FI58215B - REFERENCE FOR A PARTICULAR ANALYSIS OF A PARTICULAR ANALYZER I AND UPPER SAMPLING ANALYZER FOR A REFERENCE OF A PARTICULAR ANALYZER - Google Patents

Description

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Patmtti · And the Registry Board (44) MhtMlulp-κχ,]. to date

Patant- oc reg regttttyrelMn Antdktn utltjd oeh utl.skrlfMn publtc * r »d 29.08.80 (32) (33) (31) Pyydetty« tuolkwi —Begird prlorltet (71) Outokumpu Oy, Outokumpu, FI; Töölönkatu U, 00100 Helsinki 10, Finland-Finland (FI) (72) Seppo Juhani Uusitalo, Espoo, Georg Christian von Alithan, Espoo,

Tor Sven Andersson, Luoma, Väinö Armas Paukku, Espoo, Lasse Sakari Kähärä, Espoo, Erkki Sakari Kiuru, Espoo, Finland-Finland (Fl) (7M Berggren Oy Ab (5 ^) Method for determining the average grain size of a sludge and analyzer for performing the method - For the purpose of testing the genome-specific particulate matter and the analysis of the same analyzer for the purpose of the analysis

The present invention relates to a method for determining the average grain size of a slurry, in which at least one ultrasonic beam having a certain frequency is sent to the slurry, attenuated ultrasonic radiation passing through the slurry is detected and a first signal corresponding to its intensity is generated, and another signal. The invention also relates to an analyzer for carrying out the method.

Ultrasound is attenuated in the slurry by viscosity losses and scattering. The attenuation depends mainly on the size and volume fraction of the sludge particles and the frequency used. The specific gravity of the particles also has a small effect. In addition to attenuation, the scattering portion can be directly measured by placing the receiver outside the transmitter-Tur beam. By combining the scattering and attenuation data, the effect of sludge density can in theory be completely eliminated and an average grain size can be obtained, which with different volume proportions emphasizes different size classes. If the shape of the grain size distribution is approximately retained, the result can be further converted to a residual percentage of the screen.

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Ultrasonic attenuation and associated scattering have also been studied quite extensively for grain size measurement. Direct scatter measurements are hardly present »but the theory has still been discussed quite extensively in the literature.

Finnish patent application 3274/72 uses two ultrasonic frequencies, which are selected according to the criteria given in the description so that both the grain size and the solids content of the sludge are determined. Although in some cases a different frequency is chosen so that the defining mechanism of the attenuation is scattering, it is characteristic of this known method that it always detects and further processes only the transmitted radiation intensity and not the scattered ultrasonic radiation.

According to the present invention, it has been found that the average grain size can be determined more accurately and independently of the rather wide variation in the solids content of the sludge by measuring and utilizing the radiation scattered in the chosen direction as well. The method according to the invention is characterized by what is set out in the appended claim 1, while the main features of the grain size analyzer according to the invention are clear from claim 4.

Before describing the method according to the invention and the analyzer applying it in more detail, it may be appropriate to take a closer look at the theory underlying the invention, i.e. ultrasonic absorption and scattering in the slurry.

Scattering and attenuation of planar waves

The amplitude of the ultrasound is attenuated in the slurry of the formula

According to Aa = A0e (1), where x is the distance traveled as sound sludge. The attenuation coefficient a is at reasonable sludge densities a = Pua, (2) where P = solids content (volume fraction) of the sludge = Specific attenuation.

a

The intensity is accordingly

Ia = I0e'2c, x (3) 3,58215

On the other hand, the attenuation coefficient can be written in the form α = av + l / ia ”1 + a” 1), (4) where a = P 18Y <* + lii? 6 ~ _U-ϊίΖΥ-- 8.68 dB / cm 81 (1 + γ) ^ + γ2 / 9 + 4γ (SG + 1 / 2J / = viscose loss term (5) “s“! <?> 4 a3 [gK + 3gs (1- γ (2SG + 1) ^ j 8'68 = scatter loss term (6) ad = P (w / v) 1/3 / 4ua 8.68 dB / cm (7) = diffraction loss term w = 2 tr f γ = a / w / 2vk f = frequency / Hz 2 v = kinematic viscosity of water / cm / sv = speed of sound / cm / s SG = specific gravity of particles = a = radius of particles / cm

K-K

gK ~ K1, K0 = Compressibility of particles and water _ _ SG-1 9s 2SG + 1 *

As a function of the grain size, the attenuation is shown by the two-peak wooden curve according to Fig. 1.

