Slurry measurement techniques

Page 1

SYMPOSIUM ON CONCENTRATED SLURRY TECHNOLOGY, RMIT, Der. 1986

SLURRY MEASUREMENT TECHNIQUES = B

by A.D. Thomas Slurry Systems Pty Ltd

1 INTRODUCTION

The previous paper has discussed measurement of fine particle homogeneous slurries in which there are no settling effects. This paper is concerned with slurries which have a settling tendancy i.e. they require a suspension. These are heterogeneous slurries. The simplest heterogeneous slurries consist of near mono sized discrete grannular particles certain degree of agitation or movement to maintain the coarsest particles in tot of nee in water e.g., beach sand in water. More complicated slurries are those cause an increase in the viscosity above that of water. The coarser particles involving both fine and coarse particles. In this case the fine particles can be envisaged as being suspended in a high density viscous carrier fluid.

the coarsest particles when the slurry is static. This may be

sufficient to suspend However once the slurry is sheared this supporing mechanism is lost and dynamic effects must prevent particles settling.

2 SLURRY CHARACTERIZATION

The major measurements required to characterize a slurry are: Particle size Solids SG Viscosity Concentration Slurry abrasivity Surface Chemistry aspects - pH, Coagulants, flocculants, thinners Particle Size A particle size distribution • is required. For particle sizes above about 50 microns standard sieving methods can be used. Below this other methods must be used such as sedimentation techniques, cyclosizer, microscopy, and most recently the laser beam instrument. An important point to note however is that sub-sieve particles mainly influence slurry behaviour through their effect on the viscosity and so their actual size is relatively unimportant. This means a sieve analysis is generally all that is required. Solids Density The density of the dry solids needs to be determined by standard density bottle techniques. The likely variation in to sample is important as is any variation with particle size. With this density from sample coal particles the density varies strongly with ash content and this relationship should be determined. When pumping coal slurries some ash can be leached out with a consequent reduction in solids density. This effect can be conveniently studied in a wheelstand test rig (Thomas, 1986). A further complication with coal slurries is that the effect of inherant moisture on particle density needs to be addressed.

15

Page 2

Viscosity S Particles less than 50 microns can strongly influence the viscosity. Viscosity measurement relies on shearing of the slurry. The most convenient instrument is the rotational viscometer with the cup and bob arrangement. To obtain high shear rates the gap width is usually small, of the order of a few millimetres. To avoid jamming, all particles greater than about one third of "thin" slurries reducing the gap width should be screened out. roducing to around 100 to 20 to around 100 to 200 sec-1 for thicker slurries. Typically this might mean screening at W 3 0.5mm. Maximum shear rates generally in the range 500 to 1000 sec -1 for

These relatively low shear rates are not a serious limitation for pipeline hydraluic prediction. The thicker slurries will be pumped under laminar flow f conditions with the relevant shear rates at typical pumping velocities being relatively low. This means the viscometer data can be used directly to a predict laminer flow pipe performance. The situation with thinner slurries is n not so straight forward since they will often flow under turbulent conditions at normal pumping velocities. There are a number of proposed theories for T predicting pressure gradient of non-Newtanian slurry flow. Some of these e.g- Hanks & Dadia (1971) are based on the Bingham plastic model which requires P at high shear rates to accurately determine the parameters. A more F analysis (Wilson and Thomas, 1985) also requires high shear rate data since it requires laminar flow data up to shear stress values equal to those present in turbulent flow. Obtaining this high shear rate data can be a problem with rotational viscometers. Tube viscometers can generally attain P high shear rates so they can be used although they are less convenient. The heterogeneous effects associated with the settling tendancy of the plus size particles still have to be allowed for. For turbulent flow m conditions sliding bed theory of Wilson (1976), is most relevant. The question of relevant viscosity to use when calculating the particle settling velocity immediately arises. Use of the Bingham plastic viscosity, which equals the effective viscosity at infinite shear rate, tends to overestimate the settling velocity and the correct viscosity value appears i to be somewhere between this viscosity and the effective viscosity at the wall shear stress in question. The effect of coarse particles under laminar pipe flow conditions is a poorly m understood area. Work of Kenchington (1976), Thomas (1979) and Duckworth et a at (1983) is relevant.

a

Concentration

For slurry characterization i concentration is most easily U determined by drying a sample. Viscometer tests ar three or four different concentrations will generally provide sufficient data to allow interpolation of properties to intermediate concentrations. To aid

interpolation it is convenient to fit rheological models the Bingham plastic, the power law and the yield power law. to the date such as F

Surface Chemistry Aspects As the size of particles is reduced body forces between particles D became more predominate as do surface forces. Addition of coagulants such as Alum, thinners such as polyphosphates and pH changes, will all alter the degree of attraction between particles. This alters the viscosity. In terms of the Bingham plastic model these chemical changes tend to alter the yield stress with the plastic viscosity remaining constant. Polymer flocculants are commonly used in mineral process plant thickeners. These act by the polymer molecules forming bridges between the particles. Flocculation increases the yield stress but usually not by the same degree as it increases the settling rate. These flocculated slurries are not very stable with the flocs being easily broken by shearing.

