Iron oxide suspension

The stability of iron oxide suspensions is relevant to fields as varied as the paint industry, extraction of iron from its ores, the structure of soils, hydrometallurgy and waste water treatment. The ease of homogensisation of paint, for example, is controlled by proper adjustment of the stability of the pigment suspensions. In ground waters, the settling behaviour of small iron oxide particles influences transportation of trace elements and radio-nuclides. The stability of a dispersion of magnetic particles can determine the quality of ferrofluids and magnetic tapes. 

Navarrete, R.C. Scriven, L.E. Macosko, C.W. (1996) Rheology and structure of flocculated iron oxide suspensions. J. Colloid Interface Sd. 180 200-211... 

Tombacz, E., Csanaky, C., and Illcs, E., Polydisperse fractal aggregate formation in clay mineral and iron oxide suspensions, pH and ionic strength dependence. Colloid Polym. Sci., 279, 484, 2001. 

The experimental results were shown in three graphs. The first graph was a comparison of results for a precipitate with a 1 1 ratio of iron to vanadium and for pure hydrous ferric oxide. The concentration of NH3 in the vanadium-iron oxide suspension increased from about 50 to 200/iM over the first 24 h, then decreased over the next 12 h to almost 145/iM. The concentration of NH3 in the undoped ferric oxide suspension increased from about 5 to 18/zM over the first 8h, then increased more slowly to approximately 25/zM at 36 h. 

Many protein films are intensely "stained" (coated) by chromium oxide or iron oxide suspensions (2, 29). 

Figures 2.5.3 shows Bingham plots of data for an iron oxide suspension. Data over a wide shear rate range are shown in Figure 2.5.3a, while the lower shear rate range of data is shown in Figure 2.5.3b, both on a linear scale. The constants of the Bingham model...

Bingham fits for experimental data of 6.0 vol % iron oxide suspension in mineral oil (replotted from Navarrete, 1991) in (a) the higher shear rate range, (b) the lower shear rate range, and (c) all data on a log-log scale. 

Comparison of Bingham and Casson fits to the iron oxide suspension data over the entire range of experimental data obtained parameters for the Bingham model are t) = 0.25 Pa-s and T,. 1.66 Pa, while for the Casson model they are O.IS Pa-s and 1.66 Pa, respectively. 

Data of Figure 2.5.3 replotted as (a) viscosity versus shear rate and (b) viscosity versus shear stress for 6.0 vol % iron oxide suspension. The latter clearly shows the dramatic drop in viscosity over a very narrow shear stress range. 

The iron oxide suspension data, including even lower shear rates, are shown in Figure 2.5.4. Curves of the Bingham and Casson models are also shown Bingham model parameters fix>m Figure 2.5.3b are used instead of a best fit. Note that values of the parameters for the Casson model also depend on the range of shear rates considered. 

Using a critical shear rate rather than shear stress as a yield criteria makes application to numerical calculations much easier (Beverly and Tanner, 1989). Equation 2.5.6 with stress yield criteria (eq. 2.5.3) is known as Herschel-Bulkley model (Herschel and Bulkley, 1926 Bird et al., 1982). From Figure 2.5.5 we see that the two-viscosity models will better describe the iron oxide suspension data illustrated here. 

The liquid redox gas desulfurization processes grouped in tliis section are selected mainly for their historical value. They comprise three separate broad categories and are differentiated by the senibbing solution chemistry. These process categories are I) polythionate solutions, 2) iron oxide suspensions, and 3) iron cyanide solutions. Of the three types, only the iron oxide suspension processes were used extensively at one time. Except in very rare cases, most of these plants have since been shut down and replaced with more modem gas desulfurization units. 

The iron oxide suspension family of processes is of historical interest because they constitute transitional processes that evolved from the traditional iron-oxide dry-box processes and were forerunners of the chelated-iron based processes that currently share the U.S. market for liquid redox processes together with the vanadium based Stretford process. 

The chemistry of all iron oxide suspension processes is based on the reaction of H2S with an alkaline compound, either. sodium carbonate or ammonia, followed by the reaction of the hydrosulfide with iron oxide to form iron sulfide. Regeneration is effected by converting the iron sulfide to elemental sulfur and iron oxide by aeration. This portion of the cycle involves essentially the same reactions as those occurring in dry-box purifiers. The following equations represent the reaction mechanism ... 

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