CA2053184A1 - Process and device for cleaning of bulk goods - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention concerns a process and a device for cleaning the grain surfaces of contaminated granular bulk material. The invention is particularly suited to final-cleaning of, in particular, thermally pre-regenerated sand (for example used foundry sand). According to the method of the invention, the bulk materials which are to be cleaned or final-cleaned are charged with a high pressure gas flow which maintains a fluidised bed of the bulk material, and creates a condition of turbulence within the fluidised bed in such a way that cleaning takes place while avoiding excessive impact or rebound stresses on the grains. Cleaning of the grains occurs primarily at the walls (which may be fixed or movable) only by friction or scouring stress on the grain surfaces. The device of the invention is equipped with a container having an inlet and outlet for bulk materials and a gas-permeable flow bottom for forming a fluidized bed. The device includes at least one nozzle, which is located above the flow bottom, and which is connected to a high-pressure gas source. The nozzle serves to direct the high-pressure gas flow into the interior of the fluidized bed.

Description

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The present invention relates to a process and device for the cleaning of the grain surfaces of contaminated bulk materials. In particular, the invention relates to the mechanical final cleaning of thermally pre-regenerated sand, for example used foundry sand.
The present invention further relates to a device, for carrying out the above process, which includes a container equipped with a bulk materials inlet and a bulk materials outlet which, for the purpose of creating a fluid bed, is equipped with a flow bottom permeable to gas, which flow bottom is to be charged from a fluidizing gas source with fluidizing gas.
As is well known in the pertinent branches of technology, there are widely differing technological processes using bulk materials (for example sand). These bulk materials are contaminated during the respective process, and accordingly were formerly not only classified as waste products but also handled as such by being taken to dump sites or disposed of in some other way (thereby preventing the possibility of even partial re-use).
This has been the case with, for example, large amounts of used foundry sands which, being casting materials, were initially treated with inorganic and/or organic-chemical bonding agents. After the casting process, and in particular as the result of the previously added bonding agents, these foundry sands are typically so badly contaminated that used foundry sand as such (i.e. without regeneration) cannot readily be re-used. Regeneration, and thus at least partial recovery of contaminated bulk materials, requires (in the above sample case) that binder shells and other contaminations of the sand grains, which essentially consist of quartz, be separated from the grains, and then removed.
These types of binder shells exist (in the case of used foundry sand) in inorganically bonded molding sands, particularly because the bentonite (which was added as the molding material - depending on the casting heat) is essentially fixed in the form of shells on the sand grain 2 0 ~

surface by oolithization. Other contaminants of used molding sand (or used foundry sand) include lustrous carbons and residual additives which form lustrous carbons during the casting process.
As is shown by way of summary, the publications DE-OS 4 008 849, DE-OS 31 03 030, 38 15 877, 30 19 069 and US-PS 2,783,511 already suggest technologies for regenerating used foundry sands which, however, do not show satisfactory results either from a technical or a cost point of view. DE-OS 4,008,849 suggests a process and a device for the regeneration of used foundry sands which results in an improvement over the former state of the art. However, it has been shown that this technology, too, can be improved, which becomes apparent from the following:
The suitability of the regenerated material for potentially unlimited re-use for various molding processes is determined, among other things and in particular, by the parameters of - pH value - oolithization - elutriation material - total C content - loss on ignition and - granulation < 90 ~, whereby the (permissible) limit values of these parameters for each molding process are defined or determined, on the basis of experience, for the respective process. If these parameter limit values are exceeded, the re-use of regenerated used foundry sand is still possible in principle, but only in suitable so-called molding material cycles.
Therefore, the specific requirements for each case must be investigated or, rather, it must be determined which limit values have to be observed in each practical application.
The regenerated material from a thermal separation process of used foundry sand has, in general, a relatively high quality, but still contains components of carbonized 2 ~

