WO1987006274A1

WO1987006274A1 - Metal recovery

Info

Publication number: WO1987006274A1
Application number: PCT/GB1987/000258
Authority: WO
Inventors: Geoffrey Howard Kelsall
Original assignee: Imperial College Of Science & Technology
Priority date: 1986-04-16
Filing date: 1987-04-16
Publication date: 1987-10-22
Also published as: AU7302987A; GB8609340D0; ZA872757B

Abstract

A method and apparatus for extracting metal from scrap, ore or other material wherein an oxidant is generated electrochemically in the anode compartment of an electrochemical cell, the oxidant is reacted with said scrap, ore or other material in a reactor to form soluble species of the metal, said soluble species being reduced to metal in the cathode compartment of said electrochemical cell. The invention is described in particular in relation to reecovery of gold using chlorine and chloride solutions.

Description

METAL RECOVERY

This invention relates to the recovery of elemental metals from sources amenable to chemical oxidation. Particularly, but not exclusively, the intended application is the recovery of gold from gold-bearing ores, scrap articles and like sources.

The classic method of recovering gold is complexation with cyanide and dissolved oxygen to solubilise the gold, followed by isolation of metallic gold from the solution. However, an earlier process, which the cyanide process displaced around the end of the nineteenth century, used chloride ion, instead of cyanide, as the complexant and chlorine, instead of oxygen, as the oxidant. It has been reported that the chloride/chlorine process achieves rates of leaching which may be more than two orders of magnitude greater than those achievable with cyanide/oxygen. However, previous attempts to utilise the chloride/chlorine system have involved the use of free chlorine gas which limits the applicability of the process on a commercial scale.

Another process used for leaching gold which is used to a minor extent industrially utilises thiourea under acid conditions to complex the gold as a soluble species.

An object of the present invention is to provide an improved metal, particularly gold, recovery process. According to the present invention there is provided a method of extracting metal from scrap, ore or other material comprising the steps of: electrochemically generating an oxidant in the anode compartment of an electrochemical cell; reacting the oxidant with scrap, ore or other material in a reactor to form a soluble species of the metal; and reducing said soluble species to metal in the cathode compartment of said electrochemical cell.

Preferably, the metal is gold and the oxidant is electrochemically generated chlorine or hypochlorite ion.

The anolyte, from which the chlorine may be generated is, preferably, sodium chloride at pH equal to or greater than 7.

The invention further provides metal recovery apparatus comprising said electrochemical cell, an external reactor, and a pump arranged such that: electrolyte from the anode compartment may be passed to the external reactor and from the external reactor to the cathode compartment; and electrolyte from the cathode compartment may be passed to the anode compartment.

The electrode reactions involved in the process of the present invention are as follows:-

AuCl^" + e = Au + 2Cl^" (1) ^•

E_χ/V = 1.152 + 0.0591og(AuCl2) - (0.1181og(Cl^")

AuCl^" + 3e = Au + 4Cl^" (2)

E₂/V = 1.001 + 0.01971og(AuCl~) - 0.0791og(Cl^")

SUBSTITUTESHEET At concentrations greater than those given by equation (3) below, Au(I) species are thermodynamically unstable to disproportionation by the reaction

3AuCl₂ —5^» AuCl^ + 2Au + 2Cl^" (3) G° = -43.713 kJ (mol AuCl^")^"1

31og(AuCl~) ^s -7.66 + log(AuCl~) + 21og(Cl^")

In spite of the standard electrode potential for reaction (1) being greater than that for reaction (2), the different gold concentration dependences of the potentials results in a potential-pH area of predominance in which AuCl₂ ions are more stable than AuCl ions for those low concentrations given by equation (3). Thus, in that potential range, Au(III) species could be reduced by the reaction

AuCl^ + 2e ^» AuCl₂ + 2Cl^" (4)

E₄/V * 0.925 + 0.0291og(AuCl4) -

0.291og(AuCl₂) - 0.0591og(Cl^")

In the gold leaching and recovery process of this invention chlorine evolved at an anode of, for example Ti/Ru0₂, in a cation-exchange membrane divided cell:

