Expert SX/EW Consultants
Copper SX EW was the majority of PPM‘s global metallurgical consulting/problem-solving business focus, study reviews and SX/EW commissioning, predominately in a number of counties including Africa and Australia. Afterward, PPM moved into other minerals processing services within the minerals industry.
What is Solvent Extraction?
The purpose of Solvent Extraction (SX) is to receive an impure solution (from heap leaching) containing dissolved copper and remove (extract) that copper. Then transfer it to a synthetic and relatively pure solution from which the metal can be electrowon with minimal contamination by other metals.
SX Mixer Settlers
Solvent extraction equipment consists of a series of mixers and settlers. Here’s where the mixers receive the solvent from a previous settler in the process. This would be an aqueous solution: either a leach solution or electrolyte from the electrowinning process. The two solutions are mixed then discharged by overflow to the adjoining settler. The two solutions are immiscible (will not remain mixed) and separate naturally by coalescence. The aqueous solutions are denser than the organic solution and gravitate to the bottom layer of the settler while the organic accumulates on the surface. Settlers require no active components, the solution feeding into the settler causes the settler to overflow at the discharge end. Organic overflows a weir from the surface of the settler into a launder which drains to the next tank or mixer in the process. The aqueous solution passes under a baffle then over a weir before passing to the next tank or stage. As a result of the density differential between the organic and aqueous the height of the overflow weirs differs to establish a stable depth of the two components within the settler. The difference in weir heights controls the depth of the phases and is usually controlled by an adjustable aqueous weir.
The key component of the SX process is the Solvent which is a combination of a kerosene product and an extractant product. The Kerosene used has a narrow range of molecular chain lengths and consequently is sometimes referred to or marketed as ‘Narrow Cut Kerosene’, the preferred name is ‘Diluent’ as the role of the kerosene is to dilute the extractant and consequently reduce the viscosity of the mixture. Diluent does not participate in the SX chemical reactions. The Extractant used at WCP is a modified aldoxime with the following chemical name, 2-hydroxy-5-nonlybenzaldoxime. The extractant is capable of extracting copper selectively over other cations (metal elements). Following copper, the next metal which is susceptible to extraction is Ferric Iron (Fe3+), under typical conditions the extractant will collect copper preferentially over ferric at a ratio of better than 2000:1.
There are two essential stages to the SX process, Extraction, and Stripping, a further washing or scrubbing stage is included in the WCP SX plant. Extraction includes two mixer-settler units (E1, E2). The solution process flows through extraction are counter-current. That is, the aqueous moves from E1 to E2, while the organic flows from E2 to E1. Stripping also includes two mixer-settler units (S1,S2) and is also a counter-current arrangement. The solvent extraction circuit is completed by a Wash Mixer Settler (W1). This serves to clean the Organic solution of contaminants before the stripping stage.
The extractant is a very mild organic acid which will exchange Hydrogen Ions for Copper Ions when contacted with the relatively low acidity leach solution, according to the following basic reaction;
SX Process Reaction – 2RH(ORG) + CuSO4 ↔ R2Cu + H2SO4
The Pregnant Leach Solution (PLS) is fed to the extraction mixer-settlers (2 stages, E1 – E2). After the extraction process, the leach solution, now depleted in copper concentration, is discharged from the SX to a settling pond and referred to as raffinate.
The Organic discharged from the extraction stage is referred to as ‘Loaded Organic’. The extractant molecules are ‘loaded’ with copper. Entrainment or contamination of the loaded organic must be minimised to optimize the efficiency of the electrowinning process. The Wash stage dilutes and displaces physical entrainment by mixing the organic with a recycled cleaning solution. The cleaning solution is composed of Raw Water and spent electrolyte. The wash solution recycles in a closed circuit within the mixer-settler. Additional water and electrolyte which are added in small continuous quantities displace surplus wash solution to maintain the level of impurities in the Wash.
The Organic solution from the Wash stage (Loaded Organic) is passed to the stripping stage first mixer and contacted with lean electrolyte (Spent Electrolyte). The reaction in the strip mixers is the reverse of the extraction reaction. The reversal of the reaction is driven by the high acid concentration maintained in the electrolyte solution. There are two mixer-settlers in the stripping stage (S1 – S2). The electrolyte discharging from the stripping stage is now referred to as ‘Strong Electrolyte’ or ‘Advanced Electrolyte.
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