GB2257927A - Removal of chemical coatings from sand by combustion - Google Patents

Abstract

Chemical coatings are burned off used foundry sand in a fluidised bed (6) of the sand within a furnace (8). The bed is created in a highly conducted sheet metal can (7) of heat and oxidation-resistant nickel based alloy suspended within the furnace chamber (8'). The can is heated externally, for example by electric heaters or gas burners within the chamber, side and bottom walls of the can (above, below and at the level of the fluidised bed) being exposed for direct heating from the furnace. Sand is added to the bed by being raised down, well-dispersed, from above and overflows, after an average residence time of one or two minutes, to pass through a heat exchange unit (15) and on into a cooling unit. <IMAGE>

Description

REMOVAL OF CHEMICAL COATINGS FROM SAND BY COMBUSTION The present invention relates to removal of chemical coatings from sand by combustion and has particular, but not exclusive, application to reclamation of foundry sands used for making moulds for metal casting.

Many moulds for metal casting are made of foundry sand. Such foundry sand may be chemically treated to bind the sand grains together and give strength to the mould. It is desirable that the sand used to make a mould should be reclaimed for re-use after the mould has been used, but in order to do this the chemical binder must usually first be removed from the used sand grains.

If the products of combustion of the binder aLe entirely gaseous (e.g. carbon dioxide, sulphur dioxide or nitrogen dioxide) then a thermal reclamation process may be used. In such a process the binder is burned from the sand grains.

The most common types of binder for which thermal reclamation may be used include organic resins such as urethanes, phenolic furane with sulphuric or sulphonic acid catalyst, and air-drying oils such as linseed oil. Thermal reclamation could be used for binders which contain materials which are not gaseous on combustion but some additional processing would be required to remove the incombustible residue.

Usually, prior to reclamation, moulds and cores are shaken free from the castings and subjected to a crushing or attrition process to break down lumps of sand to individual sand grains, each grain having residual binder adhering to it. The sand is then passed to a thermal reclamation unit where the sand is heated to a temperature sufficiently high for the residual binder to be burned off.

There are several known types of thermal reclamation plant, but one common arrangement involves generating one or more hot fluidised beds of the sand grains in which heating of the sand and combustion of the coatings takes place.

In the most common types of such fluidised bed reclaimers, heating of the bed is achieved by combustion of powdered fuel, gas, or oil within the bed. This approach introduces an important source of inefficiency because the added fuel competes with the resin on the sand for the oxygen available in the air.

The introduction of extra air to provide the proper combustion of both fuel and binder increases heat loss because of the high content of inert nitrogen in the air.

Fluidised beds for sand reclamation are also known in which heating is provided by immersed electrical resistance elements, but these are particularly troublesome in a foundry casting aluminium alloys.

This is because small fragments of aluminium metal which may accompany the sand (evading a screen usually provided to retain them) fall onto the elements and melt. The molten alloy then attacks the element, which may eventually fail through liquid-metal embrittlement. To minimise that problem, elements in such beds have to be run at temperatures below the melting point of the alloy, which is typically around 5500C. Such a low temperature results in relatively inefficient operation.

Slow heating of the binder can lead to two major problems. First, the binder tends to give off copious amounts of smoke and fume. Secondly, the binder may partially melt and cause the whole mass to become sticky, with the possibility that it will fuse into a solid lump bonded with a blackened coke-like residue.

In this condition, of course, the bed solidifies and ceases to fluidise. Any immersed electrical elements would usually burn out at this juncture.

Another operational problem with heated fluidised beds can arise from instability if different parts of the bed are caused to operate at significantly different temperatures. There is a tendency for this to happen in many units because cold sand is added in bulk at one point and is then required to travel through the bed, while being subjected to heating, to a remote overflow exit. The fluidising air in the colder part of the bed is more viscous, so the flow in this part is reduced correspondingly. In the hotter parts of the bed, the lower viscosity of the air results in a relatively increased flow. The consequence of this can be that the hotter part becomes wildly turbulent, whilst the cooler part substantially shuts down and becomes a stagnant heap of sand.

