Cyanide Oxidation

Cyanide (CN~)-based plating baths exhibit the unique property of producing fine crystalline, shiny metal coatings. They are therefore widely in use despite the high toxicity of cyanide and the greater control effort necessary in pretreatment.

Cyanide destruction is commonly achieved through oxidation in the alkaline region using chlorine gas or sodium hypochlorite. Peroxide oxidation and treatment with ozone are reported to be feasible. Electrolytic oxidation has advantages in treating highly concentrated cyanide solutions stemming from heat treatment melts and spent cyanide baths [9].

1. Alkaline Chlorination

Oxidation of cyanide (CN-) to cyanate (CNO~) in the alkaline region is achieved either through chlorination with chlorine gas or through treatment with hypochlorite. The highly toxic cyanogen chloride (CNC1) is formed as an intermediate. The overall chemical reactions are

The cyanogen chloride in reaction (1) is formed instantly, and the formation rate is independent of pH. The hydrolysis reaction, reaction (2), however, is pH- and temperature-dependent. Ent-wistle [9] reports that at 15°C and pH 11 the cyanogen chloride hydrolyzes to about 0.1% of its original value within 10 min. The CNC1 decomposition rate slows appreciably at lower pH values. The cyanide oxidation is therefore performed at pH values of 11 or greater to avoid buildup of cyanogen chloride and the escape of toxic CNC1 gas.

The use of gaseous chlorine (Cl2) necessitates the installation of a chlorinator. This is avoided by adding the chlorine in the form of sodium hypochlorite (NaCIO). Sodium hypochlorite is commercially available as an aqueous solution with 15% available chlorine. The overall reactions using hypochlorite are

2 Cl2 + 2 NaCN 2 CNC1 + 2 NaCl 2 CNC1 + 4 NaOH -» 2NaCNO + 2 NaCl + 2 H20

2 NaOCl + 2 NaCN + 2 H20 ^ 2 CNC1 + 4 NaOH 2 CNC1 + 4 NaOH 2 NaCNO + 2 NaCl + 2 H20

Table 1 Dissociation Constants of Metal Cyanide Complexes at 25°C

Complex

Kd

Equilibrium expression

Cd(CN)!-

1 X

10"

6

Cd(CN)2" Cd2+ + 4 CN-

Zn(CN)|-

1 X

10

■17

Zn(CN)2- ^ Zn2+ + 4 CN"

Ag(CN)2-

1 X

10"

21

Ag(CN)2" ^ Ag+ +2 CN~

Fe(CN)g~

1 X

10"

24

Fe(CN)g- ^ Fe2+ + 6 CN"

Cu(CN)i-

1 X

10"

25

Cu(CN)!- Cu2+ + 4 CN-

Ni(CN)2-

1 X

10"

30

Ni(CN)!- Ni2+ + 4 CN~

Fe(CN)|-

1 X

10"

-31

Fe(CN)ö- ^ Fe3+ + 6 CN"

Source-. From Refs. 10 and 11.

Source-. From Refs. 10 and 11.

The cyanate (CNO ) formed in reactions (2) and (4) is much less toxic than cyanide itself. It decomposes slowly in water to form ammonia and carbonic acid:

Local regulations may allow direct discharge of cyanate (CNO-). If cyanate (CNO-) discharge is prohibited, its decomposition into C02 and N2 can be accomplished in a second chlo-rination step using chlorine [reaction (6)] or hypochlorite [reaction (7)].

3 Cl2 + 6 NaOH + 2NaCNO 6 NaCl + N2 + 2 NaHC03 + 2 H20 (6)

These reactions take place at a pH 7.2-7.7. At higher pH values, other oxidation products such as nitrates are also formed. This causes additional chlorine consumption.

In the presence of heavy metals, e.g., zinc, cadmium, or copper, cyanide is complexed and the initial cyanide-chlorine reaction rate is slowed down. This necessitates reaction times of 30-40 min in the cyanide reactor. Nickel cyanide complexes require 24 h for destruction, while Fe cyanides are only oxidized to the ferricyanide complex (Fe(CN)6)3~. For this reason it is essential to strictly separate cyanide-containing streams from nickel- and iron-containing rinses. Further destruction of ferricyanide is reported to be feasible with hydrogen peroxide and ultraviolet light irradiation [5].

The dissociation constants of the heavy metal cyanide complexes give an indication of the difficulty of cyanide destruction through chlorination. Table 1 lists these constants for the metals commonly encountered in the plating industry [10,11].

2. Electrolytic Oxidation

Special problems arise during treatment of highly concentrated cyanide solutions such as spent electroplating and heat treatment baths. Heat treatment baths typically contain BaCl2 (26%), NaCN (40%), Na2C03 (13%), NaCl (21%), and graphite.

Cyanide destruction can be achieved in a process described by Entwistle [9]. The process equipment consists of five elements: spray dissolution chamber, settling chamber, electrolytic cell, crystallizer, and scrubber.

The destruction of heat treatment salts is possible in a closed-loop operation. The salts are dissolved in a spray dissolution chamber, and insolubles are removed through settling. The electrolytic cell is constructed of a mild steel tank, graphite anodes, and mild steel cathodes. A dc voltage of 2.8-3.1 V is applied. The current reaches several thousand amperes depending on the cell size. Chloride must be present in sufficient quantity to generate hypochlorite within the cell.

Products generated in the electrolytic cell are sodium carbonate and bicarbonate, sodium formate, sodium oxalate, and ammonium carbonate. Sodium cyanate is present as a minor component (0.5%). Some ammonia escapes with the off-gas and is controlled by wet scrubbing.

The water in the feed stream can be balanced through evaporation losses and water of crystallization, which is mainly removed with the sodium carbonate decahydrate. Barium forms insoluble BaC03. Heavy metals like Cd, Cu, and Zn can be recovered a metal deposits on the cathodes. The process efficiency depends on the NaCN concentration. Direct current power consumption ranges from 0.004 to 0.019 kWh/g CN- for cyanide concentrations of 100 g/L and 5 g/L, respectively. At lower cyanide concentrations the efficiency drops sharply (Figure 1). Iron and nickel cyanides are reported to be destroyed in the process [9],

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