IE52614B1 - Aqueous coal suspension - Google Patents

Abstract

An aqueous coal suspension contains two separate groups of coal particles, preferably with the particles of the first group having an average size of from 210 to 60 mu m, the maximum size not exceeding 300 mu m, and the particles of the second group having an average size of from 1/6 to 1/20 of the average size of the particles of the first group. When plotted on a bilogarithmic scale, the cumulative particle size distribution curve (see for example Figure 1) has a flat zone lying between the values representing the average sizes of the two groups of coal particles. In addition to coal and water, the suspension contains a non-surface active anionic polyelectrolyte having a molecular weight exceeding 500, and possibly a stabiliser of the organic gel type.

Description

This invention relates to an aqueous coal suspension. More particularly, the invention relates to an aqueous coal suspension of very high concentration, which suspension facilitates the transportation of coal, and which suspension can be used directly as a fuel.

There are many known methods for preparing aqueous coal suspensions.

Thus, U.S. Patent No. 3 762 887 describes a coal suspension having a particle size from 4 to 325 Tyler mesh, which suspension has a maximum coal concentration of 69% by volume.. Most of the coal particles have a particle size of from 4 to 28 Tyler mesh. This means that the particles can have sizes of more than 4 mm.

The suspension has two drawbacks, namely the coal particles are mostly of rather large size, and the concentration of coal is relatively low. These two drawbacks, and especially the first, means that the suspension can be used directly as a fuel only in special equipment such as cyclone burners, and it cannot be used in conventional combustion equipment. It should also be noted that even with special combustion equipment, i.e. with a long dwell time of the type heretofore described, there is always the drawback of having to discharge the unburnt coal together with the smoke and/or ash in the case of a malfunction for any reason.

German Offenlegungsschrift No. 2 823 568 describes an aqueous suspension of coal particles having a size less than 100 pm. The preparation of this suspension is however complicated due to the fact that, as the preparation is carried out simultaneously with the separation of inert material from the coal by the action of a polyelectrolyte which gives the coal particles an electrical charge different from that of the particles of the inert material, the coal has to be dispersed in water at a concentration not exceeding T0% by weight, with the consequence that after separating the inert material, the coal suspension must be concentrated, this having the drawbacks of a low filter efficiency due to the presence 30 of fine coal particles, and of a relative high separation cost.

According to the present invention, there is provided an aqueous suspension of coal particles having a size not exceeding 300 pm, the particles being divided into two groups of particles of different sizes, the coal particles of the first group having an average size of from 210 to 60 pm, and the coal particles of the second group having an average size of from 1/6 to 1/20 of the average size of the particles of the first group; and the suspension containing a non-surface active anionic polyelectrolyte.

It has now been surprisingly found that the drawbacks of the known art as heretofore described can be easily obviated by choosing the coal particles in two suitably different size ranges, and by adding to the suspension a polyelectrolyte preferably chosen from a class of compounds of particular chemical structure.

The average particle size of any given group of particles is that size corresponding to 50% of the cumulative mass distribution of that given group.

The particles of the first group preferably form at least 40% by weight, more preferably about 60% by weight, of the total particles.

As the cumulative particle size distribution curve represents the resultant of two fractions (i.e. two separate groups of coal particles), it preferably shows when plotted on a bi logarithmic scale (log-log diagram) a flat zone between the values representing the average sizes of the component fractions. A flat zone means a portion of the curve at which the derivative, when calculated on a bi logarithmic scale (log-log·diagram), is less than 0.4, preferably less than or equal to 0.1, and more preferably zero.

The cumulative particle size distribution must therefore be such that there are always two particle sizes dj and d2, lying between the average diameters of the two fractions, for which the numerical value of the expression; 4 log (%CM1 - log (%CM2) log d-j - log d2 is less than 0.4, preferably less than or equal to 0.1, and more preferably zero. In this expression, %CM1 and %CM2 indicate the total percentages of the mass of particles of size less than dj and d2 respectively. The numerical value of the expression is obviously independent of the unit of measurement (e.g. micrometres or millimetres) in which the particle sizes are expressed.

In addition to coal of the aforesaid sizes, the coal suspension according to the invention contains a non-surface active anionic polyelectrolyte, in particular a polyelectrolyte containing alkyl-substituted polynuclear aromatic groups, and having a molecular weight greater than 500, more preferably from 800 to 3000. In particular, monovalent cationic salts of polymerised alkylnaphthalenesulphonic acids with a molecular weight of around 2000 can be used, examples thereof being the chemical compounds known by the commercial names *DAXAD 15 and *0AXAD 19, made by W.R. Grace, and *Reoplast 203, made by Fratelli Lamberti S.p.A.

The amount of polyelectrolyte in the suspensions is generally from 0.1 to 1% by weight, preferably from 0.3 to 0.5% by weight, more preferably about 0.4% by weight, with respect to the entire suspension.

The polyelectrolyte is preferably added in the form of a concentrated aqueous solution. The polyelectrolyte suspension is added either to *DAXAD and *Reoplast are Trade Marks 4 the water before dispersing therein the coal particles of both groups, or to the aqueous suspension of the coal particles of the second group when prepared for example by wet grinding, before adding the particles of the first group.

