Sampling from falling streams

Fig. l.n Sampling from falling streams, (a) bad sampling technique, (b) good sampling technique, (c) sampling procedure to be adopted for high mass flow rate. 

Before a size analysis can be carried out, it is necessary to collect a representative sample of the solids, and then to reduce this to the quantity which is required for the chosen method of analysis. Again, the work of Allen gives information on how this is best carried out. Samples will generally need to be taken from the bulk of the powder, whether this is in a static heap, in the form of an airborne dust, in a flowing or falling stream, or on a conveyor belt, and in each case the precautions which need to be taken to obtain a representative sample are different. 

P. Gy and L. Marin, Unbiased sampling from a falling stream of particulate material, Int. J. Miner. Process, 5, 297-315 (1978). 

Stream sampling and flow sampling are terms usually reserved for the collection of sample increments from a free-falling stream of coal as opposed to the collection of increments from a motionless (stopped) conveyor belt. Coal that passes from one belt to another at an angle tends to become segregated because... 

Fig. 1.12 Ladle suitable for sampling from a falling stream of powder.

Especially for process control it is often necessary to determine particle size distributions of particulate solids that are in motion. The material may be transported either on or by a moving device, e.g. belt, vibrating, or drag chain, screw, bucket, and other conveyors, or in a carrier medium such as gases or liquids it may also fall freely or slide down chutes of various configurations. The best method to obtain a representative sample from such situations is to temporarily divert the entire stream into a container. If the particulate matter... 

Manual sampling is usually performed in low capacity handling and when the top size of the material is low. It can be done from a falling stream, from a stopped belt or from a stationary pile or hopper. 

The first option, from a falling stream, is usually used at a transfer point between conveyor belts, from under a discharging hopper or from the end of a pneumatic conveying system. Open ended scoops or shovels are unsuitable for manual sampling... 

For material flowing down chutes or from hoppers, a sample collector may be placed in the path of the outlet stream. Open-ended scoops or shovels are unsuitable in this case because they allow coarser particles to roll out of the sample and thus bias the sample toward the finer fractions. A ladle is the only recommended device for such sampling (Plowman, 1985) since it does not allow this rolling out to happen, imless overfilled. The falling stream is traversed with the laden to collect the sample, and if the whole of the stream... 

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