Лако-красочные материалы — производство

Naturally occurring iron oxides and iron oxide hydroxides were used as pigments in prehistoric times (Altamira cave paintings) [3.3]. The Egyptians, Greeks, and ancient Romans also used them as coloring materials.

Hematite (a-Fe2O3) has attained economic importance as a red pigment, geothite (a-FeOOH) as yellow, and the umbers and siennas as brown pigments. Deposits with high iron oxide contents are exploited preferentially. Naturally occurring magnetite (Fe3O4) has poor tinting strength as a black pigment, and has found little application in the pigment industry. The natural iron oxides and hydroxides are described in detail by Cornell and Schwertmann [3.2].

Hematite is found worldwide but in large quantities in the vicinity of Malaga in Spain (Spanish red) and near the Persian Gulf (Persian red). The Spanish reds have a brown undertone. Their water-soluble salt content is very low and their Fe2 O3 content often exceeds 90%. The Persian reds have a pure hue, but their water-soluble salt content is disadvantageous for some applications. Other natural hematite deposits are of only local importance. A special variety occurs in the form of platelets and is extracted in large quantities in Karnten (Austria). This micaceous iron oxide (MIO) is mainly used in corrosion protection coatings.

Goethite is the colored component of yellow ocher, a weathering product mainly of siderite, sulfidic ores, and feldspar. It occurs in workable amounts mainly in the Republic of South Africa and France. The Fe2O3 content gives an indication of the

iron oxide hydroxide content of the ocher, and is ca. 20% in the French deposits and ca. 55% in the South African ones.

Umbers are mainly found in Cyprus. In addition to Fe2O3 (45-70%), they contain considerable amounts of manganese dioxide (5-20%). In the raw state, they are deep brown to greenish brown and when calcined are dark brown with a red undertone (burnt umbers).

Siennas, mainly found in Tuscany, have an average Fe2O3 content of ca. 50%, and contain < 1% manganese dioxide. They are yellow-brown in the natural state and red-brown when calcined [3.4].

The processing of natural iron oxide pigments depends on their composition. They are washed, slurried, dried, ground, or dried immediately and then ground in ball mills, or more often in disintegrators or impact mills [3.5].

Siennas and umbers are calcined in a directly fired furnace, and water is driven off. The hue of the products is determined by the calcination period, temperature, and raw material composition [3.6].

Natural iron oxide pigments are mostly used as inexpensive marine coatings or in coatings with a glue, oil, or lime base. They are also employed to color cement, artificial stone, and wallpaper. Ocher and sienna pigments are used in the production of crayons, drawing pastels, and chalks [3.7].

For standards see Table 1.1 (Iron, manganese oxide pigments: “Methods of anal­ysis”; “Natural, specification”; “Sienna, specification”; and "Umber, specification"). The economic importance ofthe natural iron oxide pigments has decreased in recent years in comparison with the synthetic materials.

3.1.1.2