Spinning rifflers

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... 

Fig. 4 Spinning riffler used to collect powder samples.

Experiments with silica powder chosen from five depths in a two-pound container gave surface areas from top to bottom of 9.8,10.2,10.4,10.5 and 10.7 m g . When the same sample was poured into a conical pile, five random samples produced surface areas of 10.3, 11.0, 10.4, 10.0 and 10.6 m g . However, when the sample was riffled in a spinning riffler with three size reductions, the subsequent analysis of the contents of five collectors gave 10.2,10.1,10.2,10.2 and 10.1 m g as the specific surface... 

There are many laboratory devices available for the reduction of the primary sample to an analysis sample. The three most important methods used in the pharmaceutical industry are (i) scoop sampling, (ii) cone and quartering, and (iii) the spinning riffler or rotary sample divider (Fig. A-3). 

FIGURE 7 Subsampling (a) coning and quartering (b) table sampler [6] (c) oscillating hopper (paddle) sample divider [6] (d) spinning riffler (BSI) (e) riffle/chute splitter (BSI) if) revolving sample splitter [14]. 

The rotary sample divider or spinning riffler was first described in 1934 [20] and conforms to the golden rules of sampling. The preferred method of using this deviee is to fill a mass flow hopper in such a way that segregation does not occur. The table is then set in motion and the hopper outlet opened so that the powder falls into the collecting boxes. The use of a vibratory feeder is recommended to provide a constant flowrate... 

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. 

Binary mixtures of coarse and fine sand (60 40 ratio) were examined [39] with a spinning riffler to give the data in Table 54. The experiment was repeated using other sampling techniques to give the data in Table 1.6. In every case 16 samples were examined to give the standard deviations shown in column 2. It may be deduced that very little confidence can be placed in the first three techniques and that the spinning riffler is so superior to all other methods that it should be used whenever possible. 

A variation of this system is the installation of 0.5 in diameter sieves mounted on a spinning riffler to obtain small, representative powder samples, which can be sieved in situ without removing the miniature sieves from the riffler system [113]. By mounting several different aperture sieves around the riffler, a confederation of miniature sieves could replace nest sieving. 

A different kind of riffler that generally produces better samples, that is, samples that are more accurate and precise and thus more representative, is a spinning riffler (Allen and Kahn, 1970). Material is automatically fed by gravity... 

Fig. 1 Two devices that can be used to obtain a small representative sample from a larger supply. (A) Systematic representative sampling of a powder can be achieved with the spinning riffler (From Ref. ) (B) the spinning riffler is composed of several removable sample cups (C) chaos-generating devices, such as the free-fall tumbling mixer, can be used for powder homogenization and sampling (From Ref.t f) and (D) exploded view of the sample carrier.

Information on the Spinning Riffler System is available from Microscal Ltd., 20 Mattock Lane, Eahng, London, W5 SBH. 

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. ... 

Two important methods are the spinning riffler and the free-fall tumbling mixer. The first has been proven to be the most superior method of sampling in experiments and should be used whenever possible. The second has not been investigated fully but deserves... 

FIG. 21 -6 Examples of commercial splitting devices. Spinning riffler and standard splitters. Courtesy ofRetsch Corporation.)... 

It is very likely that the re-combined, primary sample taken from the whole is going to be too large for most powder tests and it, therefore, needs to be sub-divided into secondary or even tertiary sub-samples. This sub-division may be built into the primary sampling system or it may be achieved with a separate sample divider. Allen1 reviewed and tested most methods available for sample splitting and found the one based on the spinning riffler to be the best. 

The principle of the spinning riffler is shown in Fig. 1 and it embodies both golden rules of sampling. The sample is slowly conveyed by a vibratory feeder from the feed hopper onto a... 

The primary samples are usually collected by a primary cutter which cuts the full cross-section of the stream in a way similar to that of each compartment in a spinning riffler (Fig. 1) cutting across the falling stream of powder. The design of such cutters is subject to guidelines laid down in several recent publications.2 3 4 5,6 Thus, for example2 ... 

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