Method for removing cationic contaminants from beverages

Process for production of anionically charged filter media sheet including pretreatment of filter elements with cationic charge modifier, preferably employing inorganic colloidal silica charge modifiers. The resulting filters are used for the removal of haze or haze formers from beverages.

BACKGROUND OF THE INVENTION 
This invention relates to methods for the preparation of filter media and 
more particularly, to the provision of anionically charged media of 
enhanced wet strength from anionic filter elements. 
The filtration of fine particle size contaminants from fluids has been 
accomplished by the use of various porous filter media through which the 
contaminated fluid is passed. To function as a filter, the media must 
allow the fluid, commonly water, through, while holding back the 
particulate contaminant. This holding back of the contaminant is 
accomplished by virtue of the operation, within the porous media, of one 
or both of two distinctly different filtration mechanisms, namely (1) 
mechanical straining and (2) electrokinetic particle capture. In 
mechanical straining, a particle is removed by physical entrapment when it 
attempts to pass through a pore smaller than itself. In the case of the 
electrokinetic capture mechanisms, the particle collides with a surface 
face within the porous filter media and is retained on the surface by 
short range attractive forces. 
With the exception of microporous polymeric membranes, the porous filter 
media known to the art as being suitable for the filtration of fine 
particle size contaminants are comprised of fiber-fiber or 
fiber-particulate mixtures formed dynamically into sheet by vacuum felting 
from an aqueous slurry and then subsequently drying the finished sheet. In 
those fibrous filter media that depend upon mechanical straining to hold 
back particulate contaminants, it is necessary that the pore size of the 
filter medium be smaller than the particle size of the contaminant to be 
removed from the fluid. For removal of fine, submicronic contaminant 
particles by mechanical straining, the filter media need have 
correspondingly fine pores. Since the pore size of such a sheet is 
determined predominantly by the size and morphology of the materials used 
to form the sheet, it is necessary that one or more of the component 
materials be of a very small size, such as small diameter fibers. See for 
example, any of Pall U.S. Pat. Nos. 3,158,532; 3,238,056; 3,246,767; 
3,353,682 or 3,573,158. 
As the size of the contaminants sought to be removed by filtration 
decreases, especially into the submicron range, the difficulty and expense 
of providing suitably dimensioned fiber structures for optimum filtration 
by mechanical straining increases. Accordingly, there is considerable 
interest in the use of fine particulates such as diatomaceous earth. 
However, for such materials it is necessary to provide a matrix in order to 
present a coherent handleable structure for commerce and industry. Thus, 
at least one of the component materials in the sheet is a long, 
self-bonding structural fiber, to give the sheet sufficient structural 
integrity in both the wet "as formed" and in the final dried condition, to 
allow handling during processing and suitability for the intended end use. 
Unrefined cellulose fibers such as wood pulp, cotton, cellulose acetate or 
rayon are commonly used. These fibers are typically relatively large, with 
commercially available diameters in the range of six to sixty micrometers. 
Wood pulp, most often used because of its low relative cost, has fiber 
diameters ranging from fifteen to twenty-five micrometers, and fiber 
lengths of about 0.85 to about 6.5 mm. 
In addition to controlling the dispersion characteristics (and therefore 
the porosity of the sheet) and providing wet strength, charge modifiers 
are employed to control the zeta potential of the sheet constituents and 
maximize performance in the electrokinetic capture of small charge 
contaminants. In practice, cationic charge modifiers are employed since 
most naturally occurring contaminant surfaces are anionic at fluid pH of 
practical interest. Thus, a melamine-formaldehyde cationic colloid is 
disclosed for filter sheets in U.S. Pat. Nos. 4,007,113 and 4,007,114. 
Biological fluids present a specialized problem in that certain natural 
substances, commonly of proteinaceous character, are typically present in 
the system and are preferably, and even preferentially, removed in the 
course of a filtration operation. Unlike the submicronic impurities more 
typically encountered in other systems, these materials are cationic in 
nature at applicable pH values, i.e., below the isoelectric point for such 
contaminant. 
