Gold Chemistry Leaching Processes Chemistry Of The Cip Process Cyanide

Chapter 7 CHEMISTRY OF THE CIP PROCESS Richard E Browner ____________________________________________________________________ INTRODUCTION Gold, the only yellow metal, derives its name from the old English word Geolu meaning yellow. Its chemical symbol Au is from the Latin word for gold, Aurum. Atomic No Atomic wt Density

79 197 19.32 g cm-3

The units used to describe gold content and purity are numerous. The ore is usually referred to in grams per tonne or ounces per tonne where 1 gram = 0.03215 ounces (troy). Assay values are given in parts per million by weight which are equivalent to grams per tonne. Bullion is expressed as fineness, that is, parts per thousand. For example the Australian nugget gold coins have a fineness of 999.9. Caratage is used when the gold is alloyed with another metal. Twenty-four carat is pure gold or 1000 fine. The proportion of 24 gives the gold content by weight. Figure 7.1 World gold supply 1989: 1948 Tonnes Brazil 5% Other Latin America

Other Africa 3%

USSR Sales 15%

7%

Europe 1 Other Asia 2 Philippines 2 Other Oceania 1 PNG 2

Australia 10% 31% South Africa Canada

8% 13% USA

7.1

The world gold supply for 1989 is shown in figure 7.1 and the world gold demand for the same year is shown in figure 7.2.

Figure 7.2 World gold Demand 1989: 1948 tonnes Medals 1%

Investment 19%

Industry 9%

Coins 5%

Jewellery 66%

This chapter will look at the chemistry of the extraction of gold by the CIP process. We will progress through the unit processes found on a typical CIP plant; 1) leaching, 2) carbon adsorption, 3) elution, 4) electrowinning, 5) smelting 6) carbon regeneration.

7.2

LEACHING The CIP/CIL process as used today is a relatively new technology with the first full scale plant being commissioned in 1973. HISTORY OF GOLD LEACHING

Pre 1890

-

Chlorine/Chloride - Zinc cementation.

1880

-

Davis patented wood charcoal to recover gold from chlorinated solutions.

1891

-

Davis process used at Mount Morgan, Australia.

1887

-

MacArthur and Forrest Brothers used cyanide to dissolve gold.

1889 Crown

-

First commercial use of cyanide Mine, New Zealand.

1894

-

Johnson patented use of wood charcoal for the recovery of gold from cyanide solution.

1916

-

Gold extraction using carbon at Yuanmi Mine, Australia.

1939

-

Chapman patented flotation of carbon after adsorption.

1950

-

Fruit pit carbon used experimentally, Getchell Mine, Nevada, USA.

1952

-

Zadra elution process developed.

1973

-

Carbon-in-Pulp introduced at Homestake Mine, South Dakota, USA.

Developed from Marsden and House (1992) and McDougall (1991).

7.3

pH Potential Diagrams Figures 7.3 and 7.4 show the stability fields for gold species as a function of Eh and pH. Figure 7.3 is for the gold water system. The dashed lines are the boundaries of the stability region of water. An oxidising potential can be applied by a variety of oxidising agents. Some common ones being oxygen in air or pure oxygen, hydrogen peroxide, chlorine, etc. It can be seen from figure 7.3 that an oxidising potential in excess of 1 at a pH of 8 is required to dissolve gold. Unfortunately this is greater than the potential required to decompose water. The addition of cyanide produces a stable gold cyanide species, Au(CN)2-. Figure 7.4 shows the region where gold can be leached by a cyanide solution to form gold cyanide.

Figure 7.3

Figure 7.4

From the gold-cyanide Eh-pH diagram it can be seen that gold will leach from a pH of 0 to 14. The lowest Eh value occurs at a pH of about 10.5. This corresponds to maximum cyanide as CN- and not HCN.

7.4

DISSOLUTION OF GOLD: When gold dissolves in a cyanide solution it does so by the reaction: GOLD + SODIUM + OXYGEN + WATER Û CYANIDE 8 CN-

4 Au +

+

O2

+ 2 H2O

=

GOLD + CYANIDE

CAUSTIC SODA

4 Au(CN)2- + 4 OH-

From this equation it can be seen that in order to dissolve gold oxygen and water as well as cyanide are required. The gold present in many gold ores will dissolve in cyanide solutions without any problems. These are called non-refractory ores and are usually oxidised ore that consist mainly of minerals that do not react with cyanide such as quartz and other silicates. Ores that contain gold that does not dissolve readily in cyanide solutions are called refractory ores. There are many types of refractory ores. One major type is where the gold is locked in a mineral such as pyrite and the cyanide solution can not reach the gold to dissolve it. These ores need to be roasted to disrupt the pyrite structure and allow the cyanide solution to reach the gold. Another major type of refractory gold ore are those which contain minerals that will also react with the cyanide. These minerals (pyrrhotite, chalcopyrite, pyrrhotite, etc) are known as cyanide consumers, cyanocides or cyanicides. CYANICIDES: 1.

