US Patent 7632347 Ultramarine pigment synthesis process

The present invention relates to an ultramarine pigment synthesis process which includes a calcination step in the absence of air of a mixture including zeolite A, possibly sodium sulfide and/or sulfur, possibly hydroxide ions, and polysulfides of composition Na2Sn, n being a number greater than 1, the product resulting from the calcination reaction being cooled in the absence of air, leading to obtaining a raw product which includes the synthesized ultramarine pigment, polysulfides and possibly sulfur. The Na2Sn polysulfides taking part in the calcination reaction come at least in part from the recycling of the polysulfides and possibly of the sulfur present in excess in the aforesaid raw product.