Chromium III Oxide Pigments

All the industrially important production processes for chromium(III) oxide pigments start from chromates or 

Chromium hydroxide or chromium oxide hydrate are formed when chromates are used as oxidizing agents in organic chemistry. These can be calcined to a-Cr203 or directly used as a pigment. Such processes have declined in importance, due to the increasing use of catalytic oxidation in organic chemistry. 

Sodium dichromate is reduced with sulfur by intensively mixing the components and then calcining in rotary kilns or rotary-plate furnaces at 800 to 1000°C. 

The calcination product, which contains sodium sulfate in addition to chromium oxide and also possibly unreacted chromate, is purified by mashing with water, filtering and washing. 

The presence of fluxes such as NaCl or Na2S04 during the decomposition of ammonium dichromate ensures that Cr203-pigments are formed in addition to nitrogen and water, instead of extremely fine powders. The isolation of ammonium dichromate can therefore be dispensed with by calcining mixtures of sodium dichromate and ammonium chloride or ammonium sulfate at 800 to 1000°C. 

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Chromium(III) oxide pigments are thermally stable and insoluble in water. They are not classified as harzardous materials and are not subject to international transport regulations. As long as they are kept dry their utility as a pigment is practically unlimited. 

Alkali dichromates are used as starting materials for the production of chromium(III) oxide pigments. They are available as bulk industrial products in the required purity. High impurity levels have an unfavorable effect on the hue. 

The +3 state of chromium is best represented by chromium(III) oxide, Cr2Os, which is a green, inert solid used as a green pigment. It can be made in rather spectacular fashion by heating ammonium dichromate. Once started, the reaction... 

Oxidative dissolution processes can also be used for identifying solid materials. This is the case of chromium (III) oxide—a green pigment that produces a well-defined anodic signal in acidic electrolytes near to -fl.O V, as can be seen in Fig. 2.15. This corresponds to the electrochemical process [147, 149] ... 

Chromium(III) oxide crystallizes in the rhombohedral structure of the corundum type space group D3d-R3c, Q 5.2 g/cm3. Because of its high hardness (ca. 9 on the Mohs scale) the abrasive properties of the pigment must be taken into account in certain applications [3.44], It melts at 2435 °C but starts to evaporate at 2000 °C. Depending on the manufacturing conditions, the particle sizes of chromium oxide pigments are in the range 0.1-3 pm with mean values of 0.3-0.6 pm. Most of the particles are isometric. Coarser chromium oxides are produced for special applications, e.g., for applications in the refractory area. 

Since chromium(III) oxide is virtually inert, chromium oxide green pigments are remarkably stable. They are insoluble in water, acid, and alkali and are thus extremely stable to sulfur dioxide and in concrete. They are light, weather, and temperature resistant. A change of the tint only occurs above 1000 °C due to particle growth. 

Reduction of Ammonium Dichromate. Chromium(III) oxide can be obtained by thermal decomposition of ammonium dichromate. Above ca. 200 °C, a highly voluminous product is formed with elimination of nitrogen [3.48]. The pigment is obtained after addition of alkali salts (e.g., sodium sulfate) and subsequent calcination [3.49]. 

The use of chromium(III) oxide as a pigment for toys, cosmetics, and in plastics and paints that come in contact with food is permitted in national and international regulations [3.62] -[3.68]. Maximum limits for heavy metals or their soluble fractions are usually a prerequisite. Because pure starting materials are used, these limits are satisfied by most types of chromium oxide. 

These reactions are carried out continuously in furnaces. The reacted mixture is leached with water to remove soluble products, filtered, dried and ground. Very pure chromium(III) oxide (99%) of pigment quality is thereby obtained. 

The possibility of coloured casing pigments and fillers creation was demonstrated on the basis of Si02 (Aerosil 175) with chromium(III) oxide jacket ( 15% wt. in calculation for Cr203) [80]. 

Write the chemical formulas that correspond to the following names (a) aluminum chloride (used in cosmetics), (h) chromium(III) oxide (a pigment for coloring pottery glazes), (c) calcium nitrate (provides a red-orange color in fireworks), and (d) ammonium sulfide (used to make synthetic flavors). 

A large number of CICPs contain transition metal oxides without a significant amount of a colorless base oxide. These pigments, listed in Table 5-3, employ either green chromium(III) oxide, red iron(III) oxide, or a combination of the two as a base. They are thus referred to as chromites or ferrites, for the chromium and iron bases, respectively. Most adopt the spinel configuration, with the other important structures being those of corundum and hematite. 

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