38.Explain the basic electrochemical reactions for electroextraction of gold.
39.Explain the basic electrochemical parameters, anode – cathode reactions for electroextraction of gold.
All of the hydrometallurgical gold extraction
routes utilize a leaching step to produce a gold-bearing solution as an intermediate product while the recycling of secondary gold from the recycling of secondary gold from electronic and precious metal scrap using selective and fast dissolution of the precious metal constituents. Therefore, either leaching or dissolution operations require highly corrosive media due to the well-known chemical inertness of the noble metal toward most acids and bases. Several reagents are known to leach gold from gold-bearing ores. It has been known for more than a century that alkaline solutions of alkali-metal cyanides (e.g., NaCN, KCN) dissolve gold under aerated conditions. The dissolution of metallic gold is due to the strong complexing capabilities of cyanideanionscombinedwiththeoxidizing properties of the dissolved molecular oxygen. Upon dissolution, gold forms the stable dicyanoaurate(III) complexanion [Au(CN)-], as follows: 4 Au + 8 CN- + O2 + 2 H2O = 4 Au(CN)2-+ 4OH-. Cyanide leaching of gold (Au) is an electrochemical procedure that can be described by the following reactions: Anodic: 4 Au = 4 Au+ + 4e-; 4 Au+ + 4 CN- = 4 AuCN; 4 AuCN + 4 CN- = 4 Au(CN)2-
Cathodic:O2 + 2 H2O + 2 e- = H2O+ 2OH-
H2O2+2 e- = 2OH; That yields:4 Au + 8 CN- + O2 + 2 H2O = 4 Au(CN)2-+ 4 OH
In aqueous, alkaline cyanide leaches, Au oxidizes and dissolves forming Au (I) cyanide complexes in three stages where the species causes temporary passivation of the Au surface leading to a complex with free cyanide to form a Au(CN)2- ion and finally a passivating Au (III) oxide layer that only is available at a high positive potential that is not common.
37 Explain the electrochemical extraction of gold. its features, parameters, and structure of cell. While many types of cell designed are currently in use, the Zadra type of cell has been favoured by industry since its inception in 1952. The design utilizes three concentric cylinders. The inner one, which serves as the cathode compartment, is a perforated polymer containing a central feed tube, current distributer and steel cathode. The anode – a circular stainless steel mesh – lies outside the cathode. Gold-rich liquor enters the cell from the bottom of the central feed tube and flows upwards, distributing radially through the steel wool. Electrolyte temperatures are usually maintained at 70-85C. typically, at a relatively low electrolyte flow rate of 10 dm3 min-1, the nominal residence time in the cell is approximately 5 min. depending on the electrolysis conditions and electrolyte composition, gold deposits in the steel wool matrix as a sponge, compact layer or powder. In any case, the Fe-Cr stainless steel wool may be removed from the gold by fluxing and melting followed by extraction of the Fe-Cr oxides from the slag. The Zadra cell, however, suffers from several drawbacks: 1. Electrolyte flow is unevenly distributed throughout the porous cathode; the resulting non-uniform mass transport gives rise to uneven deposition and wastage of electrode area. 2. The effective interelectrode spacing is excessive, resulting in a relatively high cell voltage. 3. The volume of the cell is not utilized effectively.
Fig. 4.7 gold electrowinning cell. a)Zadra cell design, b)a modified Zadra cell, the improved mass-transfer cell.
The circular improved mass transport cell. (Figure 4.7) has recently been designed to overcome these problems. The solution-distribution tube is stainless steel and serves as a second anode, decreasing the effective interelectrode spacing and improving current distribution. The cell hydrodynamics are rendered more uniform by a factor of up to 2. Prototype work was conducted using a circular cell, typically operating at ambient temperatures (20-21C), a cell voltage of -3.0V and a cathode packing density of 0.018 g cm-3, with an internal circulation rate of 2 dm3min-1. The cathode volume is 888 cm3 and operating currents are c. 1A. a combined Ag-Au feed yields a cathode current efficiency up to 7% for Au and 25% for Ag at deposition rates of 8 and 16 mg min-1 respectively.
38.-36.
39.-36.
40. -27.
41. – 27.