31pyrometallurgical refining of copper in an anod 153012

7/30/2019 31Pyrometallurgical Refining of Copper in an Anod 153012

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Title of PublicationEdited byTMS (The Minerals, Metals & Materials Society), Year

Pyrometallurgical Refining of Copper in an Anode Furnace

H. Antrekowitsch1

, C. Wenzl1

, I. Filzwieser2

, D. Offenthaler2

1Christian-Doppler-Laboratory for Secondary Metallurgy of Nonferrous Metals

Franz-Josef-Strae 18; Leoben, 8700, Austria2Department of Nonferrous Metallurgy

Franz-Josef-Strae 18, Leoben, 8700, Austria

Keywords: copper, thermodynamic, anode furnace, refining, refining electrolysis

Abstract

The decreasing quality of the input materials in copper recycling leads to a higher content ofimpurities in the anode copper. Therefore an improvement of the pyrometallurgical refining

process is necessary to produce high quality anodes for the copper refining electrolysis. In order

to improve the metal/slag reactions as well as the volatilisation by selective oxidation in theanode furnace, the behaviour of the most important accompanying elements (e.g. nickel, tin,

lead, zinc etc.) at different reaction conditions has to be investigated. This requires knowledge

about thermodynamic conditions like the reaction order and the activity coefficient at the

copper refining process. Additionally the interactions between different elements, but especiallythose for nickel, have been investigated as a function of the temperature, the content of the

elements and the slag composition. These investigations were done at the Christian DopplerLaboratory for Secondary Metallurgy of Nonferrous Metals.

Introduction

The raw materials for copper winning contain besides copper also numerous other elements like

nickel, lead, tin, zinc and iron. During the refining procedure of the copper these elements are

removed by using different techniques like the selective vaporization and oxidation as well asthe refining electrolysis. Nearly all copper which is won by the pyrometallurgical way (about

85 %) passes through the copper refining electrolysis, even so is most of the secondary copper.

In the electrolysis the impure copper is anodically dissolved and crystallized at the cathode

without impurities. The space time yield (currently about 0.03 t/m3) and the specific energy

consumption (about 0.4 kWh/kg Cu) represent the main key figures of the process. In order to

guarantee an economical process operation it is therefore necessary to optimize those two

operating figures to the highest possible extent. By increasing the current density we face theproblem of anode passivation, so that the electrochemical dissolution nearly stops [1]. The

consequences are a lower electricity yield as well as higher potential drops which in turn result

in an increased specific energy consumption. Due to the necessity of remelting the remaininganodes a large amount of copper has to be fed again to the anode furnace. The passivation

behavior of the anodes is strongly dependent on their chemical composition, in this context the

contents of accompanying elements like As, Bi, Sb, Pb, O and Ni are of great importance. In

many companies but especially in recycling plants the removal of those elements is verydifficult since the raw material and further also the accompanying elements are more or less

given. To economically process scrap it is often even necessary to feed low grade material [2].

Considering these aspects, there is an absolute necessity to realize further optimizations in the

field of pyrometallurgical refining in the anode furnace. In this context the behavior andreactions between metal and slag as well as the conditions for a volatilization are of great


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importance. This is because those conditions directly influence the composition of the refined

copper and the anodes respectively and as a consequence also the composition of the anodeslimes.

During the converting period of the black copper and during the following pyrometallurgical

refining all base metals as well as a part of the copper are oxidized, so that slags with high

contents in different metal oxides are generated. The latter together with the oxygen potentialstrongly influence the liquidus area of the slag. At present these slags are recycled to the shaft

furnace where the accompanying elements either accumulate in the flue dust or are transferred

into the black copper. In order to break up this closed loop of several elements and to dischargethem from the process it should be tried to reduce the metal oxides contained in the anode

furnace- and converter slag. This reduction step is becoming of increasing importance since the

quality of the scrap is continuously decreasing and therefore makes, with respect to thenecessity of unloading the refining electrolysis, a further optimization of the pyrometallurgical

refining absolutely necessary. Additionally it is of special interest, that the slags are very

homogenous and have a low viscosity, so that a high mass transfer, ensuring high reaction rates,can be guaranteed [3].

