The trick is to reduce the air entrainment on the thickener feed to stop it from forming. Otherwise you just need to blast it with water. see

http://magra.co.za/A%20METHOD%20FOR%20PREVENTING%20FROTH%20FOR%20A%20FLOTATION%20PROCESS%20TO%20C[1].doc

The water spray on the top of your thickener is the most effective way to reduce them if it's necessary. However, by reducing the amount of frother in the flotation unit, amount of froths will become lower.

Our operations use high pressure water sprays to knock down the forth, breather pipe on thickener feed pump discharge and/or baffle plate in feed well to minimize air entrainment.

Certainly reducing air entrainment in the first place is a positive move. Still, froth management is usually required on tenacious frothing applications such as zinc, lead, coal float cons. We have a number of systems for Froth control such as booms, sprays and collection systems plus de-aeration tanks for thickener feeds.

The sites I have been to control the build up of mineralized froth on top of the thickener with high pressure water sprays but make sure they are correctly positioned. I have seen these sprays installed but don't do the job properly. I have seen a scum skimmer collection system operating in Phubia mines (Laos). Not there long enough to draw a conclusion. Also tried the froth buster. Experience frequent blockages due to the hard scales present in the feed slurry. Also tried filtering to get rid of the scales.. After a while it gets too hard.

Prevention is always better than cure, although rarely easy. I try to look upstream to see where the air is coming from. Often you see large feed tanks prior to thickeners that have a slurry pipe discharging into from a great height, creating massive turbulence - I've seen it where froth in the thickener will be bad when the feed tank level is low, better when high (if the slurry entry is submerged).

Injection of air via high velocity downward slurry flows (not submerged) or hoppers running low and gulping air would be the two main culprits. If this air can escape from the slurry between this injection point and the feed well, you eliminate the problem. Unfortunately, this situation occurs when the solids are coated with float reagents so bubbles are harder to detach - prevention is definitely better than cure. If the bubbles are still attached when flocculant is dosed, the bubbles build up up in the flocc aggregates and these then float. VERY sticky strongly bound froth (via flocculant and or forth reagents) can be very hard to break down. Flocculant should only be added (ideally) at the feed well. I have seen a situation where flocculant was added to a high density slurry (not ideal either - dilution prior to flocculation is very important too) in a collection box at the periphery of the thickener with high velocity jetting simultaneously occurring. Someone believed this was a good idea. Its a good idea to discuss issues with thickener experts prior to changing pipe work, flocculant dose points etc as the changes may actually be detrimental for otherwise unseen reasons.

Most, if not all frothing issues on thickeners originate from air entrainment before the slurry enters the thickener. The big misconception is that the thickener produces froth, which couldn't be further from the truth and typically comes from slurry being jetted into feed tanks prior to the concentrate thickener.

Most suppliers have a boom type arrangememt on the long rake arms that floats on the surface and pushes the froth on top of the thickener aroud and under sprays mounted on the bridge. Another is a collecting system that skims the top and pushes the froth into a collectig tray and out of the thickener. But the best is to use a de-aeration device to remove the entrained air before it becomes an issue.

I have seen significant improvement in froth reduction by use of a high shear mixing blade within a transfer pump box. Slurry is fed into the top of the pump box via launder where the flow then travels through the fast moving blade which breaks the froth down considerably. Additionally Maelgwyn Mineral Services have a number of articles on centrifugal froth removal with their IMHOFLOT G-Cells which may also be of interest. Refer http://www.maelgwyn.com/downloads/Maelgwyn_ImhoflotG-Cell_(Randol).pdf or http://www.saimm.co.za/Journal/v107n10p623.pdf for further information.

We have successfully designed and commissioned feed tanks which accept slurry at a position below 50% of the tank and then using a semi cyclone design remove the excess air. The slurry then enters a vendor feedwell and the additional dilution water and flocc tends to create an enviroment that is heavier then the bubbles. We did this on a nickel application. great discussion all - agree with many of the points.

I agree with previous comments that the best solution is to try and prevent air entrainment in upstream process. Failing that, drip addition of a defoamer (surfactant) to the feedwell can significantly reduce foam.

Can you describe in more detail the technologies that you refer to?

How do they work and what are the advantages and disadvantages of each technology?

I have seen many thickeners in different industries that have been installed with little or no ability to handle froth.

As discussed above the first step to remove froth would be to fit sprays along the thickener feedwell bridge to break-up the froth back into the water phase.

Retrofitting froth booms (with the optional duck or crocodile attached by the helpful maintenance personal) to the rakes would further improve the removal of froth by moving the froth under the sprays on the thickener feedwell bridge.

Has anyone Retrofitted froth booms to the rakes on old thickeners?
Are there details that need to be investigated first? Eg: rake loadings and extra torque on the rakes and gearbox/motors? What type and style of froth boom is recommended?

Most de-aeration tanks that I have seen supplied with thickeners are small circular tanks with a tangential feed that are mounted at the periphery of the thickener on the feed pipe to the thickener.

