Agitation Leaching Theory and Practice

Agitation leaching is a chemical process where the soil that is to be mixed or slurried is kept in contact for a certain period of time with fluid to be extracted. The metal solubility rate is reduces quite noticeably, and the extraction gets completed on the approach of equilibrium between the metal present in the solution and the metal contained on the surface of the soil is approached

Excess metal will not be extracted from the surface of the soil unless the soil is accessed by fresh extraction solution and the contact time increases when the system is at equilibrium. On reaching equilibrium, the soil is separated from the extraction fluid using sedimentation, thickening, or clarification. An agitation vat coupled with a solid-liquid separation vessel (typical processes like clarification or sedimentation) is considered to be a single stage The process of extraction is then generally continued in a separate extraction vat and the clear solution obtained from the extraction process is used to speed up the rate of extraction [1].

Agitation Leaching Practices

Cyanide and the Gold Industry

Introduction

One of the most widely used industrial practices is the cyanidation process in the gold industry. Amount of gold present in ores typically occurs at very low concentrations in ores which generally range from less than 10 gm/tonne. At the low level of the gold concentrations the most predominant method used extensively and one that is cost effective is the aqueous hydrometallurgical extraction processes to extract the gold from its ore. Typical hydrometallurgical gold recovery involves an agitation leaching step where 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 electrodeposition or precipitation.

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Gold is one of the noble metals and is not very much soluble in water. Complexes, like cyanide, is known for stabilizing the gold species in solution, along with an oxidant preferably oxygen thereby dissolving the required amount of gold. The amount of cyanide in solution required for complete dissolution may be typically of very low concentrations such as 350 mg/l which accounts for around 0.035% of 100% sodium cyanide

Alternative complexing agents for gold, such as chloride, bromide, thiourea, and thiosulfate 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 justifies the dominance of cyanide as the primary reagent for the leaching of gold from ores since its introduction in the latter half of the 19th century.

Manufacture, Transport and Storage of Cyanide

Approximately 1.1 million metric tons of hydrogen cyanide is produced annually worldwide, with approximately 6% used to produce cyanide reagents for the processing of gold. The remaining 94% is used in industrial applications including production of plastics, fire retardants, cosmetics, adhesives pharmaceuticals, food processing and as an anti-caking additive for table and road salts.

Cyanide is manufactured and distributed for use in gold mining industries in a variety of physical and chemical forms, including solid briquettes, flake cyanide and liquid cyanide. Sodium cyanide is supplied as either briquettes or liquid, while calcium cyanide is supplied in flake form and also in liquid form. The strength of bulk cyanide reagents vary from 98% for sodium cyanide briquettes, 44-50% for flake calcium cyanide, 28-33% for liquid sodium cyanide and 15-18% for liquid calcium cyanide. The product strength is quoted on a molar basis as either sodium or calcium cyanide.

The form of cyanide reagent chosen for use typically depends on availability, distance from the source and cost. Where liquid cyanide is used, it is transported to the mine by tanker truck or rail car and is off-loaded into a storage tank. The truck or rail car may have a single or double walled tank, and the location and design of the discharge equipment varies by vehicle.

Solid briquette or flake cyanide is transported to the mine in drums, plastic bags, boxes, returnable bins and ISO-containers. The mine generally designs and constructs the necessary equipment to safely dissolve the solid cyanide in a high-pH solution considering the packaging of the reagent. The pH value of cyanide solutions during dissolution must be maintained above pH 12 to avoid the volatilization of the hazardous hydrogen cyanide (HCN) gas. The resulting cyanide solution is then pumped to a storage tank prior to introduction into the process.

The cyanide solution is fed from the storage tank into the metallurgical process stream in proportion to the dry mass of solids in the process stream. The feed rate of cyanide is controlled to maintain an optimum cyanide level as demanded by the metallurgy of the ore being treated.

Ore Preparation

Preparation of the ore is necessary so that it can be presented to the aqueous cyanide solution in a form that will ensure the optimal economic recovery of the gold. The first step in ore preparation is crushing and grinding, which reduces the particle size of the ore and liberates the gold for recovery.

Ore that contains free gold may not yield a sufficiently high recovery by sole use of cyanide leaching, due to a very long dissolution time for large gold particles. Such ore may first be subject to a gravity recovery process to recover the free gold before being subjected to cyanide leaching.

