Extractive Metallurgy of Copper Part 12 pptx

CHAPTER 18 Solvent Extraction Transfer of Cu from Leach Solution to Electrolyte Written with Jackson Jenkins, Phelps Dodge, Morenci, AZ The pregnant leach solutions produced by most l

CHAPTER 18

Solvent Extraction Transfer of Cu

from Leach Solution to Electrolyte

(Written with Jackson Jenkins, Phelps Dodge, Morenci, AZ)

The pregnant leach solutions produced by most leaching operations are:

(a) too dilute in Cu (1-6 kg Cu/m3)

and:

(b) too impure (1 - 10 kg Fe/m3)

for direct electrodeposition of high purity cathode copper Electrowinning from these solutions would give soft, impure copper deposits

Industrial electrowinning requires pure, Cu-rich electrolytes with >35 kg Cu/m3 This high concentration of Cu:

(a) ensures that CU++ ions are always available for plating at the cathode surface

(b) gives smooth, dense, high purity, readily marketable cathode copper

Solvent e,xtraction provides the means f o r producing pure, high Cu" electrolytes from dilute, impure pregnant leach solutions It is a crucial step in the production of -2.5 million tonnes of metallic copper per year It continues to grow in importance as more and more Cu ore is leached

18.1 The Solvent Extraction Process

Copper solvent extraction (Fig 18.1) entails:

307

organic

Mixer

Fig 18.1 Schematic plan view of copper solvent extraction circuit The inputs are pregnant leach solution and Cu-depleted electrolyte The products are Cu-enriched electrolyte and low-Cu raffinate Fig 18.3 shows an industrial mixer-settler Fig 18.4

shows the most common industrial circuit

(a) contacting pregnant aqueous leach solution (1-6 kg Cu++/m3, 0.5 to 5 kg H2S04/m3) with a Cu-specific liquid organic extractant - causing extraction of Cu++fyom the aqueous solution into the organic extractant

(raffinate) from the now-Cu-loaded organic extractant

(c) sending the low-Cu raffinate back to leach

(d) sending the Cu-loaded organic extractant to contact with strong-H2S04 electrowinning electrolyte (170-200 kg H2SO4/m3) - causing Cu to be stripped from the organic into the electrolyte

(e) separating by gravity the now-Cu-stripped organic extractant from the now-Cu”-enriched aqueous electrolyte

( f ) returning the stripped organic extractant to renewed contact with pregnant

leach solution

(8) sending the Cu++-enriched electrolyte to electrowinning where its Cu* is

(b) separating by gravity the now-Cu-depleted aqueous leach solution

electrodeposited as pure metallic ccpper

The process is continuous It typically takes place in ‘trains’ of 2 extraction mixer-settlers for steps (a) and (b) and 1 strip mixer-settler for steps (e) and (0

An extraction system typically consists of 1 to 4 ‘trains’ (Jenkins et a/., 1999)

Solvent Extraction Transfer of Copper 309 The organic extractants are aldoximes and ketoximes (Kordosky et al., 1999) They are dissolved 5 to 20 volume% in purijied kerosene

extractant leach solution organic (0.3 kg Cu/m3)

(1 to 6 kg Cu/m3 )

where RH is the aldoxime or ketoxime extractant

Loading of organic extractant with Cu is seen to be favored by a low concentration of sulfuric acid () in the aqueous phase So contact of dilute HzS04 aqueous pregnant leach solution with organic gives extraction of Cu from the aqueous phase into the organic phase

After this organic loading step, the organic and aqueous phases are separated The Cu++-depleted raffinate is sent back to leach to pick up more Cut+ The Cu- loaded organic phase is sent forward to a 'strip' mixer-settler where its Cu is stripped into Cu*-depleted aqueous electrolyte

The strip reaction is the reverse of Reaction 18 I , Le.:

(-185 kg H2S04/m', extractant extractant (-165 kg H2S04im3,

It is pushed to the right by the high sulfuric acid concentration of the aqueous electrolyte It strips Cu from the organic extractant and enriches the electrolyte

to its desired high-Cu++ concentration

In summary, the organic extractant phase is:

