New calibration material for particle size analysis

Powder Technology, 8 (1973) 307-310 © Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

Short Communication New calibration material for particle size analysis F.J. COLON, W.J. FIJMAN, P.C . TUINMAN and Chr. VELDT

Central Technical Institute TNO, Apeldoorn (The Netherlands) (Received June 12, 1973 ; accepted July 3, 1973)

A survey is presented of materials currently available for the calibration of particle size analysis equipment, along with a new method for the preparation of calibration material in the size range between 1 and 15 µm. The material obtained by this method consists of glass spheres with a size standard deviation of up to 8'/-c . Material in powder form is used in a wide range of industries ; the particle size distribution of the material is then usually an important factor . This is either since a good understanding and control of the process is required, or because particle size distribution is closely related to the quality of the product . A large number of analysis techniques are known for the determination of particle size distribution. There is a trend especially towards sub-standard, automatically measuring equipment. With this equipment the measurement is usually based on a change in electrical conductivity or in dispersed light intensity during the passage of a particle through a narrow opening, resulting in an electric pulse . The pulse height or the pulse area are a measure of the size of the passing particle . Before the measurement, the equipment is calibrated in order to determine the relationship between pulse height or area and particle size, making the technique sub-standard . The usual requirements as to the properties of a calibration material are presented in Table 1 . The size distribution that can be accepted depends on the applied measuring technique . For instance, with a Coulter counter the signals produced during the passage of particles

TABLE 1 Calibration material requirements

Requirement

Satisfied by

Wear resistant Inert Homogeneous Density not extremely high or low Simple, regular form Narrow size distribution

metal, glass (polymers) glass low porosity polymer, glass sphere, cube uniform growth or accurate fractionation

through a narrow opening are proportional to the volume of the particles . In order that single particles may be distinguished from doublets, the volume of the largest particle present in the calibration material must be smaller than twice the volume of the smallest particle present. When the particle size distribution is normal and the condition is satisfied that 95% of the particles meet this requirement, then it may be written dmax -dmin ' 4a,

where a is the standard deviation (see Fig . 1), so that

T

U C

N

AIL +2 a

2 a~

diameter

smallest d largest particle particle Fig. 1 . Particle size distribution_ d

30S TABLE 2 Spherical calibration materials _Slaterial

Range (p n)

Std. deviation (`;)

Supplier

Glass Latex Glass Pollen

4 -12 0.1-20 >50 3 -80

ca. 6 ca. 5 10-15 7-10

CTI-TNO Dow NBS Coulter Electronics

Steel ball bearings

>400

< 1

Various

* Address CTI-TNO : P.O . Box 541, Apeldoorn, The Netherlands .

d max-dmi" a,4- =

E--1 X 100 = 6 .5%. 4

The various spherical calibration materials available at present are summarised in Table 2. Of these materials, glass has the advantage that it is not affected by most chemicals and organic solvents . It may also be used at high temperatures, while it has the advantage over metal that its density is only about 2 g/cm'3 . The glass spheres in the smallest size range (4--12 µm) were prepared in our laboratory by a new procedure, as follows . At first, we tried to make glass spheres by dispersing glass powder in hot air in an oven . This process ; developed by Witzman i , may be carried out continuously, but, because of serious problems with the upscaling of the process, we looked for a different approach .

Fig. 2 . Photomicrograph of glass spheres obtained by the chalk method . (X 380)

