EP0337080A2 - Method for producing moulds from clay bonded moulding sand - Google Patents

Abstract

A process for the production of moulds from clay-bonded moulding sand for gray cast iron, malleable cast iron, spheroidal cast iron and heavy metal castings is characterised by the combination of the following steps: (a) production of a mould from a moulding sand which is essentially free of lustrous carbon formers and pyrolytically decomposable moulding sand containing organic components and (b) application of a mould dressing which contains the customary refractory inorganic components and is essentially free of pyrolytically decomposable organic components, at least to those surfaces of the mould which come into contact with the cast metal.

Description

The invention relates to a method for producing casting molds from clay-bound molding sand for gray cast iron, malleable cast iron, spheroidal cast iron and heavy metal castings.

In the production of molds for steel, gray cast iron, malleable cast iron, spheroidal cast iron and heavy metal castings, the clay-bound molding sand is still dominant with about 95% of the total molding sand requirement (see e.g. R. Weiss and U. Kleinheyer, Gießerei, 74 , 1987, no 21, page 629).

In addition to the environmentally friendly constituents, clay-bound molding sands contain quartz sand, clay and water, mostly also shiny carbon formers, e.g. Coal dust, to improve the casting surface and to condition the molding sand system (molding material). During casting, glossy carbon is formed in the heat under a reducing atmosphere, which deposits on the mold wall and reduces or prevents penetration of the liquid metal into the sand mold. The disadvantage of glossy carbon-containing molding sands is the formation of pyrolysis products during casting. These pollutants in the workplace, in the exhaust air and in the old sands of foundries can only be tolerated to a limited extent today and are increasingly becoming an environmental problem for many foundries with economic and sometimes existential consequences. It is therefore an urgent task to reduce or completely prevent the pollutants that arise during casting.

An environmentally friendly molding sand system, in which no gaseous, liquid or solid pollutants arise during casting, is clay-bound molding sand, which consists only of sand, in particular quartz sand, an inorganic binder such as clay (e.g. bentonite and / or kaolin) and water. When using such a molding sand system, however, sand build-ups and a poorer and rougher casting surface must be used be accepted, which is not possible in practice for quality reasons.

For this reason, it has not been possible to work with molding sands that are free of shiny carbon formers, especially for gray and malleable cast iron. The glossy carbon formers, which are usually in the form of coal dusts or synthetic coal dusts, pitches, bitumen, synthetic resins such as polystyrene, etc., are also added to the entire molding sand system, although the desired effect has only a limited effect on the mold surface when the liquid metal is poured in.

On the other hand, it is known to coat the casting molds with mold coating materials. The purpose of the coating materials is to influence the surface of the molded part, to improve the appearance of the casting, to influence the casting metallurgically and / or to avoid casting defects. The form coating materials contain e.g. Clays, talc, quartz, mica, zirconium silicate, magnesite, aluminum silicate and chamotte. In a broader sense, coke and graphite can also be used as inorganic raw materials. These raw materials are the intended portion of the form coating; they cover the mold surface and close the sand pores against the penetration of the casting metal.

Furthermore, the mold coating materials generally contain a carrier liquid, such as water or alcohol, which form a suspension with the base materials so that the mold coating material can be processed. The mold coatings generally also contain organic binders, such as celluloses, alginates and stearates. With the help of these substances, the suspensions are thickened and / or stabilized, whereby the settling of the solid particles is inhibited. In addition, they can contribute to the bond between the base material particles and to the adhesion to the molded part surface in the coating.

Finally, the mold coatings contain binders such as starch derivatives, lignin derivatives, resins and plastics. In the coating, these serve to bond between the base material particles and to adhere the coating to the surface of the molded part.

Both the suspending agents and the binders are decomposed during the casting to form pyrolysis products, so that they represent an environmental impact at the workplace and in the exhaust air.

A summary of the coating materials can be found, for example, in VDG leaflet R 150, August 1973, published by the Association of German Foundry Professionals.

Mold coatings which do not contain pyrolytically decomposable organic suspending agents and / or binders are also known for special applications; however, these mold coatings have hitherto only ever been used in conjunction with molding sand systems which contain glossy carbon formers and / or pyrolytically decomposable organic constituents, so that the formation of environmentally harmful pyrolysis products could not be avoided.

