Spinning riffler efficiency

When a sample of ca 100 g has been obtained, a representative sample for use in size characterization equipment must then be taken. Some of the more modem methods of size characterization require as Htde as 1 mg of powder, thus obtaining a representative sample can be quite difficult. If the powder flows weU and does not contain too many fines, a device known as the spinning riffler (Fig. 4c) can be used. A spinning riffler consists of a series of cups that rotate under the powder supply. The time of one rotation divided into the time of total powder flow should be as large a number as possible. Although this device has been shown to be very efficient, problems can be encountered on very small (1 mg) samples, and the powder must be processed several times. Moreover, in order to avoid cross-contamination, cleanup after each of the sampling processes can be quite difficult. Furthermore, if the powder is cohesive and does not flow weU, the equipment is not easy to use. A siUca flow agent can be added to the powder to enable the powder to flow... 

Figure 1.26). In 1959 Pownall [21] described the construction and testing of a large laboratory spinning riffler and a year later Hawes and Muller [22] described the construction of a small instrument. They also examined the riffler to find how various factors influence its efficiency using quartz and copper sulfate crystals of the same size. 

In another set of experiments, the relative efficiencies of different samplers were compared, and it was concluded that very little confidence could be placed in cone and quartering, scoop sampling, or table sampling. The spinning riffler emerged as so superior to the others that it should be used as often as possible. With the riffler, a minimum of 35 presentations are needed for optimum results to be obtained, and if the speed of rotation is too great, the efficiency falls as a result of air currents setup. ... 

A final bias has to be avoided when the samples are subjected to composition analysis. A sample withdrawn from a mixture is a point sample at a carefully chosen scale of scrutiny. When this sample is analysed for quality it is unlikely that the quantity required for analysis is the same as the scale of scrutiny for the process and it is vital that any sub-sampling of the point sample is carried out in an efficient spinning riffler. That is, for analytical purposes a bulk sample has to be taken from a point sample. Alternative methods ofobtaining a bulk analysis of the point sample would be to dissolve the entire point sample in a liquid or to grind the sample down to a fine cohesive state and elimate segregation. 

Figure 1.4. Several devices are available for taking a small representative sample from a large supply of powder [31, 32, 34]. a) A thief sampler consists of two concentric cylinders, b) The spinning riffler can efficiently produce small, representative samples.

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