Reduction as the Basis for Enhanced Solubility

The reduction of hematite was discussed in Section 7.6. We will use those results in describing methods of formation of ceramics here. 

The required solubility equations and the corresponding potential-pH relations of FeO, Fe203, and Fe304 in an acidic solution are summarized in Table 12.1 (see also Ref. [8]). 

Equations (12.3)-(12.14) govern dissolution of the oxides and, hence, formation of their phosphate ceramics. Equations (12.3) and (12.4) indicate that iron itself may be used as a reductant in the process. Equations (12.7) and (12.8) indicate that FeO is sufficiently soluble in an acidic solution that it may be dissolved directly in phosphoric acid to form a ceramic. In contrast, Fc203, being insoluble in water, will require reduction, and Eqs. (12.9) and (12.10) or Eqs. (12.11) and (12.12) provide means to reduce this oxide to produce Fe (aq) ions. In fact, adding each side of Eq. (12.3) and (12.9), we obtain 

Metal Solubility Equation Potential—pH Relation 

Equations (12.15) and (12.16) exhibit the stoichiometry to use elemental iron as a reductant and produce aquosols of Fe (aq) in an acidic environment. These sols will subsequently react with phosphate anions H2PO4 or HP04 and form the hydrophosphate bonding phases Fe(H2P04)2 and FeHP04. The complete reactions are given by 

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