There is no secret that “problem with rock fines" remains one of the most critical for non-metal industry companies. Hundreds of thousands and even million tons of fraction 0-5 mm over the years are in the dumps, "stealing" large area on the land allotment. Major task while turning rock fines into productive sand is removal of dust and clay particles, the content of which is strictly regulated. Three main solutions of the problem out of the whole range can be identified; they are washing, high-frequency screen and air classification. Each non-metal industry product engineer knows about washing and high-frequency screen, but process of "dry" classification is a particularly hard question. To understand this process, we turned to the international practices, trying to find out whether there is an alternative and what are the current world trends in the production technology of fine aggregates.
Equipment of the American company Buell that was founded in 1960 and is now considered the world leader in the production of centrifugal, inertial and gravitational air classifiers appears and is actively discussed in world specialized publications in sections for dry classification. Quantity of successfully operating equipment in 2011 is more than 200 machined, the majority of which is in the USA. Buell became part of Metso Mining and Construction Corporation in 2010; this allowed Metso to secure leadership as the supplier of crushing and screening equipment. On the example of this manufacturer we will review the technology of dry classification of the rock fines.
Air gravitational classifier operating principle is very simple and is based on the difference in mass ratio and specific surface of the particles. Material enters the classifier, where it passes through the air stream, which blows out dust particles and clean rock fines discharge into the lower part of the classifier. Secondary air supply is also used to achieve higher efficiency of separation; this gives few more chances for correct classification to "undecided" particles. The entire process takes place directly in the classifier chamber; it has shape of half- heart. The machine is also equipped with dust filter and fan that provides airflow in the system. All components are interconnected by piping.
The machine is designed for the classification of dust particles ranging in size from 50 to 300 microns and is able to take the feed size up to 8 mm. Classification reduces the amount of dust particles from 12 - 20% to 1 - 4%, which fully meets the requirements of State Standards of Ukraine (DSTU) to the sands. To do accurate prediction of the particle size distribution of final product technological tests and calculations are carried out for each project. The possibility of collecting and marketing of mineral dust, which is impossible at washing technology, is an additional plus for the classifier.
Low humidity is an important condition for the dry separation process. The classifier efficiency drops sharply if increase of this indicator is higher than 2%. Therefore, it should be installed and fed with material straight from the process line of crushing and screening - in the majority of cases rock fines leave the plant dry. This factor reduces the possibility of machine usage for material processing from dumps, if material was on damps for a long time. Moisture content of such material is usually within 4 - 8%. However, the classifier - is the perfect solution in terms of operating costs, which will allow keeping down the growth of these dumps and making profit from the production waste. Abrasion is minimized due to the absence of moving parts in the machine; ceramic chamber lining on the granite lasts from two to five years. Sleeve filters as well as chamber lining are the components that are to be replaces from time to time. We should also consider that at dry classification we have no problems with water treatment and slurry disposal as well as building of huge sumps. We can say with confidence that from an economic point of view, this technology is leading.
Summerthing mentioned above it can be concluded that air classification technology is worth of precise attention of non-metal industry companies, due to such advantages as high classification efficiency, low operating costs, environmental friendliness and additional income from the sale of mineral dust. We are confident that in the near future will see this technology at local pits.
BETON №1, 2012