FR2472433A1 - PROCESS AND APPARATUS FOR THE PREPARATION, ESPECIALLY COOLING AND MIXING OF MOLDING SAND - Google Patents

Abstract

THE INVENTION RELATES TO A PROCESS AND AN APPARATUS FOR THE PREPARATION, IN PARTICULAR THE COOLING AND MIXING OF MOLDING SAND. PROCESS CHARACTERIZED IN THAT THE COATING OF THE SURFACE OF THE GRAINS OF SAND WITH THE BINDER IS PERFORMED IN A VACUUM.

Description

The invention relates to a process for the preparation, in particular the

  cooling and mixing of molding sand, in which the used sand from the sand ball is cooled, optionally purified, mixed and homogenized with other bodies by adding water and agitating vigorously, the surface of the sand grains being at least partially coated with a binder. The invention

  also relates to an apparatus for carrying out the method.

  The main purpose of sand-making for foundry molds is to achieve the correct mixing ratio of grain sizes and the correct proportions of quartz sand, binder, coal dust, and used and new sand. , to homogenize the mixture, to coat the binder grain to a large extent, to establish the correct humidity, to eliminate unusable constituents such as sand lumps, iron, dust, to establish the temperature correct and route the sand to the points of use. The recycled fraction of used sand from

  of the sand ball can go up to 80% and more.

  Since the sand from the root ball is obtained at times

  high temperatures, for example from 100 to 140C, it is necessary

  cool the used sand before introducing it into the

  Width. It is often enough to cool it down to

  temperatures from about 35 to 450C. Sand is most often cooled with moisture and large amounts of air injected through the sand so that

  take advantage of evaporative heat for cooling.

  The quantities of air required for this purpose are extremely

  large. In a preparation facility that processes

  80 tons of sand per hour, sufficient cooling requires several hundred thousand cubic meters of air per hour and an energy expenditure of up to 160 kWh and more. Sand cooling facilities have a footprint

all the more so.

  The cooled sand is introduced into a mixer.

  than other bodies, especially new sand and binder.

  This is usually a batch mixer. We can do

  kill the mixing and homogenization using rotating wheel mixers that exercise on the sand not only a

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  mixing action, but still a kneading action. Toute-

  times, mixers with

  a tank which has, at its bottom, a rotary stirring blade

  continually transporting the material in the direction of

  upward and obliquely upward, as well as two or more rotating tourbillon groups

  from above just above the upper edge of the blades

  tutors, grab the sand and the constituents and make them

  swirl strongly. The role of this whirlpool is to homo-

  generate the charge in a very short time, for example 90 seconds, and superficially coat a binder each

grains of sand.

  More precise research has shown that a complete coating is not obtained. The binder, most often clay,

  under the effect of agitation the form of small tongues

  pastes that only partially apply against the surface of the grains of sand and also coat them only partially. To swirl the load, we use the most

  often agitating stars that arrive in close proximity to

  diate of the stirring blades rotating on the

  at speeds of 1500 rpm and more

ge.

  The object of the invention is to provide a method which enables

  not only to improve the mixing process, but also to ensure cooling and mixing with a significantly lower energy expenditure and bulk

generally.

  According to the invention, this problem is solved by virtue of the fact that the sand and the binder are mixed under vacuum, that is to say in a mixing zone where the pressure is notably lowered.

  relative to the ambient atmosphere.

  Advantageously, sand and water are introduced into a zone of tight mixing with respect to the external atmosphere, they are swirled, a vacuum is generated in the zone of

  mixing and evacuating gases and vapors from the mixing zone.

  ge. This process offers significant advantages. First, it appeared that the molding sand thus treated, undergoing

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  Moreover, the same treatment and having the same composition, present during molding a noticeably lower elasticity, thus a greater precision of molding and easier demolding. The explanation could be that in the known mixing processes, not only sand, binder and other corresponding additives are mixed and mixed together in the mixing zone, but that air also forms in these processes a constituent of the mixture. Particularly in case of strong turbulence, the air reaches high speeds in the mixing zone and can thus further impede the direct application of the binder tongues against the surface of sand grains. In particular, it is possible that small amounts of air are each time trapped tightly between the surface of the sand grains and the binder tongues and can not be expelled during molding, even under high pressures. As a result, the adhesion of the binder layer to the surface of the sand grains is less good, as well as the adhesion of the sand grains to each other. In addition, small amounts of trapped air increase the elasticity of the molds

of sand made.