The invention has for its object to avoid the formation of environmentally harmful pyrolysis products in the production of casting molds from clay-bound molding sand, which are provided with coatings from mold coating materials.

• (a) Herstellung einer Form aus einem Formsand, der im wesentlichen frei von Glanzkohlenstoffbildnern und pyrolytisch zersetzbaren organischen Bestandteilen ist, und
• (b) Aufbringung eines die üblichen feuerfesten anorganischen Bestandteile enthaltenden Formüberzugstoffs, der im wesentlichen frei von pyrolytisch zersetzbaren organischen Bestandteilen ist, zumindest auf diejenigen Oberflächen der Form, die mit dem gegossenen Metall in Berührung kommen.

In the method defined at the outset, this task is solved by combining the following steps:

• (a) producing a mold from a molding sand which is substantially free of glossy carbon formers and pyrolytically decomposable organic constituents, and
• (b) Application of a mold coating containing the usual refractory inorganic components, which is substantially free of pyrolytically decomposable organic components, at least on those surfaces of the mold which come into contact with the cast metal.

With the aid of the method according to the invention, casting molds are obtained which are coated uniformly on the one hand with a mold coating material, while on the other hand no or only a small amount of environmentally harmful pyrolysis products are formed during the casting.

The statement that the molding sand or the coating material is essentially free of pyrolytically decomposable organic constituents does not exclude that small amounts of organic impurities may be present.

The mold coating materials used according to the invention can contain one or more of the following refractory inorganic constituents: clays, talc, quartz, mica, zirconium silicate, magnesite, aluminum silicate and chamotte. In a broader sense, graphite or coke can be used as inorganic constituents. These raw materials do not release any environmentally harmful pyrolysis product when cast.

Mold coatings are preferably used in which the particle size of the refractory inorganic constituents or base materials is less than about 75 μm, preferably less than about 60 μm. There is no limit to the particle size. For example, the primary particle size of bentonite and kaolin can range down to about 0.1 µm, e.g. in the case of bentonite, a maximum of the primary particles is in the range of approximately 1 μm.

A mold coating material in the form of an aqueous dispersion having a solids content of about 300 to is preferably used 500 g / l, in particular from 450 to 500 g / l.

The method according to the invention is explained below.

1. Production of the form coating materials

First, a liquefying agent (eg 0.5% of a combination of high polymer phosphates) is mixed into the mixing water. Then an inorganic anti-settling agent (eg 7.5% of an alkaline activated bentonite; trade name TIXOTON ^(R) ) is stirred in and the base material (eg graphite, kaolin etc.) is added. A laboratory stirrer can be used to prepare the dispersion. The solids content of the dispersions is generally adjusted so that a viscosity which is favorable for processing is obtained. The viscosity can be easily determined using a viscosity measuring cup. For example, a DIN viscosity measuring cup that holds 100 ml and that has an outlet diameter of 4 mm can be used for this purpose (Ford cup). The dispersion generally has the correct consistency if the flow time in the Ford cup is 15 DIN seconds. is. During this run-out time, the solids content of the dispersions is generally between about 300 and 500 g / l, preferably about 450 to 500 g / l. The most favorable solids concentration depends, among other things, on the type, nature and particle size of the raw materials used. For example, when using swellable clays, the solids concentration of the dispersions is at an expiry time of 15 DIN seconds. lower than the solids concentration when using quartz or graphite.

The laboratory tests for mold coating materials can be carried out in accordance with VDG leaflet P79 "Testing of mold coating materials", March 1976 edition (published by the Association of German Foundry Experts). This leaflet includes the Determination of bulk density, loss on ignition, volatile constituents, grain size, solids content, chemical composition, viscosity, settling behavior and durability.

2. Production of the molding sand mixtures

The molding sand mixtures used in the examples are composed of 100 parts by weight of F32 quartz sand (a quartz sand from Frechen quartz sand with an average grain diameter of 0.23 mm), 9 parts by weight of alkaline activated foundry bentonite ⁽ GEKO ^(R) ) and so much water that the bulk density as a control variable after a total mixing time of 3 minutes in the Eirich swirler R07 was 0.800 kg / l (cf. VDG leaflet P 37, edition March 1976). The following mixing sequence was chosen: The quartz sand was mixed with the greater part of the required water (coarse water) for 0.5 min. premixed. After adding the bentonite, 1.5 min. mixed on. The required amount of residual water (amount of fine water) was calculated from the bulk density then determined, and after its addition, 1 min. ready mixed.

