CUSTOMISING COPPER-IRON SELECTIVITY USING MODIFIED ALDOXIME EXTRACTANTS: PILOT-PLANT EVALUATION K.C. Sole 1 , K. Viljoen 1 , B.K. Ferreira 1 , M.D. Soderstrom 2 , O. Tinkler 2 and L. Hoffmann 2 1 Anglo Research, Anglo Operations Limited, Crown Mines, South Africa ([email protected]) 2 Cytec Industries Inc., Phoenix, Arizona, U. S.A. ABSTRACT In most copper solvent-extraction operations, an important consideration in choice of extractant is the requirement for high selectivity of copper over iron. Ester- modified aldoxime extractants presently offer the best Cu:Fe selectivity. In some plants, however, the process feed liquors contain significant amounts of manganese but little iron. Although manganese is not chemically extracted by oximes, trace amounts are carried over to the electrowinning circuit by physical entrainment. This may result in anodic oxidation of manganese which, in turn, can cause oxidative degradation of the organic phase when returned to the solvent-extraction circuit in the spent electrolyte. Many of these operations prefer to operate with higher levels of iron reporting to the advance electrolyte, as appropriate control of the Fe(II)/Fe(III) couple will ensure that manganese remains in the benign Mn(II) form. A new reagent, LS 4202, designed by Cytec, now offers the possibility of tailoring the iron co-extraction, so that appropriate Cu:Fe selectivity can be achieved in the solvent-extraction operation to maintain an electrolyte Fe:Mn mass ratio of >10:1, while retaining the other benefits offered by ester- modified aldoximes. The results of an integrated solvent extraction-electrowinning pilot- plant trial are presented, in which the performances of the conventional extractants ACORGA ® M.5774 and LIX ® 984N are compared with LS 4202. The Cu:Fe selectivity is demonstrated, along with results on the transfer of other impurities to the advance electrolyte.
Transcript
CUSTOMISING COPPER-IRON SELECTIVITY USING MODIFIED ALDOXIME EXTRACTANTS: PILOT-PLANT EVALUATION
K.C. Sole1, K. Viljoen1, B.K. Ferreira1, M.D. Soderstrom2, O. Tinkler2 and L. Hoffmann2
1Anglo Research, Anglo Operations Limited, Crown Mines, South Africa ([email protected])
2Cytec Industries Inc., Phoenix, Arizona, U. S.A.
ABSTRACT
In most copper solvent-extraction operations, an important consideration in choice of extractant is the requirement for high selectivity of copper over iron. Ester-modified aldoxime extractants presently offer the best Cu:Fe selectivity. In some plants, however, the process feed liquors contain significant amounts of manganese but little iron. Although manganese is not chemically extracted by oximes, trace amounts are carried over to the electrowinning circuit by physical entrainment. This may result in anodic oxidation of manganese which, in turn, can cause oxidative degradation of the organic phase when returned to the solvent-extraction circuit in the spent electrolyte. Many of these operations prefer to operate with higher levels of iron reporting to the advance electrolyte, as appropriate control of the Fe(II)/Fe(III) couple will ensure that manganese remains in the benign Mn(II) form. A new reagent, LS 4202, designed by Cytec, now offers the possibility of tailoring the iron co-extraction, so that appropriate Cu:Fe selectivity can be achieved in the solvent-extraction operation to maintain an electrolyte Fe:Mn mass ratio of >10:1, while retaining the other benefits offered by ester-modified aldoximes. The results of an integrated solvent extraction-electrowinning pilot-plant trial are presented, in which the performances of the conventional extractants ACORGA® M.5774 and LIX® 984N are compared with LS 4202. The Cu:Fe selectivity is demonstrated, along with results on the transfer of other impurities to the advance electrolyte.
INTRODUCTION
The two main classes of commercial copper extractants are the ester-modified aldoximes and aldoxime:ketoxime mixtures. Ester-modified aldoximes are characterised by strong copper loadings from low pH (< 2) solutions, weaker stripping by conventional acid strength electrolytes, and excellent selectivity for copper over iron. Ketoximes are weaker extractants that operate better at higher pH values, have poorer Cu:Fe selectivity, but slightly better hydrolytic stability.
