Liberation and its role in flotation-based flowsheet development

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HIHERiil, PROtESSlH6 ELSEV![ER

Int. J. Miner. Process. 51 (1997) 39-49

Liberation and its role in flotation-based flowsheet development (3. Morizot *, P. Conil, M.V. Durance, F. Gourram Badri BRGM, Research Division, Avenue Claude Guillemin, 45060, Orleans Cedex 2, France

Accepted 14 April 1997

Abstract

Particle liberation plays an important role in flotation processes as demonstrated by studies on detachment of particles during bubble coalescence, or by comparison of kinetic parameters for liberated[ or locked particles. On the other hand, the phenomena occurring in the froth layer in columns, or in mechanical cells working with a thick froth layer enhance selectivity. This has direct consequences on circuit design both in the general flowsheets, such as the relative position of grinding, or regrinding and flotation stages, and in the choice of flotation equipment. The scale of the operation also plays an important role. In floc flotation, as in Ca F 2 flotation, obtaining high-grade concentrates may require mechanical agitation to eliminate impurities interlocked and trapped within the f l o c s . © 1997 Elsevier Science B.V. Keywords: flotation; column flotation; mechanical cells; flowsheeting; flotation circuit conception; floatability of locked particles; flotation kinetics

1. Introduction

The size to which ore to be processed should be ground in order to optimize processing profitability while taking into account metallurgical balance and size-reduction cost is a most important parameter. The economic importance of the size-reduction in a commercial operation should be an incentive to minimize the quantity of product to be redutced at the final grinding mesh by introducing various grinding stages alternating with flotation stages. This will necessitate flotation of locked particles, which will have to be reground before obtaining the final concentrate. It becomes now evident that the flotation of locked particles is not independent of the flotation equipment. The more

*

Corresponding author. Tel.: + 33 0 238-643636; fax: + 33 0 238-643680; e-mail: [email protected]

0301-7516/97/$17.00 © 1997 Elsevier Science B.V. All fights reserved. PII S0301-75 16(97)00017-3

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G. Morizot et a l . / Int. J. Miner. Process. 51 (1997)39-49

important role of the froth layer in newly developed equipment such as flotation columns or modified mechanical cells has a direct impact on flowsheet development, not only by reducing (and in some cases even eliminating) cleaning stages, but also in the order of succession of regrinding and cleaning stages. BRGM has developed flowsheets without any cleaning stage at both pilot-plant and industrial scale, as well as methodologies and devices both at laboratory and pilot-plant scales, enabling more precise observation of the effects of liberation on flotation results. The present paper is the result of BRGM's experience in various laboratory-scale studies and in flowsheet development at pilot-plant and industrial scales, rather than of specific research.

2. Production of a final concentrate in a single flotation stage

Despite the difficulty of precisely defining mineral liberation by characterization of mineralogical texture, as recently summarized by King (1994), various authors, such as Morizot et al. (1988, 1991a,b); Frew and Davey (1993) and Nice and Brown (1995) have compared the flotation performance of a pilot-plant or a commercial operation with a theoretical grade-recovery curve based on liberation characteristics; this method is described by McIvor and Finch (1990). It is interesting to note that the authors of the five papers, in spite of very different liberation characteristics of the ores, in every case observed a substantial gap between actual flotation performances and theoretical curves: such observations could be an indication of the still significant inefficiency of industrial flotation processes and of the potential for improvement.