Gold bearing ores that contain gold associated with sulphide or carbonaceous minerals require additional treatment, other than size reduction, prior to gold recovery. Gold recovery from sulphide ore is poor because the cyanide preferentially leaches the sulphide minerals rather than the gold, and cyanide is consumed by the formation of thiocyanate. These ores are subject to a concentration processes such as flotation, followed by a secondary process to oxidize the sulphides, thereby limiting their interaction with the cyanide during the gold leach. Carbonaceous minerals adsorb gold once solubilised; oxidizing the ore prior to leaching prevents this. To counter this affect, the leaching process may also be modified by the addition of activated carbon to preferentially adsorb the gold.

Leaching with Aqueous Cyanide Solutions

When gold is leached in an aqueous cyanide solution it forms a gold-cyanide complex by oxidizing with an oxidant such as dissolved oxygen and cyanide complexation. This complex is very stable and the cyanide required is only slightly in excess of the stoichiometric requirement. However, in practice the amount of cyanide used in leach solutions is dictated by the presence of other cyanide consumers, and the need to increase the rate of leaching to acceptable levels.

Typical cyanide concentrations used in practice range from 300 to 500 mg/l (0.03 to 0.05% as NaCN) depending on the mineralogy of the ore. The gold is recovered by means of either heap leaching or agitated pulp leaching.

In heap or dump leaching the ore or agglomerated fine ore is stacked in heaps on a pad lined with an impermeable membrane. Cyanide solution is introduced to the heap by sprinklers or a drip irrigation system. The solution percolates through the heap leaching the gold from the ore, and the resultant gold bearing solution is collected on the impermeable membrane and channelled to storage facilities for further processing. Heap leaching is attractive due to the low capital cost involved, but is a slow process and the gold extraction efficiency is a relatively low 50-75%.

In a conventional milling and agitated leaching circuit, the ore is milled in semi-autogenously ball or rod mills until it is the consistency of powder. The slurry is conveyed to a series of leach tanks. The slurry is agitated in the leach tanks, either mechanically or by means of air injection, to increase the contact of cyanide and oxygen with the gold and enhance the efficiency of the leach process. The cyanide then dissolves gold from the ore and forms a stable gold-cyanide complex.

The use of oxygen or peroxy compounds instead of air as an oxidant increases the leach rate and decreases cyanide consumption, due to the inactivation of some of the cyanide consuming species present in the slurry.

The pH of the slurry is raised to pH 10-11 using lime, at the head of the leach circuit to ensure that when cyanide is added, toxic hydrogen cyanide gas is not generated and the cyanide is kept in solution to dissolve the gold. The slurry may also be subject to other preconditioning such as pre-oxidation at the head of the circuit before cyanide is added.

Highly activated carbon is used in the dissolved gold recovery process, either by introducing it directly into the CIL (carbon-in-leach) tanks or into separate CIP (carbon-in-pulp) tanks after leaching. The activated carbon adsorbs the dissolved gold from the leach slurry thereby concentrating it onto a smaller mass of solids. The carbon is then separated from the slurry by screening and subjected to further treatment to recover the adsorbed gold.

When carbon is not used to adsorb the dissolved gold in the above-mentioned leach slurry, the gold bearing solution must be separated from the solids components utilizing filtration or thickening units. The resultant solution, referred to as pregnant solution, is subjected to further treatment (other than by carbon absorption) to recover the dissolved gold.

The waste from which the gold was removed by any means is referred to as residue or tailings material. The residue is either dewatered to recover the solution, treated to neutralize or recover cyanide, or is sent directly to the tailing storage facility.

Recovery of Dissolved Gold

Gold is recovered from the solution first using either cementation on zinc powder or concentrating the gold using adsorption on activated carbon, followed by elution and concluding with either cementation with zinc or electro winning. For efficient cementation, a clear solution prepared by filtration or counter current decantation is required.

The most cost-effective process is to create adsorption of the dissolved gold onto activated carbon, resulting in an easier solid-solid separation based on size. To achieve this; the ore particles must typically be smaller than 100 µm while the carbon particles must be larger than 500 µm. Adsorption is achieved by contacting the activated carbon with the agitated pulp. This can be done while the gold is still being leached with the CIL-process, or following leaching with the CIP-process. The CIL-process offers the advantage of countering the adsorption of gold on carbonaceous or shale ore particles, but is more expensive due to less efficient adsorption, increased gold inventory and increased fouling and abrasion of the carbon.

