Cinética Malaquita

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Dissolution kinetics of malachite in sulphuric acid

D. Bingola, M. Canbazoglub,*

aDepartment of Mining Engineering, Dumlupnar University, Kutahya, Turkeyb

Department of Mining Engineering, Cumhuriyet University, Sivas, Turkey

Received 28 May 2003; received in revised form 1 October 2003; accepted 5 October 2003

Abstract

A kinetic study of the sulphuric acid leaching of oxidised copper ore, primarily malachite, was carried out. The effects of

leaching time, stirring speed, acid concentration, solid to liquid ratio, reaction temperature, and particle size of the ore were

investigated. Using the best conditions, the copper recovery was nearly 94% at 25 jC and 99% at 80 jC after 180 min leaching.

Dissolution of malachite during the leaching was described by a logarithmic function, y = aln(x) + b. The data obtained for the

leaching kinetics indicated that the initial dissolution of malachite is a diffusion controlled reaction.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Malachite; Dissolution; Copper; Sulphuric acid; Leaching; Kinetics

1. Introduction

Copper was the first metal to which hydrometal-

lurgical processes including leaching, solvent extrac-

tion and electrowinning (L-SX-EW) were applied

(Seward, 1997, 1999; Hopkins, 1994). During the

leaching of copper minerals, such as chalcopyrite

(CuFeS2) or tenorite (CuO), sulphuric acid and am-

monia are generally the most used leaching mediums(Bingol et al., 1994; Amores et al., 1997; Navarro and

Alguacil, 1999; Alguacil, 1999). Copper oxide min-

erals contain copper in the divalent state (e.g. azurite

(Cu3(OH)2(CO3)2), malachite (Cu2(OH)2CO3), tenor-

ite (CuO) and chrysocolla (CuSiO3.2H2O)), and these

are completely soluble in sulphuric acid at room

temperature. Typical reactions of oxidised copper ores

with sulphuric acid can be given as follows (Blazy,

1970; Barlett, 1992),

Cu3OH2CO32 3H2SO4

! 3CuSO4 2CO2 4H2O 1

Cu2OH2CO3 2H2SO4 ! 2CuSO4 CO2 3H2O

2

CuO H2SO4 ! CuSO4 H2O 3

CuSiO3:2H2O H2SO4 ! CuSO4 SiO2 3H2O

4

The cost of acid may be the most important

economic factor in the leaching of oxidised copper

ores. Carbonate rocks, such as limestone and dolo-

0304-386X/$ - see front matterD 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.hydromet.2003.10.002

* Corresponding author.

E-mail address:[email protected] (M. Canbazoglu).

www.elsevier.com/locate/hydromet

Hydrometallurgy 72 (2004) 159165


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mite, are often significantly found in the ores and

consume additional acid according to the reaction

given below(Blazy, 1970; Barlett, 1992):

CaCO3 H2SO4 H2O ! CaSO4:2H2O CO2

5

To determine the leaching parameters, the dissolu-

tion of malachite in sulphuric acid solutions has been

recently studied by the Taguchi method (Ata et al.,

2001). In this study, the kinetics of malachite leaching

was not evaluated and the optimum leaching condi-

tions were determined as: temperature, 40 jC; solid to

liquid ratio, 0.1 g cm 3; sulphuric acid concentration,

10%; reaction time, 45 min; particle size, 10 mesh and

stirring speed, 480 rpm. Under these optimum work-ing conditions, the recoveries of copper and iron from

malachite ore were 100% and 58%, respectively.

The aim of this present study is to determine and

discuss the dissolution kinetics of malachite in sul-

phuric acid. For this purpose, the initial dissolution

rate and activation energy were determined and the

effect of other minerals on leaching reactions were

taken into the consideration.

