Pigment production cooling
Use of BUCO crushed ice from our ice machines in pigment production in the aniline application case
Aromatic amines, such as aniline, react with nitrous acid as well as sulfuric acid as catalyst to form diazonium salts. These diazonium salts are very easily decomposed thermally, which is why diazotization must be carried out at temperatures between 0°C and 5°C. This is done by the direct addition of BUCO crushed ice. Our crushed ice has the ideal addition temperature of -0.5°C to -1°C and is produced very economically at evaporation temperatures of T0= -8°C, which corresponds to half the operating costs of current drum ice generators in use with evaporation temperatures of T0= -20°C.
Most pigments belong to the group of azo dyes, which is the largest class of dyes. All these substances are characterised by the azo group (-N=N-) between two aromatic residues. At the end of the 19th century, it was possible to synthesise azo dyes, which gave the dye industry an enormous boost, as it was now possible to produce dyes in all shades and of great brilliance. The synthesis of azo dyes takes place in two steps:
- the diazotisation of aniline, a component of coal tar, and
- the subsequent azo coupling with an aromatic compound.
1. Diazotisation of aniline
Aromatic amines, like aniline in this case, react with nitrous acid and sulphuric acid as catalyst to form diazonium salts. Since these diazonium salts decompose very easily thermally, the diazotisation must be carried out at temperatures between 0°C and 5 °C, which is done by direct addition of BUCO crushed ice.
2. Azo coupling
Azocoupling is an electrophilic substitution, the typical reaction type of aromatics, where the diazonium ion acts as an electrophilic particle. Activated aromatics such as phenol or aniline derivatives are used as coupling components. Due to the possible variations of the diazo component and the coupling component, a multitude of colour nuances and properties can be achieved. A sulfonic acid or hydroxy group, for example, makes the substances water-soluble, which is an important property of food dyes, among other things because they are also used to colour beverages such as lemonades. The azo dyes that are allowed to be used in foodstuffs must also meet strict criteria and are closely examined for allergenic effects, for example.
Technical apparatus for the production of azo pigments
The apparatus consists of the diazotizing vessel, i.e. an acid-resistant stirring vessel, the dissolving vessel for dissolving the coupling component and the central coupling stirring vessel. Corresponding to the sizes of approx. 20 to 80m3 prevailing in technology, couplings of approx. 0.5 to 2.5 t per batch can be carried out. Clarifying filters or clarifying presses are connected between the diazotising vessel or dissolving vessel and the coupling vessel. A filter press is located downstream of the coupling vessel and is used to filter the crude pigment suspension.
A pressure-resistant stirring vessel for thermal post-treatment with a connected filter press completes the actual synthesis apparatus. Suitable post-treatment not only optimises or improves application properties such as colour shade, colour strength, gloss, transparency or hiding power, dispersibility and flowability, but often also significantly improves the lightfastness, weather fastness, solvent fastness and migration fastness of the pigments. The filter contents in the form of moist press cakes are fed to a continuous (belt) or discontinuous (cabinet) drying process, possibly via granulation, and then ground.
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