Powders sampling

Before any characteristics of a powder can be measured it is imperative to have a representative sample of the powder. This problem can be viewed in its true magnitude by considering that several tons of material will be analyzed on the basis of less than 1 gm of material. The ultimate that may be obtained in a representative sample is called the perfect sample the difference between this perfect sample and the bulk can be established by a statistical method, described in the following problem. 

A glaze formulation has poor color when a finely groimd silica powder has a fraction of iron impw ties larger than 50 ppm by weight. Let us assume that a 10 gm sample is taken from a 10,000 kg batch. In this lO gm sample, we find 40 ppm iron particles greater than 44 /um by sieving. 

It is very difficult to avoid bias in the selection and retrieval of powder samples. This is especially so for free flowing powders which are liable to segregate. If a considerable amount of experimental cost and effort is not to be wasted then a great deal of thought has to be put into the problems of where to sample and how to sample the powder mixture. 

The selection of samples, or where to sample, is the easier of the two problems. The selection of samples should not be left to the convenience of the operator as the convenient sample is usually heavily biased in a segregated powder. The sample at the top of the drum or first out of the mixer is not likely to be representative of the whole. The use of random numbers to identify sample locations is exemplified in section 2.3 and this technique removes the chance of operator selection bias. The systematic sample technique is useful as a routine industrial technique and involves taking a sample at regular time or space intervals. Care has to be exercised not to select an interval which coincides with a process interval such as batch size or shift change. 

Having selected the size and location of an individual sample this point sample has to be retrieved. The following rules should be followed to retrieve any sample of powder and especially those to be taken from a free-flowing powder.  

Avoid sampling from bulk powder and look for points in the process where the mixture is flowing. For example, sample a batch mixer as it is discharging rather than from the mixer itself. 

Sample the entire section of the flowing powder stream. 

2 Noninvasive Methods. Other, more technologically complex techniques have also been developed for visualizing the interior of granular beds. These include  

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This description is traditional, and some further comment is in order. The flat region of the type I isotherm has never been observed up to pressures approaching this type typically is observed in chemisorption, at pressures far below P. Types II and III approach the line asymptotically experimentally, such behavior is observed for adsorption on powdered samples, and the approach toward infinite film thickness is actually due to interparticle condensation [36] (see Section X-6B), although such behavior is expected even for adsorption on a flat surface if bulk liquid adsorbate wets the adsorbent. Types FV and V specifically refer to porous solids. There is a need to recognize at least the two additional isotherm types shown in Fig. XVII-8. These are two simple types possible for adsorption on a flat surface for the case where bulk liquid adsorbate rests on the adsorbent with a finite contact angle [37, 38]. 

Transparent solid samples can be analyzed directly by placing them in the IR beam. Most solid samples, however, are opaque and must be dispersed in a more transparent medium before recording a traditional transmission spectrum. If a suitable solvent is available, then the solid can be analyzed by preparing a solution and analyzing as described earlier. When a suitable solvent is not available, solid samples may be analyzed by preparing a mull of the finely powdered sample with a suitable oil. Alternatively, the powdered sample can be mixed with KBr and pressed into an optically transparent pellet. 

The crystal stmcture of glycerides may be unambiguously determined by x-ray diffraction of powdered samples. However, the dynamic crystallization may also be readily studied by differential scanning calorimetry (dsc). Crystallization, remelting, and recrystallization to a more stable form may be observed when Hquid fat is solidified at a carefully controlled rate ia the iastmment. Enthalpy values and melting poiats for the various crystal forms are shown ia Table 3 (52). 

Absorption of x-rays by a powdered sample of soHd fat has been a useful method for determination of polymorphic character as discussed eadier. The a, and P forms may be distinguished however, interpretation is made more difficult because subsets of the P and P forms have often been encountered. Also, a fat may contain mixtures of polymorphic forms and properties may therefore be difficult to relate to the spectra. 

Colorimetric Method. A finely powdered sample treated with sulfuric acid, hydrobromic acid [10035-10-6] and bromine [7726-95-6] gives a solution that when adjusted to pH 4 may be treated with dithi one [60-10-6] ia / -hexane [110-54-3] to form mercuric dithi2onate [14783-59-6] (20). The resultant amber-colored solution has a color iatensity that can be compared against that of standard solutions to determine the mercury concentration of the sample. Concentrations below 0.02 ppm have been measured by this method. 

Values given are typical of iron powder samples. Numbers in parentheses represent mesh sizes. 

The need for skill and experience on the part of sample designers and persoimel cannot be overemphasized in chemical plant sampling. Safety precautions are of the utmost importance. Necessary steps must be taken to document the hazards involved in an operation and to ensure that the staff are weU-trained, informed, protected, and capable. Except for bulk powder sampling, most chemical plant sampling is hazardous and difficult and must be designed with care. The following discussions are based on the assumptions that most of these decisions have been made and a satisfactory sampling procedure has been planned. 