The surfaces of such filter elements as diatomaceous earth, cellulose fiber 
and the like may also be characterized as weakly anionic in nature, hence 
it might be expected that these materials would naturally provide the 
desired electrokinetic properties for enhanced capture potential of the 
cationic impurities. However, even to take advantage of this somewhat 
limited effect it is necessary to provide a coherent integral filter 
medium of controlled, uniform porosity comprised of interengaged filter 
elements, ordinarily requiring a binding agent for adequate wet strength. 
Also, higher levels of anionic charge are desired for optimization of the 
electrokinetic capture mechanism. The use of filter media comprising 
binders or charge modifiers in filter systems with biological fluids poses 
special problems, among them the possibility of introducing to the fluid 
impurities resulting from loss of or a breakdown in filter elements. While 
certain levels of particular impurities may be tolerable in some systems, 
organic extractables pose especially sensitive problems in the filtration 
of foods and pharmaceutical products. In filter systems composed of 
cellulose fiber as a matrix for particulate filter aids modified with an 
organic charge modifying resin, organic extractables are naturally 
primarily traceable to the resin. Selection of the charge modifying resin 
can alleviate the problem, even under relatively stringent conditions of 
use including sanitization and sterilizable procedures. Even in the 
absence of meaningful levels of extractables, however, many resins of 
choice are subject to discoloration in use, tending to limit their 
marketability for food and drugs. 
Further, even low levels of certain organic extractables are unacceptable 
in some systems, and accordingly it is desirable for this reason and that 
of aesthetics to wholly remove the organic charge modifier resin from the 
filter construction. At the same time, it is desirable for the removal of 
submicron charged contaminants to retain the charge potential afforded by 
a charge modifying resin. 
The surfaces of the filter elements may be treated with an inorganic charge 
modifier such as anionic colloidal silica, but by reason of the repulsive 
effect of these commonly charged materials, only a modest amount of charge 
modification is effected, and a coherent structure of adequate wet 
strength may not be conveniently prepared, even at high levels of charge 
modifier. 
U.S. Pat. No. 3,253,978 to C. H. Dexter & Sons Inc. describes a method of 
preparing a porous, inorganic sheet product of high strength, free of 
organic binders, composed of inorganic fibers or flakes, e.g., glass or 
mineral wool bound with colloidal silica in which a cationic agent, e.g., 
cationic starch, is added to the aqueous slurry containing the anionic 
colloidal silica binder shortly before deposit upon an inclined 
Fourdrinier wire. No cellulose containing systems are employed. The 
patentee compares performance to the similar use of dicyandiamide 
formaldehyde condensates with cellulose or asbestos in U.S. Pat. No. 
3,022,213, evidencing the slow drainage rates experienced. It is probable 
that the slow drainage rates are a result of the mutual coagulation of the 
anionic colloidal silica and the cationic starch. 
It is accordingly an object of the present invention to provide charge 
modified filter media sheets of enhanced filtration performance, 
especially for the removal of submicron contaminants from aqueous systems 
at high efficiency. 
Another object is to provide charge modified filter media characterized by 
low organic extractables over a wide range of filtration conditions. 
A still further object is the provision of filter media effective across 
the spectrum of biological liquids and, particularly, ingestables such as 
food and drugs. 
A specific object is the provision of anionically charged media of enhanced 
wet strength from anionic filter elements. 
These and other objects are achieved in the practice of the present 
invention as described hereinafter. 
GENERAL DESCRIPTION OF THE INVENTION 
The method of the invention involves the utilization of an inorganic 
cationic charge modifier in the treatment of cellulose pulp and 
particulate filter aid, to reduce or reverse the surface charge of the 
anionic filter elements, whereby anionic charge modification may be 
effected to a desired level, in one or more stages. For example, in order 
to reduce the repulsive effect of the anionic character of the cellulose 
fiber or particulate filter aids to the deposition of anionic charge 
modifier, sufficient cationic charge modifier may be deposited on at least 
one of said filter or particulate to reduce or reverse the latent charge 
in affected regions or sites thereon to permit deposition of a level of 
inorganic anionic charge modifier to provide enhanced electrokinetic 
charge potential and adequate wet strength. The process may be iterated as 
desired to accomplish a selected level of charge modification. 
