A review of effects of silver, lead, sulfide and carbonaceous matter on gold cyanidation and mechanistic interpretation

Raising social awareness and environmental specifications on cyanide application force gold industry to search for alternative leaching reagents. Therefore, researchers worldwide investigate cyanide alternatives for gold recovery since several decades. Often the research activities cannot be compared directly, since different input materials and experimental conditions are used. Over the course of this study, different promising cyanide alternative reagents were investigated in terms of their capability of pure gold dissolution at different temperatures. All experiments took place under identical conditions by using uniform samples of 99.99% gold disks, to enable a comparability. Thiosulfate as one of the most promising reagent thiosulfate according to literature revealed an insufficient leaching behavior. The gold dissolution was hindered due to the formation of a sulfide passive layer. Also in the thiourea trials, a surface precipitation took place, though gold dissolution did not stop. The halogens iodine, bromine and the well-known gold solvent aqua regia dissolved gold very fast (up to ~1,000 mg·h−1·cm−2). Methanosulfonic acid (MSA) was not capable to extract any gold. The experiments were compared with cyanide trials at identical conditions. The average dissolution rate of investigated reagents at 25 °C shows following order: aqua regia > iodine > bromine > cyanide > thiourea > thiosulfate > MSA.

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Kinetic Investigation and Dissolution Behavior of Cyanide Alternative Gold Leaching Reagents

Bottom-up deposition of gold in fine trenches, also called "superfill," was recently demonstrated using a submonolayer coverage of preadsorbed, deposition-rate-accelerating lead followed by gold electrodeposition. The present study has used experiments on planar substrates to quantify the effect of Pb adsorption on the Au deposition rate and the rate at which the adsorbed Pb was consumed during the Au deposition process. The values obtained have been incorporated into the curvature enhanced accelerator coverage mechanism of superfill where they were used to quantitatively predict the nonconformal, bottom-up deposition observed during filling of the patterned features. The results indicate the potential of the process for damascene interconnect fabrication in GaAs, GaN, and related technologies.

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Gold Superfill in Submicrometer Trenches: Experiment and Prediction

This study compares the behaviour of direct and mediated electrochemistry of horseradish peroxidase (HRP) immobilised on screen-printed carbon electrodes (SPCEs), screen-printed carbon electrodes modified with carboxyl-functionalised multi-wall carbon nanotubes (MWCNT-SPCEs) and screen-printed carbon electrodes modified with carboxyl-functionalised single-wall carbon nanotubes (SWCNT-SPCEs). The techniques of cyclic voltammetry and amperometry in the flow mode were used to characterise the properties of the HRP immobilised on screen-printed electrodes. From measurements of the mediated and mediatorless currents of hydrogen peroxide reduction at the HRP-modified electrodes, it was concluded that the fraction of enzyme molecules in direct electron transfer (DET) contact with the electrode varies substantially for the different electrodes. It was observed that the screen-printed carbon electrodes modified with carbon nanotubes (MWCNT-SPCEs and SWCNT-SPCEs) demonstrated a substantially higher percentage (≈100 %) of HRP molecules in DET contact than the screen-printed carbon electrodes (≈60 %). The HRP-modified electrodes were used for determination of hydrogen peroxide in mediatorless mode. The SWCNT-SPCE gave the lowest detection limit (0.40 ± 0.09 μM) followed by MWCNT-SPCE (0.48 ± 0.07 μM) and SPCE (0.98 ± 0.2 μM). These modified electrodes were additionally developed for amperometric determination of phenolic compounds. It was found that the SWCNT-SPCE gave a detection limit for catechol of 110.2 ± 3.6 nM, dopamine of 640.2 ± 9.2 nM, octopamine of 3341 ± 15 nM, pyrogallol of 50.10 ± 2.9 nM and 3,4-dihydroxy-l-phenylalanine of 980.7 ± 8.7 nM using 50 μM H2O2 in the flow carrier.

Direct and mediated electrochemistry of peroxidase and its electrocatalysis on a variety of screen-printed carbon electrodes: amperometric hydrogen peroxide and phenols biosensor

A review on electrochemical dissolution and passivation of gold during cyanidation in presence of sulphides and oxides

The depolarization of the gold electrodeposition in the presence of lead ions depends on their concentration and the duration of electrode contact with solution preceding a potential scan in an extremum fashion. At constant coverages of the gold surface by lead adatoms θ, the process rate depends on the overvoltage in accord with the Tafel equation. Effective values of the exchange current i
0 and transfer coefficient α increase with θ from i
0 ≅ 3 × 10–5 A cm–2 and α = 0.23 in pure solutions to 3 × 10–4 A cm–2 and 0.53 at θ ≅ 0.4. The reaction order by cyanide ions is independent of θ and equals nearly –0.9. Effects of lead adatoms on the kinetics of cathodic and anodic processes are compared and the obtained data may be brought to conformance given that their mechanisms in pure solutions differ and converge in the presence of lead adatoms.