3. Making the molds

The molding sand specified above was used to produce molds according to the 3-bar model on a Künkel-Wagner vibratory press molding machine (APM-0). The lower case of this model is shown in plan view and in section in FIG. 1. It contains three bars 1, 2 and 3 of different depths, which are connected to one another by a common casting channel 4. The liquid metal is poured through an opening 5 in an upper box, not shown. With the help of this model, a differentiated surface stress is achieved in such a way that the liquid metal penetrates into the molding sand at different depths due to its different metallostatic pressure and the different levels of heat. The casting weight including the pouring system (channel 4 and opening 5) is 12.5 kg. With a sand weight of 30 kg per mold, this results in a ratio of metal to sand of 1: 2.4.

The coating material dispersion was applied with the aid of a water atomizer (nozzle diameter 1.5 mm). The amount of coating applied was determined by weighing (difference formation from the weight after and before application, subtraction of the water content).

4. Pour and empty

A cast iron melted in a medium-frequency induction crucible furnace was cast at a casting temperature of approximately 1420 ° C. The castings cooled in the mold overnight and were emptied the next morning. The loose sand adhering to the cast grapes was removed by hammer blows. Then the adherent amount of sand, which had to be removed by steel gravel blasting, was measured by weight.

5. Roughness depth measurement

The surface roughness of the castings was measured on the surfaces in the mold box below. The measurements were carried out with a roughness depth measuring device, model M4 from Microtechnik, Liederbach. The roughness was measured five times on each casting, and the averaged roughness (Rz) was calculated from the measured values.

6. Execution and evaluation of emission measurements during casting

The mold box, from which gaseous pollutants are emitted after pouring, was surrounded by a cuboid metal hood, which had an opening at the top for introducing the measuring lines. The metal hood was immediately after the Pouring over the molding box. By attaching the hood, the exhaust gases could be measured from a homogeneous, continuously changing gas atmosphere.

The measuring arrangement is shown in Fig. 2. The cover 2 is slipped over the molding box 1, through the upper opening 3 of which three measuring lines are guided. The measuring line 4 leads from a CO measuring probe 5 inside the cover to the CO measuring device 6. The measuring line 7 leads via an adsorption tube 8 filled with silica gel for the aromatic hydrocarbons to a suction pump 9. The measuring line 10 leads via a bottle 11, in the water vapor is condensed to a flame ionization detector 12 (FID) with built-in suction pump. The flame ionization detector is calibrated using a calibration gas (C₃H₈), which is removed from the calibration gas container 13. Furthermore, the fuel gas container 14 (N₂ / 0₂) and 15 (H₂) and the inert gas container 16 (N₂) are connected to the flame ionization detector. The following gaseous pollutants were determined:

a) Measurement of non-aromatic volatile hydrocarbons (KW)

A flame ionization detector (model RS 5 from Ratfisch, Munich) with connected X, Y recorder was used for the continuous registration of the measured values.

The measurements were carried out under constant conditions: Gas inlet temperature 160 ° C Trial print 200 hPa Measuring range 0-1000 ppm or 0-10000 ppm KW Calibration gas (from container 13) 800 ppm C₃H₈ Fuel gas (from containers 14 and 15) O₂ / H₂ Carrier gas (from container 16) N₂ Sample flow 3-4 ml / min Measuring time 25 min.

The measurement curves registered under constant conditions were cut out for evaluation, and the respective areas were weighed and converted to area units. The areas determined were compared in relation to the zero sample (clay-bound molding sand without further addition); the maximum occurring hydrocarbon concentrations are given in absolute values.

b) Measurement of carbon monoxide (CO)

The measurement was carried out using an infrared gas analyzer (model EFAW 215, Bosch, Stuttgart). The CO values were measured at 1 min intervals. registered.

Measurement conditions:

Gas inlet temperature 25 ° C Measuring range 0-5% CO Measuring time 25 min.

