1、INTERNATIONAL STANDARD IS0 7936 First edition 1992-04-l 5 Hard coal - Determination and presentation of float and sink characteristics - General directions for apparatus and procedures Houille - D6termination et prhsentation des caractkistiques de flottation et denfoncement - Principes directeurs re
2、latifs B Iappareillage et aux modes opbatoires IS0 7936:1992(E) Foreword IS0 (the International Organization for Standardization) is a worldwide federation of national standards bodies (IS0 member bodies). The work of preparing International Standards is normally carried out through IS0 technical co
3、mmittees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, govern- mental and non-governmental, in liaison with ISO, also take part in the work. IS0 collaborates closely with
4、the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an Inter- national Standard requires approval by at least 75 % of t
5、he member bodies casting a vote. International Standard IS0 7936 was prepared by Technical Committee ISO/TC 27, Solid mineral fuels, Sub-Committee SC 1, Coal preparation, Terminology and performance. Annexes A and B form an integral part of this International Standard. Annex C is for information onl
6、y. 0 IS0 1992 All rights reserved. No part of this publication may be reproduced or utlllzed In any form or by any means, electronic or mechanical, Including photocopying and mlcrofllm, wlthout permission In wrltlng from the publisher. International Organization for Standardization Case Postale 56 C
7、H-1211 Gen 1,5; 1,6: 1,7; 1,8; I,9 and 2,O. Relative densities less than I,3 and above 2,0 may also be required. Additional separation at intermediate relative den- sities will be found useful where cumulative ash is increasing rapidly in relation to the cumulative yield. As stated in 4.2, each rela
8、tive density fraction should weigh at least 20 g and should contain at least 10 discrete particles. Where it is known or suspected that the sample will disintegrate or otherwise react on contact with water or aqueous solutions, separations are to be carried out using organic liquids. However, the fa
9、ct that the raw coal will react with water will affect its behav- iour in the cleaning process, and any information which will provide guidance should be obtained for reference purposes. NOTE 4 Where water quality problems are suspected, water of at least “potable” grade should be used to pre- pare
10、aqueous suspensions and inorganic solutions. 6,1.2 Organic liquids Where the separation is critical, particularly in finer sizes, the use of organic liquids is preferred (see note 5) because of their low viscosity, low volatility and inertness towards shales. Some organic liquids and their physical
11、properties are listed in table 3. NOTE 5 Some organic liquids may influence subsequent analyses. Table 3 - Typical physical properties of organic liquids used in float and sink analysis Relative Distillation range or Organic liquid Vapour pressure density boiling point at Viscosity at 20 “C Flammabl
12、e 100 kN/m (100 kPa ) at 20 “C “C mPa*s (mNs/m*) kPa (kN/m*) White spirit 0,77 30 to 200 - Yes Petroleum spirit) 2) 0,73 37 to 185 0,548 2533 Yes Toluene 0,87 110,7 0,588 2,93 Yes Kerosene 0,75 165 to 230 1,365 0,ll Yes o-Xyiene 0,88 144,4 0,810 0,68 Yes m-Xylene 0,86 139,0 0,620 0,85 Yes p-Xylene 0
13、,86 138,4 0,648 0,92 Yes Bromoform 2,79 150,o 2,152 (at 15 “C) 0,60 No (tribromomethane) Sym tetrabromoethane 2,96 239 l2,O 0,Ol No (acetylene tetrabromide) Tetrachloroethyiene I,61 120,8 190 1,83 No (perchloroethylene) SAFETY PRECAUTIONS Particular attention is drawn to. the fact that many solvent
14、vapours present a serious health hazard and hence adequate ventilation, preferably down-draught, is essential. As the use of some organic liquids is governed by safety regulatlons, the user Is urged to ensure compliance with the relevant statutory regulations. 1) Mixtures of air and petroleum spirit
15、 vapour are highly explosive. 2) if petroleum spirit is used it should be ieadfree. IS0 7936:1992(E) Where relative densities of I,6 and less are re- quired, mixtures of perchloroethylene and one of the less dense liquids may be used. Where relative densities of I,6 to 2,9 are required, mixtures of
16、perchloroethylene and one or more of the more dense liquids may be used. NOTE 6 Tetrabromoethane and bromoform have ex- tremely love vapour pressures. After use, it is therefore necessary to rinse them from the sample by a more rap- idly evaporating solvent. Organic liquids are costly but are freque
17、ntly pre- ferred to aqueous solutions, since the products of the separation are easier to deal with and prolonged washing and drying times are unnecessary because of the volatility of the solvents. They should be used sparingly and it is recommended that solvent re- covery be practised, particularly
18、 by drainage, after the coal is removed from the separation tank. SAFEN PRECAUTIONS Particular attention is drawn to the fact that many solvent vapours are toxic and present a serious health hazard, and hence adequate ventilation Is es- sential, preferably down-draught (see figure 3). Suit- able pro
19、tection to avoid contact with the skin Is also required. Many countries have statutory require- ments concerning the use of organic liquids with respect to toxicity and fire; these should be ob- served. Equation (2) may be used to calculate the volumes of liquids required in formulating a mixture at
20、 the desired relative density. It is important that the rel- ative density of the resultant mixture be checked, for example by means of a hydrometer with maximum scale divisions of 0,002. Pt - PP vm = v, pm - pp where vm is the volume of the liquid with higher relative density; v, is the volume of m
21、ixture desired; Pm is the relative density of the denser liq- uid: Pt is the desired relative density of the mix- ture; PP is the relative density of the less dense liquid. 6.1.3 inorganic solutions Inorganic solutions may often be used in place of organic liquids, but not where any portion of the s
