PART I PROJECT DEVELOPMENT
6. PLANT SAMPLING
By far the best way of sampling a moving stream of ore, concentrate or slurry is using a correctly designed sample cutter at the discharge point of a conveyor belt, chute or slurry pipe. The stream can be intersected at pre- selected times or tonnages, and representative samples, or increments, can be collected by taking a complete cross-section of the stream with a sample cutter (Gy, 1982a, b; Pitard, 1993; Holmes, 1997, 2002, 2004), thereby elim- inating the increment delimitation and extraction errors. A schematic of a suitable falling stream cutter taken from an Australian sampling standard Fig. 4. Automatic sample extraction system mounted on a reverse-circulation blast-hole drill rig, which utilizes a cone divider for sample division.
Sampling procedures 11
(AS 4433.1, 1997) is shown in Fig. 5. On the other hand, sampling devices that take only part of the stream,e.g., a bleed from a slurry pipe in a con- centrator, are likely to introduce serious bias and are not recommended.
When sampling from moving streams, the stream is divided into strata of equal time or mass, and increments are then taken from a fixed point of time or mass in each stratum (systematic sampling) or at random within each stratum (stratified random sampling). However, if periodic variations in qual- ity are present in the stream, stratified random sampling should be used to eliminate possible bias.
For mass-basis sampling, the increment masses should be almost uniform to ensure that each increment carries the correct weighting information, for example, by using a variable speed cutter that adjusts its speed increment by increment so that it is proportional to the flow rate at the time the increment is taken. For time-basis sampling, the increment mass must be proportional to the flow rate, so a fixed speed cutter is required. Unless these conditions are met, weighting errors will be introduced. Time-basis sampling is the easier to implement, but variations in flow rate should not be excessive, otherwise the increment mass may not be proportional to the average flow rate during the sampling interval.
Sample cutters for sampling moving streams must satisfy the following design rules to eliminate increment delimitation and extraction errors:
Fig. 5. Schematic of a falling stream sample cutter (from AS4433.1, 1997).
R.J. Holmes 12
The cutter must collect a complete cross-section of the stream, both the leading and trailing edges of the cutter (lips) completely clearing the stream at the end of each traverse. Cutters that collect only part of the stream on a continuous or periodic basis, e.g., shark-fin cutters (see Fig. 6), pressure- pipesamplers, T-pieces orbleedsfrom the side of a pipe (see Fig. 7), must be avoided at all costs.
The sample cutter must be non-restrictive and self-clearing, discharging completely each increment without any reflux, overflow or hang-up. This is particularly important for reverse-spoon type cutters, where the ore, con- centrate or slurry has to change direction as it strikes the back of the cutter body, otherwise sample reflux may occur. These comments also apply to shark-fincutters in mineral concentrators, although they are usually fixed in position as well, which is also incorrect as already discussed above. Gen- erously large cutter bodies and chutes are therefore required to avoid reflux and overflow from the cutter aperture. For sticky materials, steep chute angles (>601) and stainless-steel or polythene chute linings are recom- mended to reduce adhesion, and the cutter aperture may also need to be enlarged to prevent bridging.
The geometry of the cutter opening must ensure that the cutting time at each point in the stream is equal. Hence, for linear-path cutters, the cutter edges must be parallel, while for cutters travelling in an arc, such asVezin cutters, the cutter lips must be radial. Flap or diverter type cutters are always incorrect in this regard, because they divert one side of the stream for a longer period than the other (seePitard, 1993).
The cutter should intersect the stream either in a plane normal to, or along an arc normal to, the mean trajectory of the stream.
Fig. 6. Shark-fincutters are unsatisfactory sampling devices, because they do not take a full cross-section of the slurry stream.
Sampling procedures 13
The plane of the cutter lips must not be vertical or near vertical, because particles that strike the inside edge of the cutter lips, and which should appear in the sample, are deflected away from the cutter aperture by gravity into the reject stream.
