Sand recycling process

Abstract

The invention relates to a sand recycling process comprising the following general steps: carrying out dry attrition; carrying out wet attrition; clarifying; calcinating to eliminate organic matter; and adjusting pH, wherein the sand recycling process has the following advantages: produces thermally expanded sand, thereby reducing veining defects; produces chemically conditioned sand, which has high resistance; reduces moulding waste by up to 90%, which represents an improvement in disposal costs and has a positive environmental impact; and reduces the use of carbon / bentonite slurry, thereby lowering the costs of new inputs by up to 80%.

Description

[0001] PROCESS FOR RECYCLING SAND

[0002] BACKGROUND OF THE INVENTION

[0003] A. FIELD OF THE INVENTION

[0004] The present invention relates to a process for recycling sand from molding processes, and more particularly to a process for recycling sand by which up to 100% recovery of molding sand is achieved.

[0005] B. DESCRIPTION OF RELATED ART

[0006] Sand casting is a fundamental process in metal casting. It is used to create complex parts or parts with hollow internal shapes, such as cylinders, pipes, and other intricate internal geometries. This process is commonly known as sand core casting or sand casting with cores.

[0007] The sand used in the molding process can be recovered and reused. After the casting is removed, the mold sand is cleaned, sieved to remove impurities, and mixed with fresh sand before being used in future molds.

[0008] However, current core sand recycling processes have certain disadvantages:

[0009] • Sand degradation: As sand is used repeatedly in the casting and recycling process, it tends to physically degrade and lose its original properties. This can result in less compactable sand, negatively impacting mold quality and the accuracy of the resulting parts.

[0010] • Accumulation of impurities: Throughout the recycling cycles, sand can accumulate impurities and contaminants, such as metal residue, ceramics, and chemicals used in the smelting process. These impurities can affect the quality of the molten metal and the integrity of the final part.

[0011] • Need for additives: As sand degrades, additives such as binders or new reinforcing materials may be required to maintain adequate compaction and cohesion properties. These additives can be costly and may also introduce new environmental and health challenges.

[0012] • Quality control: The quality of recycled sand can vary widely, making it difficult to consistently monitor mold quality and potentially affecting the quality of the castings. This may require additional testing and analysis to ensure the recycled sand meets the required standards.

[0013] • Reduced lifespan: As sand is recycled repeatedly, its lifespan is significantly reduced. Eventually, the sand becomes unsuitable for use in molds due to degradation and the accumulation of impurities. This can lead to additional costs associated with purchasing new sand and disposing of the worn-out sand.

[0014] • Separation complexity: In the demolding process, the sand and cores must be separated from the molten metal and the resulting parts. As part designs become more complex, separation can become more difficult and laborious, increasing the possibility of damage to the parts or the recycled sand.

[0015] • Environmental impact: Although sand recycling can reduce the amount of waste generated in the smelting process, it can still have an environmental impact due to the accumulation of impurities and the depletion of natural resources needed for the manufacture of new sand.

[0016] While recycling molding sand into cores can be a useful practice for reducing costs and waste, it presents challenges in terms of quality, degradation, and impurity accumulation. Therefore, currently, between 30% and 100% of the molding sand is confined, and 100% of the molding fines are also confined.

[0017] Examples of processes for recycling sand from previous art include:

[0018] Patent CN103406490B describes a foundry waste sand recycling process and reuse method comprising the following steps: pre-treatment of used sand: waste and garbage are removed by means of a closed grid, then through a magnetic separator scrap is removed, again through a linear vibrating screen bulky grains of used sand are removed, finally through a magnetic separator, iron filings waste is removed, and the sand is sent to a used sand storage tank.The sand cleaning process comprises the following stages: the pre-treated used sand is transported to the cleaning area by means of a sand pump, first it passes through a sand-water separator and the water portion is removed, then the sand is sent to a washing cylinder, and subsequently to a buffering sedimentation concentration tank, finally, it goes to a dewatering process; the recovered sand is dried at low temperature or roasted at high temperature after the aforementioned process, thereby obtaining recovered sand which is sent to a recovered sand storage tank.