A similar effect can occur in internally heated beds if a part of the bed suffers a reduction in fluidisation due, for instance, to a local blockage or blower failure. The bed then cools in this region, with the consequences referred to above.

There have been a number of thermal reclamation plants incorporating fluidised beds which have been built on a multi-level principle, in which sand from one bed cascades down to the next. Three levels are commonly used: one to preheat the incoming sand, the next to carry out the majority of the combustion of the binder, and the last to cool the sand and preheat the incoming air. The first, pre-heating, -bed can be difficult to operate; the slow heating of the sand tends to cause copious amounts of smoke and fume to be evolved, but furthermore the sand usually becomes sticky and this can lead to solidification and collapse of the bed. In those plants which are built around a tower containing such a cascade, the troublesome nature of fluidised beds is especially evident.When the tower is shut down for maintenance, or during holiday periods to conserve energy, restarting is a lengthy and frustrating exercise which is not guaranteed to have a successful outcome. Such towers are also extremely expensive to reline with refractory, which may be required every few years.

Other known systems suffer from similar disadvantages and are often inefficient and unstable in operation.

It is an object of the present invention to enable thermal reclamation to be effected in a manner which overcomes, or at least ameliorates, such disadvantages of known systems.

In one of its aspects the invention provides a method of removing combustible coatings from sand, such as chemical binders from used foundry sand, in which the sand is caused to dwell in a combustion supporting atmosphere of a fluidised bed, heat for heating the bed to a sufficiently high temperature to effect combustion of the coatings being generated by heating means located externally of the bed.

By heating the bed from an external heat source, rather than utilising internal heating by means of immersed heated elements or burners, a relatively uniform temperature can be maintained throughout the bed in a well-oxygenated atmosphere which supports efficient and rapid combustion.

Preferably sand to be treated is rained into the bed so as to be introduced into the bed with the grains well distributed. This can ensure that the individual grains are heated as quickly as possible on entering the bed and the creation of cool zones as a result of localised cooling of areas of the bed where sand is introduced is minimised.

For most efficient operation to remove common binders on foundry sand, it is ordinarily necessary to maintain the bed at a temperature not significantly less than 650"C and more preferably at a temperature of at least 7600C. It is usually necessary for the sand to have a residence time in the bed of at least twenty seconds; a residence time within the range of one to two minutes is normal.

The heating of the bed may be principally by radiant heating, and in a preferred construction of plant the heating means is arranged to heat a vessel containing the fluidised bed, wall portions of the vessel above the level of the bed serving as radiant heating elements heating the bed. The temperature within the vessel above the bed, to achieve the kind of bed temperatures referred to in the last preceding paragraph, would ordinarily be at least 8700C and optimally would be around 1000"C.

Whilst some pre-heating of the sand grains may be provided (for example where the sand is rained down into the bed through a radiant vessel as referred to in the last preceding paragraph) the efficiency of operation of the fluidised bed can be such that significant pre-heating of the sand is unnecessary.

This can mean that apart from delivery of the sand to the fluidised bed, and any provision for cooling and further handling of the treated sand leaving the bed, a single stage reclamation process may be achieved.

From another aspect the invention provides thermal reclamation plant for removing combustible coatings from sand comprising means for creating a fluidised bed of the sand and heating means located externally of the bed to generate heat for heating the bed to a sufficiently high temperature to effect combustion of the coatings in a combustion-supporting atmosphere provided by the bed.

The heating means may be arranged to heat the bed principally by radiant heating. In a preferred construction of such plant, the fluidised bed is created within a heated vessel which contains the bed, the vessel comprising heated wall portions above the level of the bed which form radiant heating elements for heating the bed. The vessel can be of metal (for example, Inconel) and may conveniently be in the general form of an open-topped can. There may be a substantial free volume above the fluidised bed within the vessel, since the vessel needs to have a sufficient upstanding wall area, above the bed, for the radiant input of heat into the body of the vessel to be at a required rate for heating the bed, without exceeding the temperature at which the wall can survive. The heating means may comprise electrical heating elements or one or more gas burners to heat the vessel.

