**2. Gold extraction process using cyanide**

Cyanidation is the predominant gold extraction technique since the late nineteenth century. The dissolution of gold in aqueous cyanide is commonly described using Elsner's equation [1]:

$$\mathsf{4A\mathsf{A}} + \mathsf{8CN}^{-} + \mathsf{O}\_{2} + \mathsf{2H}\_{2}\mathsf{O} \to \mathsf{4} \left[ \mathsf{Au} \left( \mathsf{CN} \right)\_{2} \right]^{-} + \mathsf{4O} \mathsf{H}^{-} \tag{1}$$

Gold dissolution is an electrochemical process in which oxygen is reduced at the cathodic zone, while gold is oxidised at anodic regions. The precise overall dissolution of gold in alkaline, aerated cyanide solutions taking place at cathodic and anodic regions is represented in Eqs. (2) and (3).

$$2\mathsf{Au} + 4\mathsf{CN}^{-} + \mathsf{O}\_{2} + 2\mathsf{H}\_{2}\mathsf{O} \to 2\left[\mathsf{Au}\left(\mathsf{CN}\right)\_{2}\right]^{-} + \mathsf{H}\_{2}\mathsf{O}\_{2} + 2\mathsf{O}\mathsf{H}^{-}\tag{2}$$

$$\mathbf{2\mathsf{A}u} + \mathbf{4\mathsf{CN}^{-}} + \mathsf{H}\_{2}\mathbf{O}\_{2} \to \mathsf{Z} \left[ \mathsf{A}u \left( \mathsf{CN} \right)\_{2} \right]^{-} + \mathsf{Z}\mathsf{O}\mathsf{H}^{-}\tag{3}$$

The main merits of cyanidation are the high selectivity of free cyanide for gold dissolution compared to other metals and an extremely high stability constant (2 × 1038) of the gold cyanide complex [2].

Dilute sodium cyanide solutions within concentration ranges of 0.01–0.05% are used in mines for gold leaching [3]. Gold ore is subjected to physical processes such as milling, grinding and gravity separation prior to the addition of aqueous sodium cyanide to form slurry. The pH of the resulting extracting solution is increased by adding slaked lime or sodium hydroxide to prevent generation of toxic hydrogen cyanide [4]. The slurry pH is maintained at not less than 10.5 during cyanidation to prevent excessive loss of cyanide by hydrolysis through volatilisa‐ tion of hydrogen cyanide. Oxygen an important component during cyanidation is continu‐ ously pumped into the slurry resulting in the formation of dicyanoaurate (I) complex.

Several methods are employed for cyanide leaching of gold ore [5]. However, agitation leaching is commonly used for most ores due to its commercial viability [6]. Leaching is typically done in steel vessels, and the solids are maintained in suspension by air or mechanical agitation.

The gold complex NaAu(CN)2 is then extracted from leach solutions by adsorption onto solid adsorbents such as activated carbon or a synthetic ion exchange resin [7–11]. Activated carbon is the most commonly used adsorbent for gold extraction due to several favourable properties such as high adsorption capacity, good reactivation capabilities, low cost, readily available, high mechanical strength and wear resistance [12].

Gold complexes adsorbed onto activated carbon are eluted to produce concentrated high‐ grade gold solutions suitable for final gold recovery. Eluents such as sodium hydroxide [13] and organic solvents in aqueous solutions [14] have been used for desorption or stripping of gold from activated carbon.

Gold is extracted from solution into a concentrated solid form by a process termed recovery. Zinc precipitation [15] and electrowinning [16] have been used to treat concentrated gold solutions produced from activated carbon stripping. Eq. (4) represents the electrochemical reduction process for gold.

$$\text{2}\left[\mathsf{Au}\left(\mathsf{CN}\right)\_{2}\right]^{-}+\mathsf{Zn}+\mathsf{4CN}^{-}\to\mathsf{2PA}+\mathsf{4CN}^{-}+\left[\mathsf{Zn}\left(\mathsf{CN}\right)\_{4}\right]^{2}\tag{4}$$

The gold recovered from crude undergoes refining to produce crude bullion containing between 90 and 99.5% pure gold [17]. Refining involves roasting the crude gold to convert base metals such as iron, lead, copper and zinc to their respective oxides. This process is then followed by smelting, which removes base oxide impurities in form of slag. The bullion produced can be upgraded further to higher purity platinum group metals by processes such as pyrorefining, hydrorefining and electrorefining [18, 19]. These extraction processes leave behind toxic cyanide tailings.
