EP3164234B1

EP3164234B1 - Method of reclaiming the used foundry sand and equipment for its implementation

Info

Publication number: EP3164234B1
Application number: EP14752777.4A
Authority: EP
Inventors: Alois BURIAN
Original assignee: Sand Team Spol S RO
Current assignee: Sand Team Spol S RO
Priority date: 2014-07-03
Filing date: 2014-07-07
Publication date: 2019-09-11
Anticipated expiration: 2034-07-07
Also published as: EP3164234A1; WO2016000665A1; CZ305717B6; CZ2014462A3

Description

Field of the Invention

The invention relates to a method of reclamation of used foundry sands, especially with an inorganic geopolymer binder system, and equipment for its implementation, wherein the reclamation takes place in batches, so that friction elements located on two shafts rotate inside each dose of the used sand.

Background of the Invention

The castings are cast into moulds made of sand and a binder system, in order to create a solid mould for casting of the metal. After casting of the metal and producing of the casting is the used sand removed from the casting.
For ecological and economic reasons, it is necessary to reclaim the used sand for its use for the manufacture of other moulds. This prevents the pollution of the environment, improves the economy of foundries, saves the resources of new materials-sand, reduces the necessity of transport and protects the environment.
For the production of moulds are used binder systems on organic and inorganic basis. For economic and, especially, environmental reasons it is preferred to use binder system based on an inorganic binder for mould production. However, a certain disadvantage of these binder systems is that they are difficult to reclaim compared to systems with an organic binder system.
In general, the principle of reclaiming is in the removal of binder envelope of grains of the used sand. This already cured binder envelope prevents or impedes further use of the sand, because pollutants contained therein prevent the curing reaction from processing correctly during a new application.
Generally used systems for reclaiming foundry are based on mechanical, thermal and wet principle of reclaiming. Thermal and wet reclaiming is energy demanding and its use is limited. Therefore, the focus is on the mechanical reclaiming.
For reclaiming of inorganic binder systems it is generally known to use a dry centrifugal reclamation. The inorganic binder is mainly the sodium silicate. Dry centrifugal reclamation rubs off grain by the mutual rubbing of grain and alternatively by the friction elements.
There is a known method for reclaiming foundry sands, where two rotating disks rubbing the binder from the surface of grain are placed in the cylindrical container. The efficiency of such rubbing of the grains is low and the method can be used for reclaiming of sands which contain bentonite as a binder. The adhesion strength of the binder to the grain surface in such sands is low and therefore the grain envelope can be rubbed more easily. This method cannot be used for the inorganic binder of sodium silicate type with high adhesion strength to the grain surface which cannot be removed easily.
For foundry sands with sodium silicate is used a device which uses four rotating blades placed one above the other. The shaft is placed in two bearings and on the shaft are positioned wheels with attached blades. The device operates continuously. The efficiency of such rubbing method is low and limited new usability of such reclaimed sand is related to it. Typically, only use 50% of the reclaimed sand can be used for the preparation of new sand. To improve the reclaiming effect, it is therefore necessary to preheat the used sand.
EP 0343272 B1 discloses a process of reclaiming of used foundry sands, where the used foundry sand is in the first stage deprived by heat treatment of all carbon-containing materials, wherein the heat treatment is carried out at temperatures from 500 °C to 900 °C. After the heat treatment, the used sand is transferred into a cylindrical container. In the cylindrical container a shaft with rods is rotating which treats the grains of sand and rubs off the binder from their surface. The disadvantage of this method lies in the necessity of heating the used sand before reclamation and this fact strongly complicates the energetic and economic aspects of the reclamation.
The newly developed inorganic geopolymer binder systems do not require preheating of the used sand and can be easily reclaimed. The reclamation of such used sand with the geopolymer binder is considerably easier. The connective envelope can be rubbed without preheating. A cylindrical container with a shaft according to the patent EP 0343272 B1 could be used, but the rubbing efficiency is low.
Patent application EP 2666562 A1 discloses more suitable arrangement of a rotating shaft with the applicability of such reclaimed sand being higher than 60 %, in some cases higher than 85 %, and most preferably more than 95 %. The application discloses two shafts with rotating elements in a cylindrical container. The arrangement of a container with a cylindrical cross-section and two shafts arranged inside excludes a considerable portion of the volume ("a dead zone") of the reclaimed sand from the area reachable by the rotating elements, up to 50 % of the volume. The reclamation method therefore requires longer reclamation times, because the sand grains in the excluded volume do not move or benefit from the rubbing effect. Equally, the fact that the rotating elements within one shaft are arranged one above the other, rotated at a mutual angle of 0° and overlapping with rotating elements of the other shaft, decreases the rate of mixing of the reclaimed sand and prolongs the reclamation times.

