PART I PROJECT DEVELOPMENT
4. COMPLEX ORE PROCESS OPTIONS 1. Overview
Complex ores are intermediate between free-milling and refractory ores. As such, they give rise to high usages of cyanide and oxygen and/or are preg- robbing. Speciation of cyanide complexes within leach liquors especially for feeds known to contain copper, zinc, thiosulfates and other complexes is recommended. Flowsheet selection issues are discussed under the following headings.
4.2. Treatment of high-copper ores
Adams (1999)has proposed a classification of gold–copper ores according to the relative concentrations of the two metals. Ores with high copper values are invariably exploited via flotation and smelting with gold recovered in a refinery. The following discussion centres on high gold ores or concentrates treated by cyanidation (see Chapters 32 and 33).
Copper minerals exhibit varying solubilities in cyanide solutions as illus- trated inTable 4.
Copper complexes that can be present in cyanide liquors include Cu(CN)2–, Cu(CN)32–and Cu(CN)43– with the proportions dependent on the pH, copper concentration and free cyanide concentration. The main issue facing the process engineer is the fact that minimization of cyanide consumption is Table 3
Flowsheet options for high-silver ores SilverGold
Ratio
Flowsheet Options
Low Maintain typical CIL, elution, electrowinning circuit. Require elevated elution temperature,e.g., Bottle Creek, Pajingo
Moderate Consider CIL with elution followed by zinc precipitation,e.g., Rawas. Has the potential to produce separate gold and silver bullions
High May have to opt for full Merrill-Crowe if carbon movement rate becomes impractically high for CIL
Process flowsheet selection 79
favoured by the formation of species with a low copper:cyanide mole ratio, e.g., Cu(CN)2–. However, these species will also load readily onto activated carbon and, at high loadings, can interfere with gold adsorption. In situa- tions where the cyanide-soluble copper levels are low it may be economically feasible to simply raise the cyanide concentration in the leach and thereby minimize the loading of copper. Copper rejection can be further increased by utilizing a cold cyanide elution to preferentially strip the species from the carbon.
At high cyanide-soluble copper levels, however, more specific flowsheet measures will be needed. These can generally be divided into processes that seek to suppress cyanide consumption and those that aim to recover cyanide, and possibly copper as well, from the CIL residue.
The former includes pre-treatment by acid leaching of highly soluble copper species such as oxides and reactive sulfides. Another innovative flow- sheet that was originally developed for vat leaching of gold–copper ores in Western Australia involves the use of an ammoniacal cyanide leachant. GRD Minproc also introduced this technique to the Akjoujt gold project in Mauretania.
Cyanide recovery processes (see also Chapter 29) are exemplified by the AVR (acidification–volatilization–regeneration) technology that was intro- duced at the Beaconsfield gold project in Tasmania (Kitney, 1998). A variant of this process has recently been employed at the Telfer project in Western Australia in the form of the SART (sulfidization–acidification–recycling–
thickening) process that recovers both copper as sulfide and cyanide for reuse (Barteret al., 2000).
Table 4
Cyanide solubility of copper species
Copper Mineral Solubility in CN at 231C (%)
Azurite 2CuCl2O3Cu(OH)2 94.5
Bornite Cu5FeS4 96.0
Chalcocite Cu2S 90.2
Chalcopyrite CuFeS2 5.6
Chrysocolla CuSiO32H2O 11.8
Covellite CuS 96
Cuprite Cu2O 96.6
Enargite Cu3AsS4 65.8
Malachite 2CuCO3(OH)2 99.0
Native copper Cu 90.0
Tetrahedrite (Cu,Fe,Ag,Zn)Sb4S13 21.9
D. Lunt and T. Weeks 80
4.3. Preg-robbing ores
Carbonaceous ores that exhibit a propensity to adsorb soluble gold onto the naturally occurring carbon pose difficulties (see Chapter 38). Prior to the Penjom project, the process options were largely restricted to two options,i.e.:
The addition of blinding agents such as kerosene with the objective of passivating the carbonaceous material sites.
