Cyanide has a key role in the mining industry to extract gold. It has been used for this purpose since 1887 to leach gold from ore into solution which is then recoverable. About 80% of the world’s gold production directly utilises cyanide in its extraction. However the vast majority of the remaining 20% is recovered by gravity processes which are used in part to recover gold in conjunction with a cyanide leach system for the non gravity gold. Very few whole gravity treatment plants are economically viable. There has been considerable research into the use of non cyanide lixivants to leach and recover precious metals. To date nothing has proved to be either efficient in recovery or cost effective to allow its widespread use in the mining industry. Most of the alternatives also present challenges due to their toxicity or their potential to damage the environment.

Recent concerns about the use of cyanide has lead the mining industry to introduce a voluntary code of conduct called The International Cyanide Management Code which is administered by the independent International Cyanide Management Institute. One of the criteria of the code is not to allow Weak Acid Dissociable “WAD” cyanide levels exiting the processing plant to the tailings storage area to be greater than 50 parts per million (ppm). On most gold operations this is a difficult target to achieve so cyanide destruction methods have started to be implemented. However all of the currently commercially available processes have a considerable cost implication to the mine.

In light of the Cyanide Code and the 50ppm plant exit limits, MMS have developed a cost effective cyanide destruction process called MMS CN-D Process™.  The process utilises MMS’s proprietary mass transfer reactors, the Aachen Reactors, to use oxygen, carbon, selected reagents and catalysts in a process to efficiently and economically destroy all forms of cyanide, including WAD cyanide to levels well below 50ppm. The process is completely complementary to standard Carbon in Pulp and Carbon in Leach (CIP/CIL) process plants and whilst it destroys cyanide at the exit of the plant it also optimises kinetics in the leach train whilst minimising gold losses to tailings. The MMS CN-D Process is offered to clients under a licence agreement arrangement.

Typical MMS CN-D process results

The table below indicates what levels of cyanide destruction are achievable.

Cost

The cost of the MMS CN-D process is significantly less than that of competing processes. The actual cost although significantly lower can vary significantly within this much lower range due to a number of factors including:

• Feed tenor
• Residual tenor required
• Specific residue specification requirements
• The upfront leach process
• Site oxygen cost
• Site electricity cost
• Availability of existing equipment
• Whether upfront leach is optimised through use of Aachen reactors
• Existing plant soluble loss

For a meaningful costing to be determined it is essential to perform testwork.

Testing of the MMS CN-D process

MMS currently has testing facilities in Johannesburg, South Africa and Perth Australia. Test work can be undertaken to determine various levels of destruction of cyanide and the cost implications. Larger scale pilot work can be undertaken utilising the MMS CN-D Test Column.

Leachox™ and Cyanide Destruction (CN-D) Pilot Plant Facility

• PLC controlled self contained test unit
• Ideal for on-site & Laboratory test work
• Online recording of parameters
  (Pressure, pH, rH, CN, Temperature & Dissolved Oxygen)
• Typical test sample 40-50Kg