PHYSICO-CHEMICAL PROCESSES OF CREATION OF ALUMINUM PHOSPHATE AND SILICON PHOSPHATE BINDERS FOR CORE MIXTURES

Procesi littâ, 2025, Vol 3 (161), 62-71

Abstract

The purpose of the presented work is a theoretical and practical study of the possibilities of using pre-synthesized silicon pyrophosphate and aluminum hydrogen phosphate as binders for core mixtures in foundry production. The mechanism of strength formation of core mixtures with pre-synthesized silicon and aluminum phosphates has been investigated for the first time. Aluminum phosphates are insoluble in water, but soluble in orthophosphoric acid. Due to this, the combination of aluminum hydrogen phosphate with orthophosphoric acid in the mixture ensures hardening after heating to 150 °C. Silicon pyrophosphate is insoluble in orthophosphoric acid, so adding it to the mixture in its pre-synthesized form does not provide hardening. Therefore, presynthesized powdered aluminum hydrogen phosphate re-exhibits binding ability in combination with orthophosphoric acid, therefore it belongs to binders with a reverse hardening nature. Silicon pyrophosphate does not exhibit binding ability again and belongs to binders with an irreversible hardening nature. Based on the results of theoretical research and the conclusions presented, a mixture based on pre-synthesized powdered aluminum hydrogen phosphate was created for the first time. This phosphate, compared to previously known liquid aluminophosphates, has a long shelf life, stable properties, and lower cost. With a content of 3 % aluminum hydrogen phosphate and 3 % orthophosphoric acid, after drying at a temperature of 150 °C, the mixture samples have a compressive strength in the range from 2.0 to 2.5 MPa. Core mixes with both binders provide a clean, non-sintered surface for iron-carbon alloy castings with a roughness of about 30 microns. An important advantage is the ability to use cores without a non-stick coating. The residual strength of the core mixture with silicon pyrophosphate after heating in the temperature range from 500 to 1000 °C and cooling was also investigated, and the reasons for the different labor intensity of knocking out of cores from cast iron and steel castings were established. This result was confirmed by direct experiments on determining the work of knock-out.