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In many processes in the food, pharmaceutical, paper, plastic and rubber industries, the mixing of powder and granular materials is critical to the quality and performance of various products. In order to make the final product meet the required attributes, mixed products need three important characteristics: fluidity, homogeneity, and mixture sampling to evaluate the blendability of the product. Powder mixers (tumbling, convection or high shear) are classified according to their mixing mechanism: diffusion, convection and shear. The choice of mixer depends on the characteristics of the particles, such as shape, particle size, density and the amount of each component. Considering these factors will minimize the possibility of separation. The filling level of the mixer, the mixing duration and the number of revolutions (in the case of a drum mixer) are critical.

Liquidity can never be expressed as a single value or indicator. Flowability is the result of the combination of the physical properties of the material that affect the flow of the material and the equipment used to process, store, or process the material. The main motivation for producing free-flowing powders is to establish a consistent feed from the bulk storage container to the feed mechanism of subsequent processing operations. All these transfer mechanisms require proper powder flow behavior, which depends on the material properties and process equipment, as well as the transfer equipment design. Interparticle forces, including van der Waals forces, capillary (liquid bridge) forces, electrostatic forces, forces that cause sintering and solid bridge formation, frictional forces, etc., can affect powder flow systems (such as hoppers, risers, packed beds, and fluidized beds, and Pneumatic conveying. These forces are responsible for the cohesive properties of fine powders and their tendency to form aggregates or agglomerates. Even the introduction of a relatively small amount of moisture can turn the free-flowing powder into something more difficult to handle. Therefore, , Understanding the impact of humidity on the materials being processed and stored is critical to developing cost-effective operating strategies.

Due to the cohesive nature of powders, many companies in the chemical processing industry (CPI) encounter flow problems and core flow behaviors. In many cases, modifying the design of the discharge hopper helps to promote flow. This is especially important in the pharmaceutical industry. In the pharmaceutical industry, a uniformly flowing powder mixture must be fed into the tablet press to achieve uniform dose delivery. The mass flow hopper is designed to prevent rat holes and pulsating flow of powder. The standard method to characterize the flow characteristics of solid materials is the shear test, which provides information about the yield trajectory of the solid. All other flow characteristics of the solid (internal friction angle, cohesion, flow function, wall friction angle of motion, etc.) are also determined by the yield trajectory.

Whenever the bulk material moves, segregation tends to occur. Therefore, every treatment or processing operation creates conditions under which separation may occur. Segregation occurs when different forces act on different parts of the bulk material mass. The different forces at work are determined by the prevailing mechanics in specific material handling operations. In some cases, the difference in the properties of most particles will result in non-random movement of the particles. When the difference in particle characteristics causes the particles to preferentially move to certain areas of the mixer, separation occurs in the mixer. The difference in particle size, density, shape, and elasticity is the most important cause of segregation. The most commonly observed types of segregation are percolation or sieve separation, trajectory segregation, fluidization segregation and powder spraying segregation.

Separation usually occurs during unloading of the powder or granular mixture from the mixer or product storage bin. In osmotic separation, smaller particles pass through a mixture of larger particles. Generally speaking, the greater the difference in particle size, the greater the range of segregation. When the finer particles are aerated during loading or unloading of the product, fluidized separation occurs. Therefore, dust separation occurs due to the difference in particle size between coarse particles and fine particles and their ability to suspend in the air when the mixed material is discharged from the mixer or charged and discharged from the storage silo. There are several situations where overmixing in one form or another is not only a waste of energy but also counterproductive.

To determine the quality of the mixture and characterize it, multiple samples need to be collected and analyzed. If you know the mode of action of the mixer, you can choose the sampling location so that you can consider slow moving areas or parts with a tendency to separate. The sampling method aims to give a representative sample theoretically, and then assume that any sampling error is negligible. Because the variation of the powder mixture sample may be a function of the particle size distribution, it is impossible to measure the absolute efficiency of the technique. The relationship between the number of samples taken and the accuracy of the estimated true standard deviation is the standard statistical measure of the Gaussian distribution. Generally, sampling problems increase as the quality of the mixture decreases. If you use random numbers to select potential samples, you can avoid biased sample selection. If the sample is taken from the moving stream of the mixture instead of particles of static mass, it will generally reduce the deviated samples taken from the mixture. After determining the component distribution, report the result as the variance.

In the pharmaceutical industry, mixing uniformity is essential to ensure uniform distribution of the drug in the powder/granule mixture. The most common technique for sampling drug mixtures is the use of sampling thieves. The advantage of Thief Samples is that the samples can be collected in a large mixer and then mixed until the optimal mixing time is reached. Stream sampling is also an alternative to using sampling thieves. Stream sampling cannot provide a location for poor mixing in response to suspicion. The goal of stream sampling is to obtain a representative sample, not to a specific location. When the active pharmaceutical ingredient in the mixture is found to be within specifications, the mixture is considered homogeneous. The results obtained are usually expressed in milligrams of active ingredients per gram of the drug mixture and the standard deviation or relative standard deviation of the drug content. In order to obtain a reliable estimate, the number of samples needs to be drawn. The variability of individual sampling and the variability that may occur during analysis requires time and effort to determine the quality of the mixture. The use of near infrared (NIR) to measure the mixing curve in real time provides an opportunity to study powder mixing kinetics. Advances in sensor technology such as NIR and data processing have enabled more and more parameters to be fully monitored using online programs. Due to this automation, the amount of test data that can be collected has increased significantly, enabling more comprehensive statistical analysis.

In short, the mixing of powders requires a good understanding of the physical properties of the particles, which enables the composition of the mixture, equipment design and appropriate sampling techniques to ensure that the specified mixture quality is achieved. 

Dilip M. Parikh is the President and CEO of Dpharma Group Inc., a pharmaceutical technology consulting organization based in Ellicott, Maryland. For more information, please call 410-900-8489.

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