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Free milling ore evaluation & optimisation

Comprehensive worldwide experience

Most gold projects fall into the free milling category, whereby the ore is milled to liberate the gold prior to leaching and recovery processes. Free milling process work includes the evaluation and optimisation of all relevant process parameters that influence the CAPEX and OPEX for a gold project. ALS Metallurgy, in Perth Western Australia, has conducted thousands of free milling gold projects and no other laboratory group can offer this sort of experience to meet your needs.

Free milling process testwork services

Overwhelmingly, most gold projects centre around extraction via cyanidation and gold recovery via either carbon in pulp (CIP) or carbon in leach (CIL). The journey of process development usually starts small with a few tests on a blended composite to ascertain general amenability of the ore to these processes. Typically, thereafter, scopes of work expand to much larger programs to encompass wide ranging testing on various ore types and mine domains and leading to comprehensive variability testing once key processing parameters have been established.

After sample preparation, the general order of free milling gold evaluation is as follows:

Head Characterisation. Typically consisting of comprehensive head assays on representative samples, size-by-assays, and mineralogy. These data will provide valuable insight into the direction of the subsequent test program.
Comminution testing
Grind establishment. Determining the time taken to mill the ore to a target grind P₈₀.
Gravity gold recovery
Direct leach extraction (cyanidation) amenability, usually by bottle roll tests. This part of the program is usually divided into discrete test series to optimise: grind size, pH, cyanide dosages, %solids, residence (leaching) time, air versus oxygen, pre-oxygenation, and accelerant additions (e.g. lead nitrate). The test series form a matrix to determine the optimum condition set that generates the best leach extraction and/or leach kinetics (rate of leaching).
Tests to determine ancillary parameters such as lime demand, oxygen uptake/demand, and gold robbing.
Following from direct leaching, and in cases where the ore is gold-robbing, is gold recovery testing (CIL and CIP). Essentially, these tests are extensions of the direct leach tests, and once again are conducted in a test matrix to determine optimum conditions. These tests can also be completed in bottle rolls, but are often conducted in open topped agitated vessels (i.e. small scale versions of industrial leach tanks).
Specific carbon adsorption tests such as the triple contact sequential carbon loading test (to determine Fleming constants k and n), and the equilibrium carbon loading test. Carbon characterisation test to determine the adsorption and regenerative capacity of carbon.
Testing to determine auxiliary parameters such as: cyanide detoxification, rheology, settling and flocculation, and filtering.

It doesn’t stop there when your ores are not directly suited to standard processes. This might be due to gold that is finely disseminated in pyrite and requires prior flotation and ultra-fine grinding prior to cyanidation. Ores containing soluble copper can have very high cyanide consumption. Ammoniacal cyanidation can dramatically reduce the overall level of cyanide consumption and we can carry out leach tests to optimise reagent conditions. Ores rich in silver may be better suited to gold recovery from solution via the Merrill-Crowe zinc cementation method. Whatever the requirement, our expertise covers all aspects of processing non-standard ores, from flotation to selective leaching, CIP, elution and fully integrated electrowinning circuits.

Here are some frequently asked questions

How much sample mass do I need for a free milling gold testing program?

This will depend on the number of tests required and the outcome of those tests, but at typically 1 kg per test, a parcel of 100 kg should be sufficient for a standard program. Keep in mind, though, requirements for preliminary comminution testing, and if significant gravity and/or flotation testing is required, that perhaps 200-300 kg for ore type may be more appropriate.
What is the difference between bottle rolls and agitated vessel (vat) tests?

Bottle rolls are a cheaper method for leaching and CIP/CIL testing that can be done very efficiently in large batches. In many instances bottle rolls can suffice for most of the program. The main shortcoming of the bottle roll is that whilst on the rolls, the test is a closed system. The main advantage an open topped agitated vessel test is that the slurry is always accessible, and thus enables constant air or oxygen sparging through the duration of the test.
Can ALS do a single test on a small sample?

Yes, of course. Large or small, we cater for all customer requirements.
Can you test our plant carbon?

Yes, no problem.
Can I send plant slurry samples?

Yes, ALS can filter your plant slurry samples as required or treat them as received.
Can we test our monthly plant samples to compare against our plant performance?

Yes, this is a very common service and extends to all plant samples.
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.
Can my associates and I view the testwork at ALS's laboratory?

Yes, clients are always welcome to visit our facilities and observe samples or tests in progress. Inductions are required and other rules may apply to visitors.
Will ALS dispatch my samples to third parties?

Yes, if a testwork program is in progress, ALS will happily dispatch testwork products and feed samples to vendors, third party laboratories, and customers as required. Fees apply.
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

Gold ores and the associated free milling metallurgical testwork results very often don’t fit into nice categories and packages and will often require other testwork, such as:

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

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