Behavior of the accompanying elements in the anode copper

If the accompanying elements arent completely removed during the pyrometallurgical refiningdifferent compounds are formed in the anodes, which cause problems at the electrolytic refining

process [4]. The anodes now represent multiphase alloys, since the various elements form solid

solutions as well as intermetallic compounds. Whereas Cu-Ag, Cu-Sb, Pb-Bi and Pb-Sb are

representative binary systems for the solid solutions, Cu-Sb or Cu-Se at a higher contents of Sband Se are examples for intermetallic compounds.

In figure 1 the behaviors of the various phases that occur in the anodes of a secondary copperplant are observable. The framed phases represent products, which can be found in the anode

mud, in the electrolyte or in the cathodes.

Figure 1: Behavior of different phases at the refining electrolysis [4]


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If the impurities are phase separated with the copper matrix or in the form of a solid solution or

intermetallic compounds respectively, is of great importance for the qualitative and quantitativeelectrochemical dissolving. The mechanically inserted impurities are formed by suspension of

impurity phases in the electrolyte. In figure 2 the characteristics of the anodic dissolving

process in the electrolyte close to the anodes are described schematically. For this special

reasons an optimization of the pyrometallurgical refining has to be one of the main objectivesof further investigations [5].

Figure 2: Schematic description of the electrochemical dissolving of the copper anode at the

refining electrolysis

Thermodynamic fundamentals

Activity

The activity of a metal oxide (aMO) is the driving force for the dissolution of the corresponding

metal in the slag [6]. The activity coefficient is indirectly proportional to the solubility. At lowcontents of the metal (M) in the copper and of the metal oxide (MO) in the slag Henry`s law isvalid [7]. The activity coefficient of a metal oxide is a function of temperature, oxygen potential

and slag composition. The influence of the temperature on the activity coefficient is shown in

figure 3.


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0

2

4

6

8

10

12

14

16

18

1000 1100 1200 1300 1400 1500T[C]

CuO0,5,

NiO,

SnO

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

ZnO,

PbO

CuO0,5 NiO SnO PbO ZnO

Figure 3: Activity coefficient of the metal oxide in a fayalitic slag [6]

Values for the activity coefficient of several metals in liquid copper are summarized in table 1

and shown as a function of temperature in figure 4.

Table I: Activity coefficients in liquid copper

metal RTln M0

[cal] M0 references

Fe(l) 9300-0.41T 15.95(1573K) [4]

Fe 12.6 [5]

Ni(l) 2340 2.11(1573K) [4]

Pb(l) 8620-2.55T 4.37(1573K) [4]Sn(l) -8900 0.058(1573K) [4]

Zn(l) -5640 0.165(1573K) [4]

0

5

10

15

20

25

30

35

1000 1100 1200 1300 1400 1500T[C]

Fe,

Ni,

Pb

0,0

0,1

0,1

0,2

0,2

0,3

Sn,

Zn

Fe(l) Ni(l) Pb(l) Sn(l) Zn(l)

Figure 4: Activity coefficient of several metals in liquid copper


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Distribution coefficient

The distribution coefficient (equation 1) describes the distribution of the accompanying

elements between slag and metal and is therefore an indicator for the efficiency of the metal

extraction [8].

( )[ ]MMLM

%%= (1)

In the slag the different metals M (e.g. Cu, Ni, Zn, Pb, Sn etc.) exist in the form of oxides, asdemonstrated by the reaction in equation 2. Equation 3 shows the corresponding equilibrium

constant to this reaction.

[ ] ( ) ( )nMOgOn

M + 22

(2)

2

2

n

n

n

OM

MO

MOpa

a

K = (3)

The distribution coefficient is directly proportional to pO2n/2

if the behavior of the metal M in

the copper and of the metal oxide MO in the slag obeys Henry`s law.

Accompanying elements of copper

The impurities that are fed with the input materials (scrap, sludge, dust, slag etc.) have to be

removed from the liquid copper during smelting by converting and refining [9].

Among the accompanying elements it has to be differentiated [10] between:

Base metals with a high enthalpy of formation of the oxide, that have to be transferred intothe slag in several process steps (e.g. Fe, Al, Si, P, Zn Sn and Be).

