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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

Chapter 22

Processes Used to Separate Ceramic Materials

objectives
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
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
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
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
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
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
  • 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
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
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
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
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
Home Work
  • 1. What are the raw materials for bone china tableware?
  • 2. What is slip?
  • 3. What is a filter cake?