  Another notable advantage of the new process lies

  in that a significant part of the cooling process

  may occur during the clean mixing process.

  said. This is because as a result of the application of the vacuum to the mixing zone, the moisture contained or introduced into the quantity of sand does not vaporize only but

  evaporates by boiling, so that it is able to em-

  add significantly more heat to the amount of sand used and dissipate it. Thanks to the cooling of the used sand within the mixing zone, it becomes possible to completely do without the usual sand cooling devices or to give them a considerably smaller dimension, for example simply in the form of a precooling device . Since, furthermore, in the new process, the heat dissipation is carried out by means of steam in a closed system, that is to say the vacuum system, it is possible, in a simple way, to recover

  usefully the heat thus removed from the quantity of sand.

  For this purpose, it is advantageous to cool and to condense

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  on condensing surfaces the vapors removed from the

  mixing zone, preferably keeping the vacuum.

  It is important to bring the vacuum into the mixing zone to a level which ensures that at the residual pressure and the temperature of the sand the introduced moisture does not simply vaporise, but evaporates by boiling. It has been found that a pressure of less than 17 torr, preferably about

torrs, was advantageous.

  Cooling and condensation of the atmosphere

  almost gas-free steam is advantageous.

  the heat-absorbing part of a heat pump, which transfers the heat thus recovered to a

heat consumption circuit.

  In an exemplary practical embodiment, it has been found advantageous that the used sand is introduced into the mixing zone at a temperature of 50 to 90.degree. C., preferably 55 to 60.degree. In the case of a load of 2 t, for example, in a mixing time of 60 to 180 seconds, the temperature can be lowered.

sand at a level of 30 to 40C.

  In some cases, it appeared advantageous, after filling the tank, to start by swirling the mixture and then to evacuate the tank, after which, after a predetermined mixing time, for example 30 seconds Water is introduced, preferably by injection or spray, into the swirl mixture. In other cases, it may also be advantageous to start by swirling the used sand with water and evacuating the mixing zone and adding only the binder to the mixture.

  after a predetermined mixing time.

  To increase the cooling power in econo-

  placing more energy and space, it is advantageous to provide one or more pre-cooling zones before the mixing zone. Each pre-cooling zone can also be fed discontinuously. It is advantageous to also give each of the pre-cooling zones the shape of an isolated zone of the external atmosphere, in order to recover the quantity of heat that it is necessary to evacuate here by the moisture brought, thanks to condensation of moisture on cooling surfaces, heat pump

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  for example. Cooling can be done by vaporisation

  tion. However, advantageously, the cooling again results from an evaporation of the moisture by boiling, a vacuum being applied also to the pre-cooling zone with swirling of sand and water, the vapors formed being evacuated by the application of vacuum and condensing on

condenser surfaces.

  In an example of practical execution, it has proved advantageous to provide two precooling zones, in

  cooling the sand in the first precooling zone

  from a temperature of 120WC to about 80 ° C and in a second precooling zone, from about 80WC to about 550C.

  Preferably, the vessel is completely enclosed.

  the precooling tank or the pre-cooling tank, with the adjoining arrangements, in a casing forming a vacuum chamber, with the exception of the inlets and outlets used for feeding and

  evacuation and hermetically sealed from the outside. ad-

  the tank has several large surface openings distributed circumferentially, leading to

  the empty annular chamber and which, controlled by means of

  valve devices, can be cleared and closed.

  mées. Each tank has, in addition, internally at least

  an injection or spray head for the supply of water.

  The corresponding valve devices, devices and drives used for agitation, can all be placed

  inside the outer envelope, so in the void.

  The outer casing has a volume large enough to generate inside the tank, within the desired time,

  when the connection openings are unobstructed

  the desired vacuum, sufficient to evaporate the liquid.

  Advantageously, the cooled condensing surfaces serve

  The deposition of moisture is also arranged inside the envelope. The deputy cooling device, in particular the heat pump, can be arranged outside the casing. The establishment of the necessary vacuum inside the vacuum chamber can be effected by a connected vacuum apparatus, during the evacuation and supply of the tank. In such a case, it is irrelevant that when

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  releases the connection openings between the vacuum chamber and the mixing zone, sand particles are entrained by the resulting stream. The sand particles entrained in the vacuum chamber, are deposited on the bottom-thereof and can be easily evacuated by an airlock. It is also possible to prevent, by means of filters or baffles, the entrainment of sand from the mixing zone to the vacuum chamber. By spraying or injecting water into the swirling sand layer, the mixing zone or the

  pre-cooling zone, it is possible to control the supply of moisture

  with a much greater accuracy than before-

  is lying. It should be added that by direct spraying in the swirling sand layer, moisture comes into contact much more rapidly with the large total surface of the sand grains, can absorb heat and immediately evaporate again. It is important only to ensure that the water is introduced in the form of droplets to prevent as much as possible the evaporation of water before contact

with the surface of the sand.

