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Solvent Extraction and Ion Exchange

ISSN: 0736-6299 (Print) 1532-2262 (Online) Journal homepage: http://www.tandfonline.com/loi/lsei20

Review on the Recent Developments in the SolventExtraction of Zinc

Akash Deep & Jorge M. R. de Carvalho

To cite this article: Akash Deep & Jorge M. R. de Carvalho (2008) Review on the Recent

Developments in the Solvent Extraction of Zinc, Solvent Extraction and Ion Exchange, 26:4,375-404, DOI: 10.1080/07366290802179267

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Review on the Recent Developments in the Solvent

Extraction of Zinc

Akash Deep, and Jorge M. R. de Carvalho

Centre of Chemical Processes, Department of Chemical Engineering, Instituto

Superior Técnico, Av. Rovisco Pais 1049-001, Lisbon (Portugal)

Abstract: Solvent extraction (SX) of zinc is useful for the recovery of high purity

zinc from the leaching solutions of its sulphide minerals, several low-grade ores,

and secondary materials. The technique is fast, environmentally sustainable, and

can be tailored to treat aqueous solutions of diverse compositions. It is

particularly useful in the cases where the level of contamination is high and the

upgrading of the desired metal is necessary. The present paper reviews the use of several acidic, basic, and solvating extractants for the recovery of zinc from

different acidic media. The important aspects of the extraction processes have

been discussed and some of the noteworthy applications of the SX in the

treatment of ores and secondary materials are presented.

INTRODUCTION

Solvent extraction of Zn(II) is one of the most popular hydrometallurgi-

cal ventures. The technique is expected to gain more popularity with the

latest trends in the recovery of Zn(II) from various low-grade ores and

industrial wastes. The conventional process of the recovery of zinc

employs roasting, leaching, and electrowinning (RLE) steps. For decades,

sphalerite or zinc blend (ZnS) has been one of the most widely exploited

raw materials to meet the global demand of zinc. This material is easily

roasted to be converted into ZnO, which can then be leached with

Received 12 September 2007, Accepted 22 January 2008

Address correspondence to Jorge M. R. de Carvalho, E-mail: [email protected].

Tel.: +351-21-8417311, Fax: +351-21-8499242

Solvent Extraction and Ion Exchange, 26: 375–404, 2008

Copyright # Taylor & Francis Group, LLC

ISSN 0736-6299 print/1532-2262 online

DOI: 10.1080/07366290802179267

375

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sulphuric acid to produce zinc leachates. Further purification involves

Fe(III) precipitation (jarosite, goethite, hematite), cementation, and

electrolysis. Lately, the precipitation of Fe(III) during the hydrometal-

lurgy of Zn(II) is seen as a step which not only creates huge piles of solidwaste but also co-precipitates environmentally, and to some extent

economically unacceptable amounts of some elements, including zinc,

copper, cadmium, indium, silver, etc. The disposal of the solid waste

requires the designing of special landfill sites. Selective SX of zinc in the

presence of iron can be a solution of the above problem. With natural

depletion of the sulphidic concentrates, a significant fraction of the global

zinc production is to be met through the use of other non-sulphidic ores

and industrial by-products. Mineral mixture of zinc silicate and

carbonate falls into the category of usable non-sulphidic zinc ores.

Some of these minerals include hemimorphite [Zn4Si2O7(OH)2.H2O],

sauconite [Na0.3Zn3(Si,Al)4O10(OH)2.4H2O], willemite (Zn2SiO4), smith-

sonite (ZnCO3), and hydrozincite [Zn5(CO3)2(OH)6]. These ores may

contain 10–20% of zinc, and they are not easily upgradeable by flotation.

Leaching solutions of these ores contain various impurities. SX is a

potential solution to the production of high quality zinc from such a type

of complex aqueous stream. Solvent extraction of zinc is also useful in the

processing of secondary sector materials, such as electric arc furnace dust,

Waelz oxides, galvanizing industry waste, etc.

An earlier review article by Cole and Sole[1] (published in 2003)

discussed the status of solvent extraction in process industries. Some

important literature has accumulated since the publication of the above

article. The present review article covers the updated information on the

scientific research in the area of the solvent extraction of zinc. The

applications of different extractants from different acidic media have

been cited. Some significant and recent SX applications in the processing

of primary and secondary zinc are highlighted.

EXTRACTION FROM SULPHATE MEDIA

Extractants used and Important Concepts

Di-(2-ethylhexyl)phosphoric acid (DEHPA), 2-ethylhexyl phosphonic

acid mono-2-ethyl hexyl ester (PC-88A), bis-(2,4,4-trimethylpentyl)

phosphinic acid (CYANEX 272), and bis-(2,4,4-trimethyl pentyl)dithiophosphinic acid (CYANEX 301) have been the most commonly

suggested Zn(II) extractants[2–5] from the aqueous sulphate media. The

chemical structures of these extractants are given in Fig.1. These

organophosphorus extractants primarily behave as cation exchangers

(eq 1) and the extraction of Zn(II) is pH dependent.

376 A. Deep and J. M. R. de Carvalho

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Figure 1. Structures of some important zinc extractants.

Recent Developments in the Solvent Extraction of Zinc 377

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Zn2zaqð ÞzmHA orgð Þ [ ZnA2HA m{2ð Þ orgð Þz2 H

z ð1Þ

where HA is extractant molecule, and m may vary from 2 to 4.

Figure 2 shows the extraction patterns of Zn(II) with DEHPA, PC-88A, CYANEX 272, and CYANEX 301 as a function of equilibrium pH.

Nathsarma and Devi,[3] Mellah and Benachour,[6] Ocio and Elizalde,[7]

Devi et al.,[8–10] and Nayak et al.[11] have presented experimental data on

the extraction properties of the above highlighted extractants. The

stripping of Zn(II) from the organic phase is simple and can be achieved

with sulphuric acid. The temperature also affects the rate of extraction,

and generally the extraction reaction is endothermic.

CYANEX 301 is particularly selective for Zn(II) over calcium and

magnesium. Separation of Zn(II) from Fe(III) is one of the biggestchallenges in the zinc processing industry. DEHPA and CYANEX 301 form

strong organic phase complexes with Fe(III), but the stripping is not easy

and requires the use of concentrated HCl[12–14]. This difficulty in the Fe(III)

stripping limits the utility of DEHPA and CYANEX 301 on the industrial

scale. DEHPA can, however, be used to simultaneously extract Zn(II) and

Fe(III) at low pH (1–1.5), followed by the selective stripping of Zn(II) using

Figure 2. Extraction behaviour of Zn(II) in DEHPA, PC-88A, CYANEX 272

and i. CYANEX 301 extractants (Vaq/Vorg.51) as a function of equilibrium pH,

ii. (Source: Cole and Sole, 2003).


