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Tonnage vs Volume

Volume vs weight

A Weighty Subject that speaks volumes or what is the true plant capacity

What is the capacity of your operations? Ask this question at any mine operation and you will get an answer of so many tons (or tonnes) per hour, day, or year.

While it is easy to measure the weight of the material being feed to a plant by using a scale, most (if not all) processing equipment are basically volumetric in operation. The equipment has fixed dimensions, which allow it to process a certain volume over time. And yes even crushers are designed to pass a certain volume per unit time. Taking the volume over time and a given (or assumed – more late) builk density (more precisely, the volumetric mass density, the mass per unit volume) of the material you have the weight capacity.

Some equipment is often sized by its volumetric capacity. Belt conveyors and feeders are close to pure volumetric process and often have their capacity expressed in volumetric units (or as a certain mass per unit time at a given bulk density). Flotation cells are usually given as having a certain volumetric capacity (80 cubic feet, 300 cubic meters, etc.) and then using the actual (or assumed) retention time and (again given or assumed) bulk density, you have the weight capacity.

Stockpiles have a capacity, often expressed in tons, but actually a volume based on the length, width, height, and bulk density. And this can change depending on a lot of factors.

Now the issue of given or assumed bulk density. A problem occurs because a material will have a different bulk density depending on where it is in the process. Before mining, during mining, or after sitting in a stockpile, and then during processing as it is concentrated. This can affect the apparent bulk density by a significant amount. Variations of + and – 10% are common. This is often taken into account in the mining operations where the bulk density in place, loose, and compacted are accounted for. But the changing in size of the material also impacts the bulk density. As the material is reduced in size (crushing and grinding) the space between particles is also reduced generally increasing the bulk density. Adding water to create a slurry also impacts it.

So understanding the actual plant capacity is more than just looking at the ratede tonnage capacity. Actually many operational problems can be related to not taking this in to account. Feeding a piece of equipment at a higher volumetric rate than it can handle or at a lower volumetric rate than the material needs can lead to poor performance. While the over feeding is an easy one to spot as grade and recovery often suffer, the under feeding can be harder to identify. It most often is noted in grinding (over grinding), but can effect other processes. In flotation producing a lower grade than expected, byt floating more gangue than planned maybe harder to spot.

As ore changes this whole issue can change, which requires periodic review and checking.


MIke Albrecht, P.E.

o   40+ years’ experience in the mining industry with strong mineral processing experience in precious metals, copper, industrial minerals, coal, and phosphate

o   Operational experience in precious metals, coal, and phosphate plus in petrochemicals.

o   Extensive experience performing studies and determining feasibility in the US and international (United States, Canada, Mexico, Ecuador, Columbia, Venezuela, Chile, China, India, Indonesia, and Greece).

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