THICKENER MUD GAUGE

Abstract: An apparatus for determining the concentration of solids or for measuring the interface between a liquid and settled solids in a settling vessel, comprising: (a) electrical conductivity or resistivity measuring means adapted to monitor electrical conductivity or resistivity within a liquid in a vessel; and (b) monitoring means which is capable of recording electrical conductivity or resistance levels within the vessel and which is capable of determining the concentration of solids from the electrical conductivity or resistivity results. Where the apparatus consists of a conductivity sensor (1) which is in electrical communication with and suspended by a cable (2) from a winch (3). The winch is controlled by the operation of a motor (4) which is controlled by a monitoring means (5). The conductivity sensor signals are transferred from the cable (2) by sliprings (6) to a conductivity meter (7). The conductivity data is then passed from the conductivity meter to the monitoring means (5) which measures positional data from a shaft encoder (8) and controls the position of the conductivity sensor by instructing the winch (3) to raise, lower or hold the sensor in any position inside the settlement vessel. The monitoring means is adapted to continuously examine conductivity data from the sensor.

Description: "THICKENER MUD GAUGE" THIS INVENTION relates to a method and an apparatus for the determination of the concentration of solids in a liquid. The method and apparatus are particularly useful in the alumina processing industry for determining the interface between red mud and green liquor. There a number of different types of detectors used in processing plants to measure and record the solid content of liquid slurries. In this respect, measurement of the level of the settled solids and of the solids concentration profile in settling tanks, thickeners or like vessels is essential for the effective control of such plants. Measurements based on turbidity are most common and give excellent results in clean liquids but have a limited application range in liquids commonly encountered in, for example, mineral processing. One explanation for this is that they require frequent maintenance in processes where precipitate scales form on the detector. Systems based on ultrasonic absorption are also used extensively but are unreliable in many processing industries due to temperature limitations and an uncontrollable instability often due to air entrainment on the sensor or the effect of currents in the liquid. More recently, natural radioactivity has been used in, for example, mineral processing industries as an alternative measure to determine settled solids levels and solids concentration profiles in processes where there are radionucleotides present in the solids. However, the determination of the solids content using such a measure is often difficult to achieve, since the radioactivity present in minerals and ores is generally low and these devices require time to accumulate sufficient counts for measurement purposes. Further, they are expensive to manufacture and are subject to significant amounts of electrical noise which has been found to produce false readings or skew or distort the results obtained. Another system that has been recently adopted as a means to measure levels of settled solids and solids concentration profiles in settlement tanks and the like has been based on detectors which measure sonic reflections. However, such systems have not proved reliable. Reasons for failure are unknown but could be due to air entrainment on the detectors. It has long be appreciated that the determination of the solids content in liquids used in many processing industries is essential to determine the throughput of a particular process. The presence of excess solids in a liquid often results in considerable production loss and high costs in cleaning up contaminated solution. Hitherto there has been no reliable method or instrument available to measure the concentration of solids or to detect the interface between liquids and settled solids in settling tanks, thickeners or like vessels. In accordance with the present invention there is provided a method of determining the concentration of a solid in a liquid in a vessel the method comprising the steps of: (a) monitoring the electrical conductivity or resistivity of the liquid in the vessel; and (b) determining the concentration of solids in the liquid from the electrical conductivity or resistivity thereof after correction for any temperature changes in the liquid. In a preferred form of the invention there is provided a method of measuring the interface between a liquid and settled solids in a vessel the method comprising the steps of: (a) monitoring the electrical conductivity or resistivity of the liquid in the vessel at different levels therein; (b) determining the concentration of solids in the liquid at the different levels by using the electrical conductivity or resistivity of the liquids or the solids after correction for any temperature changes in the liquid in the vessel; and (c) selecting the level where