Summary of the Invention

The aim of the invention is to improve the method of reclaiming used foundry sand, particularly using a geopolymer inorganic binder system, in which the reclamation occurs in batches so that impact bars placed on the two shafts rotate within each batch of the used sand. Another aim of the invention is to provide an equipment for the above-mentioned improved method.
The above-mentioned deficiencies are rectified by the use of an equipment for reclamation of used foundry sand, especially with an inorganic geopolymer binder system. The equipment comprises a container casing, in which shafts provided with impact bars are arranged at a certain distance between them. Spaced from the bottom of the container casing, there is a permeable bottom of a fluidized bed attached on a carrier. Further, the container casing is in its upper part provided with a dosing opening, wherein in its lower part the container casing has an unloading opening. Moreover, there is a pair of vertically oriented shafts arranged in the container casing, and there are cylindrical impact bars attached on the vertically oriented shafts. The inventive feature of the container casing has in its ground plan section the shape of an oval, and the inside of the container casing is provided with a wear-resistant lining.
Preferably, the container casing has in the ground plan section the shape of a flat oval.
In one embodiment of the invention, the rotating shafts with impact bars are arranged in the centres S₁ and S₂ of two semicircles of the flat oval of the fluidized bed.
Furthermore, the bottom of the fluidized bed can be porous, wherein the pore size is in the range from 0.05 to 0.10 mm, and it can be advantageously formed by a laminated porous material comprising a combination of wire screens, metal powders and metal foils.
In another embodiment, a dosing container placed on gauge scales is connected to an ending of the dosing opening.
In another embodiment, the impact bars are arranged in the shaft) one above the other and are rotated at an angle of 25° to 55° to one another.
The above-mentioned deficiencies are further rectified by the use of a method of reclamation of used foundry sand, especially with an inorganic geopolymer binder system, wherein the reclamation takes place in batches, so that, impact bars arranged on two shafts rotate within each batch of the used sand. The inventive feature of the method is performed by rotating impact bars in the foundry sand which is moving in a fluidized bed, wherein the ratio of the impact bars volume to the batch volume of the sand is in the range from 0.03 to 0.10. In addition, air flows through the used foundry sand at a velocity ranging from 1.5 to 4 times the threshold velocity of fluidization at which the fluidized bed of used sand is formed. The threshold velocity of fluidization for quartz sands is equal to 0.17 m/s.
To enhance the effect of the reclamation device, it is preferable to place the used foundry sand in the fluidized bed, the cross section of which has the shape of a flat oval where the width is determined by the radius of the circle r and the length is determined by the sum 2r+ a, where a is the distance between the centres of circles S₁ and S₂.
For further improvement of the reclamation effect it is preferable to place the foundry sand in the fluidized bed, the cross section of which has the distance between the centres of circles S₁ and S₂ smaller than twice the circle radius 2r.
The summary of the invention is an increased reclaiming effect, which is achieved by the rotation of the impact bars which are rotating in the used foundry sand placed in the fluidized bed formed by flowing air at a defined speed. The fluidized bed in which the reclamation occurs is made of grains of used sand and flowing air. Defined air velocity ensures optimal distribution of the grains in the fluidized bed, with an optimal porosity between individual grains. Porosity is defined by the ratio of the volume between grains and the entire volume of the fluidized bed. Suitable distribution of grains in the fluidized bed allows the rotating elements to optimally remove grain envelopes created from the cured, now inapplicable binder. Fluidized beds, adapted this way, with suitable porosity of fluidized layer allow besides the use of rubbing effect of bars also the use of the percussion effect. The percussion effect is caused by dynamic forces. The percussion effect manifests itself with rapid reduction of the binder envelope thickness on grains. By the combination of these effects emerges a process, which loosens the grain envelope more efficiently and in less time.