A much better method was found in the heating of glass powder dispersed in a high melting powdery material, e.g. chalk. This method, which was developed by Muta 2 for the preparation of small metal spheres, was used in a batch process . The upscaling was fairly easy . The initial problems with respect to the choice of materials for the containers in which the heating was done, and the separation of the chalk from the glass spheres, could be solved . In Fig. 2 a photograph is shown of the material obtained with this method . The material, which contains particles as small as 0 .5 Mm, turned out to be nicely spherical . For the preparation of fractions with a narrow size distribution, several techniques are available . The simplest method is separation between two consecutive sieves of a series . Glass spheres prepared in this way, with diameters larger than 50 pm and with standard deviations between 10 and 15%., are commercially available. When the particles are not quite spherical, a much larger standard deviation is obtained . A much smaller standard deviation is obtained (e.g. 64 ± 3.5 Mm) by separating a fraction between two sieves of nominally equal size . The size distribution then depends on the difference between the average size of the sieve openings, the standard deviation of the sieve openings of the individual sieve, and, as in any sieving process, on the separation efficiency. A drawback of this method is its poor yield . Even narrower fractions of spherical materials can be obtained by superficially cleaning a sieve after sieving and subsequently collecting the particles remaining in the sieve openings. In this way very narrow fractions were prepared by Hudson et al. 3 . The yield is, however, an order of magnitude smaller than with fractionation between two sieves . The standard deviation of the spheres depends on that of the sieve openings, which for sieves with openings smaller than 100 Mm is about 3 pm, and also on the way in which the particles are stuck in the sieve . This method is therefore not fully reliable . With a rotating zigzag sifter relatively narrow fractions can be prepared, but the standard deviation of the material obtained is too large to use it as a calibration material . If sift-





309

ing is combined with sedimentation, very good results may be obtained, as will be shown later . Elutriation by gravity gives results that are far worse than with sieving . Elutriation in a centrifugal field can give much better results . Equipment based on this principle is at present being developed (see Colon 4 ) . Very narrow fractions can he obtained by careful sedimentation, as is done in the manufacture of diamond powder fractions for pol. Themtodasnrw-ihgpuose back : it is very time-consuming . By a combination of sedimentation and sifting in a rotating zigzag sifter, the process can be accelerated substantially . With this method, which was developed by the authors in the Central Technical Institute TNO, it turned out to be possible to obtain fractions of glass spheres with sizes smaller than 15 gm and standard deviations ranging between 6 and Particle size distributions* of particles prepared by a combination of sifting and sedimentation are presented in Figs . 3, 4 and 5 . The following fractions are at present available** : 4.7 ± 0 .4 gm, 8 .6 ± 0 .6 pm, 11 .1 0.7 gm . Apart from the calibration of particle size analysis equipment, such as that manufactured by Coulter Electronics, Royco and Bausch and Lomb, the particles can also be used in the following applications :

13-

12-

i

d "m

7O "C

'M

d=e615Vmicc.--ezt 6- v-

-

75

Fig. 4 . Particle size distribution of the S .6-um fraction .

15

d yrn 'S

y„

,2~

n~2 -~-

94 9

9 99 -- 95

-

---

Fig. 5. Particle size distribution of the 11-1-pm fraction.

491±0.35 um d=4 .7 . 04 ym(correcLOn :actor=0 .96)

.,-a °1.) (arc

I 999

99

95

75

50

25

°).> 1

01

Fig. 3 . Particle size distribution of the 4 .7-µm fraction . * The correction factor of 0 .96 is based on the measuring technique (optical to photopositive) employed . *~ The particles are now commercially available . Write to : CTI-TNO, P-O_ Box 541, Apeldoorn, The Netherlands.

Fig. 6 . R.E_M. photomicrograph of the 4 .7-µm fraction . (X 2200)

310

Calibration of turbidimeters ; Determination of particle trajectories, and of the accuracy and selectivity of sedimentation methods ; Preparation of model suspensions and model aerosols (for measurement of dust retention in the lungs) ; Study of polynary mixtures . Conclusion Glass spheres smaller than 15 µm have been prepared by heating a mixture of glass particles and chalk above the softening temperature of the glass . After removal of the chalk,

very narrow fractions have been separated by a combination of windsifting and sedimentation- The standard deviation of the diameter of the particles ranges from 6 to 8% . REFERENCES 1 2

a 4

H. Witzman, Staub, 22 (1) (1962) 2-7 . A. Muta et at., Particle size analysis, Proc. Conf Soc. AnaL Chem., London, 1967, p . 215 . R.B. Hudson, M.L. Jansen and P .B. Linkson, Powder Technol., 2 (1968) 230 . F.J. Colon, J.W. van Heuven and H .M . van der Laan, Particle size analysis, Proc. Conf. Soc. Anal. Chem., London, 1970, p . 42 .