The use and development of synthetic polymers as flocculants has progressed and evolved since their introduction in the early to mid 1950's. In use flocculants provide the ability to flocculate solids suspended in a liquid medium, usually aqueous, to form a distinct entity capable of being separated from that medium.
Generally it is believed that the mechanism whereby the destabilization of ionic suspended solids occurs is by the neutralization of the charge on such solids which contributes significantly to their suspension stability. Mere neutralization of the charge of such solids, due to their small size, is often insufficient to enable their efficient separation from the liquid medium in which they are suspended and it has been long recognized that synthetic polymers are helpful in agglomerating such solids into entities commonly referred to as flocculated solids or flocs. These precipitate like flocs may then be separated from the liquid medium in which they are contained for various reasons and purposes. In the area of water purification, such techniques are used to remove materials undesirable for inclusion in discharge waters and hence find their utility in the removal of materials from the final product of a purification process. In contrast, in the area of paper manufacture, such techniques are used in a similar way to include within the paper slurry solids, and hence the resulting paper, materials that would otherwise be lost from the paper during the water removal phase of its manufacture and which may also inhibit the efficiency of dewatering during that phase.
Most effectively, the source of charge destabilization of suspended solids may be incorporated into the synthetic polymer molecule by the use in the formation of the polymer of monomers having moieties, or by modifying polymers to provide such moieties on the polymers, which moieties contain an ionic pair which upon addition of the polymer to an aqueous medium, dissociate to result in the polymer itself carrying a charge. Thus, polymers useful in flocculation have developed to present day being high molecular weight polymers which in the presence of water form a polymeric flocculent medium to provide a high content of cationically charged sites combining the ability to both destabilize suspended solids and to physically link them together into insoluble entities thereby separating them from the liquid medium in which they were contained.
As indicated above, a clear line of distinction must be drawn between charge bearing synthetic polymeric flocculant materials and synthetic polymers having charge contributing moieties for it is not until such moieties undergo disassociation that such polymers can have the desired flocculant activity.
One of the most significant advances in synthetic polymers useful in flocculant applications was disclosed in commonly assigned U.S. Pat. No. 4,956,399. In that patent there was disclosed a process using an inverse microemulsion to form Mannich acrylamide polymers, their use as flocculants and specific compositions comprising emulsion microparticles containing (alk) acrylamide polymers substituted with tertiary aminomethyl groups and having an average particle size of from about 200 to about 4000 .ANG., about 0.02 to about 0.4 .mu.m, in diameter. Theretofore, high molecular weight Mannich polyacrylamides (Mannich PAMs), while well known and used in a variety of flocculant applications, were associated with major drawbacks arising from cross-linking of the Mannich polyacrylamides which was particularly severe when polymer solids were increased.
Several approaches had been tried to overcome these problems. One approach was to make the Mannich PAMs at the site of use by inverting high solids inverse emulsion PAMs In water containing dialkylamines and formaldehyde. U.S. Pat. No. 4,021,394 and U.S. Pat. No. 4,022,741 describe continuous processes for the preparation of Mannich PAMs which entails inverting an inverse emulsion PAM in a process stream containing formaldehyde and a secondary amine and subjecting the stream to turbulence by in-line mixing to produce a 1-15% aqueous solution of Mannich PAM. This approach, however, suffered from the need to store multiple chemicals on site and from the problems inherent in running chemical reactions at such locations. Another approach had been to prepare dry Mannich PAMs, as described is U.S. Pat. No. 3,864,312; U.S. Pat. No. 3,539,535 and U.S. Pat. No. 3,790,529 or blends of dry PAMs with dry, low-molecular weight Mannich-base forming compounds which, when dissolved in water, react to produce Mannich PAMs, as described in EPO Patent No. 0,210,784. These approaches, in general, suffered from cross-linking problems, the reversibility of the Mannich reaction, the difficulty and length of time required to dissolve high molecular weight polymers, and other problems. Another approach was to make the Mannich PAM in inverse emulsions, such as described in U.S. Pat. No. 3,979,348; U.S. Pat. No. 4,093,542 and U.S. Pat. No. 4,010,131. While this approach produces a product with substantially higher solids, the average emulsion particle size thereof ranges from about 10,000-20,000 .ANG., about 10 to about 20 .mu.m, in diameter, and consequently, cross-linking of the many polymer chains in each emulsion particle renders the polymers less effective. The cross-linking rate of such polymers can be reduced somewhat by adding fairly large quantities of stabilizers, such as described in U.S. Pat. No. 4,113,685 and U.S. Pat. No. 4,073,763, but cross-linking continues and such products thus possess a very limited shelf life.
