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).
さまざまな形状やサイズのバルク品を特性評価するためには、その粒度分布の知識が不可欠です。粒度分布、すなわち異なるサイズの粒子の数は、溶解性、流動性、表面反応などの重要な物理的・化学的特性に影響を与えます。
食品、製薬、化学などの多くの業界では、粉体や顆粒の製造や品質管理のために、従来のふるい分け分析が標準となっています。ふるい分析の利点は、取り扱いが容易であること、投資コストが低いこと、比較的短時間で正確かつ再現性の高い結果が得られること、粒子径画分を分離できることなどが挙げられます。そのため、この方法はレーザー光や画像処理を用いた分析方法の代替として受け入れられています。
高度な再現性と信頼性を保証するために、ふるい振とう機とその付属品は、国内および国際的な規格の要件を満たさなければなりません。つまり、試験ふるい、ふるい振とう機、その他の測定機器（スケールなど）は、品質管理システムの一環として校正され、テストエージェントによるモニタリングを受けなければなりません。それ以外にも、サンプルの準備を細心の注意を払って行うことが絶対に必要です。そうすることで初めて、製品の信頼できる特性評価を可能にするふるい分け結果を得ることができるのです。
タップ式ふるい振とう機では、ふるい分けの際に試料に垂直方向の動き（振動ふるい分け）と水平方向の動き（水平ふるい分け）を与えます。タップ式ふるい振とう機では、この2つの動きが重なっています。このプロセスでは、粒子はすべてのふるいの開口部と比較されます。粒子がふるいを通過する確率は、粒子径とふるいの目開きの比率、粒子の向き、粒子と目開きの接触回数によって決まります。適切なふるい分け方法は、試料の細かさの度合いによって異なります（図1）。40 µmから125 mmまでのサイズ範囲では、乾式ふるい分けが好ましい方法です。しかし、測定範囲は、凝集傾向、密度、静電気などの試料の特性によって制限されます。
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.
水平式ふるい振とう機では、ふるいは平面上で水平方向に円を描くように動きます。水平式ふるい振とう機は、針状、平たい、長い、繊維状の試料に適しています。水平方向の動きのため、ふるいの上で向きを変える粒子はほとんどありません。
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.
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 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 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.
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.
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%.
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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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.
粒度分布の影響を受ける特性の例をいくつか挙げてみましょう。
これらの例は、特に生産工程におけるバルク品の品質保証の観点から、粒度分布を知ることがいかに重要であるかを明確に示しています。生産工程で粒度分布が変化すると、製品の品質も変化してしまいます。
