EP3093082B1 - Method for preparation and reuse of used foundry sand - Google Patents

Description

The invention relates to a process for the preparation and reuse of foundry sands, wherein the used sands are subjected to mechanical pulping by comminution and subsequently to a classification.

The foundry industry is one of the most important sub-sectors of the metal industry in Germany. Their economic importance stems mainly from the fact that the products of the industry are important supplies for other industries. Also in Saxony and Saxony-Anhalt, the foundry industry is of great importance for the local business location. In 2013, around 4000 employees generated a total turnover of 2.5 billion euros in 40 foundry companies. (Source: Statistisches Landesamt Sachsen).
In foundries, the local production process generates considerable amounts of contaminated sand. The reason for this is the process of the "lost form". In this process, the Gießformen- and cores are formed from sand, which are stabilized by means of binders for the duration of the casting process and then destroyed again. Compared to the amount of new sand used, the material recycling (regeneration) of used sand in the foundry process is currently a shadowy existence. The internal regeneration processes used by individual foundries are usually uneconomical, energy-consuming and often do not provide the desired sand quality.
The handling of only slightly contaminated core residual sand is uneconomical, since it is usually disposed of together with the heavily contaminated molding sand.
The disposal of used sand is associated with high costs, environmental pollution (CO2 emissions from transport) as well as environmental risks (in the case of improper landfill). The question of long-term available capacities for securing landfill of used sand is also unclear. The demand for new and ever larger landfills is growing steadily - and with it the associated environmental impact. The rising costs for the disposal of used sand as well as the transport costs for the delivery of new sand make the economic operation of the foundries based in Saxony and Saxony-Anhalt difficult.

Another significant complication of the current economic situation of the foundries is the so-called replacement building materials regulation of the Federal Ministry for Environment, nature conservation and reactor safety. The subject of this ordinance is the regulation of mineral substitute building materials in production, which includes, among other things, a tightening of the required material standards. According to the Federal Association of German Foundries (BDG-Umwelttag - 16.05.2013), the Substitute Building Substances Ordinance will increase disposal and landfill costs by up to approx. 250%.
Looking at the current state of the art in the field of GAS regeneration, one finds a wide range of mechanical and thermal plant concepts.
The effectiveness of these plant concepts in terms of energy use and product quality diverges greatly. Common to all previous approaches that an economic operation (compared to the purchase and use of new sand) is not given.

A process is known for the recycling of mineral wastes of various compositions which, after comminution to a particle size of <3 mm, are delivered as bulk material to cement plants ( DE 198 15 205 A1 ).

From the DE 38 25 361 A1 a method and a plant for the regeneration of foundry sand mixtures is known, wherein in a first regeneration step, the mixture mechanically and / or pneumatically, then thermally regenerated in a second regeneration step and then mechanically and / or pneumatically in a third step.

In the DE 31 03 030 C2 describes a method for recovering foundry sand from used used sand, wherein the used foundry sand successively passes through a magnetic separation zone, a thermal treatment zone and a final purification zone and the cleaning by countercurrent impact at air velocities of 30 to 50 m / s.

The DE 43 21 296 A1 describes a method and a plant for the wet regeneration of contaminated with impurities and pollutants granular bulk materials, especially clay bonded foundry sands, the bulk materials are subjected to mechanical pretreatment of at least one stage combination of Attritionsreinigung with the addition of water and a subsequent classification and the fines of the Suspension are subjected to a cross-flow filtration.

• Zerkleinerung der Kerne unter Bildung von Gießerei-Altsand und(oder Altsand
• Thermischer Aufbereitung durch Ausbrennen der organischen Binder und Bestandteile
• Nasschemische Aufbereitung durch Auswaschen des anorganischen Binders mit einer Waschlösung
• Abtrennung der Waschlösung und des in der Waschlösung gelösten und/oder suspendierten anorganischen Binders
• Trocknung und Klassierung des regenerierten Gießereisandes unter Bildung von Regeneratsand.

