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Optimising fine grinding for Mkango Resources

GSL investigated optimising the fine grinding circuit design on Mkango Resources‘ Songwe Hill project in Malawi. To do this, GSL looked at mill selection, classification and variability in the deposit.


Initial evaluation of the fine grinding circuit for the Songwe project had indicated that the energy and equipment requirements were very high. Following discussions with Mkango management and their engineering consultants, a test program was proposed to determine, in a more defined way, the energy and equipment requirements for the plant.


  • Stirred mill signature plots using two different mill technologies.
  • Determination of energy requirements to grind to various sizes.
  • Investigating means of reducing feed to the mills by classification.
  • Variability testing on 22 samples to determine energy requirements to a given size.


  • The signature plots carried out on the material showed that the previous estimates for energy requirements had been considerably overstated and potential power estimates could be reduced by 83%, reducing the capital cost and number/size of the mills.
  • There was a difference in the energy requirements between the two different mill types used, one being approximately half the energy requirement of the other.
  • For a small increase in energy it was shown that the grind size could be reduced further and increase the subsequent flotation grade and recovery of REE minerals, with the consequence of reducing the subsequent leach plant size and capital requirement.
  • Classification trials indicated that fine material could be removed before the fine grinding stage, significantly reducing the amount of material reporting to the mills. The specific energy required to grind the coarser material increased but this was balanced by the reduction in mass, further reducing the installed power requirement.
  • The variability testing indicated that the energy requirements varied between 5.5 kWh/t to 14.9 kWh/t to achieve the target grind size across the 22 samples.


GSL investigated optimising the fine grinding circuit planned for the Songwe deposit flowsheet. The study compared two different find grinding mills and carried out a variability study covering 22 samples. There was a significant difference in the energy requirements between the two mill types and the variability test work indicated that the specific energy varied between 5.5 kWh/t and 14.9 kWh/t across the samples. The outcome was a reduction in the required installed power of 83% compared to the original engineering estimate. Grinding finer and classification also showed significant process advantages.


The value to our client is significant potential reduction in both CAPEX and OPEX achievable through specific mill selection and fine grinding optimisation.

For more information on our comminution testing services, click here.