29 августа, 2015 
Pokraskin During precipitation, the above solutions are intimately mixed so as to form a mixture of stoichiometrically balanced sulfides that will form a pigment on firing. It must be remembered that the initial products formed on precipitation do not possess final pigmentary properties. Batch-wise production is the more common route for precipitation, although continuous precipitation has been used.
As with many other pigments, the choice of route and process conditions under which the products can be precipitated is extensive. Order ofaddition, concentration, temperature, pH and mixing conditions all have an effect on particle size and shape, and hence pigment properties. Each manufacturer has their own methods ofmanu — facture, thus making their pigments slightly different from competition.
The following points are worth noting:
Cadmium sulfide can be formed under both acid or alkaline conditions. Cadmium sulfide is more insoluble than most other cadmium salts. It can therefore be formed by reacting an alkali sulfide solution with an insoluble cadmium salt such as a carbonate, phosphate or hydroxide. For example:
CdCO3 + Na2S ^ CdSi + Na2CO3
During precipitation of the red pigments, cadmium selenide is not precipitated. Instead, selenium dissolved in sodium sulfide is precipitated as the sodium sulfide is used up in the reaction. Cadmium selenide is then formed on calcining.
If lithopones are to be precipitated, an equimolar amount of barium sulfate will be precipitated for each mole of sulfide used. For example:
CdSO4 + BaS ^ CdSi + Ba SO4 і
3.4.6
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