TSINGHUA SCIENCE AND TECHNOLOGY
ISSN 1007-0214 01/18 April 7-152
pp13
Volume 11, Number 2, 2006
Solvent Extraction in Hydrometallurgy: Present and Future
Gordon M. Ritcey**
G. M. Ritcey and Associates Inc., Ottawa, Canada
Abstract: During the past 10 years, there have been incremental advances in the application of solvent ex-
traction to process hydrometallurgy. The most cited areas in the literature include chemistry, chemical engi-
neering, pilot plants, and plant operation. Within these areas, there were considerable interest in synergism,
diluents, degradation, contactors, surfactants, hydrometallurgical applications, environmental and secondary
applications, and health and safety. The summary to the present is followed by a prediction for the future in
the above areas of interest. These include the use of speciation; improved understanding of the role of sur-
factants on the system; optimization through modelling, pilot plants, and contactor selection; improvements
in plant operation; further new applications; and plant safety. The review has indicated that considerable
knowledge is now available to optimize and improve on process design and plant applications.
Key words: chemical engineering; coalescence; degradation; diluents; dispersion; droplet size; electrostatic
pseudoliquid membrane; environmental and secondary recovery; hydrometallurgical applications
Throughout the succeeding years, considerable
Introduction
research on the chemistry and engineering of the
solvent extraction process was achieved. Many
Solvent extraction as applied to refining operations,
reagents have been developed, and mechanisms for
particularly uranium, commenced in the late 1940s.
metals extraction proposed and numerous contactors
These plants were small by present plant sizes. Shortly
have been developed and researched for the mass
after, and almost 50 years ago now, the first solvent
transfer reaction to take place. There has also been an
extraction plant was installed to treat hydrometallurgi-
improvement in design through the years as new
cal solutions at the mine site for the recovery of ura-
design and operating knowledge was attained, together
nium. The success in this first generation of uranium
with improved materials of construction and sensing
operations in the 1950s and 1960s eventually led to the
and control devices. Thus, although there has been
application of solvent extraction to copper operations
considerable research performed on all aspects of SX
in 1969, almost 15 years after the initial uranium sol-
from hydrometallurgical solutions, nevertheless limited
vent extraction (SX) plant. These plants were also very
knowledge appears to have been applied to actual
successful, and continue to be in the generations since.
design and operating systems, resulting in only
Following copper, there have been hundreds of SX
incremental improvements in the overall SX process.
plants installed for the recovery of many metals. With
This paper will briefly outline the present status and
only a couple of exceptions, all plants were mixer set-
continue on to where we should be in the future in SX
tler in design.
as applied to hydrometallurgy.
Received: 2005-10-09
E-mail: [e-mail]
aþ
aþ
138 Tsinghua Science and Technology, April 2006, 11(2): 137-152
1.2 Chemistry
1 Present Situation
The common commercial extractants, comprised of
At the present, in this early part of 2005, we have at-
cation, chelating, anion, and solvating continue to be
tained considerable knowledge on all aspects of metals
used extensively. Some of the research activities in-
extraction and recovery. Extremely large plants are op- volve the design of new compounds[1] and the evalua-
erating, as well as many small plants processing most tion for application to possible systems. So there is
of the metals of the Periodic Table of Elements. So much scientific information, but still a way to com-
where are we really now as regards the science and mercialization of those new possible extractants. One
new commercial COGNIS extractant (LIX 79) is a
technology of SX as applied to hydrometallurgy?
guanidine derivative for the extraction of Au from
Selected literature during the past several years has
cyanide liquors[2]. There was one mention of the design
been briefly surveyed to determine the present situa-
of a new extractant to extract both the anion and cation
tion of solvent extraction in hydrometallurgy. The ref-
as metal sulphates so this would eliminate the close pH
erenced material included the Proceedings of the Inter-
control required in many cation exchange systems[3].
national Solvent Extraction Conferences (ISEC 96,
There were a few papers on the use of resin-
ISEC 99, ISEC 02), the Proceedings of 3-SX Work-
impregnated extraction systems.
shops that were held in Canada in 1997, 2000, and
Many studies have been reported including the fol-
2003 sponsored by the International Committee for
lowing: chelate formation mechanisms, ion association
Solvent Extraction; the past 5 years of the journal of
effects in mass transfer, kinetics and mass transfer
Hydrometallurgy and also the recent edition of the
formation of third phases[4], the application of electric
Handbook of Solvent Extraction, plus Proceedings of
fields and the effect on mass transfer due to the inter-
Aus. IMM and ALTA conferences held in Australia,
facial tension and droplets size[5,6], and electrochemical
Proceedings of Copper 95, Proceedings of Chloride
processes to affect redox and enhance metals separa-
Metallurgy Symposium (Montreal 2002), the Proceed-
tion (galvanic stripping)[7] in which a less noble metal
ings of the South African Inst. of Mining & Metal-
is used to affect a reduction precipitation or cementa-
lurgy Copper, Cobalt, Nickel, and Zinc Recovery
tion of the more noble metal directly from the organic
Conference (2001), and the Proceedings of VI South-
to leave a purified organic for recovery of the desired
ern Hemisphere Meeting on Mineral Technology,
metal. Interesting studies on purification of nickel-
Riode Janeiro (2001). It is recognized that this has not
cobalt leach solutions using electrostatic pseudoliquid
been an exhaustive search of the literature, but hope-
membrane (ESPLIM) have been published[8-11].
fully a reasonable cross section. Each of the major ar-
Investigations were reported on the use of SX cou-
eas of solvent extraction of metals is discussed below.
pled with stripping-crystallization to produce compos-
1.1 Applications
ite powders[12-14].
