WO2019202376A1

WO2019202376A1 - A method of regenerating foundry sand

Info

Publication number: WO2019202376A1
Application number: PCT/IB2018/059493
Authority: WO
Inventors: Bartolomeo Tosco; Paolo BOCCA
Original assignee: Fata Aluminum S.R.L.
Priority date: 2018-04-17
Filing date: 2018-11-30
Publication date: 2019-10-24
Also published as: IT201800004618A1

Abstract

Sand to be regenerated is provided, the grains of which are at least partially covered with residual inorganic resin from previous production cycles. The sand is first subjected to a mechanical attrition treatment for superficially abrading the grains of sand so as to at least partially remove the residual inorganic resin therefrom. The sand is then transferred to a dedusting device to remove from the sand the resin and sand dust produced in the previous step of mechanical attrition. The sand is then subjected to a thermal treatment to neutralize the reactivity of resin residues remaining on the grains of sand.

Description

A method of regenerating foundry sand

Technical field

The present invention refers to a method for regenerating sands previously used in a foundry for the production of cores.

Background art

For the production via casting of complex metallic pieces, molten metal is cast in molds where cores are positioned that reproduce the inner shape of the pieces. Sands are mixed with special resins and suitable catalysts that allow hardening to give consistency to the cores.

It is economically convenient to recycle the sand used for the production of the cores, after neutralizing and/or partially or totally eliminating the residues of binding resins, catalysts and other impurities contained therein.

Foundry sands are regenerated by means of thermal-type systems or mechanical-type systems. The thermal-type systems burn off the organic resins that cover the surface of the grain of sand. Mechanical treatments produce a mechanical abrasion of the grain surface, obtained by attrition. Most foundry sand regeneration systems are currently of the thermal type, and the resins used to form the cores are predominantly organic (phenolic, furanic resins, etc.).

A problem related to the use of organic resins is that, when casting (at temperatures of 700- 800° C in the case of aluminum alloys), they develop gases that need to be removed, so as not to be imprisoned in the casting, and abated, as they are harmful. To abate these gaseous emissions, abatement systems are required, for example, post-combustion chambers, which have extremely high running costs. This has prompted some resin producers to consider using inorganic-type resins as binders for the cores, which do not develop gases or develop them in an extremely small amount. Inorganic -type resins are predominantly silicates, phosphates, borates or the like, to some extent water-soluble, which, through hot forming, yield the water of solubilization and harden. Since inorganic resins are non-combustible, their removal from the surface of the grain of sand requires a mechanical treatment of surface abrasion of the grain. Laboratory analyses show that the mechanical treatment alone does not result in a product that is suitable as is to be reused to form cores in place of using new sand.

Moreover, the fact that the resin is not combustible does not mean that a thermal treatment is not useful. In effect, the thermal treatment neutralizes the reactivity of the main components of the resin, making them unavailable for the subsequent reaction of the sand regenerated with new resin. However, thermal treatment alone is insufficient to make the sand immediately reusable to form new cores.

The most commonly used practice at present is to first carry out a mechanical treatment, for the removal of a part of the resin, and then to carry out, on the mechanically treated product, a subsequent thermal treatment to neutralize the resin residue. With this mixed treatment a regenerated sand is obtained that has, as it is, characteristics inferior to those of a corresponding new sand.

It is well known that the presence of dust in the sand used to make foundry cores negatively affects the resistance characteristics of the core produced. A standard foundry sand has average grain sizes ranging from about 0.1 mm to about 0.3 - 0.4 mm. The term“dust” means any granule having a size of less than approximately 0,1 mm. In effect, in various parts of the world, different standards are used, which however indicate the limit of division between sand and dust at values lower than 0.106 mm, or lower than 0.104 mm, or lower than 0.1 mm. The fact that the dust has a negative impact on the resistance characteristics of the cores is due to its specific surface value, which is higher than the standard grains of sand. Dust particles therefore tend to absorb more resin in relation to weight than standard diameter sand grains, and thus leave less resin available as a binder between grains. With the same quantity of resin, calculated by weight relative to sand, dustier sands produce cores with lower quality characteristics. It is therefore known that regenerated sand must contain as little dust as possible. Generally accepted percentage values are 1% by weight of dust out of the total weight of the sand, but even lower values, for example 0.5% by weight or less, are sometimes required.