The DE 10 2007 008 104 A1 also describes a process for the regeneration of foundry sands, in which case inorganic or organically bound core sands of a combination of thermal and wet-chemical processing with the steps

• Crushing of the cores to form foundry used sand and (or old sand
• Thermal treatment by burning out the organic binders and components
• Wet-chemical preparation by washing out the inorganic binder with a washing solution
• Separation of the wash solution and the inorganic binder dissolved and / or suspended in the wash solution
• Drying and classification of the regenerated foundry sand to form regenerated sand.

The DE 42 37 838 A1 again describes a method and an apparatus for regenerating foundry sand, wherein in a sub-step of the overall process, a method is described, with which the binder coats are to be removed from the sand particles by friction. For this purpose, the material is presumably forcibly guided through the process space via a plug screw (not described in more detail in the patent) and thus, given a given geometry, predetermines the residence time of the pretreated used sand in the process space. The material passes through a friction gap between the stationary container of the process chamber and above the bottom "Reibarmen" which rotate centrally in the container and reach up to the inner wall of the container (no precise definition and / or description of the friction and the Mahlspalt). Due to the presumed positive guidance, in addition to the frictional stress, a partial compressive stress can also be assumed. This process is similar to a known agitator ball mill in which the charged material is forcibly pumped through a dense ball packing agitated by an agitator to achieve very high comminuting intensities. For this reason, it can be assumed that the procedure described is a very intensive stress, which goes beyond the claims for a disagglomeration task (to remove sand particles from binder shell), and therefore noticeably comminutes primary particles of the quartz sand fraction. Unfortunately, it is not clear how large the "grinding gap" to the inner wall of the container exactly and how the machine is constructed in detail. Furthermore, no information is provided on the process parameters (rotor speed: slow or high-speed, throughput, etc.). That is why in this method and the device an accurate statement on the crushing intensity and rate of the machine so not possible.

The material is sucked in negative pressure on the transport medium air through the crushing unit disc mill and passes through the grinding path from the inside out and is guided by fluted crushing discs (rotor-stator principle), which for further influencing the stress intensity over a distance to each other (Mahlspalt ) can be adjusted.

From the DE 21 59 092 A1 For example, a method and an apparatus is known, wherein the method described here is based on a cooling of the starting material in a cooling device. It is used for the simple processing of foundry sand particles inside the foundry.
The pneumatic crushing takes place in a container with baffles on different floors without the use of different mills in successive stages. Simple pneumatic impact crushing destroys a significant amount of primary material, making re-use impossible. This makes this method less effective.

In the US 4,050,635 A1 it is obviously a ball or general tube mill, where the sand between the balls of a rotating tube packing is worked up in the cataract or cascade principle. The binder sleeves are to be processed by friction between the balls. Even with this method, a significant amount of primary material is destroyed so that it can not be meaningfully reused.

The US 2012/0000997 A1 describes an apparatus for separating binders from foundry sand by means of ultrasonic waves, wherein the surface is broken up by cracking, the sand is then comminuted, washed, dried and sieved. The basic technology corresponds to a conventional refining process. No information is given on the concentration of solids or other process parameters. The sand is first placed in some form of dosing unit and wet pre-crushed in a crusher (not described in detail) and then introduced with a vacuum pump via a sieve unit in an ultrasonic apparatus. Subsequently, the sand treated therein is dehumidified and separated into coarse and fine components in a filter. Thereafter, the transport takes place in a designated as crushing unit and not described in more detail process space, where the binder should be crushed again. Following this mixture is again passed through a sieve.

The entire process is cumbersome, technically complex and connected by the wet processing with an expensive drying process. At no point will it be further discussed which constituents of the foundry sand (foundry sand or core sand, mixture thereof or special sand) are used.

The invention has for its object to perform a determination of the optimum operating point of the regeneration plant, taking into account maximum Regenerat quality of various Ausgangsande while reducing the amounts of waste and waste. It is intended to achieve a deagglomeration of the binder shells from the sand particles for complete digestion of the abandoned used sand fraction with a commercial comminution method by varying the possible process parameters.