conductivity or resistivity changes substantially relative to the conductivity or resistivity of a clean liquid and/or a liquid containing solids. Also according to the present invention there is provided an apparatus for determining the concentration of solids in a liquid in a vessel, the apparatus comprising in combination: (a) electrical conductivity or resistivity measuring means adapted to monitor electrical conductivity or resistivity within the liquid in the vessel; and (b) monitoring means associated with the electrical conductivity or resistivity measuring means, the monitoring means being capable of recording electrical conductivity or resistance levels within the vessel and being capable of determining the concentration of solids from the electrical conductivity or resistivity results. The term "liquid" used herein is intended to include, but is not limited to, any liquid solution, homogenised liquids, slurries and particulate material in gaseous suspension which behave as a liquid. It has also long been known that varying levels of natural electrical conductivity or resistivity is present in all geological material. Surprisingly it has been found that bulk electrical conductivity or resistivity of a liquid at a given temperature is a function of the electrical conductivity of the liquid, the electrical conductivity of the solids and the concentration of the solids in the liquid. Therefore, when conductivity measurements are corrected for temperature changes or when temperature is constant, the concentration of solids can be estimated from measurement of liquid conductivity relative to that of clean liquid. The method and apparatus of the invention may be employed in any process where it is important to monitor the solids content in a liquid in a vessel and where the solids exhibit some electrical conductive or resistive properties. For example, the method and apparatus may be applied to brine tanks, settling tanks, slurry tanks, any tank with a solid/liquid interface, calcium carbonate clarifiers and the like. It is convenient, therefore to explain the present invention, by way of example, in the context of its application to the alumina processing industry where it is used to determine the interface between red mud (i.e Bayer mud) and green liquor (i.e the alumina-bearing solution). Alumina is typically produced from bauxite by the Bayer process. In this method, aqueous caustic soda is used to treat bauxite under high temperature and pressure. During this process aluminium hydroxide present in the bauxite dissolves in the caustic solution forming a sodium aluminate. The insoluble residues form red mud which is primarily composed of the oxides of iron, silica and clay. In stages subsequent to the Bayer process, sodium aluminate solution and caustic soda are separated from the red mud and other insoluble materials. Coarse waste particles (e.g sand etc.) are separated from solution in large settling vessels. The red mud is then separated from the green liquor by a gravity sedimentation method in large diameter vessels known as thickeners. The underflow from these vessels is known as "red mud" while the overflow is known as "green liquor". Red mud removed as underflow is washed in large washing tanks to remove valuable caustic soda for reuse. Information concerning the interphase between red mud and the green liquor is important to improve efficiency and to institute process controls and automation. Electrical conductivity or resistivity may be measured by any means known in the art. Preferably, that means is capable of withstanding high temperature and a caustic environment. Electrical conductivity may, for example, be measured using electrodes or toroidal sensors. The concentration of solids in the liquid in a vessel has been found to be a function of the electrical conductivity or resistivity and of the temperature of the liquid in that vessel. Where the temperature of the liquid is consistent throughout the volume of the vessel, electrical conductivity or resistivity bares a direct inverse relationship to the concentration of solids in the liquid. Thus, temperature measurements are not generally required where temperature is constant. However, the temperature of the vessel is preferably monitored to ensure temperature consistency throughout the vessel whilst conductivity resistivity measurements are taken. Process fluids encountered in the alumina industry are generally conductive but solids such as mud and sand behave as insulators. The presence of dispersed solid particles reduce the fluid conductivity by a ratio which is dependent on the volumetric concentration of solids. This principle applies for any combination of solids and fluid where the ratio of their conductivities exceeds a value known as the Critical Conductivity Ratio (CCR). The theoretical conductivity of a slurry or mixture containing spherical particles of any size distribution is given by the relationship: 3/2 K mixture = K fluid (1 - F) where K is conductivity and F is solids fraction by volume. The effect of solids on fluid conductivity is depicted in Figure 1. In practice, the reduction