In many parts of the world, it is hard to find a clean quarry for manufacturing concrete, asphalt, or road chippings. So, chippings are generally obtained from materials extracted from stream beds by means of a method comprising crushing, washing, screening, and dewatering.
Crushing is usually done by means of the following:
• Jawed crushers
• DMK-type impact crushers
• TBK-type vertical-shaft crushers
• Conical crushers
Showering Over Screen
Showering over screen is a method applied for materials easily washable. While the materials is moving on the screen curtain, they are exposed to pressurized water by means of nozzles screwed on pipes. Nozzles are placed in a way to expose the entire materials to pressurized water all along the screen width.
0.5 m³ to 1.5 m³ is required for 1 ton of materials (depending on the impureness degree of the material).
Washing drums are generally used for materials hard to wash. While the material is moving from one side to the other side of a drum rotating at 10-12 rounds/minute, water enters into the drum from the material outlet side and discharged from material inlet side.
The length of the drum is 3-4 time of its diameter. There are some blades within the drum to direct materials. The inside surface of the drum is coated with steel (ERD3990 or HARDOX) or rubber resistant to abrasion.
Drum is driven by pneumatic wheels and rotates over these wheels.
Dewatering Screw and Bucket Systems
Water ratio of materials dewatered by these systems is between 18% and 25%. This ratio is higher for fine materials since they absorb more water.
Undersize materials washed in screens by means of showering method (up to 7 mm in general) are fed along with dirty water to these systems. Materials are separated from the basin by means of a screw or rotating bucket group and either discharged from the system or fed to the hydrocyclone system to catch the escaping fine material.
NA-CE Dewatering Screens
Dewatering screens are used to further reduce the water content of the washed material.
These screens are generally upward sloped (approximately 5o), and driven linearly with 2 vibromotors. 200-micron spaced polyurethane screen curtain is used.
While the 10-30 cm thick material is moving upward from one side, it is compacted from the other side, so water passes through the 200-micron space. The water content of the material decreased to 13%-17%.
NA-CE Clay Removers
One of the most important equipments for recovering the washing water is the clay remover.
Clay is separated from water by means of the state-of-the-art flocculence technology and the water is returned to the washing system. In this way, a very large sedimentation pool is not necessary.