• in einer ersten Zerkleinerungsstufe der Gießereialtsand mittels einer Hammermühle desagglomeriert und teilweise marginal mechanisch vom Binder getrennt wird,
• in einer zweiten Zerkleinerungsstufe die vereinzelt vorliegenden Fromsandpartikel mittels einer Scheibenmühle weitgehend von ihren Binderhüllen befreit werden und
• in einer dritten Zerkleinerungsstufe die restlichen Binderhüllen mittels einer Stiftmühle entfernt werden,

der aufgeschlossene Gießereialtsand anschließend einer Klassierung unterzogen wird, wobei das Grobgut als Regenerat oder Recyclat in eine Lagereinheit transportiert und das Feingut einer Entsorgung und/oder Deponierung zugeführt wird.This is inventively achieved in that
the foundry sand is subjected to a multi-stage pulping grinding, wherein

• in a first crushing stage, the foundry sand is deagglomerated by means of a hammer mill and partially mechanically separated from the binder,
• in a second crushing stage, the isolated sand particles are largely freed from their binder casings by means of a disk mill and
• in a third crushing stage the remaining binder sleeves are removed by means of a pin mill,

the digested foundry sand is subsequently subjected to a classification, wherein the coarse material is transported as regenerated or recycled material in a storage unit and the fines are sent for disposal and / or landfill.

The foundry sand can be fed to a preclassification before the multi-stage pulping grinding.

It is passed through a bulk material receiving station and an intermediate storage facility prior to pre-classification.

To preclassify a Schwingsiebmaschine is used.

The screen overflow is fed as coarse material to a color sorting system, wherein a separation into dark sand and tubers is performed.

The dark tubers are fed to the multi-stage pulping process, while the bright core sand tubers are fed to a landfill.

In the first crushing stage, a hammer mill is used, followed by a disc mill in the second crushing stage, followed by a pin mill in the third crushing stage.

The classification is performed by a stream classifying method using air as a medium. It is based on the principle of cross flow classification with superimposed countercurrent classification.

For classification, a zig-zag sifter with cascading in several classifier stages is preferably used, wherein the screening process is performed by means of gravity or centrifugal force direction.

In the case of a gravity direction, the coarse material is transported as recyclate via a trigger member in a storage unit or further processed directly. The fine material is fed in this case to a solids separator, preferably a cyclone separator.

The separated fines are fed via a discharge organ of an encapsulated container loading. It is preferably the task of the goods to be sighted in the upper or lower third of the safety cascade.

The coarse material is examined for loss on ignition, with a loss on ignition of 0.2 to 0.7%, preferably 0.5% of the starting material, a thermal aftertreatment is performed.

The thermal aftertreatment is carried out in a fluidized bed furnace or a rotary kiln.

A dedusting of the sand fraction is carried out after the thermal aftertreatment, wherein the coarse fraction (> 70 to 150 microns) used as a recyclate and the fines (<70 microns to 150 microns) a disposal and / or landfill, preferably as a higher loaded mono section is supplied.

Advantages of the invention:

• Production of a product sand fraction from foundry sands generally usable for all foundries
• First opportunity to offer a complete sand cycle to foundries as a package solution
• Due to a special process concept, very careful treatment and separation into the desired components while obtaining large quantities of reusable sand components and relatively small amounts of landfill
• Significant reduction in disposal costs for foundry sands
• Increase the profitability of foundries
• Reduction of landfill costs through the possibility of disposal in a less expensive landfill class
• Thermal treatment only necessary in specific cases
• Low processing costs of used sand

embodiment

The invention will be explained in more detail with reference to an embodiment.

• Figur 1 - das Verfahrensschema der Aufbereitungsanlage

Showing:

• FIG. 1 - The process scheme of the treatment plant

For recycling, the used sand in seizure condition (1) is pre-classified in a first optional process stage (2). For this purpose, the material flow through a bulk material receiving station and an intermediate storage, as a buffer, passed to a vibrating screen (2) and classified at a mesh size of 800 microns. To protect the Klassierdecks still a protective deck is provided with a mesh size of 5 mm above it. The so-called sieve overflow (2a) is fed to a further optional process stage for sorting the used sand tubers.