The cutter must travel through the stream at a uniform speed, accelerating up to its cutting speed before entering the stream and then decelerating to a stop after leaving the stream. The cutter drive must also have sufficient power to ensure that the cutter does not slow down as it enters the stream and speed up as it leaves the stream.
The cutter aperture must be at least three times the nominal top size (d) of the ore or concentrate being sampled to prevent preferential loss of the larger particles, subject to a minimum of 10 mm for slurries and fine dry solids. For wet solids, a minimum of 50 mm is recommended to stop bridging.
Bucket-type cutters must have sufficient capacity to accommodate the in- crement mass obtained at the maximum flow rate of the stream without any reflux or overflow of material from the cutter aperture.
No materials other than the sample must be introduced into the cutter or the sample delivery chute. If necessary, both need to be covered when the cutter is parked between increments to eliminate ingress of dust or spillage.
If a belt scraper is required to remove material adhering to the belt, the scraped material must fall within the area traversed by the cutter.
When sampling slurries, the cutter lips must be sharp and the sample cutter streamlined to minimize turbulence as it cuts the slurry stream (Bartlett, 2002).
In addition, loss of sample material due to dribbling must be avoided. For Fig. 7. Taking ableedsample from a T-piece on a pipe in a concentrator is poor sampling practice and will not provide a representative sample.
R.J. Holmes 14
example, pipes must be bent to prevent slurry flowing back along the outside of the pipe, and the slope at the end of a launder must be sufficient to prevent slurry flowing back underneath the launder.
While both cross-stream and cross-belt cutters are used in industry for sam- pling ore and concentrate from moving streams, cross-stream cutters are recommended for two principal reasons. Firstly, cross-belt cutters tend to leave a layer of material on the conveyor belt if the skirts at the bottom of the cutter are not correctly adjusted as they gradually wear out or alternatively, maintenance staff tend to increase the gap due to fears of damaging the conveyor belt, so the increment extraction is incorrect. Consequently, cross- belt cutters can be seriously biased, because the material on the bottom of the belt is usually different in grade from the bulk of the material on the con- veyor belt. Secondly, it is virtually impossible to check visually whether a cross-belt cutter is performing correctly. In contrast, cross-stream cutters can be checked visually during operation to confirm correct increment delimi- tation and extraction, and visual inspections are far more economically effective than conducting statistically sound bias tests.
The cutter speed is important for cross-stream cutters andGy and Marin (1978)showed that it should not exceed 0.6 m/s at the minimum cutter ap- erture w0 (3d or 10 mm, whichever is the greater) to avoid extraction bias.
However, if the cutter aperture (w) is increased above this minimum, the maximum cutter speed (vc) can be increased as follows, subject to an absolute maximum of 1.2 m/s:
vc¼0:3ð1þw=w0Þ ð3Þ
Cutters can be found in mineral processing plants with speeds in excess of these limits to reduce the sample mass collected. However, the effective cutter aperture decreases as the cutter speed increases, and this can preferentially exclude the coarser particles and thereby introduce bias.
Theincrement mass mI(kg) taken by a cross-stream cutter is determined by thecutter aperture A(m), thecutter speed(vc) and thestream flow rate G(t/h) as follows:
mI ¼GA=ð3:6vcÞ ð4Þ
Consequently, for a given flow rate, the minimum increment mass that can be correctly taken to avoid bias is determined by the minimum cutter aperture and the maximum cutter speed. It is not possible to take unbiased increments of smaller mass unless the flow rate is reduced or the ore is crushed prior to sampling so that the cutter aperture can be reduced accordingly.
A very useful parameter for checking the design and operation of sample cutters is theextraction ratio, which is the ratio of the actual increment mass
Sampling procedures 15
collected to the theoretical increment mass calculated using Eq. (4). If this ratio is significantly less than 1, then the cause needs to be identified and corrective action taken to rectify the problem, e.g., reflux of ore from the cutter aperture or hang-up in the cutter chute due to capacity problems or blockages in the cutter chute. The extraction ratio should be determined as a function of flow rate, because problems with reflux become more serious as the flow rate increases.
7. SAMPLING FROM STATIONARY SITUATIONS