Sampling Stored Nonflowing Material

Nonflowing material is a term for very fine cohesive powders, sticky material, moist material, or fibrous solids. These may be stored in small containers such as drums or bags, or in large containers such as trucks or railway wagons. Surface sampling is usually carried out with a scoop because of its simplicity: A presupposition is that the powder at the sampling point is representative of the bulk (i.e., that the powder was mixed before storage). Accuracy is increased by taking more than one sample, and the samples should be analyzed separately and combined in later analyses if the variation between samples is at an acceptable level.

Sampling Spears. Sampling accuracy is improved if samples from the body of the material are included, and this may be carried out with the aid of a sampling spear (thief). Three types are available (Fig. 1). Type 1 consists of an inner and outer tube with a closed end and longitudinal window so that the sampling chamber runs the full length of the spear; this generates a core sample. In type 2 the sampling chamber is at the end of the spear; this generates a spot sample. In type 3 there are separate chambers along the length of the spear; this generates several spot samples simultaneously. With a type 2 sampling spear it is usual to extract several samples at different depths, typically top, middle, and bottom, and blend these to obtain a composite sample. This procedure presupposes homogeneity within the container and should only be used when this assumption has been verified (i.e., the three samples should be analyzed separately a sufficient number of times to establish an acceptable level of statistical confidence).

Fig. 1 Sampling spears

The spear is thrust into the powder with the inner chamber closed off, and when in position the outer tube is rotated to allow powder to fall into the inner chamber. When the chamber is full, the inner tube is turned to the closed position and the spear is withdrawn. Possible segregation throughout the bed may be investigated with type 3, an average value for the length of the spear with type 2, and a spot sample with type 3. Frequently the spears are vibrated to facilitate filling, and this can lead to an unrepresentative quantity of fines entering the sample volume. The sampling chambers also have a tendency to jam if coarse particles are present, because they can get lodged between the inner and outer chambers.

Coning and Quartering. In industry it is common practice to sample small heaps by coning and quartering. The powder is formed into a heap, which is first flattened at the top and then separated into four equal segments with a sharp-edged board or shovel. The segments are drawn apart and, frequently, two opposite quadrants are recombined and the operation is repeated until a small enough sample has been generated. This practice is based on the assumption that the heap is symmetrical, and since this is rarely so, the withdrawn sample is usually nonrepresentative.

Sampling from Trucks or Wagons. Consignment sampling is carried out on a single consignment (e.g., a truck or wagon load). In sampling from a truck or a wagon it is recommended that eight samples be extracted (Ref 1). No increment should be taken at less than 12 in. below the surface; this avoids the surface layer, in which segregation can have occurred due to vibration (Fig. 2). Care needs to be taken to prevent powder sliding down the slope created due to removal of surface material.

Fig. 2 Sampling points for a wagon or a truck

Sampling Stored Free-Flowing Material. It is practically impossible to representatively sample stationary free-flowing powder because of the severe segregation that has almost certainly occurred. There is only one sound piece of advice to give regarding sampling such material: Don't! If there is no alternative, several samples should be taken and analyzed separately so that an estimate can be made of the degree of segregation.

Sampling from Containers. Suppose an analysis is required from several tons of material that is available in bags or small containers. Several of these containers should be selected systematically or, preferably, using a table of random numbers. The recommended number of samples depends on the number of containers (Table 1). The whole of each bag should then be sampled using a full stream or Vezin-type splitter so that the golden rules of sampling are obeyed. This is the only way to obtain a representative sample from each bag. Where this is not possible a sampling thief may be used. It is preferable to obtain a sample as the containers are being filled or emptied.

Table 1 Recommended number of containers to be sampled from a packaged lot

Number

of

Number of containers to

containers in lot

be sampled

1-5

all

6-11

5

12-20

6

21-35

7

36-60

8

61-99

9

100-149

10

150-199

11

200-299

12

300-399

13

Note: For every additional 100 containers, one additional container should be sampled. Source: ISO 3954

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