resins, additives and/or fire-clays on the grain surface. In addition, the regenerated material usually contains undesirable (since interfering) fine components. For the optimal adjustment of the actually desired quality of the regenerated material, it would therefore be necessary (or most highly desirable) to substantially rid the regenerated material of these impurities. Similar considerations apply to other types of regeneration processes.
If one considers mechanical cleaning for such a final cleaning process, one has to note that it is well known that during a mechanical cleaning of the bulk materials, which is always desirable because it protects the grains, the cleansing effect is obtained essentially by friction or scouring action, as the individual bulk good components (i.eO
the sand grains in the above example) move in an abrading manner relative to one another. Here, an excessive relative speed of the bulk good grains to one another or to other cleaning elements is critical inasmuch as movement which is too intensive can result in a decrease in grain size, damage to the grain surfaces and/or grain destruction. In addition, excessive grain speeds can lead to considerable eroding destruction of cleaning components because of rebound effects with corresponding (excessive1 impulse energy.
Several processes and devices are known with which a mechanical removal of contaminants is achieved. In these processes, rebound and friction effects on the sand grains themselves, or on fixed or moving equipment components, are brought about. As impact examples, we shall name rebound and ball mills, vibrating containers and scouring drums, as well the rebound cleaning devices according to Jacob, Simpson and Webac, which are especially frequently used. In order to avoid grain damage and destruction as much as possible, one generally attempts in these processes to keep the rebound forces lower and to increase the friction effects by using suitable measures, such as inclining the rebound surfaces etc.

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All of these devices, or the processes to be carried out with them, have in common the considerable disadvantage of relatively high grain destruction. Furthermore, any increase in the requirements on the quality of regenerated material, leads to increased grain destruction. This is due to the fact that in order to satisfy increased quality requirements, it is logical and thus consequential to correspondingly increase the rebound speed, or to increase the number of cleaning cycles. In either case, in addition to achieving a relatively good cleaning effect, a high degree of grain destruction inevitably results.
A main object of the invention is, therefore, to create a process of the above-described type, as well as a device which is suitable and designed to implement this process, with which (process and device) the contaminated grain surfaces of bulk materials are mechanically cleaned or final-cleaned effectively, while minimising grain destruction and erosion of the components of the cleaning device.
According to an aspect of the invention, there is provided a process for mechanically cleaning the grain surfaces of contaminated granular bulk materials, wherein said process comprises charging said bulk materials with a flow of high-pressure gas so as to form a fluidized bed of said bulk materials, said high pressure gas flow being adapted to cause regions of turbulence within the fluidized bed wherein the surfaces of said grains of said bulk materials are subjected to frictional stresses while minimising impact stresses, whereby said bulk material is cleaned without excessive grain destruction.
According to another aspect of the invention there is provided a device for mechanically cleaning the grain surfaces of contaminated granular bulk materials, said device comprising: a container having a bulk material inlet and a bulk material outlet; a gas permeable flow bottom disposed in a lower portion of said container, said flow bottom being adapted to supply a charge of gas for creating a fluidised bed of said bulk material in said container above said flow 2 ~
bottom; and means for injecting high pressure gas into said container above said flow bottom, said means for injecting high pressure gas comprising at least one nozzle adapted to direct a flow of high pressure gas into said fluidised bed of bulk material, so that turbulence within said fluidised bed is created such that the grains of said bulk material are subjected to frictional stresses while minimising impact stresses, whereby to clean said bulk material without excessive grain destruction.
The process of the invention provides that the bulk materials to be cleaned or final-sleaned are charged with an high pressure gas flow and placed by this gas charge into a turbulent condition in such a way that cleaning occurs solely at fixed or moving walls by friction or scouring (surface) stress of the grains while avoiding impact or rebound stresses. The device of the invention is characterized by at least one nozzle located above the flow bottom, whose pressure gas flow is directed into the interior of the fluidized bed in which the bulk materials to be cleaned or final-cleaned are located.
Accordingly, the invention proposes that, as a most advantageous supplement to thermal cleaning as per DE-OS 40 08 849 (see, in particular, the technology described in example 2 described therein), the bulk materials to be cleaned or final-cleaned are placed in a fluidized bed condition, as is explained in greater detail below.
It should be noted in this respect that the sound velocity of the gas in the nozzle is attained with a supply pressure at the nozzles of approx. 2.3 bar abs. ~with a fluidized bed counter-pressure of approx. 1.15 bar abs. and a nozzle diameter of approximately 3.4 mm) and that therefore as the supply pressure is increased, the flow velocity remains constant, while the inflowing gas volume (and thus the impulse) rises (in spite of this).
It is of special importance here that, in contrast to the above-described state of the art processes, the dwell time, during which the scouring effects in the grains are 2;~

