Magnetite, solubility

Fig. 17.2 Correlation between mild steel corrosion and magnetite solubility (by permission of G. M. W. Mann)...

An even more serious problem can arise when dissolved species expected to predominate at high temperatures are undetectable at 25°C or are only present at concentrations which are too low for them to be adequately characterized thermodynamically. Examples are certain transition metal chloro-complexes (9,10) and mixed complexes of such metals with hydroxide and another ligand (11,12). Thus it seems that chloride complexing so alters the aqueous chemistry of copper and gold that supposedly inert gold components in autoclaves are reversibly oxidized by Cull (10) and it is likely that mixed oxine and hydroxy complexes of Fell contribute considerably to the gross under-estimation (by a factor of up to 10 ) of magnetite solubility in oxine (12,14). 

Ziemniak, S. Jones, M.E. Combs, K.E.S. (1995) Magnetite solubility and phase stability in alkaline media at elevated temperatures. J. Solut. Chem. 24 837—877... 

Figure 4.6 Magnetite solubility as a function of temperature at 25 MPa showing the contributions of ionic and uncharged species [22].

Mechanistically chromium additions have been shown to significantly enrich (lOX) in the magnetite oxide layer and, it has been suggested that this lowers its solubility. Additions of small amounts of oxygen to the water, increases the metal s potential and promotes the formation of haematite... 

In this process the reduction of the ferric components of the scale is coupled to oxidation of the base metal, both reactions yielding ferrous species readily soluble in the acid. For magnetite the processes are as shown in equations 11.1 and 11.2. 

In the absence of sufficient hydrogen, the solubility of magnetite is markedly temperature dependent, which provides an explanation for some of the problems in high-temperature circuits. Most studies in boilers relate to high temperatures. Thus the work of Bloom " and of Potter and Mann has reproduced the types of corrosion found in high-pressure boilers. The relationship between corrosion rates and iron solubility and pH are given in Fig. 17.2. Note that the pH range about the neutral point (relative to 300 C... 

When soluble iron is present in FW that also contains silica, the iron may be transported through to the boiler, where insoluble ferrous metasilicate (FeSi03) preferentially forms instead of magnetite. 

Erythorbic acid and sodium erythorbate are very safe products, widely used in the food industry as antioxidants and alternatives to vitamin C. In the water treatment industry, they are strong reducing agents that reduce metal oxides and hydroxides to their more soluble ferrous forms and promote the passivation of boiler waterside surfaces (magnetite formation). 

Where erosion is a problem, the FW pH level generally is increased from a typical level of 9.0 to 9.2 up to 9.4 to 9.6 because this improves the solublization of potential erodents and strengthens the magnetite film. 

Wood, S.A., Crerar, D.A. and Borcsik, M.P. (1987) Solubility of the assemblage pyrite-pyrrhotite-magnetite-sphalerite-galena-gold-stibnite-bismuthinite-argentite-molybdenite in H20-NaCl-C02 solutions from 200°C to 350°C. Econ. Geol, 82, 1864-1887. 

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). 

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-... 

Wan, J., Cai, W., Meng, X. and Liu, E. (2007) Monodisperse water-soluble magnetite nanoparticles prepared by polyol process for high-performance magnetic resonance imaging. Chemical Communications, (47), 5004—5006. 

Li, Z Sun, Q. and Gao, M.Y. (2005) Preparation of water-soluble magnetite nanocrystals from hydrated ferric salts in 2-pyrrolidone mechanism leading to Fe304. Angewandte Chemie International Edition, 44 (1), 123-126. 

Sweeton and Baes (1970) measured the solubility of well-crystallized magnetite in dilute acid and base at a H2 partial pressure of 0.1 MPa and over a temperature range of 50-300 °C. The value of log of 12.02 corresponds to the reaction... 

In moderately alkaline solutions (pH >8) oxidation of Fe" solutions proceeds via Fe(OH)2 and usually yields magnetite (David Welch, 1956 Sidhu et al., 1977). Under these conditions the solubility product of magnetite is exceeded so the mixed oxide is more stable than the pure Fe " oxides (see Chap. 8). Tamaura et al. (1981) monitored the transformation of Fe(OH)2 at pH 11 and 65 °C. Initially both goethite... 

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