[0019] Patent FR2584313A1 describes a process for purifying granular inorganic materials by attrition. According to the invention, attrition takes place in the absence of water as follows: the grains are sorted by their average diameter, distinguishing between those with a diameter less than a certain value “e”, called grains to be purified, and those with a diameter at least equal to “2e”, called cleaning grains; the mixture is introduced into a fluidized bed, the mixture is mixed for the time required for purification, and then the cleaning grains are separated from the grains to be purified.

[0020] Patent JPH0386351A describes a method for reconditioning green foundry sand, comprising the following steps: The used green sand, with its particle size regulated, is fed into a drying / calcining device where the moisture content is reduced to zero and the clay rises from a bed of sand. The separated sand is then fed from a hopper into a fluidized bed thermal calciner where the sand temperature is raised and the organic binder is removed. The calcined sand is then dropped onto a cooling / separation device to separate the clay. The sand is then released into a blasting chamber where the bond between the sand and clay is broken. The sand, from which the clay particles have been removed, is transferred to a feed hopper, metered, and fed into a mixer. Simultaneously, a metering pump pours an aqueous acid solution into the mixer and mixes it, thereby separating the clay particles.The pH-regulated sand is fluidized in the drying / sizing device so that the clay particles are captured in an air passage tube. The result is dry, pH-regulated sand that does not contain clay.

[0021] Patent ES2044449T3 describes a method for reconditioning wet foundry sand that has clay as a residue, which comprises the following steps: subjecting the sand to pre-screening and drying to approximately zero moisture; the sand is calcined to burn off the organic matter; the sand is cooled; the cooled sand is mechanically cleaned to mechanically release the clay and a portion of the clay magnetically attached to the sand; an acid and water solution is mixed with the sand to react with the remaining clay, magnetically attached, to allow the clay to be released from the sand; the sand is dried to release the loose clay; and the clay is removed as it is released from the sand.

[0022] Most sand recycling processes for pre-existing art include dry attrition and calcination stages, which leave the sand grains with a high amount of impurities. Furthermore, these processes result in up to 70% return to cores, requiring additional resin consumption, and the processed sand must be mixed with fresh or cold box reclaimed sand.

[0023] In order to overcome the aforementioned problems, the applicant developed a sand recycling process, whereby up to 100% recovery of molding sand cores is achieved, thus reducing the purchase of new sand.

[0024] The sand recycling process of the present invention comprises the following general stages: dry attrition; wet attrition; clarification, calcination to remove organic load and pH adjustment.

[0025] Additionally, the sand recycling process of the present invention offers the following advantages:

[0026] • It produces thermally expanded sand, thereby reducing poisoning defects.

[0027] • It produces chemically conditioned sand, which has high strength. • It achieves a reduction of up to 90% in molding waste, which represents an improvement in disposal costs and a positive environmental impact.

[0028] • The consumption of bentonite / coal slurry is reduced, resulting in a reduction of up to 80% in the costs of new inputs.

[0029] SUMMARY OF THE INVENTION

[0030] Therefore, a primary objective of the present invention is to provide a process for recycling sand whereby up to 100% recovery of molding sand in cores is achieved, thereby reducing the purchase of new sand.

[0031] It is still a primary objective of the present invention to provide a process for recycling sand of the nature described above, which comprises the following general stages: dry attrition; wet attrition; clarification, calcination to remove organic load, and pH adjustment.

[0032] It is still a primary objective of the present invention to provide a process for recycling sand of the nature described above, which produces thermally expanded sand, thereby achieving a reduction in poisoning defects.

[0033] An additional objective of the present invention is to provide a process for recycling sand of the nature described above, which produces chemically conditioned sand, which has high strengths.

[0034] It is yet an additional objective of the present invention to provide a process for recycling sand of the nature described above, by which a reduction of molding waste of up to 90% is achieved, which represents an improvement in disposal costs and a positive environmental impact.