In the form of scattering, the intensity exiting the beam per travel unit is proportional to the scattering loss term a and the intensity of the beam ti.in. The scattered intensity is unevenly distributed over different scattering angles. It can be written in the form I = 2ka I_e ”2ax s s O, (8) 4 58215 where k is the geometry-dependent factor and x is the total distance traveled by the sound before and after scattering. The expression should be integrated over the scattering volume because scattering from different locations has different lengths of distance. At this point, however, scattering is assumed to occur in the center of the measurement chamber, where the beams of the transmitter and receiver intersect.

The transmitted and scattered radiation have then traveled the same distance in the test arrangement used. Their intensity ratio is Ϊ- = 2kas. (9) a

On the other hand, α = 2x ln T ~ 'di where Iq is the intensity obtained with water alone. Dividing these in half gives the test variable 2 = k '^ = Ialn (Vla) (11) which no longer depends on the sludge density, but only on the grain size at a certain frequency, geometry, etc.

The grain size dependence of the characteristic Z is as shown in Figure 2. The dependence is very strong at a sufficiently low frequency. On the other hand, the frequency must not be unnecessarily low, because then the scattering intensity drops too much and the measurement becomes inaccurate. Scattering and absorption can also be measured at different frequencies.

Effect of grain size distribution

The granules have a certain size distribution in the slurry. When calculating damping, etc., the corresponding quantities of different grain sizes must be added together by weighting the grain size distribution. A log normal distribution with a density function of π f ln ((M - Mn) (il - Mn) / (Moo - M) M) Ί 2 f (M) = —J., - exp - --O-- is often used. --- ^ 1ησ L VTlna (12) 5 58215 where σ = standard deviation of the geometry M = diameter of the granules = 2a Mq = minimum grain size M = maximum grain size

OO

M = geometric mean of the grain size.

The specific damping with a grain size distribution is thus Moo M - M

a = / f (M) aad In - ^, (13) M0 and scattering are obtained in a corresponding manner. The general features of the damping curve remain, but the curve tapers and the minimum position shifts slightly. The grain size distribution (12) holds quite well with the measured sieve analyzes with suitable parameters.

The characteristic variable (11) is quite linear as a function of the residual percentage of the screen over a large range, as shown in Fig. 3. It is not very sensitive even to the shape of the distribution? a two-peak distribution, M = 50 ym and 100 ym, caused less than 2% error.

accuracy Estimate

Theoretically, the effect of the variations of different factors on the results given by the device has been studied and the optimal frequencies of one and two frequency cases at which the errors become the smallest have been determined. The results are shown in the following table.

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Screen Analysis Relative Error%

Large Setpoint Variation 1 freq. 2 freq.

Pp 4.6 0.2 1.4 1.4 T 15 ° C 3 ° C 1.2 1.2 f 2.4 MHz 5% 2.3 fx 1.2 MHz 5% 6.0 f2 1.8 MHz 5% 10

Aal (Aa) 1% 4.6 0.2

Aq1 (A0) 1% 0.7 0.2

Aa2 1% 1.5

As 2 (As) 1% 4.0 1.7 and 2.5 0.5 2.4 3.2

The specific gravity p and temperature T of the granules have the same effect in different cases. The two-frequency method seems to be more sensitive to frequency, but the frequency is an accurately reproducible quantity, 5% describes the variation of the resonant frequencies of different crystals at the same nominal list frequency. When the crystal is replaced, a new calibration is obviously necessary.

For the amplitude measurement error, the two-frequency method shows more sensitive. When the range of amplitudes to be measured is n.

10 times (a factor of 100 in intensity), the amplitude measurement error will apparently be a factor limiting accuracy, which would favor the use of two frequencies. There is not much difference in the variance σ of the distribution. Its variations set a limit on the achievable accuracy of about 3% at the residual percentage.

The plane wave analysis corresponds to the case that the transmitter and receiver are far from the scattering area (compared to the sensor size of 15 mm and the wavelength of about 1 mm) and that the scattering area is small. This is not true in practice, but the difference is included in the geometric factor k 'in formula (11). The factor is determined from the test results. As a function of the grain size, curves that were well compatible with the experiments were obtained at that time, and this is the most important thing in practice. However, as a function of frequency, compatibility was worse. This is natural, because the geometric factor depends on the wavelength and thus the frequency.

