Hematite solubility

Harrichourry, J.-C., and Escalier, J. (1999) Iron(III) solubility and speciation in aqueous solutions. Experimental study and modelling part 1, Hematite solubility from 60 to 300 °C in NaOH-NaCl solutions and thermodynamic properties of Fe(OH) (aq). Geochim. Cosmochim. Acta, 63, 2247-2261. 

In a modification the conversion process, the jarosite residue is hydrothermaHy decomposed to hematite by autoclaving at 220—250°C. This solubilizes zinc and other metal values and the hematite has a potential for iron recovery. Hematite stockpiles are less of a problem than jarosite because hematite is denser and holds up less of the soluble metals. 

Fig. 2.44. logaoi-pH range.s for Kuroko ore fluid.s and midoceanic ridge hydrothermal. solution. I Kuroko 2 Axial Explorer 3 2I°N, Southern Juan de Fuca 4 2I°N, Endeavour 5 Guaymas. Temperature = 250°C, ESr (total reduced sulfur concentration) = 6.6 x 10 m. HM hematite, MT magnetite, PY pyrite, PO pyrrhotite. Dotted line Au solubility (ppm) (Shikazono, 1988). 

This hematite is not soluble in the cyanide solution. The oxidative pretreatment of gold ores thus reduces the cyanide consumption. Some impurity elements inhibit leaching reactions, examples include elements, carbon, sulfur and arsenic in gold ores are such impurities, but these can be removed by heating in air. 

In the ultimate analysis it may be pointed that the aforesaid hydrolysis processes are no doubt technically very satisfactory and tolerable, but environmentally this is not the case. The different processes yield jarosite, goethite and hematite, all of which retain considerable amounts of other elements, especially, zinc and sulfur. The zinc originates mainly from undissolved zinc roast in the iron residues, and sulfur from sulfate, which is either embodied into the crystal lattice or adsorbed in the precipitate. As a consequence of the association of the impurities, none of these materials is suitable for iron making and therefore they must be disposed of by dumping. The extent of soluble impurities present in the iron residues means that environmentally safe disposal not an easy task, and increasing concern is being voiced about these problems. An alternative way of removing iron from... 

The migration of iron mineral fines, primarily hematite and magnetite, is a common occurrence in portions of the Appalachian Basin. The phenomenon often occurs after well stimulation and can result in the continuing production of iron mineral fines which pose a significant disposal problem. The migration of iron mineral fines through propped fractures can substantially reduce the fracture flow capacity. Many of these are mineral fines are native to the formation and are not formed by precipitation of acid-soluble iron salts present in injection waters during or after acidi-... 

The Rate of reductive Dissolution of Hematite by H2S as observed between pH 4 and 7 is given in Fig. 9.6 (dos Santos Afonso and Stumm, in preparation). The HS" is oxidized to SO. The experiments were carried out at different pH values (pH-stat) and using constant PH2s- 1.8 - 2.0 H+ ions are consumed per Fe(II) released into solution, as long as the solubility product of FeS is not exceeded, the product of the reaction is Fe2+. The reaction proceeds through the formation of inner-sphere =Fe-S. The dissolution rate, R, is given by... 

The solubility plots for lepidocrocite, ferrihydrite and hematite (Fig. 9.2) and for goethite, ferrihydrite and soil-Fe (Fig. 9.3) show only the total Fe activity. They were obtained in the same way as that for goethite using the appropriate constants from Tables 9.1, 9.2 and 9.4. 

The solubility and the hydrolysis constants enable the concentration of iron that will be in equilibrium with an iron oxide to be calculated. This value may be underestimated if solubility is enhanced by other processes such as complexation and reduction. Solubility is also influenced by ionic strength, temperature, particle size and by crystal defects in the oxide. In alkaline media, the solubility of Fe oxides increases with rising temperature, whereas in acidic media, the reverse occurs. Blesa et al., (1994) calculated log Kso values for Fe oxides over the temperature range 25-300 °C from the free energies of formation for hematite, log iCso fell from 0.44 at 25 °C to -10.62 at300°C. 

Fig. 9.8 Particle size effect on the solubility products of goethite and hematite (Langmuir and Whittemore, 1971, with permission).

The solubility of hematite was measured in NaOH/NaCl (0.007-2 m) at between 60 and 300 °C at saturated water vapour pressure with the dissociation reaction being described by ... 

The value of log Kg4 = -19.64 + 0.06 at 60 °C and -14.82 + 0.14 at 300 °C (Diako-nov et al., 1999). Solubility rose with rising [OH ]. In very concentrated solutions of NaOH or KOH, ion pairs can form between Fe(OH)4 and the cation and at temperatures >200°C, these ion pairs increase the solubility of hematite in such solutions. In alkaline media, the solubility of hematite depended upon the alkali hydroxide used in the order (Ishikawa et al., 1997) ... 

The maximum adsorption (deposition) of three proteins on rhombohedral hematite (SA = 19 m g ) ranged from 5 to 20 mg m (Johnson and Matijevic, 1992 a), and, for ovalbumin and y-globuline occurred near their iep 4.7 and 6.8, respectively. For lysozyme, however, the highest adsorption occurred at pH 9, much below its iep (11.0) and was probably due to strong intermolecular association of the protein (minimum solubility) at this pH. Protein uptake may depend on the number and kind of amino acids in the protein as well was on their association and conformational behaviour. 

Fig.14.20 Effect of various clay minerals on the transformation of 2-line ferrihydrite to goethite and hematite at 25 °C and pH 5 after 16 yr as measured by the ratio of oxalate to dithionite soluble Fe (Feo/Fed) (Schwertmann et al.

Owing to their extremely low solubilities in an aerobic environment, goethite and hematite remain unchanged over geological time spans. They may, therefore, store information about the environment in which they formed. Al substitution may be one such piece of information. Thus, medium to high Al substitution has been observed in goethites from tropical and subtropical soils, bauxites and saprolites (Fitzpatrick Schwertmann, 1982 Schwertmann Kampf, 1983 Curi Franzmeier,... 

---