Thermal regeneration of foundry sand

Abstract

The invention relates to a method for regenerating used foundry sand, which is contaminated with soluble glass, wherein: used foundry sand is provided, which is tainted with a binding agent made of the soluble glass, to which a particle-shaped metal oxide is added; and the used foundry sand is subjected to a thermal treatment, wherein the foundry sand is heated to a temperature of at least 200° C., thereby obtaining regenerated foundry sand. The invention further relates to regenerated foundry sand, as that obtained from using the method.

Claims

1. A method for regenerating used foundry sand with binding agent based on dissolved sodium silicate or potassium silicate or both adhered thereto, said method comprising: i) obtaining used foundry sand in the form of a used casting mould previously cured at least with dissolved sodium silicates or potassium silicates or both as a binding agent in the presence of heat and at least a particulate metal oxide, wherein the particulate metal oxide comprises at least amorphous silicon dioxide; ii) subjecting the used foundry sand obtained in step (i) with adhered binding agent based on dissolved sodium silicate or potassium silicates or both to a thermal treatment, wherein the used foundry sand is heated to a temperature of higher than 300° C., to obtain regenerated foundry sand; and iii) preparing a moulding material mixture comprising at least a portion of the regenerated foundry sand obtained in step (ii) and dissolved sodium silicates or potassium silicates or both as a binding agent and curing the moulding material mixture in a moulding tool wherein the curing exclusively consists of applying a heat treatment to the moulding material mixture by heating the moulding tool to 100 to 300° C. and/or by blowing heated air at a temperature of 100 to 180° C. into the moulding tool to obtain a cured casting mould comprising regenerated foundry sand, wherein the method further comprises adding as part of step (iii) at least one particulate amorphous silicon dioxide.

2. The method according to claim 1, wherein the thermal treatment is carried out until the acid consumption of the foundry sand, measured by the consumption of 0.1 n HCl in a 50 g quantity of foundry sand, has decreased by at least 10%.

3. The method according to claim 1, wherein the used foundry sand is present in the form of a casting mold.

4. The method according to claim 3, wherein the used casting mold comprises a casting.

5. The method according to claim 4, wherein the casting mold is separated from the casting before the thermal treatment.

6. The method according to claim 3, wherein the casting mold is broken at least into coarse pieces before the thermal treatment.

7. The method according to claim 1, wherein a mechanical treatment of the foundry sand for grain separation is carried out before or after the thermal treatment.

8. The method according to claim 3, wherein the casting mold is transferred to a furnace for the thermal treatment.

9. The method according to claim 1, wherein the used foundry sand is agitated during the thermal treatment.

10. The method according to claim 1, wherein the thermal treatment is carried out under admission of air.

11. The method according to claim 1, wherein the regeneration is carried out dry.

12. The method according to claim 1, wherein the dissolved sodium silicates or potassium silicates or both has an SiO.sub.2/M.sub.2O modulus in the range of 1.6 to 4.0 wherein M denotes sodium ions and/or potassium ions.

13. The method according to claim 12, wherein the dissolved sodium silicates or potassium silicates or both has a solid content of SiO.sub.2 and M.sub.2O in the range of 30 to 60 wt. %.

14. The method according to claim 1, wherein amorphous silicon dioxide is selected from the group of precipitated silicic acid and pyrogenic silicic acid.

15. The method according to claim 1, wherein an organic additive is added to the molding material mixture.

16. The method according to claim 15, wherein the organic additive is a carbohydrate.

17. The method according to claim 1, wherein a phosphorus-containing additive is added to the molding material mixture.

18. The method according to claim 1, wherein the dissolved sodium silicates or potassium silicates or both has an SiO.sub.2/M.sub.2O modulus range of 2.0 to 3.5 and M denotes sodium ions and/or potassium ions.

19. The method of claim 1, wherein the particulate metal oxide is synthetically produced amorphous silicon dioxide.

20. The method of claim 1, wherein the particulate amorphous silicon dioxide and at least the regenerated foundry sand are mixed and thereafter the water glass is added.

Description

EXAMPLES

(1) 1. Production and Curing of Moulding Material Mixtures Bound with Water Glass

(2) 1.1 Moulding Material Mixture 1

(3) 100 parts by weight of quartz sand H 32 (Quartzwerke GmbH, Frechen) were vigorously mixed with 2.0 parts by weight of the commercially available alkali water glass binder INOTEC® EP 3973 (Ashland-Südchemie—Kernfest GmbH) and the moulding material mixture was cured at a temperature of 200° C.

(4) 1.2 Moulding Material Mixture 2

(5) 100 parts by weight of quartz sand H 32 was first vigorously mixed with 0.5 parts by weight of amorphous silicon dioxide (Elkem Microsilica 971) and then mixed with 2.0 parts by weight of the commercially available alkali water glass binder INOTEC® EP 3973 (Ashland-Südchemie—Kernfest GmbH) and the moulding material mixture was cured at a temperature of 200° C.

