Reproducible Sieve Analysis

Sieve analysis (also known as sieving analysis or test sieving) is used to determine the particle size distribution of various bulk materials. Its handling and evaluation is described in a variety of international standards. It is also considered an important and indispensable quality assurance procedure to this day. Sieve analysis is divided into dry sieving and wet sieving. The sieving motion can be based on the principles of throw sieving, plan sieving, tapping sieving, air jet sieving and ultrasonic sieving. Manual sieving is not easily reproducible due to the individual influences of the operator (stamina, speed, strength).

Для определения характеристик сыпучих продуктов различных форм и размеров необходимо знание их гранулометрического состава. Гранулометрический состав, т.е. количество частиц разного размера, отвечает за важные физические и химические свойства, такие как растворимость, текучесть и склонность к поверхностным реакциям.

Sieve analysis

Во многих отраслях промышленности, таких как пищевая, фармацевтическая и химическая, традиционным методом анализа при производстве и контроле качества порошков и гранулята является ситовой анализ. Среди преимуществ данного метода можно назвать удобство выполнения, низкие затраты, точные и воспроизводимые результаты, получаемые относительно быстро, а также возможность разделения материала на фракции. Таким образом, ситовой анализ сопоставим с такими методами анализа, как лазерная дифракция или анализ изображений.

Для обеспечения высокой воспроизводимости и надёжности результатов анализа, просеивающие машины и аксессуары должны отвечать требованиям национальных и международных стандартов. Это означает, что лабораторные сита, просеивающие машины и все другие измерительные приборы (например, весы), используемые для определения распределения частиц, должны быть откалиброваны и подвергнуты контролю со стороны испытательных лабораторий в рамках системы управления качеством. Кроме того, проводит пробоподготовку необходимо очень тщательно. Только в этом случае можно получить результаты рассева, позволяющие достоверно охарактеризовать продукт.

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Методы ситового анализа

При рассеве проба подвергается вертикальному движению (вибрационный рассев) или горизонтальному движению (горизонтальный рассев). В ударных просеивающих машинах горизонтальные циркуляционные движения сочетаются с вертикальными ударами. Во время рассева частицы ударяются о ситовое полотно, «сравнивая» свои размеры с размером ячеек. Вероятность прохождения частицы через сито определяется соотношением размера частицы и отверстий сита, ориентацией частицы и количеством встреч между частицей и отверстиями сита. Выбор подходящего метода рассева в целом зависит тонкости пробы (Рис. 1). Для анализа проб с размером частиц от 40 мкм до 125 мм лучше всего подходит сухой рассев. Однако диапазон измерений ограничивается такими свойствами пробы, как склонность к агломерации, плотность или накопление электростатических зарядов.

ВИБРАЦИОННЫЙ РАССЕВ

The sample is thrown upwards by the vibrations of the sieve bottom and falls back down due to gravitation forces. The amplitude indicates the vertical oscillation height of the sieve bottom.

Due to this combined motion, the sample material is spread uniformly across the whole sieve area. The particles are accelerated in vertical direction, rotate freely and then fall back statistically oriented. In RETSCH sieve shakers, an electromagnetic drive sets a spring/mass system in motion and transfers the oscillations to the sieve stack. The amplitude can be adjusted continuously to a few millimeters.

Горизонтальный рассев

При горизонтальном рассеве сита совершают горизонтальные циркуляционные движения. Горизонтальные просеивающие машины особенно эффективны для рассева проб с частицами игольчатой, продолговатой или плоской формы, а также волокнистых проб. Благодаря движению сит в одной плоскости частицы сохраняют свою ориентацию в пространстве.

Tap sieving

In a tap sieve shaker a horizontal, circular movement is superimposed by a vertical motion generated by a tapping impulse. Tap sieve shakers are specified in various standards for particle size analysis.

The number of comparisons between particles and sieve apertures is substantially lower in tap sieve shakers than in vibratory sieve shakers (2.5 s-1 as compared to ~50 s-1) which results in longer sieving times. On the other hand, the tapping motion gives the particles a greater impulse, therefore, with some materials, such as abrasives, the fraction of fine particles is usually higher. With light materials such as talcum or flour however, the fraction of fine particles is lower.

AIR JET SIEVING

The air jet sieve is a sieving machine for single sieving, i.e. for each sieving process only one sieve is used. The sieve itself is not moved during the process.