Patent publication US 2014/0166226 Al discloses a method for the regeneration of foundry sand, summarized in the flow diagram of FIG. 5. Sand to be regenerated is provided (step A) the grains of which are at least partially covered with residual inorganic resin from previous production cycles. The sand is first subjected to a mechanical attrition treatment (step B) for superficially abrading the grains of sand so as to at least partially remove the residual inorganic resin therefrom. The sand to be regenerated is accelerated by a jet of air against an impact surface and falls into a cleaning chamber. Dust and binder particles which are present in the air in the cleaning chamber and which have formed as a result of the attrition step are removed by drawing in air from the top of the cleaning chamber. The suction, which takes place at the same time as the mechanical treatment, is intended to prevent dust which is harmful to health and the outside environment from leaving the cleaning chamber. The suction compensates and cancels the pressurization caused by the introduction into the cleaning chamber of the air jet used to accelerate the particles of sand to be cleaned. In systems of this type, the suction takes place to such an extent as to maintain the cleaning chamber in a low vacuum in order to prevent the escape and uncontrolled dispersion of the dust. After the mechanical treatment, the sand is transferred to a furnace and subjected to a thermal treatment (step C) to neutralize the reactivity of any resin residues remaining on the grains of sand. Finally, the sand is cooled (step D).

Other methods for the regeneration of foundry sand are known from US 4700766 A and US 5291935 A.

Summary of the invention

It is an object of the present invention to obtain a regenerated product having characteristics superior to those of sand derived from traditional mixed mechanical-thermal treatments of the type discussed herein above.

This and other objects and advantages, which will be better understood hereinafter, are achieved by a sand regeneration method that provides for the steps set forth in the accompanying claims.

In a mixed mechanical-thermal treatment, as the regenerated product is the one deriving from the final thermal part of the process, it is necessary that the sand at the end of the final thermal process is well dedusted.

In short, the present invention proposes to dedust in an optimal way the intermediate product, which comes from the mechanical treatment and which is still to be subjected to the final thermal treatment. After a mechanical attrition treatment and before a subsequent thermal treatment, the sand is transferred to a dedusting device and subjected to a forced dedusting step, wherein the dust still mixed with the sand is removed.

In one embodiment, the method comprises the steps of:

providing sand to be regenerated covered at least partially with inorganic resin, subjecting the sand to a mechanical attrition treatment for superficially abrading the grains of sand to at least partially remove the inorganic resin therefrom,

after the mechanical attrition treatment, transferring the sand to a dedusting device and subjecting the sand to a dedusting treatment to remove, from the sand, resin and sand dust produced in the previous mechanical attrition step,

after the dedusting treatment, transferring the sand to a furnace and subjecting the sand to a thermal treatment to neutralize the reactivity of any resin residues remaining on the grains of sand.

Brief description of the drawings

Some of the preferred but non-limiting embodiments of methods for the regeneration of foundry sand will now be described. Reference is made to the accompanying drawings, wherein:

FIG. 1 is a schematic view of an attrition chamber;

FIG. 2 is a schematic view of a dedusting device;

FIG. 3 is a schematic view of a furnace;

FIG. 4 is a schematic view of a cooler;

FIG. 5 is a flow diagram illustrating a method of a known type for the regeneration of foundry sand; and

FIG. 6 is a flow diagram that illustrates a method for the regeneration of foundry sand according to an embodiment of the present invention.

Detailed Description

A few preferred but non-limiting embodiments of a method for the regeneration of foundry sand according to the invention will now be described.

The present method concerns the regeneration of sands obtained from sand cores.“Sand cores” means conglomerates of sand grains, held together by inorganic resins, which reproduce the inner shape of metal parts produced by casting molten metal.

The flowchart of FIG. 6 indicates the steps of a method according to an embodiment of the invention. In an initial step (step A), a sand to be regenerated is provided, with the grains of sand covered with inorganic resin. Sands to be regenerated by this method may include silica sand, olivine sand, zirconium sand, chromite sand or synthetic sands, e.g. mullite.

