Copper Solvent Extraction

1

SOLVENT EXTRACTION More than half of the world’s copper is produced by solvent extraction. Solvent extraction was first discovered in ICI’s Manchester Laboratory in 1940’s. Its commercial use was mainly restricted to uranium beneficiation. The first commercial copper extraction plant built was Nchanga Tailings Leach Plant in Zambia by Anglo American Corporation, of South Africa. *In the extraction mixer-settler the copper bearing liquor is contacted with the appropriate extraction reagent to form an organic soluble complex. The copper-loaded organic is then advanced to the stripping mixer-settler.

Figure 1: Sketch showing heap leaching/SX/EW plant’s layout *The stripping mixer-settler is where the copper-loaded organic solution is contacted with acidic electrolyte, arising from the tankhouse. This allows re-extraction of the copper by sulphuric acid to form a concentrated, purified electrolyte solution. This solution is fed to the electrowinning tankhouse. THE ACID GENERATED IN THE TANKHOUSE IS RETURNED IN COPPER-DEPLETED ELECTROLYTE TO STRIP MORE COPPER FROM THE CIRCULATING LOADED ORGANIC SOLUTION. THE ORGANIC REAGENT IS REGENERATED DURING THE STRIPPING PROCESS AND IS RETURNED TO THE EXTRACTION SECTION TO EXTRACT MORE COPPER.

2 Extraction

CuSO4(aq) + 2LH(org)  CuL2(org) + 2H+(aq)

Stripping

CuL2(org) + 2H+(aq)  CuSO4(aq) + 2LH(org)

Equilibrium constant

Distribution

by

Log D

(K)

(D)

= [CuL2] . [H+]2 [CuSO4] . [LH]2 = Cu(org) Cu(aq)

is given

= Log K = 2 Log [LH] + 2pH

Figure 2: Diagrammatic representation of extraction and strip mixer/settlers Clearly, the extraction reaction is favoured by conditions of low acidity and stripping proceeds under conditions of high acidity. Hydroxy-oximes and Aldoximes have a performance advantage in the copper extraction process over Keytoxime in their ability to: (1)

Treat effectively a wider range of feed solution compositions in regard to both copper and acid concentration.

(2)

Extract copper rapidly across the solution interface and maintain this excellent kinetic performance, even under low temperature conditions.

(3)

Reject ferric ions during the extraction process in all respects except one, Aldoximebased extractant systems can match or improve the performance of products containing Keytoximes. Because of their intrinsic high strength, Aldoximes are more difficult to strip than Keytoximes, although the difference tends to reduce at acid concentrations above 170 g/l. Some chemical manufacturers (Cytec and Cognis), have a range of commercially available organic chemical compounds which can modify the stripping performance of the Aldoxime ligand e.g esters and alcohols.

*In an operating SX process, the ligand is in contact with concentrated acid during the 2 minutes of mixing with the spent electrolyte in the strip mixer i.e 10% of the time spent in the SX circuit.

3 ester loss is approximately tridecanol alcohol

0.016 % per day 0.057 % per day

*In an operating SX plant, losses of circuit organic arise from several sources. One of the major sources of such losses is by entrainment of the organic phase in the aqueous streams exiting the circuit, and from the enriched electrolyte steam feeding the EW tankhouse. The former is normally considered more important than the latter as – (1) (2)

Flow rates involved are much larger The organic recovery streams for the latter, tend to be more efficient

Modifiers can help to attenuate these losses. *Organic surge tank location. – Best is between extraction and strip. *To maximise production, flow rate of aqueous stream feeding the extraction stages must be maximised. The limiting factor here is almost invariably the rate of separation of the phases after mixing. Aldoximes, plus modifiers, have been shown to have very fast phase separation rates. *If phase separation is too rapid, “secondary haze” results. In this case, very fine particles of organic can remain suspended in the aqueous phase if phase separation is too rapid. THESE FINE PARTICLES HAVE GREAT DIFFICULTY IN COALESCING. The longer the mixing time, at high shear, the worse this problem becomes. The use of a variable-speed mixer would permit the adjustment of drop size. *A relatively deep dispersion band in the settler can act as a filter, assisting coalescing and reduce organic in aqueous entrainment levels. *The dispersion band disappears approximately one-third to half-way down the settler. *IT IS NECESSARY THAT THE LEVEL OF SOLUTION IN THE AQUEOUS AND ORGNIC WEIRS BE MAINTAINED AT A HIGH LEVEL TO PREVENT AIR ENTERING THE SUCEEDING MIX CHAMBER. Increased loss of organic can result, as well as promoting undesired continuity changes in the mix chamber.

Figure 3: Block diagram showing the solvent extraction process *Impurities such as dissolved silica, suspended solids and polyacrylamide can influence phase separation characteristics.

4 *The ingress of solids must be minimised to prevent crud formation To promote coalescence of the organic bubbles, picked fences are used.

Figure 4: Simplified picket fence arrangement *Crud (Chalk River Unidentified Deposit), from an area in Canada where it was first noticed, is most easily controlled in operating plants by regularly removing it from the oil/water interface as part of planned operational maintenance. Failure to do so will result in higher-than-necessary losses. *Acid concentration of 165-180 g/l H2SO4 is required to achieve a suitable compromise between corrosion rate and stripping efficiency.

Figure 5: Diagram of a typical mixer/settler

5

*Crud collects in the settlers. They can collect at the bottom or at the interface, or can sometimes be seen floating on the surface of the organic layer. It is found in strip as well as in extraction settlers. Centrifuging can remove crud. It is far superior to filtering using pressure filters. CRUD EVENTUALLY LEADS TO HIGH AQUEOUS AND ORGANIC ENTRAINMENTS. This can be detrimental to tankhouse operation as the transfer of iron impacts adversely on current efficiency and cathode quality. Crud is oil/water emulsion mixed with fine solid particles.

Figure 6: Typical pressure filter used for crud filtration *Leach solution can vary from 1.8 g/l to 3.9 g/l Feed Solution O:A Organic (g/l) Cu H2S04 Extract Loaded Stripped 3.25 8.6 1.22:1 4.60 2.20 3.04 6.8 1.22:1 4.50 2.20 3.09 2.6 1.11:1 4.65 2.00 2.98 3.7 1.54:1 3.92 2.10 *Lead-alloy anode Stainless steel cathode *Mixing flow rate 4.4 – 6.1 m3/h/m2 *Settling flow rate 2 – 6 m3/h/m2 R Seecharran Concentrator Manager

Raffinate Cu 0.32 0.22 0.17 0.17

Spent Cu H2SO4 42.6 172 49.1 170 42.8 173 42.8 173

Recovery 90.2 92.8 94.3 94.3