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H2SO4. Extraction of Zn(II) at low pH is also advantageous in minimizing

interferences due to copper, cobalt, nickel, and cadmium. Nonetheless, some

impurities, such as indium, tin, and bismuth, may still interfere, and a

continuous buildup of Fe(III) in the organic phase is never avoided. Somealternative stripping routes have been suggested for the removal of Fe(III)

from the loaded DEHPA phase. Reduction of Fe(III) to Fe(II) in the

organic phase has been suggested as one of the options[14–16]. This has been

achieved using H2 or SO2, maintaining high temperature and pressure. Lupi

and Pilone[17] suggested the reductive stripping of Fe(III) from DEHPA in

vacuum using zinc powder as a reducing agent. More than 89% iron (when

initial concentration of Fe in DEHPA was 5 g/L) could be stripped at room

temperature under a pressure of 80 kPa. The galvanic stripping of Fe(III) has

been proposed by Moats et al., [18] Barrera Godinez et al., [19] and O’Keefe

et al. [20]. An advantage of galvanic stripping is the production of a

concentrated iron solution (90–130 g/L Fe). However, this process is not

continuous. Van Weert et al.[21] suggested the use of 6 N nitric acid for the

stripping of Fe(III) from DEHPA (in kerosene), with a view of producing

spheroidal hematite by the autoclave treatment of the recovered ferric nitrate

solution. The complex of Fe(III) was very stable when exposed to nitric acid

solutions up to 6 N and temperatures up to 70uC over a period of five days.

But this process has disadvantages. The stripping became more difficult with

increasing DEHPA concentration. Also, it was not possible to generate

ferric nitrate solution having more than 10 g/L of iron.

Another interesting way of tackling the problem of difficult DEHPA

regeneration is the mixing of the extractant with other reagents, like with tri-

n-butyl phosphate (TBP), tri-n-octyl phosphine oxide (TOPO), CYANEX

923, and amines[14,22–25]. Mixing of DEHPA with the above reagents may

enable the use of sulphuric acid as a stripping reagent of Fe(III).

PC-88A and CYANEX 272 offer easy stripping of Fe(III) with

sulphuric acid

[1,26]

. CYANEX 272 also provides selective separation of Fe(III) from Zn(II)[26] at pH around 1.8. The co-extracted fraction of Zn(II)

can be scrubbed using dilute H2SO4, followed by the stripping of Fe(III)

using 2 M H2SO4. A successful selective separation of Fe(III) from Zn(II)

may be followed by the extraction of Zn(II) using DEHPA or CYANEX

301; the latter extractant being more selective toward Ca(II) and Mg(II). A

significant feature of employing Zn(II) extraction with DEHPA or

CYANEX 301 after the removal of Fe(III) is the flexibility in choosing

the range of working pH. A high concentration of DEHPA and CYANEX

301 can be used so as to achieve the extraction at pH as low as 1, therebyrequiring little pH adjustment during the extraction stages.

Some recent studies by Principe and Demopoulos[27,28] have

indicated the use of octylphenyl acid phosphate (OPAP) as Fe(III)

extractant from ZnSO4-H2SO4 solutions. OPAP was claimed to provide

better kinetics of iron extraction, and offers greater pH functionality than


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DEHPA. OPAP was also useful in keeping the sulphate co-extraction

lower than DEHPA; however, slightly higher co-extraction of Zn(II) was

observed. Stripping of the loaded iron could be achieved with 6 N HCl. A

stripping solution of 70 g/L of Fe(II) in 6 N HCl provided 100 g/L of metal ion build-up in the final solution at 20uC.

Industrial Applications

An industrially tested application of the SX of Zn(II) with DEHPA is the

treatment of bioleaching liquor at MIM Holdings Ltd. (now Xstrata Plc)[29].

The leaching of zinc sulphide concentrate was performed in stirred-tank

reactors with a mixed bacterial population of Thiobacillus ferrooxidans,

Leptospirillum ferrooxidans, Thiobacillus thiooxidans, Sulfobacillus strains,

Thiobacillus caldus, Acidiphilium cryptum, Acidiphilium organovorum,

and some heterotrophic microorganisms (pH 1.6–1.7) for a residence time of

3 days maintaining the temperature at 40–45uC and providing the reactors

with 2% v/v CO2. Some nutrients were introduced during the leaching. The

whole process gave a pregnant leach solution containing 25–30 g/L Zn, 3–

4 g/L Fe (mostly ferric), and around 100 mg/L Cd(II). This solution was

adjusted to pH 4.0 with limestone slurry to precipitate iron. After reducing

the iron concentration to less than 10 mg/L, a two-stage SX was employed

using 25% (w/w) solution of DEHPA in Shellsol 2046 (O/A53). The co-

extracted Ca(II) was removed by scrubbing with dilute electrolyte (O/

A520). Finally, Zn(II) was stripped (O/A54) using the spent electrolyte

(60–70 g/L Zn, 180 g/L H2SO4). The obtained advanced electrolyte (80–

100 g/L Zn) was used in electrolysis cells. MIM bioleaching, combined with

SX, has been tested on a pilot-plant level[30] using a commercial zinc

concentrate (48.6%, w/w Zn, 2.7%, w/w Pb, 8.6%, w/w Fe, 32.6%, w/w S)

and a mixed zinc-lead concentrate (43.8%

, w/w Zn, 11.3%

, w/w Pb, 6.1%

, w/w Fe, 31.0%, w/w S).

Teck Cominco’s HydroZinc process (Fig. 3) is also based upon the

solvent extraction of Zn(II)[31,32]. The process involves the leaching of

volcanogenic massive sulphides and Sedex deposits with mesophile,

thermophile, or extreme thermophile microorganisms. Enough aeration

(5 L/min/m2) is provided from the heap bottom and iron precipitation is

avoided by keeping sufficient acid content (15–30 g/L H2SO4) at the lower

heap levels. The temperature can range from about 35uC to above 60uC.

The leaching solution containing 10–50 g/L Zn has to be neutralized topH 4 with limestone slurry to remove Fe(III). Strong oxidation

conditions are not used as a small amount of Fe(II) can be tolerated in

the DEHPA extraction circuit. A 20% DEHPA solution in kerosene (v/v)

was employed to extract around 30–50% of initial Zn(II). The pH

adjustments during the extraction process were cut to a minimum by


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avoiding the use of an expensive caustic reagent to ensure a low cost of the

process. This was the reason for a low percentage of the Zn(II) extraction.

