What is Raffinate?
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:
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.”
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.
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