Silver loss during the oxidative refining of silver–copper alloys

Minerals Engineering 24 (2011) 514–523

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Silver loss during the oxidative refining of silver–copper alloys C.A. Pickles ⇑, C. Harris, J. Peacey Robert M. Buchan Department of Mining, Queen’s University, Kingston, Ontario, Canada K7L 3N6

a r t i c l e

i n f o

Article history: Available online 24 December 2010 Keywords: Oxidation Pyrometallurgy Extractive metallurgy

a b s t r a c t The high temperature oxidative refining of molten silver–copper alloys is accompanied by silver losses to the slag. Typically, borosilicate slags are utilized as a flux and air or oxygen is injected into the molten alloy. The majority of the copper can be selectively oxidized to copper oxide, but some silver is also lost to the slag. In this research, the mechanism of silver loss has been investigated. Firstly, TGA and DTA studies of molten silver–copper alloys were performed, in order to elucidate the oxidation processes occurring at the gas–metal interface in the bubble. Secondly, the silver losses to the slag in the high temperature refining tests were quantified. Thirdly, the slag was characterized using XRD and the morphology of the silver in the slag was examined. Fourthly, a pseudo-equilibrium model of the refining process was developed and this was utilized to suggest methods for minimizing the silver loss to the slag. Finally, possible methods for recovering the silver from the slag are discussed. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Alloys containing silver and copper arise from a number of sources such as the treatment of Miller chlorides from gold refining (Pickles et al., 1995) and also during the processing of anode slimes (Sanmiya and Matthews, 1981; Swinbourne et al., 1996, 1997). The thermal separation of the copper from the silver is typically performed by reacting oxygen with the molten alloy and the copper is selectively oxidized. One of the earliest studies of this process was by Kohlmeyer and Sprenger (1948), who utilized two approaches to investigate the equilibrium compositions in the Ag– Cu–O ternary system at about 1500 K. Firstly, molten alloys of silver–copper were oxidized and the reaction products were examined. Secondly, mixtures of Ag2O, Cu2O and CuO were heated and the results were similar to those obtained by oxidizing the silver–copper liquid. Alloys containing less than 30 mass% silver were completely oxidized to Ag2O and Cu2O, while for alloys containing greater than 30 mass% silver, a silver-rich metallic button was obtained in addition to the Ag2O and Cu2O. Further improvements in the separation of the copper from the silver were achieved through the utilization of a slag phase and this was investigated by Atmore et al. (1971) for alloys containing 2, 5 and 8 mass% copper. Borosilicate slags were chosen for the following reasons: the activity of copper oxide is low, the slags possess both low liquidus temperatures and viscosities, both refractory corrosion and slag volatilization are minimal and the slags are relatively inexpensive. The removal of copper was associated with a loss of silver to the slag, with the loss increasing with the silver ⇑ Corresponding author. Tel.: +1 613 533 2759; fax: +1 613 533 6597. E-mail address: [email protected] (C.A. Pickles). 0892-6875/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2010.11.006

content of the alloy. An increasing oxygen ion activity in the slag increased the loss of silver to the slag and decreased the amount of copper removed. More recently, studies have been performed on the oxidation of silver–copper alloys at 1413 K under a borosilicate slag and a number of variables were investigated (Pickles and Toguri, 1995; Pickles, 1998). Three silver alloys (7.5, 11.2 and 12.7 mass% copper) were studied and tests were performed with both air and oxygen injection into the slag or metal. Also, both the borax to silica ratio of the slag and the amount of the slag were varied. About 90% of the copper could be removed with silver recoveries of over 95%. Copper levels in the metal of less than 2 mass% could readily be achieved. The silver losses to the slag increased with the copper content of the slag. Based on the experimental results and the thermodynamic information available at the time, a model of the process was proposed. It was postulated that silver oxide–copper oxide forms on the inside of the gas bubble and this oxide is transferred with some silver–copper alloy into the slag phase where a slag–metal exchange reaction occurs. Lately, there has been renewed interest in the silver–copper oxide system, as knowledge of this system is of considerable significance in the development of silver-clad cuprate-based superconductors (Balachandran et al., 1995) and also for air brazed ceramic to metal joints (Kim and Weil, 2003). This has resulted in an improved understanding of the phase equilibria in the Ag–Cu–O system (Assal et al., 1998; Nishiura et al., 1998; Darsell and Weil, 2007). In the present research, the recent equilibrium studies reported in the literature and additional experimental results on higher copper alloys are utilized to provide a more in-depth understanding of the oxygen refining of silver–copper alloys. Firstly, the literature regarding the Ag–Cu–O system is reviewed. Secondly, TGA and DTA results of the high temperature oxidation of silver–copper