[0035] A further objective of the present invention is to provide a process for recycling sand of the nature described above, thereby reducing the consumption of bentonite / coal slurry and achieving a reduction of up to 80% in the cost of new inputs. These and other objectives and advantages of the present invention will become apparent to persons of ordinary skill from the following detailed description of the invention.

[0036] DETAILED DESCRIPTION OF THE INVENTION

[0037] The process of the present invention will now be described in relation to a preferred embodiment thereof.

[0038] The process of the present invention aims to process green molding sand, and generally comprises the following stages:

[0039] Dry Attrition

[0040] In this stage, the molding sand that will be recycled enters and the aim is to eliminate any contaminants that may come from the process, as well as to remove silica, carbon and bentonite fines, through primary screening, secondary screening, dry attrition and fluidized beds.

[0041] Wet Attrition

[0042] To complement the sand cleaning process, the sand is added to a tank of water and the mixture is pumped to a hydrocyclone to separate the silica sand from the remaining bentonite and carbon adhering to the grain.

[0043] Slurry Solution

[0044] From the previous stage, a portion of the solution is collected to recycle the bentonite and carbon present in the water-based solution.

[0045] Burned

[0046] The sand that comes out of wet attrition is added to a calciner so that with the help of high temperature the organic remains of the washing process are eliminated, in addition to the heating gives a finish to the grain that changes its chemical phase and eliminates the effects of thermal expansion.

[0047] Chemical Treatment

[0048] The pH is adjusted with an acidic solution according to the specifications. Each stage of the process of the present invention will be described in detail below, comprising: a) Tempering the sand: for the sand to be properly processed by the method of the present invention, it must have a temperature below 75°C. Due to the variability of the receiving temperature, it is necessary to measure the temperature of the received sand. If the temperature is above 75°C, the sand to be processed must be left to rest for a period of time to temper it until it reaches a maximum temperature of 75°C before processing; b) Dry attrition: b1) Before starting dry attrition, the sand is passed through a primary screening using a rectangular screen No.1) where the contamination carried by the sand, such as wood, wires, and agglomerated clumps of sand formed in previous processes, known as cores, is removed. Once the sand is subjected to primary screening, it is stored in a hopper; b2) the sand from the primary screening hopper is subjected to secondary screening, which is carried out in a screening machine, preferably with a 2.5 HP motor, working at 1,160 RPM, using a No. 4 mesh with a diameter of 60 mm made of 304 grade stainless steel.The screening machine is fed by a bucket elevator at a rate of 20 tons / hour; b3) the sand from the secondary screening undergoes a first mechanical attrition process, where the sand particles collide with each other, causing grain-by-grain impact and thus cleaning their surface of fine sand and agglomerated particles, taking advantage of the mechanical energy of the process, as well as breaking up agglomerates. Mechanical attrition is carried out by passing the sand through a turbine equipped with two curved-blade impellers, rotating in opposite directions at 1,500 to 1,800 RPM, one clockwise and the other counterclockwise; b4) the sand from the first mechanical attrition process passes to a first fluidized bed to separate unwanted fine elements (sand dust), which are then sent to a first pneumatic dust collector.The first fluidized bed operates with an airflow at an average rate of 3,750 CFM and 9 psi of pressure; b5) the sand from the fluidized bed undergoes a second mechanical attrition process. The second mechanical attrition is carried out by passing the sand through a second turbine equipped with two curved-blade impellers, rotating in opposite directions at 1,500 to 1,800 RPM, one clockwise and the other counterclockwise; b6) the sand from the second mechanical attrition process passes to a second fluidized bed to achieve the separation of unwanted fine elements (remaining sand dust), which will be sent to a second pneumatic dust collector. The second fluidized bed operates with an airflow at an average rate of 3,750 CFM and 9 psi of pressure; b7) the sand from the second fluidized bed undergoes final screening using a screen with a 2 HP motor.5 HP, working at 1,160 RPM, using two screens, No. 20 and 60, said screens have a diameter of 60 and are made of 304 grade stainless steel; b8) the sand from the final screening is sent to storage by means of a conveyor belt;.

