THE EFFECT OF AGGLOMERATE STRENGTH ON ATTRITION DURING PROCESSING

Agglomerates of sand particles bound with polyvinylpyrrolidone (molecular weight 44 000) have been prepared; the binder concentration was varied so as to produce agglomerates with a range of strengths. The breakage rate of these agglomerates has been measured in a fluidised bed with a perforated plate distributor. In this test, the primary breakage mode is attrition rather than fragmentation and the attrition rate is greatest in the region of the gas distributor. Attrition appears to occur as a result of impacts between agglomerates at low relative velocities and at low angles of incidence. A wear simulation test has been employed, and the dependence of wear on agglomerate strength determined. Fracture mechanics theory has been shown to apply to porous agglomerates such as these, and both the attrition rate during fluidisation and the wear rate in the abrasion test have been shown to be inversely proportional to the critical stress intensity factor K(c), as measured in separate fracture mechanics tests. A confined uniaxial compression test has been used in order to study the breakage behaviour of many agglomerates simultaneously. A newly-developed theory for this test enables a characteristic single agglomerate strength to be obtained, which is also a function of K(c) and consequently provides a simple means of assessing the tendency to breakdown by attrition.