A preliminary step of the method may provide for a crushing step, in which sand cores or pieces thereof resulting from previous casting operations are broken up and finely crushed. The crushing step produces single grains of sand still covered with resin, and/or small aggregates of grains of sand held together and at least partially covered with resin, with dimensions of a few millimeters. Crushing may be carried out with machines that are known per se in the art, for example, crushers that crush by means of vibration. Alternatively, jaw crushers, hammer crushers with a rotating part, or rotary drum crushers, may be used.

The sand, or aggregates of sand grains resulting from crushing, is then subjected to a mechanical attrition treatment (step B, FIG. 6) in an attrition chamber 10 (FIG. 1). With the mechanical attrition treatment, the grains of sand are superficially abraded to remove at least part of the inorganic resin from previous production processes.

The mechanical treatment applied provides for moving the sand by means of a pneumatic device by means of which the sand is transported at high speed through a vertical tube 12 to a target 13 placed at the top of the tube 12.

The movement of the sand is obtained with a fan 11 of adequate flow and pressure to accelerate the sand from the base of the tube 12 towards the target 13. The target 13 may be made as a metal bell with high wear resistance, inside of which is created a sort of sand cushion held in suspension by the same transport air of the sand that creates the fluidization.

The grains of sand carried by the tube are then immersed in the sand cushion, where they are slowed down gradually by attrition with the grains of sand of the same cushion. The grain of sand is then cleaned superficially by attrition between the same grains of sand, in the sense that attrition does not occur by direct impact of the grain against the metal bell but rather occurs in a cushion of fluid sand that acts as a means of mutual surface abrasion between the grains.

The choice of the device that carries out the mechanical attrition step is not to be considered as limiting for the implementation of the present method. Mechanical attrition may alternatively be produced by other machinery, for example by mechanical attrition cells comprising mills in which the sand is agitated to remove contaminants through the force of attrition.

In the embodiment schematically depicted in FIG. 1, the attrition treatment may be carried out in a continuous manner. The attrition chamber may have an inlet (to the left for the sand to be treated, and an outlet (to the right) with a chute beneath the bell 13. The dust-laden air resulting from the mechanical attrition step is sucked in and filtered through a suction tube 14 at the top of the attrition chamber 10.

Downstream of the mechanical attrition station, the sand is introduced into a dedusting device 20 (FIG. 2) to remove from the sand the resin and sand dust produced in the previous attrition step.

The dedusting device may be a fluidized bed dedusting device. Advantageously, the dedusting device may be mounted in cascade or connected in series with the attrition chamber 10 to exploit a portion of the flow rate and pressure generated by the fan 11 already used in the mechanical attrition step to generate the air jet in the attrition chamber.

In the dedusting device, a series of nozzles 21 blows air upwards through a bed of fluidized sand. The air blown from the nozzles 21 removes resin and sand dust from the sand bed. Above the fluidized bed, an air suction system 22 is provided that produces the evacuation of resin and sand dust. The suction in the dedusting device produces the removal of most of the dust, i.e. of the particles having dimensions less than 0.106mm.

At the end of the dedusting step, which precedes a thermal treatment step, the dedusting levels reached by the present method are such as to guarantee at most the presence of 1% by weight of dust in the mechanically treated sand. Dedusting may also be pushed to lower values, e.g. 0.5% by weight or preferably 0% by weight.

The dedusted sand, resulting from the previous dedusting step, is subjected to a thermal treatment (step C, FIG. 6) in a furnace 30, (FIG. 3) to neutralize the reactivity of the resin residues left on the grains of sand.

The temperatures reached in the thermal treatment step, and its duration, are variable depending on the type of resin and the degree of final purity required. By way of example, temperatures may vary between approximately 600-650°C and up to 800°C, with treatment times ranging from approximately 30 to approximately 60 minutes.

The thermal treatment, which is preferably carried out on a bed of fluidized sand, is not per se appreciably different from the thermal treatments already conventionally applied in the art for the regeneration of foundry sand. The thermal treatment produces a calcination of the sand by chemically deactivating the residues of the resins, making such residues no longer combinable with new resin in subsequent production cycles.