The material on the sieve is moved by a rotating jet of air: A vacuum cleaner which is connected to the sieving machine generates a vacuum inside the sieving chamber and sucks in fresh air through a rotating slit nozzle. When passing the narrow slit of the nozzle the air stream is accelerated and blown against the sieve mesh, dispersing the particles. Above the mesh, the air jet is distributed over the complete sieve surface and is sucked in with low speed through the sieve mesh. Thus the finer particles are transported through the mesh openings into the vacuum cleaner or, optionally, into a cyclone.

In air jet sieving, only a single sieve is used at a time, and it is not moved during the sieving process. A rotating nozzle below the sieve directs a jet of air onto the material to be sieved, causing particles to deagglomerate and then be sucked through the sieve. Air jet sieving is suitable for size ranges from 10 µm to 4 mm.

Dry sieving

Dry sieving is the most popular method of reproducible sieve analysis, including vibration, horizontal and tap sieving. Air jet sieving is also considered a dry sieving method, but it is a special process (see below). If necessary, the sample is dried in advance to avoid clumping. Before sieving, the sample is weighed, then placed in the sieving system and weighed again at a later point in time.

Sieving is used to determine the percentage of the sample that remains on the sieve or is smaller than the selected mesh size. If a particle size determination of the various fractions is to be carried out (set sieving), a sieve stack is used that contains several sieves with different mesh sizes (40 µm – 125 mm).

However, to ensure that the results are reproducible beyond doubt, the machine should be set up completely digitally. Furthermore, the integrated control unit should be constantly monitored to avoid unintentional changes and deviations during the test.

Wet sieving

Wet sieving is used to determine particle sizes in moist, greasy or oily samples. It is also the method of choice when the material to be analyzed is already present as a suspension and cannot be dried, as well as for particles that tend to agglomerate (usually < 45 µm), which would otherwise clog the sieve openings.

The material to be sieved is suspended and, as with dry sieving, applied to the uppermost sieve and then rinsed with water under vibration until the liquid emerging from below the sieve stack is unclouded. Wet sieving is carried out in the range 20 µm - 20 mm.

Grain size analysis

The formal size of individual particles in a mixture is referred to as the “grain size”, and grain size analysis is used to determine this size. The subsequent size distribution of the particles has a significant influence on the properties of a material, both scientifically and technically.

Due to numerous differentiations and even different methods of determination, grain size analysis is considered an independent discipline of granulometry.

Methods of grain size analysis

Although there are different methods for analyzing and determining grain sizes, the equivalent diameter is always determined in all variants. Which method is ultimately used depends heavily on the question, possible regulations and the grain size range itself.

Larger particles, from a size of about 40 mm, are usually measured by hand or on the basis of photos, while sieving is often used for the particle size analysis of very small particles, down to a size of 10 µm. For sieving, sieves of different sizes are first stacked on top of each other and clamped in a sieving machine. The sample is then placed in the top sieve (with the largest hole size) and subjected to a defined sieving motion for a certain period of time to ensure precise sieving.

The particles of the sample are separated according to their size on the sieves. After that, the percentage of the individual fractions remaining on the sieves with different hole sizes is determined. The percentage mass fractions of the individual fractions are referred to as p3. The cumulative distribution curve Q3 provides information about the added masses of the individual fractions. It is common to provide information about the size of the sample smaller than 90%, 50% and 10%.

Optical particle characterization

The particle size analysis can also be carried out using optical measurement technology. Depending on the measurement variant, statements can also be made about the particle shape. The measuring range is between 0.3 nm and 30 mm, depending on the system. The particle characterization can be carried out in suspensions, emulsions, colloidal systems, powders, granules and bulk materials.

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Анализ распределения частиц по размерам - Обзор оборудования

SIEVE ANALYSIS FOR QUALITY CONTROL

We all know the term “quality”. It is widely used to describe a product of particularly high value. However, the exact definition of quality is as follows: Quality is the compliance of defined properties with the detected properties of a product as determined by performing tests. A product can be described as high-quality if a test measurement ascertains that the desired properties lie within a given tolerance. If the measured values deviate too much, the quality is lower. Many materials, whether natural or artificial, occur in dispersed form (material which does not form a consistent unity but is divided into elements which can be separated from each other, e.g. a pile of sand). The particle sizes and their distribution within a material quantity - i.e. the fractions of particles of different sizes – have a crucial influence on physical and chemical properties.

Несколько примеров свойств, на которые может влиять гранулометрический состав:

прочность бетона
вкус шоколада
свойства растворения таблеток
порошкообразность и растворимость стиральных порошков
поверхностная активность фильтрующих материалов

Эти примеры наглядно показывают, насколько важно знать гранулометрический состав, особенно в контексте обеспечения качества сыпучих продуктов для производственных процессов. Если гранулометрический состав изменится в процессе производства, то изменится и качество продукта.

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