Elements that are partly reduced with the copper and therefore have to be separated byaccumulation in semi products or by the electrolytical refining process. Such metals are

beside the noble metals elements like As, Sb, Ni and Pb that have a enthalpy of formation of

the oxide which is similar to that of copper.

It should be ensured that the accompanying elements are not distributed in several phases

(metal, slag, dust) but accumulate in just one of those phases [11].

During the smelting process most impurities are at least partly transferred into the slag or thedust. As an exception merely Ni and the precious metals (Au, Ag, platinum group metals), with

their noble character avoiding oxidation, are solved in the copper and so form the anode mud inthe refining electrolysis.

During converting, the base accompanying elements are either volatilized or transferred into theslag by selective oxidation. The formed converter slag is then recycled to the shaft furnace

again. Due to this practice all accompanying elements are either reinserted or pass on to further

process steps. Since also the off gas is cleaned there are hardly any losses of elements [10].

The accompanying elements can be removed from the copper by injection of air on the one

hand or by the use of oxidizing slags on the other hand [11]. If air injection is used theoxidizing behavior of the elements depends on the gas flow rate and the temperature.

After the oxidation of Zn and Sn the oxygen content of the liquid copper increases continuously

until a critical value is reached, that also allows the oxidation of lead.


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In a liquid Cu-Pb-O metal phase the oxidation rate of lead is lower than in a Cu-Zn-Sn-Pb-O

phase, indicating that the activity coefficient of lead oxide (PbO) has a lower value in slagswhere Cu2O coexists with ZnO and SnO [11].

Even though by increasing the flow rate higher oxidation rates of the accompanying elements

can be realized, this practice generates larger amounts of slag and therefore causes higher

copper losses [11]. The oxygen content of the liquid copper increases rapidly with temperature.Whereas the oxidation reactions of Zn and Sn are exothermic and therefore an increased

temperature decreases the oxidation rates of those elements, the oxidation rate of Pb is hardlydependent on temperature [11].

In this context the most important aim of the pyrometallurgical refining process is to produce a

slag with a low copper content and a high absorption potential for the accompanying elementoxides. In order to be able to describe the behavior of each element it is necessary to determine

the activity and the distribution coefficient of the corresponding elements in thermodynamic

experiments and calculations.

Copper losses in the slag

In the slag copper exists in form of enclosed metal drops, Cu0,as

well as

in the solved form of

Cu+.

Enclosed metallic copper

It are physical slag properties like density, surface tension and viscosity that determine the

amount of enclosed copper. These copper losses can either be decreased, by giving the metallic

drops enough time for sedimentation or by lowering the slag-viscosity by reducing the content

of magnetite in the slag. At higher temperatures the influence of the slag melting point and the

slag-viscosity vanishes certainly, but fuel consumption and as a consequence also process costs

increase. From this point of view the overall objective of the investigations should be toguarantee a low slag-viscosity at lower process temperatures.

The sedimentation rate of the enclosed metal particles can be estimated by Stoke`s law [12].

( )

S

DSD rgv

=

2

9

2(4)

v sedimentation velocity [m/s]g acceleration of gravity [m/s]

D, S density of the enclosed particles and of the slag [kg/m]

rD radius of the enclosed particles [m]

S viscosity of the slag [kg/m.s]

According to Stokes law small metal particles will settle rather slow. By the injection of gas

into the slag/metal system more metal is transported into the slag by the gas bubbles. The rising

gas bubbles are covered with a layer of liquid metal that bursts when the bubbles are enteringthe slag. Thats why the amount of enclosed metal increases with the flow rate. By the higher

turbulences the sedimentation of bigger particles is retarded too[12].

Solved oxidic copper

The content of copper oxide in the slag depends first of all on the pO2 but also on the

temperature and the slag composition [13].Oxidic copper dissolves in the slag according to the following reaction (5).


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( ) ( ) ( )lCuOgOlCu 5,024

1+ (5)

( )

( )4

1

2

5,0

OlCu

lCuO

pa

aK

= (6)

The temperature dependence of the equilibrium constant is determined by equation (7) andgraphically shown in figure 5 [14].

639,27361

ln =T

K (7)

0

5

10

15

20

25

1000 1100 1200 1300 1400 1500

T[C]

K(CuO0,5

)

Figure 5: Equilibrium constant for the oxidic dissolution of copper as a function of temperature.

