Лако-красочные материалы — производство

Iron blue pigments are produced by the precipitation of complex iron(II) cyanides using iron(II) salts in aqueous solution. The product is a whitish precipitate of iron(II) hexacyanoferrate(II) MII2FeII[FeII(CN)] or MIIFeII[FeII(CN)6], (Berlin white), which is aged and then oxidized to give the blue pigment [3.176].

Sodium or potassium hexacyanoferrate(II) or mixtures of these salts are used in most cases. When the pure sodium salt or a calcium hexacyanoferrate(II) solution is used, the pigment properties are obtained by adding a potassium or ammonium salt during the precipitation of the white paste product or prior to the oxidation stage. The iron(II) salt used is crystalline iron(II) sulfate or iron(II) chloride solution.

The oxidizing agent can be hydrogen peroxide, alkali chlorates, or alkali dichro­mates. Industrial precipitation is carried out batch-wise in large stirred tanks by si­multaneous or sequential addition of aqueous solutions of alkali hexacyanoferrate(II) and the iron(II) compound to a diluted acid. The filtrate from the white paste product must contain a slight excess of iron. Temperature and concentration of the starting solutions have a decisive influence on the size and shape of the precipitated particles. The suspension of white paste is aged by heating. The ageing period varies in length and temperature depending on the required properties of the finished pigment. This is followed by oxidation to form the blue pigment by adding e. g. hydrochloric acid and sodium or potassium chlorate [3.177]. Finally, the suspension of the blue pigment is pumped into filter presses, either immediately or after washing with cold water and decanting. After filtering, it is washed until the absence of acids and salts. The washed filter cake (30-60% solids) is carefully dried in suitable driers to form a solid which is eventually ground, packed into bags, or stored in silos. Serve tunnel or belt driers as well as spray — or spin-flash driers are used. Another possibility is to form

rods or pellets by extruding the washed filter cake with a granulator. After drying a dust-free iron blue pigment is obtained.

Dispersibility can be improved by adding organic compounds to the pigment sus­pension before filtering to prevent the particles from agglomerating during drying [3.178, 3.179]. In another method (the Flushing process), the water in the wet pigment paste is replaced by a hydrophobic binder [3.180]. Although these and other methods of pigment preparation produce fully dispersible products consisting mainly of iron blue and a binder, they have not become established on the market [3.181-3.183].

A “water-soluble” blue can be manufactured by adding peptizing agents (the latter improve the water solubility via an emulsifying action). This forms a transparent colloidal solution in water without the use of high shear forces [3.172].

3.6.3