Attenuation ultrasonic

The attenuation of ultrasound by a slurry depends upon the particle size distribution and concentration of the solid phase. In order to separate these two variables it is necessary to carry out analyses at two different wavelengths, one of which is strongly dependent on concentration and the other on particle size distribution. The attenuation is also dependent on the spacing of the transmitter and receiver and other physical parameters in a predictable manner [3,4]. 

The first commercial ultrasonic on-line particle size analyzer was developed in the 1970 s and was based on the measurement of ultrasonic attenuation at two frequencies with an empirical model to predict particle size and concentration [5]. Instruments based on this patent are available as the Denver Autometrics PSM-100, 200, 300 and more recently 400. These are pre-calibrated for the selected mesh size (100, 200, 300 and 400) and the mesh read-out is proportional to the mass percentage less than this. These instruments can operate at extremely high concentrations, up to 60% by weight, and have found their widest application in mineral processing plants for improved grinding circuit control. 

A major problem in the early development of the Autometrics system was that traces of air could lead to substantial attenuation losses. The air must be stabilized or removed to allow accurate measurement of particle size. Removing the air with a device that utilizes a combination of centrifugal force and reduced pressure solved the problem. The need to remove air increases the cost of the overall system significantly and makes it an expensive instrument when compared with other instrumentation often installed in grinding circuits. Nevertheless, it appears to be perfectly compatible with other approaches when its inherent reliability and longterm stability as an accurate size analyzer is taken into account. Several articles have been written describing applications of the PSM systems [6-9]. 

The limitations of the system 100 are (a) the percentage solids should be less than 60% by weight (b) the particle size distribution should be within the range 20% to 80% less than 270 mesh and (c) the slurry particles should not be magnetized. The PSM systems 200 and 400 are later instruments designed to overcome these limitations. 

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Condensed phase vibrational or vibronic lineshapes (vibronic transitions create vibrational excitations of electronic excited states) rarely provide infonnation about VER (see example C3.5.6.4). Experimental measurements of VER need much more than just the vibrational spectmm. The earliest VER measurements in condensed phases were ultrasonic attenuation studies of liquids [15], which provided an overall relaxation time for slowly (>10 ns) relaxing small molecule liquids. 

F Babick, F Hinze, S Ripperger. Dependence of ultrasonic attenuation on the material properties. Colloids Surfaces A Physicochem Eng Aspects 172 33-46, 2000. 

Fig. 2.6 Temperature dependence of ultrasonic attenuation at fixed frequency

Hinrichs and Thuen [28] used ultrasonic attenuation to determine the proper time for pressure application during an otherwise traditional pre-established cure cycle. Because dielectric is an electrical property, it is influenced by moisture content and temperature as well as viscosity, so it may vary quantitatively. Ultrasonic measurements are also affected by other parameters (i.e., void content), but they are a mechanical measurement rather than an electric one. The ultrasonic sensors used by Hinrichs unfortunately were less reliable than the dielectric sensors. 

Papadakis, E. P. (1968). Ultrasonic attenuation caused by scattering in poly-crystalline media. In Physical acoustics IV-B (ed. W. P. Mason), pp. 269-328. Academic Press, New York. [78,198]... 

Experiments to distinguish between these two possibilities have often involved measurements of ultrasonic attenuation (ref. 5,9,31,32). The popularity of this approach derives in part from the fact that small impurities in liquids, such as suspended particles, have negligible influence on attenuation in comparison with even a very small concentration of microbubbles (ref. 9). (Microbubbles, in contrast to solid particles, appreciably increase the compressibility of a liquid, introducing forms of viscous losses and nonreversible energy exchanges that do not exist in the case of solid particles.) It is therefore of considerable interest that all fresh tap water samples measured by Turner (ref. 9) showed substantial and persistent abnormal (ultrasonic) attenuation, amounting to a minimum of 44% over that of distilled water it was concluded that this result stemmed from the presence of stabilized micron-sized bubbles. 

