (process dependent) 1500 m3
Carbon Concentration (CIL) 50 g/L
Minimum residence time during hyper-oxygenation To be defined minutes
Carbon Concentration (CIL) 50 g/L
Loaded carbon for regeneration To be defined t/month
Acid (HCl) dosing capacity, ratio controlled maximum addition 1000 g/t
Screens to feed Aachen of next stage