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Chapter 22. Processes Used to Separate Ceramic Materials. Objectives. Mills to grind clay for mixing as a slurry. Filter pressing and spray drying to remove excess water. Stock for dry pressing. Grinding to remove excess material from sintered work pieces. Introduction.
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Chapter 22 Processes Used to Separate Ceramic Materials
Objectives • Mills to grind clay for mixing as a slurry. • Filter pressing and spray drying to remove excess water. • Stock for dry pressing. • Grinding to remove excess material from sintered work pieces.
Introduction • Raw clay must be ground and refined before it is useful in manufacturing. • The initial refining is done by processing plants that mine mineral deposits from the earth. • The purpose of refining (done near the mine to reduce transportation costs) is to remove impurities from the clay. • The material is then packaged in the form of blocks of clay.
Grinding Clay to Unify the Mixture • Pulverizing machines called mix mullers crush the material to reduce the clay particles to the desired fineness. • They are then passes through screens to remove particles that are oversized or undersized. • Manufacturers purchase stock that has already been crushed, screened, and refined. • Sometimes the clay is mixed with water and the resulting slurry shipped by tank truck. • When clay is used for extrusion, air bubbles must be removed using a de-airing pug mill that contains knives that cut and fold the stock. • The kneading action compresses and compacts the clay.
Milling • The grinding process is called milling. • Compositions of clay will differ depending on the type of ceramic product to be made. • For bone china tableware, the raw material consists of raw calcinated cattle bones (50%) mixed with china clay (25%) and feldspar stone. • All of these materials must be thoroughly milled and mixed to ensure that the body will have uniform composition throughout. • Milling is accomplished in a tumbling or rotating ball mill.
Filter Pressing • Clay that is transformed into liquid by adding water is called slip. • Filter presses used to dewater slip consist of an iron frame with nylon filters. • Hydraulic pressure is used to close the filter press over the slip. • What is left after pressing is called a filter cake.
Pugging • A process called pugging is often used to remove air bubbles from the cake and further refine the clay body. • The cake is fed into the pug mill is shredded by rotating auger blades (fig 22-6, page 325). • The clay is then extruded. • The “worms” that are produced are cut in short lengths.
Spray Drying • Spray drying is another method for reducing the amount of water in slip. • Keeping the amount of water, binder, and lubricant to a minimum is important because these elements are burned out when the product is fired thereby resulting in shrinkage. • Spray drying is used to produce granular particles with a moisture content of 7 percent or less. • The solid granules do not contain any air bubbles or dust.
Grinding for Material Removal • Ceramic work pieces may have to go through a grinding process to remove unwanted material. • The use of cutting fluids generally improves the cutting efficiency while helping prevent overheating that can produce microscopic cracking. • Diamond belts are used to grind glass.
Green Machining • Ceramic parts are often made by compacting powder under high pressure. • Machining is done on the part before firing (green machining). • If final machining is done on parts that have been fired it is called hard machining. • Hard machining required diamond tooling and is slow and time consuming.
Etching Process • Material removal from glass resulting in frosted or opaque appearance is done by etching. • Sandblasting and etching cream are used for etching. • The cream is left on for a few minutes and then washed off with water.
Summary • For bone china tableware, the raw material consists of raw calcinated cattle bones (50%) mixed with china clay (25%) and feldspar stone. • Clay that is transformed into liquid by adding water is called slip. • What is left after pressing is called a filter cake. • Keeping the amount of water, binder, and lubricant to a minimum is important because these elements are burned out when the product is fired thereby resulting in shrinkage. • The use of cutting fluids generally improves the cutting efficiency while helping prevent overheating that can produce microscopic cracking.
Home Work • 1. What are the raw materials for bone china tableware? • 2. What is slip? • 3. What is a filter cake?