Activated carbon in contact with a pulp containing gold can typically recover more than 99.5% of the gold in the solution in 8 to 24 hours, depending on the reactivity of the carbon, the amount of carbon used and the mixer’s efficiency. The loaded carbon is then separated from the pulp by screens that are air or hydro dynamically swept, thus preventing blinding by the near sized carbon particles. The pulp residue is then either thickened to separate the cyanide containing solution for recovery/destruction of the cyanide, or sent directly to the tailings storage facility from which the cyanide containing solution is recycled to the leach plant.

The gold adsorbed on the activated carbon is recovered from the carbon by elution, typically with a hot caustic aqueous cyanide solution. The carbon is then regenerated and returned to the adsorption circuit while the gold is recovered from the eluate using either zinc cementation or electro winning. If it contains significant amounts of base metals, the gold concentrate is then either calcined or directly smelted and refined to gold bullion that typically contains about 70 – 90% gold. The bullion is then further refined to 99.99% fineness using smelting, chlorination, and electro-refining. High purity gold is taken directly from activated carbon eluates, using recently developed processes that utilize solvent extraction techniques to produce intensive leaching of gravity concentrates [2].

Agitation leaching Applications

Commonly applied to a wide range of ore types, agitation leaching has been in use for well over 200 years. Leaching is typically performed in steel tanks, and the solids are kept in suspension by air or mechanical agitation. Air agitation in carried out in conical-bottomed leach tanks (Browns or Pachuca tanks) was widely practiced in the early years of cyanidation but has been overtaken in recent times by more efficient mechanical agitation with reduced energy requirements and improved mixing efficiency. Well-designed systems can approach perfectly mixed flow conditions in a single reactor, which help to optimize reaction kinetics and make the most of available leaching equipment.

Particle size.

The material to be leached is ground to a size that optimizes gold recovery and communition costs. In a few cases, whole ore is being ground to very less particle sizes for optimal processing, either by oxidative pre-treatment and/or leaching. Agitation leaching is rarely applied to material at greater coarse sizes because it becomes increasingly difficult to keep coarse solids in suspension, and abrasion rates increase. Increasingly, agitation leaching is being considered to treat very finely ground materials and, with the advances in ultrafine milling equipment have been ground to lesser particle sizes to liberate gold contained in refractory along with the sulphide mineral matrices prior to processing by agitation leaching and/or oxidative pre-treatment.

Slurry density.

Leaching is usually performed at slurry densities of between 35%and 50% solids, depending on the solids’ specific gravity, particle size, and the presence of minerals that affect slurry viscosity (e.g., clays). Mass transport phenomena are maximized at low slurry densities; however, solids retention time in a fixed volume of leaching equipment increases as the density increases. In addition, reagent consumptions are minimized by maximizing slurry density, since optimal concentrations can be achieved at lower dosages, because of the smaller volume of solution per unit mass of material.

Modification of pH

Alkali, required for slurry pH modification and control, must always be added before cyanide addition to provide protective alkalinity, which prevents excessive loss of cyanide by hydrolysis. Most leaching systems operate between pH 10 and 11. Staged addition of alkali may be required throughout the leaching circuit to maintain the desired operating pH, particularly when treating ores containing alkali-consuming materials. pH control is achieved by manual or automatic (on-line) measurement at various stages in the process. Calcium hydroxide (slaked lime, Ca (OH),), or sodium hydroxide can be used for pH modification. Calcium hydroxide (slaked lime) is the cheaper of the two but is less soluble and produces solutions that are much more susceptible to salt precipitation and scale formation. Unslaked lime (CaO) is used occasionally because it is less costly than slaked lime, but it is less effective for pH modification.

For nonacidic- or non-alkali-consuming ores, calcium hydroxide concentrations of 0.15 to 0.25 g/L are typically required to achieve the desired pH range for leaching (i.e., pH 10 to 11). This represents typical lime consumptions of 0.15 to 0.5 kg/tonne for non-acidic ores. Sodium hydroxide is known to be more effective than calcium hydroxide at dissolving a variety of minerals, particularly at high alkalinities, and it is a highly effective dispersant. This may result in the dissolution of ore constituents, such as silicates, to produce various solution species, which can subsequently precipitate in a number of undesirable forms, potentially affecting downstream processes, including filtration, gold precipitation, or carbon adsorption. Consequently, calcium hydroxide is generally the preferred method of pH control in agitated leaching systems.