Most of the elements in the Periodic Table now can be
1.3 Synergism
recovered by solvent extraction. Thus, during 1995-
1.3.1 Reagents of the synergistic systems
2005, the metals most cited in connection with
Synergism was a major topic of interest during the past
separations were: alkaline metals (Rb, Cs); alkali
10 years, and many reagent combinations have been
earths (Be, Mg, Ca); transition metals (Sc, Ti, V, Cr,
proposed for enhancement of mass transfer as well as
Mn, Fe, Co, Ni, Cu, Zn, Cd, Hg); rare metals (Zr, Hf,
Nb, Ta, Mo, W, Tc, Re, Al, Ga, In, Tl, Si, Ge, Sn, As, the kinetics. Some of the synergistic systems include
Bi, Se, Te); precious metals (Au, Ag, Ru, Ir, Pt, Pd,
the following:
Rh); actinides (U, Th); lanthanides. Although sulphate
1) Alkaline earths
media was still the most common, nevertheless there
TTA + TOPO for Sr from alkaline earths[15];
were a surprising number of chloride-based systems.
Organophosphorus and bifunctional phospho-
139
Gordon M. RitceyÿSolvent Extraction in Hydrometallurgy: Present and Future
nates for Ca[16] view in separation and recovery of cobalt and nickel
2) Aluminum from laterites, there were numerous publications. Proc-
DNNSA + EHPA from nitrate for Al and
esses were developed to separate and recover cobalt
Fe[17]
and nickel from sulphate, alkaline, and from chloride
3) Cadmium media. Pilot plants were described and plants designed
DEHPA + MEHPA with TBP for separation of
for mixer settlers as well as pulse columns. A good re-
Cd-Zn[18]
view was published on the developments in SX and the
4) Cobalt-nickel
plants commissioned for Co and Ni[32].
LIX 63 + Cyanex 272[19];
The aspect of recovery of acids from effluents[33]
Separation of Ni/Co from nitrate with a car-
was of increased importance and the number of papers
boxylic acid (4-tert-butylbenzoic acid) + pyri-
reflected this concern to decrease processing costs
dine (4-(5-nonly)pyridine in xylene[20];
through recovery by solvent extraction and recycle.
Versatic acid + 4-nonyl pyridine for Co+Ni
Columns and mixer settlers were proposed.
extraction from bioleach solution containing
Many papers described the recovery of metals from
high Ca[21]
chloride media in what appears to be a significant in-
5) Copper
creased interest in that medium.
Cu from chloride using mixed extractants, e.g.,
1.4 Diluents selection
Alamine 336 + LIX 54; Acorga CLX 50 +
LIX 54[22, 23]
Some knowledge of diluent selection criteria has been
Cu from pickling bath with Cyanex 302 + LIX
achieved over the years and there is a recognized dif-
860[24]
ference in the performance relative to aliphatic vs.
6) Gallium
aromatic diluents for a specific system. The aspect of
TTA + TOPO for Al, Ga, and In[25]
diluent oxidation by a small amount of cobalt in lat-
7) Iron
eritic cobalt-nickel processes has been of some concern
Primene JMT + EHPA from sulphuric for Fe;
for the impact on the process[34]. Given the fact that
MEHPA + primene better but not for strip-
systems containing highly oxidative speciation at much
ping[26];
higher cobalt concentrations have been present for
DEHPA + Cyanex 923 for In and Fe from sul-
decades in many processes without concern for diluent
phate to give easy stripping[27];
oxidation-degradation makes this somewhat a question.
Selective Fe extraction from Zn liquors with
Of course, the plant experience over time has shown
derivatives of ammonomethylene phosphinic
that higher oxidation states of Cr, Mn, V, and Fe, for
acids[28]
example, can act adversely on extractants as regards
8) lanthanides
degradation. Some plants add alcohol anti-oxidants to
Mixed hydroxyquinoline+EHP mono-
the diluent. Probably more basic research is required
ethylhexyl phosphonic acid for rare earths[29];
on possible effects on the diluent.
