System and method for recovering foundry sand

Abstract

The invention relates to a system for recovering foundry sand comprising: a rotary drum open at its upper base for depositing the sand to be recovered therein, a cover which can adopt an open position for allowing passage through the upper base of the rotary drum, and a closed position for closing passage through said upper base, and means for generating electromagnetic radiation installed on the cover, such that when the cover is arranged closing the upper base of the rotary drum, the electromagnetic radiation generated is directed towards the inside of the rotary drum. This technique for recovering foundry sand based on the use of microwave energy successfully cleans the sand at great speed, preventing CO2 emissions, and with low energy consumption.

Description

OBJECT OF THE INVENTION

[0001] The present invention is comprised in the foundry sector corresponding to the valorization and implementation of environmental engineering systems, more specifically it consists of the development of valorization techniques for foundry sand originating from disposed molds once they have been used, melted, and removed.

[0002] More specifically, the present invention provides a technique for recovering foundry sand based on the use of microwave energy, which successfully cleans the sand at great speed, preventing CO2 emissions, and with low energy consumption.STATE OF THE ART

[0003] Foundry sand recovery is a challenge facing the metallurgical industry in the coming years in order to comply with new environmental legislations. Traditionally, it has been carried out by means of conventional mechanical and thermal methods. These methods have drawbacks such as high energy consumptions, processes that raise the temperature of the recycled sand, which then has to be cooled, generation of sand fines, long process times, among others, in addition to the generation of polluting emissions.

[0004] It should be noted that sand in molds has been shaped by adding a binder that allows binding sand grains together and giving shape to the mold. At present, there are different organic and inorganic, chemical or non-chemical sand binders (furanic, phenolic - acid, alkaline, urethane -, silicate - ester / CO2, bentonite, among others) with different percentages depending on the type of part, resistance, quality, characteristics of the sand, etc. This means that the system for recovering or cleaning this sand is adapted to each type of sand and binder.

[0005] Binder is the linker element for sand grains to achieve the binding thereof and the sufficient hardness that the mold must withstand in order to contain the broth that forms the part that is cast at high temperatures after melting. This binder material is the critical point that makes it necessary to apply the energy required to "separate" it from the base (silica sand, olivine, zirconium, or chromite and in general all types of sand used in foundry). The technology to enable separating said binder can be applied mechanically and / or thermally.

[0006] With respect to sand recovery systems, currently the various known processes and systems for treating sand after use thereof in foundry consist of: Mechanical: This process allows grains to collide and rub together so that the binder becomes detached. However, the cleaning is not completely effective and causes the grain to increase in size with respect to its original state. Thermal: This consists of heating the sand with gas while an air stream is generated in the furnace, burning the resin at a temperature above 700°C. The system must be complemented with a subsequent cooling to bring the temperatures below 30°C, otherwise the sand will clump together, will not flow for proper filling, and will not be moldable for shaping. Attrition: This system generates a more intense rubbing, which causes the silica grain to break and, therefore, a higher percentage of fines is generated, which corresponds to the finest particles (dust type) that are highly detrimental to the shaping of the mold and the quality of the cast part.

[0007] Foundry sand for molds must meet several essential physical characteristics to ensure the quality and precision of the cast parts, these characteristics being: Permeability: The ability of the sand to allow the passage of gases during the casting process, preventing defects in the final part. This is related to the size and shape of the grain, the amount of binder and the amount of fines in the mold. High temperature resistance: The sand must withstand the high temperatures of the molten metal without decomposing or melting. Low thermal expansion: It is crucial that the sand has low thermal expansion in order to maintain the dimensional precision of the mold during heating and cooling so that the actual dimensions of the part can be obtained. Compactability: The sand must be suitably compacted to form solid, stable molds that maintain their shape during the casting process. This characteristic is also defined by the grain morphology. Good surface finish: The sand must provide a smooth and uniform surface finish on the cast part, minimizing the need for subsequent machining.

[0008] The recovery methods described above, well known to one skilled in the art, require a large amount of energy to the process which is necessary to raise the temperature so that the binder can detach from the grain by means of melting or friction.