Here, the Formsandknollen (dark) and Kernsandknollen (Hell) on a so-called. Color Sorting (3) in bright (3a) and dark (3c) tubers are separated. The interfering component, preferably the core sand (3a), is thereby levered out of the flow of material, for example via compressed air nozzles, and sent for disposal on a landfill (5). The dark nodular tubers (3c) are fed back to the material flow of the lower filter undercurrent (2b) and move together into the next process stage.
In this, the formaldehyde is once deagglomerated in a multi-stage pulping grinding (4) and then gradually freed from the enclosing the quartz sand grains envelopes of binder. The casings are chamotte casings around the quartz grains of a bentonite bonded foundry sand due to high thermal load during casting. This degree of influence on the binding behavior of the molding sand is referred to as oolitization.
In a first crushing stage, for example a hammer mill, the used sand to be processed, predominantly foundry sand, is deagglomerated and partially marginally mechanically separated from the binder. Responsible for this are, for example, in a hammer mill, the crushing hammers which straighten up as a result of the rotating shaft and coarsely crush the used sand particles and agglomerates by impact and impact. The residence time is specified during this process by a sieve in the grinding path of the mill. Other influencing factors are the speed of the rotor and the material throughput. The crushing success is known to increase with increasing speed to a maximum possible. In combination with a maximum possible throughput through a so-called oversupply at the technical limit of the mill and a critical speed optimal deagglomeration is adjustable.
In the next size reduction step, the isolated sand particles are freed from their binder sheaths (chamotte sheaths). For this purpose, the material passes through a so-called disc mill. In this mill, the centrally deposited material is transported to the outside and must pass through a rotor-stator system. This consists of both sides of corrugated surfaces whose distance and thus the resulting grinding gap is adjustable. Further parameters for the regulation of the comminution intensity are the speed and the material throughput. Background of this process stage is to work out an optimum gap size, rotor speed and material throughput, in order to achieve as gentle as possible grinding of the binder shells of the quartz grains, without destroying the quartz grains per se.
After the second process stage, a further crushing stage is provided by means of a so-called pin mill, wherein a rotor-stator system with concentrically mounted pins, which centrally introduced into the process space and claimed Material claimed by impact and impact and between the in operation past each other passing pins, a possible remainder of the binder sleeves is removable. For this purpose, the focus on the process parameters is again on the right combination of rotor speed, material throughput and pin geometry. To view the pin geometry and arrangement as a constant, the choice for a gentle grinding of the remaining binder sleeves on a minimum rotor speed at the same time as possible throughput. As with the first two digestion reduction stages, the right combination of parameters is dependent on the material system and is empirically adjusted.