Sirotyuk (ref. 25) found that the complete removal of solid particles from a sample of water increased the tensile strength by at most 30 percent, indicating that most of the gas nuclei present in high purity water are not associated with solid particles. Bernd (ref. 15,16) observed that gas phases stabilized in crevices are not usually truly stable, but instead tend to dissolve slowly. This instability is due to imperfections in the geometry of the liquid/gas interface, which is almost never exactly flat (ref. 114). Medwin (ref. 31,32) attributed the excess ultrasonic attenuation and backscatter measured in his ocean experiments to free microbubbles rather than to particulate bodies this distinction was based on the fact that marine microbubbles in resonance, but prior to ultrasonic cavitation (ref. 4), have acoustical scattering and absorption cross sections that are several orders of magnitude greater than those of particulate bodies (see Section 1.1.2). 

Stone DE, Clark B. Ultrasonic attenuation as a measure of void eontent in carbon-fibre Reinforeed Plastics. Non Destructive Testing 1975 July 137-45. 

Jeong H. Effects of Voids on the Meehanieal Strength Ultrasonic attenuation of laminated eomposites. J. Comp. Mater. 1997 31(3) 276-92. 

M. O Donnell, E. T. Jaynes, J. G. Miller, Kramers-Kronig Relationsliips Between Ultrasonic Attenuation and Phase Velocity , J. Acoust. Soc. Am., 1981, 69,696. 

Figure 9.12. Ultrasonic attenuation (A) and velocity (B) spectra for samples measured at temperatures up to 15CPC. (a) 0.81% insoluble particles suspension ( ) 4.4% siiica particies and ( ) distiiied water. The decreased attenuation of the industriai particies above 70°C is due to a change in soiubiiity. (Reproduced with permission of Eisevier, Ref [53].)...

Obtaining these desired output parameters involves two steps. In the first step, tests are conducted on the target system in order to obtain a set of measured values for macroscopic properties such as temperature, pH, ultrasonic attenuation, etc. In the second, the previously measured data are analysed in order to compute the desired microscopic properties. Such analysis requires three tools a model dispersion, a prediction theory and an analysis engine. 

In-line measurements of particle size distributions are essential in order to maximize production capacity and product quality. Ultrasonic attenuation is emerging as a technique, with capabilities beyond those of light scattering. In addition to the needs of industry for compact, robust instrumentation, this method is capable of operating at high concentrations, thus eliminating the need for an expensive dilution step, which may alter the very properties one wishes to measure [225,226]. 

Sympatec Opus The OPUS (Figure 10.15) is based on ultrasonic attenuation in the regime where attenuation is proportional to the total projected area concentration of the particles and the attenuation is governed by the Lambert-Beer law. For this to be valid, the particles must be considerably larger than the wavelength of the incident radiation. 

CSIRO Minerals has developed a particle size analyzer (UltraPS) based on ultrasonic attenuation and velocity spectrometry for particle size determination [269]. A gamma-ray transmission gauge corrects for variations in the density of the slurry. UltraPS is applicable to the measurement of particles in the size range 0.1 to 1000 pm in highly concentrated slurries without dilution. The method involves making measurements of the transit time (and hence velocity) and amplitude (attenuation) of pulsed multiple frequency ultrasonic waves that have passed through a concentrated slurry. From the measured ultrasonic velocity and attenuation particle size can be inferred either by using mathematical inversion techniques to provide a full size distribution or by correlation of the data with particle size cut points determined by laboratory analyses to provide a calibration equation. 

Aeration in a slurry cause large ultrasonic attenuations making accurate particle size measurement impossible. Aerated slurry can be fed to a holding tank and ultrasonic measurements made once the aeration level has dropped low enough to allow meaningful measurements. Clearance times were found to be short enough for a baffled tank passive de-aeration system to be feasible. 

For composite samples the method discriminated separate Ti02 and CaC03 components and accurately determined their proportions. In addition, in combination with ultrasonic attenuation measurements, the size fractions of iron ore slurries below 10 and 30 pm were determined to within 1.3% and 1.0% respectively when compared with laser diffraction measurements [272]. According to Coghill et. al. velocity measurements are complementary to attenuation methods but better suited to the finer size fractions. A description of the analyzer and the results of plant feasibility tests and on-line installation has been presented [273]. 

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