Pease Desiging Flotation Circuits

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Designing Flotation Circuits for High Fines Recovery J D Pease1, D C Cu Curry rry2 and M F Young3 ABSTRACT

Liberated Fines:

Many flotation operations need to improve liberation to increase recovery, yet often plant operators are concerned that finer grinding will produce ‘slimes’ with low flotation recovery. Yet simple circuits can be designed to give good liberation and excellent fines recovery. The economics of fine fi ne gri grindin nding g has chan changed ged fund fundame amental ntally ly wit with h new sti stirre rred d mil millin ling g processes like the IsaMill. This has also transformed flotation design due to the clean surfaces produced in the inert grinding environment and the narrow size distribution produced by the IsaMill. Past poor experiences with ‘overgrinding’ in flotation circuits was often due to the wide size distri dis tribut bution ion and the deg degrad radati ation on of min minera erall sur surfac faces es prod produce uced d by conventional devices using steel media, like ball mills and tower mills. The correct practice is to design both grinding and flotation together to suit the liberation characteristics of the ore. This may result in several stages sta ges of gri grindin nding g and flotation flotation,, rec recov overi ering ng min minera erall as soo soon n as it is liberated, then applying further grinding energy only to those particles that still need liberation. liberation. Flotat Flotation ion then occurs in relati relatively vely narrow size bands and with minerals with simila similarr libera liberation tion characteristics. characteristics. Though the circuit may look more complex on paper, in reality it is far simpler to operate since each section performs predicably. Circuit design also needs to address materials handling. Finer particles affect froth handling and dewatering, but can still be easily handled by standard products with carefu car efull att attenti ention on to des design ign of lau launder nders, s, pump pumpss and pum pumpbox pboxes, es, and de-aeration before thickening and filtering. Excellent examples of designing circuits to include ultrafine grinding include Mt Isa and McArthur River (MRM). In the last decade these operations have produced over ten million tonnes of concentrate with an average grain size below 10 µm and above 80 per cent recovery. At Mt Isa, zinc recovery was increased by ten per cent by applying fine grinding to existing poorly liberated streams. In fact, grinding finer in the IsaMill actually improved the recovery of fine particles by ten per cent, and maximum particle flotation recovery occurs for 10 µm particles. A more recent example is the flotation of Platinum Group Metals (PGMs) (PG Ms).. The use of con conven ventio tional nal ball or to tower wer mil millin ling g to imp improv rovee liberation libera tion is sev severely erely limited by the impact of commonl commonly y associated chromitee on class chromit classifica ification, tion, and degrad degradation ation of flotat flotation ion performance by the steel grinding environment. The ability of open circuit IsaMills to deliver a narrowly sized ultrafine product with clean mineral surfaces is transforming the design of platinum flotation circuits. The science and practice of fine-grained flotation is now thoroughly proven. The availability of the 2.6 MW IsaMill has made ultra fine grindin gri nding g a prac practic tical al too tooll for flo flotat tation ion pla plant nt des design ign.. It is exp expect ected ed to become a standard choice for future applications of both fine-grained ores and for minerals with d iffi ifficult cult flotation characteristics.

INTRODUCTION: FINES DO FLOAT! FLOAT! The conventional view of flotation of different size fractions is shown sho wn in Fig Figure ure 1. Sam Sampli pling ng of mos mostt flo flotati tation on pla plants nts will produce a similar curve. Not surprisingly this leads to the view thatt ‘fi tha ‘fines nes don don’t ’t flo float’, at’, and oper operator atorss are car carefu efull to av avoid oid overgrinding or ‘sliming’ of feed. Howe Ho wever ver for fi finene-gra grain ined ed ore oress the there re is no cho choice ice – cle clean an concentrates can only be made from liberated particles. In some cases achieving liberation means grinding to sizes below 10 µm. For example, at Xstrata’s McArthur River Mine (MRM), grinding 1.

MAusIMM, MAusIM M, Xst Xstrat rataa Techn echnolo ology gy,, Lev Level el 2, 87 Wi Wickha ckham m Terra errace, ce, Brisbane Qld 4000. Email: [email protected]

2.

Xstrataa Technology Xstrat Technology,, Level 2, 87 Wic Wickham kham Terrace, Terrace, Brisbane Brisbane Qld 4000. Email: [email protected]

3.

