Method for preparing a foundry sand mixture

Abstract

A method for recovery of moulding sand from a foundry sand mixture, which includes at least one proportion of moulding material fragments or loose moulding material grains, which accumulates when a cast part is demoulded from a casting mould as a result of the destruction of casting cores which have been formed from the moulding sand and an inorganic binder. The method includes: a) mixing the foundry sand mixture with cleaning water to form a slurry in order to dissolve the inorganic binder residues contained in the foundry sand mixture and optionally present additives from the moulding sand and to rinse them from the foundry sand mixture, and b) separating the cleaning water contaminated with the inorganic binder residues from the moulding sand contained in the slurry, wherein the process temperature of the slurry formed in step a) is 50 to 200° C.

Claims

1. A method for recovering moulding sand from a foundry sand mixture, which comprises at least one proportion of moulding material fragments or loose moulding material grains, which accumulates when a cast part is demoulded from a casting mould as a result of destruction of casting cores or moulded parts representing the cast part which have been formed from the moulding sand and an inorganic binder and optionally one or a plurality of additives to set properties of the moulding material, wherein the method comprises the work steps: a) mixing the foundry sand mixture with cleaning water to form a slurry, in order to dissolve inorganic binder residues contained in the foundry sand mixture and optionally present additives from the moulding sand and to rinse them out of the foundry sand mixture, and b) separating the cleaning water contaminated with the inorganic binder residues from the moulding sand contained in the slurry, characterised in that a process temperature of the slurry formed from the cleaning water and the foundry sand mixture (work step a)) is 80 to 200° C. and the slurry is mixed for a dwell time of 5-60 minutes, and the contaminated cleaning water accumulating in work step b) is reused at least once for work step a).

2. The method according to claim 1, characterised in that the process temperature of the slurry is 80 to 120° C.

3. The method according to claim 1, characterised in that the moulding material fragments contained in the foundry sand mixture are mechanically separated into grains prior to mixing with the cleaning water (work step a)).

4. The method according to claim 1, characterised in that the foundry sand mixture passes through a heat exchanger prior to work step a), through which cleaning water that is contaminated, still hot and separated from the moulding sand in work step b) is channelled in order to pre-heat the foundry sand mixture.

5. The method according to claim 1, characterised in that the contaminated cleaning water separated from the moulding sand in work step b) passes through a heat exchanger in which cleaning water flowing in for work step a) is heated.

6. The method according to claim 1, characterised in that reuse of the contaminated cleaning water is repeated until solubility of binder in the water is reached or a proportion of suspended materials contained in the water prevails.

7. The method according to claim 1, characterised in that a pH value of the moulding sand obtained in work step b) is set to a pH value of 5 to 9 by rinsing or wetting with a neutralisation solution.

8. The method according to claim 7, characterised in that a diluted acid is used as the neutralisation solution.

9. The method according to claim 1, characterised in that the moulding sand obtained in work step b) is mechanically dewatered.

10. The method according to claim 1, characterised in that the moulding sand obtained in work step b) is dried at a drying temperature of 80 to 800° C.

11. The method according to claim 1, characterised in that the foundry sand mixture contains a proportion of fragments or grains of casting cores or moulded parts, which have been formed from a moulding material, which has been formed from the moulding sand and an organic binder and optionally one or a plurality of additives to set the properties of the moulding material.

12. The method according to claim 11, characterised in that the moulding sand obtained in work step b) is heated to a temperature of at least 500° C. in order to burn organic binder residues adhering to the moulding sand.

13. The method according to claim 12, characterised in that burning of the organic binder residues takes place during drying of the moulding sand.

14. The method according to claim 1, characterised in that the moulding sand obtained in work step b) is subjected to classification by which the sand is divided into at least two portions, each portion containing particles having sizes falling within a predetermined size range.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a schematic view illustrating an exemplary embodiment of the invention.