[0009] Therefore, there is a need in this technical field for foundry sand recovery techniques that operate with low energy consumption and high speed, preventing CO2 emissions. The speed of recovery allows foundry to have a smaller amount of sand accumulated in the silos (space), as well as reduced purchasing costs.DESCRIPTION OF THE INVENTION

[0010] The present invention proposes a technique for recovering foundry sand based on the use of energy from electromagnetic currents (microwaves). This technology offers a series of significant advantages, such as a drastic reduction in process time, lower energy consumption, heating according to the sand to be recovered, and greater efficiency in separating the sand, the binder and other added elements. Furthermore, the influence of the waves on the binder through selective heating allows maintaining its original properties and reducing the generation of fines. The electromagnetic current technology maintains the properties of the base (silica and other sand types) as it does not cause morphological changes in the structure of the base and thereby does not cause it to lose its properties.

[0011] More specifically, one aspect of the invention relates to a system for recovering foundry sand, comprising: a rotary drum open at its upper base for depositing the sand to be recovered therein, and a cover which can adopt an open position for allowing passage through the upper base of the rotary drum, and a closed position for closing passage through said upper base.

[0012] The system further comprises: means for generating electromagnetic radiation installed on the cover, such that when the cover is arranged closing the upper base of the rotary drum, the electromagnetic radiation generated is directed towards the inside of the rotary drum. Therefore, the means for generating electromagnetic radiation and the cover are arranged together having a rotational movement at one end of the cover, and allowing coupling with the static drum when it is closed, and for starting the sand recovery process, and with the opening thereof, the removal of the recovered sand.

[0013] The electromagnetic radiation induces excitation in the binder, causing the binder to heat up and allowing the separation thereof from the base grain that is in turn heated by heat transmission. In this way, a quick, efficient and clean recovery thereof is achieved compared to conventional methods.

[0014] The loaded sand does not need to be treated with either prior breakage or heating, nor does it need to be cooled after the operation since the working temperatures are below the working limits for reusing molding sand.

[0015] The required energy source is obtained through solar energy provided by solar panels which allow generating the energy necessary for the operation of the system.

[0016] The means for generating electromagnetic radiation are suitable for heating or melting any type of foundry sand binders, so the invention is applicable to the recovery of foundry sand with all types of binders (inorganic binders, organic binders and binders with bentonite) derived from the sand mold gravity casting process.

[0017] Preferably, the system further comprises a static drum open at its upper base, such that the rotary drum is housed inside the static drum concentric thereto, and the cover can be coupled in the closed position thereof to the open base of the static drum. The cover can be pivotably attached to the static drum.

[0018] By providing two concentric drums, one being a static drum and the other being a rotary drum inside the static drum, personal dangers such as entrapments, blows, etc., are prevented, and at the same time waves are prevented from coming out with traps that are in controlled areas.

[0019] In a preferred embodiment, the axis of rotation of the rotary drum is inclined with respect to the ground, and the means for generating electromagnetic radiation are arranged to generate electromagnetic radiation directed with an angle of inclination similar or equal to the inclination of the axis of rotation of the rotary drum. In this way, the sand is subjected to electromagnetic waves homogenously for a specific estimated time so that the binder juxtaposed to the sand detaches from said sand. Preferably, the angle of inclination is 45° and can be adapted depending on the space and volume of the recovered sand.

[0020] The means for generating electromagnetic radiation are adapted to generate electromagnetic fields with frequencies typical for such uses, such as 2450 MHz ± 50 MHz and 915 MHz, which can be implemented, for example, by means of one or more cavity magnetrons.

[0021] To fill and unload sand with respect to the rotary drum once the sand recovery and cleaning process has ended, it has been envisaged for the static drum and the rotary drum to be able to pivot together with respect to an axis orthogonal to the axis of rotation of the rotary drum.

[0022] Another aspect of the invention relates to a method for recovering foundry sand, which comprises applying electromagnetic radiation suitable for heating or melting foundry sand binders on a moving mass of foundry sand to be recovered. The electromagnetic radiation is applied with a frequency of 915 / 2450 MHz ± 50 MHz.BRIEF DESCRIPTION OF THE FIGURES

[0023] Figure 1 shows a schematic depiction of a cross-section of the system of the invention. Figure 2 shows a schematic depiction of a top perspective view of the system of the Figure 1 with the cover closed. Figure 3 shows a depiction similar to that of Figure 2 with the cover open. Figure 4 shows several views of a preferred embodiment of the system of the invention, in which Figure 4A is a front elevational view, Figure 4B is a side elevational view, and Figure 4C is a top plan view. PREFERRED EMBODIMENT OF THE INVENTION

[0024] In view of the figures, it can be seen that the system (1) consists of two concentric drums, a static outer drum (2) and a rotary inner drum (3) rotating with respect to an axis (X) and housed inside the outer drum (3). The inner drum (3) is open at its upper base and the outer drum (2) further has a hinged cover (4) to allow passage to the inner drum (3) for depositing or removing sand (5) with respect to the inner drum (3).

