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Basic methods of ceramic products manufacture

 

There are three basic methods of preparation of ceramic mixture – soft-mud (plastic), dry and slip.

At a soft-mud method ceramic mixture has humidity 20-30%. It can be obtained directly from clay with quarry humidity, mixing up with admixtures, or from dry powders with the next moistening.

At a dry method mixture has humidity of 6-12%, it is obtained by preliminary drying, grinding and mixing up the components.

At a slip method ceramic mixture in the form of suspension (slip) with the humidity 30-35% can be used.

A softmud method is the most effective at the using of clays with enhanced quarry humidity, which gets wet in water well and dry - at the dense structure of clay raw material and low initial humidity. Slip method is used when it is needed to attain the especially careful mixing of initial components (porcelain-faience production, production of facings tiles).

The brick manufacturing process has six general phases: 1) mining and storage of raw materials, 2) preparing of raw materials, 3) forming the brick, 4) drying, 5) firing and cooling and 6) de-hacking and storing finished products (Fig. 7.5, 7.6).

Surface clays, shales and some fire clays are mined in open pits with power equipment. Then the clay or shale mixtures are transported to plant storage areas.

Continuous brick production regardless of weather conditions is ensured by storing sufficient quantities of raw materials required for many days of plant operation. Normally, several storage areas (one for each source) are used to facilitate blending of the clays. Blending produces more uniform raw materials, helps control color and allows raw material control for manufacturing a certain brick body.

Fig. 7.5. Diagrammatic representation of brick manufacturing process  

To break up large clay lumps and stones, the material is processed through size-reduction machines before mixing the raw material. Usually the material is processed through inclined vibrating screens to control particle size.

Tempering, the first step in the forming process, produces a homogeneous, plastic clay mixture. Usually, it is achieved by adding water to the clay in a pug mill, a mixing chamber with one or more revolving shafts with blade extensions. After pugging, the plastic clay mixture is ready for forming. There are three principal processes for forming brick: stiff-mud, soft-mud and dry-press.

In the stiff-mud or extrusion process, water in the range of 10 to 15 % is mixed into the clay to produce plasticity. After pugging, the tempered clay goes through a deairing chamber that maintains a vacuum of 15 to 29 in. (375 to 725 mm) of mercury. De-airing removes air holes and bubbles, giving the clay increased workability and plasticity, resulting in greater strength.

Next, the clay is extruded through a die to produce a column of clay. As the clay column leaves the die, textures or surface coatings may be applied. An automatic cutter then slices through the clay column to create the individual brick. Cutter spacings and die sizes should be carefully calculated to compensate for normal shrinkage that occurs during drying and firing. About 90 % of brick in the United States are produced by the extrusion process.



The soft-mud or molded process is particularly suitable for clays containing too much water to be extruded by the stiff-mud process. Clays are mixed to contain 20 to 30 % water and then formed into brick in molds. To prevent clay from sticking, the molds are lubricated with either sand or water to produce “sand-struck” or “water-struck” brick.

Dry-press process is particularly suited to clays of very low plasticity. Clay is mixed with a minimal amount of water (up to 10 percent), then pressed into steel molds under pressures (3 to 10 MPa) by hydraulic or compressed air rams.

Fig.7.6. Chart of ceramic bricks manufacture by the method of the dry forming: 1- excavator and trolleys for raw materials transporting; 2- batcher; 3- grinding mill; 4- mixer; 5- extruder with a cutting device; 6- trolley-platform for adoby bricks; 7- tunnel drier; 8- rotating device; 9- tunnel kiln; 10- finished-products storage; 11- transportation of wares    

Wet brick from molding or cutting machines contain 7 to 30 % moisture, depending upon the forming method. Before the firing process begins, most of this water is evaporated in dryer chambers at temperatures ranging from about 100 °C to 200 °C .The extent of drying time, which varies with different clays, usually is between 24 to 48 hours. Although heat may be generated specifically for dryer chambers, it usually is supplied from the exhaust heat of kilns to maximize thermal efficiency. In all cases, heat and humidity should be carefully regulated to avoid cracking in the brick.

Hacking is the process of loading a kiln car or kiln with brick. The number of bricks on the kiln car is determined by kiln size. The brick are typically placed by robots or mechanical means. The setting pattern has some influence on appearance. Brick placed face-toface will have a more uniform color than brick that are cross-set or placed face-to-back.

Brick are fired between 10 and 40 hours, depending upon kiln type and other factors. There are several types of kilns used by manufacturers. The most common type is a tunnel kiln, followed by periodic kilns. Fuel may be natural gas, coal, sawdust, methane gas from landfills or a combination of these fuels.

