What is a sample mill?

Overview

Sample mills differences
Sample mills with cutting action
Sample mills using rotors for sample crushing
Sample mills using balls for sample crushing
Achieving good homogeneity with sample mills

A sample mill, commonly utilized in sectors such as food, agriculture, and mining, is engineered for pulverizing or grinding small material samples for compositional, physical property analysis, or other testing objectives. These devices can prepare samples to a uniform size or texture, ensuring consistency for precise evaluations. Various sample mill models exist, each tailored for distinct materials and particular uses.

The selection of an appropriate sample mill depends on the material's characteristics and the intended result of the sample processing. For analytical techniques like AAS, NIR, ICP, or XRF, it is crucial that the sample is thoroughly homogenized to a suitable level of analytical fineness. Only through consistent sample preparation dependable and accurate analyses are assured. RETSCH provides an extensive selection of sample mills and crushers for initial, fine, and ultrafine size reduction of virtually any substance. The portfolio of grinding instruments and accessories guarantees that our devices facilitate contamination-free and reliable sample preparation before laboratory testing.

What is the difference between a sample mill and a grinding mill?

In essence, sample mills specialize in preparing small quantities for detailed analysis with focus on precise control over the particle size and consistency of the resulting sample to ensure it meets specific testing or analytical requirements. Grinding mills focus on bulk processing for various production applications. In summary, the main difference between a sample mill and a grinding mill is their scale of operation and primary purpose, with sample mills being more suited for small-scale, precise sample preparation for analysis, and grinding mills being used for larger-scale production processes requiring material size reduction.

What is the difference between a sample mill and a pulverizing mill?

The primary distinction between a sample mill and a pulverizing mill lies in their operational objectives, the mechanical actions they perform to comminute materials, and the contexts in which they are typically employed. A sample mill is specifically designed for the preparation of small material samples for analytical or testing purposes, aiming to achieve a certain particle size or consistency for precise analytical outcomes. It is utilized in laboratory settings or research environments where exact control and repeatability of the grind size are paramount. On the other hand, pulverizing mills are dedicated to converting materials into a fine powder, serving various industries that require processing of raw materials into a more manageable or further processable form. The focus here is on attaining extensive material breakdown rather than sample preparation for analysis. In summary, while both types of mills are involved in material size reduction, sample mills are specialized in creating small, accurate samples for analysis with a high degree of particle size control, whereas pulverizing mills aim to produce fine powders.

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Sample mills with cutting action

1. Molinos de corte

Cutting mills are adept at the efficient primary size reduction of mixed heterogeneous materials and are equally effective for processing soft, medium-hard, elastic, or fibrous samples. The size reduction in these sample mills is achieved through cutting and shearing forces. Upon hitting the the rotor the material is reduced in size between the blades and the stationary double-acting cutting bars within the housing. The 6-disc rotor features spirally arranged, reversible hard metal plates that sequentially cut the sample. The parallel section rotor's knives deliver a robust cutting action for comminution. The V-rotor is particularly suited for processing light and fluffy samples. The sample stays only briefly in the grinding chamber; once it is sufficiently small to pass through the sieve's openings at the bottom, it exits through the gravity outlet and is collected. The optional cyclone-suction-combination not only cools the sample but also enhances material discharge from the grinding chamber.

Molinos de corte

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Principio de funcionamiento

A rotor speed of 1,500 min^-1 ensures rapid yet gentle size reduction. These sample mills are known for their high operational safety and convenience, with a broad range of accessories for easy adaptation to various applications. They can process initial particle sizes of up to 80 mm. The SM 100 is RETSCH's budget-friendly basic model, ideal for samples that do not require extreme forces. Designed for routine tasks, it is user-friendly and can be installed on a sturdy table or an optional base frame. The most powerful SM 300 sample mill boasts a 3 kW drive with high torque and RES technology, excelling in challenging tasks where other mills might fail. This mill offers perfect adaptability to application requirements with variable speeds ranging from 100 to 3,000 rpm.