The filter media sheet is preferably formed by vacuum felting of the 
anionically disperse aqueous slurry comprising beaten cellulose fibers and 
fine particulate to provide a uniform, high porosity, and fine pore size 
structure with excellent filtration and flow characteristics. The filter 
media, comprising cellulose fiber as a matrix, and particulate filter aid, 
the surfaces of at least one of which have been modified with inorganic 
anionic colloidal silica, are free of extractables, such as formaldehyde 
or amines originating with organic resinous charge modifiers, and are free 
of discoloration, such that the sheets are usable under any sterilizing 
conditions and may be employed safely and effectively with potables or 
ingestables such as food or drugs; and exhibit a wet strength of at least 
2.5 kg/in. 
The filter sheets so prepared may be used in the treatment of fluids for 
the removal of submicronic impurities therein, alone or in conjunction 
with other filter media. In one such combination, the anionically charged 
medium of the present invention may be employed in concert, as in tandem, 
with a cationically charged medium, as disclosed in copending and commonly 
assigned application Ser. No. 027,568 of Hou, et al., now abandoned, for 
the efficient removal of differentially charged contaminants. A 
representative use, for removal of hazes and haze precursors in alcoholic 
and fruit beverages is described in copending and commonly assigned 
application Ser. No. 065,258 of Green, et al. filed Aug. 9, 1979, now 
abandoned, incorporated herein by reference. 
The inorganic anionic colloidal silica is an aqueous dispersion of 
negatively charged colloidal particles, as disclosed in any one of U.S. 
Pat. Nos. 2,224,325; 2,285,477; 2,574,902; 2,577,485; 2,597,872; 
2,515,960; 2,750,345; or 2,573,743; incorporated herein by reference, and 
available commercially for example as Ludox LS, MS and HS, all aqueous 
sols containing about 30 percent solids sold by E. I. duPont de Nemours 
and Co.; and Cab-O-Sil, a colloidal silica powder sold by Cabot 
Corporation of Boston, Mass. 
While the principal requirement for this component of the filter system is 
that it functions as a charge modifier and dispersion agent, it should 
also be able to interact with the matrix to provide strong bonding as by 
cross-linking. It is surprising, in that the bonding of organic to 
inorganic materials is more difficult to achieve, to find that strong 
bonds are achieved with cellulose fiber upon curing with colloidal silica. 
However, the bonding once achieved is relatively more inert to its 
surrounding environment, and loss of the material by hydrolytic or 
solvolytic action, believed to be among the causes of impurities or 
extractables generated in other systems, is obviated. The maximization of 
wet strength, even at low levels of cationic modifier, is believed to be 
related to the essential structural identity of the inorganic cationic and 
anionic modifiers employed. 
In preferred embodiments of the invention, relatively high loadings of fine 
particulates such as diatomaceous earth or perlite, to 50-70 percent of 
more by weight of the sheet, are employed. Without wishing to be bound by 
an essentially hypothetical elucidation, it is believed that surface 
modification of these materials with silica colloid, particularly at these 
high loadings, contributes to the integrity of the overall structure, and 
may be attributable to the formation of some siliceous, or inorganic 
interbondings, interengaging the relatively low level (10-20%) of 
cellulose fibers comprising the total sheet weight in such embodiments, 
with the particulates by way of the cross-linking action of the active 
hydroxyl sites provided by the colloidal silica.

DETAILED DESCRIPTION OF THE INVENTION 
The filter media sheets of the invention are prepared from anionically 
modified filter elements, usually in the form of an anionically disperse 
aqueous slurry comprising cellulose fiber and optimized levels of fine 
particulate such as diatomaceous earth or perlite. The filter elements may 
be anionically modified in the slurry and the sheet prepared dynamically 
by vacuum felting, and drying, or the filter elements may be pretreated 
and formed into sheet media. A special feature of the invention is the 
provision of filter media sheet in which the level of particulate retained 
is enhanced as compared to sheet prepared conventionally. 
