Calibrating material density to match your representative and not your actual PSD

Properly calibrating your particle set within the Rocky DEM particle simulation software is critical to obtaining accurate and usable results. In general, the particle calibration process includes defining a particle size distribution (PSD), particle shape(s), material interactions and material bulk density to match as closely as you can to the actual material properties. 

If your simulated PSD can be set up to closely match your actual PSD, then the particle calibration process can be fairly straight forward. But if it is not feasible to have your simulated PSD exactly match your actual PSD—due to large distributions, small particles, hardware limitations and the like—then particle calibration becomes a bit more complicated as you are no longer simulating reality, but a representative of it.

As an example, if the material you needed to simulate has a PSD with a maximum particle size of 75 mm, 50% passing at 35 mm, and a flow rate of 6,800 mtph, it would be very expensive computationally to simulate particles below a certain minimum size due to the resulting large number of particles. In these instances, one of the more common approaches in the mining industry for speeding up simulation time while still achieving useable results is to increase the minimum particle size and thus create a representative PSD. The minimum particle size you select will likely depend on your particle shape(s), flow rate, whether the material is dry or wet, and your available processing power. But regardless, with this representative method a portion of the actual PSD will not be simulated and thus the actual bulk density should no longer be used.

The reason for this is because the smallest particle size in the representative PSD is now accounting for all the particles smaller than that size in the actual PSD, which means that the bulk density of the representative material is now different than the actual material. If you were to apply the actual bulk density to the representative PSD the resulting simulated volumetric flow would be incorrect. To correct for this, you need to calibrate the representative bulk density.

Below is the process CDI uses to calibrate the bulk density of a representative material in Rocky.

1. Define the representative PSD.
2. Set/calibrate the friction and cohesion parameters, including rolling resistance.
3. Create a 3D model of a 1.5 m³ test box with an open top.
4. Import the test box into Rocky and fill it with the representative material. Ensure the material has enough time to completely settle.
5. Create a 1 m³ User Process Cube in the middle of the material (Image 1) and measure the mass of material inside the Cube to get the bulk density (kg/m3).

Analysis cube measuring material mass inside the box
Image 1: Analysis cube measuring material mass inside the box

6. Adjust the particle density to achieve the target mass of material in the 1 m³ analysis cube.
7. Refill the test box using the new particle density setting and validate that the mass of material in the Cube is correct.
8. If the target mass in the Cube is not achieved, repeat steps 6 – 7 until the target is reached within the desired tolerance.

Once the representative bulk density is calibrated, CDI simulates the material on a horizontal feed conveyor to validate the bulk or volumetric flow. The input tonnage and feed conveyor should be configured to match the study you are going to be conducting, and it should only be long enough to allow the material to reach steady state and full speed (Image 2).

material simulated at steady state on feed conveyor
Image 2: Material simulated at steady state on feed conveyor

At this point, there are several ways you can go about verifying the bulk flow. A quick-and-dirty check is to measure the height of the material on the conveyor. Using the bounds of a User Process Cube at the end of the conveyor provides the approximate simulated material profile height (Image 3).

material profile on conveyor
Image 3: Material profile on conveyor

You can calculate what the material profile height should be by hand or you can use conveyor design software, such as BeltStat. If the material height is roughly where it should be, then the next step is to evaluate the material profile area.
The actual material profile area can be manually calculated by using the actual material bulk density, input tonnage, and speed of the conveyor (formula below).

Material profile area formula

The way CDI goes about evaluating the simulated material profile area is to first save an image of the profile in Rocky (as shown in Image 3, above). We then import it into our drafting software and scale it to be the actual size. Next, we draw a line around the material and measure the area. If the profile area is within the preferred tolerance for the project, then the calibration is complete. If not, then we go back and repeat the process until it is.

Image 4 shows how CDI overlays the Rocky simulated material profiles onto a material profile generated by BeltStat. While not necessary as part of the calibration process, this type of image is a nice visual check and we sometimes use it in our material calibration reports. As you can see, the profile of the material with the actual bulk density is approximately 13% greater than the profile of the material with the calibrated bulk density. This could be substantial if you are designing a transfer chute with a design tonnage that is already 20% greater than the normal operational tonnage.

Material profile comparison using BeltStat and Rocky
Image 4: Material profile comparison using BeltStat and Rocky

Author Jason Aldrich

Jason Aldrich

Project Engineer at Conveyor Dynamics, INC.

Jason Aldrich is a Project Engineer at Conveyor Dynamics, INC. (CDI), a leading engineering consulting firm specializing in the design and optimization of mining conveyor systems. Jason has been focused on using Discrete Element Modeling (DEM) to optimize bulk materials handling equipment, including transfer chutes and comminution mills. He holds a Bachelor's degree in Construction Management, Associate's degrees in both Engineering Sciences and Engineering Graphics, and is a certified Project Management Professional (PMP).

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