(6) 2. Regeneration of the Cured Moulding Material Mixtures Bound with Water Glass

(7) 2.1 Mechanical Regeneration (Comparison, not According to the Invention)

(8) The cured moulding material mixtures produced according to 1.1 and 1.2 are firstly coarsely crushed and then mechanically regenerated in a regeneration system from Neuhof Giesserei—und Fördertechnik GmbH, Freudenberg, which operates according to the impact principle and is provided with a dust removal system, and the dust fractions produced are removed.

(9) The analytical data, AFS number, average grain size and acid consumption of the two regenerates are listed in Table 1. For comparison, the granulometric data of the initial mould material H32 and the acid consumption of the two cured moulding material mixtures before regeneration are given. The acid consumption is a measure for the alkalinity of a foundry sand.

(10) TABLE-US-00002 TABLE 1 Moulding Moulding Mechanical Mechanical material material regen- regen- H32 mixture 1 mixture 2 erate 1.sup.(a) erate 2.sup.(b) AFS number 45 — — 44 45 Average 0.32 — — 0.34 0.32 grain size (mm) Acid — 43.7 41.0 38.7 32.9 consumption (ml/50 mg of moulding material) .sup.(a)Starting from moulding material mixture 1 .sup.(b)Starting from moulding material mixture 2
2.2 Thermal Regeneration

(11) Approximately 6 kg each of mechanical regenerates 1 and 2 were exposed to temperatures of 350° C. or 900° C. in a muffle furnace from Nabertherm GmbH, Lilienthal.

(12) The cured moulding material mixtures 1 and 2 were thermally treated in the same way at 900° C. after coarse crushing without preceding mechanical regeneration.

(13) After cooling, the sands were used without screening for the further tests. For this reason the AFS number and the average grain size were not determined.

(14) The acid consumption of the thermal regenerates were determined analytically (see Table 2).

(15) TABLE-US-00003 TABLE 2 Treatment Acid Thermal Starting Treatment time temperature consumption regenerate material (h) (° C.) (ml/50 g) 1 Mechanical 3 900 2.8 regenerate 1 2 Mechanical 3 350 18.2 regenerate 1 3 Mechanical 6 350 9.9 regenerate 1 4 Cured 3 900 4.3 moulding material mixture 1* 5 Mechanical 3 900 2.0 regenerate 2 6 Mechanical 3 350 14.4 regenerate 2 7 Mechanical 6 350 7.8 regenerate 2 8 Cured 3 900 3.7 moulding material mixture 2* *Sample was crushed but not mechanically regenerated.
3. Core Production Using Regenerated Foundry Sands
3.1 Mechanically Regenerated Foundry Sands (Comparison)

(16) So-called Georg Fischer test bars were produced for testing the mechanically regenerated foundry sands. Georg Fischer test bars are understood as rectangular test bars having dimensions of 150 mm×22.26 mm×22.36 mm.

(17) The composition of the moulding material mixtures is given in Table 3.

(18) The following procedure was followed to produce the Georg Fischer test bars:

(19) The components specified in Table 3 were mixed in a laboratory paddle mixer (Vogel & Schemmann AG, Hagen). For this purpose, the regenerate was first supplied. Then, if specified, the amorphous silicon dioxide (Elkem Mikrosilica 971) was added whilst agitating and after a mixing time of about one minute, the commercially available alkali water glass binder INOTEC® EP 3973 (Ashland-Südchemie—Kernfest GmbH) was added lastly. The mixture was then agitated for another minute.

(20) The freshly prepared moulding material mixtures were transferred to the storage bunker of an H 2.5 hot box core shooter from Röperwerk—Giessereimaschinen GmbH, Viersen, the moulding tool being heated to 200° C.

(21) The moulding material mixtures were introduced into the moulding tool by means of compressed air (5 bar) and remained in the moulding tool for a further 35 sec. To accelerate the curing of the mixtures, hot air (2 bar, 120° C. on entry to the tool) was passed through the tool for the last 20 seconds; The moulding tool was opened and the test bars removed.

(22) In order to test the processing time of the moulding material mixtures, the process was repeated three hours after producing the mixture, the moulding material mixture being kept in a closed vessel during the waiting time to prevent the mixture drying out and CO.sub.2 from entering.

(23) In order to determine the flexural strengths, the test bars were inserted in a Georg Fischer strength testing apparatus, fitted with a three-point bending apparatus (DISA Industrie AG, Schaffhausen, CH) and the force resulting in rupture of the test bar was measured.

(24) The flexural strengths were measured according to the following system: 10 seconds after removal (hot strengths) approx. 1 hour after removal (cold strengths)

(25) The measured strengths are summarised in Table 4.

(26) TABLE-US-00004 TABLE 3 Composition of the moulding material mixtures (comparative examples) Amorphous silicon Binding Sand dioxide.sup.(a) agent.sup.(b) Example 1 100 parts by wt. — 2.0 parts by wt. H32.sup.(c) Example 2 100 parts by wt. 0.5 parts by wt. 2.0 parts by wt. H32.sup.(c) Example 3 100 parts by wt. 0.5 parts by weight 2.0 parts by weight mechanical regenerate 1 Example 4 100 parts by wt. 0.5 parts by weight 2.0 parts by weight mechanical regenerate 2 .sup.(a)Elkem Microsilica 971 .sup.(b)INOTEC ® EP 3973 (Ashland-Südchemie-Kernfest GmbH) .sup.(c)Quartzwerke GmbH, Frechen

(27) The weight of the test bars were determined as a further test criterion. This is also given in Table 4.

