The purpose of milling in raffinate is to liberate valuable minerals while initiating leaching by recycling an acidic stream generated in the downstream processes, prior to the actual leaching stage. This process is particularly amenable to liberating copper and other minerals using circuits that traditionally have a milling stage, a leaching stage and a solvent extraction stage.

What is Raffinate?


  • An acidic barren solution after copper extraction in solvent extraction
  • Contains acid and residual metals in solution
  • Most of the solution is usually sent to leach and some of it is bled out to the tailings dam via the wash thickeners (CCDs)


The Milling in Raffinate Process


While conventional milling uses raw water in grinding of ore to a slurry state, milling in raffinate replaces the raw water with a recycled acidic stream from the downstream processes. According to Mponda Masaninga, Principal Process Engineer at SENET, “in this process, we use an aqueous stream from the extraction stage of a solvent extraction circuit, which is high in acid.”

Nick Dempers, Principal Process Engineer at SENET, describes the comminution process as follows:

  • Obtain rock from the ground
  • Rock crushed to a certain, required size
  • Mill the crushed rock to reach a fine particle size that is amenable to leaching, this is where milling in raffinate can be used

Benefits of Milling in Raffinate


Masaninga confirms that milling in raffinate reduces fresh acid use, eliminates some stages from the usual process (for example, allows mines to skip the pre-thickening stage prior to leaching), reduces the size of some downstream equipment, and potentially improves leach recovery.

Dempers notes that milling in raffinate allows mines to reduce acid consumption. “This is notable because acid consumption is usually the biggest cost driver on many projects. Therefore, if you mill in raffinate you substantially reduce the quantity of acid required, reducing the cost of the process.”

Benefits include:

  • Reduction in the consumption of neutralising agents used to treat the acidic plant effluents
  • Improvement in the conservation of raw water resources, with improved water balance
  • Reduction in plant operating costs
  • Improved copper cathode income due to an improved leach recovery of about 1 to 1.5% above conventional milling

The Relationship Between Raffinate and Copper

Milling in raffinate has been successfully used for copper and associated mineral extraction. “This is largely due to the fact that copper extraction using solvent extraction technology generates a high acid raffinate stream - which you can recycle to the milling area,” confirms Masaninga.

Capex vs Cost Savings

The biggest perceived drawback to milling in raffinate is that the mill and associated equipment must be made of exotic materials. “This can be a stainless steel of some sort, or a specially rubber lined mill. As a result, the capital expenditure of the milling section is quite a bit higher than in conventional milling processes,” says Dempers.

However, despite this initial outlay, Masaninga confirms that it is becoming popular to mill in raffinate because newer special stainless steels have proven to be reliable. The special stainless steels are comparable in terms of cost to traditional stainless steels and have stronger mechanical strength. “This makes milling in raffinate more attractive. In addition, although the mill costs more than conventional milling, one stage of the process is removed (the pre-leach stage), and reduced acid consumption offset the initial high capital costs,” he concludes.

For more information from SENET’s milling experts, contact Nick Dempers ( or Mponda Masaninga (


- ENDS -

Suggested Articles

17 Jul 2023
Fatal incident at Moyeath Project

DRA Global is deeply saddened to advise that an incident occurred at approximately 4pm (AST) on 15 July 2023 at the M... Read More

19 Oct 2021

According to Pieter Theron, Senior Vice President at SENET, a DRA Global Group Company, the very successful, SENET Se... Read More

17 Sep 2021

Across the globe, electrolytic manganese plays a critical role in electrochemistry. Its use in modern alkaline, lithi... Read More