The state of refinement of a wood pulp fiber is determined by means of a 
"freeness" test in which measurement as the flow rate through a forming 
pad of the fibers on a standard screen is determined, most commonly 
utilizing the "Canadian Standard Freeness Tester". In this method, the 
quantity which is measured is the volume of water (expressed in ml.) which 
overflows from a receiver containing an orifice outlet at the bottom. The 
Canadian Standard Freeness measurements are employed in the present 
specification. Coarse unbeaten wood pulp fibers produce high drainage 
rates into the receiver from the screen resulting in large overflow 
volumes, and hence record a high freeness. Typical wood pulps show 
Canadian Standard Freeness values ranging from +400 ml. to +800 ml. In 
paper or filter media manufacture, such pulps may be subject to mechanical 
refining processes such as beating which tends to cut and/or fibrillate 
the cellulose fibers. Such beaten fibers exhibit slower drainage rates, 
and, therefore, lower freeness. 
In accordance with the present invention, such beaten pulp is preferably 
employed in the self-bonding matrix for the filter media. The Canadian 
Standard Freeness of the pulp system will vary with pulp selection, and 
may be reflective of varying states of subdivision or refinement, as where 
different pulps or differently beaten pulps are combined for sheet 
formation, but the beaten pulp will be employed to provide a composite or 
average value ordinarily ranging from 100 to 600 ml., with lower values 
e.g., 200-300 ml. or less being preferred for higher solids retention. 
The wood pulp may comprise as little as 10 percent by weight with up to 20 
to 30 percent, by weight of the total, being preferred to provide filter 
media sheet with structural characteristics suitable for industrial 
filtration applications. 
Performance is enhanced by maximizing the amount of fine particulate in the 
filter media sheet. While as little as 10 percent of a fine particulate 
will result in noticeable improvement in filtration performance of either 
type of media, optimum performance is achieved by utilizing the maximum 
amount of fine particulate. For industrial filtration, structural 
characteristics suggest a practicable maximum of about 70 percent by 
weight. Of course, for less demanding applications, somewhat higher levels 
will be possible. Generally, levels of 50-70 percent by weight are 
employed. 
There are various types of fine anionic particulates that are suitable for 
the intended purpose, including diatomaceous earth, perlite, talc, silica 
gel, activated carbon, molecular sieves, clay, etc. Functionally, the fine 
particulate should have a specific surface area in excess of one square 
meter/gram and/or particle diameters of less than 10 microns. In a broad 
sense, any fine particulate may be suitable (such as J. M. Filter Cel, 
Standard Syper Cel, Celite 512, Hydro Super Cel, Speed Plus and Speedflow; 
Dicalite 215 and Dicalite 416 and Dicalite 436) and may be evaluated by 
techniques well-known to the art. Siliceous materials are preferred, and 
from the standpoint of size, morphology, cost, fluid compatibility and 
general performance characteristics, the finer grades of diatomaceous 
earth/perlite for example, in proportion by weight of from about 80/20 to 
20/80 give better filtration performance or better cost/performance 
characteristics than that achieved by the use of any single type by 
itself. Similarly, mixtures in all proportions of relatively coarse and 
fine particulates, e.g., 50/50 parts by weight of 10 and 5 micron diameter 
particulates may be used. 
In paper production, where charge modifiers are sometimes used, the 
objective is reduction of charge to approximately the isoelectric point to 
maximize efficiency in interfelting of fiber. For filtration, maximum 
charge is desired to enhance removal of charged particles by 
electrokinetic mechanisms. In the present case the surface charge of at 
least one of the negatively charged filter elements, i.e., cellulose and 
particulate is first reduced to render the surface less electronegative 
and receptive to deposition of the desired amount of anionic charge 
modifier, whereupon the surface is rendered even more electronegative 
providing at least certain more highly electronegative regions or sites 
within the filter sheet. 
In the first stage of the process, the particulate filter aid and beaten 
cellulose pulp is dispersed in an aqueous medium, and treated with an 
inorganic cationic charge modifier, preferably cationic silica colloid. 