Cl₂ + 2e » 2C1 (5)

E₅/V = 1.395 + 0.0291og(Cl₂) - 0.0291og(Cl^")

is allowed to react with gold ore (or electronic scrap etc.) in an external reactor in which gold dissolution then occurs at a mixed potential by the reactions:

SUBSTITUTE SHEET Au + 0 .5C1₂ + Cl -?» AuCl₂ (6 )

A E₆/V = 0 .243 + 0 .0291og(Cl ₂ ) -

0 .0591og( (AuCl₂ ) + 0 .0891og(Cl ^" )

Au + 1.5C1₂ + Cl^'-^AuCl^ (7) Δ^{E V} " °-^{395 +} 0.0291ogCGl₂) -

0.01971og(AuCl~) + 0.04941og(Cl^")

The solubilised gold may then be electrowon from dilute solution by reactions (1) and (2), using a three dimensional (packed or fluidised bed) cathode in the same electrochemical cell to achieve high mass transport rates, cross sectional current densities and space-time yields. While chlorine/hypochlorite is preferably used as the oxidant in this invention, in principle other soluble oxidants which can be electro-(re)generated, and which have an adequately high reversible potential (^E may be used, with chloride as the

complexant. Indeed, subject to the same oxygen solubility constraints as the cyanide process, the concomitant reduction of oxygen by the reactions

0₂ + 2H⁺ = H₂0₂ (8)

H₂0₂ + 2H⁺ +2e = 2H₂0 (9)

enhances the rate of gold leaching by chlorine. The solubility of chlorine is much greater than that of oxygen:

Cl₂ (aq) = Cl₂ (g) (11)

SUBSTITUTE SHEET though in an open system, chlorine would be lost to the atmosphere. However, such is the depassivating effect of chloride ions on gold (Figs. 3 and 4) that pH's up to about 7 could be used in the leaching of gold in chloride media, so that hypochlorite rather than chlorine could be used as the oxidant:

Cl₂ + H₂0 = HC10 + Cl^" + H⁺ (12) log(HClO) = -3.332 + log(Cl₂) + pH -log(Cl^')

HC10 = H⁺ + CIO^" (13) log(Clθ^') = -7.55 + pH + log(HClO)

2HC10 = C1₂0 (g) + H_£0 (14) log p_{cl 0} - -3.59 + 21og (HC10)

2 HC10 + H⁺ + 2e = Cl^" + H₂0 (15)

E₁₅/V = 1.494 - 0.0295pH - 0.02951og(HC10) - 0.02951og(Cl^")

CIO^" + 2H⁺ + 2e » Cl^" + H₂0 (16)

E₁₆/V = 1.718 - 0.059pH + 0.0291og(Clθ^") - 0.0291og(Cl^")

Au + 1.5HC10 + 1.5H⁺ + 2.5Cl^" = AuCl^ + H₂0 (17)

- 0 .02951og (HC10) -

- 0 . 0295pH - 0 .04951og (Cl ^" ) ^•

Au + 0.5HC10 + 0.5H⁺ + 1.5Cl^" =» AuCl₂ + H₂0 (18)

SUBSTITUTESHEET • 5A-

ΛB_{l 8}/V = 0 .342 - 0 .02951og(HC10) -

0.0591og(AuCl₂) - 0.0295pH + 0.0881og(Cl^")

In a preferred embodiment the present invention is used to recover, from chloride electrolytes, the low concentrations (say 1-100 ppm) of gold which are generated in a leaching step from the gold source material. As aids to interpreting the results, E, -pH and Au(III) activity-pH diagrams were calculated from available thermodynamic data and UV spectrophotometry used to discriminate between Au(I) and Au(III) species

The invention will now be described, by way of example, with reference to the accompanying drawings of which Fig_* 1 is a drawing of a membrane-divided electrochemical reactor, and

Fig.2 is a flow diagram of an electrochemical system for gold recover

SUBSTITUTESHEET ^■ 6 -

Re erring to Fig.l, a flow-through electrochemical reactor 1 has anolyte inlet 2 and outlet 3 and catholyte inlet 4 and outlet 5.

