Thickener Control

S

Thread Starter

Samuel

Hi 2 All! I would like to know is there is someone who can provide more information on the existing control strategies for thickeners/settling tanks. If neccessary a flocculant can be added. This aim is water conservation. I am working on a project on which some mines in my country wants to decrease the amount of water used/wasted. What needs to be done, as far as I know, is to get the underflow concentation as high as possible to still be able to pump the underflow "product". The less water it contains the easier the disposal, but the more difficult the pumping. Please give guidelines/comments.
 
Hi,
Our thickener controls are mostly manual. The underflow feed pumps use a common discharge with a flow meter. The variable speed pumps are controlled within a 55%-100% range, based on a flow controller (PID) in DCS. We have underflow density meters in place, that value is used as input to a density controller (PID) that cascades to the setpoint of the flow controller. Our density meters are very inaccurate, and are only useful for trending a change in density, so the controller is set manually, for now.
We take sample density readings at the suction of the underflow pump four times per day, and adjust the flow setpoint to maintain a constant density. We set the flocculant addition flow manually based on overflow water turbidity and bed density. Once per day, we check bed level using graduated taps on the side of the thickener tank, and adjust the flow setpoint if necessary. Our thickener also has a rake torque value that we monitor so the bed is not allowed to get too dense/deep.
- John
 
J
One: Instrumentation
Two: Method

Instrumentation can help you monitor the solids conetnt and/or viscosity at any time either in the tanks (at the "teeter zone") or in the underflow pumping line to control the pump.
Commonly used are vibrating element density and/or viscosity meters.

Method: which will give you the best conttrol? a continuous underflow pumping at variable rate controlled by the instruments or on/off pump control based on build up of deposits?
You can also look at the optimum way to use the instruments to control floculent addition.

Don't be trapped by history. Many plants and methods are based on historical solutions that relied on schemes that were based on an absence of instumentation. One you have good instruments, radical rethinks become possible.

Density measurement is used in a wide variety of such operations from china clay slurries, settlement tanks, granite slurry washing plants, seperators etc.

In the cement industry chalk is made up into a slurry for pipeline transport. The higher the solids content, the less water to be removed at the cement plant. Inline visometers are used to control the addittion of sodium lignosulphonate which is a viscosity modifier. This reduces the viscosity for a given solids content and thus allows a higher solids content for the pumping capacity.

In sand and gravel plants density measurement in the underflow provides a more accurate control. This is a benefit as in many such operations the underflow pumping is based on a timer. This means that at the end of the timing cycle clean water is pumped away. The density meter will shut the pump down at a specified density or solids content.
Pumping can be initiated by an intank density measurement.
 
I would suggest that vibrating density device is not the best way to go.
Contact = wear.
A gamma gauge or coriollis meter is recommended. Gamma gauge best.
 
J
Most of the mineral mines in my area are using Milltonics bed level sensor model DPS300. It is an ultrasonic level sensor which works very well providing there is an defined interface and no aeration in the thickener. The higher the bed level the denser the underflow. For more information see http://www.prts.com.au
 
Hi 2 All! I would like to know is there is someone who can provide more information on the existing control strategies for thickeners/settling tanks. If neccessary a flocculant can be added. This aim is water conservation. I am working on a project on which some mines in my country wants to decrease the amount of water used/wasted. What needs to be done, as far as I know, is to get the underflow concentation as high as possible to still be able to pump the underflow "product". The less water it contains the easier the disposal, but the more difficult the pumping. Please give guidelines/comments.
Hi,
We have a Settling Rate Monitor, for automatic dosing of flocculant & coagulant into thickeners.
Nalco Water - Ecolab
https://www.scribd.com/document/511383420/Storm-Apr-2020-1
 
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