Rare earths Cyanex 272+C274 9a synthesized
1.5 Analyses
new compound from China to give high load-
ing, good SF, and stripping[30];
A number of analytical papers on metal analyses as
Synergism for separation of trivalent lantha-
well as organic constituents have been reported in
nides from trivalent actinides[31];
aqueous and organic solutions. The determination of
Organophosphorus extractants with 2 func-
organics in aqueous raffinates (entrainment losses) has
tional groups to improve on SF in lantha-
been of considerable interest, and finally some analy-
nides[3]
ses of organics have commenced regarding the identi-
1.3.2 Systems
fication of degradation products of the process. In only
Because of the high interest during the period in re-
140 Tsinghua Science and Technology, April 2006, 11(2): 137-152
a few cases have crud compositions been partially
regeneration or  cleaning of the solvent may be
analyzed. Some on-line as well as off-line analyses
required to restore the system to the conditions prior to
have been developed. the fouling of the solvent. In some of the chelate sys-
tems, where significant degradation has occurred, a
1.6 Speciation
few plants use in-situ chemical regeneration of an ex-
tractant, but the overall effects of the added chemicals
To date, the use and understanding of speciation in the
and degradation by-products are not clear. Although
SX process and its effects on mass transfer and separa-
there could be a possible savings in reagent costs, the
tion have been minor. Speciation of mass transfer
practice may be more costly in the long term if adverse
complexes does not always correspond to those gov-
effects in the process are produced. The presence of
erning distribution, e.g., effect of pH. As for speciation
manganese in some copper circuits was shown to cause
in crud samples, very little crud characterization and
reagent degradation if permitted to enter the electrow-
analyses have been performed.
inning (EW) circuit where Mn was then oxidized to
1.7 Degradation permanganate. If there was sufficient Fe/Mn ratio of
10/1, the effect was minimized[38].
Degradation is system dependent. Degradation may be
1.8 Membrane technology
caused by one or more of the following: elevated tem-
perature, high acidity, high Eh, sunlight, and bacteria.
Considerable work on both liquid and supported mem-
The effect of sunlight on hydroxyoxime degradation
branes has been reported. Although membrane tech-
was documented, indicating that some reagents are
nology is still a possible option in hydrometallurgy in
more amenable to re-oximation than others[35]. Also, in
specific cases, such as in clean solutions, possibly in
other research, the data indicated that the presence of a
the strip circuit, the numerous papers on the subject
copper chelate stabilized the extractant from sunlight
still remain in the scientific area of development.
oxidation[36].
Kordosky et at.[37] have reported on the degradation
1.9 Chemical engineering
of the beta-diketone, LIX 54 in the extraction of Cu
1.9.1 Fundamentals
from ammoniacal solution in the Escondita copper
Numerous papers still appear on droplets, single drop,
plant in Chile, forming a degradation product of
and micelle experiments and determining diffusional
ketimine. Research by Cognis has resulted in two new
mass transfer, kinetics, and interfacial phenomena and
alternative extractants that could replace the LIX 54,
reactions.
the XI-N54 or XI-57, both resistant to degradation in
1.9.2 Contactors
the presence of high concentrations of ammonia.
There are now considerable designs available for mixer
The extractants Cyanex 301 and 302 are both known
settlers as they have been promoted as being inexpen-
to degrade, by oxidizing to the disulphides, and tests in
sive, easy to design, and easy to operate. However, due
a mini-pilot plant have shown that Cyanex 301 de-
to high entrainment losses, work was necessary on im-
grades faster (8 days) than Cyanex 302 (40 days)[24].
provements in the impellor design by Lightnin[39,40]
The degradation is reversible and can be restored by
and the vertical smooth flow (VSF) mixer settler tech-
contact with reducing agents, such as hydrogen, nickel,
nology of Outokumpu[41] to decrease entrainment. The
and zinc.
design has been very successful, with several success-
Any degradation that occurs in an SX circuit may af-
ful plants in Chile operations[42].
fect the chemistry of mass transfer and discrimination
There are now many other contactor designs avail-
as well as the physical operation as regards dispersion
able that are designed on sound engineering principles.
and coalescence. This is due to the fouled or poisoned
Although many columns have been in use in the
solvent by the impurities resulting from the degrada-
chemical, pharmaceutical, pertrochemical industries
tion, together with other surfactants. A
for many years, only a few columns have been applied
141
Gordon M. RitceyÿSolvent Extraction in Hydrometallurgy: Present and Future
as to the significant effect of surfactants on the process.