Accordingly, there existed a need for a Mannich acrylamide polymer which could be prepared at high solids levels without extensive interpolymer cross-linking such that it could be economically transported and easily handled by the end user without the need for any on-site preparation.
As discussed in U.S. Pat. No. 4,956,399, it was discovered that Mannich acrylamide polymers, produced in the form of inverse microemulsions, gave superior performance relative to the Mannich acrylamide polymers of the then prior art and could be conveniently prepared at high solids content while maintaining a very low bulk viscosity. As disclosed, in contrast to solution and inverse emulsion Mannich acrylamide polymers of the then prior art which contained large quantities of polymer molecules in the same aqueous environment, the Mannich acrylamide polymers as manufactured in the microemulsion process in the aforesaid patent are isolated as individual, or at most, several, polymer molecules in each aqueous microemulsion micelle. Thus, the problem of large scale debilitating interpolymer cross-linking inherent in the solution and inverse emulsion products of the prior art was overcome.
Additionally, in contrast to the high bulk viscosities of the more stable dilute solution acrylamide polymers of the prior art, the microemulsion produced Mannich acrylamide polymers disclosed in that patent could be made at high solids levels while still maintaining an extremely low bulk viscosity.
Such a Mannich acrylamide polymer composition satisfied a long felt need and constituted a notable advance in the art. The methods of manufacture of such polymers, the polymers and their use are incorporated herein by reference to the aforesaid U.S. Pat. No. 4,956,396 as well as U.S. Pat. Nos. 4,956,400; 5,037,863; 5,037,881 and 5,132,023.
Interestingly, it has been observed that the manufacture of the above discussed polymers resulted in them being in what is believed to be a predissolved form within the aqueous microemulsion particle. By this is meant that the polymer is present in the microemulsion droplet in a hydrated form which is advantageous in the make-up of a synthetic polymeric flocculent medium for use in flocculent applications.
As indicated above, a line of distinction must be drawn between charge bearing synthetic polymer flocculant materials and synthetic polymers having charge contributing moieties for it is not until the latter is contacted with an aqueous environment that the charge neutralizing function of the polymer molecule is available for use. A number of researchers have made observations regarding the availability of theoretical charge, some postulating that availability as a measure of polymer dissolution. While there might be a relationship between the two, for a given molecule under a given circumstance, the inavailability of theoretical charge has no predictive value as to degree of dissolution, as factors such as stearic hindrance of the titrant on an otherwise soluble material could yield data indicative of a degree of charge unavailably. Similarly, accumulation of the titrant on the otherwise water-soluble polymer can itself render it insoluble in much the same way as the mechanism of flocculation.
It has long been known that the availability of the theoretical charge on a polymer can be affected by stearic hindrance. Thus, branching that has occurred in prior art water-soluble polymers, to the extent such branching exists, affects the degree to which ionic sites can be accessed especially if the moiety of opposite charge is of any size. This is true whether or not the polymer is soluble or is cross-linked to the point that it becomes insoluble which, in the case of the latter, has been present to varying degrees in prior art polymers for decades.
As discussed above in the case of Mannich polyacrylamides, excessive cross-linking, in particular, intermolecular cross-linking, has been long seen as detractive of polymer performance and it is believed that a major contribution was the unavailability of charge sites or the formation of water-insoluble water swellable gels.
The polymers produced by reverse phase microemulsion polymerization are conveniently employed as flocculants prepared in the form of dilute aqueous solutions to form a synthetic polymeric flocculant medium. These solutions are prepared by inverting the microemulsion into water, optionally in the presence of a breaker surfactant, or by recovering the polymer from the microemulsion, such as by stripping or by adding the microemulsion to a solvent which precipitates the polymer, e.g. isopropanol or acetone, filtering off the resultant solids, drying and redispersing in water. The microemulsion can also be stripped to increase the percentage of polymer solids thereof.
Concentrating dispersions of suspended solids can be carried out by adding an effective amount of the polymer in solution form (i.e. a flocculant medium) to the suspension to remove water therefrom and produce an effluent of desired characteristics.
The flocculent media are useful in facilitating a wide range of solid-liquid separation operations. The cationic polymers may be used in the dewatering of biologically treated suspensions, such as sewage and other municipal or industrial sludges, the drainage of cellulosic suspension such as those found in paper production, e.g. paper waste, and the settlement of various suspensions, i.e. refinery waste, food waste etc.