Effect of Lead Ions on the Kinetics of Gold Deposition from Cyanide Electrolytes

The possibility of determining kinetic parameters of the gold electrodeposition in the presence of thallium adatoms is considered. The coverage θ of the electrode surface by thallium adatoms is monitored. The steady-state values of potential used correspond to current densities i that are directly proportional to the concentration of thallium ions in solution. The procedure is based on the assumption that the rate of incorporation of adatoms is proportional to the product iθ. With θ increasing to ∼0.25, the exchange current and transfer coefficient increase from 5 × 10–5 A cm–2 and 0.23 in pure solutions to1.5 × 10–4 A cm2 and 0.6 at θ = 0.25–0.3, whereas the reaction order by cyanide ions remains practically invariant with increasing θ. Variations in the kinetic parameters with θ are compared to similar measurements obtained earlier for the anodic process. These may be made consistent by assuming that mechanisms of cathodic and anodic reactions differ in pure solutions and are identical in the presence of catalytically active adatoms. An explanation to the above regularities is given.

Effect of Thallium Adatoms on Electrode Processes in the System Gold–Cyanide Solutions

Effect of lead hydroxy compounds on the process of electrodeposition of silver from cyanide electrolytes is studied on an electrode whose surface is renewed in solution by cutting off a thin layer of metal. This permitted to perform the study on both the freshly renewed electrode and at controlled values of the time of the electrode contact with solution Δt. Shown is that on the freshly renewed electrode (Δt<1 s) the presence in the solution of lead ions in concentrations c1 on the order of 10–5 M leads to the process depolarization only in the initial portion of a polarization curve. With c1 increased to 10–4 M the effect of depolarization extends on the entire polarization curve. Keeping the electrode in solution after the renewal of the metal surface magnifies depolarization, and the greater the concentration c1, the shorter the time period Δt required to achieve the same effect. These regularities are attributed to catalytic influence of lead adatoms, whose surface concentration depends on c1 and Δt, as well as on the intensity of their incorporation in the silver deposit.

Silver Electrodeposition from Cyanide Solutions: Effect of Lead Ions

The peculiarities of the effects of upd thallium, lead, bismuth, and mercury on the dissolution rates of gold and silver in cyanide electrolytes are compared. In general, they feature the abrupt acceleration of the dissolution of gold and, to a lesser extend, silver in the chemisorption range of mentioned ions. As the potential increases, the gold dissolution rates passes through a maximum the height of which is comparable with the limiting current of this process associated with limitations in the delivery of cyanide ions to the electrode surface. The current decay after the maximum is due to desorption of catalytically active adatoms. The chemisorption rate of thallium, lead, and bismuth ions at potentials more negative than the current peak is controlled by their diffusion to the gold surface, whereas the chemisorption rate of mercury is controlled by the adsorption kinetics. With the increase in the surface coverage with adatoms θ, the catalytic activity of all considered adatoms passes through a maximum. The sharp increase in the effective transfer coefficient in the presence of these adatoms makes the main contribution into the acceleration of the gold dissolution, while the increase in the exchange current has a smaller effect. The chemisorption of mentioned atoms on gold not only accelerates the dissolution but also changes its mechanism. For gold dissolution, the catalytic activity of upd thallium, lead, and bismuth increases in the following sequence: Tl ≪ Pb < Bi and the effect is additive in their simultaneous presence. For silver, the increase in the exchange current makes the main contribution into the acceleration of dissolution, whereas the transfer coefficient and the reaction order with respect to the ligand change insignificantly. Explanation of the observed peculiarities is given.

Electrocatalysis by adatoms at the gold and silver dissolution in cyanide solutions

Regularities of silver dissolution in acid thiocarbamide electrolytes are studied. The kinetics of the process is shown to be severely affected by the admixture of hydrogen sulfide molecules that form upon inserting sodium sulfide or accumulate in electrolyte with the passage of time elapsed since its preparation. Catalytic effect increases with increasing length of time of the electrode’s contact with solution prior to the beginning of experiment or following an increase in the concentration of sulfide ions. Experiments with the surface renewed in the course of potential scans show that the catalytic effect is connected with the adsorption of sulfide ions on an interface. At large values of the surface coverage with sulfide ions, the dissolution rate increases so much that the dissolution process starts to be limited largely by the process of supply of thiocarbamide molecules toward the electrode surface.

L. I. Shuraeva

Papers

Effect of Lead Ions on the Kinetics of Gold Deposition from Cyanide Electrolytes

Effect of Thallium Adatoms on Electrode Processes in the System Gold–Cyanide Solutions

Silver Electrodeposition from Cyanide Solutions: Effect of Lead Ions

Electrocatalysis by adatoms at the gold and silver dissolution in cyanide solutions

Silver dissolution in acid thiocarbamide solutions containing sulfide ions