Powder sampling
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Powder sampling


DisciplinaProcessamento de Minerais I206 materiais2.052 seguidores
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Powder sampling 
1.1 Introduction 
There are many instances where estimates of population characteristics 
have to be made from an examination of a small fraction of that 
population and these instances are by no means confined to the field of 
powder technology. Regrettably, there are many workers in this field who 
still believe that sample selection procedure is unimportant. This results 
in the analyst being presented with hastily taken biased samples on which 
a great deal of energy is devoted to get precise results which do not reflect 
the characteristics of the bulk powder. Non-representative sampling 
results in incorrect analyses, process failure, unacceptable products and 
customer dissatisfaction. It is essential that the samples selected for 
measurement should be representative of the bulk in particle size 
distribution and the relative fractions of their various constituents, 
irrespective of whether a physical or chemical assay is to be carried out, 
since these characteristics are frequently inter-dependent. The magnitude 
of the problem may be realized when one considers that the characteristics 
of many tons of material may be assumed on the basis of analyses carried 
out on gram, or even milligram, measurement samples so the chances of 
carrying out the measurement on a non-representative sample are 
considerable. Two case studies illustrate problems that may arise. 
In case 1 a binary mixture of fine and coarse granules was blended to a 
satisfactory end-point. The blend was next emptied into a bucket elevator 
and thence to a hopper through a central feed point. The material was then 
fed, in 60 kg lots, to an extruder to produce an unacceptable finished 
product. The process started with a mixing operation followed by a 
segregating operation in the hopper. The central region (core) of the 
hopper contents was rich in fines and the outer regions rich in coarse, 
hence the initial feeds to the extruder contained an excess of fines whereas 
later feeds contained an excess of coarse. Since a bimodal mixture such 
Powder sampling and particle size determination 
as this has a high tendency to segregate it is better to weigh-feed the two 
streams concurrently into the extruder: This also has the advantage of 
eliminating process steps resulting in lower operating costs and a less 
expensive plant. The second case was a tabletting operation where a well-
mixed fine powder was emptied into a core-flow hopper in such a way 
that segregation occurred. The resulting tablets were well outside 
specification. The solution was to replace the hopper with a properly 
designed mass-flow hopper. 
The total sampling error is made up of errors due to the primary 
sampling, subsequent sample dividing and errors in the analysis itself 
Sampling is said to be accurate when it is free from bias, that is, the error 
of sampling is a random variable about the true mean. Sampling is precise 
when the error variation is small irrespective of whether the mean is the 
true mean or not. The ultimate that may be obtained by representative 
sampling may be called the perfect sample; the difference between this 
sample and the bulk may be ascribed wholly to the expected difference on 
a statistical basis. Errors in particle size analysis may be due to: 
\u2022 instrument limitations; 
\u2022 improper procedure e.g. inadequate dispersion, particle fracture during 
handling; 
\u2022 operator errors e.g. improper instrument set-up or poor calibration; 
\u2022 incorrect sampling. 
Two types of sampling errors are possible [1] 
\u2022 Errors due to segregation of the bulk; this depends upon the previous 
history of the powder and can be minimized by suitable mixing and 
building up the sample from a large number of increments. 
\u2022 Statistical errors that cannot be prevented. Even for an ideal random 
mixture the quantitative distribution in samples of a given magnitude is 
not constant but is subject to random fluctuations. It is the only 
sampling error, which cannot be suppressed and occurs in ideal 
sampling. It can be estimated beforehand and reduced by increasing 
the sample size. 
1.2 Sample selection 
Samples are withdrawn from a population in order to estimate certain 
characteristic of that population and to establish confidence limits for that 
Powder sampling 3 
characteristic. The characteristic may be particle size, composition or 
quality; a measure of the spread of the distribution may also be required. 
The objective may be to set up limits between which the quality of a final 
product is acceptable, to decide whether the characteristics of a given lot 
meets preset criteria, or it may be to estimate the variability within a lot or 
between lots. 
If the material comes in containers, or can be viewed as discrete units, 
the objective may be to estimate the number of units outside of 
specification. The value of the estimate is largely dependent on the 
sampling technique adopted. It is of little value, and could impart false 
information, if it is biased or imprecise. 
It is usually impossible to measure the size distribution of a bulk 
powder and so it is necessary to carry out measurements on a sample 
extracted from the bulk. This sample is itself frequently too large and has 
to be further sub-divided. 
The probability of obtaining a sample that perfectly represents the 
parent distribution is remote. If several samples are taken their 
characteristics will vary and, if these samples are representative, the 
expected variation can be estimated from statistical analysis. However, 
the sampling equipment will introduce a further variation, which may be 
taken as a measure of sampler efficiency. Imposed on this there may also 
be operator bias. The stages, in reducing from bulk to measurement 
samples may be conveniently divided into the five stages illustrated 
below: 
bulk or process 
stream 
(10" kg) 
gross 
sample 
(>kg) 
laboratory 
sample 
(<kg) 
test 
sample 
(g) 
measurement 
sample 
(mg) 
The gross sample is either one of a series of spot samples that have been 
extracted in order to determine the variability of the bulk or process 
stream with location or time or it is made up of sub-samples to be 
representative of the bulk as a whole. In some cases the gross sample is 
too large to send to the laboratory and has to be reduced in quantity. This 
reduction needs to be carried out in such a way that the laboratory sample 
is fully representative of the gross sample. When this reduction is 
unnecessary the gross sample is also the laboratory sample. 
The laboratory sample may be required for a number of tests, so it is 
sometimes necessary to further sub-divide it into test samples. 
Finally, the test sample may be used in its entirety or further sub-
divided to form the measurement sample. 
4 Powder sampling and particle size determination 
The object of sampling is to gain knowledge of the characteristics of the 
whole from measurements impracticable to apply to the whole; bias at any 
of the reduction stages adversely affects the final analysis. Problems arise 
due to inhomogeneities in the parent distribution. If the bulk powder is 
homogeneous, or can be mixed prior to sampling in order to generate a 
homogeneous powder, sampling problems do not arise. In order to 
establish homogeneity it is necessary to examine samples taken from the 
bulk, either at random or according to some pre-determined pattern. If 
homogeneity is established a single sample is representative of the bulk. 
The definition of homogeneity requires specification of the portion or 
sample size between which the variability is sufficiently small to be 
neglected. 
Temporal or spatial variability of inhomogeneous powders may be 
random, i.e. there is no discernible pattern and it is impossible to predict 
the value at any one point from knowledge of the value at any other point. 
In this case it is necessary to examine a number of samples in order to 
establish a mean and a standard deviation [2]. Non-random