(28) TABLE-US-00005 TABLE 4 Strengths (N/cm.sup.2) and core weights (g) (Comparative example) Hot Cold Core Hot Cold Core strength strength weight strength strength weight (fresh (fresh (fresh (3 h old (3 h old (3 h old mixture) mixture) mixture) mixture) mixture) mixture) Example 1 60 350 127.0 50 300 126.2 Example 2 155 440 127.6 140 420 126.9 Example 3 125 420 120.3 40 200 117.2 Example 4 120 410 117.9 (n) (n) (n) (n): no longer shootable

(29) In the mechanically regenerated foundry sand used in Example 3, which was produced from foundry sand which had been hardened with water glass containing no particulate amorphous silicon dioxide (mechanical regenerate 1), a 3 h old mixture is still shootable. However, test bars which exhibit a poorer strength compared to Example 1 and 2 are obtained.

(30) If the mechanically regenerated foundry sand contains a binding agent which contains amorphous silicon oxide (Example 4), the 3 h old mixture is cured and can no longer be shot. This shows that used foundry sands containing water glass as binding agent mixed with a particulate metal oxide are not suitable for mechanical regeneration.

(31) 3.2 Thermally Regenerated Foundry Sand

(32) For testing the thermally regenerated foundry sands, the procedure was similar to that for the mechanically regenerated foundry sands.

(33) The composition of the moulding material mixtures is given in Table 5, the strengths and core weights are summarised in Table 6.

(34) TABLE-US-00006 TABLE 5 Composition of the moulding material mixtures (according to the invention) Amorphous silicon Binding Sand dioxide.sup.(a) agent.sup.(b) Example 5 100 parts by wt. 0.5 parts by wt. 2.0 parts by wt. thermal regenerate 1 Example 6 100 parts by wt. 0.5 parts by wt. 2.0 parts by wt. thermal regenerate 2 Example 7 100 parts by wt. 0.5 parts by wt. 2.0 parts by wt. thermal regenerate 3 Example 8 100 parts by wt. 0.5 parts by weight 2.0 parts by weight thermal regenerate 4 Example 9 100 parts by wt. 0.5 parts by weight 2.0 parts by weight thermal regenerate 5 Example 100 parts by wt. 0.5 parts by weight 2.0 parts by weight 10 thermal regenerate 6 Example 100 parts by wt. 0.5 parts by weight 2.0 parts by weight 11 thermal regenerate 7 Example 100 parts by wt. 0.5 parts by weight 2.0 parts by weight 12 thermal regenerate 8 .sup.(a)Elkem Microsilica 971 .sup.(b)INOTEC ® EP 3973 (Ashland-Südchemie-Kernfest GmbH)

(35) TABLE-US-00007 TABLE 6 Strengths (N/cm.sup.2) and core weights (g) Hot Cold Core Hot Cold Core strength strength weight strength strength weight (fresh (fresh (fresh (3 h old (3 h old (3 h old mixture) mixture) mixture) mixture) mixture) mixture) Example 5 145 450 124.4 135 410 123.6 Example 6 135 425 123.3 125 385 121.9 Example 7 140 435 123.4 125 390 122.2 Example 8 130 415 123.1 130 400 122.4 Example 9 150 445 123.1 135 405 122.7 Example 10 140 420 122.9 130 395 122.3 Example 11 140 430 123.1 125 405 122.6 Example 12 135 425 123.2 130 390 122.5

(36) Thermal regenerates originating from moulding material mixture 1 were used in Examples 5 to 8. This moulding material mixture used a water glass as binding agent containing no amorphous silicon dioxide. The moulding can still be shot very well after 3 hours. The test bars show very good strength.

(37) The same result is achieved with thermal regenerates 5 to 8, as Examples 9 to 12 show. The regenerates used in these example originate from moulding material mixture 2 which contains water glass as binding agent mixed with amorphous silicon dioxide. Even after a standing time of 3 hours, the moulding material mixture can be shot very well. The test bars obtained show very good strength.

(38) Result:

(39) Comparison of Tables 1 and 2:

(40) It can be seen that the acid consumption of the moulding materials is reduced considerably more substantially by the supply of heat than by mechanical regeneration. The determination of the acid consumption is at the same time a simple method of tracking the progress of the thermal regeneration.

(41) Comparison of Tables 4 and 6:

(42) It can be seen that the processability of the moulding material mixtures when using thermally regenerated foundry sands is significantly longer than when using mechanically regenerated foundry sands and this is regardless of whether the thermal treatment was preceded by mechanical regeneration or not.

(43) It can also be seen that the weight of the test bars produced using the thermally regenerated foundry sands is higher than that of those test bars which were produced using mechanically regenerated foundry sands, i.e. the flowability of the moulding material mixtures has increased due to the thermal regeneration.