*7.2.1.3. Hydrogen peroxide oxidation*

Oxidation of cyanide tailings by hydrogen peroxide is more suitable for solutions rather than slurries. The oxidation process is maintained at pH of 9–10 to avoid formation of hydrogen cyanide [47]. The oxidation reaction is catalysed by copper (II) sulphate resulting in the production of carbonate and ammonium (Eqs. (10) and (11)).

$$\text{CN}^{-} + \text{H}\_{2}\text{O}\_{2} \xrightarrow{\text{Cu}^{2-}} \text{CNO}^{-} + \text{H}\_{2}\text{O} \tag{10}$$

$$\text{CNO}^- + 2\text{H}\_2\text{O} \rightarrow \text{CO}\_3^{\cdot 2} + \text{NH}\_4^+ \tag{11}$$

### *7.2.1.4. Ozonation*

Among the methods used in removing cyanide from wastewater include photocatalysis [38], biotreatment [39], copper‐catalysed hydrogen peroxide oxidation [40], ozonation [33], electrolytic decomposition, alkaline chlorination [22], reverse osmosis, thermal hydrolysis and adsorption [41]. Most of these methods have limited applications due to the high cost, production of toxic residues and incomplete degradation of all cyanide complexes [42, 43]. However, biodegradation of aqueous cyanide ions is cheaper than chemical and physical

This process was developed by a Canadian company, Inco metal limited, in 1984 [44]. The process makes use of air and sulphur dioxide in the catalytic oxidation of free and complexed cyanide to cyanate [37, 45, 46] as shown in Eq. (6). The process is catalysed by aqueous copper (II) ions under controlled pH of 8–10. The pH is normally maintained by addition of lime.

> - - -+ + + + ¾¾¾® + + +2 Cu

After completion of the oxidation process, previously metal ions complexed with cyanide such as Zn+2, Cu+2 and Ni+2 are precipitated as metal hydroxides. This process effectively treats

In this process, cyanide is oxidised by alkaline chlorine. The process converts all acid dissoci‐ able cyanide except for iron cyanide complexes and more stable metal‐cyanide complexes. The process is a two‐stage process. The first stage involves initial oxidation of free cyanide to cyanogens chloride followed by hydrolysis of cyanogens chloride to cyanate (Eqs. (7) and (8))

> ClCN+Cl - 2


During the second stage, cyanate is further oxidised to hydrogen carbonate and nitrogen as

 2H Cl

> 6Cl

3

O OCN


2

 N 2HCO

O SO

 H

CN +Cl

> H

 6OH

2

ClCN

shown in Eq. (9). The reaction occurs at pH 8.5.

 2OCN

3Cl

2 2 2

4

 OCN  2H

® (7)

(8)

 2H O

2

(9)

(6)

methods [30].

230 Water Quality

*7.2.1. Chemical oxidation methods*

*7.2.1.1. Sulphur dioxide/air (INCO) process*

SO

cyanide in slurries and solutions.

*7.2.1.2. Alkaline chlorination*

at pH 11.

2 2

 O CN Ozone is a superior oxidant to oxygen and has been extensively studied in the oxidation of cyanide [48–51]. Two oxidation mechanisms of cyanide to cyanate by ozone have been proposed, namely simple (Eq. (12)) and catalytic (Eq. (13)).

$$\rm{CO}\_3 + \rm{CN}^- \rightarrow \rm{OCN}^- + \rm{O}\_2 \tag{12}$$

$$\rm{^3O\_3 + 3CN^+ \to 3OCN^-} \tag{13}$$

Catalytic ozonation rarely occurs and has only been observed under high acidic conditions. Continued addition of ozone results in the formation of hydrogen carbonate and nitrogen (Eq. (14)).

$$2\text{OCN}^{\circ} + \text{O}\_{3} + \text{H}\_{2}\text{O} \rightarrow 2\text{N}\_{2} + 2\text{HCO}\_{3}^{\circ} \tag{14}$$

### *7.2.1.5. Peroxymonosulphuric acid*

Peroxymonosulphuric acid (H2SO5) or Caro's acid [52] is used for cyanide treatment in gold tailings. Caro's acid is prepared in situ by the reaction of hydrogen peroxide with sulphuric acid since it easily decomposes. This acid is mostly used in situations where sulphur dioxide/ air cannot be used. Caro's acid oxidises cyanide to cyanate as shown in Eq. (15).

$$\rm{\bf H}\_{2}\rm{\bf SO}\_{6} + \rm{\bf CN}^{-} \rightarrow \rm{\bf OCN}^{-} + \rm{\bf SO}\_{4}^{2-} + \rm{\bf 2H}^{+} \tag{15}$$