Use of large quantities of activated carbon in a full CIL circuit,i.e., a brute force approach to persuading soluble gold to adsorb onto the activated carbon added to the system. While relatively effective this method has disadvantages, in that the circuit requires high carbon inventories and a large elution and regeneration treatment capacity.
The Penjom project employs kerosene but combines this with the use of a commercial ion-exchange resin to adsorb the gold. The advantages of the resin over activated carbon are the much higher equilibrium gold loadings that can be achieved coupled with a greater resistance to fouling in the presence of kerosene. The kerosene addition rates at Penjom were reported to be up to 8 l/t of ore and this compares with usages of up to 500 l/t for activated carbon systems. Further discussion on this option may be found in Chapter 25.
4.4. Oxygen-consuming ores
A number of species, sulfides in particular, consume oxygen to a degree in CIP or CIL circuits and some, such as stibnite (see Chapter 40) and pyr- rhotite, can evidence high rates under the conditions that prevail. It is im- portant, in terms of achieving completion of the gold dissolution reaction, to maintain a high level of dissolved oxygen in the leach pulp. A technique that was employed in a number of earlier projects was the use of a high-intensity, air-sparged agitation stage ahead of CIL. This aimed to oxidize the surface of the sulfide mineral thus negating the impact of oxygen depletion in the main CIL system.
Oxygen demand or consumption rate is generally measured by a test procedure originally developed by Lightnin. It entails sparging air or oxygen into a mini cyanidation reactor and monitoring the rate of loss. A minimum concentration of 4 ppm O2in an air-sparged system is mandatory and ideally the value should be 8–9 ppm O2, although some schools of thought target the lower range of dissolved oxygen levels in high oxygen-consuming ores such as from UFG mills to minimize cyanide consumption (see Chapter 20). Table 5 provides a decision-making matrix according to the measured values.
Process flowsheet selection 81
It should be noted that there might be other imperatives for the use of oxygen, for example, improved gold dissolution rates. Of the strong oxidizing agents, gaseous oxygen is often preferred as it can be generated on-site and is cheaper than the alternatives if efficient gas dispersion is employed. An ad- vantage of peroxides, such as hydrogen or calcium, is that they can enhance downstream detoxification operations such as the removal of arsenic from solution. The Salsigne gold project in France evidenced exceptionally high oxygen demands and utilized a combination of oxygen in the first two gold leach tanks followed by peroxide in the CIL. This project also employed an aeration device external to the leach tanks to intimately contact pulp and oxygen. A number of equipment types are commercially available.
4.5. Issues associated with mercury
The presence of species other than copper and sulfides may necessitate the incorporation of specific measures to mitigate their impact. Mercury, par- ticularly in the form of cinnabar, leaches readily in cyanide and is of course highly toxic. One method of reducing its dissolution is to limit the cyanide addition as was done at Minahasa but this may not be possible in all sit- uations. Several North American gold projects have faced the issue and have generally used sulfidizing agents to precipitate mercury sulfide from the leach liquor. Precipitants include sodium sulfide and sodium hydrosulfide. Since these are reductants and the gold leach requires oxidizing conditions, they tend to be used towards the end of the leach by which time some of the mercury will have adsorbed onto carbon. Carbamates have also been em- ployed particularly on heap leach liquors where removal of mercury at this point minimizes the extraction onto activated carbon.
Mercury has a low vapour pressure and hence volatilizes at various points in the elution, electrowinning and carbon regeneration operations. The de- sign will need to address this issue in the form of extensive hooding and scrubbing facilities.
Table 5
Process options by oxygen demand Classification Oxygen Demand (mg/l/
min)
Options
Low o0.05 Air injection
Medium 0.05–0.4 Consider oxygen injection
High 40.4 Oxygen injection via external aerators and/or
peroxide addition D. Lunt and T. Weeks
82