CARBON-IN-PULP PROCESS

Activated carbon is employed in many cyanidation plants to recover gold and silver fro m cyanide leach solutions from several processes: carbon in pulp (CIP), carbon in leach (CIL), and carbon in column (CIC). The activated carbon used in mining is called many times absorbent activated carbon and it’s a solid with high porosity and superficial area of more than 1000 m²/g. Each pore is about 10-20 Angstrom.

The CIP process usually comprises the following operations: extraction, elution, electrowinning, and thermal reactivation. The absorption process is perhaps the most important part of the process since the optimization of extraction determines no only the amount of soluble gold lost in the residues from the plant, but also the amount of gold locked up in the plant, and the quantity of carbon reporting for elution and reactivation. Unlike most hydrometallurgical processes, the rate of extraction of gold from a pulp is slow, and the kinetics is very important in order to determine optimum conditions for the operation.

 Gold Cyanidation

Gold Cyanidation.: 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 a gents 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 reaction is expressed in its simplest form by the 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, but is usually expressed in the following manner:

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

The reaction of silver Sulphide with sodium cyanide is reversible and does not proceed far unless sodium sulphide is remover from the system or an excess of free sodium cyanide is available to drive the reaction to the right.

Fortunately sodium sulphide is sensitive to oxidation, discomposing via side reactions to sulphocyanate and thiosulphate. The point is that oxygen is an indispensable ingredient for successful cyanide leaches of gold as well as the sulphide compounds of silver. Nevertheless, silver halides are attacked by cyanide without the necessity of oxygen.

Although the affinity of cyanide for gold is such that it is extracted preferentially, cyanide will also form complexes with other metals from the ore, including copper, iron and zinc. The formation of strongly bound complexes such as those with iron and copper will tie up cyanide that would otherwise be available to dissolve gold.

Copper cyanides are moderately stable; their formation can cause both operational and environmental concerns, as wastewater or tailings from such operations may have significantly higher cyanide concentrations than would otherwise be present in the absence of copper. High copper concentrations in the ore increase costs and lower recovery efficiencies by requiring higher cyanide application rates to compensate for reagent that complex with copper rather than gold.