Iron sulfate, illustration

Figure 8.19 A schematic illustrating the mechanism of atmospheric corrosion of steel (a) shows the inital phase of the corrosion of a metallic surface covered by a thin water film (b) shows the reactions that take place on a wet surface in the presence of a rust layer and (c) shows the oxidized state of a dry rusted surface with precipitated iron sulfates.

The strong influence of nonhalide anions on the passivation of iron is illustrated by experiments in which borate was added to sulfate solution. In pure sulfate, passivation occurs only after formation of a salt film, which requires the passage of considerable anodic charge. In borate solution, as mentioned earlier, the passive film forms with essentially 100% current efficiency, the addition of borate to sulfate Copyright 2002 Marcel Dekker, Inc. 

The basic flow sheet for the flotation-concentration of nonsulfide minerals is essentially the same as that for treating sulfides but the family of reagents used is different. The reagents utilized for nonsulfide mineral concentrations by flotation are usually fatty acids or their salts (RCOOH, RCOOM), sulfonates (RSO M), sulfates (RSO M), where M is usually Na or K, and R represents a linear, branched, or cycHc hydrocarbon chain and amines [R2N(R)3]A where R and R are hydrocarbon chains and A is an anion such as Cl or Br . Collectors for most nonsulfides can be selected on the basis of their isoelectric points. Thus at pH > pH p cationic surfactants are suitable collectors whereas at lower pH values anion-type collectors are selected as illustrated in Figure 10 (28). Figure 13 shows an iron ore flotation flow sheet as a representative of high volume oxide flotation practice. 

More recendy, the molten caustic leaching (MCL) process developed by TRW, Inc. has received attention (28,31,32). This process is illustrated in Eigure 6. A coal is fed to a rotary kiln to convert both the mineral matter and the sulfur into water- or acid-soluble compounds. The coal cake discharged from the kiln is washed first with water and then with dilute sulfuric acid solution countercurrendy. The efduent is treated with lime to precipitate out calcium sulfate, iron hydroxide, and sodium—iron hydroxy sulfate. The MCL process can typically produce ultraclean coal having 0.4 to 0.7% sulfur, 0.1 to 0.65% ash, and 25.5 to 14.8 MJ/kg (6100—3500 kcal/kg) from a high sulfur, ie, 4 wt % sulfur and ca 11 wt % ash, coal. The moisture content of the product coal varies from 10 to 50%. 

Fig. 2.43. Graphical illustration of sulfur isotope values of HiS (left axis and. solid line) produced during basalt-seawater interaction at various water/rock ratios. Calculations assume that seawater sulfate is mostly removed as anhydrite, that any residual sulfate is reduced by iron oxidation in reacting basalt, and that there is quantitative leaching of basaltic sulfide and homogeneous mixing of both sulfides. Dashed line...

Two processes are used in the manufacture of titanium dioxide pigments the sulfate process and the chloride process. The chemistry of the sulfate process, the longer established of the two methods, is illustrated schematically in Scheme 9.1. In this process, crude ilmenite ore, which contains titanium dioxide together with substantial quantities of oxides of iron, is digested with concentrated sulfuric acid, giving a solution containing the sulfates of Ti(iv), Fe(m) and Fe(n). Treatment of this... 

Let us now include an additional component to the Fe-0 system considered above, for instance S, which is of relevance for oxidation of FeS and for hot corrosion of Fe. In the Fe-S-0 system iron sulfides and sulfates must be taken into consideration in addition to the iron oxides and pure iron. The number of components C is now 3 and the Gibbs phase rule reads Ph + F = C + 2 = 5, and we may have a maximum of four condensed phases in equilibrium with the gas phase. A two-dimensional illustration of the heterogeneous phase equilibria between the pure condensed phases and the gas phase thus requires that we remove one degree of... 

Ferrous sulfate is a commonly used reducing agent for chelated waste streams that provides an illustration of the above problem. Although it is effective in removing process metals from complexes, the iron in the ferrous sulfate precipitates out along with the process metal. Since ferrous sulfate is typically added in sufficient quantity to raise the iron/process metal ratio to 8 1, considerable extra sludge is generated (Couture 1984). 

Depositional and Diagenetic Behavior of Sulfur. The key controls on sulfur behavior in freshwater-lake, saline-lake, and marine sediments include the concentration of reactants for bacteriogenic H2S formation and sulfide-mineral formation. Table IV illustrates the relative importance of the independent reactants (organic matter, dissolved sulfate, and iron) in each of the lacustrine environments studied. We have shown that in saline... 

In summary, signihcant numbers of bacteria, detected with several different approaches, are present in landhll leachate plumes. Methanogens, sulfate reducers, iron reducers, manganese reducers, and denitrihers are believed to be widespread in leachate plumes. Microbial activity seems to occur throughout leachate plumes, although the actual activity (as measured by ATP, PLFA, and redox processes) is low compared to activity in topsoil. Several redox processes can take place in the same samples adding additional diversity to the concept of redox zones illustrated in Figure 3. 

A similar study to the one just described was undertaken by Jorgensen (1977c) on the coastal marine sediments of Limfjorden, Denmark. Sulfate reduction (on average 9.5 mmol m d ) was largely confined to the upper 20 cm of sediment and over 90% of the HjS produced appeared to be reoxidized at the sediment-water interface only about 7% of HjS was converted to iron sulfides. The development of the sulfur cycle in sediments has been described by Jorgensen and Fenchel (1974) using the experimental system described earlier (see p. 303). The results, illustrated in Fig. 6.1.3, showed that the rate of sulfate reduction remained almost constant over the period studied and was accompanied by increasing pools of HjS, FeS and S ... 

As illustrated in Fig. 6, ground water with low (<10 pg/l) concentrations of arsenic (Type B) has similar chemistry to non-impacted ground water, with pH ranging from 7 to 8, iron concentrations less than 10 mg/1, and sulfate concentrations below 200 mg/1. Althoughalkalinity concentrations... 

As illustrated in reactions (1) and (2), oxidation of pyrite by dissolved oxygen (DO) or nitrate releases sulfate and iron in a molar ratio of 2. Ground water samples with high arsenic exhibit a strong positive correlation between sulfate and iron the slope of the trendline, which represents the molar ratio of sulfate to iron, is approximately 2 (Fig. 7b Table 1). These data strongly support sulfide oxidation. In contrast, ground water samples with low and moderate arsenic do not exhibit a strong correlation (Fig. 7a Table 1). 

Ferric iron in solution exists in several forms. The more important forms ass Fe(SO)f. FeSOd+. FE OH), and FeOH2+. The sulfate complexes are greatly favored over the hydroxyl complexes, To illustrate the imponance of iron complex formation, a typical leach solution will be considered in equilibrium with precipitated hydrogen jarosile, having an approximate ftee-sulfate activity of 0.02. The equilibrium is represented by... 

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