Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2000-06-26
2003-11-04
Michl, Paul R. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
Reexamination Certificate
active
06642351
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to methods for improving the dispersal of finely divided dry polyacrylamide particles in water and to methods of using the resulting aqueous compositions.
Polyacrylamides are widely used in a variety of industrial applications, including the flocculation of suspended solids such as biological solids, e.g., sewage sludge, mineral slurries, oily water and cellulosic slurries. The user typically employs the polyacrylamide in the form of an aqueous composition containing from about 0.01% to about 1% of dispersed polyacrylamide. The polyacrylamide may be water-soluble, in which case the aqueous composition may be a polyacrylamide solution comprised of dissolved polyacrylamide, or the polyacrylamide may be water-swellable, in which case the aqueous composition may be comprised of swollen polyacrylamide particles. The aqueous composition may also be comprised of a mixture of dissolved polyacrylamide and swollen polyacrylamide particles. Flocculation may be accomplished by adding the aqueous composition to the suspended solids in a manner that is generally well known to those skilled in the art.
The manufacturers and suppliers of polyacrylamides typically provide the polymer to the user in a concentrated form in order to reduce shipping costs. Concentrated forms include concentrated polymer solutions, dry polymers, water-in-oil emulsions, water-in-oil microemulsions, aqueous dispersions, dispersions of the polymer in oil, etc. In order to convert the concentrated form into the dispersed form required for the particular application, the user generally intermixes the concentrated form with water to the extent necessary to provide an aqueous composition having the desired level of dispersed polyacrylamide.
Each form of concentrated polymer has advantages and disadvantages. Dry forms in which the polymer is less hydrated generally contain from about 5 to about 30% water and thus tend to have an advantageously high concentration of active polymer. However, in many cases they are disadvantageously more difficult to dissolve than other forms containing greater amounts of water in which the polymer is more fully hydrated, particularly when the molecular weight of the polymer is high, e.g., about 100,000 or greater.
The dispersal rate of dry particulate polymers may also be influenced by the size of the particles. In commercial practice dry forms are generally supplied to the user in the form of particulate polymer compositions having a number average particle size in the range of about 500 microns to about 1500 microns. When the particles are too large, dispersal may be too slow for practical commercial use. Optimum dispersal rates are generally observed at intermediate particle sizes. Smaller particle sizes are generally disfavored because of handling problems, e.g., dusting, caking etc.
Polymer compositions which contain polyacrylamide particles having a particle size of about 300 microns or less may be especially problematic because these small particles generally disperse more slowly in water than larger particles. Examples 44-49 of U.S. Pat. No. 5,849,862, the entire disclosure of which is hereby incorporated herein by reference, demonstrate that the smaller particles of precipitated and spray-dried polyacrylamides dissolved more slowly than the corresponding agglomerated forms of the same polyacrylamides having a larger particle size, see Table 9 of U.S. Pat. No. 5,849,862. It is believed that smaller particles disperse more slowly because they have a strong tendency to stick together upon contact with water to form globules or so-called “fisheyes.” Formation of these fisheyes is generally undesirable not only because of their tendency to disperse slowly but also because the fisheyes may not pass easily through small orifices and thus may undesirably clog equipment and slow production.
One approach to the problem of slow dispersal of small polymer particles has been to agglomerate the smaller dry particles to form larger dry particles which disperse more quickly, see, e.g., U.S. Pat. No. 5,849,862. Other approaches to this problem include dispersing the polymer into a liquid carrier in which the polymer is insoluble to form a suspension, then adding the suspension to water, see, e.g., U.S. Pat. No. 4,499,214; and dispersing the polymer into an aqueous solution of an equilibrating agent, see, e.g., U.S. Pat. No. 4,673,704. However, these approaches have not been entirely satisfactory because they may involve additional inconvenient and/or expensive processing steps. Fisheye production may be avoided or reduced by adding the polymer to the water at an extremely slow rate, but the resulting delays in production and/or low level of dispersed polymer in the resulting aqueous composition may be commercially impractical. The fisheyes may be stirred in water for extended periods until they disperse, but this may also undesirably delay production. The fisheyes may be removed e.g. filtered from the aqueous composition, but removal may be cumbersome and the resulting aqueous composition may have a lower polymer content than desired. The dispersal problem may be mitigated somewhat by preparing the dry polymer in such a way as to avoid or reduce the formation of small polymer particles, but in some cases the natural tendency of the process may be to produce small particles, see, e.g., the spray-drying process of U.S. Pat. No. 5,849,862.
A great variety of mechanical devices and techniques have been disclosed for dispersing polymers in water, see, e.g., U.S. Pat. Nos. 4,077,612; 4,086,663; 4,390,284; 4,507,470; 4,518,261; 4,529,794; 4,531,673; 4,603,156; 4,643,582; 4,688,945; 4,845,192; 4,874,588; 5,171,781; 5,344,619; 5,368,385; 5,407,975; 5,599,101; 5,626,422; 5,660,466; 5,879,080; 5,947,596 and WO 97/34945. However, to our knowledge, none of these disclosures is particularly directed to the problem of rapidly dispersing commercially significant quantities of very small polyacrylamide particles in water to provide uniform aqueous compositions containing dispersed polymer.
Manufacturers may occasionally produce batches of dry polyacrylamide particles which have a particle size below that specified by the user i.e., “off-spec” batches. These batches frequently present a problem for the manufacturer who must often choose to either dispose of them, at significant cost, or provide them to the user at a substantial discount in price, at possible risk to the manufacturer's reputation for quality. To our knowledge, no manufacturer is currently supplying the flocculation market with commercially significant quantities of dry polyacrylamides having a number average particle size below about 300 microns as part of its regular “on-spec” production. It would be beneficial for manufacturers to be able to supply small polyacrylamide particles directly to the market. In the aforementioned spray-drying process, where there is a natural tendency to produce small polyacrylamide particles, the need for agglomeration could be avoided or reduced. Also, manufacturers could profitably sell “off-spec” batches to the market without damaging their valuable reputations. However, there appears to be a lack of demand for small polyacrylamide particles that is directly related to the various aforementioned problems, and particularly the problem of dispersing small polyacrylamide particles in water. Therefore, there remains a need for a method which provides a solution to the dispersal problem.
BRIEF SUMMARY OF THE INVENTION
This invention provides a method for improving the dispersal of small polyacrylamide particles in water to form aqueous compositions comprised of commercially useful amounts of homogeneously dispersed polyacrylamide. The aqueous compositions formed by this method have such a low fisheye content that greater than about 90% by weight of the aqueous composition passes through a 50 mesh screen within about 15 minutes after being formed. This rapid and efficient dispersal of polymer particles in water is achieved by separating the dry polyacrylamide particle
Harlukowicz John
Lewellyn Morris
Ryles Roderick G.
Cytec Technology Corp.
Mallon Joseph J.
Michl Paul R.
Negron Liza
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