2.1. Chessy example Flotation treatment of the Chessy polymetallic ore (comprising chalcopyrite, galena, sphalerite, pyrite, barite and silicates) has been comprehensively studied at laboratory and pilot-plant scale, both in mechanical cells and in columns. Satisfactory liberation after grinding at d80 = 85 /,m is indicated by the recovery versus concentrate-grade curve obtained on the basis of liberation analysis (Fig. 1). After initial definition of a flowsheet based on mechanical cells, the quest for profitability improvement led to test the introduction of flotation columns, mainly to improve copper- and zinc-concentrate grade. Development of the flowsheet has been described in various papers by Morizot et al. (1988, 1991c). Pilot-plant results have not yet been confirmed at the industrial scale, as additional reserves are required before the operation can be launched commercially. The flowsheet based on mechanical cells was very classical for copper and zinc flotation, including roughing and scavenging stages for both circuits, with two cleaning stages; the column pilot-plant tests demonstrated the possibility of eliminating both cleaning stages for copper and for zinc, and producing a final concentrate in both cases by roughing flotation, with improved grades, compared to concentrates produced in mechanical cells. At pilot-plant scale, a scavenging stage in mechanical cells has been included for Cu flotation, which was not required for the zinc circuit. In both flowsheets (mechanical cell and columns) no regrinding stage was included in the circuit, the final

G. Morizot et al. l i n t . J. Miner. Process. 51 (1997) 39-49

41

Cu c o n c e n t r a t e 35 34 33

A

~=

Cu concentratetheoretical grade recovery relationship based on liberation

A

Mechanicalcells actual results

•

Columnflotation actual results

=

Zn concantrate theoretical grade recovery relationship based on liberation

A

Mechanical sells actual results

®

Column flotation actual results

32 31 30

O

o

29

A

28 27 80

85

90

95

100

Recovery (%) Zn c o n c e n t r a t e 65 64 63

.~"

62

~"

61

~

60

R

59 58 57 56 9O

92

94

96

98

100

Recovery(%)

Fig. 1. Ca and Zn concentrate grade versus recovery relative to Cu and Zn circuits feed based on liberation. Actual results with mechanical cells (pilot-plant operation).

grindinlg mesh of d80 = 80/.tm being attained before chalcopyrite flotation. Recovery of chalcopyrite or sphalerite versus particle size and particle liberation is shown in Fig. 2, and coJTesponding metallurgical results in Table 1. Economic comparison of the two flowsheets for a daily treatment capacity of 900 Mt demonstrates a saving of about US $ 3 / M t treated, which corresponds to a 54% increase in revenue, a 20% saving in investment, and a 26% reduction in operating costs (including concentrate transport). Ano'Iher zinc-bearing ore from an existing flotation plant, with grinding at d80 = 200 /xm, one roughing, one scavenging and three cleaning stages, was processed at a pilot-plant scale by a simple column flowsheet comprising a single column flotation stage ~ithout scavenging. For the same recovery (94%) as produced by the commercial operation a final concentrate was produced grading 2% higher in Zn than the commer-

42

G. Morizot et al. lint. J. Miner. Process. 51 (1997) 39-49 Free and locked chalcopyrite recovery 100

80 60 ~.,

8 4o 20

I

0 -20

I

+20-32

+32-4,0

I

+40-80

I

I

+80-100

+100

Particle size class (micron) .t

1O0 % chalcopydte

I

•

50-75 % chalcopydte

I

0-50 % chalcopyrite

Fig. 2. Recovery by size and liberation for chalcopyrite flotation in column. cial concentrate. The industrial circuit has not been modified to introduce a column, because the operating mine was short in reserves. 2.2. A n t i m o n y ore flotation example

B R G M compared a mechanical-cell circuit with a column circuit on an antimony ore at pilot and industrial scales. In a plant treating about 400 M t / d a y in a flotation circuit composed of two roughing banks (one after grinding and the other after regrinding), three cleaning banks and one scavenging bank, a single column, scaled up as the basis of pilot-plant experiments produced a final concentrate of much improved quality (a change of reagents also contributed to this improvement). The final grinding size is d90 = 90 /zm. The column produces two thirds of the final concentrate, the remaining being produced after scavenging and cleaning in mechanical cells.

