EP2191909B1 - Method and installation of recycling used green foundry sands - Google Patents

Description

The present invention relates to a process and a plant for converting sand to green spent foundry, also called purge sands, for the recycling of sand, clay binder and any inorganic black type additives they contain.

The sand foundry uses for the manufacture of molds and cores a mixture of new or recycled sand with various additives (binders, additives, etc.). For the molds, a so-called sand with synthetic green sand or silico-clay sand composed of silica, bentonite (clay) and addition elements, the main one being mineral black, is used. The mixing causes the coating of the sand grains by sheets of bentonite. The addition of water allows the separation of the leaflets. Clamping by means of molding machines brings the bentonite sheets closer to the formation of bridges between the sand grains, thus giving cohesion to the sand / bentonite system.

For cores, sand is generally agglomerated with polymerized synthetic resins.

After casting of the liquid metal, the blank is separated from the sand (hot or after a cooling period). This operation, called shakeout, sometimes manual, is most often performed using vibrating grids. The shakeout operation makes it possible to obtain, on the one hand, the cast and hardened metal parts and, on the other hand, the unbracked sand composed of the molding sand, a part of which has undergone a transformation related to the rise in temperature at the contact between the molten metal and the surrounding sand, and finally pieces of stones.

The so-called green sand molding uses reclaimed recycled sand after shaking. This return sand, obtained from sand untreated and recycled in the smelter of the foundry, is the basic raw material for the preparation of sands to green. Casting losses are compensated by adding new sand, clay, water, carbon additive. This process is called "sand to green" not because of its color but because the sand contains water, by analogy with wood, which is said green when it is still wet. The major components of this technique are sand, clay and water. Rehabilitation of the unchecked sand and the preparation of the sands to green are carried out in the sablerie which includes all the necessary equipment. Founders generally recycle their silico-clayey sand to more than 90%. The average composition of sand to green is 88% siliceous sand, 8% clay (bentonite) and 6% mineral black.

The setting sands used in the manufacture of cores are composed of a refractory material, mainly of the silica but also zircon, chromite or olivine type, agglomerated by chemical-based binders, of organic or inorganic type. The amount of binder is still very low compared to sand, of the order of a few percent maximum.

The chemical analysis of a typical new reference sand gives a percentage of SiO ₂ greater than or equal to 99.5, of Fe ₂ O _{3 of} less than 0.02, of Al ₂ O _{3 of} less than 0.07 and of K ₂ O less than 0.015, with the following characteristics: Fire Loss (at 1000 ° C) 0.15% max. Pyroscopic resistance 1750 ° C Acid application at pH = 3 3.5 Acid application at pH = 5 2.0 AFS index (granulometry) 57 to 59

The loss on ignition is the loss of relative mass of a sample after calcination at the indicated temperature. It represents the residual moisture.

The particle size is given by the fineness index according to the AFS (American Foundrymen Society) standard, for example explained in the document US3874073 ; it is an integer corresponding to a dummy sieve number having a mesh size corresponding to 50% of the cumulative distribution curve of refusals. The higher the value of this index, the finer the sand.

The acidic application consists of bringing sand into contact with a known volume of 0.1 N hydrochloric acid, and then the unreacted acid with the sand is titrated with 0.1 N sodium hydroxide (return assay). . The results are expressed on 50g of sand in ml of 0.1 N HCl consumed to reach pH 3 or 5 (DA3, DA5).

During the casting of the liquid metal in the mold, the clay portion of the molding sand which has undergone high temperatures is calcined and its structure changes. It turns into chamotte (dead clay). This Chamotte has a very low coefficient of thermal expansion, its major disadvantage is its open porosity, which consumes the binder. The chamotte has no water of constitution and strongly adheres to the grain of sand in concentric layers (phenomenon of oolithization).

The oolithization layer comprises all the residues of binders and molding aids, degraded and sintered by the thermal action of the casting of the metal in the mold. Its volume relative to the grain of sand is of the order of 15 to 30%. The calcined clay content is determined after subtraction of the various elements (silica, fines with particle size <20 μm, loss on ignition, etc.).

The recycling of sand to green requires a rehabilitation of the sand unchecked because it is more or less dry, dusty and hot. In addition, it contains a number of foreign bodies that must be removed (metal burrs, stone cores, refractory waste, slag).

• ~ séparation des particules métalliques,
• ~ réduction des mottes,
• ~ criblage.

The restoration of the unchecked sand goes through the following operations:

• ~ separation of metal particles,
• ~ reduction of clods,
• ~ screening.

During these operations, dust is removed from the sand. The sand debris generated by the screening are pieces of cores and clumps of sand to green.

The sand under the thermal effect becomes dusty and therefore requires dedusting. The fines are collected throughout the sand restoration circuit. Some of these fines can be reintroduced into the circuit.

The sand purge is made necessary by a too important supply of stone sand which, on the one hand, increases the volume of return sand, and on the other hand, lowers the quality of the recycled sand.

Sand purges are directly related to the process and type of parts manufactured. The volume of the purges depends mainly on the size and quantity of nuclei used.

These purges often contain a high proportion of sand to green and therefore have a high clay content. To recycle and use this sand in the manufacture of cores, as is the wish according to the present invention, it is necessary to return the grain and clean it, which is particularly difficult.

This is why spent green sands, or purging sands, are most often removed for example by burying, which is harmful to the environment, or possibly recovered in cement plants, which involves handling, transport and recovery costs. by the cement companies.

The spent green sand which is the subject of the recycling process of the invention has a high acid demand characterizing an end of life cycle:> 15 ml at pH = 5.

