This invention relates to the treatment of clay minerals and, more particularly, is concerned with a method of treating clay minerals in order to improve the rheological properties of the clay.
Clays for use in a slip casting process for manufacturing ceramic wares should form slips, or suspensions, in water which are sufficiently fluid to be poured, even when containing a high percentage by weight of solids, without requiring a large addition of a deflocculating agent, such as sodium silicate. The clay should also be such that when the casting slip is poured into a plaster mould to form a ceramic article, the water in the slip should flow rapidly through the walls of the mould and through the cake which is being formed on the walls of the mould.
Clays for use as pigments, for example for use in coating paper, must generally have good whiteness and fine particle size and, in addition, must have good rheological properties, e.g. they must be capable of forming an aqueous suspension of high solids content which can flow through a paper coating apparatus so that the minimum quantity of water has to be removed from the coated base paper by evaporation. In modern paper coating apparatus a paper coating composition is subjected to shear rates which may be as high as 10,000 sec .sup.-1, or even higher, and it is important that the rheological properties of a coating pigment are such that the paper coating composition is still fluid under these conditions. It is also an important advantage if, when a paper coating pigment is made into a deflocculated aqueous suspension at a high solids content, it remains fluid even after storage for several days so that the pigment is capable of being transported in a slurry form and of being successfully pumped on arrival.
Also, one of the most commonly used methods of dewatering dilute aqueous suspensions of clay pigments is that of filtration, and it is important that the clay pigment should form a filtercake having good permeability so that it can be dewatered rapidly by this method. A clay pigment which forms a filtercake having poor permeability presents severe production problems due to the long time required for the filtration stage.
There are often found natural deposits of clays which have good whiteness and fine particle size, but which have inferior rheological properties so that they are unsuitable, for example, for use in slip casting processes or as paper coating pigments. It is clearly desirable to be able to improve the rheological properties of such clays in order to bring them to the standard required to make them suitable for use in slip casting processes or as paper coating pigments; and it is an object of the present invention to provide a method of treating a clay which has naturally poor rheological properties in order to obtain therefrom a material whose rheological properties are such as to make it useable as a paper coating pigment or in a slip casting process.
In British Patent Specification No. 1,228,538 there is disclosed a method of forming a coated kaolinite which has improved physical characteristics, especially as regards its use in organic polymer systems, the method comprising forming a slurry of a kaolinite containing dissolved therein a compound or compounds capable of forming, either alone or in combination, an inorganic gel when the pH of the slurry is changed, and adjusting the pH of the slurry to form the inorganic gel and cause deposition of said gel on the surface of the kaolinite. The specification states that suitable inorganic gels are magnesium silicate, aluminum silicate, silica and alumina and the method is illustrated by a number of Examples, in one of which (namely Example 4) a kaolinite is treated with aluminum hydroxide in a quantity which exceeds 10 mg. of Al per g. of clay.