Ultrafine-size range

More recently, the importance of silica fume particle size on toxicity has been noted. Specifically, particles of the ultrafine size range may be expected to have higher toxicity compared with particles of larger size. 

Concentration of particles in the air as well as particle size distributions can be considered either in terms of particle number or mass. In terms of number, the vast majority of airborne particles are in the ultrafine range. For example, in urban outdoor air where motor vehicle emissions are a dominant pollution source, over 80% of particulate matter in terms of number is in the ultrafine range [17]. Since outdoor particles contribute significantly to indoor particle concentrations, also in indoor air particle number concentration is usually dominated by the smallest particles. However, most of the mass of airborne particles is associated with large particles since the mass of ultrafine particles is often very small in comparison with the mass of larger particles. The particle surface area in turn is largest for particles somewhat above the ultrafine size range. 

Tr ash removal—Removal of extraneous foreign matter from a processed material. Essentially a form of scalping operation. Type of screen employed will depend on size range of processed material—coarse, fine, or ultrafine. 

Particle sizes are conventionally described in different units depending on the size range involved. Coarse particles are measured in inches or centimeters fine ones, in terms of sieve size very fine ones in microns or millimicrons. The sizes of ultrafine particles are sometimes specified in terms of their surface per unit mass, this being commonly expressed in square meters per gram. 

Particles with the lowest specific gravity are carried with the water towards the outside wall of the spiral. The spiral separates at its greatest efficiency when used in the size range of 10 to 200 mesh. Some particles will be recovered both above and below these size ranges, but occasionally, ultrafine and very coarse heavies will be lost in the tailings, as will be middlings or unliberated ore particles. The spiral will benefit, therefore, from the use of hydraulic classification as a feed preparation step. 

Scheibel, H.G. and J. Porstendorfer, Penetration Measurements for Tube and Screen Type Diffusion Battery in Ultrafine Particle Size Range, J. Aerosol Sci. 15 673-679 (1984). 

It has been found that the "unattached" fraction is an ultrafine particle aerosol with a size range of 0.5 to 3 nm. In order to initiate studies on the formation mechanism for these ultrafine particles, a series of experiments were made in the U.S. Bureau of Mines radon chamber. By introducing SO into the chamber, particles were produced with an ultrafine size distribution. It has been found that the particle formation mechanism is supressed by the presence of radical scavengers. These experiments suggest that radiolysis following the decay of Rn-222 gives rise to the observed aerosol and the properties of the resulting aerosol are dependent on the nature and the amount of reactive gas present. 

It has been found that the activity which is conventionally referred to as the "unattached" fraction is actually an ultrafine particle aerosol with a size range of 0.5 to 3 nm. The hydroxyl radical from water molecule radiolysis is a key element to the particle formation mechanism. By injecting different concentrations of S02 into the test chamber, a possible particle formation mechanism has been suggested as follows Oxidizable species such as S02 reacts promptly with hydroxyl radicals and form a condensed phase. These molecules coagulate and become ultrafine particles. 

Polymerization in microemulsions allows the synthesis of ultrafine latex particles in the size range of 5 to 50 nm with a narrow size distribution [33], The deposition of an ordered monolayer of such spheres is known to be increasingly difficult as the diameter of such particles decreases [34], Vigorous Brownian motion and capillary effects create a state of disorder in the system that is difficult... 

As we have seen in our earlier discussion of the size distribution of tropospheric particles, the chemical components are not generally distributed equally among all sizes but, rather, tend to be found in specific size ranges characteristic of their source. Generally, the smallest ultrafine particles are produced by homogeneous nucleation and hence tend to contain secondary species such as sulfate and likely organics (see Section A.2). Particles in the Aitken nuclei range are produced... 

In a similar study, Allen and co-workers (1996) determined the particle size distribution for 15 PAHs with molecular weights ranging from 178 (e.g., phenan-threne) to 300 (coronene) and associated with urban aerosols in Boston, Massachusetts. As for BaP in the winter (Venkataraman and Friedlander, 1994b), PAHs with MW >228 were primarily present in the fine aerosol fraction (Dp < 2 /Am). A study of 6-ring, MW 302 PAH at the same site showed bimodal distributions, with most of the mass in the 0.3- to 1.0-/zm particle size size range a smaller fraction was in the ultrafine mode particles (0.09-0.14 /xm) (Allen et al., 1998). For PAHs with MW 178—202, the compounds were approximately evenly distributed between the fine and coarse (D > 2 /am) fractions. Polycyclic aromatic hydrocarbons in size-segregated aerosols col-... 

Waldie 56, 57), in an attempt to prepare ultrafine powders from coarser materials, obtained spheroids of oxide powders in low-power RF torches. When silica powder (50-72 p.m) was injected into a 2.5-kW, 34-MHz argon plasma at 15 gm/hr, a 15% conversion to ultrafine particles (0.015-0.15 / m) and coarse spheroids were obtained. Ultrafine powders of barium oxide (50% <0.1 ju.m) and alumina spheroids were also prepared by this technique. When alumina was injected cocur-rently into a 3.5-kW, 10-MHz argon plasma 57), 48% spheroidization of a 180-250 /xm powder was obtained at a feed rate of 36 gm/hr. Waldie obtained better results by use of countercurrent particle flow similar to the technique used by Haiti 26). Up to 26% spheroidization of a 300-500 /xm powder was measured for an alumina feed rate of up to 140 gm/hr. It is evident from this work that countercurrent spheroidization can achieve not only higher yields of spheroids but also spheroidization of a larger size range of solid. 

Synthetic Fe oxides are usually in the micron or submicron size range. Ultrafine particles with dimensions in the nanometer range frequently require more specialized techniques than do the larger crystals. The two basic requirements for a monodisperse system must be met and in addition, coagulation and/or contact recrystallization of the primary particles must be prevented. 

On a mass basis, nano or ultrafine particles (i.e., <100nm) are considered to produce greater pulmonary toxicity when compared to fine-sized particles (i.e., size range from 100 nm to >3 pm) of identical composition. This conclusion has been derived based only on comparisons of two or three particle types. 

---