Base Metals Recovery From Copper Smelter Slag by Oxidising Leaching and Solvent Extraction

Base metals recovery from copper smelter slag by oxidising

leaching and solvent extraction

A.N. Banza a,*, E. Gock a, K. Kongolo b,c

aInstitute for Mineral Processing and Disposal Technology, Technical University of Clausthal,

Walther-Nernst-Strasse 9, D-38678 Clausthal-Zellerfeld, GermanybUniversity of Lubumbashi, Lubumbashi, D.R. Congo

cGecamines Metallurgical Research Centre, Likasi, D.R. Congo

Received 19 February 2002; received in revised form 20 August 2002; accepted 21 August 2002

Abstract

Due to its amorphous structure, smelter slag cannot efficiently be leached with sulphuric acid; the formation of silica gel

induces an increase of leach liquor viscosity, difficult pulp filtration and crud formation during solvent extraction. The problem

was solved by leaching with sulphuric acid under hydrogen peroxide, which also performs simultaneous iron oxidation and

removal.

A copper smelter slag from Lubumbashi, Democratic Republic of Congo, containing 1.4% Cu, 0.7% Co, 8.9% Zn and

20.9% Fe(II), has been used in this study. The leaching tests have been carried out at normal pressure on ground slag samples

( < 100 Am). Silica gel-free solutions containing copper, cobalt and zinc were produced, so that pulp filtration could be easily

carried out.

The dissolved base metals were successfully extracted from solution by solvent extraction using kerosene Shellsol D70 as

diluent. Copper was extracted with LIX 984 and stripped with sulphuric acid solution. Thereafter, cobalt and zinc were

collectively extracted with D2EHPA and then separated by selective scrubbing with sulphuric acid solutions of different

dilutions. This method provided an overall recovery of 80% Cu, 90% Co and 90% Zn in separated solutions which could be

further treated by electrowinning or salt precipitation.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Copper smelter slag; Copper–cobalt –zinc recovery; Sulphuric acid leaching; Hydrogen peroxide; Solvent extraction

1. Introduction

Various hydrometallurgical methods using lixi-

viants such as acids, bases and salts have been

developed for base metal extraction from smelter

slags. Aydogan et al. (2000) obtained more than

92% Cu from Hafik–Madentepe copper slag contain-

ing 2.62% Cu by sulphuric and ammonia leaching at

95 jC. Gbor et al. (2000) achieved 77% Co and 35%

Ni from INCO nickel smelter slag containing 0.13%

Co and 0.29% Ni by aqueous sulphur dioxide leach-

ing. More than 80% of Cu and Co could be extracted

from different slags by leaching with H2SO4, FeSO4,

(NH4)2SO4, FeS2, NaCl or FeCl2 after roasting (Her-

ros et al.,1998; Anand et al., 1981; Tumen and Bailey,

1994; Sukla et al., 1986). Anand et al. (1983)

extracted more than 90% of Cu, Co and Ni from

0304-386X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0304 -386X(02 )00138 -X

* Corresponding author. Fax: +49-5323-72-2353.

E-mail address: [email protected] (A.N. Banza).

www.elsevier.com/locate/hydromet

Hydrometallurgy 67 (2002) 63–69

Ghatsila smelter copper slag containing 4.03% Cu,

0.48% Co and 1.98% Ni by pressure leaching at 130

jC. Information on iron coextraction and silica gel

formation is missing in most of these studies. During

sulphuric acid leaching of slag, silica gel is formed

according to reaction (1). The presence of silica gel

not only prejudices metal extraction and pulp filtra-

tion, but also causes crud formation during solvent

extraction.

2FeO � SiO2 þ 2H2SO4 ! 2FeSO4 þ H4SiO4 ð1Þ

Nonferrous metal extraction, iron removal and

production of good filterable pulp have been per-

formed in this study by sulphuric acid leaching in

the presence of hydrogen peroxide at normal atmos-

phere. Sulphide and iron oxidation takes place in

leaching system according to reactions (2) and (4).

Dissolution of iron(II) from slag is expressed by

reaction (3).

