GB2025914A - Method for improving the Viscosity Characteristics of Kaolinitic Clay Suspensions - Google Patents

Abstract

It is known to coat paper with kaolinitic clays using aqueous suspensions of kaolinitic clay applied by means of a high speed coating machine. Problems are commonly encountered because of increase in viscosity of the suspensions at high rates of shear, especially at high solids contents commonly employed in paper coating operations. The invention overcomes these problems by subjecting the clay suspension to mechanical shearing and, either prior to or after mechanical shear, treating the suspension with a cation exchange resin.

Description

SPECIFICATION Method for Improving the Viscosity Characteristics of Kaolinitic Clay Suspensions The invention relates to a method of reducing the viscosity of aqueous suspensions of kaolinitic clays, particularly but not exclusively aqueous suspensions of kaolins useful in paper coating.

Modern high-speed paper coating machines demand supplies of kaolins which can be incorporated into suspensions of high solids content without creating problems related to the viscosity of such suspensions at high rates of shear. The world market provides few sources of kaolins which will satisfy the viscosity requirements which are normally specified.

As will be appreciated from the specific Examples which follow, the invention provides a method whereby at high shear rates the viscosity characteristics of kaolins in general can be improved.

In its broad sense the invention provides a method for improving the viscosity characteristics of a kaolin suspension including the step of mechanically shearing a suspension of the kaolin and the step of treating the suspension with a cation exchange resin, characterised in that the steps are carried out successively in either order.

Preferably, the first step is the mechanical shearing and the suspension may be subjected to repeated treatments.

In a particular embodiment of the invention, the process is conducted repeatedly, for example by continued shearing whilst the clay of the suspension is in contact with a cationic exchange resin, conveniently by conducting the shearing operation in a first process stage, bringing the resin into contact with the suspension and continuing shearing.

In the method of the invention, the clay used conveniently is clay refined by the known steps of removing substantially all non-clay material, then deflocculating with a suitable agent such as tetrasodium pyrophosphate so as to produce a suspension, or "slip", having a solids content of the order of 60% to 72% by weight.

Conveniently, this suspsnsion is subjected to a period of intense shearing whereby the aggregates of kaolin crystals are dispersed, but the crystals are not delaminated to an appreciable extent. Suitable intense shearing may be effected by treating the slip in a macerator having a shaft rotating at rates between 5,000 and 10,000 revolutions per minute for a treatment period of from 5 to 30 minutes. Passing the slip through an extrusion machine such as the Gaulin homogenizer at pressures between 2,000 and 10,000 p.s.i. produces comparable shearing effects. Prolonged ball milling of the slip will also produce the required degree of shearing.The solids content of the slip and the period of treatment will vary according to whichever shearing technique is used, but the objective of this step remains the same in so far as it is necessary to break down the clusters of individual kaolin crystals that are common in suspensions prepared by levigation of kaolinitic deposits. These clusters may be more or less difficuit to break down, depending on such factors as the nature of the cementing impurities, that exist in a particular kaolin deposit; in practice it is necessary to determine, by experiment, the optimum combination of rate and period of shearing that suits a particular kaolin.This optimum state exists when clusters of kaolin crystals have been sufficiently dispersed by shearing to expose most of the surfaces of the individual crystals to contact with the aqueous fluid phase of the suspension, but before the applied intensity and period of shearing has been such as to substantially delaminate the individual crystals by cleaving along the direction of the basal plane of the kaolinite structure. A simple practical method of identifying this optimum state of dispersion for any particular kaolin suspension is to subject samples of the slip to varying times and degrees of shearing and then treat each sample according to the other essential step of this invention as described hereinafter, checking the resultant viscosity of the slip and identifying the shearing conditions that have produced minimum viscosity characteristics.The progressive breakdown of crystal clusters permits the progressive reduction of the viscosity of a slip, but the delamination of crystals produces exceedingly fine flakes which have the effect of increasing slip viscosity. The optimum shearing treatment is thus a compromise between these factors for any particular kaolin. Normally, an energy input of from 50 to 100 k.w.h. per tonne is to be expected in order to produce optimum results, assuming an efficient machine is used.

In the other step of the method of the invention, the cation exchange may conveniently be the sodium or hydrogen form of the resin.

As already mentioned, the shearing step is preferably conducted prior to the contact with the resin. In normal practice, the resin will be introduced into the sheared suspension and allowed to remain in contact, preferably with slight agitation, for a period which is best determined by experiment for any particular kaolin, but is normally not less than 5 minutes nor greater than 1 5 hours. The period of contact required for best results depends on a number of factors including the nature of the kaolin being treated, the amount and variety of the cations in exchange positions on the surfaces of the kaolin crystals, the accessibility of these cations to the suspending aqueous phase, and resin characteristics such as particle size and permeability.The amount of resin required also depends on some of these factors and can therefore vary widely, though it is normally in the range 0.5% to 60% by weight of the kaolin. A typical cation exchange resin which gives good results is Zerolit 225 manufactured by the Permutit Company.

