Abstract:
The plant for continuously regenerating foundry sand comprises a combustion chamber, in which a combustible gas is added to the sand, and a cooling chamber, in which the sand coming from the combustion chamber is cooled, both chambers being provided with systems for maintain the sand in a fluidized state. The chambers are also in direct communication with one another in the manner of communicating vessels in such a way that, during normal operation in equilibrium, the free surface of the sand may be at the same level in the two chambers.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a plant for continuously regenerating sand which has previously been used in foundries for the manufacture of cores. 
     According to techniques generally used for the production of complicated metal components by melting, the molten metal is injected into modules containing cores which reproduce the internal shape of the components. 
     To go into more detail, special resins, for example of the phenolic type, and suitable catalysts which permit the curing thereof, are mixed with the sand in order to give the cores firmness. 
     In recent years, it has been found to be economically advantageous to recycle the sand used for the production of such cores, after removing the residues of resins, catalysts and other impurities contained therein. 
     Thus, plants have already been produced for the regeneration of foundry sand, as illustrated, for example, in EP-A-0 055 230. Some of those plants operate continuously and may comprise a combustion chamber, in which a combustible gas is added to the sand in order to enable the above-mentioned impurities to be burnt, and a cooling chamber, in which the sand coming from the combustion chamber is cooled to ambient temperature or slightly above ambient temperature. 
     According to a first known technique, those chambers, in which the sand is maintained in a fluidised state, are arranged inside separate containers between which the transfer of sand takes place by gravity, via suitable valves and without the provision of any additional device for thermal recovery. 
     That arrangement has the disadvantage of high operating, costs owing to the substantial consumption of energy (of the order of 50 m 3  of combustible gas per tonne of treated sand). In addition, the fact that the cooling is carried out in a single stage means that it is of long duration, in order to enable the sand to be cooled from a temperature of approximately 700-800° C., which is characteristic of the combustion stage, to the final desired temperature of approximately 30-35° C. 
     Furthermore, the movement by gravity from one chamber to the other means that the plant must have a substantial vertical bulk (of the order of 7-10 m) which is not always compatible with the space available. 
     According to a further known technique, illustrated by EP-A-0 089 927, regeneration plants of the type indicated above are completed by thermal recovery devices which use the considerable heat of the streams of material leaving the combustion chamber to preheat the incoming streams of material. 
     Plants of the last-mentioned type are thus much more efficient in terms of energy than are the previous plants because they provide for a consumption of combustible gas of the order of 30 m 3  per tonne of treated sand, but they are also much more expensive and functionally complex. This is due to the fact that the heat exchange devices, in addition to constituting additional components that have to be manufactured using valuable materials, such as stainless steels, are also intrinsically delicate, being subject to a very high degree of wear and thus requiring careful maintenance which involves not inconsiderable machine stoppage times. 
     Moreover, plants of the last-mentioned type also have the disadvantages of having a substantial bulk in the vertical direction and poor versatility. 
     SUMMARY OF THE INVENTION 
     In order to overcome the above-mentioned disadvantages of the known techniques, the present invention relates to a plant for regenerating foundry sand and to the associated method of operation which have the characteristics claimed specifically in the following claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Advantages and characteristics of the present invention will become clear from the following detailed description which is given with reference to the appended drawings provided by way of non-limiting example, in which: 
     FIG. 1 is a diagrammatic representation of a plant of the invention, 
     FIG. 2 is a diagrammatic representation on an enlarged scale of a detail of the plant of FIG. 1, and 
     FIG. 3 is a diagrammatic representation of an alternative embodiment of the detail of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A plant for continuously regenerating foundry sand comprises (FIG. 1) a combustion chamber  10  which is arranged inside the top of a first column  12  and a cooling chamber  14  which is arranged inside a second column  16 , the base of which is connected by a communicating duct  18  to the base of the first column  12 . 
     Respective fans  20 ,  22  are arranged in the area of the bases of the columns  12 ,  16  and are capable of blowing air through feed nozzles  24  in such a manner as to maintain the sand in a fluidised state inside the combustion chamber  10  and the cooling chamber  14 . As will become clear from the following description of operation, the two columns  12 ,  16  constitute a system of communicating vessels in which the fluidised sand, which is comparable to a liquid, tends to reach the same level. 
     A device  26  for introducing the sand to be regenerated is arranged above the first column  12 , while an opening  28  for discharging the regenerated sand is formed at the top of the second column  16 . A duct  30  for discharging the combustion gases extends from the top of the column  12 . A duct  31  for discharging the fluidisation air from the chamber  14  extends from the top of the column  16 . 
     A perforated transverse baffle  32  for separating the combustion chamber  10  from the underlying portion  34  of the column  12  is also arranged in the first column  12 . 
     The baffle  32  has holes of a diameter such as to permit a rate of passage of the fluidisation air of not greater than 1 m/s and thus such as to permit the descent by gravity of the sand in the grain sizes normally used in foundries for the manufacture of cores. 