I find these units supplied are small and undersized for most duties, but ok if you have a weak and brittle froth structure, but this froth type is normally removed in the pumpbox that feeds the thickener. They can be useful, if the froth feed is flowing by gravity from flotation to the thickener, and has a weak and brittle froth structure.

For the strong and tenacious froths other improved systems have also been used in the past.

At McArthur River Mine in the NT Australia, they have a particularly tenacious froth, due to the P80=7um regrinding. They break the froth in the flotation circuit with “froth bashers” which are stirrer/impeller systems that are installed in the pumpbox boxes to break the froth. These work OK but consume a moderate amount of energy, and are difficult to access and maintain, so should be the solution of last resort.

The Mount Isa lead-zinc concentrator also has a strong and tenacious froth, I designed a de-aeration sump/tank system in-between the final product inventory sampling system and the pump feeding the thickener, to allow the froth enough residence time to detach from the slurry and then be broken up by the energy of the incoming slurry, with no extra energy required.

When operated well and as designed this mostly eliminates the thickener frothing issues that had been occurring for many year.

A system similar to this has also been installed at the Phu Kham copper concentrator.

Most of the ones that I have seen are homemade by the site personnel.
Is there a standard supplier and design of froth booms for thickeners?
Do different styles of froth boom have different benefits and advantages?

If you remember in 1999 we purchase a Superflo feed-well from you to retrofit on our Zinc thickener. At the time I liked this system, as the auto-dilute feedwell slots allowed the froth booms to push the any froth buildup back into the feedwell. This retrofit did work well and then allowed us to start adding floc to the thickener, as it was a very old un-floculated thickener that was overloaded compared to original design.

Have there been further improvements and innovations in this Superflo feedwell design?
In recent years I have seen systems for the froth removal that have been supplied with thickeners that have a slot near the periphery of the thickener mounted at the water level and with a pipe that feeds out the side wall of the thickener to an external pumpbox. The Froth boom moves and then scraps the froth into the slot and the froth flows to the external pumpbox, to be reclaimed and pumped to the most appropriate part of the plant circuit. This system seems to operate well.

I think this was installed/supplied with EIMCO thickeners.
Have people used and operated these systems?
Do they operate well?
Are there any issues with these systems?

Can this froth removal system be retrofitted onto existing thickeners to improve the froth removal?
Has anyone retrofitted this system to an existing thickener?
Are there any issues in the retrofit that need to be improved in the design?

Are there any other improvements and innovations in thickener design that have helped with froth removal?

Outotec has made many improvements to the Supaflo feedwell that are now combined into the Vane Feedwell design. Outotec also has a number of froth control / management systems including deaeration tanks, froth booms and scraping systems, froth removal and various spray configurations. Many of these can be fitted to an existing thickener with minor modifications.

I would say prevention is better than a cure so minimising air entrainment is the first step. Removing the air through our new deaeration tank before the thickener is also a positive fix.

Treatment of surface froth once it's on the thickener can still have advantages as it can manage the froth load / buildup and this can have a positive impact on thickener overflow clarity as well as the practical issues such as chocking launders. It also helps the moral of personnel - make people feel they are winning the war against froth!

You could examine the agricultural industry's use of boom sprays and apply the technology to the mineral sector, if no-one has already done so. One major pitfall is the blockage of the jets. Usually fertiliser products are used in boom spray systems, and insolubles are a critical component in product specification for fertigation. That said the feed water should then be highly filtered, such as used in gland water systems, otherwise you face the peril of them working for a very short period. before becoming blocked.

There's no overcoming a poorly designed feed system, so step one is getting that part right to minimize air entrainment in the first place. The biggest issues generally happen on concentrate thickeners, where the objective is to get the solids into the underflow. Because of that, along with abrasion and poor froth flow characteristics, froth collection and removal systems on minerals concentrate thickeners have been problematic. Instead, FLS typically supply concentrate thickeners with froth baffles and peripheral spray systems. We've also used bridge mounted sprays along with a boom to move the froth. In either case, the sprays need to be accessible for maintenance. If it's difficult to arrange access for peripheral sprays, the bridge mounted system will work.

In concentrate applications FLS frequently use the Dynafroth feedwell, designed to allow for froth in the feed to be hit with sprays prior to the flocculation stage, to minimize air entrainment in the floc structure.

I have had great luck with NS 9541 a product we commonly use as a filter/ dewatering aid. We also commonly use it as a defoamer in flot cell launders as it does not break bubbles through mechanical means.

I had some experience for the froth. De-aeration is needed for the flotation agent. Another way is to use the skimmer. It can rotate around the shaft. Skimmers are used in the municipal clarifier and thickener a lot.

In addition to Sprays, we have recently installed a modulated valve after the tank feed entering the feed well to minimize the entry of froth. A loop level control valve modulates to maintain a level in the feed tank and thus prevent the entry of air.

I agree with the general theme that the best way is to prevent the froth getting to the thickener in the first place. An audit of the thickener feed flow will often show points at which the slurry is falling or bends in pipes where air entrainment can occur. A well designed deaeration tank will work if there is room. Thickeners do make surprisingly good flotation cells. Feed pipe modifications are available also.