Scattering takes place over the entire area of the measuring chamber. The area is delimited by the use of finite length transmitter pulses and a reception window. The widths of the radiation beams of the transmitter and receiver also delimit the area. Finally, time discrimination is taken into account, which excludes scattering that is too far from the assumed range.

The above theory has thus been applied to the present invention. As a function of the grain size, the characteristic quantities describing scattering and absorption behaved in accordance with the theory in the measurements. Achieving concordance as a function of frequency required consideration of the finite geometry of the measurement space and the temporal length of the transmitter pulses and the receive window. A small addition was made to the expressions to account for the effect of the specific gravity and compressibility of the granules on scattering. Thereafter, the concordance of theory and experiments was good for all successful measurements. However, there are relatively few reliable test results so far.

The invention will now be described in more detail by way of example and with reference to the accompanying drawings, in which Figure 1 shows the specific attenuation as a function of grain size, Figure 2 shows the dependence of test quantity on grain diameter, Figure 3 shows comparison of test quantity and screen residual percentage for two different sieves, S = 75 μm and S = 150 μm Fig. 4 schematically shows a test apparatus for applying the invention, Fig. 5 shows the structure of an ultrasonic sensor, Fig. 6 shows a geometry of a measuring cell, Fig. 7 schematically shows a schematic diagram of an ultrasonic grain size analyzer according to the invention, Fig. 8 shows a block diagram of a grain size analyzer block diagram, Fig. 11 shows a block diagram of the calculation part of the analyzer, and Figs. 12 to 14 show the measured values obtained with the analyzer according to the invention and the curves drawn therefrom.

Of the images listed above, Figures 1-3 have been described previously.

Figure 4 thus shows 8 58215 test apparatus for applying the invention. Reference numerals 1a and Ib denote alternative filling vessels from which the sludge to be examined is fed to the pipe loop 2. The sludge is circulated by a pump 3, and a measuring cell 4 with sensors 5 is placed in the vertical part of the loop. The sensors 5 are further connected to the analyzer electronics 6.

Figure 5 shows an ultrasonic sensor specially designed for this analyzer. The number 7 means the metal body of the sensor, the number 8 an ultrasonic crystal and the number 9 a titanium plate glued to the end of the sensor to protect the crystal. The connecting wire 10 extends through the sensor, and the interior of the body is filled with araldite 11. The sensors can be placed in the measuring cell, for example as shown in Fig. 6. It has a sensor acting as a transmitter marked 5t, the ultrasonic radiation that has passed through it, some of which is absorbed into the sludge, the measuring sensor is 5 = and the scattered radiation

CL

the measuring sensor is 5g. To eliminate annoying echoes, the inside of the cell is lined with rubber 12.

Figure 7 shows a schematic diagram of an ultrasonic grain size analyzer. There, the sensors connected to the transmitters 13 and 14 are transmitter sensors, and in addition, the cell has two receiving absorption sensors and one scattering sensor. I are the intensities, k and 1 are the constants defining the calibration lines, p is the slurry density, φ is the volume fraction of the granules and G is the quantity describing the sieve analysis. The ringed quantities have to be determined by means of calibration measurements, i.e. using fractions of known grain size suspended in the slurry.

Figure 7 corresponding to the block diagram shown in Figure 8. It is indicated by a master oscillator 15 which supplies the 1 MHz frequency signal, and transmitters 13 and 14 and the timing circuit 16, which operates the transmission side and the reception side to synchronize. Numeral 17 denotes a multiplex device which receives the signal frequencies and f2 and from which these are passed on to the synchronous detector 18. The signal of the scattered ultrasonic receiving sensor is also fed to the synchronous detector 19. The signals from the detectors 18 and 19 are passed to the calculation unit 20.

9 58215 From the transmitters, pulses with a length of approx. 20 ps and a frequency of 0.5-4 MHz with a repetition frequency of approx. 1 kHz are received alternately at each sensor. In the receiver section, these high-frequency pulses are expressed synchronously, and the voltages corresponding to their amplitudes are formed in the calculation section as data proportional to the residual percentage and the slurry density.

