Pumice attrition in an air-jet

doi:10.1016/j.powtec.2016.11.051

Powder Technology

Volume 308, 15 February 2017, Pages 298-305

https://doi.org/10.1016/j.powtec.2016.11.051 Get rights and content

Highlights

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Attrition test on pumice in an air-jet (ASTM D5757–00) produce substantial fines.
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Residence time and feed grain size control rates and amount of fines production.
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Wet collection methods recover total grain size distributions (TGSDs) of run-products.
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Transient parent-daughter GSDs constrain an attrition model for pumice.
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A new fine production rate equation is developed with a long residence limit.

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 (< 2 mm) 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 (~ 15 min) 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 500 μm vs. highly efficient attrition for a 250 μm 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: $\frac{m_{fines}}{m_{bed}^{0}} = 0.291 (1 - e^{- 0.312 t})$

where m⁰_bed 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.

Graphical abstract

Introduction

Particle attrition is a process that operates in a diverse range of engineering and natural environments to cause particle size reduction, as well as, reshaping and resurfacing of particles. In the engineering sciences, particle attrition studies are commonly experimental in nature and concern the mechanisms, rates, and consequences of attrition in fluidized and conveyed systems. The experiments involve diverse materials such as: fluid cracking catalysts [1], limestone particles [2], and CO₂ sorbent pellets [3], [4], tested under conditions relevant to the engineering environment. Attrition is also widespread in geological processes including sediment transport (e.g., stream beds, beach sand), volcanic eruption (e.g., xenolith milling), and glaciation (e.g., till deposits). There are, however, few experimental studies of attrition in geologically relevant systems or on geological materials. Exceptions include, but are not limited to: secondary fragmentation of crystal rich ash [5]; rounding of pumice clasts during transport in pyroclastic density currents [6]; wear of kimberlitic minerals [7], [8], [9], [10]; milling of lithic material within volcanic conduits [11] and abrasion of geological materials by eolian action [12].

Pumice is a naturally occurring resource produced through explosive volcanic eruptions. It is commonly defined as a highly vesicular silicic to mafic glass foam, having a bulk density less than water (i.e. floatable) [13], [14]. Pumice represents an interesting material because it has unique properties: high vesicularity, low density, and a contiguous glass (i.e. not crystalline) framework. This porous volcanic material is of interest to both engineering and geological sciences. Some existing uses of pumice in industry include: a natural pozzolan for cement [15]; abrasives in skin products and dentistry; water filtration [16]; a chemical or catalyst carrier in fluidization systems [17], [18] and as an inert fluidizing solid [19]. These latter applications are of particular relevance to this study; if pumice is to be used in a fluidized system it is important to understand how grain size may evolve with residence time in the fluidized apparatus. On its own, pumice has low strength due to its highly vesicular nature and is easily broken down by crushing and fracturing of the thin, typically interconnected, glass bubble walls. Its low density has made it an ideal aggregate in cement to reduce the density of concrete; it does so without reducing the strength of the concrete significantly.

Yet, despite its widespread industrial uses and its importance in geology, its susceptibility to attrition remains poorly known [20]. Pumice attrition has rarely been studied experimentally. Previous experimental work on pumice has shown the grain size reduction of pumice by ball milling [21], [22], [23] and the decrease in fines production with increased milling duration by rock tumbling [6], [24]. These experiments inform on attrition processes typically involving continual particle-particle contact whereas air-jet experiments feature much shorter durations of particle-particle contact. A small amount of experimental work involving fluidization of pumice [25], [26] has been done in volcanology with the aim of understanding: grain size distributions and sorting within natural deposits; the degree of particle segregation during flow; and elutriation of fine particles produced by attrition.

Here we present a suite of attrition experiments involving particles of pumice within an ASTM standard device providing a particle-laden jet. Our experiments are designed to further our understanding of how pumice (i.e. porous glassy material) undergoes grain size reduction in a gas jet and have relevance to fluidized beds using pumice as a catalyst support [27], [28], [29]. The experiments use well-characterized pumice particles having a known initial total grain size distributions (TGSDs) and bed mass are subjected to jet attrition for fixed amounts of time. We then collect the experimental run-products and process them for their TGSD and use the data to establish rates/mechanisms for pumice attrition and the evolution of grain size with residence time.

