Iron reduction zones

SL/RN Process. In the SL/RN process (Fig. 4), sized iron ore, coal, and dolomite are fed to the rotary kiln wherein the coal is gasified and the iron ore is reduced. The endothermic heat of reduction and the sensible energy that is required to heat the reactants is provided by combustion of volatiles and carbon monoxide leaving the bed with air introduced into the free space above the bed. The temperature profile in the kiln is controlled by radial air ports in the preheat zone and axial air ports in the reduction zone. Part of the coal is injected through the centerline of the kiln at the discharge end. The hot reduced iron and char is discharged into an indirect rotary dmm cooler. The cooled product is screened and magnetically separated to remove char and ash. 

The iron oxide circulates between oxidation and reduction zones. The following reactions are typical, of those occurring in the steam-iron reactor section at temperatures in the 1500° to 1600°F range. 

Lovely D. R. and Phillips E. J. P. (1987) Competitive mechanisms for inhibition of sulfate reduction and methane production in the zone of ferric iron reduction in sediments. Appl. Environ. Microbiol. 53(11), 2636-2641. 

These microbially mediated redox processes utilize electron acceptors and produce reduced species. This will generate more reduced environments as long as there are electron donors available. The microbial population thus strongly affects their environment in the core of the plume. At the boundaries of the plume, complex microbial communities may exist, and steep redox gradients are created when dissolved electron acceptors are consumed. In addition, reoxidation of sulfides or ferrospecies by oxygen diffusing into the plume may increase the concentration of sulfate and ferric iron, which can stimulate sulfate and iron reduction in these zones as observed at Norman Landhll (Cozzarelli et al., 2000). 

At the surface of some marine sediments, organic sulphur can comprise as much as 50% of the total sulphur present (Francois, 1987) due to biosynthesis which incorporates sulphur of all oxidation states, but also, because of the reactivity of sulphides and polysulphides, by chemical addition. There is usually an increasing S/C ratio with depth in sediments, partly associated with humic substances, and most of this increase occurs in the oxic and suboxic zones. This organic repository may be the source of the sulphur required to convert metastable iron sulphides, formed in the lower part of the sulphur reduction zone, to framboidal pyrite, which is often found closely associated with organic matter. 

In fully marine systems siderite formation is probable to occur below the sulfate reduction zone where dissolved sulfide is absent, if reactive iron is still present and the Fe/Ca-ratio of pore water is high enough to stabilize siderite over calcite (Berner 1971). The coexistence of siderite and pyrite in anoxic marine sediments was shown by Ellwood et al. (1988) and Haese et al. (1997). Both studies attribute this observation to the presence of microenvironments resulting in different characteristic early diagenetic reactions next to each other within the same sediment depth. It appears that in one microenvironment sulfate reduction and the formation of pyrite is predominant, whereas at another site dissimilatory iron reduction and local supersaturation with respect to siderite occurs. Similarly, the importance of microenvironments has been pointed out for various other processes (Jorgensen 1977 Bell et al. 1987 Canfield 1989 Gingele 1992). 

Bioturbation is an effective transport mechanism to replenish freshly precipitated, highly reactive iron oxyhydroxide from the sediment surface to the zone of iron reduction. Vice versa, reduced iron phases, e.g. FeS, is transported to the oxic / suboxic zone, where it becomes oxidized. 

A low redox potential - as in bog soils -leads to the formation of H2S, which may be bound to sulfides of iron or may volatilize. Fe-sulfides are the cause for the black color in reduction zones of soils. Sulfide is the stable form under strong reducing conditions, but when changing to aerobic conditions sulfuric acid is formed, and this leads to soil acidification. Both the oxidation and reduction of sulfur compounds involve autotrophic bacteria (sulfur bacteria). 

The present curve does not show the maximum found by the previous authors at — 78°C. Private communication from Dr. Paul Emmett indicates that the possible explanation for this is that the reduction procedure here employed may have failed to reduce the iron in those areas where the low temperature chemisorption of hydrogen occurs. According to Emmett these areas are only uncovered by reduction at around 500 C. in a rapid stream of hydrogen freed from oxygen and well dried before passing into the reduction zone. The data in Fig. 7 do, however, show that from room temperature upwards there is alwa s evidence of desorption on raising the temperature followed by readsorption, and the... 

Figure 7-3. Plasma fiimace for iron reduction from oxides. Reduction in slag zone by thermal arc electrically attached to the melt (1) plasma generator, (2) electrodes, (3) arc.

In the oxidation environment in most cases is identified only one zone called oxidation or oxygenic zone. Within transitional environment are usually identified two zones the nitrate zone and the iron (ferrous) zone. East, in the reduction environment, which practically does not contain... 

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