GB1589076A

GB1589076A - Reclamation of foundry sand

Info

Publication number: GB1589076A
Application number: GB2494277A
Authority: GB
Original assignee: Fordath Ltd
Current assignee: Fordath Ltd
Priority date: 1978-04-11
Filing date: 1978-04-11
Publication date: 1981-05-07

Description

(54) RECLAMATION OF FOUNDRY SAND (71) We, FORDATH LIMITED, a British Company of Brandon Way, West Bromwich, West Midlands, B70 8JL, do hereby declare this invention for which we pray that a Patent may be granted to us to be particularly described in and by the following statement: This invention relates to a method and apparatus for the reclamation of used foundry sand.

Foundry sands are usually treated with a binder to make them adhere together in use.

Such binders frequently include some form of organic resin which becomes partly degraded during the moulding process.

Although the used sand can simply be disposed of and fresh sand and binder used, this is essentially wasteful and there are potential environmental hazards to the dumping of foundry sand, in view of the possibility of phenol and similar chemicals being leached into the water courses. It is therefore desirable to reclaim and re-use the sand.

All traces of the original binder should desirably be removed before the sand is reused, to prevent reduction in bond strength in moulds and cores made from the reclaimed sand and to reduce the risk of increased gas evolution during the moulding process.

The sand may be broken up into lumps and reduced to grain size by various mechanical means but this still leaves the grains of sand with a carbonaceous coating. It is known to remove this coating by burning and the principle has been put into effect using various types of furnace such as fluidised bed, rotary drum, shaft or multiple hearth furnaces, in which the sand grains are heated in bulk with excess air to about 800 C.

The capital cost of the equipment is relatively high and the fuel consumption is large relative to the output of reclaimed sand, since the operation requires the entire bulk of the sand to be raised to combustion temperature and to remain in the combustion zone for anything from 15 to 30 minutes or more. After this heating stage, the body of the sand then has to be cooled which again requires expensive equipment and takes a substantial amount of time.

U.S. Patent Specification No. 3 686 034 proposes to improve the efficiency of thermal reclamation by achieving substantially complete combustion of the coating on the sand grain with a relatively short retention time in the combustion zone. The proposed method is to allow the sand to gravitate downwardly through a vertical furnace having a number of vertically spaced sand grates, the grains being exposed to radiant heat from burners in the sides of the furnace.The process described envisages that the sand should spend a period of at least 15 seconds passing through the apparatus to achieve substantially complete combustion of the grain coating, based on the burners operating in a temperature range between about 900 C and 1000 C. The temperature of 1000"C is regarded as an upper limit to avoid the temperature of the sand grain itself exceeding about 650"C which is alleged to be a critical temperature for the sand.

The apparatus described in U.S. Specification No. 3 686.034 would have a height of about five metres, involving substantial capital outlay and, in operation, critical temperature control of the burners is required if the process is to be reasonably effective.

It is an object of the present invention to provide a method and apparatus for thermal reclamation of foundry sand which is more economical and efficient than processes and apparatus referred to above.

According to the invention there is provided a method of thermal reclamation of used foundry sand of substantially grain size having a spent organic binder coating thereon, the method comprising impelling the coated grains rapidly, in the presence of a supply of oxygen, through a radiant heat combustion zone wherein substantially complete combustion of the organic grain coating takes place, the speed of travel of the grains being high enough to ensure that a grain of sand takes no more than one second to pass through the combustion zone and the temperature of the zone being such that burning off of the organic grain coating occurs without substantial heating of the grains themselves.

The coating on a grain of used foundry sand is extremely thin compared with the size of the grain itself and, provided that the temperature of the combustion zone is sufficiently high and that sufficient free oxygen is present to ensure substantially complete combustion of the coating, the coating can be burned off without raising the temperature of the sand grain significantly in a minimum amount of time.

Preferably, the time taken by the sand grains to pass through the combustion zone is between one twentieth and one second. A preferred time is approximately one fifth of a second.

The temperature of the combustion zone may be between 1000 C and 2000 C. A preferred temperature is approximately 1500 C.

The invention also provides apparatus for carrying out the method for the thermal reclamation of used foundry sand of grain size, and having a spent organic binder coat thereon, the apparatus comprising a combustion chamber having an inlet and an outlet, radiant heat generating means operable to heat the combustion chamber to a temperature greater than approximately 1000 C, oxygen supply means and used sand grain supply means for supplying sand to said inlet, the sand being transported rapidly, during operation of the apparatus, to said outlet and being discharged therefrom.

The radiant heat may be generated by burning of combustible fuel. This fuel may be gas or oil for example. In this case, the oxygen supply means may comprise means for supplying excess oxygen gas or air to the fuel burner or burners.

Alternatively, the radiant heat may be generated electrically.

The oxygen supply means may alternatively comprise a supply of oxygen gas or air via the sand supply means.