Choice of extractant is usually governed by the nature and origin of the feed liquor (copper concentration, pH, impurity elements, ionic strength, presence of halides or oxidising species, heap or agitation leaching) [1, 2]. In most North and South American operations, high Cu:Fe selectivity is an important consideration, tending to favour the selection of modified aldoximes. In Africa and Australia, however, many plants treat liquors that contain significant Mn but little Fe. Unlike Fe, Mn is not chemically extracted by oximes, but small amounts of Mn are transferred from the pregnant leach solution (PLS) to the electrolyte via physical entrainment of the aqueous phase in the loaded organic (LO) phase. This is exacerbated in circuits that do not include a wash stage or that do not use a LO holding tank between the extraction and strip circuits.
Mn reporting to the electrowinning (EW) circuit can be oxidised at the anodes to Mn(III), Mn(IV), or Mn(VII), creating a highly oxidising species that is returned to the strip circuit in the spent electrolyte (SE) [3]. This contributes to oxidative degradation of the extractant and diluent, ultimately leading to increased organic losses, deterioration in phase separation, increased crud formation, and lower organic loading capacity. Industry convention indicates that the total Fe:Mn mass ratio in copper electrolytes should be maintained at ~10:1 to ensure that all Mn remains in the divalent state [4]. For this reason, an increasing number of operations add Fe(II) to the electrolyte, usually as FeSO4 or by passing the electrolyte through scrap iron. Where Mn is present in feed liquors, a recent trend has been towards the choice of ketoxime-based extractants that are less selective for Fe.
A new reagent, LS 4202, designed by Cytec, now offers the possibility of tailoring the iron co-extraction so that Cu:Fe selectivity can be manipulated to maintain an appropriate electrolyte Fe:Mn ratio. LS 4202 is based on ACORGA M.5774, so the other advantages of ester-modified aldoximes are maintained, while ensuring that sufficient Fe is transferred to the electrolyte to offset the corresponding Mn transfer by entrainment.
In this study, the copper- and iron-transfer characteristics of LS 4202 were compared with those of two of the most widely used competitive products, ACORGA M.5774 and LIX 984N (an equivolume blend of C9 aldoxime and ketoxime), in identical closed-loop solvent extraction-electrowinning (SX-EW) circuits in a nine-day continuous pilot-plant trial.
EXPERIMENTAL
Reagents
The extractants used were LS 4202, ACORGA M.5774, and LIX 984N, each dissolved in Shellsol 2325, a partially (~ 20 %) aromatic diluent. Since LIX 984N transfers slightly less copper per unit volume, LS 4202 and ACORGA M.5774 were used at 20 vol.% concentration, while LIX 984N was employed at 21.5 vol.%, thereby ensuring that copper transfer to the organic phase was equivalent for the three systems.
The PLS was supplied from a Zambian operation, the average composition of which is shown in Table 1. Synthetic SE (35 g/l Cu, 180 g/l H2SO4) was made up to start the stripping and EW circuits.
Table 1 – Average composition of PLS
Element Conc. (g/l) Element Conc. (g/l) Element Conc. (g/l) Al Ca Cu Cr Fe
0.188 0.300 6.50
0.0006 0.158
Mg Mn Pb Si U
0.776 0.452
0.0004 0.243
0.0006
V Zn
H2SO4 pH Eh
< 0.0002 0.016 1.66 2.0
640 mV Pilot Plant
The flowsheet comprised two extraction stages and one strip (2E-1S) in the SX circuit, integrated with an EW circuit. The basic flowsheet of a single circuit is shown in Figure 1, while a photograph of the entire plant is shown in Figure 2. The design operating conditions are summarised in Table 2.
Figure 1 – Simplified flowsheet of a single SX-EW circuit
E2 E1 S1
SO tank
Raffinate
PLS
EW feed tank EW cell
SO
LO
AE
SE
RE
EW feed + – +
E2 E1 S1
SO tank
Raffinate
PLS
EW feed tank EW cell
SO
LO
AE
SE
RE
EW feed + – +
Figure 2 – Photograph of pilot plant, showing SX in centre, EW cell on the right, and combined raffinate holding tank on the left-hand side
Table 2 – Design operating conditions of SX-EW circuit
Circuit Parameter Value Extraction PLS flowrate
SO flowrate Number of stages Advance O:A Mixer residence time Continuous phase
140 ml/min 170 ml/min
2 1.2
3 min Organic
Stripping SE flowrate Number of stages Advance O:A Mixer residence time Continuous phase
70 ml/min 1
2.4 3 min
Organic Electrowinning Advance electrolyte
�Cu across EW Single-pass �Cu RE flowrate Current density Current Cell voltage
48 g/l Cu 13 g/l Cu 4 g/l Cu
214 ml/min 220 A/m2
45 A 2.0 V
Conventional mixer settlers, manufactured of clear polyvinylchloride with
polypropylene interstage piping and valves, were configured for counter-current flow of the aqueous and organic phases. Each stage had a square mixing box with an active
volume of 1000 ml and settling area of 0.01 m2. Mixing was achieved using 19-mm diameter, flat-vaned pumping impellers. The EW cells were constructed of polypropylene. Each cell contained one stainless steel cathode (330 x 330 mm) and two Pb-Sn-Ca anodes, with an electrode spacing of 40 mm. The electrolyte was piped into the base of each cell, and overflowed at the top of the cell on the opposite side. The cells were connected in series to ensure that the same current was passed to each. The direct current was supplied by a 60 A rectifier and measured hourly using a current clamp.
