Iron oxides, deposition

There are several means by which boiler water can become highly concentrated. One of the most common is iron oxide deposition on radiant wall tubes. Iron oxide deposits are often quite porous and act as miniature boilers. Water is drawn into the iron oxide deposit. Heat appHed to the deposit from the tube wall generates steam, which passes out through the deposit. More water enters the deposit, taking the place of the steam. This cycle is repeated and the water beneath the deposit is concentrated to extremely high levels. It is possible to have 100,000 ppm of caustic beneath the deposit while the bulk water contains only about 5—10 ppm of caustic. 

In addition, organics also may be found in boiler section iron oxide deposits taken from large process industry boiler plants. Here, the organics are more likely to be hydrocarbons, such as oil, tar, or petroleum coke, rather than inhibitor treatments. The various mixtures that form all become very insoluble with age and quite dense. 

Some basic BW treatment objectives include keeping boiler surfaces clean and corrosion-free to minimize fuel bills and managing variable quality FW smoothly and efficiently to limit upsets and other downstream problems. But the nature of potential boiler deposition problems changes with increases in pressure and, simply put, is primarily concerned with a reduction in simple, hardness-related deposits and an increase in complex, iron oxide deposits. The effect of dirty boilers on fuel costs can be seen in Figure 10.4. 

A number of solvents are used to dissolve and remove iron oxide deposits. These are detailed in the following sections. 

Natural Iron Oxides. The earth s crust contains about 7 wt % iroii oxides, but only a few deposits are rich enough in iron to be suitable for mining pigmentary-quality iron oxides. Deposits that are a suitable source of natural iron oxide pigments are usually hydrated aluminum silicates that contain various amounts and forms of iron oxide. 

The observations have shown that consideration must be given to several different processes of carbonate deposition and/or silica or iron oxide deposition in contact which such bacterial mats. Obviously some important lithification processes take place within the decay zone below the active photosynthetic zone. In most of the cases where lithification was observed there, it was carbonate lithification of a type not related to the photosynthetic depletion of C02. Different filamentous and coccoid cyanobacteria can become more or less lithified depending on slime production, mobilization, outer morphology and microenvironments. 

St. Pierre, T. G. Chan, R Bauchspiess, K. R. Webb, J. Betteridge, S. Walton, S. Dickson, D. P. E. Synthesis, structure and magnetic properties of ferritin cores with varying composition and degrees of structural order models for iron oxide deposits in iron-overload diseases. Coord. Chem. Rev. 1996, 151, 125-143. 

Catling DC, Moore J (2000) Iron oxide deposition from aqueous solution and iron formations on Mars. Lunar Planet Sci Conf XXXI, Houston, TX. Abstract 1517... 

Low-pH cleaners are typically used to address calcium carbonate scale and iron oxide deposition. These cleaners are usually formulated using only acid, such as acetic, hydrochloric, or sulfamic. Figure 13.5 shows the effects of temperature and pH on the removal of calcium carbonate from a membrane.7 As the figure shows, lower pH, and higher temperatures are more effective at restoring permeate flow than higher pH and lower temperatures. 

Shearing force increase at the initial instant was mainly due to in-gap iron oxide (Fe304) depositions on surfaces of the tested SFAs after preliminary hold up in water. However no further densification of iron oxide depositions was revealed. Thus the performed tests confirmed the possibility of SFA imloading after long-term hold up inside imwatered reactors. 

Duhig, N. C., Davidson, G. J. Stolz, J. (1992). Microbial involvement in the formation of Cambrian sea-floor silica iron-oxide deposits, Australia. Geology, 20, 511-14. Edwards, J., Chamberlain, D., Brosnan, G. et al. (1998). A comparative physiological... 

As indicated above, the products of the well-studied iron-oxidizing neutrophiles have high potential as mineralogical biosignatures, and in fact have been used as such (e.g., Alt 1988 Hofmann and Farmer 2000). Cambrian sea-floor silica-iron oxide deposits were described by Duhig et al. (1992), and a Jurassic hydrothermal vent community was described by Little et al. (1999). 

Kennedy, C.B., Scott, S.D. Ferris, F.G. (2003) Characterization of bacteriogenic iron oxide deposits from Axial Volcano, Juan de Fuca Ridge, Northeast Pacific Ocean. Geomicrobiology Journal 2d, 199-214. 

Hanert, H. H. (2002). Bacterial and chemical iron oxide deposition in a shallow bay on Palaea Kameni, Santorini, Greece microscopy, electron probe microanalysis, and photometry of in situ experiments. Geomicrobiol. J. 19, 317-342. 

Pneumoconiosis A condition of the lung in which there is permanent deposition of particulate matter and the tissue reaction to its presence. It may range from relatively harmless forms of iron oxide deposition to destructive forms of silicosis. 

The geochemical conditions that result in arsenic contamination of water are often associated with the presence of iron, sulfur, and organic matter in (alluvial) deposits produced by water. Iron released from rocks eroded by river water forms iron oxide deposits on rock particle surfaces. 

There are a very large number of solvents for removing iron oxide deposits. They include mineral acids, organic acids, chelating agents, and mixtures of these chemical classes. The choice of a particular solvent system depends on the metal of construction, exact type and density of the oxide, and cost and disposal considerations. 

There were also unique corrosion problems associated with foam cleaning by using air as the gas phase. When air was used, a cyclical type corrosion process could occur. In the presence of an iron oxide deposit and an acidic foam, the ferric ion would spend on base metal to become reduced to the ferrous ion. Exposure to the oxidant air, present as the gaseous phase of foam, regenerated the ferrous ion to the ferric ion and the corrosion process was repeated. If recycle of the foam cleaning was attempted, the corrosion could become quite severe. The same type of cyclical process could occur in the presence of cupric copper. Cupric copper would reduce on admiralty metal and form the cuprous ion. In the presence of air, the cuprous would be regenerated into the cupric ion for a repetition of the corrosion process. Equations (4)-(7) show the corrosive processes. 

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