1.Overview
(1) Sodium silicate, also known as silicate of soda, it is a dispersion system of sodium silicate, potassium silicate, lithium silicate and quaternary ammonium silicate in water with the coexistence of ions, molecules and colloidal particles of silicate.
(2)The appearance of pure sodium silicate is a colorless transparent viscous liquid. It becomes colorful, like yellow, green, gray and white, when containing the oxide of iron, manganese, aluminum and calcium.
(3) The modulus of sodium silicate used in casting is usually 2<M<4.
(4) Sodium silicate sand will have good compromise and can reduce the hot cracking defects of the casting when they are heated to more than 800℃. However, the residual strength of the casting sand is high and the collapse is poor if the volume of sodium silicate sand is too much.
2. Characteristics of sodium silicate
(1) Hardening & Strengthening:
The sodium silicate began to harden when the it’s viscosity exceeds the critical value of concentration – modulus combined, hardening sodium silicate is then got hardened due to further water loss, known as the strengthening stage. We can use the heating drying, microwave drying, CO2 gas hardening method, ferrosilicon powder self-hardening sand and other methods to promote the hardening of sodium silicate.
(2)
Apping strong dehydration and less reaction to increase the bond strength during the sodium silicate binder hardening processing.
(3) Modulus adjustment:
Adding the NaOH aqueous solution (mass fraction is 10%-20%) to reduce the modulus of sodium silicate; Adding NH4Cl aqueous solution (mass fraction 10%) or amorphous Si02 to increase the modulus. We can also reach a medium modulus by mixing high and low modulus proportionally.
(4) Concentration adjustment:
It can be achieved by thermal dehydration or anstau. Density is commonly used to measure the concentration of sodium silicate by baume degree°Be’.
(5) Aging and physical modification:
Aging refers to the viscosity and bond strength of sodium silicate decreased significantly and the condensed gel speed got accelerated during the storage process. In fact, this is the process of slow release of internal energy. The aging process can be eliminated by physical modification such as magnetic field treatment, ultrasonic oscillation, reflux heating and hot autoclave heating.
(6) Sodium silicate sand breakdown agent:
polysaccharide, resin, oil, cellulose, carbon, inorganic, mineral, etc.
(7) Sodium silicate hardener:
gas (such as CO2), solid (ferrosilicon powder, etc.), liquid (carbonate acrylate). China’s supply mouth common MDT series of organic esters for MDT-901 (slow ester), MDT-903 (fast ester), MDT-800 (very slow), MDT-Q (very fast).
3. Moulding sand and core sand with sodium silicate as binder
(1) CO2 air hardening sodium silicate sand: according to different ratio can be applied to cast steel (core) sand, cast iron sand, etc.
(2) Sodium silicate self-hardening sand: organic ester sodium silicate self-hardening sand.
(3) Drying and hardening sodium silicate sand.