The requirement for good level control of flotation machines is well established and documented. In this article we will focus on the more common level measurement options available for the flotation plant today, including the pros and cons associated with each one.
All level control loops consist of essentially three main components: the level measurement sensor, the level controller and the level control valve. In order to achieve good level control each of the components need to work well.
The problem
A well-known expression amongst control and instrumentation engineers is, “The most reliable measurement will always be non-contact and have no moving parts.”
The reason for this is linked to the robustness and reliability of the instrument. Any instrument that is in contact with the measured medium and has moving parts is going to be far more prone to failure, more so, if that medium happens to be slurry.
Unfortunately, this is not always achievable and in this case we generally settle for as close to the ideal as is practically possible.
Level measurement of a flotation cell is not as straightforward as it first sounds because we actually measure the depth of the froth. Of interest in a flotation cell is how deep the froth is above the slurry, as this determines the grade of the resulting concentrate. The typical range for a froth depth is 600 mm from the top of the flotation cell, the launder, down to the slurry interface. Here we are actually trying to measure the depth of a froth, or foam layer. Since the level we are interested in is covered in froth, measuring it is a lot more challenging.
Measurement techniques available today
All the measurement techniques share a few common requirements. The ability to transmit the measured level back to the control room reliability, often through a hostile environment, resist EMI and RFI, and update within an acceptable time frame. The loop powered current signal which works on a range of 4-20 mA is still by far the most common way of transmitting a level signal to the control room. It has proven itself as being reliable, stable and has the useful ability to indicate a wire break or fault. As the current signal would then fall away to zero.
Modern digital bus technology is gaining popularity due to the minimising of signal cables required, even more recently wireless transmitters are being introduced, which eliminate signal cables altogether. Even though changes in level in the average flotation cell are relatively slow you still need to update the signal frequently. For control, you always want your sample rate to be at least ten times faster than your loop update rate, and you need to refresh the level signal at worst once per second.
One of the oldest level measuring techniques is via a rotary potentiometer connected to a float arrangement. The float at the slurry/froth interface is connected to the rotary potentiometer by a set of mechanical linkages not unlike a pantograph. This is a simple instrument, but it has been so unreliable that it is seldom used in modern plants. The mechanical linkages become stiff and jammed with froth and eventually the instrument does not respond to the changes in level. These devices are maintenance intensive. In addition the conversion from the linear movement of the float to a rotary potentiometer introduces a non-linear error as well. With all float systems, if a cross current exists in the flotation machine it will be necessary to install a stilling well around the float.
An ultrasonic level measurement alone cannot be used as the froth absorbs the ultrasonic signals, likewise with infrared and radar.
Probably the most common, although not ideal, method of measuring level in a flotation cell is via a combination of an ultrasonic level transmitter and a target plate assembly. This technique involves connecting a ‘target plate’ to a float and then measuring the changes in height of the target plate using an ultrasonic instrument.
The ultrasonic transmitters are now a mature technology having been in the market for over 15 years and supplied by a number of world-class instrumentation suppliers. Generally they work well and are cost-effective.
However, the problem with this technique is once again with the mechanical components that are in contact with the froth and slurry. They suffer from coming into contact with the froth, causing them to stick or jam, and the slurry build-up changes the buoyancy of the float, all resulting in increased maintenance and wear rates and decreased reliability.
Guided wave radar is a step closer to the ideal instrument in that it does not have any moving parts. These instruments use a metal tube as the antenna, or wave guide, for the radar. The wave guide is extended into the slurry interface; the radar signal travelling along the wave guide is reflected back off the slurry interface to be detected by the instrument. The time the signal takes to return to the instrument is directly proportional to the level. A slurry build-up on the wave guide used to be a problem for this technique instrument, however the introduction of modern instruments that feature multiple frequency radar waves have virtually eliminated this problem. Even so, these units are not popular for the simple reason that they are costly when compared to the alternatives.
The customer’s sensitivity to cost per level point stems from the fact that flotation concentrators often have in excess of fifty flotation machines requiring level measurement.
Another approach, which is fairly common, is based on the magneto-resistive principle. Here a magnetic sensor within a sealed pipe moves in relation the slurry level in the flotation cell, up and down a wire sensor. The signal transmitted down the wire is reflected by the induced magnetic field in the wire sensor and therefore indicates the level in the flotation machine. This type of sensor is accurate and repeatable and it also has extremely fine resolution, in the order of 0,5 mm. However, as the magnetic sensor is coupled to a float on the slurry interface, it is susceptible to slurry build-up as in all float systems. Despite this, it is now one of the more successful techniques in use.
Conductance type level sensors can also be found in this market. Here the sensors make use of the electrical conductivity to indicate the slurry level. As the slurry level changes, so does the conductance of the sensor. (Sometimes these are also referred to as resistivity type instruments, as conductance is the reciprocal of resistance, or resistivity.) As the ore is never consistent, these sensors suffer from the fact that ore changes, changes in the conductivity of the water, or reagent changes in type and strength will result in a change of conductance measured, resulting in frequent re-calibration requirements. More modern instruments include more than one measurement technique which has resulted in more reliable measurements.
There is a lot of exciting development work being done using this measurement principle. These instruments have been developed to the extent where they can give an output signal of ‘froth density’ over the length of the measured range and the froth/slurry interface as well as the froth height, other than the simple slurry interface point. Again, it is the sophisticated instruments that work fairly well, which come at a cost.
Nothing is ideal
Theoretically, it is even possible to use X-ray to determine the slurry level interface. However, cost and safety considerations make this an impractical option. Personally, I have never seen such an instrument in use in flotation.
As you can see there is no shortage of choice for flotation level measuring instruments, but as yet none of them meets the full requirements for the ideal instrument – non-contact and no moving parts. The search continues.
For more information contact Richard Rule, eDART Slurry Valves, +27 (0)11 823 6620, [email protected], www.edart.co.za
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