Exploiting supramolecular chemistry in metal recovery: Novel zwitterionic extractants for nickel(II) salts

Article abstract of DOI:10.1039/b300176h

Hexadentate tris-salicylaldimine ligands bearing ortho-N-dialkylaminomethyl substituents have been shown to function as ditopic ligands for NiSO ₄ or NiCl₂. The incorporation of the Ni-ion into the N₃O₃^3- site templates the pendant alkylammonium groups to allow them to hydrogen bond to the attendant anion(s). Formulation as complexes of the trianionic/tricationic ligand is supported by X-ray structure determinations of solvated forms of the complexes [Ni(L)SO₄] and [Ni(L)Cl]Cl, where L = tris-1,1,1 - {2-hydroxy-3-(piperidin-1-ylmethyl)-5-tert- butyl-benzaldiminomethyl}ethane. The kerosene-soluble ligand, tris-2,2′,2″-{2-hydroxy-3-(di-n-hexylaminomethyl) -5-nonylbenzaldiminomethyl}amine, functions as a good extradant for nickel salts, showing high selectivity for recovery of NiCl₂ over NiSO ₄. This reagent can be stripped and recycled directly using aqueous ammonia or by first displacing the Ni²⁺ with acid and then neutralising the pendant alkylammonium groups. The acid-stripping is accompanied by hydrolytic degradation of the reagent, but in many other respects this novel class of reagent shows promise in opening up novel flowsheets for nickel recovery, based on transport of the metal salt as opposed to ion-exchange based processes. The Royal Society of Chemistry 2003.

Full text of DOI:10.1039/b300176h

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Exploiting supramolecular chemistry in metal recovery:
novel zwitterionic extractants for nickel(II) salts†
Nilgun Akkus,c John C. Campbell,b James Davidson,a David K. Henderson,a Hamish A. Miller,a
Andrew Parkin,a Simon Parsons,a Paul G. Plieger,a Ronald M. Swart,b Peter A. Tasker*a and
Lee C. Westa
a School of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh,
UK EH9 3JJ. E-mail: p.a.tasker@ed.ac.uk
b Research Centre, Avecia, Blackley, Manchester, UK M9 3ZS
c Department of Chemistry, Yildiz Technical University, 80270 Sisli, Istanbul, Turkey
Received 7th January 2003, Accepted 24th January 2003
First published as an Advance Article on the web 23rd April 2003
Hexadentate tris-salicylaldimine ligands bearing ortho-N-dialkylaminomethyl substituents have been shown to
function as ditopic ligands for NiSO4 or NiCl2. The incorporation of the Ni-ion into the N3O33Ϫ site templates
the pendant alkylammonium groups to allow them to hydrogen bond to the attendant anion(s). Formulation as
complexes of the trianionic/tricationic ligand is supported by X-ray structure determinations of solvated forms of
the complexes [Ni(L)SO4] and [Ni(L)Cl]Cl, where L = tris-1,1,1-{2-hydroxy-3-(piperidin-1-ylmethyl)-5-tert-butyl-
benzaldiminomethyl}ethane. The kerosene-soluble ligand, tris-2,2Ј,2Љ-{2-hydroxy-3-(di-n-hexylaminomethyl)-5-
nonylbenzaldiminomethyl}amine, functions as a good extractant for nickel salts, showing high selectivity for recovery
of NiCl2 over NiSO4. This reagent can be stripped and recycled directly using aqueous ammonia or by first displacing
the Ni2ϩ with acid and then neutralising the pendant alkylammonium groups. The acid-stripping is accompanied
by hydrolytic degradation of the reagent, but in many other respects this novel class of reagent shows promise in
opening up novel flowsheets for nickel recovery, based on transport of the metal salt as opposed to ion-exchange
based processes.
solution (pls). The most commonly used are based on equilibria
Introduction
involving liquid–liquid (solvent extraction) and liquid–solid
The processes used in extractive metallurgy have evolved over
(ion-exchange and chelating resins) systems. Solvent extraction
several thousand years1 and are based on the unit operations:
has the advantage of being easily engineered for continuous
concentration, separation, reduction, and, when high purity
operation and has proved remarkably robust since its introduc-
metal is required, refining.2–4 The concentration and separation
tion for uranium production.5 For recovery of base metals the
of base metals such as Cu, Ni and Zn present remarkable
preferred water-immiscible phase is a high boiling hydrocarbon
technical challenges. Typically the production of 1 tonne of
on grounds of both cost and safety. The use of such non-polar
electrical grade copper requires the processing of 800 tonne
of rock (Fig. 1).
solvents facilitates assembly reactions based on hydrogen-bond-
ing and on ion-pair and ion–dipole interactions of solutes. The
exploitation of such supramolecular chemistry is a central
theme of this paper, providing the basis for organisation of
metal- and anion-binding sites in relatively simple ligands. An
essential criterion for any commercial exploitation of organic
complexing agents in base metal recovery is that they are
inexpensive. A large extractant inventory makes a substantial
contribution to capital costs and even when such reagents are
recycled in the flowsheet there are always some losses through
chemical degradation or entrainment of droplets in the
aqueous phases.
Fig. 1 Flowsheet for recovery of copper from sulfidic ores.1
The last four decades have seen a move from pyro- towards
hydro-metallurgical processes for the recovery of such metals,
providing an opportunity for the development of selective
metal complexing agents to effect the concentration and separ-
ation operations. In general, the design criteria for such metal
extractants are defined by the “front end” (leaching) and “back
end” (reduction) processes in the simplified flowsheet shown in
Fig. 2.