[0049] The dry attrition process (substages b1 to b6) retains a maximum of 3% fines in the primary screening of stage b1); a maximum of 0.15% fines in the secondary screening of stage b2; and a maximum of 0.10% fines of silica, coal, and bentonite in the final screening of stage b6. The grain-to-grain collision effect during mechanical attrition causes subangular grains to become more rounded, increasing their mechanical strength and thus providing better quality results when used in a foundry process. c) Wet attrition carried out in at least four wet attrition stages, each stage comprising processing a fluid paste using a hydrocyclone and subsequently in a different attrition cell: c1) The sand treated in stage b), from storage (stage b8), is fed into a mixing tank at a sand flow rate of 20 t / h. In one embodiment of the process,The mixing tank is 1.22 m in outside diameter and 2 m high, made of A-36 grade carbon steel with a thickness of 3 / 16 in; it has a 3-blade agitator on three levels made of 304 stainless steel with a thickness of 3 / 16 in, driven by a linear gearbox with pulleys and belts; c2) add 700 liters of water to the mixing tank for every 300 kg of sand at ambient temperature to prepare a fluid paste that can be pumped; c3) pump the fluid paste to a first separation hydrocyclone lined with high-density rubber, to continue shaping the particles and removing fines. Pumping to the hydrocyclone is done at a flow rate of 52 m³ / ha and a pressure of 15 psi; c4) separate the heavy and light elements of the fluid paste by means of the first separation hydrocyclone, which is achieved with an inclination of 60°, at 52 Hz and 1,603 RPM, thus achieving the separation of a sludge composed of water,bentonite and coal from the fluid paste; c5) pumping the fluid paste obtained in stage c4) to a first wet attrition cell, said equipment having a set of blades, each having a forward inclination of 30° and a backward inclination of 15° interchangeable in each section, each agitator working at a rotational speed of 250 RPM to 300 RPM and said fluid paste passing continuously through the attrition cell at a preferred flow rate of 20 tons / hr. c6) pumping the fluid paste obtained in stage c5) to a second hydrocyclone to further separate the heavy and light elements from the fluid paste obtained in the previous attrition cell and subsequently to a second wet attrition cell, said second hydrocyclone and said second wet attrition cell being able to have the same characteristics as those of stages c3), c4) and c5), and where: the pumping to the second hydrocyclone,The processing in the second hydrocyclone and the processing in the second wet attrition cell may have the same operating conditions as those of stages c3), c4) and c5), producing a fluid paste; c7) pumping the fluid paste obtained in stage c6) to a third hydrocyclone to further separate the heavy and light elements from the fluid paste obtained in the previous attrition cell and subsequently to a third wet attrition cell, wherein said third hydrocyclone and said third wet attrition cell may have the same characteristics as those of stages c3), c4) and c5) and wherein: the pumping to the third hydrocyclone, the processing in the third hydrocyclone and the processing in the third wet attrition cell may have the same operating conditions as those of stages c3), c4) and c5),producing a fluid paste; c8) pumping the fluid paste obtained in stage c7) to a fourth hydrocyclone to further separate the heavy and light elements from the fluid paste obtained in the previous attrition cell and subsequently to a fourth wet attrition cell, wherein said fourth hydrocyclone and said fourth wet attrition cell may have the same characteristics as those of stages c3), c4) and c5), and wherein: the pumping to the fourth hydrocyclone, the processing in the fourth hydrocyclone and the processing in the fourth wet attrition cell may have the same operating conditions as those of stages c3), c4) and c5), producing a fluid paste; c9) depositing the fluid paste obtained in stage c8) into a single-pass classifier to separate the sand from the water. Preferably, the single-pass classifier is made of A-36 grade carbon steel with a thickness of 5 / 16 in, 1.50 m wide x 8 m long,Single-pass helical classifier manufactured with a 12 in hollow shaft, with A-36 grade carbon steel blades 3 / 8 in thick, fixed to the shaft by channel support, secured by clamps bolted to the shaft with interchangeable 3 / 16 in thick 304 stainless steel plates. The classifier has a 25° incline and operates at 5 RPM; c10) the