MANAGEMENT BRIEF ON PRESSURE OXIDATION

Compared to pyrometallurgy (smelting and roasting), hydrometallurgy is a relatively

new discipline. Pressure hydrometallurgy of gold ores and concentrates is the

newest development which came into practical application in the 1980’s.

In a hydrometallurgical process, for the extraction of metal values from ores and

concentrates, there are three basic procedures, namely:

Dissolution of the metal value from the ore or concentrate into a leach solution

Purification and upgrading of the leach solution, and

Subsequent recovery of the metal from the purified solution

Besides these three basic procedures, there are processes in hydrometallurgy that

are utilised, for example, as a pre-treatment step. Such is the case in the pressure

oxidation of refractory gold ores.

Treatment options for refractory gold ores are shown in Figure 1.

Figure 1: Treatment options for treating refractory gold ores

Often, there are advantages to be gained by operating at temperatures above the

normal boiling point of the solution. A pressurised environment facilitates this. In

such cases, the term “pressure hydrometallurgy” is used. In the gold industry,

pressure oxidation is synonymous to pressure hydrometallurgy. An increase in

temperature will in nearly all cases, increase the rate of a chemical reaction to a

significant extent. For every 10˚C rise in temperature, the specific rate of the

dissolution reaction could increase by a factor of 2.

In pressure oxidation of ores and concentrates, temperatures above 175˚C are

aimed for. At lower temperatures, elemental sulphur forms. The production of

elemental sulphur is to be avoided as it has the ability to depress gold recovery by:

Adsorbing or encapsulation gold and shielding it from attack by cyanide in the

subsequent gold-cyanidation step

It has the ability to coat unoxidised sulphide particles, preventing completing

oxidation and thereby inhibiting the release of the locked gold particle, and

It reacts with cyanide in the gold-leach step, consuming it, and increasing

operating cost

Pressure oxidation refers to the oxidation of sulphides such as pyrite (FeS2),

marcasite (FeS2) and arsenopyrite (FeAsS) at elevated temperatures and pressures.

This process is carried in a pressure vessel called an autoclave.

Pyrite/Marcasite Oxidation:

2FeS2 + 7O2 + 2H2O 2FeSO4 + 2H2SO4….1

2FeSO4 + H2SO4 + ½O2 Fe2(SO4)3 + H2O….2

Fe2(SO4)3 +3H2O Fe2O3(↓) + 3H2SO4….3

Arsenopyrite Oxidation:

4FeAsS + 11O2 + 2H2O 4HAsO2 + 4FeSO4….4

4FeSO4 + 2H2SO4 + O2 2Fe2(SO4)3 + 2H2O….5

2HAsO2 + O2 +2H2O 2H3AsO4….6

Fe2(SO4)3 + 2H3AsO4 2FeAsO4 + 3H2SO4….7

From the above reactions, the following should be noted:

The conversion of ferrous sulphate to ferric sulphate in the autoclave,

equations 1 and 2, is highly desirable because ferrous sulphate consumes

cyanide in the cyanidation step and increases operating cost, and

The ferric arsenate produced, equation 7, is considered to be crystalline and

does not pose an environmental hazard

Oxidation releases locked/occluded gold, Figure 2

Figure 2: Gold grains trapped in a sulphide crystal matrix

The pretreatment leaching step in gold hydrometallurgy involves the use of gaseous

oxygen. When gaseous reagents are used in a leaching reaction, the gas must be

transferred to the solution as rapidly as possible. This can be achieved by:

Increasing the partial pressure of the gaseous reagent

Vigorous agitation to increase the surface area of the of the gas-liquid

interface to assist transfer, and

Vigorous agitation to shorten the diffusion path so that the rate-determining

step is likely to be in the solid-liquid reaction boundary

Vigorous agitation also has the side benefit in solutions containing a high proportion

of fines. Any protective layers formed on the solid surface can be abraded due to the

agitation, thereby allowing reaction rates to proceed unimpeded.

However, agitation cannot be too vigorous or agitator impellers will wear out

prematurely. Tip speed of impellers must be kept at a maximum of about 4ms-1.

Otherwise, accelerated wear of the blades drastically reduces the on-line availability

of the autoclave.

A sulphide concentrate is floated ahead of the autoclave circuit, followed by acid

pretreatment. Prior to autoclaving, the slurry is thickened in a thickener. The acidic

overflow passes to a waste treatment plant, Figure 3.

Figure 3: Campbell concentrator showing acid pretreatment, autoclaving,

waste treatment and countercurrent decantation

The sulphide concentrate has sufficient sulphur to allow autogenous reaction in the

autoclave. Autoclave discharge passes to a countercurrent decantation (CCD) wash

circuit, where the acid in the thickener overflow is used to acidify feed prior to

autoclaving.

After the CCD wash circuit, the slurry is neutralised with lime and forwarded to gold

recovery by normal cyanidation.

The two main producers of autoclaves are Outotec and Tenova.

Acid autoclaving uses exotic materials of construction, which increases capital and

operating costs. The lining of the autoclave vessel is 8mm lead on the carbon steel

shell. This lead lining is overlaid by 3mm fibrefrax paper on which 23cms acid bricks

are laid, Figure 4. Valves on the autoclave are made of titanium.

Figure 4: Autoclave construction

Operating pressures could be as high as 2.9bar and temperatures 220˚C, and as low

as 2.2bar and 200˚C. Positive displacement pumps are used to feed the autoclave.

The list of some mines practicing concentrate pressure oxidation is shown in Table

1.

Table 1: A list of some mines practicing pressure oxidation

Plant Location Capacity (td-1)

São Bento* Brazil 240

Mercur USA 680

Porgera Papua New Guinea 1 215

Campbell Canada 71

Con Canada 90

Lihir** Papua New Guinea 8 100

Hillgrove Australia 24

Macraes New Zealand 20

* The first pressure oxidation plant

** A mixture of ore and concentrate treated

Figure 5: A modern Outotec autoclave being delivered to a Russian customer

Ramoutar (Ken) Seecharran

Senior Group Metallurgist