The present invention relates to a method and to an arran¬ gement for regulating the flow of crushed materials, such as ore or limestone, from a hopper or like construction of the kind which comprises a downwardly narrowing container having located at its lower end an outlet opening which co-acts with a swing gate in a manner to define a material outflow opening, wherein the gate seals-off the outlet opening when in its closed position.

A storage hopper provided with a swing gate of this kind is known from SE-D-8601161-6 , to which reference is made with respect to the constructional principle of the swing gate.

Material is normally intended to fall from storage hop¬ pers onto a conveyor belt which delivers the material to an intended material processing site, material storage site or for further transportation. These conveyer belts move at a substantially constant speed and the speed at which the material is poured from the hoppers is con¬ trolled by adjusting the size of the outflow opening, i.e. by adjusting the position of the swing gate. This control method, however, is encumbered with certain drawbacks. For example, in the case of finely crushed and freely flowing materials, significant amounts of dust are generated when the material flows out through the outflow opening and falls down onto the conveyer belt. In the case of coarser materials, the materials are liable to form self-locking arched conglomerations when stored for a longer period of time in a closed hopper, preventing the materials from flowing when the hopper is opened. This condition is norm-

ally rectified by means of devices which vibrate the walls of the hopper. In such cases, however, there is a danger that the conveyer belt will be overloaded and damaged by the weight of material falling onto the belt when the arch suddenly disintegrates as a result of such vibration.

Admittedly, this danger can be reduced by decreasing the size of the outflow opening, but this will result in the danger of a new arch building-up.

The object of the present invention is to reduce or eliminate these drawbacks and to provide an outflow arrangement for hoppers and like storage columns which can be readily controlled for the emptying of materials of varying natures.

This object is achieved in accordance with the invention by means of a method and an arrangement having the charac¬ terizing features set forth in Claim 1 and Claim 2 respec¬ tively. By locating the conveyer belt close to the outflow opening, the fall height of the material is reduced and therewith the extent to which dust will be developed by dust generating materials, while by varying the belt speed the flow of freely flowing materials can be throttled, by decreasing the belt speed, so as to reduce dust genera- tion. while overloading of the belt by material falling from the hopper when a self-locking arch of material breaks-up can be prevented by temporarily increasing the speed of said belt.

These features of the invention and advantages afforded thereby will become more apparent from the following description of an exemplifying embodiment of the invention made with reference to the accompanying drawings, in which Fig. 1 is a schematic, longitudinal sectional view of an arrangement according to the invention.

Fig. 2 illustrates schematically and in longitudinal section the outflow of material with a partially open

swing gate and with the conveyer belt moving at a high speed,

Fig. 3 illustrates schematically and in longitudinal sec¬ tion the outflow of material with a partially open swing gate and the conveyer belt moving at a slow speed. Fig. 4 illustrates schematically and in longitudinal sec¬ tion the outflow of material with a fully open swing gate, and

Fig. 5 is a cross-sectional view of the arrangement illu¬ strated in Fig. .

The arrangement illustrated in Fig. 1 comprises a contai¬ ner 1, of which the lower part is illustrated in the Figu¬ re and which has a rectangular cross-section with sloping front and rear walls 2 and 3 respectively, and also slo¬ ping side walls. The container walls are terminated by lower vertically extending parts 4 and 5 of the front and rear walls, these parts being connected by vertically extending side-wall parts having partly circular-arcuate shaped edges such as to form a vertical container outlet channel. A swing gate 6 is pivotally carried by two arms journalled on the side walls of the container. The gate 6 has a bottom 7 and side walls 8 and its forwardly located part is of circular-arcuate configuration and intended to co-act with the circular-outlet edges of the container side walls. Fig. 1 shows the gate in its fully open position in full lines and in its closed position and an intermediate position in chain lines. A variable speed conveyer belt 9 is arranged at a short distance beneath the hopper outflow opening configured by the container 1 and the gate 6.

The direction in which the belt 9 moves is indicated by arrows in the Figures.

Figs. 2 and 3 illustrate schematically those instances when the material is discharged from the hopper with the

gate partially open. This operational mode is used when the material is a freely flowing or freely pourable mate¬ rial, such as granular material or aluminium pellets or iron ore pellets. The flow pattern of the material above the gate will then correspond to so-called funnel flow, i.e. a stationary ring of material is formed in the con¬ tainer around a conical flow core, in which the velocity components of the particulate material include a horizon¬ tal component. This stationary ring of material is indica- ted by the sectioned parts in Figs. 2 and 3 and the mate¬ rial flow in the conical core of the material configura¬ tion is indicated by arrows.

The thickness or height to which material is deposited on the conveyer belt 9 is adjusted by varying the belt speed, and the rate of flow of material from the hopper is con¬ tingent on the angle to which the gate is open, provided that the belt is travelling at a speed sufficient to en¬ able the material to fall freely onto the belt.

In the case of the example illustrated in Fig. 2, the belt 9 is driven at a high speed, therewith engendering the flow pattern illustrated in said Figure. In the case of the Fig. 3 example, on the other hand, the belt 9 is driven at a speed which is so low that the material will heap on the belt to an extent such as to prevent material from falling freely onto the belt, therewith throttling the flow of material from the hopper. In this case, the speed at which material is discharged from the hopper is solely dependent on belt speed. The operating method in which the flow of material is throttled in accordance with Fig. 3 is preferred in the case of dust generating mate¬ rials, since the slow speed at which the material is dis¬ charged from the hopper will greatly decrease the extent to which dust is formed in comparison with methods in which the material is allowed to fall freely onto the conveyer belt.

The method illustrated in Fig. 4 utilizing a fully open gate is applied to prevent the risk of arching when dis¬ charging materials which do not flow freely. Provided that the belt speed is sufficient to prevent throttling from occurring, the material in the region above the gate will have a flow pattern which can be considered to be a plane free mass flow, i.e. the velocity components of the coarse particulate material in this region will not include a horizontal component. This is highly beneficial, since the risk of arching is muc smaller in the case of free mass flow than in the case of funnel flow. Once the material has started to flow, the probability of arching occurring is very low when proceeding in accordance with the method illustrated in Fig. 4.

However, it is possible that a self-locking arch has for¬ med during the transportation of material stored in a clo¬ sed hopper, for instance during a long sea voyage. Conse- guently, conventional vibrators (not shown) are mounted on the hopper for the purpose of vibrating the container walls and therewith disintegrate the material arch. In the case of hoppers eguipped with conventional outfeed devices with which the conveyer belt is driven at a substantially constant speed, the material which leaves the hopper upon disintegration of said arch and which will thus have a high rate of flow is liable to overload the belt to such an extent as to stop the belt from moving. In order to decrease this risk, the gate is closed partially so as to obtain an outflow aperture of given width. Although this will admittedly reduce the risk of overloading the belt, the risk of a self-locking arch forming in the outfeed opening is created at the same time, since, as beforemen- tioned, the probability of arching is much greater in the case of funnel flow than in the case of free material flow. Such overloading of the belt is avoided in accordan¬ ce with the invention, by increasing the belt speed prior to disintegrating the arch and then decreasing the belt

speed to a speed required to obtain the desired material layer thickness on the conveyer belt. This method of pro¬ cedure ensures that funnel flow will never occur in the outfeed region of the hopper.

Finally, it is mentioned that when practising the inven¬ tive control method, the set position of the gate is not changed during the process of discharging the material from the hopper but that those changes occurring in the properties of the material with time and which in the case of conventional methods are compensated by adjusting the width of the outflow opening are compensated by changing the speed of the conveyer belt.