Water-washed clay

Airfloat clay, the type most used in rubber compounds, is dry-ground hydrous kaolin that has been air-separated to reduce impurities and control particle-size distribution. However, some water-washed clay (slurried in water and centrifuged or hydrocycloned to remove impurities) is used, because it contains a lower level of impurities and gives compounds that are more colorable. The water-washed clay also causes less die wear during extrusion. 

Standard water-washed kaolins come in a variety of particle size distributions, ranging from 0.2 to 9.5 pm in median particle size. These wet-processed clays tend to have more uniform physical properties and are preferred for uniformity and specific properties such as increased loadings and improved molding characteristics. Water-washed clays can be used as rheological modifiers in conjunction with other extenders to optimize flow characteristics and reduce cost... 

Titanium dioxide is by far the most effective white pigment a compounder can use in a rubber compound. It has superior opacity, brightness, and hiding power over all other white pigments or fillers. However, it is also more expensive than other white fillers such as special water-washed clays, calcium carbonate, and talcs. Occasionally, organo-titanates are used in rubber formulations to improve filler dispersion, that is, the compound s rheology. 

Air-floated clays are less expensive than those produced from the water sedimentation process. However, those clay fillers obtained from water sedimentation, sometimes called water-washed clays, can be brighter in color, possess less iron impurities, have better particle size control, and have a lower level of impurities such as alpha quartz (which can be a hazard to workers if inhaled). 

Water-washed day - Water-washed kaolin has been slurried in water and centrifuged or hydrocycloned to remove impmities and produce specific particle size fractions. The refined slurry is either dewatered (to reduce soluble impurities) and dried, or concentrated to 70% solids and sold in slurry form. Water-washed clays are often treated to improve brightness. This includes chemical bleaching and/or high-intensity magnetic separation to remove iron and titanium impurities. 

Adhesives and sealants - Kaolin is used to control flow, penetration, and specific adhesion on application, and adhesive strength, tear strength, tensile strength, and elongation after cure. For aqueous systems the choice of airfloated vs. water-washed clay is dictated by cost, color, abrasion, and rheological properties pre-cure and reinforcing properties post-cure. Stearate-coated clays are used for improved compatibility in nonaqueous systems. 

Some producers beneficiate the phosphate ore prior to agglomeration to increase the phosphate content and remove undesirable contaminants. One approach uses a water wash to remove fines which are lower in assay and higher in contamination. If too much clay is removed from the ore by the beneficiation, a binder such as bentonite clay must be added back to the ore to faciUtate agglomeration. 

The presence of polysulfonates in petroleum sulfonates used in lube formulations has a destabilizing effect on the formulation stabiUty and function of the sulfonate in motor oils, etc. Special techniques are utilized to help reduce the carryover of residual sludge components, including the use of hydrocarbon solvents such as hexane or heptane to faciUtate separation of sludge, often with centrihigation. Other desludging procedures include water wash, H2SO4 wash, clay percolation, and filtration. 

Fig. 2. (a) Brightness improvement obtained by the use of kaolin as a filler in paper and (b), opacity improvement obtained as pulp fibers are replaced with various kaolins. In both cases, A represents fine-particle calcined clay B, high brightness No. 1 kaolin C, coarse-particle water-washed kaolin and D,... 

Treatment of water-washed hard china clays with silanes with pendant amino or mercapto groups enables them to give improved reinforcement to rubber compounds. Silane-treated clays give compounds with lower viscosity, considerably higher modulus and tensile strength and lower compression set than compounds containing untreated clays. 

The unique design of the tube press allows for this cycle to be amended, however, to include air pressing and/or cake washing. With air pressing, once the initial filtration is complete, air is introduced between the membrane and the cake. The pressure cycle is then repeated. Typically an air press will further reduce the moisture content of china clay by 2.5-8 per cent. The final moisture contents with other materials are shown in Figure 7.25. Water washing, which is used for the removal of soluble salts, is similar to air pressing, except that it is water that is introduced between membrane and cake. 

In commercial installation, the acid is applied to the deasphalted oil in propane solution and the sludge is removed by continuous settling. The acidic oil-propane solution is neutralized continuously with a solution of caustic and water-washed for removal of the last traces of caustic. The oil after removal of propane may be finished by conventional percolation through clay to produce a green cast finished bright stock (4). 

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