The PLS, SE, and stripped organic (SO) phases were fed to the SX circuit and the advance (AE) and recirculating electrolytes (RE) to the EW circuit using peristaltic pumps. Flowrates were measured hourly and adjusted as necessary. The settler interface heights, mixer O:A, and phase continuity of each mixer were also monitored. Electrolyte potential was measured relative to a Pt-ring Ag/AgCl (3 M) electrode (0.222 V).
Aqueous- and organic-phase samples, taken on an eight-hourly basis from all streams, were analysed directly by inductively coupled plasma optical emission spectroscopy (ICP-OES) using matrix-matched standards for calibration.
RESULTS AND DISCUSSION
Stability of Control
The three circuits were evaluated under identical conditions of flowrate and current, so it was essential to maintain stable operating conditions. Figure 3 shows the variation in the advance organic-to-aqueous volumetric flowrate ratios (O:A) in the SX extraction and strip circuits as a function of running time. Similar reproducibility of flowrate control was achieved in the EW circuit.
Figure 3 - Variation in the advance O:A of the three circuits, showing excellent stability of operation (closed symbols – extraction circuit; open
symbols – strip circuit)
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Strip
Extraction
Extraction and Stripping of Copper
Figure 4 compares the extraction and stripping efficiencies. All three systems demonstrated extraction efficiencies close to or above the target value of 93 %. Mixing efficiencies of 100 % were calculated, based on comparison of the measured stage-wise performances with the equilibrium extraction isotherms. Stripping efficiencies varied slightly, depending on the strength of the extractant, but also fell close to the target values. Table 3 compares the copper transfer characteristics of the three SX systems.
Figure 4 – Comparison of extraction and stripping efficiencies as a function of running time for the three extractants
Table 3 – Comparison of average copper extraction and stripping performance
As designed, very similar loading performances were seen for the three systems.
The aldoxime-based extractants showed slightly higher copper transfer. There was more
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Strip
variation in the stripping characteristics: in the aldoxime systems, ACORGA M.5774 showed slightly higher copper transfer, but stripping was enhanced with LS 4202.
Deportment of Impurity Elements
The PLS composition remained constant throughout the trial, however the concentrations of all species in the electrolyte increased in an approximately linear manner with running time (Figures 5 and 6). It is well known that species such as Al, Si, Ca, Mg, and Mn are not chemically extracted by oximes, so their transfer through the circuit occurred by physical entrainment of the PLS in the LO, followed by subsequent transfer to the electrolyte in the strip circuit. As shown in Table 4, the entrainment values for all three extractants were very similar, although LIX 984N consistently showed higher entrainment, particularly towards the end of the campaign (Figure 5).
Table 4 – Entrainment of impurity elements
Entrainment (% of PLS)* Extractant
Al Ca Mg Mn Si
LS 4202 ACORGA M.5774 LIX 984N
0.0283 0.0286 0.0312
0.0164 0.0180 0.0196
0.0158 0.0165 0.0227
0.0090 0.0092 0.0148
0.0101 0.0120 0.0173
* average mass/hour in the AE compared to the PLS, expressed as a percentage
Figure 5 – Deportment of entrained impurity species to the advance electrolyte
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rity
dep
ortm
ent t
o A
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/l)
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Mn
Si
Mg
Al
Figure 6 – Deportment of iron to the advance electrolyte
In contrast to the above elements, iron is carried across to the electrolyte by both physical and chemical means, as trace quantities of Fe(III) are co-extracted by oximes. Ketoximes show less selectivity against iron than aldoximes: Figure 6 confirms that LIX 984N transferred more iron than ACORGA M.5774. Although LS 4202 is an aldoxime-based extractant, the low-selectivity modifier allows increased transfer of iron. The relative iron transfer by entrainment and co-extraction are shown in Table 5. It is evident that the low-selectivity modifier allows the extractant to be tailored for specific chemical transfer of iron to the electrolyte.