in conductivity caused by the introduction of solids such as red mud to Bayer process fluids has been found to be slightly greater than that predicted by the model for spherical particles. At low volumetric concentrations of solids in thickeners and washers and for the purpose of mud level measurements the following simple linear relationship may be used to determine solids concentration: Solids in gpl = A(K fl U jd - K mixture) /K fluid where A is a experimentally determined constant. If the temperature of the liquid in the vessel is known to vary, the monitoring means is preferably connected to a temperature measuring device which is capable of determining temperatures of the liquid at different levels within the vessel. Where a temperature measuring device is used that device should be capable of withstanding high temperatures and a caustic environment. Preferably, the temperature measuring device, when present, is in communication with the monitoring means which in turn directs temperature data to be recorded at the same time that electrical conductivity or resistivity readings are taken. The temperature measuring device may, for example, be a thermistor, a thermocouple, a resistance temperature device (RTD) or some other thermal detector. Further, the temperature measuring device may be positioned in combination with the conductivity or resistivity measuring means as a single sensor unit. The present invention will now be described by way of example only, with reference to the accompanying drawings, of which: Figure 2 illustrates one form of the electrical conductivity measuring device of the present invention; Figure 3 is a graph showing the output data from 5 different liquid concentrations measured over varying electrical conductivity and temperature regimes; and Figure 4 illustrates an alternative form of the electrical conductivity measuring device of the present invention. Figure 2 depicts an apparatus for determining the concentration of solids or for measuring the interface between a liquid and settled solids in a settling vessel, consisting of a conductivity sensor 1 which is in electrical communication with and suspended by a cable 2 from a winch 3. The winch is controlled by the operation of a motor 4 which is controlled by a monitoring means 5. The conductivity sensor signals are transferred from the cable 2 by sliprings 6 to a conductivity meter 7. The conductivity data is then passed from the conductivity meter to the monitoring means 5 which measures positional data from a shaft encoder 8 and controls the position of the conductivity sensor by instructing the winch 3 to raise, lower or hold the sensor in any position inside the settlement vessel. The monitoring means is adapted to continuously examine conductivity data from the sensor. In one mode of operation the conductivity sensor 1 is lowered at a predetermined speed into the settling vessel. The monitoring means 5 records the position of the conductivity sensor 1 and the electroconductivity at different points. If a temperature sensor is present the monitoring means would also measure temperature at each point. From such data, the interface between green liquor and red mud may be identified as indicated by a substantial change (e.g decrease) in conductivity. The monitoring means 5 then indicates the position of the interface by any suitable means. The present invention may also be used to control the level of red mud in a settling vessel. For this purpose the conductivity sensor 1 may be variably positioned within the settling vessel. The location of the conductivity sensor 1 will correspond to the maximum height that the red mud may rise to according to process control requirements. The monitoring means 5 continuously examines electrical conductivity data from the conductivity sensor 1. When electrical conductivity reaches a predetermined value which corresponds to the maximum height to which the red mud may rise the monitoring means would produce a signal which automatically initiates withdrawal of red mud from an outlet in the bottom of the settling vessel. The method of the present invention may also be used in rake- equipped tanks, for example to control the level of red mud in a liquid (i.e tanks equipped with automatic stirrers which are capable of maintaining solids liquid). For this purpose, the monitoring means 5 is in electrical communication with the drive motor for the rake and is adapted to raise the conductivity sensor 1 when the rake approaches, and holds the output signal constant until the rake passes. The monitoring means may then reposition the sensor at the last point of measurement prior to the rake approaching and resume recording data. Alternatively the monitoring means may reintroduce the sensor in to the liquid and commence continually recording data until it reaches the bottom of the vessel whereupon it returns to the surface. When electrical conductivity reaches a pre- determined valve which corresponds to the maximum concentration of suspended solids in a liquid the monitoring means would signal that the pre-determined concentration was reached. In addition to or in the