2. Experimental

Oxidised copper ore was used for the leaching

experiments. Mineralogical analysis, performed using

a Rigaku DMAX IIIC model X-ray diffractometer

using CuKaradiation at 35 kV and 15 mA, indicated

that malachite, pyroxene group minerals, quartz, goe-

thite and magnetite were the major mineral phases in

the ore. The chemical analysis of the malachite ore

sample is presented in Table 1. Copper, iron, nickel,

cobalt and chrome value were determined by using

Atomic Absorption Spectrophotometer (AAS). Sul-

phur and carbon content were determined by using

LECO-444 SC analyser. The carbon in the ore orig-

inates from carbonate content of malachite mineral,

and the sulphur from the base metal minerals. Quartz

was determined using the conventional hydrofluoric

acid method. The ore was first crushed, ground and

then screened to obtain the desired particle sizefractions for the leaching experiments.

The leaching experiments were performed in a 600

mL Pyrex beaker in a thermostatically controlled

water bath, and equipped with a thermometer and

Teflon stirrer agitated mechanically. The ground cop-

per ore was added into the agitated sulphuric acid

solution at the required temperature. After pre-deter-

mined time intervals, 5 mL of leaching solution

sample was withdrawn from the reactor and 5 mL

of fresh sulphuric acid solution was added immedi-

ately to leaching medium. After filtration, the clear

leach solution sample was analysed for copper by

AAS. The weight loss during leaching was calculated

by difference. The acid consumption was determined

by titration of the leach solution with NaOH, using

dimethyl yellow + methylene blue as indicators. For

the leaching experiments where the acid concentration

and solid to liquid ratio were examined, the ore

sample has a P80 of 0.65 mm was used, the 100

150 Am size fraction was used to test the effect of

temperature.

3. Results

3.1. Effect of leaching time

The effect of leaching time on the dissolution of

malachite is shown in Fig. 1. Clearly, the copper

recovery increased with increasing leaching time with

the initial dissolution of copper being very rapid.

After 5 min, 77% copper recovery was achieved and

this increased to 97% after 210 min. In practice, 30

min of leaching time was found to be optimum. It isclear that chemical reaction of malachite with sulphu-

ric acid determines the dissolution rate of malachite

during the leaching.

3.2. Effect of sulphuric acid concentration

The effect of sulphuric acid concentration on the

dissolution of malachite was investigated. The copper

recovery, acid consumption and weight loss are given

in Fig. 2. It can be seen that there is a direct

Table 1

Chemical analysis of ore

Component (%)

Cu Fe Ni Co Cr S C SiO2

28.36 15.74 0.044 0.010 0.019 0.10 2.72 15.34

D. Bingol, M. Canbazoglu / Hydrometallurgy 72 (2004) 159165160


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relationship between the copper recovery and the acid

consumption. The consumption was high until 1.02

mol/L sulphuric acid was used, e.g. the copper recov-

ery was 87% and the acid consumption was about85% when 1.02 mol/L sulphuric acid was used. Acid

in the leaching solution (97%) was consumed while

0.61 mol/L sulphuric acid was used and the copper

recovery was approximately 57%.

For the acid concentrations higher than 1.02 mol/L,

both acid consumption and copper dissolution rate

decreased. When 2.04 mol/L of sulphuric acid was

used, the acid consumption was 20% and the copper

recovery remained 91%. The weight loss increased

with acid concentration and reached 48% when 2.04

mol/L of sulphuric acid was used. It was found thatthis value was also equivalent to the malachite disso-

lution from the sample.

The experimental results showed that the acid

consumption was 0.92 mol/L for 90% copper recov-

ery. From reaction (2), the theoretical amount of acid

consumed during leaching can be calculated. Under

the experimental conditions, 0.80 mol/L acid is re-

quired for malachite dissolution. It is suggested that

the slightly higher value in practice is due to acid

consumption by gangue minerals.

3.3. Effect of stirring speed

Fig. 3 shows that stirring speed has a significant

effect on the dissolution of malachite. The copperrecovery obtained under the same experimental con-

ditions without agitation was 39%, while it was nearly

87% when 170350 rpm stirring speed was applied.

