Scale removal chemical

Sulfamic acid has a unique combination of properties that makes it particularly well suited for scale removal and chemical cleaning operations, the main commercial appHcations. Sulfamic acid is also used in sulfation reactions, pH adjustment, preparation of synthetic sweeteners (qv), and a variety of chemical processing appHcations. Salts of sulfamic acid are used in electroplating (qv) and electroforrning operations as well as for manufacturing flame retardants (qv) and weed and hnish killers (see Herbicides). 

The furnace scales which form on alloy steels are thin, adherent, complex in composition, and more difficult to remove than scale from non-alloy steels. Several mixed acid pickles have been recommended for stainless steel, the type of pickle depending on the composition and thickness of the scale For lightly-scaled stainless steel, a nitric/hydrofluoric acid mixture is suitable, the ratio of the acids being varied to suit the type of scale. An increase in the ratio of hydrofluoric acid to nitric acid increases the whitening effect, but also increases the metal loss. Strict chemical control of this mixture is necessary, since it tends to pit the steel when the acid is nearing exhaustion. For heavy scale, two separate pickles are often used. The first conditions the scale and the second removes it. For example, a sulphuric/hydrochloric mixture is recommended as a scale conditioner on heavily scaled chromium steels, and a nitric/hydrochloric mixture for scale removal. A ferric sulphate/ hydrofluoric acid mixture has advantages over a nitric/hydrofluoric acid mixture in that the loss of metal is reduced and the pickling time is shorter, but strict chemical control of the bath is necessary. 

The mechanisms of oxide dissolution and scale removal have been widely studied in recent years. This work has been thoroughly reviewed by Frenier and Growcock who concluded, in agreement with others", that oxide removal from the surface of steel occurs predominantly by a process of reductive dissolution, rather than by chemical dissolution, which is slow in mineral acids. 

In general there does not appear to be any direct correlation between the rate of the chemical dissolution of oxides and the rate of scale removal, although most work on oxide dissolution has concentrated on magnetite. For example, Gorichev and co-workers have studied the kinetics and mechanisms of dissolution of magnetite in acids and found that it is faster in phosphoric acid than in hydrochloric, whereas scale removal is slower. Also, ferrous ions accelerate the dissolution of magnetite in sulphuric, phosphoric and hydrochloric acid , whereas the scale removal rate is reduced by the addition of ferrous ions. These observations appear to emphasise the importance of reductive dissolution and undermining in scale removal, as opposed to direct chemical dissolution. 

Before entering the perraeator for salt removal, the feed is either acidified or modified with proprietary chemicals for scale control. While both approaches have their followers, despite its handicaps, acid dosing today appears predominant in the industry in fact some RO permeator manufacturers will only warranty their product for acid. Studies to be initiated shortly at our laboratory should hopefully quantitatively evaluate the relative merits of these approaches. It is hoped that at least equivalence if not superiority for proprietary scale control chemicals will be unequivocally established so that one can drop acid use and satisfy many who are justifiably reluctant to use acid, for reasons familiar to all. 

Thermal and photochemical reactions of other, stronger reductants with Fe(III) oxides, including oxalate (Baumgartner et al., 1983 Blesa et al., 1987), ascorbate (Zinder et al., 1986), and citrate (Waite and Morel, 1984b) have been studied. The oxidation of mercaptans by Fe(in) oxides has been examined (Baumgartner et al., 1982 Waite et al., 1986 Waite and Torikov, 1987) in order to understand chemical transformations of mercaptans in the environment, and to improve formulations for rust and scale removal. 

Spatial scales characteristic of various atmospheric chemical phenomena are given in Table 1.1. Many of the phenomena in Table 1.1 overlap for example, there is more or less of a continuum between (1) urban and regional air pollution, (2) the aerosol haze associated with regional air pollution and aerosol-climate interactions, (3) greenhouse gas increases and stratospheric ozone depletion, and (4) tropospheric oxidative capacity and stratospheric ozone depletion. The lifetime of a species is the average time that a molecule of that species resides in the atmosphere before removal (chemical transformation to another species counts as removal). Atmospheric lifetimes vary from less than a second for... 

Still, all of the required conditions are present to make the photolysis of pesticides in natural waters inevitable. It seems that final proof of its extent and significance must await either more sophisticated methods for detecting and measuring transient chemical species or the actual application of photochemical principles to the practical-scale removal of pesticides from water. 

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