22、ample is subject to disintegration in water to an extent which will influence the accuracy of the test. For samples of less than 8 mm particle size, be- cause of the effect of viscosity on the separation process, longer times of separation are required. The quantity of sample immersed at any one tim
23、e therefore has to be controlled to achieve complete separation. Zinc chloride is a commonly used inorganic salt, but it has several disadvantages which should be con- sidered carefully. Zinc chloride solutions are cor- rosive and hence care should be exercised in the choice of the container used in
24、 the test. Further- more, the pores of the sample frequently become permeated by the zinc chloride solution which is difficult to remove even with prolonged washing with fresh water. The presence of residual zinc chloride may introduce errors in mass and may also affect the analysis of the ash. WARN
25、ING - Zinc chloride must not be allowed to contact the skin. Where contamination by zinc chloride is likely to af- fect any of the coal analysis results, a measure of the level of zinc chloride in the wash water should be established. 6.1.4 Solids in aqueous suspension Insoluble material with a medi
26、um to high relative density and the correct particle size distribution may be used to give a relatively stable suspension of low viscosity. Examples of suitable materials are given in table4. IS0 7936:1992(E) Table 4 - Suitable solids for aqueous suspensions Material Relative density Nominal top siz
27、e w Comments +ely ground shale Froth flotation :alllngs Barytes Magnetite Ferrosilicon 250 250 63 38 38 Discard from a coal preparation plant Brlckwork shales Commercial barium sulfate As used In coal preparation plants Ground or atomized NOTES 1 All of these solids can be used separately or in mixt
28、ures. Bentonite may be used to stabilize a sus- pension. 2 For separations with relative densities above 1,5, ground shale or froth flotation tailings will require an addition of higher relative density materials to avoid viscosity problems. 3 Aqueous suspensions require continuous mixing to keep th
29、e solids from settling and to keep the mixture homogeneous, which in turn can affect the gravimetric separation of the coal particles. The procedure for using aqueous suspensions is similar to that recommended for use with inorganic solutions. The separated fractions are thoroughly washed with water
30、 to remove adhering medium. It is important to have a device for accurately deter- mining the relative density of the suspension. A suitable apparatus, which should be calibrated with water at 20 “C, is shown in figure I. Aqueous suspensions are non-toxic and non- volatile, and therefore fume extrac
31、tion is not re- quired. Provided that products are washed free of medium, no adverse effects on product quality oc- cur. Aqueous suspensions are not recommended for testing particles below 4 mm. 6.2 Apparatus The apparatus should be unaffected by the liquids or suspensions involved and should be con
32、venient for use. Types of apparatus which have been found to be suitable are shown in figures 2 to 5. The apparatus for separating the float fraction from the sink fraction may consist of a basket with a movable partition which allows the float and sink fractions to be dealt with simultaneously whil
33、e keeping them separate. Alternatively, a tank with a fine mesh base made to fit inside another tank, forms a useful means of recovering floats and sinks separately. In thls case, the floats are skimmed off the liquid surface using a fine mesh scoop and the sinks are recovered by raising the inner p
34、erforated base tank and draining off the liquid. NOTE 7 The mesh used for both the scoop and the basket should have openings small enough to ensure that solid particles are retained. 6.3 Test procedure 6.3.1 Basic method The separation tank is partly filled with the required medium, the relative den
35、sity of which is then checked using a suitable hydrometer. The actual relative density should be adjusted to the correct reading, and frequent checking should be carried out to ensure that it remains within the range of f 0,002 of the desired relative density throughout the test. An increment of the
36、 size fraction under test, which is sufficient to form a thin layer, is introduced into the tank containing the medium and gently agitated. Care needs to be taken not to overload the tank, as this is liable to interfere with the separation of en- trained near-density material. After allowing suf- fi
37、cient time for separation, the float material is removed and collected on a draining platform or tray. The settled sinks material Is agitated to release any entrained float material. This process is re- peated until all of the test sample has been separ- ated. The time for separation will vary accor
38、ding to the type of separating medium used and the particle size. NOTE 8 To reduce the time required for gravimetric separation of coals finer than 1 mm, a centrifugal float and sink procedure may be employed, as described in annex A, provided that the requirements of table1 are met. The centrifugal
39、 float and sink procedure is rec- ommended for minus 0,063 mm material, for improved separation efficiency and reduced separation times. This method gives reliable results only if certain precautions are taken and the liquids are correctly selected. 6.3.2 Testing in ascending order of relative densi
40、ty This sequence is used where it is known from pilot testing or previous experience that the majority of the sample is of low relative density. The sample is introduced into the medium of lowest relative den- sity. All float material is washed free of medium if necessary, dried In air, weighed and prepared for analysis when required. The sink material is well 6
