GB2368349A

GB2368349A - Electrolytic extraction of metals; recycling

Info

Publication number: GB2368349A
Application number: GB0026390A
Authority: GB
Inventors: Geoffrey Howard Kelsall; Nigel Peter Brandon
Original assignee: Imperial College of London
Current assignee: Imperial College of London
Priority date: 2000-10-27
Filing date: 2000-10-27
Publication date: 2002-05-01
Also published as: GB0026390D0

Abstract

A process for extracting a metal from a composition comprising the metal comprises i) leaching the metal from the composition by contacting the composition with an oxidant electrolyte; ii) subjecting the separated electrolyte to electrolysis between an anode and a cathode such that the metal is liberated at the cathode and oxidant at the anode; and iii) recycling the liberated oxidant to the electrolyte of step i). The oxidant electrolyte may be acidic brine and the metals may be selected from Au, Ag, Pd, Cu, Ni, Sn or Pb. Between steps i) and ii) some of the composition may be separated from the electrolyte.

Description

METHOD

The present invention relates to the recovery of metals from electronic scrap by leaching and electrowinning.

This work is concerned with developing a novel process for the recovery of metals from electronic and similar scrap, estimated to be worth more than E80 million pa in the UK alone. The volume of such scrap is expected to increase sharply, and its disposal in landfill sites discouraged, over the next few years. Hence it is timely to develop new technology in this area which overcomes the problems associated with current pyrometallurgical based processes, namely the generation of noxious gaseous emissions The process uses a suitable electrolyte and oxidant, for example an acidic chloride electrolyte plus chlorine, to leach metals such as Au, Ag, Pd, Cu, Ni, Sn and Pb out of electronic scrap in a leach reactor. This produces a liquor containing all the metals in the form of chloro-complexes. Thermodynamic calculations predict that is should then be possible to selectively recover the metals from solution by electrowinning, based on differences in their respective electrode potentials (summarised in Fig. 1). During the electrowinning process, which is carried out using a suitable cathode, for example a three dimensional graphite felt cathode, to maximise the recovery of metals present at low concentrations (such as Au and Ag), chlorine is evolved at the anode of the cell.

This chlorine can then be passed into the leach reactor to act as an oxidant in the leaching process. Fig 2 describes the overall process chemistry. An ion exchange membrane allows selective chloride ion transport from the catholyte into the anolyte. In this manner, the chloride is effectively recycled around the process, with the only inputs being the electronic scrap and electrical energy, and the only output the recovered metals.

Figure 3 describes the overall process flowsheet.

Laboratory testing has demonstrated the feasibility of the process, including the possibility of selectively recovering metals, even when operating the electrochemical reactor under constant current rather than constant potential conditions, which is the preferred method of operation for industrial scale electrochemical reactors. These two

embodiments are discussed in the following : Potential Control The primary and important metals dissolved in leaching process are listed in Figure 1. It shows that the standard potential of redox couple AuCI ;/Au is much higher than that of redox couple ZnCl2/Zn. The difference between them is more than 1.7 V. From the thermodynamic point of view, if the standard potential between two redox couples is greater than 0.18 V, it suggests that these two metals can be selectively electrodeposited. Therefore, the metals listed in Fig. 1 are likely to be selectively recovered in a few groups. Indeed, the experimental results have demonstrated this. For example, Fig. 4 describes the timeline of an experiment in which a leach liquor produced by the dissolution of electronic scrap was held at a series of controlled potentials for 50 hours. The current was monitored, and a number of samples taken for analysis. Figs. 5, 6 and 7 respectively show the impact of electrode potential on the fraction of metal recovered from the liquor, the quantity of metal recovered from the liquor, and the composition of the metal recovered from the liquor, as a function of the applied potential.

It can be seen that when the electrode potential was controlled at 0.25 V (SCE), gold was first deposited on the cathode with a recovery around 0.8 (i. e. 80% of the gold within the liquor was recovered at the electrode). As the electrode potential decreased to-0.1 V (SCE), most of palladium and silver were recovered. As the electrode potential further decreased to-0.4 V (SCE), silver and copper recovery approached 1.0. When the electrode of potential reached-0.7 V (SCE), tin and lead were electrodeposited with a recovery of more than 0. 99. In practice, it is anticipated that a series of plating electrochemical reactors will be used to recover the dissolved metals, each operating at a controlled potential. Alternatively, that the reactor would be operated in a batch mode, with the electrode being replaced at the end of each controlled potential deposition. To represent this, Fig. 8 illustrates the composition that would be expected from such a process, i. e. assuming the each electrode was removed from the bath before changing the potential. This Figure illustrates that is should be possible to sequentially recover the precious metals, a pure copper deposit, and then a lead/tin alloy.

