Bulk material

The term “bulk material” is used to refer to a granular or lumpy mixture existing in a free-flowing form. A bulk material’s properties are defined by its grain size and grain size distribution, as well as by its bulk density, angle of repose, moisture, and temperature.
Bulk materials are classified into two groups:

  • Cohesionless, free-flowing bulk materials
  • Cohesive bulk materials 


The following are some important characteristics of bulk materials:

  • Bulk density
  • Angle of repose
  • Grain size
  • Grain size distribution
  • Grain shape
  • Cohesion
  • Adhesion


The German equivalent of the term “bulk” is defined on the basis of DIN standards concerning transportation, as well as on the principles of process engineering insofar as the latter deals with the handling of bulk materials. Bulk materials, especially loose materials, are materials that can move freely in a container or that are not otherwise secured in their position.
The field of bulk material handling mechanics studies the storage and transportation conditions of bulk materials, e.g., their angle of repose (angle of friction), fluidization, bulk densities, the discharge behavior of silos, etc. After a specific activation energy, bulk materials can behave like a fluid, i.e., begin to flow. When this occurs, the corresponding particles remain essentially the same or do not change in form on the conveyance route. The most important characteristic within this context is pourability. Finally, in terms of physics, the properties of bulk materials are analyzed in the body of theories that studies granular materials.
Bulk materials include construction materials such as plaster, sand, gravel, and cement, as well as raw materials such as ore, slag, and de-icing salts. In addition, foodstuffs such as grains, nuts, sugar, salt, coffee, and flour are included in the category. Powdery materials such as pigments, as well as fillers, granular material, and pellets can also be classified under it.
Due to their material properties, bulk materials are often stored in silos or hoppers. A distinction is made between free-flowing and cohesive bulk materials. The former can be discharged with rotary airlocks or discharge gates (for example) without a problem. However, in the case of difficult (complex) bulk materials, such as cohesive, hygroscopic, segregating, wear-causing, non-pourable, and paste-like bulk materials, the discharging process (especially when requiring precise metering) is much more demanding. Nevertheless, materials with the aforementioned properties can be discharged without a problem by using live bottom feeders.  Materials that are not sensitive to weather can also be stored outdoors.
In regard to a material’s conveyance, a distinction is drawn between continuous (i.e., flowing without interruption) conveyance and intermittent conveyance methods. Continuous conveyance methods include simple items such as hoses and tubes and complex systems such as belt conveyors (generic term), chain conveyors, screw conveyors, bucket elevators, proper belt conveyors, and tubular drag chain conveyors. In addition, devices such as rotary airlocks are used for discharging and feeding purposes, while double dump valves are used to convey coarse bulk materials. Intermittent conveyance methods, on the other hand, include bulk material silos and hoppers; dumpers and high-sided vehicles and cars; silo trucks, silo wagons, and semi-trailers with sliding bottoms; and bagged cargo and big bags.