• % Poids en particules de granulométrie < 150 µm élevé (hors fines < 20 µm): > 15 % (AFS > 65),
• % Poids de particules de granulométrie < 20 µm élevé (fines): 6à8%
• Teneur en argile active (BM > 25 mg/g) et en carbone élevées (PAF > 5%)

From a granulometric point of view it has the following characteristics:

• % Weight of particles with a particle size <150 μm high (excluding fines <20 μm):> 15% (AFS> 65),
• % Weight of particles with particle size <20 μm high (fine): 6 to 8%
• Active clay content (BM> 25 mg / g) and high carbon (PAF> 5%)

It also has casting residues, agglomerated blocks related to the molding process and core residues containing a few resins.

It may also contain, depending on its origin, residual chromite, zircon or olivine particles.

The principle and benefits, in terms of cost and environment, of foundry sand recycling for reusing sand and clay are already well known.

So, we know from the document WO2002 / 092259 a process for recycling foundry sands and foundry dusts by mechanically separating sand from impact or abrasion to separate clay and additives to obtain a heavy fraction consisting essentially of directly recyclable sand after drying and a light fraction consisting of residual sand, clay and additive that is sent, together with the foundry dusts, to form a mud with water (at least ten times in water the weight of dry matter) . This sludge is the subject of two successive hydraulic separations, for example in hydro-cyclone, so as to obtain on the one hand a still active bentonite, and on the other hand materials consisting either of recyclable sand or waste, depending on the particle size obtained. Although this method makes the recovery of part of the active clay by the hydraulic separations very well known, it does not optimize it and above all does not optimize the recycling treatment of the sand itself.

In addition, the document WO2006 / 134099 discloses a process for recycling foundry sands also for recycling the clay, wherein, after a first screening, the material is kneaded in 1 to 8 times its weight of water and a separation is made between the sand removed clay and clay carried in the water. The sand is dried for recycling. The clay mud is settling. Again, there is no optimization of sand recycling treatment.

The documents DE 43 21 296 A1 , DE 41 09 167 A1 and Zanetti MC et al. "Conservation and Recycling Resources, Elsevier, Science Publisher, Amsterdam, NL, Vo1.38, June 1, 2003, pp. 243-254. , describe an installation and process for converting purge sands into recyclable sand.

These processes have opened the way for sand and clay recycling, but do not make the most of purge sands.

This is the purpose of the invention.

• A) mise en pulpe du sable de purge pour détacher les parties d'argile encore active entourant les grains de sable,
• B) criblage du sable de purge à une valeur voisine de 850 µm pour éliminer les rejets grossiers,
• C) classification hydraulique à une maille de coupure granulométrique de 40 µm à 80 µm en une fraction fine de boue diluée traitée aux étapes H et suivantes et une fraction de sable déschlammé traitée aux étapes Dà G,

• D) attrition par voie humide de la fraction de sable déschlammé pour nettoyer les grains de sable des couches oolithisées,
• E) classification hydraulique à une maille de coupure granulométrique comprise entre environ 150 µm et 250 µm en une fraction de rejets fins et une fraction de sable attrité de granulométrie désirée pour le recyclage,
• F) séchage et refroidissement du sable attrité,
• G) séparation magnétique sur le sable attrité séché pour éliminer la chamotte résiduelle et la chromite et les autres minéraux paramagnétiques et obtenir le sable recyclable pour la fonderie,
• H) classification hydraulique à une maille de coupure granulométrique de 5 µm à 20 µm de la fraction fine de boue diluée obtenue à l'étape C pour ne conserver que la fraction ultrafine contenant l'argile et le noir minéral éventuel et rejeter la fraction moins fine,
• I) Déshydratation de cette fraction ultrafine pour récupérer l'argile et le noir minéral éventuel recyclables.

The invention achieves its goal by means of a process for converting purge sands into recyclable sand in coring or molding and in mold-reclaimable clay, whether this binder is used alone or in admixture with mineral black, of the type comprising steps mechanical separation and hydraulic separation, characterized by the orderly succession of the following steps A to G on the one hand and AC, H, I on the other hand:

• A) pulping the purge sand to remove the still active clay portions surrounding the sand grains,
• B) screening purge sand to a value close to 850 microns to eliminate coarse releases,
• C) hydraulic classification with a mesh size of 40 μm to 80 μm in a fine fraction of sludge diluted treated in steps H and following and a fraction of debloamed sand treated in steps D to G,

• D) wet attrition of the fraction of sand de-ignited to clean the grains of sand of the oolithized layers,
• E) hydraulic classification with a mesh size of between about 150 microns and 250 microns in a fraction of fine rejects and an attrition sand particle size fraction desired for recycling,
• F) drying and cooling of the sand,
• G) Magnetic separation on the dried dried sand to remove residual chamotte and chromite and other paramagnetic minerals and obtain recyclable sand for the foundry,
• H) hydraulic classification at a mesh size of 5 μm to 20 μm granulometric size of the thin fraction of diluted sludge obtained in stage C to keep only the ultrafine fraction containing the clay and the possible mineral black and reject the fraction less fine,
• I) Dehydration of this ultrafine fraction to recover the clay and the possible mineral black recyclable.

The order and nature of the steps according to the method of the invention ensure the treatment of sand purges so that an optimal amount of sand and other active minerals is effectively recycled.

• par la mise en pulpe du sable de purge réalisée très en amont du procédé et permettant de séparer efficacement sensiblement toute l'argile active,
• par l'attrition en voie humide qui s'ensuit du sable débarrassé de l'argile active de manière à décaper efficacement la surface des grains de sable sans être gêné, dans ce traitement, par la lubrification que produirait la fraction fine,
• Par la séparation magnétique qui s'ensuit et permet d'éliminer les particules contenant de la chamotte résiduelle. On notera que des procédés de séparation magnétique sont déjà connus, par exemple par le document US 5 526 937 . Mais cette séparation magnétique est effectuée généralement très en amont du procédé (cf. également WO 2002/092259 ) alors que selon l'invention sa réalisation en aval du procédé permet d'augmenter considérablement la quantité des produits recyclés.