Figure 6 shows the correlation between the copper content and the oxygen potential. For the

dissolution of copper in siliceous slags Henry`s law can be applied, whereby the activitycoefficient of CuO0,5 shows marginal dependence of the slag composition. The solubility

decreases with increasing content of SiO2 and CaO. Furthermore also the addition of CaO,

MgO and Al2O3 to SiO2 saturated fayalitic slags decreases the copper solubility[8].


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Figure 6: Solubility of copper in FCS slags as function of pO2 [21] (R and Q represent the slag

composition and the basicity respectively)

Equation (8) results from the validity of Henry`s law and the limited solubility of copper:

( )5,0

% CuOakCu = (8)

The activity coefficient of copper oxide can be calculated according to equation (9):

( ) ( )( )Cu

pnaMK OTlCuCuCuO

%

4

1

2

5,0

= (9)

In the system of CaO-FeOx-SiO2 high values of5,0CuO

(maximum 13) are achieved at values of

Q between 0.45 to 0.55 and of R of about 0.2 [21]

Total copper losses

The total copper loss depends not only on the copper solubility of the slag but also on the total

slag amount, that is indirect proportional to the iron content of the slag [13]. If copper exists in

oxide form, the copper content in the slag and the total amount of in fayalitic slag dissolvedcopper can be reduced by decreasing the content of SiO2.

During melting and refining copper losses can be minimized by:

Reducing conditions in the melting aggregates (but the formation of solid metallic iron has

to be absolutely avoided)

An oxidic atmosphere at the refining process, that is as low as possible but high enough to

remove the impurities.

A slag composition that guarantees a low copper solubility as well as a small amount ofcopper inclusions.


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Nickel losses in the slag

As an important alloying element for copper nickel is introduced by feeding secondary material.

Although nickel is oxidized easier than copper it is removed as nickel sulphate in the refining

electrolysis by precipitation from the electrolyte.

The nickel containing slags from the converter and the anode furnace are fed to the shaftfurnace. The losses of nickel through the shaft furnace slag are with 0.5% rather low. The

highest nickel contents can be found in the black copper and the converter slag [10].

The oxidic dissolution of Ni in the slag takes place according to the following reaction:

( )

( )2

1

2OlNi

lNiO

pa

aK

= (10)

( ) ( ) ( )lNiOgOlNi + 22

1(11)

Experimental

The number of experiments were calculated by the software MODDE 7.0. Furthermore astoichiometric amount of reducing agent was used for the reactions. For the calculation of this

amount it was assumed that all non ferrous metal oxides (CuO0.5, NiO, PbO, SnO, ZnO) in the

slag are reduced to metal. Although iron should remain in the slag, it should be reduced from

FeO1.5 to FeO. The addition of iron was necessary for the exact ratio of Fe/SiO2. Additionallyiron served as reducing agent for the oxides of Zn, Pb, Sn, Ni and Cu whereas the iron itself is

oxidized to FeO, which is produced also by the reduction of FeO1.5. The ratio of Fe/SiO2 and of

CaO/SiO2 were the investigated parameters in these experiments. The composition of the slag

for the investigations is given in table II.

Table II: Composition of the anode furnace slag

The investigations were carried out in an induction furnace. For the experiments a sintered

alumina crucible with a height of 38 mm, a diameter of 32 mm and a wall thickness of 1 mmwas used. The crucible was fed with the anode furnace slag (table II) and several additions(SiO2, CaO, Al2O3, MgO, Graphite). The reaction time for each experiment was 4 hours.

After 4 hours the thermocouple was removed and the crucible was cooled down in the furnace.

To investigate the viscosity of the slag the temperature was kept constant at 1300C during theexperiment. It turned out that a higher basicity increases the viscosity of the slag at this

temperature, which in turn leads to a higher metal content in the slag.