The cation exchange resin will conveniently be used in bead form.

The choice between a sodium and a hydrogen resin may be influenced by the fact that a resin in the hydrogen form generally produces better viscosity characteristics at low shear rates than a sodium resin, though at high shear rates there is little difference between the results produced by either form of resin. The effectiveness of both types of resin is improved by acidifying the slip to a pH near 2.5 and maintaining this condition throughout the period of contact.

At the end of the period of contact with the exchange resin, beads of resin can be separated from the slip by screening and the aqueous phase can then be separated from the kaolin by filtering and drying.

The method lowers both the high shear viscosity as measured on a Hercules viscometer and the low shear viscosity as measured on a Brookfield viscometer, as is shown in the graphs shown in Figures 1 to 4 of the accompanying drawings. Figures 1 and 2 show the results obtained with two clay samples from the Gabbin area of Western Australia, Curve A representing the results before treatment and Curve B representing results after homogenization and resin treatment along the lines described in Example II below. Figure 3 shows the results obtained with a Hydrafine clay from the United States following treatment along the lines described in Example I below and Figure 4 shows the results obtained with a Dinkie A English china clay when treated as described in Example 1 below.

A useful, but less effective result is obtained if the clay slip is first treated with the exchange resin and then sheared. This is illustrated in the graph which comprises Figure 5 of the drawings, the treatment of the sample being described in Example III below. The Figure 5, line A represents no treatment, line B represents treatment with shearing only, line C represents treatment with resin only, line D represents treatment with resin followed by shearing and line E represents shearing followed by resin treatment. It will be appreciated that the results obtained with the two stage treatment are superior to treatment with resin aione or shearing alone.

The various aspects of the invention will be better understood by reference to the following specific examples.

Example I (a) Procedure:-- Homogenization followed by resin treatment. Material:-- Dinkie A English China Clay. An initial dispersion was prepared by the procedures described below The clay was dispersed in distilled water using 0.5% by weight of tetra sodium pyrophosphate as a dispersant and after the solids were adjusted to 70% by weight the viscosity was measured with the following results: High sheared 8x 105 dyne cm. (Hercules 210 rpm) (Curve A of Figure 4) Low shear-602 cp. (Brookfield No. 3 Spindle 100 rpm) The solids content was then lowered to 68% by weight and the slip homogenized in a Gaulin homogenizer for one minute at 9000 p.s.i.The sample was then dried and redispersed to 70% solids by weight and the viscosity again measured with the following results: High shear:-1 8x 105 dyne cm. (Hercules 625 rpm) (Curve A of Figure 4) Low shear:-- 384 cp. (Brookfieid No. 3 Spindle 100 rpm.) The homogenised slip was diluted to 20% solids and treated with approximately 60% by weight, the hydrogen form of a cation exchange resin. The resin treatment extended overnight with stirring for a short initial period and then just before separating.The sample was separated from the resin, dried and redispersed to 70% by weight using 0.45% of tetra sodium pyrophosphate and the viscosity again measured with the following results: High shear 4.2x 1 05 dyne cm. (Hercules at 1100 rpm) (Curve B of Figure 4) Low shear217 cp. (Brookfield No. 3 spindle 100 rpm).

It will be seen that the two stage treatment results in a substantial and unexpected improvement in the viscosity of the clay suspension at the 70% solids level.

(b) The procedures described in part (a) were repeated using Hydrafine clay from the United States instead of the English China Clay. The results are shown in Figure 3, Curve A representing results prior to treatment according to the invention (ie immediately following preparation of the initial dispersion) and Curve B representing results after the homogenization and resin treatment steps described in part (A).

Example II Procedure:-- Homogenization followed by resin treatment Material-Gabbin "A" Kaolin (a) A nominal minus 2 micron clay fraction was prepared from a first specimen, of crude ore and an initial dispersion prepared by dispersing to 70% by weight of solids in distilled water using 0.25% by weight of tetra sodium pyrophosphate. The viscosity was measured with the following results: High shear-i 8x 10 (dyne cm. (Hercules 800 rpm) (Curve A of Figure 1) Low sheared 93 cp (Brookfield No. 3 spindle 100 rpm).

The slip was homogenized in a Gaulin homogenizer at 70% solids for one minute at 9000 psi.

The sample was then diluted to 20% by weight and treated with 60% by weight of the hydrogen form of a cation exchange resin for 1 6 hours.

After separation from the resin the sample was dried and then redispersed to 70% solids by weight using 0.32% of tetra sodium pyrophosphate by weight and the viscosity measured with the following results: High shear-3.2 105 dyne cm. (Hercules 1100 rpm) (Curve B of Figue 1) Low shear 07 cp (Brookfield No. 3 spindle 100 rpm).

