Slurry sampling errors

In order to reduce sampling errors, during the preliminary tests, in the sampling process, the slurry was sampled with a device, consisting of a rigid tube, 3cm in diameter and 3m in height, into which a plasticcoated steel cable was placed, to which a rubber sphere, 6cm in diameter was anchored. The sphere closes the lower end of the tube itself. 

Several systematic errors were investigated, mainly related to the sample pretreatment steps (microwave acid digestion, ultrasound acid leaching and slurry sampling), calibration mode and number of repeats. Experimental designs and principal components were used... 

Thin L-shaped probes are commonly used to measure solids concentration profile in slurry pipelines (28-33), However, serious sampling errors arise as a result of particle inertia. To illustrate the effect of particle inertia on the performance of L-shaped probes, consider the fiuid streamlines ahead (upstream) of a sampling probe located at the center of a pipe, as shown in Figure 2. The probe has zero thickness, and its axis coincides with that of the pipe. The fluid ahead of the sampler contains particles of different sizes and densities. Figure 2A shows the fluid streamlines for sampling with a velocity equal to the upstream local velocity (isokinetic sampling). Of course, the probe does not disturb the flow field ahead of the sampler, and consequently, sample solids concentration and composition equal those upstream of the probe. 

Direct solids sampling, however, be it directly with powders or with slurries, should be used very carefully. Indeed, the amounts of sample used are often of the order of a few mg and accordingly sampling errors may occur as a result of sample inhomogeneity. Therefore, the use of fairly large furnaces in which larger amounts... 

Different analytical procedures have been developed for direct atomic spectrometry of solids applicable to inorganic and organic materials in the form of powders, granulate, fibres, foils or sheets. For sample introduction without prior dissolution, a sample can also be suspended in a suitable solvent. Slurry techniques have not been used in relation to polymer/additive analysis. The required amount of sample taken for analysis typically ranges from 0.1 to 10 mg for analyte concentrations in the ppm and ppb range. In direct solid sampling method development, the mass of sample to be used is determined by the sensitivity of the available analytical lines. Physical methods are direct and relative instrumental methods, subjected to matrix-dependent physical and nonspectral interferences. Standard reference samples may be used to compensate for systematic errors. The minimum difficulties cause INAA, SNMS, XRF (for thin samples), TXRF and PIXE. 

Substances being measured—caffeine and theobromine in this case—arc called analytes. The next step in the sample preparation procedure was to make a quantitative transfer (a complete transfer) of the fat-free chocolate residue to an Erlenmeyer flask and to dissolve the analytes in water for the chemical analysis. If any residue were not transferred from the tube to the flask, then the final analysis would be in error because not all of the analyte would be present. To perform the quantitative transfer, Denby and Scott added a few milliliters of pure water to the centrifuge tube and used stirring and heating to dissolve or suspend as much of the chocolate as possible. Then they poured the slurry (a suspension of solid in a liquid) into a 50-mL flask. They repeated the procedure several times with fresh portions of water to ensure that every bit of chocolate was transferred from the centrifuge tube to the flask. 

Most mills control the cooking cycle by automatic time-temperature controllers and recorders. The rate of temperature rise to the conversion plateau must be slow to prevent hot pockets or cold areas. The rate of temperature increase to the inactivation plateau must be rapid to prevent excessive depolymerization in the intermediate temperature range. The viscometers operate according to different mechanisms time to expel paste from a sample device (Norcross) vibration of a probe in the paste (Dynatrol) torque readings (Brookfield) or pressure drop on passage through an orifice (Escher Wyss). Potential errors in viscosity can result from variations in starch solids due to differences in moisture content of the starch, errors in slurry preparation and the quantity of condensate added by the steam. The process yields a maximum paste concentration of about 32%. 