separated sand, obtained in stage c9) is sent to storage and left to settle until it reaches a moisture content of between 2% and 4%; d) Slurry Solution: d1) the slurry containing water, bentonite and carbon from each hydrocyclone is sent to a clarification process, by means of a 1.22 x 1.22 x 3.05 m pumping box,where all the water and fines mixtures resulting from the hydrocyclones are concentrated, and a polymer and a coagulant are applied to facilitate the settling of bentonite and carbon from the water in the next sub-stage, producing treated sludge. The polymer comprises polydiallyl dimethyl ammonium halide in a 0.1–1.0% solution applied at rates of 20 to 80 liters per ton. The coagulant comprises ethylene carboxamide homopolymer in a 1.0–10% solution applied at rates of 40 to 100 liters per ton. d2) The treated sludge obtained in sub-stage d1) is sent to at least one silo where the separation of bentonite and coal from water is carried out by decantation, where the treated sludge is left to settle in the at least one silo until a sediment forms at the bottom of the at least one silo and the resulting liquid has from 7% to 12% solids,a density of between 1.04 and 1.06 g / cm2 and a Baumé degree of 10 to 30; d3) the liquid resulting from stage d2) is sent to at least one storage silo; e) Calcination: e1) feed the sand obtained in stage c10) to a preheater. The feed is carried out by means of rubber belts. Said preheater increases the temperature of the sand to a temperature of 75° C to 80° C in order to remove its moisture content until it reaches a moisture content of a maximum of 2%, at which point the sand is transferred to a hopper. In the preferred configuration, the preheater is a rotary dryer 1.35 m in diameter x 10 m long x 3 / 4 in thick, with 1 / 2 in torispherical heads, made of A / SA 516 Gr.70 grade carbon steel with 3 / 8 in thick helical internal blades, and uses hot air from a fluidized bed located at the outlet of a respective calciner. The air reaches the preheater at a temperature ranging from 160 to 200°C.with a flow rate of between 8,500 and 10,000 CFM and a pressure of 0.5 psi. The air exits the preheater at the same pressure, and its temperature drops to at least 90°C. e2) feed the sand obtained in stage e1) to a rotary calciner to remove the remaining organic load in the sand. At the inlet, the calciner has a temperature of 400°C, and with the tilting and rotation of the calciner, the sand advances towards a burner at the other end of the calciner, where a temperature of up to 1,000°C is reached. Preferably, the calciner can process 5 to 10 t / h of sand, has a diameter of 1.35 m x 20 m length x 3 / 4 in thickness, and features a 1 / 2 in diameter torispherical lid. It is constructed of A / SA 516 Gr.70 steel, with an interior lining of 4.5 in thick insulating bricks in the first layer; 2.5 in thick working bricks in the second layer; and refractory concrete in the third layer. It has one burner that uses natural gas to produce 900 kcal / h.000 to 1,500,000 BTU / t of energy. To calcine the sand to achieve an organic load of 0.05 to 0.10%, as well as to eliminate the effects of thermal expansion of the silica, the calciner temperature varies from 750 to 1,000°C. This ensures that the silica no longer undergoes thermal expansion with the temperature applied during the molding process, thus preventing deformation defects in the casting process; e3) Transfer the sand to a fluidized bed where it is cooled with a flow of 8,500 CFM of air at 5 psi with an area of ​​13,951 in2, until the sand reaches a maximum temperature of 180°C. As mentioned previously, this air is used in the preheater. d) Chemical treatment (optional): f1) Transfer the sand obtained in stage e3) to a mixer where it undergoes chemical treatment by applying a 3-4% acid solution.to adjust the pH from 7.0 to 7.5 as requested by the customer and cool the sand to a maximum of approximately 80°C. The acid solution comprises a 1-20% aqueous solution of phosphorus oxoacid; f2) transfer the sand treated in stage f1) to a screening process to remove unwanted elements (sometimes parts of the calciner refractory are found) and adjust the particle size to a size required by the end customer. The transfer is preferably carried out by means of a bucket elevator with a capacity of 5 to 10 t / h, and the screener preferably has a 2.5 HP motor, operates at 1,160 RPM, using a No. 16 mesh, said mesh has a diameter of 60 mm and is made of grade 304 stainless steel; f3) the screened sand obtained in stage f2) is transferred to a hopper and finally packaged.