(a) loaded with Cu from weak H2S04 pregnant leach solution

(b) separated from the pregnant leach solution

(c) contacted with strong H2S04 electrolyte and stripped of its Cu

It is the different H 2 S 0 4 strengths of pregnant leach solution and electrolyte

which make the process work

18.3 Extractants

The organic extractants used for Cu are oximes, Fig 18.2 Two classes are used: aldoximes and ketoximes, Table 18.1 They are dissolved in petroleum distillate

to produce an organic phase, 8 to 20 volume% extractant This organic is (i)

immiscible with CuSO4-H2SO4-H*0 solutions and (ii) fluid enough (viscosity =

0.01 to 0.02 kg/m.s) for continuous mixing, gravity separation and pumping around the solvent extraction circuit

A successful Cu-extractant for any leach project must (Kordosky, 1992; Kordosky et al., 1999):

(a) efficiently extract Cu from the project’s pregnant leach solution

(b) efficiently strip Cu into the project’s electrowinning electrolyte

(c) have economically rapid extraction and strip kinetics

(d) disengage quickly and completely from leach solution and electrolyte, i.e

not form a stable emulsion

Solvent Extraction Transfer of Copper 3 1

(e) be insoluble in the project’s aqueous solutions

(f) be stable under extraction and strip conditions so that it can be recycled many times

(g) not absorb sulfuric acid

(h) extract Cu preferentially over other metals in the pregnant leach solution, particularly Fe and Mn

(i) not transfer deleterious species from pregnant leach solution to electrolyte, particularly C1

(i) be soluble in an inexpensive petroleum distillate diluent

(k) be nonflammable, nontoxic and non-carcinogenic

Ketoxime and aldoxime extractants satisfy these requirements

Table 18.1 Properties of Cu solvent extraction extractants (Kordosky et ul., 1999)

Aldoxime-ketoxime extractants are customized by adjusting their relative quantities

Aldoxime- Property Ketoxime Aldoxime with modifier ketoxime

mixtures, no modifiers Extractive strength

very fast

very good low

LIX 84-1

strong good excellent very fast

very fast

very good low

LIX 984N

~

* Depends on modifier **Depends on pregnant leach solution and modifier

Aldoximes are strong extractants However, their Cu can only be stripped by contact with 225+ kg H2S04/m3 electrolyte This level of acid is too corrosive

for industrial electrowinning It also tends to degrade the extractant For these reasons, aldoximes are only used when mixed with ketoximes or modifiers, e.g highly branched alcohols or esters

The most common extractants in 2002 are ketoxime-aldoxime and ester- modified aldoxime solutions

18.3.2 Diluents

Undiluted ketoxime and aldoxime extractants are thick, viscous liquids They are totally unsuitable for pumping, mixing and phase separations They are, for this reason, dissolved 8 to 20 mass% in moderately refined high flash point petroleum distillate (purified kerosene), hydrogenated to avoid reactive double bonds (Bishop et a/., 1999)

Commercial diluents typically contain -20 volume% alkyl aromatics, -40% naphthenes and -40% paraffins (Chevron Phillips, 2002)

18.3.3 Rejection of Fe and other impurities

An efficient extractant must carry Cu forward from pregnant leach solution to electrolyte while not forwarding impurities, particularly Fe, Mn and CI This is a critical aspect of efficient electrowinning of high purity copper Fortunately, ketoxime and aldoxime extractants have small solubilities for these impurities Ester-modified aldoximes are especially good in this respect (Cupertino et al.,

1999, Kordosky et al., 1999)

Impurities may, however, be carried forward to electrolyte in droplets of pregnant leach solution in the Cu-loaded organic This carryover can be minimized by (i) coalescing the pregnant solution droplets on polymer scrap; (ii) filtering and (iii) washing the loaded organic (Jenkins et al., 1999)

18.4 Industrial Solvent Extraction Plants

Solvent extraction plants are designed to match the rate at which Cu is leached in the preceding leach operation They vary in capacity from 20 to 600 tonnes of

Cu per day Table 18.2 gives operational details of five solvent extraction plants Additional details are given in Jenkins et a/., 1999

The key piece of equipment in a solvent extraction plant is the mixer-settler, Fig