alternative the monitoring means would also direct an outlet means to open to release the liquid suspension. The same monitoring means may also direct the delivery of further liquid and or solid into the vessel. The liquid in this instance preferably consists of one or more flocculants (ie. a settling and clarification aid). While the foregoing method is described for use in rake- equipped vessels it would be appreciated that the same method would be applicable to vessels devoid of a rake apparatus. Figure 3 illustrates graphically test results showing the relationship between temperature, electrical conductivity, and solids concentration from a mud slurry in a sodium aluminate solution. The points on the graphs of Figure 3 show that at a constant temperature electrical conductivity decreases as solid concentration increases. Figure 4 illustrates an alternative form of the invention wherein there is provided a doughnut style toroidal conductivity detector 10 suspended on a cable 12 which may be insulated with high temperature teflon, from a winch 14. The detector is connected to the cable by an underwater breakaway connector 16. The cable is connected to the winch and is in electrical communication with a conductivity transmitter 18. The conductivity transmitter is mounted in the winch drum 20 to avoid the need to pass low level detector signals across slip rings. The conductivity signal wiring passes along the inside of the winch main shaft 22 to a rotary connector 24 mounted on the opposite end of the shaft. From there the signal is connected back to a small modular style controller (not shown) such as PLC, which is used to control the detector and record data from the detector. The winch 14 is driven by a motor 26 having, an integral worm reduction gearbox 28 which serves to reduce the speed and prevent runaway of the winch under the weight of the detector. Further speed reduction is achieved through a timing belt and/or pulleys to give a winch speed of approximately 3m/min. The present apparatus has a pulley on the motor output shaft which is mounted to a slip clutch 30 so that in the event of the detector being caught up on something inside the fluid containing vessel then the slip clutch will allow the cable to peel off the drum and disconnect therefrom. A quick release socket (not shown) is used to wire the detector cable to the winch drum. A secondary shaft 32 may be used to drive a rotary limit switch 34 with upper and lower winch settings. Also mounted on this shaft are gears which operate a proximity switch 36 for position measurement by the controller. Preferably the conductivity transmitter 18 is mounted centrally in the winch drum on a flanged collar (after removing the terminal cover from the transmitter). The conductivity transmitter is preferably a microprocessor based transmitter. The detector is preferably able to operate reliably in caustic solution at 100- 120°C (eg 106°C). The cable should also be able to withstand the same environment. Additionally the present invention includes a wash system (not shown) to keep the winch cable clean and minimises scale build up on the detector. For example the wash system may have two different types of wash nozzles. Straight nozzles should be used to wash the cable while deflection nozzles should be used to wash the detector. The nozzles are mounted to the apparatus in close proximity to the cable and the detector to provide a washing thereof. Preferably there is provided a plurality of deflected spray washers. One opposing pair should be adjusted to deflect upwards and the other pair downwards. When the present invention is used in rake mounted vessels, limit switches need to be mounted on the rake drive mechanism to provide an interlock so that the unit will operate in the safe zones between rake passes. In operation the apparatus initiates a scan each time the rake limits make a transition from an unsafe zone to a safe zone. The winch lowers the detector into the vessel updating a reference position as the detector leaves the upper limit setting. If there is a clear zone within the thickener fluid the controller records the fluid conductivity in that zone to use as a reference to calculate solids. Once in the scan range the controller continuously calculates the solids concentration from the conductivity readings and the reference reading. The controller calculates the position of the detector by counting the proximity pulses from the gear teeth. It counts on both positive and negative transitions so that each pulse represents 10mm or 1 cm. The outputs are updated during the scan and are held until the next scan. When the output reaches a preset point the unit ceases scanning and returns to the upper limit position. A wash system cleans the cable when the detector is being raised from the vessel and operates after each cycle to minimising scale growth on the detector. Those skilled in the art will appreciate that although a specific embodiment of the present invention has been illustrated and described above, variations in the form of this embodiment can be made without departing from the present inventive concept.

