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Gravity gold recovery

Process design & development

Gold is one of the few metals that occurs in metallic from in natural geology and it makes sense to recover it prior to any chemical processing. Given its very high SG of 19.3 comparative to host rocks in the 2.8-4.0 range, gold is very amenable to separation using gravity techniques. ALS offers a range of gravity recovery processes, which can contribute to process design criteria for green fields projects (i.e. in terms of mass yield and grade) or process optimisation for existing operations.

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Gravity gold recovery testwork service

In industrial gold plants, gravity processes are usually situated within the grinding circuit to process either the Hydrocyclone underflow or mill discharge. In bench scale testwork, gravity is routinely implemented immediately after grinding and prior to other beneficiation or extraction processes. Typically, the entire feed sample (usually 5kg) undergoes gravity separation as a batch via a centrifugal concentrator. Large masses of higher-grade material can also be processed via a shaking table (Wilfley or Gemini table).

In terms of centrifugal concentrators, ALS employs both the Knelson KC-MD3 (supplied worldwide by FLSmidth) and the Falcon L40 (Supplied Worldwide by Sepro Systems).

For recovered gravity gold concentrates, the two main treatment methods are mercury amalgamation and intensive cyanidation. Due to the tendency to over recover precious metals on a bench scale relative to full scale operations, mercury amalgamation is the preferred method as it generally will only recover the coarse/free gold, whereas intensive cyanidation will also extract fine grained or partially liberated gold particles, and thereby further overestimate the Gravity Recoverable Gold (GRG) content.

When a more comprehensive method of determining GRG is required, ALS conducts the LaPlante (2000) test that has become the industry accepted method for the determination of GRG, whereby 50-100kg of sample is passed sequentially through the Knelson concentrator at three successive grind sizes ranging from P80:850µm to P80:75µm. At each stage, the Knelson concentrate is collected and subjected to size by assay gold analysis. The final stage Knelson tailings is also subjected to size by assay gold analysis, and using these data an accurate GRG content can be determined.

It should be noted that these techniques can also be implemented for other naturally forming higher SG metals/minerals, such as Native Copper and Chromite.

Need more information or want to devise a specific gravity program?

ALS has access to a full range of equipment and can also leverage of vendors for larger scale testwork.

No problem. Please contact us for further details.

 

Here are some frequently asked questions

  • How much sample mass do I need for gravity testing?

    Gravity is one unit process that usually requires very little mass to achieve a result, as follows:

    Pre-gravity stage 1 kg plus and in batches there after
    Wilfley table 1 kg plus and in batches there after
    Half GRG test 20-30 kg
    Full GRG test 50-100 kg
  • Why are benchscale gravity results often higher than industry practice?

    Centrifugal concentrators are a batch process whereby the spinning bowl will capture the high SG material in the sample feed and progressively displace according to the highest SG particles. The bowl of a Knelson KC-MD3 will typically capture 60-100 g of mass. If the feed mass is only 1 or 2 kg, this represents 3-6% of feed mass. Whereas in industrial plants, the bowl of an industrial grade centrifugal concentrate will capture 1—20 kg from hundreds or thousands of tonnes processed, which only represents a mass yield of fractions of 1%. Another important factor is that full-scale gravity circuits on a mine site are capturing liberated gold grains from a continuous closed-circuit system that is not replicable in a batch testing arrangement. However, most engineering companies will have proven plant design scaling factors for batch test results.

  • Will ALS treat hazardous samples?

    Generally, yes. ALS has facilities and procedures to treat fibrous and radioactive samples. Surcharges do apply, however, to cover safety equipment, monitoring and decontamination.

  • Our samples are from overseas, what do I do?

    ALS is a quarantine approved premises and treats many samples of oversea origin. Strict regulatory requirements are in needed around packaging and paperwork, however. ALS can advise you on these requirements.

  • Can I do my own testwork in ALS’s laboratory?

    Generally, no. Insurance and safety protocols typically prevent renting laboratory space and this is not part of ALS’s business model. Clients are able to visit and witness testwork but this is to observe only. Typically, clients may attend on 24/7 piloting campaigns and interact with senior staff members for technical and operational issues.

  • Does ALS do design engineering and consulting?

    No, as it is not our core business and would put us in direct competition with many of our clients. We can, however, put you in contact with a range of reputable engineering providers and consultants.

Associated testwork services

Related testwork

Gravity testwork rarely happens in isolation and typically many other processes are required pre and post gravity testwork, as follows:

Click on the above links or contact us for further information. We can integrate a full program for you if required.