This is made possible according to the invention:

• by the pulping of the purge sand carried out very far upstream of the process and allowing effective separation of all the active clay,
• by the ensuing wet attrition sand freed from the active clay so as to effectively strip the surface of the grains of sand without being hindered, in this treatment, by the lubrication that the fine fraction would produce,
• By the magnetic separation which follows and allows to eliminate the particles containing the residual chamotte. It will be noted that magnetic separation methods are already known, for example by the document US 5,526,937 . But this magnetic separation is generally carried out very far upstream of the process (see also WO 2002/092259 ) while according to the invention its implementation downstream of the process can significantly increase the amount of recycled products.

In addition, thanks to its step of dehydration of the ultrafine fraction recovered after the hydraulic separation of the diluted sludge, the mineral additives are recovered in a form allowing further handling or storage.

Advantageously, the amount of water added to the purge sand for the pulping of step A is of the order of 0.5 to 1 liter per kilogram of purge sand, that is to say that a solid concentration of at least 50% (mass of solid to total mass) is kept, which also distinguishes the process of the invention from the two documents WO2002 / 092259 and WO2006 / 134099 where the concentration was much lower, of the order of 10% for example.

Advantageously, the attrition step D is carried out in two attrition cells with a passage time greater than 15 minutes in total.

Advantageously, the magnetic separation step G is performed in two separation stages to separate the high and low paramagnetism particles.

Advantageously, the dehydration stage I comprises a centrifugal decantation.

• A) des moyens de mise en pulpe du sable de purge pour détacher les parties d'argile encore active entourant les grains de sable,
• B) des moyens de criblage à une valeur voisine de 850 µm du sable de purge pour éliminer les rejets grossiers, reliés aux moyens de mise en pulpe,
• C) reliés auxdits moyens de criblage, des premiers moyens de classification hydraulique à une maille de coupure granulométrique de 40 µm à 80 µm en une fraction fine de boue diluée et une fraction de sable déschlammé,
• D) reliés auxdits premiers moyens de classification hydraulique, des moyens d'attrition par voie humide de la fraction de sable déschlammé pour désagréger l'argile,
• E) reliés auxdits moyens d'attrition, des deuxièmes moyens de classification hydraulique à une maille de coupure granulométrique comprise entre environ 150 µm à 250 µm en une fraction de rejets fins et une fraction de sable attrité de granulométrie désirée pour le recyclage
• F) reliés auxdits deuxièmes moyens de classification hydraulique, des moyens de séchage et refroidissement du sable attrité,
• G) reliés auxdits moyens de séchage, des moyens de séparation magnétique du sable attrité séché pour éliminer la chamotte et la chromite et les autres minéraux paramagnétiques et obtenir le sable recyclable pour la fonderie,
• H) reliés auxdits premiers moyens de classification hydraulique, des troisièmes moyens de classification hydraulique à une maille de coupure granulométrique de 5 µm à 20 µm de la fraction fine de boue diluée pour ne conserver que la fraction ultrafine contenant l'argile et rejeter la fraction moins fine,
• I) reliés auxdits troisièmes moyens de classification hydraulique, des moyens de déshydratation de cette fraction ultrafine pour récupérer l'argile recyclable.

The invention also relates to an installation for the implementation of the above process for the transformation of purge sands into recyclable sand in foundry and clay and black mineral eventually recyclable foundry, this installation comprising:

• A) means for pulping the purge sand to detach the still active clay portions surrounding the sand grains,
• B) screening means at a value close to 850 μm of the purge sand to eliminate coarse rejects connected to the pulping means,
• C) connected to said screening means, first hydraulic classification means with a mesh size of 40 .mu.m to 80 .mu.m in a fine fraction of diluted sludge and a debloamed sand fraction,
• D) connected to said first hydraulic classification means, means for wet attrition of the fraction of de-ignited sand to disintegrate the clay,
• E) connected to said attrition means, second hydraulic classification means with a mesh size of between about 150 microns to 250 microns in a fraction of fine rejects and an attrition sand fraction of desired particle size for recycling
• F) connected to said second means of hydraulic classification, means for drying and cooling the sand,
• G) connected to said drying means, means for magnetic separation of the dried dried sand to remove the chamotte and the chromite and the other paramagnetic minerals and to obtain the recyclable sand for the foundry,
• H) connected to said first hydraulic classification means, third hydraulic classification means to a mesh size of 5 μm to 20 μm particle size of the thin sludge fraction diluted to retain only the ultrafine fraction containing the clay and reject the fraction less fine,
• I) connected to said third means of hydraulic classification, means for dehydration of this ultrafine fraction to recover the recyclable clay.

• Environ 65% du sable de purge entrant qui est recyclable en application pour le noyautage.
• Environ 5% du sable de purge entrant sous la forme d'argile et de noir minéral éventuel actifs qui sont recyclables en application moulage pour le sable à vert.

The invention makes it possible to recover the waste constituted by the purge sand by:

• About 65% of the incoming purge sand is recyclable in application for coring.
• About 5% of the purge sand comes in the form of clay and optional mineral black which are recyclable in molding application for sand to green.

The invention therefore makes it possible to reuse about two-thirds of the mineral waste associated with the purge, and to reduce the consumption of new raw materials (siliceous sand, bentonite clay, mineral black) and to reduce the road traffic dedicated to the transport of these materials. first.

• la figure 1 est un organigramme des principales étapes du procédé de l'invention
• la figure 2 est un schéma d'un exemple d'installation de recyclage pour la mise en oeuvre du procédé de la figure 1 conforme à l'invention.