One of the most successful base-metal recovery processes
involving solvent extraction is illustrated in the flowsheet in
Fig. 3. This circuit accounts for more than 25% of worldwide
Fig. 2 Basic flowsheet for hydrometallurical recovery of base metals.
Ion-exchange reagents and metal ion transport
A range of separation technologies can be used to effect
transfer of the desired metal from the aqueous pregnant leach
Fig. 3 A simplified flowsheet and materials balance for the recovery of
copper from oxidic and transition ores by heap leaching, solvent
extraction and electrowinning.
† Based on the presentation given at Dalton Discussion No. 5, 10–12th
April 2003, Noordwijkerhout, The Netherlands.
D a l t o n T r a n s . , 2 0 0 3 , 1 9 3 2 – 1 9 4 0
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
1932

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copper production.6 Its commercial success is based on the very
good materials balance which is obtained when processing
oxidic ores, using the phenolic oxime “pH-swing” extractants in
conjunction with a conventional electrolysis process.6–8 Acid
released on extraction is re-used in leaching and the acid needed
for stripping and recycling the extractant is generated by the
electrolysis process (see eqns. in Fig. 3).
The relatively “strong” commercial phenolic oxime reagents
1 allow high recovery from pls’s which can have pH values as
low as 1.56 and effect the key separation of Cu from Fe which is
often present in comparable or higher concentrations.
Reagents transporting metal salts
Whilst pH-swing extractants such as
1
and D2EHPA
(di-2-ethylhexylphosphate) find extensive use in hydrometal-
19–24
lurgy9,
they have limitations when treating certain pls’s.
When leaching does not consume acid equivalent to metal
transferred to the aqueous phase, as is the case in many
new oxidative high pressure processes25,26 to treat sulfidic ores,
neutralisation of the raffinate will be required, leading to
poor materials balances. Also, the pH-swing extractants
are inefficient when treating feeds with high metal contents
because the acid released on extraction displaces the
equilibrium and only limited metal transfer can be obtained
without inter-stage neutralisation. A solution to this problem is
provided by using a different mechanism for metal extraction
from the aqueous phase, avoiding an exchange process and
transporting metal salts. This has been proposed for the
recovery of base metals by oxidative chloride-leaching of
sulfidic ores.27,28 For the recovery of copper from chalcocite and
related ores by ferric chloride leaching this is illustrated in the
flowsheet in Fig. 4.
The “strength” of a formulated reagent can be tuned to meet
the requirements of a particular feed solution. It can be reduced
by selecting ketoximes (1, R1 = Me or Ph) which are generally
weaker than the aldoximes, or by the addition of “modifiers”
which are materials that hydrogen-bond to the extractants in
the hydrocarbon diluents. The competitive formation of lig-
and–ligand, ligand–modifier and modifier–complex assemblies
in the organic phase influences the efficiency of extraction
and provides a very practical application of supramolecular
chemistry.9–14
In the solid state H-bonding between the oximic OH and
phenolate oxygen atoms within a complex unit is almost invari-
ably observed in 2 : 1 complexes, leading to pseudo-macrocyclic
structures 2 with N2O2 donor sets.15 A pseudo-macrocyclic
2Ϫ
Fig. 4 A simplified flowsheet and materials balance for the recovery of
copper from sulfidic ores by chloride leaching, solvent extraction and
electrowinning, using for example reagents such as 4.
structure 3 is also present in the solid state forms of some
free ligands15 and is retained in solution.16,17 As a consequence
they are partially preorganised for complex formation and
the dimers should be considered in modelling extraction
equilibria.16,18
Neutral, ‘solvating’ ligands‡ such as 4 and 5 have been used in
trials for copper29–31 and zinc32–34 recovery from chloride media.
[(LH)2](org) + M2+
[ML2](org) + 2H+
(1)
The low activity of water in these feed solutions which have
high chloride content results in high proton activity. In addition
to the problems this creates with corrosion of plant, there is a
tendency for even very weakly basic ligands to become proton-
ated, allowing extraction of Fe() by an ion-pairing mechan-
ism,§
Also, interactions of the monomeric, dimeric or other forms
of the extractant with modifiers are important in interpreting
extraction equilibria,16,18 e.g. as in
Ϫ
L(org) + H+ + FeCl4
[LH][FeCl4](org)
(3)
LH(org) + Y
[LHY](org)
(2)
where the modifier competes with metal cations for the donor
functionalities. Work in this area has been reviewed7,16 and the
importance of the preorganised dimer in recovery of copper
has been established, particularly in saturated hydrocarbon
solutions.18
It is timely and challenging to investigate the roles of second-
ary bonding in metal extraction. Assembly processes are essen-
tial to modification of reagent strength and probably account
for many of the observations of synergistic extraction shown by
combinations of reagents. Most importantly for base metal
recovery, they provide the possibility of using relatively
simple cheap reagents to obtain organised donor sets with high
selectivity and strength of extraction.
‡ Extraction with neutral or ‘solvating’ ligands typically involves form-
ation of neutral organic-soluble complexes containing coordinated
anions, and the processes use anion concentration to control equilibria,
e.g.,
UO22+ + 2NO3Ϫ + 2TBP
[UO2(NO3)2TBP2]
where tri-n-butylphosphate (TBP) is used as the solvent and also is
involved in the second coordination sphere.
