EFFECT OF VARIOUS PROCESS PARAMETERS
The effect of various process parameters on the efficiency of gold leaching in cyanide solution has been widely discussed and studied.
Effect of Еh-pH on Cyanidation
Figure 4.6 presents the aqueous chemistry of precious metal in cyanide solution and shows that Au(CN),, aurous-cyanide ions occur predominantly in the wider pH range while the Au-,+. auric, ions are in a very limited area. The stability field of Au° at relatively low potential value (Eh) covers the whole pH range, as does the stability
FIGURE 4.6 Eh-pH diagram of the Au-CN-H,0 system at 25 C and 1 atm.
of water. At high Eh values. Au° can form gold peroxide or hydrated auric oxide (insoluble oxides) which are unstable thermodynamically and hence powerful oxidants. The oxidizing power of these compounds depends on the acidity of the system and declines with an increase in pH. At very low Eh values, hydrogen cyanide (HCN) and cyanide ions (CN~) are stable species, the latter being predominant at pH > 9.24, whereas cyanate (CNCT) is the only stable species at a higher Eh value.
As Figure 4.6 shows, [Au(CN)2]~ (aurous cyanide complex), has substantial stability, extending into a large area of the Au-H20 stability fields. The presence of extensive stability fields for this compound, especially at pH > 9.2, where the formation of HCN can be totally avoided, makes leaching in cyanide solution feasible. Although solid aurous cyanide, AuCN, and the auric cyanide complex, [Au(CN)4]_, have been reported, the aurous cyanide complex. [Au(CN)2]“ is the only stable complex of gold cyanidation. It has been found that the introduction of cyanide in aqueous systems drastically reduces the stability fields of zero-valence gold and its oxides.
Effect of Cyanide Concentration on Cyanidation
The gold cyanidation rate increases linearly with an increase in cyanide concentration until the maximum point is reached (Wadsworth et al., 2000; Marsden and House, 1992; Kondos et al., 1995; Ling et al., 1996; Deschtmes et al., 2003). A retarding effect on gold solubilization was observed with further increase in cyanide concentration (Figure 4.7) (Maclaurin, 1893; Barsky et al., 1934).
FIGURE 4.7 Effect of cyanide (KCN and NaCN) concentration on leaching rate of gold. Source: Modified from Ilyas and Lee (2018)
At a high concentration cyanide ion undergoes hydrolysis, resulting in an increase in alkalinity which suppress the leaching of gold.
Further investigations by various researchers indicated that the use of 400 to 500 ppm sodium cyanide could result in a similar rate of gold leaching that is independent of the cyanide concentration, if it exceeds 0.075% potassium cyanide or 0.06% sodium cyanide in the lixiviant solution (Ling et al., 1996; Deschenes et al„ 2003; Ellis and Senanayake, 2004). In fact, an excess of cyanide causes needless consumption of cyanide, which does not favour leaching reactions (Kondos et al., 1995). Large amounts of cyanide would create cyanide complexes of impurities. On the other hand, a higher cyanide concentration may be needed in view of its competition with other kinds of associated impurities (Marsden and House, 1992), but a decrease in cyanide concentration ultimately controls the extra cost of effluent treatment at industrial scale (Ling et al., 1996; Deschenes et al., 2003). In conclusion, therefore, a lesser or higher concentration of cyanide than the optimal level may negatively affect the gold cyanidation process. Like gold, the dissolution rate of other precious metals increases with increasing cyanide concentration until it reaches a plateau. There was a small decrease in recovery of Pt, Pd, and Rh at further increased concentrations of NaCN (Figure 4.8).
The reason for the decreased leaching with the increase in NaCN concentration shown in Figure 4.8 is believed to be due to the surface chemical reaction process whereby both CN' and 02 need to be absorbed on the surface of metal. If the concentration of CN" is too high, many active sites on the metal surface will be occupied by CN", which will not favour the absorption of CL. The existence of an optimum ratio between the concentration of cyanide and oxygen was also observed in the cyanide leaching of gold (Gu, 1994).
FIGURE 4.8 Effect of cyanide concentration on leaching of Pt. Pd. and Rh.
Source: Adopted with permission from Chen and Huang (2006).
