Iron oxide, hydrated form FeOOH

The residue after sulfur distillation contained all the iron from the pyrite and also much sulfur. It was heaved out onto pot mountains and weathered there for twenty years. In this process two layers were formed - one lower, hard and salty one upper, loose as mold. The lower was iron(ii) sulfate, from which iron vitriol was prepared for use in textile dyeing. The upper layer contained iron oxide hydrate FeOOH, iron ocher, and was the raw material for the manufacture of red ocher paint. During the weathering processes in the pot mountains, sulfur dioxide was evolved, evilsmelling but disinfecting. In periods of plague in the country, prominent persons, such as those from the King s court in Stockholm, arrived at Dylta for protection on or near the pot mountains. 

When steel or iron is exposed to an atmospheric environment, a thin layer of magnetite, Fe304, is formed, covered by a layer of FeOOH. Atmospheric oxygen then penetrates though the almost water-free, porous outer layer of FeOOH and oxidizes the magnetite to hydrated ferric oxide, Fe203, or FeOOH. The presence of Fe " in the electrolyte initiates the precipitation of various corrosion products. The electrochemical mechanism of atmospheric corrosion of iron suggested by Evans is briefly summarized in this chapter [8]. 

In the goethite process, the precipitation of iron from solution occurs in the form of hydrated ferric oxide, FeOOH. The commercial development of the process was due to Societe de La Vielle Montagne. The process basically involves the reduction of iron to the ferrous state, and this is followed by oxidation by air at a temperature of around 90 °C and at a pH controlled at around 3.0. The reaction can chemically be shown as ... 

Simple cations such as [Fe(H20)6]3"" undergo a certain amount of primary hydrolysis, depending upon the pH of the solution. The ion is in the hexaaqua form only at pH values lower than 2.0. Above that value the hydroxopentaaquairon(III) ion, [Fe(H20)50H]2 +, is predominant. Further increase in the pH of the solution causes more hydrolysis, until a complex solid material sometimes described erroneously as iron(lll) hydroxide is precipitated. The solid does not have the formula Fe(OH)3, but contains iron(III) oxohydroxide (FeOOH) and iron(III) oxide in various states of hydration, Fe203.xH20. 

The selective hydrolysis of metal ions to produce various forms of hydrated oxides is the most widely used form of precipitation. In particular, the removal of iron from hydrometallurgical process streams is a continuing problem. Iron enters the circuit as a constituent of a valuable mineral, such as chalcopyrite (CuFe2), or an impurity mineral, such as the ubiquitous pyrite or pyrrhotite. So far, effective removal of the iron has been achieved by the precipitation of iron(III) as jarosite (MFe3(S04)2(OH)6),401 goethite (FeOOH)402 or hematite (Fe203).403... 

Three polymorphous crystalline ferric hydroxides, a-, /k and y-FeOOH are known as the major components of naturally occurring oxidized products of iron, which, together with Fe3Oj. and various amounts of amorphous hydrated oxides, constitute rust on steel surfaces. The properties of each component and the state of the composite formed from the small crystal... 

The slow hydrolysis of iron(III) chloride solutions yields a yellow hydrous precipitate containing an amount of chloride that varies with the conditions of formation. The X-ray diffraction pattern of this yellow precipitate is distinct from the patterns of Q -Fe203 H20, 7-Fe203 H20, and FeOCl. The dehydration isobar for the yellow compound is characteristic of a hydrous 1-hydrate. This form of iron(III) oxide has been designated as 8-Fe20s-H20 (or /3-FeOOH) in order to distinguish it from the alpha and gamma 1-hydrates. 

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