Sand reclamation

Details Sand reclamation Sand reclamation

1999-06-11

2001-09-11

Dunn, Tom (Department: 1722)

Metal founding

Process

Including recycling of process material

C164S520000, C164S525000

Reexamination Certificate

active

06286580

ABSTRACT:

This invention relates to the reclamation of sand, for example silica sand, which has been used to produce moulds and cores in foundries, and in particular to the reclamation of sand which has been bonded with an alkaline resol phenol-formaldehyde resin in order to produce the moulds and cores.
When used to make foundry moulds and cores sand is mixed with one of a variety of binders such as bentonite clay, sodium silicate or a resin. Due to the effect of exposure to metal casting temperatures and contact with molten metal the sand becomes contaminated with binder decomposition products, metallic particles and other debris. The sand must therefore be replaced by new sand when making further moulds and cores, or if the sand is to be reused it must first be treated to remove at least some of the contaminants.
Due to the cost of virgin sand and the cost of disposal of used sand, and also due to the strict regulations which now exist governing the disposal of waste materials in land fill sites, foundries now wish to increase the level of reclaimed sand which they use.
If sand is to be reclaimed successfully the reclamation process must not only restore the condition of the sand by breaking down agglomerates and removing particles of metal, but the process must also enable the reclaimed sand to be reused, preferably with the same type of binder as before.
In recent years there have been introduced a number of methods of mould and core production which utilise an aqueous alkaline solution of a resol phenol-formaldehyde resin as the binder. In one such method the resin is cured by means of an ester which is mixed with the sand and the resin. In another method the mixed sand and resin is formed to the desired shape, and the resin is cured by passing a vaporised ester such as methyl formate through the formed shape. In another method in which curing of the resin is achieved by complexing the resin with borate ions, the binder used contains both the resin and the borate ions and the alkalinity of the binder solution is such that complexing is prevented. After forming of the sand-binder composition carbon dioxide gas is passed through the formed shape, thereby reducing the pH of the binder and triggering cross-linking by the borate ions.
One process which is commonly used to reclaim foundry sand is a dry attrition process in which the sand is subjected to a rubbing or abrasive action, which breaks up agglomerates into individual particles, and which removes adhering binder residues from the sand particles. The binder residues and fine sand particles are then removed by classification. The dry attrition process on its own is insufficient as a viable process for reclaiming sand which has been bonded with an alkaline resol phenol-formaldehyde resin. The attrition process does not remove all the resin residues from the sand particles, and the re-bonding properties of the reclaimed sand are inferior when compared to the bonding properties of new sand. As a consequence the dry attrition process usually allows re-use of only up to about 80% of the resin bonded sand, so this means that the remainder has to be disposed of. As the used sand contains a high level of phenolic and alkaline residues disposal is more of a problem and more costly compared with the disposal of some other used foundry sands.
Another process which is commonly used to treat used foundry sand is a thermal reclamation process in which the used sand is heated to a sufficiently high temperature to remove any binder residues which are present. In one type of thermal reclamation process a rotary unit is used and in this process lumps of agglomerated used sand or crushed used sand are fed to the unit. In another thermal reclamation process the thermal treatment is done in a furnace having a fluidised bed, and the used sand which is fed to the furnace is first subjected to an attrition process so as to break down agglomerates into individual particles. A thermal reclamation process which utilises a fluidised bed is described in GB-A-2244939. Thermal reclamation is usually carried out at a temperature of the order of 400 to 800° C.
In practice there are problems in reclaiming foundry sand which has been bonded with an alkaline resol phenol-formaldehyde resin, by a thermal process, particularly when the thermal treatment is done in a fluidised bed, because the individual particles of used sand tend to re-agglomerate during the process. Due to the presence of an appreciable quantity of alkali in the resin binder, the used sand contains sodium or potassium compounds (usually potassium compounds, because it has been found that potassium is more beneficial than sodium in such resin binders), and it is believed that during the thermal treatment these alkaline compounds on the surface of the sand particles decompose or melt and cause the sand particles to fuse together.
In rotary thermal reclamation units there may be sufficient sand on sand attrition to prevent fusion between individual sand particles from taking place. However, although the fused bond is relatively weak, it is not readily broken in a fluidised bed reclamation unit, and the agglomerates formed by the fusion of the particles prevent the fluidising gas from maintaining an effective fluidised bed. As a result blockage, and ultimately failure of the unit can occur.
It would be possible to remove the alkaline compounds from the used sand by washing and drying the sand prior to thermal reclamation. However the washing treatment and subsequent drying would add considerably to the cost of the reclamation process, and would be uneconomic.
It has also been proposed to incorporate additives in the used sand prior to the thermal reclamation process in order to prevent fusion of the sand particles. WO 94/05448 describes the use of an additive, for example a halogen acid, sulphuric acid, boric acid or an ammonium salt of such acids such as ammonium chloride, which will convert the potassium hydroxide and other salts in used sand which had been bonded with an ester-cured phenolic resin into a potassium compound having a melting point above 550° C. WO 94/26439 describes the use of a clay of particle size less than 0.5 mm, such as a kaolin or a montmorillonite, which will react with elutable alkali contained in the used sand.
These additives suffer from disadvantages. They themselves, or compounds which they produce by chemical reaction with the alkali, remain in the sand after the reclamation process, and can have deleterious effects when the reclaimed sand is used to make moulds or cores. Acidic additives suffer from the additional disadvantage that they can corrode components of the reclamation equipment.
It has now been found that the thermal reclamation of used foundry sand which has been bonded using an alkaline resol phenol-formaldehyde resin can be improved if prior to the thermal treatment of the used sand a carbohydrate material is mixed with the sand.
According to the invention there is provided a process for thermally reclaiming sand which has been used to make foundry moulds or cores and which has been bonded using an alkaline resol phenol-formaldehyde resin, comprising subjecting lumps of the used and bonded sand to attrition in order to break up the lumps into individual sand grains, adding a carbohydrate to the sand grains in the amount of 0.25% to 5.0% by weight based on the weight of the used sand, and subjecting the sand to thermal treatment in a thermal reclamation apparatus, such that the carbohydrate is removed from the sand by combustion.
The thermal reclamation treatment may be done in other equipment, such as a rotary thermal reclamation unit, but the treatment is preferably done in a fluidised bed reclamation unit, and prior to the addition of the carbohydrate additive, the sand is subjected to dry attrition to break down lumps and agglomerates of used sand into individual particles, and then classified. The fluidised bed reclamation unit may be apparatus of the type described in GB-A-2244939.
Accordingly, in a preferred embodiment of the invention the sand which

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