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Recovery

Recovery processes, as defined in this chapter, remove gold and other metal value from solution into a concentrated solid from. The high- grade solid product may then either be sold directly or treated further on- side by refining (chapter 10) to improve the grade. Recovery may be achieved from a wide range of solutions, varying dramatically in gold concentration and purity, both of which can greatly affect the efficiency of the various recovery processes avaliable.

Prior to the widespread adoption of carbón adsortion processes for treatment of dilute leach slurries and solutions in the late 1970s (Chapter 7), zinc precipitation was used almost exclusively for direct recovery of gold from clarifield leach solitions. Subsequently, both electrowinning and zinc precipitation have been used to treat the more concentrated gol solutions produced by carbón elution. Direct recovery from dilute solutions by zinc precipitation is still preferred over carbón adsorption for treatment of some ores, for example:

Ores with high silver content Ores containing species that inferfere with carbón adsrption (e.g., clay minerals, organic

material) Small orebodies that cannot justify the generally higher costs of carbón adsorption,

elution, and regeneration systems at that scale of operation

The factors affecting this process selection are considered further in Section 3.2.8. The chemistry of both electrowinning and zinc precipitation processes are considered in detail in this chapter.

Although no comercial processes now use aluminum dust for gold precipitation, it has been used in the past, and the chemistry of this recovery system is briefly reviewed. Activated carbón has also been used in a few instances for final gold recovery, usually with incineration of the loaded carbón to produce a high- grade ash for refining (see Chapter 7).

Posible schemes for gold recovery from noncyanide leaching systems are also considererd.

8.1 ZINC PRECIPITATION

Precipitation, or “cementation”, of gold with zinc was introduced commercially for the treatment of cyanide leach solutions in 1890 [1], and has subsequently been applied widely in the industry. The process, which is commonly referred to as Merrril- Crowe precipitation after its pioneers (Chapter 1), has evoived to be highly efficient, with gold recoveries from solution >98% routinely achieved and sometimes as high as 99.5%. The Merril- Crowe process has cost advantages over the carbon adsorption process for treatment of solutions containing high silvr content (see Chapter 3) and has the advantage of being able to more easily handle fluctuations of gold grade in the feed. For te treatment of low silver content solutions and slurries, the use of carbon adsorption as an intermediate step has emerged as the preferred process route, mainly due to lower capital and operating costs.

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Zinc precipitation was first used to treat hot, high- grade solutions produced by carbon elution in 1981 in the United States and South Africa, and has subsequently been applied widely around the world as an alternative to electrowinning.

8.1.1 Reaction Chemistry

8.1.1.1 Anodic Behavlor of Zinc in Cyanide Solution

The anodic oxidation of zinc in aqueous solutions is given by:

Zn2+¿¿+ 2e =Znyy; E0=¿ -0.763 (EQ 8.1)

Where

E=−0.763+0.0295 log ¿¿

In cyanide solutions zinc forms a stable cyanide complex:

Zn2+¿¿+ 4CN =Zn(CN )42−¿ ¿ (EQ 8.2)