MAKING AND MOLDING"

The present Patent of Invention proposes a treatment process by regeneration for surplus sand from casting aiming for use in core making and molding, a wet regeneration process which purpose is to reach a technical and economical solution for the problem of muds generated by wet cleaning of sand as per conventional processes.

As known by the experts in the casting field, the casting process for green sand basically uses two kinds of sands, as follows: a) a molding sand, used to make a mold, which will form the external parts of the casted part. This sand, usually of silica, is agglomerated with bentonite and water, also including the additives coal powder and eventually corn starch and sawdust; and b) a core making sand, used to make the cores, which parts of sand will form cavities or internal parts of the casted part. This sand of silica or another refractory aggregate is agglomerated with a chemical ligand, which may be organic (usually a resin) or inorganic (usually a silicate), and may also contain additives such as iron oxide, chromite, mulite, zirconite, wood powder and others. In other casting processes, said sand with chemical ligand is also used for molding.

With the metal leaking, solidification, part cooling and demolding process, it is normal and desirable that the core ligand is thermally disaggregated, so that, when demolded, sand from the core is mixed with sand from the mold. In the molding sand, heat causes partial deactivation of bentonite, making it become inert clay, also thermally deactivating coal powder, generating gases and cock.

This mixture of sands is then sent to sand regeneration for use in molding and consists in the steps of metal removal, declustering, cooling and re-preparation, when, in this latter step, bentonite, water and additives are again included. Sand as regenerated is sent to the step of molding and the process is repeated.

However, due to the incorporation of core sand into the molding sand, the mass of the system increases, so that a part of the molding sand must be taken off the system. This aliquot of sand, called surplus sand from casting, is taken to other industrial uses (cement, civil architecture) or to landfills.

Core confection requires new sand to be used, since the presence of residues from molding sand (bentonite, burnt sand, coal powder and cock) avoids the good operation of the resin ligand film. Therefore, at each casting cycle, a quantity of new sand corresponding to the weight of cores is used and, depending on the kind of the produced part, it may be equivalent to the weight of the casted parts.

A few processes have been proposed to regenerate surplus sand from casting, so that it can be used again for core making. Those processes aim to take off, as much as possible, bentonite, inert clay, coal powder and cock included in the surplus sand from casting, making it become as similar as possible to the initial base sand.

A group of regeneration processes for the surplus sand from casting are mechanical processes, wherein the surplus sand from casting suffers friction, to remove particles adhered to sand granules, and is later submitted to exhaustion, wherein said fine particles are separated.

Results as obtained by these processes are not always satisfactory, since the rate of cleanness as obtained is not very high. Some casters try to take off surplus sand from casting in some special place while demolding, not removing an average aliquot, but rather an aliquot richer in core sand, with less inorganic fines, thus allowing better results for these kinds of mechanical processes. Friction may be reached by means of impact between sand grains, impact against metal plates or also by the action of abrasive grindstones.

Another group of regeneration processes for surplus sand from casting are thermal, wherein the surplus sand is submitted to a heating step under a temperature between 500 and 800 °C so to remove hydroxyls from bentonite and burn all organic components (coal powder, cock, resins, sawdust), wherein just inert clay remains together with sand granules.

As associated to a mechanical process, said inert clay is separated from sand granules and removed by exhaustion. The mechanical process is eventually effected in two steps, one before and the other one after the thermal process. This kind of process supplies regenerated sand in good quality, which may be used again to make cores, being however a process involving high costs, especially for investment and energy (thermal and mechanical).

Yet another alternative is to wet process surplus sand from casting, wherein the sand suffers wet friction, so to remove particles as adhered to sand granules, wherein fine particles can be removed by different processes, such as centrifugation or flotation. Very similar processes to those used by base sand processors are then used, by taking off the nature deposits of contaminated sand with clays and organic materials, and purifying them by using wet processes.

Table 1 shows comparison results as obtained by the different regeneration processes. We can verify that the wet regeneration process results in appropriate resistance values for the cores. Table 1 - Characterization of Regenerated Sands by Various Processes

One of the difficulties of this kind of process is the cost of water as used and its cleanness (removal of fine particles as taken off the surplus sand from casting), since the quantities of mud as generated are huge. The high cost of deposition of said fine particles as removed from sand is also important. Another important cost item is the drying of the wet processed sand. Sand as regenerated may be used in core making and molding, with chemical ligands, despite still having considerable quantities of organic materials.