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melts are discussed in terms of the oxidation processes occurring at the gas-bubble interface. Thirdly, experimental results obtained from the refining process and a thermodynamic model, are utilized to minimize the silver loss to the slag. Finally, potential methods for recovering the silver from the slag are examined. 2. THE Ag–Cu–O system In the oxidizing gas bubble in the refining process, the equilibrium is determined by the Ag–Cu–O system and therefore this system is of interest. First the two relevant binaries, Ag–O and Cu–O, are discussed and subsequently the ternary Ag–Cu–O system. There have been a number of studies of the Ag–O system (Assal et al., 1997; Karakaya and Thompson, 1992). It is generally accepted that Ag2O is the only stable oxide phase under ambient conditions and it is considered to be stoichiometric. Ag2O is relatively unstable and decomposes into silver and oxygen at 420 K. Other higher valence oxides such as AgO, Ag3O4 and Ag2O3 have been reported at low temperatures and high oxygen partial pressures. Assal et al. (1997) showed that an ionic two-sublattice model could describe the Ag–O liquid and the calculated thermodynamic properties were in good agreement with the available experimental data. The Cu–O system is more complex due to the presence of a large miscibility gap and a rapidly rising activity of oxygen as the Cu2O composition is reached. There is considerable interest in this system both for metallurgical and superconductor applications (Hallstedt et al., 1994; Clavaguera-Mora et al., 2004). The two wellestablished oxides are CuO and Cu2O and again they are considered to be stoichiometric. The phase diagram above 1300 K consists of a eutectic reaction at about 1340 K and a monotectic at about 1500 K. In this system there is a large miscibility gap, with an upper critical solution temperature of about 1620 K and a lower critical solution temperature of about 1500 K. The critical composition occurs at an oxygen mole fraction of about 0.2 and the gap extends from almost the copper terminus to nearly the composition corresponding to Cu2O. Again an ionic two-sublattice model has been utilized to describe the liquid phase (Hallstedt et al., 1994). More recently, Clavaguera-Mora et al. (2004) developed a partially associated regular solution model to describe the Cu–Cu2O portion of the phase diagram. They claim that since their model utilizes fewer parameters to determine the excess free energy than the other models, then it is more easily extended to higher order systems. A recent version of the phase diagram for the Ag–CuOx system in air at temperatures above 1100 K is shown in Fig. 1 (Hallstedt and Gauckler, 2003). The diagram is characterized by three reactions as follows:

Eutectic : L1 ðX Ag =ðX Ag þ X Cu Þ ¼ 0:99Þ $ CuO þ Ag T ¼ 1215 K

ð1Þ

Monotectic : L2 ðX Ag =ðX Ag þ X Cu Þ ¼ 0:36; X O =X Cu ¼ 0:80Þ $ CuO þ L1 ððX Ag =ðX Ag þ X Cu Þ ¼ 0:98Þ T ¼ 1242 K

ð2Þ

Monotectic : Cu2 O $ L2 ðX Ag =ðX Ag þ X Cu Þ ¼ 0:10Þ þ CuO T ¼ 1302 K

ð3Þ

The large miscibility gap observed in the liquid state in the Cu– O system is similarly a major feature in the Ag–Cu–O system. The exact shape of the miscibility gap is still being investigated. Based on an extrapolation of their results, Nishiura et al. (1998) and Shao et al. (1993) suggested that the miscibility gap should have an almost symmetrical parabolic shape. On the other hand, research

Fig. 1. Calculated CuOx–Ag phase diagram in air, from Hallstedt and Gauckler (2003). The dashed lines were calculated with the parameters from Assal et al. (1998) and the experimental data from Nishiura et al. were included.

by Assal et al. (1998), based on a thermodynamic assessment and computational analysis, concluded that the miscibility gap should be asymmetrical towards the CuOx side. They developed both an associated solution model and an ionic liquid model and concluded that although both models could be applied, the ionic model was more reliable in predicting the miscibility gap as shown in Fig. 1. At the typical operating temperatures utilized in the refining of silver–copper alloys (1373–1573 K) and for most alloy compositions, which span the miscibility gap, both silver and copper oxides do not exist in the liquid state. Instead, a silver-rich ionic liquid containing small amounts of oxygen and copper (L1) is in equilibrium with a copper-rich ionic liquid containing oxygen and some silver (L2). 3. Experimental 3.1. Refining experiments on silver–copper alloys In addition to the three silver–copper alloys (7.5, 11.2 and 12.7 mass% copper) studied previously (Pickles et al., 1995; Pickles, 1998) an additional three higher copper alloys (26.9, 38.7 and 49.7 mass% copper) were investigated in this work. In each test, 200 g of the alloy in bulk form was utilized and a borosilicate slag was employed with a borax to silica mass ratio of three to one. For the alloys with less than 30 mass% copper, the slag mass was 200 g, while for the 38.7 and 49.7 mass% copper alloys, the slag masses were 300 and 400 g, respectively. The slag and the alloy were placed in a No. 14 fireclay crucible and melted in a gas-fired muffle furnace. After heating for about 40–45 min, the slag and metal were both molten and a quartz lance (inside diameter of 0.6 cm) was placed in the molten metal (in some cases in the molten slag). Oxygen or air was bubbled for various time periods and flow rates. On completion of the test for the selected conditions, the melt was allowed to slowly cool in the furnace in order for the metal particulates in the slag to separate and to promote the growth of the various phases in the slag. Once at room temperature the crucible was broken open and samples of metal and slag were obtained for analysis. In some cases, quenched slag samples were obtained from the liquid slag by inserting an aluminium rod into the slag and then