Table 1 Comparison of mechanical-cell and column flotation results in Chessy ore Cu concentrate Cu grade% Zn grade% Cu recovery% Zn concentrate Zn grade% Zn recovery: versus Cu circuit feed versus Zn circuit feed

Mechanical cells

Columns

28.3 4 92

29 3.5 92

56

58

94.5 98

96 98.3

G. Morizot et aL l i n t . J. Miner. Process. 51 (1997) 39-49

43

o

% Cu

% Pb

% Zn

% Sb

Fig. 3. Comparison of industrial concentrate (three cleaning stages in conventionnal cells) with column concentrate (one cleaning stage in column).

Befo:re the column was installed, the plant was producing an antimony concentrate grading 56% Sb and 0.6% As; after column installation and the change of reagents, the grade of the global concentrate (column concentrate + mechanical cells concentrate) was 65.9% Sb with 0.1% As. The economic improvement in commercialization of the concent~rate due to increase in revenue is obviously considerable. 2.3. Complex sulphide ore example

During a research programme supported by the European Union, the possibility of obtaining a final concentrate in a single stage of column flotation after grinding to about d80 = 40 /zm was also been investigated on Boliden Apirsa ore in Spain. It was concluded that the liberated but not the locked particles can be floated by column. The high grinding cost for a complex ore fully liberated only after grinding to about 2 0 / z m makes obtaining a liberation size for roughing flotation uneconomical; as a consequence a roughing column can only float a low percentage of the valuable minerals and is generally not suitable for roughing flotation of complex sulphide ores. When the ore is properly liberated, as it is after the final regrinding stage, the column demonstrates its ability to produce very selective concentrates, as shown in Fig. 3. 2.4. CoJ~clusions on producing a final concentrate in column in one stage

The results obtained with the USBM sparger (in a 6-cm-diameter column for pilot-plant tests, and in a 2.3-m-diameter column for industrial scale operation) demonstrate that at both pilot-plant and industrial scales a final concentrate can be produced in a single stage if the ore is sufficiently liberated. In the single case mentioned above in which a pilot-plant experiment was followed by an industrial-scale operation, the recovery of concentrate produced in a single flotation stage was not as high as expected on the basis of pilot-plant results, probably due to convection flows in the industrial column~;, which do not function as piston reactors, but partially as perfectly agitated reactors. The :results obtained at both pilot-plant and industrial scales, with the new Microcel TM bubble :generator demonstrate better recovery increase (Cazorla et al., 1996; see Fig. 4).

G. Morizot et al. / Int. J. Miner. Process. 51 (1997)39-49

44

• [] & A • O • ©

100

80

V

.....

Microce[- LKAB P Cleaning Porous- LKAB P Cleaning

Microcel- Apirsa Zn Cleaning TurboAir- Apirsa Zn Cleaning

MicroceI-ApirsaZn Roughing Turbo Air- Apirsa Zn Roughing Microcel- Guemassa Cu Cleaning TurboAir- Guemassa Cu Cleaning Microcel - Zinkgruvan Pb Cl,/Scav. Porous- Zinkgruvan Pb Cl./Scav,

Maximum Grade-Recovery Curves

=~ 6O E', .

o

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4o

"; z~:

:

<>lx'.~

I

I

I

I

I ',

I

10

20

30

40

50 '*

60

70

Grade (%)

Fig. 4. Grade/recovery curves obtained from pilot-plant operations.

Possible decrease in recovery from pilot-plant scale to industrial scale could therefore be eliminated by using the Microcel sparger. However, when the products to be floated are flocs and not individual particles, as for example in the case of barite or chromite flotation, BRGM authors (1991, 1996) have observed in various cases that flotation in column (with USBM sparger) is not capable of eliminating impurities as efficiently as flotation in mechanical cells: the explanation is that due to the strength of the flocs the elimination of impurities trapped within them is not possible, either by froth washing or by mechanical agitation (which does not exist in columns): after two column flotation stages (one roughing and one cleaning) it has been possible to produce a barite concentrate grading 98% BaSO 4, 0.2% Fe203, less than 0.5% SiO 2 and 2000 ppm F, with a recovery of 94%. In mechanical cells, the same type of concentrate was obtained with a recovery of 87.5%, after seven flotation stages, but with a F grade of 500 ppm. Additional column flotation stages cannot lower the F grade. Perhaps another type of sparger in which particles are submitted to sharing forces (such as Microcel) might induce re-arrangement of such flocs.

