Silicate production powders

The rotacurrent dryer can handle particulate and non-sticky products such as aluminum silicate, copper powder, pyrite ore, charcoal, granulated rubber, chopped straw, detergents, and the like [75]. Performance data on the rotacurrent dryer are given in Table 21.21. 

A commercial design based on semicontinuous operation was developed for manufacture of silicate powders (27). A slurry, prepared containing the feed materials and water, is fed to the reactor tank and heated by circulating a heat-exchange fluid in channels located on the outside vessel wall. A six-bladed stirrer is operated at about 100 rpm in order to keep reagents well mixed. Once the slurry reaches the operating temperature, the vessel heat is maintained until reaction is complete. For most fine-particle products, this time is less than 1 hr. 

Zirconia, ZrOj, is made from the natural hydrated mineral, or from zircon, a silicate. Silicon carbide and silicon nitride are made by reacting silicon with carbon or nitrogen. Although the basic chemistry is very simple, the processes are complicated by the need for careful quality control, and the goal of producing fine (<1 jiva) powders which, almost always, lead to a better final product. 

An intimate mixture of ferro-silicon and powdered caustic soda or lime is packed in strong cylinders communicating with a high pressure storage. By means of a fuse the temperature is locally raised so that chemical reaction takes place, with the production of hydrogen and sodium and calcium silicates. 

The name Iron minium is given to a product composed essentially of ferric oxide mixed with day and sometimes also with siliceous sand and forming a fine, heavy powder of a deep red colour this also may be adulterated with gypsum or chalk and may contain admixed organic colouring matter. 

The dry product is ground to a powder and then a little calcium sulfate (CaS04) is added to slow down the setting rate of the cement. When water is added to the mixture, slow complex chemical changes occur, resulting in the formation of a hard interlocking mass of crystals of hydrated calcium aluminate and silicate. 

Al203, are dissolved in 2,300 ml of water, to which a solution comprising 272 g of caustic soda with 800 ml of water, is slowly added with stirring. Thereafter, a solution comprising 414 g of sodium silicate (Na20, 9%, Si02, 29%) with 100 ml of water is added, the mixture is heated and is stirred for five hours, while keeping the temperature at 60°C. When the reaction mixture becomes neutral it is allowed to cool and to stand, the supernatant fluid is withdrawn and the white gel-like precipitate is washed by decantation to remove the impurities and dried at 105°-110°C and 350 g of white powder of fine particle size is obtained as the final product (almasilate). 

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