The combustion chamber may comprise an elongate tubular structure having the inlet and outlet at or adjacent opposed ends.

Preferably, the tubular combustion chamber is inclined downwardly towards the outlet end.

An expansion chamber may be provided, communicating with the outlet and having a reclaimed sand collector. The invention will now be described in more detail by way of example only with reference to the accompanying drawing which shows an apparatus for thermal reclamation of used foundry sand.

Referring to the drawings, the apparatus comprises a combustion chamber generally indicated at 10, an expansion chamber 11, a sand inlet hopper 12 and a burner 13.

In operation, used foundry sand broken down to grain size is supplied to the sand inlet hopper 12, is transported rapidly along the combustion chamber 10 and falls into the expansion chamber 11 in a reclaimed condition, substantially free of the resin binder which was adhering to the grains supplied to the inlet. During passage through the combustion chamber 10, the resin coating on the grains is heated very rapidly by the burner 13 with an excess of oxygen supplied through the burner, the combustion gases expanding on reaching the expansion chamber 11 and being discharged to atmosphere, via a chimney 14 may discharge directly to the atmosphere or may conduct the fumes to suitable cleaning apparatus.The reclaimed sand is discharged via a discharge hopper 15 for re-use and is in a relatively cool condition having a temperature below about 200 C. It will be necessary to cool the sand to room temperature before re-use.

Referring to the apparatus in more detail, the burner 13 is supplied with combustible gas or vapourised fuel oil, together with air or oxygen to support the combustion of the fuel and an excess of air or oxygen gas to enable burning off of carbonaceous resin from the sand grains to be treated.

The combustion chamber 10 is approximately three to four metres in length annd is lined with a suitable refractory material 16. The combustion chamber is of square cross-section having dimensions of approximately 0.2 metres. It is fabricated from mild steel in two sections, joined together at the flanges 17. The burner 13 is sealed into one end of the combustion chamber as shown, below the sand inlet hopper 12.

The combustion chamber slopes downwardly from this inlet end towards the expansion chamber 11, at an angle of approximately 45 .

This ensures that sand cannot build up on the base of the combustion chamber in use.

It will be appreciated that the dimensions given for the combustion chamber can be varied and that the cross-section could be circular or of any other chosen shape. The description given is by way of example only.

The sand supplied through the hopper 12 is propelled rapidly through the combustion chamber by the burning gases coming from the burner 13. Additional air or other propellent gas may be provided if desired but we have found that this may cause blowing back of the burner flame and for this reason we prefer to allow the sand to be drawn into the burner gases in the manner described. Also for this reason, we prefer that excess air or oxygen gas is supplied through the burner to ensure combustion of the resin coating.

The temperature of the flame is preferably approximately l500 C and, with a combustion chamber length of three to four metres, the time taken for the sand to pass from the inlet to the outlet end is approximately one fifth of a second.

The coating is burned off during this time but the core temperature of the sand is elevated only slightly. The higher the temperature in the combustion chamber the greater is the heat transfer rate to the resin coated sand grains, thus the higher the temperature, the shorter is the time required to burn the resin layer so avoiding heat up of the sand grains.

The combustion gases reaching the outlet 18 of the combustion chamber reach the expansion chamber and expand into its relatively large volume. At this stage, the speed of the combustion gases drops rapidly and the speed of travel of the sand particles is reduced. A baffle plate 19 is provided within the expansion chamber 11 to prevent undue wear from the abrasive sand grains and deflects the grains downwardly into the discharge hopper 15 which may be controlled by a valve. An observation window 20 enables the sand level to be monitored. Additionally, this enables the operator to ensure that the flame from the burner 13 is confined within the combustion chamber.

It will be appreciated that the combustion of the resin on the sand grains is exothermic and adds to the heating effect obtainable from the burner alone.

The apparatus illustrated can be modified in various ways and in particular, the combustion chamber can be disposed horizontally. The apparatus can be extended to provide greater sand treatment capacity. However, it is estimated that a through-put of approximately one ton per hour can be dealt with using a combustion chamber having the dimensions referred to above.

The burner could be replaced by using electric radiant heaters disposed in the walls of a refractory chamber. However, in this case, a separate supply of air or oxygen is required to support combustion of the binder.

The following actual results were obtained using a test furnace having a length of approximately 1.2 metres and a square crosssection of approximately 0.2 metres in dimension. A sand treated with a furane coldsetting resin and then used in a foundry process was broken down to approximately grain size and supplied to the combustion chamber in which a gas burner maintained a temperature of between 1000 C and 1100 C.

Before reclamation, the adherent binder, as measured by the loss on ignition, was approximately 1.2% by weight of the sand. After treatment, the loss on ignition dropped to 0.25% approximately and the discharge temperature of the sand from the apparatus was about 150 C.