3. Flotation modelling for free and locked particles Steady state simulation of unit operations, as embedded in the USIM PAC 2 simulator could prove to be an effective aid for design and optimisation of mineral processing plants, as explained by Villeneuve et al. (1994). Among the various models available for simulating flotation processes, predictive models could take into consideration not only the size of flotation equipment, but also the parameters describing flotation

G. Morizot et al. l i n t . J. Miner. Process. 51 (1997)39-49

45

Rougher and Scavenger Zinc Circuit Size class (- 23 IJm, + 11 pm) 100

80

o • 60 oo (1) n,"

20

y

ET:.o. Cell

Fig. 5. Kinetic curves of the zinc circuit.

kinetics of each mineral species; parameters such as size distribution and particle liberation (size by size if necessary) could be included in the model; the curves presented in Fig. 5 show the importance of the difference in kinetic behaviour for locked and for free particles in mechanical cell flotation of a complex sulphide ore (chalcopyrite flotation in a zinc circuit). Table 2 shows the results obtained by Vedrine et al. (1990) on flotation kinetics of free and locked particles (chalcopyrite, sphalerite, pyrite) in the rc,ughing and cleaning banks of a flotation circuit, using the same ore as shown in Fig. 5. For the cleaning bank, flotation kinetics were determined with and without a regrinding stage. The importance of two parameters can be seen: (1) liberation of particles; (2) regrinding (free particles of hydrophobic species such as chalcopyrite and sphalerite have very different behaviour with and without regrinding for the same size distribution). Table 2 Mixture and regrinding effects on kinetic constants for flotation of a complex ore in mechanical cells Size (/xm)

Sphalerite

~ee locked

Pyrite

free locked

Chalcopyrite

free locked

Kinetic constants in mi n- I roughing

1st. cleaning

cleaning after regrinding

-44 -23 -44 -23

+23 + 11 +23 + 11

0.258 0.618 0.226 0.232

0.700 0.622 0.237 0.242

1.480 1.890 0.324 0.452

-44 - 23 -44 -23

+23 + 11 +23 +11

0.032 0.037 0.121 0.205

0.033 0.061 0.077 0.107

0.041 0.050 0.539 0.283

-44 -23 - 44 -23

+23 + 11 + 23 +11

0.196 0.204 0.084 0.076

0.141 0.192 0.063 0.067

0.019 0.500 0.263 0.105

G. Morizot et al. l i n t . J. Miner. Process. 51 (1997)39-49

46

Table 3 Nature o f bubble load without and after coalescence Nature o f particles collected by the bubble

Without coalescence (%)

After coalescence (%)

Percentage of increase or decrease after coalescence (%)

79.2 17.2

80.6 13.5

+ 2 -20

6.1 2.5 8.6

3.8 1.0 4.8

- 35 - 60 - 45

Free particles CuFeS 2 ZnS

Locked particles ZnS-CuFeS 2 FeS 2 - C u F e S 2

Total

Villeneuve et al. (1996) presented the same modelling method for column flotation, but without taking particle liberation into consideration. 4. Development of new tools to observe selectivity in the froth layer Two types of new tools have been developed at BRGM, at laboratory or pilot-plant scale in order to enable better observation of the effect of the froth layer on selectivity: (1) equipment enabling observation of the effect of bubble coalescence under conditions similar to those prevailing during industrial flotation; (2) a tool for sampling loaded froth within the froth layer, at different froth heights. 4.1. Bubble coalescence studies

Gourram-Badri et al. (1997) described a device enabling laboratory-scale observation of the detachment of particles due to coalescence of nearly fully loaded bubbles: the

Teflon seal

--V-3

Froth zone

it .2,

~ .-~.. ~

.3

~. ~

5

t n-1

~

T

4

n

Operation

terfac_ee

6 .7,

~.-~.