After a three-stage scrubbing of the organic phase with the dilute spent

Figure 3. Teck Cominco’s HydroZinc process.


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electrolyte to remove co-extracted impurities (Ca(II) and Cd(II)), Zn(II) was

recovered by stripping with the spent electrolyte to produce an advanced

electrolyte. Despite prior iron decontamination of the aqueous phase, some

fraction of this impurity transferred into the organic phase during theextraction step. The iron build-up in the organic phase was proposed to be

removed by reductive stripping with zinc dust[33]. The HydroZinc process

has been tested on pilot plant level treating zinc sulphide ore (15% Zn) from

the Red Dog mine (Alaska, USA).

Skorpion mine (Namibia) use the modified ZINCEX process, which

can be termed an important success story in the development of Zn(II)

SX[34]. A detailed flow sheet of Skorpion process is given in Fig. 4.

Skorpion mine produces a silicate ore with a composition of 8–14% Zn,

2–3% Fe, 22–27% Si and 4–6% Al[35,36]. The ore was milled and then

leached with dilute sulphuric acid at 50uC. This extracted more than 95%

of Zn(II). The leach solution was neutralized to pH 4.2 with limestone,

lime, or basic ZnSO4. Some impurities like iron, silica, and aluminium

were precipitated and a pregnant leach liquor containing 30–40 g/L

Zn(II), along with other metal ions, such as Cu(II), Cd(II), Ni(II), and

Co(II), was obtained. SX of the above leaching solution was done in three

or four stages at 40uC with 30% (v/v) DEHPA[37,38]. Inter-stage pH

adjustments were made using lime or basic ZnSO4

. However, those

optional inter-stage pH adjustments may be avoided by using higher

concentration, e.g. 40%, of the organic extractant. After the extraction,

the organic phase was scrubbed subsequently with water and the dilute

spent electrolyte to remove co-extracted metals, such as Cd(II) and

Cu(II). Zn(II) was finally recovered from the organic phase by stripping

(4 stages) with the spent electrolyte (O/A53). A bleed of the organic

phase was separately contacted with 4–8 M HCl solution to remove

entrained iron and aluminium. SX in the Skorpion process helps in

upgrading the zinc concentration and the recovered zinc sulphateelectrolyte becomes appropriately concentrated for its use in electrolytic

cells. Anglo American Plc. is the present owner of the Skorpion project.

The company has ramped up the production of special high grade (SHG)

electrolytic zinc to about 150,000 metric tons per annum.

EXTRACTION FROM CHLORIDE MEDIA

Extractant used and Important Concepts

Chloride hydrometallurgy has been of research interest due to the

effective leaching characteristics of this aqueous medium for some of the

zinc ores, such as oxides of zinc and secondary materials. Several

extractant systems have been explored for use in chloride media. Acidic


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Figure 4. Skorpion process for the recovery of zinc from the silicate ore.


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extractants extract Zn(II) from chloride media according to equation (1).

DEHPA has been one of the most frequently studied acidic extractants

for Zn(II)[39,40]. At low Zn(II) concentration, ZnR2.HR and ZnR2.(HR)2are the prominent extracted complexes. Macro concentration of Zn(II)forms (ZnR2)n, where n ranges between 2 and 3.5. A thio- derivative of

CYANEX 272, bis (2,4,4-trimethyl pentyl) monothiophosphinic acid

(CYANEX 302) extracts the (ZnR2)2.(HR)3 species from chloride

media[41]. This reagent has been reported as a stronger Zn(II) extractant

than DEHPA[42] and also provides selectivity over Ca(II).

Amongst the chelating agents, KELEX 100 is one of the most

notable Zn(II) extractant from the chloride media[43,44]. The extraction

with protonated extractant, [RH2][Cl]2, is proposed to distribute the

[RH2]2.ZnCl4 species into the organic phase. Co-extracted chloride ions

can be scrubbed by ammonia solution at equilibrium pH of 6.5 to 8.0.

During this scrubbing step, the complex in the organic phase changes to

R2Zn. Finally, stripping with 0.5 N H2SO4 recovers Zn(II) as ZnSO4solution. Jia et al.[45] have described the extraction of Zn(II) by a mixture

of primary amine N1923 and PC-88A. A low concentration of the amine

used enhances the extraction of Zn(II), while a high concentration

displays an antagonistic effect. The extraction reaction is exothermic.

Basic extractants have also been frequently used for the extraction of

Zn(II) from the chloride media. The secondary and tertiary amines form

extractable complexes with anionic Zn(II) chloride species, ZnCl422 and

ZnCl32. A typical reaction for the extraction of Zn(II) chloride anionic

species with secondary amines can be given as

ZnCl2{4 aqð Þz2 R2NH2Cl orgð Þ [ R2NH2ð Þ2ZnCl4 orgð Þz2 Cl{

aqð Þ ð2Þ

About three decades back, a secondary amine was proposed for the

treatment of pyrite cinder leach liquor containing 20–30 g/L Zn[46,47]. This

process involved the extraction of Zn(II) from the chloride solution usingAmberlite LA-2. The extraction step also transferred some associated

metals, such as Fe(III), Cu(II), and Cd(II) into the organic phase, while

other impurities, like Co(II) and Ni(II) were left in the aqueous phase.

The loaded organic phase was scrubbed with water, followed by the

stripping of Zn(II) with acidified water (pH 5–6). These steps recovered

the Zn(II) solution, which is again subjected to the extraction with

DEHPA (kerosene solution). The stripping of Zn(II) from the loaded

DEHPA phase is carried out with sulphuric acid to finally obtain a

sulphate electrolyte. This process was called the ZINCEX process, and itwas used in the plants at Bilbao, Spain in 1976, and at Quimigal, Portugal

in 1980 for the production of zinc from pyrite cinders obtained from

Longmaid-Henderson nonvolatilizing chloride roast and Kowa-Seiko

volatilizing roast processes. Both of these plants worked for about fifteen

years. The ZINCEX process was modified during the early 1990s and is


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now used at the Skorpion mine in Namibia. Wassink et al.[48] have

reported the use of Aliquat 336 for the extraction of Zn(II) from the

chloride medium. This reagent is a quaternary amine and performs the

selective extraction of Zn(II) over Co(II) and Ni(II). 30% (v/v)concentration of Aliquat 336 is used to achieve 18 g/L Zn(II) concentra-

tion in the organic phase. The stripping of the extracted Zn(II) is carried

out with ammonia solution.

