8/20/2019 Particle Size Analysis (An003) Sampling Techniques

http://slidepdf.com/reader/full/particle-size-analysis-an003-sampling-techniques 1/2

 

 AN002 Sampling Techniques rev 0.doc

IntroductionReliable powder sampling is obligatory for allparticle characterisation procedures. The samplecollected from the bulk must be representative ofthe entire physical and chemical characteristics ofthe system in order for derived data to contain anymeaning. According to the ‘Golden Rule ofSampling’

1: 

1. A powder should be sampled when in motion2. The whole sample stream should be taken over

many short time increments, rather than part ofthe stream being taken for the whole of thetime.

Bulk SamplingSampling issues arise due to powder segregation.Variation exists in different regions of a heap orbag and also between sample containers. If staticsampling must be employed, samples should betaken from many random positions in different

bags or containers, and later combined to form asrepresentative a sample as possible. In the caseof heap sampling, if possible the heap should bemixed prior to sampling.

Sub-samplingThere are a variety of different sub-samplingmethods which can be used to turn a bulk sampleinto a more manageable laboratory sample.

Cone and quartering The powder is formed into a cone shaped heap ona flat surface, before being flattened with aspatula and divided into four identical volumes.One portion is taken and the entire procedure isrepeated until only 1/16th of the original volumeremains. This method is heavily operatordependant and is only suitable for powders withpoor flow behaviour and thus little segregation.Large particles will tend to roll to the outsideedges of the heap as it is made, while finematerial will adhere to the working surface, thusnarrowing the final overall size distribution.

Scoop Sampling The operator manually scoops a small portion ofpowder from the bulk sample. Only appropriate

for homogenous materials with poor flowcharacteristics.

Table Sampling The sample moves down an inclined plane,prisms divert and control the flow so that aliquotsof sample fall through holes in the table. Thepowder that is collected at the end of the table isthe required sub-sample (figure 1). Initial feed

should be representative and mixed thoroughly, toeliminate the main source of variation.

Figure 1: Table sampling

Chute The sample is introduced into a V shaped trough,feeding through a number of chutes, and collectedin two trays on either side of the trough. Theprocedure is repeated with the powder in one tray,until the desired amount is obtained. The troughmust be loaded without causing samplesegregation.

Spin Riffling  A spinning riffler (see figure 2) divides the sampleinto a series of glass containers. Powder istransported from the hopper, via a vibratory chute,into a series of containers rotating at a constantspeed. The sample flows into the receivers at aconstant rate causing them to be filled uniformly.The drawback to this method is that the samplevolume is relatively small, handling powders ingrams and milligrams.

Sampling Techniques

8/20/2019 Particle Size Analysis (An003) Sampling Techniques

http://slidepdf.com/reader/full/particle-size-analysis-an003-sampling-techniques 2/2

 AN002 Sampling Techniques rev 0.doc  Page 2 of 2 

Figure 2: Spinning riffler

For ease of comparison, a summary of theadvantages and disadvantages of each samplingtechnique is given in table I.

Table I: Sampling technique comparison

Method  Advantages Disadvantages

Cone &quarter

Good forpowders withpoor flowcharacteristics

Operatordependent

Scoop Reliable forhomogenous andnon-flowingpowder

Particlesegregation

Table Separates alarge quantity ofmaterial

Initial feeddependant

Chute Reduce powdersample by halfafter one pass

Operator bias

Spinningriffler

Reliable for freeflowing powders

Inability to dolargequantitiesefficiently

 A paper by Allen and Khan2  details a study

regarding the reliability of each of the samplingdevices mentioned in this application paper, andthe results are given in table II. It is clearly seenthat the spinning riffler is the most reproduciblemethod of sub-sampling, with cone and quarteringgiving the largest standard deviation.

Table II: Sampling technique reliability

Method Relative StandardDeviation (%)

Cone & quarter  6.81

Scoop 5.14

Table 2.09

Chute 1.01

Spinning riffler  0.125

ConclusionsThe most reproducible method of sub-sampling is

governed by the characteristics of the material.Using an incorrect method will result in a particlesize distribution not representative of the bulkmaterial.

References1 Allen, T., Particle Size Measurement, ed. R.Davies, 1996, Kluwer Academic Publishers, 824

2 Allen, T. and A.A. Khan, Critical evaluation ofpowder sampling procedures, The ChemicalEngineer, 1970, pp. 108 - 112

Escubed LtdLeeds Innovation Centre103 Clarendon RoadLeeds LS2 9DF UKtel +44 (0)870 126 3200fax +44 (0)870 126 3201www.escubed.co.uk