The corresponding net current efficiency at various electrode potentials is shown in Fig. 9. The net current efficiency represents the proportion of current passed that resulted in metal deposition. This indicates that when the electrode potential was first controlled at

0. 25 V (SCE) and then-0. 1 V (SCE), the current efficiency was 0. 75 and 0. 88 respectively (where a current efficiency of 1 represents 100%). The loss of current efficiency may be due to the reduction of residual oxidant species, such as HCIO and Cx, consuming electrical energy and decreasing the current efficiency. However, when the electrode potential were controlled at-0.4 and-0.7 V (SCE), the current efficiencies reached 0.96 and 0.95 respectively. This means that copper, tin and lead, the three main metal ions in the leach solution, were electrodeposited with current efficiencies > 0.95.

Fig. 9 also indicates that when the electrode potential further decreased to-0.85 and-1.1 V (SCE), the current efficiency significantly decreased, suggesting that another cathodic reaction, hydrogen evolution, became significant.

Current control In industrial practice, electrolysis is often more conveniently operated at constant current rather than by controlled potential. Therefore, electrowinning experiments were carried out at different constant currents to explore the possibility of selectively recovering metals from the leach solution. Fig. 10 illustrates the change in measured potential with time as three constant current densities. Samples were taken for analysis as shown.

Figs. 11, 12 and 13 respectively show the impact of the resulting electrode potential on the fraction of metal recovered from the liquor, the quantity of metal recovered from the liquor, and the composition of the metal recovered from the liquor at a constant current density of-484.8 A m-2. Fig. 13 assumes that the electrodes were either removed from the electrochemical reactor after a specific time (indicated by a sharp change in electrode potential) or that a series of reactors were used. Figs. 15 to 18 present the same data, but against time rather than potential. Fig. 15 indicates that gold was deposited first with a recovery around 0.8. Then palladium, silver and copper were electrodeposited together. The palladium recovery was around 0.7, but the recovery for silver and copper reached 0.98 and 0.99 respectively. Finally, tin and lead were electrodeposited with the recovery of nearly 1.0. Fig. 14 shows the net current efficiency of the electrowinning process as a function of constant current density. It indicates that the current efficiency decreased with an increase in current density. However, even when the current density increased to 484.8 A m', the current efficiency was higher than 0.8.

Leaching

The chlorine produced at the anode of the electrochemical cell is passed into a leach reactor, containing a well mixed acidic brine solution (typically 3 M HCI and 1 M Nazi), and shredded (sub 4 mm) electronic scrap. The chlorine gas acts as an oxidant, dissolving the metals within the scrap as soluble chloro-complexes. In this manner, more than 99% of the metals within the scrap are put into solution and are available for recovery within the electrowinning step.

Overall process Table 1 summarises the overall performance of the process, based on experimental data. It indicates that when electronic scrap is treated by the proposed process, the chlorine and electricity costs fall below : EO. 15 kg-1, whereas the value of the recovered metals is more than 3. 4 kg-1. This suggests that this novel process may offer both environmental and economic advantages over conventional pyrometallurgical processes.

Claims

1. A process for extracting a metal from a composition comprising the metal, the process comprising i) leaching the metal from the composition by contacting the composition with an oxidant electrolyte ; ii) subjecting the separated electrolyte to electrolysis between an anode and a cathode such that the metal is liberated at the cathode and oxidant at the anode; and iii) recycling the liberated oxidant to the electrolyte of step i).

2. A process according to claim 1 further comprising the step, between steps i) and ii) of separating at least some of the composition from the electrolyte.

3. A process according to claim 1 or 2 wherein the oxidant electrolyte is a chlorine containing acidic electrolyte.

4. A process according to claim 1,2 or 3 wherein the metal is selected from Au, Ag, Pd, Cu, Ni, Sn, Pb and combinations thereof.

5. A process according to any one of claims 1 to 4 wherein the cathode is a three dimensional graphite felt cathode.

6. A process according to any one of claims 1 to 5 wherein electrolysis is performed across an ion exchange membrane wherein the membrane allows selective chloride ion transport from catholyte to anolyt.

7. A process according to any one of claims 1 to 6 wherein electrolysis is performed at a constant current.

8. A process according to any one of claims 1 to 7 wherein electrolysis is performed at a constant potential.

9. A process as substantially hereinbefore described.