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An Introduction to Cyanidation

The cyanide process has allowed the development of large, low grade precious metal deposits more than any other process.   The early development of the process is attributed to a Scotsman, John Stewart MacArthur, in collaboration with Dr Robert and Dr William Forrest. The method was introduced into South Africa in 1890.  From there it spread to Australia, Mexico and the United States. Now it is used in practically all the major gold mining areas of the world.

 The reasons for its widespread acceptance are economic as well as metallurgical. It usually obtains a higher gold recovery than other processes, especially for fine gold.  Cyanidation is easier to operate than the other leaching processes, and is safer to use than mercury amalgamation. It produces the final product in the form of practically pure metal. Thus the production from a large cyanide mill will be represented by a comparatively small gold bar, which is easy to transport. Accordingly gold mines can be located in relatively inaccessible districts served only by airplane or truck.

 However, the gold metallurgist must be familiar with the other processes of gold treatment, such as gravity concentration and flotation (and, in some locations still, amalgamation), as they are frequently used as an auxiliary to the cyanide process.

 General Theory

Gold does not oxide (tarnish) at ordinary temperatures nor is it soluble in sulphuric, nitric or hydrochloric acids. It does dissolve in aqua regia (a mixture of nitric and hydrochloric acid) also in some chlorine and bromine compounds.  The chlorine and bromine reaction was the base for the bromo-cyanide method used on some refractory ores in the early days of gold mining in Australia.  Gold is also soluble in mercury, uniting with it to form amalgam. However, the main chemical property of commercial interest is that gold is soluble in dilute cyanide solutions.

 The basis of the cyanide process is that weak solutions of sodium or potassium cyanide have a preferential dissolving action on small particles of metallic gold and silver over other materials common in gold ores. However, there are a few minerals known as cyanicides that have deleterious effects which are discussed later.

 Cyanide is the general descriptive term applied usually to sodium cyanide (NaCN). However, the early work in cyanidation was based on the use of potassium cyanide and strengths of solution as well as basic formulae are still in terms of that chemical.  It is to be noted that the cyanogen radical (CN) actually has the dissolving power, the alkaline base of potassium, calcium or sodium merely giving a chemical stability to the compound.

 The main difference between the alkaline cyanides, aside from their cost, is their relative dissolving power. This depends entirely on the percentage of cyanogen radical present.

 Elsner's equation is generally accepted as expressing the action of gold in dilute cyanide solutions; 4 Au + 8 KCN + 0₂ + 2 H₂0 = 4 K[Au (CN)₂] + 4 KOH (or 4 Au + 8 NaCN + 0₂ + 2 H₂0 = 4 Na[Au (CN)₂] + 4 NaOH).  When fresh surfaces of gold are exposed to the action of cyanide in an aqueous solution containing free oxygen, a gold cyanide compound will be formed and a hydroxide (alkaline).

 Solutions

Strength of solution is usually about one pound of cyanide to a ton solution (water) (or 0.02% -0.05% NaCN). This is usually sufficiently strong for most straight cyanide circuits and experimental work has shown that maximum dissolving power is obtained at this strength. Furthermore, a weak solution is less affected by cyancides, and danger of poisoning from fumes formed by evaporation in hot weather is decreased.

 Gold sulphide concentrates, obtained by table concentration or flotation, are frequently treated by higher strength solutions. These concentrates usually require very thorough study.

 Strength of solutions is usually expressed in pounds of equivalent potassium cyanide per ton of solution. 1 lb. cyanide to 1 ton of water = 0.05% solution;  2 lb cyanide = 0.10%, etc.

 

Temperature of solution is also important in maintaining efficient dissolving action. Especially in cold climates, the solutions are frequently heated to about 70°F. Above this temperature the loss of cyanide by decomposition becomes a serious factor Theoretically, gold dissolves fastest in a solution at a temperature of 138°F.

 Slurry (Pulp) Density

To maintain maximum capacity and minimum loss of valuable material in solution, it is usually advisable to maintain the highest densities in the mill circuits. It should be kept in mind that for every ton of water added to the mill circuit, a ton of water ends up in the tailings, which needs to be handled.  This tailings solution not only contains reagents, such as lime and cyanide, but also dissolved gold, even if only in minute quantities.

The higher the density of the feed to the leach circuit the greater the capacity of the circuit, or conversely, smaller or fewer tanks and agitators are required. Assuming an ore where the solids have a specific gravity of 2.6, one ton of solids as 30% solids (70% solution) will occupy 86.7 cubic feet, while at 50% solids will only occupy about half that space, namely, 44.3 cubic feet. Also there is apt to be more settling of sand fractions which may cause mechanical difficulties when treating a dilute pulp. Accordingly agitator densities are usually kept between 30% to 60% solids.  Grinding capacity in a ball mill is also limited if the density drops below 70% solids.