Suitable cationic colloidal silica materials include those of U.S. Pat. 
Nos. 3,007,878; 3,252,917; 3,620,978; 3,719,607; and 3,956,171 all 
incorporated by reference. 
These are aqueous dispersions of negatively charged colloidal particles 
consisting of a dense silica core coated with a negatively charged 
polyvalent metal-oxygen compound, e.g., of the class consisting of metal 
oxides, metal hyxroxides and hydrated metal oxides of metals having a 
valence of 3 or 4, preferably aluminum and titanium. Most preferably the 
dispersion is acidic, and the coating is of polymeric alumina species. 
Typically, the mole ratio of aluminum to silica on the surface is about 
1:1, and the dispersion (which has been commercially available as Ludox 
Positive Sol 130M, from E. I. duPont de Nemours & Co.) is stabilized with 
a counterion, as described in the aforesaid U.S. Pat. No. 3,007,878. The 
dispersion has been supplied at 30% solids stabilized with chloride ion 
(1.4%, as NaCl) for use in the pH range 3.5 to 5.5. 
The colloidal particles exhibit a surface area of about 150-225 m.sup.2 /g 
by nitrogen adsorption, a particle diameter of about 15-16 mu, and a 
molecular weight of about 5 to 18 million by light scattering. 
In its preferred form, the characteristics of the silica aquasol are 
further modified to higher levels of polymeric alumina species calculated 
as alumina from 13 to 15% or more, in the stable range based upon the 
colloidal solids. The coating, or overcoating, may be achieved by simply 
treating with an appropriate aluminum compound, e.g., basic aluminum 
chloride, as described in U.S. Pat. No. 3,007,878, or another source of 
polybasic aluminum cations. The alumina in such systems exists as a 
surface coating and, to the extent it exceeds the available surface area, 
as free alumina in solution. The free alumina may, of course, also serve 
as a coating for virgin filter elements, e.g., particulate present and 
systems so prepared offer improved resistance to autoclaving and hot water 
flushing conditions together with added wet strength. When desired, the 
resulting colloidal dispersion may be, and customarily is treated to 
remove excessive electrolyte, as by dialysis, in order to achieve storage 
stability. 
The amount of inorganic cationic colloidal silica employed in the initial 
treatment of the filter elements is in general an amount rendering the 
surfaces of the cellulose pulp and/or particulate receptive to the 
deposition of the inorganic anionic colloidal silica to a level providing 
enhanced electrokinetic capture potential for positively charged 
submicronic contaminants and adequate wet strength i.e., at least 2.5 
kg/in. in the sheet. Preferably, an amount of inorganic cationic colloidal 
silica just sufficient to modify the surface charge of the filter elements 
electroneutral or slightly electropositive is employed. Usually a 
relatively small amount of inorganic cationic charge modifier in the range 
of 0.1 to 1.0 wt. percent by weight of the filter elements proves 
sufficient. This is somewhat surprising, considering the surface area 
involved, but may be explicable in the sense of providing a minimum number 
of receptive bonding sites for the anionic charge modifier. Of course, 
larger amounts of inorganic cationic modifier may be used but, given the 
objective of providing an anionically charged sheet, are contraindicated 
since this will necessitate employing correspondingly larger amounts of 
inorganic anionic modifier. 
The inorganic cationic charge modifier is essentially fully sorbed onto the 
surfaces of at least one of the cellulose pulp and particulate filter aid 
within a very short period of time i.e., essentially within one minute, 
whereupon the second stage treatment may be commenced. It will of course 
be understood that the filter elements may be pretreated with inorganic 
cationic charge modifier, and then modified anionically in the sheet 
forming slurry. 
In this stage, employing the inorganic anionic charge modifiers described 
above, the amount of charge modifier employed is preferably that 
sufficient to at least provide an anionically disperse system, i.e., a 
system in which no visible flocculation occurs at ambient conditions in 
the absence of applied hydrodynamic shear forces. The system therefore 
comprises essentially discrete fiber/particulate elements exhibiting a 
negative charge or zeta potential relatively uniformly or homogeneously 
distributed in and throughout the aqueous medium. The specific level will, 
of course, vary with the system and the modifier selected but will be 
readily determined by one skilled in the art. 