§ Outersphere complex formation, often involving hydrogen bonding, is
important in ion-pairing extractants e.g. Alamine 36 is an effective
extractant for Fe() in acidic chloride media:
+
Fe3+ + 4ClϪ + H+ + R3N(org)
FeCl3–ClϪ ؒ ؒ ؒ H–NR3
(org)
D a l t o n T r a n s . , 2 0 0 3 , 1 9 3 2 – 1 9 4 0
1933

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which reduces the otherwise very high Cu/Fe or Zn/Fe selec-
tivity of 4 or 5. These problems¶ and the engineering costs
associated with handling chlorine from the electrolysis of metal
chlorides have contributed to the limited commercial exploit-
ation of the chloride-stream technology. Most of these prob-
lems would be overcome if a sulfate process stream were used
and extractants were available to transport metal sulfates. As
2Ϫ
Ϫ
SO4 and HSO4 are relatively poor ligands for base metal
cations it will be difficult to exploit a transport mechanism based
on formation of inner-sphere complexes as used in the chloride
flowsheet, Fig. 4. Consequently we have recently developed
2Ϫ
ditopic ligands with separated metal cation and SO4 binding
sites.35,36 The prototypes were based upon tetradentate H2salen-
type molecules bearing tertiary amine groups which on proton-
ation provide the SO42Ϫ binding sites (Scheme 1).
Proof-of-concept studies37–39 have established that: (i) incor-
poration of Ni() or Cu() into the salen N2O22Ϫ site templates
the pendant dialkylammonium groups to facilitate binding of
SO42Ϫ, (ii) the binding of the metal sulfate into a zwitterionic
form of the ligand provides a convenient pH-swing based proto-
col for sequential stripping of the cation and anion, recycling
the ligand and generating a concentrated metal sulfate solution
for electrowinning, and (iii) hydrophobic forms of the ligands
are easily prepared from commercial 2-hydroxy-5-alkylbenz-
aldehydes.This paper deals with the development of systems
which could be used to recover nickel from oxidative leaching
of sulfidic ores based on a flowsheet as outlined in Fig. 5.
Scheme 1 Formation of metal sulfate and “metal only” complexes
from ditopic salenH2 ligands with pendant dialkylaminomethyl groups.
Experimental
Instrumentation
Fast atom bombardment (FAB) mass spectra were obtained
on a Kratos MS50TC spectrometer in thioglycerol (thio) or
3-nitrobenzyl alchol (3-noba) matrices and electrospray mass
spectra on a Finnigan Mat LCQ spectrometer. Inductively
coupled plasma atomic emission spectroscopy (ICP-OES)
analysis for Ni and S was performed on Thermo Jarrel Ash
IRIS ICP-OES equipment.37,38 C, H, N contents were obtained
on a Perkin-Elmer 2400 elemental analyser by the University of
Edinburgh Microanalytical Service.
Fig. 5 A flowsheet and materials balance for recovery of metals from
sulfidic ores using oxidative leaching, solvent extraction of metal sulfate
and electrowinning.
¶ Phenolic-oxime reagents such as 1 have also been used63 to recover
copper from chloride leach solutions,
Solvent and reagent pre-treatment
2LЈH(org) + CuCl2
CuLЈ2(org) + 2HCl
All reagents and solvents were used as received with the
exception of triethylenetetramine which was distilled before
use. Orform SX7 was obtained from Avecia.
but the high proton activity in these feeds limits the efficiency of these
processes and steps have to be taken to adjust and control pH.29,64
A creative solution to this problem based on using a mixture of a
“pH-swing” extractant LЈH and either a solvating or an ion-pairing
extractant has been reviewed recently.64,65 On contacting the aqueous
feed the majority of the copper extracted is transferred to the organic
phase by the solvating or ion-pairing reagents, e.g.
Ligand synthesis
The 2-hydroxy-5-alkylbenzaldehydes and ethoxy-N-piper-
idinylmethane were prepared by the methods described previ-
ously by Levin40 and Fenton and co-workers.41 cis,cis-1,3,5-
Triaminocyclohexane (tach) was synthesised according to the
method of Bowen and co-workers.42
Cu2+ + 2ClϪ + 2L(org)
Cu2+ + 2ClϪ + 2[R4N]Cl(org)
Cu2+ + 2ClϪ + 2[R3NH]Cl(org)
[CuL2Cl2](org)
2[R4N][CuCl4](org)
[R3NH]2[CuCl4](org)
Scrubbing the loaded organic phase with water or ammonia removes
HCl, transferring the copper to the “pH-swing” extractant as in
Compound
6.
2-Hydroxy-3-(morpholin-4-ylmethyl)-5-
methylbenzaldehyde (3.4 g, 14.3 mmol) in diethyl ether (30 ml)
was added to a solution of triethylenetetramine (tren) (0.7 g,
4.8 mmol) in ethanol (30 ml). The resultant yellow solution was
stirred at room temperature for 72 h then concentrated in vacuo
to give a yellow oil which on tritutation in pentane/diisopropyl
ether at Ϫ78 ЊC gave a yellow solid which was collected by
filtration and dried in vacuo (2.7 g, 71%); MS (electrospray) m/z,
798 (MHϩ 100%). Crystals suitable for X-ray diffraction were
obtained by slow diffusion of pentane into a dichloromethane
solution.