applied, can practically be extended at will. The result is that the process according to the invention can attain the preferred type of mechanical sand regeneration - exclusively by abrasion of the adhering binder shells or other contaminations.
In the device of the invention, the nozzles may be mutually offset in height and/or laterally, so that sand flows which are accelerated by counter-current air jets cannot collide.
In addition, the process of the invention also offers the possibility of conducting the cleaning process under heat. This permits the thermal removal of organic components, e.g. from remnants of plastic bonding agents, during the mechanical cleaning cycle. In this way, it is also possible to burn out, and thereby reduce to harmless residual materials, organic compounds which contaminate the dust which is removed from the fluidized bed with the exhaust gas. Thus, with this embodiment, the process of the invention offers the possibility of combining the thermal and mechanical steps of a thorough sand regeneration into one single step.
This economically very positive possibility is further supplemented by several even more significant advantages. It should be especially noted here that the fluidized bed obeys its own laws, which deviate from those of pneumatic conveying. This results in a considerably more favourable operating mode, since only relatively low energy expenditure is required for the fluidization of solids.
Likewise, very little energy suffices for the air cross currents in order to produce the above-described flow phenomena in the fluidized bed. The sum-total of the energy expenditure of the invention's process amounts to only approximately 20~ of the energy consumption required for conventional pneumatic processes. These economic advantages are supplemented by a large yield of qualitatively high-grade sand regenerate with low grain losses.
However, the more important gain can be found in the reduced grain destruction and consequently the reduced 8 ~

costs for transportation, new sand, and disposal.
Approximately 15% (by weight) less grain is destroyed, as compared to conventional processes. The average grain size per share >0.09 is reduced during cleaning by about 10% during one hour of cleaning.
In a preferred embodiment of the process the pressure gas flow is introduced on/into the bulk materials to be cleaned through at least o~e charging point. Preferably the (at least) one pressure gas charging point is located in the peripheral area of the bulk materials to be cleaned, but it may also be located in the interior of the bulk materials.
In addition, it has been found to be practical to place essentially opposite from at least one first pressure gas charging point at least one second pressure gas charging point. This creates the result - possibly at several points or generated by several "casual" points - that the bulk materials to be cleaned are located within the range of (at least) two essentially mutually opposite gas flows and is consequently cleaned in a particularly intensive manner. A
precisely dosed impact effect can be superimposed on the friction or scouring effects between the grains (rather than at the walls or components) by selecting the fluidization condition of the bed (i.e. more or less aerated), and the gas jet impulse (i.e. supply pressure at the nozzle) only in accordance with varying gas penetration depth (into the bed) required.
In an embodiment of this invention, it also proved to be especially practical to direct the pressure gas from at least one pressure gas charging point essentially tangentially onto the outer surface of the bulk materials to be cleaned, with the result that (especially if this is done at two or more points) the bulk materials to be cleaned can be put into a rotational motion which positively affects the desired cleaning, while the friction effect can be increased at heretofore suitable abrasion walls or components.
Furthermore, it has proven highly practical if the charging intensity of at least one (preferably all) pressure 2 0 ~ 4 gas charging points is ad~ustable, so that pressure charging may be optimally adapted to each respective technological problem. Preferably compressed air is used for the charging gas, which not only solves the problem under discussion in a most practical manner, but is also available, or can be made available, cheaply.
Accordingly, it is preferred in the device of the invention to place at least a second nozzle on a side of the fluidized bed (or the bulk materials to be cleaned) which is opposite at least one first nozzle, and the flow intensity of at least one (preferably all) nozzle is preferably adjustable, i.e. can be controlled or regulated.
As already discussed above with regard to the process of the invention, several nozzles can preferably be provided in the device of the invention, which nozzles are preferably arranged in pairs located essentially opposite one another, in such a manner that the central axes of the nozzles are parallel, angled to one another, or are located on a common line.
Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which:
Figure 1 is a diagrammatic representation, partially in cross-section, of a device according to the invention; and Figure 2 is a schematic diagram which serves to illustrate the distribution of the pressure gas flow.

