Iron oxide, synthetic

About 60% of the natural iron oxide pigments is used to color cement and other building materials (qv). About 30% is consumed in the production of paints. For coloring plastics and mbber, synthetic iron oxide pigments are preferred. The main advantage of the natural iron oxide pigments, as compared to the synthetic ones, is cost. However, the quaHty is inferior, and in most cases, they are consumed in close proximity to the mines. As colorants, the natural iron oxides are about 50% weaker than synthetically produced iron oxides. 

Synthetic Iron Oxides. Iron oxide pigments have been prepared synthetically since the end of the seventeenth century. The first synthetic red iron oxide was obtained as a by-product of the production of sulfuric acid from iron sulfate containing slate. Later, iron oxide pigments were produced direcdy by the thermal decomposition of iron sulfates. In the 1990s, about 70% of all iron oxide pigments consumed are prepared synthetically. 

Iron Oxide Yellows. From a chemical point of view, synthetic iron oxide yellows, also known as iron gelbs, are based on the iron(III) oxide—hydroxide, a-FeO(OH), known as goethite. Color varies from light yellows to dark buffs and is primarily determined by particle size, which is usually between 0.1 and 0.8 p.m. Because of their resistance to alkahes, these are used by the building industry to color cement. Thermally, iron oxide yellows are stable up to 177°C above this temperature they dehydrate to iron(III) oxide ... 

Iron Browns. Iron browns are often prepared by blending red, yellow, and black synthetic iron oxides to the desired shade. The most effective mixing can be achieved by blending iron oxide pastes, rather than dry powders. After mixing, the paste has to be dried at temperatures around 100°C, as higher temperatures might result in the decomposition of the temperature-sensitive iron yellows and blacks. Iron browns can also be prepared directiy by heating hydrated ferric oxides in the presence of phosphoric acid, or alkaU phosphates, under atmospheric or increased pressure. The products of precipitation processes, ie, the yellows, blacks, and browns, can also be calcined to reds and browns. 

Synthetic Iron Oxide. This colorant is one or a combination of various synthetically prepared iron oxides, including the hydrated forms. The naturally occurring oxides are unacceptable as color additives because of the difficulties frequendy encountered ia purifyiag them. 

These sections are mainly concerned with synthetic iron oxides. The morphologies of Fe oxide crystals in rocks, soils and biota are described in Chapters 15, 16 and 17. Table 4.2 provides an overview of the common crystal habits of the various Fe " oxides. 

The specific surface area of a solid is the surface area of a unit mass of material, usually expressed as m g . There is an inverse relationship between surface area and particle size. Massive crystals of hematite from an ore deposit (e. g. specularite) may have a surface area 1 m g". As particle size/crystallinity is governed largely by the chemical environment experienced during crystal growth, the surface area of a synthetic iron oxide depends upon the method of synthesis and that of a natural one, upon the environment in which the oxide formed. 

Like the natural iron oxide pigments, the synthetics are used for colouring concrete, bitumen, asphalt, tiles, bricks, ceramics and glass. They are also used extensively in house and marine paints. Because the shapes of the particles can be accurately controlled and the particle size distribution is narrow, synthetic iron oxides have a greater tinting strength than the natural ones and so, are chosen where paint colour is important, i. e., for top coats. Red iron oxides are used in primers for automobiles and steel structures. 

Schwandt, FI. Kdgel-Knabner, I. Stanjek, FI. Totsche, K. (1992) Sorption of an acidic herbicide on synthetic iron oxides and soils sorption isotherms. Sci. Total Environ. 123/124 121-131... 

Schwertmann, U. (1985) The effect of pedo-genic environments on iron oxide minerals. Adv. Soil Sci. 1 172-200 Schwertmann, U. (1988) Some properties of soil and synthetic iron oxides. In Stucki,... 

The manufacture of synthetic iron oxides is carried out by three different methods, using raw materials such as scrap iron or by-product hquors from TiO production and steel pickhng. 

The continually increasing importance of iron oxide pigments is based on their nontoxicity chemical stability wide variety of colors ranging from yellow, orange, red, brown, to black and low price. Natural and synthetic iron oxide pigments consist of well-defined compounds with known crystal structures [3.1] ... 

Iron oxide pigments produced from pure starting materials may be used as colorants for food and pharmaceutical products [3.42], Synthetic iron oxides do not contain crystalline silica and therefore are not considered to be toxic, even under strict Californian regulations. 

All synthetic iron oxides possess good tinting strength and excellent hiding power. They are also lightfast and resistant to alkalis. These properties are responsible for their versatility. The principle areas of use are shown in Table 24. 

Accurate production figures for natural and synthetic iron oxide pigments are difficult to obtain, because statistics also include nonpigmentary oxides (e.g., red mud from bauxite treatment, intermediate products used in ferrite production). World production of synthetic iron oxides in 1995 was estimated to be 600000 t ... 

Table 24. Main areas of use for natural and synthetic iron oxide pigments...

Synthetic Iron Oxides. Advantages of synthetic iron oxides over their natural counterparts include chemical purity, more uniform particle size and size distribution, and in the case of precipitated oxides the ability to prepare the pigment in predispersed vehicle systems by flushing techniques. 

Iron Browns. Iron browns are often prepared by blending red. yellow, and black synthetic iron oxides to the desired shade. The most effective mixing can be achieved by blending iron oxide pastes, rather than dry powders. 

Desai DS, Abdelnasser MA, Rubitski BA, Yaria SA. Photostabilization of uncoated tablets of sorivudine and nifedipine by incorporation of synthetic iron oxides. Int J Pharm 1994 103 69-76. 

Synthetic iron oxide Pigment red 101 Very slight... 

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