Other features and advantages of the invention will emerge from the following description of the method of the invention and an example of installation. Reference will be made to the accompanying drawings in which:

• the figure 1 is a flowchart of the main steps of the method of the invention
• the figure 2 is a diagram of an example of a recycling installation for the implementation of the method of the figure 1 according to the invention.

• A) mise en pulpe du sable de purge pour détacher les parties d'argile encore active entourant les grains de sable,
• B) criblage du sable de purge mis en pulpe pour éliminer les rejets grossiers, typiquement ceux qui sont supérieurs à la valeur de criblage voisine de 850 µm (en pratique entre environ 850 µm et 1200 µm),
• C) classification hydraulique du sable criblé en une fraction fine traitée aux étapes H et suivantes et une fraction de sable déschlammé traitée aux étapes D à G ; la fraction fine est constituée de boues d'argile et de noir et de silice fine, typiquement de granulométrie inférieure à une coupure choisie entre 40 et 80 µm;
• D) attrition de la fraction de sable déschlammé pour désagréger l'argile et donner un sable attrité
• E) classification hydraulique du sable attrité en une fraction de rejets fins (boues fines de sable et chamotte) et une fraction de sable attrité de granulométrie désirée pour le recyclage, typiquement une granulométrie de type AFS 58,
• F) séchage et refroidissement du sable classé
• G) séparation magnétique sur le sable séché pour éliminer la chamotte et la chromite et les autres minéraux paramagnétiques et obtenir le sable recyclable pour la fonderie
• H) classification hydraulique de la fraction fine obtenue à l'étape C pour ne conserver que la fraction ultrafine sous forme de boue diluée contenant l'argile et rejeter la fraction moins fine
• I) Déshydratation par décantation des boues diluées pour enlever l'eau de cette fraction ultrafine afin de récupérer l'argile recyclable

The figure 1 illustrates in flowchart form the steps of the process of the invention for the conversion of purge sands into recyclable sand in foundries and foundry recyclable clay, characterized by the ordered succession of the following steps A to G on the one hand and AC , H, I on the other hand ::

• A) pulping the purge sand to remove the still active clay portions surrounding the sand grains,
• B) screening of the pulped purge sand to eliminate coarse discharges, typically those which are greater than the screening value close to 850 μm (in practice between about 850 μm and 1200 μm),
• C) hydraulic classification of sand screened into a fine fraction treated in steps H and following and a fraction of debloamed sand treated in steps D to G; the fine fraction consists of clay sludge and black and fine silica, typically of particle size less than a cutoff selected between 40 and 80 microns;
• D) attrition of the fraction of debloamed sand to disintegrate the clay and give an attrited sand
• E) hydraulic classification of the sand attrited in a fraction of fine discharges (fine sludge of sand and chamotte) and a fraction of sand of particle size desired for recycling, typically a particle size AFS 58 type,
• F) drying and cooling of classified sand
• G) magnetic separation on the dried sand to remove the chamotte and chromite and other paramagnetic minerals and obtain recyclable sand for the foundry
• H) hydraulic classification of the fine fraction obtained in step C to keep only the ultrafine fraction in the form of dilute sludge containing the clay and reject the less fine fraction
• I) Dewatering by settling diluted sludge to remove water from this ultrafine fraction to recover recyclable clay

• Les grains de sable usés à nettoyer sont des grains de quartz. Ils présentent une structure oolithique c'est-à-dire qu'ils présentent en surface un recouvrement de couches concentriques d'argile et de carbone +/- actifs et sous différentes formes en fonction de leur état de transformation à l'issue du procédé de moulage : noir minéral actif, coke ou cendres).

Thus, according to the invention, the establishment from the beginning of the pulping step A allows a good recovery of active clay and unpolluted mineral black which can be reintroduced as a molding additive. This follows from the following observation:

• The grains of sand used to clean are quartz grains. They have an oolitic structure that is to say that they have on the surface an overlap of concentric layers of clay and active +/- carbon in different forms depending on their state of transformation at the end of the process molding: active mineral black, coke or ash).

• un recouvrement peu lié à la surface, friable et facile à enlever (argile active et additif carboné), que l'étape A de mise en pulpe conforme à l'invention permet de détacher,
• une croûte transformée thermiquement, plus adhérente au grain de quartz et plus difficile à enlever (chamotte contenant des grains de carbone ou coke et de la silice), qui sera enlevée lors de l'étape d'attrition par voie humide D.

In general we find the following layers:

• a poorly bonded, friable and easy-to-remove surface covering (active clay and carbonaceous additive), which pulping stage A according to the invention makes it possible to detach,
• a thermally transformed crust, more adherent to the quartz grain and more difficult to remove (chamotte containing carbon grains or coke and silica), which will be removed during the wet attrition step D.

The baked clay crust called chamotte consists of concentric beds from a few μm to a few tens of μm thick. This chamotte is also present in the interstices within the grains, related to the intrinsic quality of the quartz grains and the thermal shocks to which the quartz grains are subjected during the successive stages of contact with the liquid metal.

The grains of sand must be cleaned of their oolitic layer as well as the interstitial chamotte, as much as possible, without the size, the shape and the surface of the grains being affected, in order to be reintroduced in place of a part of the new sand in the coring process. This is done in the wet attrition step D of method of the invention, supplemented by the magnetic separation step G which makes it possible to remove the particles containing the residual chamotte.

The installation for carrying out this process essentially comprises a section 100 for recycling the sand and a section 200 for recycling the active mineral additives. There is also shown a section 300 for recycling water.

The purge sand is discharged into a feed hopper 2 of determined capacity depending on the desired treatment rate. Given the agglomerating power of the product, one or two vibrators can be placed on the hopper 2 and operated continuously or discontinuously depending on the clogging observed.

