Testwork: SAG grindability index

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Testwork: SAG Grindability Index (SGI)

The SAG Grindability Index (SGI) test, or alternatively the trademarked SAG Power Index (SPI), is a SAG-specific test involving a small batch SAG mill that is charged with a two kilogram ore sample and ground until the product has achieved an 80% passing size of 1.7 mm. The test result is reported as time required to achieve the target product size (in minutes).

For tests performed at SGS laboratories, companion determination of the SPI is the CI (Crusher Index), which should not be confused with a Crushing Work Index. The CI is used as a parameter in determining the feed size of a sample in the CEET2 model, and can be thought of as an "amenability to crushing" index. Higher values indicate finer material (which usually means "softer").

Sample Requirements

An SGI determination (no CI) requires 2 kg of material with F80 of at least 12 mm.

A combined SPI and CI determination requires about 8 kg of material at least 20 mm top size (normally, split NQ-diameter or split HQ-diameter drill core is used).

Laboratory Outputs

The laboratory will report the following information:

  • ci: The Minnovex crushing index, a measure of the amenability to crushinge)
  • sgi: The SAG grindability index (in minutes), the time required to grind a sample to 80% passing 1.7 mm in a standardized SGI mill.

Extra field available for modelling

  • synthetic indicates whether this is a real test result, or just a synthetic one that should only be used for modelling. If this column contains a value of '1' (boolean=true) for a test, then that test is understood to not be a real test result and is therefore not shown on the testwork comparison charts. Synthetic values are available when running circuit model simulations and do show up in the list of model results.

Differences: Starkey SAG Test and Starkey SAGDesign Test

The SPI test was originally named the "Starkey SAG Test" in a paper presented at the Canadian Mineral Processors meeting in January, 1994. The inventor of the test, John Starkey, was associated with Minnovex Technologies Inc. and assigned the test's patent to them. Minnovex was subsequently sold to SGS Mineral Services, a major operator of metallurgical laboratories, and the capability to perform SPI tests was transferred to SGS laboratories world-wide.

John Starkey left Minnovex to form Starkey and Associates, Inc. and developed a different test with superficial similarities with SPI; this new test was named the "SAGDesign Test".

The SGI or SPI test is still occasionally referred to as the "Starkey SAG Test", but this causes confusion with the "Starkey and Associates" SAGDesign test. The term "SPI" is a trade mark of SGS and can not be used by laboratories who haven't licensed it from SGS, whereas the term "SGI" was proposed by Peter Amelunxen in a paper published in Minerals Engineering as a generic name for the SPI test to avoid infringing the SGS trade mark.

To avoid confusion between types of "Starkey test" (SGI, the generic form of SPI and SAGDesign), check the units that the test result are reported in. If the results are reported in "minutes", then the test is SGI or the generic version of SPI. If the results are reported in "revolutions" or "kWh/t", then the test is SAGDesign.


The Amelunxen SGI model built into sagmilling.com is based on the method by Amelunxen et al (2013). It is a power-based model that predicts Esag using the SAG grindability index (should be compatible with SPI™), and predicts Eball using the Bond ball mill work index with a manual calibration factor CFball to account for effects like the phantom cyclone.

The SPI™ test is used in the CEET model (Comminution Economic Evaluation Tool) developed by Minnovex and now marketed by SGS. There are two forms of CEET - the older "CEET 1" model is a power-based model that predicts grinding specific energy consumption for a SAG mill (Esag) using a formula with superficial similarity to a Bond equation.

The newer "CEET 2" model is a hybrid of a population-balance model and a power-based model that uses SPI to estimate Esag (SPI also is a parameter in feed size, pebble crusher circulating load and T80). The CI value is used as a parameter to estimate the SAG feed particle size distribution.

Trademarks & Licensing

SPI is a trade mark of SGS Mineral Services who is the exclusive provider of the test in Canada, the USA and other countries where it is patented.

Minnovex (Technologies) is a trade mark of SGS Mineral Services. Note that the laboratory name "Minnovex" in the SAGMILLING.COM database of historic testwork samples refers to the original Minnovex laboratory located in Toronto prior to the take-over by SGS.

SAGDesign is a trade mark of Starkey and Associates, Oakville, Canada.