The same effect of stirring speed was observed on the

dissolution of the iron minerals. There was no appar-

ent iron dissolution in the absence of agitation, but it

was 21% at 300 rpm. It can be concluded that this

effect occurs by the increased diffusion rate of ions in

leaching medium with increasing agitation.

3.4. Effect of solid/liquid ratio

Fig. 4gives the copper recovery and the weight loss

that occurred during leaching, as the function of the

solid to liquid ratio (weight of solid/volume of liquid).

According to experimental results presented inFig. 4,

it was found that the copper recovery increased until

the solid/liquid ratio was 1:5, where 87% of copper

was recovered. At ratios higher than 1:5, there was no

significant increase in the copper recovery. During the

Fig. 2. Effect of sulphuric acid concentration on the recovery of

copper from the malachite ore, and on the acid consumption and the

weight loss during the leaching (temperature, 25 jC; particle size,

80% 650 Am; the amount of ore, 20 g; solid to liquid ratio, 1:5 g/mL; stirring speed, 250 rpm; time, 30 min).

Fig. 1. The effect of leaching time on the recovery of copper from

the malachite ore (acid concentration, 2.04 mol/L; temperature, 25

jC; particle size, 80% 650 Am; the amount of ore, 20 g; solid toliquid ratio, 1:5 g/mL; stirring speed, 250 rpm).

D. Bingol, M. Canbazoglu / Hydrometallurgy 72 (2004) 159165 161


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leaching, the weight loss pattern was found similar to

malachite dissolution. Consequently, it was suggested

that the weight loss be related to the dissolution of

malachite. The recovery was 90% and the weight loss

was 40% for a solid to liquid ratio of 1:5.

3.5. Effect of temperature

As seen from Fig. 5, the copper recovery in-

creased as the temperature was increased. The re-

covery after 120 min reached 94% and 98% for 25

Fig. 3. Effect of the stirring speed on the recovery of copper and iron from the malachite ore (acid concentration, 1.02 mol/L; temperature, 25

jC; particle size, 80% 650 Am; weight of ore, 25 g; solid/liquid, 1:10; time, 30 min).

Fig. 4. Effect of solid to liquid ratio (w/v) on the recovery of copper

from the malachite ore and on the weight loss during the leaching

(acid concentration, 1.02 mol/L; temperature, 25 jC; particle size,

80% 650 Am; weight of ore, 20 g; stirring speed, 250 rpm; time,30 min).

Fig. 5. Effect of leaching time on the recovery of copper from the

malachite ore (acid concentration, 1.02 mol/L; temperature, 25 jC;

particle size, 100 150 Am; weight of ore, 25 g; solid/liquid ratio,

1:10; stirring speed, 350 rpm).



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jC and 80 jC, respectively. However, the malachite

dissolution hardly increased after a rapid dissolution

in 5 min.

3.6. Effect of particle size

The effect of particle size was studied using eight

crushed and five ground size fractions. For the copperrecovery calculations, the copper grade of each frac-

tion was considered. These samples were all leached

using 1.02 mol/L sulphuric acid used at 25 jC. The

results are given inFigs. 6 and 7. From these figures,

it is clear that copper recovery is rapid at the begin-

ning. If the particle size decreases, the dissolution is

more rapid. However, the particle size effect decreases

with increasing leaching times.

4. Discussion

Malachite can be easily dissolved by acidic reac-

tions. The experimental results showed once more

that, as the amount of acid consumed is increased,

the dissolution of malachite increases (Fig. 8).

According to reaction (2), only about 90% of the

acid is used for malachite dissolution. The acid

consumption per mol malachite was found to be

2.222.37, the extra 10% is consumed in the disso-

lution of other minerals found in the ore, notably the

iron oxides.