The suspension according to the invention can also contain a stabiliser of the organic gel type, preferably a polysaccharide such as those known under the name *Kelzan, made by Kelco.

The stabiliser, which is preferably used in an amount of from 0.01 to 0.03% by weight of the total suspension, more preferably 0.02% by weight, can be added at the same time as or after the polyelectrolyte.

The stabilizer is preferably used only when the coal suspension is to be stored for a long period. This depends on the nature of the coal, and for example the period can be of the order of one month.

The aqueous coal suspension according to the present invention may also comprise any inert material which accompanies the coal. In this respect, the method for preparing the coal suspension according to the invention differs from other methods, which latter methods require the previous separation of the inert material, the coal-water suspensions being prepared only after this latter operation. Consequently the inert material does not constitute an obstacle to the preparation of a stable, fluid suspension of high concentration.

The coal content can attain very high values, for example 80% by weight or higher.

With regard to the type of water to be used for preparing the coal suspension according to the invention, it has surprisingly been found that this can be of any type, and thus normal fresh water or demineralised water *Kelzan is a Trade Mark or even salt water containing up to 40 g/litre or more of dissolved salts, and in particular sea water, can be used.

The coal suspension according to the present invention is preferably prepared as follows. The coal is ground to two particle size levels, the grinding being carried out either dry or wet (i.e. in the absence or presence of water), to obtain either the two groups of coal particles in the case of dry grinding or aqueous slurries of the two groups of coal particles in the case of wet grinding. The grinding is preferably carried out dry to obtain the particles of smaller size.

The polyelectrolyte can be present during wet grinding. The two groups of particles are mixed together in the described proportions when prepared both dry or both wet. The polyelectrolyte is added either to the water or, if only the coal particles of smaller size are prepared wet, to the aqueous suspension of the coal particles of smaller size. Suitable quantities of either the two mixed groups of coal particles are added to the aqueous polyelectrolyte solution, or the coal particles of greater size are added to the aqueous suspension of the coal particles of smaller size containing the polyelectrolyte. If desired, a stabilizer is added either with the polyelectrolyte or after the addition of the polyelectrolyte but before the addition of the mixed coal particles of the two groups or of those of greater size, or after the two groups of coal particles have already been fed into water.

Examples of coal suspension of the invention are given in the following Table 2, which suspensions contain coals of the types given in the following Table 1. The percentages in Table 1 are by weight.

CHARACTERISTICS OF TESTED COALS South African o CO H 4 Ο -3* · · · Ό H * N N £3 cd nd 3 3 O § O rd N O rd rd Π M3 H 1 Chinese O \O Jt o tn · _ ό ο · σ> to o \o o At t\i r-t Polish 2 1 o 1 Polish 1 o c\ _ o c*- co ο · N N · xO rd rd 0Λ O f Type of coal ϊ L.C.V. (KCal/kg) Moisture % Ash $ Volatile natter $ Fixed carbon $ Sulphur $ 58614 <<<<<<<<<<<<<^ 5§'>>'§-'3''l'>> CXI ο pH ο γΗ Ο pH Ο Ν Ο • ° 6 ο ο ο ο ο ο ο ο Ο Ο Ο Ο ο ο Ο Ο ο ο ο ο ο .’Λ όόο’όαοόόόόόόόόοόοοόοααο £s.\0\0\Q\0\0\OvOOO £ £ $ £ C< £ £ Ό C^ Ά Ά ΙΑ Ά χθ Ά 00 ^ CM<> gs ??sssn3ssss?s ????nsss TABLE 2 Sj^A O's?*· To 3 3 m S s 3 ? 3 ϊ ί Ϊ ? n 3 2 3 ιηΆΙΑΟΟΟΟΟΟ ΆΆχΟΌΌΌΌΌ Ό g Ο Ό V\ ΧΑ Ά Ά Ο Ο Ο Ο Ο ιΑΙΑΙΑΙΑχΟχΟΌΌχΟ Jits 0 Ο fr* O fr* Ά fr- O fr- O O & ΙΑ Ά O Εχ- ζχ- cm O «3* O xO O r- O fr- O fr- O 0* O c*- O 0- O Cx- O fr- O O O CX- Ά CM 3 Α 8 8 O CM Ά pH Ά pH s 8 Ά pH Ά pH CM rs- 00 IA rH 8 O CM s 8 Ά pH v\ rH Ά rH Ά pH W\ H U\ H pH pH jA cm CM C*- pH e- c* pH OX O- pH fr. CM pH K rH *A fr- pH OX O- pH OX pH 0 Ά 0- CM rH CM & CM e» pH CM IS pH CM 0- H CM H CM CX- rH Cx- CM rH Ά & Ά rH Ά & ex- CM rH 3 XX ^Α 0 rH CM O f! O pH CM O pH CM O Ά pH O Fl O Fl O rH CM O Fl CM CO O Ά pH 0 $ O Fl O O pH CM O rH CM O 3 £ pH 0 CM O 0 rH CM $ pH CA OO ' a pH CM 5 C re υ >* O i. Ρ £ (fl _ - 2 2 J2 Π — 2 2 s- M O £ = = 2 2 •rt β d pH pH «Η .c δ • δ & fi P co • Ά XO O'- 00 Ox © pH CM CA tS CM -=f iA xO CO Ox 0 pH CM CA -4- pH pH rH Pl pH CM CM CM CM pH pH pH rH rH SS614 LEGEND dml : average particle size first group (nm) dm2 : average particle size second group (pm) Cwl : weight percentage of particles of first 5 group (with respect to dry matter) Cw2 : weight percentage of particles of second group (with respect to dry natter) Cw (total) : weight percentage of coal in the aqueous suspension 10 Cw DAXAD 15 : weight percentage of DAXAD in the aqueous suspension Cw KSLZAN : weight percentage of EELZAN in the aqueous suspension A/- : viscosity between 350 and 700 cP 15 3/- : viscosity greater than 700 and up to 1000 cP 0/- : viscosity greater than 1000 cP -A : Newtonian or weakly Bingham plastic behaviour -A .: 3ingham plastic behaviour 20 W/- : excellent stability 2/- : good stability 1/- : acceptable stability -/1 stability measured after 1 day -/30 : stability measured after 30 days. 53614 Figures 1 and 2 of the drawings are examples of the possible forms of the curve of cumulative percentage against particle size plotted on a log-log scale. The ordinate axis represents %CM, and the abscissa axis represents the diameter in microns. In the case of Figure 1, the average particle size of the first group is 83 pm, the average particle size of the second group is 11 pm, d-| is 44 pm, d2 is 25 pm, %CM1 is 40, and %CM2 is 40 (Example 23). In the case of Figure 2, the average particle size of the first group is 82 pm, the average particle size of the second group is 7.2 pm, d^ is 50 pm, d2 is 20pm, %CM1 is 69.6 and %CM2 is 63.6 (Example 10).