DESCRIPTION OF THE INVENTION

(2) The FIGURE schematically shows a work process when preparing a foundry sand mixture, as typically occurs in a casting operation in which from a lightweight metal melt, in particular from an Al or Al alloy melt, cast parts not shown here, such as components for vehicles, can be produced using casting technology in a conventional manner with the aid of casting moulds also not shown here.

(3) A part of the casting moulds here comprises casting cores or moulded parts which are formed from a moulding material mass containing a moulding sand tried and tested in practice for this purpose and an inorganic binder similarly tried and tested, for example water glass. During the production of the respective casting core or moulded part, the binder is activated as usual by heat application in order to ensure the rigid cohesion of the grains of the moulding sand.

(4) Another part of the casting moulds, in contrast, contains casting cores or moulded parts which are formed from a moulding material mass containing a moulding sand tried and tested in practice for this purpose and an organic binder also tried and tested. During the course of the production of the respective casting core or moulded part, a chemical reaction of the binder is caused by adding a reaction medium, for example a gas, by way of which the binder develops its solidifying effect and ensures the rigid cohesion of the grains of the moulding sand.

(5) Upon demoulding the cast parts, the casting cores or the moulded parts are destroyed in a known manner by thermal or mechanical treatments. The moulding material fragments falling away from the cast part in this case and loose moulding material grains form a foundry sand mixture G, in which moulding sand F, hardened inorganic and organic binder and possibly also combustion residues are present which are the result of the combustion or disintegration of parts of the binder present in the respect core or moulded part which occurs as a result of the heat application during the casting operation or the subsequent thermal treatment. Similarly, common additives can also still be present in the foundry sand mixture G which are added in practice to produce moulding material masses provided by cores or moulded parts in order to ensure for example an optimal flow behaviour during the forming of the respective core or moulded part (“core shooting”).

(6) In order to recover the moulding sand the foundry sand mixture G, of which a proportion FAB of fragments or grains originate from moulded parts or casting cores made of moulding material with inorganic binder and a proportion FOB of fragments or grains originate from moulded parts or casting cores made of moulding material with organic binder, is fed into the preparation process illustrated in FIG. 1.

(7) In this case, the foundry sand mixture G firstly passes through a grain separation apparatus 1 in which the coarse fragments contained in the foundry sand mixture G are crushed in a manner known per se until only grains and smaller fragments are still present.

(8) The foundry sand mixture G, which is grain-separated and optionally pre-heated in a heat exchanger not illustrated here is introduced into a mixing apparatus 2 with the aid of gravity or for example by pressurised air support.

(9) In order to form a slurry S, in the mixing apparatus 2 the foundry sand mixture G is flown through by or stirred with cleaning water RW by using a fluidised bed or a stirrer, the cleaning water RW being previously, for example, heated in a flow heater. In the slurry S, the inorganic binder residues adhering to the grains dissolve in the cleaning water RW. The slurry S formed in the mixing apparatus 2 is circulated intensively in order to ensure turbulence supporting the removal of the inorganic binder and the other impurities. If necessary, heat is supplied in order to bring the slurry S to a process temperature which is in the optimal range of 80 to 100° C. Excess cleaning water RWK contaminated with inorganic binder residues and other dirt, such as moulding material additives and combustion residues is channelled out of the mixing apparatus 2.

(10) Through the increased process temperature, the mixture of the cleaning water RW with the foundry sand mixture G occurs so intensively that in particular the inorganic binder is substantially completely dissolved within a short time in the cleaning water RW. At the same time, the combustion residues and possibly present additive residues are absorbed by the cleaning water RW from the foundry sand mixture G. The dwell times provided for this purpose of the slurry G in the mixing apparatus 2 are 5 to 60 mins.

(11) From the mixing apparatus 2, the slurry S arrives to a rinsing apparatus 3 in which it is rinsed with cleaning water RW in order to rinse away the inorganic binder residues dissolved in the slurry S from the moulding sand grains and other impurities from the moulding sand grains F of the slurry S.

