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Xth International Congress on Rheology, Sydney, 1988
ON-LINE VISCOSITY MEASUREMENT FOR THE MINERAL PROCESSING
INDUSTRY
A. D. THOMAS N. T. COWPER P. B. VENTON
Slurry Systems Pty. Ltd Cammeray, N.S.W. 2062, Australia
1. INTRODUCTION
processing generally involves crushing and grinding of ore to release the valuable minerals. Although water is usually added during grinding and from this stage on the
processing steps involve slurry handling and pumping. Within the constraints of the various process requirements
the slurry be as concentrated as possible to minimise both the size of the
(capital cost) and pumping power (operating cost). Slurry solids concentration is usually monitored by nuclear absorption density gauges and the various are controlled using these density readings. As long as the ore remains uniform this is a satisfactory However with the inevitable trend towards lower grade and more difficult ores it is becoming increasingly apparent that density control is not sufficient to obtain maximum efficiency. There are a number of reasons, including:- Because the ore is low grade there is more gangue material present with the possibility of significant variations can result in considerable differences in concentration. Even some high grade ores may exhibit a large variation in viscosity and may have for this very reason been judged too difficult to mine in the past. c) Low grade ore requires processing of much larger quantities of material. This makes it more imperative that the highest possible concentration be used. all process steps it is slurry viscosity and not solids concentration per se which is viscosity affects such such things as ball mill grinding performance, performance, and agitation, thickening and
in increasing the future efficiency all mineral processing using chemical viscosity Slurry Systems have developed an instrument and accurate, making process control by slurry viscosity a practical alternative to traditional control
2. TRADITIONAL MINERAL PROCESSING
The traditional approach has been to operate at a conservative solids concentration such that any variations in slurry properties do not effect plant performance. This generally means operating equipment such that the is fully turbulent. Figure 1 illustrates the situation for pipe flow of a typical slurry. Consider Slurry A of The lower sloping curve on the left represents the laminar flow portion whilst the represents the turbulent flow regime. Transition between the two regimes occurs at 1 m/s. I= operating region between 1.5 to 2.5m/s flow is therefore turbulent. This slurry behavior would of a traditional mineral slurry. As the slurry concentration is increased the behavior changes from through to E. Between A and C there is about a 4 fold increase in the laminar pressure gradient but only about a turbulent pressure gradient. Thus even large changes in viscosity have operating pressure gradient. However as the concentration is further increased from laminar flow now prevails in the operating velocity The operating pressure gradient now increases rapidly
Operation in this region requires monitoring it was good enough to operate with slurry A with its large "factor of safety"
slurry C without major effects on the plant performance, this is no longer the case. Increasingly there will be the need to operate continuously with slurry D in or near the laminar regime. Similar arguments apply to
3. ON-LINE VISCOMETER REQUIREMENTS
A nineral processing plant presents an extremely harsh environment to any equipment. An on-line viscometer must be accurate, repeatable, and reliable, under all service conditions, requiring a robust design. With the presest visconeter this robustness is achieved by keeping the instrument as simple as possible and by using well
extensively in the industry. The instrument must not clog in
VoL. 2
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must minimise wear, self drain upon shutdown. and if wear occurs, it should not affect the instrument calibration. Any wearing Abrasive slurries can cause severe wear problems. The instrument design because of its inherent simplicity. should be low cost and their replacement effected as simply and quickly as possible. - tube viscometers and rotational viscometers. We chose the tube There are two most
4. DESCRIPTION OF VISCOMETER AND PRINCIPLE OF OPERATION
viscometer consists of a measuring U section is measured by a differential pressure tube in the shape of an inverted U mounted in the vertical plane. prevent a build up of solids. The diameter of this U section is typically 50mm and the flow rate through it such that the mean They are at the same elevation to eliminate static velocity is around 0.5m/s. The vertical configuration means any solids segregation has minimal influence on the discussed diameter magnetic meter and a flow control valve are installed downstream of the measuring tube Because of the higher velocity in this resistance tube the flow smaller As appropriate choice of length of resistance tube the pressure drop over this section can be made much larger than the pressure drop over the measuring tube section making the total resistance of the resistance under turbulent flow is relatively insensitive to By insensitive to viscosity changes. in pressure at the tapping point also alter the flow through the instrument. only slightly. Slurry is normally supplied to the instrument by tapping into the process stream and fluctuations Thus as the slurry viscosity alters, the flow through the instrument changes programmable controller to maintain constant flow through the instrument irrespective of process changes. Figure tube means the flow is more inherently stable and it reduces the work required of the control valve. of turbulent flow velocity varies only as the square root of pressure drop. changes in pressure causes large changes in velocity under laminar flow Thus incorporation of the resistance The control valve is operated by a viscosity. This instrument meets all the aforementioned criteria:valve this resistance tube is selected to suit the feed pressure and the slurry viscosity range. keeping the flow rate constant the measured pressure drop is a direct indication of the apparent slurry - All components are well proven in the industry It is self draining upon shutdown - Mease are There are no moving parts (except for the flexing of the control valve muscle) wear is neglible due to the low velocity. The resistance tube may wear but can valve has to work harder. cheaply be replaced. Control valves of this type are renowned for their wear resistance. The sleeve is easily wear of the resistance tube does not affect the calibration, it will only mean