MAusIMM, MAusIM M, Xstrata Xstrata – Mt Isa Isa Mines, C/O- PO Box 1902, 1902, Mount Mount Isa Qld 4825. Email: [email protected]

Centenary of Flotation Symposium

High surface area Need high collector and low depressant

Intermediate: fast floating lower collector need, Coarse Particles

composites need depressant

Low liberation

y r e v o c e R

5

10

30

50

80

100

150

200

Size (microns) FIG 1 - The conven conventional tional view – ‘fines don’t float’.

to P80 of 7 µm is essential to produce a saleable concentrate. At Mt Isa, grinding streams to P80 of 12 µm and 7 µm is essential to get adequate recovery. In these plants, creating ‘slimes’ is absolutely essent ess ential ial fo forr goo good d flo flotat tation ion rec recov overy ery.. Bet Betwee ween n the them, m, the these se operations produce around 1 Mt/y of concentrates by flotation of particles mostly finer than 10 µm, at over 80 per cent recovery. In fact, the best flotation recovery recovery is in the ‘slimes’. At MRM, 96 per cent of the individu individual al particles recovered are finer than 2.5 µm. So fine particles do float – and they float very well, as shown by Figure Figu re 2, the reco recover very-si y-size ze curv curvee for sph sphaler alerite ite fro from m roug rougher her concentrate at Mt Isa. 50%

100% % n o i t c a r f e z i s n i y r e v o c e R c n i Z

Recovery

90%

45%

80%

40%

70%

35%

60%

30%

Size Distribution

50%

25%

40%

20%

30%

15%

20%

10%

10%

5%

0%

0.0%

C7

C6

C5/C4

C3-C1

38/53

75

0-4 µm

4 -8 -8 µm

8-16 816 µm

16-38 1638 µm

38-75 3875 µm

Size fraction

FIG 2 - Mt Isa zinc recovery from rougher concentrate by size.

Are fine particles different? There is much discussion about the different behaviour of fines in flotation, and the need for special attention – high energy flotat flo tation ion mac machine hines, s, spe specia ciall imp impell ellors ors,, sma small ll bu bubble bble siz sizes, es, attritioning, different reagents, entrainment. In our experience, after making over ten million tonnes of fine concentrate in the last decade, there is nothing special about fines, they just respond differently because:

Brisbane, QLD, 6 - 9 June 2005

905

J D PEASE, D C CURRY and M F YOUNG

•

they have higher surface area per unit mass, so need relatively higher reagents;

•

they have less momentum, so tend to follow water more easily than coarse particles – less energy for bubble attachment, more tendency for entrainment;

•

as a result, flotation rates will be slower, and lower cleaning densities or froth washing may be needed to counter the extra entrainment;

•

they tend to be more affected by surface coatings – perhaps because the high surface area ratio makes them more reactive, or perhaps because their low momentum means it less likely that loose surface deposits are abraded off by other particles;

•

they tend to be more affected by water chemistry and ions in solution;

•

the high surface area to volume means that froths can be tenacious and thickening and filtering is more difficult; and

•

their flotation kinetics can be slower and may be similar rate to coarse composite particles. They can still float with big bubbles, but smaller bubbles increase their flotation rate.

Though these effects become stronger as particles become finer, there is no mysterious sharp distinction between ‘coarse’ and ‘fine’ particles, just a steady gradation as particles get finer.

Bursting the bubble myth on flotation cell design We believe that efforts to design flotation impellors and cell energy input specifically to suit fines are misdirected. Mt Isa and MRM successfully float more fines than any other sites, and we use a range of different flotation equipment – flotation columns, Jameson cells, several different makes of conventional cell. Our selection criteria for flotation cells have been on the basis of: •

ensure adequate residence time,

•

ensure adequate lip length,

•

apply froth washing if beneficial, and

•

choose the cheapest cell available that meets these criteria.

Design principle for fines flotation The reason that fines don’t float well in many circuits is because they are mixed with coarse particles and with composites. The fines need more collector and more flotation time – but the flotation conditions usually have to be set to suit the coarser fractions. Designing different flotation impellors distracts from the real problem – fines and coarse will never float well in the same cell, since the reagents can’t be optimised for both. This is particularly relevant when composites have to be rejected to maintain concentrate grade. For example, a 40 µm composite that includes a 15 µm grain of sphalerite has to be rejected – but the conditions to do so will probably also reject a 15 µm liberated sphalerite particle. Texts as old as Taggart (1927) described the benefits of ‘sand/slimes’ splits into separate circuits. This simple concept has been largely overlooked in the push for circuit simplification and larger flotation cells. Surface analysis of fines lost to tailings almost invariably shows that they are there because they are not hydrophobic enough – either they have a hydrophilic surface coating, or there simply isn’t enough collector on the surface. (Grano et al, 2004). Adding extra collector will float the latter particles, but at the cost of floating other diluents – ie the operator can’t afford to float the fines because he will lose selectivity. Figure 3 shows a simple conceptual solution – tailor the flotation conditions to different size distributions to achieve high recovery across many size ranges. Fines performance when