The loosened grain envelope which is made of an inapplicable cured binding agent has the form of fine dusty particles. These particles have to be immediately removed from the used sand. Otherwise, the electrostatic forces re-attach themselves to the surface and a loss of the reclamation effect occurs. Immediate removal of dust particles from the grain envelope is secured by optimal velocity of the air flowing through the fluidized bed between purified particles-grains of sand.
Percussion and rubbing effect of the rotating bars and the immediate removal of loose dusty parts from the grain envelope are associated with the porosity of the fluidized bed. Porosity is defined by the ratio of the volume of the pores in the layer and the volume of the whole fluid layer. It was proved that the optimal porosity in the fluidized bed ranges from 0.50 to 0.70. To secure the porosity within the optimal range an adjustment of the speed of the air flow can be used. Air velocity to provide an enhanced reclamation effect according to the invention ranges from 1.5 to 4 times the threshold velocity of the fluidization.
For quartz sands with medium grain size, the threshold velocity of fluidizing air responsible for formation of the fluidized bed is equal to 0.17 m/s.
It was also proved that a fluidized bed created according to the invention have other positive effects on reclamation of the foundry sand. The contact of rotating rods with the grains of the reclaimed foundry sand creates a collision and eliminates grain envelope and moreover, the sliding of grains on the surface of the rotating bars affects the grain surface. It causes not only the removal of mechanically adhered particles, but also the formation of the thin transformed layers directly on the quartz surface. The so called Beilby layer is being created, which has quite different characteristics differing from the original crystalline material. The surface of quartz grain is normally generated by the spatial binding of co-ordinational tetrahedrons (SiO₄)^4- and Si and O ions alternate on the surface having free valences and this way active centres for reaction with the binder are formed. These active centres of the common grains are "occupied" by foreign ions, water molecules or OH^- groups and therefore their activity is limited. In used sands, the active places are "occupied" with residues of the binder. If active places are covered with e.g. sodium ions from the binder, the interface grain - binder has a low binding energy. It was proved that in a fluidized bed under conditions according to the invention the surface of the grains activate, the number of active places increases. The depth of the deformational hardened Beilby layer is affected by the speed of impact and sliding and also by the temperature.
Such activated grain surface allows the increase of adhesion bonding forces between the grain surface and the binder. The consequence is the increase of the strength of sands. The result of such reclamation process is that the surface quality of the reclaimed sand is higher than the quality of the surface of new sand. It was demonstrated that the use of reclaimed sand prepared using the process according to the invention can consume much lower binder content than new sand.
According to another aspect of the invention, the increase in efficiency is achieved by the use of a more preferred form of the fluidized bed. While using the previously known cylindrical container with two shafts is the material during a substantial part of the container outside the rubbing effect of bars and reclamation efficiency is therefore low, and there is no effect on the activation of the grain surface. The space which is not affected by the action of bars comprises, in this case, 50% of the cross section of the cylindrical container. It can be observed that this often creates the so-called dead space, in which the regeneration of the grains does not occur.
The method of reclaiming foundry sands according to the invention uses a fluidized bed (in which impact bars move), which has in a ground plan section the shape of an oval, which is preferably flat (see Fig. 1). Fluidized bed shaped in this way has two axes of symmetry which intersect at the point S. The cross-section of the fluidized bed is determined by the width which is equal to the circle radius rand the length which is equal to the sum of 2r + a , where a is the distance between the centres of circles S₁ and S₂ as is evident from Fig. 1.
The reclamation method of foundry sands using a fluidized bed with the shape of a flat oval shows significantly higher efficiency. Contacts between grains and impact bars are more frequent. The grain envelope is removed earlier and by the contact of cleaned grains with bars occurs also an impact, the removal of the envelope from the grain surface and moreover, the sliding of grains along the surface of bars forms the Beilby layer and the surface of the grains activates. Such adapted bed is more efficient because in the case the distance a = 2r, the space that is not directly affected by the bars constitutes only 12% of the cross section of the fluidized bed. This does not create any dead zones without moving of sand grains.