Claims: TH E CLAIMS defining the present invention are as follows: 1. A method of determining the concentration of a solid in a liquid in a vessel comprising the steps of: (a) monitoring the electrical conductivity or resistivity of the liquid in the vessel; and (b) determining the concentration of a solid in the liquid from the electrical conductivity or resistivity thereof after correction for any temperature changes in the liquid. 2. A method of measuring the interface between a liquid and a settled solid in a vessel comprising the steps of: (a) monitoring the electrical conductivity or resistivity of the liquid in the vessel at different levels therein; (b) determining the concentration of solids in the liquid at the different levels by using the electrical conductivity or resistivity of the solids after correction for any temperature changes in the liquid in the vessel; and (c) selecting the level where conductivity or resistivity changes substantially relative to the conductivity or resistivity of a clean liquid and/or liquid containing solids. 3. A method according to any one of the preceding claims wherein the interface between the liquid and settled solids is calculated from conductivity or resistivity results according to the formula: solids = A(K fluid - K mixture)/K fluid where A is an experimentally determined constant and K is the conductivity of the fluid in the vessel. 4. A method according to any one of the preceding claims in which the solids form a mineral processing slurry. 5. A method accordingly to claim 4 wherein the mineral processing slurry is the underflow from a settling vessel or suspended solids in a thickener vessel. 6. A method according to claim 5 wherein the slurry is a red mud slurry from an aluminium processing operation. 7. An apparatus for determining the concentration of solids in a liquid in a vessel, said apparatus comprising: (a) electrical conductivity or resistivity measuring means adapted to monitor electrical conductivity or resistivity within a liquid in a vessel; and (b) monitoring means which is capable of recording electrical conductivity or resistance levels within the vessel and which is capable of determining the concentration of solids from the electrical conductivity or resistivity results. 8. An apparatus according to claim 7 wherein the electrical conductivity or resistivity measuring means is in communication with the monitoring means through a winching device that is capable of altering the position of the measuring means within the vessel. 9. An apparatus according to claim 8 wherein the winching device is controlled by the monitoring means 10. An apparatus according to anyone of claims 7 to 9 wherein the electrical conductivity or resistivity measuring means is capable of withstanding high temperatures and a caustic environment. 11. An apparatus according to anyone of claims 7 to 10 wherein the monitoring means is in communication with a temperature measuring device which is capable of determining temperatures of the liquid at different levels within a vessel. 12. An apparatus according to anyone of claims 7 to 11 wherein the temperature measuring device is capable of withstanding high temperatures and a caustic environment. 13. An apparatus according to claim 11 or 12 wherein the temperature measuring device is in communication with the monitoring means and is attached to the electrical conductivity or resistivity measuring means, wherein the monitoring means directs temperature data to be recorded at the same time that electrical conductivity or resistivity readings are taken. 14. An apparatus according to claim 7 for use in rake-equipped vessels, wherein the monitoring means is triggered by the rake in the rake-equipped vessel to raise the conductivity sensor from inside the vessel when the rake approaches the sensor, hold the output signal constant until the rake passes the sensor and then take one or more further measurements as the sensor is introduced back into the liquid in the vessel. 15. An apparatus according to any one of claims 7 to 14 wherein the monitoring means is adapted to control the concentration of solids in a liquid suspension by instructing an outlet means to open or close in response to the concentration of suspended solids in the liquid. 16. An apparatus according to any one of claims 7 to 14 wherein the monitoring means is adapted to control an outlet means at or near the base of the vessel which is capable of releasing solids from the vessel. 17. An apparatus according to anyone of claims 7 to 16 wherein the monitoring is adapted to control the delivery of further liquids or solids into the vessel. 18. An apparatus according to claim 17 wherein the further liquid is a flocculant. 19. An apparatus according to any one of the preceding claims wherein the apparatus includes a wash system which is capable of washing the apparatus after it has been removed from the liquid. 20. A method of measuring the interface between a liquid and settled solids in a vessel substantially as hereinbefore described with reference to the examples and/or the accompanying drawings. 21. An apparatus for determining the concentration of solids in a liquid in a vessel substantially as hereinbefore described with reference to the examples and/or the drawings.

Inventors: DOOLEY VINCE ROBERT, DOOLEY, VINCE, ROBERT
Applicants: ALCOA AUSTRALIA, DOOLEY VINCE ROBERT, ALCOA OF AUSTRALIA LIMITED, DOOLEY, VINCE, ROBERT
Application year: 1995
Main classification: G01F23
IPC codes: G01F23, G01N27