Hi Joe,

you are correct.  Heavy/dense froth is generally the result of overdosing reagents (collector / frother).

At 200 g/t, are you maintaining the required water rator to keep maximum working coagulant dilution?  If you only increase the coagulant without needed dilution, you'll lose effectivity.  

+ Upping the coagulant should increase the settling rate but with not 'kill froth'.

Is your Question 1) about underflow density formulated OK? I do not follow. Did you mean density would INcrease?

If that is the case, it would just reflex you have reached maximum compression or the underflow pipe is pumping as fast as it can without allowance for a thicker U/F.

Hi David,

First, thanks for the prompt reply.

The addition of coagulant, typically is to improve clarity. But as coagulant is added, depending on how much dose rate, it will decrease the U/F %w/w density. My expectation is that at 200 g/t coag or even at 500g/t, would decrease the U/F density significantly, but not in this case and what more surprising, is that the U/F doesn't seem to be affected at all. 

and at 200 g/t coag, I also would expect the O/F clarity would be clear water, but in this case, better still a bit cloudy.

(by the way, operators increase the coag to combat  very poor clarity, very milky. but typically running at 60 g/t coag)

(also, the tails feed tank also increased significantly in froth quantity when very poor clarity happen, even up to overflow froth to launder, and the operators would increase the coag to 200-500g/t) 

My suspicious is that the persistent froth might be carrying all the coagulant and not reacting with the tailings feed slurry as it is added above the froth, and not fully submerge into the slurry. (Both tails thickeners nearly covered with froth, also the boom spray is not working correcty)

Also, i think the coag is dose at 1% solution without any dilution, to a feed slurry of 5-6%w/w tails feed slurry. I think this is reasonable correct dilution.

any feedback is much appreciated.

thanks

Hi Joe,

I have never worked with Coagulants myself but I do know that Flocculants will not perform great at 1% solution. It  cannot hurt to further dilute.  I run my thickeners at 0.1% In some cases, I was able to reduce raw flocculants doses by 50% just be dilution the floc prior to addition.

If the U/F doesn't seem to be affected at all, it could be that the 200-500 g/t is not effective at all (or little).
Yes it is possible a large portion of your 'thick coagulant' gets carried into the 'thick froth'.

Could you add/inject the coagulant earlier in the system?  

How many TPH of solids are you looking at settling? 

What family of frother are you using?  Glycol?

I have a case where it was not until I changed the flother type before getting improvements in O/F clarity.  The thick froth kept passing to the O/F.

Hi David,

again thanks,

Coagulant are typically added to clear the O/F clarity but usually needs a lot of mixing and time to react. unfortunately, it doesn't happen in this case,

I was thinking, is it possible if the coagulant is actually being carried away by the thick froth. and it is typically about the right dilution for coag at 1% although you are right the more dilution is usually better. the coag, is definitely the correct one, as it is tested find in cylinder test

I will also try different position of coagulant addition,

Also, how effective is high pressure boom spray, to destroy froth? is position, and direction of spray are important?

see attached picture, boom spray only spraying one side of the tank and sideways directions rather than directly, what u think?

regards,

joe

Thanks

Froth needs to break and release its air ahead of the thickener.  I have had success in installing a de-aeration box ahead of the thickener.  Am attaching a concept sketch for you.  This only works if the froth is not too persistent.  It is great when using alcohol based frothers. The box needs to be tall enough to allow your froth to break under its own weight and release the air.

Your sprays need to be positioned to insure the froth bed does not just go around in circle on top of your thickener.

I also include some of my favorite sprays. Also see http://www.tecpro.com.au/technical-info/spray-engineering/flat-fan-nozzle-technology/

https://www.911metallurgist.com/blog/thickener-froth-removal-sprays

hmm, no picture.

in any case, 30 sec is so little...you have no way to insure it gets well mixed?

OK, well. I would say your coagulant if being taken up into the froth and rendered ineffective. Stay in touch.

Joe, is it possible to find out some defoamers (like kerosene for example, or fatty acid soaps) instead of using coagulant? Otherway you may try to understand the zeta-potentials of particles - controlling it helps to change settling properties of slurry dramatically.