CA2046508C - Process for the regeneration of used foundry sand - Google Patents

Abstract

A process is disclosed for the regeneration of used foundry sand, whose original molding sand contained as molding material inorganic bonding agents such as bentonite (= "inorganic used sand") and/or organic bonding agents such as phenolic and/or furan resins (= "organic used sand"). The used sand to-be-cleaned is soaked with water and subsequently moved into a heated fluidizing bed in its wet condition.

Description

2U46~08 This invention relates to a process for the regeneration of used foundry sand whose original molding sand contains as molding material inorganic bonding agents such as bentonite (= "inorganic used sand") and/or organic bonding agents such as, for example, phenolic and/or furan resins (= "organic used sand").
In inorganically bonded molding sands the bonding agent bentonite or a similar material is fixed in shell-form at the surface of the sand grains by oolithization, depending on the degree of heat treatment during casting, while molding sands with organic bonding agents suffer thermal decomposition during casting and thus acquire at the grain surface firmly adhering residues of carbon-rich derivatives from the organic bonding agents. In addition, the used sand is further contaminated because of additional additives, such as the formation of lustrous carbons from the additives.
While used (foundry) sand was formerly at least predominantly simply disposed of, acute scarcity of available deposit sites and also cost considerations today have led to a demand for the regeneration, i.e. cleaning, of used sand, so that it may be re-used. However, this requires that the above-mentioned bonding agent shells and other contaminations of the used sand be separated from the quartz bodies and then eliminated.
A known method of regenerating used foundry sand which consists at least predominately of organic used sand is to treat it thermally by annealing at a temperature of ca. 800 degrees C and/or possibly pneumatically. In this process the bonding agent shells and other contaminations of the used sand are generally so thoroughly removed that the used sand can be reused as molding sand. On the other hand, this thermal cleaning process does not give rise to satisfactory results with inorganic used sands.
The thermo-mechanical process known from German Patent No. 31 03 030 has useful results only if the --- 2~!~6~8 proportion of inorganic sand is relatively small.
Consequently, this process does not solve the above problem, since used foundry sand generally consists of more than two thirds of inorganic used sand.
In view of these conditions, German Patent No. 38 15 877 has proposed (e.g. according to German Patent No. 31 03 030) that the thermo-mechanically pre-treated or treated regenerate be subsequently treated ultrasonically in water (as a coupling layer). In this way the bonding agent shells, which are sintered onto the sand grains, are caused to split off and the degree of oolithization is lowered to normal values, while simultaneously the basic pH value of the regenerate is adjusted to a near neutral value.
Quite apart from the complex and costly equipment needed for implementing the above process, as well as the high operating costs, further costs arise because the cleaned sand still has to be dried, which makes this multi-step process very uneconomical. In addition, it should be noted that, due to a relatively high wear factor for the sand grains, only a relatively low regenerate yield of sufficient quality can be achieved.
For the regeneration of used inorganic sand processes have been developed in which it was attempted to remove the bonding agent shells by a wet process from the surface of the sand grains. This was accomplished by means of mechanical stirring in a watery sand suspension resulting in intensive friction among the sand grains.
This process step, which is also called attrition, is generally repeated several times and possibly affected or intensified by creating special sand-water-mix conditions (see e.g. U.S. Patent No. 27 83 511 or German OLS 30 19 096).
One disadvantage of this wet process is the fact that hydrocarbon compounds of organic used sand portions, as well as lustrous carbons and their generating agents, cannot be sufficiently removed. These materials act - 3 20465~ 8 practically as lubricants (as, e.g. argillaceous components) and prevent the desired abrasion, so that sufficiently satisfactory regenerating results cannot be obtained even with multi-step attrition.
In order to solve this problem, Australian Patent No. 387 921 suggested conducting a sludge removal (by means of classifiers) after each attrition step, so that the "lubricants" are thus removed from the circulating water and the desired abrasion is intensified. However, even if a satisfactory result could be obtained from a technical point of view with three-, four- or manifold attrition, this result would be considerably burdened with the attendant costs. In addition, in this process an adherent structure of finely distributed, porous silicic acid remains on the grain surface, which necessitates increased amounts of bonding agents when the cleaned used sand is reused as new foundry sand.
It is an object of the present invention, while avoiding the above-mentioned and other disadvantages, to create a process for the cleaning of used foundry sand which is satisfactory both from a technical and an economical point of view, in which process (as compared with the state of the art) a relatively low technical and financial effort produces a regenerated material which is extensively and generally usable and which contains relatively few harmful components, and can be reused without problems in place of new sand for mold and core production in foundries.
Accordingly, the present invention provides a process for the regeneration of used organic or inorganic foundry sand formed from original molding sand which contained, as the molding material, inorganic binders in the case of inorganic used sand or organic binders in the case of organic used sand, said binders being fixed in a shell form at the surface of the sand grains of said used sand, said process comprising the steps of:

, ~

2 ~ 8 a) impregnating the used sand to be reconditioned with water to such an extent to substantially fill the pores of the grain shells of the sand with water;
b) providing a heated fluidized bed consisting of sand;
c) maintaining said fluidized bed at a steady temperature of about 750 to 950~C;
d) feeding said used sand impregnated with water into the heated fluidized bed; and e) discharging the sand from said fluidized bed.

-- 20~6~08 Thus, the used sand to-be-cleaned is (to begin with) soaked in water and then placed in its wet condition into the heated fluidized bed, where the shock-heating of the used sand causes a spontaneous evaporation of the water located in the pores thereof. The resulting considerable increase in volume causes the shells (at least) of the (inorganic) bonding agent to split off or at least to be loosened to an extent that they can be directly separated from the quartz grains in the fluidized bed. the traditional drying of wet-regenerated used sand by hot air at temperatures of ca. 150-300 degrees C in various devices does not achieve such a splitting-off or loosening effect, as is proven by the above-mentioned bonding agent requirements.
The heat removal by water evaporation in the immediate proximity of the quartz grains also prevents possible changes of crystal modification and/or grain decomposition, due to the shock-like heating of the quartz grains. In addition, the sintering onto the grain surface of mullite, which has been created from bentonite due to the effect of heat, is prevented or at least inhibited.
Furthermore, it is not necessary in the process of the invention that the used sand, which may consist of any mixture of organically and inorganically bonded used sands, must during wetting or soaking pass through an attrition step which is otherwise required for wet regeneration. The simplified operation and the less costly equipment result in considerable economic advantages.
Consequently, it is not at all necessary in the process of the invention (even though it may be practical in certain cases) to assure a basic separation of organic and inorganic used sand in the preparatory stages of the cleaning, while it is indeed practical to clean the organic and inorganic used sand essentially separately from one another if they are not already separate, as this is often the case.