Intermediate and coarse behaviour

treated by themselves

If you find this controversial, look again at Figure 2. Flotation at Mt Isa and MRM is done in five different types of cells, usually chosen because they were cheap (often because we already had them in the plant or in the scrap yard). We put a lot of science into achieving the results in Figure 2, but none of it was about flotation mechanisms, it was about achieving the correct level of liberation and the right surface chemistry. Once this is achieved flotation is easy in any device. Put simply, flotation impellors do not grind composites and they do not make particles hydrophobic. They just make bubbles. The choice between cells is purely one of what capital is needed to achieve the required residence time, and to reduce entrainment (either by dilution cleaning or by froth washing). In Mt Isa’s case, new Jameson cells would have been a great technical solution, using fine bubbles to quickly recover fines in a small space that makes froth washing easy. While this would have been a low capital solution, it could not compete with using the existing unfashionable old cells. Of course, smaller bubbles have much higher surface area, so fines that are already hydrophobic will be collected in a smaller volume. In an existing plant with fixed flotation capacity, this faster flotation rate converts into higher recovery in the fixed equipment. This is probably the source of the misunderstanding that fines need small bubbles. Big bubbles will still float hydrophobic fines, just more slowly so more cells are needed. So a device that creates smaller bubbles like the Jameson cell or other pneumatic devices will have a smaller footprint and lower capital to achieve the same recovery, and the ability to froth wash

906

a small volume can further reduce the amount of cells needed to reduce entrainment. Low footprint and low capital and froth washing are good, but they should not be confused with the task of making particles hydrophobic – the right reagents on clean surfaces do this.

y r e v o c e R

Fines peformance when treated with coarse particles

5

10

30

50

80

100

150

200

Size (microns)

FIG 3 - Conceptual staged grind-float circuit performance.

The Mount Isa circuit developed into an excellent balance of the needs of different minerals, relying on several stages of grinding and flotation. The design principals are: •

achieve the correct mineral liberation you need to make target grade and recovery;

•

apply the most efficient grinding method , in the place that needs least grinding power to achieve target liberation ;

•

make clean surfaces in grinding, and float as soon as possible before surfaces are oxidised again;

•

float minerals in narrow size distributions ;

•

minimise circulating loads – grind cleaner feed rather than cleaner tail, and open circuit where possible;



DESIGNING FLOTATION CIRCUITS FOR HIGH FINES RECOVERY

•

•

design launder and pumping systems for the more tenacious froths made by fine particles; and design thickening and filtering for the finer particles. These principles are explained below.

TABLE 1 Comparison of various grinding technologies independent laboratory data. Feature

IsaMill

Tower

Vertical

Mills

Pin Mills

Achieving the correct liberation, in the most efficient way

Grinding intensity (kW/L)

0.54

0.005

0.15 - 0.18

Residence time to 15 µm (min)

0.6

154

7-9

Selection of the right grind size is invariably determined by economics. If a high-grade concentrate is required, or if a contaminant (eg silica) must be eliminated from concentrate, then the grinding must achieve high liberation of the target mineral. For high value products where recovery is much more important than concentrate grade (eg platinum), liberation only needs to be enough to expose enough valuable mineral surface for flotation recovery (though more liberation to increase concentrate grade for smelting may still be economic). Since required grinding power increases exponentially as target size reduces, it is important to limit the tonnage sent to finer grinding stages. For Mt Isa and MRM the best technical solution would be to grind all the ore to 7 µm. But these are relatively low value orebodies, so the economic compromise is to only grind rougher feed fine enough to give most values a chance to report to rougher concentrate, then apply fine grinding to smaller tonnage streams that have a concentration of composites.

Power usage to 15 µm (k Wh/t)

17 .4

59 .6

37. 5 - 3 9.0

Media material

Various

Steel

Steel

Media size (mm)

0.8 - 1.6

9 - 12

6-8

Use the most efficient grinding method – high intensity stirred mills versus tower mills Fine grinding is capital intensive and energy intensive, so it is crucial to get good power efficiency at the full plant scale. As Figure 4 shows, this broadly means tumbling mills to around 40 µm, and stirred milling for finer sizes. Tower Mills are more efficient than ball mills for sizes below about 40 µm, mainly because they typically use smaller balls and operate at slower speeds that favour attrition grinding over impact breakage. However Tower Mills struggle to grind below about 25 µm, and are less efficient than the new high intensity stirred mills like IsaMill or detritors. These stirred mills are more efficient, and can grind to much finer sizes, eg below 7 µm, because they can operate with very fine media (eg 1 - 2 mm for the IsaMill), and with very high intensity, both of which greatly accelerate the attrition mechanism. Table 1 (Gao and Weller, 1993) shows the very high intensity in a stirred mill, assisting grinding and cleaning surfaces. 140

IsaMill -2 mm media 120

t / 100 h W k 80 y g r e n 60 E t e N 40

Ball Mill, 9 mm media

20 0

0

10

20

30

40

50

60

70

80

90

100

110

Grind Product P80 microns

FIG 4 - Grinding energy versus product size for a pyrite concentrate.

