Лако-красочные материалы — производство

Titanium oxide hydrate is precipitated by hydrolysis at 94-110 °C. Other sulfuric- acid-soluble components of the raw material are precipitated simultaneously, mainly niobium as its oxide hydrate.

Hydrolysis is carried out in brick-lined, stirred tanks (n) into which steam is passed. The hydrolyzate does not have any pigment properties. The properties of the hy — drolyzate are strongly influenced by the particle size and degree of flocculation of the hydrolyzate (mean primary particle size of hydrolyzate is ca. 5 nm, and the particle size ofTiO2 pigments is 200-300 nm).

The properties of the hydrolyzate depend on several factors:

1. The hydrolysis of concentrated solutions of titanium sulfate (170-230 gTiO2 L-1) proceeds very sluggishly and incompletely (even if boiled) unless suitable nuclei are added or formed to accelerate hydrolysis. The nuclei are usually produced by two methods. In the Mecklenburg method, colloidal titanium oxide hydrate is precipitated with sodium hydroxide at 100 °C, 1% of this hydrate is sufficient. In the Blumenfeld method a small part of the sulfate solution is hydrolyzed in boiling water and then added to the bulk solution [2.33]. The particle size of the hydrolyzate depends on the number of nuclei.

2. The particle size and degree of flocculation of the hydrolyzate depend on the intensity of agitation during the nuclei formation by the Blumenfeld method and also during the initial stage of the hydrolysis.

3. The titanium sulfate concentration has a great influence on the flocculation of the hydrolyzate. It is adjusted, if necessary by vacuum evaporation, to give a TiO2 content of 170-230 g L-1 during hydrolysis. Lower concentrations result in a coarser particle size.

4. The acid number should be between 1.8 and 2.2. It has a considerable effect on the TiO2 yield and on the particle size of the hydrolyzate. For a normal hydrolysis period (3-6 h) the TiO2 yield is 93-96%.

5. The properties of the hydrolyzate are affected by the concentrations of other salts present, especially FeSO4. High concentrations lead to finely divided hydrolyzates.

6. The temperature regime mainly affects the volume-time yield and hence the purity of the hydrolyzate.