20~6~ ~8 Furthermore, it is advantageous that practically no harmful residues occur, that in accordance with the above explanations it is not necessary to maintain a certain proportion of different used sands for cleaning, and that quite clearly no separate process branches are required and have to be adjusted to one another, which, as is well known, can generally cause considerable difficulties especially in a hot process.
As a rule it has proved to be highly practical to soak the used sand so thoroughly with water (while the water may be given additives for lowering surface tension) that the pores located between the grain shells are at least essentially filled with water in order to optimize the above-mentioned effect and, in the final analysis, to optimize the entire used sand cleaning process and the quality of the regenerate.
In a preferred embodiment of the invention the soaking of the used sand is carried out with circulated water in order to eliminate harmful components, especially alkalis. This operating method is practical in cases where the used sand contains, as a result of the use of alkali-silicates or phenolates as bonding components, significant amounts of such ions. If these were allowed to remain in the sand regenerate, the resulting high basicity would significantly limit reusability.
If necessary, the water used for soaking the used sand may be kept at a pH value between 2 and 5 by continuous addition of acid and is kept in circulation until a maximum salt concentration (to be determined in each case) has been reached. Furthermore, the exchange with fresh water may be carried out continuously. Before each entry into the soaking device the circulating water passes through a settling tank and, if necessary, a filter, where the solids absorbed by the water from the sand in the form of sludge can be separated and the sludge can be returned to the fluidized bed. The shock-like heating of 20~6508 these solids, which are continuously soaked with water, also prevents a sintering onto the sand grains, and their organic components burn out.
In another embodiment of this process variant, carbon dioxide is used as an anionic component of the decomposition of the alkali-silicates instead of mineral or carboxylic acid. Here the circulating water acquires a basic pH value; however, the alkalis are removed from the used sand just as well, namely in the form of carbonates.
The alkaline solution which still adheres to the used sand is rinsed from the grains by spraying on the drainage screen with the fresh water required as replacement. The resulting soda or potash solution can be evaporated with exhaust or waste heat, crystallized and the alkali can thus be recovered therefrom in a reusable form.
However, the process is extremely satisfactory not only from a technical, but also from an economical point of view, especially since the heated fluidized bed itself can consist of sand (even of already cleaned sand, if necessary).
In order to have available a fluidized bed with sufficient heating capacity to obtain the desired results, the ratio of the fluidized bed sand quantity to the amount of wet used sand which is added per minute should be in the range of about 50 to 100.
In accordance with the invention the fluidized bed is most preferably heated from the top, preferably with high-speed burners, for which gaseous fuels have proven to be especially practical as additional fuels (beyond fuel already contained in the used sand).
In order to obtain the above-explained effects, the fluidized bed is preferably maintained at a stable temperature in the range of about 750 to 950 degrees C.
According to a further preferred embodiment of the invention, the temperature in the solid material, on the one hand, and in the gas chamber above the fluidized -- 20~6508 bed, on the other, can be variably adjusted, as can the dwell time of the used sand in the fluidized bed, i.e.
depending on its composition.
In this manner complete burn-out of organic substances even in flying dust, and thus is decontamination, is achieved.
In order to intensify and/or accelerate the wetting of the used sand it can be advantageous if the used sand which is to be wetted and cleaned is wetted in a vacuum.
It is also quite practical as a rule if the propelling speed or velocity of the fluidized bed is adjustable over a wide range, so that the operating conditions can be optimally suited to the respective requirements of the used sand.
Furthermore, it can be practical to separate sand in a separator or similar device downstream of the fluidized bed, in which case it may be further practical to return this separated sand partially to the fluidized bed.
The used sand (regenerate) cleaned in this manner can be subjected to subsequent mechanical cleaning if desired.
A preferred embodiment of the invention will now be described with reference to the accompanying drawing which illustrates diagrammatically an apparatus for carrying out the process for regenerating used foundry sand.
The drawing shows as a diagrammatic representation a bin 1, in which the used sand (which is routinely a mixture of inorganic and organic used sand) is collected. The used sand to-be-cleaned is moved from the bin 1 into used sand pre-treatment equipment 2, consisting of for example means for magnetic separation of Fe particles, a lump crusher and a sifter, and is then moved into an intermediate bin 3.

2o~6so8 -From the intermediate bin 3 the pre-treated used sand is filled into a mixer 5 by means of a dosing device 4. Into this mixer 5 are added dosed water from a dosing device 6 and materials for the reduction of surface tension. The water is heated to a maximum of 95 degrees C
with exhaust or waste heat.
The wet sand is rotated in a conditioner 7 until complete wetting of the pores of the grain shells is attained and then reaches a fluidized bed oven 8 in dosed form.
As a result of the extremely fast heat transfer (and the good mixing in the fluidized bed) the wet used sand is heated abruptly to an operating temperature of 800 to 850 degrees C, while the above-described formation of steam splits the fire-clay shells off the (quartz) sand grains, or at least loosens them considerably, and harmful organic materials burn off completely. The separated shells are trapped in a separator 9.
The oven exhaust gas is cooled in a heat exchanger 11 by preheating the fluidizing air and then passes through a filter 10.
To cool the oven discharge the latter first passes through a fluidizing bed cooler 12, in which the water needed for soaking is pre-heated to 70 to 95 degrees C and the perceptible heat of the sand is recovered in the form of steam or hot water.
A concluding treatment of the resulting regenerate occurs in a pneumatic cleaning device 13 and by fractionation into (at least) two grain sizes ("coarse" and "fine"). The cleaned used sand is universally reusable, while the granulating of the new sand which is to be formed from the cleaned used sand (if necessary by adding certain amounts of new sand) can be achieved by dosing from the various fractionations in order to obtain used sand with pre-determined average granulation or a certain grain bond.
After cooling and pneumatic cleaning of the 2Q~650~

treated sand, regenerate and filter dust are obtained. The filter dust is free of harmful substances and may be disposed of or used as construction material or additive without any problem.