A device adapted for performing the reclamation of foundry used sands according to the invention has a shaft with impact bars placed in the centre of circles S₁ and S₂.
Another method of improving the efficiency of reclamation of foundry sands is achieved by creating a fluidized bed in the same shape as in the previous case, but with the difference that the distance a of the shafts with impact bars moving in the fluidized bed is less than 2r, as seen in the fig. 2. This allows the overlapping of impact bars and significantly increases the effect of reclamation, wherein the bars arranged on both shafts are located at a different height so that they pass each other during rotation.
Another variant of the embodiment of the fluidized bed, resulting in the improvement of the reclamation effect, is the location of the rotating bars in the fluidized bed one above the other. According to this aspect of the invention are the rotating bards placed above each other in a helix. The bars move in a fluidized bed with a certain angular rotation. The grains are brought in a helical motion and are rubbing easier. It was demonstrated that appropriate rotation angle of the bars is 45 degrees (Fig. 3). The buoyancy of grains improves, removal of the grain envelope is more efficient and the engine load reduces.
The effect of the impact of the rotating bars causes the activation of grains. It was demonstrated that for an optimal effect of the activation it is preferable that the impact surface was in a certain ratio to the batch. It means that the volume of the rods is in a certain ratio to the volume of the fluidized bed. It was proved that the optimal activation effect is achieved if the ratio of the volume of the rotating rods to the volume of the used sand is in the range from 0.03 to 0.10. The result of such a reclamation is the ability to use 100 % reclaimed sand for preparing new sands. When preparing the sands it is only possible to use the reclaimed sand as an opening material and it is not necessary to use new sand. Moreover, this enhanced activating effect on the grain surface allows preparing the sand with the dosage lower than that of the sand with new sand. The benefit is saving the binder system and the subsequent easier reclamation due to the smaller amount of reacted binder in the used sand.
For achieving an optimal reclamation and activation effect it is necessary to maintain constant conditions in the fluidized bed. One of these conditions is a constant air flow through the fluidized layer. To achieve this, it is necessary to provide an even distribution of air across the entire layer and to maintain a constant air velocity throughout the reclamation cycle. Another condition is always dosing an appropriate quantity of used sand for creating of the fluidized bed.
In the even distribution of the air in the fluidized bed plays the bottom of the fluidized bed an important role. The bottom (grate) of the fluidized bed mechanically separates the fluidized layer from the space without particles, and also distributes the air throughout the cross section of the fluidized bed. Optimal conditions for the reclamation effect are achieved via another aspect of the present invention with the use of the bottom which ensures even distribution of air throughout the cross section of the fluidized bed and even distribution of the reclaimed particles of the sand throughout the volume of the fluidized bed. This effect is achieved by the use of the bottom that contains a large number of small pores for the passage of air. The bottom of the fluidized bed is formed by a laminated porous material made by a combination of wire screens, metal powders and metal foils, wherein the pore size is between 0.04 to 0.15 mm. Moreover, such a bottom is highly abrasion resistant and ensures a long durability.
Providing a constant air flow velocity through the fluidized layer is made possible, according to another aspect of the invention, by a device for performing the method according to the invention which comprises means-source of air which maintains the amount of air supplied to the reclamation's container at a constant value. A good source of air is the blower with frequency converter that allows the control of the amount of air supplied to the reclamation's container.
Keeping the size of the processed dose constant is provided, according to another aspect of the invention, by a device for performing the method according to the present invention, which includes a dosing container, which is placed on the gauge scales and thus allows accurate dosage of the used sand.