Preferably, fumes emerging from the bed have to travel a substantial distance up through the hot interior of the vessel, giving plenty of opportunity for them to be consumed prior to escape through an exhaust flue. A large free volume in the vessel can also be effective in reducing the carry-over of sand fines into the exhaust.

For feeding sand to the bed, the plant may comprise feeding means having an outlet provided above the bed and arranged to disperse the grains being added to the bed, the sand consequently being rained down into the bed. Ensuring that the sand falls on to the bed in a diffuse rain, can prevent the formation of localised concentrations of cold sand in the bed. The sand-feeding arrangement may be such that the incoming sand falls a considerable distance through the vessel before entering the fluidised bed. This can help to ensure that the sand enters the fluidised bed well dispersed and at speed.

The bed-creating means may comprise one or more toroidal sparge tubes positioned below the operating level of sand in the bed to provide air to the bed.

The plant may further comprise a heat exchanger to which sand exiting the fluidised bed is passed and through which air to fluidise the sand is drawn. The sand exiting from the bed can thus conveniently be used to preheat the incoming oxidising air used for fluidisation. In this way the efficiency of the reclaimer is improved. A small fluidised bed has been found to be a suitable form of heat-exchanger.

The heat exchanger bed may also be useful for conveying the hot sand to a cooling unit situated some distance away. There the sand may be cooled to room temperature or thereabouts, as required, by conventional means such as allowing it to flow through a fluidised bed with immersed tubes containing cooling water.

An embodiment of the invention will now be described in detail, by way of example, with reference to the accompanying drawings in which: Figure 1 is a sectional view of reclamation plant embodying the invention; and Figure 2 is a perspective view showing a heat exchanging bed of the plant in more detail.

Used foundry sand which is coated with combustible chemical binder and which has been crushed to free the individual sand grains is introduced into a hopper 1.

A sliding gate 2 is provided at the outlet from the hopper to control the rate at which sand drains from it.

From the hopper, the sand falls on to a horizontal vibratory conveyor 3 which deposits the sand into a vertical charging pipe 4 which extends down through a roof 9 of a reclaiming furnace 8. The sand falls from the pipe 4 on to a deflector 5 which causes the sand to become distributed conically to fall as a fine rain on to a fluidised bed 6 within the furnace 8.

A thin-walled vertical cylindrical can 7 of Inconel metal is provided within the furnace beneath the charging pipe 4. The can 7 has an open mouth 7' at its upper end. Its bottom end is closed and provides a floor 7" beneath the fluidised bed 6.

The can is suspended within a chamber 8' of the furnace, the mouth 7' of the can being positioned immediately beneath the roof 9. The can 7 is arranged to be heated by means of electrical resistance-heated elements (not shown) which embrace the can externally.

The charging pipe 4 extends through one aperture in the roof and an exhaust flue 10 extends through another aperture above the can to provide for extraction of fume from the can.

Coaxial apertures are provided through a bottom wall 8" of the furnace 8 and through the floor 7" of the can 7. An open-ended overflow pipe 11 extends through these apertures and projects some way upwards above the floor of the can. The level of sand in the fluidised bed 6 can rise until it reaches the open top end of the overflow pipe whereupon sand will flow into the pipe and fall downwards out of the can 7. The deflector 5 is arranged so that sand being fed into the top of the can does not fall directly into the overflow pipe 11 but instead falls on to the fluidised bed radially outwards of it.

An air feed pipe 12 of smaller diameter than the overflow pipe 11, is mounted coaxially within the overflow pipe 11 and projects out of the top of it Branch tubes 13 carry air radially outwards from the top of the air feed pipe 12 to coaxial, coplanar, inner and outer toroidal sparge tubes 14' and 14" which supply air from holes in their undersides within the fluidised bed. At a point below the furnace the air feed pipe 12 passes out through the wall of the overflow pipe 11.

Sand passing down the overflow pipe 11 is led to a second fluidising unit 15 (see Figure 2) forming a heat-exchanger unit. The second fluidising unit 15 comprises a container 16 in which the sand is collected and a sparge tube arrangement 17 having a plurality of horizontal tubes 18 which- supply fluidising air downwards to fluidise the sand. Sand enters the container 16 near one end from the overflow pipe 11 and travels through a labyrinth (not shown) to an exit pipe 19 at an opposite end of the container.