Table 5 – Iron deportment to the electrolyte by chemical and physical means
LS 4202 ACORGA M.5774
LIX 984N
Average entrainment (% of PLS)* Total carryover of Fe to AE (% of PLS) Fe transfer by chemical extraction (%) Fe transfer by physical entrainment (%)
0.0159 0.4100
96.1 3.9
0.0168 0.2139
92.1 7.9
0.0211 0.2220
90.5 9.5
* Average of values in Table 4 Fe:Mn Ratio in the Electrolyte
Under conventional EW conditions (Table 1), copper was electrowon from the AE with a measured current efficiency of ~97 %. During the trial, it was observed that the redox potential of the SE increased steadily with running time (Figure 7). On Days 3 and 4, the electrolyte reservoir was topped up by addition of synthetic SE, causing the
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Running time (days)
Fe d
epor
tmen
t to
AE
(g/l)
LS 4202ACORGA M.5774LIX 984N
potential to temporarily drop again. Allowing a SE with a high potential to return to the SX circuit poses a risk of oxidation to the organic phase. This problem was overcome by passing the SE through a packed bed of metallic copper ahead of the SX circuit. This reduced the electrolyte potential, with minimal impact on the copper concentration. As soon as the copper bed was bypassed, the potential again increased rapidly (Figure 7).
Figure 7 – Variation in redox potential of the spent electrolyte
It is widely accepted in the industry that a total Fe:Mn mass ratio of 8:1 to 10:1 in the electrolyte is adequate to ensure sufficient ferrous iron in solution so that Mn remains in the divalent state [4]:
Mn7+ + 5 Fe2+ � 5 Fe3+ + Mn2+ (1)
or: Mn4+ + 2 Fe2+ � 2 Fe3+ + Mn2+ (2)
The South American rule-of-thumb is represented as Fe(II):Mn = 5:1. In this trial, however, the presence of Mn(III) or Mn(VII) was observed, even at total Fe:Mn ratios above 30:1 (Figure 8). This was evident in a change of colour of the electrolyte to a deep indigo-purple, indicative of the presence of the Mn3+ or permanganate species.
Miller [4] reports that the concentration of Mn in the electrolyte is dependant on the level of iron in solution: for an electrolyte containing 2 g/l Fe, Mn can be tolerated to 200 mg/l, while for an electrolyte containing 500 mg/l Fe (such as at Chuquicamata, Chile [5]), the maximum advised Mn concentration is 40 mg/l. In fact, there are several operations that are known to run without problems at conditions very different to this. For example, the Senyati plant, Zimbabwe, ran successfully at Fe:Mn = 1:4 for many years before closure; Kanshanshi 3, Zambia, currently operates at 1.5:1, while Bwana
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Eh
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ele
ctro
lyte
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)
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rent
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LS 4202ACORGA M.5774LIX 984NCurrent
Dilution of RE with synthetic SE
Copper bed in place
Mkubwa, Zambia, operates at < 1:1 [6]. The present study demonstrates that conditions may exist where an Fe:Mn ratio well in excess of 20:1 may be inadequate to avoid the formation of oxidised Mn species.
Figure 8 – Fe:Mn ratio in PLS and advance electrolytes
Degradation of the Organic Phase under Oxidising Conditions
The effect of the high redox potential initially manifested as an increase in the phase disengagement time (Figure 9). Although ketoximes are generally considered more resistant to hydrolytic degradation than aldoximes, in this study the LIX 984N system showed the fastest deterioration in phase separation due to oxidative degradation. Other properties of the organic phases were similarly affected by exposure to the oxidising environment and concentration of the extractant by diluent evaporation (Table 6).
