Removal of Iron Contaminants

Cementation is the process of recovery of metals from dilute aqueous solution by reductive precipitation using another metal with a more negative electrode potential, e.g., Cu + Fe° Cu° + Fe. The product, in this case cement copper, is relatively impure because of iron contamination. However, cementation can be used in conjunction with a solvent extraction flow sheet to remove small amounts of a metallic impurity, for example, removal of copper from a nickel solution by cementation with nickel powder. Here the dissolved nickel conveniently augments the nickel already in solution. 

Contamination of urban soils by POPs such as PCBs is of concern for human and ecological health. Soil contamination by metals and persistent organics disrupts nutrient cycling and is associated with declines in density of sod microfauna and lower rates of decomposition of organic compounds (Carreiro et al., 1999 Pouyat et al., 1994), an important process for the removal of organic contaminants from the environment. Ironically, urban warming may counteract this effect in some locations (McDonnell et al., 1997). 

It was also found that As removal by FeCla required only a very low concentration of FeJ p) (10 mg/L). The need for greater amounts of Fe p) for complete removal of As(III) from held-collected groundwater is attributed to the presence of anions such as HCOa and SO and possibly trace amounts of silica and H2P04. The actual surface site on Fe p) that is reactive towards As(III) appears to be either an iron(II), mixed iron(II)/(III), or iron(III) oxide, hydroxide, or oxyhydroxide corrosion product. Similar studies with As(V)-contaminated water samples clearly established the potential of Fe p) for adsorptive removal of this contaminant as well (54). 

The material condition, fabrication, and finish can tdso affect its service. Corrosion resistance, particularly SCC, can depend upon whether a material is annealed, hardened, or cold-worked, etc. Welding or stress relieving can affect intergranular or SCC resistance. Smoother surfaces, free of scale and other foreign jjarticles, generally exhibit better resistance. Passivation can be useful for stainless steels to remove free iron contamination, which can cause rusting of resistant materials. 

In summary, quantitative results indicate a strong influence of the reduction conditions on the composition of the resulting surface. The most abundant element on the active surface is oxygen. Iron must be present in several different chemical states, and a large fraction is present as elemental iron. A common observation is the efficient removal of carbon contamination during reduction. There are indications of two types of carbon contaminants, each with a different chemical reactivity. 

Use of excess sodium drives the reaction, usually done under an argon or helium blanket, to completion. After cooling, the excess sodium is leached with alcohol and the sodium and potassium fluorides are extracted with water, leaving a mass of metal powder. The metal powder is leached with hydrochloric acid to remove iron contamination from the cmcible. 

Condensate Polishing. Ion exchange can be used to purify or poHsh returned condensate, removing corrosion products that could cause harmful deposits in boilers. Typically, the contaminants in the condensate system are particulate iron and copper. Low levels of other contaminants may enter the system through condenser and pump seal leaks or carryover of boiler water into the steam. Condensate poHshers filter out the particulates and remove soluble contaminants by ion exchange. 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

Chemical pretreatment is often used to improve the performance of contaminant removal. The use of chemical flocculants is based on system efficiency, the specific DAF application and cost. Commonly used chemicals include trivalent metallic salts of iron, such as FeClj or FeSO or aluminum, such as AISO. Organic and inorganic polymers (cationic or anionic) are generally used to enhance the DAF process. 

In critical applications, if stainless steel is to be used near its limit (in terms of corrosion), and for cases such as welds, where a good finish cannot be otherwise achieved, additional passivation is required. Nitric acid (10-15 per cent by volume) is the best passivator. It also dissolves iron contamination. In circumstances where the use of nitric acid is not possible for safety or physical reasons (such as the underside of vessel roofs) passivation paste is appropriate. Both materials are used at ambient temperature and require a contact time of approximately 30 minutes. They must be removed by thorough rinsing with low chloride-content water. 

Platinum, palladium and the normal alloys of platinum used in industry are easily workable by the normal techniques of spinning, drawing, rolling, etc. To present a chemically clean surface of platinum and its alloys after fabrication, they may be pickled in hot concentrated hydrochloric acid to remove traces of iron and other contaminants —this is important for certain catalytic and high-temperature applications. In rolling or drawing thin sections of platinum, care must be taken to ensure that no dirt or other particles are worked into the metal, as these may later be chemically or elec-trolytically removed, leaving defects in the platinum. 

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