[0025] There are installed on the cover (4) means for generating electromagnetic radiation, specifically, a group of cavity magnetrons (7) arranged to generate electromagnetic radiation with an angle of inclination similar to the inclination of the axis (X) of rotation of the inner drum (3) and towards the inside thereof, when the cover (4) is in a closed position such as that shown in Figures 1 and 2. The cover (4) has a hole for each cavity magnetron and a glass pane to prevent dust from entering the cavity magnetron, but allows the passage of electromagnetic waves (11) to excite binder molecules, which are passed through waveguides manufactured from aluminum alloy with an inner coating as a dielectric element.

[0026] The assembly described above is inclined with respect to the ground (6) as can be seen particularly in Figure 1, such that the axis (X) in the embodiment shown in the figures forms an angle of 45° with respect to the ground (6).

[0027] The cavity magnetrons (7) are suitable for heating or melting organic and inorganic, chemical or non-chemical foundry sand binders, for example: furanic, phenolic - acid, alkaline, urethane -, silicate - ester, CO2, bentonite. Preferably, the cavity magnetrons (7) are adapted to generate electromagnetic fields with a frequency of 915 / 2450 MHz ± 50 MHz. The nominal power of the cavity magnetrons (7) will be suitable for the space and number of cavity magnetrons to be placed for improving wave distribution inside the inner drum (3), and with individual air cooling by means of fans (12). The frequency of the cavity magnetrons (7) is 915 / 2450 MHz, with a SHF (super-high frequency) wave type and without interference with other frequencies such as cellular frequencies.

[0028] These waves cause binder molecules to vibrate and / or rotate, which generate an increase in temperature. Thereafter, the sand (5) is not induced by the electromagnetic currents given the dielectric value thereof, and instead, the binders are subjected to excitation and sand-binder separation is thus achieved.

[0029] The effect of the radiofrequency energy applied to the binders allows a differentiating behavior, applying an induction effect only on the binder that at the same time does not generate any effect whatsoever on the base grain and, therefore, does not cause any transformation, any change in the base component; this being one of the requirements essential for the recovery thereof.

[0030] The electromagnetic waves consists of a wavelength in a range between 10 mm and 250 mm radiofrequency energy emitted by means of the cavity magnetrons (7) located in the upper part of the cover (4) such that the cavity magnetrons are static together with the cover (4). The cover has a seal with respect to the inner drum (2) where the sand is located, such that the waves can only leak out through the shaft system which is sealed with an insulating material.

[0031] Waves are applied within the margin of 45°C with respect to the direction of rotation given that, when rotating at low speeds, the sand (5) moves at an angle of 45°C as a result of the Coriolis phenomenon, being the area where the waves have an effect on the binders. This application of electromagnetic waves utilizes the different dielectric capacity of the binders and the "transparent" behavior of the sand so that it can be heated and separated from the sand. The temperatures reached by this binder vary with respect to its morphology, but they can be comprised between 150°C and 500°C.

[0032] Moreover, fumes are collected in the upper area of the inner drum (3) by means of a conduit (8) for discharging emissions derived from binder resins and fines derived from the breakage of the sand which are captured through filters and cyclones and directed to additional containers (BigBag) where this waste is deposited for subsequent treatment.

[0033] As depicted in Figure 4, the assembly formed by the two drums (2, 3) and the cover (4) are mounted on a support (10), such that the assembly can pivot with respect to an axis (Y) orthogonal to the axis of rotation of the rotary drum for loading and unloading sand with respect to the inner drum.

[0034] The system (1) is inclined 45° with respect to the vertical of the equipment. The cover (4) is then opened completely, leaving the mouth of the inner drum (3), that is stopped, free. Loading of the sand (5) to be treated is started in this position and state. The filling system is for sand originating from the foundry after being used, being in any condition (blocks, lumps, mixed with other elements, etc.), which means that there is no initial pretreatment. The loading limit is calculated through a weighing cell which determines the loading amount based on the design of the system.