The experiments mainly focused on the behavior of Ni, which can strongly influence theelectrolysis, and Cu. Since the Ni distribution in the anodes depends on the solidification

conditions, the Ni content fairly varies at the cross section of the anodes, which then causes

different conditions during the electrolysis. The distribution of the element Ni along the cross

section of an anode as a function of the solidification conditions is shown in figure 7. As it can

[%] [%]

Cu 28.500 As 0.016

Fe 6.9 Ag 0.035

Pb 4.9 SiO2 11.0

Sn 3.1 Al2O3 4.5

Ni 2.6 MgO 1.8

Sb 0.2 CaO 2.0


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be seen it is very important, if the form of the primary copper crystals is either dendritic (figure8) or globulitic (figure 9).

0,97

0,99

1,01

1,03

1,05

1,07

1,09

1,11

1,13

0 0,5 1 1,5 2

cross section of the anode in cm

nickelin%

spheriodal

dendritic

Figure 7: Ni distribution along the cross-section of the anode as a function of the solidification

conditions

Figure 8:Dendritic microstructure of anode copper

Figure 9: globulitic microstructure of anode copper


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The copper and nickel content of the slag as a function of the CaO/SiO2 und Fe/SiO2 ratio is

given in figure 10. It has to be considered, that these investigations have been carried out at a

temperature of 1300 C. Therefore the viscosity of the slag influence the reaction and the

sedimentation of the liquid metal. In industrial operation these circumstance have to beconsidered at higher basicities to get a slag with a low viscosity.

Figure 10: Cu and Ni content as a function of the CaO/SiO2 and Fe/SiO2 ratio at a temperature

of 1300 C and a stoichiometric amount of the reduction agent.

The copper and nickel content of the slag under the investigated conditions can be calculated

according to equations (12) and (13):

( )2222

7056.42606.33432.60670.16%SiO

CaO

SiO

Fe

SiO

CaO

SiO

FeCu += (12)

( )2222

3278.02773.08055.13085.2%SiO

CaO

SiO

Fe

SiO

CaO

SiO

FeNi ++= (13)

Further experiments at higher temperatures (1400 C) have shown, that the content of theelements but especially that of copper decreases significantly. Due to this fact in future

investigations also the temperature will be a parameter that has to be varied. Although this

practice will increase energy costs, it is justified by an increased process yield and an improvedslagging of the accompanying elements.

Conclusion

Due to the lower quality of the input materials and the necessity for a continuous increase of the

space time yield the pyrometallurgical refining step in the primary and secondary copperindustry has to be optimized. The changed process conditions also change the behavior of the

accompanying elements at selective oxidation and evaporation reactions. For the description of

the process conditions and for further investigations knowledge about parameters like viscosity,

temperature and basicity of the slag is essential. The investigations showed, that thetemperature is one of the most important parameters for influencing the slag viscosity.


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The continuous improvement of the pyrometallurgical refining process in copper secondarymetallurgy is of great importance for an unproblematic operation of the final refining

electrolysis. The different elements strongly influence the refining electrolysis where they cause

e.g. passivation of the anodes which results in a lower yield of the whole process. Further

investigations should enable the production of anode copper, which can be inserted into the

refining electrolysis without any restrictions, although low quality scrap is used.

References

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passivation of anodes in the process of copper electro-refining. Hydrometallurgy 28

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solubility, ferric/ferrous ratio and minor-element behavior of iron-silicate slags.Metallurgical and Materials Transactions B, Vol.29B, June 1998, 583-590

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11. Nakashima K., K. Yamamoto, E. Shibata, H. Tahori und K. Mori: Oxidation rate of

impurities in liquid copper by gas and slags. Second International Conference onProcessing Materials for Properties, San Francisco, November 2000

12. Poggi D., R. Minto und W. G. Davenport: Mechanisms of metal entrapment in slags.

JOM, Vol.21, November 1969, 40-4513. Reddy R.G. und C.C. Acholonu: Activity coefficient of CuO0,5 in alumina saturated iron

silicate slags. Metallurgical and Materials Transactions B, Vol.15B, December 1984,

345-34914. Yazawa A., Y. Takeda und S. Nakazawa: Ferrous calcium silicate slag to be used for

copper smelting and converting. Copper 99, Vol.VI., The Minerals, Metals and

Materials Society, 1999, 587-59915. Vartiainen A. und M. Kyt: Olivine slags - the ultimate solution to low copper slags?.

Scandinavian Journal of Metallurgy, Vol.31, Issue 5, October 2002

31pyrometallurgical refining of copper in an anod 153012

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