Copper Minerals: Copper minerals such as Chalcocite (Cu2S) and covellite (CuS) dissolve in cyanide solutions and cause excessive consumption of cyanide. This slows down the dissolution of gold as there is less cyanide at the gold surface to dissolve the gold.

CuS

+

H2O + 4CN- Û

CuCN42- +

SCN- + 2OH-

Covellite + water + cyanide Û copper cyanide + thiocyanate + hydroxide 2.

Sulphide Minerals: Some reactive sulphide minerals such as marcasite and pyrrhotite will dissolve in cyanide solutions. The effect is the same as that for copper minerals.

3.

Flotation Reagents: (Copper sulphate): Copper added in the form of copper sulphate as a flotation reagent will react with cyanide and consume it.

7.5

RATE OF DISSOLUTION OF GOLD: The rate or speed at which gold will dissolve depends on many variables. Firstly the gold must be liberated so that the cyanide solution can reach the gold particles. Once this is achieved the following factors also play and important part in the speed of leaching. 1.

Coatings: Gold can be coated with a variety of substances that cause the gold to dissolve much slower. Iron oxides or tarnish, clay and elemental sulphur may coat gold. Flotation reagents designed to coat minerals and make them water repellent so they float can also coat gold particles and make them water repellent which will also repel the cyanide solution reducing the rate at which the gold will dissolve.

2.

Size of Gold Particles: The rate that gold particles dissolve depends directly on their size. Larger particles take longer than smaller ones. This is why coarse gold is often separated by gravity methods before the remainder of the ore is sent to cyanidation.

3.

Gold Compounds: Compounds of gold can dissolve slowly in cyanide solutions. Gold tellurides dissolve very slowly and often require a roasting or chlorination step prior to cyanidation.

4.

Cyanide: At low cyanide concentration, cyanide can be the limiting reagent that slows down the leaching of gold. This usually is not the case in CIP circuits where a large excess of cyanide is usually used.

5.

Oxygen: Oxygen is normally the reagent that limits the rate at which gold will dissolve. CIP leach tanks need to be kept well oxygenated by sparging with air or oxygen.

6.

Temperature: The rate that gold will dissolve in cyanide solutions increases with increasing temperature up to about 85oC. It then begins to decrease due to less oxygen being dissolved in hot solutions.

7.6

CYANIDE SOLUTIONS: When solid sodium cyanide dissolves in water, some of it will react with the water to form hydrogen cyanide gas (HCN): SODIUM + WATER Û HYDROGEN + CAUSTIC CYANIDE CYANIDE SODA NaCN

+

Û

H2O

HCN(g)

+

NaOH

How much hydrogen cyanide gas is formed will depend on the acidity of the water. The lower the pH or the more acidic the water, the more hydrogen cyanide gas is formed. Some highly saline bore water can have a high acid content with a pH as low as 4. Water that has an acid nature needs to be neutralised or made alkaline before sodium cyanide is added. Lime or caustic soda is usually added to adjust the pH and make the water alkaline. The following diagram demonstrates the percentage cyanide, as hydrogen cyanide gas as a function of pH in fresh water. If excess aeration occurs or the pH is allowed to drop, HCN(g) will be formed and cyanide lost.

90

10

80

20

70

30

60

40

50

50

40

60

30

70

20

80

10

90

% cyanide as CN

% cyanide as HCN gas

0

-

100

100

0 4

5

6

7

8

9

10

11

12

13

14

pH

Figure 7.5 Cyanide ion as a function of pH in fresh water. From figure 7.5 it can be seen that a pH in excess of 10 is required to keep most of the cyanide in solution as the hydrogen cyanide ion (CN-). A pH of 10.5 is typically used in a CIP plant. If the plant is using saline water, lower operating pH values of about 9.0 can often be used. The pH is increased to higher values by adding alkalis.

7.7

ALKALIS: Alkalis are substances that make water basic or alkaline. By removing acid and producing alkalinity they can prevent the formation of hydrogen cyanide gas. Common alkalis used in the CIP process are quicklime (calcium oxide), hydrated lime (calcium hydroxide) and caustic soda (sodium hydroxide). In addition to preventing the loss of cyanide as hydrogen cyanide gas, alkalis also: a)

Neutralise acid constituents in mill water and ore before addition to cyanide circuit.

b) Aid in extraction of gold from tellurides which decompose more readily at highly alkaline solutions. WATER: Many gold operations in arid areas use saline bore water in their treatment plants. This salt water can have up to 250000 ppm total dissolved salts or 6 times the salinity of sea water. It is saturated with dissolved salts and very easily forms solid salts. The two main salts present are sodium chloride, or common salt (~90%) and magnesium sulphate, or magnesia (~10%). The sulphate in the salt water reacts with calcium from lime or the ore to form calcium sulphate (gypsum). Most of the scale in pipes and on screens is due to gypsum. This problem may be reduced by using caustic soda instead of lime for protective alkalinity. CALCIUM MAGNESIUM HYDROXIDE + SULPHATE (LIME) Ca(OH)2

