WO2003061873A1

WO2003061873A1 - A method of treating sand

Info

Publication number: WO2003061873A1
Application number: PCT/GB2003/000219
Authority: WO
Inventors: Vaughan Wray
Original assignee: Pec Technologies Limited
Priority date: 2002-01-23
Filing date: 2003-01-23
Publication date: 2003-07-31
Also published as: GB0201420D0; GB0416249D0; GB2399528A

Abstract

An apparatus for regenerating foundry sand comprising; a first chamber (13) having a sand input (16) and a sand output (17), and a source (14) of microwave radiation, the first chamber further comprising a waveguide for microwave radiation, and the microwave source (14) supplying the first chamber with microwave energy such that substantially all of the sand passing through the chamber is heated to a temperature of at least 580°C, the apparatus further comprising a second chamber (23) associated with the first chamber (13) and positioned such that sand exiting the first chamber (13) via the sand output enters the second chamber (23), the second chamber (23) serving as a phase conversion chamber.

Description

A METHOD OF TREATING SAND

This invention relates to a method of, and apparatus for recycling sand, and particularly, but not exclusively to a method and apparatus for regenerating foundry sand.

In this specification the term "binder" is to be understood as meaning material used to bond sand together in the production of moulds pre-used in a foundry industry. Such materials include compounds such as phenolic and furanic resins, clays, slurrys, coatings or any other bonding system used for the manufacture of moulds for the foundry industry.

The term "mould" is to be understood as including also the term "core" which terms also include, where appropriate, surface coatings applied to cores or moulds.

In this specification the term "sand" means sand comprising silica, zirconium, chromite or cereabeads, or another particulate material used with a binder system for the manufacture of cones or moulds for the foundry industry.

Sand is used to make cores or moulds that are employed in the casting process in the foundry industry. The cores or moulds are formed with appropriate dimensions to form a cavity of the desired shape and size in the casting. In order to ensure that a cavity of desired dimensions is formed, it is necessary for the core or mould to maintain its shape, and to remain substantially in one piece until the casting process is complete, and the casting has solidified. Once the process is over, it is necessary to remove the sand from the cavity. This is usually achieved by breaking up the core. In order to ensure that the sand maintains its shape during the casting process, but at the same time may be broken up at the end of the process, it is known to mix the sand with an organic resin or clay that serves as a binder. At the end of the casting process, however, a resin or clay residue remains located on grains of sand.

From an environmental and economic point of view, it is preferable to be able to recycle the sand used in foundries. However, once the sand is covered with the resin residue, in general it cannot be used to form moulds, without the resin residue being removed. In the past, therefore, large amounts of sand have been discarded, resulting in the need to mine large amounts of high purity sand. Not only is this wasteful, but the mining process itself can be hazardous, causing pollution of the environment, and in some cases silicosis related deaths. In addition, the sand which is discarded as waste is passed to landfill every year causing significant pollution problems including heavy metal and furanic contamination of leachates.

Primary recycling of sand and other materials is known. This is where the sand is recycled without the removal of the resin residue. As a result additional resins or clay are added each time the sand is recycled, resulting in a build up of contaminants. This means that only a limited number of cycles can be achieved. Typically it is possible to recycle the sand for a maximum of four times before it must be discarded.

It is known to use secondary recycling methods through which most if not all of the binder elements forming the resin residue are removed from the sand particles. Known secondary recycling methods generally make use of mechanical methods, wet scrubbing or thermal oxidation. A disadvantage of mechanical methods can be that although such methods remove a high proportion of the resin residue coating the grains of sand, the removal is often incomplete, and fracturing and powdering of the grains of sand may also occur. In order for such processes to achieve effective treatment, the resin residue coating the grains of sand has to be brittle with low bond interface between the binder and the sand grains. This is often not the case especially in regions where binder elements have not been exposed to casting temperatures, or when high proportions of residual solvent results in increased elasticity.

Wet scrubbing methods can be used only if the binder residues are solvent in water. Within the foundry industry, this is often not the case.

Thermal oxidation methods tend to be more efficient than either mechanical or wet scrubbing methods. However, in general, the apparatus required to carry out thermal oxidation must be large. This means that such methods are often inaccessible to smaller companies that may not have the space or financial resources to carry out these methods.

According to a first aspect of the present invention there is provided an apparatus for regenerating foundry sand comprising: a first chamber having a sand input and a sand output; and a source of microwave radiation, the first chamber further comprising a waveguide for microwave radiation, and the microwave source supplying the first chamber with microwave energy such that substantially all of the sand passing through the chamber is heated to a temperature of at least 580°C. the apparatus further comprising a second chamber associated with the first chamber and positioned such that sand exiting the first chamber via the sand output enters the second chamber, the second chamber serving as a phase conversion chamber. In use, sand enters the first chamber via the sand input, and exits via the sand output, having been resident in the first chamber for a predetermined length of time.