The reactor is divided internally by a membraneous partition 6 of an ion-exchange membrane, into cathode 7 and anode 8 compartments

An anode, Ti/Ru0₂, referenced 9, extends into the anode compartment 8, and a cathode feeder electrode 10, of Ti/Pt, extends into cathode compartment 7 to feed a packed (or fluidised) bed particulate graphite electrode therein.

Flow distributor 11 is provided to distribute flow of liquid through the reactor 1. A compartment 12 is provided for placement of a reference electrode, for example, a standard calomel electrode (s.c.e.) to enable potential control of the bed electrode.

The reactor 1, shown in Fig.l is, on a preferred embodiment, a Perspex packed/ luidised bed electrode cell, which incorporates a Ti/Ru0₂ mesh anode (IMI Ltd) and a Nafion 425 cation exchange membrane (Du Pont

Inc.). The cathode feeder electrode 10 is a Ti/Pt mesh contacting a 10 mm thick packed bed electrode which consisted of 2-3 mm graphite chips (estimated projected

2 area 0.09 m ). The cross sectional active area of the electrode and membrane was 0.145 m x 0.046 m = 6.67 x 10 -3 m2. The membrane prevented transport of anionic gold species to the anode, at which Au(I) species would otherwise have been oxidised, and, more importantly, minimised transport of anodically generated chlorine to the cathode, at which its reduction would have decreased the current efficiency for gold deposition and lowered the chlorine utilisation

Referring now to Fig.2, a flow system for electrochemical recovery of gold includes the following components: an electrochemical reactor 1 (as described hereinabove with reference to Fig.l), an anolyte reservoir 20, a catholyte reservoir 21 and a leach vessel 22. Suitable pipework, pumps, values and flowmeters are provided to interconnect the component parts of the system.

Anolyte from reservoir 20 is pumped by pump 23 via line 24 and flowmeter 25 to the anode compartment of the membrane-divided reactor 1. At the anode of reactor 1, chlorine is liberated and dissolves in the anolyte.

Chlorine containing anolyte leaves reactor 1 via line 26 and may be returned to reservoir 20 via line 27 or diverted, wholly or in part, via lines 28 and 29 to the catholyte reservoir 21.

Catholyte from reservoir 21 is pumped by pump 30 via line 31 and flowmeter 32 to diverted valve 33 whence it may be sent by lines 34 and 36 to the reactor 1 or via line 35 through leach vessel 22, containing a gold source material, and hence via line 36 to the cathode compartment of the reactor 1. Catholyte flows from reactor 1 via line 37 to return to reservoir 21.

In a preferred embodiment the flow circuit shown in Fig.2 was constructed from uPVC pipework, valves and fittings (G. Fischer Ltd.) and incorporated 5 dm aspirators as reservoirs, Totton Electrics Ltd. EMP 50/7 ^■ 8 -

magnetically coupled polypropylene pumps and flow meters with acid resistant ceramic floats (Fisher Controls Ltd.). The only corrodible material in the flow circuit was the gold source, in particulate form, in the leach vessel 22.

In the accompanying drawings, the remaining Figures 3 to 11 are as described below.

The potential-pH diagram shown in Fig 3. was generated by computer using published free energy of formation data of the gold species considered in

"Standard Electrode Potentials in Aqueous Solutions", Ed. A.J. Bard et.al., Dekker, New York 1985, pp 313-320. For the chosen dissolved gold concentration (0.5 mol m^" ), AuClI had no area of predominance, in agreement with the prediction of equation (3). Comparison with the corresponding diagram for the Au/H-0 system shows the powerful depassivating and solubilising effect of Cl^" ions, due to the reaction:

Au(0H)₃ (s) + 3H⁺ + 4Cl^" » AuCl^" + 3H₂0 (20)

log(AuCl^') ^» 18.38 - 3pH + 41og(Cl^")

However, the standard electrode potential (equation (2)) for the AuCl^/Au couple is 1.001 V, so that tetrachloroaurate ions are very easily reduced.