X-ray diffraction consists of the measurement of the coherent scattering of x-rays (phenomenon 4 above). X-ray diffraction is used to determine the identity of crystalline phases in a multiphase powder sample and the atomic and molecular stmctures of single crystals. It can also be used to determine stmctural details of polymers, fibers, thin films, and amorphous soflds and to study stress, texture, and particle size. 

The values of m given above conform to Hemng s scaling law (1950) which states that since the driving force for sintering, the transport length, the area over which uansport occurs and the volume of matter to be transported are proportional to a, and respectively, the times for equivalent change in two powder samples of initial particle size ai q and 2,0 are... 

Usually, particle size has relatively little effect on Raman line shapes unless the particles are extremely small, less than 100 nm. For this reason, high-quality Raman spectra can be obtained from powders and from polycrystalline bulk specimens like ceramics and rocks by simply reflecting the laser beam from the specimen surface. Solid samples can be measured in the 90° scattering geometry by mounting a slab of the solid sample, or a pressed pellet of a powder sample so that the beam reflects from the surface but not into the entrance slit (Figure 3). 

Hgure 3 Schematic of a polyethylene slug die used to compact powder samples. When... 

W. E. Petit and G. Horlick. Spect. Acta. 41B, 699, 1986. Describes an automated system for direct sample-insertion introduction of 10-pL liquid samples or small amounts (10 mg) of powder samples. 

Porous samples would appear on the surface to provide such a complex and uncontrolled local environment for deformation of solids that they would be of little interest in scientific investigations. Indeed, the principal interest in their responses is technologically driven they are very effective attenuators of wave profiles and much of materials synthesis and processing is carried out on powders. Duvall [86D01] has summarized the difficulty of work with porous powder samples as follows ... 

In this chapter The background of shock-induced solid-state ehemistry eonceptual models and mathematical models chemical reactions in shock-compressed porous powders sample preservation. 

Table 6.2 summarizes the low pressure intercept of observed shock-velocity versus particle-velocity relations for a number of powder samples as a function of initial relative density. The characteristic response of an unusually low wavespeed is universally observed, and is in agreement with considerations of Herrmann s P-a model [69H02] for compression of porous solids. Fits to data of porous iron are shown in Fig. 6.4. The first order features of wave-speed are controlled by density, not material. This material-independent, density-dependent behavior is an extremely important feature of highly porous materials. 

A schematic drawing of the system is shown in Fig. 6.5. A powder sample is pressed in place in a cavity in a copper capsule. The cavity is closed with... 

From the results obtained by thermal decomposition of both low-molecular weight vicinal dichlorides in the gas phase [74,75] and of the copolymers of vinyl chloride and /rthermal instability of PVC to the individual head-to-head structures. Crawley and McNeill [76] chlorinated m-1,4-polybutadiene in methylene chloride, leading to a head-to-head, and a tail-to-tail PVC. They found, for powder samples under programmed heating conditions, that head-to-head polymers had a lower threshold temperature of degradation than normal PVC, but reached its maximum rate of degradation at higher temperatures. 

Measured with powder sample in tablet from under high pressure... 

Figure 19. Model fo the sudden volume increase of Mn02 powder sample at EPV (effective pore volume) point, (a) water fills 50% of the pores (b) water fills almost 100% of the pores (c) when excess water (1-2 mL more than the pore volume) is added, the Mn02 volume suddenly increases by 5-10 mL since the particles stick to each over. The sudden increase (far more than the amount of water added) is shown as H in (3), stage (c), above.

The deviations of Class I, here called absorption and enhancement effects, are known in the literature also as matrix effects, as self-absorption, and as interelement effects. The authors consider the most important objection to each of the last three names to be as. follows. To matrix effect the element sought (ncrt included in the matrix) contributes to the absorption effect for the sample in the same kind of way as any element (free or combined) in the matrix. To self-absorption the name makes no provision for enhancement effects. To interelement effects it fails to recognize that an absorption effect occurs even when only one element is present (Equation 7-4). The term matrix is useful but requires precise definition. What is the matrix when an internal standard is added, or when a powdered sample is dissolved ... 

It is sometimes convenient to use a properly chosen scattered line in the background for comparison to avoid having to add an internal standard to the sample.37 The experience of the Applied Research Laboratories shows that the effects of variations or fluctuations in the equipment and of particle size in powdered samples can be eliminated satisfactorily in this way. In some cases, absorption and enhancement effects are also adequately compensated. We have already mentioned (7.8) that scattered reference line and the analytical line will be subject to considerably different absorption effects if the two lines differ appreciably in wavelength. Everything depends upon the selection of a satisfactory scattered reference line, and this is done empirically. 

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