According to a second aspect of the present invention there is provided a method of regenerating foundry sand comprising the steps of: exposing sand to microwave radiation in a fluidised bed such that the sand is exposed to temperatures in excess of 600°C; causing the sand to exit the first chamber and enter a second chamber which second chamber is also fluidised and wherein the temperature of the sand drops, whereby the sand undergoes an allotropic transformation from the α phase to the β phase.

Conveniently, a fluid passes through the first chamber causing the sand to behave like a fluid whilst passing through the first chamber. This ensures that agglomeration of the sand is reduced or completely avoided as the sand passes through the first chamber.

Preferably a controlled volume of the fluid travels through the first chamber at a predetermined velocity.

Preferably the fluid passing through the first chamber is air, although other fluids containing of oxygen may also be used.

The source of microwave radiation generates microwaves within the chamber, and the sand is exposed to the microwave radiation during its time in the first chamber. The microwave energy together with the fluid in the first chamber causes the sand to heat up to temperatures in excess of 600°C. At this temperature, any binder coating grains of sand will be incinerated and will become brittle. This allows any resin residue to be detached from grains of sand via a separation process after being exposed to the microwave energy for an appropriate length of time.

The effect ofthe fluid passing through the first chamber in conjunction with the microwave energy produced by the microwave radiation, is that the temperature of the sand stabilises at a temperature within the range of 350 to 600°C within seconds. There then follows a gradual decay in temperature due to thermal losses.

In addition, when sand is heated to temperatures in excess of 580°C there is an allotropic transformation of the sand from the α to the β phase. Virgin sand when extracted from the ground is present in the α phase. During the casting process, the sand is heated to temperatures in excess of 580°C, and therefore the sand transforms to the β phase. This transformation results in a significant expansion in the volume of the sand due to the different crystalline structure ofthe sand in the β phase to that ofthe α phase.

This means that when virgin sand is used to form cores or moulds during the casting process, the rapid expansion of the sand at temperatures over 580°C may result in imperfections in the solidifying castings, and undesirable cracking of moulds and cores. Sand treated in accordance with the invention will give improved control and the opportunities of simpler formulation chemistries in mould and core preparation.

The first chamber acts as a wave guide for microwave radiation generated by the microwave source, and the microwave source supplies the first chamber with microwave energy. The first chamber is thus dimensioned so that microwave radiation generated by the source is guided appropriately within the chamber so that heating ofthe sand passing through the chamber is maximised.

The rate at which the fluid forming the fluidised bed flows through the first chamber may be used to determine the residence time of the sand within the first chamber.

By means of the present invention it is possible to subject grains of sand to intense microwave energy for a relatively short period of time as the sand passes through the first chamber. Very high surface temperatures on the grains of sand may be achieved often in access of 1800°C.

Typically, grains of sand which have been used to form cores in casting processes have a resin residue layer which is approximately 10 microns thick coating the grain of sand. It is believed that the resin residue layer which has a high organic content and residual water, absorbs approximately two thirds of the microwave energy that the sand is exposed to. The sand which is largely transparent to microwave radiation, absorbs considerably less despite representing a higher volume of material.

The transformation of the sand from the α phase to the β phase takes place within the second chamber.

The second chamber is preferably fluidised and serves as a β phase conversion chamber thus optimising the allotropic transformation of the recycled sand whilst also avoiding any agglomeration effect.

Preferably, the apparatus further comprises a sand guide positioned within the first chamber for channelling the sand in a controlled manner as it passes through the first chamber. This reduces the risk that a blockage will be formed by the sand.

Preferably, the apparatus further comprises a sand classifier which classifies sand before it enters the first chamber via the sand input. In other words, the sand classifier is used to substantially evenly distribute the sand particles thus ensuring that individual sand particles receive an even and consistent dosage of microwave radiation. This further reduces the risk of a blockage.

The relative dimensions of the first chamber and the sand guide, and the position of the sand guide within the chamber ensure that sand has optimum exposure to the microwave radiation.

An advantage of the apparatus and method of the present invention is that relative energy density is low compared to other known methods and apparatus since in the present invention, there is no high thermal conduction barrier between the sand and surrounding air. Microwave energy causes a heating effect which works from the inside out. A thermal effect is therefore generated from within the resin layer itself. In time, the thermal energy dissipates from the surface into the sand to give temperature stabilisation.

Further substantially all of the produced microwave energy is absorbed in the reaction.