Fig.4 shows an Au(III) activity (solubility)-pH diagram calculated from the same data source as Fig.3 and for unity Cl^" activity. Again the strong coraplexing effect of Cl ions is evident, giving rise

SUBSTITUTE SHEET to significant gold solubility at high Cl ion activity, even in the neutral pH region, so that rigorous pH control is not required in the process of the invention.

Fig.5 shows a typical cyclic voltammogram of a platinum electrode in a solution containing predominantly AuCl ions at pH zero. The key features are:

(i) an anodic current peak at +0.95 V corresponding to the stripping of previously deposited gold principally as AuClZ by reaction (1); (ii) a second anodic current peak at about 1.12 V due to reaction (2) in parallel with reaction (4); (iii) the onset of chlorine evolution at E 1.2 V; and,

(iv) a single reduction wave at potentials 0.95 V due to deposition of gold by reactions (1) and (2) in parallel with reaction (4).

Fig.6 shows the consequence of the instability to reduction of AuClT ions, and possibly of their electrostatic adsorption on flow circuit surfaces (uPVC, carbon/graphite etc) which were likely to have been positively charged due to protonation of surface groups.

Fig.6 also shows that there was very significant depletion of AuClT ions from solution by the previously unused corbon packed bed with no applied potential.

Fig.7 shows the flow rate dependence of the exponential decay of currents and dissolved gold concentrations by the recirculation of the electrolyte - 10-

through the packed bed electrode (PBE), the potential between the feeder electrode and solution at the membrane of which was held at 0.529 V, on the mass transport limited reduction wave observed in the voltammetry (Fig.5). Although the lowest gold concentration shown in Fig.7 is 3 g m , levels below the detection limit (say, 0.1 g m ) of atomic absorption spectrophotometry were achieved routinely. Initial decay rates were sensitive to the history of the bed, reflecting the changing area for reaction on the graphite particles. The concentration decay has the general form of the steady-state stirred tank reservoir, plug flow rector model equation:

c(t) = c(o)exp[-t/T(l-exp(-kAaL/Q))3 (21)

though as discussed below the reaction mechanism is more complex than a single mass transport controlled process assumed by the model. However, using such a model as a first level approximation, then the inlet (c.) and outlet (c ) concentrations are related by the equation:

^Co " c_iexp(-kAaL/Q) (22)

The linear current density vs concentration relationships shown in Fig.8 are supporting evidence that the reduction process(es) were transport controlled, and as the lines passed through the origin, this implied high current efficiency even at very low concentrations. Analysis of solution samples taken during depletion experiments showed (Figs. 9 to 11) that while the total dissolved gold concentration (Au_™) decayed exponentially with time (Fig.7), the

SUBSTITUTESHEET - 11 -

Au(I)/Au(III) molar ratio also decreased initially from a value of 0.35 to 0.25, depending on the particular solution used, the former being the equilibrium value given by equation (3). That ratio then increased, passing through a maximum value which increased with flow rate, before decaying to zero at long times, though prior to the dissolved gold (Au_τ) being totally depleted. This behaviour was particularly pronounced at the highest flow rate used (Fig.11), at which the Au(I)/Au(III) molar ratio showed a sharp peak after

700 s, when the total dissolved gold had decreased to <0.1 mol m^"3 (19.7 g m^"3)

As the solutions contained both Au(III) and Au(I) species, a figure of merit of Faradays per mole of gold deposited was used, rather than the more usual current efficiency (%). The data for F (mol Au)^" corresponding to the depletion results in Fig. 7. are given in Figs. 9-11. These show an increase from a short time ( <200 s) value of about 1 F (mol Au)^" (deposition via reaction (1)), with the cumulative values reaching a plateau of about 3 (deposition by reaction (2)) at long times. The incremental F (mol Au)^'1 values reflected (Figs.9-11) the Au(I)/AU(III) molar ratio data, particularly in the region of the peak, which presumably arose from the production Au(I) species by reaction (4), and decayed due to their deposition by reaction (1), which required 1 F (mol Au)^'1.

With an applied potential of + 0.529 V between the feeder electrode and solution at the bed/membrane interface precluding hydrogen evolution, the extra Faradaic requirement (Fig. 11) above the value of 3 F • 12 -

(mol Au)^" for reaction (2) was probably due to the reduction of oxygen (reactions (8) and (9)). The anolyte would have been supersaturated with dissolved oxygen and published data [T. Sakai et. al. J. Electrochem, Soc, 133(1) (1986) 88] showed that grades of Nafion membrane to have high solubilities and diffusion coefficients for oxygen. Adventitous ingress through the various joints in the flow circuit would have constituted a secondary source of dissolved oxygen.

As the AE values for reactions (6) and (7) are only 0.243 and 0.295 V respectively, typical operating voltages for the cell were about 1 V, even with a large anode-membrane gap (Fig.l). The cumulative specific charge requirements of about 3 F (mol Au)^" , shown in Figs. 8-11, correspond to a specific energy requirement ooff aabboouutt 440000 kkWWhh ((ttoonnnnee AAu)~ for gold electrowinning at a cell voltage of 1 V.

The behaviour of the system described in Figs. 8-11 could be explained by the following tentative model:

Au(III) + 2e ^Au(I)surface (4)

^AuCl)surface ^Au^bulk (24)

^AuCl)surface⁺ Au (25)

^AuCl)bulk ^{+ e} Au (1) 3Au(I) 2Au + Au(III) (3)

The rapid initial decay of the Au(I)/Au(III) molar ratio, and the initial F (mol Au)^" values of 1-3, imply that while reaction (1) was transport controlled, and as shown in Fig. 5 for a solution containing predominantly Au(I) species, reactions (4) and (25) were only partially so, for the range of potentials • 13 -

applied. Having depleted the Au(I) species from bulk solution to concentrations well below their equilibrium value given by equation (3), reaction (4) became increasingly significant, causing a rise in the Au(I)/Au(III) molar ratio, which increased with increasing flow rate due to mass transport dependent dispersion process. However, depletion of the Au(III) species, causing decreasing rates of formation of Au(I) species by reaction (4), and the transport controlled removal of Au(I) species by reaction (1), would cause the Au(I)/Au(III) molar ratio to pass through a maximum and decrease to zero, prior to total depletion of the total dissolved gold, as observed

It is not immediately apparent why the further reduction of Au(I) species formed from Au(III) species (reaction (4) should be comparatively slow, enabling their dispersion to the bulk solution (step 24), while reaction (1) is fast. Nicol et al [NIM Report No. 1846 (7.7.76) Mintek, Randberg, South Africa] found a chloride ion reaction order for reduction of AuCl~4 ions to be -1, but did not determine a value for the reduction of AuCl₂ ions. If a similar inverse order were operative, the high local chloride ion concentration resulting from the reduction of AuClT ions would inhibit the further reduction of the Au(I) intermediate, whereas the reduction of AuCl₂ ions could still be fast. If there was insignificant potential drop in the particulate graphite phase, so that the potential applied appeared near the bed/membrane interface, even then the remaining bed volume would have operated as lower overpotentials. This would have had the effect of further favouring the faster of the elementary steps in the overall process. At lower applied potentials than used in the experiments reported here both Au(I) reduction processes may be fast.

Under the low Au- concentration conditions of these experiments, the rate of reaction (3) can probably be neglected, since Nicol et al have shown it to be slow even in concentrated solutions. From the Tafel slopes for gold deposition and the chloride activity effect on the kinetics, Nicol et al proposed the following^' reaction mechanism for reaction (2):

AuCl^ = AuCl₃+ Cl^"

AuCl₃ + 2e = Au(I) (slow)

AuCl(I) + e ■ Au (two orders of magnitude faster than the slow step)

However, this would appear to be incongruent with their detection of Au(I) intermediates by their oxidation at a Pt ring electrode, while reducing Au(III) species under constant current control at a Pt disc electrode.

