Leaching

    Cyanidation

    When we have to mention hydrometallurgical treatments, the cyanidation process must be mentioned initially due to has been employed in the gold mining industry for a long time. The cyanidation process was invented in 1887 when the gold recovery started to have some problems due to the lack of good technology. Basically the process is supported by two facts, gold is soluble in dilute solutions of cyanide, and second, the pregnant solution can be processed successfully by using zinc powder or activated carbon and electrowinning. In other words, the process involves several steps from crushing until obtaining a Dore bar.

    Gold typically occurs at very low concentrations in ores - less than 10 g/t or 0.001% (mass basis). At these concentrations the use of aqueous chemical (hydrometallurgical) extraction processes is the only economically viable method of extracting the gold from the ore. Typical hydrometallurgical gold recovery involves a leaching step during which the gold is dissolved in an aqueous medium, followed by the separation of the gold bearing solution from the residues, or adsorption of the gold onto activated carbon. After elution from the activated carbon the gold is further concentrated by precipitation or electrodeposition. Gold is one of the noble metals and as such it is not soluble in water. A complexant, such as cyanide, which stabilizes the gold species in solution, and an oxidant such as oxygen are required to dissolve gold. The amount of cyanide in solution required for dissolution may be as low as 350 mg/l or 0.035% (as 100% NaCN).
     
     Alternative complexing agents for gold, such as chloride, bromide, thiourea, and thiosulphate form less stable complexes and thus require more aggressive conditions and oxidants to dissolve the gold. These reagents present risks to health and the environment, and are more expensive. This explains the dominance of cyanide as the primary reagent for the leaching of gold from ores since its introduction in the later part of the 19th century. The introduction of cyanide leaching two centuries ago revolutionized the processing of gold and silver ores. Gold is dissolved as an aurocyanide complex in oxidizing alkaline cyanide solutions. The basic principle of the cyanidation process is that alkaline cyanide solutions have a preferential dissolving action on the precious metals contained in an ore. The reaction generally accepted is shown below and is known as Elsner equation.
     

     4Au + 8NaCN + O2 + 2H2O = 4Na[Au(CN)2] + 4NaOH


    The reaction of silver sulphides, commonly associated with gold ores, is not quite so straight-forward,



    Ag2 + 4NaCN = 2NaAg(CN)2 + Na2S

     

    The gold dissolution rate is dependent on the concentration of NaCN and the alkalinity of the solution, the optimum pH is around 10.5. For efficient leaching, the gold should occur as free, fine-size, clean particles in an ore. The presence the certain minerals such oxidized copper minerals is a poison for the process due to copper will be dissolved in first instance and the free cyanide will not be used for gold dissolution. Also, it is important to mention that an adequate supply of dissolved oxygen must be present in the cyanide process.The mechanisms of oxidation and cyanidation of gold together with the consumption of oxygen and cyanide by gangue minerals is a serious problem. Commonly air or oxygen can be used as an oxidant in the leaching of auriferous ores because it’s readily available, cheaper and much less aggressive than most of the chemical oxidants (e.g. H2O2, Na2O2, O3, CaO2), but , the rate of gold cyanidation with molecular oxygen is slow compared with the chemical oxidant.

     Conventional mill practice involves the sparging of oxygen or air through the pH adjusted gold leach pulp preferably prior to the addition of cyanide. This preareation oxidizes soluble sulphide into thiosulphate and finally sulphate.  If cyanide is present in the pulp, it would be oxidized to thiocyanate. These reactions obviously indicate that soluble sulphide is a major oxygen consumer. This oxidation process actually prevents the formation of passive film on the gold surface, thus gold cyanidation rate remains unaffected. The preareation operation not only meets oxygen demand for the oxygen consumers but also produces a leach slurry saturated with dissolved oxygen which is being utilized in oxidation of gold in the cyanidation circuit. If the concentration of oxygen consumers are high (more than 1000 ppm) in the leach feed, the preareation alone would not to be enough to oxidize all of them. Then, oxygen demand would remain even high even in the cyanidation circuit. Consequently, the gold dissolution rate will drop if no additional supply of oxygen is maintained in the cyanidation circuit. Some gold mill operations maintain air or oxygen sparging in the cyanidation circuit. The maximum utilization of oxygen in the gold mill circuit depends on its solubility in the leach pulp. The solubility of oxygen in water is temperature and pressure dependent. The solubility of oxygen increases with the decrease of temperature or increasing the oxygen pressure. The concentration of oxygen in an aqueous medium at a ambient pressure could be increased by injecting oxygen through a diffuser. The injection mechanism disperses oxygen through the aqueous medium in the form of fine bubbles.
     
     The presence of reactive sulphides such as marcasite, pyrrhotite, realgar or chalcocite in the cyanidation feed often inhibits gold dissolution by forming a protective film on the gold surface. Nevertheless, this effect could be eliminated or minimized by intensive preareation or adding a promoter in the cyanidation pulp. Salts of lead (e.g. lead nitrate) or mercury (e.g. mercury acetate) serve as promoters and remove soluble sulphide (S-2) from the cyanidation solution as sparingly soluble PbS or HgS, thus keeping the gold surface clean. It is also known that these promoters accelerate the rate of gold cyanidation possibly through the development of local galvanic cells between gold and lead or mercury. A small quantity of promoter (e.g. 0.4 to 0.8 g/t) is often sufficient to counteract the harmful effect of sulphides. The addition of a lead salt to cyanidation pulp is practiced in some operations and it is also reported that an excess may cause retardation in the rate of cyanidation.
     

    The Merrill-Crowe Process |  Activated Carbon Carbon-in-Pulp  |  Heap Leaching  |  Carbon-in-Leach  |  Stripping circuit |  CIP vs. CIL  |  Carbon-in-Column |  Importance of Cyanide and Lime Dosage |  Effect of Temperature, Particle Size and Agitation on Cyanidation |  Effect of Base Metals on Cyanidation | Gold Passivation in Cyanidation  | Counter Current Decantation Circuit (CCD)  |  Primary Grind and Gold Cyanidation  |  Application of Sequential Cyanidation  |  Continuous Vat Leaching  |  Methods of Agitation in Cyanidation Plants   |  Use of Chemicals in Cyanidation  |   Recovery of Silver and Gold by Cyanidation  |   Mercury and Cyanidation  |   Tips to Design a Cyanidation Circuit   |  Pachuca Tanks |  Cyanidation of Siliceous Gold Ores  |   Thickeners and Cyanidation  |   Influence of Gold Characteristics on Cyanidation by Agitation  |   Mineralogy and Cyanidation Performance   |  Cyanidation of Clayey Gold Ores  |  Importance of Agitation in Cyanidation  |   Effect of NaCl and Na2CO3 on Cyanidation of Gold Ores   |  Cyanidation of Calcines   |  Problems Associated to the Cyanidation of Calcines  |  Addition of Sodium Cyanide in the Ball Mill  |  The Dissolution of Gold in Cyanide Solutions