Thickeners are used to separate a solid-liquid mixture into two products, a clear liquor in the overflow and a concentrated and thick slurry in the underflow by gravity. Flocculants are also fed to the system to help aggregate the solids and increase their settling rate.
What is bed level and why is it important?
The term “bed level” is widely used in the industry to refer to different settling phases in the thickener (figure 1). Some people use this term to describe the mud level, others use it to refer to the interface level or layer. To avoid any confusion, this article will use the terms mud level and interface level as defined below.
During the settling process, a zone with higher solid concentrations than the input slurry stream is created at the bottom of the tank and is known as the mud layer.
Figure 1. Thickener and its settling zones
The interface layer, also known as hindered settling region or interface, is defined as the height of the zone between the mud layer or compacted zone at the bottom of the thickener and the clarity layer or clarification zone (layer with low suspended solids content) at the top.
The thickening process is generally controlled by manipulating process variables that result in an increase in the clarity of the overflow (aiming to achieve zero solids reporting to the overflow) and an increase in the density of the underflow (with the objective of getting all the solids reporting to the underflow with the least amount of water or liquor).
Control is generally achieved by manipulating the discharge rate of the underflow and the flocculant addition. Mud level is a variable that can be used to control the underflow pump speed to maintain a constant underflow density. Interface can be used to control settling rate of the solids by manipulating flocculant concentration or flow rate.
As mud and interface levels are used to control underflow density and flocculant addition, incorrect measurement can result in water (or liquor) being drawn out through the underflow, solids reporting to the overflow or flocculation problems. All these issues in turn will translate into extra costs associated with wasted flocculant or re-processing.
How are mud and interface levels measured?
There are several methods for measuring mud and interface levels in thickeners. Some are more sophisticated than others. Some measure either mud or interface levels or both manually or automatically. The most popular methods are:
- Manual core samples
- Hydrostatic pressure
- Buoyancy-based system
- Fixed position ultrasound
- Mud diver
The selection of technology to measure mud and interface levels in thickeners depends on the process, environment and budget. Below is a summary of each technique with pros and cons to help you make the selection.
Manual core samples
A large tube is used to extract a “core” sample from the thickener manually. This sample is then visually inspected and underflow pumping and flocculant rates are varied on this basis. This test is difficult, risky and labor intensive.
Figure 2. Manual thickener sampling
The total mass of the solids is calculated using hydrostatic pressure measurements, commonly referred to as “bed pressure” from a pressure sensor in the bottom of the tank. If the tank is full of water or clear liquor, the pressure will be at its lowest. As solids start to build up in the tank, the pressure increases as the tank contents are denser.
When used in conjunction with rake torque, the mud level height can be estimated. This method is simple, cheap and generally reliable. Suitable for tanks with predictable settling behaviors.
However, it does not tell you the exact mud height so operators normally err on the side of caution and keep a low bed pressure even if the underflow density is low.
Depending on rheology of the mud, the rake torque and the bed pressure do not always align.
This method can also require frequent maintenance.
Buoyancy based system
A floating ball (figure 3) typically with the same density as the material in the layer to be measured (interface or mud) is run down the thickener via a wire. The ball will sink until the density of the material in the ball and in the thickener is the same e.g. if the ball is set up to measure the mud, the ball would sink through the clear zone, through the interface and float on the mud. Once the ball has touched mud, this point will be recorded as the mud level.
Figure 3. Floating ball
Using a floating ball is an inexpensive way to measure mud and interface levels. However, they can be a high maintenance item. Balls also leak and get filled with what they are supposed to float on and sink through. They can be difficult to fill with the correct density fluid.
Interface is hard to detect with this method as the density difference between clarity and interface layers can be very small.
Fixed position ultrasonic
These devices are submerged in a fixed position in the top of the tank. They send an ultrasonic pulse and analyse what is reflected back. The reflection is analysed in its intensity and time. The end result is that the height of the reflection (interface or mud) can be calculated. Some newer devices use dual frequencies. One frequency picks up the interface, the other should penetrate the interface and detect the mud.
This measurement technique is relatively inexpensive and the device is easy to install and maintain. However, it does not work on all applications. It has difficulty detecting the interface if the interface layer is not well defined and not dense. Fixed position ultrasonic devices will pick up the mud level well only if the interface does not attenuate all the energy before it gets to the mud layer. Hence, the method can be unreliable when there is a thick layer of interface.
In addition, the measurement tends to drift after initial set up. Fixed position ultrasonic devices are not suitable for applications with high temperature and scale and have difficulty measuring in non-water-based slurries.
A mud diver is an automated device that mechanically lowers and retracts a probe into a thickener. The probe can use different sensing techniques (e.g. optical, ultrasound, etc) to provide the depths of the layers in the tank in real time. Automatic control of flocculant dosing and underflow pumping based on these measurements can then be implemented.
Mud diver dips are synchronized with the rotation of the rake, preventing entanglement of the probe in the rake arms. They are also commonly equipped with a washing station that cleans the probe after each dive preventing fouling.
Because mud diver sensors are actually penetrating through the zones, the readings are actual readings of the physical property of the solids at that point in the tank and not indirect/inferred measurements.
Mud divers can be used in applications with high temperatures and scale. They are also suitable for water based and non-water-based slurries.
However, mud divers can be expensive and in high scale applications, they can also be a high maintenance item.
PLA manufactures a mud diver variant called the SmartDiver (figure 4). The SmartDiver is a ruggedized automatic mud diver. The system has been designed to work in harsh environments with temperatures up to 150C.
SmartDiver’s probe uses ultrasonic techniques to measure accurately suspended solids and density simultaneously in real time all the way from the top to the bottom of the tank.
The system provides accurate real time readings of levels of all zones in the tanks and provides a solids vs depth profile. It detects the interface and the mud levels and give an approximate reading of the clarity.
Historical data of the behavior of the layers in the thickener is collected, trends are displayed and alarms integrated to the DCS. This allows the user to visualize and monitor the thickener operation performance. The system is ideal for processes prone to disturbances from changes in ore properties or varying flow rates where automatic control is essential to keep the operation of the thickener stable.
Figure 4. PLA SmartDiver
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