[R3NH]2[CuCl4](org) + 2LЈH(org)
[CuLЈ2](org) + 2[R3NH][Cl](org) + 2HCl
or
[R3NH]2[CuCl4](org) + 2LЈH(org) + 4NH3
[CuLЈ2](org) + 2R3N(org) + 2[NH4]Cl
which then can be stripped with sulfuric acid to give a sulfate electrolyte
for conventional electrowinning.64–70
D a l t o n T r a n s . , 2 0 0 3 , 1 9 3 2 – 1 9 4 0
1934

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Compound 7. 2-Hydroxy-3-(piperidin-1-ylmethyl)-5-tert-
butylbenzaldehyde (2.3 g, 8.3 mmol) in diethyl ether (30 ml)
was added to a solution of triethylenetetramine (tren) (0.4 g,
2.7 mmol) in ethanol (25 ml). The resultant orange solution was
stirred at room temperature overnight then concentrated in
vacuo to give an orange oil. The yellow oil was recrystallised
from hexane to give a yellow solid (0.5 g, 20%).
4, 1 : 6 and 1 : 10 at room temperature for 24 h. Extraction
experiments were performed on a minimum of a 20 ml scale
(i.e. for a 1 : 1 contact 10 ml of organic and 10 ml of aqueous
was used). The organic and aqueous phases were separated and
both analysed for nickel and sulfur content by ICP-AES.
Acid stripping isotherm. A Ni-loaded organic solution of 8 in
Orform SX7 containing 10 vol% n-decyl alcohol was generated
by 2 contacts with the mock Bulong feed solution and analysed
for nickel (4.5 g LϪ1). This loaded Orform SX7 solution was
intimately mixed with 20 g LϪ1, 35 g LϪ1 or 70 g LϪ1 sulfuric acid
solutions in the ratios 10 : 1, 5 : 1, 2 : 1 and 1 : 1 at room
temperature for 1 h. The organic and aqueous phases were
separated and both analysed for nickel content by ICP-AES.
Compound 8. Triethylenetetramine (tren) (22 g, 151 mmol) in
ethanol (200 ml) was added to a solution of 2-hydroxy-3-
(dihexylaminomethyl)-5-nonylbenzaldehyde (200 g, 450 mmol)
in dichloromethane (400 ml). The resultant orange solution was
stirred at room temperature for 72 h then concentrated and
dried in vacuo to give an orange oil (192 g, 89%) (Found C, 76.9;
H, 11.6; N, 6.9. Calc. for C93H165N7O3: C, 78.1; H, 11.6; N,
6.9%). MS (FAB, 3-noba) m/z, 1429 (MHϩ 74%), which was
used without further purification.
Collection and reduction of X-ray data
Data for 6 were collected on a Stöe Stadi-4 four-circle diffract-
ometer equipped with an Oxford Cryosystems low temperature
device using graphite monochromated Cu-Kα sealed tube X-ray
radiation. An absorption correction was applied using the
method of Blessing44 with the program SADABS.45 Data for
[Ni(9)SO4]ؒ5.25H2O and [Ni(9)Cl]Clؒ2.5CH2Cl2 were collected
on a Bruker SMART APEX CCD equipped with an Oxford
Cryosystems low temperature device using graphite mono-
chromated Mo-Kα sealed tube X-ray radiation. An absorption
correction was applied using a semi-empirical method based on
azimuthal measurements46 for [Ni(9)SO4]ؒ5.25H2O, whilst for
[Ni(9)Cl]Clؒ2.5CH2Cl2 the method of Blessing44 was used with
the program SADABS.45
The structures of 6 and [Ni(9)SO4]ؒ5.25H2O were solved
using direct methods and the Patterson method respectively
and were refined against F 2 using the program SHELXTL.47
For 6 a Flack parameter48 and an extinction parameter were
refined. All H-atom positions were calculated and refined as
riding or rotating groups. All fully-occupied non-H-atoms were
refined anisotropically. The space group for 6 was tested using
the symmetry search programs49 within PLATON.50 Although
the majority of atoms match the symmetry of P21/c, one of the
arms did not and the structure refined better in the lower sym-
metry space group Pc. One tert-butyl group (C41A to C44A) in
[Ni(9)SO4]ؒ5.25H2O is rotationally disordered over two posi-
tions in a 25 : 75 ratio and the sulfate is disordered over two
positions in a 36 : 64 ratio. Three water molecules (O1W, O2W
and O3W, one, O1W, on an inversion centre) were refined
as half-occupied. All partially occupied atoms, excluding the
sulfate, were refined isotropically. No water H-atoms were
found on difference maps or inserted in calculated sites in
refinement.
Compound 9. 2-Hydroxy-3-(piperidin-1-ylmethyl)-5-tert-
butylbenzaldehyde (0.6 g, 2.0 mmol) in diethyl ether (25 ml) was
added to a solution of tameؒ3HCl (0.15 g, 0.7 mmol) (tame =
1,1,1-tris(aminomethyl)ethane) and NaOH (0.2 g, 5 mmol) in
water (10 ml) and the dark orange mixture stirred for 24 h. The
organic phase was separated and dried in vacuo to give a yellow
solid (0.58 g, 98%), which was used without further purification
for the preparation of complexes. MS (FAB, 3-noba) m/z, 889
(MHϩ 57%).