 Aeration

Another of the prime requisites of successful cyanidation is free oxygen.  For efficient dissolving, it is necessary that the oxygen (air) come in actual physical contact with the gold particles. As these particles are usually very sparsely distributed through the pulp, it means that the air bubbles should be thoroughly dispersed and a huge excess be used beyond theoretical requirements of air.

 Oxidizing agents have also been used. These oxidizers may be sodium peroxide, potassium permanganate or manganese dioxide. They act in two ways; by a nascent or active condition and so accelerating the dissolving of gold, and by oxidizing impurities that may be present in the ore or solution.

It has been found in some mills that due to increases in tonnages or changes in the ore, that extra aeration is necessary. Various methods have been used for this, such as placing a ring of air jets around the circumference of each of the agitators. High pressure compressed air was used, giving violent aeration and agitation.  Another method is based on the dispersal of the flow of pulp as it enters the various tanks in one large thick stream.  Another method noted in the field is the insertion through the sides of the tank, well below the top of the pulp.

 Decomposition of Reagents

The amount of reagents actually required for dissolving the gold is extremely small. However, frequently the amount of reagents used is much higher and certain causes should be recognized and, if possible, remedied. These may be briefly listed as follows:

·         Water Quality

·         Cyanicides

·         Mechanical losses

 Water Quality

The source of water is very important, not only from the viewpoint of the quantity, but also the quality. In some districts the only water available is from small lakes or ponds and, as such, is frequently contaminated with organic material and soluble salts. This water may be highly reducing in its action. Extra lime treatment may be necessary before this water joins the mill return stream. Flocculants and percipatates may be added to aid in the precipitation of the soluble salts. Chemical oxidizers such as potassium permanganate are also used. Some of these problems are also discussed further under the heading of clarification.

 Cyanicides

Certain materials known as cyanicides may be present in the ore. A cyanicide may be defined as a natural occurring material that destroys cyanide.  Pyrrhotite is one of the best known. It combines with the cyanide giving ferro-cyanide and sulphocyanide.  It is stated that stibnite requires extremely low alkalinity to prevent its solubility in solution.  The reverse is true in the case of sphalerite, where high lime tends to reduce zinc solubility.  Ores often contain copper, antimony, arsenic, cobalt or nickel sulphides which go into solution through the action of the cyanide.

Although the dissolving rate of these materials may be controlled to some extent, the solutions in time will lose their potency due to being re-cycled.  Then it is necessary to bleed off part of the fouled solution and restore the balance by the addition of fresh stock. After the solutions have been de-aerated and precipitated, it is also necessary to positively aerate them before being used.  This aeration not only restores the free oxygen to the solution, but also partially regenerates some of the combined cyanide.

 Mechanical losses

They occur in two ways:

(a) Accidental losses.

(b) Inherent losses.

 The first is due to spills and leaks on account of poor launder design and tank spillages. Losses also occur when it is necessary to dump agitators, classifiers or thickener tanks, due to power failures or mechanical difficulties.

 Inherent losses may also be considered from two view-points, namely, those occurring only in a new circuit, and those occurring continuously. The first is due to solutions soaking into the fresh wood tanks and may occur over a period of two or three months. The second is due to losses from filter discharges, etc. Naturally it is desired to keep these losses at a minimum, as they are a flat charge against the cost of operation. For example, a filter cake may have 10 to 12'10 moisture in it. A heavy water wash on the filter will reduce the amount of chemicals in this moisture, while re-pulping and second filtration may be advisable in some cases. A cost analysis in every case is desirable.

 pH Control

To improve metal recovery and to reduce the amount of cyanide needed, the slurry pH is adjusted, normally by adding lime to maintain a "protective alkalinity." It is usual to keep this alkalinity at from ½ to 1 ½ Ibs. per ton of solution. Lime has a further beneficial effect of hastening settlement of finely ground rock, or slimes, in thickeners, and it further precipitates certain undesirable substances.

 In order that the lime will begin its protective action as soon as possible, it is usually added with the fresh ore in the ball mill (if the ore is ground). It may be added dry or as a milk of lime. Frequent and systematic sampling of mill solutions at various predetermined points in the dissolving circuit is advisable. Then the operator can control the lime and cyanide strength and be certain at all times that minimum required strength is being maintained. Cyanide is usually added in the freshly aerated solution pumped to the grinding circuit, although sometimes blocks of cyanide may be suspended in baskets in a dissolving circuit to remedy some local trouble.

For moreinformation on the pros and cons of leaching methods see Cyanide Leaching Methods: An Overview in the topics section.