Effect of some salts on the viscosity of slip casting
Introduction
Forming, by means of casting, is an important process which has been used in traditional and modern ceramic production for more than 200 hundred years (Scharrer, 1994). Rheological properties of slip casting are of importance because they are one of the significant parameters which control the production. The rheological properties depend on physical and chemical properties of the raw material and on the conditions under which the slip casting is prepared. Since the properties of the raw material are variable, depending on the area where the raw material is produced, the rheological properties of the slip casting change depending on the raw material. Therefore, the rheological properties must be redetermined for each new raw material in the casting process.
The wall thickness of the product and the casting speed are mainly controlled by the viscosity of the slip in a casting process. The viscosity of the slip must, therefore, always be constant within statistical limits. The viscosity of the slip depends on particle size, shape and type of the raw material, and the viscosity of the medium, the speed of mixing and the purity of the water used.
The slip should not precipitate during the process. The water used in preparing the slip must not contain excess Na+, K+, Fe2+, Ca2+, Mg2+ Cl−, SO2−4 PO3−4 and CO2−3 ions; they cause precipitation of the slip and spoil its stability. It is well known that some ions effect the colloidal clay suspension (Schulle et al., 1983; Brandenburg and Lagaly, 1988; Singh et al., 1992). The zeta (ζ) potential of colloidal clay decreases with adding counter ions (Hunter, 1992). In the meantime, the clay particles aggregate due to surface–surface, edge–surface and edge–edge interactions (Demiralp et al., 1987). Variations in the shape and size of the colloidal particles cause changes in their rheological properties.
In this work, the effect of some salts on the viscosity of a casting slip with a viscosity of 400 mPa s was studied.
Section snippets
Methods and materials
The composition of the slip used in sanitary ware production is approximately as follows: feldspar 20%, quartz 15%, kaolin 32% and ball clay 33% by weight. The density and the viscosity of the slip are about 1800 g l−1 and 400 mPa s, respectively. These viscosity values are rearranged by adding Na2CO3 and Na2SiO3.
The viscosity was measured by using LVTD Model Brookfield Digital Viscometer and the particle size was measured by using Sedigraph S100 V3.03 model. Chloride and sulfate salt solutions
Results and discussion
The solid particle size distribution in the slip used is shown in Table 1. It is observed from Table 1 that 97.5% of the particles are smaller than 80 μm and 50% of them are smaller than 3.6 μm. As it is shown in Table 1, 16% of all particles are over the range of 2–3 μm and 16.9% are smaller than 1 μm.
The salt solutions were firstly added with increasing amounts into the slip and the viscosity of which was prefixed by means of Na2CO3 and Na2SiO3. The viscosity of the suspension was finally
Conclusions
High amounts of soluble salts containing di- or trivalent cations or SO42− or Cl− anions are injurious to the casting slip. It does not matter whether they are introduced in the water or come from the raw materials. Such ions make deflocculation difficult and the properties of the resultant slip inferior.
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