Compound 10. 2-Hydroxy-3-(piperidin-1-ylmethyl)-5-tert-
butylbenzaldehyde (2.0 g, 7.3 mmol) in diethyl ether (25 ml) was
added to a solution of tachؒ3HBr (0.9 g, 2.4 mmol) in NaOH
(0.5 g, 12.5 mmol) in water (10 ml) and the mixture stirred for
72 h. The organic phase was separated and dried in vacuo to
give a yellow solid (1.7 g, 78%) (Found C, 75.6; H, 9.1; N,
9.1. Calc. for C57H84N6O3: C, 76.0; H, 9.4; N, 9.3%). MS (FAB,
thio) m/z, 902 (MHϩ 30%), which was used without further
purification for the preparation of complexes.
Complexes for X-ray diffraction
[Ni(9)SO4]ؒ5H2O. A solution of 9 (0.1 g, 0.1 mmol) in
methanol (40 ml) was added to NiSO4ؒ6H2O (0.03 g, 0.1 mmol)
in methanol (20 ml) to give a green/brown solution which was
stirred at room temperature for 78 h. Solvent was removed in
vacuo to give a brown solid which was washed with diethyl
ether/pentane and collected by filtration (0.09 g, 86%) (Found
C, 57.3; H, 8.0; N, 7.6. Calc. for C56H94N6NiO12S: C, 59.3; H,
8.3; N, 7.4%). MS (FAB, 3-noba) m/z, 1044 (MHϩ 15%). Crys-
tals of [Ni(9)SO4]ؒ5.25H2O suitable for X-ray diffraction were
obtained by slow diffusion of hexane into a dichloromethane
solution.
[Ni(9)Cl]Clؒ2.5CH2Cl2 was solved using the Patterson
method with the program Dirdif51 and refined against F 2 as
described above. One tert-butyl group (C41D to C44D) is rota-
tionally disordered over two positions in a 85 : 15 ratio. The
thermal parameters of the major component of the disorder
were refined anisotropically whilst C42G to C44G, the minor
component of the disorder, were refined isotropically. Dis-
ordered solvent regions were treated in the manner described by
van der Sluis and Spek,52 as 445 e per cell, approximating to
2.5CH2Cl2 (= 105 e) per formula unit. Details of data collection
and structure determination are given in Table 1.
[Ni(9)Cl]Cl. A solution of 9 (0.1 g, 0.1 mmol) in methanol
(10 ml) was added to NiCl2ؒ6H2O (0.03 g, 0.1 mmol) in
methanol (20 ml). The resultant yellow/brown solution was
heated under reflux for 2 h, cooled and the solvent removed in
vacuo to give a brown powder (0.09 g, 88%). Crystals of
[Ni(9)Cl]Clؒ2.5CH2Cl2 suitable for X-ray diffraction were
obtained by slow diffusion of hexane into a dichloromethane
solution.
CCDC reference numbers 200690–200692.
See http://www.rsc.org/suppdata/dt/b3/b300176h/ for crystal-
lographic data in CIF or other electronic format.
Solvent extraction studies
Loading isotherms. An aqueous feed with composition simi-
lar to that in the Bulong circuit43 was prepared by dissolving
CoSO4ؒ7H2O 49.3 mg, NiSO4ؒ6H2O 26.3 g, CaSO4ؒ2H2O 4.2 g,
MgSO4 138.4 g, MnSO4ؒ4H2O 9.8 mg, ZnSO4ؒ7H2O 1.6 mg,
and NaCl 170.9 g in distilled water (2 L) and adjusting to pH 4
using NaOH. A 0.1 M solution of 8 in Orform SX7 containing
10 vol% n-decyl alcohol was intimately mixed with the mock
Bulong feed solution in the ratios 4 : 1, 2 : 1, 1 : 1, 1 : 2, 1 : 3, 1 :
Results and discussion
Synthesis of ligands and complexes
Whilst the bifurcated “salen-like” ligands in Scheme 1 have
been shown37 to form neutral low spin [NiLSO4] complexes, the
D a l t o n T r a n s . , 2 0 0 3 , 1 9 3 2 – 1 9 4 0
1935

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Table 1 Crystallographic data
Parameter
6
[Ni(9)SO4]ؒ5.25H2O
[Ni(9)Cl]Clؒ2.5CH2Cl2
Empirical formula
M
C45H63N7O6
798.02
C56H94.5N6NiO12.25
1138.65
S
C58.50H89Cl7N6NiO3
1231.22
Crystallographic system
Space group
Monoclinic
Pc
Monoclinic
P 21/n
Triclinic
P1
¯
a/Å
b/Å
c/Å
α/Њ
22.870 (5)
18.410 (4)
10.360 (2)
90
93.26 (3)
90
4354.9 (15)
4
1.217
13.4209 (15)
20.963 (2)
23.789 (3)
90
103.006 (2)
90
6521.2 (12)
4
1.160
15.5235(13)
16.4482(15)
28.230(3)
96.448(2)
105.731(2)
108.900(2)
6403.7(10)
4
β/Њ
γ/Њ
V/[Å3
Z
ρcalcd/g cmϪ3
µ/mmϪ1
T /K
1.277
0.641
150(2)
0.654
150(2)
0.388
150(2)
Unique data (Rint
Unique data with Fo>4σ(Fo)
R1, wR2
)
7782 (0.0000)
7161
0.0434, 0.1216
8533 (0.1433)
4019
0.1051, 0.2791
21930 (0.0545)
11688
0.0678, 0.1711
The hexadentate ligands 6, 7, 9 and 10 with methyl
or tert-butyl substituents were isolated as solids with accept-
able purity for the preparation of metal salt complexes by
adaptation of the methods described previously.37,39 The
more hydrocarbon soluble ligand 8 with mixed isomer/
multi-branched nonyl substituents was required in large
quantities (>500 g) for solvent extraction experiments and
proved more difficult to obtain in high purity. The precursor
2-hydroxy-3-(dihexylaminomethyl)-5-nonylbenzaldehyde was
obtained in a substantially pure form by chromatography
on a large scale and 8 was obtained in ca. 90% purity from
the Shiff base condensation with tren. Purification of a 0.1 M
solution in 9 : 1 SX7 : decyl alcohol was effected by transfer-
ring water soluble Ni-complexing impurities into water, taking
the ligand solution through a full load/strip cycle. The solu-
tion of 8 was loaded using a 1 M NiSO4 solution, then
stripped in 2 stages using concentrated ammonia, to give a
residual Ni concentration of 0.5 g LϪ1. After washing with
water this partially loaded solution was used to generate iso-
therms. The ligands 6–10 show NMR, IR and mass spectra
comparable to those of their bifurcated analogues reported
earlier.37,39 The structure of 6 was confirmed by X-ray
crystallography.