This includes the metals loading onto an extractant[62],
commercially at this time in the mining industry. In the
past 10 years of this review, pulse columns were in- chelation effects, mass transfer, kinetics, discrimina-
stalled at the Western Mining Olympic Dam operations tion, and dispersion and coalescence[63]. These surfac-
in Australia[43], in which the authors compared the tants include the chemicals that may be added
Bateman pulse column with the performance of mixer
throughout the plant. Also, perhaps more adverse ef-
settlers that the columns were replacing. The Bateman
fects are the organic acids, which result from humic
columns have been successfully evaluated for cobalt[44]
and fulvic acids as well as degraded reagents that are
and these contactors will be incorporated into a cobalt
recycled[63,64]. These organic acids have the capability
plant, and were evaluated on a pilot plant for copper[45]
of complexing many metals in the water[63]. Work by
and other applications such as nickel[46] and zinc[47].
others has shown that lathanides can be complexed by
There have been numerous papers on columns, indi-
humic acids[65]. The adverse effect of flocculents was
cating a very significant interest in this type of contac-
also researched on a cobalt-nickel circuit[66].
tor design. These column designs include the following:
1.9.5 Models
spray, packed, pulsed[48,49], RDC, Oldshue-Rushton
Mathematical modelling of contactors continues to be
(Mixco column)[50], Khuni[51,52], and design for scale-
an area of interest, but much has been from synthetic
up of the Schiebel, Mixco, and Karr reciprocating col-
systems. Although physical/chemical mathematical
umns was presented[52]. Performance characteristics of
models exist, they are under-utilized
a Karr column were described[53]. Parameters for their
1.10 Pilot plant
operation have been described, and this includes drop
formation, drop sizes, and phase disengagement. De-
Pilot testing is conducted to confirm chemistry, num-
sign parameters for new columns were proposed[54].
ber of stages, recycle requirements, as well as the
Centrifugal extractors have also been cited[55]. In-line
physical aspects of dispersion/coalescence, crud for-
mixers have suddenly become of more interest, with
mation, and to compare and evaluate differing flow-
possible applications described for U[56], Ni[57], and
sheet configurations. Most pilot plants to this time only
Cu[58]. Use of electrostatics to enhance coalescence in
consider mixer settlers. The scope of a pilot program is
columns was studied, as well as electric fields in gen-
limited to the amount of feed solutions and budget. The
eral, including effect on mass transfer[59], interfacial
size usually ranges from 1.0 to 10.0 L/m for assessing
tension, and droplets formation and size. Some new
process chemistry, and is in the range of 20-40 L/m to
contactor designs have been proposed. Finally, mixing
obtain scale-up design data. The duration of pilot
studies in mixer settlers were described[50].
plants varies considerably, from a few days to several
1.9.3 Dispersion / coalescence
months, often depending upon budget constraints.
There have been numerous papers on dispersion and
coalescence, so that much scientific data exist but there
1.11 Design and engineering of plant
has been relatively little application to plant design and
The design considerations for solvent extraction cir-
operation. Flocculants can seriously affect the rate of
cuits were discussed considerably in three SX work-
coalescence[60]. Some emulsion breaking methods have
shops[67-69], as well as in a couple of papers[64,70,71].
been proposed, and some are in use, but optimum de-
Some plants have been built with limited bench data
sign and control in the plant are still required. The in-
and without any pilot plant testing prior to plant design
fluence of plate (material) wettability upon dispersion
and construction. New technologies and equipment
and throughput has been cited periodically in the past,
continue to be difficult to accept in the short term.
but there was some additional information published[61].
Process performance guarantees by the contractor re-
1.9.4 Surfactants
quire proven process technology and equipment. De-
The influence of impurities, or surfactants, in the proc-
sign data are provided to engineering companies for
ess has reached a more accepted level of appreciation
142 Tsinghua Science and Technology, April 2006, 11(2): 137-152
scale-up. Many plants are designed without adequate
treatment, and crud have been described at the
concern for materials of construction, and often there is
Chuquicamata[74] and the Nchanga Consolidated Cop-
over-design, because of the possibility that expansion
per Mines[75]. Some plants measure interfacial tension
will occur within a short time following start-up. Set- as a guide to solvent fouling.
Crud recovery/treatment is practiced somewhat, but
tler design is usually accepted, based on a similar plant,
at a high operating cost. Treatment of crud includes: a)
but many of the more recent plants have been designed
settling in tanks; b) vacuum off from settler and filtra-
based on settler tests. However, kinetics in the design
tion; c) 3-phase centrifuge; d) send to special pond for
have been virtually ignored in the design. Therefore,
settling and gradual recovery; or e) discard to tailings
the sizing and design of the mixer boxes are the main
and skim the organic for recovery. There is some lim-
concern. Because of the excess time in the mixer at
high shear, if over-design has occurred because of ig- ited understanding of the role of surfactants, bacteria,
noring the kinetics required for mass transfer, the ex- organic acids, and amphoteric compounds in the for-
mation of crud[64,76]. Certain systems with high dis-
tremely small droplets will result in slow coalescence.
solved silica in the aqueous solution can exhibit gel
In settler designs having a reverse flow and therefore
formation during extraction. This is particularly true in
an external launder, the launder is not always easily
the Zr-HNO3-TBP process, and some of the difficulties
accessible. The distribution of flows in a settler may
have been described[77]. Investigations on a nickel cir-
not be equal. And the materials of construction in the
cuit showed that addition of a coagulant to the leach
pilot plant are usually different from the eventual plant
feed to SX could decrease the adverse effects of col-
design. This aspect must be improved.
loidal silica, and that anti-scalants could inhibit gyp-
1.12 Plant operation
sum formation[66].
