Quantitative analysis of powder mixing quality

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Source: internal company

In the fourth stage of GMP certification of EYH Series two-dimensional motion mixers or other powder mixing equipment performance confirmation(PQ), the mixing uniformity test(RSD) is mentioned. There is no doubt that the mixing uniformity is the most important indicator to evaluate the performance of the mixer.

The meaning of powder mixing

Powder mixing is a process in which more than two components are mixed in a dry state or in the presence of a small amount of liquid, with external force, so that their non-uniformity is continuously reduced. The so-called two or more components can be different substances or the same substance with different physical characteristics, such as different moisture content, particle diameters, colors, etc.

Powder mixing is a complicated random process,and the evaluation and determination method of mixing quality has always been a thorny problem that puzzles people. With the development of the times, the days of judging the uniformity of mixing based on human senses are over. The scientific and quantitative form to determine the mixing uniformity is the quantitative analysis of power mixing. There must be several processes of sampling, testing, and statistic analysis(data processing), so as to obtain a single value to express the uniformity of the mixture.

Sampling

A small amount of material is taken from a certain spot in the mixture, which is called ‘sampling’. This small amount of material is called ‘observing sample’, and it is also called ‘point sample’, and the sampling spot is called ‘sampling point’.

In the same container and at the same time, samples taken at different sampling points constitute the‘sample’. The number of spot samples is the size of the sample.

Regarding the size of the sample: under the premise of satisfying the amount required for testing and possibly representative of the materials around the sampling point, the sample is the smaller the better. If it is too big, it will not only wastes materials but also has negative effect on the accuracy of quantitative analysis.

The analysis will be more reliable if samples were taken in a big number. But for now, there is no evidence saying the optimized number of samples is. According to the suggestion from American Chemical Engineering Association, the samples should be 5-15,while others believe that it should be at least 20-50. While what we do in China is to take 5-10 samples. So where the samples should be taken from? If it is under rest state, the taking spots should be evenly allocated. The result accuracy can be higher when taking the samples from a mixed material under flow condition.

Testing

Samples will be taken and the content (X) of each component (especially the key component-tracer) will be determined by chemical or physical methods. If there are 5 samples, then the corresponding result will be: X1, X2, X3, X4 and X5. The testing method is determined by the components nature, mixing purpose and actual conditions.