ISO 20904:2020 download.Hard coal – Sampling of slurries.
ISO 20904 sets out the basic methods for sampling fine coal, coal rejects or tailings of nominal top size <4 mm that is mixed with water to form a slurry. At very high ratios of fine solids to water when the material assumes a soft plastic form, the mixture is correctly termed a paste. Sampling of pastes is not covered in ISO 20904.
The procedures described in ISO 20904 primarily apply to sampling of coal that is transported in moving streams as a slurry. These streams can fall freely or be confined in pipes, launders, chutes, spirals or similar channels. Sampling of slurries in stationary situations, such as a settled or even a well- stirred slurry in a tank, holding vessel or dam, is not recommended and is not covered in ISO 20904.
ISO 20904 describes procedures that are designed to provide samples representative of the slurry solids and particle size distribution of the slurry under examination. After draining the slurry sample of fluid and measuring the fluid volume, damp samples of the contained solids in the slurry are available for drying (if required) and measurement of one or more characteristics in an unbiased manner and with a known degree of precision. The characteristics are measured by chemical analysis or physical testing or both.
The sampling methods described are applicable to slurries that require inspection to verify compliance with product specifications, determination of the value of a characteristic as a basis for settlement between trading partners or estimation of a set of average characteristics and variances that describes a system or procedure.
Provided flow rates are not too high, the reference method against which other sampling procedures are compared is one where the entire stream is diverted into a vessel for a specified time or volume interval. This method corresponds to the stopped-belt method described in ISO 13909-2.
2 Normative references
The following documents are referred to in the text In such a way that some or all of their content constitutes requirements of ISO 20904. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 1213-1, Solid mineralfuels — Vocabulary — Part 1: Terms relating to coal preparation
ISO 12 13-2, Solid mineralfuels — Vocabulary — Part 2: Terms relating to sampling, testing and analysis
ISO 13909-1, Hard coal and coke — Mechanical sampling — Part 1: General introduction
4 Principles of sampling slurries
4.1 General
For the purposes of ISO 20904, a slurry is defined as fine coal, coal rejects or tailings of nominal top size <4 mm that is mixed with water, which is frequently used as a convenient form to transport coal, rejects or tailings though plant circuits by means of pumps and pipelines and under gravity in launders or chutes or through long distances in slurry pipelines. Tailings from wet plants are also discharged as a slurry through pipelines to the tailings dam. In many of these operations, collection of increments at selected sample points is required for evaluation of the coal or rejects in the slurry.
A lot or sub-lot sample is constituted from a set of unbiased primary increments from a lot or sub- lot. The sample container is weighed immediately after collection and combination of increments to avoid water loss by evaporation or spillage. Weighing is necessary to determine the mass percentage of solids In the lot or sub-lot sample. The lot or sub-lot sample can then be filtered, dried and weighed. Alternatively, the lot or sub-lot sample may be sealed in plastic bags after filtering for transport and drying at a later stage.
Except for samples for which their characteristics are determined directly on the slurry, test samples are prepared from lot or sub-lot samples after filtering and drying. Test portions may then be taken from the test sample and analysed using an appropriate and properly calibrated analytical method or test procedure under specified conditions.
The objective of the measurement chain is to determine the characteristic of interest in an unbiased manner with an acceptable and affordable degree of precision. The general sampling theory, which is based on the additive property of variances, can be used to determine how the variances of sampling, sample preparation and chemical analysis or physical testing propagate and hence determine the total variance for the measurement chain. This sampling theory can also be used to optimize mechanical sampling systems and manual sampling methods.
If a sampling scheme is to provide representative samples, it Is necessary that all parts of the slurry in the lot have an equal opportunity of being selected and appearing in the lot sample for testing. Any deviation from this basic requirement can result in an unacceptable loss of accuracy. A sampling scheme having incorrect selection techniques, i.e. with non-uniform selection probabilities, cannot be relied upon to provide representative samples.
6.1 Minimizing bias
Minimization of bias in sampling and sample preparation Is vitally important. Unlike precision, which can be improved by collecting more slurry increments, preparing more test samples or assaying more test portions, bias cannot be reduced by replication. Consequently, sources of bias should be minimized or eliminated at the outset by correct design of the sampling and sample preparation system. The minimization or elimination of possible bias should be regarded as more important than improvement of precision.
Sources of bias that can be eliminated include sample spillage, sample contamination and incorrect extraction of increments, while a bias source that cannot be fully eliminated is that arising from variable settling rates of particles with different size and density during sample division prior to filtration.
The guiding principle to be followed is that increments are extracted from the lot in such a manner that all parts of the slurry have an equal opportunity of being selected and becoming part of the test sample that is used for chemical or physical testing, irrespective of the size, mass or density of individual particles in the slurry. In practice, this means that it is necessary to take a complete cross-section of the slurry when sampling from a moving stream.
The requirement of equal-selection probabilities shall be borne in mind when designing a sampling system and the practical rules that follow from this principle are as follows.
a) A complete cross-section of the slurry stream shall be taken when sampling from a moving stream.
b) There shall be no loss or spillage of the slurry sample.
c) The cutter aperture shall be at least three times the nominal top size of the particles in the slurry, subject to a minimum of 10 mm.
d) The cutter-slot length shall be substantially longer than the maximum depth of the falling slurry stream relative to the direction of cut to intercept the full stream.
e) The cutter lips on straight-path cutters shall be parallel, while the cutter lips of Vezin cutters shall be radial from the axis of rotation.
16 Packing and marking of samples
Samples for further preparation and/or analysis should be placed in airtight containers, with relevant information shown on the label and on a card placed in the container. The following are examples of such information:
a) identification of the lot, e.g. shift;
b) identification of sampler;
c) type, quality and nominal top size of the solids content of the slurry;
d) time duration of the lot or sub-lot;
e) sample number or portion of the lot or sub-lot the sample represents;
1) place and date of sampling;
g) method of sampling. e.g. mechanical or manual;
h) any special purpose or test for which the sample is taken.