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Recovery of copper, cobalt, and zinc from copper smelter

and converter slags

Cuneyt Arslana,*, Fatma Arslan b

a Extractive Metallurgy Division, Metallurgical and Materials Engineering Department, Istanbul Technical University, 80626 Maslak,

Istanbul, Turkey b Mineral and Coal Processing Division, Mining Engineering Department, Istanbul Technical University, 80626 Maslak, Istanbul, Turkey

Received 4 February 2002; received in revised form 7 June 2002; accepted 9 June 2002

Abstract

A study on the recovery of copper, cobalt, and zinc from copper smelter and converter slags by roasting with sulfuric acid has

been conducted. Acid roasting of slags followed by hot water leaching was carried out to bring the metal values into solution. In

the leaching experiments, the effects of roasting time, acid/slag ratio, roasting temperature, and application of thermal

decomposition prior to leaching on the metals dissolution extents were investigated. A total of 88% of copper, 87% of cobalt,

93% of zinc, and 83% of iron were extracted in 2 h of roasting at 150 jC and 3:1 acid/slag ratio. Increasing acid roasting

temperature and time did not have increasing effect on the Co and Zn dissolution extents, while significant improvements were

observed in Cu dissolution. Application of thermal decomposition prior to leaching gave small decreases in metal extractions, but since there was no iron dissolution, it was favoured from the viewpoint of metal recoveries from the leachates due to the

elimination of an iron removal step.

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

Keywords: Copper smelter; Converter slags; Cobalt; Zinc

1. Introduction

The flash smelting process has been used in copper

industry for a number of years and has replaced most of

the reverberatory applications, known as ‘‘conven-tional’’ copper smelting processes. In this process,

due to high copper lockup in the slag, a separate

processing system consisting of electric furnace slag

cleaning, converter for copper matte blowing and fire

refining is required (Traulsen et al., 1982; Jones and

Deneys, 1998). Recycling copper converter slag back

to the flash furnace has been practised in some plants

and it leads to an increase in the matte grade (Antonioni

et al., 1982).

In some plants, flash smelting furnace slag (1.6–

2% Cu) and converter slag (5–7% Cu) are cleaned inan electric arc furnace, where coke is used as reductant

and copper content of waste slag decreases to 0.5–

0.8% (Kim, 1982; Suziki et al., 1982). Ausemelt top

entry submerged lance technology is also used to

recover Cu, Ni, and Co from slags (Hughes, 2000;

Vernon and Burks, 1997).

The flotation process plays an important role in the

evaluation of slags and is utilized in some copper

smelting processes (Bota et al., 1995; Edlund and

Hussey, 1972).

0304-386X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.P I I : S 0 3 0 4 - 3 8 6 X ( 0 2 ) 0 0 1 3 9 - 1

* Corresponding author.

E-mail address: [email protected] (C. Arslan).

www.elsevier.com/locate/hydromet

Hydrometallurgy 67 (2002) 1–7

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There are also some hydrometallurgical methods

given in the literature for the treatment of slags, such as

leaching in nitrate, perchloride, chlorate, and sulfuric

acids with and without pressure, ferric chloride, cya-nide, ferric sulfate, and ammonia solutions (Lindblad

and Dufresne, 1974; Jia et al., 1999; Shelley, 1975;

Prater et al., 1970; Kayadeniz and Sagdık, 1980).

Smelter slag and sulfur dioxide are the waste prod-

ucts of nonferrous pyrometallurgical processes. Disso-

lution behavior of Co, Cu, Fe, Ni, and Zn from smelter

slag in aqueous sulfur dioxide has also been studied

(Ahmed et al., 2000). Dissolution behavior of Cu is

found to be fast, but subsequent precipitation lowers its

dissolution.

Studies on the recovery of Cu, Co, Ni, and Zn from

copper converter slags by roasting with ferric sulfate,

pyrite, ammonium sulfate, and sulfuric acid followed

by leaching have been reported (Altundogan and Tu-

men, 1997; Tumen and Bailey, 1990; Sukla et al., 1986;

Tumen, 1994).

In the Black Sea Copper Works (Samsun, Turkey)

about 35,000 tons of blister copper is produced annu-

ally along with approximately 100,000 tons of slag,

which is dis posed to a slag area and left there for 24 h

for cooling (Sirkeci, 1998). Then, slag is treated by

flotation after being crushed and ground. The flotation

concentrate is fed to the flash smelting furnace. How-ever, important metals such as Co and Zn are recycled

continuously from slag to concentrate (and vice versa)

and some of them are lost in tailings. Metal contents of

smelter and converter slags, which materialize in cop-

per smelting plants, are very high and should be recov-

ered from an economic viewpoint. The aim of this

research is to investigate the possibility of recovering

Cu, Co, and Zn present in the slag through a pyro-

hydrometallurgical route as an alternative to flotation.

