Bulk solids are goods that are present in a pourable form. Bulk material is present in lumpy, granular or powdery form. Bulk solids are primarily differentiated according to how flowable they are. There are free-flowing (cohesionless) bulk solids and cohesive (cohesive) bulk solids.
Important quantities of bulk material are:
- Bulk density
- Angle of repose
- Grain size
- Grain size distribution
- Grain shape
The handling of bulk materials is a DIN standard defined process, the “pouring”. A bulk refers to such goods that can move freely in the storage or transport container or are not further secured.
Bulk cargo include building materials such as gypsum, sand, gravel and cement, and also raw materials such as road salt, ore or slag. Likewise, foodstuffs such as grain, salt, sugar, flour or coffee count as bulk material. Various pellets, granules, chips, feedstuffs and other powdery goods are also referred to as bulk goods.
Storage and discharge of bulk materials
Bulk cargo are stored in silos, bunkers or, in the case of insensitive goods, in the open air. In practice, the storage and especially the discharge of bulk materials are sometimes complex due to the unpredictability of the bulk material. Therefore, the specification of the bulk material in question is an important point. This includes the determination of bulk density, particle size, particle shape, moisture and other relevant parameters.
When discharging the bulk material, it is not only the previously suitable storage location or container that is decisive, but also the availability of the appropriate plant technology in order to avoid problems such as spillage. Often, a bulk material unpredictably shows a different behavior, although otherwise all parameters of storage and bulk material mechanics have remained the same. The application of the appropriate bulk solids technology and conscientious planning of flowability represent critical variables in the handling of bulk solids.
Likewise, accurate batching of certain bulk solids is very demanding. Here, too, appropriate equipment is used. These include, above all, dosing screws, which are often used in conjunction with vibration support. With the so-called coarse flow, high metering capacities are achieved, whereas in the fine flow, metering is very precise. Dosing can also be discontinuous (individual batches) or continuous (constant flow). Metering systems operate either volumetrically, dispensing the bulk material by volume, or gravimetrically, weighing the bulk material and dispensing it by mass.
In the case of cohesive, moisture-binding, segregating, abrasive or strongly cohesive bulk solids, the discharge and, above all, the exact metering is quite complex. In this case, a tilt-beam bottom is used for discharge during storage and transport. These are usually placed under silos or containers. In combination with the suitable geometry of the silo and a continuous activation of the (rotating) tilting beams, even very poorly flowing bulk solids can be discharged in this way.
In the case of difficult (complex) bulk solids, for example, cohesive, hygroscopic, segregating, abrasive, non-flowing or even pasty bulk solids, the discharge (especially the precisely metered discharge) is more than demanding. However, materials with the described properties can be discharged without any problems with a tilting beam bottom.
Transportation of bulk materials
The type of bulk material determines the means of transport. Construction debris, gravel or sand can be transported in open dump trucks or high-sided trucks. Weather-sensitive goods such as salt or sugar require a silo vehicle for transport. Vehicles with push floors are also used or the bulk goods are transported as bagged goods or in big bags. In sea freight, bulk goods are transported in bulk holds or bunkers on the ship.
The carrier decides which means of transport is suitable for the particular bulk cargo. Bulk cargo transports therefore require a high level of expertise and extensive experience for the optimal execution of the transport.