Management Brief on Pressure Oxidation
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Transcript of Management Brief on Pressure Oxidation
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