Details of the transmitter section are shown in Figure 9. To ensure sufficient frequency stability, the transmitters are synchronized to a 1 MHz main oscillator. From this fundamental frequency, all necessary frequencies can be generated by the synthesizer pair 21a, 21b. Switches Sg have four frequency ranges to choose from: 250-500 kHz, 500-1000 kHz, 1-2 MHz and 2-4 MHz. Within each range, fine tuning is possible with the switch set SA · The size of the smallest frequency step varies according to the frequency range, being 3.125 kHz in the lowest range and 25 kHz in the highest range.

The signals from the synthesizers 21a and 21b are compared in a phase-locked loop to the VCU voltages (blocks 22a and 22b), the frequency and phase difference are corrected, and the signal frequency is divided in circuits 23a and 23b by N. The gates 24a and 24b are connected to ua sensors. to power amplifiers 25a and 25b. The gates are controlled by the timing circuit 16 via terminals 26a and 26b.

The power amplifiers are able to supply a voltage of approx. 40 V to the load,

PP

which in practice has proved sufficient. Due to the structure of the sensors, their impedances are very reactive even at their resonant frequency. Therefore, when supply line lengths longer than 1 m are used, 50 Ω: η terminating resistors must be installed inside the transmitter sensors so that the resulting standing waves do not damage the terminating stages.

Amplitude stabilization can be easily arranged when the amplifier is constructed with a so-called kytkinvahvistimeksi. In this case, according to the input signal, either a positive or a negative supply voltage is connected to the load, whereby the amplitude stabilization can be performed as a direct voltage control.

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When the control signal of the transmitter sensors is a square wave, it also contains the odd harmonics of the fundamental frequency. Experiments have shown that in pure water, depending on the type of sensor, the transmission of the 3rd harmonic is at worst about 10% of the transmission of the fundamental wave. However, the amplitude of the 3rd harmonic is only 33% of the fundamental wave, so no more than about 3% of the received signal is from the third harmonic. In sludge measurement, this value continues to decrease significantly.

The receiver part is as shown in Fig. 10. Because the received ultrasonic signals, especially scattering, are random in nature, special attention has been paid to detection. In the experiments performed, it was found that the measurement results are not significantly affected by whether the intensities of the signals are measured as rms values or as arithmetic time averages. Based on this observation, the structure of the receiver could be simplified. The preamplifier 27, the rectifier 28 and the synchronous detector 18 form one receiver channel, of which the whole receiver includes three: two for scattering and absorption measurement at ff and one for absorption measurement at ff. Since the frequencies and f2 are transmitted as pulses at different times, the absorption signals A 1 and A 1 can be measured by the same rectifier 28 by multiplexing the signals with a multiplexer 17.

Timer 16 is also controlled by a 1 MHz main oscillator. With the switch sets Sc and SD, the repetition frequency of the pulses is adjusted to suit and the time position of the measurement window is adjusted to the synchronous detectors 18. It has been found that the interval between successive transmitter pulses must be at least 500 ys. The transmitter pulse length is set to 20 ys. A shorter pulse cannot be used because receiver filtering would integrate the pulse height too much. A longer transmission pulse is out of the question, as it would in turn cause interference at the receiver due to the relatively short (about 60 ys) travel time of the signal.

The test variable Z and the slurry density S are constructed with the unit of calculation shown in formula 11. It has two dividers 29, 30 and one log ratio module 31, as well as a square former 32. During the calibration with water, the A 1 potentiometer is adjusted. The frequencies are chosen so that a is a quantity approximately proportional to the slurry density. The percentage of residue is obtained from the test variable Z.

11 5821 5 by approximating it to a line whose slope and origin are adjustable. The same procedure is used to scale the sludge density output.

The most important factor influencing the accuracy of the calculation part is the sludge density. According to the specifications, the sludge density changes at worst in a ratio of 1: 3.

