Ultramarine Pigments

Production

Ultramarine is produced from simple, relatively inexpensive materials, typically Chinese kaolin, feldspar, anhydrous sodium carbonate, sulfur and a reducing agent (fat, pitch, coal, etc.). Clay Activation: The conversion of kaolinite to metakaolin is achieved by heating the clay to approximately 700°C, thus causing hydroxyl ions to be removed as water. The dehydroxylation rate has been examined as a heating function. Accordingly, the reaction can be carried out either as a batch process with clay in crucibles in a directly heated kiln or as a continuous process in a tunnel kiln, rotary kiln or other kiln. Raw Material Mixing and Heating: The activated clay is typically dry-ground and mixed with other raw materials in batch or continuous ball mills to an average size approaching 15 μm. Typical composition (by weight %):   The mixture is heated under reducing conditions, normally to approximately 750°C in a batch process. The traditional way of doing this is to use directly heated kilns or covered kilns with mixing in crucibles with controlled porosity lids. To increase efficiency, the application in Holliday Pigments' production facilities involves consolidating the base material to form bricks, which are then stacked in a predetermined manner and fired in chambers heated indirectly using gas-fired burners. Sodium carbonate is reacted with sulfur and a reducing agent at 300°C to form sodium polysulfide. At higher temperatures the clay lattice transforms into a three-dimensional framework, which at 700°C converts to a sodalite structure coated with sodium and polysulfide ions.

Oxidation

Air is supplied in controlled amounts and the kiln is cooled to 500°C. Oxygen reacts with excess sulfur to form sulfur dioxide, exothermically oxidizing di- and triatomic polysulfide ions S2- and S3- free radicals, leaving sodium sulfoxites and sulfur as by-products. Exothermically oxidizing di- and triatomic polysulfide ions to radicals not containing S2- and S3-, leaving sodium sulfoxites and sulfur as by-products. When oxidation is complete the kiln is cooled and emptied; the complete kiln cycle takes between 3 and 4 weeks. The "crude" ultramarine product typically contains 75% by weight blue ultramarine, 23% by weight sodium sulfoxite and 2% by weight some unreacted free sulfur combined with iron sulfide.

Purification and Refining:

Purification and refining operations can be batch or continuous processes. Crude ultramarine blue is crushed and ground, made into a slurry in warm water, then filtered and washed to remove sulfoxites. Re-slurrying and wet grinding removes sulfurous impurities and ultramarine is generally reduced to particle sizes down to 0.1-10.0 μm. Impurities are removed by floating using hot or cold foam flotation similar to techniques used in the mining industry. The liquid is then separated into fractions of different particle sizes by gravity or centrifugal separation; fine particles remaining are recovered by flocculation and filtration. The separated fractions are dried and grouped to give pigment types of different particle sizes. These are blended as standard sales types with adjustments made to hue, brightness and color intensity to achieve the specified color tolerance. Purple ultramarine can be prepared by heating a medium ultramarine blue with ammonium chloride at approximately 240°C in the presence of air. The purple pigment is processed with hydrogen chloride gas at 140°C to obtain various pink derivatives. A good ultramarine pigment should provide the following properties:

Applications

The stability and safety of ultramarine pigments form the basis of the wide range of applications given below:

Plastics in Brightening Whiteness:

Blue ultramarine can be used in all types of polymers; the maximum process temperature for purple ultramarine is 280°C and for pink ultramarine is 220°C. If color fading occurs during processing with PVC, acid-resistant types are used. Surface-treated types are available for improved dispersion. Ultramarines do not cause shrinkage or warping of polyolefins. Ultramarine pigments are permitted worldwide for coloring plastics in contact with food.

Paints:

Ultramarine pigments are used in decorative paints, baked-on paints, transparent lacquers, industrial paints and powder coatings. The transparent nature of the pigment leads to impressive and eye-catching paints when combined with effect pigments such as mica.

Printing Inks:

Ultramarine pigments can be used in inks for most printing processes, particularly hot foil stamping. High-performance pigment types are required for printing such as letterpress, flexography and gravure; lithographic printing requires water-repellent types; any of these pigment types are suitable for screen printing inks, fabric printing and hot foil stamping inks. Advanced durable types prepared in a high-solids aqueous dispersion are finding increasing application in flexographic printing applications.