Solvating extractants for Zn(II) mainly include tri-n-butyl phosphate

(TBP), dibutylbutyl phosphonate (DBBP), and CYANEX 923 (a mixture

of four trialkyl phosphine oxides). Due to their electron donor properties,

the above reagents can form complexes with the neutral Zn(II) chloride

species. TBP is by far the most extensively studied extractant. In the

1980s, Ritcey et al.[49,50] developed an extraction process for the recovery

of zinc from a leaching liquor containing 30 g/L Zn(II). The researchers

used 60% (v/v) TBP solution in Solvesso 50. The stripping of Zn(II) from

the loaded TBP phase was achieved by the spent electrolyte solution

containing 15 g/L Zn(II) (HCl media) at pH 1.0. The composition of

extracted Zn(II) complexes in TBP was determined as ZnCl2.2TBP from

dilute HCl solutions. At HCl acidity higher than 0.10 mol/L, Zn(II) is

distributed in the organic phase as an acidic complex HZnCl3[51]. The

investigations on TBP were continued at the Cato Research Corporation,

USA[52] with special emphasis on the stripping of the extracted metal ion

with ammonium chloride solution. The objective was to finally produce

zinc chloride product by the decomposition of the stripped liquor. The

studies on the stripping of Zn(II) from TBP with ammonia/ammonium

chloride solution have recently been revisited by Mishonov et al.,[53] who

observed that a vigorous shaking of 2–3 min provided equilibrium

stripping conditions. The pregnant strip solution can be concentrated up

to 53 g/L Zn(II). This recovered solution is suitable for use in

electrowinning cells. Recently, Dessouky et al.

[54]

have reported therecovery of Zn(II) (25 g/L) from the spent pickling solution of the hot

dipping galvanizing bath by extraction with undiluted TBP. The aqueous

phase, also containing 158 g/L Fe(II), 2 g/L Fe(III), and 10% (v/v) HCl,

and traces of Cd(II), was filtered and then subjected to reduction by

granulated zinc to reduce ferric to ferrous and to precipitate cadmium.

Two-stage extraction by undiluted TBP (A/O51) transferred 96% Zn(II)

into the organic phase. The co-extracted traces of Fe(II) were removed

from the organic phase by scrubbing with distilled water (A/O50.25).

Zn(II) was finally stripped using distilled water at an A/O ratio of 1.Dibutylbutyl phosphonate[55] has been used to extract Zn(II) from a

leach solution containing 5 g/L Zn at 25uC. The loaded Zn(II) was

stripped at 60uC using an electrolyte of 30 g/L Zn in 116 g/L NaCl so as to

produce pure solution of 65 g/L Zn. The purified solution was used for

the recovery of electrolytic zinc. This process, called ZINCLOR, has been


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tested on a pilot plant level and it was reported to consume low energy

due to high current efficiency and low cell voltage. The extraction of

Zn(II) with DBBP was exothermic and the standard enthalpy D Ho was

228.4 kJ/mol[56]. The performance of DBBP vs. TBP has been evaluated[57] to show that DBBP has a higher Zn(II) extraction efficiency than

TBP. It has also been proposed that DBBP forms a 1 (metal ion): 2

(extractant) complex at extractant concentration higher than 40% (v/v);

whereas the stoichiometry of this complex changes to 1 (metal ion) : 1

(extractant) at lower extractant concentrations. Zn(II) can be separated

from Fe(II) with a separation factor of more than 103. The stripping of

the extracted metal ion was achieved in three stages using water.

CYANEX 923 is another solvating extractant suggested for the

recovery of Zn(II) from the chloride solutions[58–60]. This extractant

formed a 1 (metal ion) : 2 (extractant) complex. The extraction reaction

was exothermic (DHu5255.2 kJ/mol). The stripping of Zn(II) from the

organic phase was achieved by water. CYANEX 923 has been used for

the extraction of 0.23 g/L Zn(II) from mixed chloride (176.3 g/L) and

sulphate (48.9 g/L) solution, also containing 11.8 g/L Fe(III) and 24.8g/L

Cu(II)[58]. This was done by first removing Fe(III) and Cu(II) in two

successive extraction circuits using TBP (1 M) + MIBK (20%, v/v) and

LIX 84I (70%, v/v) reagents, respectively followed by the extraction of

Zn(II) with 0.05 M CYANEX 923. The extracted Zn(II) was recovered by

stripping with water. Some extensive studies on the use of CYANEX 923

for different 3d transition metal ions have indicated that this extractant

performs fast kinetics in extraction and has the selectivity for Zn(II) in

the presence of associated metal ions, such as Ti(IV), Mn(II), Co(II),

Ni(II), and Cu(II)[61]. The selective extraction of Zn(II) was achieved at

1 mol/L chloride ions, wherein Ti(IV), Mn(II), Co(II), and Ni(II)

remained in the raffinate.

The co-extraction of Fe(III) has been a problem in the chloride mediaalso. DEHPA, TBP, and CYANEX 923 strongly extract Fe(III). The

regeneration of DEHPA and TBP requires the use of concentrated HCl.

To avoid this situation, some suggestions have been given to reduce ferric

ions to ferrous ions prior to the extraction step. The distribution of Fe(II)

in DEHPA and TBP is negligible. Thus, the contamination of the organic

phase can be avoided[54,59]. CYANEX 923 is a better extractant in terms

of its regeneration capacity. This reagent can be easily recycled from iron

by contacting with only moderately concentrated (1 mol/L) sulphuric acid

solution[62]

.Bis-benzimidazole (ACORGA ZNX 50) has been reported as a selective

Zn(II) extractant in the presence of Fe, As, Ca, Cr, Pb, Mg, Mn, and

Ni[63–65]. Copper was the only important co-extractable impurity, which was

removed by scrubbing with water. The stripping of the extracted Zn(II) was

achieved with the spent electrolyte solution (30 g/L Zn, 2 mol/L NaCl, 5 g/L


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HCl) in a closed loop SX-EW circuit. Smith et al.[66] and Flett and

Anthony[67] have used ACORGA ZNX 50 to treat ferric chloride leachates

of a complex sulphide ore (7.13%, w/w Zn, 37.5%, w/w S, 32.1%, w/w Fe,

2.63%, w/w Pb 0.75%, w/w Cu), mined from the province of NewBrunswick, Canada. The extracted Zn(II) was recovered by stripping with

the spent electrolyte. This process was suggested to be economically suitable

only in the case of a low cost mining and a high yield production. Despite its

highly selective property, ACORGA ZNX 50 was never fully commercia-

lized due to a lack of demand.

Ammonium chloride and ammonium carbonate leaching of zinc

offer selectivity advantages. Harvey[68] has recently reviewed the history

of ammonium carbonate leaching of zinc. He has cited the advantages of

this leaching medium. The process is simple and economical. Thesolubility of Zn(II) in the reagent is high under mild reaction conditions.

