It is therefore critically important, that when evaluating gold extraction techniques, the ore deposit and the mineralogical factors are well known prior to making any decision Marsden amp House, 2009. Although there is a large set of mineral processing technologies to select from, the list is distinct Stewart, 2012. The ore classification techniques however, are not distinct there is no universal characteriation technique that is applicable to all gold-bearing rocks. La Brooy et al. 1994 provided a useful framework that is still used by many today outlined in Figure 1.Get A Quote
Ball mills can operate in either wet or dry conditions. They are advantageous because they can achieve a very fine particle product sie refer to Table 1. Generally ball mills are utilied after an initial material crushing or grinding has occurred. Thus, a disadvantage of the ball mill is that the feed sie must already be small. Table 2 outlines some typical ball diameters found in ball mills dependent on the staging or purpose of the mill.
At the most basic level, the response that an ore has to conventional cyanidation is the key factor in determining its gold ore classification. From La Brooy et al. 1994, there are three basic factors affecting response to conventional cyanidation.
The degree to which each of these factors is relevant in the ore response to cyanidation determines its classification, and thus the processing strategy for gold extraction.
The complete definition of a free-milling ore is ore sied 80160 lt 75 m from which cyanidation can extract 90160 of the gold, and has not sustained high reagent consumption John. O. Marsden, 2006. The gold is also commonly recovered through gravity concentration. The main classes of free-milling ores are placers, quart veined gold ores, oxidied ores, and silver-rich ores. Epithermal deposits may also be free-milling in the oxidied portion, although frequently have higher concentrations of sulfide minerals wherein the ore is considered refractory.
Among free-milling gold ores placer gold is a special case, as there is no pre-treatment required for gold extraction. Gold can be extracted from placer ores using physical separation techniques, particularly through gravity treatment. In modern practice, concentrators use centrifugal force in order to liberate finer gold particles.
Complex ores require augmented cyanide or oxygen addition in order to be able to attain economic extraction. The main categories for complex ores are cyanide consuming, oxygen consuming, and preg-robbing. The various difficulties and extra considerations for extracting gold from each type of complex ore will be discussed below.
Reactive sulphides containing gold, such as pyrrhottite, can require high amounts of oxygen as the iron II is oxidied to iron III, and sulphide to sulphate Brooy, 1994. Oxidant addition is employed in these cases to meet oxygen demands, and can be controlled by an oxygen electrode inserted into the leach tank. The main concerns with this process is that saline water can cause meter calibration issues Komosa, 1991. The process options for oxygen consuming ores are thus to use alkaline pre-oxidation, or aeration and flotation, followed by a cyanide leach with oxygen addition.
Refractory ores are classified as to their level of refractoriness, as shown in the table below.
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S.L. Chryssoulis, J. McMullen, in Gold Ore Processing Second Edition, 2016.
D. Lunt, T. Weeks, in Gold Ore Processing Second Edition, 2016.
Ultrafine grinding UFG has continued to evolve in terms of equipment development. A number of specialist machines are commercially available including Xstratax27s IsaMill, Metsox27s Vertimill, Outotecx27s High Intensity Grinding HIG mill, and the Metprotech mill. UFG equipment has been developed with installed powers of up to 5MW.
Compared with conventional ball or pebble milling, the specialist machines are significantly more energy efficient and can economically grind to 10m or lower, whereas the economical limit on conventional regrind mills was generally considered to be around 30m. Coupled with improvements in downstream flotation and oxidation processes, the rise of UFG has enabled treatment of more finely grained refractory ores due to a higher degree of liberation in the case of flotation or enhanced oxidation due to the generation of higher surface areas.
Ultrafine grinding is used to liberate gold finely disseminated in metallic sulfides. KCGM is the first gold mine using ultrafine grinding followed by cyanidation Ellis and Gao, 2002. The gold sulfide concentrate is grind with an IsaMill to a P80 of 1012m. In the first 3years of operation, high consumption of cyanide and high gold content in leach residues were experienced with difficulty overcoming these issues Deschnes etal., 2005.
Figure26.15. Effect of duration of pretreatment on gold extraction from the Eleonore flotation concentrates. Pretreatment 0.25Lminkg oxygen, pH 11.0 cyanidation 2000ppm NaCN, pH 11.0, DO 35ppm, 20C, 35 pulp density Deschnes and Fulton, 2013.
For the addition of oxygen in the pretreatment, it was found that increasing the flow rate of oxygen addition from 0.13Lminkg to 0.25Lminkg reduced the cyanide consumption from 6.4kgt to 6.0kgt Figure26.16, as well as the gold content of the leach residue from 2.52 to 1.92gt Au. When the oxygen addition was increased to 0.53Lmin, the gold content of the leach residue increased to 2.46, while the cyanide consumption went to 5.5kgt NaCN Figure26.16.
Figure26.16. Effect of oxygen flow in the pretreatment on gold extraction from the Eleonore flotation concentrate. Pretreatment 2.00kgt lead nitrate, pH11.0, 16h cyanidation 2000ppm NaCN, pH 11.0, DO 35ppm, 20C, 35 pulp density Deschnes and Fulton, 2013.
Figure26.17. Effect of lead nitrate in the pretreatment on gold extraction from the Eleonore flotation concentrate. Pretreatment 0.25Lminkg oxygen, pH 11.0, 16h cyanidation 2000ppm NaCN, pH 11.0, DO 35ppm, 20C, 35 pulp density Deschnes and Fulton, 2013.
Figure26.18. Effect of cyanide concentration on gold extraction from the Eleonore flotation concentrate. Pretreatment 0.25Lminkg oxygen, pH 11.0, 16h, 5.5kgt lead nitrate cyanidation 0.5kgt lead nitrate, pH 11.0, DO 35ppm, 20C, 35 pulp density Deschnes and Fulton, 2013.
Refractory gold ores may be defined as those which yield low recoveries of gold when treated by the conventional grinding and cyanidation process. This refractory behaviour can vary from mild to severe with gold recoveries of 85 down to less than 30 1.
The Western Australian goldfields, like many others, contain numerous orebodies which are refractory in their primary ones. The mineralogy of these ores shows great variation, as does the gold recovery by various processing options. 2 The best process can only be chosen with confidence after a careful metallurgical evaluation programme based on good sampling, process mineralogy and testing various processing options eg UFM, biological oxidation, pressure oxidation, roasting, etc., coupled with environmental and economic factors.
For the past few years, Ammtec has been involved in numerous refractory gold projects. This paper presents some of the results from these process development programmes.
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