Mixer settlers are a class of mineral process equipment used in the solvent extraction process. A mixer settler consists of a first stage that mixes the phases together followed by a quiescent settling stage that allows the phases to separate by gravity.
The mixer may consists of one or multiple stages of mixing tanks. Common laboratory mixers consist of a single mixing stage, whereas industrial scale copper mixers may consist of up to three mixer stages where each stage performs a combined pumping and mixing action. Use of multiple stages allows a longer reaction time and also minimizes the short circuiting of unreacted material through the mixers.
The settler is a calm pool downstream of the mixer where the liquids are allowed to separate by gravity. The liquids are then removed separately from the end of the mixer.
- extraction (moving an ion of interest from an aqueous phase to an organic phase),
- washing (rinsing entrained aqueous contaminant out of an organic phase containing the ion of interest), and
- stripping (moving an ion of interest from an organic phase into an aqueous phase).
Copper Example 
In the case of oxide copper ore, a heap leaching pad will dissolve a dilute copper sulfate solution in a weak sulfuric acid solution. This pregnant leach solution (PLS) is pumped to an extraction mixer settler where it is mixed with the organic phase (a kerosene hosted extractant). The copper transfers to the organic phase, and the aqueous phase (now called raffinate) is pumped back to the heap to recover more copper.
In a high-chloride environment typical of Chilean copper mines, a wash stage will rinse any residual pregnant solution entrained in the organic with clean water.
The copper is then stripped from organic phase in the strip stage into a strong sulfuric acid solution suitable for electrowinning. This strong acid solution is called barren electrolyte when it enters the cell, and strong electrolyte when it is copper bearing after reacting in the cell.
See also 
- University of Illinois in Chicago (Fall 1999) by Zachary Fijal, Constantinos Loukeris, Zhaleh Naghibzadeh, John Walsdorf, URL: http://vienna.bioengr.uic.edu/teaching/che396/sepProj/Snrtem~1.pdf as found on the 21st November 2006