The purge sand is dumped by a dump truck 1 or a loader in the case of an off-center treatment plant. If necessary a buffer stock is made upstream. A grid of 50 x 50 mm mesh at 150 x 150 mm depending on the quality of sand can be installed above the receiving hopper, to avoid the presence of large pieces of cores, scrap or other waste likely to clog the base of the hopper and penalize the flow.

An installation directly integrated in a foundry can work in conjunction with the sand shop of said foundry: the purge sand usually discharged by pulsed air conveyor or screw is stored or feed directly into the hydraulic recycling facility.

The sand is extracted from the hopper 2 by means of a screw 3 whose motor is equipped with a speed variator making it possible to adapt the treatment flow rate to the desired output flow rate and to the characteristics of the feed: if the feed contains a lot of fines, the flow rate is slowed down for an equivalent flow of water treatment which promotes the pulping step A which will follow; if the sand contains a lot of coarse impurities, the flow is slowed so as not to disturb the screening that follows.

The screw 3 directly feeds a trough or chute 4 provided with garden ramps equipped with nozzles 5 injecting water against the flow of sand. In order to allow the recovery of clay and mineral black "active" that is to say non-oolithized and covering the grains of sand in a friable way, the sand is mixed with water so energetic. A first separation of the fine particles and oolithized sand is performed during this pulping step A.

If the fine particles are strongly agglomerated with the purge sand of the feed, provision can be made to pulp in rotary sludge or washing tube (horizontal cylindrical piece full slightly inclined relative to the horizontal and driven by a rotational movement variable speed depending on the quality of the feed, high input which are introduced purge sand and water) possibly equipped downstream of a horizontal cylindrical grid in extension of the 50x50 mesh tube 150x150 mm which allows to eliminate before the screening coarse rejects.

The diameter of the sludge is a function of the flow rate to be treated and the length of the efficiency in terms of separation of the fine particles at the surface of the sand grains. The solid concentration (solid mass over total mass) for settling is at least 50% because a lubrication phenomenon related to the presence of mineral black can affect the performance if the water content is greater than the solid content.

If necessary (presence of agglomerates), grinding bodies such as rollers or steel balls can be used during settling to facilitate disagglomeration.

The quantity of water required is adjustable according to the degree of agglomeration of the sand to be treated and varies from 0.5 to 1 1 per kg of purge sand.

This step pulp on the one hand the "active" fine fraction mixed with other particles of sand of greater particle size, which will be recycled in section 200, and on the other hand a largest possible fraction of mineral black mixed with the fine fraction "active" to be removed with the fines in the next step of classification so as not to disturb, by lubricating effect, the attrition step which will be seen in section 100.

1. 1. d'achever une séparation des particules fines non adhérentes aux grains de sables,
2. 2. de réaliser une coupure granulométrique des grains de sable à une taille supérieure à environ 850 µm (en pratique entre 850 µm et 1200 µm).

This pulping step A is followed by a screening step B. The pulped purge sand is discharged upstream of a vibrating screen 6 inclined relative to the horizontal or not. It is provided with one or more garden ramps provided with nozzles 7 injecting water against the current and thus allowing:

1. 1. to complete a separation of the fine particles not adhering to the grains of sand,
2. 2. Carry out a granulometric cut of the sand grains to a size greater than about 850 microns (in practice between 850 microns and 1200 microns).

The cutoff> at about 850 μm is achieved by means of multiple grids or a single grid, polyurethane or other abrasion-resistant material, square mesh or longitudinal.

The fraction> about 850 μm flows along the screen and is discharged via a trough 8, by gravity, into a storage container.

The fraction <850 μm passes through the screen and is stored in a tank 9.

New sand theoretically does not contain grains larger than 850 μm. The recycled sand must therefore respect this granulometric condition.

Given the effectiveness of the sieve 6 and its technical characteristics (inclination, frequency of vibration, optimum watering rate), a standard cut-off mesh of 850 μm, 1 mm, 1.25 mm or even 1.6 mm can be used. to be used.

The screen used can also be a trommel type rotary screen, especially if the pulping stage is carried out by means of a sludge, in which case the pulp / screening step at 850 μm can be carried out using of one and the same equipment.

The quantity of water required is adjustable according to the degree of agglomeration of the sand to be treated and varies from 0.5 to 1 1 per kg of purge sand.

This granulometric cutting step B therefore makes it possible to eliminate, on the one hand, particles larger than 850 μm in size so as to obtain the upper part of the AFS granulometric curve of the recycled sand, and on the other hand the coarse rejects of cores and cores. scrap particles that can be mixed with the feed.

This screening step B is followed by a classification step C of the fine fraction in a device 10. This fine fraction deagglomerated and pulped is separated from the rest of the sand by hydraulic classification at by means of a submerged screw or a hydrocyclone equipped with a feed pump.

A screw classifier 10 consists of one or two Archimedean screws freely rotating in an elongate tray: this movement produces a classification of the solid particles according to their particle size and / or their density: the most grainy particles or the denser sediment on the bottom and are brought back in the upper part by the screw or screws.

A hydrocyclone is a cylindrical-conical static centrifugal classifier fed tangentially under pressure into the cylindrical portion with a tubular overflow outlet in the axis of the cylindrical portion and a vent opening at the tip of the cyclone.

The objective of this step C is to isolate the "active" mineral additives and to eliminate the fine silica which disturbs the following stages of sand cleaning (notably attrition, disturbed by the lubrication produced by the fines) and which represents a particle size unsuitable for recycled sand that can be used in molding.

The cut is located in a range of 40 to 80 microns depending on the content and the particle size of the fine silica to be removed. "Active" bentonite and mineral black have a particle size of 20 μm maximum.

