In this blog post, we take a closer look at why uniformity is important in the powder mixing process and how it is measured and managed.

Meaning and importance of mixing uniformity

Uniformity refers to the degree to which the ratio of ingredients in a product remains constant when powders of different ingredients that do not react chemically with each other are mixed to create a complex chemical product. Uniformity is a key factor in determining product quality, especially in mass production, where large quantities of raw materials are mixed and then divided into equal masses to create the final product.
This uniformity must be thoroughly managed throughout the entire raw material mixing process, and it is important to ensure accuracy at the initial stage, as it is almost impossible to make corrections after the process is complete. To this end, the raw material powder must be weighed in advance to ensure accurate content ratios before being fed into the mixer, and repeated analysis must be performed during the mixing process to check that each component is well mixed in accordance with the target ratio.

Method for measuring mixing uniformity: Dispersion analysis

Dispersion analysis is a commonly used method for measuring mixing uniformity in mixing processes. The mixer is stopped at regular intervals, and a certain number of samples with the same mass are randomly collected to analyze the component content of each sample. The dispersion value for each component is calculated from the content ratios obtained in this way, and the smaller the dispersion value, the higher the uniformity of the entire mixture. By repeating this test at regular intervals, it is possible to check the changes in the dispersion value according to the stirring time. Usually, the dispersion value gradually decreases as the stirring time increases, and after reaching a certain level, it converges without any further significant changes. Based on this convergence point, it is common procedure to check whether the dispersion value has reached the predetermined standard, and if so, to terminate the mixing process.

Considerations for sample design

In order to accurately check uniformity, the number, mass, and sampling location of samples must be carefully determined based on statistical standards. If the number of samples is too small, the accuracy of the dispersion value estimation will be low, and the mass per sample must be set based on the upper limit of the mass per final product. The sample collection location must use a random sampling method that considers both horizontal and vertical directions, which is essential for ensuring representativeness of the entire powder in the agitator.

Limitations of traditional methods and analysis delays

However, these traditional analysis methods have the disadvantage of repeatedly stopping the agitator, requiring sample pretreatment for analysis, and taking time for chemical reactions, which reduces the overall efficiency of the process. Sample pretreatment requires a process of converting the sample into a liquid state for analysis, and additional time is required for chemical analysis, such as reagent reactions, to analyze the content of each component.
In order to shorten the process time, the agitator is sometimes restarted before the analysis results are available, but this is highly inappropriate. Not only do the analysis results fail to reflect the real-time status of the agitator, but simply increasing the agitation time does not necessarily result in good mixing.

Relationship between powder properties and mixing uniformity

Powder properties also play an important role in affecting uniformity. Ideally, powders with similar physical properties, such as particle size, density, shape, and surface adhesiveness, should be mixed together. However, in reality, it is often difficult to artificially control physical properties such as powder density and shape, and these differences can cause particles with similar physical properties to clump together during mixing (particle separation), resulting in poor mixing uniformity. Some physical properties, such as particle size, can be controlled in advance, so these characteristics must be taken into account during the preliminary preparation of the powder to ensure uniform mixing.

Latest technology: Real-time analysis using a spectrometer

To overcome these limitations, real-time mixing uniformity analysis technology using a spectrometer has recently become widely used. This method collects data through a spectrometer attached to a specific location (usually a transparent window) in the agitator while the agitator is in operation, and then transmits the data to a computer for real-time analysis. As the powder inside the agitator rotates and passes in front of the transparent window, it is exposed to light from the spectrometer. This light is reflected by the powder and returns to the spectrometer, where the intensity of the light of a specific color is measured to obtain information related to the content of each component. Based on the light intensity data for each component obtained in this process, the content ratio can be calculated by applying the component content-light intensity relationship obtained in advance. Since the spectrometer collects data almost continuously, the content ratio data for each section is grouped at regular intervals to calculate the dispersion value, and when the dispersion value reaches the set standard, the mixing process is terminated.

Conclusion

Uniformity is a key indicator that determines the quality of complex chemical products, and the design and management of the mixing process and the selection of analysis methods are very important to ensure uniformity. Conventional chemical analysis-based methods ensure high reliability but can be inefficient in terms of time and cost. As an alternative, real-time analysis technology using spectrometers is emerging. In addition, it is necessary to understand and approach mixing uniformity not simply as “how long the mixture was stirred,” but as the result of the comprehensive interaction of complex factors such as particle properties, sample design, and the timing and method of analysis.