The charge modification effected is demonstrable in measurements of surface 
zeta potential, and in improved filtration efficiency for positively 
charged particles in liquid systems. 
The slurry of pulp and particulates is formed in any suitable manner. The 
sequence of adding these components to water to form the initial slurry 
appears to be relatively unimportant, except that, as aforesaid, the 
inorganic cationic charge modifier is essentially fully sorbed, deposited 
or coated onto the filter elements prior to addition of the inorganic 
anionic modifier. The consistency of the slurry will represent the highest 
possible for a practical suspension of the components, usually about 4 
percent. The system is subjected to hydrodynamic shear forces as by a 
bladed mixer, and the charge modifier is then added to the slurry. 
The shear level is not critical, i.e., any otherwise suitable shear rate or 
shear stress may be employed having regard for available equipment, 
preferred processing times etc. but is selected and employed simply to 
break up the flocs as required and maintain the system in a dispersed 
condition during treatment. Of course, upon the formation of an 
anionically disperse slurry, the system is free of floc formation even in 
the absence of applied shear. 
After charge modification, the slurry is diluted with additional water to 
the proper consistency required for vaccum felting sheet formation, 
ordinarily 0.5 to 21/2 percent, depending upon the type of equipment used 
to form the sheet, in a manner known to the artisan. The slurry is formed 
into a sheet and oven dried in standard manner. The performance of the 
sheet as related to the drying parameters and optimized conditions may 
reflect energy considerations or desired thermal history consistent with 
minimization of unnecessary exposure to elevated temperatures, especially 
as the decomposition or scorch point for the system is approached. 
In accordance with a preferred embodiment of the invention, filter media 
sheets are formed from filter elements, i.e., particulate and a 
self-bonding matrix of beaten cellulose pulp, at least one of which is 
charge modified, the pulp being a system incorporating beaten pulp to 
provide a Canadian Standard Freeness of up to 600 ml., preferably less 
than 300 ml. e.g., 100-200 ml. the charge modifier consisting of inorganic 
anionic silica and being applied in a proportion to enhance 
electronegativity of the surface and secure a wet strength of at least 2.5 
kg/in. Filter media sheets so prepared may be autoclaved, hot water 
flushed or otherwise treated at elevated temperature to sanitize or 
sterilize the structure. 
Most preferably, both cationic and anionic charge modifiers comprise the 
same basic silica material which is believed to contribute to the 
maximization of performance characteristics, especially wet strength. 
While the invention has been principally described by reference to filter 
media sheet, it will be understood that the principles set forth will have 
similar applicability to the construction of other fiber and 
fibre-particulate structures including depth filters in wound or compacted 
form. 
Filter media sheets in accordance with the invention may be employed alone 
or in combination with other such media to treat fluids containing 
proteinaceous contaminants wherein the pH of the fluid is of sufficiently 
low value so that the proteinaceous contaminants are essentially cationic. 
Specifically, such filter media sheets are effective in the removal of 
submicron protein chill hazes which are formed in distilled spirits, 
beers, fruit juices and other such low pH generally acidic fluids. 
The present invention is representatively illustrated in the following 
examples, in which certain tests are performed as described hereinafter. 
Oil Flow Test 
As a measure of the porosity of the filter media sheets, 100 ssu oil is 
pumped through the sample sheet until a differential pressure drop of 5 
psid is attained, at which point the flow rate (ml./min.) is recorded. 
Wet Strength 
Wet strength is determined via a tensile test on a 2" wide test specimen 
which has been presoaked in distilled water for 5 minutes, employing UTM 
Chatillon Model UT-SM. 