+

MgSO4

CALCIUM SULPHATE (GYPSUM)

= =

CaSO4(s)

+

+

MAGNESIUM HYDROXIDE Mg(OH)2

The magnesium present in salt water usually prevents the pH of the process water from raising above a pH of 9.0 to 9.2. At about this pH the solution usually consumes all extra alkali added to form magnesium hydroxide. MAGNESIUM + SODIUM = SULPHATE HYDROXIDE MgSO4

+

NaOH

=

7.8

MAGNESIUM + SODIUM HYDROXIDE SULPHATE (SOLID) Mg(OH)2

+

Na2SO4

Figure 7.6 shows a typical pH rise for saline water as an alkali reagent such as lime is added. At a pH of about 9.0 to 9.2 the pH stops increasing as more alkali is added.

Figure 7.6 pH Modification of Salt Water with an alkali.

7.9

OXYGEN: At cyanide concentrations above 2.5 x 10-3 mol dm-3 the rate of leaching is controlled by the transfer of oxygen to the gold surface. The solubility of oxygen (5-10 mg dm-1) in water therefore plays an important part in gold leaching. A variety of minerals will remove oxygen from a leach solution. The most common are arsenic minerals and reactive sulphides such as pyrrhotite. Naturally occurring organics such as lipids and humates can also consume oxygen. Oxygen consumers can be overcome by using one or more stages of air, oxygen or hydrogen peroxide pretreatment, prior to the introduction of cyanide. The use of oxygen and peroxides in gold leaching is becoming accepted. Hoeker (1992) indicates that about thirty gold plants in Australia are using oxygen injection. The claimed benefits of using oxygen include: Increased gold recovery. Reduced residence time. Reduced cyanide consumption. Lower cyanide levels required. Lower operating costs compared with compressed air. Figure 7.7 shows the rate of gold dissolution for a given oxygen concentration in the aeration gas being bubbled through the solution.

Rate of gold dissolution mg/sq.cm/hour

14 12 10 8 6 4 2 0 0

10

20

30

40

50 60 Oxygen / %

70

80

90

100

Figure 7.7 Rate of gold dissolution versus oxygen concentration. Data from American Cyanamid Company, 1968.

7.10

Table 7.1 gives the maximum effective oxygen concentrations for given cyanide levels (Nicol, Fleming and Paul 1987). Table 7.1. Maximum effective oxygen concentration at various cyanide levels NaCN concentration / % 0.005 0.01 0.02 0.03 0.04

Oxygen concentration / mg/l 3.2 6.5 13.1 19.6 26.1

Table 7.2 summaries the oxygen addition options, which are available to maintain oxygen levels in the pulp to reasonable values.

Table 7.2. Summary of use of different oxygen supplements and plant observations (After La Brooy, Muir and Komosa 1991). Air Oxygen

CILO H2 O2

CaO2

(Max. O2 concentration 9 mgL-1) Can strip HCN(g) from the solution especially at pH about 9.2 with saline water. (Max. O2 concentration 40 mgL-1) Accelerates leaching kinetics. Cyanide savings in the presence of reactive sulphides. Reduced Pb(NO3)2 requirements. (Max. O2 concentration 25-40 mgL-1) Accelerates leach kinetics. Cyanide savings in 5h lab test. No Pb(NO3)2 required. (Max. O2 concentration 40 mgL-1) Accelerates leach kinetics. No stripping of HCN(g) from solution. Degussa PAL application rate 0.2-0.5kgt-1. Less copper leaching, reduction in cyanide consumption with increase in gold recovery. (Max. O2 concentration 40 mgL-1) Accelerates leach kinetics more than recovery. No stripping of HCN from solution. Active in heap leach for > 20 days. Maximum O2 release at pH 9. Application rate 0.1-0.8kgt-1.

7.11

CARBON ADSORPTION ACTIVATED CARBON HISTORY Egyptians

- Purifying agent

Ancient Hindus

- Filter drinking water

1847

First recorded use to adsorb gold from chloride solutions.

1880

Davis patented wood charcoal to recover gold chloride.

1890

MacArthur and Forrest Brothers found cyanide was a good solvent for gold.

1894

Johnson patented wood charcoal to recover gold cyanide.

1917

Wood charcoal in columns, Yuanmi Mine, WA, Australia.

1961

First fully integrated CIP Plant, Cripple Creek, Colorado, USA.

Developed from Marsden and House (1992) and McDougall (1991). STRUCTURE Activated carbon is a very porous carbon based product based on a graphite structure. The normal structure of graphite is shown below.

Figure 7.8 Schematic representation of the structureof graphite (after Bochris, 1969).

7.12

Activated carbon is formed when part of the graphite structure is burnt away leaving a very porous product.

Figure 7.9 Schematic representation of the proposed structure of activated carbon. During activation, functional groups are formed along the edges of the broken graphite planes. These functional groups serve the purpose of rendering the carbon hydrophilic. The carbon will therefore be wetted by water and gold cyanide adsorbed from solution.

Figure 7.10 Structure of some surface oxides: (a) carboxylic acid (b) phenolic hydroxyl (c) quinone-type carbonyl groups (d) normal lactone (e) fluorescein-type lactones (f) carboxylic acid anhydrides (g) cyclic peroxides (after Mattson and Mark, 1971).

7.13

MANUFACTURE OF ACTIVATED CARBON: Activated carbon can be manufactured out of numerous materials. Some materials include: coal (peat, lignite, bituminous coal or anthracite), coconut shells, peach and apricot pits, wood, etc. The physical characteristics, structure and pore size distribution of the resultant carbon are determined by the type of raw material used. Activity of the carbon is determined by conditions used in the manufacturing process. Coconut shell activated carbon is most often used in the adsorption of gold cyanide from solution. A flow sheet for its manufacture is shown below.

Raw coconut shell ò Carbonisation ò Optional crushing/screening ò Activation ò Cooling ò Crushing/screening ò Bagging

Figure 7.11 Flowsheet for the manufacture of activated carbon from coconut shells (after Yehaskel, 1978). The carbonisation step involves heating the carbon in an inert atmosphere to about 600oC. In this process volatile components in the coconut shell are driven off producing a char.

7.14

Activation is conducted by heating the char to between 800 and 1100oC in the presence of steam, air, carbon dioxide or a combination of these. Preferential etching occurs, removing weak spots in the char, and creating a large internal surface area. 2-4 m 2 / gram

WOOD CHARCOAL

THERMAL ACTIVATION

ACTIVATED CARBON

1000 m2 / gram

Figure 7.12 Surface area increase during activation. A surface area in excess of 1000 m2g-1 may be produced. This is the primary advantage of activated carbon over wood charcoal. Both when activated have the same activity per square metre of surface area but activated carbon has a much larger surface area. To achieve maximum adsorption, the pores in the activated carbon used for gold cyanide adsorption should be larger than the gold cyanide molecule to allow easy access but small enough to allow maximum pore volume. A pore size distribution is shown below for coconut shell and coal-based carbons. The gold cyanide molecule fits easily into the majority of coconut shell carbon pores shown at about 10Å radius.

Figure 7.13 Pore size distribution for a typical thermally activated coal-based carbon and a coconut shell-based carbon (after Fornwalt et al., 1963). 7.15

ADSORPTION OF GOLD CYANIDE ONTO ACTIVATED CARBON: No consensus has been achieved amongst researchers regarding the mechanism by which gold cyanide loads onto activated carbon. One proposed method of adsorption will be examined. This method, although not universally accepted, aids in the understanding of fouling of activated carbon and other processes that occur in the CIP/CIL circuit.

Ion-pair adsorption

Au(CN) -2 Carbon CaAu(CN) 2

Ca2+ Mg 2+ Figure 7.14 Proposed mechanism of ion pair adsorption.

According to the ion-pair adsorption theory, a cation forms a bond with the negatively charged gold cyanide ion which is then adsorbed onto the carbon particle. From the ion-pair mechanism, it can be seen that: 1)

the loading capacity of activated carbon will increase with increasing concentration of cation in the order Ca2+ > Mg2+ > H+ > Li+ > Na+ > K+

2)

anions in solution will decrease the loading capacity of activated carbon with increasing concentration in the order CN- > S2- > SCN- > SO32- > OH- > Cl- > NO3Nicol et al. (1987).

It is thought that anions such as CN- compete with gold cyanide for active sites on the carbon.

7.16

FOULING OF ACTIVATED CARBON: Carbon is said to be fouled when substances other than gold cyanide have adsorbed onto the carbon preventing gold cyanide from adsorbing. Fouling of carbon can be classified into two types. Inorganic fouling by substances such as calcium carbonate, silica and magnesium hydroxide and organic fouling by oil, flotation agents and other organic molecules. Fouling by inorganics Many anions other than gold cyanide will form ion-pairs and adsorb onto activated carbon, competing with gold cyanide for active sites.

Mg(OH)2 CaCO3 Fe(CN)6

Carbon

Silica

Figure 7.15 Fouling of activated carbon by inorganics.

A second mechanism by which inorganics can deactivate carbon is by precipitation in pores reducing the available surface area. Calcium carbonate is believed to act in this way. Haematite, silica and finely divided clay and shale material may also lodge in pores, block them, and reduce the surface area available for gold cyanide adsorption.

7.17

Fouling by organics Fouling by organics can be divided into two groups. The following tables (after La Brooy et al., 1986) show the effect of organics on carbon. The first table shows the effect of large organic molecules on carbon. These molecules are assumed to be too large to enter the pores on the carbon used for gold cyanide adsorption. As a result they adsorb mainly on the outside surface of the carbon particle not significantly reducing the surface area or activity of the carbon. Organic reagent (@ 20 ppm unless stated)

% activity vs distilled water, after ageing Calgon GRC 22

Pica G210 AS

100

100

Control Distilled water Grinding aid Dow XFS 4272.00 (10 ppm)

102

Viscosity modifiers Sodium pyrophosphate (2 x 10-3 M)

94

Sodium hexametaphosphate, 'Calgon' (130 ppm)

84

Freevis 528

117

Flocculants Magnafloc E24

88

Cyanamid A2120

88

Table 7.3 Fouling of carbon by large molecule (after La Brooy et al., 1986).

