Overflow-control system for a hydrocyclone battery

Overflow-control system for a hydrocyclone battery

Int. J. Miner. Process. 74S (2004) S339 – S343 www.elsevier.com/locate/ijminpro Overflow-control system for a hydrocyclone battery M. Schneider, Th. ...

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Int. J. Miner. Process. 74S (2004) S339 – S343 www.elsevier.com/locate/ijminpro

Overflow-control system for a hydrocyclone battery M. Schneider, Th. Neege* Lehrstuhl fu¨r Umweltverfahrenstechnik and Recycling, University Erlangen-Nuremberg, Germany

Abstract In practice, the advantages of high-quality hydrocyclone separation are often restricted due to varying feed suspension properties, solids concentrations and particle size distributions. Control systems of large hydrocyclones are usually fitted with controllable underflow nozzles, which is not practical for small hydrocyclone batteries. Therefore, a new control system for a battery of hydrocyclones with a nominal diameter of 150 mm was invented at the LUR in co-operation with AKW Apparate und Verfahren [Computergesteuerte Hydrozyklonbatterie erfolgreich eingesetzt, Aufbereitungstechnik 42 (2001) Nr. 12, Th. Neege, F. Donhauser, Advances in the theory and practice of hydrocyclone technique, Proceedings of the XXI International Mineral Processing Congress, Rome, Italy Juli 2000, V.A., p A4 – 74., Th. Neege, F. Donhauser, M. Schneider, Computer-controlled hydrocyclone battery, Advances in Filtration and Separation Technology, Science and Technology of Filtration and Separations for the 21st Century, Editors Shiao-Hung Chiang and Samuel E. Lee, American Filtration and Separation Society, Vol. 15, Pittsburgh, 2001., M. Schneider, Th. Neege, F. Donhauser, Mechatronik fqr eine Hydrozyklon-batterie, Aufbereitungstechnik 40 (1999) Nr. 2, S. 79 – 83 AT., M. Schneider, T. Neege, B. Schricker, F. Donhauser, Overflow-control system for a hydrocyclone battery, Proceedings of the International Congress for Particle Technology PARTEC 2001, Nqrnberg, March 2001] This process was developed for bentonite slurry regeneration associated with tunnel excavation using bentonite slurry as a transport aid and lubricant. D 2004 Published by Elsevier B.V. Keywords: hydrocyclone; control; separation; tunnel; bentonite

1. Introduction An important function of a process control system for a hydrocyclone plant is the stabilization of the cut point or slurry density in the overflow, respectively, for feeds with varying solid concentrations and * Corresponding author. E-mail address: [email protected] (Th. Neege). 0301-7516/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.minpro.2004.07.037

particle size compositions. So far, appropriate regulation of the volume split has been restricted to single hydrocyclones of larger diameters in which adjustable underflow nozzles have been fitted. For smaller hydrocyclones, interconnected in batteries, this concept is not feasible. Moreover, the following two preconditions must be given: –

measurable process variables for the characterization of hydrocyclone separation

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controlled process quantities, without manipulation of the individual hydrocyclone.

penetrates into the underflow resulting in more fines being swept into the underflow with low solid content.

2. Principle of the hydrocyclone control

3. Detection of the operational state

It is known that the maximum solids recovery of coarse solids in the hydrocyclone depends on the form of the underflow discharge. The maximum recovery and a high solid content in the underflow will occur when this discharge has the form 2 shown in Fig. 1, obtained when the hydrocyclone is operated at the critical point between rope discharge (3) and umbrella discharge (1). The control principle should stabilize the hydrocyclone operation near that critical point Neehe et al., 2001. When the discharge takes the form of a strand, solid material is dammed up in the cone of the cyclone and is forced into the overflow. In comparison, with umbrella discharge, the core of air formed inside the cyclone, owing to the action of the centrifugal forces,

At the core of the control system are the sensors, which detect the three different flow conditions (Fig. 1) of a hydrocyclone. The following methods to detect of the flow conditions were tested Schneider et al., 1999: (a)

Optical method. Contact free scanning of the shape of the discharge using a laser beam. (b) Gravimetrical method. The recurrence of rope or umbrella discharge is determined by the quantity of solids stored in the cyclone. The solids mass or the hydrocyclone weight, respectively, can be measured with a gravimetric cell using a special fastening means for the hydrocyclone. (c) Capacitive method. The best results were achieved using a capacitive sensor which was

Fig. 1. The three different flow conditions in a hydrocyclone.

M. Schneider, Th. Neege / Int. J. Miner. Process. 74S (2004) S339–S343

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Fig. 2. Hydrocyclone regulation scheme.

modified to a slurry resistant sensor at the LUR. The transition between rope and umbrella discharge is registered by contact of the discharge spray and the sensor.