Claims (24)

1. A process for the regeneration of used organic or inorganic foundry sand formed from original molding sand which contained, as the molding material, inorganic binders in the case of inorganic used sand or organic binders in the case of organic used sand, said binders being fixed in a shell form at the surface of the sand grains of said used sand, said process comprising the steps of:
a) impregnating the used sand to be reconditioned with water to such an extent to substantially fill the pores of the grain shells of the sand with water;
b) providing a heated fluidized bed consisting of sand;
c) maintaining said fluidized bed at a steady temperature of about 750 to 950°C;
d) feeding said used sand impregnated with water into the heated fluidized bed; and e) discharging the sand from said fluidized bed.

2. A process according to Claim 1, wherein the water used for impregnating the used sand is circulated and harmful alkali metal ions are thereby dissolved out.

3. A process according to Claim 2, wherein said circulated water is maintained at a pH of between 2 and 5 by addition of mineral acids or carboxylic acids and is replaced by fresh water to such an extent that a salt concentration, which is to be pre-determined, in the circulated water is not exceeded.

4. A process according to Claim 2 or 3, wherein said circulated water is treated with CO2 in the alkaline range and the alkalis are thus recovered as carbonates.

5. A process according to Claim 2, 3, or 4, wherein said circulated water is purified and filtered and the sludge thus obtained is recycled into the fluidized bed.

6. A process according to Claim 1, 2, 3, 4 or 5, wherein the ratio of the rate of fluidized-bed sand to the rate of moist used sand fed per minute is about 50 to 100.

7. A process according to Claim 1, 2, 3, 4, 5 or 6, wherein inorganic and organic used sands in any desired mixtures with one another are impregnated with water and these mixtures are then fed to the fluidized bed.

8. A process according to Claim 1, wherein used sand with at least predominantly organic binders is introduced separately into the fluidized bed.

9. A process according to Claim 8, wherein only the inorganic used sand is impregnated with water.

10. A process according to Claim 8, wherein exclusively organic used sand is separately introduced into the fluidized bed which has been impregnated separately with water.

11. A process according to any one of Claims 1 to 10, wherein the water is preheated to 70 to 95°C.

12. A process according to Claim 9, wherein surface tension-reducing agents are added to said water.

13. A process according to any one of Claims 1 to 12, wherein the fluidized bed is heated from above.

14. A process according to Claim 13, wherein said heating from above is achieved by using a gaseous fuel.

15. A process according to any one of Claims 1 to 14, wherein the temperature in the fluidized bed and the temperature in the gas space above the fluidized bed are adjusted to different levels.

16. A process according to any one of Claims 1 to 15, wherein residence time of the used sand to be reconditioned in the fluidized bed is adjustable.

17. A process according to any one of Claims 1 to 16, wherein said impregnation is enhanced by means of a vacuum.

18. A process according to any one of Claims 1 to 17, wherein the fluidizing velocity of the fluidized bed is adjustable within wide limits.

19. A process according to any one of Claims 1 to 18, wherein sand, which has been precipitated in a separator or the like downstream of the fluidized bed, is partially recycled into the fluidized bed.

20. A process according to any one of Claims 1 to 19, wherein the regenerated used sand product is finally purified mechanically or pneumatically or mechanically/pneumatically.

21. A process according to any one of Claims 1 to 20, wherein fluidizing air is preheated by heat transfer from the exhaust gas from the fluidized bed.

22. A process according to any one of Claims 1 to 21, wherein heating is operated close to the stoichiometric fuel-air ratio.

23. A process according to any one of Claims 1 to 9, 11 to 22, wherein the original molding sand contained bentonite as an inorganic bonding agent.

24. A process according to any one of Claims 1 to 23, wherein the original molding sand contained a phenolic or furan resin or mixture thereof as an organic bonding agent.