A study of some factors affecting the rheology of coal-water slurries

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A.D. THOMAS, B.E., Ph.D., Consultant, Newcastle A Study of G.R. RIGBY, B.E., Ph.D., Some Factors Affecting the Rheology of Coal-Water Slurries. F.I.E. Aust., BHP Central Research Laboratories, Shortland, N.S.W.

Cheneca Cafenn, Let 1985, her,

different Australian coals having a range of properties were tested. The rheology of coal-water slurries has been investigated using a rotational viscometer. Each coal was ground to a number of a number of different concentrations. different gizes having median particle sizes ranging from 75 to 23 microns, then each sample was tested at As a result the effect on the Bingham plastic paraneters of such have been tested. factors as coal type, mineral matter content, types of clays present, particle size, concentration and pH varied with representative particle size, ds0, approximately as den-1.5 Colst the plastie Viscosity varted The most significant finding was that for all of coals the Bingham yield stress approximately as d50-0.5. Both the yield stress and plastic viscosity were strongly dependent on concenlevel and the degree to which the clays were disseminated throughout the coal. Care must therefore be Mineral matter and type influenced the rheology in a complex manner depending on pH, electrolyte exercised in extrapolating rheological results from one coal to another.

kept constant.

increased in recent years. Interest in the rheology of coal-water slurries has The concept of using The slurries for testing were prepared by adding the coal slowly to the required amount of tap fine coal-water mixtures as an alternative to oil water contained in a beaker fitted with a high for boiler fuel is being pursued in a number of speed stirrer. Baffles were provided in the beaker countries (see for example Ist European Symposium to allow good mixing. Stirring was continued for The rheology of such on Coal Liquid Mixtures, Inst. Chem. Engrs. (1983)). slurries is of crucial 30 minutes to ensure that all the particles were wetted and a homogeneous slurry was achieved. The importance in assessing and designing pumping and combustion equipment. In the slurry transportation addition of sodium hydroxide or hydrochloric acid. pH of the slurry was adjusted where required by the field, apart from its importance in the pumping of pumping of stabilized mixtures of coarse coal in a slurries, the current interest in the Size analyses were carried out by firstly rinsing fine coal vehicle relies on the rheology of the The plus 45 micron material was air dried and a a sample of the slurry through a 45 micron screen. fine coal portion (Thomas (1979), Rigby et al (1982), Duckworth et al (1982)). screen analysis carried out. The minus 45 micron the treatment of the fine coal fraction can involve analyser. material was analysed using a Quantimet 720 image Figure I shows typical size analyses in equipment such as centrifuges for the Appin coal. For recovery and beneficiation of ultrafine coal the oil agglomeration process is being considered (Elkes et al, 1983). slurry rheology is of importance in evaluating the various process options. The present paper reports on a rheological investi- 99gation into the effects of concentration, particle size, mineral matter and surface chemistry using five different Australian coals.

Samples of coal were prepared for chemical analyses and rheological studies in accordance with rheological studies to ensure that representative Australian Standard 1676 (1975). samples were used. exercised in preparing sub samples for the EQUIPMENT AND EXPERIMENTAL TECHNIQUE Special care was PERCENTAGE UNDERSZE Rheological measurements were made using a Contraves Rheomat 30 viscometer. of various sizes were prepared by firstly crushing the coal to a nominal top size of I mm in a small laboratory hammer mill. 10 x0Los0 200 500 was then further ground to the required size in a SIZE, microns Siebtechnik laboratory mill for varlous times ranging from 5 to 60 seconds. The sample size was Figure 1 Size Analyses for Appin Coal

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AM THOMAS and CR RICRY

A.D. THOMAS and G.R. RIGBY

A Study of Some Factors Affecting the Rheology of Coal-Water Slurries.