5. WHAT DOES THE INSTRUMENT READ?
For a Newtonian fluid the pressure gradient over a straight length of pipe is given by:
H/B = 32u (1)
Where u is the viscosity, transmitter signal by an appropriate factor will give an instrument read out directly in viscosity units. constant flow rate the viscosity is directly V is mean viscosity, and D is the pipe diameter. proportional to the pressure gradient. Hence in a given instrument for Multiplying the pressure legitimate and is entirely suitable for control purposes. non-Newtonian fluid this readout represents the effective Newtonian viscosity at an apparent shear rate of 8V/D reciprocal seconds. Although this may not be a conventional measure of non-Newtonian viscosity it is quite For a is given by the following approximation to the Buckingham equation, valid at higher shear rates. plastic viscosity and Ty is the yield stress. Consider the special case of a Bingham fluid which is often used to model slurry behavior. The pressure gradient Here n is the
= 320[ + D (2)
oncentration ranges of interest. Assuming Ty/N=K then; not generally realized that for mineral slurries the ratio Ty/n is relatively constant over typical
AP/I = Ty (32 V/KD2 + 16/3D)
For a constant flow rate the term in brackets is therefore approximately constant and so the yield directly proportional to the pressure drop. Hence for a Bingham plastic the instrument can be calibrated to read used to characterise directly in units of yield stress. Obviously this is not strictly correct but if yield stress is viscosity or yield stress. course the control capability of the instrument is unaffected whether the read out is in a particular industry then it may
6. THE EFFECT OF THE U BEND
The above considerations apply to a straight length of pipe. However, this instrument includes a bend as well as a 90 degree bend upstream of the first tapping. and will disturb the flow and result in additional pressure Also for practical reasons the pressure experimentally that the increased resistance due to a bend is a function of the Dean number given by: De = ReVD/Da (4) where ile is che Reynoids number and De is the diameter of the bend centre line. Using his relationship it can be shown that for a given pipe size and constant velocity the pressure drop around the bend (AP, ) is given by:
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АРЬ = Кр и'-п pn
can be ignored. where P is the density of the fluid and Kb is a constant for a given pipe size velocity and bend small range, typically about 15%. This fact, between 0 and 0.4. In any particular application the slurry density will only vary combined with the small value of n means the influence of density (5) similarly directly proportional to viscosity Approximating each pressure tapping as a dead tee the additional pressure drop due to the tappings can be directly proportional to viscosity and the second proportional to u'-n. to be directly proportional to viscosity. (equation 1). The pressure drop in Hence the total pressure drop is given by two terms, the straight sections of insignificant so that to a good approximation the total pressure drop is directly proportional to viscosity. the tapping geometry, By appropriate states that the losses in fittings, Devarajan [4] studied the flow of power low fluids though bends. All of the foregoing applies to Newtonian fluids. bend is somewhat greater than in the case of an equivalent Newtonian fluid. For a Bingham fluid Cheng [5] For non-Newtonian fluids information is sparse. Mashelkar and Their analysis and data indicate the influence the plastic viscosity. number and can be a factor of ten higher when the Reynolds number is low. give the same pressure gradient in a straight pipe as the Bingham plastic, If an effective Newtonian Reynolds number is used based on the Newtonian viscosity which that of a Newtonian fluid, increase with a decrease in Reynolds But he uses a Reynolds number based on viscosity approach. the present instrument indicate the losses are similar to that predicted by an the factor will be much less. equivalent Newtonian
7. ACTUAL PERFORMANCE
yield stress compared to that measured in the laboratory on a Contraves RM15 rotational viscometer. Figure 2 shows results of commissioning tests on a limestone slurry at a cement plant. to correct for this so to be reading consistently about 5% low. as to give the true reading. The instrument calibration It shows the indicated can simply be adjusted most likely give a similar degree of scatter. between data from two different viscometers. Measurements in two different laboratory viscometers would There is some scatter of the data but no more than would be
8. CONCLUSIONS
stress and has been shown directly proportional to fluid viscosity. processing plants. been described. This robust instrument is designed to withstand the rigorous For a Newtonian fluid an analysis has shown that 9. REFERENCES to give readings in good agreement with laboratory data. For a Bingham plastic the instrument displays the yield 5. Cheng, 2. White, iM. and ear. J.D. 4.3645. 11,19,01,85 (1973 4. 2. Austin, L.R. 3. Jamison, D.K. and Villemonte, and Devarajan, G.V. Trans Instn Chem Engrs 645 (1929) Jnl of Hyd.Div, July (1971) p 1045. 54, 100(1976). D.C.H. Proc Hydrotransport 1 Conf., BHRA, Cranfield, England (1970).
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PRESSURE GRADIENT (ka/m) 0.1 0.2 0.6 0.2 105mm PIPE 0,4 VELOCITY (m/s) 0.6 08 1 WATER 2 3 4 YIED STRESS (Pa) COMPARISON BETWEEN ON-LINE READING ( ON-LINE MEASUREMENT ) 20 10 40 30 10 (MEASURED IN LABORATORY ) YIELD STRESS ( Pa) 20 30 40 50 60