Manufacturing of Bricks

Manufacturing of Bricks
The manufacturing process has six general phases.
                             
                           

1.)  Mining and Storage. 
Surface clays, shales and somefire 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 colour and allows raw material control for manufacturing a certain brick body.

    2.) Preparing Raw Materials. 
     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.

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

·         Stiff-Mud Process - In the stiff-mud or extrusion process (see Photo 3), water in the range of 10 to 15percents is mixed into the clay to produce plasticity. After pugging, the tempered clay goes through a deairingchamber that maintains a vacuum of 15 to 29 in. (375 to 725 mm) of mercury. De-airing removesair 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 Anautomatic cutter then slices through the clay column to create the individual brick. Cutter spacing and diesizes must be carefully calculated to compensate for normal shrinkage that occurs during drying and firing. About 90 percents of brick in the 
      United States are produced by theextrusion process.

·         Soft-Mud Process - The soft-mud or moulded process is particularly suitable for clays containing toomuch water to be extruded by the stiff-mud process. Clays are mixed to contain 20 to 30 percents waterand then formed into brick in moulds. To prevent clay from sticking, the moulds are lubricated with eithersand or water to produce “sand-struck” or “water-struck” brick. Brick may be produced in this manner bymachine or by hand.

·         Dry-Press Process - This process is particularly suited to clays of very low plasticity. Clay is mixed witha minimal amount of water (up to 10 percent), then pressed into steel moulds under pressures from 500 to1500 psi (3.4 to 10.3 MPa) by hydraulic or compressed air rams.

    4.) Drying. 
      Wet brick from moulding or cutting machines contain 7 to 30 percent moisture, depending upon the formingmethod. Before the firing process begins, most of this water is evaporated in dryer chambers at temperaturesranging from about 100 ºF to 400 ºF (38 ºC to 204 ºC). 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 issupplied from the exhaust heat of kilns to maximize thermal efficiency. In all cases, heat and humidity must becarefully regulated to avoid cracking in the brick.

     5.) Hacking.
      Hacking is the process of loading a kiln car or kiln with brick. The number of brick on the kiln car isdetermined by kiln size. The brick are typically placed by robots or mechanical means. The setting pattern hassome influence on appearance. Brick placed face to face will have a more uniform colour than brick that are cross-set or placed face-to-back.

    6.) Firing. 
      Brick are fired between 10 and 40 hours, depending upon kiln type and other variables. 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, andthe 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.

   7.)  Cooling.
      After the temperature has peaked and is maintained for a prescribed time, the cooling process begins. Cooling time rarely exceeds 10 hours for tunnel kilns and from 5 to 24 hours in periodic kilns. Cooling is an important stage in brick manufacturing because the rate of cooling has a direct effect on colour.

    8.)  De-hacking. 
      De-hacking is the process of unloading a kiln or kiln car after the brick have cooled, a job often performed by robots. Brick are sorted, graded and packaged. Then they are placed in a storage yard or loaded onto rail cars or trucks for delivery. The majority of brick today 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.