BACKGROUND
The ultramarine blue pigment is the synthetic form of the semi-precious stone lapis lazuli. For centuries, a blue pigment has been prepared from this stone. This very high-quality pigment was, however, very expensive and so its chemical synthesis was sought. It was only in 1828 that J. B. Guimet discovered the ultramarine blue pigment synthesis process.
The ultramarine blue pigment is the synthetic form of the semi-precious stone lapis lazuli. For centuries, a blue pigment has been prepared from this stone. This very high-quality pigment was, however, very expensive and so its chemical synthesis was sought. It was only in 1828 that J. B. Guimet discovered the ultramarine blue pigment synthesis process.
The current synthesis process for the ultramarine blue pigment, based on the process discovered by Guimet, consists of heating a mixture of meta-kaolin, sodium carbonate, sulfur and a reducer. These reagents are mixed in the desired proportions, then are calcined in furnaces. The temperature cycle is comprised of several steps: heating to 800° C. in the absence of air, followed by an oxidation step at a lower temperature.
This process produces ultramarine blue pigment of good quality, but presents several disadvantages:
- the high temperatures and the duration of synthesis make this process costly in energy;
- during heating in the absence of air and during cooling, a large quantity of gas is released, namely sulfur dioxide and carbon dioxide; SO2must be treated in order to adhere to local environmental protection standards, which increases the cost of production;
- the raw pigment wash water has a high COD (chemical oxygen demand) due to the presence of residual polysulfides and sodium thiosulfate. To adhere to local environmental protection standards, this water must be treated before being discharged;
- the quality of the blue pigment synthesized varies as a function of the various parameters and as a function of the temperature cycle, in particular during oxidation.
Shortly after the discovery of the synthesis of the ultramarine blue pigment, pink and green pigment having the same structure as ultramarine blue pigment and also containing sulfur species were synthesized.
Ultramarine green was synthesized by the calcination, in the absence of air, of raw materials similar to those used for the manufacture of ultramarine blue. During its production, problems were encountered related to the discharge of the raw pigment wash water which contains a large quantity of polysulfides.
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