Since the dissolution of metal is achieved at pH higher than 3, iron is

rejected in the precipitate. The process is not directly usable on the zinc

sulphide concentrate, but it can be considerable after the roasting or

fuming of the concentrate. Other zinc ores, such as zincite (ZnO),

hydrozincite (Zn5(CO3)2(OH)6), calamine (H2Zn2SiO5), and smithsonite

(ZnCO3) can be directly leached. Amer et al. [69,70] have proposed the

extraction of Zn(II) from the ammonium chloride solutions with

DEHPA. The following extraction reaction was proposed

Zn NH3ð Þ2Cl2 aqð Þzn HAð Þ2 orgð Þ [ ZnA2 HAð Þ2n{2, orgð Þz2 NH4Cl aqð Þ

ð3Þ

where the value of n depends upon the equilibrium pH according to the

equation:

n~1:62{0:1pH:

Release of ammonia in the above solution served to neutralize the

protons released from the acidic extractant during the extractionreaction, which eliminated the need of an external neutralizing agent.

Though the extraction was done from the chloride media, stripping could

be achieved with sulphuric acid. This may consequently permit the use of

the recovered solution in the conventional electrolysis cells. CYANEX

923 has also been proposed as a Zn(II) extractant from the ammonium

chloride media[71].

Industrial Applications

So far, only a few industrial plants have used the extraction of zinc from

chloride media. Técnicas Reunidas’s ZINCLOR process was based upon

this route. Important features of the process have already been discussed.

Combination of SX with CENIM (Centro Nacionál de Investigaciónes


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Metalúrgicas)-LNETI (Laboratório Nacional de Engenharia e

Tecnologia Industrial) leaching process is another significant develop-

ment. In this process, a mixed copper-zinc-lead sulphide ore was

subjected to leaching by 6 M NH4Cl solution at 105uC and 1.5 atm O2.

ZnS sð Þz2 NH4Cl aqð Þz0:5 O2 gð Þ [

Zn NH3ð Þ2Cl2 aqð ÞzS sð ÞzH2Oð4Þ

Several researchers such as Amer et al., [69,70] Limpo Gil et al., [72]

Limpo et al., [73,74] Figueriredo et al. [75] have presented the data on the

above leaching process. The high pH (6–7) of the leaching solution helped

in dissolving Zn(II) selectively over Fe(III). The reaction also depended

upon the cupric/cuprous ion ratio in leaching solution. The minimum

required concentration of cupric ions was 1 g/L. A single stage leaching

provided only incomplete (80–85%) recovery of Zn(II), and the residue

needed further treatment in an acid leach step to increase the recovery of

Zn(II) to more than 95%. The two stages, neutral and acid leaching, ran

in countercurrent mode. The leaching solution, already free from iron,

bismuth, antimony, and arsenic, was first treated by Zn or Cu dust

cementation to remove entrained silver and mercury. Pb(II) was then

separated as insoluble lead chloride by cooling and vacuum crystal-lization at 50uC. It was followed by the precipitation of sulphates as

gypsum with the addition of lime. Zn(II) was finally extracted using 20%

(v/v) DEHPA at 50uC. The scrubbing of the loaded organic phase with

the zinc chloride solution, derived from the lead cementation, removed

the co-extracted fractions of Cu, Ca, and Pb. The washing of the organic

phase with water removed chlorides. Zn(II) was recovered by stripping

with the spent electrolyte. The finally recovered solution contained 85 g/L

Zn(II). The CENIM-LNETI process has an advantage of rejecting iron

during the leaching itself; however, even the smallest chloride carryoverto the final electrolyte can deteriorate the cathode quality.

EXTRACTION FROM PHOSPHORIC ACID

Extractants used, Important Concepts, and Applications

The industrial wet process phosphoric acid (WPA) production is carried

out through the leaching of phosphate rocks using sulphuric or nitricacid. The leaching liquor is a complex solution containing around 30%

P2O5 (4.5 M H3PO4) and other organic and inorganic impurities. Zn(II) is

also present in an average concentration of 335 mg/L[76]. The extraction

of Zn(II) from the phosphoric acid solution is important for obtaining an

impurity free acid product. Likewise, cleaning of the rusted steel


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hardware with phosphoric acid may results into the leaching of Zn(II)

and other metal ions. The removal of Zn(II) is necessary to regenerate the

used phosphoric acid. Though, over the years, the liquid-liquid extraction

of inorganic impurities, such as Cd(II), Pb(II), Cu(II), U(IV), U(VI),Fe(III), and lanthanides, from the phosphoric acid medium has been

investigated by several researchers[77–83], the removal of Zn(II) does not

find many references.

In 1997, Riveros and Dutrizac[84] investigated DEHPA, CYANEX

301, CYANEX 302, and CYANEX 272 for the removal of Zn(II) from

the spent phosphoric acid leaching solutions used for the cleaning of

galvanized pole line hardware. The liquid-liquid extraction approach was

assessed to be a better option than selective zinc precipitation or direct

zinc electrolysis. Amongst the four extractants tested, DEHPA and

CYANEX 301 were found to have practically the same maximum

loading capacities (25 g/L for CYANEX 301, 24.6 g/L for DEHPA).

CYANEX 272 could offer a loading capacity of only 8.5 g/L, and

CYANEX 302 suffered with the problem of emulsification. Further tests

on the removal of Zn(II) from the spent leaching solution containing

106 g/L of Zn(II) and 84.8 g/L of H3PO4 using DEHPA and CYANEX

301 revealed the loading capacity of DEHPA to be 24.2 g/L after five

consecutive contacts. Interestingly, the maximum loading capacity of

CYANEX 301 under the given condition was on a lower side, 18.5 g/L.

DEHPA further proved to be a better choice, as the 66% of the loaded

Zn(II) was readily recovered by stripping with 1 mol/L H2SO4 (O/

A53.33/1). Whereas, the maximum stripping efficiency from the loaded

CYANEX 301 was observed to be only 33% even after using a higher

(3 mol/L) concentration of H2SO4. This Zn(II) extraction process with

DEHPA was tested in continuous extraction circuits (three extraction

stages) to separate Zn(II) from feed solutions containing 103–120 g/L

Zn(II), and 76–84 g/L H3PO4. The final concentration of Zn(II) wasreduced to 22–26 g/L after around 38 h of the extraction experiments. The

concentration of H3PO4 was also upgraded to 256–291 g/L. The complete

(100%) removal of Zn(II) was not possible. The stripping of the Zn(II)

loaded DEHPA was carried out with 1 mol/L H2SO4 in two counter

current stages, which finally produced about 110 g/L Zn(II) solution.

