Rheology Measurement
3.4 On-line viscosity measurement
As mentioned in this section, the main problem with measuring the viscosity of dense medium suspensions is the high settling rate of these suspensions, particularly at low specific gravities. To measure the viscosity of these suspensions, industrial suspensions need to be able to overcome this settling nature. Over the years, the most commonly used on-line viscometer has been the bob and cup viscometer. This type of viscometer is a modification of the concentric-cylinder viscometer described in section 3.2.1. Almost all industrial bob and cup viscometers work under Searle's principle. Recently there has also been a tendency to use capillary on-line viscometers.
This section will give an example of both capillary and rotational on-line viscometers.
CHAPTER 3 RHEOLOGY MEASUREMENT
3.4.1 An industrial capillary on-line viscometer
Goosen et al. (2004) have developed a tube viscometer which was specifically designed for use with fme segregating slurries such as dense medium suspensions. This viscometer has been termed the FeSi viscometer. A schematic diagram of the viscometer configuration is shown in Figure 3.l1.
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... ----_ ... -_ ... -_ ... -_ ...... . , Figure 3.11 Schematic of the FeSi viscometer Apparatus (Goosen et al. (2004»)Feed to the viscometer is supplied through a pressurised slurry source, which could be a tapping close to a pump discharge in the dense media circuit [Goosen et al. (2004)]. The valve configuration is important for the proper pressure measurement across the capillary. To reduce the settling rate of the slurry common under laminar flow conditions, the measurement tube is coiled in vertical loops. The differential pressure across the measurement tube is measured using a differential pressure transmitter and appropriate tapping points. The differential pressure is used to calculate the shear stress at the capillary wall using methods similar to those described in Section 3.1. The flowrate of the slurry through the measurement tube and into the receiving vessel
CHAPTER 3 RHEOLOGY MEASUREMENT
is detennined from the rate of change of vessel pressure. These results are used to calculate the bulk shear rate in the measurement tube.
3.4.2 An industrial on-line rotational viscometer
Reeves (1985) describes an industrial viscometer used to measure the viscosity of dense media and other mineral suspensions. The viscometer can be used on-line to monitor the viscosity contillually on a plant or off-line to investigate the rheological behaviour of suspensions. The viscometer (Debex viscometer) is of rotational design and employs a specially designed bobbin that is rotated in a sample of the suspension under investigation. The bobbin is submerged in the fluid and is rotated at a ftxed rate by a DC micromotor and ancillary electronic equipment. The rate of rotation may be selected manually and, once chosen, is maintained by the electronics. If a retarding force is applied to the motor by the viscous resistance of the fluid, additional voltage is required to maintain a constant rate of rotation. The voltage is measured and displayed continually electronically. The motor has been found to be sensitive to changes in temperature, which causes the rotation rate of the bobbin to drift. To overcome this, the motor is surrounded by a heating jacket that maintains a constant, higher than ambient temperature.
CHAPTER 3 RHEOLOGY MEASUREMENT
E'
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Figure 3.12 Debex on-line viscometer [Reeves (1986), Napier-Munn et al. (1996)]
A schematic section ofthe viscometer is shown in Figure 3.12. Feed to the viscometer is degritted by a cyclone or a screen, the undersize passing to a constant head tank B. the sample then flows by gravity into the 'V-shaped' section, where a proportion is drained through the pipe C' to reduce the flow rate. This pipe also allows the constant-head tank to drain when the viscometer is switched off (an important for rapidly settling solids). The remainder of the sample flows upward through the annulus, C, before overflowing both inwards into the measuring chamber, D, and outwards to drain E. For rapidly settling suspensions the upward velocity of the suspension is greater than the settling velocity of the solid particles. The annulus, C, and the measuring chamber are both fitted with baffles to eliminate rotational movement of the fluid.
For the measurement of Newtonian fluids the viscometer is calibrated with aqueous glycerine solutions of known viscosity. The measuring chamber, D, is plugged and filled with solution. The solution viscosity is measured by means of another suitable viscometer and compared with the on-line viscometer reading. The procedure is repeated to produce a calibration graph that correlates the viscometer reading with viscosity. For non-Newtonian fluids the calibration becomes more difficult and tedious because of the settling out of solids from suspensions [Napier-Munn (1996), Reeves (1985)].
CHAPTER 3 RHEOLOGY MEASUREMENT
Napier-Munn et al (1996) extensively covers the calibration of both Newtonian and non- Newtonian fluids. The derivation of the shear stress equations assumes that the flow past the bobbin is laminar. However, the flow in the Debex viscometer is turbulent in even for a static fluid. In turbulent flow the internal motion of the fluid is not smooth but includes many whorls and vortices. Since irregular flow leads to greater dissipation of energy than occurs in laminar flow, the apparent shear rate is diminished by turbulence to a value below that expected for laminar flow at a given applied shearing stress, resulting in a higher apparent viscosity. If this is not corrected the resulting flow curves take on the appearance of dilatancy, even though the fluid may be Newtonian.
These problems were believed to be associated with the wide gap between the measuring cup and the rotating bobbin, and the existence of baffles, both of them causing Taylor vortices and turbulence. When the inner cylinder is rotating and the outer cylinder is at rest, the fluid near the inner wall experiences a higher centrifugal force and shows a tendency to be propelled outwards.
When a certain Reynolds number is exceeded, vortices appear in the flows whose axes are located along the circumference and which rotate in alternately opposite directions.
CHAPTER 3 RHEOLOGY MEASUREMENT