Normalized Streaming Potential 
The measurement of streaming potential is a conventional means of 
determining zeta potential i.e., the electric potential excess of the 
surface, and the surrounding fluid to the hydrodynamic shear plane, over 
the bulk potential of the fluid. In the present test, streaming potential 
values are determined, and normalized for differing pressure drop in the 
media being tested, expressing the results in units of millivolts per foot 
of water. The filter media is evaluated by flushing out the filter media 
with water until the measured streaming potential achieves a relatively 
stable maximum value. At this point, the filter media has ceased to 
contribute any significant ionic species to the water, i.e., the inlet 
resistivity equals the outlet resistivity. 
The filter media test cell is based on the design of Oulman, et al. JAWWA 
56:915 (1964). It is constructed from Lucite having an effective area of 
3.14 square inches (2" diameter) and is equipped with platinum black 
electrodes. Water and mercury manometers are used to measure the pressure 
drop across the media being evaluated. Streaming potential values (by 
convention, of opposite sign to the zeta potential and the surface charge) 
are measured with a high impedance volt meter. The influent and effluent 
resistance are monitored with conductivity flow cells (cell 
constant=0.02/cm) using a resistance bridge. 
Upon the attainment of equilibrium streaming potential (i.e., after flush 
out) contaminant challenge tests may be performed in the same system. The 
tests above are described in more detail in a paper presented at the 71st 
Annual AICHE meeting (1978): "Measuring the Electrokinetic Properties of 
Charged Filter Media", Knight, et al. 
In the following examples which further illustrate this invention; 
proportions are by weight, based upon total pulp and particulate, 
excluding charge modifier. 
EXAMPLE I 
In each of the following runs, cellulose pulp having an average Canadian 
Standard Freeness of 130 ml. (in an amount to constitute 31% by weight) 
was dispersed in water to a consistency of about 4%, 426 perlite 
(diatomaceous earth supplied by Grefco, having a mean particle size of 4.2 
microns) was added (in an amount to constitute 69% by weight) while 
maintaining the consistency with the addition of water, and the inorganic 
anionic silica colloid charge modifier added, while the system was 
maintained under agitation (hydrodynamic shear applied by action of a 
Hei-Dolph stirrer by Polyscience Inc., having 4 propeller blades, rotating 
at about 700 ppm on setting 2). The slurry was subsequently diluted to 0.5 
percent consistency and vacuum felted into a sheet ranging from about 
0.160 to 0.200 inch thickness (depending upon retention) in a nine inch by 
twelve inch hand sheet apparatus utilizing a 100 mesh screen. The sheet 
was subsequently removed, dried in a static oven at 350.degree. F. until 
constant weight was achieved, and the final weight recorded. 
To demonstrate the effect of pretreating the filter elements with inorganic 
cationic silica colloid, in runs 5-11 the slurry was first dispersed with 
the cationic silica colloid, and the system agitated for 15 minutes to 
effect deposition of the modifier on surfaces of the particulate/fiber 
components, whereafter anionic silica colloid was dispersed in the system, 
deposited on the filter elements and the sheet forming completed as above. 
The vacuum filtered sheets were compared in respect of flow properties and 
wet strength as follows: 
TABLE I 
______________________________________ 
Pulp Anionic Cationic 
Free Silica Silica Oil Wet 
Sheet ness Colloid, Colloid, 
Flow Strength 
No. (CSF) Wt. % Wt. % (ml/min.) 
(kg/in.) 
______________________________________ 
Control 
130 0 0 1.20 
1. 130 3 0 21.0 2.15 
2. 130 5 0 20.0 2.30 
3. 130 6 0 18.0 2.60 
4. 130 10 0 15.0 2.75 
5. 130 5 0.1 20.0 3.27 
6. 130 5 0.2 18.0 3.60 
7. 130 6 0.2 18.0 3.65 
8. 130 5 0.4 18.0 4.00 
9. 130 3 0.1 -- 2.6 
10. 130 6 0.1 -- 3.3 
11. 130 3 0.2 -- 2.7 
______________________________________ 
As will be seen from the foregoing results, graphically represented in 
accompanying FIG. 1, pretreatment with the inorganic cationic colloidal 
silica prior to treatment with the inorganic anionic colloidal silica 
enhanced the wet strength of the sheet considerably, while maintaining 
comparable flow rates. 