There are also a few processes allowing to recover sand as used for casting, wherein the water coming from washing is treated and used again by the system and the residue as separated from the water generates mud which is used in other industrial fields, such as to produce bricks, pipelines and tiles, as disclosed by the patent documents PI9002540 and MU7100137. However, said processes require the use of new sand to make new cores, thus increasing core manufacturing cost and damages to the environment.

Within this context, the present process to treat surplus sand from casting for regeneration is proposed, aiming to be used in core making and molding, being said process a wet regeneration process, bringing in an economical and technical solution for the problem of muds as generated in wet sand cleaning.

The present Patent of Invention will be disclosed with reference to Figure 1 , which is a flow diagram of the process as disclosed herewith.

According to the illustrations of Figure 1 as disclosed above, the process for treatment of surplus sand from casting for regeneration aiming for its use in core making and molding consists of a set of steps, which can be disclosed as follows: a) feeding of the surplus sand from casting into a silo (1 ); b) initial screening, which may be made dry or even wet on a first screen (2); c) processing the mixture in first hydrocyclone (4), which mixture comes from the first screen (2) and is previously guided to a first tank (3), from where it is discharged, wherein the first removal of fines from the sand takes place in the hydrocyclone (4) and from where the water with fines is guided to the first discharged tank (5); d) processing in the attritor (6), and wet mechanical shaking therein to separate particles from grains; e) second wet screening step, taking place on the second screen (8), and the mixture coming from the attritor (6) is previously sent to the second tank (7); f) processing of the mixture in a second hydrocyclone (10), wherein the second separation of fines from the sand takes place. The water with fines is guided to the second discharged tank (1 1 ). The mixture as submitted to the second hydrocyclone (10) is previously sent to the third tank (9); g) grading step, wherein the separation of silica fines takes place in a grader (12). Water with fines is guided to the first discharged tank (5); h) mixture processing with a third hydrocyclone (15), wherein the third separation of fines from the sand takes place. Before entering the third hydrocyclone (15), the mixture is previously guided to a fourth tank (14), wherein the water is sent to a third discharged tank (13); i) processing washed sand in a collecting gutter (16), which may include a press filter, collecting table and a vacuum-, gravity- or centrifugation-assisted belt. Still another option is to pile the mixture in open air. The water as extracted on the collecting gutter (16) is guided to the third discharged tank (13); j) drying in a dryer (17), wherein the final removal of water from the regenerated sand takes place, which can be made in a rotating stove or also in a fluid bed stove; k) processing in a refrigerator (18), wherein the sand temperature is reduced to near room temperature. This can be made with a fluid bed or tubes for water circulation, or with a combination of both techniques.

Optionally, the mixture in the hydrocyclones (4, 10 and 15) of steps c, f and h can be processed by means of a grader or a sedimentation tank with a trap. In the same fashion, the mixture in the attritor (6) of step d can be processed by an ultrasound tank.

Water as separated in the discharged tanks (11 ) and (13) may recirculate in the system and return to screening as made in the second screen (8), the attritor (6) and the grader (12). Water from the discharged tank (5) has a large quantity of solids and then would be, in a classical process, submitted to a particle removal procedure, with the addition of polymers or chemicals, followed by a physical separation process, i.e. decantation or centrifugation.

The present invention gives an economical purpose to said mud, by sending it to casting to be used to recover casting sand and adding it to cooling and/or re-preparation.

Said mud should be appropriately treated so that, when re-introduced into the sand system, it does not harm mold properties. Said treatment is an innovation, since said mud contains a polyelectrolyte harming sand properties.

Treatment with sodium carbonate, in quantities between 0.013 and 0.52% (by weight) neutralizes the negative effect of the polyelectrolyte. The whole fraction of active components as present in that mud, such as bentonite and coal powder, is then recovered and the water as normally used in cooling and re-preparation of molding sand is substituted, thus representing an important economical advantage for this process. Therefore, the addition of mud brings in a 5.0% to 20% reduction in bentonite consumption and between 6.0% and 17% in coal powder consumption, thus bringing in important savings.

The sand treatment process as discussed herein allows to produce cores wherein 100% of that sand is used, thus showing the technical viability of the process. The parts are casted and sent to machining and assembly of various components, such as for use in automobiles.

This process makes it technically and economically viable to regenerate surplus sand from casting, thus allowing about 80% of the requirements of new sand to be supplied this way. This represents an important step to reduce the consumption of natural resources, by not adding further water consumption, since all the water as used in the regeneration is recirculated in the process itself or substitutes water in cooling and repreparation of molding water in casting.