Ocio and Elizalde[76] reported the extraction of Zn(II) (1.661024 –

6.361024 mol/L) from phosphoric acid (0.4–7.3 mol/L) using CYANEX

301. Based on the graphical and numerical data analyses, they

determined the composition of the extracted species as ZnR2(R5bis(2,4,4-trimethylpentyl)dithiophosphinate). They also proposed

that the 50% removal of phosphoric acid (initial concentration53.0-

mol/L) could be achieved using only 1.561022 mol/L of CYANEX 301.

Recently, Mellah and Benachour[85–87], have investigated TBP, DEHPA,

and KELEX 100 as reagents for the extraction of Zn(II) from phosphoric


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acid solutions. In the case of TBP, the extraction of Zn(II)

(4.561023 mol/L) increased with the increase in the phosphoric acid

concentration, and rose to a maximum at 5.5 mol/L H3PO4 (O/A51,

t530 min). The further increase in the phosphoric acid concentrationdecreased the extent of Zn(II) distribution in the organic phase. This

decrease was attributed to the formation of less extractable Zn(H2PO4)+

species. The extraction of Zn(II) with TBP also depended upon the

temperature. The standard molar enthalpy of the reaction was calculated

to be 51.11 kJ/mol. DEHPA was used for the extraction of Zn(II)

(4.561023 mol/L) from 5.5 mol/L phosphoric acid solution. The equili-

brium constant of the extraction reaction was 0.2861022 mol1/2 L21/2.

Around 95% extraction of Zn(II) was observed after contacting the

aqueous and the organic phases (0.3 mol/L DEHPA) for 25 min.

Extraction of Zn(II) with KELEX 100 was a pH depending process. A

higher aqueous phase pH offered an increased extraction of Zn(II). The

kinetics of extraction was slow, and it took 240 min of contact time to

achieve equilibrium conditions. The addition of n-decanol improved the

rate of extraction. Only 30 min of contact time was sufficient to achieve

equilibrium conditions in the presence of 10% (v/v) modifier. The pH0.5for the extraction of Zn(II) (4.561023 mol/L) (extractant50.1 mol/L

KELEX 100) was 2.0¡ 0.1. The extraction improved from 65% (0.1 mol/

L) to 83% (0.4 mol/L) with the increasing extractant concentration.

IMPORTANT CHARACTERISTICS OF THE ZN(II) SOLVENT

EXTRACTION FROM SULPHATE, CHLORIDE, AND

PHOSPHATE MEDIA

The selective and quantitative extraction of Zn(II) can be achieved from

sulphate, chloride, and phosphate media. A number of commerciallyavailable extractants, including organophosphorus acids and oxides, high

molecular weight amine, and chelating reagents, are able to serve the

purpose. Organophosphorus extractants are definitely the widely

recognized Zn(II) extractants. Some of the reagents of this category,

namely DEHPA, PC-88A, CYANEX 272, and CYANEX 301, offer

strong extraction of Zn(II) from sulphate media. The extraction is pH

dependent, and the exchange of cations governs the mechanism of mass

transfer. The stripping of the extracted metal ion can be achieved with

dilute (1–2 mol/L) H2SO4. DEHPA, and CYANEX 301 are very goodreagents in terms of their selective nature with respect to a number of

generally associated metal ions, such as Mg(II), Cu(II), Co(II), Ni(II),

and Cd(II). However, the co-extraction of Fe(III) poses problems.

Though Zn(II) can be selectively stripped in the presence of Fe(III), the

regeneration of the extractant solution requires the removal of the ferric


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impurity by employing special steps. Most of the zinc processing

industries prefer the use of sulphuric acid as the leaching medium;

therefore, the liquid-liquid extraction of Zn(II) from the sulphate medium

has been thoroughly investigated. Xstrata Plc process, Teck Cominco’sHydroZinc process, and the modified ZINCEX process have been

successfully used on industrial scale.

The chloride leaching provides the effective recovery of zinc from

sulphides and oxides minerals and from secondary sources. The separation

of Zn(II) from this medium is achievable by a variety of extractants,

including organophosphorus acid and oxides, chelating reagents, and

amines. The mechanism of the extraction, and thus, the choice of the

extractant for the extraction of Zn(II) from chloride medium depend upon

the formation of different chloride species at different Cl2 concentrations.

Below 1 mol/L of Cl2, the cationic species Zn2+ dominates, which is

extractable by acidic extractants, such as DEHPA and CYANEX 302.

Above about 1 mol/L of Cl2, the main species are ZnCl422 and ZnCl3

2, and

these species can be extracted by basic extractants, such as Amberlite LA-2,

and Aliquat 336. Neutral species at higher acidity are extractable by

solvating reagents, e.g. TBP, DBBP, and CYANEX 923.

The extraction of Zn(II) from the phosphoric acid medium is a

relatively new concept. The removal of Zn(II) from the phosphoric acid

has been studied by the cation exchange and solvating reagents. Based on

the currently available information, DEHPA can be termed as the most

useful extractant providing effective extraction of Zn(II), coupled with a

convenient stripping.

AN OVERVIEW OF THE SOLVENT EXTRACTION OF ZINC

FROM THE PRIMARY RESOURCES

Currently, about 90% of the world zinc production is obtained through

hydrometallurgy. The conventional roasting-leaching-electrowinning

(RLE) method is still the main technology in use for the metal

production. Nevertheless, with the emergence of strict environmental

regulations, the feasibility of the roasting step is under question. It is also

no more a hidden fact that the available zinc sulphide resources are

rapidly exhausted and the current zinc market is facing supply deficit.

The exploitation of mixed sulphide and zinc oxide ores and secondary

sector materials has become quite significant. The integration of SX withthe new direct leaching techniques is a prospective solution to use the

leaner sources for a viable recovery of zinc. The SX also helps in

preventing the losses of valuable metals that occur with the traditional

precipitation methods. The technology can also be helpful in upgrading

the leaner electrolytes for their use in electrowinning cells. A successful


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implementation of SX at Skorpion mine, Namibia, has verified the

impetus of the technology.

Another useful aspect of SX in the zinc process industry may be the

separation of iron from the leaching solution. Current practice of jarosite

or goethite precipitation is flawed with the recurring landfill cost and

possible contamination of the soil.

The integration of new leaching and SX has been considered in

several recent projects. A test work is in progress at Mintek, South

Africa, on the direct sulphuric acid leaching and solvent extraction of zinc

from Mexican Sierra Mojada zinc oxide concentrate[88]. Outotec is all set

to design and supply technology for a 100,000 t/y zinc plant for Iran Zinc

Production Company in cooperation with Kahanroba (Iran), ABB

(Switzerland), and TAIM (Spain). This plant will employ a direct

leaching technique and SX with DEPHA[1]. Accha reserves of Peru are

being exploited by Southwestern Resources Corp. The modelling work is

in progress to test the compatibility of Skorpion process to the Accha

material. The experimental work is yielding favorable results as per the

latest available information[89].

