Heavy media separation in aggregate beneficiation

Abstract

The importance of using high quality aggregates in concrete and road construction is now well recognized. More rigid aggregate specifications and depletion of existing resources, however, have created a serious supply problem in many areas. Beneficiation of poor quality sand and gravel by use of heavy media separation has proved a remedy in a number of cases. A laboratory investigation is described which illustrates both benefits achieved and problems encountered in heavy media separation of gravel aggregate. Aggregate evaluation studies were completed on the gravel before and after heavy media separation (HMS), including petrographic examination, physical tests, and concrete durability studies. Concrete beams containing the aggregate products were exposed to accelerated freezing and thawing for a period of about 700 cycles between 0 and 40°F. Separations were made at media densities of 2. 51 and 2. 55 on a feed sized between 1/4 and 1 1/2 in. The chief deleterious constituents proved to be argillaceous limestone, clay and shale, schists. and highly weathered The argillaceous limestone was found particularly troublesome to remove by HMS because of porous weathered surfaces and sound interiors. Results of the investigation showed that it was possible to obtain a marked improvement of the aggregate by HMS. However, sufficient deleterious constituents remained in the beneficiated product to render questionable its use in concrete exposed to water saturation and freezing conditions. Heavy media separation has proved an inexpensive and effective method of improving the quality of gravel aggregate since it was first used commercially for this purpose in 1948, and its use is expected to increase greatly as demand for high quality aggregates grows. The results of the investigation reported herein serve to show the importance of careful laboratory study before a decision is reached on the applicability of HMS to a particular problem.