Batch mixing of solid granular substances or materials of other consistencies is one of the most frequently applied technological operations in this industry. Although a homogenization process depends on several parameters (physical features of the substance, type of mixer, and operation mode), the most frequent question is how
long a mixture in the applied mixer must be mixed to achieve the required homogeneity.

To answer this question, one component of the mixture is labeled with a radio­active tracer prior to the study. The labeled component is placed in the mixing equipment in accordance with the normal technological process and the mixing process is launched. Depending on the required information (e. g., if homogeneity is needed to describe with a quantitative metric number or only information on the sufficient homogenization time is needed to achieve the best homogeneity which cannot be improved with longer mixing), sampling or an outer detection technique is applied.

For sampling, the mixing process is suspended in certain time intervals and a statistically sufficient number of samples are taken from the mixture, followed by measuring the count rates of the samples and the statistical processing of the count rates. The mixture homogeneity is determined by calculating the relative standard deviation of the sample counts:

where a is the count rate of a sample, a is the average of the counts, and n is the serial number of the sample.

By plotting the obtained relative standard deviation values as a function of time, the optimal homogenization time (where homogeneity does not change further) can be determined (as shown in Figure 11.7).

When applying the outer detection technique (Figure 11.8), the detector(s) are installed outside the mixing equipment wall, which will follow movement of the radioactive tracer and concentration equalization of the mixture. The time change of the detector signal does not provide quantitative information on the homogeneity of the mixture; it only shows how much time is needed for the equali­zation (for ending the signal fluctuation). The outer detector technique requires

approximately 10 times more activity than the sampling technique, but it is much faster and it is a noninvasive method.

Most of the homogenization studies have been performed in Hungary in the field of porcelain manufacturing and cement production using fluidization raw meal mixers.

Examples for homogization studies carried out with radiotracer technique include:

• Homogenization of components (Co and W metal powder) of hard metal production.

• Homogenization of components of porcelain mixture with labeling a powder fraction.

• Homogenization of a wet porcelain mixture with labeling the wetting agent.

• Study of the raw meal fluidization type homogenization in cement factories (with Au-198 colloid tracer).