EXAMPLE II 
The procedure of Example I was repeated except that the pulp freeness was 
modified under constant conditions, as follows: 
TABLE II 
______________________________________ 
Pulp Anionic Cationic 
Free- Silica Silica Oil Wet 
Sheet ness Colloid, Colloid, 
Flow Strength 
No. (CSF) Wt. % Wt. % (ml/min.) 
(kg/in.) 
______________________________________ 
1. 241 6 0.1 42.5 2.35 
2. 130 6 0.1 18.0 2.60 
3. 62 6 0.1 12.5 3.25 
4. 22 6 0.1 7.0 3.45 
______________________________________ 
The lower freeness values (more highly beaten pulp) were preferred for wet 
strength, but affected flow. From the results of this and the prior 
example, it can be seen that lower levels of anionic silica colloid, e.g., 
5% with higher levels of cationic silica colloid, e.g., 0.3% are preferred 
for the best balance of wet strength and flow. 
EXAMPLE III 
In this Example, the performance of a prior art cationic silica colloid 
(Wesol PA) charge modified filter media sheet (see copending application 
Ser. No. 027,568) was compared to filter media sheet prepared from filter 
elements pretreated with cationic silica colloid, and then charge modified 
with anionic silica colloid in accordance with the invention. 
A. Filter media sheets were prepared containing 30% by weight of a 
cellulose pulp system (C.S.F. about 130) and 70% by weight of particulate 
(The filter sheet of the invention employed a mixture of perlites, and 
whereas that of the prior art utilized a 50/50 admixture of diatomaceous 
earth and perlite) and were each formed in identical manner by preparing 
an ionically disperse aqueous slurry charge modifying, vacuum felting and 
oven drying, except that in the case of the invention, the filter elements 
were first dispersed with cationic silica colloid (Wesol PA, 0.1%) for a 
period of 15 minutes sufficient to permit essentially complete deposition, 
and thereafter the filter elements were charge modified with anionic 
silica colloid (Ludox HS-30, 5%) whereas in accordance with the prior art 
the filter elements after deposition of cationic modifier (Wesol PA, 6%) 
thereon. 
Employing test conditions detailed above, normalized streaming potential 
values were determined over time, and equilibrium flush out curves plotted 
for the respective filter media, compared in FIG. 2. As will be seen, the 
prior art media exhibits an increasingly negative normalized streaming 
potential which stabilized at a high equilibrium valve indicative of a 
high positive surface charge. The media prepared in accordance with the 
invention exhibits a positive normalized, equilibrium streaming potential 
which corresponds to its anionic nature. 
B. The anionic filter media sheet of the invention was subjected to 
contaminant challenge in a manner illustrative of the removal of 
submicronic proteinaceous substance. Blended whiskey containing an 
abundant well dispersed very fine haze (8 NTU, initial pH, 4.1 ambient 
temperature) was passed through the anionic filter media. Effluent 
turbidity values were reduced to about 1 NTU, evidencing the removal of 
cationically charged haze formers. Similar results were achieved with 
unstable rum, and other liquids containing proteinaceous impurities of 
cationic nature below the isoelectric point. 
While the invention has been described hereinabove with reference to the 
use of a single filter sheet media, it will be understood that multiple 
sheets may be used to provide further depth, or the anionic filter sheet 
may be used in coordination, as in tandem with a cationic filter sheet to 
remove differentially charged impurities. In a preferred embodiment this 
invention constitutes an improvement to the process of stabilizing 
unstable beverages against haze development disclosed and claimed in 
copending and commonly assigned application Ser. No. 065,258 of Green, et 
al., filed Aug. 9, 1979, now abandoned, in which the beverage is conducted 
through a first filter medium, the surfaces of which are modified with a 
polyamido polyamine epichlorhydrin cationic resin, forming haze in the 
beverage, the improvement comprising thereafter conducting the beverage 
through the anionic filter sheet of the invention to remove haze or haze 
formers. Hazes formed in such beverages by the conventional chilling 
process may of course also be removed in this manner.