This paper considers the issue of recycling of waste printed circuit boards (WPCBs) containing precious and base metals in appreciable amounts. High-pressure oxidative leaching (HPOL) with dilute sulfuric acid resulted in removal of a significant amount of base metals from a WPCB ash sample obtained by incineration at 800°C. The parameters investigated in the precious metal leaching from WPCB residue after HPOL included the sulfuric acid concentration, thiourea concentration, oxidant concentration, leaching temperature, and leaching time. Recovery of gold, silver, and palladium of 100%, 81%, and 13% from the WPCB residue sample was achieved by thiourea leaching under optimized conditions. The results show that the efficiency of precious metal dissolution from the WPCB sample using thiourea solution depended strongly on the concentration of both thiourea and oxidant.

Recovery of Precious and Base Metals from Waste Printed Circuit Boards Using a Sequential Leaching Procedure

The cathodic reduction of gold from mixed sulfite-thiocarbamide electrolytes is studied as a function of the electrolyte composition. In the absence of thiocarbamide in the gold-plating sulfite solutions, gold is deposited at the cathode at high overpotentials. The equilibrium composition of the electrolyte is calculated at various ratios between the amounts of sulfite and thiocarbamide; it is shown that an addition of 10−5 to 0.5 M thiocarbamide does not change considerably the solution composition, and gold ions are present, predominantly, in the form of the complex with sulfite ions. However, an addition of thiocarbamide to the solution leads to a decrease in the overpotential of metal deposition by approximately 0.5 V. A possible mechanism of the catalytic effect of thiocarbamide on the cathodic reduction of gold from mixed sulfite-thiocarbamide electrolytes is proposed. It is shown that sulfite ions have a stabilizing effect on the decomposition of thiocarbamide in the alkaline solutions.

Peculiarities of cathodic reduction of gold from sulfite-thiocarbamide electrolytes

Numerical simulation and experiment in the systems of labile complexes of metals. Silver reduction from the thiourea electrolyte

Effective values of reaction order with respect to ligand P, transfer coefficient α, and exchange current i
 0 at constant silver surface coverages θ by sulfide ions are measured. The employed solutions contained from 0.4 to 0.05 M thiocarbamide, 0.5 M HClO4, 10−4 M AgNO3, and from 10−5 to 10−4 M Na2S. It is shown that the exchange current grows approximately linearly from 10−5 to 1.5 × 10−4 A/cm2 at θ increase in the range from zero to 0.8, while α and P values grow negligibly in the ranges of 0.4–0.45 and 0.9–1.1, accordingly. The obtained results are compared with the data of similar studies of the gold behavior in acidic thiocarbamide solutions. The possible reasons for the different effects of sulfide ion chemisorption on the anodic dissolution of gold and silver in the studied solutions are discussed.

Kinetics of silver dissolution in sulfide containing thiocarbamide electrolytes

Potentiodynamic polarization and potentiostatic electrolysis techniques were employed to study the anodic oxidation of silver and a number of other metals (copper, iron, nickel, and tin) in sulfite media, both individually and in Ag-M binary electrodes. The conditions in which silver is dissolved with a nearly 100% current efficiency were found.

On anodic dissolution of silver coatings deposited on base metals

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.

Silver dissolution in acid thiocarbamide solutions containing sulfide ions

Continuous polarographic control of the concentration of industrial solution components by computer-aided solid renewable electrode and camac apparatus

R. Yu. Beck

Papers

Silver dissolution in acid thiocarbamide solutions containing sulfide ions

Continuous polarographic control of the concentration of industrial solution components by computer-aided solid renewable electrode and camac apparatus