     A gas feed duct  36  leads into the combustion chamber  10  through a plurality of nozzles  38 , the openings of which are provided with means for preventing the penetration of sand into the inside. 
     Those means may be constituted (FIGS. 2 and 3) by diaphragms  40  of ceramic material permeable to gas but not to sand, or, in variants which are not illustrated, by plates having holes of a diameter of preferably from 0.1 to 0.15 mm. 
     As illustrated in FIG. 2, each nozzle  38  has a U-shaped path  42  for the flow of the gases, on which path the diaphragm  40  is arranged in such a manner as to divide it into at least two series-arranged portions  44 ,  46  in order to constitute a double barrier to undesired penetration of sand into the inside thereof. 
     A structure of that type thus offers better protection against such penetration than does the structure illustrated in FIG. 3, which shows two diaphragms  40  arranged in parallel along the path  42  of flow inside the nozzle  38 . 
     The functioning of the plant just described is as follows. The sand which has been used in a foundry and is charged with residues of resins, catalysts and various impurities is fed by the device  26  to the top of the first column  12  and into the combustion chamber  10  which also receives combustible gas through the nozzles  38  and air blown by the fan  20  through the nozzles  24  arranged on the base of the column  12 . 
     The thermodynamic and fluid-dynamic conditions necessary for triggering and maintaining combustion reactions of the impurities mixed with the sand are produced in the combustion chamber  10 . In particular, it is possible to provide that the inflow of combustible gas into the chamber  10  is controlled automatically in such a manner that it decreases or even stops when the temperature tends to rise above the desired value of approximately 700° C., and increases in the opposite case. 
     The fact that the openings of the nozzles  38  are provided with diaphragms  40  or other similar blocking-off means prevents the pressure that prevails inside the combustion chamber  10  from forcing grains of sand into the nozzles, even when the supply of combustible gas ceases. 
     A critical parameter of the regeneration process is the rate of air flow inside the column  12  which has to be such that the grains of sand are maintained in a state of fluidisation. 
     Experimental tests have shown that sand grain sizes of, for example, from 0.15 to 0.5 mm require an air flow rate of from 0.2 to 1 m/s because, at rates higher than 1 m/s, the downward fall of the sand is prevented while, at values lower than 0.2 m/s, the granules of sand are not fluidised. 
     The passage of material (sand and air) between the combustion chamber  10  and the underlying portion  34  of the column  12  is regulated by the holes formed on the baffle  32  and by the state of fluidisation which is established in the column. 
     The dimensions of those holes are such as to permit the descent of the grains of sand and, at the same time, to bring about a rate of upward air flow in the range of from 0.2 to 1 m/s, bursting any air bubbles generated in the course of the ascent of the fluidisation air, which would otherwise explode in an undesired manner in the combustion chamber  10 . Such an explosion would drive the grains of sand randomly in all directions and in particular also towards the bottom beyond the baffle  32 , without the grains remaining in the chamber  10  for the time necessary to complete the combustion reactions. 
     The portion  34  of the column  12  acts as a heat exchanger permitting the preheating of the air blown upwards by the fan  20  at the expense of the considerable heat of the sand coming from the combustion chamber  10 . 
     The sand then continues to flow, still in a fluidised state, through the duct  18  and the second column  16  where it is driven upwards towards the discharge opening  28  by the air blown by the fan  22 , while continuing to cool gradually. 
     The two columns  12 ,  16  thus act as a system of communicating vessels in which the admission of sand at the top of the column  12  there creates a head  48  and consequently an overpressure which causes sand to flow towards the column  16  which is filled until the sand, which is now cold, is discharged through the opening  28 . Under normal operating conditions, the level of the free surface of the sand in the two columns  12 ,  16  is equal and is determined by the height at which the opening  28  is arranged. 
     Owing to the presence of the two separate fans  20 ,  22  which are capable of producing the same pressure in the two chambers  10 ,  14 , the fluid-dynamic conditions of normal operation of the plant are those of stable equilibrium and tend to be restored even if there are disturbances such as any stoppage of fluidisation. 
     Whenever such a disturbance takes place in the cooling chamber  14 , sand is caused to flow towards the combustion chamber  10 , with a reduction in the quantity of sand present in the chamber  14 , until the stoppage is removed and the fluidisation conditions are restored. 
     As a whole, the plant of the invention, while not providing for the presence of expensive additional devices for thermal recovery, such as heat exchangers or other fluidised beds of stainless steel, nevertheless succeeds in effecting such recovery so that a more than satisfactory energy efficiency is obtained with low operating and maintenance costs. 
     Nor does the plant of the invention, which operates on the principle of communicating vessels, have to be extended in height, so that its bulk in the vertical direction is substantially reduced compared with that of conventional plants. 
     Finally, the fact that the plant of the invention does not require heat exchangers and stainless steel valves not only reduces its cost but also increases its versatility, because those devices have to be produced precisely for each specific plant size. 
     Naturally, the principle of the invention remaining the same, the details of construction and forms of embodiment may be varied widely with respect to those described purely by way of example, without thereby departing from the scope of the invention.