Hydrometallurgy in Extraction Processes, Volume II

C. K. Gupta, T. K. Mukherjee

• Publisher: CRC Press
• Year: 2017
• eBook ISBN: 9781351439602
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

This two-volume set provides a full account of hydrometallurgy. Filled with illustrations and tables, this work covers the flow of source material from the mined or concentrate state to the finished product. It also highlights ion exchange, carbon adsorption and solvent extraction processes for solution purification and concentration. The extensive reference list-over 850-makes this set a valuable resource for extraction and process metallurgists, researchers, and practitioners.

Chapter 1

OTHER LEACHING PROCESSES

I. GENERAL

In addition to the specific leaching processes described in the previous volume, there are also a number of others which are of considerable importance in the field of hydrometallurgy. Among such miscellaneous processes, special mention may be made of (1) cyanide leaching, (2) chlorine leaching, (3) hypochlorite leaching, (4) hypochlorous acid leaching, (5) dichromate leaching, and (6) electrochemical leaching. This chapter has been organized to provide an account of these leaching processes.

A. Cyanide Leaching

Processing of gold and silver ores by cyanide leaching is one of the prominent examples of early hydrometallurgy-based processes realized on a commercial scale. Today, one finds the cyanide leaching process as successfully taken and applied to other areas. In this context, one may cite as a major example the successful application of cyanide leaching to sulfidic resources of copper. Elaboration of this particular application area of cyanide leaching appears in a later text of this section.

1. Basics of Cyanide Leaching of Au and Ag Ores

Gold and silver readily dissolve in dilute cyanide solutions and this fact was known almost two centuries ago. The reason, however, remained unexplained for quite some time. It was not understood why noble metals like Au and Ag could be so easily leached in dilute cyanide solutions and also why rates of leaching showed a dependence on cyanide concentrations only up to a certain critical limit. In 1846, a publication from Elsner¹ proposed for the first time that Au dissolved in cyanide solution only in the presence of O₂ according to following reaction

$4\text{Au}+8{\text{CN}}^{-}+{\text{O}}_2+2{\text{H}}_2\text{O}\to 4\text{Au}{\left(\text{CN}\right)}_2^{-}+4{\text{OH}}^{-}$

(1)

Dissolution of Ag in cyanide solution can be similarly shown as:

$4\text{Ag}+8{\text{CN}}^{-}+{\text{O}}_2+2{\text{H}}_2\text{O}\to 4\text{Ag}{\left(\text{CN}\right)}_2^{-}+4{\text{OH}}^{-}$

(2)

More than 40 years later, Janin² suggested that dissolution of Au and Ag took place not by reduction of O₂, but by liberation of H₂

$2\text{Au}+4{\text{CN}}^{-}+2{\text{H}}_2\text{O}\to 2\text{Au}{\left(\text{CN}\right)}_2^{-}+2{\text{OH}}^{-}+{\text{H}}_2$

(3)

$2\text{Ag}+4{\text{CN}}^{-}+2{\text{H}}_2\text{O}\to 2\text{Ag}{\left(\text{CN}\right)}_2^{-}+2{\text{OH}}^{-}+{\text{H}}_2$

(4)

The support for the leaching reaction (1) as proposed by Elsner came from Bödlander.³ He, however, pointed out that the reaction proceeded in two stages as shown here

$2\text{Au}+4{\text{CN}}^{-}+{\text{O}}_2+2{\text{H}}_2\text{O}\to 2\text{Au}{\left(\text{CN}\right)}_2^{-}+2{\text{OH}}^{-}+{\text{H}}_2{\text{O}}_2$

(5)

$2\text{Au}+4{\text{CN}}^{-}+{\text{H}}_2{\text{O}}_2\to 2\text{Au}{\left(\text{CN}\right)}_2^{-}+2{\text{OH}}^{-}$

(6)

FIGURE 1. Potential-pH diagram for Ag-H₂O and Ag-H₂O-CN, Ag-10⁻⁶ mol/1; H₂O₂-10⁻⁶ mol/1; temperature − 25°C.

$2\text{Ag}+4{\text{CN}}^{-}+{\text{O}}_2+2{\text{H}}_2\text{O}\to 2\text{Ag}{\left(\text{CN}\right)}_2^{-}+2{\text{OH}}^{-}+{\text{H}}_2{\text{O}}_2$

(7)

$2\text{Ag}+4{\text{CN}}^{-}+{\text{H}}_2{\text{O}}_2\to 2\text{Ag}{\left(\text{CN}\right)}_2^{-}+2{\text{OH}}^{-}$

(8)

Other investigators^{4, 5} thought that intermediate agents like cyanogen gas [(CN)₂] and potassium cyanate (KCNO) were responsible for the dissolution of Au and Ag. Such assumptions were, however, later proved⁶, ⁷, ⁸ to be wrong. Deitz and Halpem⁹ suggested that all these reactions could basically be considered as oxidation and reduction steps. The oxidation step can be represented as

$\text{Ag}+2{\text{CN}}^{-}\to \text{Ag}{\left(\text{CN}\right)}_2^{-}+\text{e}$

(9)

and the reduction step as

$2{\text{H}}_2\text{O}+2\text{e}\to 2{\text{OH}}^{-}+{\text{H}}_2$

(4a)

${\text{O}}_2+2{\text{H}}_2\text{O}+4\text{e}\to 4{\text{OH}}^{-}$

(2a)

${\text{O}}_2+2{\text{H}}_2\text{O}+2\text{e}\to {\text{H}}_2{\text{O}}_2+2{\text{OH}}^{-}$

(7a)

${\text{H}}_2{\text{O}}_2+2\text{e}\to 2{\text{OH}}^{-}$

(8a)

The potential-pH diagrams for Ag-H₂O and Ag-H₂O-CN⁻ are shown in Figure 1. This diagram shows that in the absence of cyanide, an insoluble Ag₂O phase is formed at a pH higher than 12.3. The equilibrium potential between Ag and Ag⁺ is 0.446 V. The addition of cyanide lowers the concentration of free Ag⁺ ions through the formation of argentocyanide complex as given in the equation

$\text{Ag}+2{\text{CN}}^{-}\to \text{Ag}{\left(\text{CN}\right)}_2^{-}$

(10)

and allows the Ag₂O phase to disappear. These findings are further summarized in Figure 2, which presents the potential-pH diagram for various dissolution reactions of silver in aqueous cyanide. It can be seen that Equations 2a, 7a, and 8a correspond to positive potentials(i.e., negative free energy change) and are therefore thermodynamically feasible, whereas Equation 4a is unfavorable because it is associated with negative potential. Barsky et al.¹⁰ determined the equilibrium constants for these reactions and arrived at similar conclusions regarding feasibilities of these reactions. Based on experimental observations, Kudryk and Kellogg¹¹ argued that the product of the reduction of oxygen was hydroxyl ions and not hydrogen peroxide. But Kameda¹² and later Lund¹³ showed that for every 2.2 equivalents of metal dissolved, 1 mol of O₂ was consumed and 1 mol of H₂O₂ was produced. Similarly, for every 1 equivalent of metal dissolved, 2 mol of cyanide were consumed. Based on these findings, Habashi¹⁴ pointed out tha...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Chapter 1: Other Leaching Processes
• Chapter 2: Solution Purification
• Chapter 3: Metal Recovery Processes
• Index