Effect of Alkali on Cyanidation
The addition of alkali in cyanide solution has both accelerating and retarding effects on gold dissolution. The purposeful addition of alkali has following advantages: (i) it prevents the hydrolysis loss of cyanide; (ii) it prevent cyanide loss by the action of carbon dioxide in air; (iii) decomposition of bicarbonates occurs in the mill water prior to cyanidation; (iv) it neutralizes acidic compounds (such as ferric salts, ferrous salts, and magnesium sulphate) in the mill water before adding to the cyanide circuit; and (v) neutralization of acidic constituents—pyrite, alkalis (CaO/NaOH/Na,CO,)— added in the gold cyanidation process. The use of lime additionally promotes the settling of fine particles to obtain a clear leach liquor that can be separated easily from the pulp.
Although the use of an alkali is essential in cyanidation, several researchers have found that the addition of alkalis like sodium hydroxide and particularly calcium hydroxide retards the cyanidation leaching of gold. Barsky et al. (1934) investigated the effects of calcium hydroxide and sodium hydroxide on the rate of gold dissolution in cyanide solutions containing 0.1% NaCN and found a decreased leaching rate when calcium hydroxide was added with a solution pH ~ 11. Leaching was almost nil at pH > 12.2. The effect of NaOH was less pronounced, with the leaching rate starting to be slower at pH > 12.5. However, leaching was more rapid at pH 13.4 using NaOH than in a solution containing a similar concentration of cyanide by using calcium hydroxide at pH 12.2. The effect of calcium ion on leaching was then investigated by adding CaCl, and CaS04 to a cyanide solution. Neither of these salts affected the leaching rate to any appreciable extent. The solubility of oxygen in cyanide solutions with various amounts of Ca(OH)-, also did not show any appreciable difference.
It is thus concluded that a decrease in leaching rate using NaCN solutions caused by the addition of Ca(OH), is not due to either lower solubility of oxygen or the presence of calcium ions. Habashi (1967) attributed the retarding effect of Ca(OH)2 to formatting calcium peroxide onto the gold surface, which prevents the cyanidation reaction. Notably, calcium peroxide is supposed to form by the reaction of lime with H,02 accumulating in the solution.
Moreover, cyanide exits as HCN gas in the less alkaline region (pH < 9.2) where the formation of insoluble AuCN along with hydrogen peroxide is possible, as per the reaction below:
To avoid the formation of AuCN, therefore, the cyanide solution should be alkaline, which can control the decomposition of cyanide ions via hydrolysis and also in the presence of atmospheric CO,.
To prevent the formation of insoluble AuCN, and avoid the adverse effect on the leaching rate of a very high pH, it is necessary to carefully optimize the leaching pH, which is usually maintained between 11 and 12.
Effect of Dissolved Oxygen on Cyanidation
Pure oxygen was first introduced in the gold cyanidation process by Air Products, South Africa in the 1980s (Stephens, 1988) and then also practised in Canadian plants (McMullen and Thompson, 1989). The Lac Minerals plants were the first to demonstrate a faster leaching rate associated with dissolved oxygen and lead nitrate. Deschenes et al. (2003) investigated the effect of dissolved oxygen as presented in Figure 4.9, from which it was concluded that dissolved oxygen concentration is not much related to cyanide consumption; rather, rapid leaching kinetics can be achieved.
Oxygen-assisted leaching was adopted quickly in industry with the technological advancement for improving oxygen mass transfer, e.g., the use of Degussa’s peroxide-assisted leach or pressure acid leaching, PAL, and Kamyr’s carbon-in-leach- with-oxygen process, CILO (Loroesch et al., 1988; Elmore et al., 1988; Revy et al. 1991; Kondos et al., 1995; Liu and Yen, 1995). Numerous efforts have been made to develop an efficient device for enhancing oxygen dispersion (Jara and Harris 1994; Sceresini, 1997; McLaughlin et al., 1999). The practice of continuous oxygen monitoring and control stabilizes process performance and compensates for disturbances related to changes in oxygen requirements. A better design of oxygen probes has added further robustness to the control and operation strategy in leaching (McMullen and Thompson, 1989). The FILBLAST Gas-Shear Reactor has been employed to improve gas mass transfer efficiency via an improved dissolved oxygen concentration and reduced oxygen consumption, which could enhance the leaching rate of gold in cyanide solution.