In a tunnel kiln, brick are loaded onto kiln cars, which pass through various temperature zones as they travel through the tunnel. The heat conditions in each zone are carefully controlled, and the kiln is continuously operated. A periodic kiln is one that is loaded, fired, allowed to cool and unloaded, after which the same steps are repeated. Dried brick are set in periodic kilns according to a prescribed pattern that permits circulation of hot kiln gases.

Firing may be divided into five general stages: 1) final drying (evaporating free water); 2) dehydration; 3) oxidation; 4) vitrification; and 5) flashing or reduction firing. All except flashing are associated with rising temperatures in the kiln. Although the actual temperatures will differ with clay or shale, final drying takes place at temperatures up to about 200 0C, dehydration from about 450 °C to 700 °C, oxidation from 550 °C to 950 °C and vitrification from 850 °C to 1200 °C.

Bricks are sorted, graded and packaged. Then they are placed in a storage yard or loaded onto rail cars or trucks for delivery. The most of bricks are packaged in self-contained, strapped cubes, which can be broken down into individual strapped packages for ease of handling on the jobsite. The packages and cubes are configured to provide openings for handling by forklifts.

Drainage and sewage pipes are made as a usual by a soft-mud method. The pipes of small diameter are formed on horizontal band presses, and large - on vertical vacuum.

From the powdered mixtures products are formed on the presses of high-pressure (10-30 MPa and anymore). At the pressing drying of raw - protracted and complicated process is abbreviated. Wall brick products, refractories, and different thin-walled products are made of the powdery mixtures.

Slips are used for making of the thin-walled products with complicated configuration (technique, decorative, chemically proof ceramics and others like that). This method of forming is based on property of gypsum forms to absorb part of water from slip in itself inundated in it.

Formed products are dried, for giving required strengths to them at a firing. Distinguish the convection and radiation drying. At the convection drying warmth is passed from heat-carrier - smoke gases or air, and at radiation it emanates from the heated surfaces. For drying of products there is selected an optimum temperature condition for which a drying device has the best production, and the term of drying and amount of spoilage are minimal. The tunnel and conveyer dryers of continuous action, which provide a high productivity of manufacture, are widespread.

The determinative stage of ceramic technology, at which the properties of ceramics are formed, is a firing. Formation of liquid phase as fusions begins at a temperature close 700°Ń and intensively develops as far as the increase of temperature of firing. Glassy fusions are glued together separate grains of ceramic mixture in the unique monolith. Sintering of ceramic mixtures can be also due to reactions in a solid phase.

The most important crystalline compound at the firing of ceramic mixtures is a mineral - mullite 3Al203×2Si02, which most intensively forms in the interval of temperatures 1000-1200°Ń.

For the firing of products of every kind set a certain temperature condition. If ceramics are fired at a temperature, below than required, it has enhanced porosity and low strength, and at a higher temperature strength grows, but products can be deformed and melt.

Ceramic materials are formed as a result of high temperature processes, going at firing of various mineral materials, possessing with ability to sinter. Under a sintering ability in general case understand ability of matters at firing to make a more compact with formation of stony solids. Along with crystalline phases at sintering usually there is the partial melting, that results in formation of glassy phase. Correlation of crystalline and glassy phases determines physical - mechanical properties of materials in a great deal.

Sintering is carried out on air or in a protective gas environment at a temperature, as a rule, not below 0.6-0.7 from the temperature of melting of material.

Rough ceramic products are firing usually at temperature 950-10500. Noticeable formation of fusion takes a place at temperatures 850-900 and more high. From a physical side the action of fusion shows up in shrinkage of products, declining of its porosity. Such ceramics are characterized with a coarse-grained structure, often by high porosity (7-20%).

Firing of products of thin ceramics (facings tiles, sanitary ware) on traditional technology is conducted at temperatures to 1250-12800 C with formation of densely caked mass (water absorption to 4%). For firing on the speed conveyers of facings tiles at a temperature to 1100 0C it is required to utilize the proper compositions of raw material charge. The special types of hard porcelain are fired at temperatures to 14500 C. As a result of firing to sintering the structure of thin ceramics is characterized with sweating of superficial areas of grains of quartz, feldspar with formation of grains of mullite 2Al2O3.3SiO2; voids of such ceramics are mainly closed, its size is about 10 mm, amount does not exceed 5%. The amount of glass mylite phase is 22-28%.

 


Date: 2015-12-18; view: 3156


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