Pumice attrition in an air-jet

Abstract

We present the results from a series of jet-attrition experiments performed using a standard ASTM device (ASTM D5757-00) on naturally occurring ash-sized (<2mm) pumice, a product of explosive volcanic eruption comprising highly porous silicate glass. We investigate the effect of both feed grain size and attrition duration on the production of fines. We utilize a wet methodology for fines collection to ensure recovery of the total grain size distribution for each experimental run. The experiments convert a restricted size range of pumice particles to a bimodal population of parent and daughter particles. The bimodal distribution develops even after short (~15min) attrition times. With increased attrition time, the volume of daughter particles increases and the mode migrates to finer grain sizes. Jet attrition efficiency depends heavily on the particle size of the feed; our data show little attrition for a feed of 500μm vs. highly efficient attrition for a 250μm feed. Our rates of attrition for pumice are extremely high compared to rates recovered from experiments on limestone pellets. Fines production data are well modeled by:mfinesmbed0=0.2911−e−0.312twhere m0bed is the initial mass of particles in the bed, t is in hours, and the two adjustable coefficients dictate the long time limiting behaviour (0.291) and the rate at which the limit is reached (−0.312). This functional form provides more realistic limits in time while preserving a zero intercept and defining a plateau for long residence times.