Cross-contamination, in plants that have more than a
Operators are concerned about the reliability and pre-
single metal recovery system, has been demonstrated
dictability of solvent extraction circuits in achieving at
to be extremely serious and the potential problems
the end the desirable product with an acceptable cost
were essentially ignored[78]. Also, the use of modifiers
and minimum adverse impact on the environment. One
has not been fully appreciated (loss of reagent inven-
of the major economic and environmental concerns has
tory to 3rd phase formation). There are now many
been the often-high solvent losses (misting, entrain-
plants producing high purity compounds for the elec-
ment, crud, and evaporation). Breakdown estimates of
tronic, ceramics, and other industries.
these losses are: entrainment 50% of total; evaporation
The aspects of plant design and operation problems
and misting 25%; crud 25%; and solubility, spillage,
in solvent extraction plants have been documented[64].
and sampling are usually small.
Entrainment recovery has been with after-settlers,
1.13 Hydrometallurgical applications
and some plants have incorporated coalescing media in
There are many possible SX applications to hydro-
the settler to reduce entrainment[72]. Dual media filters
metallurgy as a result of R&D, pilot plants, and plant
have also been used for solvent recovery. Air flotation
has been used since the early copper operations in Ari- operations where, because of the knowledge, better
plants will be designed and commissioned in the future.
zona, at Bluebird Mine, and more recently, the
Some examples follow: a) some citations regarding
Jameson flotation cell was introduced for capture of
hydrometallurgical plants for primary metal recovery,
most of the entrained solvent[73]. Losses by evaporation
and b) a listing of secondary or environmental process-
have been reduced in some operations by installation
of off-gas scrubber systems. Solvent treatment to re- ing. The lists below are not intended to be complete
move poisons has been used to varying degrees of but does indicate the variety of systems that have been
performance, in many plants, by slurry with clay. proposed and those adapted for commercialization.
Various technologies for optimizing coalescence, or-
ganic treatment, and crud have been described at
143
Gordon M. RitceyÿSolvent Extraction in Hydrometallurgy: Present and Future
With 2-ethylhexanol[129]
1.13.1 Primary metal recovery 6) Lanthanides
Isolation of Nd from rare earth chloride solu-
1) Arsenic
As and H2SO4 with TBP[79,80], Cyanex 923, tion with Ionquest 801[130];
925[81]; U, Th, and lanthanides using Cyanex 923 from
As, Sb, and Bi from tankhouse electrolyte with HNO3/ H3PO4[131]
Cyanex 923[82] 7) Manganese
Zn, Mn, and Ca from Co, Ni in sulphate with
2) Cobalt-nickel laterites
Sulphate[44,83-96]; PC-88a or Cyanex 272[90]
Alkaline media[96-102]; 8) Platinum group
Pt group metals a review[132];
Chloride[103];
Separation of Pd from Pt in chloride solution
Ni laterite processing[104];
with substituted pyrazole compounds[133]
Pilot plant for Ni[105];
Pilot plants were described and plants de- 9) Scandium
Sc from sulphate with Cyanex 272[134]
signed for mixer settlers[98] as well as pulse
10) Silver
columns;
Ag and Au from thiourea using Cyanex 302[135];
Separation of Mn from Co in sulphate with
Ag from chloride with tri-n-butyl- and tri-n-
DEHPA[106]
octylphosphine sulphides[136]
3) Copper
11) Tantalum/niobium
Cu by SX-review of 40 years[107];
Ta/Nb separation in high purity recovery by
Cu, Zn, and Cd from waste streams DEHPA,
Alamine 336+isodecanol in Shellsol AB dilu-
carboxylic acids, Cyanex 272, and Cyanex
ent in a fluoride medium[137];
471 from sulphate[108];
Ni and Zn with LIX 984 from nitrate[138]
Impurities from Cu tankhouse liquors with
12) Zinc
Cyanex 923[109];
Zn plants treating various feed materials[139];
Cu from sulphate[110, 111], the Girilambone
Recovery from primary and secondary
plant experience[112]; comparison of LIX 984
sources[140,141];
and Acorga M5640 for Cu in China[113];
Recovery from battery waste[142];
Cu from ammoniacal carbonate leach with
Updated technologies[143] for ZINCEX, ZIN-
LIX 54-100[114];
CLOR, CUPREX, MERCUREX, and ARSEP;
Cu-world operating data[115];
Acid recovery processes[143];
Cu from chloride[116-118];
Modified ZINCEX process[144];
Cu from processing of concentrates[119,120];
DEHPA+MEHPA for Zn-Cd separation with
Extractant evaluation for 2 pilot plants[121];
TBP modifier[18]
Cu and Ni from tankhouse effluent[122];
13) Nuclear
Chloride metallurgy review for complex sul-
Chemical flowsheets for the Purex process[145];
phides[116, 123, 124]
Treatment of radioactive waste for separation
4) Chromium
of transuranic elements;
Chromium extraction from sulphate by or-
New U operations and expansions on older
ganophorus reagents[125]
plants
5) Gold
1.13.2 Environmental and secondary recovery
Au from chloride refining liquors with Cyanex
The application of solvent extraction to processing ef-
925 or 923[126];
fluents and residues has still continued to be of interest,
With Cyanex 471X from chloride[127];
and the followings are some of the typical referenced
Gold refinery[128];
144 Tsinghua Science and Technology, April 2006, 11(2): 137-152
applications: to identify the causes. Electrostatic generation or
1) Acid sparks are always possible causes. A few plants have
HCl extraction by Cyanex 921, 923, 925[125];
adequate grounding for dissipation of charges that
H2SO4, HNO3, HCl recovery from electroplat- build-up. High shear mixing and pumping large vol-
ing, pickling acids, and diluted acids with
umes through pipes will generate a charge if not
Cyanex 923[33,143,146]
grounded. The use of high flash point diluents is now
2) Chromium almost universal, there being few exceptions where a
Extraction with Alamine 336 from industrial
low flash point aromatic diluent is used. Adequate
plating waste[147] and waste waters[148];
grounding of solvent storage tanks and diluent tanks
Cr from ground water with Aliquat 336[147]
are necessary. Papers on the subject have been pub-
3) Cobalt and nickel
lished[161,162].