3 PROPERTIES OF COALS TESTED the slurry the more deviation from the Bingham

model at low shear rates. During testing a

Five coals having a range of properties were "thixotropic" effect was generally observed. This selected for this study. None of the coals had meant that for a given shear rate the shear stress been washed and therefore contained all of the original mineral matter contained in the run-ofmine coal.

decrease observed with a higher initial yield

Table I lists the chemical analyses for the five stress. This phenomenon is almost certainly samples. Comprehensive mineral matter analyses associated with settling of particles under shear effect of various mineral species on rheological were undertaken to allow an assessment of the onditions. Thomas (1979(2)) showed that particle hich are non-settling under static conditions ca behaviour. readily settle once shearing begins. He also

TABLE I

ANALYSES OF COALS USED IN THE STUDY

Chemical Analysis Colliery Seam Wongaw1111 Wongawill1 Appin Bullt Southern* Blend Dudley Burwood Griffin Hebe

Moisture Ash Mineral Matter, %d.b. %d.b. %a.d.b. 22.79 25.32 1.38 11.30 12.96 1.20 21.2 23.7 1.30 28.12 25.94 2.82 14:33 3.02

Hydrogen Volatile Matter mE Carbon 25.15 89.09 5.00 21-79 4.70 89.8 19.9 4.79 37.40 84.49 5.57 38.30 76.08 1.35

Nitrogen Sulphur 1.86 1.80 1.68 2.16

Specific Energy (MJ/kg) Oxygen " 36.01 4.00 0.46 36.30 0.28 36.71 0.43 35.21 0.33 29.35 17.75 0.15

2.92 7.01

Mineralogical Analyses of Mineral Matter (X-Ray Diffraction)

7

Pyrite = = 1

Calcite Feldspars Apatite Siderite Quartz Dolomite Jarosite Bassanite KaolinitetChlorite" = = 52 - 1+1-11% 10 n.a. [=11y 70 6

Illite Expandable clays **.. 19 4 9 24 -

* 40% Wongawi111 (Nebo); 30% Bul11 (Appin); 30% Bulli (Kemira ** Montmorillonite group and mixed layer clays

4 RHEOLOGICAL RESULTS

4.1 Method Rheology measurements were performed on the five Monogel! coal samples for various combinations of particle size and concentration. In most cases the leasuring System A was used (largest bob and cup)

to be employed. tates et 1o sec stered loved tests to sear whereas System B was limited to

195 sec-1. although for the thicker slurries the B system har SHENA STRESS (Pa)

Figure 2 shows typical flow curves. The Bingham model,

du 950 = 21mm

Corte

where o is the shear stress °o is the Bingham yield stress /dy is the shear rat 1 is the plastic viscosit SHEAR RATE (DoC-") 200 and flts the high shear rate data well. The thicker Figure 2 Typical rheograms

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A.D. THOMAS and G.R. RIGBY

A Study of Some Factors Affecting the Rheology of Coal-Water Slurries

100

Ippir

50

50)

YIELD STRESS (Pa) Southern Blend Wongawilli YIELD STRESS (Pa) 30- 20- Wongawilli

10H

0.1l 10 20 100 Burwood. 500 Figure 7 Effect of pH on Yield Stress for 50% Concentration Slurries for (nominal d50=23 microns) for 60 Second Grind dso (um) This is to be expected. low clay content, exhibits the lowest yield stress.

Figure 5 Effect of Particle Size on Yield Stress For Appin and Burwood coals pH is seen to have a

dramatic effect. However for Wongawilli coal, which has not too dissimilar properties of the

1000 g different clay types from these two, there was no effect of pH. The clays are known to be very

finely disseminated throughout the Wongawilli coal

500 567. would consequently be locked within coal particles and this could be a factor since some of the clays

and so not influence rheology.

This is exploited in the use of Alum to clarify water. Coagulation occurs because

PLASTIC VISCOSITY (mPas) 100 50 Wongawilli Southern Burwood Long Blend However previous work by the authors (not reported the tve ions compress the electrical double layer allowing Van der Waal attractive forces to predominate. exhibited a very low yield stress at its natural Electrolyte concentration can also have Additien of tve ions is known to cause In the present tests Burwood coal

here) using water from the Burwood mine resulted in a much higher yield stress. This is because of the high salinity levels in the mine water. It is obvious that the effect of clay proportions and clay types on theology is very complex and beyond the scope of the present paper. It is

10 20 30 ) 40 50 60 70 difficult to make generalizations and it would seem best to test each particular coal to determine 050 (um) extrapolating rheological results from one coal to the effects. Care must therefore be exercised in

another.