Claims (22)

1. An aqueous suspension of coal particles having a size not exceeding 300 μπι, the particles being divided into two groups of particles of different sizes, the coal particles of the first group having an average size of from 210 to 60 ym, and the coal particles of the second group having an average size of from 1/6 to 1/20 of the average size of the particles of the first group; and the suspension containing a non-surface active anionic polyelectrolyte.

2. A suspension as claimed in claim 1, wherein the particles of the first group form at least 40% by weight of the total weight of particles.

3. A suspension as claimed in claim 2, wherein the particles of the first group form about 60% by weight of the total weight of particles.

4. A suspension as claimed in any of the preceding claims, wherein the cumulative particle size distribution curve of the particles is such as to comprise two particle size values, d-j and dg, lying between the average diameters of the two groups of particles, for which the numerical value of the expression: (a) log (%CM1) - log (%CM2) log d-j - log dg is less than 0.4, where %CM1 and %CM2 indicate the total percentages of the particle mass having sizes of less than d^ and dg respectively.

5. A suspension as claimed in claim 4, wherein the numerical value of the expression (a) is less than or equal to 0.1.

6. A suspension as claimed in claim 5, wherein the numerical value of the expression (a) is zero.

7. A suspension as claimed in any of the preceding claims, wherein the non-surface active anionic polyelectrolyte contains alkyl-substituted polynuclear aromatic groups.

8. A suspension as claimed in any of the preceding claims, wherein the polyelectrolyte has a molecular weight exceeding 500.

9. A suspension as claimed in claim 8, wherein the polyelectrolyte has a molecular weight of from 800 to 3000.

10. A suspension as claimed in claim 9, wherein the polyelectrolyte is a monovalent cationic salt of a polymerised alkyl-naphthalenesulphonic acid having a molecular weight of about 2000.

11. A suspension as claimed in any of the preceding claims, wherein the polyelectrolyte is present in an amount of from 0.1 to 1$ by weight of the entire suspension.

12. A suspension as claimed in claim 11, wherein the polyelectrolyte is present in an amount of from 0.3 to 0.5$ by weight of the entire suspension.

13. A suspension as claimed in claim 12, wherein the polyelectrolyte is present in an amount of about 0.4$ by weight of the entire suspension.

14. A suspension as claimed in any of the preceding claims, the suspension further containing a stabiliser.

15. A suspension as claimed in claim 14, wherein the stabiliser is present in an amount of from 0.01 to 0.03$ by weight of the entire suspension 15. A suspension as claimed in claim 15, wherein the stabiliser is present in an amount of about 0.02$ by weight of the entire suspension.

16. 17. A suspension as claimed in any of claims 14 to 16, wherein the stabiliser is an organic gel.

17. 18. A suspension as claimed in claim 17, wherein the stabiliser is a polysaccharide.

18. 19. A suspension as claimed in any of the preceding claims, being a suspension in salt water. 53614

19. 20. A suspension as claimed in sea water.

20. 21. A suspension as claimed in described with reference to Tables

21.

22. A suspension as claimed in substantially as hereinbefore described. claim claim 1 and claim 19, being a suspension in 1, substantially as hereinbefore 2. 1, when prepared by a method