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removing the rod with some attached solidified slag. A portion of each slag sample was ground and characterized by X-ray Diffraction (XRD) using a Philips X’Pert Pro MPD diffractometer. Another portion was mounted in epoxy, polished and examined in an optical microscope. The copper contents of the metal and slag were determined by electrolysis, while the silver content of the slag was determined by the fire assay method. The silver content of the metal was calculated by difference. The silver and copper recoveries were calculated using the silver and copper analysis of the metal and the initial and final masses of the alloy. 3.2. Characterization of the Ag–Cu–O system In order to elucidate the processes occurring during the oxidative refining of silver–copper alloys, portions of the Ag–Cu–O system were investigated. Both a low copper alloy (11.2 mass% copper) and a high copper alloy (49.7 mass% copper) were studied and in order to facilitate oxidation, the samples were ground into a powdered form. Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) studies were performed on a Netzsch STA-409 high temperature (1848 K) DTA up to about 1273 K. The powdered alloy samples, weighing a few hundred milligrams, were placed in an alumina crucible and the heating rate was 2 K/min in air. 3.3. Modelling of the slag–metal system The slag–metal system was modelled using the equilibrium module of Outokumpu HSC Chemistry 6.1 (Roine, 2006). The control conditions were similar to those utilized in the refining tests. The amount of the initial alloy (10 mass% copper) was 100 g and 200 g of slag was utilized with a borax to silica mass ratio of three to one. The amount of air utilized was 100 Nm3 at 1 bar. First, the various elements were inputted and for the Cu, Ag, O, Na, B, Si system this generates a list of 83 species. Many of the gaseous species and some of the silver oxides are not stable under the present conditions. Some species such as silica have numerous crystal structures but only the common form was included. Additionally, only one stable phase was considered for both copper and silver and elemental boron, while silicon and sodium were not accounted for. Oxygen and nitrogen were also included and the three phases and the 21 species utilized in the slag–metal calculations are shown in Table 1. The input information consisted of the amounts of anhydrous sodium tetraborate and silica and the amounts of metallic copper and silver in the alloy. The Equilibrium module calculates the multicomponent equilibrium composition under isothermal and isobaric conditions, using the Gibbs free energy minimization method. The various phases are considered to be ideal solutions and the activity coefficient of each species is unity. However, if values of the activity coefficients are known or expres-

Table 1 Species considered in the model calculations of the slag– metal system. Oxides

Oxides

Ag2O Cu2O B2O3 NaBO2 NaBO3 NaB3O5 Na2B4O7 Na2B6O10 Na2B8O13 Na2O Na2OB2O3

Na2O2SiO2 Na2O3SiO2 2Na2OSiO2 3Na2O2SiO2 Na2SiO3 SiO2 Metals Ag, Cu Gases O2, N2

sions are available as a function of composition and/or temperature then these can be inputted. Golonka et al. (1979) have measured the activities of silver and copper in silver–copper alloys and the activity coefficients can be represented as follows:

