The ceramic industry, at first glance, you might think that this industry is only limited to tiles, kitchenware and porcelain parts, yes in the past it was limited to that, but with the development of material science and due to the physical and chemical properties of ceramic materials, this industry began to expand to include many areas such as electronics, nuclear, aerospace, biomedical and mechanical.
The word ceramic is inspired by the Greek word (κεραμικός) (keramikos), which means pottery or clay. The ceramics industry is considered one of the oldest crafts in the history of mankind, in antiquity, clay was used with water to obtain a plastic paste in which were made different shapes, which was then dried to solidify and retain its shape. And with the Renaissance, earthenware began to be made in Italy and then spread throughout Europe. Today, ceramics are used in many industrial products such as: refractories, piping, orthopedic prosthesis, mechanical motor elements, furnace linings, high temperature optical lenses, dental implants and prostheses, floor tiles, etc.
A ceramic material is a product made from mineral (inorganic) powders, and by the action of heat becomes coherent and solid, generally a ceramic product is a poly crystalline product with ionic bond or / and covalent (strong and stable bonds ).
Raw materials used:
There are three types of ceramic raw materials:
They are represented by the family of clays, clays have the property of absorbing water in their pores and forming with it a pasty mass that can be shaped either with the hands or with the aid of a machine. .
A clay (SiO2, FeO, ……., A little organic phase, clay mineral) is interesting if it contains a high percentage of clay mineral. clay minerals include all phyllosilicates (kaolinite, halloysite, montmorillonite, vermiculite).
2- anti-plasticity substances:
With the use of pure clay, the plasticity would be too great (too deformable a paste) and drying becomes difficult (cracking of the products). In this case, anti-plasticity substances must be added to reduce plasticity and facilitate drying.
The most frequently used anti plasticity are chamotte and silica SiO2 (in the form of sand).
Silica sand (SiO2). Source: Pixabay
-The chamotte is a product broken after cooking, we will recycle it, divide it in the form of grains and introduce it into the mixture.
- Silica is the fundamental oxide of industrial ceramics, it is especially in the form of quartz sand (SiO2 rate equal to or greater than 90 percent).
Without fondants, a high cooking temperature will be necessary in order to eliminate the porosity (filling the pores), and therefore a lot of energy (fuel) and a high production cost.
The addition of the fondant reduces the production cost by filling the internal pores (maximum densification), so that the product does not need a very high cooking temperature and therefore less energy (fuel) and cost. The role of fondant begins only during cooking, at around 1100 ° C it generates an amorphous phase and fill the pores, it reacts as a binder cementing the particles, consequently the mechanical resistance of the product is improved.
Among the fondants used, we mention feldspars and dolomite CaMg (CO3) 2.
The two most used feldspars are:
Potassium feldspar: 6SiO2.Al2O3.K2O
Sodium feldspar: 6SiO2.Al2O3.Na2O
Preparation of the ceramic paste:
First of all, it is necessary to dry and reduce the raw materials in the form of grains to obtain a homogeneous mixture.
a-Raw materials division:
The soft raw materials (Plastics) are crushed by a clod cutter (machine), this device works by crushing and tearing, it essentially consists of two cylinders armed with teeth.
The hard materials are divided by a ball mill, this type of mill consists of a metal cylinder rotating horizontally, the inside of the cylinder is filled to about half of its volume by flint balls (variety of silica ) or alumina. In this type of device, wet grinding is carried out (in the presence of water), the raw material to be ground + water + grinding elements (flint or alumina balls) is introduced into the cylinder, at the outlet of the grinder we obtain a very divided solid mixed with the mass of a liquid without being dissolved (heterogeneous mixture).
(sieve. Self made picture)
This is the process we perform to separate the different particle size fraction.
c- Iron removal:
Ferruginous particles are often found torn off from crushing and grinding devices, before mixing, the ferrous particles are removed by means of electromagnets (a coil subjected to an electric field).
The crushed raw materials are suspended in water in large capacity muckers, this operation concerns the plastic raw materials as well as the chamotte. As a result of mixing the hard raw materials with the soft raw materials, we get a mixture called ceramic-slip.
The slip must be dehydrated by a filter press, the filter presses are made up of a series of vertical plates, the slurry is compressed between these plates, the water is impelled and a paste is deposited on the canvas, this paste contains 20 to 30 percent of water, pressing is complete when the flow stops.
The air trapped inside a ceramic paste causes many inconveniences if it remains included in the paste, the air will decrease the plasticity of the paste and during drying the air will hamper the transport of the paste, the air will expand and cause defects in the product.
To carry out this operation (mixing), deaerating mixers are used, the principle of these machines is to reduce the dough into small pieces which are then directed into a vacuum chamber, the air is forced and the mixture is deaerated.
Shaping by casting is widely used in many branches of ceramics: sanitary ware, earthenware, porcelain. This simple and economical process is used to reproduce complicated shapes. The slip is poured into plaster molds, these molds have the shape of the object to be obtained. The dough in the mold will continue to dry.
It is necessary to remove the water used for shaping, when the quantity of water present in a dough is large, drying is accompanied by shrinkage:
L0: initial length
Ls: length after drying
During drying the water must pass from the inside of the ceramic piece to the surface to evaporate freely.
Drying takes place in two periods:
- The 1st period during which the departure of water is accompanied by weight loss and shrinkage .
- The 2nd period:
The departure of water is accompanied by a loss of weight without shrinkage (creation of porosity).
Firing gives ceramic products their final dimensions, and mechanical resistance necessary for their use, there will be a formation of an amorphous phase which coats and binds the dispersed crystals.
- from 370 ° C: formation of alumina and mullite germ.
- During a rise in temperature, kaolinite (2SiO2.Al2O3.2H2O) is transformed into kaolin metal (2SiO2.Al2O3) with elimination of the water of constitution:
2SiO2.Al2O3.2H2O--------------2SiO2.Al2O3+ H2O (400°C-700°C)
- Towards 1200 ° C: formation of mullite (SiO2.Al2O3)
-The best cooking temperature is between: 1200 ° C-1260 ° C
The field of the ceramics industry is very wide and not everything can be mentioned in one article.
For more information on this industry, you can read the following references:
LES céramiques industriels
Fabrication des céramiques
Céramique — Wikipédia
Comprendre ce que sont les céramiques en chimie
Industrial Ceramics by F.SINGER (BOOK)
Mandal, H. (1999). New developments in α-SiAlON ceramics. Journal of the European Ceramic Society, 19(13-14), 2349-2357.