How To Measure Angle Of Repose Of Powder?

The powder is composed of many small particles. The fluidity of powder is essential for its production processes, such as mixer, hopper, and other devices. If the powder fluidity does not meet the requirements, it may cause unevenly mixing of various components.

The fluidity of powder is related to some factors, including powder raw materials and the actual production process, such as the ability of powder can flow out of containers (hoppers, funnels, cylinders, etc.) or the compressibility when making tablets. There are four ways to test the flow of powder:

• Flow characteristics through different orifices.
• Angle of repose
• Shear cell
• Compression index and Hausner ratio.

What is the angle of repose?

The angle of repose refers to the maximum inclination angle at which the horizontal plane of one side slope can maintain a natural and stable state when cohesionless bulk materials are stacked in a loose condition. After the bulk materials are stacked to this angle, they will slide down naturally if they continue to pile. The slope angle of bulk materials will remain unchanged, which will only increase the height and bottom area. The soil slope is in the limit equilibrium state, and the corresponding slope angle is equal to the internal friction angle of cohesionless soil, the natural angle of repose of cohesionless soil slope. The angle of repose is one of the critical parameters of bulk materials and the primary basis for the design of material stacking, slope, and dust removal equipment.

There are motion repose, net repose, water angle, and underwater angle of repose.

The angle of repose ranges from 0 ° to 90 °. The angle of repose is related to the type, particle size, shape, and moisture content of bulk materials. For the same kind of dust, with the increase of particle size and edge angle, the natural angle of repose also increases slightly. The smoother the surface or, the closer it is to the sphere, the smaller the particle; The greater the moisture content, the greater the angle of repose.

We can use the angle of repose in the granular solid treatment equipment design. For example, design a suitable hopper or silo to store materials or adjust the conveyor belt’s size to transport materials. It can also be used to determine whether slopes are likely to collapse. The angle of repose is also crucial for correctly calculating vascular stability.

Therefore, if you want to test the fluidity of powder, we can find the angle of repose to know. The smaller the angle of repose, the smaller the friction and the better the fluidity. In general, If the angle of repose θ ≤ 40 °, that can meet the flow requirement of production.

What are the factors affecting the angle of repose?

The angle of repose is related to the particle size and its distribution, shape, friction coefficient, adhesion, electrostatic voltage, porosity, compressibility, moisture, temperature, and humidity in the air.

1) The influence of particle size, shape and type of materials

The repose angle of materials is related to their particle size, appearance shape, type of materials, and other factors. For the same material, the smaller the particle size, the larger the repose angle; The smoother the surface or, the closer to spherical particles, the smaller the repose angle.

The angle of repose of the material is related to the design of many types of equipment, such as the design of the ash bucket (conical bucket body) of the dust collector. The included angle of the steel plate corresponding to the ash bucket of the dust collector should be slightly smaller than the angle of repose of the material at 3 ~ 5 °. Suppose it is more than the angle of repose of the material. In that case, the material dust filtered by the dust remover will not be discharged out of the ash hopper in time, and the material will accumulate in the ash hopper, failing the dust removal equipment to work normally. In bad cases, if it is pulverized coal, the long-term accumulation may cause an explosion, resulting in many losses.

When the material is transported, its repose angle will also change. Due to movement, its repose angle is slightly smaller than that when stationary. We should consider the factor when considering conveying materials with conveying equipment because it will affect the conveying volume of materials in a certain factor.

2)The influence of the moisture of the material

The greater the moisture content of the material, the greater the angle of repose. The moisture content of the material will increase the adhesion between the materials and the relative friction between the materials. However, For some materials, If too much moisture content or the material itself can flow at a slight inclination angle, the material’s angle of repose will become smaller.

In short, the angle of repose of materials is a vital property characteristic of various materials. We can better store and transit them when entirely using this property characteristic.

How to measure the angle of repose?

Injection method: add the powder slowly from the top of the funnel, and the materials leaked from the bottom will form a conical angle on the horizontal plane.

The angle of repose meter: the instrument can measure the angle of repose.

Incline: add 1 / 2 ~ 1 / 3 of the powder into the cylindrical container rotating slowly around the horizontal axis. When the surface of the powder slides, measure the angle of the surface.

Discharge: Add the powder into the cylinder container to keep the bottom of the cylinder horizontal. When the powder flows out of central hole at the bottom of cylinder, it will form the inclination angle of inverse conical in the cylinder.

Bottom area injection: The powder particles flow out of the funnel mouth and fall into the powder pile formed by the plane and the cone of the powder pile. The bottom angle of the cone is the angle of repose.

Height measurement: measure the height of the cone formed by the powder and calculate the angle of repose through the formula.

Image: By using a CCD device, the software automatically calculates the average value of the angle of repose.

FAQ:

What is the natural angle of repose for soil?

The natural angle of repose is slightly smaller than the sliding angle at which the landslide begins to slide and is about 5 ° ~ 10 ° smaller than the internal friction angle of the same material. The angle of natural repose angle on a natural slope is about 33 ° ~ 37 °, which is less than 30 ° or more than 39 °.

The size of the angle of repose depends on the soil’s friction characteristics. With the increase of soil particle size and edge angle, the natural angle of repose also increases slightly.

How to measure the bulk density

Bulk density (ρ B) refers to the density by dividing the mass of powder by the volume V of the container, i.e., ρ b=W/V。 When filling powder, the density measured after regular vibration or tapping is called tap density (ρ bt).

Put the powder into the container, and the measured volume includes the true volume of the powder, the gap of the particles, the gap between the particles, etc. Therefore, the shape and size of container, the filling speed, and the filling method of material affect the volume of the powder. The measured density is the loosest when the powder is filled into the measuring container without applying any external force. When the external force is used to make the powder in the tightest filling state, the measured density is called the tightest loose density—the compaction density changes with the number of tapping. When the final oscillation volume remains unchanged, measured compaction density is the tightest bulk density.