Complexes of 6, 7, 9 and 10 were formed almost immediately
upon mixing alcoholic solutions of the ligand and nickel()
sulfate. Elemental analysis and FAB mass spectrometry are
consistent with the formation of simple 1 : 1 : 1 metal : ligand :
sulfate assemblies which exhibit characteristic absorbancies
for the sulfate ion in their IR spectra. The formulation as
complexes of the trianionic/tricationic ligand zwitterions is
confirmed by X-ray structure determinations of [Ni(9)SO4]ؒ
5.25H2O and [Ni(9)Cl]Clؒ2.5CH2Cl2.
Fig. 6 Trifurcated ligands providing tricationic sulfate-binding and
trianionic six-coordinate metal-binding sites.
slow complexation/decomplexation rates in 2-phase systems||
and the poor organic phase loading levels do not make them
good candidates for development.53 The trifurcated analogues
6–10 shown in Fig. 6 were selected for study on the grounds that
they are likely to be stronger ligands for metal() sulfates. A
Extraction results
The aqueous mixed metal salt solution used in the loading iso-
3Ϫ
therm experiments [Co 5.3 mg LϪ1, Ni 2.8 g LϪ1, Ca 0.5 g LϪ1
,
zwitterionic form of the ligands 6–10 has a trianionic N3O3
Mg 14 g LϪ1, Mn 0.4 mg LϪ1, Zn <0.2 mg LϪ1, S (as SO42Ϫ) 22 g
Ni-binding site and a tricationic trisalkylammonium sulfate-
binding site and will give an overall neutral NiSO4-complex
which is needed to achieve high solubility in non-polar water-
L
Ϫ1 and Cl (as ClϪ) 65 g LϪ1] has a composition similar to that
in the proposed Ni-recovery circuit in Bulong Australia, after
cobalt removal using Cyanex 272.43 This has an unusually high
chloride content which arises from the saline bore water used in
the leaching process.
Loading isotherms obtained using a 0.1 M solution of
unpurified 8 in Orform SX7/decyl alcohol gave a maximum
Ni-loading of only ca. 4.2 g LϪ1, representing 71% of the
theory. Ni-transfer to the organic phase was particularly poor
when high organic : aqueous ratios (>2) were used and was
accompanied by a colour change in the aqueous phase from
2Ϫ
immiscible solvents. It is also likely to bind both Ni2ϩ and SO4
more strongly than the bifurcated systems as a result of the
higher charges on the complexing sites. It was also hoped that
formation and decomposition of Ni() complexes in the solvent
extraction process would proceed more rapidly than the low
spin complexes in Scheme 1.
|| The kinetics of loading and stripping are very poor (>24 hours is
needed to approach equilibrium).
D a l t o n T r a n s . , 2 0 0 3 , 1 9 3 2 – 1 9 4 0
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green to blue. This suggested that impurities in the extractant
form stable water-soluble Ni–amine complexes. These impur-
ities were readily removed by putting the extractant through
a load/strip cycle (see Experimental section), and the loading
isotherm for the purified extractant (Fig. 7) shows a maximum
loading of 5.3 g LϪ1 Ni, close to 100% of theory.
Fig. 9 The stripping isotherm for Ni-loaded 8 (0.09 M in SX7 ϩ 10
vol% n-decyl alcohol after equilibration with excess mock Bulong
aqueous feed43) with H2SO4 (35 g LϪ1) containing NiSO4 (30 g LϪ1).
Also illustrated is the McCabe–Thiele construction (triangle) assuming
a stage efficiency of 90% and an aqueous : organic phase ratio of 1 : 1,
demonstrating that efficient Ni-stripping is achievable in a single stage.