At the end of this fine classification step C, two fractions to be valorized are obtained: the "active" mineral additives contained in the fine fraction directed towards the recycling section 200 and the recycled sand of the coarse fraction directed towards the recycling section. recycling 100.

The person skilled in the art knows how to choose the mesh size which is optimal between the elimination of the fine silica which disturbs the sand treatment steps (coarse fraction) - superior criterion, and the obtaining of a quantity of fine silica too important so as not to disturb the isolation of mineral additives later - lower criterion.

The recycling operations of "active" mineral additives will now be described in section 200.

The fraction 0 - 40 to 80 μm obtained during the fine classification is again classified by hydraulic classification H by means of one or several hydrocyclones 202 fed by a pump 201 so as to isolate the ultrafine fraction containing predominantly "active" mineral additives of the fine fraction containing predominantly silica. The "active" mineral additives are evacuated by the overflow 203 of the cyclone 202 and the coarser and denser elements mainly composed of fine silica are removed at the level of the void 204.

The separation mesh is between 5 and 20 microns depending on the additives present.

Several operating parameters make it possible to adapt to the composition of the fraction: the apex and vortex diameters used, the solid concentration of the pulp supplying the hydrocyclone and the injection pressure. The apex diameter is between 10 and 30 mm and the maximum solid concentration (dryness) is 10% maximum to ensure effective separation.

The fine fraction discharged at 204 is a waste product of the process and is stored in pulp form in an intermediate tank, not shown.

The ultrafine fraction evacuated by the overflow 203 contains the "active" mineral additives, residual fine silica and water.

1. 1. pour diminuer le volume à transporter jusqu'à l'atelier de moulage,
2. 2. pour diminuer la quantité d'eau qui sera de facto ajoutée dans la préparation de sable à vert en atelier de moulage,
3. 3. pour augmenter le volume d'eau de procédé à recycler de façon à consommer le moins possible d'eau « fraîche »

This fraction is in the form of a diluted pulp or sludge which must be dehydrated, for example by decantation I:

1. 1. to reduce the volume to be transported to the molding shop,
2. 2. to reduce the amount of water that will be de facto added in the preparation of sand to green in molding workshop,
3. 3. to increase the volume of process water to be recycled so as to consume as little fresh water as possible

Dehydration must be optimal between reduction of water necessary for reuse operations of "active" additives and conservation of part of the water necessary for handling the product: beyond a dryness greater than 30%, the mud becomes solid and is difficult to manipulate and reintroductible in the circuit. The optimal dryness is between 15 and 30%.

This dewatering by decantation I is carried out by means of a vertical or horizontal centrifuge 205 with high acceleration: 3000 to 4500 G taking into account the difficulties of sedimentation of the particles and in absence of flocculant so as not to modify the chemical properties of the particles to be reused.

This step produces an eluate at 206 to be clarified as a filter press before returning to the process and a 207 centrifugate which contains about 50% bentonite and 30% "active" mineral black, the remainder being composed of fine silica and other particles.

The eluate obtained at 206 has a significant amount of suspended solids which will be removed by filter press in section 300 with those of the other process water outlets to obtain reusable recycled water in the process.

The centrifugate obtained in 207 is composed of 80% recyclable mineral additives in molding application (bentonite and mineral black); added in place of "new" bentonite, it allows the molding sand to acquire better mechanical characteristics (compression, shear and compressibility) than in the case of "new" additives because the recycled "active" bentonite develops rapidly its binding properties. The reducing and non-sticking effect of mineral black is not affected by the presence of water.

If the melter wishes to store the ultrafine fraction for a long time, a filter press, a belt filter or a volute separator will be preferred to a centrifuge to obtain a greater dehydration (greater than 30% screw or belt conveyor and closed tank storage. The reuse of the sludge thus conserved will require a step of re-homogenization in water with violent stirring, sludge dryness greater than 30% are indeed agglomerated and compact, so difficult to manipulate in solid form.

Otherwise, the sludge containing the "active" mineral additives is stored at the outlet of the centrifuge 205 in a tank 208 with high stirring between 75 and 750 revolutions per minute depending on the proportion of bentonite, mineral black and fine silica in mud.

The sludge is then either injected directly into the kneader of the molding workshop via a volumetric metering pump 209, or transported in the tank and used after re-homogenization by stirring still via a volumetric metering pump.

Sand recycling operations will now be described in Section 100.

The fraction 40 to 80 μm - 850 μm formed of deschlamed sand from the classifier screw 10 or the hydrocyclone feeds a bank of two attrition cells 101 with a total passage time of at least 15 minutes. The solid concentration in feed is 65% minimum to obtain a grain friction and an effective stripping of the oolithized layers.

The attrition of step D is a stirring technique, in closed hexagonal tanks, by means of a shaft with three sets of blades and which causes a concentric circulation of the particles against each other. This friction is at the origin of a surface etching of the grains.

Attrition generates fine particles that are pieces of oolite containing "cooked" or "chamotte" bentonite, indurated mineral black.

At the end of the attrition step, a sand of particle size 0 - 850 μ is obtained due to the generation of fine particles, which must be eliminated in a special stage of hydraulic classification E.

In fact, the fine particles generated by attrition are pollutants of the sand for coring in terms of chemical composition (bentonite = presence of alumina, sodium and / or calcium, coke and indurated carbon) and particle size composition.

The maximum content of particles smaller than 150 μm in size is 3% for a sand that can be used for coring: a removal of the fines by hydraulic classification is carried out in the equipment 102.

A cut-off mesh of between approximately 150 and 250 μm (for example in practice between 100 μm and 300 μm) makes it possible to obtain a sand of particle size AFS 55 to 59. The mesh of cut is optimized taking into account the granulometry of the sand. initial purge so as to obtain an acceptable final grain size and a good final product recovery.