-15-

LEGENDS FOR FIGURES 3 to 12

Fig.3 Potential-pH diagram for the Au/-H₂0- Cl^" system at 298°K and total dissolved gold concentration of 0.5 mol m ;

Fig.4 Au(III) activity-pH daigram for 298°

Fig.5 Cyclic voltammogram of Pt electrode in 1 kmol HC1 m^"3, [Au(I)]+[Au(III)] = 51.4 g m^"3, [Au(I)]/[Au(III)] =41.8, potential sweep rate 5 mV s , starting potential 0.54 V;

Fig.6 Depletion of total dissolved gold by adsorption on flow circuit surfaces (A ), and unused

2 carbon bed particles ( # ) of area 0.082m ;

Fig.7 Total dissolved gold concentrations (open symbols) and current densities (solid symbols) as functions of time and flow rate. Feeder-membrane potential 0.53 V, bed area 0.094 m², flow rates (O , ♦) 1.9 x lθ^"6 m³ s^"1, (□ , ■ ) 8.4 x lθ^"6 m³ s^"1, ^■( © , # ) 16.2 x 10^"6 s^'1;

Fig.8 Total dissolved gold concentration dependence of the PBE cross-sectional current density. Feeder-membrane potential 0.53 V, bed area 0.094 m³, flow rate 1.9 x lθ^"6 m³ s^"1 (< ),8.4 x 10^"6 m³ s^"1 (13 ), 16.2 x 10^"6 m³ s^"1 ( • );

SUBSTITUTE SHEET - 16-

Fig.9 Total dissolved gold concentration ( β ), [Au(I)/[Au(III)] molar ratio ( • ), and incremental ( 13 ) and cumulative ( © ) Faradays per mole of gold deposited in the PBE operating under the conditions specified for Fig.7 (O);

Fig.10 Total dissolved gold concentration ( ■ ), [Au(I)/[Au(III)] molar ratio ( • ), and incremental (£3 ) and cumulative (© ) Faradays per mole of gold deposited in the PBE operating under the conditions specified for Fig.7 (<^);

Fig.11 Total dissolved gold concentration (■ ), [Au(I)/[Au(III)] molar ratio (• ), and incremental (Q ) and cumulative (0 ) Faradays per mole of gold deposited in the PBE operating under the conditions specified for Fig.7 (O");

Fig.12 Effect of rotation rate and pH on the mixed potential laeching rate of a gold-plated rotating disc electrode at 295°K, constant chlorine concentration (2 mol m ) in 1 kmol Cl^", (0) pH = 0, (Δ.) pH » 2, (•) pH = 4. Theoretical leaching rates assuming a three electron reaction ( WL ) and a one electrode reaction (Q ), calculated from the Levich equation for mass transport controlled chlorine reduction.

SUBSTITUTE SHEET

Claims

- / 7 -CLAIMS

1. A method of extracting metal from scrap, ore or other material comprising the steps of:- electrochemically generating an oxidant in the anode compartment of an electrochemical cell; reacting the oxidant with scrap, ore or other material in a reactor to form soluble species of the metal; and _f reducing said soluble species to metal in the cathode compartment of said electrochemical cell.

2. A method as claimed in claim 1, in which the oxidant is chlorine or hypochlorite ion, generated from a chloride-containing anolyte.

3. A method as claimed in claim 1 or claim 2, in which the metal is gold.

4. Metal recovery apparatus for performing the method of claim 1 comprising said electrochemical cell, an external reactor and a pump arranged such that:- electrolyte from the anode compartment may be passed to the external reactor and from the external reactor to the cathode compartment; and electrolyte from the cathode compartment may be passed to the anode compartment.

SUBST_ITUTE _SHEET - -

5. Apparatus as claimed in claim 4 in which anode and cathode compartments of the electrochemical all are divided by an ion-exchange membrane as a partition wall.

6. Appartus as claimed in claim 4 or claim 5 in which the cathode is a three-dimensional electrode.

7. Apparatus as claimed in claim 6, in which the cathode is a packed or fluidised bed electrode.

8. Apparatus as claimed in claim 7, in which the cathode is a packed bed of particulate graphite. _f

SUBSTITUTE SHEET