Paper and Paper Coatings:

Ultramarine pigments are used to enhance the color of white paper or to intensify the color of colored paper. They can be added directly to the paper pulp or used in applied coatings. They are particularly suitable for colored papers for use by children.

Detergents:

Ultramarine pigments are widely used in enhancing the effects of optical brightening agents in improving the whiteness of fabrics to be washed. They do not stain or accumulate even with repeated use.

Cosmetics and Soaps:

Ultramarine pigments are widely used in cosmetics. Pink ultramarine is not recommended for toilet soaps because the color tends to turn purple. Complete safety, non-staining properties and compliance with all important regulations are advantages of these pigments.

Artist Colors:

This traditional use of ultramarine in all types of binders remains an important application area. Its unique color properties, stability and safety are extremely valued.

Toys and Other Children's Materials:

Ultramarine pigments are widely used in toy paints, children's paints and finger paints, modeling compositions, colored papers and colored pencils in plastics and surface coatings. These comply with important regulations and standards.

Toxicology and Environmental Effects

Ultramarine pigments have a unique safety record. No other pigment has been shown to be exposed to such prolonged and widespread human and environmental use without any reported instances of disease effects. In early times, ultramarine's primary applications were as a sugar additive sold worldwide, creating a whitening effect, and as a fabric whitening agent for use in household laundry, thus establishing a worldwide human safety test. All populations regularly consume sugar. The use of ultramarine for whitening clothes remains widespread. This demonstrates a safety test in human digestion and with skin contact on a large scale. The only known hazard is the release of hydrogen sulfide in the event of acid interaction. Tests supported by Reckitt's Ultramarine Colors (now Holliday Pigments) confirm that acute oral toxicity in rats and mice (LD50) is higher than 10,000 mg kg-1. Fish toxicity (LC50 in trout) exceeds 32,000 mg L-1. Ultramarine is non-mutagenic, non-irritating and does not sensitize skin. There is no threshold limit value or maximum effect retention limit listed for this pigment. In normal application, a TLV value of 10 mg m-3 is considered to make it an irritating dust. The pigment is not listed as a hazardous substance in the European Union or in any similar national or international classification; nor is it classified as hazardous for disposal. The production process generates 1 ton of gaseous sulfur dioxide and 0.3 ton of water-soluble sodium sulfoxite per metric ton of pigment produced. These must be disposed of in an environmentally acceptable manner. If the soluble salts are completely oxidized, they can be safely discharged to tidal waters. Regulations worldwide requiring reductions in sulfur dioxide discharge have led many ultramarine producers to cease production. The required investment is significant, but can result in a dramatic reduction in emission levels. With the solution applied at the world's largest ultramarine plant, sulfur dioxide emissions have been reduced by more than 99.5%. This was achieved by converting SO2 to SO3 using a vanadium pentoxide catalyst and subsequently concentrating the sulfur trioxide to produce pure sulfuric acid.

Economic Aspects

Ultramarines can be categorized as laundry-use types, which are lower strength and sometimes lower purity materials, or industrial/technical types, which are higher strength, higher purity pigments. Factories in India and the People's Republic of China produce laundry-grade ultramarines for supply worldwide. There are only three major producers of high-grade ultramarine pigment. These are: Holliday Pigments (United Kingdom, France), DKK (Japan) and Nubiola (Spain, Romania, India, Colombia). In 2000, total ultramarine production worldwide was approximately 25,000 t a-1. M.Namık Kayaalp Chemical Engineer Ecelak Boya Kimya San. Tic. Ltd. Şti.    

References: 1. Industrial Inorganic Pigments, G. Buxbaum and G. Pfaff, Wiley-VCH Verlag GmbH & Co. KGaA 2. Pitture e vernici European Coatings16 (2003), 3. 157 Booth, D. G., Dann, S. E., Weller, M. T., Dyes and Pigments 58 (2003), 4. Reckitt's Colours Ltd., The Cost of Whiteness, Hull, United Kingdom 5. Ullmann Encyklopädie der Technis-chen Chemie, 4th edn., Vol. 18, Verlag Chemie, Weinheim, 1979,