Fig. 6. Effect of particle size of the crushed ore for the dissolution of

malachite (acid concentration, 1.02 mol/L; temperature, 25 jC;

weight of ore, 20 g; solid/liquid, 1:5; stirring speed, 250 rpm; time,

30 min).

Fig. 7. Effect of particle size of the ground ore for the dissolution of

malachite (acid concentration, 1.02 mol/L; temperature, 25 jC;

weight of ore, 25 g; solid/liquid ratio, 1:10; stirring speed, 350 rpm).

Fig. 8. The graph showing the direct relationship between the acid

consumption and the copper recovery.



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Accordingly, XRD analysis of the leaching resi-

dues showed malachite to be completely dissolved,

whereas goethite, magnetite, pyroxenes and quartz

were not dissolved in the acid leaching conditionsapplied. It was also determined that the amount of

these gangue minerals in the residue increases as the

copper recovery or the amount of dissolved mala-

chite increased.

Combining the chemical analysis and XRD anal-

ysis, it can be estimated that the ore consisted of

49.35% malachite, 22.56% goethite and magnetite,

27.36% quartz and pyroxene group, leaving only

0.73% unaccounted for by XRD analysis.

The kinetic evaluation of reactions occurring

during the acidic leaching of malachite indicated that

neither chemical nor diffusion models applied. These

models are unable to explain the dissolution of

malachite for the rapid initial reaction and the

complete leaching period studied. However, the

initial dissolution can be modelled using a logarith-

mic function: y = a ln(x) + b; where y is the copper

recovery, x is the leaching time, a and b are the

constant coefficients. A significant part of dissolution

has been achieved at the leaching times within 15

min and the values can be expressed by logarithmic

curves as presented in Fig. 9. It was also observed

that the values ofaand bincrease as the temperature

increases. The value ofa increased from 3.539 at 25

jC to 3.735 at 80 jC and the value of b increases

from 81.683 to 86.141 over the same temperature

range.It is possible to calculate the dissolution rate after

0.1 min of leaching at different temperatures by

using the model. From these rates, an Arrhenius

curve can be drawn(Fig. 10). It was determined that

the activation energy, Ea, was 1.3 kJ/mol. This value

is very low and indicates that the dissolution of

malachite is solution diffusion-controlled during the

initial dissolution. This is supported by the signifi-

cant increase in dissolution due to stirring (Fig. 3).

After the initial dissolution, the availability of readily

accessible malachite had been greatly reduced and achange in rate determining step is likely.

5. Conclusions

The following conclusions can be drawn from the

kinetic study of the malachite dissolution:

(i) Malachite was readily dissolved by sulphuric

acid giving high recoveries of copper, other

Fig. 9. Effect of time on the dissolution of malachite at 40 jC

leaching temperature (acid concentration, 1.02 mol/L; temperature,

25 jC; particle size, 100150 Am; weight of ore, 20 g; stirring

speed, 350 rpm).

Fig. 10. The Arrhenius curve obtained for the dissolution of

malachite ore.



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minerals, such as iron minerals, were only

slightly affected from sulphuric acid medium.

(ii) The most important parameters affecting the

leaching were time, acid concentration, stirringspeed and the temperature of medium. Acid

concentrations of less than 1.02 mol/L were

insufficient for complete dissolution, for higher

concentrations there is a direct relationship

between the acid concentration and the dis-

solution of malachite. Leaching was found to

comprise two steps, an initial, very rapid

dissolution with f85% copper recovery with-

in f15 min and a slower second step. After

210 min of leaching, the effect of particle size

on dissolution was found to be negligible. The

malachite dissolution increased slightly with

temperature, going from 94% to 98% as the

temperature increased from 25 to 80 jC.

(iii) The initial dissolution kinetics may be modelled

by a simple logarithmic function. From this

model, the activation energy was found to be

1.3 kJ/mol, clearly malachite dissolution is

controlled by solution diffusion.

(iv) Sulphuric acid consumption for 90% copper

recovery was computed to be about 450 kg per

ton ore.

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