Control of water glass to reduce sand hole and air hole defects of large steel castings

1. Quality control of sodium silicate.

“Modulus, concentration” and “guest salt” (NaCI, Na2SO4, Na2CO3, etc.) are important parameters of sodium silicate, which will have a great impact on the quality of sodium silicate. “Modulus” and “concentration” have been recognized by everyone; The content of “guest salt” has not been paid enough attention, or even very little attention, by both water glass producers and foundry workers. “Guest salt” content has significant adverse effects on the concentration, viscosity, surface tension, aging speed, hardening speed and bonding strength of sodium silicate, which must be strictly controlled.

How to control the salt content? The following two measures can be taken:

① Try not to use water glass produced with original clear powder (anhydrous mirabilite Naz SO4) as raw material. The sodium silicate produced with the original bright powder as the raw material has high impurity content, less effective bonding components at the same concentration, and more residual salt Na2SO4 that promotes the aging of sodium silicate, so the bonding strength is reduced.

② Do not adjust the modulus with hydrochloric acid (HCI) and ammonia chloride (nh2ci). Increase the modulus with hydrochloric acid or ammonia chloride to produce harmful salt NaCI and promote the aging of sodium silicate. “Amorphous 5120 (silica gel powder)” can be added under heating conditions to increase the modulus of sodium silicate.

2. Aging phenomenon and Countermeasures of sodium silicate.

The aging of sodium silicate is a spontaneous polymerization process with slow release of energy. As a result, the polymerization degree of silicic acid disproportions to both ends, the viscosity decreases, the surface tension increases, and the bonding strength decreases by 30-35%.

The aging phenomenon of sodium silicate is inevitable, but if properly controlled, it can slow down the aging speed of sodium silicate, or control the aging phenomenon to a lesser extent, so as to minimize the loss of bond strength. For example, from production to use, it is best to control within 15 days, and not more than 30 days at most.

Professor Zhu Chunxi of Shanghai Jiaotong University elaborated on “aging and modification of sodium silicate sand” in his book “basic theory of sodium silicate sand”. However, some foundry workers and water glass producers still do not pay enough attention.