The air feed pipe 12 passes through the container 16 at a level within the fluidised sand. This causes the air passing through the pipe to be warmed by the sand, so pre-heating the air before it enters the fluidised bed 6 in the can 7. The feed pipe 12 may be provided with internal and/or external fins to aid heat transfer from the bed to heat the air.

The second fluidising unit 15 provides for only slight cooling of the sand in pre-heating the fluidising air supply to the main bed 6. A modest amount of reclamation takes place within the second unit since the sand is still very hot and there is plenty of air available for combustion. The unit 15 also serves a practical purpose in conveying the sand horizontally from beneath the furnace 8 to a cooling unit (not shown) beyond the exit pipe 19.

The fluidised bed 6 is of a size to provide a sufficient dwell time of one to two minutes for the sand in the oxidising environment of the bed; the air for fluidisation also provides the oxygen for combustion of the binder. The bed 6 receives some heat through the side walls and closed end 6' of the can 7 below the level of the bed, but principally receives heat by radiation from hot wall portions of the can forming radiant heating elements above the level of the bed.

When managed optimally the process consumes approximately 100kwh per 1000 kg of sand containing 1 per cent binder. For higher calorific binder contribution the energy input is correspondingly reduced, as it is for better insulation around the body of the furnace, and for more effective heat exchange between the outgoing sand and incoming air. It is possible that 50 kwh per 1000 kg might eventually be attainable, particularly in larger units where there will be benefits of scale.

The infalling sand enters a relatively large mass of fluidised sand at high temperature, so that the temperature of the added sand is quickly raised above that at which smoke and fume will form. Any slight release of fume may be dealt with by oxidation of the fume itself in the hot free volume above the fluidised bed so assuring complete combustion of the fume.

Rapid combustion of the binder assists the effectiveness of the over all process by contributing additional heat.

For coated silica sand containing approximately 1 per cent binder the requirement of air is approximately 0.5 cubic metres per second (100 cfm) to within a factor of 2. For 2 per cent binder the air requirement is doubled, and generally the requirement for air increases with the content of binder. Less air than this optimum will inhibit combustion of the binder.

More air will require more heat and reduce the total efficiency of the reclaimer. The volume of air required for combustion has been found to be almost exactly the volume required to support the fluidisation of a bed of sand of the order of 1 metre square and 300 mm deep.

The furnace shown in the figure is capable of reclaiming approximately 750kg per hour of sand, for a useful electrical input of approximately 75kw.

This is a rather small unit with an exterior volume of about 1 meter cubed. (It could alternatively be heated by a gas-fired burner within the chamber 8' (but outside of the can 7) the products of combustion being kept separate from the oxidising environment required for reclamation by the can 7 and exhausted through an additional flue). The whole furnace 8 and can 7 are designed for easy disassembly and maintenance.

Maintenance of the plant requires occasional attention to the fluidising sparge tubes 14 to remove sand which has found its way into them. This is only required when the fluidised bed is seen to be severely disabled by extensive heaps of unfluidised sand.

Maintenance is also required for the furnace. In the case of an electrically heated furnace, the elements may require occasional replacement. In some designs of furnace this can be conveniently carried out by removing exterior panels. Usually, however, it is necessary to remove the can and change the elements from inside the furnace.

Claims (4)

1. A method of removing combustible coatings from sand in which the sand is caused to dwell in a combustion-supporting atmosphere of a fluidised bed, heat for heating the bed to a sufficiently high temperature to effect combustion of the coatings being generated by heating means located externally of the bed.

2. A method according to claim 1 in which heating of the bed is principally by radiant heating.

3. A method according to claim 2 in which the heating means is arranged to heat a vessel containing the fluidised bed, wall portions of the vessel above the level of the bed serving as radiant heating elements heating the bed.

4. A method according to any preceding claim in which sand to be treated is rained on to the bed so as to be introduced into the bed with the grains well distributed.