Processes Used to Separate Ceramic Materials 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?