In practice, the divider modules achieve an accuracy of 0.1% of the full output value, and the log ratio module achieves an accuracy of 0.5% reduced to the input signal. Consider the effect of sludge density through these errors: - Grain size 40 et al

Aal / AQ1 max value is approx. 0.75 A _ / A - range is approx. 2 S 2 3.2 a max error is 0.4% sa "0.9% total error 1.3% - Grain size 100 ym The value of A ^ / Aq ^ max is about 0.6 A »/ A - the range is about 2 s2 a2 ctg max error is 0.4% a" 0.5% total error 0.9%

It is stated that the calculation part meets the given 2% requirement. measurement Examples

Ultrasonic absorption and scattering measurements related to the development of the ultrasonic grain size analyzer with both water and sludge have been performed using the sludge recycling system described above. Since a proper deaerator has not yet been used, the air has been allowed to leave itself through an open container. Such deaeration requires about half an hour.

Sensors as shown in Figure 5 were used with a glued titanium plate to protect the crystal. 12 582 1 5 transducers operating at different frequencies were used, because the two-frequency method considerably improves the resolution of the characteristic quantity (Z) characterizing the grain size. Figures 12 and 13 show the values of the characteristic quantity (Z) calculated on the basis of the measurement results.

The result according to Figure 12 was obtained using frequencies = 0.95 MHz and f2 = 1/9 MHz, in Figure 13 the frequencies = 1.16 MHz and f2 = 1/9 MHz. The values of the theoretical plots have been calculated by assuming that the granules of the test material are of the same size. The experimental results have been adapted to the theoretical ones by setting the value of the characteristic value given by the experiments corresponding to the volume fraction of the 2% slurry density of the middle fraction to coincide with the theoretical single-grain size plot at 89 μm.

When performing the measurements of Figures 12 and 13, the electronics of the analyzer were still deficient, especially with respect to the scatter measurement. Figure 14 shows the measurement result obtained using improved electronics and new sensors. These sensors had a stainless steel plate to protect the crystal, which was soldered to the crystal with a special alloy. The frequencies used were = 1.3375 MHz and f2 = 1.8625 MHz. The fitting of the experimental and theoretical values took place in the same way as in Figures 12 and 13.

Claims (7)

A method for determining the average particle size of a slurry, wherein the slurry is applied to at least one ultrasonic beam of a certain frequency, the attenuated ultrasonic radiation penetrating the slurry is detected and a first signal corresponding to its intensity (I) is generated, the ultrasonic radiation scattered in the slurry in a selected direction and a second signal corresponding to the intensity of the scattered radiation (Ig) is formed, characterized by the scattering attenuation factor and by the penetrated and the intensity (IQ) obtained with pure water is determined by the total attenuation factor, and by means of the spread attenuation factor, the grain size or a similar descriptive quantity is determined, whereby in order to eliminate the slurry density, the delay between the spread attenuation factor and the total attenuation factor is formed. 15 5821 5 2. A method according to claim 1, characterized in that two signals with different frequency (f 1, f 2) are used to operate signal transducers and measure with one frequency (f 2) the permeated damped radiation. Method according to claim 2, characterized in that with the second frequency (f2), both the penetrated attenuated radiation and the scattered radiation are measured. The particle size analyzer for performing the method of claim 1, which analyzer comprises at least one ultrasonic transmitter (13, 14) adapted to transmit an ultrasonic cluster of a specified frequency (f 1) into the slurry to be analyzed, at least one ultrasonic transmitter. for receiving the penetrated attenuated ultrasonic radiation, and detector means (18) for providing a first signal corresponding to the intensity (I) of the received ultrasonic radiation, characterized in that it has at least one ultrasonic transmitter for receiving scattered ultrasonic radiation. detector means (19) for forming a second signal corresponding to the intensity (I) of the scattered ultrasonic radiation, and circuit means (20) for comparing S of the first and second signals with each other and forming a particle size descriptive magnitude on the basis of the comparison result . Analyzer according to claim 4, characterized in that it comprises two at different frequencies (f, functioning transmitters (13, 14), which via sensors in turn transmit ultrasonic radiation in the slurry, two synchronously functioning detectors (18, 19), which is connected to the receiver encoder and forms a first signal cl corresponding to the intensity (I) of the transmitted radiation and a second signal corresponding to the intensity of the scattered radiation (Ig), whereby a common main oscillator (15) is arranged for the transmitter and receiver side. a timing circuit (16) synchronizes the transmitters (13, 14) with the detectors (18, 19). 6. An analyzer according to claim 5, characterized in that it comprises, for receiving the penetrated radiation, two sensors having a different frequency (f ^, f ^) operating sensors, which are connected to a transmitter via a multiplex device (17).