Following the multi-stage disintegration comminution (4), the digested used sand is separated from the mechanically separated contaminations in a classification plant (5). The separating cut should be selected so that the usable sand fraction is separated as completely as possible from the interfering components in the fine fraction. Due to the properties of the interfering components after the digestion, the logical choice of the separation process results in a stream classifying process in the transport medium air, since the physical properties of the components that intervene for such a process. The quartz sand as a component to be recovered has a true density of 2.6 t / m ³ and, with the reusable particle sizes, lies in the range between about 100 and about 500 μm. The chamotte shells have a microporous structure, their density is much lower (p = 1.80 t / m ³ ) than that of the quartz sand, and can be separated well after mechanical splitting of the quartz grains in the fine fraction (<150 microns). The lustrous carbon still contained in the molding sand can also be deposited very well in the fine fraction with a true density of 1.35 t / m ³ and its primary particle size present.
The abandoned bulk material is separated according to the separation property particle size. The separation is effected by the forces acting on the individual particles in the process space (field force versus the flow force) according to the principle of cross flow classification with superimposed countercurrent classification. This can be embodied, for example, as a so-called zig-zag sifter, which represents a cascading of several sifter stages. Background of the cascading is a maximization of the selectivity depending on the abandoned bulk material. Such a classifier can be designed as a so-called gravitational classifier or centrifugal classifier. With regard to the separation follows, depending on the design, the so-called Sinkgut or coarse material of the field force and the so-called climbing material or fines of the transport fluid flow. This resistance of the flow is in direct Correlation with the flow velocity and particle shape (inflow surface) as variable parameters.
In the case of a gravity separator, this would mean that the coarse fraction with particle sizes greater than the resulting separating cut is discharged downwards as sinking material and the fine material upwards as rising material. The debris is then transported to a storage unit or further processed directly via a suitable extraction device (eg belt conveyor) for further processing. The climbing material follows the flow upwards and is fed via the subsequent pipeline system a Feststoffabscheideeinheit. Such a separator may be, for example, a cyclone separator. In this case, the solid contained in the flow is discharged as a result of the acting downward potential vortex flow down and fed, for example via a discharge organ of an encapsulated container loading. The cleaned air stream is deflected by the constriction in the lower part of the cyclone and leaves the process space in an upward eddy flow upwards. The air is basically generated by a corresponding fan (eg centrifugal fan) which operates in suction or pressure mode. Furthermore, with regard to the process air guidance, it is possible to drive the classifying system as a whole in a closed circuit or in an open system. The advantage of a closed mode of operation is that one obtains a more even flow in the sifting chamber of the sifter, which in turn has a positive effect on the selectivity. The open system implies the acceptance of false air at the non-encapsulated sites, such as B. Sinkgutauslass and material task classifier. In this case, flow fluctuations must be expected, which can have a negative effect on the separation. Further material-related parameters influencing the classification process are the particle density and particle shape.
With regard to the particle density, the influence is to be understood in terms of so-called simultaneity classes. For example, two different materials to be separated with spherical or spherical-like particle shape and different particle sizes (light matter: larger particles and heavy particles: smaller particles) at different particle densities have the same mass and are each highly likely to be discharged in the unintended direction.
The particle shape is influenced by the drag force of the flow as a function of the flow area of the process fluid. For an ideal separation according to the particle size would have to be ideal spherical particles, the smallest particles of the sinking material must be specifically heavier than the largest particles of the climbing material.

In the case of old sand classification, ball-like particles are assumed, which may result in the following phenomenon. Depending on the load on the solids, the interaction of material and air throughput, and the possible dispersion of the feed material in the sifter, miscleaning in both directions occurs during classification in the sifter.
In order to realize an optimization in this connection, according to the invention cascaded, as already mentioned, by means of several classifier stages in order to achieve a prevention of this phenomenon. The exact geometry of the step and number of stages is to be determined empirically based on the nature of the fraction to be separated. In this context, it is essential according to the invention to determine the manner in which the material reaches the process space. This should ideally be realized for an effective separation process across the entire width of the process space of the sifter to obtain maximum dispersion. The width of the process space results from the total throughput of solids. In the case of a closed system, the task can be realized, for example, effectively with a rotary valve as a pass-gate.
Furthermore, according to the invention essential for optimum operation of the plant is in the case of using a gravity zig-zag sifter, the position of the feed point (English FEED). This can be done depending on the task in the upper or lower third or alternatively in the middle of the classifier cascade. From the point of view of an effective and economic separation, the two variants named first play a superordinate role. If now a maximum of clean detritus is in the foreground of the separation task, a task in the upper third can be achieved by exploiting the cascade-related increase in selectivity. Conversely, with the maximum purity separated material to be picked, the feeding point in the lower third of the gravity zigzag classifier should be selected.
The coarse material fraction (5a) between about 100 μm and about 500 μm resulting from the separation process of the classifier represents the recyclate, which can be used wholly or in portions as standard fraction (5b) in the foundry process. Should the material state of the coarse material fraction (5a) be insufficient as a recyclate with respect to its loss on ignition, the material may be given an optional thermal residual treatment (6) in order to finally convert the product sand fraction into a fully usable state. After dedusting (7) the thermally treated sand fraction under the same process conditions as in step (5a), the produced coarse fraction can be used as the recyclate (7a).