At the end of the thermal treatment step, the regenerated sand may be introduced in a cooler 40 (FIG. 4) for a final thermal conditioning (step D, FIG. 6). Optionally and preferably, the sand may be reintroduced into a dedusting device (not shown) for further reduction of the finer dust fraction. Optionally, the final dedusting and cooling step may take place simultaneously in a same apparatus 40 where a bed of regenerated sand is penetrated by a flow of cold air moving upwards. A fan 41 generates a flow of air which, by means of a series of nozzles 42, passes through a bed of fluidized sand wherein a cooling coil 43 is arranged.

Experimental tests carried out by the Applicant have shown that a forced dedusting carried out before sending the product to thermal treatment allows a regenerated sand to be obtained with which it is possible to produce cores with characteristics at least equal to or superior to the corresponding cores produced with new sand.

Although without wishing to be bound to any specific theory in this regard, the experiments conducted by the Applicant show that, as a result of the intermediate dedusting step between the mechanical and thermal treatment steps, a final regenerated product with better characteristics than the one treated only mechanically and then thermally is obtained. The effectiveness of the present method is likely to be due to the fact that the absence of dusty parts during the thermal treatment improves the effectiveness of the same thermal treatment.

Different aspects and embodiments of the method have been described. It is understood that each embodiment may be combined with any other embodiment. The invention, moreover, is not limited to the described embodiments, but may vary within the scope defined by the accompanying claims.

Claims

1. A method of regenerating foundry sand, the method comprising the steps of:

providing sand to be regenerated, the sand having grains at least partly covered with inorganic resin left from previous production cycles;

subjecting the sand to a mechanical attrition treatment for superficially abrading the grains of sand so as to at least partially remove the residual inorganic resin therefrom;

after the mechanical attrition treatment, transferring the sand to a dedusting device and subjecting the sand to a dedusting treatment to remove, from the sand, resin and sand dust produced in the previous mechanical attrition step;

2. A method according to claim 1, wherein at the end of the dedusting step, the percentage by weight of dust in the sand having sizes smaller than 0.l06mm is less than 1%.

3. A method according to claim 2, wherein at the end of the dedusting step, the percentage by weight of dust in the sand is less than 0.5%.

4. A method according to claim 3, wherein at the end of the dedusting step, the percentage by weight of dust in the sand is equal to 0%.

5. A method according to any one of the preceding claims, wherein the thermal treatment step is followed by a second dedusting step.

6. A method according to any one of the preceding claims, wherein after the thermal treatment step, the sand is introduced into a cooler and subjected to a final thermal conditioning step.

7. A method according to claims 5 and 6, wherein the second dedusting step and the final thermal conditioning step take place simultaneously in a same apparatus wherein a bed of sand is crossed by a flow of upwardly moving cold air.

8. A method according to any one of the preceding claims, wherein

the dedusting step is carried out in a fluidized bed dedusting device, wherein a plurality of nozzles blows air upwards through a fluidized bed of sand, whereby the air blown by the nozzles removes resin and sand dust from the fluidized bed of sand, and wherein

an air suction is provided above the fluidized bed for removing resin and sand dust from the dedusting device.

9. A method according to claim 8, wherein the fluidized bed dedusting device is mounted near an attrition chamber in which the mechanical attrition treatment is performed, wherein the fluidized bed dedusting device exploits a part of the pressurized flow of air generated by a fan used in the mechanical attrition step to generate the air jet.

10. A method according to any one of the preceding claims, wherein

the dedusting step is carried out in a fluidized bed dedusting device having a series of nozzles that blow air towards the other through a fluidized bed of sand, wherein above the fluidized bed an air suction is provided which removes resin and sand dust.

11. A method according to any one of the preceding claims, wherein the thermal treatment step is carried out in a furnace at temperatures ranging from 600°C to 800°C, for a time comprised between 30 minutes and 60 minutes.

12. A method according to any one of the preceding claims, wherein the mechanical attrition step is preceded by a preliminary crushing step, wherein sand cores or pieces thereof, resulting from previous production operations, are broken and finely crushed so as to obtain single sand grains still partly covered with resin, and/or small aggregates of grains of sand held together and at least partially covered with resin, the aggregates having a size of a few millimeters.