Kaolin clay slurries of reduced viscosity

Aqueous refined kaolin clay slurries of reduced low shear viscosity are prepared by dispersing the clay under controlled pH conditions in water containing a combination of water-soluble citrate and water-soluble polyacrylate within a specified range of concentrations.

BACKGROUND OF THE INVENTION 
This invention concerns a method for preparing aqueous kaolin clay slurries 
of reduced low shear viscosity. 
Kaolin clay has found many uses in the chemical and related industries. For 
example, it is employed extensively by the paper industry as a filler and 
coating pigment. For such uses, however, the naturally occurring kaolin 
must be refined. 
In the typical refining procedure, the crude kaoline is slurried in water 
to a solids content of from about 35 to 45 weight percent in the presence 
of a primary dispersant such as sodium hexametaphosphate. The resulting 
slurry is screened to remove grit and then subjected to delamination and 
centrifugal classification to yield a fine fraction containing primarily 
particles of less than two microns. The fine fraction is flocculated with 
such as alum and sulfuric acid, bleached with such as sodium hydrosulfite, 
filtered and washed with water. The filtered refined clay, which has a 
solids content of about 60 weight percent, is then either dried to a 
powdered product, such as by spray drying, or redispersed in water in the 
presence of a secondary dispersant to a slurry having a solids content of 
from about 60 to 72 weight percent, using dried product as needed. 
Numerous secondary dispersants have been disclosed for the dispersion of 
refined clay. The condensed phosphates such as sodium tripolyphosphate and 
tetrasodium pyrophosphate have been found effective and are commonly used 
for this purpose. However, these agents tend to readily degrade by 
hydrolysis in water, causing the clay slurry to thicken and finally gel on 
standing. Dispersants purported to alleviate this problem include citric 
acid as disclosed in U.S. Pat. No. 4,144,083 and sodium polyacrylate as 
disclosed in Canadian Pat. No. 854,271. Sodium polyacrylate has further 
been disclosed in U.S. Pat. No. 3,736,165 for use as a primary dispersant 
which is purported to carry through the clay refining process and act 
along with the added secondary dispersant such as citric acid to disperse 
the refined clay. 
It is the primary objective of the present invention to provide a more 
effective dispersant system for preparing a refined kaolin clay slurry of 
reduced viscosity and improved viscosity stability. 
SUMMARY OF THE INVENTION 
I have now found that, within a limited range, the combination of citric 
acid with sodium polyacrylate offers an unexpected improvement both in the 
reduction of the low shear viscosity of high solids aqueous slurries of 
refined kaolin clay and in the stability of such slurries. 
Accordingly, the present invention entails a method of preparing an aqueous 
clay slurry of reduced low shear viscosity, which comprises dispersing 
from about 60 to 72 percent by weight of particulate kaolin clay in water 
at a pH of from about 6 to 8 in the presence of from about 0.01 to 0.05 
percent of water-soluble citrate, expressed as anhydrous citric acid, and 
from about 0.02 to 0.08 percent of water-soluble polyacrylate, expressed 
as sodium polyacrylate, in a ratio of from about 0.4 to 6 grams of the 
polyacrylate per gram of the citrate, the combined amount of the citrate 
and polyacrylate being from about 0.06 to 0.11 percent, each of the 
percentages being expressed by weight based on the weight of the clay. 
Preferably, the citrate is citric acid or sodium citrate; the polyacrylate 
is sodium polyacrylate; the ratio of polyacrylate to citrate is from about 
0.5 to 3 grams per gram with the citrate percentage from about 0.02 to 
0.04, the polyacrylate percentage from about 0.02 to 0.07 and the combined 
amount of the citrate and polyacrylate from about 0.06 to 0.10 percent; 
and the pH of the prepared slurry is from about 6.8 to 7.7. 
DETAILED DESCRIPTION OF THE INVENTION 
The method of the present invention, in using the combination of 
water-soluble citrate and water-soluble polyacrylate within a specified 
limited range, offers to the clay producer and clay user a superior system 
for preparing, storing and transporting dispersed refined kaolin clay 
slurries. The combination not only permits preparation of refined kaolin 
clay slurries of reduced low shear viscosity using lesser amounts of the 
two dispersants, but, more importantly, also insures that the slurry 
viscosity will not rise appreciably during storage or shipment. 
The method is primarily applicable to refined kaolin clay slurries of high 
solids content containing from about 60 to 72 weight percent clay. 
Although more dilute slurries in the order of about 50 percent solids may 
be employed, economics dictates the use of slurries with a minimum clay 
content of about 60 percent. Likewise, while dispersed slurries having a 
clay content of as high as 75 percent may be realized, the practical upper 
limit is about 72 percent. The preferred clay solids content is about 70 
weight percent. 
In one convenient method of executing the present invention, the 
particulate refined kaolin clay is dispersed in water in the presence of 
the water-soluble citrate and the water-soluble polyacrylate in the total 
amount of from about 0.06 to 0.11 percent by weight of the clay, the 
concentrations of the individual components being from about 0.01 to 0.05 
percent, expressed as anhydrous citric acid, for the citrate and from 
about 0.02 to 0.08 percent, expressed as sodium polyacrylate, for the 
polyacrylate with the weight ratio of the polyacrylate to the citrate 
being from about 0.4 to 6. Preferably the combined amount of the 
dispersants is from about 0.6 to about 0.10 percent by weight of the clay 
with the citrate from about 0.02 to 0.04 percent, the polyacrylate from 
about 0.02 to 0.07 percent and the ratio of polyacrylate to citrate from 
about 0.4 to 3. 
The soluble citrate may be added in the form of, for example, citric acid 
or one of its alkali metal or ammonium salts. The preferred forms of the 
citrate are anhydrous citric acid, added as a 50 weight percent aqueous 
solution, and granular trisodium citrate dihydrate. 
By water-soluble polyacrylate is meant acrylic acid or methacrylic acid 
polymer having a weight average molecular weight of from about 1000 to 
20,000, as well as its monovalent cation salts. Preferably the 
polyacrylate has a weight average molecular weight of from about 1500 to 
5000 and is sodium polyacrylate. Suitable commercial sodium polyacrylates 
includes, for example, DISPEX.RTM. N40, a 40 percent aqueous solution 
supplied by Allied Colloids Inc., Ridgewood, New Jersey; SUN-KHEM.RTM. 
505, a 45 percent aqueous solution supplied by Sun Chemical Company, 
Charleston, South Carolina; Colloid 211, a 43 percent aqueous solution 
supplied by Colloids, Inc., Newark, New Jersey; NOPCOSPERSE.RTM. 44, a 
42.5 percent aqeuous solution supplied by Diamond-Shamrock, New York, New 
York, and GOOD-RITE.RTM. K-759, a powdered product supplied by the B F 
Goodrich Company, Chemical Division, Cleveland, Ohio. 
To realize the desired reduced low shear viscosity stability of the present 
invention, the pH of the dispersed slurry is adjusted during the slurry 
preparation to within the range of from about 6 to 8, preferably from 
about 6.8 to 7.7. Since the refined kaolin clay is usually slightly 
acidic, this adjustment is most readily accomplished by the careful 
addition of a strong base such as sodium hydroxide or preferably sodium 
carbonate.