h i log cAg ¼ 700 ð1  X Cu Þ1:9  2:1ð1  X Cu Þ0:9 þ 1:1 =T

ð4Þ

h i log cCu ¼ 700 ð1  X Cu Þ1:9 =T

ð5Þ

These equations for the activity coefficients of silver and copper were utilized in the equilibrium calculations. With regards to the slag phase, the oxide species containing boron, silicon and/or sodium were assumed to behave ideally. Deviations from ideality were taken into account in terms of the activity coefficients of the copper and silver oxides, which were determined from the data generated from the refining experiments. 4. Results and discussion 4.1. Refining of Molten Ag–Cu alloys Fig. 2 shows the silver grade versus recovery curves for the refining tests performed on a number of the silver–copper alloys utilized in the present research. It can be seen that the silver grade decreases with the silver recovery. For the lower copper alloys or for the removal of small amounts of copper from the higher copper alloys, reasonable silver recoveries can be achieved. However, for the higher copper alloys where considerable copper removal is required to achieve a high grade, then the silver recoveries are significantly reduced. Therefore, the silver loss to the slag increases with the copper removal. Additionally, the maximum silver grade for a given alloy, decreases with increasing initial copper content of the alloy. In order to further investigate the mechanism of loss of silver to the slag, samples of the liquid slag were taken at selected time intervals during the refining process. Also samples were obtained after the bulk metal and slag had been allowed to slowly cool. These samples were analysed for silver and copper and a comparison provides information on the behaviour of silver and copper in the slag. The results for silver and copper are shown in Figs. 3a and 3b, respectively. At any given time, both the silver and copper contents in the liquid slag are higher than in the slowly cooled slag. Additionally, the silver content of the slag increases with refining time and thus with the copper oxide content of the slag. Clearly, silver and copper are transferred into the slag by the gas bubbles and are present in metallic and/or oxide forms. The lower silver

SILVER GRADE (mass %)

516

100

100

90

90

80

80

70

70

60

50 70

7.5 mass %Cu 12.7 mass %Cu 26.9 mass %Cu 49.7 mass %Cu

75

60

80

85

90

95

50 100

SILVER RECOVERY (%) Fig. 2. Silver grade (mass%) versus recovery for the refining of various silver–copper alloys.

517

5

0.12 Quenched Slowly Cooled

2

MOLE FRACTION Ag 2O (XAg O)

SILVER CONTENT OF SLAG (mass %)

C.A. Pickles et al. / Minerals Engineering 24 (2011) 514–523

4

3

2

1

2

4

6

8

10

12

14

0.08 0.06 0.04 Lance in Slag Lance in Metal Lance in Metal and Vary Slag/Metal

0.02 0.00 0.0

0 0

0.10

16

0.1

Fig. 3a. Silver content of quenched slag and slowly cooled slag (mass%) as a function of refining time.

0.3

0.4

0.5

Fig. 4. Mole fraction silver oxide ðX Ag2 O Þ in the slag as a function of the mole fraction of copper oxide ðX Cu2 O Þ in the slag.

rate increases and as a result the Cu2O contents are higher. Lower slag masses result in increased copper oxide contents in the slag. In all cases, the Ag2O content of the slag increased with the Cu2O content and the results were fitted to the following relationship:

12 Quenched Slowly Cooled

10

X Ag2 O ¼ 0:10X Cu2 O þ 0:40X 2Cu2 O

8

ð6Þ

This dependency of the mole fraction of Ag2O on the square of the mole fraction of Cu2O, results in proportionately higher silver losses to the slag, with increasing copper oxide content. Additionally, this would indicate that the activity coefficient of Ag2O becomes lower at higher copper oxide contents.

6 4 2

4.2. Refining mechanism 0 0

2

4

6

8

10

12

14

16

REFINING TIME (mins) Fig. 3b. Copper content of quenched slag and slowly cooled slag (mass%) as a function of refining time.

and copper contents of the slag after slow cooling can be attributed to both the settling of any entrained metallic particles and the redistribution of the silver or the copper, as a result of a slag/metal exchange reaction. Earlier research has shown that the amount of silver dissolved in metallic form (AgO) in molten silicate slags or borosilicate-type glasses is relatively low (Richardson and Billington, 1956). In contrast to the Ag–Cu–O system, where the silver is mainly considered to be present with some copper and oxygen as L1, as discussed previously, the silver in slags and glasses is believed to be present in monovalent ionic form as Ag+ and is associated with oxygen ions (O2). Therefore, the silver in equilibrated slags can be considered to be present as Ag2O (Willis and Hennessy, 1953; Maekawa et al., 1969). Similarly, the solubility of metallic copper in these types of oxide melts is considered to be low (Richardson and Billington, 1956) and the copper is generally considered to be present as Cu2O at the high temperatures utilized in the refining experiments. Fig. 4 shows the mole fraction of silver oxide ðX Ag2 O Þ in the slag as a function of the mole fraction of copper oxide ðX Cu2 O Þ in the slag for many of the silver–copper alloys studied, for three different test conditions: injection into the slag, injection into the metal and varying slag to metal ratio with injection into the metal. With the lance in the slag, only limited oxidation occurs at the metal surface and thus the amount of Cu2O in the slag is restricted. On the other hand, with injection directly into the metal, the oxidation

In order to elucidate the oxidation processes occurring at the gas–metal interface in the bubble, high temperature TGA and DTA studies were performed on a low copper alloy (11.2 mass% copper) and a high copper alloy (49.7 mass% copper) in air. The eutectic in the Ag–Cu system occurs at 1053 K and therefore above this temperature there is liquid metal present for all compositions. Fig. 5 shows the TGA results and it can be seen that initially the mass continuously increases, due mainly to the conversion of the copper into Cu2O and/or CuO. The oxidation rate of the high copper alloy is significantly higher than the low copper alloy. In all cases there is an abrupt mass increase just above 1200 K and then a sudden decrease in mass at about 1250 K. The increase in mass in the

112 49.7 mass %Cu 11.2 mass %Cu

MASS CHANGE (%)

COPPER CONTENT OF SLAG (mass %)

0.2

MOLE FRACTION Cu2O (XCu2O)

REFINING TIME (mins)

110

108

106

104 1000

1050

1100

1150

1200

1250

1300

1350

TEMPERATURE (K) Fig. 5. TGA results for the low and high copper alloys.