2. Molino de cuchillas

Knife mills ensure quick, uniform and reproducible size reduction, allowing for representative sampling from any part of the grinding container. The GRINDOMIX GM 200 knife mill is an excellent sample mill for grinding and homogenizing food and feed. Equipped with two sharp, sturdy blades and a robust 1000 W motor, it processes up to 0.7 liters of sample volume efficiently. The GM 200 excels at homogenizing substances with high water, oil, or fat content and is also ideal for grinding dry, soft, and medium-hard products. Its rapid homogenization process prevents significant temperature increases, preserving volatile components in the sample. The mill's reproducible settings for parameters, programs, and sequences ensure minimal standard deviation in analytical results. This sample mill, with its array of lids and containers for customization, surpasses any commercial household mixer in professionalism.

Molino de cuchillas

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Principio de funcionamiento

The GRINDOMIX GM 300 laboratory knife mill, with its special cutting knife system and adjustable grinding chamber volume, processes a wide range of samples quickly and reproducibly into homogeneous analytical samples. Its 5000 ml chamber volume can homogenize large items like a loaf of bread or a pizza in one go. Like the GM 200, the GM 300 sample mill guarantees analytical results with minimal standard deviation, offering a selection of lids and containers for tailored application needs, setting it apart as a professional-grade device beyond any commercial household mixer.

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Sample mills using rotors for sample crushing

Rotor mills are utilized for the rapid size reduction of materials ranging from soft to medium-hard, as well as temperature-sensitive or fibrous substances. In these sample mills, size reduction is achieved through impact and shearing forces between the rotor and the fixed (ring) sieve. Materials enter the rotor through the hopper and are swiftly crushed by impact. Subsequently, they are finely ground either between the rotor and the sieve by shearing forces or on a grinding insert by friction.

This two-step grinding process is particularly gentle yet efficient. The material remains in the grinding chamber for only a brief period, ensuring that the sample's defining characteristics are not altered. Cyclones that assist in discharging the sample materials also have a cooling effect. These sample mills may come with an integrated cyclone or have one added optionally. The rotor mill series includes Ultra Centrifugal Mills, Rotor Beater Mills, Cross Beater Mills and the cyclone mill Twister, each designed for specific sample processing needs.

Molinos de rotor

1. Molino ultracentrífugo ZM 300

The powerful Ultra Centrifugal Mill ZM 300 is an ideal sample mill as it provides maximum grinding performance combined with ease of use. The variable speed from 6,000 to 23,000 rpm allows for gentle, neutral-to-analysis sample preparation with initial feed size up to 10 mm in a very short time. Thanks to an integrated temperature monitoring system, reproducibility is guaranteed even for long grinding processes or pulverization of large sample volumes. The wide selection of rotors, ring sieves and cassettes makes the ZM 300 a true allrounder which meets the requirements of a great variety of size reduction tasks. Very small particles down to 40 µm can be achieved.

Molino ultracentrífugo ZM 300

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Principio de funcionamiento

2. Molino de impacto de rotor SR 300

The Rotor Beater Mill SR 300 is suitable for coarse and fine size reduction. It can process dry, soft, medium-hard, organic and inorganic substances. This sample mill is suitable for a defined final fineness due to bottom sieves with aperture sizes from 0.08 - 10 mm, accepting larger initial feed size up to 25 mm. The SR 300 has an optional grinding insert 180° for grinding of hard-brittle materials by additional impact. With the adjustable speed up to 10,000 rpm, easy cleaning thanks to exchangeable push-fit grinding insert, rotor and cassette and the optional distance rotor to reduce frictional heat, the SR 300 is the ideal sample mill for batches up to 30 l.

Molino de impacto de rotor SR 300

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Principio de funcionamiento

3. Molino ciclón TWISTER

The cyclone mill TWISTER is specifically engineered for processing food and feed samples for subsequent NIR analysis. Its design, featuring an optimized rotor and grinding chamber, creates an air jet that transports the ground sample through the integrated cyclone into the sample bottle. This air jet helps maintain the sample's temperature, thus preserving its moisture content, a premise for a reliable sample mill.