Small organic molecules can, however, penetrate into the pores of the activated carbon used to adsorb gold cyanide and block them. Small organic molecules such as oils, frother, and collectors are far more detrimental to a CIP/CIL circuit than larger organic molecules.

7.18

Collectors Sodium ethyl xanthate

44

Sodium iso-butyl xanthate

22

Potassium amyl xanthate

6

Aerofloat 208

42

Frothers Teric 401

17

17

Teric 402

22

25

Dowfroth 200

20

24

MIBC

42

32

Oils Mobil ALMO 527

31

Diesel

30

Multigrade

14

Vegetation products Swamp water

19

13

No ageing Standard kinetic test

188

190

Table 7.4 Fouling of carbon by small organic molecule (after La Brooy et al., 1986).

7.19

CARBON REACTIVATION (REGENERATION): The reactivation of carbon takes two forms. Inorganic foulants can be removed by acid washing in a 3 to 10% hydrochloric acid solution prior to elution. Organic foulants have to be burnt away. This is usually done in a rotary or vertical kiln at temperatures between 600 and 750°C. During kiln regeneration, a small part of the carbon is burnt away releasing haematite and silica that may have lodged in the pores and deactivated the carbon. The surface of the carbon is also reactivated in the kiln regeneration step. FOULANT

SOURCES

REACTIVATION

1.

Calcium carbonate

Lime, ore, water

Acid wash

2.

Magnesium hydroxide

Water

Acid wash

3.

Silica

Lime, ore

Kiln

4.

Iron oxides, cyanides

Ore calcine

Acid wash

5.

Copper compounds

Flotation, ore

Acid wash

6.

Organics

Flotation, wood, oils, etc.

Kiln

Table 7.5 Some carbon foulants, and suggested means of removing them.

7.20

ELUTION OF GOLD FROM CARBON: Gold cyanide at concentration used in CIP/CIL plants is physically and reversibly adsorbed onto activated carbon. Physical adsorption is a thermodynamically reversible process that can be reversed by disruption of the equilibrium between gold in solution and gold on the carbon. High gold cyanide and cation concentrations, low temperatures, low anion concentrations and active carbon drive the gold cyanide onto the carbon. During elution, lack of cations (Ca2+, Mg2+, etc), high anion concentration (CN-, OH-) and high temperatures will disrupt the equilibrium and remove gold cyanide from the carbon.

Figure 7.16 Effect of the concentration of hydroxide and cyanide in the eluant on the elution rate (after Adams and Nicol, 1986).

While an increase in hydroxide and cyanide will increase the rate of elution, an increase in cation concentration will decrease the rate of elution.

7.21

Figure 7.17 The effect of the ionic strength of the eluant (NaCl) at the eluant rate (after Adams and Nicol, 1986). The most critical factor in the elution of gold cyanide from activated carbon is temperature. Thermodynamically the equilibrium loading on activated carbon will be decreased with increasing temperature. The rate of kinetics of elution also increases with increasing temperature.

Figure 7.18. Effect of temperature on the rate of gold extraction (after Fleming and Nicol, 1984).

7.22

Optimum elution can be achieved by high temperatures, high anion (hydroxide and cyanide) concentrations, low cation (Mg2+, Ca2+, etc.) concentrations and low gold cyanide concentrations in solution. A problem is encountered when cyanide decomposes rapidly at elevated temperatures.

Figure 7.19 The decomposition rate of cyanide in aqueous solution as a function of temperature (after Adams and Nicol, 1986).

CN-

+

2 H2O

Û

H CO2-

+

OH-

Û

NH3 + H CO2CO32- + H2

CO32-

+

H2O

Û

CO2 + 2OH-

The decomposition rate of cyanide and the subsequent detrimental effects on elution can be minimised by adding cyanide continuously throughout the elution cycle.

7.23

Build-up of gold cyanide in the eluant will also decrease the rate of elution.

Figure 7.20 The effect of gold in the eluant feed on the rate of elution (after Adams and Nicol, 1986).

Several methods have been developed to minimise the effects of gold build-up in the eluant. These will be examined as we have a look at three elution processes: Zadra, AARL and Micron elution procedures.

7.24

Zadra Elution Process

95-100 oC atmospheric 100-150 oC pressure

1% NaOH 0.2-1% NaCN pH ~ 13 Elution

Electrowinning 80-90 oC Figure 7.21 Flowsheet for the Zadra elution process.