As the resin residue layer heats, an incineration process takes place. When the fluid passing through the first chamber comprises air or other oxygen containing fluid, the incineration process is enhanced due to the presence of oxygen. Once the resin residue has been incinerated there may be a resulting char that is friable and may be removed by simple grading.

A further advantage of the present invention is that sand treated according to the present invention returns to a condition comparable with virgin sand, with the benefit that it has been converted into the β phase. This means that a foundry can treat the sand as virgin sand, adding binders in normal quantities. This results in more uniform results when using sand regenerated by the present invention, compared to sand regenerated by other methods.

Preferably, the first chamber is positioned substantially vertically, so that sand entering the chamber via the sand input may fall under the influence of gravity through the first chamber.

The velocity of the sand falling through the first chamber will be determined and controlled by the velocity of the air stream or other fluid passing through the first chamber.

Alternatively, the apparatus further comprises one or more baffles within the first chamber.

The invention will now be further described by way of example only with reference to the accompanying drawings in which;

Figure 1. is a schematic representation showing a grain of sand coated with a resin residue;

Figure 2. is a graph showing the expansion of sand in each of the α and β phases; and Figure 3. is a schematic representation of a first embodiment of an apparatus according to the present invention;

Figure 4. is a schematic representation of a second embodiment of an apparatus according to the present invention.

Referring to Figure 1, a grain of sand which has been used to form part of a mould in a casting process in the foundry industry is designated generally by the reference numeral 10. The grain of sand 10 is coated with a layer of resin residue 11 resulting from the casting process. The resin layer 11 is typically 10 microns thick, whereas the grain of sand 10 is typically 320 microns in diameter.

Turning now to Figure 2, the percentage of expansion of sand when in the α phase compared with the β phase is illustrated graphically. It can be seen that the transformation between the α phase and the β phase occurs at about 580°C.

It can also be seen that during the transformation from the α phase to the β phase, a rapid expansion occurs, and that the conversion is permanent, and does not reverse on cooling.

Referring now to Figure 3 an apparatus according to the present invention is designated generally by the reference numeral 12. The apparatus 12 comprises a first chamber 13 in the form of a wave guide for microwave radiation. The apparatus further comprises a source of microwave energy 14 comprising a magnetron, and a microwave sink 15. The chamber 13 comprises a sand input 16 and a sand output 17. The sand input 16 and sand output 17 form part of a sand guide 18 which serves to channel sand passing through the first chamber 13 through the chamber along a predetermined path. A stream of air passes through the chamber 13, entering the sand guide 18 via the sand output 17. The air is supplied to the chamber 13 by port 19. Air travels through the sand guide 18 generally in the direction of arrow 20 and exits via sand input 16.

The chamber 13 is substantially vertical, and sand entering the chamber via the sand input 16 will pass through the chamber 13 within sand guide 18 manner assisted by gravity.

The velocity of air in the air stream forming the fluidised bed which runs in substantially the opposite direction to the movement of sand will slow down the sand as it travels through the chamber, and thus the length of residency time of sand within the chamber 13 may be determined by appropriately choosing the velocity of the air stream and the length of the chamber. A secondary gate arrangement could be employed in cases where residence times are required to be longer.

The sand is contained within a feed hopper 21 and passes through a self- cleaning sand classifier 22 and heat exchanger to precondition the sand and ensuring that the sand moisture content remains constant before entering the chamber 13. The sand classifier reduces the agglomeration of the sand. When the sand passes through the chamber, it is exposed to microwave energy produced by the magnetron 14. The microwave energy, coupled with the controlled air flow, causes flash oxidisation of the coated sand which causes incineration of the resin residue layer 11. As the sand passes through the chamber 13 temperatures in excess of 1800°C may be achieved. Within seconds however the temperature of the grains of sand falls to a temperature within the range of 350 to 600°C. Because the temperature of the sand has risen above 580°C, the sand will undergo an allotropic transformation from the α phase to the β phase.

As the sand exits the chamber 13 via sand output 17 it enters a second chamber 23 which is also fluidised. Sand entering the chamber 23 will still be at an elevated temperature, and the temperature will gradually decay within the second chamber 23 in a pre-determined manner. The second chamber also supports further oxidation of the binder until either the source of fuel is exhausted or the temperature drops below the threshold required for self combustion. The second chamber serves as a fluidised β phase conversion chamber and heat exchanger, allowing complete transformation from the α to β phase while the sand is contained within the second chamber 23. This allows the maximum amount of sand to be recovered.

A fluidised air stream enters chamber 23 via port 24 and exits via port 25. The sand contained in chamber 23 gradually moves towards output 26 where it is discharged from the apparatus. The chamber 23 further comprises a baffle plate 27 which regulates the speed at which the sand travels through the chamber 23. The baffle plate 27 is also used to control the direction of flow of the air. More than one plate 27 may be used to improve energy recovery.