CuSþ 2H2SO4 þ 2H2O2

! CuSO4 þ 2H2SO3 þ 2H2O ð2Þ

FeOþ H2SO4 ! FeSO4 þ H2O ð3Þ

2FeSO4 þ H2O2 þ 2H2O ! 2FeOOH þ 2H2SO4

ð4Þ

After filtration, the leach liquor was treated by

solvent extraction using classic extractants such as

LIX 984 for copper and D2EHPA (di-2-ethylhexyl

phosphoric acid) for cobalt and zinc.

2. Experimental

2.1. Material

The slag sample used in this investigation is a

composite from different levels of the copper slag

dump in Lubumbashi, Katanga, Democratic Republic

of Congo, whose chemical composition is given in

Table 1. A mineralogical study by Banza et al.

(2002a,b) showed that this slag has a fayalite struc-

ture. More than 80% of copper is in the form of

sulphides. Cobalt is finely disseminated in the slag

matrix. Zinc is present as ferrites and silicates.

2.2. Leaching of the slag

The leaching batch tests of ground slag ( < 100 Am)

with a combination of H2SO4 and H2O2 (50 vol.%)

were done in a 1-L round bottom glass reactor with

four openings serving for a pH electrode, a condenser

tube, a thermometer and a mechanical stirrer. Hydro-

gen peroxide was continuously added to the pulp

during the leaching. The effect of leaching time,

temperature, oxidant dosage and potential (Eh) has

been investigated. The leach liquor and the residues

were analysed for copper, cobalt, zinc and iron by

AES/ICP or AAS.

2.3. Solvent extraction

Selective metal extraction from aqueous phase

containing 1.99 g/L Cu2 + , 1.04 g/L Co2 + , 12.58 g/

L Zn2 + and 2.57 g/L Fe3 + from the above-described

oxidising leaching was carried out in glass vessels.

The extractants LIX 984 for copper and D2EHPA for

zinc and cobalt were used in this investigation. Ker-

osene Shellsol D70 was used as diluent in both cases.

These reagents were supplied by Henkel KG Dues-

seldorf, Germany. After copper extraction, iron was

removed from solution with calcium carbonate.

Cobalt and zinc were then simultaneously extracted

and thereafter separated by selective stripping with

sulphuric acid solution of different concentrations.

3. Results and discussion

3.1. Leaching of the slag

3.1.1. Sulphuric acid leaching of copper smelter slag

Leaching tests were carried out at pH 2.5 on

ground slag samples ( < 100 Am). Figs. 1–4 show

Table 1

Chemical composition of the sample from the Lubumbashi copper

slag dump

Constituent Cu Co Zn Fe S Si Ca Mg Al

Amount (%) 1.43 0.72 8.90 20.70 0.59 15.37 6.26 2.53 2.56

A.N. Banza et al. / Hydrometallurgy 67 (2002) 63–6964

the reacted fraction of copper, cobalt zinc and iron

during leaching in the temperature range between 24

and 80 jC.About 60 min was required to obtain high copper,

cobalt and zinc extraction. Temperature increase

from 24 to 60 jC significantly increased metals

leaching recoveries, especially for copper. The effect

of further temperature increase, from 60 to 80 jC,

was negligible in all cases. Filtration of the resulted

pulp from the leaching at 24 and 60 jC was very

difficult because of presence of silica gel. The silica

content in the leach liquor varied between 3 and 8 g/

L depending on acid content. Best leaching results

were obtained at 80 jC with metals recoveries of

around 60% Cu, 90% Co, 90% Zn and 90% Fe after

2 h.

Fig. 2. Temperature effect on cobalt leaching recovery (10% solid,

particle size: 100% < 100 Am, pH 2.5, H2SO4 consumption: 500 kg/t

slag).

Fig. 3. Temperature effect on zinc leaching recovery (10% solid,


slag).

Fig. 4. Temperature effect on iron leaching recovery (10% solid,


slag).

Fig. 1. Temperature effect on copper leaching recovery (10% solid,


slag).

A.N. Banza et al. / Hydrometallurgy 67 (2002) 63–69 65

3.1.2. Leaching at constant flow rate of hydrogen

peroxide

The H2O2 flow rate was varied from 0 to 125 L/t

slag. Fig. 5 shows the metals leaching recoveries at 70

jC and pH 2.5 as a function of H2O2 consumption.