This elution procedure was developed by Zadra at the U.S. Bureau of Mines in the early 1950's. A hot aqueous solution of 1% sodium hydroxide and 0.2 - 1.0% sodium cyanide (3-4 k litres per tonne of carbon) is circulated through the carbon, to an electrowinning cell, and back through the carbon. Gold cyanide build-up in the eluant is prevented by circulation through the electrowinning cell. Most Zadra circuits operate at atmospheric pressure and 95-100oC with times usually ranging between 30 and 48 hours for stripping of the carbon. As temperature is such a critical parameter, increasing the temperature above 100oC will greatly aid elution. Pressurised Zadra circuits have hence been introduced. These operate at 5-6 atmospheres and 150oC. Elution can be achieved in as low as 6 hours. Flow rate through the elution column is not critical. A flow rate of about 1-2 bed volumes per hour seems to be reasonable for a Zadra elution circuit. If a low length to diameter elution column is used, care must be taken to avoid channelling in the column. Zadra elution has the advantage that it requires smaller volumes of potable water than does the AARL elution process. A bleed off about 33% eluant can be done to limit the build-up of ionic strength in the circuit.

7.25

Anglo-American elution procedure (AARL) 1-3%NaOH 2-5%NaCN

Demin Water 100

to o 120 C

Electrowinning 40- 60o C

Elution

Diluted NaOH Diluted NaCN

Variable flow rate Figure 7.22 Flowsheet for the AARL Process.

The loaded carbon is pre-soaked with 0.5-1 bed volume at hot 2-5% sodium cyanide and 1-3% sodium hydroxide for about 1 hour. The procedure can be operated at atmospheric pressure and 95100oC or at a slightly elevated pressure and 110oC; elution is complete in 8-12 hours at 110oC. Pre-soaking is followed by elution with pure water (about 8 bed volumes) at a flow rate of 2 bed volumes per hour. This is a batch process and the spent eluant is not recycled after electrowinning. If fresh water is at a premium, the AARL process can be run as a "split" elution. In this variation, only the first four bed volumes of eluant advance to electrowinning. The last four bed columns, which contain only a low gold tenor, are stored in an intermediate tank and used in the next elution. This has the advantages of reducing water usage, heating costs and electrowinning capacity. The AARL process relies on the strong influence of the ionic strength to desorb the gold cyanide from the carbon. The rate of desorption increases as water quality improves; cations are decreased.

7.26

Figure 7.23 Effect of different cyanide cations on the rate of desorption of gold (after McDougall et al., 1980) Elution with Organic Solvents 2% NaOH 10% NaCN

80oC Methanol

Electrowinning NaOH NaCN

Figure 7.24 Flow sheet of the Micron Elution Process.

7.27

The loaded carbon is firstly soaked in a solution of caustic soda (2%) and sodium cyanide (10%) at a temperature of 25oC. Methanol is then refluxed through the carbon from a hot still below the column. The condensed methanol carries the desorbed gold back to the still. Elution is complete in 4-6 hours and a very concentrated eluant is produced which makes the electrowinning procedure fast and effective. The advantages of this system are the low usage of high quality water and the production of a stripped carbon free of organic contaminants such that kiln reactivation is not required for periods up to 6-12 months. Hence for projects with limited upfront capital, this system may be preferred. A kiln can be fitted after a year when a cash flow is established. The running costs of a Micron system are, however, much higher than ZADRA or AARL (Kyle 1987).

Figure 7.25 Comparison of elution procedures with extruded and coconut carbon (after Muir et al., 1984)

7.28

REGENERATION / REACTIVATION OF ACTIVATED CARBON The organic molecules that adsorb on carbon reducing its activity need to be removed. Activated carbon is, therefore, periodically subjected to thermal treatment to selectively remove the organic adsorbates and thus restore its activity. REGENERATION STEPS The following steps (Van Vliet 1991) occur when fouled carbon is heated in a regeneration kiln. 0-200oC

Drying which eliminates water and highly volatile adsorbates.

200-500oC

Vaporisation of volatile adsorbates, and decomposition of the unstable adsorbates to form volatile fragments.

500-700oC

Pyrolysis of the non-volatile adsorbates that results in the deposition of a carbonaceous residue on the surface of the activated carbon.

700oC

Selective oxidation of the pyrolysed residue by steam, carbon dioxide, or a combination of oxidising agents.

7.29

ACTIVATED CARBON LOSS Calcium, magnesium and iron are known catalysts for the steam carbon gasification reaction. Acid washing before kiln regeneration removes most of the calcium and some of the magnesium and iron thus reducing the amount of carbon that is lost in kiln regeneration. THE EFFECT OF ACID WASHING ON CARBON QUALITY

Test parameters measured Mass loss, % Acid wash before regeneration Acid wash after regeneration

0.36

3.15

3.25

0.73

3.14

6.23

0.9

1.0

1.7

1.8

1.4

2.6

Kinetic Activity+, % Acid wash before regeneration Acid wash after regeneration

52 (43)

53 (48)

56 (56)

51 (50)

51 (53)

56 (56)

Carbon capacity for gold+, kg/t Acid wash before regeneration Acid wash after regeneration

20 (18)

20 (18)

19 (19)

24 (17)

24 (17)

24 (17)

