CS276562B6 - Method of making a foundry mixture - Google Patents

Abstract

By mixing the polysaccharide with the core and only then adding an aqueous solution of sodium hydroxide to the mixture, the powdered polysaccharide is activated directly on the core grains, which creates a significantly thicker layer of the binding film on the grains. For the manual production of more complex cores for drying and cores requiring exceptionally high strength, bentonite and/or sodium silicate can finally be added to the mixture.

Description

The invention relates to a process for the production of a foundry composition with a binder based on polysaccharides, intended for hot curing in a hot core or dryer.

Hitherto, binder systems based on various organic resins have been used to prepare thermosetting compositions; eg phenol formaldehyde and urea formaldehyde resins. Although these mixtures have high strength, the main disadvantage of their binder system is the hygienic defect both in the preparation of the mixture and the production of cores, and in casting. The presence of harmful substances such as phenol, formaldehyde and urea also has a negative effect on the quality of wastewater and air. Another significant disadvantage is the dependence on imported raw materials and resins. To overcome these shortcomings, polysaccharide-based binders, especially starches, have been used in foundry mixes, which are activated into the binder mold before mixing with the slag. However, the disadvantage of the mixtures thus prepared is the formation of an imperfect binder film. As a result, foundry mixtures with polysaccharides have insufficient physical and mechanical properties, such as very low strengths - max, 0.5 MPa compressive strength - and high abrasion.

These disadvantages are eliminated by a process for the production of a foundry mixture containing a slag, a binder based on polysaccharides, a 1.4 to 25.4% aqueous sodium hydroxide solution and, if appropriate, a bentonite and / or sodium silicate. The essence of the invention is that a binder film is formed on the grains of the slag premixed with the polysaccharide by the action of an aqueous solution of caustic soda added to the mixture, and then other components of the mixture are mixed.

The advantage of preparing the composition according to the invention is that the activation of the powder binder takes place directly on the grains of the slag, whereby a substantially thicker binder film is formed directly on the grains. As a result, the mixture acquires surprisingly high strength values and high abrasion resistance. When using only starch in a mixture with slag and the preparation process according to the invention, the mixture reaches a compressive strength of 4.0 to 5/0 MPa. This strength is ten times higher than with hitherto known mixtures with starch. By adding bentonite to the mixture, a bonding mixture is prepared from which molds with a complex and broken surface can be made well, while the environment from the preparation of the mixtures to the hammering is completely environmentally friendly. By adding alkali silicate to the mixture, the mixture thus obtained can be used very well for the production of cores in hot cores. The strengths obtained after curing are equal to the flexural strengths of mixtures with organic resins and reach 5.0 MPa. These strength properties are at least comparable to binders such as phenol formaldehyde; however, the mixture with the polysaccharide binder is completely hygienically safe due to the absence of harmful substances. For the same reason, foundry waste can be stored in commonly used landfills without the risk of groundwater contamination. A significant advantage of these mixtures is the replacement for the phenolic resins imported so far, especially for the bitter nucleation method. Another advantage of the process according to the invention is the easy dosing of the polysaccharide powders by means of dispensers and the perfect, yet easy miscibility with the slag.

The following examples illustrate the use of the invention.

Example 1

A mixture was prepared with the composition:

quartz sand with a medium grain of 0.27 mm ............ 98 ^ 9% by weight, wheat starch ................... 1 , 0% by weight, caustic soda 0.1% by weight, water .................................... .................... 2.5 wt.

The sand was mixed with starch in the dry state, then a solution of sodium hydroxide solution in water was added and the mixture was stirred for at least 2 minutes. 50 x 50 mm test specimens were made from the mixture and dried at 150 ° C. The compressive strength after drying was 4.5 MPa.

CS 276 562 86

This mixture is suitable for manual production of cores for drying, in addition to high strength and low abrasion, it excels in high disintegration after casting. The great advantage of this mixture is the excellent ecological conditions - only starch is the binder.

Example 2

A mixture was prepared with the composition:

quartz sand with a medium grain of 0.27 mm ............... 98.35% by weight, wheat starch ................. ............................. 1.5 wt.

caustic soda ......... 0.15% wt.

water ........................... 2.5% by weight, sodium silicate .............. ..... 2.0 wt.

The procedure for preparing the mixture was the same as in Example 1, but after stirring a solution of sodium hydroxide solution in water, sodium silicate was added to the mixture, and the mixture was stirred for at least 1 minute. Then, test beams were produced in a hot core at a temperature of 200 ^D C, the flexural strength of which was 4.0 MPa.

The mixture prepared in this way has a very good flowability and is suitable for the production of cores in hot kernels. The cores have a high strength, as well as cores made of resin mixtures, good disintegration and are completely hygienically harmless. In the same way, the waste from the cores after casting is free of harmful substances.

Example 3 '

A mixture was prepared with the composition:

quartz sand with a medium grain of 0.27 mm ............................... 94.8% by weight, wheat starch. .................................................. 2.0% by weight, caustic soda ............ 0.2% by weight, water ...... 3.0% by weight, bentonite .......... ................. 3.0 wt.

The procedure for the preparation of the mixture was the same as in Examples 1 and 2, except that instead of sodium silicate, bentonite was added to the mixture and stirred for at least 3 minutes. The 50 x 50 mm test specimens had a raw bond at 40 kPa and a compressive strength after drying at 150 ° C of 2.0 MPa.

The mixture prepared in this way is suitable for the manual production of more complex cores for drying. Sufficient bonding allows you to work without shaped washers.

Example * 4

A mixture was prepared with the composition:

quartz sand ........ 97.4 wt.

dextrin ....... 1.5 wt.

caustic soda ............. 0.1 wt.

water ......... ..................................... 1.0 % wt.

sodium silicate ................. 5.0 wt.

bentonite ............ 1.0 wt.

The procedure was the same as in Example 2, but finally bentonite was added and the mixture was stirred for at least 3 minutes. The 50 x 50 mm test specimens had a raw bond

CS 276 562 B6 kPa and the flexural strength after drying at 150 ° C was 7.0 MPa.

This mixture is suitable for the production of cores with a requirement for extremely high strength, which can only be achieved with a mixture with a high content of phenolic resins or core oils. However, both types of organic binders cause an unbearable deterioration of air hygiene directly in the foundry and in the surroundings.

Claims (1)

PATENT CLAIMS A process for the production of a foundry mixture comprising a slag, a binder based on polysaccharides, a 1.4 to 25.4% aqueous sodium hydroxide solution and, if appropriate. bentonite and / or sodium silicate, characterized in that a binder film is formed on the grains of the slag premixed with the polysaccharide by the action of an aqueous solution of caustic soda added to the mixture, and then the other components of the mixture are mixed.