The sand was suitable for re-use and was slightly blackened in colour, indicating the presence of carbon and hence indicating that the remaining binder residue had been carbonised, leaving little or nothing of the original binder unchanged.

It is believed that, by use of a higher temperature, these figures could be substantially improved.

WHAT WE CLAIM IS:- 1. A method of thermal reclamation of used foundry sand of substantially grain size and having a spent organic binder coating thereon, the method comprising impelling the coated grains rapidly, in the presence of a supply of oxygen, through a radiant heat combustion zone wherein substantially complete combustion of the organic grain coating takes place, the speed of travel of the grains being high enough to ensure that a grain of said takes no more than one second to pass through the combustion zone and the temperature of the zone being such that burning off of the organic grain coating occurs without substantial heating of the grains themselves.

2. A method according to Claim 1 wherein the time taken by the sand grains to pass through the combustion zone is between one twentieth and one second.

3.A method according to Claim 2 wherein the time taken is approximately one fifth of a second.

4.A method according to any preceding claim wherein the temperature of the combustion zone is between approximately 1000 C and 2000 C.

5. A method according to Claim 4 wherein the temperature is approximately 1500"C.

6. Apparatus for carrying out the method for thermal reclamation of used foundry sand of grain size and having a spent organic binder coat thereon comprising a combustion chamber having an inlet and an outlet, radiant heat generating means operable to heat said combustion chamber to a temperature greater than approximately 1000 C, oxygen supply means, and used sand grain supply means for supplying sand to said inlet, the sand being transported rapidly during operation of the apparatus to said outlet and discharged therefrom.

7. Apparatus according to Claim 6 wherein the radiant heat is generated by burning of combustible fuel.

8. Apparatus according to Claim 7 wherein the fuel is fuel gas.

9. Apparatus according to Claim 7 wherein the fuel is fuel oil.

10. Apparatus according to any one of Claims 7 to 9 wherein the oxygen supply means comprises means to supply excess oxygen gas or air to the fuel burner or burners.

11. Apparatus according to Claim 6 wherein the radiant heat is generated electrically.

12. Apparatus according to any one of Claims 6 to 11 wherein the oxygen supply means comprises a supply of oxygen gas or air via the sand supply means.

13. Apparatus according to any one of Claims 6 to 12 wherein the combustion chamber comprises an elongated tubular structure having the inlet and outlet at or adjacent opposed ends thereof.

14. Apparatus according to Claim 13 wherein the combustion chamber is inclined downwardly towards the outlet end thereof.

15. Apparatus according to any one of Claims 6 to 14 wherein an expansion chamber is provided communicating with the outlet and having a reclaimed sand collector.

16. A method of reclaiming used foundry sand according to Claim 1 and substantially as hereinbefore described.

17. Apparatus for reclaiming used foundry sand substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. monitored. Additionally, this enables the operator to ensure that the flame from the burner 13 is confined within the combustion chamber. It will be appreciated that the combustion of the resin on the sand grains is exothermic and adds to the heating effect obtainable from the burner alone. The apparatus illustrated can be modified in various ways and in particular, the combustion chamber can be disposed horizontally. The apparatus can be extended to provide greater sand treatment capacity. However, it is estimated that a through-put of approximately one ton per hour can be dealt with using a combustion chamber having the dimensions referred to above. The burner could be replaced by using electric radiant heaters disposed in the walls of a refractory chamber. However, in this case, a separate supply of air or oxygen is required to support combustion of the binder. The following actual results were obtained using a test furnace having a length of approximately 1.2 metres and a square crosssection of approximately 0.2 metres in dimension. A sand treated with a furane coldsetting resin and then used in a foundry process was broken down to approximately grain size and supplied to the combustion chamber in which a gas burner maintained a temperature of between 1000 C and 1100 C. Before reclamation, the adherent binder, as measured by the loss on ignition, was approximately 1.2% by weight of the sand. After treatment, the loss on ignition dropped to 0.25% approximately and the discharge temperature of the sand from the apparatus was about 150 C. The sand was suitable for re-use and was slightly blackened in colour, indicating the presence of carbon and hence indicating that the remaining binder residue had been carbonised, leaving little or nothing of the original binder unchanged. It is believed that, by use of a higher temperature, these figures could be substantially improved. WHAT WE CLAIM IS:-

1. A method of thermal reclamation of used foundry sand of substantially grain size and having a spent organic binder coating thereon, the method comprising impelling the coated grains rapidly, in the presence of a supply of oxygen, through a radiant heat combustion zone wherein substantially complete combustion of the organic grain coating takes place, the speed of travel of the grains being high enough to ensure that a grain of said takes no more than one second to pass through the combustion zone and the temperature of the zone being such that burning off of the organic grain coating occurs without substantial heating of the grains themselves.

8. Apparatus according to Claim 7 wherein the fuel is fuel gas.

9. Apparatus according to Claim 7 wherein the fuel is fuel oil.