Co and Ni separation from HCl with Cyanex
923 from spent catalysts[149]; 2 Projections to the Future
Ni from plating baths with LIX 84I[150];
2.1 Chemistry
4) Copper
Cu from pickling baths with Cyanex 302 +
Speciation chemistry should be used to optimize
LIX 860[24];
chemical separations and product purity. Also, the na-
Cu from ammoniacal etch solutions with LIX
ture and effect of Eh-pH in the process require optimi-
84[151,152]
zation. The development of novel and improved
5) Cyanide
 scrubbing techniques will enhance the product purity.
Cyanide recovery from effluents with Cyanex
It would be useful to have extractants available to use
923 instead of by volatilization[153]
below the pH of the hydrolysis of Fe. There could be
6) Gallium
the use of  applied synergism in the process other
Ga and In from Zn residues in chloride with
than the intentional  blends for the manufacture of
TBP, DEHPA, and EHPNA[154]
many chelating extractants. More extractants for appli-
7) Indium
cation to nuclear waste may appear. With further re-
In from Zn residues, using DEHPA from sul-
search, an improved understanding of the organic deg-
phate[155,156]
radation should be resulted in within the process, as
8) Iron
well as an improved understanding of the analyses and
Fe and Zn with PC88A and DEHPA;
chemistry relative to crud and its formation.
Fe and In from zinc residues; H2SO4 and Fe
2.2 Physical chemistry
and Ni from pickling bath[157];
Fe, Ti, and V recovery from waste chloride
It will be necessary to decrease the shear in mixing to
liquors with TBP + EHPA[158]
minimize creation of stable emulsions and other prob-
9) Silver
lems associated with fine droplets and air. There is a
Ag from waste nitrate with Cyanex 301, 302,
requirement for an increased examination and charac-
Aliquat 336[159]
terization of cruds to reduce crud formation by a better
10) Zinc
understanding on how they are formed. Also, an im-
Zn from chloride solution with TBP[160]
proved understanding of the role of  surfactants in the
1.13.3 Health and safety
SX process is required.