(12) The rinsing apparatus 3 can be designed as a conventional sieve machine, in which the slurry S is placed onto a sieve and sprayed with cleaning water RW which is applied by means of nozzles arranged above the sieve.

(13) The cleaning water RWK resulting here and contaminated with inorganic binder residues and other dirt is collected and supplied to a pre-cleaning apparatus 4 in which the insoluble inorganic binder residues are separated from the contaminated cleaning water RWK. Similarly, the excess contaminated cleaning water RWK channelled away from the mixing apparatus 2 is supplied to the pre-cleaning apparatus 4. A partial flow RWKV′ of the pre-cleaned contaminated cleaning water RWK can be reused by supplying it as cleaning water RW to the mixing apparatus 2. In this case, the total volume flow of the cleaning water RW supplied to the mixing apparatus 2 can be composed of a partial flow of fresh cleaning water RWF and the partial flow RWKV′ of the pre-cleaned cleaning water RWK.

(14) Equally, another partial flow RWKV″ of the pre-cleaned contaminated cleaning water RWK can be supplied to the rinsing apparatus 3 to rinse the slurry S. The total volume flow of the cleaning water RW supplied to the rinsing apparatus 3 can be composed here of a partial flow of fresh cleaning water RWF and the partial flow RWKV′ of the pre-cleaned contaminated cleaning water RWK and a further partial flow RWK of contaminated cleaning water RWK which originates from one or a plurality of the process steps explained below.

(15) Contaminated cleaning water RWKE, which is so significantly soiled that it can no longer carry out a cleaning function, is channelled away from the process and supplied to a separate preparation step.

(16) If the moulding sand F separated in the rinsing apparatus 3 from the slurry S is supposed to be used for the production of moulding material, which comprises an organic binder, then the moulding sand F passes through a treatment apparatus 5, in which it is wetted with an acid-containing neutralisation solution NL in order to set its pH value to a value of 7 to 8 that is optimal for this purpose. Then, the moulding sand F set in regards to its pH value is rinsed in a rinsing apparatus 6 with fresh cleaning water RWF in order to remove excess neutralisation solution NL. The cleaning water RWN accumulating in this case and contaminated with neutralisation solution is collected and disposed of.

(17) The setting of the pH value in the treatment apparatus 5 and the subsequent rinsing in the rinsing apparatus 6 can be skipped if the moulding sand F is exclusively intended for the production of moulding material, which comprises an inorganic binder.

(18) The moulding sand F still loaded with cleaning water RW is transported after rinsing in the rinsing apparatus 3 or the optionally cycled stations “treatment apparatus 5 and rinsing apparatus 6” to a dewatering apparatus 7, in which dewatering is carried out with mechanical means. The dewatering machine 7 can be configured as a sieve machine known for this purpose in the prior art, as a vacuum belt dryer or as a press. Through the mechanic dewatering, the moisture of the moulding sand F is reduced to such an extent that during the subsequent thermal drying notably less energy is needed to reach the required degree of dryness.

(19) The contaminated cleaning water RWK accumulating during mechanical dewatering is for example supplied to the rinsing apparatus 3 as a further partial flow of the cleaning water RW fed in there.

(20) For the thermal drying, the mechanically dewatered moulding sand F is supplied to a drying apparatus 8, which may be a rotary furnace, a belt dryer or the like. In the case where the foundry sand mixture G used comprises a proportion of moulding material fragments and grains containing organic binders or binder residues, the temperature Tw, at which the thermal drying takes place, is set to >500 to 700° C. such that the organic binder residues still adhering to the corresponding proportion of the moulding sand F burn.

(21) In contrast, if the moulding sand F no longer contains any organic binder constituents, then the thermal drying can be carried out at temperatures in the range of 100 to 300° C.