The separated fines (5c) and thus primarily the interfering contamination is disposed of (5d), e.g. as a higher loaded mono section fed. The separated after the thermal treatment fine dust (7b) is in this respect also supplied to a corresponding disposal.

List of reference numbers used

1

- Old sand in the seizure state

2

- pre-classification

2a

- Screen overflow

2 B

- Screen underflow

3

- Color sorting

3a

- Kernsand tubers bright

3b

- landfill

3c

- Formsand tubers dark

4

- Multi-stage pulping

5

- Classification

5a

- Grobgutfraktion

5b

- Standard fraction

5c

- fine material

5d

- Disposal / dumping

6

- thermal residual treatment

7

- dedusting

7a

- Recyclate

7b

- Particulate matter

Claims (18)

1. A method of treating and reutilising used foundry sand, wherein the used sand is sent for mechanical disintegration through fragmentation and then classification
   characterised in that
   the used foundry sand undergoes multiple-stage fragmentation grinding (4), wherein

   • in a first fragmentation step the used foundry sand is disagglomerated by means of a hammer mill and partially marginally mechanically separated from the binder,

   • in a second fragmentation stage the individually present moulding sand particles are largely free of their binder shell by means of a disc mill and

   • in a third fragmentation stage the remaining binder shells are removed by means of a pin mill,

   the disagglomerated used foundry sand then undergoes classification (5), wherein the coarse material (5a) is transported to a storage unit as reclaimed or recycled material (7a) and the fine material (5c) is taken taken to a disposal facility and/or dump (5d).
2. The method according to claim 1, characterised in that before the multiple-stage fragmentation grinding (4) the used foundry sand is taken for pre-classification (2)
3. The method according to claim 2, characterised in that before pre-classification the material flow is channelled via a bulk material receiving station and intermediate store.
4. The method according to claim 2, characterised in that for pre-classification (2) a shaking screen machine is used.
5. The method according to claim 2 to 4, characterised in that a screen overflow (2a) is added as coarse material to a colour sorting system (3), wherein separation into dark moulding sand clumps (3c) and light core sand clumps (3a) is carried out.
6. The method according to claim 5, characterised in that the dark moulding sand clumps (3c) are supplied to the multiple-stage fragmentation grinder (4).
7. The method according to claim 5, characterised in that the light core sand clumps (3a) are transported to a dump (3b).
8. The method as denned in claim 1, characterised in that classification is carried out by means of a flow classification method using air as the medium.
9. The method as denned in claim 8, characterised in that classification is carried out in accordance with the cross flow classification principle with superimposed counterflow classification.
10. The method according to claim 9, characterised in that for classification a zig-zag sifter with cascading into several sifter stages is used.
11. The method according to claim 10, characterized in that the sifting process is carried out by means of gravity and centrifugal sifting.
12. The method according to claim 11, characterised in that in the case of gravity sifting the coarse material is transported as recycled material via an extraction element to a storage unit or undergoes direct further processing.
13. The method according to claim 11, characterised in that in the case of gravity sifting the fine material is supplied to a solids separator, preferably a cyclone separator.
14. The method according to claim 13, characterised in that the separated fine material is conveyed via an extraction element to an encapsulated container loading system.
15. The method-according to claim 10, characterised in that the addition of the material to be sifted is carried out in the upper or in the lower third of the sifter cascade.
16. The method according to claim 12, characterised in that the coarse material is examined for loss on ignition and in the case of a loss on ignition of 0.2 to 0.7 % of the initial material thermal after-treatment is carried out.
17. The method according to claim 16, characterized in that the thermal after-treatment (6) is carried out in fluidised bed combustion or a rotary kiln.
18. The method according to claim 17, characterised in that de-dusting of the sand fraction of the thermal after-treatment (6) is carried out, wherein the coarse fraction > 70 to 150 µm is used as recycled material (7a) and the fine material < 70 µm to 150 µm is transported to a disposal facility and/or dump (5d), preferably as as a higher contaminated monosection.

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