Attrition* resistance, % Acid wash before regeneration Acid wash after regeneration

* +

Regeneration temperature (30 min contact time + steam) 750oC 800oC 850oC

Attrition was measured after 5 days by use of the standard AARL procedure Kinetic activity and carbon capacity were measured by use of AARL procedures both before and after extended attrition Table 7.6 The effect of acid washing on carbon quality (after Davidson and Schoeman, 1991)

7.30

REACTIVATION TEMPERATURE VS CARBON ACTIVITY The optimum temperature for kiln regeneration depends on the type of organic foulants on the carbon. If no flotation reagents, frothers or organics are present regeneration at 600°C is satisfactory. In the presence of organic agents and vegetation decomposition products, temperatures of up to 750°C are required. The following graph shows the effect of reactivation temperature on carbon activity.

450 400

Carbon activity (k)/hr-

350 300 250 200 150 100 50 0 300

400

500

600

700

800

900

Reactivation temperature/ C

Figure 7.26 Laboratory study of carbon activity vs reactivation temperature.

A reactivation temperature about 750°C will not greatly increase the activity of the carbon. A higher temperature will burn away more of the activated carbon, making the structure weaker. Thus breakdown of the carbon is more likely to occur with the resultant loss of gold on the fine carbon. The temperature needed to achieve reactivation will depend on the type of organic material fouling the carbon.

7.31

CARBON REGENERATION AT VARIOUS CARBON-BASED PLANTS

Circuit Ergo Simmergo Daggafontein

Pre-acid wash (Yes/No) Yes No Yes

Predrying (Yes/No) Yes No Yes

Kiln type Rotary Rotary Rotary

Regeneration conditions Carbon Approx. temp. /°C residence time at temp. min 730 40 650 15 700 20

New Brand Brand Calcine

Yes Yes

No No

Rotary Rotary

600-650 660-730

NA NA

City Deep Crown Mines Crown Mines Harmony Western Areas Doornkop

Yes Yes Yes

Rintoul Rintoul Rotary Rintoul Rotary

700 700-750 750 700 600-650

30 90

No

Yes Yes No Yes No

No

No

Rotary

720

10

Grootvlei Beatrix St Helena Kinross

Yes No No No

No Yes No

Rotary Rotary Rotary Rotary

650 600 710 590-610

Af. Lease VR No. 8 VR No. 9

Yes No No

No No No

Rotary Rotary Rotary

600 600 400

3-5 10 10

WDL No. 1 WDL No. 3

Yes Yes

Yes Yes

Rintoul Rintoul

650

30

Table 7.7 Carbon regeneration at various carbon-based plants (after Davidson and Schoeman, 1991)

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60

RECOVERY OF GOLD FROM ELUANTS Gold can be recovered from eluant solutions by two different processes: a)

Electrowinning

b)

Zinc cementation

The most common procedure is to electrowin the gold onto steel wool. ELECTROWINNING Gold can be recovered from eluants by passing an electric current through the eluant solution. This process is called electrowinning. GOLD SOLUTION

+

ELECTRIC CURRENT

Û

GOLD SLIME

Electrowinning is performed in cells, constructed of a nonconducting material such as fibreglass or plastic that contain the anode and cathode assemblages. Various types of electrowinning cells are used including the Zadra, Mintek, AARL and NIM cells. In the Zadra Cell, the gold collects on the negative electrode or cathode that is usually a cylindrical polypropylene basket filled with steel wool. This produces a high surface area for reaction and increases the speed of the electrowinning process. The positive electrode or anode does not collect gold but is necessary for the current to flow. It is usually made of stainless steel. A Zadra cell will typically draw 150 Amps at 3-4 Volts and exhibit current efficiencies of 5-10% (Kyle 1987). In the Mintek cell, cathodes consist of mild steel wool packed into rectangular polypropylene baskets. The best steel wool loading is about 5-15g per litre. These fit snugly into a rectangular tank to prevent liquid bypass. The anode is usually stainless steel mesh. Typically, a Mintek cell of volume 1 cubic metre can produce 20kg of gold per 24 hour period with a loading of 15kg gold per kg of steel wool. Steel wool cathodes can be loaded up to 20 times their weight with gold but this is rarely done as efficiency decreases as the gold coating gets thicker. The cell requires about 300-800 Amps at up to 10 volts at current efficiencies of 5-10% (Kyle 1987). In general, electrowinning efficiency depends on the eluant concentration. Hence efficiencies increase in the order Zadra ® Anglo ® Micron. For Micron eluants, aluminium foil cathodes (plain or shredded) are often used. These produce a higher barren eluant that is returned to the carbon adsorption circuit. Expanded aluminium, however, produces a very low barren eluant that can be rejected.

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ZINC CEMENTATION Zinc cementation or precipitation can be used as an alternative to electrowinning for the recovery of gold from eluants. Zinc dust is added to the gold eluant. The zinc dust dissolves and the gold precipitates out of solution. ZINC + DUST

GOLD SOLUTION

Û

GOLD + ZINC SLIME SOLUTION

Eluants from AARL elution circuits are usually suitable for zinc cementation. Eluants for Zadra elution circuits however contain higher concentrations of caustic soda and may be at a higher temperature. Zadra eluants need to be cooled and diluted before zinc cementation.