Clothing has varied from synthetics and all-cotton to
mixtures. As regard static electricity, the cotton cloth-
2.3 Chemical engineering
ing would be recommended as the least potential haz-
Numerous contacting equipments have been studied
ard. Recent fires in several solvent extraction plants
the next stage is the application to specific situations
have been major, and therefore, there is a great concern
145
Gordon M. RitceyÿSolvent Extraction in Hydrometallurgy: Present and Future
where comparisons are made of mass transfer, kinetics, diluent selection) as well as the method of achieving
and equilibria. Pilot plants will examine contactors mass transfer. Therefore, the contactor selection is of
other than only mixer settlers that are the present phi- major importance where plant performance enhance-
losophy. Only a few plants to date have been success- ment may result, e.g.,
ful in adapting other than mixer settlers but this will a) it is essential to have sufficient bench data fol-
change. Models of multi-component systems are re- lowed by several mini-pilot plants to establish capital
quired with mass transfer and kinetics on real solutions, and operating costs and determine possible problems;
including the validation of the model. Increased b) the operator needs to take a risk with new process
knowledge of micelles, interfaces, and stable emul- equipment;
sions and cruds will assist in the design of contactors c) concern by the operator of the large, and expen-
based on drops relative to mass transfer and coales- sive, solvent inventory or the necessity for maintaining
cence. Design of robust monitoring equipment and a certain oxidation state may lead to other contactors
techniques will assist in an improved understanding of use;
the plant, and obtaining data for modelling. Validation d) contactors and their operation will be determined
of computer models requires information from the con- by the drop size required for mass transfer, the kinetics,
tractor (e.g., columns) designers and manufacturers for and the effect on phase disengagement;
scale-up. e) various contactors will be compared in pilot scale
tests, including mixer settlers, columns, and in-line
2.4 Pilot plants and company philosophy
mixers. An early decision in the project is required;
f) mixer settler designs will have improved distribu-
The size of a pilot plant will be designed on the basis
tion of flows in settler; launders more accessible, re-
of flows, kinetics, phase disengagement and not over-
duced shear in mixing, reduced air in droplets, etc.;
designed as in the case of present plants. Cost has al-
g) more applied design/research will improve on the
ways been one of the most critical factors in decisions
design based on kinetics and equilibria and not major
on the size of pilot plants, but this has to change so that
 over-design ;
the decision is based also on the information required
h) the time of mixing vs. effect on drop
for the design of the optimum plant. That is, we have
size/coalescence and metals discrimination will be
to question what we want from the pilot plant. Contac-
considered in the optimum design;
tor selection will be based on the considerations of the
i) design will also be determined by the phase sepa-
chemical/physical aspects. The duration of piloting
ration characteristics of the particular process and stage,
will still be site-specific and depend on the complexi-
taking into account any possible surfactant affects; and
ties of the process as well as the relative experience of
j) materials of construction will be more important,
the technical personnel. The decisions to scale-up to
particularly to enhance phase disengagement, as well
plant or abandon the process will be determined by the
as for piping and containers.
quality of the product, the recovery, the economics,
and the environmental impact. More input is required
2.6 Operation
from the operator and the engineering contractor in or-
der to define the total integrated flowsheet by identify- To improve the plant operations, there will be more
ing all streams, compositions, flows, process variables, controls for process optimization, including the follow-
environmental, health, safety impacts, materials of ing, in no particular order of importance:
construction, and product specifications. a) increased use of columns and possibly in-line
mixers where drop size can be controlled to provide
2.5 Design engineering
increased throughputs and increased rates of coales-
cence, and therefore, reduced solvent inventories and
The plants in the future should be designed based on
reduced solvent entrainment losses;
the chemistry (PLS composition, extractant choice,
146 Tsinghua Science and Technology, April 2006, 11(2): 137-152
b) improved coalescing devices will be used if v) monitoring and control of speciation and Eh-pH
potential fire hazards can be eliminated; where applicable to process;
c) run the plant at the design flows or higher not w) more measurements for interfacial tension to
lower; control the impurity accumulation on the solvent, as
d) control the time in mixing that is dictated by the well as viscosity and density measurements; and
kinetics required for mass transfer and optimization of x) on-line monitoring and analyses of organics in
coalescence; aqueous discharges to ensure environmental limits of
e) control phase continuity to optimize both mass discharges.
transfer (as well as discrimination) and phase separa-
2.7 Applications
tion;
f) minimize air in the mixing and settling through
In the next few years, we should see more emphasis on
proper design and operation;
operating cost reduction by accepting technology that
g) minimize turbulence in the settler zone to reduce
has been proven scientifically and in subsequent pilot
solvent losses and misting;
plant trials. The days of accepting old technology
h) dedicated stage for continuous treatment of any
based on the fact  that it has worked and been used in
bleed streams (e.g., regeneration) instead of treatment
a number of plants should turn to  using the best
of a whole stream on a campaign basis;
technology to achieve the objectives of economics, en-
i) crud characterization/analyses;
vironment, and products . There are already many pos-
j) improved understanding of the role of bacteria in
sible options available to future plants, from the proc-
the solvent extraction process;
ess side as well as the equipment and methodology.
k) role of organic surfactants in process, and the
Some of the new advances could include the following:
possible requirement for treatment of feed water by
a) solvent-in-pulp systems; b) additional metals and
diluent scrubbing;
by-product recovery; c) recovery of very high purity
l) destruction of bacteria in feed water by addition of
metals; d)  chemical refineries at the mine site; e) in-
biocides, UV, ozonation;
creased treatment of tank house bleeds, waste dusts,
m) elimination of alcohols in the system because of
scrap for recycle; f) removal/recovery of noxious gases
their degradation to organic acids;
from smelters and refineries; g) treatment of nuclear
n) improved settler design with the use of dissimilar
wastes; h) more acid recovery plants; i) treatment of
materials for the plates will result in improved coales-
effluents for contaminant removal to meet environ-
cence and decreased solvent losses;
mental; and j) coupling of IX with SX to assure safe
o) minimizing of air entrainment will reduce or
discharge of effluents as well as possible activated car-
eliminate the misting problem;
bon treatment.