(22) The water vapour accumulating during the thermal drying is collected, condensed and supplied as fresh cleaning water RWF to the process. In this case, the fresh cleaning water RWF obtained during thermal drying for example also forms a partial flow of the cleaning water RW fed into the rinsing apparatus 3.

(23) After the thermal drying in the drying apparatus 8, the moulding sand F passes through a dedusting apparatus 9, in which fine dust FS present in the moulding sand F is separated from the remaining grains of the moulding sand F. The fine dust FS can no longer be used for the casting-related purposes and is therefore deposited in the usual manner or supplied for another use. The dedusting apparatus 9 is for example based on the principle of flow classifying, in which air is used as the separating medium (so-called “wind winnowing”). The air used here can be reused or dissipated to the environment.

(24) The dedusted moulding sand F lastly arrives to a classification apparatus 10 in which the moulding sand F is divided in accordance with at least two moulding sand classes into at least two moulding sand partial quantities Fk, Fm of which the one moulding sand partial quantity Fk comprises the part of the moulding sand F, whose grains do not exceed a certain limit size, while the other moulding sand partial quantity Fm contains the part of the moulding sand F, whose grains have a size which is at least the same as this limit size. The classification step can also be carried out in combination with the dedusting. To this end, fluid basins are normally used in which the moulding sand F is supplied from above, air flows through a sintering plate applied to the base and is set into vibration with the aid of unbalanced motors. At the same time, the fine dust FS is removed by means of the air via a suction apparatus. The grain classes are pulled at the opposing ends of the basin. The finer proportions rise higher and have to overcome a barrier. The coarse proportions do not rise as high and are therefore extracted under a barrier.

(25) The fresh cleaning water RWF required in the preparation process according to the invention and reused contaminated cleaning water RWK or the cleaning water RW possibly formed therefrom through mixing can, if required, be pre-heated via heat exchangers not shown here, in which exhaust heat released in the process itself according to the invention or in other processes is used in order to heat the respective cleaning water RWF, RWK, RW to a temperature that is optimal for the respective process step.

(26) In FIG. 1 the process flow, which the foundry sand mixture G, the slurry S formed therefrom and the moulding sand F obtained therefrom follow, are represented in continuous lines.

(27) In contrast, the flow of the cleaning water RW, the fresh cleaning water RWF, the contaminated cleaning water RWK, the pre-cleaned contaminated cleaning water RWKV, the neutralisation solution NL and the cleaning water RWN contaminated with neutralisation solution are represented with dashed lines.

(28) New moulding material FA, containing inorganic binder, and new moulding material FO, containing organic binder, is produced from the moulding sand partial quantities FK, FM obtained following the classification, by mixing with organic binder or inorganic binder and the respectively required additives.

(29) From the moulding materials FA, FO, cores or moulded parts can be produced in a conventional manner for casting moulds.

REFERENCE NUMERALS

(30) 1 Grain separation apparatus 2 Mixing apparatus 3 Rinsing apparatus 4 Pre-cleaning apparatus 5 Treatment apparatus 6 Rinsing apparatus 7 Mechanical dewatering apparatus 8 Thermal drying apparatus 9 Dedusting apparatus 10 Classification apparatus F Moulding sand FA New moulding material containing inorganic binder FAB Proportion of fragments or grains with inorganic binder in the foundry sand mixture G FK,FM Moulding sand partial quantities FO New moulding material containing organic binder FOB Proportion of fragment or grains with inorganic in the foundry sand mixture G FS Fine dust G Foundry sand mixture NL Neutralisation solution RW Cleaning water RWKE Contaminated cleaning water RW to be disposed of RWF Fresh cleaning water RWK Contaminated cleaning water RWN Cleaning water RW contaminated with neutralisation solution NL RWKV′ Partial flow of the pre-cleaned contaminated cleaning water RWKV″ Partial flow of the pre-cleaned contaminated cleaning water S Slurry