Grain size

The term describes particle size (also called grains) in a mixture the size of individual particles. The grain or particle size distribution has a significant influence on the material properties in many technical and scientific fields such as construction, sedimentology and soil science as well as in metallurgy (see grain growth and grain refinement ). There a variety of grain or particle mixtures used. These grain mixes include all types of bulk materials such as sand and gravel, flour, cocoa, plastic granules and pigments. In metallurgy, the microstructures are referred to within metallic materials as grain.

Due to the variety of methods for the identification, description and interpretation of grain size and other particle properties according to EN ISO 14688 (form, roundness and surface ), the particle size has developed as a discipline.

• 5.1 Absolute grain size
• 5.2 Relative grain size

Equivalent diameter

Moreover, to accept that grains or particles were available as perfect spheres, one could simply use the sphere diameter as a measure of the grain size. However, this assumption is inadequate in practice, since naturally formed or engineered particles are present in different forms. For a description of the size of one therefore uses the equivalent diameter. This means that you can determine another measurable property and relates the measurements to the same size (equivalent) balls.

A simple example of an equivalent diameter is the sieve diameter. Through the square hole of a sieve with 1 mm square, for example, both a ball of 1 mm in diameter and fits into an elongated grain shape of a pencil with 1 mm diameter. This applies on the diagonal of the Sieblochs for a flat particle, in the form of a coin with significantly more than 1 mm in diameter. All three grain obtain the same equivalent diameter of 1 mm.

Other examples of equivalent diameter are hydrodynamic diameter (same falling velocity in a column of water as a sphere ) and aerodynamic diameter (the same speed in the case of air as a sphere).

Grain size analysis

To determine the grain size can be selected from a variety of methods, which ultimately always an equivalent diameter is determined. The appropriate method depends on the particle size range, the question or regulations.

Very large particles (about a size of 63 mm) are measured individually by hand or it will be the size determined from photos.

For particles in the range of 10 microns to button size, the size can be determined by sieving. Here, a set of down progressively finer screens will be set on each other. The sample to be analyzed is poured into the top sieve and the nest of sieves and then clamped in a screening machine. The machine shakes or vibrates the screen set for a certain period. With a high proportion of fine grain sieving is performed with running water ( wet sieving ). The thus determined grain sizes are usually given in millimeters. In Anglophone countries, the unit mesh is often used.

For very fine particles (<10 microns ) are methods used, which allowed to settle the particles in a water column ( coarse particles fall faster than fine ) and regularly the density of the suspension is determined ( using a hydrometer ) or the mass of the deposited particles determines (sediment balance). Modern methods work with the laser light scattering on the particles, which varies depending on the particle size, or with digital image processing. In soil science, the hydrometer analysis is applied from a particle size of 0.063 mm = 63 micron (and smaller ) in Materials Science, the determination is carried out in the wash-out.

In the agricultural soil investigation, the finger test is used to determine the soil type and grain size in routine operation for the classification of analytical values ​​in nutrient content classes. According to the proportions of sand, silt and clay of the lime requirement is determined and created a proposal for the environment and need-based fertilizer.

Grain size distribution

The result of a grain size analysis, the particle size distribution, ie, a frequency distribution in the form of a bar or line chart. Against classified equivalent diameter (abscissa), the percentage (by weight) is applied to the grains. The usual statistical parameters such as mean, median, percentiles, dispersion or skewness of the distribution, as well as the uniformity coefficient can, calculated and thus characterize the sample with respect to their particle size.

In production processes, which depend on the raw materials or the product to defined particle sizes, particle size analysis and particle size distribution are an essential part of quality control. In sedimentology and Soil Science the particle size distribution is a very important feature for the characterization of soils and sediments. It serves their classification and is property -determining, for example, in water management, compaction potential or slope stability.

Grain size in the sedimentology and Soil Science

In sedimentology and pedology grain size analysis is a fundamental method of investigation of sediments, sedimentary rocks and soils. The grain size distribution is used for classification and nomenclature as well as the derivation and interpretation of properties. In soil science, soil type is defined by the mixture proportions of the different grain sizes, which is addressed in the context of soil mapping can be established on the finger test ( bottom ).

In principle, the broad spectrum is divided into the geosphere occurring particle sizes of well below one micron up to several meters in logarithmic classes. In detail, the division within the various geoscience disciplines from author to author, or between different countries varies. In German-speaking classification according to DIN 4022 has the largest circulation.

Grain

When considering flooring is to distinguish between the Siebkorn and the Schlämmkorn. The Siebkorn can be seen with the naked eye and has a particle size of greater than 0.063 mm. In contrast, the Schlämmkorn can be made visible only under the microscope. The particle size range is between 0.0002 mm and 0.063 mm.

Grain size classification

The classification as they are 4022 ( naming and description of soil and rock) in about the DIN. The DIN 18196 ( Soil classification for civil engineering purposes ) is thus largely compliant, but features some different and sets other conditions. Depending on the author, and especially in the U.S., the class boundaries are slightly to significantly different, and even the names of the major groups are internationally uniform.

Grain size in the petrology and mineralogy

In petrology ( lithology ) and mineralogy, a distinction between the absolute and the relative grain size in the microstructure of rocks and mineral aggregates.

Absolute grain size

The absolute grain size can already be roughly estimated with the naked eye or a microscope. For the concepts of macro- crystalline ( with naked eye ), microcrystalline (only visible under the microscope ) and cryptocrystalline ( under the microscope no longer resolve ) be used to differentiate.

Coarse - grained to giant crystal structure correspond with an average grain size of 5-30 mm in about the class of the gravels. Middle Grain crystal structure having an average grain size of 1-5 mm ( transition between gravel and sand rocks ). A fine-grained structure of grains with less than one millimeter in diameter is present in a medium-fine sandstone. In microliths or crystallites, the average grain size is only a few micrometers.

Relative grain size

The relative particle size analysis compares the overall structure of the individual crystallites ( grains) with each other. In an even-grained microstructure show the corresponding single crystals only small size differences.

Unequal granular structure can be designed very differently about it. Larger crystals are embedded in a fine-grained crystalline matrix and are called porphyritic texture. Vitrophyrische fabric beds larger crystals in a glassy matrix. In glomerophyrischen structures are crystal aggregates present in a fine-grained matrix.

In addition, the grain size within a rock can vary. This means that it changes in serialen structures in certain continuous intervals, or it changes irregularly or intermittently in hiatalen structures.