The evaluation of the measured values was carried out as described under a) for the volatile hydrocarbons by determining the area units via the basis weight of the recorded individual values. Here, too, the relative representation was based on clay-bound molding sand as standard and the maximum values were given as absolute values.

c) Measurement of aromatic hydrocarbons

To determine the aromatic hydrocarbons, a gas stream was passed over 10 g of granulated silica gel in the adsorption tube 8 using a membrane suction pump 9 (model 3 from Hartmann and Braun, output about 250 l / h) with a gas meter to adsorb the aromatic components. The silica gel granules were then eluted with diethyl ether (25 ml) and the eluate was examined by gas chromatography for benzene and phenol; Benzene and phenol were determined in separate steps. The gas chromatograph used (Carlo Erba) was equipped with a flame ionization detector (FID) as in the determination of the volatile hydrocarbons.

Measurement conditions

Column filling 5% FFAP (Free Fatty Acid Phase = mixture of polyethylene glycol and terephthalic acid) on wide-pored SiO₂, treated with dichlorodimethylsilane (Volaspher A2 ^(R) ) Temperature (benzene) Inj. 150 ° C, oven 50 ° C Temperature (phenol) Inj. 275 ° C, oven 200 ° C Purge gas Helium 2 bar.

The measured values for the aromatic gas fractions obtained by comparison with standard solutions were given in parts by weight of benzene (adsorbed on 10 g of silica gel); Phenol could not be found in any of the samples examined.

The compositions of the molding sand systems are shown in Table I, the composition of the molding sand systems and the pollutant emissions during casting are given in Table II.

Notes on Table I

All percentages (with the exception of% VB) relate to the weight. The loss on ignition was determined according to VDG leaflet P 33, January 1976 edition (published by the Association of German Foundry Experts). The compressibility (% VB) was determined according to the VDG leaflet P 37, March 1976 edition.

Summary of casting attempts

As Table I shows, with the conventional molding sand systems, which are pyrolytically decomposable carbon-containing substances, such as e.g. Coal dust and natural asphalts were added (system No. 2), good casting results were achieved with smooth surfaces of the castings.

An essential feature of this process is that only a fraction of the total amount of glossy carbon former present in the sand system is required to improve the casting surface, but the greater part of the glossy carbon former is only carried along. However, in order to comply with existing regulations for air pollution control, it is absolutely necessary to limit the proportion of pyrolysis products to a minimum, i.e. to reduce possible sources for the formation of pyrolysis products (pyrolytically decomposable carbon-containing substances).

The formation of pollutants can be avoided by completely dispensing with the use of pyrolytically decomposable carbon-containing compounds, ie by using inorganic mold coatings (which also include graphite and coke powder according to the invention) in aqueous systems. These systems are listed in Tables I and II under Nos. 5 to 10. These systems also show good casting surfaces (low surface roughness). The molding sands without mold coating materials and without glossy carbon formers (systems No. 1 and 4) show relatively large roughness depths. The commercial mold coatings (systems 11 and 12) show relatively low roughness, but relatively high emissions of volatile hydrocarbons.

With the aid of the method according to the invention, the pollutant load measured in the casting test can thus be significantly reduced, the roughness depth values of known molding sand systems being reached or even being fallen below.

Claims (4)

1. Process for the production of casting molds from clay-bound molding sand for gray cast iron, malleable cast iron, spheroidal cast iron and heavy metal casting, characterized by the combination of the following steps: (a) producing a mold from a molding sand which is substantially free of glossy carbon formers and pyrolytically decomposable organic constituents, and (b) Application of a mold coating containing the usual refractory inorganic components, which is substantially free of pyrolytically decomposable organic components, at least on those surfaces of the mold which come into contact with the cast metal. 2. The method according to claim 1, characterized in that one uses a mold coating with one or more of the following refractory inorganic components: clays, talc, quartz, mica, zirconium silicate, magnesite, aluminum silicate, chamotte, graphite or coke. 3. The method according to claim 1 or 2, characterized in that a mold coating is used in which the particle size of the refractory inorganic constituents is less than about 75 microns, preferably less than about 60 microns. 4. The method according to any one of claims 1 to 3, characterized in that one uses a mold coating in the form of an aqueous dispersion with a solids content of about 300 to 500 g / l, preferably from 450 to 500 g / l.

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