  If the pre-cooling of the used sand is not done under vacuum, the air which absorbs moisture is circulated in closed circuit, the recycled air circuit including the cooling and condensing surfaces of a device

  cooling, in particular a heat pump.

  In this way, not only the coating of sand grains by the binder is noticeably favored and improved, but with a very compact construction, it saves a

large amount of energy.

  The pre-cooling zone and the mixing zone can also be provided in the same vacuum envelope, so that the cooling and condensing surfaces provided therein are used to the same extent to condense the moisture.

from both areas.

  It is also possible to add airlock to

  filling and evacuation of the pretreatment tank.

  cooling and mixing tank, so that the inside of the container can remain constantly under vacuum. In this case, the vacuum chamber can communicate freely with

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inside the tanks.

  Here "vacuum" is understood to mean air pressure distinctly

  lowered in relation to the environment, and not an atmosphere

  phere also free of vapors.

  The following invention is more specifically explained in

  a simple execution example.

  The single figure shows in vertical section a device

according to the invention.

  The new apparatus 1 comprises a mixing chamber 2 having in its upper region a filling opening

  3, more precisely located at the lower end of uo gou-

  fillet 4 to which is added a drawer 5 provided with a passage opening 5a and can be actuated by a jack 6. In the figure, the drawer is shown in the closed position. Above the drawer, the fill-widening duct

hopper shaped 7.

  The mixing chamber 2 is closed at the bottom, but present

  a bottom opening through which one can empty the contents.

  At the bottom opening is attached a shutter 8 which is pivotally mounted at 9 and can be brought to the closed position

  or in the open position by the actuating device 10.

  In the middle of the mixing chamber 2 is mounted a mixing shaft 15 having a pin 11 protruding outwardly through a bottom hub and carrying a pulley 12 with

  belt 13. The mixer shaft is equipped with mixing tools.

  Their main role is to maintain the material contained in the chamber 2 in continuous turbulent motion, in order to continuously release a particle surface as large as possible and at the same time, to act energetically on the

  material. In the example shown, on the shaft 15 are

  adapted for this purpose blades 16a, 16b subdivided into two groups in which the oblicity of the blades is opposite. The arrangement is such that the lower set of blades 16a picks up the material in the bottom region and projects it upward, while the upper set of blades 16b intercepts the projected material and accelerates it, both in the circumferential direction and in

  opposite direction down. This results in an indistinct swirl

  tense of the material inside the mixing chamber 2.

  Above the mixing shaft 15 is provided a device

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  concentrically surrounding the filling opening 3 and serving to bring moisture in the form of steam or water or to bring other liquid additives. For this purpose is provided an annular tube 20 provided with multiple fine outlets and can be connected by pipes 21 to a

  corresponding source of moisture, possibly through

of controlled valves.

  The actual mixing chamber 2 is closed upwardly by a filter plate, for example a sintered metal plate 22 which holds the solid particles in the chamber 2, but allows the vapors and the air to pass almost unhindered. To the sintered plate can be added if necessary a pneumatic or mechanical cleaning device. The

  cleaning of the sintered plate can be carried out intermittently

  this, for example by means of a directed compressed air pulse

to bedroom 2.

  The mixing chamber 2 is surrounded by an annular chamber

  concentrically 24 closed externally by an envelope

  23. The annular chamber 24 communicates with the chamber of

  mixture 2 through openings 26 and with the cavity

  above the sintered plate 22. In 25 is indicated a connecting pipe for connecting the annular chamber

  24 to a source of depression or emptiness.

  Inside the annular chamber 24 are cooling coils 27 constituting, for example, the

  heat absorption side of a heat pump. The serpents

  are connected, by pipes 28, 29 exceeding the ex-

  inside the refrigerant circuit of a cooling device

  especially of the heat pump. A device

  not shown at the bottom of the annular chamber.

  24, allows to evacuate the liquid that collects in

the annular chamber 24.

  During commissioning, the mixing chamber 2 is filled with the hot mixture, through the hopper 7, the drawer 5 being

  open. Then, the drawer 5 is closed in a largely

  tick. We can then mix together the constituents introduced

  first, in a substantially dry state. Preferably, after sealing the mixing chamber 2,

  apply a vacuum through the annulling chamber

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  24, so that the air is evacuated more or less abruptly.

  to a large extent, of the mixing chamber 2 and

  the amount of material introduced. The dry mix is

  kills under vacuum so that any binders can be applied

  uniformly layer or film, without disturbance,

  on the open surface of the grains of sand.