FIGURE 4.9 Effect of dissolved oxygen concentration on gold cyanidation at pH 11.2 with 500 ppm NaCN concentration and time 24 h.
Source: Modified from Deschenes et al., 2003; Ilyas and Lee. 2018.
Overall, the role of oxygen is critical in gold cyanidation and the maximum dissolved oxygen content of a dilute cyanide solution is 8.2 ppm at room temperature and atmospheric pressure that corresponds to 0.27x 10mole per litre. Mostly cyanide leaching of gold is performed at pH ~11, where the dissolved 02 concentration remains ~6 ppm. A decrease in O, concentration (particularly below 4 ppm) reduces the leaching rate drastically. In contrast, leaching increases remarkably when the concentration of dissolved oxygen rises above 10 ppm. An oxygen- enriched operation (12-18 ppm 02) is beneficial for achieving high throughput at the commercial level.
The effect of O, pressure on platinum dissolution was investigated by Chen and Huang (2006) for a fixed NaCN concentration. In this series of experiments, 02 pressure ranging from 0.02 to 2.5Mpa was used. The rate was observed to increase with increasing 02 pressure, subsequently reaching a plateau value that is independent of O, pressure. The reason for the increased dissolution with the increase of 02 concentration is believed to be the surface chemical reaction process where 02 needs to be absorbed on the surface of the metal. If the concentration of 02 is high, the Pd dissolution increases. In aqueous solution, the cyanide ion was oxidized successively to cyanate and carbon dioxide. However, more rapid oxidation of free cyanide may have an effect on the leaching rate. Cyanide oxidation was found to be first order with respect to hydroxyl ion and zero order with respect to cyanide ion in the rate-determining step. Optimum dissolution was obtained with oxygen pressure between 1.1 and 1.3 MPa (Figure 4.10).
Technically, neither the concentration of dissolved oxygen alone nor the concentration of free cyanide ions alone are important in practice but it is the molar ratio of two concentrations that should be 6.
FIGURE 4.10 Effect of oxygen on Pt. Pd and Rh cyanidation at 6.25 g/L NaCN, 160 C and 1 h. Source: Adopted with permission from Chen and Huang (2006).
Effect of Temperature on Cyanidation
Generally, reaction kinetics are expected to increase with a rise in temperature but the temperature has a dual effect on gold cyanidation. An increase in temperature increases the activity of cyanide solution, thus increasing the rate of gold leaching but decreasing the amount of dissolved oxygen in the solution, which ultimately decreases the gold leaching rate. Therefore, there should be an optimum temperature for the maximum rate.
Maximum cyanide leaching of gold in 0.25% KCN at 85°C showed half the amount of dissolved oxygen at this temperature compared with room temperature (Figure 4.11).
Interestingly an increase in temperature to 1 00 C slightly reduced leaching, although no dissolved oxygen was observed at that temperature. This leaching trend can be attributed to the lower capacity of an electrode to adsorb/retain hydrogen in a heated solution. Hence, the maximum opposing electromotive force (EMF) due to polarization reduces for the heated solution until the EMF of gold leaching exceeds the polarization EMF, allowing gold leaching even in the absence of dissolved oxygen. Polarization can be prevented either by oxygen oxidizing the hydrogen at the gold surface to favour leaching at low temperatures, or by heat dislodging the hydrogen from the gold surface to favour leaching without oxygen. The activation energy of gold and silver leaching ranges from 2-5 kcal/mole, which is typically the diffusion-controlled reaction.
For Pt, Pd, and Rh, when the reaction temperature was higher than 160 C, the percentage of Pd leached decreased rapidly because Pd(CN)42“ is not stable at high
FIGURE 4.11 Effect of temperature on gold cyanidation with 0.25% KCN solution under aeriation.
Source: Modified from Julian and Smart (1921).
temperature and is easily decomposed to Pd metal. In contrast, the Pt and Rh cyanide complexes remained relatively stable in solution at 180 C but showed a slight decrease in concentration (Figure 4.11).
At these high temperatures, free cyanide is readily hydrolyzed and oxidized and decomposes more quickly than complexed cyanide. The thermal decomposition reactions of Pt. Pd, and Rh are as follows.
