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Lecture 19A - Gold Processing

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Page 1: Lecture 19A - Gold Processing

GOLD PROCESSING

Gold is usually present as a metal, alloyed with metallic silver and perhaps, copper. The high S.G. (19.3)

of gold means that that gold particles, even when of sub-sieve size, settle readily from pulps in which

the main gangue mineral is silica. Gold is malleable, and during grinding to liberate the gold from

associated gangue mineral, the particles of gold become flattened without being reduced in size. In

point of fact considerable agglomeration can occur during grinding which leads to an increase in the

particle size of the gold. This differential grinding effect can assist in the recovery of gold by gravity

separation within the grinding circuit itself. Against this however, the weight and malleability of gold

particles can lead to significant retention in the pump and sump boxes in a closed grinding circuit.

In addition to "native" gold, the element may occur as inclusions with sulphide minerals such as pyrite,

pyrrhotite, stibnite, arsenopyrite, and galena at sizes as small as only 1 micron in diameter. Such

minerals are called "auriferous". It is not always practicable to grind these sulphides to the fineness

required to liberate this finely-disseminated gold. In such a case, the usual practice is to concentrate the

gold-bearing sulphides at a relatively coarse size, then regrind the concentrate, and finally extract the

gold by Cyanidation.

A third class of gold ores has its values in the form of tellurides. These compounds are not malleable and

they slime readily. Gold can be extracted directly by leaching with cyanide but a lower pH level than

normal is required (about 10). Process selection depends on whether the gold can be freed from its

gangue at a coarse size, or whether it is carried in sulphides which can be similarly liberated. If the ore

contains other valuable minerals, it may also be necessary to provide for their recovery. A notable

example is the flowsheet used in a number of mills in South Africa, where half the gold is recovered

from strakes (a form of sluice), and the balance by Cyanidation after which the tailings are re-treated to

recover uranium.

Gravity separation of gold is practiced on strakes, shaking tables, and in sluices and jigs. Mercury

amalgamation can be used in these circuits but is losing its appeal because of environmental concern.

Froth-flotation can be employed to remove gold and sulphides from a finely ground pulp.

Hydraulic traps can catch coarse gold in a spigot product. This type of hydraulic classifier is usually

placed between the mill discharge and the mechanical classifier. It removes particles of gold and other

metallics from the circuit. Jigs, unit flotation cells, and tables are similarly employed. Even when the

entire mill feed is to be "cyanided" (treated with cyanide to dissolve the gold), it is important to trap

large particles of gold by gravity methods, since dissolution proceeds slowly and would be incomplete

for such gold particles even after many hours of leaching.

Cyanidation for gold recovery is used world-wide for various ore types. The process of Cyanidation

proceeds in four stages:

1. Preparation of the ore to expose its gold.

2. Dissolution of gold using low strength NaCN solutions (0.05%).

3. Separation of gold-rich liquid from residual solids.

4. Recovery of gold from pregnant solution.

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The gold is recovered from the loaded solution by precipitation with Zn dust in a process known as

Merrill-Crowe. This process demands a clarified and de-aerated liquor.

Gold Processing Flowsheet using the Merrill-Crowe Process to recover gold from solution.

Today, modern practice utilizes Activated Carbon to strip the gold from solution. This process, known as

Carbon-in-Pulp or Carbon-in-Leach can replace the expensive dewatering steps required of conventional

methods. Gold is extracted from the carbon using elution with the eluant treated by electrolysis for

recovery onto steel-wool cathodes.

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Carbon-in-Pulp Recovery of Gold from Solution

CYANIDATION

GOLD DISSOLUTION

Cyanidation is a relatively simple process that exploits the solubility of gold in dilute cyanide solutions

and its ease of recovery by cementation with zinc dust. The reaction chemistry can be represented by

the following equation proposed originally by Elsner in 1852:

4Au + 8CN-

+ O2 + 2H2O = 4Au[CN]2-

+ 4OH-

The major features of the process as indicated by this equation are the formation of a gold-cyanide

complex ion, the requirement of oxygen, and the sensitivity to pH.

The formation of cyanide complexes is not unique to gold and silver. Cyanide can also complex with

numerous other heavy metals such as copper, iron, mercury and zinc. Thus, the presence of other ions

derived from sulphide or oxide minerals in the ore can present difficulties in processing gold ores.

Typical cyanide concentration ranges from 0.02 to 0.2 percent but the presence of "cyanicides" can

significantly increase consumption rate required to maintain this concentration. Leaching times also vary

from ore to ore with 24 hours being a typical minimum requirement but, up to 72 hours being necessary

in some cases.

Cyanicides derive from chalcocite, bornite, oxide-copper minerals, nickel and zinc minerals, pyrrhotite,

arsenopyrite and graphite. Usually, pyrite is not a problem, but marcasite can be trouble. Pyrrhotite acts

to deplete dissolved oxygen from the pulp preventing Elsner's reaction from taking place while graphite

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and other carbonaceous material can adsorb gold and silver complex ions onto their surfaces depleting

the solution of gold and silver values. The other minerals act to promote cyanide consumption.