Section snippets

Review of attrition in a gas jet

Attrition processes comprise two primary mechanisms of particle size reduction: fragmentation and abrasion. Fragmentation refers to particle fracturing wherein the original particles (i.e. parent or mother particles), subjected to critical collisions are mechanically broken into smaller particles (i.e. daughter particles). Collisions causing fragmentation typically result from direct impact with other particles or a hard surface at, or above, a critical threshold velocity. Commonly the parent

Pumice samples from Mount Meager, British Columbia

The Mount Meager volcanic complex is a calc-alkaline stratovolcano complex situated approximately 150 km north of the city of Vancouver, in southwestern British Columbia and belongs to the northernmost extension of the Cascade Volcanic Arc [35]. The most recent eruption of the Mount Meager is dated to 2360 BP [36] and produced explosive and effusive dacite volcanic deposits including: pyroclastic fall deposits, pyroclastic flow deposits, and lava [37]. For this study, blocks (> 10 cm) of pumice

ASTM fines collector

Results of the nine experiments using the 250 μm grain size pumice show rapid and pronounced attrition (Fig. 2). The rate and extent of attrition are summarized in Fig. 2 as, both, the production of fines and the reduction in grain size of the starting material with the duration of the attrition experiment. The 250 μm starting material reduces from 20 g to 6.1 g in just 15 min (represented by solid triangles in Fig. 2). In a similar manner the mass of material ~ < 4 μm collected in the fines collection

Attrition model

Our experimental results clearly demonstrate the extreme vulnerability of pumice particles to mechanical attrition. The rate of production of fines during attrition is exponential and can be described by the Gwyn model (Eq. (1); Table 2). To a first order the Gwyn model with best-fit parameters K_a = 0.09402 h⁻^b and b = 0.3901 fits our data well (Fig. 7A). The Gwyn model assumes that particle attrition is achieved solely through abrasion and that fragmentation does not occur. In our experiments,

Summary

In this paper we adopted the methodology of the ASTM D5757-00 standard attrition test, with particular emphasis on the recovery of an entire grain size distribution from each experiment. By a thorough water flush collection, we were able to analyze the total grain size distribution produced, rather than just the mass recovered in the fines collection bin. This technique allows for further insights into the attrition method operating (abrasion or fragmentation). We found that for Mt. Meager

Acknowledgements

JKR is supported by the Natural Sciences and Engineering Research Council of Canada through the Discovery Grants and Discovery Accelerator Supplements programs. TJJ was funded by NERC NE/L0025901, part of the IAPETUS doctoral training partnership.

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Studies based on fluidized bed apparatuses are most widely reported (Arena et al., 1983; Chen et al., 2008; CHI RONE et al., 1992; Chirone et al., 1985; Jia et al., 2007; Maurer et al., 2016; Pis et al., 1991; Ray et al., 1987; Scala et al., 2000; Tomeczek and Mocek, 2007; Vleeskens and Roos, 1989; Bemrose and Bridgwater, 1987; Chourasia and Alappat, 2018). Some researchers have developed annular shear cell (Carr and Walker, 1968; Paramanathan and Bridgwater, 1983a) and air-jet apparatus (Xiao et al., 2011; Jones et al., 2017). Zhi Tang developed an experimental test method by combining static combustion and cold sieving (SCCS) (Zhi et al., 2001), in which coal is first burnt-out in a muffle furnace at a certain temperature, and the ash produced is collected and sieved by a sieve shaker with a preset shaking amplitude to simulate the attrition process.
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Pumice attrition in an air-jet

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Review of attrition in a gas jet

Pumice samples from Mount Meager, British Columbia

ASTM fines collector

Attrition model

Summary

Acknowledgements

Powder Technol.

Powder Technol.

J. Geochem. Explor.

Russ. Geol. Geophys.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Int. J. Miner. Process.

Phys. Chem. Earth

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

Powder Technol.

Powder Technol.

Powder Technol.

Chem. Eng. Sci.

Adv. Powder Technol.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

Attrition characteristics of fluid cracking catalysts

Ind. Eng. Chem.

J. Mater. Chem. A

Primary and secondary fragmentation of crystal bearing intermediate magma

J. Volcanol. Geotherm. Res.

Rounding of pumice clasts during transport: field measurements and laboratory studies

Bull. Volcanol.

Morphology and surface features of olivine in kimberlite: implications for ascent processes

Solid Earth

Experimental abrasion 4: eolian action

J. Geol.