Despite the many advantages of the microemulsion formed polymers referred to above, commercial experience has shown opportunities for improvements. In particular, the amino methylated acrylamide polymer microemulsions have suffered from insufficient stability at elevated temperatures and problems associated with ageing of the microemulsion synthetic polymer flocculant material independent of the temperature or pH of said medium making them unacceptable for some specific applications depending on local conditions.
As indicated above, the polymers produced by microemulsion reverse phase polymerization can be employed as dilute solutions to form a synthetic polymeric flocculent medium. In practice, the microemulsion is inverted into a large volume of water stripping the continuous oil phase from the aqueous micro droplets containing the polymer which is then mixed to form the diluted solution. The resulting solution is then aged to maximize the expected performance from the medium. While in most cases this can be accomplished in a relatively short period of time, it has been found that under certain conditions of temperature and alkalinity longer periods of aging are required which may necessitate the installation of storage tanks that may not be easily accommodated or desired at the user's site. In the alternative, it may be required to adjust either the temperatures and/or pH of the diluent water, one or both of which may be unacceptable to the user. The alternative of using the product without sufficient aging is that less than optimum flocs are obtained with the flocs sometimes being somewhat fragile or at the least an uneconomic use of the polymer. In such cases, the polymer is generally rejected for that particular application. Interestingly, it has been found that this diminished performance is not related to a lack of solubility of the polymer, which is believed already solubilized in its microparticle environment prior to inversion. Thus a need exists for an improved polymer having less dependence on temperature and pH or alkalinity for proper aging.
U.S. Pat. No. 3,988,277 teaches the stabilization of aqueous solutions of Mannich polymers against viscosity increase and gelation by the addition of an aldehyde scavenger thereto. Suitable scavengers include hydrazine, ammonia, morpholine, guanidine, dimethylamine and urea. The patent, however, fails to teach microemulsions and does not recognize the necessity of adjusting the pH to the range claimed herein. The patent also is silent with respect to the polymer concentration and the need for further heating after the scavenger addition.
Phillips et al., U.S. Pat. Nos. 4,010,131, and 4,079,027 disclose treating inverse emulsions or solutions of quaternary modified acrylamides with halogen free oxygen containing inorganic acids such as sulfurous acid, followed by heating to stabilize the quaternary modified emulsions. The patentees teach that sulfurous acid is used both to adjust the pH of the emulsion and as a formaldehyde scavenger. The references do not teach that the heat-treated inverse emulsion will invert in water independent of temperature and alkalinity. The references do teach, however, that improved storage stability and cationic charge are obtained, however, the inverse emulsions described must be inverted-at alkaline pH of 8.0. The patents teach that an adjustment of the emulsion pH between 0-6 is required, however, when this teaching is applied to quaternary Mannich microemulsions (QMM), the product fails to fully age independant of temperature and pH. Thus, these references fail to teach the critical pH range claimed herein and also are devoid of any teaching of the necessity to adjust the water concentration of the aqueous phase of the microemulsion.
U.S. Pat. Nos. 4,113,685 and 4,179,370 disclose the stabilization of Mannich acrylamide polymer emulsions by adding thereto 1) a water-soluble salt of an amine, alone or 2) in association with an amine or ammonia, or 3) an ammonium salt of a mineral acid plus ammonia or 4) a carboxylic acid amide. The patentees, however, are silent with regard to the pH to which the emulsions are adjusted by the addition of these additives and do not recognize the advantages exhibited by the instant invention by including a formaldehyde scavenger, adjusting the polymer solids content of the aqueous phase of the emulsion and heating the resultant emulsion for a specific length of time. The referenced patents thus fail to teach the instantly claimed process.
U.S. Pat. Nos. 4,120,840 and 4,195,003 disclose that the use of orthophosphorous acid provides an odorless and effective formaldehyde scavenger as well as pH adjuster. The invention teaches a method for stabilizing water-in-oil emulsions of polytrimethylaminomethylol acrylamide using orthophosphorus acid, but did not produce a satisfactory emulsion when applied to quaternary Mannich microemulsions as judged from the standard discussed hereinafter.
Canadian Patent 1,204,535, teaches the use of sodium bisulfite as both an acid pH adjuster and formaldehyde scavenger prior to alkylating partially cationically modified acrylamide polymer emulsions. The products disclosed in this patent invert in water having a pH about 8.0 or greater, however, when the teachings of this patent are applied to microemulsions, they fail to produce products which invert in water of any temperature and alkalinity.
None of the prior art teachings provide a method for producing a quaternized tertiary aminomethyl acrylamide polymer microemulsion (QMM) which successfully inverts in water independent of the pH and temperature of the water and also provide improved dewatering characteristics.