18.3 (Lightnin, 2002) Mixer-settler operation consists of

( a ) pumping aqueous and organic phases into a mixer at predetermined rates

Pregnant leach solution

Fig 18.3 Copper solvent extraction mixer-setter The two mixing compartments, the large settler and the organic overflow/aqueous underflow system are notable Flow is distributed evenly in the settler by picket fences (not shown), Table 18.2

Table 18.2 Details of five Cu solvent extraction plants, 2001 Details of the

Cathode production, tonnesiyear

Total pregnant solution input rate, m'lhour

SX plant detals

plant type

number of SX 'trains'

extraction mixer-settlers per train

strip mixer-settlers per train

Mixer-settler details

Mixers

round or square

number of mixing compartments

compartment size: depth x width x length, m

mixer system

construction materials

liquids residence time, minutes

length x width x depth, m

flow distributor system

construction materials

organic depth, m

aqueous depth, m

estimated residence time, minutes

estimated phase separation time, minutes

aqueous removal from organic

crud removal system

crud treatment system

organic cleaning system

organic removal from raffinate

Settler

Flowrates per train, m31hour

pregnant solution input rate

organic flowrate, extraction to strip

depleted electrolyte input rate

% of electrolyte flow sent to SX

organic removal from electrolyte

electrolyte treatment before tankhouse

130 000

4000 series

4

3 LIX 860-NIC/LIX 84-IC

13

Orfom SX-12

no wash none pneumatic pump Chuquicamata mechanical breakage clay treatment with Sparkle filter skimmer

750

1040 I80

218 000 5000-7500

2 series

2 series-parallel

4 series 2; series-parallel 3 series 2; series-parallel 1

square

3 3.1 x 3.7 x 12.7 suction mixer polymer concrete 2.4

28 x 29 x 1.1

2 picket fences

HDPE-lined concrete 0.27 0.63

3 PT5050-LIX 984NC

2 I 4%Ll, 15.8% L2 Conosol 170ES water wash to pH I I Wemco coalescers pneumatic pump centrifuge and pressure filter zeolite treatment Wemco coalescers

1400 series

2400 series-parallel 1500-l650 450-500

25

c u His04 4.94 6.44 0.70 12.10 3.33

7.40 36.33 171.41 40.29 170.9 Wemco pacesetter coalescence sand/garnet/anthracite

Solvent Extraction Transfer of Copper 3 15

equivalent leach and electrowinning plants are given in Chapters 17 and 19

one wash mixer-settler

8 Disep garneuanthracite filters

15

0.5 Acorga M5640

8 Escaid I 10

no wash aqueous entrainment pumps

in loaded organic tank 2.5 cm diaphragm pump bentonite mixing-recovery filter press pumping from pond

12

70 seconds LIX 984

21 Conosol 170

I wash stage mixer-settler drain loaded organic tank interface pumping and settler dumping clay mixing and filter press clay mixing and filter press skimmed from organic recovery tanks

3.80 9.80 38.0 200.0 50.0 185.0 organic IS floated from rich booster tank

6 anthracite garnet filters

(b) mixing the aqueous and organic with impellers

(c) overflowing the mixture from the mixer through flow distributors into a flat settler where the aqueous and organic phases separate by gravity (organic and aqueous specific gravities, 0.85 and 1.1 respectively [Spence and Soderstrom, 19991)

(d) overflowing the organic phase and underflowing the aqueous phase at the far end of the settler

Typical mixer-settler aqueous and organic flowrates are 500-4000 m3 per hour (each)

The mixer is designed to create a well-mixed aqueous-organic dispersion Modem mixers consist of two or three mixing chambers They create the desired dispersion and smooth forward (plug) flow into the settler Mixer aqueous/organic contact times are 2 to 3 minutes - which brings the liquids close

to equilibrium Entrainment of very fine droplets is avoided by using low tip- speed (<400 &minute) impellers (Spence and Soderstrom, 1999)

The settler is designed to separate the dispersion into separate aqueous and organic layers It:

(a) passes the dispersion through one or two flow distributors (picket fences

or screens) to give smooth, uniform forward flow

(b) allows separate layers to form as the dispersion flows smoothly across the large settler area

