Tips on maximum gold recovery from ore.
By Ashley Manjonjo
Gold recovery takes places across all mines in the world. Like every other industry, gold extraction must strike the intricate balance between controlling operating costs and maximizing the return on investment (ROI) whilst also performing the mandatory duty of keeping the environment safe. Maximizing gold recoveries can ensure high profitability rates which in turn increases the return on investment.
The key to maximizing gold recovery lies in understanding what type of gold ore is being processed. Gold deposits can be categorized into two primary groups: placer deposits, which are areas that have highly concentrated heavy minerals caused by erosion of rivers or streams, and lode deposits, where the gold is still interlocked within the host rock and needs to be liberated firstly by crushing and grinding. Gold recovery techniques for these two types of ore deposits differ immensely from simple panning to an intricate network of crushing, leaching, gravity concentration, flotation concentration, etc. The million-dollar question then is with so many different methods to recover gold from ore deposits, how does one choose the optimal process for maximum gold recovery? The answer to this question lies in fully comprehending the type of gold deposit one is dealing with and then coming up with a process which will ensure maximum gold recovery from the ore.
Understanding Gold Deposits
Placer deposits result from weathering and release of gold from lode deposits, transportation of the gold, and concentration of the gold dominantly in stream gravels. Before a placer mining operation can begin, the first task is to identify where drainage has deposited gold into sediment layers and to do this, prospectors use pans to evaluate the surface materials. After locating a deposit, placer deposits can easily be mined without the need for blasting, crushing, or grinding. Gold panning is the preferred method of recovering placer gold ore deposits and is predominantly popular amongst Artisanal miners or small-scale miners (ASM) particularly because of its relative simplicity.
Whilst gold panning can be used to separate gold from sediments in small scale mining, it is not a very practical method for gold recovery on a large scale.
The most cost-effective method for maximum gold recovery from placer deposits is using a gravity concentration circuit. Gravity concentration is a technology which is based on the differential movement of mineral particles in water due to their different density and hydraulic properties. First the valuable mineral particles (in this case gold) must be liberated, they must be separated from the gangue particles, and this is done by exploiting the physical properties of the different minerals. The most important factor for successful gravity separation is liberation of the gold particles from the gangue minerals. It is not easy to establish the degree of liberation of low-grade minerals such as gold and the most recommended technique to establish the optimum gold liberation size is grinding for different times (or grain size distributions) and applying gravity concentration to the ground products. This is a very important procedure to recommend any type of gravity concentration process and because mot artisanal miners do not screen the crushed material (i.e. they work in open circuit), their chance to improve gold recovery are very limited.
After mineral liberation, sluices or jig are then used to recover the gold. Sluices are inclined, flat-bottom troughs that are lined on the bottom with a trapping mechanism that can capture particles of gold and other heavy minerals. Sluices work on the principal that heavy particles tend to sink to the bottom a stream of flowing water while the lighter particles tend to be carried downstream and discharged off the end of the sluice. In the case of deposits with high clay content, the rocks are cleaned with a trammel or rotary scrubber to remove contaminants, and then gold recovery can take place.
Lode deposits, otherwise known as hard rock deposits, have gold trapped inside the surrounding rock. Blasting and comminution (a combination of crushing and grinding) are necessary to liberate the gold for recovery. Lode gold deposits have three sub-classes: free-milling, sulfide-associated and refractory.
Gold is considered free milling when it is easily liberated by grinding and separated from the other minerals. These are ores of gold from which the precious metal can be recovered by concentrating techniques without resorting to pressure leaching or other chemical treatment. These free-milling gold ores can be further classified into low grade and high grade which helps determine the most economical and optimal gold recovery method.
Low grade – Low grade free-milling gold deposits contain less than 1 g/t of gold. This low-grade ore is crushed and heap leached to recover the gold. While heap leach recoveries are lower than in conventional gravity-cyanidation, the savings in capital expenditure may justify this route.
High grade – Blasted rocks are crushed and ground to a size less than one inch. They are then sent to a hydrocyclone, which sorts the particles based on their size and density. The fine, light particles are sent to the cyclone overflow, and the coarse or dense particles are directed to the cyclone underflow.
The fine and light particles collected in the overflow are sent to the carbon in leach circuit (CIL) for cyanide leaching and carbon adsorption to extraction the gold. A high-grade gold solution is then produced for electrowinning which is followed by smelting to create doré bars. A doré bar is a semi-pure alloy of gold and silver.
The dense particles from the cyclone underflow are sent back to the grinding mill, but before they get there, these particles pass through a Concentrator. This machine can collect gold from the cyclone underflow, which is then sent to a shaking table or leach reactor for further upgrading. All other particles not collected by the concentrator are sent back to the mill for further grinding.
A typical flow sheet for free-milling gold is shown below.
Gold is present in refractory sulfide gold ores mainly in chalcopyrite, pyrite and arsenopyrite, where it occurs in both the chemically bonded state and as nano-size grains of metallic gold. Sulfide-associated gold is often very fine in nature and difficult to separate from the sulfides. So then how is gold recovered from this type of ore in an economical and effective way?
In many cases, portions of sulfide-associated gold can be considered free-milling and can be recovered using the gravity recovery process described above.
For the portion of sulfide-associated gold that is not free-milling, different tactics are needed to extract the gold. Typically, this type of gold is recovered with froth flotation.
The gravity-flotation combination to recover sulfide-associated gold begins the same way as the free-milling process, with crushing and grinding. The exception here is that instead of sending the cyclone overflow for cyanide leaching, it is sent instead for flotation processing. The particles are mixed with chemical reagents to render the surfaces of the gold-containing sulfide mineral hydrophobic. Froth flotation cells bubble air through the slurry. These air bubbles attach to the now hydrophobic gold and float them to the surface. The concentrated minerals float above the solution and can be collected and sent for smelting.
The cyclone underflow is still sent to a Concentrator (preferably a knelson concentrator) for fine gold recovery, and the gold is still sent to a shaking table or leach reactor for further upgrading. The other particles go back to the start of the circuit for further grinding as well.
A typical flow sheet for the sulfide-associated gold ore extraction is shown below.
The last type of gold deposit is called refractory gold, and it is the most difficult type to extract. It can be defined as any ore that responds poorly to a conventional gravity-cyanidation process with examples including copper porphyry, complex sulfides, arsenopyrite and lead-zinc associations. Ultra-fine gold particles can be trapped within the mineral matrix, even at extremely fine grind sizes.
Three primary factors cause gold to be considered refractory. Each factor needs to be addressed with a different type of pre-treatment before gold can be recovered by conventional cyanidation. These factors are physical encapsulation, chemical interference, and preg-robbing elements.
Extremely fine (<10μm) particles are locked within an impervious, unreactive mineral. This can be overcome by ultra-fine grinding. However, this is often not economically viable. If the gold is encapsulated in sulfides, oxidation of the sulfide minerals by roasting, pressure oxidation, or bio-oxidation is used.
This occurs when there are minerals in the ore that consume large quantities of cyanide or oxygen. These minerals interfere with the leaching process and need to be removed or altered before conventional gold recovery can take place. After identifying the obstructing mineral, it can be dealt with accordingly.
Carbon is a preg-robbing mineral and can be managed in a few different ways. It can be burned off, treated with pre-flotation, or deactivated with other chemicals. In recent years gravity concentration has been used as an effective treatment for dealing with carbonaceous ores.
For maximum gold recovery, it is vital to know the type of ore you are dealing with. Only after identifying the type of ore deposit you are dealing with, then one can come up with the appropriate procedure of gold extraction.