Particle size detection (2)
2023-04-28 15:11:57
Semi A2 by the microbalance (weighing 200 grams, minimum reading 0.1 mg) adapted balance, it is a gem blue seat, ruby and the like as the balance arm blade. 
The liquid medium and the suspension are filled in the two settling bottles, respectively. If there is a difference in density between them, the difference in buoyancy between the two sinkers suspended at the ends of the balance beam causes the balance to deflect. This deflection is detected by two photocells coupled to the differential end, resulting in an output signal proportional to the deflection of the balance arm.
This signal is amplified by the DC amplifier and returned to the feedback coil mounted on the balance arm.
A feedback coil is in the magnetic field of permanent magnets, and therefore, under the action of the electromagnetic force, the balance arm and returned to its original position.
Since the feedback current entering the feedback coil is proportional to the electromagnetic force, this current is recorded and the density change can be recorded and indicated.
(2) Recorder
The input to this instrument is the output of the density difference measuring amplifier. On the horizontal axis of the paper, the particle size distribution of the particles is directly recorded as a percentage. The vertical axis on the paper indicates the radius of the particles. The instrument has an automatic advancement mechanism that drives the recording paper to always measure simultaneously.
The block diagram of this part is shown as 6.
(3) Ascending and descending mechanical systems
Measuring the difference in density between the liquid medium at each position and the suspension placed separately for a certain period of time, two heavy balls will be hung down at the ends of the balance arm, sinking them into the settling bottle, and moving the settling bottle up and down , relative movement with the heavy ball in the bottle.
The drive uses a synchronous motor and the settling bottle moves at a constant speed. The microswitch relay is used to detect position and control up and down motion, and has an indicator light to indicate its position.
A block diagram of this system is shown in Figure 7.
Figure 6 recorder block diagram
Figure 7 Block diagram of the rise and fall mechanism
3. Application characteristics
The application characteristics of this instrument are as follows:
(1) This instrument is used to determine the particle size distribution of fine particles. According to Stokes' sedimentation method, the principle is simple and the performance is good.
(2) It is a unique method (specific gravity balance method) for measuring, especially for fine particles.
In addition, a 3A2 semi-microbalance (accuracy 0.02 mg) was used as the balance to obtain a very stable output (signal).
(3) The cumulative particle size distribution curve is directly recorded automatically, and the particle size distribution is immediately visible, and the advantages are outstanding.
(4) No need for data analysis at all, which means that there will be no errors due to the operator, saving time and effort.
(5) Sample preparation is simple because it is made into a suspension in the settling bottle by the operator as in the conventional method.
(6) The measurement time of one sample is very short, about 30 seconds (except for the settling time). If the suspension is prepared in advance in many settling bottles, the measurement time can be shortened, thus greatly increasing the measuring capacity of the instrument. [next]
(7) In a conventional apparatus, the particle size of the particles is measured under a constant temperature system. This instrument is easy to control temperature because the settling bottle can be placed outside the unit during settling.
(8) The measuring method of this instrument is based on the electro-magnetic balance method, which has a high electrical behavior. At the same time, since the deflection is small, this does not interfere with the sedimentation of the particles in the suspension.
(9) Photoelectric elements are used to detect the deflection of the balance, so some combustible samples can also be measured.
(10) The recording uses a high-efficiency automatic balance recorder to continuously record the density distribution. The instrument also has an automatic mechanism that can start and stop the measurement of the particle diameter, which is convenient for continuous measurement.
Third, photoelectric scanning particle size analyzer
Figure 8 shows the photoelectric scanning particle size analyzer produced by Dandong in China.
Figure 8 Schematic diagram of photoelectric scanning particle size analyzer
The beam of this analyzer passes through a diaphragm of 15 to 20 mm in width and 1 to 1.5 mm in height to form a sheet-like light, which is about 150 mm high, 12 mm thick and 40 mm wide. The material to be analyzed is micro-injected into the water-filled box, fully agitated to completely suspend it, stand still and start measurement. The particles settle in the box, the incident light power is P 0 , and is reduced to P by the sedimentation box, and is sent to the photoelectric element through the exit light column to be converted into an electrical signal, generally a voltage signal V, which is recorded on the recording paper by using a recording pen. Form a t-V curve.
In order to speed up the analysis process, the beam and the settling box can be moved relative to each other. If the beam does not move and moves the settling box down at a constant speed, the moving time reflects the shortened settling distance. As the settling distance is shortened, the analysis time is also shortened. If the beam is moved, it constitutes a photoelectric scan.
According to Lanbert-Beer's law, when light passes through a colored solution, the radiant power of the light will decay in a logarithmic relationship, ie
P 0
1g=----=aLC
P
(6)
Where P 0 , P —— the radiation power of the light injected into the solution and from the solution (radiation power – the energy transmitted by the light wave per unit time);
L - the thickness of the solution (the length through which the light passes);
C - the concentration of the solution;
a —— the absorption rate of light by the coloring matter in the solution, which depends on the wavelength (frequency) of the light and the material properties.
When light passes through the suspension, its energy is absorbed by the suspended particles, that is, it is to be extinction. Therefore, the extinction method cannot determine the particle size composition of the transparent particles, and the amount of matting is proportional to the concentration of the suspension, and is related to the particle size and composition of the suspended matter.
From the figure, the light with a radiant power of P becomes P-ΔP after passing through the suspension of ΔL thickness, the remaining power is: A(P-ΔP), and the power lost is AΔP=P is the total area of ​​the particles irradiated by light. .
n n
A=K Σ k i N i d i 2 =k Σ k i A ΔLCn i d i 2
i=1 i=1
(7)
Where k i is the coefficient associated with the particle size;
d i — grain size;
n — the number of levels; [next]
n i — the number of particles having a particle size d i per gram of suspension;
K - coefficient related to particle shape, direction, etc.
from that we get
â–³P n
----K △L Σ n i d i 2 k i
P i=1
(8)
Integral (8), get
P 0 n
Ln-----=KLC Σ k i n i d i 2
P ′ i=1
(9)
The concentration of the suspension at which a certain instantaneous beam passes is related to the total concentration of the suspension.
For each of the narrow-grained grades d 1 to d 2 , the change in optical power is the difference between the fractions of 0 to d 1 and 0 to d 2
P 0 P 0 n 2 n 1
Ln------ -ln------=KC 'L( Σ k i n i d i 2 - Σ k i n i d i 2 )
P ′ 2 P ′ 1 i=1 i=1
(10)
Narrow grade average particle size
Based on a large number of experiments, the following conclusions are drawn:
1
W i (1 ~ 2) = -------- E (lnP '2 -lnP' 1) d m (1 ~ 2)
K m (1 ~ 2)
(11)
Where W i(1~2) is the weight of a certain granular material;
K m(1~2 ) - a coefficient related to the shape and concentration of the particles;
E - scale factor.
Thus, the weight percentage of each fraction is
d m (1 ~ 2) ( lnP '2 -lnP' 1)
Wi=------------------------- . 100%
Σ d mi (lnP i -lnP i -1)
(12)
Figure 9 extinction [next]
Using Table 3, the particle size composition of the material can be calculated from the measured curve.
Table 3 Photoelectric scanning measurement of particle size record
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