16

Page 3

Slurry Abrasivity nient sity. pipe and pump wear rates. A popular abrasivity criteria is the Miller number The abrasivity of the slurry and the particle size affect the likely To (Miller 1974). This test was developed to predict wear rates resulting from a few rubbing type abrasion occurring in reciprocating pumps. It is sometimes used

1g at rd of wheelstand test ris is more sulted for this purpose (Thomas, 1986). predicting but it is not ideal in this regard. The

i for

ON-LINE MEASUREMENT

eline On-line measurement techniques are governed by the overiding necessity fLow for the instrument to be non-invasive. Slurries, by nature, are erosive to eing so, many instruments which are used in water and to applications are unsuitable for slurry service. Further requirements are the es is need to avoid clogging and to allow for effects of any slurry segregation. tions for The principal quantities of interest are: e.g. tires Pressure Bore Flow rate data Density inose Viscosity a tain Pressure plus transmitters For monitoring of slurry pressure standard pressure gauges used in conjunction with isolating diaphragms, suitably and Flow mounted to avoid settling blockage problems. The the Flow Rate ends The magnetic flow meter is the most commonly used flow measurement ears wall stic instrument. minerals. Even in nominally non-magnetic slurries, trace main limitation concerns slurries containing in meter amounts of magnetic periodic ultrasonic vibration are available to overcome this. can foul the electrodes. Self cleaning electrodes subjected Magnetic flow orly " et axisymmetric. Thus flow changes from laminar to turbulent flow have no effect meters are insensitive to velocity profile changes providing the profiles are on the accuracy. Non-axisymmetric velocity profiles do affect the accuracy vertical section of pipe. hetrogeneous slurries the flow meter should be placed in a tour sily Ultrasonic doppler flow meters which strap onto the outside of the pipe are also suitable for slurry use. They have the advantage of portability, but are 11ow sensitive to velocity changes and particle concentration and aid distribution. Because of this they should be field calibrated. • as For small flow rates the Coriolis effect mass flow meter is a useful instrument although very susceptible to pipe wear. For homogeneous slurries the venturi tube can be used to measure flow rae. eles Density • as erms the instrument widely used in industry for on-line density measurement. Knowing The nuclear density gauge is a reliable non-invasive strap-on type ield solids concentration can also be monitored. are This instrument measures the in-situ density. With heterogeneous slurries the mmer significantly different from the delivered the this reason the gauge should be located on a vertical ling eing section of pipe with velocities sufficiently high to ensure minimum holdup of

17

Page 4

Other less satisfactory methods of density measurement include static pressure measurement and weighing a length of horizontal pipe. The Coriolis effect mass flow meter can also measure slurry density. Viscosity On-line measurement of viscosity, although in principle simple, practice quite difficult. Reliability and avoidance of bockages are of prime consideration. The two basic approaches use either the tube viscometer or the rotational viscometer principle. Both types are currently on the market. tube viscometer is inherantly the simplest and with proper blockages ete is reliable. With rotational viscometers considerable complication is needed to sample the required quantity and avoid blockage and settlement problems.

Both types essentially measure the effective viscosity at a single shear rate. For a non-Newtanian fluid this does not fully characterize the flow curve, but this is not a serious limitation for on-line control purposes.

Direct shear stress measurement instruments are available, but these measure the product of velocity and viscosity and so require velocity measurement to obtain the viscosity. They also susceptible to wear and blockage problems.

REFERENCES

Duckworth, R.A., Pullum, L., Lockyear, C.F. and Lenard, J. - Hydraulic Transport of Coal. Bulk Solids Handling, 3, 4, (Nov 1983) Hanks, R.W. and Dadia, B.H. - Theoretical Analysis of the Turbulent Flow of non-Newtanian Slurries in Pipes, A.I.Ch.E.J., 17, 554 (1971) Kenchington, J.M. - Prediction of Critical Conditions for Pipeline Flow of Settling Particles in Heavy Medium, Hydrotransport 4 conf. (1976) Miller, J.E. - Miller Number, Chem. Engineering, July 22, (1974) Thomas, A.D. - Pipelining of Coarse Coal as a Stabilized Slurry - Another Viewpoint, 4th Int. Tech. Conf. on Slurry Transportation, Las Vegas, (1979) Thomas, A.D. - The Effect of Rheology on Pipe Wear, Proc. 4th National Conf. on Rheology, Adelaide, (1986) Wilson, K.C. - A Unified Physically - based analysis of solid-liquid pipeline flow - Proc. Hydrotransport 5 Conference, Banff (1976)

Wilson, K.C. and Thomas, A.D. - A New Analysis the Turbulent Flow of non- Newtanian Fluids, Can. Jnl of Chem. Eng., 63, 539 (August 1985)

18