(b) The procedure described under part (a) were repeated using a second clay specimen. The results are shown in Figure 2, Curve B representing the results after the homogenization and resin treatment steps and Curve A the results for the initial dispersion.

It will be seen that the resin treatment results in a substantial and unexpected improvement in viscosity.

Example Ill Procedure:-- Homogenization before and after resin treatment.

Material:-- Gabbin clay (labelled MPH).

A nominal2 micron fraction.of this clay was prepared as a suspension in the normal way then divided into three similar samples, A, B and C.

A. This sample was made up to suspension containing 70% solids by weight with 0.3% tetra sodium pyrophosphate and its high shear viscosity was measured, with the results shown in Fig. 5 curve A. Hercules 450 rpm 1 Sx 105 dyne cm.

B. This sample was treated in the Gaulin homogenizer at 70% solids at 9000 p.s.i. for one minute and the high shear viscosity measured with the results shown in Figure 5, Curve B. Hercules 760 rpm 18x 105 dyne cm.

The homogenized sample was then diluted to 20% by weight and treated with 60% by weight of hydrogen saturated resin overnight. The resin was removed and the high shear viscosity again measured at 70% solids, with the results shown in Figure 5, curve E 2.7 x 105 dyne cm. (Hercules 1100 rpm).

C. This sample was adjusted to 20% solids by weight and treated with 60% by weight of hydrogen saturated cation exchange resin. The contact time was 1 6 hours. The high shear viscosity of this sample was measured at 70% solids, with the results shown in Figure 5, curve C. 18x105 dyne cm. (Hercules 800 rpm).

The treated suspension was then homogenized at 9000 p.s.i. and its viscosity was measured with the following results: High shear-6x 105 dyne cm. (Hercules 1100 rpm) (Figure 5, Curve D) By comparing Curves D and E it will be seen that homogenization before resin treatment produces a better result than homogenization after resin treatment.

Example IV Procedure :--Maceration and resin treatment.

Material:-Gabbin clay (bulk sample).

The clay was dispersed in distilled water using 0.5% by weight of tetra sodium pyrophosphate and after the solids were adjusted to 70% by weight the viscosity was measured with the following results: High shear--l 8x105 dyne cm. (Hercules 625 rpm) (Curve A of Figure 6) Low shear-286 cp (Brookfield No. 3 spindle 100 rpm) The sample was then placed in a top drive macerator for 10 minutes and after cooling, viscosity was measured again.

High shear-7x 1 05 dyne cm. (Hercules at 1100 rpm) (Curve B of Figure 6) Low shear 57 cp. (Brookfield No. 3 spindle at 100 rpm).

The macerated sample was then treated with 1% by weight of the hydrogen form of a cation exchange resin (Zerolit 225) for 30 minutes. The resin was removed, the sample was dried and then redispersed using tetra sodium pyrophosphate and the viscosity measured again.

High shear-2.5x 105 dyne cm. (Hercules at 1100 rpm) (Cuve C of Figure 6) Low shear-(Brookfield No. 3 spindle at 100 rpm).

This example demonstrates that a suitably controlled period of maceration can be as effective as homogenization in the first step of the procedure.

Claims (12)

Claims

1. A method of improving the viscosity characteristics of a suspension of a kaolin which method comprises (a) the step of mechanically shearing a suspension of the kaolin and (b) the step of treating the suspension with a cation exchange resin, said steps (a) and (b) being carried out successively in either order.

2. A method as claimed in claim 1 and comprising the mechanical shearing followed by the treatment with the cation exchange resin.

3. A method as claimed in claim 1 or claim 2 wherein the suspension is subjected to repeated mechanical shearing.

4. A method as claimed in any one of claims 1 to 3 wherein the suspension has an initial solids content of from 60% to 72% by weight.

5. A method as claimed in any preceding claim wherein the shearing is carried out to expose most of the surfaces of the individual crystals of the kaolin without substantially delaminating the individual crystals by cleaving along the direction of the basal plane of the kaolinite structure.

6. A method as claimed in any preceding claim wherein the contact time with the cation exchange resin is from 1 5 minutes to 1 5 hours.

7. A method as claimed in any preceding claim wherein the cation exchange resin is in the sodium or hydrogen form.

8. A method as claimed in any preceding claim wherein the cation exchange resin is used in an amount of from 0.5% to 60% by weight based on the weight of the kaolin.

9. A method as claimed in any preceding claim wherein the suspension is maintained at pH 2.5 during the period of contact with the cation exchange resin.

10. A method of improving the viscosity characteristics of a kaolin suspension substantially as herein described in any one of the Examples.

11. Kaolin whenever treated according to the method claimed in any preceding claim.

12. Paper made by a process including a coating operation in which a suspension of a kaolin as claimed in claim 11 is applied to a paper substrate.