1400

518

C.A. Pickles et al. / Minerals Engineering 24 (2011) 514–523

range of about 1200–1225 K is due to the absorption of oxygen by silver. It is known from the Ag–O phase diagram that silver exhibits a freezing point depression as it absorbs oxygen at its melting point of about 1235 K (Karakaya and Thompson, 1992). The rapid mass loss in the range of about 1230–1260 K is a result of the reduction of some of the CuO to Cu2O and oxygen as follows:

4CuO ¼ 2Cu2 O þ O2ðgÞ

ð7Þ

This process continues, but at a slower pace as the temperature continues to increase. For the higher copper alloy, both the abrupt mass loss and the rate of decrease of mass above about 1250 K are higher due to the higher copper content of the alloy. Below about 1200 K it can be seen that the oxidation rate of the lower copper alloy liquid is linear. This would indicate that diffusion through the increasing amount of solid oxide product does not reduce the rate of reaction. For the high copper alloy, the rate of increase is linear at low temperatures, but begins to decrease as 1200 K is approached and therefore diffusion through the reaction product layer is limiting the reaction rate. A number of studies have shown that during the high temperature oxidation of copper, a CuO layer forms on the surface of the oxide and this reduces the oxidation rate because bulk diffusion of reacting species through the CuO layer is problematic (Zhu et al., 2004). Higher copper contents in the alloy would favour the formation of CuO rather than Cu2O and this could reduce the oxidation rate. This surface effect of CuO could even affect the oxidation process at higher temperatures, where there is a liquid oxide, since the melting point of CuO (1719 K) is almost 200 K higher than that of Cu2O (1508 K). Fig. 6 shows the DTA results for the low copper alloy and the high copper alloy, respectively. For each condition, two major endothermic peaks are observed with the lower one corresponding to the eutectic reaction (Eq. (1)) and the higher one to the monotectic reaction (Eq. (2)). The temperatures of these peaks correspond to the temperatures of the sudden mass changes as described previously. The temperatures at which the peaks occur are not dependent on the copper content of the alloy. The proposed refining process consists of two major reactions: (1) a reaction between the oxygen and the molten alloy at the gas-bubble interface and (2) an exchange reaction in the slag. The four major stages in the oxidation of the liquid alloy are shown in Fig. 7a–d. Since the gas is injected at room temperature and under pressure then it is necessary to postulate the oxidation sequence based on both kinetic and thermodynamic factors. At 1413 K, the overall composition of the alloys utilized in the refining experiments lie within the miscibility gap in the Ag–Cu–O system as shown in Fig. 1 and the two immiscible liquids, L1 and L2, would

coexist. For the alloys investigated, the compositions of L1 and L2 would remain the same but the relative amounts would change. The first stage would be the oxidation of the copper in the molten alloy to Cu2O and CuO and the oxygen content of the silver–copper alloy would increase. Then, L1 forms in equilibrium with Cu2O and CuO. Subsequently, the CuO is converted into L2, which coexists with L1. At even higher temperatures (over about 1250 K) oxygen can be evolved according to Eq. (7) and this could result in some gas bubbles in L2. 4.3. Determination of activity coefficient of Ag2O These two immiscible liquids (L1 and L2) are then transported by the gas bubble into the slag phase, where they both dissolve and a new equilibrium is established. These liquids are oxygen-enriched and once in the slag, additional oxygen can be released according to Eq. (7). As the two liquids dissolve in the slag an exchange reaction is established between the cuprous and silver oxides and the silver and copper metals as follows:

ðAg2 OÞ þ 2Cu ¼ ðCu2 OÞ þ 2Ag

The reaction is written in the direction in which it should proceed in order to minimize the silver content of the slag. The standard Gibbs free energy change (DGo) at the refining temperature of 1413 K is 117.28 kJ. The silver and copper metals are present in a finely disseminated form in the slag and if dissolved in each other, their activities are less than unity. Also, the silver and copper oxides are dissolved in the slag and their activities would be less than one. Therefore, despite the physical proximity of the reagents and their dispersed form, the reaction could be hindered if the activities of the reactants are low or the activities of the products are high. The actual reaction of the bulk metal with the bulk slag across the slag–metal interface would be expected to be very slow and an experiment in which the slag was allowed to equilibrate with the metal in air for 6 h resulted in a slag containing only 0.02 mass% silver and 0.13 mass% copper. The true equilibrium silver and copper contents of the slag in the refining process would be achieved by extending the refining times until the copper content reached a constant value. However, if the gas bubbling is stopped prematurely and the slag and metal are allowed to cool slowly then the system can be considered to be in a pseudo-equilibrium state and the reaction stops because of the limited supply of reagents. The equilibrium constant for the exchange reaction in terms of activities is as follows:

-0.2

0.2

0.0

0.4

0.2

0.6

0.4

0.8

0.6

1.0

11.2 mass %Cu 49.7 mass %Cu

0.8

1.2 1.0 1180 1190 1200 1210 1220 1230 1240 1250 1260 1270 1280

TEMPERATURE (K) Fig. 6. DTA results for the low and high copper alloys.

ðaCu2 O ÞðaAg Þ2 ðaAg2 O ÞðaCu Þ

2

¼

ðcCu2 O X Cu2 O ÞðcAg X Ag Þ2 ðcAg2 O X Ag2 O ÞðcCu X Cu Þ2

ð9Þ

If the system behaved ideally then the activity coefficients for both the metals and the metal oxides would be unity and therefore the equilibrium could be represented in terms of the slag–metal partition coefficients for silver and copper as follows:

uv/mg

uv/mg

K¼ 0.0

ð8Þ

X 2Ag ln X Ag2 O

! ¼ ln K  ln

X Cu2 O X 2Cu

! ð10Þ

This ideal equation is plotted in Fig. 8 and also included are the partition coefficients as calculated from the experimental data. It can be seen that the silver partition ratios are higher than the ideal values at high copper contents in the metal. On the other hand, as the copper content of the metal decreases then the silver partition ratio approaches the ideal value. Lower copper contents in the metal and higher copper oxide contents in the slag correspond to higher silver losses to the slag. Although the activity coefficient of silver as calculated by Eq. (4) approaches unity at low copper

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C.A. Pickles et al. / Minerals Engineering 24 (2011) 514–523

Ag-Cu Alloy + CuO/Cu 2O

Ag-Cu Alloy

(a)

(b)

Gas Bubble

Gas Bubble

(d)

(c)

Gas Bubble

L1

Gas Bubble

L2 +O2 Bubbles

L1 + CuO/Cu2 O

Fig. 7. Proposed oxidation mechanism at the bubble-metal interface, with the oxidation sequence proceeding from (a) to (d): (a) oxidizing gas bubble in liquid Ag–Cu alloy, (b) formation of CuO/Cu2O, (c) formation of L1 and CuO/Cu2O, (d) formation of L1 and L2.

The oxidation of the copper in the melt can be represented by the following equation:

10 Experimental Ideal

4CuðlÞ þ O2ðgÞ ¼ 2Cu2 OðlÞ

ð12Þ

The standard free energy change of this reaction is given as follows (Assal et al., 1998):

6

DGo ¼ 230; 759 þ 87:76T ¼ RT ln K kJ=mole

4

Also the equilibrium constant for Eq. (12) can be defined as follows:

ð13Þ

2

2

ln (XAg /XAg O)

8

K¼

2

0 -2

0

2

4

6

8

10

2

a2Cu2 O a4Cu pO2

Fig. 8. Partition coefficient of silver (in terms of mole fraction) as a function of the partition coefficient of copper (in terms of mole fraction) for the experiments and also the ideal case.

contents, the activity coefficient of copper (Eq. (5)) approaches a value of about three. In order for the experimental partition coefficient ratios to approach the ideal values then this would indicate that the ratios of the activity coefficients of the metal and slag phases as shown in Eq. (9) are approximately equal as follows:

!

c2Ag cCu2 O  cAg2 O c2Cu

! ð11Þ

c2Cu2 O X 2Cu2 O ¼ c4Cu X 4Cu pO2

! ð14Þ

For the refining temperature of 1413 K and in an air atmosphere (pO2 ¼ 0:21 bar), cCu2 O is as follows:

ln (XCu O/XCu ) 2

!

cCu2 O ¼

43:1c2Cu X 2Cu X Cu2 O

ð15Þ

where cCu is defined by Eq. (5). Values of cCu2 O were determined as a function of X Cu2 O and then these values were substituted into Eq. (9), in order to obtain cAg2 O as a function of X Cu2 O . Fig. 9 shows the results and it can be seen that the activity coefficient of silver oxide decreases with increasing copper oxide content according to the following relationship:

ln cAg2 O ¼ ð1:5  70:7X Cu2 O Þð1 þ 15:0X Cu2 O Þ1

ð16Þ

Thus, the silver losses to the slag increase with the copper oxide content of the slag. In comparison to copper oxide, silver oxide has a relatively low activity and thus its absorption into the slag is

