Free milling ore evaluation & optimisation

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.

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:

• Flotation
• Roasting
• High pressure oxidation
• Bacterial leaching
• Heap leaching

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