CS246544B1 - ) Hardener for acid curing molding compounds - Google Patents

Abstract

The solution relates to a hardener based on raw materials containing magnesium oxide, such as magnesia or chrome magnesite, intended for acid-cured molding mixtures. The essence of the solution lies in the fact that the hardener contains magnesium phosphate, formed by adding from 1 to 10 parts by weight of trihydrogenphosphoric acid with a specific gravity of from 1,050 to 1,150 kg.m-3 per 100 parts by weight of the basic raw material. The reactivity of the hardener is in the range from 150 to 200 Jg_1

Description

BACKGROUND OF THE INVENTION The present invention relates to a hardener based on magnesium oxide-containing raw materials for acid-cured molding compositions. In addition to the basic hardener powder, the binder system most often contains phosphoric acid.

It is known to use a foundry molding and core mixture in which chrommagnezite is used as the powder base hardener. The reaction of chrommagnesite and trihydrogenphosphoric acid gives rise to reaction products, which give foundry self-setting mixtures with sufficient strength properties. The composition has good resistance to high temperatures, but a major disadvantage is the short processability of the composition. This short pot life greatly limits the applicability of the mixture. The high heat resistance of the mixture is particularly suitable for large massive castings, which, however, require a long processability of the mixture due to the labor intensity of production, which the hardeners used up to now cannot provide.

These drawbacks are overcome by the hardener for the acid-cured molding compositions according to the invention, consisting of 80 to 95 wt. parts of grog, from 2 to 10 wt. % of phosphoric acid and from 2 to 15 wt. from 2 to 15 wt. parts of a hardener based on raw materials containing magnesium oxide, such as magnesium or chrommagnesite. The principle of the invention is that before the hardener is added to the mixture, the hardener contains from 1 to 10 wt. parts by weight of 1050 to 1150 kg.m ^-3 per 100 wt. parts of feedstock containing magnesium oxide. Hardener reactivity ranges from 150 to 200 Jg- ¹ ·

The advantage of the hardener prepared according to the invention is that it has a surface passivated by the magnesium phosphate formed. This achieves a significant slowdown in the curing reaction of the binder system in the molding composition and ensures a sufficiently long workability of the molding composition, as shown in the examples. Furthermore, the hardener according to the invention ensures that the basic hardener quality remains constant. Depending on the reactivity of the feedstock, passivation can be carried out by adding a suitable amount of phosphoric acid, so that the resulting hardener reactivity is always within a certain narrow interval regardless of the reactivity of the feedstock. For normal foundry use is a suitable amount of reaction heat in the range of 150 to 200 Jg ^first

Example 1

The hardener was treated as follows: In a mixer, 100 wt. 8 parts by weight of chrommagnesite were gradually added while stirring. parts by weight of 1100 kg of trihydrogenphosphoric acid. m ~ ³ . After mixing for 5 to 10 minutes, a free-flowing powder mass was formed. The powder hardener thus prepared can be stored indefinitely.

A foundry molding and core mixture was prepared with the following composition:

quartz sand 95 wt. parts of chrommagnesite hardener according to the invention 10 wt. parts by weight of trihydrogen phosphoric acid 7 wt. parts

Molds and cores can be made from this mixture for 40 min. ie the pot life of the mixture. The cores or molds have a strength of 2.0 to 3.0 MPa, and have good abrasion resistance.

For comparison a foundry mixture of similar composition was prepared, but untreated chrommagnesite was used:

quartz sand chrommagnesite concentrated trihydrogenphosphoric acid

100 wt. parts 10 wt. parts by weight parts

This comparative mixture had a pot life of only 3 to 4 minutes, which makes it impossible to produce the cores or molds in the required quality. The mixture has high abrasion and the strength of the cores produced after processing is very low, virtually zero.

Example 2

The chrommagnesite hardener was prepared as follows: First, the reaction heat of chrommagnesite - 360 Jg ^-1 was determined. Then they were in a mixer to 100 wt. 3 parts by weight of this chrommagnesite were added while stirring. parts of acid 1100 kg. m ^-3 . The resulting powder hardener having a reaction heat of 180 µg ^-1 was used to prepare the foundry mixture:

quartz sand chrommagnesite hardener according to the invention trihydrogenphosphoric acid mass. parts by weight parts

3.5 wt. part

Workability of the mixture was 40 min. The strength of the mixture after curing reaches 1.5 to 2.0 MPa. The cores and molds have good abrasion resistance.

Claims (1)

SUBJECT Hardener for acid-curing molding compositions, consisting of from 80 to 95 parts by weight. foundry slag, from 2 to 10 parts by weight. % trihydrogenphosphoric acid and from 2 to 15 parts by wt. curing agents based on magnesium oxide-containing raw materials, such as magnesite or chrommagnesite, characterized in that they contain magnesium phosphate formed by adding from 1 to 10 parts by weight of the composition prior to addition to the mixture; phosphoric acid having a specific gravity of 1 050 to 1150 kg. m ^-3 per 100 parts by mass. the magnesium oxide containing feedstock and its reactivity ranges from 150 to 200 µg ^-1 .