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SGS has earned the reputation as the world’s leading gold metallurgical testing facility.

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Our decades of experience have allowed us to assemble the most comprehensive assortment of expertise and equipment to address refractory gold issues.

The problems that result from refractory gold are caused by:

  • Physical encapsulation of disseminated gold particles (usually less than a few micrometers in size) within a mineral that is unreactive and impervious to leaching
  • Chemical interference by one or more of the ore constituents in the cyanide leaching process

Physical encapsulation

To address this, it is critical to understand how the gold occurs in the rock (gold characterization analysis). Is the gold standing alone? Is it physically surrounded or locked in another mineral? Or is it in solid solution substituting for another element? Can the gold be contacted by the cyanide leach solution? Understanding these questions is critical to optimizing gold recovery in flowsheet development and troubleshooting low gold recovery. Depending on your needs, we can bring the power of High Definition Mineralogy to provide key liberation information. SGS has developed a series of testing and mineralogy assessment protocols that allow us to determine the character of refractory gold and the mass balance between refractory and free-milling gold.

Once the deportment or textural associations of the gold is determined and the mass balance known, flowsheet options are developed. Gold in sulfide minerals could be liberated by oxidization using techniques such as roasting, high temperature pressure leaching or bacterial leaching. Also, grinding the ore to an extremely fine particle size can liberate gold encapulsated in sulfides. SGS has very extensive experience with all the unit operations involved in the processing of refractory gold ores, and in selecting the optimum flowsheet for a particular ore. Our gold metallurgy expertise is second to none. 

Chemical interference

There are two important mechanisms by which gold recovery is limited through chemical interference. The first happens when there are minerals in the ore that consume cyanide or oxygen - the two essential ingredients for effective gold dissolution. In this case, the simplest route is to increase the concentrations of cyanide and oxygen until the consuming species has reacted away to completion, but this is often not economically viable.

In assessing the economic benefit of the extra processing that needs to be done to improve recovery from refractory or partially refractory ores, it is always important to consider the incremental improvement in gold recovery versus standard grinding and cyanidation, and the incremental increase in costs to achieve this improvement. 

The second occurs when graphite or carbonaceous matter is present. In this case, the gold leaches well into the cyanide solution but then the carbonaceous minerals reabsorb the gold back into the pulp. This is known as “preg-robbing”. SGS experts will identify the best solution for your ore as there are many methods to choose from: 

  • The use of chlorine deactivates the carbonaceous matter under strongly oxidizing conditions. However, the consumption and cost of chlorine are prohibitive for many ores
  • Blinding and deactivation of the carbonaceous minerals with an organic compound such as kerosene is a cheaper alternative to chlorine and has been tested with many ores. It is generally not an effective approach, however
  • Pressure oxidation is also not an obvious choice in these situations, as the carbonaceous material has the potential to become even more active as an adsorbent for gold cyanide after this treatment
  • The effect of preg-robbing can be partially overcome by the use of CIL rather than CIP, provided the activated carbon used in CIL is more “active” than the carbonaceous matter in the ore. The rate of equilibration of gold between the carbonaceous matter and the activated carbon can be very slow, however, and this approach is seldom completely satisfactory
  • Mineral dressing techniques (gravity, flotation, etc.) have only ever been partially successful in eliminating graphitic carbon
  • Roasting of the ore will eliminate graphitic carbon, but the operating parameters have to be carefully chosen to ensure the graphitic carbon is burnt off and is not converted to a more active form

Advantages of preconcentration

The other parameter that SGS experts consider when designing refractory gold treatments is whether or not to preconcentrate the gold-bearing ore. This is usually done by flotation of arsenopyrite, pyrite, or other gold-bearing sulfides. To be economically justifiable, preconcentration should meet two objectives. First, the gold recovery to the concentrate should be sufficiently high that the tailings can be discarded without further treatment, and the mass ratio of concentrate to tailings should be low.

SGS has extensive experience with environmentally sustainable refractory gold extraction. Our experienced team of metallurgical professionals has developed bankable flowsheets for thousands of projects. These flowsheets are confirmed by bench and pilot-scale testing programs that are internationally recognized by the mining, engineering and financial communities.

SGS is the industry leader in refractory gold characterization. We ensure you get a quality product that answers to your needs, from grassroots exploration to production support, and provides significant interpretive value.