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C.A. Pickles et al. / Minerals Engineering 24 (2011) 514–523

Ag2O/Cu2O MASS PERCENT RATIO

2 1

2

lnγAg O

0 -1 -2 -3 -4 -5 0.0

0.1

0.2

0.3

0.4

1.0 10 mass %Cu 25 mass %Cu 50 mass %Cu

0.9 0.8 0.7 0.6 0.5 0.4 0.3 1300

1350

1400

1450

1500

1550

TEMPERATURE (K)

MOLE FRACTION Cu2O (XCu O) 2

facilitated by the presence of the copper oxide (Miroslaw and Zmuda, 1989; Ziolek and Szczepaniak, 1998). By using multiple slags and thus maintaining a low copper oxide content in the slag, it should be possible to reduce the silver losses. This would only be beneficial for the high copper alloys. In this regard, Freidl and Frost (1974) proposed a slag washing technique in which they considered the refining process to be a cross-flow solvent extraction problem. This also allowed an accurate prediction of the end point of refining.

4.4. Modelling of silver loss to the slag

Ag2O/Cu2O MASS PERCENT RATIO

Utilizing the activity coefficients of the silver and copper oxides, the equilibrium module of Outokumpu HSC Chemistry 6.1 was used to investigate the effect of operating variables on the silver loss to the slag. The results are presented in terms of the calculated Ag2O to Cu2O mass ratio of the slag. Lower ratios indicate a lower silver loss to the slag and a higher degree of copper removal from the metal. The effects of the copper content of the metal, the operating temperature, the amount of slag and the partial pressure of oxygen are shown in Figs. 10a–10d, respectively. An increasing copper content in the metal results in higher copper oxide contents in the slag and this results in higher silver losses to the slag and thus the Ag2O to Cu2O mass ratio increases. Since Cu2O is more stable than Ag2O, then an increase in the temperature results in a pro-

Fig. 10b. Effect of refining temperature on the Ag2O to Cu2O mass percent ratio of the slag for various initial copper contents (mass%) of the alloy.

Ag2O/Cu2O MASS PERCENT RATIO

Fig. 9. Activity coefficient of silver oxide as a function of the mole fraction of copper oxide in the slag.

1.0 10 mass %Cu 25 mass %Cu 50 mass %Cu

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0

100

200

300

400

500

600

700

SLAG MASS (grams) Fig. 10c. Effect of slag mass on the Ag2O to Cu2O mass percent ratio of the slag for various initial copper contents (mass%) of the alloy.

0.8

0.7

0.6

0.5 Fig. 10d. Effect of oxygen partial pressure on the Ag2O to Cu2O mass percent ratio of the slag.

0.4

0.3 0

10

20

30

40

50

60

COPPER CONTENT OF ALLOY (mass %) Fig. 10a. Effect of mass percent copper content of the alloy on the Ag2O to Cu2O mass percent ratio of the slag.

portionately larger decrease in the amount of Ag2O in comparison to Cu2O and the ratio decreases. With regards to slag mass, for the low copper alloy, the Ag2O to Cu2O ratio is not a strong function of slag mass. However, for high copper alloys the ratio decreases with increasing slag mass, since for a given alloy composition, the

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3000

1400 B = Borax

Ag

Cu2O

1200

2500

INTENSITY

INTENSITY

1000 B B Cu2O

800 B

Ag

B

600

Cu2O

2000

Ag

1500 CuO

1000

CuO Cu2O Ag

Cu2O

400

Cu2O Ag

Cu2O CuO

500

CuO

Ag CuOCuO

200 10

20

30

40

50

60

70

2θ

0 20

30

40

50

60

70

2θ

Fig. 11. XRD pattern of quenched slag.

Fig. 12b. XRD pattern of the lower portion of the slowly cooled slag.

1000

INTENSITY

800

600

400

Ag

Ag

200

0 10

20

30

40

50

60

70

2θ Fig. 12a. XRD pattern of the top portion of slowly cooled slag.