Combustion of coal produces ash that can be transported through the air. Slagging and fouling problems can also be predicted from elemental analysis. Therefore, elemental analysis of both the coal as well as the ash are important. Procedures for dissolution and analysis of coal and combustion products of coal have been reported [334-336]. Laser ablation sampling has been successfully used for coal and combusted materials [337,338]. The direct introduction of slurries has also been used [339]. Comparison of ICP-MS and PIXE analysis of coal combustion aerosols showed that analysis errors can occur in ICP-MS if particle vaporization is incomplete in the ICP [340]. 

Rushton (35) was the first to draw attention to the errors associated with wall sampling. Sharma and Das (37) mentioned that the mechanism of particle collection using an opening flush with the wall is different from the concept of isokinetic sampling. Moujaes (38) used wall sampling to measure solids concentration in upward vertical slurry flows. He found the sample concentration to be consistently lower than the true values in the pipe, especially with the coarse sand particles. 

Samples for trace metal analysis by A AS or ICP-OES must be presented to these instruments in liquid form. However, in some cases, samples are submitted as powders or solids (chippings, residues, etc.) requiring chemical decomposition prior to metal analysis that can lead to systematic errors in accuracy and precision of measurements. There have been many attempts to introduce samples as a slurry suspension and these were found to be successful for a limited number of samples provided that the particle sizes are suitably small. In most cases the nebulisation of the majority of samples analysed this way has shown that very low sample-introduction efficiency caused by variable particle sizes is in some cases difficult to dissociate, owing to the short residence times in the plasma. The availability of standards for this type of analysis is non-existent or difficult to obtain. 

Approximately 25 g of finely ground A-( — )D-[Co(N02)2(S-aigH)2]Cl is sieved with a 140-mesh screen, and the fines are discarded. The 20 g of complex which does not pass through the screen are slurried in toluene and packed into a chromatography column 1 cm X 50 cm. A 0.2-g sample of tris(2,4-pentane-dionato)cobalt(III)9 dissolved in 5 mL of toluene is applied to the top of the column. The column is eluted with toluene and 35 mL of eluate is collected prior to the emergence of the sample band. Sixteen fractions of 0.8 mL each (15 drops) are collected and each is diluted to 3.0 mL. From the absorbance of the solutions at 590 nm (e = 160 L mole- em1) and the optical rotations at 546 nm, molar rotations are obtained for the earliest and latest fractions collected. The middle fractions are almost inactive. There is a large error at the last two concentrations. 

Various techniques are available to measure velocity and solids concentration profiles in slurry pipeline. Sample withdrawal using an Li-shaped probe can give a representative sample at isokinetic conditions. Other sample devices will produce significant errors that must be corrected. Conductivity probes can be used to measure local velocity and concentration profiles simultaneously. However, the carrier fluid should be conductive. NMR imaging methods do not disturb the flow with a probe however, they are limited to pipes of small diameter. 

For the first live atomization sources listed in Table 8-1, samples arc usually iiilroduccd in the form of aqueous solulions (occa.sionally, nonaqueous soUilion.s are used) or less often as slurries (a slurry is a suspension of a ftnely divided powder in a liquid). For samples lhai are difficull lo dissolve, however, several methods have been used lo introduce samples into the atomizer in ihe form of solids or finely dispersed powders. Gen-crallv, solid sample-introduction techniques arc less reproducible and more subject lo various errors and as a result are not nearly as widely applied as aqueous solution techniques. Table 8-2 lists the common sample-introduction methods for atomic spcciro.scopy and the ispe of samples lo which each method is applicable. 

Before sample preparation the sample should be thoroughly blended. In the case of grains and powdered substances, liquids, and slurries, this reduces the possibility of stratification of the material, which can be a serious source of error. Stratification can occur in liquids due to differences in temperature, refractive index, composition, density, and other factors. Blending of forages means ensuring that the correct proportion of leaves to stems is maintained, and that the sample submitted... 

Certain errors can occur in evaluating the density of solids with heterogeneous mixtures. If the heavier slurry particles settle out and a sample is taken, it may reflect a greater density of finer particles. Due to these possible sources of error, the engineer is encouraged to measure the density of the slurry mixture after proper mixing, and to use the data on concentration by weight or by volume to work back to the density of the solids. 

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