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GOLD PURIFICATION The gold produced by electrowinning contains iron (steel wool) and that produced by zinc cementation contains excess zinc. These impurities must be removed before the gold can be refined. There are four common purification processes in use. ACID DIGESTION / SMELTING This method can be used with the product from both zinc cementation and electrowinning. The impurity, zinc or iron, is dissolved with a weak solution of acid at temperatures up to 60°C for 12 to 24 hours. ZINC or IRON

HYDROCHLORIC +

Û

ZINC OR IRON

ACID

HYDROGEN +

SOLUTION

GAS

Some of the iron or zinc may be completely coated with a coherent layer of gold that prevents the acid from reaching and dissolving the impurity. The gold filtered and refined further by smelting. In smelting, the gold slime is melted in a pot with a flux of silica (SiO2) borax (Na2B4O7), soda ash (Na2CO3) and nitre (KNO3). The furnace, which operates at over 1200°C, produces molten gold (M.Pt. 1063°C) and silver (M.Pt. 961°C) and a slag that contains most of the other metals that were in the gold (copper, zinc, nickel, iron etc). The slag is poured first and then the molten gold is poured into a series of moulds and allowed to cool to produce gold bars. CALCINING / SMELTING The steel wool cathodes that have been plated with gold are placed in trays and calcined at about 750°C for up to 12 hours. A good flow of air is maintained to oxidise iron and any other base metals. DIRECT SMELTING The fluxes used in smelting can be adjusted such that the gold plated steel wool cathodes can be directly smelted without any pretreatment. The use of the oxidising agent, nitre, in direct smelting requires care as excess will oxidise silver and this may be lost to the slag phase. Gold losses may also be increased by direct smelting. ELECTRO-REFINING A recent innovation involves electrowinning the gold onto stainless steel wool instead of mild steel wool as is usually the case. The gold coated cathodes are then loaded into a second cell where they act as the anodes. Flat stainless steal plate forms the new cathode. 7.35

A fresh sodium hydroxide / sodium cyanide solution is put in the cell and a current applied. The impure gold dissolves and a purer produce is plated out on the cathode. The advantages (Kyle 1987) of this method of gold purification are: (i) (ii)

the steel wool can be used many times. the gold can be scraped off the plates clearly and quickly. (iii) operating costs are reduced. (iv) a refinery ready product is produced without smelting. (v) if smelting is done, less slag is produced meaning less gold is lost in the slag.

7.36

REFERENCES Adams, M.D. and Nicol, M.J., 1986. The kinetics of the elution of gold from activated carbon. GOLD 100. Proceedings of the international conference on gold. Volume 2: Extractive metallurgy of gold. Johannesburg, SAIMM. American Cyanamid Company, 1968. Chemistry of cyanidation. Mineral Dressing Notes, Number 23. American Cyanamid Company, Wayne, N.J. Bockris, J.C., 1969. In Chemistry and physics of carbon. vol.5 Walker, P.,C. (ed). New York, Marcel Dekker. Davidson, R.J. and Schoeman, N., 1991. The management of carbon in a high-tonnage CIP operation. J. S. Afr. Inst. Min. Metall., vol. 91, no.6. Jun. 1991. 195-208. Fleming, C.A., 1992. Hydrometallurgy of precious metals recovery. Hydrometallurgy, 30 127-162. Fornwalt, H.J., Helbig, W.A. and Scheffer, G.H., 1963. Activated carbon for liquid-phase adsorption. Br. Chem. Eng., vol 8. 546Kyle, J.H., 1987 Chemistry of the CIP process. Gold Plant Operators Course, Western Australian School of Mines. Department of Minerals Engineering and Extractive Metallurgy. Kalgoorlie, Western Australia. Mattson, J.S. and Mark, H.B., 1971. Activated carbon. New York, Marcel Dekker. McDougall, G.J., 1991. The physical nature and manufacture of activated carbon. J. S. Afr. Inst. Min. Metall., vol. 91, no.4. Apr. 1991. 109-120. Nicol M.J., Flemming C.A. and Paul R.L. 1987. The chemistry of the extraction of gold. in Stanley G.G. (Ed). The extractive metallurgy of gold in South Africa. Vol2, Johannesberg, SAIMM 831-905. Verhoeven, P., Hefter, G. and May, P.M., 1990. Dissociation constants of hydrogen cyanide in saline solutions. Minerals & Metallurgical Processing, November 1990 185-188.

7.37

Van Vliet, B.M., 1991. The regeneration of activated carbon. J. S. Afr. Inst. Min. Metall., vol. 91, no.5. May. 1991. 159-167. Yehaskel, A., (ed.). 1978. Activated carbon - manufacture and regeneration. Park Ridge (USA), Noyes Data Corporation.

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