Around 62.5 L/t slag was the optimal dosage of H2O2.

This is higher than the stoichiometric need for oxida-

tion of copper sulphide and ferrous iron according to

reactions (2) and (4). Without adding of hydrogen

peroxide, pulp filtration was difficult. H2O2 addition

to the leaching system considerably decreased iron

dissolution from over 90% to less than 5%, while it

increased the copper recovery from 60% to 85%.

Cobalt and zinc recoveries were not affected.

Fig. 6 shows the metal extraction as a function of

leaching time at 35 L H2O2/(h�t). The Eh values

measured during the leaching were 292, 358, 434,

622 and 645 mV at 15, 30 60, 90 and 120 min,

respectively. Copper extraction and iron removal are

proportional to H2O2 dosage in the first 2 h. Around

180 min was required to obtain the highest extraction

of copper, cobalt and zinc and the lowest iron dis-

solution. Although the long leaching time was neces-

sary for iron and silica gel removal, no significant

increase of the base metals recoveries could be

observed after 120 min. The dissolved iron quantity

increased during the first 10 min, decreased strongly

after 15–30 min and slowly down after around 90

min. This could be due to the solution potential Eh

which was higher than 600 mV after 90 min. In fact,

low potentials prevailing before this time are not

suitable for iron oxidation. Ney (1973) reported that

some metal oxides can modify the negative zeta

potential of silica suspension. Simultaneous silica

gel formation and iron removal as goethite have

produced a good filterable pulp. The leach liquor at

pH higher than 2 contained less than 0.5 g/L Si. In

contrast to the leaching with H2SO4, no gel formation

was observed after 1 year storage of leach liquor

obtained with H2SO4–H2O2 combination. Metal

extractions of more than 80% Cu, 90% Co, 90% Zn

and 5% Fe were recorded at 70 jC and pH 2.5.

According to Dutrizac and Monhemius (1986), the

goethite stability field is situated in the pH range

higher than 2 and the temperature range 40–100

jC. Although higher cobalt, copper and zinc recov-

eries and iron removing could be obtained at pH value

lower than 2, pulp filtration became difficult in this

pH range. Similar results were recorded by treating

the same slag material with the use of digestion

method and roasting (Banza, 2001; Gock et al.,

2001; Banza et al., 2002a,b).

3.1.3. Leaching at constant potential Eh

The leaching tests have been conducted at pH 2.5

and 70 jC to investigate the effect of potential Eh on

Fig. 5. Metal recovery at different H2O2 dosages by oxidising

leaching with H2SO4 of copper slag (10% solid, particle size: 100%

<100 Am, pH 2.5, temperature: 70 jC, leaching time: 120 min).

Fig. 6. Kinetic of metal recovery by oxidising leaching with H2SO4

of copper slag (10% solid, particle size: 100% < 100 Am, pH 2.5,

temperature: 70 jC, H2O2: 35 L/(h�t)).


the leaching result. The potential was varied in the

range 450–650 mV by H2O2 addition. Fig. 7 shows

that iron was better removed at higher potential

values. According to Pourbaix diagrams, oxidation

of ferrous ion at 25 jC is favourable at high potentials.

Fig. 8 also indicates that leaching at high potentials

can reduce the leaching time: 65% copper was

extracted after 180 min at 450 mV and only 60 min

at 650 mV. At 450 mV, iron dissolution remained

constant at approximately 15% independently of the

leaching time. Copper dissolution significantly in-

creased with the potential increase from 450 to 650

mV, while iron dissolution simultaneously decreased.

3.2. Metal recovery by solvent extraction

3.2.1. Copper extraction

Ritcey and Ashbrook (1984) have reported that

the presence of both solids and colloids such as silica

in solvent extraction feed to can produce stable

emulsions and cruds during the mixing of organic

and aqueous phases. This fact decreases mass transfer

effects. Copper extraction was carried out using the

solution given in Table 2 obtained under optimal

oxidising leaching conditions. Rodriguez de San

Miguel et al. (1997) have found the selectivity of

LIX 984 as copper extractant in the pH range 0.5–

2.5. Copper extraction was carried out in three stages

with 12% v/v LIX 984 diluted in kerosene Shellsol

D70 at 25 jC, at pH 2.5 and a phase ratio A/O of

Fig. 7. Metal recovery as a function of potential Eh by oxidising

leaching with H2SO4 of copper slag (10% solid, particle size: 100%

< 100 Am, pH 2.5, temperature: 70 jC, leaching time: 180 min).

