Metallic iron pigments

Production. Metallic iron pigments are commercially produced by the reduction of acicular (needle-shaped) iron compounds [5.30], As in the production of magnetic iron oxide pigments, the starting materials are iron oxide hydroxides (see Section 3.1.1) or iron oxalates, which are reduced to iron in a stream of hydrogen either directly or via oxidic intermediates. 

Economic Aspects. The largest producers of metallic iron pigments are Dowa Mining, Kanto Denka K., and Mitsui Toatsu. 

The largest producers of metallic iron pigments are Dowa Mining and Toda Kogyo (Japan). World consumption in 2002 was ca. 1800 t, of which ca. 65% were used in the manufacture of broadcasting media, 2% for audiotapes, and 33% for data storage applications this last being expected to increase later. 

Fig. 5.2 TEM-photographs showing typical morphology of metallic iron pigments (Table 5.1). A) Iron pigment for lEC IV compact cassettes (metal operating point). B) Iron pigment, for digital audio (R-DAT) 8 mm video tape, data storage.

The world production capacity for magnetic pigments in 1991 was ca. 40 10 t, magnetic iron oxide pigments accounting for over 90% of this. Metallic iron pigments are preferred for 8 mm camcorders tapes. 

Two blue pigments can be prepared in transparent form cyanide iron blue and cobalt aluminum blue. These pigments are used in achieving a blue shade of the metal effect pigments in metallic paints. Transparent cyanide iron blue is prepared by a precipitation reaction similar to the one used for the preparation of the opaque pigment, but considerably lower concentrations of solutions are used. It is produced by Degussa (Germany), Manox (U.K), and Dainichiseika (Japan). 

Apart from alizarin lake , which is now being formulated as an aluminum/cal-cium complex [3] (Sec. 3.6.2), the oldest known metal complex pigment is an iron complex. In 1885,0. Hoffmann reported on the iron complex of l-nitroso-2-naph-thol, which under the name of Pigmentgriin B (Pigment Green 8,10006) was first industrially exploited in 1921 by BASF. 

This process is used principally in Europe. It was first developed in 1854 for the production of aniline. Nitrobenzene was reduced to aniline using metallic iron, hence the process was termed the aniline or nitrobenzene process. Iron oxides were formed as unusable, grey/black products. Around 1925, Laux found that addition of iron chloride modified the process so that iron oxides suitable for use as pigments could be produced. With this additive alone, magnetite with a high tinting strength results, i. e. 

Enough heat is generated by the reaction to keep the suspension at boiling point. It is essential to have an excess of iron metal to ensure complete decomposition of the nitrobenzene. The details of the reaction mechanism are not fully understood, but it is presumed that the nitrobenzene oxidizes Ee to Ee which is then hydrolysed. The acid is released by hydrolysis and pigment formation and dissolves the metallic iron and thus renews the supply of Ee " no additional acid other than that produced dur-... 

A number of electrochemical processes involving solid materials can be described in terms of the reductive or oxidative dissolution of such materials. Within this type of processes, one can include the stripping of metal deposits previously mentioned. In the context of archaeometry, conservation, and restoration sciences, the reductive dissolution of iron oxide-type materials is of particular interest. Thus, application of the voltammetry of the microparticles approach for identifying iron pigments has been described [108, 137-139]. 

Molybdates/lead chromates Luster pigments Mixed metal oxide pigments Iron blue pigments Ultramarine... 

Oxides and oxide -hydroxides Iron oxide pigments Chromium oxide pigments Mixed metal oxide pigments chromium oxide cob chromium oxide hydrate green alt green and b lue iron oxide red iron oxide orange chromium rutile orange iron oxide yellow nickel rutile yellow, chromium rutile yellow iron oxide brown zinc iron spinell, Mn-Fe-brown... 

Mixed metal oxide pigments containing iron oxide are also used (see Section 3.1.3). Magnetic iron oxide pigments are discussed in Sections 5.1.1 and 5.1.2. Transparent iron oxide pigments are described in Section 5.4.1. Methods of analysis and specifications of iron oxide pigments are listed in the standards given in Table 1. 

The type and quality of the pigment are determined not only by the nature and concentration of the additives, but also by the reaction rate. The rate depends on the grades of iron used, their particle size, the rates of addition of the iron and nitrobenzene (or another nitro compound), and the pH value. No bases are required to precipitate the iron compounds. Only ca. 3 % of the theoretical amount of acid is required to dissolve all of the iron. The aromatic nitro compound oxidizes the Fe2 + to Fe3 + ions, acid is liberated during hydrolysis and pigment formation, and more metallic iron is dissolved by the liberated acid to form iron(II) salts consequently, no additional acid is necessary. 

Table 36. Some quality requirements for iron oxide and metallic iron magnetic pigments... 

Mixed metal oxide pigments cobalt green and blue chromium rutile orange nickel rutile yellow, chromium rutile yellow zinc iron spinel, Mn-Fe brown... 

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