Liquid purification or separation – Processes – Preventing – decreasing – or delaying precipitation,...
Reexamination Certificate
1998-10-07
2001-02-06
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Preventing, decreasing, or delaying precipitation,...
C210S701000, C210S764000, C252S180000, C252S181000, C252S389230, C252S389540, C252S395000, C422S015000, C422S017000, C514S665000, C134S003000, C134S022190
Reexamination Certificate
active
06183649
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods for treating water-circulating systems such as systems for cooling towers. An aspect of this invention relates to compositions utilized in the treatment methods and methods for preparing them. Still another aspect of this invention relates to treatments for obtaining control over (preventing, inhibiting, removing, etc.) so-called “white rust” in water-circulating systems containing metal parts having coatings containing metallic zinc.
DESCRIPTION OF THE PRIOR ART
Virtually all water-circulating systems need repeated anti-fouling treatments, e.g. to remove corrosion or organic material that builds up and deposits inside conduits and other parts of the system. A wide variety of anti-foulants (anti-corrosion agents or anti-sealants, biocides, surface-active agents, and the like) are available for such treatments. These agents can also have a prophylactic effect by inhibiting the corrosion chemistry, preventing the deposition of organic materials, or by reducing the population of microbes (bacteria, fungi, and algae) in water circulating through the system. Indeed, the biological aspect of the fouling of water-circulating systems has extremely important consequences. In addition to microbial-induced corrosion, the formation of slimes which become “biofilms” (coatings or layers of organic material, particularly on the insides of conduits), interference with efficient heat exchange, and deterioration of wooden elements of the system or painted surfaces, there are also health concerns. Pathogenic microorganisms can thrive in water-circulating systems and can affect people and animals, e.g. by becoming airborne.
Some forms of metal corrosion are especially difficult to deal with and yet are encountered rather often in water-circulating systems. Perhaps foremost among this very troublesome corrosion chemistry is the phenomenon called “white rust”.
The formation of white rust stems from the corrosion or oxidation of metallic zinc, particularly in alkaline media. The compounds formed as a result of this corrosion are believed to include zinc oxide, zinc hydroxide, zinc carbonate, and/or physical and chemical combinations of these compounds. Since zinc is attacked by both acids and bases, it is an excellent sacrificial metal and is often used in coatings which protect other metals, e.g. the ferrous metals. The use of zinc in coatings to make galvanized metal is in fact the largest single use of this metal. The coatings can be provided by hot-dipping, electrodeposition, or by spraying atomized particles of molten zinc. Galvanized metals are widely used in water-circulating systems, hence methods for preventing or removing white rust have significant utility in the maintenance of these systems.
The methods for treating white rust which have the most promise for long-term efficacy are those which involve adding multi-component compositions containing aggressive chelating agents and a molybdate salt to the main water stream or the make-up water stream. The organo-phosphorus chelating agents are especially effective, and a molybdate commonly used is the mono-molybdate sodium salt, Na
2
MoO
4
, available commercially as the dihydrate. According to U.S. Pat. No. 5,407,597 (Busch et al, issued Apr. 18, 1995), sodium diethyl dithiocarbamate is also useful in such compositions, along with the organo-phosphorus chelating agents and a molybdate. The thiocarbamates are known to have strong complexing effects with respect to certain metals or metallic cations.
However, depending upon the degree of biofilm buildup in the water-circulating system, even so-called multi-purpose compositions can be blocked from access to the white rust. Biofilms can be controlled with the aid of various chemicals, e.g. biocides, biostats, and surface-active agents. The linear alkylbenzene sulfonates are typical of surfactants which have been employed in anti-fouling treatments. See, for example, U.S. Pat. No. 5,670,055 (Yu et al), issued Sep. 23, 1997. Biostats and biocides reduce the population of microbes which can play a major role in biofilm formation and other forms of contamination. Surface-active agents help to disperse fats and other oleaginous materials which can coat the interior of water pipes. There are two general types of biocides: oxidizing and non-oxidizing. Chlorine-releasing agents such as the hypochlorites are typical of the oxidizing type, and quaternary ammonium compounds are typical of the non-oxidizing type. Still another class of anti-fouling chemicals—which have both biocidal and surfactant properties and which are themselves oxidized then hydrolyzed in situ to relatively harmless degradation products—is described in commercial literature of AMSA, Inc. of Midland, Mich., U.S.A. and in patents such as U.S. Pat. No. 4,751,051 (Thompson et al, issued Jun. 14, 1988); see also U.S. Pat. No. 5,670,055, cited previously.
Compounds which remove scale, inhibit corrosion, or inhibit deposition of scale can be polymeric as well as monomeric, particularly if the polymer has water-compatibility, as in the case of salts of acrylic acid-type polymers. Acrylic-type polymers known to be useful as water-treatment chemicals are available in both acid and salt forms and are disclosed in trade literature of the B.F. Goodrich Company of Akron, Ohio, U.S.A. and in the Amjad et al U.S. Pat. Nos. 4,885,097 (issued Dec. 5, 1989), 4,952,326, and 4,952,327 (Aug. 28, 1990).
Water treatment is a highly developed art, and any exhaustive discussion of this field would have to be voluminous. The references mentioned above are believed to be representative of the state of the art, but the patent and trade literature is far more extensive than these few references. Notwithstanding this vast amount of work in the field of treating water-circulating systems, however, considerable further improvement is needed, particularly for truly effective, continual removal or prevention of white rust, for fully adequate cleaning of the entire water-circulating system (including not only pipes but also sumps, trays, pumps, and other elements of the system exposed to water), and effective cleaning at normal ambient temperatures, not just in parts of the system where heat is present.
SUMMARY OF THE INVENTION
It has now been discovered that a highly improved treatment of water-circulating systems is provided by treating water in the water-circulating system with an aqueous composition comprising, in combination (including any products formed by chemical or physical interaction or transformation of any ingredient or ingredients):
I) an organo-phosphorus chelating agent,
II) an acrylate homopolymer,
III) a salt of an acrylic copolymer, a portion of the repeating units of which contain sulfonate groups,
IV) a molybdate salt, and
V) a salt of a fused-ring aromatic triazole (the fused aromatic ring is preferably monocyclic, as in the case of unsubstituted or substituted benzotriazoles);
optionally, ingredients such as those designated I to III can be added in acid form and neutralized in situ.
This composition (hereafter referred to as “Composition A” or “Component A”) preferably contains a bromide salt (e.g. an alkali metal bromide) and a sufficient amount of a basic compound to insure that the pH will be at least about 10.5. To provide an aqueous medium capable of dissolving or otherwise uniformly distributing the above-listed components, Composition A is generally at least 40% by weight of water; even more typically, water is the major ingredient. The composition is surprisingly effective in controlling (preventing or removing) white rust.
In the event that the corrosion in the water-circulating system is relatively inaccessible to the above-described composition because of biofilm formation, an alternative embodiment of this invention is effective in exposing as well as essentially removing the corrosion. In this embodiment, the entire Composition A becomes one of a plurality of components used to treat the system. Thus, water in the water-circulating system is treated with aforementioned Compos
Connolly Bove & Lodge & Hutz LLP
Hruskoci Peter A.
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