Technical Bulletin

ExcellentMineralsSolutions

Weir Minerals Division

Bulletin #28 - Version 2 August 2009

Pumping Froth

Introduction

Pumping mineral froths utilising centrifugal pumps has typically presented major problems for users. Most of these problems have occurred due to incorrect application and selection of operating speeds. New horizontal froth pump designs now offer opportunities to greatly improve the pumping of froths while maintaining high effi ciencies. Simple selection procedures are presented along with tips and techniques to maximise froth pumping applications.

Background to froth pumping

To extract minerals from ores generally requires crushing and grinding of the ore. For complex ores and to maximise the recovery of minerals this grinding is carried out to quite fi ne particle sizes. The fl otation process is often utilised to liberate the mineral from the waste rock. The fi ne particles in the form of slurry are introduced into agitated fl otation cells with the addition of air and fl otation agents. The fi ne particles of mineral ore attach themselves to the bubbles and fl oat to the surface while the waste rock sinks. The froth spills over the side of the fl otation cells and is recirculated through other stages of the fl otation process to achieve the maximum effi ciency and mineral recovery.

The froth liberated from the fl otation cells consists of slurry of fi ne particles with a fi ne dispersion of air bubbles. To move the froth slurry from one fl otation cell to another and to the next process requires pumping. The characteristics of the froth depends to a large degree on the type of ore being treated, the fi neness of the particles, the concentration, the amount of air in the froth slurry and the types of reagents used.

Froths can vary from brittle (easily broken down and the bubbles are generally large) to tenacious (the air is tightly bound in the froth and will remain a froth for many hours – bubbles tend to be very fi ne). The characteristics of the froth will vary from day to day and even hour to hour depending on these many parameters, so it is important to select froth pumps for the worst pumping scenario.

2 Weir Minerals Technical Bulletin - #28 Pumping Froth

FIGURE 1 - Horizontal froth pump diagram

Types of froth pumps

Vertical and horizontal centrifugal pumps have traditionally been used for pumping froth. Due to the added bulk of the froth, the size of the pump used for froth pumping needs to be larger than if the pump is being selected for a normal slurry duty without froth. Pumps with large intakes are necessary to pump froths due to the diffi culties of drawing the froth into the pump.

Horizontal pumps are preferred as they take up less fl oor space for their capacity, are easier to maintain and pump more froth fl owrate than vertical pumps. However, vertical froth pumps generally have the pump mounted in the bottom of a conical hopper with the feed

coming in from the top. Hence vertical froth pumps do not require a separate feed hopper, as do horizontal pumps. Whatever the type froth pump utilised, the most critical item is the inlet and the feeding of the froth into the pump.

In broad terms, vertical pumps are good for pumping brittle type froths. For medium type froths to very tenacious froths, horizontal froth pumps are generally superior.

Typical froth pumping problems

Froth slurries while containing solids also contain a large volume of air, so the froth slurry density is normally less than one. The air in the slurry is equivalent to a fl uid with a very high vapour pressure.

Consequently, the available NPSH with a slurry froth is very low as the vapour pressure is close to the atmospheric pressure (refer to FIGURE 4).

Pumping froth requires extra energy to be added to the froth so that it can more easily enter the pump impeller eye. One method might be to use impellers with low NPSHr characteristics. However, this does not always assist. The other preferred method is to open the pump intake so that more froth is presented to the impeller eye. The spinning impeller is also able to induce pre-rotation and shear to the froth in the intake pipe ahead of the pump thereby assisting froth handling. Open impellers are best for inducing the swirling of the froth slurry in the intake.

For very tenacious froths, it is necessary to enlarge the pump inlet size as well as adding a fl ow inducer in the pump intake. Feed hoppers continuously overfl owing is a sure sign that the froth pumps is not coping. Spillage should be avoided as the froth contains the mineral product.

New generation froth pumps

Weir Minerals has developed new horizontal froth pumps after extensive modelling and testing including the new types AHF, MF and LF. These pumps have enlarged area inlets, open impellers and in some cases fl ow inducers. By fi tting a new open froth impeller, enlarged bore throatbush and a new cover plate in pace of the standard components on the AH, M and L pump ranges, one is able to convert a normal horizontal slurry pump into a froth slurry pump (refer to FIGURE 2).

Froth pump selection method

Refer to page 4. The main requirement is to select the correct size of pump and impeller type for the particular froth and then select the correct speed. It must be emphasised that over-speeding is one of the worst things that will destroy pump performance when handling froths. Over-speeding typically will lead to unsteady fl ow and oscillating discharge pressures from the pump. If this occurs, the speed should normally be reduced, not increased.

FIGURE 2 - Warman patented fl ow inducer impeller inlet

FIGURE 3 - Typical vertical froth pump for light/brittle froths

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Sealing, drives and hoppers

Any of the normal seal types such as gland and expeller can be utilised for froth pumping. A typical hopper for froth pumping is shown in FIGURE 3. Belt or direct drives are both suitable, but it is preferable to use a variable speed drive to cater for changes in the froth characteristics that occur due to the normal and variable output of the fl otation cells and to also allow for duty changes, different throughput of ore, grinding size and duty changes etc.

Other froth pump applications

Pumping froth highlights some of the problems that occur at the inlet of centrifugal pumps and the need for proper system design and pump selection. Generally, if the fl uid is able to enter the impeller it typically can be pumped successfully. Other types of pumping applications that can present problems at the inlet and related operability problems are high yield stress, non-Newtonian, paste slurries, such as thickened tailings and viscous froths, such as oil sands bitumen froth. While these particularly diffi cult to pump slurries may have more severe corrections than those provided

for mineral froths, the large inlet, fl ow inducing vanes and larger passageways of the Warman fl ow inducer impeller have shown the ability to handle these diffi cult slurries better than conventional slurry pumps.

Do’s and Don’ts of froth pumping

Do select the correct pump • size

Do minimise the head•

Do speed the pump correctly•

Do provide a proper • enlarged pump inlet

Do allow adequate space for • pump maintenance

Do allow a large Overfl ow in • the hopper in case of spillage’s

Don’t utilise Froth Volume • Factors over 1.8

Don’t over-speed the pump•

Don’t impede froth fl ow into • or out of the pump

Don’t allow the fl ow into the • pump hopper to entrain further air in the hopper

FIGURE 4 - Froth characteristics and NPSH

FIGURE 5 - Froth pump selection

FIGURE 6 - HRf and ERf selection chart

Note:

1. HRf & ERf are constant over fl owrange

2. HRf & ERf also include effects of fi ne solids

3. STD = Standard open froth impeller

4. QU1 = Flow inducer open froth impeller

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Copyright © 2004, 2009, Weir Minerals Division All rights reserved

WARMAN is a registered trademark of Weir Minerals Australia Ltd and Weir Group African IP Ltd; CAVEX, HAZLETON, MULTIFLO are registered trademarks of Weir Minerals Australia Ltd; LEWIS PUMPS is a registered trademark of Envirotech Pump Systems Inc; GEHO and BEGEMANN PUMPS are registered trademarks of Weir Minerals Netherlands bv; FLOWAY is a registered trademark of Weir Floway Inc.; VULCO is a registered trademark of Vulco SA; ISOGATE is a registered trademark of Weir do Brasil Ltda.

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Weir MineralsTechnical Bulletin

Contact details

Head Offi ce

Weir Minerals Division2701 S. Stoughton Rd.Madison, Wisconsin USA53029Tel: +1 608 221 2261Fax: +1 608 661 8734www.weirminerals.com

Regional Headquarters

Weir Minerals North America2701 S. Stoughton Rd.Madison, Wisconsin USA53029Tel: +1 608 221 2261Fax: +1 608 661 8734

Weir Minerals Latin AmericaSan Jose 0815 San Bernardo, Santiago, ChileTel: + 56 2 754 22 00 Fax: + 56 2 879 99 59