[0050] Although it was stated that in step c) the resulting fluid paste is processed in four different hydrocyclones and subsequently in four different wet attrition cells, it should be understood that the fluid paste can be processed in at least one different hydrocyclone and at least one different attrition cell, and all the hydrocyclones and attrition cells do not necessarily need to have the same characteristics, but rather provide the same results. Furthermore, these steps can be carried out in the same hydrocyclone and the same wet attrition cell, such that the product resulting from the wet attrition cell is fed back into the hydrocyclone, creating a cycle where the wet paste can be subjected to four cycles, or, in other configurations, to at least one cycle.

[0051] Laboratory tests are carried out throughout the process to ensure that the established requirements for the sand are met at the various stages of the process.

[0052] Sand samples are collected at different stages of the process:

[0053] • Green sand that enters the process.

[0054] • In dry attrition, at the end of the screening.

[0055] • After the classifier.

[0056] • In each bag of finished product. In each sand sample, the following tests are performed:

[0057] • Granulometry (AFS Fineness Grade, American Foundation Society)

[0058] • Loss on ignition (LOI)

[0059] pH

[0060] • Turbidity

[0061] • Magnetic particles (Magnetics)

[0062] • Acid Demand Value (ADV)

[0063] Regarding the sludge, samples are taken from each transport pipe to analyze the following:

[0064] • Density

[0065] • Baumé Degrees

[0066] • Water content (relative humidity)

[0067] It is necessary to mention that the requirements established for the sand in the different phases of the process depend on the final specifications for the sand requested by the client.

[0068] If the established requirements are not met and cannot be adjusted, the sand is reprocessed.

[0069] Likewise, the description provided above comprises a specific modality of the process, since the capabilities and specifications of the equipment used may vary depending on the amount of sand to be treated and the final specifications for the sand requested at a given time by a client.

[0070] It should finally be understood that the sand recycling process of the present invention is not limited to the modality described above and that experts in the field will be trained, by the teachings established herein, to make changes to the sand recycling process of the present invention, the scope of which will be established exclusively by the following claims.

Claims

CLAIMS 1. A process for recycling green molding sand characterized by comprising the following steps: a) measuring the temperature of the sand entering the process, and if the temperature is greater than 75°C, allowing the sand to stand until its temperature is reduced to a maximum of 75°C; b) subjecting the sand from step a) to a dry attrition process comprising the following sub-steps: b1) subjecting the sand to primary screening to remove contamination carried by the sand, such as wood, wires, agglomerated clumps of sand formed in previous processes, known as cores; b2) subjecting the sand obtained in sub-step b1) to secondary screening;b3) subject the sand obtained in sub-stage b2) to a mechanical attrition process where the sand particles collide with each other to cause grain-to-grain impact, cleaning their surface of fine sand and agglomerated particles, taking advantage of the mechanical energy of the process, in addition to breaking up agglomerates; b4) feed the sand obtained in sub-stage b3) to a first fluidized bed to achieve the separation of unwanted fine elements, which will be sent to a first dust collector; b5) subject the sand obtained in sub-stage b4) to a second mechanical attrition process; b6) feed the sand obtained in sub-stage b5) to a second fluidized bed to achieve the separation of unwanted remaining fine elements, which will be sent to a second dust collector; b7) subject the sand obtained in sub-stage b6) to a final screening; c) subjecting the sand obtained in the previous stage to a wet attrition process, comprising the following sub-stages: c1) preparation of fluid paste: the sand from stage b7) is fed into a mixing tank where an amount of water of 700 liters per 300 kg of sand at ambient temperature is added to prepare a fluid paste that can be pumped; c2) pumping the fluid paste obtained in stage c1) to a hydrocyclone to separate the heavy and light elements from the fluid paste, clean the sand of fines and thus separate a sludge composed of water, bentonite and carbon from the fluid paste; c3) pumping the fluid paste obtained in stage c2) to a wet attrition cell; c4) depositing the fluid paste obtained in the last wet attrition stage of the last cycle carried out, into a single-pass classifier to separate the sand from the water;c5) transferring the separated sand obtained in step c5) to storage and allowing it to settle until it reaches a moisture content of between 2% and 4%; d) subjecting the sand to a calcination process, comprising the sub-steps of: d1) feeding the sand obtained in step c5) to a preheater, to increase the temperature of the sand to between 75°C and 80°C in order to remove its moisture content until it reaches a moisture content of a maximum of 2%, at which point the sand is transferred to storage means; d2) feeding the sand from the storage means of step d1) to a calciner to remove the remaining organic load in the sand, where the sand is calcined at a temperature of between 750°C and 1000°C, where the sand is calcined until it has an organic load of between 0.05% and 0.10%; d3) transfer the sand obtained in stage d2) to a fluidized bed where it is cooled until the sand reaches a maximum temperature of 180°C.