Classification is crucial High grinding efficiency requires good classification; and good classification also produces narrow size distributions ideal for flotation. Too often fine grinding circuits are constrained by poor classification, causing higher energy consumption, unnecessary


production of ultrafines, and poor control of top-size. To classify sharply below 20 µm needs small cyclones, theoretically two-inch diameter. However this is expensive, and in our experience they are virtually inoperable in a minerals plant. So most operators compromise and use bigger cyclones – but the flatter size distribution compromises grinding efficiency and flotation performance (and especially leaching performance if this is the downstream process). The IsaMill addresses this by classifying within the mill using the ‘product separator’ – effectively an internal centrifuge with a tip speed of 20 m/s, giving sharper separation than even two-inch cyclones. This allows the mill to produce a very sharp size distribution in open circuit configuration without cyclones.

Make clean surfaces in grinding and float them quickly With their high surface area fine particles are highly reactive, and are sensitive to oxidation and surface precipitates. Grinding with steel media can be very harmful. This has been well reported by Frew et al, 1994, who examined fine grinding with steel media at six sites. The grinding achieved significant liberation, but this benefit was largely lost due to the negative impact on surface chemistry. Sites like Helyer were successful in applying High Intensity Conditioning (Holder, 1994) to clean surfaces after Tower Milling, but at significant additional capital cost, and operating cost of power and maintenance. Greet and Steiner (2004) reported x-ray photon spectroscopy (XPS) work on grinding product with three different media. Forged steel balls produced 16.6 per cent Fe concentration on galena surfaces (present as oxy-hydroxide species), reducing to ten per cent for high chrome media, and to below the detection limit (less than 0.02 per cent) for inert ceramic media. Using the same flotation conditions, this resulted in galena recoveries of 48 per cent, 70 per cent, increasing to 92 per cent for inert media. Of course in practice higher recovery is achieved after steel milling by increasing collector addition – the problem is that this also increases the recovery of unwanted species (ie loss of selectivity), as well as the higher cost of reagents and slower flotation times. Fine grinding creates a lot of new surface – this is an opportunity to create clean surfaces ideal for flotation rather than altered surfaces. The motto is ‘make ‘em clean and float ‘em quick ’. The IsaMill makes clean surfaces very quickly; if water chemistry and reagents are controlled to keep the surface clean long enough to get collector coverage, then the resultant minerals can be very fast floating. An excellent example of is the lead circuit at Mt Isa, where a small (1.7 m by 3.2 m) Jameson cell at the discharge of the lead IsaMills produces 30 - 40 per cent of the concentrate at higher than concentrate grade target. It takes advantage of the high flotation rates of the freshly produced -10 µm particles and small bubble size (0.3 mm) produced in the Jameson cell, and the small surface area is easy for froth washing to maintain high grades. This was a low cost way to extend the capacity of the existing flotation cells.


907


FIG 5 - Mt Isa lead regrind IsaMills and the lead Jameson cell.

Float minerals in narrow size distributions and minimise circulating loads Figure 3 demonstrates that fines should not be mixed with coarse particles in flotation, yet this is what happens when cleaner tailings are reground and sent back to roughing. If the regrind is in a conventional mill then it often doesn’t effectively liberate fine composites anyway, it just worsens the surface chemistry before adding back to the fresh stream. This helps sustain the myth that fines don’t float. The answer is a staged grind and float approach as shown in Figure 6 for Mt Isa. Note that the rule not to recirculate tails does not mean that particles only get one chance. Consider the potential path of a sphalerite particle that has been floated in roughing in Figure 6. It has three chances to make concentrate before we ‘give up on it’:

70um Primary Grind / Float Prefloat

•

It can float directly from roughers to columns to concentrate in the ‘37 µm circuit’ – if it is liberated and fast floating it will take this route. If it doesn’t, then it is slower floating either because it is composite or because its surface is altered.

•

If so it is sent to the 12 µm regrind and cleaning circuit. If it doesn’t float here, then it is still composite or has developed new surface coatings.

•

If so, it is sent to the 7 µm regrind and cleaning circu it.

If after these three stages of grinding and flotation the particle still hasn’t been recovered we give up on it – we have run out of ideas (and grinding power). Though the particle has three chances to be recovered, we would never send it back to the head of the circuit – why would we put recalcitrant 7 µm particles back with fresh new feed? What else can the circuit do with it? (Note that we do circulate tailings within cleaner stages – closed circuit cleaning is a valid way to increase grade, the crucial point is that all the particles within cleaning are in a similar size range and liberation class; we do not send a fine cleaner tailings to join a coarse rougher feed.) Another outcome is that it is better to regrind cleaner feed rather than cleaner tailing. In fact, Figure 6 shows that you may do both – the cleaner tails of one stage may be reground and become the feed of another stage of cleaning. Conventional logic is to only regrind cleaner tailing: why regrind minerals that can already make a decent concentrate? The answer to this question is a better question: why wait until you have made most of your concentrate before applying your most powerful tool for improving flotation? Certainly fast floating liberated minerals should be recovered before grinding – eg a quick float in a cell or column or Jameson cell. For Mt Isa about 30 per cent of the sphalerite is recovered this way, but the remainder is sent to IsaMilling to improve liberation and clean particle surfaces, before being floated in the next cleaning circuit.

37um Secondary Grind / Float

Pb Ro

Pb Ro / Scav

Zn Ro

Zn Ro / Scav Tailings

Ball Milling

Rod & Ball Milling

37% Zn Rec

Zn Columns Tailings

Zn Conc

3 x 1.1MW IsaMills 34% Pb Rec Pb Conc

12um Regrind / Float

Jameson Cell Pb Cleaners 2 x 1.1MW IsaMills

46% Pb Rec Pb Conc

Zn Cleaners

1 x 0.52MW Tower Mill 39% Zn Rec Zn Conc

12um Regrind / Float

3 x1.1MW IsaMills

Zn Retreatment Ro

Tailings

Zn Retreatment Cl Zn Conc

7um Regrind / Float

6% Zn Rec

FIG 6 - Mount Isa Pb/Zn concentrator flow sheet.

908




Build simpler circuits with less flotation capacity, not more The above design principles make circuits look more complex on paper. In reality the circuit is simpler, since each flotation stage can be tailored to suit its feed, and is not disrupted by circulating loads. Theoretically fines float more slowly, but in reality the staged grind float does not need more flotation capacity – in our experience it may need less capacity, since: •

with fresh clean surfaces and tailored reagents, the fines float faster than they did when mixed with coarse particles; and

•

removing circulating loads can more than halve the volumes sent to roughing and cleaning.

Designing froth handling and dewatering for finer particles As every operator knows, a technology is only as good as the materials handling constraints. As discussed before, operating two-inch cyclones to close circuit fine grinding is technically sound, but an operating nightmare, so fine grinding technology can be seriously compromised by poor classification. The advantages of fine grinding can also be rendered useless by inadequate attention to froth launders, froth pumps, pumpboxes, thickeners and filters. Usually the design features needed are not complex or expensive, but they are crucial to get right. At MRM we thought we had paid adequate attention to froth handling and dewatering for 7 µm P80 concentrates. In fact we didn’t do enough, and this seriously reduced tonnage and recovery and plant hygiene for almost four years. The solutions are very simple in hindsight, but very easy to get wrong without experience. The solutions include: •

The overall circuit design – including eliminating circulating loads.

•

Setting reagent addition rates and reagent addition points correctly. This sounds simple when you know how, but is easy to get wrong. Starting with the wrong reagents often ‘locks in’ a high reagent position, where too much collector and too much depressant compete with each other, causing high loads and spillage. The clean surfaces produced in a IsaMill require a ‘clean sheet’ approach to reagents.

•

Adequate sprays on launders, and enough room in launders for de-aeration. This isn’t as hard as it sounds, since proper circuit design reduces the load on launders by eliminating circulating loads. Also pipework design must ensure the launders don’t ‘back-up’ with froth and overflow.

•

Pumpboxes with enough surface area for de-aeration, and adequate sprays.

•

Water to spray lines should be strained, and the sprays and the strainer must be easily accessed and cleaned on-line by operators.

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For true fine grinding applications, choose pumps to run at a slow speed, and with a wide open impellor (eg the ‘Woody’ pump applied at Mt Isa, available from Warman as their horizontal froth pump).

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Control of pipe entry into pumpboxes, and control of pumpbox level, to avoid air entrainment and the ‘milkshake’ effect.

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Adequate de-aeration of froths prior to pumping to the thickener, so air is not entrained in the flocs, causing froth on the thickener.

•

Adequate thickener residence time and bed height to compress the concentrate bed to achieve the desired thickener underflow density.


•

Pumping and pipework systems that don’t entrain air into the thickened concentrate, to avoid lowering the feed density to the filtering systems.

•

Filtering systems designed to handle the range of low feed densities that may occur. That is, capacity to handle increased cycle times, if the feed density is low.

CASE STUDY 1 – MOUNT ISA LEAD ZINC CONCENTRATOR The changes to the Mount Isa circuit as part of the ‘George Fisher Project’ are detailed elsewhere (Young and Gao, 2000, Young, Pease and Fisher, 2000). In summary, the project involved adding a further six IsaMills, to regrind lead rougher concentrate to P80 of 12 µm, most zinc rougher concentrate to 12 µm, and a zinc regrind to P 80 of 7 µm (Figure 6). Lead performance increased by five per cent concentrate grade and five per cent recovery (equivalent to ten per cent increase in lead recovery at the same grade). Zinc recovery increased by ten per cent, in two steps, and zinc concentrate grade by two per cent (equivalent to 16 per cent increase in zinc recovery at the same grade). The story of zinc metallurgy is told in Figures 7, 8 and 9. The project predicted five per cent higher zinc recovery (and no extra concentrate grade) due to extra liberation. Figure 7 shows this was achieved instantly. The surprise was the ‘second wave’ of a further five per cent zinc recovery increase and the two per cent increase in zinc concentrate grade. This was because fines flotation improved after grinding finer. It took about six months to discover how much better the fines could perform because we were so used to flotation after conventional grinding rather than after IsaMilling. Our three biggest mistakes were: •

expecting to need a lot more reagents after IsaMilling due to the huge new surface area created. Some reagent additions were forecast to triple;

•

not taking the depressant (lime to pH 11) off the zinc cleaners; and

•

‘pulling’ flotation harder because we thought flotation rates of the fines would be slower.

To our surprise, even though we introduced 6 MW of extra grinding power, operating cost per tonne of feed did not increase. This was because of: •

the lower reagent additions;

•

elimination of circulating loads between roughing and cleaning – a lot of power (and flotation capacity) is wasted in conventional circuits by pumping circulating loads of 100 300 per cent; and

•

virtual elimination of spillage – due to new designs for pumpboxes and pumps, the lower reagents, and especially the reduction in circulating loads.

The reduction in reagents deserves comment. We expected the additional surface area would need more reagents – so we increased reagents when we commissioned the mills; in fact we should have reduced it. Why? Certainly additional surface needs more collector coverage. But our mistake was to assume that more collector on the surface meant more collector added to the pulp. Our zinc circuit had never seen clean mineral surfaces before – they had always been masked by iron hydroxides from grinding, or gypsum precipitation from pulp, or simply oxidation after long flotation times in the lead circuit. High collector additions to pulp were needed to diffuse through surface layers to get adequate mineral surface coverage. But the IsaMills produced very clean mineral surfaces very quickly – with the right water chemistry and adding reagents in the right place, surface collector coverage was achieved with much lower than expected collector concentrations in the pulp.


909


82.5 IsaMills Commissioned

80.0

2nd Wave +5% Zinc Recovery

77.5 % c e R n Z

+ 2% Conc Grade (not shown)

75.0 1st Wave +5% Zinc Recovery

72.5 70.0 67.5

Baseline

65.0 62.5

Reduced grinding & flotation capacity, due to equipment relocation during construction. Apr

May 1999

Jun

Jul

Aug

Sep

Oct

Nov D ec 1999

Ja n

F eb 2000

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct Nov 2000

FIG 7 - Zinc recovery increase from IsaMilling. ole circuit recovery, roug ing an cleaning, inclu ing losses to lea concentrate

CASE STUDY 2 – SOUTH AFRICAN PLATINUM INDUSTRY – CONCENTRATION OF UG2 ORES

100 Apr-99 Oct-00

80

) % ( 60 y r e v o c 40 e R

Increased recovery in fine sizes

Moved particles from low recovery and low grade fraction to higher recovery and higher grade fines fraction

20 Net result - 10% recovery increase and 2% concentrate grade increase

0

1

10

100

Size (microns)

FIG 8 - Zn recovery versus size – before and after IsaMilling.

Another advantage of the lack of circulating loads is that cleaner densities remain low, thereby assisting dilution cleaning.

Size-by-size performance in the zinc circuit Figure 9 demonstrates the power of the staged grind-float circuit at Mt Isa. Fast floating liberated particles have a chance to be recovered in the column (37 µm) circuit – column recovery is not high, and columns aren’t ideal for the job – but they were in the circuit so they were free. The intent is simply to scalp out fast floating particles, then send the remaining particles to the 12 µm circuit. In this circuit flotation conditions are tailored to 12 µm particles, so not surprisingly the highest flotation recovery (90 per cent) is in this size fraction. Particles not recovered here are probably still composite, so they are sent to the 7 µm grinding and cleaning circuit. Flotation here is tailored to 7 µm particles (a simple task using the old existing Agitair cells – flotation is easy since there is only one size range and few composites to depress, and density is low because most of the solids have already been recovered). Not surprisingly, the highest recovery (above 95 per cent) in this circuit occurs in the sub 7 µm size fraction. Overall circuit recovery from rougher concentrate is determined by adding the products from all three cleaning stages, resulting the flat size-recovery curve. Above 95 per cent recovery from rougher concentrate is achieved in all size fractions from 1 µm to 25 µm – recovery drops in the coarser fractions since these particles are frequently composite – the circuit is designed to reject them and send them to an IsaMill to grind into the high-recovery fines fractions.