4. Hydrocyclone regulation scheme The signals (1) of the operational status are transmitted to the computer together with the values for the power input (2) of the feed pump, feed pressure, and counter pressure in the overflow (3) (Fig. 2). A throttle valve (4) for control of the combined overflow of all hydrocyclones and the feed pump speed (5) are then regulated by the control system. With increased stepwise throttling of the overflow, the volume split is changed resulting in the solids discharge in the underflow increasing. To stabilize the total throughput, the feed pump delivery rate is simultaneously increased so that the pressure drop across the hydrocyclone, Dp, remains approximately constant. However, this causes the pressure inside the hydrocyclone to build up, resulting in a further intensification of the (unthrottled) underflow discharge. After the breakthrough of the air core or the

transition to umbrella discharge, respectively, detected by the capacitive probe (6), the throttle valve is shortly opened, and a new regulation interval can start again. The control concept is focused on maintaining the optimum operating state, characterized by an underflow discharge shape at the transition point between rope and umbrella-shaped discharge.

5. Technological results The presented automatic control concept has been initially developed for separation plants used with Table 1 Details for a 150-mm hydrocyclone for bentonite recovery in a tunneling project Nominal diameter of the hydrocyclone Overflow nozzle diameter Underflow nozzle diameter Feed pressure Volumetric flow rate Feed material Cut size

150 mm 72 mm 25 mm 0.7–2.5 bar 25–70 m3/h 80 t % quartz sand b0.5 mm and 20 t % bentonite 30 Am

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Fig. 3. Particle size distribution Q 3 of the feed.

tunnel driving projects, wherein the feed solids concentrations varied between 50 and 500 g/l and corresponding solids discharges varied between 2 and 16 t/h per 150-mm hydrocyclone Schneider et al., 2001. The operational conditions of a 150-mm hydrocyclone in a two-stage separation plant used in a tunneling separation operation are given in Table 1. The aim of the control process in this application was stabilizing to a cut size of ~30 Am independent of the feed solid content and, additionally, to allow replacement of the two-stage hydrocyclone plant with a single-stage unit. This would permit the elimination

of the preliminary separation of coarse particles in a 500-mm hydrocyclone prior to fine particle separation. As can be seen from Fig. 3, the particle size distribution of the feed was changing within a relatively wide range. With the control system implemented in a 150mm hydrocyclone, it is possible to stabilize separation for varying solid concentrations up to 500 g/l at a cut point of approximately 30 Am (Fig. 4) or a slurry density of 1.1 kg/m3 in the overflow. Selected technological values of this 150-mm hydrocyclone separation process are listed in Table 1.

Fig. 4. Separation curves of the 150-mm hydrocyclone with or without throttling (solid feed concentration=420 g/l).

M. Schneider, Th. Neege / Int. J. Miner. Process. 74S (2004) S339–S343 Table 2 Control of a 150-mm hydrocyclone, D=50 mm, D O=72 mm, D U=29 mm, with a solids concentration=550 g/l (b2 mm) in the feed Feed pump speed (min 1)

Counterpressure in the overflow, p1 (bar) throttling

Split V O/V (%)

Solids recovery in the underflow (t/h)

1080 1140 1200 1260 1310

0 0.10 0.25 0.30 0.45

79 78 74 73 68

54 54 65 70 79

Table 2 shows that, at a feed suspension of the particle size range minus 2 mm and feed concentrations of up to 500 g/l, a maximum solids recovery per cyclone in the underflow of up to 79% can be achieved Neehe and Donhauser, 2000. This represents such an improvement in the discharge capacity of the 150-mm hydrocyclone that the installation of a preliminary cyclone of a larger diameter is no longer necessary. With such a computer-controlled process, it is also possible to replace a two-stage hydrocyclone circuit with a single-stage plant. Possible remote control of the hydrocyclone plant on the basis of remote data transmission is another benefit.

6. Summary For small hydrocyclones connected in a battery, a computer-controlled system has been developed.

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The elements of this control system consist of sensors for measuring the operating state, a process control computer for recording and processing the measured data, and the activator, in this case a control valve downstream of the overflow collector of the hydrocyclone battery. With this system, the hydrocyclone battery can be operated at the optimum operating point, corresponding with the transition from rope to umbrella discharge of the underflow.

References Neege, Th., Donhauser, F., 2000. Advances in the theory and practice of hydrocyclone technique. Proceedings of the XXI International Mineral Processing Congress, Rome, Italy, Juli 2000, V.A., pp. A4 – A74. Neege, Th., Donhauser, F., Schneider, M., 2001. Computercontrolled hydrocyclone battery. In: Chiang, Shiao-Hung, Lee, Samuel E. (Eds.), Advances in Filtration and Separation Technology, Science and Technology of Filtration and Separations for the 21st Century, American Filtration and Separation Society, vol. 15, Pittsburgh. Schneider, M., Neege, Th., Donhauser, F., 1999. Mechatronik fqr eine Hydrozyklon-batterie. Aufbereitungs-Technik 40 (2), 79 – 83. AT. Schneider, M., Neege, T., Schricker, B., Donhauser, F., 2001. Overflow-control system for a hydrocyclone battery. Proceedings of the International Congress for Particle Technology PARTEC 2001, Nqrnberg, March 2001.