ZINC SX FROM INDUSTRIAL WASTE AND SECONDARY

MATERIALS

Around 30% of the consumed zinc is recycled and this figure is bound to

increase as the environmental regulations on the discharge of metals become

stricter. Several industrial wastes are considered as the potential sources for a

profitable recovery of zinc. These wastes include the electric arc furnace dust

(EAFD), Waelz oxides, spent batteries, and galvanizing industry sludge. The

overall scenario indicates towards a worldwide awareness in processing the

secondary zinc materials. This review article provides some important and orrelatively new published research activities dealing with the use of solvent

extraction in the processing of secondary zinc.

Cole and Sole[1] have discussed about the treatment of hydrochloric acid

leachates of pyrite cinders and other zinc secondary materials at plants in

Bilboa, Spain, and in Quimigal, Portugal. This treatment utilized the

ZINCEX process based upon the extraction of Zn(II) with Amberlite LA-2

and DEHPA. In the first extraction stage of this process, Zn(II) was

extracted from chloride solutions using Amberlite LA-2. Some impurities,

e.g. Cu and Cd were co-extracted, but other metal ions, e.g. As, Ni, Co, andPb, remained in the raffinate. The loaded organic phase was subjected to the

stripping with water. The obtained stripping solution was again treated in a

DEHPA extraction circuit to extract Zn(II) selectively over Cu(II) and

Cd(II). The loaded DEHPA phase was then washed with dilute acid to

remove entrained chlorides. Zn(II) was finally recovered by stripping with


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the spent electrolyte of the electrowinning cell. The process thus provided an

advanced electrolyte containing 60 g/L of Zn(II).

The galvanic sludge has also been treated for the extraction and

removal of Zn(II) with DEHPA and CYANEX 272[90]. The sulphuric

acid leaching solution, containing Zn(II), Ni(II), Cu(II), and Cr(III), was

first subjected to the cementation and precipitation steps to remove

Cu(II) and Cr(III), respectively. The partially purified solution was then

treated for the solvent extraction of Zn(II). DEHPA proved to be a

stronger extractant than CYANEX 272. DEHPA offered more than 95%

extraction at pH 3 from a leaching solution containing 13 g/L of Zn(II).

Kinoshita et al.[91] have reported the recovery of Zn(II) from the

hydrochloric acid leaching solution of ashes of automobile tire waste. The

researchers have tested LIX 54, LIX 84, DEHPA, Alamine 336, TOA

(tri-n-octyl amine), and Aliquat 336. TOA provided the best recovery

(67%) at an acidity of 0.7 mol/L HCl. The stripping was performed with

water. The recovered solution was also free from the following impurities,

Fe, Al, Mn, Co, Cu, and Mg. Pareira et al. [92] have proposed the use of

DEHPA for the treatment of an industrial effluent produced by

Votorantim Co. (Brazil). Apart from containing 13.4 g/L of Zn(II), the

effluent was loaded with Cd(II),Co(II), Fe(III), Pb(II), Ca(II), Mg(II),

Mn(II), and Ni(II). Around 98% of initial Zn(II) was selectively removed

from the effluent using three extraction (pH 2.5, [D2EHPA]520% (w/w)

and A/O51), and three stripping stages (O/A54). The final solution

contained 125.7 g/L of Zn(II), which was suitable for the electrowinning.

Nonetheless, some contamination by Mg and Mn could not be prevented.

Furnace dust reprocessing has also been an interesting area

involving the solvent extraction of Zn(II). Lupi et al.[93] have presented

the data on a trial plant in Italy to recover zinc from EAFD. Zn and Fe

were simultaneously leached and subjected to a five-stage extraction

with 1 mol/L DEHPA. Zn(II) was stripped in seven stages with 1 mol/Lsulphuric acid. The co-extracted Fe(III) was removed by the stripping

under reducing conditions. Cole and Sole[1] have discussed the process

of a SX based EAFD processing plant at MetMax Peñoles, Mexico.

The plant produces 5000 t/a zinc cathode of 99.99% purity. Their

process involves sulphuric acid leaching of the dust, followed by the

cementation of the leaching solution with SrCO3, KMnO4, and zinc

dust to remove Pb, Fe, As, and Cd impurities, and finally extracting

Zn(II) with 12% (v/v) DEHPA at an O/A ratio of 1.5/1 maintaining the

pH to 2.5 to 3. The loaded organic phase has to be washed with waterto remove entrained chlorides. Zn(II) is recovered by the stripping with

the spent electrolyte.

The recovery of zinc from the spent batteries and the rayon effluent has

also drawn some research interests. The R. F. Procés Plant in the Spanish

province of Cataluña uses DEHPA for the treatment of the spent domestic


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batteries[1,94]. The process is detailed in Fig. 5. The domestic batteries are

collected, classified, and then crushed to obtain the zinc-manganese dust.

The feed solution is prepared by leaching the zinc manganese dust at pH 1.8

to 2.2. Fe(III) is then removed by precipitating the solution with KOH,followed by the cementation of Ni and Hg with Zn dust. The resulting

aqueous phase is subjected to the extraction with 15% (v/v) DEHPA in three

stages. After the extraction, the loaded organic phase is treated in the two

scrubbing stages. Water is first used to remove the entrained feed solution,

followed by a small portion of loaded Zn(II) strip liquor to remove the co-

extracted Ca(II) and Mg(II). Zn(II) is finally recovered from the organic

phases by a two-stage stripping with 2 mol/L H2SO4, thus obtaining an

advanced electrolyte containing 140 g/L Zn(II).

Some researchers have discussed the treatment of the spent batteries

with CYANEX 301[95,96]. The mechanism of the Zn(II) extraction is the

same as that in the DEHPA process. However, the pH is not necessarily

Figure 5. Recovery of zinc from spent domestic batteries.