The sieves provided ensure an ideal particle size distribution, eliminating the need for recalibration of the NIR spectrometer. The rotor speed of the mill is adjustable in three stages, allowing for precise customization to the sample's requirements. Cleaning the mill is straightforward and efficient, as the air jet facilitates a complete discharge of the material from the grinding chamber.

Molino ciclón TWISTER

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Sample mills using balls for sample crushing

Ball mills are highly versatile and efficient sample mills for pulverizing hard, brittle, or fibrous materials. Their diverse grinding modes, adjustable volumes, and selection of grinding tool materials make them suitable for a wide array of applications. To comprehend the distinctions between ball mill types, we examine their common features. Fundamentally, all ball mills operate on the principle of moving the sample material, potentially with grinding balls, within a sealed container. This action produces intense blending and pulverizing effects on the material. The primary difference is evident in the jar movements. For instance, in a planetary ball mill, the jar follows a circular trajectory akin to a planet orbiting the sun, while in a mixer mill, the jar executes a horizontal oscillating motion, and in a drum mill, the jar simply rotates on its axis.

Another crucial aspect of a ball mill is its power, which greatly impacts the milling outcome. Depending on the application, jars may rotate slowly for gentle processing or rapidly for effective grinding. The maximum speed, often expressed in frequency or revolutions per minute (rpm), is commonly associated with performance. However, a more significant measure of a mill's capability is the acceleration force "g," generated by the mill's kinetic energy.

Molinos de bolas

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Dr. Lena Weigold explains how to select the most suitable ball mill

The ideal sample mill for pulverizing elastic and tough samples is the CryoMill. Cryogenic grinding is a technique for processing thermally sensitive and elastic substances by cooling them with liquid nitrogen. The CryoMill, a specialized sample mill, is engineered for this purpose. It incorporates an integrated cooling system that consistently cools the grinding jar with liquid nitrogen both before and during the grinding process. Consequently, the sample becomes brittle, and volatile components are preserved. The liquid nitrogen is supplied continuously by an autofill system, maintaining the temperature at –196 °C without any direct user contact, ensuring high operational safety. The CryoMill's adaptability, including cryogenic, wet, and dry grinding at room temperature, renders it the ideal sample mill for samples up to 1x 20 ml. The MM 500 control is a sample mill suitable for samples up to 2 x 40 ml. It enables temperature monitoring and control during the grinding process, with a range from -100 to 100 °C. For ultimate versatility, the mill operates with various thermal fluids, accommodating different cooling or heating devices. When using liquid nitrogen for cooling, the mill can be fitted with the optional cryoPad extension. This innovative technology allows for selecting and controlling a specific cooling temperature between -100 and 0 °C during the grinding process.

CryoMill

Molino mezclador MM 500 control

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Achieving good homogeneity with sample mills

Homogenization is a pivotal process in sample preparation, ensuring that the sample is uniformly mixed to achieve a consistent composition throughout. This uniformity is essential for accurate and reliable analytical results, as it eliminates the variability that could arise from analyzing different parts of a non-homogeneous sample. Homogenization is particularly crucial when the sample is heterogeneous or has multiple components, as it allows for a representative analysis of the entire sample.

Homogeneity is a critical premise for reliable sample analysis for several reasons:

Representative Sampling: Homogeneity ensures that each sample portion is representative of the entire batch or population. This is crucial in analytical testing because it means the results obtained from a small sample can be accurately extrapolated to the entire batch. Without homogeneity, there's a risk of analyzing an unrepresentative sample, leading to inaccurate conclusions about the whole.
Consistency and Reproducibility: When samples are homogeneous, repeated tests on different portions of the sample or batch yield consistent results. This reproducibility is essential for the reliability of analytical results, as it demonstrates that the findings are not due to random variation or sampling errors.
Error Reduction: Homogeneous samples reduce the likelihood of analytical errors. Inhomogeneous samples can introduce variability that may be mistaken for inherent variability in the population or for an effect due to the experimental conditions. By ensuring homogeneity, one minimizes these potential sources of error, leading to more accurate and reliable analysis.
Efficiency: Homogeneous samples streamline the analytical process. When each portion of the sample is representative, fewer samples may need to be analyzed to obtain a reliable understanding of the whole. This efficiency can save time and resources, which is particularly important in high-throughput or time-sensitive analyses.
Statistical Validity: Many statistical analyses assume that the data points (samples) are independent and identically distributed. Homogeneity supports this assumption by ensuring that all parts of the sample are similar and that the sampling process does not introduce bias. This underpins the statistical validity of the conclusions drawn from the sample analysis.

In summary, homogeneity is essential for reliable sample analysis because it ensures that samples are representative, results are reproducible and consistent, analytical errors are minimized, efficiency is maximized, and statistical analyses are valid.

La homogeneización de las muestras garantiza resultados reproducibles. La desviación típica de las muestras de molido grueso suele ser mayor que la de las muestras completamente pulverizadas. Esto puede se ilustra con el siguiente ejemplo: una muestra de salchicha con partículas de 4-5 mm y una muestra homogeneizada con partículas <0,5 mm se analizaron cinco veces seguidas para determinar su contenido en grasa mediante secado inducido por microondas combinado con espectroscopia de RMN. Para cada medición se secaron 4 g de muestra en 2,5 min. y se analizaron en 1 min. El contenido de grasa de las muestras gruesas de salchicha varió más que el de las muestras más finas. El contenido de grasa de la primera fracción se midió en un intervalo del 14,85 % al 17,12 %, con una desviación estándar del 0,88 %. En la muestra homogeneizada, la desviación típica se redujo más de diez veces, hasta el 0,07 %, y el contenido de grasa osciló entre el 15,84 % y el 16,02 % (la desviación típica relativa se redujo del 5,63 % al 0,45 %).

Ejemplo: Análisis del contenido de grasa

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En todas las áreas industriales se emplean aparatos RETSCH para preparar muestras que serán sometidas a los más diversos análisis y para realizar determinaciones granulométricas en controles de producción y de calidad.
Con el fin de satisfacer las exigencias cada vez más altas del mercado, RETSCH desarrolla todos sus productos siguiendo una filosofía clara, basada en la cita de Aristóteles:

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Sample Mills - FAQ

What is a sample mill?

A sample mill is a device used in various industries like food, agriculture, and mining for pulverizing or grinding small material samples. It prepares samples to a uniform size or texture for consistent evaluations, which is crucial for analytical techniques such as AAS, NIR, ICP, or XRF. RETSCH offers a wide selection of sample mills and crushers for different substances and purposes.

What are the different types of mills available for sample preparation?

There are several types of sample mills used for sample preparation, each designed for specific materials and purposes. Here's a brief overview of the different mill types:

Cutting Mills: These are suitable for the efficient primary size reduction of mixed heterogeneous materials and are effective for processing soft, medium-hard, elastic, or fibrous samples. The size reduction is achieved through cutting and shearing forces.
Knife Mills: Knife mills are ideal for grinding and homogenizing food and feed. They ensure uniform and reproducible size reduction swiftly, allowing for representative sampling from any part of the grinding container.
Rotor Mills: Rotor mills are used for the rapid size reduction of soft to medium-hard, temperature-sensitive, or fibrous substances. Size reduction is achieved through impact and shearing forces between the rotor and the fixed (ring) sieve.
Ball Mills: Ball mills are versatile and efficient for pulverizing hard, brittle, or fibrous materials. They offer diverse grinding modes, adjustable volumes, and a selection of grinding tool materials suitable for a wide array of applications.
Cryogenic Mills: The CryoMill is designed for cryogenic grinding, which is a technique for processing thermally sensitive and elastic substances by cooling them with liquid nitrogen. This mill is engineered to consistently cool the grinding jar with liquid nitrogen both before and during the grinding process.