Description of the Drawings

The present invention will be further illustrated by the drawing in which the figure 1 shows a cross section of the fluidized bed, figure 2 shows a cross section of the fluidized bed with overlapping rotating rods, figure 3 shows the location of bars on the shaft with the angular rotation of 45 degrees and forming a helix, figure 4 shows the reclamation's container, figure 5 shows the bottom of the fluidized bed, figure 6 shows a dosing hopper on the gauge scales, figure 7 shows the firmness of sands containing 100 % new sand and 100 % reclaimed sand after 24 hours.

Description of the Preferred Embodiments

The invention will be explained in more detail by an example of its embodiments with reference to corresponding drawings. In these drawings, the technical solution is illustrated in the exemplary embodiment of an apparatus for performing the reclamation method of used foundry sand. It is obvious that the following descriptions are illustrative expression of application of the principles of the invention.
The method, by which the reclamation of used sand is performed, especially with an inorganic geopolymer binder system, involves the following steps. The reclamation occurs in batches, so that within each batch of used sand rotate impact bars arranged on two shafts. Reclamation is performed by turning the impact bars 3 in foundry sand moving in the fluidized bed, where the volume ratio of the impact bars 3 to the volume of the batch of the used sand is between 0.03 to 0.10, wherein the air passes through the layer of the used sand at a defined velocity, which varies from 1.5 to 4 times the fluidization velocity threshold, wherein the threshold speed of fluidization for the quartz sand is equal to 0.17 m/s.
The equipment for performing the method of reclamation of the used foundry sand with a geopolymer binder is shown in Figs 3, 4 and 5. As it is apparent from these figures, the equipment consists of a casing 1 of the container, which has in a ground plan shape of a flat oval, wherein the bottom 6 of the fluidized bed is arranged in a distance from the bottom of the container. The casing 1 of the container has in its upper part a dosing opening 8 , to which the dosing container 12 is connected. In its lower part has the casing 1 of the container unloading opening 9 , wherein the inner side of the container casing 1 is provided with a wear-resistant lining 11 . In the casing 1 of the container is further arranged a pair of vertically oriented shafts 2 , on which are attached cylindrical impact bars 3 . The distance between the axes of shafts 2 is a = 0.9* 2r, wherein the impact bars 3 on the shafts 2 are placed in such a way that they are rotated by 45 º and therefore form a rising helix, as shown in Fig. 3. Further are shafts 2 attached by means of clamping mechanisms 10 , including bearings and seals, in both, the bottom as well as in the cover of the container casing 1 . Shafts 2 are driven by engines 4 located alternately both, on the cover and on the bottom, of the container casing 1 , see Fig. 3. As seen from Fig. 5, the support structure of the carrier 5 for holding its own bottom 6 of the fluidized bed creates a frame in the shape of a flat oval with bearing stiffeners inside surrounding the cylindrical openings for shafts 2 . The thickness of the bottom 6 of the fluidized bed is 1.2 mm. The bottom 6 of the fluidized bed preferably consists of a laminated porous material which is a combination of wire screens, metal powder and metal foils, wherein the pore size is 0.060 mm, wherein the porous material is placed in the space between stiffeners.
Reclamation method according to the invention is as follows. To the internal space of the container casing 1 is positioned the batch of the sand from the dosing container 12 , see Fig. 6. The dosing container 12 is placed on the gauge scales 13, which secures a constant batch size of the used sand. The batch size is chosen so that the volume of impact bars 3 rotating in the fluidized bed comprises 6% of the batch of the used foundry sand, i.e. ratio of the volume of rotating bars 3 to the volume of the batch of the used sand is 0.06.
Into the internal space of the container casing 1 is through the porous bottom 6 , which is placed on a carrier 5 , supplied the air from the blower. Thus a fluidized bed is formed, in which shafts 2 rotate with impact bars 3 and operates on the reclaiming sand moving in suspension throughout the whole fluidized bed space. The blower is configured so that the air velocity in the fluidized bed is 0.28 m/s.
This way a perfect release of grain envelope from the grain surface and its removal from the used sand occurs. The result is granular material with properties more advantageous than the original new sand.
For comparative test sands containing silica sand were prepared (with round grain of the medium size d₅₀ = 0.27 mm) and geopolymer binder with gradually decreased content in the sand in the range from 1.2 to 1.8%. An ester hardener in an amount of 14 % per binder was always used for curing. Firmness of these sands after 24 hours is shown in figure 7.
Practical experience shows that for the production of high quality casting moulds it is necessary to use a sand which after 24 hours of spontaneous curing has firmness of 2,5 MPa.
This test shows that using the new opening material requires sand a dosage of 1.8 % to the binder for achieving the desired firmness.
This sand was reclaimed after the casting using the process according to the invention and the resulting reclaimed sand was used to prepare a new sand. It was demonstrated that the firmness of sands using 100 % of the reclaimed sand have significantly higher firmness. Firmness after 24 hours is higher with the reclaimed sand than with the new sand. The result is that the surface quality of the reclaimed sand is better than the quality of the surface of new sand. To achieve the required firmness, which corresponds to 2.5 MPa, it is sufficient to dose to the sand only 1.2 % of the binder, as is evident from figure 7.