Figure 1 illustrates a cleaning device 1 for the mechanical final-cleaning of thermally pre-regenerated used foundry sand 2, with a bulk materials inlet (not shown in detail) and a container 3 equipped with a bulk materials outlet. The container 3 can be essentially rectangular or conical with respect to its vertical axis of symmetry 4.
High pressure gas is injected into the cleaning device 1 through supply lines 6 and/or 12 (as will be described in greater detail below) so as to form a fluidized 2 ~

bed of the used foundry sand 2. The high pressure gas then rises upwards through the container 3 into the upper section 3' of the container 3. The gas subsequently exits in the upward direction and finally leaves the cleaning device 1 as shown by arrow 15.

In order to create a fluidized bed in the area of the used foundry sand 2 (which is to be final-cleaned), the container 3 is equipped with a gas-permeable flow bottom 5, which consists, for example, of a ball fill, and which is charged from below via a supply line 6 from a compressed gas source (not shown) with fluidizing gas. The gas may, for example be composed of air.
Above the flow bottom 5, nozzles 9 are located in the area of the fluidized bed 8. The nozzles are located in pairs opposite one another, each pair being fed via a common throttling device 11 from a pressure gas source (not shown).
In particular, the throttling device 11 is located in a compressed air feeder line 12 and feeds a circular line 13, which is connected with the nozzles 9. The compressed air feeder line 12, and thus the circular line 13, may be connected with a single throttle device 11, so that the charging intensity of the used foundry sand 2 can be adjusted uniformly at all nozzles 9 simultaneously. Alternatively, as illustrated in Figure 2, several secondary compressed air feeder lines 12" may branch off from the compressed air feeder line 12'. In each of the secondary lines 12", a respective throttling device 11 can be located, so that the charging intensity of the used foundry sand 2 can be adjusted variably corresponding to different zones of the fluidized bed.
Clearly, a corresponding adjustment possibility does not only have to be provided in different zones of the fluidized bed (as shown in Figure 2) but any zone may also be provided with a variable charging intensity by equipping each of the nozzles 9 with a respective throttling device 11.
It must be pointed out that the container 3 does not necessarily need to have a rectangular/conical cross-section, but that the latter may also be circular, for example. A circular cross-section of the container 3 (at least in the area of the fluidized bed 8) is highly practical especially if at least some of the nozzles 9 are placed in such a way that the bulk materials 2 (or the fluidized bed 8) are to be set rotating during operation. In particular, rotation about the rotationally symmetrical axis 4 of the container 3 may frequently be particularly advantageous.
It may also be pointed out that in other embodiments of the invention, partitioning walls can be placed in the container 3, beginning in the stabilizing chamber 3' and extending to just above the flow bottom 5 of the fluidizing bed 8. These partitioning walls can be used to divide the open interior of the container 3 into two or more sectors, thereby preventing the re-mixing of largely cleansed bulk materials with freshly added material. This is equivalent to placing several fluidized beds in series.
In the case of fluidized beds with circular cross-section, partitioning may be achieved with concentric sectors or ~ith fluidized beds located on top of one another.
It may be further mentioned that the discharge diameter of the nozzles 9 in the above embodiment is 3.4 mm.
However, it will be readily apparent to those skilled in the art that the nozzles can be smaller as well as larger.
The container 3 may be given a continuous rubber lining 14 in the area of the fluidized bed 8 which serves to reduce impact-induced erosion of the interior surfaces of the cleaning device 1. Furthermore, one or more heat exchanges (not shown) can be located in the upper area of the fluidized bed 8, above the nozzles of the container 3, whereby reusable or further usable heat can be recovered from the final cleaning process, particularly, if the final-cleaning fluidized bed is connected to a thermal fluidized bed.