A mixture of flash smelting and converter slags was

subjected to the experimental study and treated by acidroasting followed by either direct hot water leaching or

thermal decomposition and then hot water leaching

techniques.

2. Theoretical considerations

Flash furnace and converter slags, usually fayalitic

based, contain Cu, Co, and Zn in the form of silicates

and ferrites, as determined from the X-ray diffraction

patterns (Arslan, 1982). Although copper in the slag is

usually ferritic in nature, appearance of bornite (Cu5

FeS4), a metastable phase, may be explained by the

matte inclusions in the slag. Moreover, metallic copper is also observed by the microscopic analysis of these

slags. Standard free energy values (Barın et al., 1977)

for the reactions, which probably occur between some

noticeable compounds existing in the slag and either

liquid or gaseous H2SO4, are plotted vs. temperature in

Fig. 1a and b, respectively.

Usually, the standard free energies of reactions

between silicates (and ferrites) and H2SO4 are negative

and decrease as the temperature rises. In contrast,DG Tj

values of some silicates, such as ZnOÁSiO2 and

2ZnOÁSiO2 approach zero as the temperature increases.

As seen from Fig. 1, thermodynamically speaking,

the possibility of the reactions to occur between metal

silicates and sulfuric acid is decreasing in the order of

2FeOÁSiO2, 2CoOÁSiO2, and 2ZnOÁSiO2 and between

ferrites and silicates increasing in the order of ZnOÁ

Fe2O3, NiOÁFe2O3, CoOÁFe2O3, and CuOÁFe2O3. On

the other hand, the standard free energy change of the

reaction between bornite (Cu5FeS4) and H2SO4(l) rap-

idly goes toward negative values at high temperatures.

In standard conditions, DG Tj values of reactions

between gaseous H2SO4 and silicates (and ferrites) are

more negative than those of with H2SO4(l). Thermody-namic possibility of reactions to occur follows the

same order in both cases. However, the possibility of

reactions to occur with H2SO4(l) seems to be more

likely since the vapor pressure of H2SO4 is not going

to reach 1 atm until its boiling temperature (Interna-

tional Critical Tables, 1978). Since the vapor pressure

of H2SO4 over 200 jC is high, the possibility of

reactions with H2SO4(g) is high. On the other hand,

the possibility of formation of water during the reac-

tions with liquid or gaseous H2SO4 is considerable.

The water formation will decrease the H2SO4 concen-tration; the vapor pressure of H2SO4 at lower H2SO4

concentrations will be lower at the same temperatures,

and thus, the reactions with H2SO4(l) seem to be more

likely up to 200 jC.

Fig. 2 shows the stability regions of various metal

sulfates, basic metal sulfates, and metal oxide phases

in relation to roasting temperature and partial pressure

of SO2 (Addemir, 1978). Considering the constant

partial SO2 pressure, increasing roasting temperature

causes the decomposition of metal sulfates to metal

C. Arslan, F. Arslan / Hydrometallurgy 67 (2002) 1–7 2

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Fig. 1. DG Tj – temperature relationship of some silicates and ferrites with liquid and gaseous H2SO4.

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oxides. Thermal decomposition of metal sulfates are in

the order of Fe2(SO4)3, CuSO4, ZnSO4, CdSO4, and

PbSO4.

3. Experimental

3.1. Material

A mixture of flash smelter and converter slagssubjected to this experimental study was taken from

the feed to the flotation unit in the Black Sea Copper

Works, Turkey. Chemical analysis of the slag sample

is: 2.64% Cu, 0.095% Co, 0.67% Zn, 47.2% Fe,

0.13% Pb, 0.004% Cd, 0.065% Ni, 8.5% Si, and

1.3% S.

By examining the smelter and converter slag sam-

ples with optical microscopy and by X-ray diffraction

analysis, it was found that copper exists as bornite and

ferrites, and also in metallic form, though in small

quantities, where zinc, cobalt, and nickel were in the

form of silicates and ferrites, while iron formed as a

fayalite-type matrix.

