Hematite precipitation processes

Wet preparation of red iron oxides can involve either a hydrothermal process (see Hydrothermal processing) or a direct precipitation and growth of iron oxide particles on specially prepared nucleating seeds of Fe202- In the hydrothermal process, iron(II) salt is chemically oxidized to iron(III) salt, which is further treated by alkahes to precipitate a hydrated iron(III) oxide gel. The gel can be dehydrated to anhydrous hematite under pressure at a temperature around 150°C. 

The alkali process uses sodium hydroxide and is well known as Bayer s process. It involves relatively simple inorganic and physical chemistry and the entire flowsheet can be divided into caustic digestion, clarification, precipitation and calcination. Although mineral assemblage in bauxites is extensive, processing conditions are primarily influenced by the relative proportions of alumina minerals (gibbsite and boehmite), the iron minerals (goethite and hematite), and the silica minerals (quartz and clays-usually as kaolinite). 

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... 

Figure 15. Illustration of possible variations in isotopic fractionation between Fe(III),q and ferric oxide/ hydroxide precipitate (Aje(,n),q.Fenicppt) and precipitation rate. Skulan et al. (2002) noted that the kinetic AF (ni)aq-Feiricppt fractionation produced during precipitation of hematite from Fe(III), was linearly related to precipitation rate, which is shown in the dashed curve (precipitation rate plotted on log scale). The most rapid precipitation rate measured by Skulan et al. (2002) is shown in the black circle. The equilibrium Fe(III),-hematite fractionation is near zero at 98°C, and this is plotted (black square) to the left of the break in scale for precipitation rate. Also shown for comparison is the calculated Fe(III),q-ferrihydrite fractionation from the experiments of Bullen et al. (2001) (grey diamond), as discussed in the previous chapter (Chapter lOA Beard and Johnson 2004). The average oxidation-precipitation rates for the APIO experiments of Croal et al. (2004) are also noted, where the overall process is limited by the rate constant ki. As discussed in the text, if the proportion of Fe(III),q is small relative to total aqueous Fe, the rate constant for the precipitation of ferrihydrite from Fe(III), (Ai) will be higher, assuming first-order rate laws, although its value is unknown.

Although the majority of attention in discussions on the origins of BIFs has been on the oxide facies, siderite facies rocks are equally important in many BIF sequences. Reaction of Fe(II)aq and dissolved carbonate with hematite to form siderite and magnetite has been hypothesized to be an important diagenetic process in marine basins during formation of some BIFs if sulfate contents were low (e.g., Klein and Beukes 1989 Beukes et al. 1990 Kaufman 1996 Sumner 1997). In Figure 18 we assume that Fe(II)aq was derived either from MOR sources or DIR, or a combination of the two, which reacted with ferric oxide precipitates to form magnetite or dissolved carbonate to produce siderite. 

How factors such as the degree of ordering of ferrihydrite, and solution conditions, particularly pH and temperature affect the goethite/hematite ratio can provide information about the details of the process and also the conditions under which these two oxides might have formed in nature. The proportion of hematite formed after 15 hr at 100 °C increased from 43% to 95 % as the temperature at which the ferrihydrite was precipitated rose from 0 to 100 °C (Schwertmann Fischer, 1966). This suggests that with increasing temperature of precipitation, ferrihydrite dissolves less readily and the rate of dissolution (and hence goethite formation) falls. 

The spherical or disk-like particles of hematite dissolved and large, octahedral crystals of magnetite precipitated. The dissolution process involved electron transfer between hydrazine and the Fe of the hematite and was promoted by complexing agents such as TEA. 

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... 

From these data it follows that when iron is precipitated in acid and neutral environments the first products should be X-ray-amorphous highly dispersed iron hydroxides, which in the course of time acquire the crystal structure of goethite or hematite. The mechanism of this process depends on kinetic factors (rate of oxidation of Fe " ), form of migration of the iron (ionic or colloidal), and acidity of the parent solution. In neutral environments ferrihydrite possibly is formed as an intermediate metastable phase, especially if the iron migrates in colloidal form or in the form of the Fe ion. The products of diagenesis of such a sediment may be both goethite (in the case of low Eh values typical of the Precambrian iron-ore process) and dispersed hematite (in the case of deposition of the oxide facies of BIF). 

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