Claims (22)

1. A process for mechanically cleaning the grain surfaces of contaminated granular bulk materials, wherein said process comprises charging said bulk materials with a flow of high-pressure gas so as to form a fluidized bed of said bulk materials, said high pressure gas flow being adapted to cause regions of turbulence within the fluidized bed wherein the surfaces of said grains of said bulk materials are subjected to frictional stresses while minimising impact stresses, whereby said bulk material is cleaned without excessive grain destruction.

2. A process according to claim 1, wherein the high pressure gas flow is introduced into said bulk material to be cleaned at at least one charging point.

3. A process according to claim 2, wherein at least one pressure gas charging point is positioned in the peripheral portion of the location of said bulk material to be cleaned.

4. A process according to claim 2, wherein a second pressure gas charging point is located opposite a respective one of said at least one first pressure gas charging point.

5. A process according to claim 1, wherein the high pressure gas is directed essentially tangentially into the peripheral region of said bulk materials to be cleaned from at least at one pressure gas charging point.

6. A process according claim 1 wherein said high pressure gas is compressed air.

7. A process according claim 1 wherein the intensity of the compressed gas charge is adjustable.

8. A process as claimed in claim 1, wherein said contaminated bulk material is used foundry sand.

9. A device for mechanically cleaning the grain surfaces of contaminated granular bulk materials, said device comprising:
a container having a bulk material inlet and a bulk material outlet;
a gas permeable flow bottom disposed in a lower portion of said container, said flow bottom being adapted to supply a charge of gas for creating a fluidised bed of said bulk material in said container above said flow bottom; and means for injecting high pressure gas into said container above said flow bottom, said means for injecting high pressure gas comprising at least one nozzle adapted to direct a flow of high pressure gas into said fluidised bed of bulk material, so that turbulence within said fluidised bed is created such that the grains of said bulk material are subjected to frictional stresses while minimising impact stresses, whereby to clean said bulk material without excessive grain destruction.

10. A device according to claim 9, wherein said means for injecting high pressure gas comprises at least one pair of nozzles disposed on opposite sides of said container in the region of said fluidized bed.

11. A device according to claim 9, wherein the flow intensity of at least one nozzle is adjustable.

12. A device according to claim 9, wherein the flow intensity of all nozzles is adjustable.

13. A device according to claim 10, wherein the center lines of respective ones of each of said pairs of nozzles are parallel to each other.

14. A device according to claim 9, wherein said gas permeable flow bottom comprises ball fill.

15. A device according to claim 9, wherein said container has an essentially circular cross-section at least in the area of the fluidized bed.

16. A device according to any of claims 9 to 15, wherein at least some of said nozzles are located in such a way that the bulk materials to be cleaned are set rotating during operation.

17. A device according to any of claims 9 to 15, wherein the container has a rectangular cross-section, at least in the region of the fluidized bed.

18. A device according to any of claims 9 to 15, further comprising dividing walls located at least in the region of the fluidized bed, which dividing walls subdivide the open interior of said container into at least two sections.

19. A device according to any of claims 9 to 15, wherein the diameter of the nozzles is in the range of approximately 1 to 4 mm.

20. A device according to claim 9, wherein the container is lined with a resilient buffering material, at least in the area of the fluidised bed.

21. A device according to claim 20, wherein said resilient buffering material is rubber.

22. A device according to any of claims 9 to 15, further comprising at least one heat exchanger adapted to remove heat from the region above the fluidised bed.