Table 6 – Comparison of some chemical and physical properties of the organic phases before and after contact with high redox potential electrolyte
Extractant Vol.% Age of organic phase
Interfacial tension (mN/m)
Viscosity (mPa.s)
Density (g/cm3)
Max. loading Cu
(g/l)
[Fe] at Cu max load
(mg/l)
LS 4202
20.0 Fresh Day 11
39.9 37.0
2.8390 2.8961
0.8474 0.8482
12.58 13.53
12.4 13.4
ACORGA M.5774
20.0 Fresh Day 11
39.5 37.8
2.8214 2.8703
0.8478 0.8485
12.57 13.58
0.35 2.34
LIX 984N
21.5 Fresh Day 11
32.5 30.8
2.6926 2.6365
0.8358 0.8365
12.24 13.64
0.69 2.84
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Fe:M
n in
PL
S
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n in
AE
PLSLS 4202ACORGA M.5774LIX 984N
Figure 9 – Rate of phase disengagement in stage E2
From the reduction in interfacial tension and increased phase disengagement times, an increase in entrainment is expected. This was confirmed in this trial, where the rate of accumulation of impurity elements in the electrolyte was seen to increase with running time, particularly for the LIX 984N system (Figure 5). A consequence of the presence of Mn(VII) is an increase in crud formation as the organic degrades. In this trial, the LIX 984N system produced ~70 % more crud (by mass) than the LS 4202 circuit.
CONCLUSIONS
This study examined the copper- and impurity-transfer characteristics of LS 4202, ACORGA M.5774, and LIX 984N under continuous operating conditions in a closed-loop SX-EW circuit. The main conclusions are summarised as follows:
(i) The iron-transfer capability of an aldoxime-modified extractant can be tailored to enable a specified amount of iron to be chemically transferred to the EW circuit.
(ii) Although iron transfer was tailored to ensure an Fe:Mn electrolyte ratio of >10:1, this was inadequate to ensure sufficient Fe(II) in solution to prevent the oxidation of manganese. Irrespective of the extractant employed, it may still be necessary to use a copper or iron reduction tower to maintain an appropriate redox potential in the electrolyte.
(iii) Impurity carryover to the electrolyte by entrainment was lower when using modified aldoximes than ketoxime:aldoxime mixtures.
It should be noted that, although iron transfer to the electrolyte is proposed as a
method of controlling Mn at certain geographical sites, there is an operating cost
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e se
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tage
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(s)
LS 4202ACORGA M.5774LIX 984N
Copper bedin place
associated with this: increasing the iron content of the electrolyte results in lower current efficiency of the copper electrowinning [6]. Fe(III) is reduced at the cathode under mass transport-controlled conditions, the exchange current density for which is dependent on the iron concentration.
ACKNOWLEDGEMENTS
Appreciation is extended to Bwana Mkubwa for providing the PLS used in this trial, to Shell Chemicals for Shellsol 2325, to Cytec for provision of LS 4202 and ACORGA M.5774, and to Cognis for LIX 984N. Excellent technical support was provided by Ntumi Baloyi, Kabelo Ledwaba, Boitumelo Molefe, and Arinao Munyai (Anglo Research), and Jan de Bruyn (de Bruyn Spectroscopic Solutions). This paper is published by permission of Anglo Research and Cytec Industries.
REFERENCES
1. G.A. Kordosky, “Copper recovery using leach/solvent extraction/electrowinning technology: Forty years of innovation, 2.2 million tones of copper annually”, International Solvent Extraction Conference ISEC 2002, Vol. 2, South African Institute of Mining and Metallurgy, Johannesburg, 2002, 853-862.
2. M. Virnig, D. Eyzaguirre, M. Jo, and J. Calderon, “Effects of nitrates on copper SX circuits: A case study”, Copper 2003-Cobre 2003, Vol. VI – Hydrometallurgy of copper, Santiago, Chile, 2003, 795-810.
3. C.Y. Cheng, C.A. Hughes, K.R. Barnard, and K. Larcombe (2000), Manganese in copper solvent extraction and electrowinning, Hydrometallurgy 58, 135-150.
4. G.M. Miller, “The problem of manganese and its effects on copper SX-EW operations”, Copper 95-Cobre 95, Vol III – Electrorefining and Hydrometallurgy of Copper, The Metallurgical Society of CIM, Montreal, 1995, 649-663.
5. L. Farías and G. Alvarez, “Experiencia de operación en las plantas de SX-EW, Subgerencia de Oxides, Chuquicamata”, Workshop Electro-Obtención de Cobre, Viña del Mar, Chile, 1993.
6. T. Robinson, J. Jenkins, S. Rasmussen, M. King, and W.G. Davenport, “Copper Electrowinning – 2003 World Operating Data”, Copper 2003-Cobre 2003, Vol. V – Copper Electrorefining and Electrowinning, Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, 421.