Figure 6 Effect of Particle Size on Plastic Viscosity It needs to be noted however that all coals tested coagulated or flocculated. The degree of lower mineral matter levels, the effects on here were unwashed coals. For washed coals of congulat lon depends on a number of factors rheology will be proportionately less. electrolyte concent rat lon. Including clay type and quantity, pll and Figure I chows ther 6 CONCLUS TONS yleld stress of four of the coals plotted agalmat pil. Note fleally that lebe coal, whleh las very The rheology of coal water slurrles has been found

1o he dependent on soldds concentracton, Partlele

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A.D. THOMAS and G.R. RIGBY

5

A Study of Some Factors Affecting the Rheology of Coal-Water Slurries.

mineral matter content and type and surface Firth, B.A. and Hunter, R.J. (1976): Flow chemistry. In the concentration range 30% to 60% properties of coagulated colloidal suspensions, by weight both the yield stress and plastic Jnl. of Colloid and Interface Science, 57, n2, p248. viscosity varied by a factor of about 100 times. At the lowest concentration the variation was Institution of Chemical Engineers, (1983).| First approximately proportional to concentration squared European conference on coal liquid mixtures, Symp. whilst at 60% the exponent was in excess of 10. Series No.83, Cheltenham, U.K. Oct. 5-6. Yield stress was found to vary with median Rigby, G.R., Jones, C.U., Mainwaring, D.E. and plastic viscosity varied roughly as d50-0.5. particle size dso roughly as d5o-1.5 whilst Thomas, A.D. (1982).| the BHP-BPA 30 tonne per hour demonstration plant, Slurry pipeline studies on

Proc. Hydrotransport 8 Conf., Johannesburg, S.A.

Mineral matter content and type Influenced the p.181. rheology in a complex manner depending on pH, electrolyte level and the degree to which the Thomas, D.G. (1963). Non-Newtonian suspensions, clays are disseminated through the coal. Pt. 1, Physical Properties and Laminar Transport of these effects no typical rheology can be given Characteristics, I.&E.C., 55, 11, p18. for coal slurries of a given size and concentra-

Thomas, A.D. (1979(1)), Pipelining of coarse coal

REFERENCES Int. Tech. Conf. on Slurry Transportation, as a stabilized slurry - another viewpoint, 4th

March 28-30, Las Vegas, U.S.A.

Duckworth, R.A., Pullum, L. and Lockyear, C.F. (1982) • The hydraulic transport of coarse coal at high concentration, 4th Int. Symp. on treight Pipelines, Atlantic City, U.S.A. Oct. 4-6.

June 1.

Elkes, G.J., Rigby, G.R. and Mainwaring, D.E.

(1983). Enhanced coal recovery through IPTACCS Proc. 2nd Aust. Coal Prepn. Conf., Thomas, A.D. (1981). Slurry pipeline rheology,

technology. 2nd National Conf. on Rheology, Sydney, Rockhampton, Oct. 10-14th, p177. May 14-15th.

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ABSTRACT

A STUDY OF SOME FACTORS AFFECTING THE RHEOLOGY OF COAL-WATER SLURRIES.

KEYWORDS: coal-water mixtures, rheology, shear stress, slurries, yield stress. rotational viscometer. ABSTRACT: The rheology of coal-water slurries has been investigated using a Five different Australian coals having a range of properties were tested. Each coal was ground to a number of different sizes having median particle gizes ranging from 75 to 23 microns, then each sample was tested at a number of different concentrations. Bingham plastic parameters of such factors as As a result the effect on the types of clays present, particle size, concentration and pH have been tested. coal type, mineral matter content, most significant finding was that for all of the coals the Bingham yield stress plastic viscosity were strongly dependent on concentration. varled with representative particle size, d50, approximately as d50-1.5 whilst che plastic viscosity varied approximately as dso Both the yield stress and type influenced the rheology in a complex manner depending on pH, electrolyte level and the degree to which the clays were disseminated throughout the coal. coal to another. Care must therefore be exercised in extrapolating rheological results from one REFERENCE: Thomas, A.D. and Rigby, G.R., "A Study of rheology of coal-water slurries". some factors affecting