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Specific gravity & bulk density

Ore & waste rock characterisation

The specific gravity or bulk density of rock is essential information for ore reserve calculations. Often, bulk density of rocks is determined using the water displacement method soon after drilling. This can be more challenging in parts of a resource where rocks are highly weathered and can disintegrate upon submersion in water. For this reason, ALS offers a range of methods to determine either specific gravity or bulk density for a variety of material types. Whether to supplement routine measurements taken during resource drilling, or to replace field measurements, confidence in results can be ensured.

Minério e caracterização de resíduos de rochas

Specific gravity

Specific gravity is a unitless measurement of a sample's density relative to water. The equation used to calculate specific gravity is the weight of a dry sample in air divided by the weight of the sample in air minus the weight of the sample in water (i.e., the weight of water displaced by the sample). It is the most common value used to characterise a sample for resource calculations. The range of specific gravity reported is 0.01 to 20 which covers all rock and mineral types.

Sample types and specific gravity measurement

Specific gravity for rock and pulp samples can be measured although by different methods. A rock sample can be submersed in water, either as submitted or covered with paraffin wax. As a pulp sample can't be submerged in water, specific gravity measurements are taken by using a pycnometer.

Bulk density

This variable is characterised by determining the volume of water that is displaced by a sample when it is submerged. The weight of the sample is then divided by the volume of water displaced to produce a density with a unit of g/cm3 . Bulk density can be determined for raw samples, those coated in paraffin, fragmented samples, and samples coated in waterproof sealant.

Calculating bulk density from specific gravity

Specific gravity is often used for ore calculations thus is the most common parameter measured. The bulk density of a sample can be calculated from specific gravity by using the density of water at the temperature the specific gravity was measured.

Specific gravity & bulk density

Code Description Range
OA-GRA08 Specific Gravity on solid objects. Reported as a ratio.
OA-GRA08b Specific Gravity on pulps using pycnometer. Reported as a ratio.
OA-GRA09 Bulk Density by water displacement 0.01 – 20g/cm3
OA-GRA09A Bulk Density after wax coating (wax removal not included). 0.01 – 20g/cm3

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