3.2. Method

Samples were first crushed and ground before the

tests so that 80% of them would pass below 0.1 mm. In

the acid roasting experiments, a Heraeus Roka type

tube furnace was used. A sample of 5 g was mixedwith varying amounts of acid and placed into a quartz

boat (10 cm length Â 1.5 cm width), which was then

inserted into the tube furnace with a known temper-

ature profile. Air was blown at a rate of 2.5 L minÀ 1 to

sweep the reaction gases out of the tube. After samples

were cooled to room temperatures, leaching was car-

ried out in 500 mL water at 70 jC and 400 – 450 minÀ 1

for 1 h. Acid leaching experiments done after thermal

decomposition were carried out in 100 mL solution

containing 20 g/L H2SO4 at 70 jC and 400–450

Fig. 2. Thermodynamic equilibrium conditions for sulfate decompositions in Fe–Cu–Zn–Cd–S–O system, under 1 atm total and P O2= 0.2 atm

partial pressure (reproduced from Addemir (1978)).


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minÀ 1 for 1 h. Two experimental routes were followed

after the acid roasting step, namely, hot water leach-

ing + filtration, and thermal decomposition + hot water

leaching + filtration. Standard glassware, a heater +magnetic stirrer, and a contact thermometer were

employed in leaching experiments. After leaching,

the pulp was filtered and all chemical analyses were

made using an Atomic Absorption Spectrophotometer.

4. Results and discussion

The effects of roasting time, acid/slag ratio, and

roasting temperature on the dissolution extents of

metals were investigated in the leaching experiments.

Increasing acid/slag ratio shifted metal dissolution

extent–roasting time curves to higher values except

copper. A 3:1 acid/slag ratio was to be the optimum

(Arslan, 1982). Fig. 3 shows the effect of roasting time

on metal dissolutions at 150 jC and 3:1 acid/slag ratio.

Under these conditions, 80% of Co, 83% of Zn, and

81% of Fe were dissolved. Increasing roasting time to

4 h increased Cu dissolution to 95%.

Fig. 4 shows the effect of roasting temperature on

metal dissolutions at 3:1 acid/slag ratio and 2 h of

roasting time. In all cases, roasting temperature of 150

jC was chosen to be the optimum. Under these con-ditions 87% Cu, 87% Co, 93% Zn, and 84% Fe were

dissolved.

Copper flash smelting and converter slags contain

considerable amounts of iron in the forms of silicate,

magnetite, and metal oxy-ferrites as observed in the

case of acid roasting experiments, and they were able

to be transformed into the soluble compounds by sul-

fatizing as a result of roasting with concentrated acid.

However, the main purpose of this study was to

dissolve Cu, Co, and Zn while keeping Fe in the solid

residue as an undissolved compound. Thus, the fol-lowing experiments aimed at producing insoluble iron

oxides by thermally decomposing iron sulfates,

formed during acid roasting stage, at a temperature at

which not all other metal sulfates are affected. For this

purpose, slag sample roasted at 250 jC and 3:1 acid/

slag ratio was subjected to the thermal decomposition

at 600, 650, and 700 jC for 0.5–4 h and by blowing

2.5 L/min air to the furnace and then dissolving in hot

water followed by filtration.

As seen from Fig. 2, increasing roasting temper-

atures caused an increase in the decomposition of metal sulfates to oxides. As a result of this, metal

dissolutions dropped significantly by increasing ther-

mal decomposition temperature. All metal decompo-

sitions at given temperatures were investigated, and

650 jC was found to give the best results in terms of

Cu, Co, and Zn dissolutions, where Fe dissolution was

the lowest (Arslan, 1982).

Fig. 5 shows the change in the metal dissolution

extents with time in the leaching experiments with 20

g/L H2SO4 at 70 jC after thermal decomposition at Fig. 3. Effect of roasting time on metal dissolutions at 150jC and 3:1

acid/slag ratio.

Fig. 4. Effect of roasting temperature on metal dissolutions at 3:1acid/slag ratio and for 2 h of roasting time.


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solutions. Lower Co and Zn dissolution extents could

be reasonable owing to their low amounts in the slag

and having higher charges for iron removal. Therefore,

a small decrease in metal dissolution extents could beacceptable and 79% Cu, 66%v Co, and 41% Zn

dissolution extents were achieved at these conditions.

As a concluding remark, it can be said that this study

has proven the recovery of Cu, Co, and Zn in the slag

by the application of acid roasting followed by hot

water leaching with acceptable metal extraction

extents.

Acknowledgements

Authors would like to dedicate this paper to thememory of their beloved teacher and adviser, Prof. Dr.

Fuat Yavuz BOR.

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