Aeration prior to Cyanidation can minimize the effects of pyrrhotite and arsenopyrite, but gold

dissolution during grinding is precluded in this case.

The need for pH control is evident from Elsner's equation but moreover, a protective alkalinity is

necessary to ensure that the reagent is ionized to be predominant as free-cyanide and not as dissolved

HCN gas. The pulp pH is usually maintained above 11 unless tellurides are present. Higher values retard

the reaction rate but lower levels reduce the free-cyanide appreciably.

Cyanidation was first used in the South African gold mines in the 1890s to treat tailing dumps.

Recoveries are typically 95 percent so the process spread rapidly around the world and today is the

single most important method for treatment of gold and silver ores. Amalgamation is rarely used today

because of environmental concern but gravity separation ahead of Cyanidation within the grinding

circuit is often used to recover coarse, free gold which may be as much as 40% of the total gold value.

If the gold is associated with sulphides such as pyrite or arsenopyrite, flotation is used to produce a

concentrate for leaching. This can reduce plant size significantly as flotation residence times are

measured in minutes compared to the days required for Cyanidation (24 to 72 hours).

If the ore is appreciably "refractory" due to the presence of cyanicides or due to the extremely fine

associations of the gold with the sulphides (<1 µm), the ore or concentrate must be pre-treated in some

fashion. Pre-treatment processes might include:

- Roasting

A process in which ore is processed under high temperature in air to alter chemically the material into

an oxidized form. Generally, this process is conducted on sulphide ores prior to leaching or to sulphide

concentrates prior to smelting. Gold ores containing sulphide minerals may also need to be roasting

prior to treatment by Cyanidation.

- Aeration

Aerating the pulp prior to leaching is done with ores containing high amounts of pyrrhotite which

consume oxygen. By oxidizing the surfaces of the pyrrhotite the oxygen is then available for cyanide to

dissolve gold and silver.

- Double-oxidation (acid-leaching)

If the ore contains a lot of sulfides that consume free cyanide ion, the sulfides can be rendered inert by

practicing a double acid-leaching operation prior to Cyanidation.

- Pressure oxidation

Barrick Gold pioneered this process in which high pressure autoclaves are used to leach the sulfides

prior to Cyanidation. The pressure is typically 100 atm and the temperatures approach 200 °C.

Residence times are of the order of 1-2 hours.

- Bio-leaching

In warmer climates, biological leaching of the sulfides can be practiced prior to Cyanidation. Typical

residence times are 24 hours.

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All of these processes act to chemically render the deleterious material inert to cyanide solution. Most

are very expensive and can contribute to environmental concerns. Roasting generates arsenic-bearing

dusts and gases which are difficult to control. the other processes dissolve metals into solution and so,

precipitation is necessary before discarding tailing water.

GOLD PRECIPITATION

Following dissolution the gold must be recovered from the loaded solution. The most common

procedure has been to separate the liquid from the solid tailing using multiple-stage thickeners or filters.

The liquor is progressively washed back into the plant from the solids in a counter-current fashion (CCD

Circuits). The loaded solution is then de-aerated and treated with zinc dust or its equivalent to achieve

the following reaction proposed by Parks:

2Au[CN]2-

+ Zn = Zn[CN]4-

+ 2Au

To ensure total gold precipitation, excess zinc is used as some zinc also dissolves according to:

Zn + 4CN-

+ 2H2O = = Zn[CN]4-

+ 2OH-

+ H2

In the past decade, this process (known as Merrill-Crowe) has been replaced in a number of mills with

Carbon-in-Pulp processing. The major advantage of Carbon in Pulp is not the reduction in the use of zinc

dust but rather the impact on the dewatering requirements. With Merrill-Crowe, dewatering occurs in a

Counter-Current Decantation (thickener) circuit or in a Counter-Current Filtration circuit. These stages of

consecutive removal and rewashing of the pulp are done to reduce the solution losses of water with the

final tailings. These processes are very expensive from both a capital and operating cost viewpoint.

Using carbon allows the gold to be recovered on coarse pellets (16 mesh) of activated carbon. Activated

carbon is made from coconut shells or peach pits to ensure the material is hard and doesn't abrade

during contact with the solid particles in the pulp. The carbon is first conditioned to remove sharp

corners and to remove fines. Then it is used in the process and recycled through an elution stage. The

elution stage removes the gold from the carbon and puts it into a very clean electrolyte solution that

goes on for gold recovery by electrowinning. The carbon can be reused several times without much loss

in effectiveness but eventually the pores become blinded with lime deposits and coatings of organic

materials (oils, etc.). So a portion of the carbon is bled off the circuit and sent for regeneration which

consists of acid-washing and pyrolysis. Some carbon is burned off and lost but the loss is small and there

is no gold associated with the regeneration loss. Some abrasion does occur during processing and there

is a gold loss associated with those fines of carbon – typically less than 0.5%.

CCD Circuit using paste thickeners for dewatering after leaching.

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CCD (counter-current-decantation) thickeners associated with a gold mill using Merrill-Crowe.

CCD Thickener Configuration.

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IAMGold's Rosebel gold mill flowsheet (Suriname).