p) possible implementation of more in-situ reagent
2.8 Plant safety and worker hazards
regeneration;
q) use of synergistic mixtures for enhanced pH and
Plant safety will be improved by: a) ensuring proper
kinetics and equilibrium;
grounding throughout the plant (tanks and lines); b)
r) treatment of the stripped solvent that has been
minimizing open tanks of solvent and evaporation in
 poisoned using a wash stage such as dilute alkali;
the solvent area; c) minimizing possible static through-
s) diluent wash between multi-circuits and on-
out the plant; d) selection of cotton clothing instead of
stream monitoring and analyses;
synthetic materials; e) ensuring adequate ventilation;
t) diluent wash treatment of final raffinates to re-
negative pressure in circuits; f) keeping hatches (in
move residual extractant;
mixer settlers) closed and minimizing venting to at-
u) carbon columns treatment to remove residual or-
mosphere by scrubbers; g) automatic discharge of sol-
ganics before discharge to environment;
vent to an external sump in case of fire; h) developing
147
Gordon M. RitceyÿSolvent Extraction in Hydrometallurgy: Present and Future
optimum operational procedures to reduce environ- [3] Goto M, Matsumoto S, Yoshizuka K, Inoue K. In: Pro-
ceedings of ISEC 96. Melbourne, Australia, March 19-23,
mental impact; and i) continued use of the present con-
1996; Pub. University of Melbourne: 281-286.
ventional safety equipment (e.g., masks, gloves,
[4] Jansen M R, Chiarizia R, Ferraro J R, Borkowski M, Nash
glasses).
K L, Thiyagarajan P, Littrell K C. In: Proceedings of
2.9 Environmental concerns
ISEC 02. Cape Town, South Africa, March 17-21, 2002:
1137-1142.
a) Water, air, soil; b) metal-organic complexes hazards;
[5] Bart Hans-Jorge. In: Proceedings of ISEC 02. Cape Town,
c) containment of wastes; d) risk assessment analyses;
South Africa, March 17-21, 2002: 45-52.
e) environmental impact data; f) oxidation/disposal of
[6] Kuipa P K, Hughes M A. In: Proceedings of ISEC 02.
unwanted organics; and g) decommissioning protocols.
Cape Town, South Africa, March 17-21, 2002: 53-57.
[7] O Keefe T, O Keefe M, Fang R, Sun J, Dahlgren E. In:
2.10 Novel systems
Proceedings of ISEC 02. Cape Town, South Africa, March
17-21, 2002: 459-466.
At this time, there is considerable technology for use
[8] Heckley P, Ibana D. Paper presented at New Generation
of membranes, but commercialization in hydrometal-
Nickel Laterites in the Eastern Goldfields, Australia.
lurgy has not yet been achieved in plant operations.
WMC Conf. Centre, Kalgoorlie, March 23-25, 2000.
The suspended solids in the leach liquor would be a
[9] Heckley P. Extraction and separation of cobalt from acidic
fouling problem. However, the very clean solutions
laterite leach solutions using electrostatic pseudo liquid
such as the strip product liquor could probably be con-
membrane (ESPLIM) [Ph.D. Dissertation]. Australia: Kal-
sidered for treatment by membrane technology for fi-
goorlie, Curtin University of Technology, WASM, 2002.
nal purification. Liquid membranes (supported, emul-
[10] Heckley P S, Ibana D C, McRae C. In: Proceedings of
sion, and electrostatically-assisted membranes) would
ISEC 02. Cape Town, South Africa, March 17-21, 2002:
be considered.
730-735.
[11] Briggs M K, Ibana D C, Cheng C Y. Paper presented at
3 Conclusions
New Generation Nickel Laterites in the Eastern Goldfields,
Australia. WMC Conf. Centre, Kalgoorlie, March 23-25,
It is now about 50 years since the first U-SX plant was
2000.
commissioned, and more than 40 years of Cu process-
[12] Shibata J, Yamamoto H. In: Proceedings of VI Southern
ing, plus the treatment of many other metals for recov-
Hemisphere Meeting on Mineral Technology. Rio de Ja-
ery. It has been a successful, exciting, and interesting
neiro, May 2001: 433-440; CETEM/MCT, 2001.
history in the development and implementation of a
[13] Shibata J, Yamamoto H. In: Proceedings of ISEC 02.
unit process in hydrometallurgy for separation and pu-
Cape Town, South Africa, March 17-21, 2002: 543-548.
rification.
[14] Sanchez-Loredo M G. In: Proceedings of ISEC 02. Cape
Because of the  bank of knowledge that is now
Town, South Africa, March 17-21, 2002: 934-939.
available on all aspects of the solvent extraction proc-
[15] Komatsu Y. In: Proceedings of ISEC 96. Melbourne, Aus-
ess, the industry should be in the position to improve in
tralia, March 19-23, 1996; Pub. University of Melbourne:
the designs, applications, and plant optimization in the
611-6160 J.S.
future.
[16] Preston J S, du Preez A C. In: Proceedings of ISEC 99.
Barcelona, Spain, Sept. 1999; Pub. Society Chemical In-
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