The vertical position of the aqueous-organic interface is controlled by an adjustable weir at the far end of the settler It avoids accidentally overflowing aqueous or underflowing organic

Modem settlers are square in plan This shape is the best for smooth flow and an adequate residence time Liquid residence times in the settlers are 10 to 20

minutes, Table 18.2 This time is sufficient to guarantee complete phase separation (laboratory separations occur in 0.5 to 2 minutes [Spence and Soderstrom, 19991) The aqueous phase is -0.5 m deep The organic phase is -0.3 m deep Advance velocities are typically 1 to 5 m per minute

18.4.1 ‘Trains

2002 solvent extraction plants consist of one to four identical solvent extraction circuits (‘trains’) - each capable of treating 500 to 4000 m3 of pregnant solution per minute Each train transfers 20-250 tonnes of Cu from pregnant solution to electrolyte per day, depending on the Cu content and flowrate of the pregnant solution

Solvent Extraction Transfer of Copper 3 1 I

18.4.2 Circuit design

Most copper solvent extraction is done in series circuits, Fig 18.4 The two extraction mixer-settlers typically transfer -90% of Cu-in-pregnant-leach- solution to the extractant (Jenkins et al., 1999) The remaining Cu is not lost It

merely circulates around the leach circuit

The single strip mixer-settler strips 50% to 65% of the Cu-in-loaded-organic into electrolyte The remainder circulates around the solvent extraction circuit, Fig 18.4 These transfer efficiencies can be increased by adding extraction and strip mixer-settlers to the circuit However, the 2 extraction, 1 strip mixer-settler configuration predominates

18.5 Quantitative Design of Series Circuit

This section describes the preliminary design of a series solvent extraction circuit It is based on the data in Table 18.3 and Fig 18.4

Table 18.3 Preliminary specifications for design of solvent extraction circuit They are

also given in Fig 18.4

Circuit type

Extractant

Specified organic/aqueous ratio in

extraction mixer-settlers ( N O )

Expected pregnant leach solution

composition and input rate

Specified composition of Cu

depleted electrolyte from

tankhouse

Specified composition of Cu-

enriched electrolyte returning to

tankhouse

Stripping data from laboratory

tests (Cognis, 1997)

series: 2 extraction mixer-settlers

expected extraction from pregnant solution into organic: 90%

LIX 984N in Orfom SX 12 diluent

Loads -0.25 kg C d m ? per volume% LIX 984N

~~

*O/A ratios of -I permit easy switching between aqueous-continuous and organic-continuous

operation Industrial O/A ratios are discussed in Biswas and Davenport (1994)

Intermediate Loaded

aqueous organic

Extract 2 Extract 1 StriD

270 m3/hour depleted electrolyte from electrowinning (35)

Fig 18.4 Plan view of series solvent extraction circuit The bracketed numbers are Cu

concentrations in kg Cu/m3 Industrial mixer-settlers are tight against each other to minimize plant area and flow distances Note the two extraction mixer-settlers (Extract 1

and Extract 2) and one strip mixer-settler This is the most common arrangement (Jenkins

et al., 1999) Flowrates and Cu concentrations are those in Table 18.3

~lntermediate

18.5.1 Percent extractant in organic

organic p

(2.3)

LIX 984N extracts up to:

0.25 kg Cu per m3 of organic phase per volume% LIX 984N in the organic

V

(Cognis, 1997) The LIX 984N strength which will extract a specified amount of

Cu is calculated by the mass balance:

V

0.25 kg of Cu extracted

per m 3 of organic per volume% LIX 984 volumetric flowrate

volume% LIX 984 in in the organic of organic, m3/hour

the organic

volumetric flowrate o f pregnant leach solution, m 3ihour required Cu extraction from

kg Cu/m

= pregnant leach solution into organic, X (1 8.3)

of pregnant leach solution

or:

Solvent Extraction Transfer of Copper 3 19

required Cu extraction from pregnant leach solution

0.25

(where A/O is the aqueous/organic volumetric ratio entering the extraction

mixer-settler)

Extraction of Cu from a 3 kg Culm’ pregnant leach solution with the Table 18.3-

prescribed N O volume ratio of 1/1 requires, therefore:

3

volume% LIX 984N in organic = -

0.25

So each train of the solvent extraction plant requires pumping of 1000 m3/hour

of 12 volume% LIX 984N in Orfom SX 12 diluent

This calculation is the first step in choosing the organic phase for a proposed solvent extraction circuit The chosen organic must then be tested with actual leach and electrowinning solutions to ensure suitability for the proposed operation

18.5.2 Extraction eficiency

Under the dynamic conditions of two industrial mixer-settlers in series, Cu extraction from pregnant leach solution is about 90% (Jenkins et al., 1999) In the case of a 3 kg Cu/m’ pregnant solution, the raffinate leaving the series of two extraction mixer-settlers will contain - 0.3 kg Cu/m3 of raffinate, Fig 18.4

18.5.3 Rate of Cu extraction into organic

The overall rate at which Cu is extracted into the solvent extraction organic phase is given by the equation:

This is also the overall rate at which metallic copper will have to be plated in the electrowinning plant It allows the electrowinning designer to calculate the cathode area and current density for the proposed electrowinning plant

18.5.4 Equilibrium strip Cu concentrations

The sulfuric acid concentration in depleted electrolyte is very high (-185 kg H2S04/m3 of electrolyte) This causes copper to strip from the solvent extraction organic into the electrolyte Table 18.3 indicates that:

the 45 kg Culm3 electrolyte specified f o r return to the electrowinning tankhouse

will be at equilibrium with:

-1.5 kg Cu/m3 organic (12% LIX984Nin OYfom 12)

These concentrations are shown in Fig 18.4 The precise equilibrium values would need to be determined with the project‘s actual electrolyte

18.5.5 Electrolyte flowrate into the strip mixer-settler

Section 18.5.3 indicates that the electrowinning plant must plate 2700 kg metallic copper per hour From the strip mixer-settler point of view, it means that 2700 kg of Cu per hour must be transferred to electrolyte

This, and the Table 18.3-specified depleted and enriched electrolyte compositions (35 and 45 kg Cuirn’), permit calculation of the rate at which electrolyte must flow into and out of the strip mixer-settler, i.e.:

electrowinning

kg Cu/hour

-

extraction, m3/hour extraction, kg C d m 3 extraction, kg Cu/m3 1

electrolyte flowrate Cu in electrolyte Cu in electrolyte

to and from solvent x leaving solvent - entering solvent

from which the electrolyte flowrate in and out of the strip mixer-setter is:

electrolyte flowrate = 2700 kg Cdhour = 270 m3 of electrolyte

(45 - 35) per hour

kg Cu/m3 of electrolyte Figure 18.4 summarizes flows and Cu concentrations in the newly designed plant

Solvent Extraction Transfer of Copper 32 1

Intermediate Loaded

18.6 Stability of Operation

M:er 4 (’”) Settler ( 4 4 ) d l

Industrial solvent extraction circuits are easily controlled and forgiving

Consider, for example, how the Section 18.5 circuit responds to an increase in

Cu concentration in pregnant leach solution (which would happen if easily- leached ore is encountered in the mine) Suppose that the pregnant solution improves from the 3 kg Cu/m3 in Fig 18.4 to 3.3 kg Cu/m3

Copper extraction from the extra 0.3 kg Cu/m3 will probably be about 65%

instead of 90% so that:

depleted electrolyte from electrowinning

(35)

(a) the raffinate will contain 0.4 kg Cu/m3 rather than 0.3 kg Cu/m3

(b) 2900 kg of Cu will be transferred to organic in the extraction mixer- settlers, Eqn (18.4)

organic

(2.3)

The resulting flows and Cu concentrations are shown in Fig 18.5

V

Of course, the rate at which copper is being plated will also have to be increased

to 2900 kg copper per hour This can be done by increasing current density and

by bringing unused cells into operation

V

Fig 18.5 Solvent extraction circuit that has been perturbed by receiving 3.3 kg Cu/m’

pregnant leach solution instead of 3 kg Cdm’ pregnant leach solution, Fig 18.4 It is

assumed that Cu electrowinning rate has been increased (by increasing current density) to match the rate at which Cu is being transferred from pregnant leach solution to electrolyte Note that the only operating variable that has to be changed is electrolyte recycle rate