The higher temperature stability of Pt(CN)42_ than of Pd(CN)42_ can be explained by the higher thermodynamic and kinetic stability of heavy platinum group metal complexes than that of light ones even with the same valence state, same com- plexing agent, and same geometrical structure (Chen, 1995). On the other hand, the higher stability of Rh(CN)63~ than Pd(CN)42_ can be explained by the chemical reactivity of the cyanide complex with different geometrical structures. Pd(CN)42~ has a square planar structure, allowing O, to attack the central ions along the Z-axis. However, Rh(CN)63~ has an octahedral structure, and the central ion is surrounded by cyanide ions. Therefore, the bond between CN~ and the central atom needs to be broken before reaction with 02 can occur (Chen and Huang, 2006).
Effect of Agitation and Particle Size on Cyanidation
The precious metal cyanidation rate depends on the mixing pattern and the thickness of the diffusive layer (Marsden and House, 1992). Consequently, the agitation rate must be sufficient to properly suspend all solid particles in the lixiviant solution. When stirring speed is increased, the leaching kinetics increases. An intense mixing pattern reduces the thickness of the diffusive layer and improves the rate of mass transfer of oxygen and cyanide, allowing feasible saturation of pulp (Ellis and Senanayake, 2004). Ling et al. (1996) investigated whether smaller particle size could enhance the rate of gold leaching due to the larger available surface area and longer contact time between solid and lixiviant. At optimal aeration and agitation conditions, the maximum rate of gold leaching was determined to be 3.25 mg/cm2/h. This equals a penetration of 1.68 microns on each side of a flat 1 cm2 gold particle, or a total reduction in thickness of 3.36 microns hourly. At this rate, a gold particle of 37 microns thickness would take about 11 h to completely leach out in the solution (Kondos et al., 1995).
Effect of Associated Metal Ions on Cyanidation
Mostly gold occurs in native form, along with varying amounts of co-existing silver. As well as silver, several other metals such as copper, lead, zinc, etc., and some carbonaceous matter, are also present in gold-bearing urban mined sources. The presence of carbonaceous matter is a cause for concern, as it significantly adsorbs the gold-cyanide complex, causing operational loss. The metals that dissolve in cyanide solution either accelerate or retard the leaching.
Dissolved Fe and Cu form different respective cyanide complexes in leach liquors. Similarly, the presence of small amounts of lead also accelerate the leaching. The electrode potential value of these metals in the cyanide solution indicates that gold can displace these metal ions. The accelerating effect in the presence of these metal ions corresponds with the alteration in the surface character of the gold by alloying with displaced ions. A change in surface character may lead to a decreasing thickness of the boundary layer through which the reactants diffuse to reach the metallic surface. Oxygen is necessary for gold cyanidation leaching. Any side reaction that may deprive gold of its oxygen content in cyanide solution will lead to a decrease in the leaching rate. Metal impurities like silver, copper, zinc, and iron associated with gold may dissolve in a cyanide solution, causing depletion of the cyanide content from the lixiviant. Aluminosilicates, if present, form colloidal silica and alumina in alkaline pH as well as precipitating the iron. These are reaction products that have a strong adsorptive capacity for sodium cyanide, thus retarding the gold leaching. A large number of lead ions cause a retarding effect by forming an insoluble film of Pb(CN)2 on the gold surface.
Although calcium ion has no effect on gold dissolution, at pH > 11.5 it retards gold cyanidation. Solutions kept alkaline by Ca(OH)2, when compared with others at the same pH kept alkaline with KOH. hinder the leaching in the case of lime, as shown in Figure 4.11. The decrease is presumably due to the formation of calcium peroxide (as in Equation 4.22) on the gold surface, which prevents the reaction with cyanide.
FIGURE 4.12 Effect of temperature on Pt, Pd, and Rh cyanidation at 6.25 g/L NaCN, 160 C .
1 h and O, pressure 1.5 MPa.
Source: Adopted with permission from Chen and Huang (2006).
The addition of ozone to the cyanide solution decreases the leaching rate. Apparently, a brick-red layer of gold oxide is responsible for the retarding effect. The oxidation of potassium cyanide to cyanate is also possible with ozone.