  When the desired state of mixing is reached, or from the beginning of the mixing, the desired humidity is brought to the desired amount of sand by the annular distributor 20. The humidification is effected extremely rapidly and uniformly, because the humidity is injected into the mass of swirling sand. The vapors formed, when it reaches the grains of sand are discharged continuously and quickly from the mixing chamber 2 and sweep the coils 27. Inside the vacuum that also reigns in the annular chamber 24, the liquid condenses and gathers at the bottom of the annular chamber 24. Of this

  way, the amount of material introduced is rapidly

  and surely at the desired temperature.

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Claims (17)

-R E V E N D I C A T I 0 N S-   1. Process for the preparation, in particular the cooling and mixing of molding sand, in which the used sand from the sand ball is cooled, optionally purified, mixed and homogenized   with other bodies by adding water and waving   geometrically, the surface of the grains of sand being at least partially coated with a binder, characterized in that the coating of the surface of the sand grains by the binder is carried out under vacuum.   2. Method according to claim 1, characterized in that the sand and the other ingredients are introduced into an isolated mixture zone of the external atmosphere, in that they are swirled, in that one generates a vacuum in the mixing zone, and in that it is removed from it gases and vapors.   3. Process according to any one of the claims   1 and 2, characterized in that the used sand is introduced into the mixing zone at an elevated temperature due to the previous casting, in that the pressure in the mixing zone is lowered at least to a minimum evaporation pressure which depends on the temperature of the sand, and that subsequently, water is introduced into the mixing zone in such a quantity that a predetermined amount of heat is subtracted from the sand with steam.   4. Method according to any one of the claims   1 to 3, characterized in that the used sand and the binder are first vacuum-mixed, and only after one hour   predetermined mixing time, water is introduced, in particular   spray, in the mixture maintained in the swirling state.   5. Process according to any one of the claims   1 to 4, characterized in that the vapor discharged from the mixing zone is cooled to the condensation temperature or below.   6. Process according to claim 5, characterized in that   that condensation of the steam takes place under vacuum.   7. Process according to any one of the claims   and 6, characterized in that the vapors removed from the mixing zone are brought to condenser surfaces of the   heat absorption side of a heat pump.   8. Process according to any one of the claims   1 to 7, characterized in that the used sand is swirled with water in a precooling zone having a heat content which results from the casting, in that by sucking gases and vapors, the sand is cooled between 50 and 900C, and then, in the mixing zone, it is   still cool to a temperature of about 20 to 40 C.   9. A method according to claim 8, characterized in that the precooling zone is isolated from the ambient atmosphere, and that the gas atmosphere is brought to a closed stream to a zone of cold and condensation. and / or   steam coming from the pre-cooling zone.   Process according to Claim 8, characterized in that a vacuum is applied to the pre-cooling zone and the condensation zone while the vortex is being swirled. sand and water.   11. Apparatus for carrying out the method according to   any one of claims 1 to 10, comprising at least   a cooling station and at least one mixing station, the mixing zone being equipped with a tank for containing the ingredients to be mixed, devices for feeding them, a stirring device, a device for supplying water and a device for removing the finished mixture, characterized in that the tank can be made hermetic or is placed in an envelope which can be sealed, and in this case   that the apparatus comprises a vacuum chamber provided with a device   tif used to generate a vacuum and at least one listening liaison.   between the tank and the vacuum chamber, which can be closed   by a valve device, which can be controlled.   Apparatus according to claim 11, characterized in that the container and the adjunct devices, with the exception of hermetically sealed filling and discharge openings, are substantially completely inside the vacuum chamber.   Apparatus according to any of the claims   11 and 12, characterized in that in the vacuum chamber is   a condensation device, in particular a condenser, 12 2472433   assistant to the cooling side of a heat pump.   Apparatus according to any of the claims   11 to 13, characterized in that the water supply device comprises at least one spray head disposed in the tank.   Apparatus according to any of the claims   -11 to 14, characterized in that the used sand supply device can be fed with hot sand resulting from the last casting.   16. Apparatus according to claim 15, characterized in that the device for supplying used sand can be fed   by a pre-cooling device.   17. Apparatus according to claim 16, characterized in that the pre-cooling device comprises a container   pre-cooling that can be isolated from the external atmosphere   higher, for the hot used sand resulting from the casting, a stirring device, a water supply device, in   in particular by spraying and a closed device for   aspiration of hot, humid gases and vapors.   Apparatus according to claim 17, characterized in that   the suction device comprises a suction fan   connected to the precooling vessel in a closed gas circuit having a cooling and condensing device. 19. Apparatus according to claim 17, characterized in that the suction device comprises a vacuum chamber -munie of a vacuum device and having a device for   cooling and condensation as well as a suction port   leading to the precooling vessel and can be   closed by valves or similar organs.