HOPPER (EN: bunker; DE: Bunker; FR: silo, trémie; ES: tolva, carbonera; RU: бункер) is the structure for the temporary accumulation and storage of the bulk loads. Within the mining craft, the hoppers are most widely used within the entire chain of the transport and technological machines. The hoppers may be made of the reinforced concrete (monolithic, assembled, and of the mixed type), metallic, combined (for example, the concrete casing, with the metallic bottom, and so on); as the hopper, there may also serve the mine workings.

According to the purpose, they sub-divide the hoppers into the technological, accumulating, and the structures for the storage of the bulk loads. The technological (which are averaging the flow of the load) hoppers have the minimal required volume capacity, which is sufficient for the stable (according to the adopted organization) work of the transport system, independent of the character of the change of the load flow. The accumulating hoppers permit to ensure the independent work of the transport segments, which are following them (during the temporary interruption of the delivery of material from the previous segments) at the expense of the accumulation within the hoppers of the certain reserve of the load, which is transported.

According to the type of installation, they sub-divide the hoppers into the stationary, semi-stationary, and mobile. The stationary (accumulating) hopper within the underground mine is the vertical or inclined mine working near the skip shaft (the volume capacity is up to 3000 cubic metres), the winzes, or the lower part of the ore chute (the volume capacity is up to 1000 cubic metres). At the surface, the stationary hoppers (depending on the techological scheme for the loading into the means of the external transport) may be receiving, receiving-loading (are situated at the junction with the previous transport segment, for example, with the skip rise), and loading (are installed at the certain distance, for example, at the boundary of the industrial site). The transport connection between the receiving and loading hoppers is performed, as a rule, with the help of the belt conveyors. In case of the large volume capacity of the hoppers, they separate them by the partitions into the cells with the volume capacity of up to 400 cubic metres, each of which has its own unloading hole. They install the semi-stationary and mobile technological (averaging) mechanized metallic hoppers onto the soil of the mine working and onto the railway track.

They distinguish the hoppers of the continuous action (during the continuous release of the load, the upper level of the materials within the hopper remains unchanged, at the expense of their new deliveries), and of the cyclic action (the bulk load moves periodically during the activation of the unloading mechanism). The latter hoppers may be not completely emptied (the level of the bulk load does not decrease lower than the specified height), and completely emptied during each opening of the gate (the metering hoppers).

They construct the rectangular (with vertical walls, pyramidal, combined), round (conical, cylindrical - silos, cylindrical-conical, parabolic), trough-shaped (prismatic and parabolic, with lateral or central unloading, slot-shaped) hoppers. The shape of the hopper in each special case ensures the possibility of its complete filling and emptying. For the facilitation of the movement of the load, they make the walls of the hopper smooth at the internal side. They coat the internal surfaces, which are subjected to the strikes and wear, because of the interaction with the moving mass, with the replaceable lining, which is made of the wear-resistant materials (the metallic sheets with the thickness of at least 10 millimetres, the diabase blocks, and so on).

In order to decrease the crushing of the coal, and for the protection of the gates against the destruction under the impact of the striking loads, they install into the hopper the special structures for the descent of the material. In case of the storage of the wet freezable loads, they make the cold-insulated and heated hoppers. They load the hoppers through the open top or hatch, and they unload the hoppers through the holes within the bottom or walls. In this case, they distinguish the hoppers with the central, single-sided, and double-sided unloadings. The flow of the load outward through the hole, as well as the movement of the load within the hopper, proceed under the impact of its own weight. The angle of inclination for the edges of the issuing funnel is at least 50 degrees for the well flowing bulk coals and ores, and at least 60 degrees for the wet coals and badly flowing bulk ores. The transversal dimension of the unloading hole must exceed the maximal size of the pieces by at least 3 times. They open and close the issuing hole of the hopper by the special gates and feeders.

According to the position relative to the level of the ground, they sub-divide the hoppers into suspended (which are mounted onto the columns or ceilings of the buildings) and deepened into the soil. The latter hoppers are mostly slot-shaped (instead of the individual holes, they are equipped with two longitudinal slots, which are blocked at the bottom by the shelves, which are preventing the spontaneous outward flow of the load). Relatively to the access railway tracks, the hoppers are situated longitudinally, transversely, and longitudinally-transversely.

The longitudinal position ensures the significant length of the front for the loading works, with minimal quantity of the manoeuvring operations, the loading of one or two grades of the useful minerals.

The transverse scheme permits to perform simultaneously the loading of several grades, but is associated with the significant volume of the manoeuvring works. There are the variety of the hoppers, like the re-loading funnels and semi-hoppers.

The re-loading funnels differ from the hoppers by the smaller sizes, and, as a rule, by the absence of the gates and feeders. Their major purpose is the concentration of the flow of the bulk load.

The semi-hoppers have small vertical dimensions (in comparison with the dimensions along the horizontal plane), and are somewhat deepened into the soil. The significant part of the load within these hoppers is situated above the edge of the structure, in the shape of the cone or of the trapezoidal prism.

The unloading is performed by the belt conveyor, which is situated within the trench under the semi-hopper. They widely use these structures at the outdoor coal depots. At the indoor depots, there are introduced the hoppers of the silo type (with the volume capacity of the individual silo of up to 400 cubic metres, with the diameter of up to 1800 millimetres); usually, there is planned the installation of up to 4 hoppers of this type, with the total volume capacity of 6000, 9000, and 12000 cubic metres.