Fluidization and attrition of pyroclastic granular solids

Pyroclastic flows have been frequently addressed in the recent volcanological literature as far as the depositional mechanisms and the field features of the related deposits are concerned. Much less attention has been paid to phenomena that control the dynamic of pyroclastic flow motion. The peculiar feature of pyroclastic flows, hence focused, is the onset of an intense upflow of gas, whose velocity may be so large that a fluidized state may be established in the granular phase. The fluidization behaviour of granular synthetic mixtures has been studied in a broad range of conditions, mostly driven by the widespread use of fluidized beds in the process industry. However, experimental studies focused on fluidization of natural granular mixtures are lacking. The aim of this paper is the characterization of the fluidization behaviour of natural pyroclastic mixtures. In particular, mixing and segregation of granular mixtures of natural origin and fluidization-induced abrasion/fragmentation of pyroclastic particles have been investigated. A pyroclastic mixture extracted from a pumice fall deposit has been used as the starting material, as it is representative of the whole material of a plinian column immediately before collapse and emplacement of the pyroclastic flow deposits. Experimental tests have been also performed using the granular material extracted from pyroclastic flow deposits. Incipient and complete fluidization velocities, onset of particle segregation due to size and density polydispersity of the samples, elutriation of attrition-induced fine particulates and attrition-induced changes of particle size distributions have been characterized. When comparing the segregation propensity of the pumice fall and of the pyroclastic flow starting material, it is concluded that only the former exhibits significant segregation. Results of elutriation and attrition experiments highlight the transient nature of particle attrition phenomena: generation of fines by attrition of relatively coarse granular solids is at a maximum in the early stage of fluidization and decays significantly thereafter.