Industrial Applicability

The method of reclamation of the used foundry sand and the device for its implementation can be used in the foundry industry and specifically for the production of castings in sand moulds by the reclamation of the foundry sand.

List of reference signs

1. Container casing
2. Shaft
3. Impact bar
4. Electric engine
5. Carrier of the bottom of the fluidized bed
6. Bottom of the fluidized bed
7. Space of the fluidized bed
8. Dosing opening
81. Ending
9. Unloading opening
10. Clamping mechanism with bearing and seal
11 .Wear-resistant lining
12. Dosing container
13. Gauge scales

Claims

An equipment for reclamation of used foundry sand, especially with an inorganic geopolymer binder system, the equipment comprising a container casing (1), in which shafts (2) are arranged at a certain distance between them and are provided with impact bars (3), wherein spaced from the bottom of the container casing (1) there is a permeable bottom (6) of a fluidized bed attached on a carrier (5), and further, the container casing (1) is in its upper part provided with a dosing opening (8), wherein in its lower part the container casing (1) has an unloading opening (9), and wherein in the container casing (1) there is arranged a pair of vertically oriented shafts (2) on which cylindrical impact bars (3) are attached, characterized in that the container casing (1) has in its ground plan section the shape of an oval, and the inside of the container casing (1) is provided with a wear-resistant lining (11).
The equipment for the reclamation method of used foundry sand according to claim 1, characterized in that the container casing (1) has in the ground plan section the shape of a flat oval.
The equipment for the reclamation method of used foundry sand according to claims 1 and 2, characterized in that the rotating shafts (2) with impact bars (3) are arranged in the centres S₁ and S₂ of two semicircles of the flat oval of the fluidized bed.
The equipment for reclamation method of used foundry sand according to claim 1, characterized in that the bottom (6) of the fluidized bed is porous, wherein the pore size is in the range from 0.05 to 0.10 mm, and it is formed by a laminated porous material comprising a combination of wire screens, metal powders and metal foils.
The equipment for reclamation method of used foundry sand according to claim 1, characterized in that a dosing container (12) placed on gauge scales (13) is connected to an ending (81) of the dosing opening (8).
The equipment for reclamation method of used foundry sand according to claim 1, characterized in that the impact bars (3) are arranged in the shaft (2) one above the other and are rotated at an angle of 25° to 55° to one another.
A method of reclamation of used foundry sand, especially with an inorganic geopolymer binder system, using the equipment according to claims 1 to 6, wherein the reclamation occurs in batches so that impact bars (3) located on two shafts (2) rotate inside each dose of the used sand, characterized in that the reclamation is done by rotation of the impact bars (3) in the used sand moving in a fluidized bed, wherein the volume ratio of impact bars (3) to the volume of the batch of the used sand is in the range from 0.03 to 0.10, wherein air flows through the used sand at a velocity ranging from 1.5 to 4 times the threshold velocity of fluidization at which the fluidized bed of used sand is formed.
A method of reclamation of used foundry sand using equipment according to claim 7, characterized in that for quartz sands the threshold velocity of fluidization is equal to 0.17 m/s.
The method of reclamation according to claim 7, characterized in that the reclamation takes place in the fluidized bed with its cross section having a flat oval shape and the width is determined by the circle radius r and the length is determined by the sum 2r + a, where a is the distance between the centres of circles S₁ and S₂.
The method of reclamation according to claim 7, characterized in that the reclamation takes place in the fluidized bed and its cross section has the distance a between the centres of circles S₁ and S₂ smaller than twice the radius of the circle 2r.