The equipment used 102 is, depending on the flow rate to be treated, a hydrocyclone, a screw classifier or a densimetric hydro-separator.

A densimetric hydro-separator is a pyramidal or frustoconical cylindrical vessel with tangential water injection under pressure above the base. The injected water creates an upward countercurrent to the pulp feed which is carried out from the top of the tank: the particles that sediment according to their density (density and / or size) either continue their downward progression or are driven upwards by the water of injection. The injection water pressure, the flow rate and the sizing of the separator make it possible to cut between small and / or light particles (overflow overflow at the top of the separator) and larger and / or denser particles. (at the tip of the basal part).

The fine discharges are removed at 103: the fine particles generated by attrition are filtered as a filter press in section 300 so as to recover the process water

We recover in 104 recycled sand from attrition, whose quality is as follows: Loss on fire at 1100% (%) 1 Acid application at pH 5 3.5 to 4.5 ml AFS granulometry 55 to 59 % <20 μm <0.1

The sand is still present in the order of 5 to 7% of materials other than silica. These materials are fired clay and carbonaceous materials trapped in the aggregates of cooked clay because several analyzes have shown that this sand no longer contained "active" mineral additives. For this reason, the fine sand, devoid of its fine particles, constitutes a cleaned purge sand whose characteristics are not yet acceptable for a direct reintroduction in coring in terms of loss on ignition and of acidic demand, therefore in terms of residual pollutants. calcined bentonite and mineral black: a final cleaning step is still needed.

An optional additional purification step for separating silica with a density of 2.6 to 2.7 from other lower density minerals (bentonite: 1.8 to 2.2 and coke: 0.4 to 0.8) can be added at the end of this classification step and before the final cleaning: it consists in passing the pulp in one or more spirals or on a shaking table so as to eliminate the light particles of the silica.

For spiral densitometric separation, the pulp flows up and down in a channel describing a helix so as to combine sedimentation and centrifugal force. The heavier and finer grains remain in the vicinity of the central axis while the coarser and lighter particles are centrifuged in areas of high flow velocity. Adjustable deflectors allow to recover at the low point of cross sections to define the heavier grains while the light ones are recovered at the base of the spiral.

For the separation by shaking tables, the principle consists in creating a floating film web on an adjustable tilt plate subjected to longitudinal asymmetrical shaking and provided with longitudinal bars. Shaking allows stratification by sedimentation and the progression and evacuation of stratified (dense) particles between the webs. The particles are recovered on the sides adjacent to and opposed to the feed that is performed at the top of the table.

1. 1. Un essorage est réalisé à l'aide d'un essoreur 105 vibrant ou à tambour ou au moyen d'un filtre à bande. Le sable présente une humidité résiduelle de 12 à 18% en sortie de l'essoreur 105.
2. 2. Il alimente ensuite un sécheur 106 rotatif ou à lit fluidisé fonctionnant au gaz naturel ou à l'électricité pour ne pas le polluer avec des résidus d'hydrocarbures plus lourds. Il est équipé du système de ventilation adéquat, d'un refroidisseur permettant une sortie du sable sec à 35°C maximum et d'un dépoussiéreur garantissant un rejet d'émissions atmosphériques conforme à la réglementation.

The final moisture content of the sand for reuse during coring is 0.3% maximum. However, the sand obtained from the hydraulic classifier 102 has a dryness of between 50 and 60%. A two-part drying step F is therefore required:

1. 1. A spin is performed using a wiper 105 vibrating or drum or by means of a band filter. The sand has a residual moisture of 12 to 18% at the outlet of the wiper 105.
2. 2. It then feeds a rotary or fluidized bed dryer 106 operating on natural gas or electricity so as not to pollute it with heavier hydrocarbon residues. It is equipped with the appropriate ventilation system, a cooler allowing a dry sand outlet at 35 ° C maximum and a dust collector guaranteeing a rejection of atmospheric emissions in accordance with the regulations.

At the end of this step, the output of the drying and cooling system is in the presence of an attrited sand with a particle size of 55 to 59 AFS, a residual moisture ≤ 0.3% and a temperature specific to the manipulation namely ≤ 35 ° C, which will be able to be subjected to the final purification stage by double magnetic separation in the separators 108.

In fact, the iron contained in the residual oolite fragments, originating from the initial bentonite, has acquired a magnetic susceptibility allowing to confer the residual oolite fragments containing a large fraction of "cooked" bentonite or chamotte a paramagnetism.

The purge sand may further contain residual chromite which is unaffected by previous treatments and has paramagnetism.

The sand obtained is therefore subjected to a high intensity magnetic separation G drum: the active body of the apparatus consists of a magnetic pulley resulting in a thin belt carrying the product to be treated. The pulley consists of a succession of permanent magnet disks separated by soft iron elements. The polarities of the magnetic disks are alternated. The soft iron discs channel the field lines by tightening them, thus ensuring a high magnetic field gradient on the surface.

• La vitesse de rotation,
• L'ajustement des déflecteurs permettant la séparation des particules magnétiques collées au tambour des non magnétiques éjectées par la force de rotation.

Two separation stages with different magnetic fields are suitable for separating different types of particles: a first stage at 0.3 - 0.8 T eliminates the highly paramagnetic particles, chromite type, then a stage at 0 , 8 - 1.6 T for low paramagnetism particles of the oolitic particle type (residual chamotte). The operating parameters are:

• The speed of rotation,
• The adjustment of the deflectors allowing the separation of the magnetic particles stuck to the drum of non-magnetic ejected by the rotational force.

The recycled sand obtained at 109 at the end of this stage represents 60 to 70% of the initial bleed sand. Mixed with 30 to 40% of new sand necessary to fill the eliminated fractions, it presents the quality required by the foundries for a use in coring.

