In this study, methods based on ultrasonic attenuation and optical time-of-flight measurements are used simultaneously in determining both the fibres and fines mass fractions, respectively, of a cellulose pulp fibre suspension. The optical measurements are done by a laser radar and the acoustical measurements are based on ultrasonic attenuation measurements in a pulse-echo set-up. Two kinds of long-fibre fractions are studied, thermo-mechanical pulp and chemical softwood pulp. Fibre and fines mass fraction ranges are 0.25-1.0% and 0-0.75%, respectively. The results show that the fibres are the predominant source for absorption and scattering of ultrasonic waves and are thus mainly contributing to the attenuation of ultrasound in the pulp. It is also found that the fines are the predominant source for optical scattering and fines are thus mainly contributing to the propagation delay of the light pulse in the laser radar set-up. By combining the ultrasonic attenuation and the optical time-of-flight measurements, it is shown that the mass fraction of fines and the mass fraction of fibres in a pulp sample could be determined, respectively.
The problem of determining the fines content of pulp in cases where consistency is not known remains to be solved. We explored the hypothesis that this problem may be solved by studying shape changes in a laser pulse after it has travelled through the pulp.A matrix was constructed of pulp samples with consistencies varying from 0 to 1.5% by increments of 0.2% and fines contents varying from 0 to 50% by 10% increments. A streak camera was used to record three pulses simultaneously. The first was a reference pulse, which was used to calibrate the measurement pulses. The second was a pulse measured at an angle of 90° to the straight light path. The third was the straight path pulse.Different fines contents form their own lines on consistency–maximum power graphs and consistency–time of 50% power fall graphs. When transmitted power is plotted against time of 50% power fall the lines representing different fines contents cross each other. These results indicate that the fines content and consistency can be measured in some cases with a single measurement. Also, if water is added in a controlled manner, measurement of the lowering in consistency allows the original consistency and fines content to be determined.
The consistency of fibres and concentration of fines need to be controlled during the production process in the paper industry. In paper pulp, fibre lengths range from less than a millimetre to several millimetres whereas fines particles have sizes of a few tens of micrometres. Therefore, the two fractions have different properties of optical scattering and acoustic attenuation, i.e., fibres produce more forward optical scattering and acoustic attenuation, while fines produce larger and more homogeneous scattering but less acoustic attenuation. Based on these facts, we specifically develop a new method, a scattering photoacoustic technique, to measure the consistency of fibres and concentration of fines simultaneously. It employs near-infrared light (1064 nm wavelength) to produce three acoustic waves with MHz frequencies. One piezoelectric transducer detects these waves, which are used to measure optical scattering and acoustic attenuation of pulp samples. The results indicate that our current apparatus successfully discerned the pure fibre and fines samples. It also proved capable of extracting the consistencies of fines and fibres in the studied samples that consisted of mixtures of fibre and fines. Finally, the scattering photoacoustic technique has a potential ability in online measurement of fibre and fines consistencies in pulp suspensions.
An innovative backward-mode photoacoustic transducer was developed, consisting of an optical fibre, a composite absorber, piezoelectric film and high impedance preamplifier. By receiving scattering light from a turbid suspension, the transducer produces a photoacoustic source in it. This source emits two photoacoustic waves travelling in opposite directions. The waves' amplitudes relate to the optical scattering properties of the suspension, and the echo of a wave returning from the suspension carries information of acoustic attenuation. By assessing the optical scattering and acoustic attenuation, fraction consistencies in a two-fractional suspension can be determined if one fraction dominantly scatters light and the other mainly attenuates ultrasound. This technique is used in this paper to investigate paper pulp suspensions. Pulp consists of wood celluloses and wood fines (or extra-added fillers in some cases), where cellulose lengths range from a few sub-millimetres to millimetres and fines/filler sizes are a few tens of micrometres or smaller. Due to their different size and shape, celluloses and fines (or fillers) have different optical scattering and acoustic attenuation properties. Experimental results showed that the transducer can measure pulp consistency with good linearity at least in the range from 0.5% to 3%, and that it can distinguish pulp cellulose from fines or fillers (TiO 2 particles). Needless to say, this technique is also suitable for determining other suspensions in the food, pharmaceutical and mineral industries.
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