Bulk Materials Flow Discover the Most Important Design Considerations for Mass Flow in Hoppers

Author / Editor: Atanu Banerjee; L.P. Biswas / Dominik Stephan

A steel plant faces various difficulties while handling raw material. This article gives an idea for designing the geometry of bins/ bunkers suitable for the mass flow. Read on to know more.

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Hoppers/bins used in a steel plant.
Hoppers/bins used in a steel plant.
(Picture: M N Dastur & Company (P))

In raw material handling systems for the steel industry, flow of the material in bins, bunkers, chute, etc., is a nightmare for material handling engineers. Specially maintaining flow of materials like iron ore, fines, flux and coal fines during the monsoon in India is an extremely difficult task. Recently, various flow aids have come into the market and these have been used at various steel plants like Tata Steel, Vizag Steel Plant (VSP) and RSP; however, the result is not up to the mark.

Various type of flow pattern Depending on the shape of bins, the roughness of its interior surfaces, properties of the stored material, several patterns of the flow are possible during the discharge of materials. The air blaster / air cannon, bin vibrator and manual hammer are examples of common types of flow aid devices.

Mass flow is defined as the flow that occurs when the entire volume of stored material is emptied from the bin. In flat-bottom bins and bins with hoppers that are not steep enough for the entire material to be emptied in one go, funnel flow occurs.

Properties of Bulk Solids

In order to find the solution for this problem, it is important to define the properties of bulk solids. The solution mentioned in this article is applicable only to bulk solids.

  • a) Bulk density: It is the weight of bulk material per unit volume. There are two types—loose bulk density and compacted bulk density. Compacted bulk density is determined by weighing a compacted sample, i.e., where compaction has been done through vibrating the container that has the sample.
  • b) Angle of repose: When an unconsolidated bulk solid falls freely to a horizontal surface from a height, the particle rolls down from a pile. The angle made with horizontal surface is called angle of repose.
  • c) Internal angle of friction: Unconsolidated material has no strength, but it gains strength when it is stored in bins or is compacted. Cohesive materials gain strength with consolidation and hence, develop the pressure, as a result one faces flow problem.
  • d) External angle of friction: Flow in a bin also depends on the co-efficient of friction between the materials and side wall of the bin.
  • e) Flow factor: Force acting on the stored material in a hopper/bin tends to compact the material and shear stress in the material tends to make it flow. Jenike & Johanson showed that for an element at any position inside a mass flow hopper/bin, the ratio of compacting stress to the shear stress is constant and he called it as the flow factor (ff).
  • f) Valley angle of hopper/bin: Valley angle in a hopper/bin has great influence in the flow of material. It is expressed as:

(Picture: Dastur)