Fig. 7 The Ni-extraction isotherm for purified 8 (0. 09 M in SX7 ϩ 10
vol% n-decyl alcohol) on contact with mock Bulong aqueous feed.43
The steepness of the loading isotherm and the associated
efficient nickel extraction is consistent with the “subtractive”
nature of the process, contrasting with ion exchange solvent
extractants. In particular it compares favourably with the iso-
therms for pH-swing extractants where metal recovery from
weakly acid feed solutions is associated with the release of pro-
ton to the aqueous phase, lowering pH and depressing the
isotherm.
The extractant 8 shows high selectivity for Ni over the other
metals in the mock Bulong feed. Ca, Mg, Mn and Zn levels
were below detection limits in the organic phase after two
1 : 1 contacts. Whilst the Co-loading is also low (<2 mg LϪ1
)
and close to the detection limit, any transfer to the organic
phase is potentially a problem for the long term operation of a
circuit because oxidation to Co() will generate complexes
which will be very difficult to strip and could lead to a steady
build up of Co in the organic phase, effectively poisoning the
extractant.
Stripping with sulfuric acid could provide a potential route
for recycling the extractant and generating an electrolyte for
Ni-production (Fig. 8).
Fig. 10 A circuit and the predicted Ni-concentrations of streams to
achieve >98% recovery of NiSO4 and generate a 55 g LϪ1 Ni electrolyte
for Ni recovery from a mock Bulong feed using 8 (0.1 M in Orform SX7
with 10% n-decyl alcohol). In the MINCHEM software54 1 : 1 organic
to aqueous ratio and a 90% approach to equilibrium in mixer settlers
was assumed.
On paper the simplicity of this circuit compares very favour-
ably with the one currently proposed for the Bulong operation
which uses the pH-swing extractant versatic acid,43 with 4
extract and 3 strip stages plus extractant recovery steps (versatic
acid has an appreciable solubility in aqueous media). However,
based on other observations of the properties of the prototype
extractant 8, two major problems arise. The extractant was
found to show a high selectivity for NiCl2 over NiSO4 when
contacted with a mixed chloride/sulfate feed of the Bulong
type. The loaded organic solutions had undetectable levels
of sulfate as judged by sulfur analysis by ICP OES, whilst
chloride levels, determined by titration with AgNO3 after dis-
solution of samples in thf/sodium hydroxide solution, corre-
sponded closely to a 2 : 1 molar ratio with Ni loadings (by
ICP OES). More seriously, in these preliminary tests stripping
with sulfuric acid was accompanied by ligand hydrolysis,
leading to very significant reductions in Ni-loadings when it
was taken through further cycles of extraction. The sensitivity
of 8 to hydrolysis was unexpected as the 5-nonyl-derivatives of
salicylaldoximes show long lifetimes when used in kerosene as
Cu-extractants.7
Fig. 8 Recovery of Ni from a sulfate stream using a NiSO4 extractant
such as 8 followed by acid stripping and reagent neutralisation.
Solutions with a range of compositions typical of spent
electrolyte from Ni-electrowinning (see Experimental section)
gave almost quantitative removal of nickel, as illustrated by the
isotherm shown in Fig. 9.
These isotherms and the assumption that the reagent 8
could be effectively regenerated in its neutral form and recycled,
for example as in Fig. 8, allow the nickel recovery to be mod-
elled using the MINCHEM software.54 A 98% recovery is pre-
dicted (Fig. 10) for a circuit with two extraction stages and one
stripping stage. This will generate an electrolyte with 55 g LϪ1
Ni, the concentration currently proposed for the Bulong
circuit.43
A number of approaches to dealing with the NiCl2-loading
from the Bulong feed can be considered. (i) The very high
selectivity for extraction of chloride over sulfate may justify the
D a l t o n T r a n s . , 2 0 0 3 , 1 9 3 2 – 1 9 4 0
1937

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development of a circuit for electrowinning from chloride solu-
tion,**allcomponentsofwhichhavebeenestablishedpreviously.
(ii) The loaded organic could be stripped with ammonia to gen-
erate [Ni(NH3)6]Cl2, a potential feedstock similar to that which
has been used for the Sherrit hydrogen reduction process.55,56
An advantage of this approach is that it avoids stripping under
acidic conditions which have so far resulted in extensive hydro-
lytic degradation of 8.
Contacting a fully NiCl2-loaded solution of 8 with concen-
trated aqueous ammonia solutions leads to efficient stripping
and regeneration of the neutral extractant, but phase dis-
engagement has been poor in the experiments performed to
date due to the low solubility of [Ni(NH3)6]Cl2 which results in
formation of a third phase and we have been unable to generate
a full strip isotherm.
2Ϫ
The high ClϪ/SO4 selectivity shown by 8 is consistent with
the Hofmeister series.57,58 Attempts to reverse this order require
an understanding of the anion binding modes in the trifurcated
ligands 6–10.
X-Ray structure determinations
3Ϫ
Incorporation of Ni2ϩ into the N3O3 donor set will influence
the disposition of the pendant teriary amine groups, and hence
the efficacy of anion binding. The free ligand 6 has a very
extended structure in the solid state (Fig. 11), with the tertiary
amine groups more than 5.2 Å from their centroid.
Fig. 12 The [Ni(9)]2ϩ units in (a) [Ni(9)SO4] and (b) [Ni(9)Cl]Cl
viewed down the pseudo-3-fold axes defined by the CH3–C bond of the
capping tame unit. The anions and all H-atoms apart from the
trialkylammonium protons have been omitted for clarity.
of the ligand (Fig. 12) and a pseudo-octahedral coordination
geometry (Table 2).
Fig. 11 The structure of the free ligand 6 showing one of the two
crystallographically independent molecules. The other was assigned
similar atom labels for its three arms D, E and F. The tertiary amine
N-atoms N62A,B,C and N62D,E,F are located 5.393(5), 6.823(5), and
6.553(5) and 5.231(5), 6.349(5) and 6.156(5) Å from their centroids
respectively in the two molecules.
Incorporation of the Ni2ϩ into the metal binding site has
constrained the tertiary amine atoms N62A etc. to lie much
closer to the ligand’s pseudo-3-fold axis (< 3.7 Å) and from
their centroids (Table 2).
The presence of water in the lattice of the [Ni(9)SO4] struc-
ture is associated with complex hydrogen-bonding patterns and
consequently does not help us to predict the mode of binding
of sulfate in non-polar, water immiscible media. The interpret-
ation is further complicated by disorder of the sulfate group.
One of the piperidinium protons (H62A) forms a bifurcated
H-bond to two oxygen atoms on the higher occupancy sulfate
ion (Fig. 13). In this hydrated structure, in contrast to the struc-
tures of the low spin NiSO4 complexes,37 two of the piper-
idinium protons H62B and H62C make close contacts to the
phenolate oxygen atoms, O1B and O1C, in the same arms of
the ligand (Fig. 13). In the NiCl2 complex in both crystallo-
graphically independent molecules all the piperidine nitrogen
atoms make fairly close contacts (2.57 to 3.15 Å) to their neigh-
bouring phenolate oxygen atoms. One of the chloride ions lies
close to the pseudo-3-fold axis and is bonded strongly to one
piperidinium group and weakly to another (N62A and N62C in
Formation of the NiSO4 and NiCl2 complexes of 9 results in
a “3-fold propeller” arrangement of the salicylaldiminato arms
** An excellent materials balance results from the sequence
D a l t o n T r a n s . , 2 0 0 3 , 1 9 3 2 – 1 9 4 0
1938

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Table 2 Bond lengths (Å) and angles (Њ) in the Ni-coordination spheres of [Ni(9)Cl]ϩand [Ni(9)SO4]a
[Ni(9)Cl]ϩ
[Ni(9)SO4]
part Ab
part B
part C
part D
part E
part F
part A
Part B
part C
Ni(1)–N(2)
Ni(1)–O(1)
2.052(4)
2.029(3)
89.23(12)
87.24(11)
91.08(11)
2.056(4)
2.045(3)
87.44(13)
2.035(3)
2.053(3)
89.78(12)
2.043(4)
2.025(3)
90.21(13)
90.18(12)
87.37(12)
2.033(4)
2.059(3)
89.85(13)
2.053(4)
2.059(3)
87.10(14)
2.059(8)
2.010(6)
89.0(3)
87.0(3)
85.7(2)
2.081(7)
2.044(6)
87.5(3)
2.069(8)
2.039(6)
87.1(3)
O(1)–Ni(1)–N(2)
cO1–Ni(1)–O1Ј
cO1–Ni(1)–O1Љ
O1Ј–Ni(1)–O1Љ
cN2–Ni(1)–N2Ј
cN2–Ni(1)–N2Љ
N2Ј–Ni(1)–N2Љ
dN(62) ؒ ؒ ؒ centroid
86.92(11)
88.84(11)
90.7(3)
87.4(3)
85.00(15
87.64(13)
86.99(15)
84.97(16)
85.2(3)
88.9(3)
89.74(13)
3.584(5)
90.21(15)
3.352(5)
3.108(5)
3.244(5)
3.069(5)
3.715(5)
3.323(12)
3.362(12)
3.447(12)
3Ϫ
a Values compare well with Ni() complexes of hexadentate ligands with N3O3 donor sets.60–62 b Parts A, B, and C refer to chemically equivalent
arms of the ligand. For [Ni(9)Cl]ϩ, parts D, E and F refer to a second crystallographically independent complex in the asymmetric unit. See Fig. 12
for the definition of atom labels. c The single prime and double prime refer to chemically equivalent atoms in parts B and C or E and F respectively.
d The e.s.d.s are estimated as the mean of the e.s.d.s for the N62 ؒ ؒ ؒ N62Ј, N62 ؒ ؒ ؒ N62Љ and N62Ј ؒ ؒ ؒ N62Љ contacts.
NiSO4 is unexpected and whilst not considered as part of a
strategy to process sulfidic ores via oxidative leaching, for which
electrowinning from sulfate media was envisaged, may prove to
be very useful in chloride-based processes.
Acknowledgements
The authors wish to thank Drs T. Higgs and C. Squires for a
supply of tame, Mr J. Miller and Mr M. W. Kerr for obtaining
NMR spectra and Mr A. Taylor and Mr H. MacKenzie for
mass spectra. The work has been supported by the EPSRC
(grants GRM/33303 and GRM/35774), SHEFC under a POC
award and by Avecia (equipment, expertise and materials). We
are grateful to Infineium UK Ltd for a studentship (J. D.) and
to Bryn Harris (Hatch) and Geoff Seward (Hydromet Services
International) for advice on circuit requirements.
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D a l t o n T r a n s . , 2 0 0 3 , 1 9 3 2 – 1 9 4 0
1940