Numerous studies and papers have been devoted to the subject of thickener design and operation to achieve specified throughput and discharge densities. Mathematical models have been developed that present methods for sizing thickeners and predicting performance under varying process conditions. The compression effect of deeper beds on dewatering performance is a recognized phenomenon. This is one variable of many that are interrelated and influence thickener performance.
For example, in the steady-state, continuous operation of a thickener, if the solids feed rate increases and the bed level remains the same, the underflow discharge rate must increase, resulting in lower solids concentration. The interactions of these parameters are at the heart of thickener models. Knowing the bed depth is critical to maintaining stable thickener operation.
Numerous methods have been employed to achieve reliable bed-level measurement, including bed pressure, ultrasonic, capacitance, nuclear, vibrating probes, divers, and floats (to name just a few). They are typically limited to measuring the level at one location in the thickener and they range from being very expensive to reasonably affordable. Each type can point to processes where it has successful operating instruments. However, without universal success of any type of instrument, many plant operators feel making this measurement accurately and reliably is difficult.
The patented MudMax™ bed-level monitoring instrument started development as a way to overcome this difficulty.
Following is a brief history of this instrument’s development.
In the process of testing, both in the laboratory and in an actual operating thickener, the instrument was found to provide data that went well beyond just bed-level detection. Data collected during tests of the instrument are presented and discussed. The application to thickener control will also be discussed.
The operating principle of the instrument was that resistance to movement of any object through a settled thickener bed increases with increasing solids concentration and that this resistance could be measured. A sensor was developed for proof-of-concept tests that were conducted on different solids concentrations to determine the relationship to sensor output. The sensor was a paddle-shaped probe of stainless steel instrumented with a strain gauge. Drag created by moving the sensor through settled solids of different concentrations created deflection of the paddle, which was measured by the strain gauge. The property of the settled solids that was chosen for comparison was yield stress. Other prototype configurations used load cells instead of strain gauges to measure the drag on the probes resulting from movement through the settled solids.
The medium ranged from mostly liquid in upper regions to settling solids to concentrated solids having a yield stress in the lower regions. As a sensor moved through the thickener, it was exposed to drag forces or pressures caused by resistance to movement through the medium.
The instrument system used a battery-powered transceiver that controlled data acquisition from the sensors and data transmission to the receiver. This eliminated the need for slip rings to transmit data and power the system. To extend battery life, sensors were only powered when a data sample was collected. The time between data samples could be set or reset as necessary while in operation.
In the lab, two different solids were prepared for testing: copper tails and clay. Different concentrations of each material were prepared by successive dilutions of the same sample. A Haake VT550 viscometer was used to determine the yield stress of each concentration. The sensor was rotated through each sample and the strain readings from the strain gauge were recorded.
The relationship between sensor output and yield stress of test samples was established. The measured strain magnitude gave a linear relationship with the yield stress over the tested range. The type of material was also a factor. The clay sample had a different linear relationship with a different slope. The clay sample was tested at lower yield stresses for the purpose of observing sensor sensitivity. The sensor produced a measurable strain value below 10 pascals (Pa) yield stress. Another objective of the testing was to gain experience using the wireless data-acquisition electronics.
The laboratory sensor accomplished the goal of proving the concept. Movement of the sensor through the samples provided a measurable response over a range of solids concentrations that would be expected in a thickener bed. The wireless electronics and data acquisition were found to work well with the test sensor. With these positive results, development of a prototype instrument to test in a full-scale thickener proceeded.
Prototype sensors were developed that could be installed and tested in an operating thickener. This design used load cells to measure the drag force caused by sensor movement. Three sensors were mounted on a structure that was placed over one of the outer dewatering pickets on top of the rake arm. The three sensors were spaced 300 millimeters (mm) apart vertically on the support, with the lowest sensor located just ahead of the rake arm at an elevation approximately 600 mm above the bottom of the tank wall. A circular disk on a tubular arm transmitted the drag force to the load cell where the output signal was generated. Wiring from the sensors was routed up the support structure to the transceiver, located in a protective enclosure on top of the support and above the liquid level of the thickener.
The thickener was a 22.86 meter (m) diameter, high-density thickener with a 3.66 m tank sidewall, producing 40-60 Pa yield stress underflow. The process dewatered insoluble material from soda ash production for either underground or surface deposition. The thickener had a peripheral walkway that allowed convenient access to the instrument and provided the ability to take manual bed-level readings at any angular position around the tank.
At the time of the prototype test, thickener control was based on the operator conducting a manual bed-level reading every four hours. These bed readings were used to control the underflow discharge rate. Additional manual readings were taken at other times as needed to correlate with instrument data.
The plant procedure for making manual measurements at the prototype test site was to lower a long PVC pipe into the tank at a shallow angle until the operator could feel the bed. Then, maintaining the feeling of the bed, the pipe would be transitioned to vertical along the tank wall. Markings on the pipe allowed operators to measure the depth of the bed below the liquid level.
Testing of the prototype instrument started with the focus of detecting bed level. During normal operation, the output signals from the sensors as the mud bed level varied demonstrated that the bed level interface could be clearly identified. Sensor signals were seen to increase from zero to maximum output before the bed level was even detected by the next higher sensor, 300 mm above it. This indicated a very distinct bed-level interface and that the instrument was able to detect it. Correlation was demonstrated to exist between manual bed-level measurements and instrument signals.
Sensor signals were proportional to bed yield stress. Test data indicated that as the bed level rose above the level of a sensor, the yield stress at that sensor also rose. This indicated that a relationship existed between yield stress at a position in the settled bed and the depth of the bed above that position.
The magnitude of sensor signals was found to have a linear relationship to the yield stress of the material. The instrument described provided data from which the bed level interface inside a thickener tank could be determined. It also detected increasing yield stress as the bed level increased above the elevation of the sensor. This provided unprecedented information from within the thickener bed that could be used for early warning of reduced thickener performance. The potential exists to develop control methods and strategies to optimize thickener performance using this real time feedback.
Controlling thickeners to achieve steady-state operation has been a major challenge for plants. Reliable control depends on the ability to properly detect and manage the bed level within the thickener. The prototype testing outlined in this blog led to the development of MudMax, a first-of-its-kind bed-level monitoring system that provides 360-degree bed-level measurements, effectively bringing thickener control within reach.
This post was originally published March 2016, and updated August 2019.