EP0521201B1 - Process for reclaiming used foundry sand - Google Patents

Description

The invention relates to a method for regenerating foundry used sand according to the preamble of claim 1, that is, used sand, the original molding sand as a molding material inorganic binders such as bentonite (= "inorganic used sand") and / or organic binders such as phenol or / and Furan resins (= "organic old sand") contained.

In the case of inorganically bound molding sands, the binding agent is fixed in a shell-like manner on the surface of the quartz grains by fireclaying (oolithization), while it is in the case of molding sands with organic Binding agents in the course of the casting process lead to their thermal decomposition and thus to adherent residues of carbon-rich degradation products of the organic binders on the surface of the quartz grains. In addition, further additives lead to contamination of the old sand, such as the formation of shiny carbon.

While (foundry) used sand used to be mostly dumped on landfills, nowadays it is necessary to regenerate used sand due to the acute shortage of available landfill space and not least for cost reasons, i.e. to process it so that it can be reused. However, this presupposes that the above-mentioned binder shells (grain shells) and other impurities in the old sand are separated from the quartz grains.

For the regeneration of used sand, which at least consists predominantly of used organic sand, it is known to treat it thermally by annealing at a temperature of approximately 800 ° C. and / or, if necessary, pneumatically. The binder casings and other impurities of the old sand are usually so far removed that the used sand can be reused as molding sand. In contrast, this thermal treatment process does not lead to a satisfactory result for inorganic waste sands.

The thermal-mechanical method known from DE-PS 31 03 030 leads at most to usable results if the proportion of inorganic waste sand is relatively small. This method therefore does not generally solve the above problem, since used sand usually consists of more than two thirds of inorganic used sand.

Taking these conditions into account, it was proposed with DE-PS 38 15 877 to subject the (for example according to DE-PS 31 03 030) thermally-mechanically (pre-) treated regenerate to an ultrasonic treatment in water (as a coupling layer). This is intended to cause the binder shells sintered onto the quartz grains to flake off and to reduce the degree of oolithization to normal values.

Quite apart from the large and complex equipment required to carry out the above-mentioned method and the high operating costs, further costs result from the fact that the processed used sand has to be dried afterwards, so that the economy of this multi-stage process is no longer given, i.ü . it should also be noted that due to the relatively high wear of the grains of sand, only a relatively low yield of regenerate of sufficient quality can be achieved.

For the regeneration of old inorganic sand, methods have been developed in which an attempt has been made to remove the binder shells from the surface of the sand grains by wet means, namely by carrying out a mechanical stirring process in an aqueous sand suspension with the result of intensive friction of the sand grains against one another, whereby this process step, also referred to as attraction, as a rule can be repeated several times and, if necessary, influenced or intensified by setting special sand / water mixing ratios (see, for example, US Pat. No. 2,783,511 or DE-OS 30 19 096).

One disadvantage of this wet process is the fact that hydrocarbon compounds contain organic waste sand as well as clanz carbon and its formers cannot be removed sufficiently. These substances (like clayey components, for example) act as lubricants and hinder the desired abrasion, so that even with multi-stage attrition, a sufficiently satisfactory regeneration result cannot be achieved.

In order to remedy this, it was proposed with AU-PS 387 921 to connect a desludging (by means of classifiers) behind each attraction stage in order to remove the "lubricants" from the circulating water and thus to intensify the desired abrasion. But even if a satisfactory result could be achieved with three, four or multiple attrition, this obviously suffers considerably from the point of view of cost-effectiveness due to the associated costs. In addition, in this process, a firmly adhering structure of finely divided, porous silica remains on the grain surface, which results in an increased need for binder when reusing the processed used sand for (foundry) new sand.

From DE 36 39 634 A1 a generic method for the regeneration of old foundry sand is known, in which the old sand to be treated is guided in a fluidized bed along high-temperature, rod-shaped heating elements in such a way that its grains repeatedly briefly on their surface, i.e. the grain shell come into contact with a heating element and be subjected to thermal shock. As a result, the grain shells made of binder should gradually become brittle or burn and fall off the quartz grains. This involves swirling in a fluidized bed in order to cause the surface of the old sand grains to hit a heating element at several points and to be subjected to thermal shock.

The present invention has for its object, while avoiding the aforementioned and other disadvantages to provide a both technically and economically satisfactory method for processing used foundry sand, in which (compared to the prior art) relatively low technical and economic outlay a largely generally usable regrind with relatively low pollutant components What can be achieved, which enables problem-free reuse instead of new sand for mold and core production in foundries.

The solution to this problem is, according to the invention, that the old sand to be treated is first soaked with water so that the pores of the grain shells i.w. are filled with water, and that the impregnated old sand is then placed in a heated fluidized bed, the water in the pores spontaneously evaporating due to the shock heating of the old sand to be treated and the grain shells being blown off or at least blown off due to the considerable increase in volume of the water that arises are loosened so much that they can be separated from the quartz grains directly in the fluidized bed. Due to the usual drying of wet regenerated used sand using hot air at approx. 150 - 300 ° C, such a blasting or loosening effect is not achieved, as the binding agent requirement mentioned above shows.

The heat dissipation by the water evaporation in the immediate vicinity of the quartz grain prevents a possible change in the crystal modification and a grain decay due to a shock-like heating of the quartz grains.

In the process according to the invention, it is not necessary that the used sand, which can consist of any mixture of organically and inorganically bound used sands, has to pass through an attraction stage that is otherwise necessary for wet regeneration when it is wetted or soaked. This results in considerable economic advantages due to the simplified method of operation and the less expensive equipment.