Fig. 8. Copper and iron recovery as a function of leaching time by

oxidising leaching with H2SO4 of copper slag (10% solid, content

size: 100% < 100 Am, pH 2.5, temperature: 70 jC).

Table 2

Copper extraction with LIX 984 in three stages and copper stripping

with H2SO4 in three stages

Element Cu Co Zn Fe

Feed (mg/L) 1986 1030 12,578 2586

Copper extraction (%) A/O 1:2 98.6 0.76 0.48 0.09

A/O 1:1 99.3 0.81 0.62 0.10

A/O 2:1 99.4 0.72 0.58 0.11

Copper stripping (%) A/O 2:1 99.2 0.79 0.59 0.08

A/O 1:1 99.0 0.80 0.52 0.06

A/O 1:2 98.2 0.76 0.54 0.07

A/O 1:4 98.0 0.78 0.46 0.05

Table 3

Cobalt–zinc extraction with D2EPHA in two stages and cobalt –

zinc stripping with H2SO4 in three stages

Element Cu Co Zn Fe

Feed (mg/L) 3.90 1010 11,800 53.8

Cobalt and zinc A/O 1:2 – 97.4 99.7 –

extraction (%) A/O 1:1 – 96.1 99.8 –

A/O 2:1 – 95.8 98.8 –

Cobalt stripping (%) A/O 1:1 – 95.0 0.09 –

A/O 1:4 – 94.4 0.07 –

Zinc stripping (%) A/O 1:1 – 0.91 97.5 –

A/O 1:4 – 0.65 95.4 –


1:1. More than 99% copper was extracted with less

than 1% coextraction of cobalt, zinc and iron. Copper

stripping was done in two stages at the phase ratio

A/O of 1:1.

3.2.2. Zinc and cobalt extraction

After copper extraction, iron was removed by

precipitation in a round glass vessel at 85 jC and

pH 3.5 with addition of a 300 g/L CaCO3 suspen-

sion. About 99% iron was removed with cobalt and

zinc coprecipitation of around 3% and 5%, respec-

tively. Residual iron content in solution was only 54

mg/L. Cobalt and zinc were thereafter collectively

extracted in two stages using 20% v/v D2EPHA in

kerosene Shellsol D70 at 25 jC, pH 3.5, and a phase

ratio A/O of 1:1. Extraction and stripping results are

given in Table 3. Cobalt stripping from organic phase

was carried out in two stages with diluted sulphuric

acid at 25 jC, pH 2.5. Afterwards, zinc was stripped

at pH 1. More than 96% Co and 99% Zn were

extracted.

Based on the present study a schematic flow sheet

is given in Fig. 9.

4. Conclusions

This study clearly demonstrates that oxidising

sulphuric acid leaching under hydrogen peroxide at

70 jC and normal pressure can effectively be used to

extract base metals from amorphous copper smelter

slags after grinding at 100% < 100 Am. By using this

method, both silica gel formation and iron coextrac-

tion are avoided. Silica gel-free solutions and good

filterable cakes are obtained.

Selective copper solvent extraction with LIX 984

followed by collective cobalt–zinc extraction by

D2EPHA with subsequent cobalt–zinc separation by

selective stripping were successfully performed with-

out any crud formation. The use of an additional

extractant for cobalt was not necessary by applying

this method.

Fig. 9. Proposed flow sheet for the retreatment of copper smelter slag by oxidising leaching and solvent extraction.


Overall recoveries of approximately 80% Cu, 90%

Co and 90% Zn in separate solutions were achieved

by applying this process. The recovery of metals from

their respective solutions can be carried out by elec-

trowinning or salt precipitation.

Acknowledgements

The authors would like to express their gratitude to

Gecamines for supplying the slag sample which was

used in this study. The award of a doctorate fellowship

to A.N. Banza by the German Academic Exchange

Service (DAAD) is gratefully acknowledged.

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Base Metals Recovery From Copper Smelter Slag by Oxidising Leaching and Solvent Extraction

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