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Convert Specific Gravity (8 replies)

Zander Barcalow
8 months ago
Zander Barcalow 8 months ago

Is an adjustment factor required to convert specific gravity to bulk density? And if so is it based upon mineralogy?

8 months ago
Hauptsturm 8 months ago

For design purposes in drill chip sampling applications, we allow a volumetric increase of 30% from specific gravity volume to bulk density volume. This will of course vary depending on what is being drilled but the 30% rule of thumb works for equipment design.

Maya Rothman
8 months ago
Maya Rothman 8 months ago

Depending on how the measurements are made, bulk density could include pore spaces and specific gravity would not. I don't think mineralogy is a factor.

Jean Rasczak
8 months ago
Jean Rasczak 8 months ago

Generally there is a lot of sloppy terminology used in mining when it comes to the terms specific gravity and bulk density (SG and BD). It is often assumed that they are the same, but they are not. SG data (for example from air pycnometer work done on RC samples) does not account for rock porosity, but would be nearly the same as mineral density (mineral density potentially having some porosity associated with the lattice structures of crystals etc). For mining and resource work, you usually need a dry bulk density for the rock including the porosity. Hence most BD determinations are generated using the Archimedean methods on sealed samples. If you knew what the average porosity of a specific rock type was (e.g. through SG/BD twins), you could possibly factor the SG values accordingly (BD should be less than SG) to approximate an average BD. But there is never going to be a nice constant factor that can be applied, as porosity in individual rock samples and between rock types is variable think sandstone versus granite or vuggy vein versus solid wallrock. Hope that helps!

8 months ago
Sturmbann 8 months ago

I agree terminology is sloppy.
From an internet site on definition of specific gravity:

Specific gravity is the density of a substance divided by the density of water. Since (at standard temperature and pressure) water has a density of 1 gram/cm3, and since all of the units cancel, specific gravity is usually very close to the same value as density (but without any units).

What I was taught was that the density of water can change, particularly due to temperature. Hence a small adjustment may be required to convert to density.

Comments by others regarding porosity are of course correct; just need to be precise about what definitions you are using.

Sugar Watkins
8 months ago
Sugar Watkins 8 months ago

I am glad that I got a clear understanding that Bulk Density is used in the resource and reserve models. How do measure BD for mineralization occurring in more than two different formations such as quartzite, sandstone and sand?

Alan Carter
8 months ago
Alan Carter 8 months ago

Your last question is a little ambiguous. If you were to try and calculate bulk density for a volume containing two or more rock types with differing SGs, then you would need a way of estimating proportions for each rock type and then back calculate a volume weighted SG. After this you would then factor in porosity / broken ground.

The other way to interpret what you have said is to consider how to apply a valid density value to mineralisation that spans two or more rock types within a model. In this instance, it is simply a matter of ensuring that separate domains are created for each rock type, and any data (block or assay drillholes intercepts or samples) that fall within a particular domain, are assigned an appropriate SG / density value.

Hopefully either one or both of these responses helps answer your query!

8 months ago
Obersturmbann 8 months ago

For some, bulk density might be determined form a cylindrical core (or core plug). Simple geometry gives the volume and the calculated density includes any porosity (the greater the porosity the lower the "bulk" density value c.f. mineralogical/grain density).

But for most, bulk density is determined after crushing, so now the bulk volume includes the volume of spaces between pieces/particles. Depending on the particle size distribution and mechanical agitation finer material might fill some of the voids between the larger particles thereby reducing the overall bulk density. If the fines and coarser material are caused to segregate the "bed" then expands again.

Tony Verdeschi
8 months ago
Tony Verdeschi 8 months ago

Couple of other pieces of information necessary: Are the place, timing and time where you want Bulk Density to be measured!

Bulk density will depend heavily on the condition of the material - pre-blast (including cavities), broken stock in pit, post relocation, stockpiled, deep stockpile, post plant crusher etc. Shape of particle and likelihood of aligning to fill gaps has a bit impact. For example shaley particles might align like plates leaving large gaps.

Also packing order makes a big difference - just get some rocks and sand and if you add the sand to a container first, it will fit fewer rocks than if you add the rocks then the sand as the sand will flow and fill the gaps.

In plants it is well known that bulk density changes with the conditions and packing. Consolidation would also be expected for longer term stockpiles.

I tend to work with bulk density as relative values rather than absolutes - i.e. best case scenario the ore is as dense as particles - often called SG but I prefer particle density - worst case scenario it could be loosely packed. I believe there are ISO standards (not really my forte) describing tests for measuring compacted and un-compacted bulk density - but don't forget to make sure you have a sufficient mass of sample for the test!