Powder Rheology and Powder Flow Testing

Powder flowability dictates the storage, chilli powderhandling and transport of granular solids across a range of industries. Flow properties of powders can drastically influence the processing of materials such as:

  • Food flavours, colours and coatings
  • Spice blends, flours and starches
  • Cheese and dairy products
  • Pharmaceutical blends for tablet, capsule and granulation
  • Building materials and cements
  • Ceramics, minerals and clays

 

We can provide powder flowability measurement and a range of related powder characterization tests. Our annular shear cell Powder Flow Tester performs a series of shear stress tests on a sample across a range of consolidation stresses to deliver the following studies:powder flow ring shear cell

  • Flow Function
  • Time Consolidation Test with Flow Function
  • Wall Friction
  • Compressibility

Typical data outputs include:

• Flow Index
• Arching Dimension
• Rat-hole Diameter
• Hopper Half Angle
• Gravity Chute (Wall Friction) Angle
• Bulk Density Curve

Powder Flow Function

Unlike liquids and semisolids, powders display flowability that is dependent upon the applied consolidating pressure. As pressure is increased from, for example, the weight of a column of material in a silo, hopper or vessel, the yield stress of the material at the bottom increases, resulting in decreased flowability. By obtaining  yield stress (known as the unconfined failure strength) across a range of consolidation stresses it is possible to generate a flow function such as those shown in the graphic below.

Flow function 2

 

A powder that fails easily, even under a high consolidation stress, is signified by a shallow flow function and will prove to be a very flowable powder. In contrast, a powder that develops high shear strength, even under moderate consolidating stresses, displays as a steep flow function and will demonstrate poor flowability.

Time consolidation of powders

When left undisturbed powders will undergo time-consolidation to varying degrees. Time consolidation can lead to rat-holing in hoppers, bins and silos. A time consolidation study can reveal the propensity for a powder to gain strength and become problematic in this way.

Compression/compaction and relaxation studies on powders and granular solids

consolidating-stress
Density as a function of consolidating stress.

We have a custom compression cell that enables studies to be performed that deliver information on how a powder or granular solid behaves under compaction at controlled temperature and humidity conditions. Furthermore, on completion of a compression step the subsequent relaxation of the compacted mass can be monitored over time. The resulting compacted core can then be subjected to a uniaxial unconfined failure strength test.

In addition to these specialist measurements, more standard tests such as tapped density, Hausner ratio and Carr’s Compressibility Index can be performed.

 

Wall Friction

steel-surface-for-wall-friction
Steel surface for wall friction measurements – can be replaced other materials of interest.

Friction between a powder and a wall can significantly influence the flowability on hoppers, where, for example, high friction requires a steep wall angle to ensure flow and vice versa. Equally, high wall friction distributes the weight of a column of powder down the vessel walls rather than concentrating it on the material at the bottom. Wall friction tests can be performed with a sample powder against a range of materials.

 

 

 

 

Share this: