The combination of operating at higher pH and saturated solutions of calcium along with sulphate or carbonate ions can lead to calcite / gypsum precipitates forming. There is considerable discussion in technical literature of this, e.g.,

Impact of gypsum supersaturated solution on the flotation of sphalerite, 2013:http://is.gd/YgRhF4

I agree that you have a precipitate forming. You should inspect your air inlets into the slurry. A rapid build up of precipitate can form at pressure change points.

Have you gotten any water analyses?

The more details of the dissolved content, the better!

At the very least you could crunch some quick calculations on calcium carbonate and sulfate scale formation, and get an idea of what you major one or two "culprits" might be - assuming it is a scaling precipitate.

Else, you might want to run a more thorough analysis using a proprietary software (one of the things I do for people fancy that!), and get a more detailed idea of the possible culprits.

As mentioned, there are mechanical drivers for scale formation, which influence the kinetics, but ultimately it comes down to the underlying thermodynamics of the water - what is suspended, what is dissolved, temperature, pressure, etc.

Once you have a good idea of what you are dealing with, you can have a better idea of how to minimise or even prevent it.

Additionally, you could consider getting a mineralogical analysis done on a representative sample of the precipitate (e.g. XRF), and have it interpreted for you.

I presume that lime is used to modulate the pH for selectivity against pyrite.

The higher the pH, the more intense is the formation of scale - most often being dominantly gypsum with other minor phases.

There are a number of compounds which are claimed to be effective in the control of scale formation. If it is desired to investigate this route, flotation tests with the anti-scaling agent at the maximum dosage recommended for use should be performed to ensure not detrimental impact on flotation.

I would add to comments that Calcium Carbonate is also an extremely common scale; depending on application type, it is often the most common scale.

The deposit is dissolved by HCL - so the basis is CaCO3. To prevent its formation you should exclude CO2 (anion of CO3) from the slurry. e.g. by blowing of nitrogen ( it is used in Cu-Mo sulfide flotation to protect S anion against oxidation.

I concur with you that the primary chemistry of the deposit (CaCO3 or CaSO4 or combination) depends on the chemistry of the water it-self.

As with many things in a process plant, it will be a trade-off between costs of preventing it from occurring vs. costs of dealing with it when needed to be removed for maintenance.

Anyone have any experience of using electro polishing as a way to reduce scale build up on stainless steel?

There are lots of examples of Ultrasonic Anti Scaling applications which are maintenance free and long lasting. This way you would not have to change the delicate balance of the chemistry.

Why do you all think it’s Calcium Carbonate - is this more prone to deposit / scale than say ZnS or other components. I have now found out that caustic is used for pH adjustment and not lime.

I must admit I would have said no, but then I remember the situation at Black Mountain when Anglo-American took over from Gold Fields. Although another Cu-Pb-Zn orebody at Aggeneys, the reason for the purchase was to treat ores from Gamsberg - a nearby sphalerite deposit. This sphalerite was soluble and absolutely played havoc with the Cu-Pb separation.

So under alkaline conditions, I would expect zinc hydroxides to precipitate in the slurry phase but not preferentially on any surfaces. If you are separating sphalerite from pyrite, then I would expect high levels of calcium in the system - any way, it sounds like our 'old friend' gypsum - calcium sulphate.