2. A process for recycling green molding sand according to claim 1, wherein the fluid paste obtained in step c3) is pumped to the hydrocyclone of step c2) and steps c2) and c3) are repeated for at least one more cycle.

3. A process for recycling green molding sand according to claim 1, wherein the slurry composed of water, bentonite and coal from the fluid paste obtained in step c2) is processed by the following steps: e1) subjecting the slurry composed of water, bentonite and coal to a clarification process by applying a polymer and a coagulant to facilitate the settling of the bentonite and coal from the water; e2) send the sludge from step e1) with the addition of a polymer and a coagulant to at least one silo where the separation of the bentonite and coal from the water is carried out by decantation, wherein the treated sludge is left to settle in the at least one silo until a sediment forms at the bottom of the at least one silo and the resulting liquid has 7% to 12% solids, a density of between 1.04 and 1.06 g / cm2 and a Baumé degree of 10 to 30; e3) send the liquid recovered in step e2) to storage means.

4. A process for recycling green molding sand according to claim 2, wherein when steps c2) and c3) are repeated for more than one cycle, the cycles are carried out in different hydrocyclones and wet attrition cells, such that the fluid paste obtained from one wet attrition cell is fed to the next hydrocyclone.

5. A process for recycling green molding sand according to claim 1, wherein the sand obtained in step d3) is subjected to a chemical treatment step comprising the sub-steps of: f1) transfer the sand obtained in stage d3) to a mixer where it is subjected to chemical treatment by applying a 3-4% acid solution to adjust the pH from 7.0 to 7.5 and cool the sand to a maximum of 80°C; f2) transfer the sand treated in stage f1) to a screening process to remove unwanted elements and adjust the particle size to a predetermined size; f3) the screened sand obtained in stage f2) is transferred to storage.

6. A process for recycling green molding sand according to claim 1, wherein the primary screening of step b1) is carried out using a rectangular screen No.

1.

7. A process for recycling green molding sand according to claim 1, wherein the secondary screening of step b2) is carried out using a screen with a No. 4 mesh.

8. A process for recycling green molding sand according to claim 1, wherein the final screening of step b7) is carried out using two meshes, No. 20 and No.

60.

9. A process for recycling green molding sand according to claim 1, wherein in the primary screening of step b1) a maximum of 3% of fines is retained; in the secondary screening of step b2) a maximum of 0.15% of fines; and in the final screening of step b7) a maximum of 0.10% of fines, said fines comprising particles of silica, coal and bentonite.

10. A process for recycling green molding sand according to claim 3, wherein in step e1), the polymer comprises Polydiallyl dimethyl ammonium halide in solution at 0.1 - 1.0% applied in ranges of 20 to 80 liters per tonne and the coagulant comprises Ethylene carboxamide homopolymer in solution at 1.0 - 10% applied in ranges of 40 to 100 liters per tonne.