Fig. 13. Optical micrograph of finely disseminated metal particles in the quenched slag.

copper oxide content of the slag is lower at higher slag masses. Again this indicates that a multiple slag refining process would be beneficial particularly for the high copper alloys. With regards to the oxygen potential, the Ag2O to Cu2O ratio increases with increasing pO2 , again reflecting the fact that Cu2O is more stable than Ag2O. 4.5. Slag characterization X-ray Diffraction studies were performed on the quenched and slowly cooled slags from the refining process. The XRD pattern in Fig. 11, shows that the rapid cooling of the slag resulted in an amorphous phase (mainly silica), metallic silver, Cu2O and borax (Na2B4O5(OH)48(H2O)). Slow cooling of the slag in the furnace resulted in separation into an upper and lower phase. The XRD patterns of the upper and the lower phases are shown in Figs. 12a and 12b, respectively. The upper phase was amorphous borosilicate glass but contained some small amount of metallic silver, while the lower phase was more crystalline and consisted of metallic silver, CuO and Cu2O. Photomicrographs of the same samples are shown in Figs. 13, 14, respectively. The consideration of these figures and the previously discussed XRD patterns provide some information on possible metal recovery methods. As shown in Fig. 13, the quenched slag shows only evidence of localized areas of a dispersion of very fine metallic particles, which the XRD pattern (Fig. 11) identifies as metallic silver. Thus the majority of the silver and copper remain

Fig. 14a. Agglomerated metal particle in the lower portion of the slowly cooled slag.

dissolved in the slag. In the slowly cooled slag, there are a number of forms of metallic silver but mainly relatively large silver–copper metallic droplets (Fig. 14a) which likely form as a result of the decomposition of the silver and copper oxides in the slag and subsequent coalescence in the liquid state. Additionally, as shown in Fig. 14b, there are some finer silver–copper particles associated with the copper oxide precipitates.

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Fig. 14b. Fine metallic particles associated with the copper oxide precipitates in the lower portion of the slowly cooled slag.

(2) DTA and TGA studies were performed on both a low copper and a high copper alloy and the results are in agreement with previous research on the Ag–Cu–O system. These results indicate that in the oxidizing gas bubble, a silver-rich ionic liquid, containing small amounts of oxygen and copper (L1), is in equilibrium with another ionic liquid, containing copper and some silver and oxygen (L2). For the low copper alloy, the oxidation rate in air was linear with temperature. However, for the high copper alloy, the oxidation rate at high temperatures decreased and this was attributed to the presence of a larger amount of CuO, which hinders the oxidation process. (3) These two liquids (L1 plus L2), containing silver, copper and oxygen in ionic form, are transported by the gas bubble into the borosilicate slag where the final equilibrium is determined by the following exchange reaction:

CONCENTRATION IN SLAG (mass %)

ðAg2 OÞ þ 2Cu ¼ ðCu2 OÞ þ 2Ag 10 Cu2O Ag2O

8

6

4

2

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

MASS OF CARBON ADDED (g) Fig. 15. Calculated silver and copper oxide contents (mass%) of the slag as a function of the mass of carbon added at the temperature of 1413 K.

4.6. Silver recovery from slag A number of possibilities could be considered for recovering the silver from the slag such as slow cooling, fragmentation and physical separation and/or leaching. In the liquid state, the slag could be reduced by various reducing agents such as carbon, aluminium or ferrosilicon. Fig. 15 shows an example of the expected behaviours of silver oxide and copper oxide in the slag as a function of the carbon added at the temperature of 1413 K. With sufficient carbon, the majority of silver oxide and some copper oxide in the slag can be reduced to metallic form. However, carbon has a relatively low density and it is difficult to achieve good contact. Ferrosilicon, which has a density closer to that of the slag could prove advantageous. 5. Conclusions (1) During the high temperature oxidative refining of silver– copper alloys under a borosilicate slag, silver is lost to the slag. Grade-recovery curves for alloys with up to about 50 mass% copper, showed that the loss increased with the amount of copper removed and increasing initial copper content of the alloy. The recent research on the Ag–Cu–O system, as reported in the literature, was utilized to develop an improved understanding of the mechanism of both the silver transport to the slag and the loss.

This reaction can occur either in the slag or at the bulk slag/ metal interface. Copper can be considered to be present in the slag as Cu2O and for thermodynamic analysis purposes the silver can also be thought of as being present as Ag2O. (4) It was shown that the silver losses to the slag increased with the copper oxide content of the slag and this was attributed to a decrease in the activity coefficient of silver oxide with increasing copper oxide content. (5) The Equilibrium module of Outokumpu HSC Chemistry 6.1 was used to predict the changes in the silver content of the slag as a function of the operating variables in the refining process. In general, lower copper contents in the metal and thus lower copper oxide contents in the slag, lower refining temperatures, lower oxygen potentials and higher slag masses for the higher copper alloys can be utilized to reduce the silver loss to the slag. (6) A number of potential processes were considered for recovering the silver from the refining slag. Calculations show that reduction processes using carbon, aluminium or ferrosilicon could be utilized to recover the silver. Also, slow cooling of the slag promotes the growth of metallic particles and physical separation processes and/or leaching could be considered to recover the entrapped silver.

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