910

Platinum flotation operations also derive significant benefits from the combination of liberation and improved surface chemistry from high intensity inert grinding (Curry, 2002). These benefits have been extensively demonstrated on the UG2 orebody, part of the Bushveld Igneous Complex located to the north of Johannesburg in South Africa. UG2 ore is characterised by repetitive layers of dark, high-density minerals (pyroxene, chromite) and light, less dense feldspars. Typically, the highest grades of Platinum Group Metals (PGMs) occur at the lower contact zone of the layers, where a strong association with base metal sulfides occurs (specifically nickel, copper and iron). The bimodal distribution of mineral densities is a significant mineral processing challenge to produce high-grade PGM rich concentrates. The PGM grain size is ultra-fine (sub 10 µm) and associated with the silicate and sulfide species, rather than the large (200 µm), dense chromite minerals. Difficult middling streams have a median mineral grain size of 20 µm and a median PGM grain size of 3 µm. The PGMs are most commonly locked in silicates, or on the grain boundaries of silicates and base metal sulfides. As well as the difficulty associated with the fine grain size, many of the sulfides coated in talc slimes that impede flotation recovery. UG2 concentrators typically use staged grinding and flotation to first recover fast floating valuable particles as soon as possible. Primary and secondary circuits are most common (MF1 and MF2), however tertiary mill/float options (MF3) are more frequently being considered. Surprisingly, the use of regrinding within each MF circuit is not common. A key part of circuit design is to reject chromite particles, which are hard and dense. Various methods are used to avoid overgrinding of chromite and reject it while it is still coarse, eg gravity separation within grinding circuits, or open circuit milling. Because primary and secondary grind sizes are much coarser than the PGM grain size, cleaning of rougher concentrates leads to high circulating loads. Extra liberation is needed to increase concentrate grade. Traditionally this is achieved by depressing composites from cleaning then regrinding cleaner tailing. However low intensity regrinding with steel media further harms PGM flotation. The result is high circulating loads, slow flotation kinetics, poor fines flotation, and poor grade/recovery performance. High intensity inert grinding is an ideal solution to provide both the liberation and improved surface chemistry to achieve a step change in circuit design and performance. Typically UG2 applications target regrind product size of 15 to




Zn Recovery

Zn Recovery

50.0%

100% 50.0%

100%

Recovery

90%

37 µm zinc circuit

70% 60%

Size Distribution

35.0%

70%

35.0%

30.0%

60%

30.0%

25.0%

50%

10.0%

20%

25.0%

Size Distribution

15.0%

30%

45.0% 40.0%

20.0%

40%

12 µm zinc circuit

Recovery

80%

40.0%

80%

50%

90%

45.0%

40%

20.0%

30%

15.0%

20%

10.0%

10%

5.0%

5.0%

10%

0.0%

0%

C7

C6

0 -4 um

C5/C4

4 -8 um

8- 16 um

C3/C2

C1/38

53

1 6- 30 um 30-53um

75

0.0%

0% C7

Size fraction

C6

0- 4u m

C5/C4

4 -8u m

8 -1 6u m

C3/C2

16-30um

C1/38

53

75

30-53um

Size fraction 70 µm Primary Grind/Float P r ef l oa t

37 µm Secondary Grind/Float

Pb Ro

Pb Ro / Scav

Rod & Ball Milling

Zn Ro

Zn Recovery

Zn Ro /Scav

Tailings

Ball Milling

50%

100%

Zn Columns 37%Zn Rec, 54% Zn

90%

Zn Conc

33% Pb rec

3 x 1.1MW IsaMills

12 µm Regrind/Float

Jameson Cell Pb Conc

Pb Cleaners*

7 µm zinc circuit

Recovery

Tailings

80%

40%

70%

35%

60%

30%

50%

25% 20%

40%

Zn Cleaners*

46% Pb Rec

2 x 1.1MW ISaMills

Size Distribution

30% 1 x 0.52MW Tower Mill

12 µm Regrind/Float

Zn Conc

34% Zn rec, 50% Zn

3 x1.1MW IsaMills

15%

20%

10%

10%

5% 0%

0%

Zn Retreat Ro

C7

0-4um

* 3 stages of closed circuit conventional cleaning

C6

C5/C4

4-8um

C3-C1

8-16um

16-38um

38/53

75

38-75um

Size fraction

Tailings

Zn Retreatment Cl

45%

Zn Conc 6% Zn rec, 47% Zn

7 µm Regrind/Float

Combine 37 µm circuit with 12 µm circuit with 7 µm circuit for overall zinc cleaning performance graph below

Overall Zinc Circuit Recovery by Size 50%

100%

Recovery

90%

% n o i t c a r f e z i s n i y r e v o c e R c n i Z

45%

80%

40%

70%

35% 30%

60%

Size Distribution

50%

25%

40%

20%

30%

15%

20%

10%

10%

5%

0%

0.0% C7

C6

C5/C4

C3-C1

0-4um

4-8um

8-16um

16-38um

38/53

38-75um

75 Size fraction

FIG 9 - Zinc recovery in cleaning stages and for overall cleaning circuit.