Weir Minerals Europe / Middle EastHalifax Road, Todmorden, OL14 5RT UKTel: +44 1706 814 251Fax:+44 1706 815 350

Weir Minerals Africa5 Clarke Street South, Alrode, JohannesburgTel: +27 11 617 0700 Fax:+27 11 617 0791

Weir Minerals Australasia1 Marden Street, Artarmon NSW 2064Tel: +61 (0)2 9934 5100 Fax:+61 (0)2 9934 5201

Web site address:www.weirminerals.com

Spanish web site:http://es.weirminerals.com

Portuguese web site:http://pt.weirminerals.com

French web site:http://fr.weirminerals.com Russian web site:http://ru.weirminerals.com

Chinese web site:http://zs.weirminerals.com

Conclusion

The new types of Weir Minerals horizontal froth pumps overcome may of the limitations of using standard pumps for handling froths. They also overcome the maintenance problems of vertical froth pumps. As the

Warman® horizontal froth pump selection procedure

Determine the type of Froth. For 1. Brittle froth select FVF = 1.1 to 1.25. For Tenacious froth select FVF = 1.5 to 1.8. For Medium froths FVF = 1.205 to 1.5.

Qs = Slurry fl ow or De-aerated fl ow (L/s). Use maximum rated design fl ow. Also check the 2. selection for the maximum duty slurry fl ow.

Qf = Froth fl ow or Aerated fl ow = Froth volume factor x De-Aerated fl ow = FVF x Qs (L/s).3.

Calculate Hf (froth system head in m) at Aerated fl ow, Qf, for concentration of solids in slurry 4. and disregarding froth. Keep discharge pipe diameters large and pipe velocities less than 2 to 2.5 m/s and static head low to maintain total head below 35 m.

Water head Hw = Hf/HRf (m) where HRf is obtained from FIGURE 6 for mineral froths. For 5. viscous froth, such as oilsands bitumen froth, consult your Weir Minerals representative for de-ratings.

Select a froth pump such that the froth duty point is to the left of the best effi ciency line (but not 6. less than 25% of Qbep fl ow) and also less than 3.5 m NPSH required. Select the next larger size froth pump if in doubt. Add the froth system curve to the selected froth pump performance curve to help establish the best operating range.

At the froth duty point, select the pump speed Nf (r/min) and the water effi ciency Ew (%). 7. Calculate the froth effi ciency Ef = (Ew* ERf) where ERf is obtained from FIGURE ure 6.

Calculate froth density Sf = (Sm / FVF) where Sm is slurry specifi c gravity disregarding froth. 8. Note: Sf may be < 1.

Calculate froth power Pf = (Qf x Hf x Sf x 0.98) / Ef (kW).9.

Check the power when pumping water at speed Nf and ‘water duty’ point as indicated on 10. FIGURE 5.

Select a motor size with 20% margin to allow for drive losses and variations in duty.11.

Units: Q = L/s; H = m: E = %, P = kW. HRf, ERf, Sm, Sf and FVF are dimensionlessFor Q = m3/hr and H = m, Pf = (Qf x Hf x Sf x 9.81)/(3600 x (Ef/100)/1000)Subscripts: f = froth; s or m = slurry or mixture (de-aerated) respectively; w = water

Vertical froth pump type ‘AF’ selection

Use mainly for brittle type froths eg copper.•

Use a FVF of 2 to 2.5 to establish froth fl owrate knowing slurry fl owrate.•

Select duty fl owrate 30% to 85% of maximum fl ow shown on vertical froth pump curve.•

Use HR = ER = 0.9 for brittle froths as the conical hopper will assist in removing air from the froth.•

Select head and speed as for horizontal froth pump.•

Review operation to ensure speed is high enough to maintain a good vortex in the hopper.•

improvements have been made to the inlet of the pump, the froth pump is also useful in other applications when diffi cult to pump slurries are encountered eg Non-newtonian viscous slurries with a yield stress.

Submitted by Kevin Burgess, Warman Pump Technology Centre, Artarmon, Australia and Michael Bootle, Madison, Wisconsin, USA