[0035] Once the inner drum (3) has been loaded, the cover (4) is closed hydraulically by means of a pair of hydraulic cylinders (9) and this prevents the possibility of manipulating the cover by manually opening same with the system in operation and of causing personal dangers. Once closed, the rotary inner drum starts to rotate at low revolutions (20 rpm) and it starts at the same time as the emission of electromagnetic waves on the residual sand for treatment thereof and the capture of emissions by starting the capture cyclones. Temperature measurement and emission measurements are started at the same time for controlling the system, once the time of use of the system for cleaning thereof is determined.

[0036] The process ends by stopping the rotation of the rotary inner drum, as well as the emissions of the electromagnetic waves and the capture cyclones for capturing fines and burnt particles. After one minute, in order to prevent the existence of harmful parasitic currents at this time point, the rotation / turning of the assembly of the two drums starts, transitioning from 45° to 100°, a position in which the recovered sand is emptied out into the capture device of the foundry sand circuit. The cover is then opened completely and unloading of the treated sand is allowed.

[0037] The method ends when all the recovered sand has been emptied out and the assembly of the two drums (2, 3) has returned to the 45° position to start the new cycle.

[0038] The type of binder, as well as its percentage, generates a difference in electric consumptions due to the difference in temperature at which treatment is performed. These temperatures range from 200 to 500°C.

[0039] The discriminating factor in the use of the system is the dielectric property and the electrical permittivity of the binder added to the sand. These properties are the critical factors for the system to be able to act on and remove same. To that end, the value thereof must be less than 1 for the electromagnetic waves to have an effect when these binders are below this figure. Furanic resins and phenolic resins have a relatively low dielectric constant, which makes them highly suitable.

[0040] The dimensions of the installation are obtained from the application of the characteristic of magnetism by applying the Maxwell's law which links the power and the dimensions of the diameter of the rotary inner drum and height, being a ratio of 1.5-2 with respect to a height of 1.

[0041] To control the entire process, different measurements sensors are installed throughout the process. To control the level of sand in the system, temperatures of exiting fumes and of the reaction chamber, as well as measurement of the particles emitted through the outlet conduit, and these with a control algorithm will be that which controls and decides the application time of the system.

Claims

1. A system for recovering foundry sand, comprising: a rotary drum open at its upper base for depositing the sand to be recovered therein, a cover which can adopt an open position for allowing passage through the upper base of the rotary drum, and a closed position for closing passage through said upper base, means for generating electromagnetic radiation installed on the cover, such that when the cover is arranged in a manner that is closed and supported on the upper base of the static drum, the electromagnetic radiation generated is directed towards the inside of the rotary drum, and wherein the means for generating electromagnetic radiation are suitable for heating or melting foundry sand binders.

2. The system according to claim 1, further comprising a static drum open at its upper base, wherein the rotary drum is housed inside the static drum, and the cover can be coupled in the closed position thereof to the open base of the static drum.

3. The system according to claim 2, wherein the cover is pivotably attached to the static drum.

4. The system according to any of the preceding claims, wherein the axis of rotation of the rotary drum is inclined with respect to the ground, and wherein the means for generating electromagnetic radiation are arranged to generate electromagnetic radiation directed with an angle of inclination similar to the inclination of the axis of rotation of the rotary drum.

5. The system according to claim 4, wherein the angle of inclination is 45°.

6. The system according to any of the preceding claims, wherein the means for generating electromagnetic radiation are adapted to generate electromagnetic fields with a frequency of 915 / 2450 MHz ± 50 MHz.

7. The system according to any of the preceding claims, wherein the means for generating electromagnetic radiation comprise one or more cavity magnetrons.

8. The system according to any of claims 2 to 7, wherein the static drum and the rotary drum can pivot together with respect to an axis orthogonal to the axis of rotation of the rotary drum, for unloading sand from the rotary drum.

9. A method for recovering foundry sand, which comprises applying an electromagnetic radiation suitable to incite molecular excitation on the binder which causes the heating thereof and therefore the separation of the binder from the foundry sand grain, on a moving mass of foundry sand to be recovered.

10. The method according to claim 9, wherein the electromagnetic radiation is applied with a frequency of 915 / 2450 MHz ± 50 MHz.

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