In the method according to the invention, it is also not necessary (even if this may be expedient at least in certain cases) to ensure, as it were, a fundamental separation of organic and inorganic waste sands in advance, although it is regularly advisable to use the organic or inorganic ones Processing used sands essentially separately from one another, if they occur separately anyway, as is often the case.

It has generally proven to be extremely useful if means for lowering its surface tension are added to the water, so that the pores are at least substantially filled with water, in order to achieve the effect set out above, and thus ultimately the entire processing of used sand and the quality of the regenerate to optimize.

However, the process according to the invention is highly satisfactory not only from a technical but also from an economic point of view, especially since the heated fluidized bed can itself consist of sand (possibly also of already processed used sand).

In order to have a fluidized bed with sufficient heat capacity available for the desired effect, the ratio of the amount of fluidized bed sand to the amount of moist used sand supplied per minute can be selected in the range from approximately 50 to 100.

According to the invention, the fluidized bed is most preferably heated from above, preferably with high-speed burners, gaseous fuels having proven to be particularly useful as the additional fuel (in addition to the fuel components already contained in the used sand).

In order to achieve the effects explained above, the fluidized bed can preferably be kept at a steady temperature of approximately 750 to 950 ° C.

According to a further preferred embodiment, the dwell time of the used sand to be processed in the fluidized bed can be adjusted depending on its nature. In this way, a complete burnout of organic substances is to be ensured.

In order to intensify or accelerate the moistening of the old sand to be treated, it can be advantageous if the old sand to be humidified and to be treated is moistened in a vacuum.

Preferred embodiments of the present invention are described in the subclaims.

The invention is explained below using an exemplary embodiment with reference to a drawing.

The drawing shows a schematic representation of a bunker 1 in which the used sand, which is regularly a mixture of inorganic and organic used sand, is collected. The used sand to be processed is fed from the bunker 1 to a used sand preparation 2 (consisting of a magnetic separator for separating Fe parts, a tuber shredder and a sieve) and from there it arrives in an intermediate bunker 3.

From the intermediate bunker 3, the prepared used sand is fed by means of a metering device 4 to a mixer 5, into which water 6 and agents which reduce the surface tension are also added. The water is heated to max. 95 ° C heated.

The moist sand is circulated in a conditioner 7 until the pores of the grain shells are completely moistened is reached and then metered into a fluidized bed furnace 8.

As a result of the extraordinarily rapid heat transfer (and the good mixing in the fluidized bed), the wet old sand to be treated is suddenly heated to an operating temperature of 800 to 850 °, whereby the steam formation described above blows the fireclay shells off the quartz grains or at least considerably loosens them and organic Burn pollutants completely.

The furnace exhaust gas is cooled in a heat exchanger 11 while preheating the fluidized air.

To cool the furnace discharge, it first passes through a fluidized bed cooler 12, in which the water required for the impregnation is preheated to 70 to 95 ° C. and the sensible heat of the sand is recovered in the form of steam or hot water.

A final treatment of the regenerate obtained takes place in a pneumatic cleaning device 13 and a fractionation into (at least) two grits ("coarse" and "fine"). The processed used sand is universally reusable, whereby the granulation of the new sand to be formed from the processed used sand (if necessary with a certain addition of new sand) can be done by dosing from the individual fractions in order to obtain used sand with a predetermined average grain size or a certain grain size range .

After cooling and pneumatic cleaning of the processed sand, regenerate and filter dust are generated. The filter dust is free of pollutants and can be safely deposited or used, for example, as a building material or aggregate.

Claims (10)

1. A method of regenerating used foundry sand consisting of quartz grains having a shell made up mainly of inorganic and/or organic binders from the original moulding sand, wherein the grain shells of the used sand, which is in a fluidised bed, are removed from the quartz grains by shock heating, characterised in that the used sand for processing is first impregnated with water sufficiently for the pores of the shells to fill substantially with water, after which the impregnated used sand is brought into a heated fluidised bed and as a result of the shock heating of the used sand the water in the pores of the shells spontaneously evaporates and the shells, owing to the resulting increase in the volume of water, burst or are so loosened from the quartz grains that they drop off them into the fluidised bed.
2. A method according to claim 1, characterised in that the heated fluidised bed is of sand.
3. A method according to claim 2, characterised in that the ratio of the sand in the fluidised bed to the amount of wet used sand supplied per minute is about 50 - 100.
4. A method according to one or more of the preceding claims, characterised in that if the used sand containing grain shells formed from inorganic binders is already separate from the used sand containing shells consisting of organic binders, the two are introduced separately into the fluidised bed, in which case only the used sand containing grain shells consisting of inorganic binders is previously impregnated with water.
5. A method according to one or more of the preceding claims, characterised in that the water for impregnating the used sand is preheated to 70 - 95°C.
6. A method according to one or more of the preceding claims, characterised in that surface tension reducing agents are added to the water.
7. A method according to one or more of the preceding claims, characterised in that the fluidised bed is heated from above.
8. A method according to one or more of the preceding claims, characterised in that the fluidised bed is maintained at a steady temperature of about 750 - 950°C.
9. A method according to one or more of the preceding claims, characterised in that the residence time of the used sand for processing in the fluidised bed is adjustable.
10. A method according to one or more of the preceding claims, characterised in that the used sand for processing is impregnated with water in a vacuum.

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