The effect of different precursors on the synthesis of ultramarine blue using a modified test furnace

The industrial pigment, ultramarine blue, has been prepared using a modified laboratory-scale test furnace. A study to investigate the reproducibility between runs of the test furnace, in terms of the colour properties of the products and impurities present in the products, has been performed using predensified precursor mix taken from an ultramarine plant. A second set of experiments was performed in single runs in the furnace to show the effect of different types of sulfur source and feldspar on the quality and colour of ultramarine blue produced. Colour measurements indicate that the ultramarine with the best colour properties is produced when sodium sulfide is used as the sulfur source in the starting mixture. Analyses of products washed with different alkali solutions show that while washing with potassium hydroxide improves the colour properties of the pigment, it is less effective than sodium hydroxide at removing impurities in the product. The structures of the ultramarine blue products have been investigated using the Rietveld refinement of powder X-ray diffraction data, A strong correlation between structural parameters and colour parameters has not been found.