Because the sand in chamber 23 is still at a higher temperature which is gradually decaying, the air entering chamber 23 via port 24 will be heated by the sand and will exit via port 25 at a higher temperature than that which it entered the chamber 23. The warm air exiting at 25 is then recycled and directed to port 19 so that heated air enters the first chamber 13. This reduces the temperature difference between the air in the chamber 13 and the heated sand. Turning now to Figure 4 a second embodiment of an apparatus according to the present invention is designated generally by the reference numeral 42. Apparatus 42 carries out a batch process as will be explained further herein below.

Parts in apparatus 42 which are similarly to those in apparatus 12 shown in Figure 3 have been given corresponding reference numerals for ease of reference.

Apparatus 42 comprises a first chamber 13 and a source of microwave energy 14 together with a microwave sink 15. Extending through the first chamber 13 is a sand guide 18. Fluid in the form of air enters the sand guide 18 by a sand output 17 and is supplied to the apparatus 42 by port 19. The air travels through the sand guide 18 generally in the direction of arrow 20 and exits via sand input 16. The apparatus further comprises a container 44 into which sand is poured prior to entering sand guide 18.

Before sand enters container 44 it is pre-processed in pre-processor 46. Preprocessor 46 comprises an input container 48 into which unprocessed sand to be regenerated is poured.

The pre-processor 46 comprises a metal fines separator 50 which separates small magnetic particles particularly ferrus metals, from the sand. Sand then enters a drying oven 52 where it is conditioned to a pre-determined moisture level.

Because the sand entering the sand guide 18 has a pre-determined moisture level, the latent heat of water required can be calculated. The chamber 13 further comprises an RF stirrer 54 which causes microwaves to evanesce around the chamber 13 and thus to pre-heat the sand before it passes through the wave guided microwave 14.

Sand collected in container 44 is released into the sand guide 18 via batch gate 56. Once all the sand has passed through chamber 13, the sand is allowed to exit into second chamber 23 by a batch out gate 58. The sand then passes along fluid bed 60 and enters a sieve section 62 where the sand is conditioned before exiting the apparatus 42. Hot fluid exiting the second chamber 23 is recycled to heat the oven 52.

Claims

1. An apparatus for regenerating sand comprising; a first chamber having a sand input and a sand output, and a source of microwave radiation, the first chamber further comprising a waveguide for microwave radiation, and the microwave source supplying the first chamber with microwave energy such that substantially all of the sand passing through the chamber is heated to a temperature of at least 580°C the apparatus further comprising a second chamber associated with the first chamber and positioned such that sand exiting the first chamber via the sand output enters the second chamber, the second chamber serving as a phase conversion chamber.

2. An apparatus according to claim 1 wherein the first chamber comprises a fluid which passes through the first chamber.

3. An apparatus according to claim 2 wherein a controlled volume of the fluid passes through the first chamber at a predetermined velocity.

4. An apparatus according to claim 2 or claim 3 wherein the fluid comprises air.

5. An apparatus according to any one of the preceding claims further comprising a sand guide positioned within the first chamber.

6. An apparatus according to claim 5 wherein the relative dimensions of the first chamber and sand guide, and the position of the sand guide within the chamber ensure that heating ofthe sand is maximised.

7. An apparatus according to any one of the preceding claims wherein the second chamber is fluidised and serves as a β phase conversion chamber.

8. An apparatus according to claim 7 wherein the second chamber is fluidised by air.

9. An apparatus according to any one of the preceding claims further comprising a sand classifier which classifies sand before it enters the first chamber via the sand input.

10. An apparatus according to any one of the preceding claims wherein the first chamber has a substantially vertical axis, and a sand guide extends substantially parallel to the axis ofthe chamber.

11. An apparatus according to claim 9 wherein the air used to fluidised the second chamber subsequently passes through the first chamber.

12. A method of regenerating foundry sand comprising the steps of: exposing sand to microwave radiation in a fluidised bed such that the sand is exposed to temperatures in excess of 600°C; causing the sand to exit the first chamber and enter a second chamber which second chamber is also fluidised and wherein the temperature of the sand drops, whereby the sand undergoes an allotropic transformation from the α phase to the β phase.

13. A method according to claim 12 further comprising an initial step of classifying the sand.

14. A method according to claim 12 or claim 13 further comprising a final step of sieving the regenerated sand.

15. An apparatus substantially as herein before described with reference to the accompanying drawings.

16. A method substantially as herein before described with reference to the accompanying drawings.