Froth pumping has long been a major engineering challenge for mine operators, and new flotation practices designed to enhance mineral reclamation in the face of declining ore grades are proving problematic in sub-optimally designed pumping setups.
Hoppers without systems for actively removing excess air are increasingly causing circuit bottlenecks and requiring more frequent maintenance as they struggle to deal with tenacious froths.
To manage these problems, it’s important to understand the factors contributing to froth composition and how froth handling equipment can be improved to prevent build-up and alleviate performance bottlenecks.
How to prevent issues in your sump and hopper
For many operators, the hoppers feeding into the pumps represent a persistent source of problems, with air bubbles and their valuable mineral product building up in the sump and interrupting the effective flow of the froth.
Frequently, these issues stem from a hopper which adheres to standard slurry pumping designs, rather than one specifically built to transport froth.
A standard slurry pump seeks to prevent solid matter from settling in corners of the tank and keep in suspension through sufficient turbulence. However, that same turbulence prevents the air in froth from escaping the slurry, reducing pressure, and causing blockages.
Effective tanks for froth pumping instead promote continuous circular movement, which pushes the solids and liquid to the outside of the sump while air is gathered in the centre via centrifugal force, where it can escape to atmosphere.
Ideally, froth pumping hoppers should be conical, or round with a conical shape at the bottom, to promote the transfer of minerals and liquid down into the pump rather than building up. Slurry should be introduced from the top of the hopper at a tangential direction to promote the whirlpool action inside the tank.
Promoting efficient transport between the tank and froth pump
As in nearly all aspects of froth pumping, intake pipes should be as large as practical to facilitate the clean movement of air and prevent blockages. They should be sloping down towards the pump, to allow air that does escape from the slurry to travel back up the suction pipe and into the sump, where it can ultimately escape. Otherwise, air can linger in the pipe and interfere with the slurry’s flow through to the pump.
The pipe should also be as short as practically possible, with just enough space for a maintenance spool and an isolation valve, alongside a water injection port to help flush any built-up solids out.
Having a dump valve on the suction side of the pipe will further streamline maintenance by making it simple to disconnect the pump from the tank and remove any residual solids from the pump.
How to prevent air lock in the eye of your froth pump
One of the key challenges in pumping slurry with air entrained is that the air will naturally centrifuge into the centre of the pump, the eye of the impeller, where it forms an ‘air lock’ which physically blocks the slurry from progressing through the pump.
Eventually, this will block enough slurry to create a surge, where the amount of slurry in the sump builds up so much that the pressure it exerts eventually forces the air through the pump, which can cause significant strain on the pump’s bearings via a cyclic axial load, and a radial load on the pump’s impeller and shaft, decreasing their wear life.
Firstly, it’s important to ensure the pump’s discharge pipe is either at the top or positioned at a 45° angle to allow air which has been trapped in the pump’s casing to escape outwards. Pumps using the horizontal discharge at the bottom will quickly build up air inside the casing, rendering them ineffective.
When this isn’t enough, the most effective way of alleviating this problem is using a pump with an integrated Continual Air Removal System (CARS), which is available with the Warman® AHF, MF and LF pumps for applications which require it. With CARS, the pump uses vent holes in the impeller to move gas into a dedicated collection chamber in the back end of the pump. From here a flow inducer facilitates the movement of the air through vent pipe and out of the pump.
This continuous removal prevents air build-up in the impeller’s eye and promotes efficient slurry transport.
How to manage tenacious froths
Mineral froths are generally classified as either brittle or tenacious. Brittle froths feature large bubbles that break easily, while tenacious froths often feature very fine bubbles which can be much harder to separate from the surrounding slurry liquid.
As ore grades drop around the world, two trends in mineral liberation are producing increasingly tenacious froths, which contribute to the pumping difficulties site operators are encountering.
Firstly, the increasing percentage of fines and ultra-fines being ground are leading to smaller bubble formations and more tenacious froths. Secondly, increasing amounts of flocculants are being added into the slurry which lock up the air into even smaller bubbles.
Together, these can result in a highly tenacious froth which forces operators to design a more accommodating froth pumping circuit by adhering to best-practice design as outlined above or reduce the flocculant of fines being deployed into the slurry.
Weir Minerals has a long history with froth pumping and its specialist engineers can help you resolve issues in your flotation circuit. To find out more, download the whitepaper below.