The recycled sand is either injected directly into the core by means of a forced-air conveyor or stored in silo by means of a screw, a belt conveyor, a bucket elevator or a forced-air conveyor.

It is injected directly into coring if the foundry has a dedicated supply of new sand: the mixture is then made to core. Otherwise, a weighting system allows from the storage of recycled sand and new sand to obtain a mixture in the proportions allowing reuse in coring.

As already indicated above, the effluents from the various treatment steps are collected in a tank which feeds in the water recycling section 300 a filter press 301 so as to reduce the rate of suspended solids. less than 100 mg / l. Solid residues are process wastes while the resulting filtrate is stored in a buffer tank before being reintroduced as process water. If necessary, a step of removal of ionized heavy metals and pH regulation is installed.

1. 1. à une installation mobile déplacée d'une fonderie à une autre,
2. 2. à une installation coopérative traitant les sables de purge de fonderies différentes mutualisant leurs coûts,
3. 3. à une installation intégrée au sein d'une fonderie dont le tonnage annuel de sable de purge représente une quantité suffisante à un tel investissement.

The hydraulic recycling process of the invention applies as well:

1. 1. a mobile installation moved from one smelter to another,
2. 2. a cooperative facility treating purging sands from different smelters pooling their costs,
3. 3. an integrated facility within a smelter with an annual tonnage of purge sand that is sufficient for such an investment.

In the case of installations 1 and 2, the recycling process will have adaptations to the sands to be treated easily put in place which will be detailed after description of the basic process.

• Quantité de fines +/- importante,
• Grains de sable siliceux +/- oolithisés,
• Proportion argile active / chamotte variable,
• Présence ou non de noir minéral de nature chimique variable,
• Présence d'argile de trois types différents : sodique, calcique ou « activée »

The process is flexible and makes it possible to recycle a feed whose following characteristics are variable:

• Amount of fines +/- important,
• Silica sand grains +/- oolithized,
• Proportion of active clay / variable chamotte,
• Presence or absence of mineral black of variable chemical nature,
• Presence of clay of three different types: sodium, calcium or "activated"

Claims (6)

1. Method for transforming purge sands into sand which can be recycled in a foundry and into clay which can be recycled in a foundry, of the type comprising mechanical separation and hydraulic separation steps, characterised by the ordered succession of the following steps A to G on one hand and A-C, H and I on the other hand:

   - A) pulping of the purge sand to detach the still active clay and possible mineral black portions surrounding the grains of sand,

   - B) sifting of the purge sand at a value of approximately 850 µm to eliminate coarse waste,

   - C) hydraulic classification at a granulometric cut-off of 40 µm to 80 µm into a fine fraction of diluted sludge treated in step H and the following steps and a deslimed sand fraction treated in steps D to G,

   - D) wet attrition of the deslimed sand fraction to disintegrate the oolites surrounding the grains of sand,

   - E) hydraulic classification at a granulometric cut-off of between 150 µm and 250 µm into a fraction of fine waste and a fraction of attrited sand of desired granulometry for the recycling,

   - F) drying and cooling of the attrited sand,

   - G) magnetic separation of the dried attrited sand to eliminate the chamotte and the chromite and the other paramagnetic minerals and obtain the recyclable sand for the foundry,

   - H) hydraulic classification at a granulometric cut-off of 5 µm to 20 µm of the fine fraction of diluted sludge obtained in step C so as to retain only the ultra-fine fraction containing the clay and reject the less fine fraction,

   - I) dehydration of this ultrafine fraction to recover the clay and the possible mineral black which can be recycled.
2. Method according to claim 1, characterised in that the quantity of water added to the purge sand for the pulping in step A is in the order of 0.5 to 1 litre per kilo of purge sand.
3. Method according to either claim 1 or claim 2, characterised in that the attrition step D is carried out in two attrition cells with a residence time greater than 15 minutes in total.
4. Method according to any of claims 1 to 3, characterised in that the magnetic separation step G is carried out in two separation stages for separating highly and weakly paramagnetic particles.
5. Method according to any of claims 1 to 4, characterised in that the dehydration step I comprises centrifugal decantation.
6. Plant for implementing the above method for transforming purge sands into sand which can be recycled in a foundry and into clay which can be recycled in a foundry, characterised in that it comprises:

   - A) means (4, 5) for pulping the purge sand to detach the still active clay and possible mineral black portions surrounding the grains of sand,

   - B) means (6, 7) for sifting the purge sand at a value of approximately 850 µm to eliminate coarse waste, connected to the pulping means,

   - C) connected to said sifting means (6, 7), first means (10) for hydraulic classification at a granulometric cut-off of 40 µm to 80 µm into a fine fraction of diluted sludge and a deslimed sand fraction,

   - D) connected to said first hydraulic classification means (10), means (101) for wet attrition of the deslimed sand fraction to disintegrate the clay,

   - E) connected to said attrition means (101), second means (102) for hydraulic classification at a granulometric cut-off of between 150 µm and 250 µm into a fraction of fine waste and a fraction of attrited sand of desired granulometry for the recycling,

   - F) connected to said second hydraulic classification means (102), means (106) for drying and cooling the attrited sand,

   - G) connected to said drying means (106), means (108) for magnetically separating the dried attrited sand to eliminate the chamotte and the chromite and the other paramagnetic minerals and obtain the recyclable sand for the foundry,

   - H) connected to said first hydraulic classification means (10), third means (202) for hydraulic classification at a granulometric cut-off of 5 µm to 20 µm of the fine fraction of diluted sludge so as to retain only the ultra-fine fraction containing the clay and reject the less fine fraction,

   - I) connected to said third hydraulic classification means (202), means (205) for dehydrating this ultrafine fraction to recover the recyclable clay.

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