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controlled in that narrow range. In this treatment, the co-extraction of

Cu(II) and Fe(III) in the organic phase deteriorates the regeneration

capacity of CYANEX 301. El-Nadi et al.[97] have worked on the recovery

of Zn(II) from the black paste of the spent MnO2-Zn cell batteries.Sulphuric acid was found to be a better leaching medium in this

particular case. Both Zn(II) and Mn(II) were extracted into the organic

phase, but the extraction of Zn(II) was more favorable. The selective

stripping of Zn(II) was achieved with 5 mol/L HCl. The overall Zn(II)

recovery was 99%. Salgado et al.[98] have discussed a hydrometallurgical

process for the recovery of Zn(II) and Mn(II) from the spent alkaline

batteries using CYANEX 272. The dry black powder of the batteries,

containing 19.5%, w/w Zn and 31.1%, w/w Mn along with some fractions

of K, Na, and Fe, was leached with 0.05% (v/v) H2SO4 (S/L ratio510%).The obtained leaching solution, containing 18.96 g/L of Zn(II) and

12.95 g/L Mn(II) was extracted in two stages with 20% (v/v) CYANEX

272 at pH 2.5 maintaining the reaction temperature to 50uC. Almost 90%

of Zn(II) was recovered leaving Mn(II) in the raffinate.

The recovery of Zn(II) from the rayon industry effluent has been

investigated by some researchers. Around three decades back, Reinhardt

et al.[99] developed the Valberg process. This process was based upon the

solvent extraction of Zn(II) with dioctyl phosphoric acid and DEHPA. For

some years, this process was in operation at two industrial plants. Therequirement of a relatively high pH of the aqueous solution for the

extraction with DEHPA is considered a disadvantage of the Valberg

process. Recently, CYANEX 272 and CYANEX 302 have been

investigated [100]. Since the waste contains Ca(II), it is considered that

CYANEX 302 is a better extractant as it provides an improved Zn(II)/

Ca(II) separation factor. More than 99% of Zn(II) was extracted from the

effluent at an equilibrium pH of higher than 3.4 (O/A51/30). The stripping

of the extracted metal ion was achieved by 10% (v/v) H2SO4. Another

report by Ali et al.[101] on the use of CYANEX 272 for the treatment of rayon industry waste discussed several optimization parameters. The

addition of ammonium sulphate in the aqueous phase was observed to

improve the extraction percentage of Zn(II) in CYANEX 272. Four stages

of extraction ensured the loading of extractant in proportion of 0.105 mol/L

metal ion per mole of CYANEX 272. The increasing temperature of the

reaction environment slowed down the kinetics of the metal extraction. A

moderately concentrated HCl or HNO3 solution provided an effective

stripping of the extracted metal ion.

CONCLUSIONS

The industrial zinc leaching solutions are invariably loaded with a number of

metal ions. The composition of leaching solutions from the leaner sources is


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even more complex. The conventional precipitation and cementation

methods for the refining of zinc cannot always be conveniently applied to

the aqueous streams of diverse compositions. Solvent extraction, coupled

with the new innovative direct leaching, ammonia leaching, and bioleachingprocesses, is quite effective for the useful exploitation of several sulphidic,

non-sulphidic, low-grade, and secondary zinc resources. The introduction of

SX in the process flow sheet provides certain advantages, e.g., minimizing

the loss of metal values, curtailing the solid waste generation and attaining a

high purity electrolyte solution. Some of the commercially available Zn(II)

extractants, such as DEHPA and CYANEX 301, are very strong

extractants, but their inability of rejecting Fe(III) is seen as a limitation.

CYANEX 301 is also susceptible of degradation in the presence of Cu(II).

However, the prior removal of these two impurities may provide an

opportunity to exploit DEHPA and CYANEX 301 in a really advantageous

way. CYANEX 272, CYANEX 923, and PC-88A also seem to be the

reagents of choice. All these three extractants have reasonably fast extraction

kinetics and can be easily regenerated from several metal ions, including

Fe(III) and Cu(II). CYANEX 272 and CYANEX 923 are selective for

Zn(II) over a number of metal ions, including Co(II), Ni(II), Mn(II), Ca(II),

and Mg(II). The problem of co-extraction of Fe(III) with Zn(II), can be

tackled by a prior removal of Fe(III) or by the selective stripping of Zn(II).

Solvent extraction is also a potential technique to exploit secondary

sector materials for the recovery of high purity zinc. Successful tests have

been carried out for the spent domestic batteries and the furnace dusts.

The feasibility studies have proven the technique to be economically

viable. CYANEX 923 is a promising option to recover pure zinc from the

industrial effluents and wastes in a chloride medium. This extractant is

selective for Zn(II) over Mn(II), Co(II), Ni(II), Cu(II), Cd(II), and

Hg(II).

ACKNOWLEDGEMENTS

AD acknowledges the postdoctoral fellowship (SFRH/BPD/20321/2004)

from the FCT, Portugal. We are thankful to the Portuguese innovation

agency (ADI; PRIME-IDEIA, Project no. 70/00191), and Somincor,

Portugal.

LIST OF ABBREVIATIONS

ACORGA ZNX 50 60% wt. of the active substances consisting of

unreacted dimethyl bibenzimidazole, dimethyl

1-mono(tridecyloxycarbonyl)-2,2’-bibenzimidazole


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and dimethyl 1,1’-bis(tridecyloxycarbonyl(22,2’-

bibenzimidazole dissolved in Cl0-Cl5 hydrocar-

bons.

ALAMINE 336 tricaprylyl amine

ALIQUAT 336 tricaprylmethylammonium chloride

Amberlite LA-2 n-lauryl-(trialkyl-methyl) amine

CYANEX 272 bis-(2,4,4-trimethylpentyl)phosphinic acid

CYANEX 302 bis-(2,4,4-trimethyl pentyl) monothiophosphinic

acid

CYANEX 301 bis-(2,4,4-trimethyl pentyl) dithiophosphinic acid

CYANEX 923 93% pure mixture of four trialkylphosphine

oxides: R3

P5O, R9R2

P5O, R2

R9P5O, and

R39P5O, where R and R9 represent n-octyl and

n-hexyl hydrocarbon chains

DBBP dibutylbutyl phosphonate

DEHPA di-(2-ethylhexyl) phosphoric acid

KELEX 100 7-(4-Ethyl-1-methylocty)-8-hydroxyquinoline

LIX 54 six isomeric compounds of 1-phenyldecane-1,3-

diones, heptane-8,10-dione, 1,3-diphenylpropane-

1,3-dione

LIX 84 2-hydroxy-5-nonyl-acetophenone oximeMIBK methyl isobutyl ketone

N1923 primary amine with the following formula:

R1R2CHNH2, the total number of carbon atoms

is 19–23

OPAP octylphenyl acid phosphate

PC-88A 2-ethylhexyl phosphonic acid mono-2-ethyl hexyl

ester

SOLVESSO 50 hydrocarbon diluents with .99% aromatics

TBP tri-n-butyl phosphateTOA trioctylamine

TOPO tri-n-octyl phosphine oxide

REFERENCES

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Documents

Review on the Recent Developments in the Solvent Extraction of Zinc