Manta Controls and PLA have been working together for the past five years. PLA supplies the Smart Diver that is used as part of the Manta Thickener Cube. worth taking a look at this.

Manta Controls (especially the owner John Karageorgous) are excellent. I have installed PLA mud diver (Smart Diver) at St Ives gold mine tailings thickener (when I used to work there), and Manta implemented the control philosophy (The Cube). It worked well. And I believe it still works well.

Another electrical conductivity probe type I’ve used in the past was a solid state probe with many sensors inside a cylindrical tube. This probe was mounted 1/3 inwards point on the feedwell bridge and was stationary in the thickener bed being connected between the bridge and the rakes.

As this was a tailing thickener with the rakes and the feedwell bridge being driven together from a motor/drive on the outside of the thickener. This was in the late 1990’s, so about 15 years ago. This was an EIMCO Thickener but I cannot remember where the probe came from.

Similar to the previous probe, this probe would profile the bed with many reading down the probe and determine the fluffy bed interface the thick bed interface, and we used this information for thickener bed control.

The probe was reliable and did not break down, but would only be able to be installed on thickener at this point in the bed, where the rakes and the feedwell bridge being driven together from a motor/drive on the outside of the thickener.

Another electrical conductivity probe type I’ve seen in the past was a solid state probe with many sensors inside a cylindrical tube, mounted at the wall of the thickener, so that the rakes and the froth booms do not hit the probe when they pass by.

But readings at the wall are a long way from the feed well and may not represent the readings of the bed in the mid point (or 1/3 point) of the thickener.

Another Bed determining device that I have used in the past, was on a set of high rate thickeners of small diameter (10m and 15m), that had sight glass windows mounted in the external wall of the thickeners from near the top to the near the bottom of the wall. This was installed almost 25 years ago.

You could actually see the level of the bed against the scale on the wall, and when the thickener bed locked onto the rakes from overflocculation/overfloccing, you could see the whole bed rotating past the window.

You can see that some of these bed measurement devices have been used and tested many years ago. 

I have used a couple of different thickener bed level measurement probes that use electrical conductivity, and they worked well, and they did tell you a lot about how the thickener bed and thickening interface were behaving.

But I have seen very few thickeners that have this type of level measurement installed, so the Industry does not seem to value the information and control that these probes can deliver.

One type I used in the past, (from Superflo, now Outotec) was installed on a Zinc Concentrate Thickener in the late 1990’s. It had three conductivity rings, similar to the Finch/Gomez probe that is used to measure gas holdup.

The Probe was on a cable on a winch drum installed at the 1/3 inwards point on the feedwell bridge and would be lowered into the thickener and profile the bed, taking readings every 100mm of depth (configurable) , for each rotation of the thickener rakes.

It need to lift out of the thickener after the cycle, otherwise the froth booms on top of the thickener would catch the probe and disconnect (break) it. On Plant power failure the probe would stay in the thickener and get broken when the thickener rakes were re-started.

The probe gave very good readings and information, but was regularly broken due to lack of care on startup after power failure.

It could determine the start of the fluffy bed interface and the start of the thick bed interface, so you could control the height of the thick bed interface by controlling underflow withdrawal rate.

Also you could see if the fluffy interface was a short or deep interface and use the flocculant addition to control the height of the fluffy interface, in a feedback control loop.

As well as using the feed forward control of the settling rate clarometer, that takes a sample from the feedwell and let the sample settle in a cylinder and measures the settling rate. This information can be used to control the flocculant addition in a feed forward loop.

This second system is more commonly used to flocculant control, as it is usually recommended and installed by the flocculant/Reagent companies. This is why the settling rate clarometer is more common installed on thickeners.

Ultrasonic probes are primarily used for thickener bed level these days. The mud diver is used a lot in alumina due to the harsh conditions and probably the most reliable probe around at the moment. They are also one of the most expensive. The other manufacturers that supply ultrasonic probes are Royce and Hawk. Mobrey do a fixed position ultrasonic head that we've used a lot on pilot units but can't see any reason why they can't be rigged up for a full scale thickener. Less prone to getting fouled compared to probe heads of the other two. Another type is a float probe that sits a cone shaped "float" on the top of the slurry bed in the thickener and has switched in the tube it sets off as it goes up and down with the bed.

I am a big supporter of the SmartDiver as above. Hawk are also producing a probe - I think it depends on how dynamic your thickeners are. Compare a base metal tails thickener against an acid CCD circuit - polar opposites in terms of dynamics and require different levels of accuracy and attention.

I also like Hawk manufactures a sonar based technology used specifically to measure bed level and floc layers in thickeners. this device is well know for it's reliability around the world and has become a standard with several manufactures of thickeners and clarifiers.

Oscillation Pty Ltd (of "Clarometer" fame) also produce some bed level devices, such as here: http://clarometer.com/Product%20Summary.html, under "Bed Level". I can vouch for the quality of their Clarometers, but cannot vouch for the quality of their Bed Level devices as I haven't used them before - though I do know that the owner/lead engineer is incredibly fastidous and has a penchant for build quality and operational robustness, so they might be worth checking out.

We have recently tested our own in house control system which uses a mud bed detector which responds to the physical properties of the mud bed. Based on the signal we control the flow of the underflow pump to keep the mud bed at a desired height so that we can control the thickener in what we call a semi steady state mode. We tested it on a 18 m diameter paste thickener treating clay tailings (diamond) over a 72 hour period with very good results.