The determination of the density of bulk powder materials is complicated by the existence of empty space surrounding the individual particles, and by any pores or voids within the particles themselves.
The method to determine the bulk powder’s tapped density (sometimes reported as tap density) reduces a significant proportion of the between-particle void space, by vibrationally settling a bulk sample of the powdered solid to produce near optimal packing of the particles. As a general rule, smaller particles will pack together more efficiently (i.e. have a higher tapped density) than would larger particles of the same material.
The tapped (bulk) density of a 250 ml volume of the +65 mesh (>210 micron) Miller graphite crystals was found to be 0.965 g/cm3. This high tapped density will have positive implications on transportation costs as it is about 20-25% higher than typical graphite concentrates in this size range. Numerous examples of "micronized" or "spherical" graphite with tapped densities approximately equal to 1 g/cm3 were found (examples provided in links below).
What is called the skeletal or the true density of the particles is a determination of the density of the solid particles themselves, excluding any voids or spaces between the particles, or on their surfaces. A bulk powder sample is first exposed to a vacuum, to remove ambient gas. Then a fluid or gas, often helium gas, is used to saturate the particle voids. This leads to the term "helium density", but that is only one method for determining the true density of a bulk sample of a porous solid; fluids which may be employed are xylene or mercury, resulting in "xylene density" or "mercury density" determinations, respectively. For graphite, each of these methods should provide similar results.
The helium (true) density of this +65 mesh (>210 microns) particle size fraction was found to be 2.12 g/cm3, approaching the theoretical density of graphite at 2.26 g/cm3 at 293K. This suggests that the Miller graphite concentrate is highly crystalline, and demonstrates very low total void volume. 1
The tapped density determination was conducted by Evans Analytical Group, Liverpool NY, in accordance with ASTM Standard Method B527-06, "Standard Test Method for Determination of Tap Density of Metallic Powders and Compounds".
The helium density determination was conducted by IMR Test Labs, Lansing NY., in accordance with ASTM Standard Method D5965-02, (2013), "Standard Test Methods for Specific Gravity of Coating Powders", method B.
References (links to commercial "micronized synthetic" and "spherical flake" graphite product specifications, for comparison purposes):