18 µm. At this size about 90 per cent of the PGMs associated with base metal sulfides are liberated and can easily be recovered with xanthate collector – if the surfaces are clean. Two major platinum producers have applied these advances by installing IsaMills in their circuits with great success. Consistent with experience at Mt Isa and MRM, Platinum producers are increasingly realising that cleaner feed is the best target stream. Inert grinding of cleaner feed increases the flotation rate and performance by removing surface layers of talc, oxidation and iron hydroxides. By improving liberation and flotation kinetics immediately, particles in rougher concentrate have an


opportunity to report quickly to a high-grade final concentrate, rather than being forced to recirculate to get to a grinding mill and then being sent back to roughing. The practicality of achieving this has been greatly enhanced by the 2.6 MW M10 000 IsaMill installed by Anglo Platinum. All rougher concentrate is sent directly to a single IsaMill before cleaning. No cyclones are needed before or after the mill, since the internal product separator allows the mill to handle a wide variety of feed densities, and produces a very sharp product size distribution. Cleaner tailings can be open-circuited – a novelty in platinum flotation, but with immense benefits for the roughing circuit. Up


911


to 50 per cent of platinum roughing capacity is often consumed by circulating loads, and frequently the roughers are operated to suit the troublesome recirculated particles rather than the fresh feed. By applying the right grinding technology in the right place, circuits can be designed with considerably less flotation capacity in both roughing and cleaning. This is the opposite of the common view that finer grinding will require more flotation. A more conservative, but still successful, application is to regrind cleaner tailings, then to float mill product in a separate small cleaning circuit tailored for the narrow, fine size distribution produced by the IsaMill. Because liberation and kinetics are good, fine cleaner tailing can report to final tailing. This practice is used by Lonmin. Plant data is not made available for publication. However testwork on a wide range of Platinum ores and a wide range of operators shows that a single IsaMill regrinding stage increases overall PGM recovery by four to 15 per cent, at the same or higher concentrate grade.

CONCLUSIONS Contrary to common perception, fines flotation is quite simple and can achieve very high recoveries. It does not require special flotation cells or exotic reagents or long residence times. It requires firstly an understanding of the size-by-size liberation characteristics of the ore, then designing a staged grinding and flotation circuit to suit. Attention to surface chemistry, water chemistry, classification and materials handling is important. The ability to design cheap effective circuits for fines recovery has been enabled by the introduction of high intensity stirred mills using inert media. The clean mineral surfaces can transform flotation performance of fines compared with conventional grinding with steel media. The development of large scale units like the 2.6 MW IsaMill, efficiently producing liberated particles with clean surfaces in narrow size distributions without the need for cycloning has provided a powerful new tool for circuit design.

912

REFERENCES Curry, D, 2002. The Impact of IsaMill technology on modern concentrator design, paper presented to MMMA New Technologies Conference, Carltonville, RSA. Frew, J A, Davey, K J and Glen, R M, 1994. Effects of fine grinding on flotation performance: distinguishing size from other effects, in Proceedings Fifth Mill Operators’ Conference, pp 263-270 (The Australasian Institute of Mining and Metallurgy: Melbourne). Frew, J A, Smart, R and Manlapig, E V, 1994. Effects of fine grinding on flotation performance: generic statements, in Proceedings Fifth Mill Operators’ Conference, pp 245-250 (The Australasian Institute of Mining and Metallurgy: Melbourne). Gao, M and Weller, K R, 1993. Review of alternative technologies for fine grinding, AMIRA Project P336, Report P336/20, November. Grano, S, Weedon, D, Akroyd, T and Wiseman, D, 2004. Application of a property based flotation model in circuit, in Proceedings Metallurgical Plant Design and Operating Strategies, pp 299-317 (The Australasian Institute of Mining and Metallurgy: Melbourne). Greet, C J and Steinier, P, 2004. Grinding – The primary conditioner, in Proceedings Metallurgical Plant Design and Operating Strategies, pp 319-336 (The Australasian Institute of Mining and Metallurgy: Melbourne). Holder, R K, 1994. Improvements in copper and silver flotation at Hellyer using high energy conditioning, in Proceedings Fifth Mill Operators’ Conference, pp 153-159 (The Australasian Institute of Mining and Metallurgy: Melbourne). Taggart, A F, 1927. Handbook of Mineral Dressing, pp 12-92 – 12-97 (John Wiley and Sons: New York). Young, M F and Gao, M, 2000. Performance of the IsaMills in the George Fisher flowsheet, in Proceedings AusIMM Seventh Mill Operators’ Conference, pp 75-81 (The Australasian Institute of Mining and Metallurgy: Melbourne). Young, M F, Pease J D and Fisher, K S, 2000. The installation of the George Fisher flowsheet in the Mount Isa lead/zinc concentrator, in Proceedings Seventh Mill Operators’ Conference, 157-163 (The Australasian Institute of Mining and Metallurgy: Melbourne).



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