Cyanide

Cyanide may be added to agitated leaching systems either prior to the leaching circuit, that is, during grinding, or in the first stage of leaching. Subsequent reagent additions can be made into later stages of leaching to maintain or boost cyanide concentrations to maximize gold dissolution. In the absence of cyanide-consuming minerals in the ore or concentrate to be leached, cyanide concentrations used in practice range from 0.05 to 0.5 g/L NaCN, and typically between 0.15 to 0.30 g/L NaCN. Typical cyanide consumptions observed in agitated leaching systems for free-milling ores vary from about 0.25 to 0.75 kg/t. In cases where the feed material contains significant amounts of cyanide consumers and/or high silver content (i.e., >20 g/tonne), higher cyanide concentrations may be applied, that is, 2 to 10 g/L NaCN. In such cases, cyanide consumptions may vary from 1 to 2 kg/t, and in some cases much higher, depending on the nature and amount of cyanide-consuming minerals. Cyanide concentrations are usually monitored by manual titration techniques or less commonly by on-line cyanide analyzers, based on titrimetric, colorimetric, potentiometric, and ion-specific electrode techniques.

Oxygen Content

Oxygen is typically introduced into leaching systems as air, either sparged into tanks as the primary method of agitation, or supplied purely for aeration. In either case, crude sparging systems are usually sufficient to provide satisfactory bubble dispersion and to ensure that adequate dissolved oxygen concentrations are maintained. Typically, the amount of dissolved oxygen concentrations can be maintained at, or even slightly above, calculated saturation levels with air sparging. The optimum sparging system depends on the geometry of the leach tanks. For example, conical-bottomed Pachuca tanks with single sparging points (common South African practice prior to about 1980) and flat-bottomed leach tanks with multiple sparging points, or simple down-the-agitator-shaft addition, have all been used. In a few cases, particularly when treating ores that contain oxygen-consuming minerals, pure oxygen [5] or hydrogen peroxide [4] have been added to increase dissolved oxygen concentrations above those attainable with simple air sparging systems.

Residence time. Residence time requirements vary depending on the leaching characteristics of the material treated and must be determined by test work. Leaching times applied in practice vary from a few hours to several days. Leaching is usually performed in 4 to 10 stages, with the individual stage volume and number of stages dependent on the slurry flow rate, required residence time, and efficiency of mixing equipment used.

Counter-current leaching.

Leaching efficiency can be enhanced by the application of Le Chatelier’s principle. In summary, the lower the concentration of gold in solution, the greater the driving force for gold dissolution to occur, although in a mass transport controlled reaction it is debatable what role this plays in gold leaching. An alternative explanation for this phenomenon is the reversible adsorption of gold cyanide onto the ore constituents. The gold adsorption is reversed when the solution is exchanged for a lower grade solution or when a material (such as activated carbon or suitable ion exchange resin) is introduced into the slurry, which actively competes for the Aurum cyanide species. This effect can be exploited in practice by performing intermediate solid-liquid separation steps during leaching to remove high-grade gold solutions, and rediluting the solids in the remaining slurry with lower-grade leach solution and/or with freshwater plus reagents. Successful applications of this principle have been used at the Pinson and Chimney Creek, Nevada (United States), and East Driefontein (South Africa) plants, and at other operations [6, 7].

At many operating gold plants, an increase in gold extraction is observed when a leach slurry can be transferred from one type of process equipment to another (i.e., between leach tanks, thickeners, filters, pumps, and pipelines).This can be explained by the different mixing mechanisms in the different equipment, coupled with other factors, such as changes in slurry percent solids, changes in solution composition, and the effects of pumping transfer (i.e., plug flow mixing).Likewise, the benefits of the carbon-in-leach (CIL) process compared with leaching and carbon-in-pulp (CIP) have been clearly demonstrated both experimentally and in practice, even without the presence of interfering constituents in the ore[8]. The CIL process results in improved conditions for gold dissolution[3].