Cleaning and liquid contact with solids – Processes – For metallic – siliceous – or calcareous basework – including...
Reexamination Certificate
2001-05-24
2004-07-13
Stinson, Frankie L. (Department: 1746)
Cleaning and liquid contact with solids
Processes
For metallic, siliceous, or calcareous basework, including...
C134S019000, C134S022130, C134S022170, C134S035000
Reexamination Certificate
active
06761774
ABSTRACT:
FIELD OF THE INVENTION
The subject invention generally relates to a composition and method for the in situ removal of scale from a substrate. More specifically, the subject invention relates to a composition and method for the in situ removal of silicate-containing scale from interior surfaces of a boiler.
BACKGROUND OF THE INVENTION
Compositions and methods for removing scale are known in the art. It is understood that steam is produced for various residential, commercial, and industrial purposes. For instance, steam is produced in a heat exchange apparatus, such as a steam boiler, to provide heat for residential, commercial, and industrial buildings. Steam is also produced to generate power for various residential, commercial, and industrial applications. In the production of steam, large quantities of water are heated to temperatures that exceed the boiling point of water. As such, minerals and salts, which are dissolved in the water, are deposited on interior surfaces of the steam boiler. These deposits are known in the art as scale.
Scale is undesirable as it decreases the thermal efficiency of the steam boiler. Scale may also possess corrosive properties that can degrade the interior surfaces of the steam boiler and shorten the useful operating life of the steam boiler. It is generally understood that there are three classifications of scale. First, there is scale, typically an iron oxide, that is generated as the steam boiler corrodes. Second, there is scale that is a phosphate, carbonate, or sulfate of either calcium or magnesium. This second classification of scale is most common. Third, there is scale that is a silicate-sulfate complex of calcium, magnesium, aluminum, and other various metals. This third classification of scale is typically the most difficult scale to remove from the interior surfaces of the steam boiler.
The conventional compositions and methods of the prior art are primarily directed at removal of the first two classifications of scale. For example, conventional compositions include acids targeted to remove phosphate, carbonate, and sulfate scale of calcium and/or magnesium. It is known throughout the art that these acid compositions are ineffective in removing the third classification of scale from the steam boilers. More specifically, these acid compositions are ineffective in removing the silicate-sulfate complexes of calcium, magnesium, aluminum, and other metals. Other conventional compositions rely exclusively on chelants, such as ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA), to remove the phosphate, carbonate, and sulfate-based scale. As with the acid compositions, these chelant only-based compositions are ineffective in removing the third classification of scale and even require several days to remove the other classifications of scale. Other conventional compositions rely on citric acid for the removal of scale. More specifically, these conventional compositions rely on citric acid in an overall acidic environment, i.e., at pHs<7, to remove only the first classification of scale, the iron oxide scale.
Due to the overall ineffectiveness of the above compositions in removing the silicate-sulfate complexes of calcium, magnesium, aluminum, and other metals, the many industries that utilize steam for heat, power, and other reasons, have had to resort to impractical, time-consuming, and expensive methods to prevent the deposition of this third classification of scale. These industries have even had to resort to physical removal of this scale. For example, some in industries must continuously treat their water to remove the minerals and salts that form the scale from the water, or to prevent the deposition of the scale. Other industries take the steam boiler out of service for certain periods of time and employ personnel to physically remove the scale while the steam boiler is out of service.
In sum, the prior art compositions described above are characterized by one or more inadequacy. Due to the inadequacies identified in the prior art, it is desirable to provide a novel composition and method that use simple ingredients, such as citric acid in an overall basic environment established by an alkali metal hydroxides, to effectively remove silicate-containing scale from the interior surfaces of a boiler.
SUMMARY OF THE INVENTION
A composition and method for the in situ removal of scale from a substrate are disclosed. Specifically, the composition and method of the subject invention are effective in removing silicate-containing scale, formed as silicate-sulfate complexes of calcium, magnesium, aluminum, and other metals, from the interior surfaces of boilers. The substrate is exposed to the composition to remove the scale. The composition includes a chelating agent (A) and a basic agent (B). The chelating agent (A) has at least two carboxylic acid functional groups and is preferably citric acid. The basic agent (B) is an alkali metal hydroxide. More specifically, the basic agent (B) is preferably potassium hydroxide, sodium hydroxide, or a combination of the two. The chelating agent (A) is present in the composition in an overall basic environment having a pH of from 7 to 14 established by the amount of the alkali metal hydroxide.
The method of the subject invention circulates the composition to contact the substrate. The method also includes other steps, such as heating the composition, filtering the composition, and, if necessary, re-circulating the composition, to ensure sufficient removal of the silicate-containing scale from the substrate. As the composition circulates to contact the substrate, the chelating agent (A) interacts with and extracts atoms of calcium, magnesium, aluminum, and other metals from the silicate-containing scale, and the overall basic environment in the composition enables these metal atoms to precipitate out of the composition as solid particulates.
Accordingly, using simple chemical ingredients, the subject invention provides a novel composition and method including a chelating agent (A) and a basic agent (B) that establishes an overall basic environment for the chelating agent (A) to effectively remove silicate-containing scale from the interior surfaces of a boiler.
DETAILED DESCRIPTION OF THE INVENTION
The composition and method of the subject invention remove scale from a substrate. More specifically, the composition and method provide for the in situ removal of scale, preferably silicate-containing scale, that is deposited on interior surfaces of various heat exchange apparatus, such as a steam boiler, during normal operation of the apparatus. To remove scale that is deposited on the interior surfaces, the substrate is exposed, in some manner, to the composition. Preferably, the composition is circulated to contact the interior surfaces. Of course, it is to be understood that the composition and method of the subject invention can be applied to any vessel in which an aqueous liquid is handled without varying the scope of the subject invention. The silicate-containing scale that is effectively removed by the composition and method disclosed herein is more completely described as a silicate-sulfate complex of calcium, magnesium, aluminum, and other various metals. An example of such a silicate-sulfate complex is calcium silicate, CaSiO
3
.
The composition includes a chelating agent (A) and a basic agent (B). The chelating agent (A) has at least two carboxylic acid functional groups, and the basic agent (B) establishes a pH of from 7 to 14 in the composition and is selected from the group consisting of alkali metal hydroxides. The chelating agent (A), the basic agent (B), and the significance of the basic pH are described in detail below.
The chelating agent (A) is selected from the group consisting of compounds having at least two carboxylic acid functional groups (A)(i), compounds having at least two functional groups, other than carboxylic acid functional groups, that are convertible into carboxylic acid functional groups (A)(ii), and combinations thereof. It is to be understood that,
Ellis Gary Don
Lusher Matthew T.
BASF Corporation
Stinson Frankie L.
LandOfFree
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