.8

,=~-~

S

Segment volume = 301.9 ml

Pulp zone Slice of solution = 9.06 ml

Pulp ,=,,, Operation After sampling

After sampling

Fig. 6. Froth s a m p l i n g device.

G. Morizot et a l . / Int. J. Miner. Process. 51 (1997) 39-49

47

Interface

30

- 20

-18 -16 -14 S= o

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-10~ -8

l

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6 Cu grade (%)

=

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=

Froth load (g/I) 4

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16

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26

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°

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46

Distance to the top (era) Fig. 7. Type of results obtained with the sampling system.

bubbles were loaded with a mixture of free (ZnS and CuFeS 2) and locked (ZnS-CuFeS 2, FeS2-CuFeS 2) particles and were collected either in the absence of or after coalescence. Particles loaded on the bubbles in the absence of coalescence or after coalescence were collected separately, and their nature defined. Selectivity of coalescence is clearly shown in Table', 3, which gives the percentages of free and locked particles for each type of particle.

4.2. Sampling equipment Labuche et al. (1996) described a pilot-plant scale column sampling device capable of cutting various slices of the froth (or pulp) layer and of collecting all their contents (water, ..solid and air). Such equipment is described in Fig. 6 and a typical example of samplin;g within the froth layer is shown in Fig. 7. The device constitutes a very powerful tool for observation of particle behaviour within the froth layer: it enables modelling the effect of particle mixture on flotation selectivity within the froth layer.

5. Conclusions (1) It is possible to produce a final concentrate in a single flotation step within a high froth layer such as occurs in flotation columns (or in mechanical cells using the froth layer principle of the columns) if the ore is fully liberated. (2) The flotation columns, due to their high selectivity, are less suitable than mechanical cells for flotation of locked particles. (3) Choice of columns or mechanical cells in a circuit will consequently be directly related to particle liberation: in small-scale operations where for reasons of investment optimization the number of grinding stages are limited, a column could produce a final

48

G. Morizot et al. l i n t . J. Miner. Process. 51 (1997)39-49

concentrate directly after the grinding. In large-scale operations, however, the tendency will be to have several regrinding stages with intermediary flotation steps, and to limit use of columns to the flotation step following the final grinding stage. (4) Regrinding of intermediate products introduces a significant increase in particle floatability, independently of liberation of the particles. (5) When floating strong flocs, the advantage of using columns could disappear if impurities are trapped within the flocs. (6) To optimise use of columns or mechanical cells in a flotation circuit, flotationcolumn modelling must take liberation of particles into account. Tools have been developed which could give the necessary data to integrate such parameters within the models. The effect of regrinding should also constitute one of the parameters in the modelling of flotation.

Acknowledgements Work on this paper (BRGM scientific publication No. 96038) was financially supported by a BRGM research project and by a European Brite Euram (BRE 2 CT 92 0301) project. Many people have contributed over a number of years to the development of flotation circuits at BRGM. The authors would specifically like to acknowledge the contribution of Maurice Save, Jacques Villeneuve and Philippe Verdier.

References Cazorla, A., Jimenez, J.-M., Monredon, T., 1996. Sparger in Spain. World Mining Equipment, May, pp. 41-47. Frew, J.A., Davey, K.J., 1993. Effect of liberation on the flotation performance of a complex ore. Proc. XVIII IMPC, Sydney, 23-28 May 1993, pp. 905-911. Gourram-Badri, F., Conil, P., Morizot, G., 1997. Measurements of selectivity due to coalescence between two mineralized bubbles and characterization of MIBC action on froth flotation. Int. J. Miner. Process. 51, 197-208. King, R.P., 1994. Mineral liberation. Notes for Icra Workshop. Stockholm, Sweden. Labuche, Y., Durance, M.V., Houot, R., Conil, P., Save, M., Morizot, G. 1996. Development of a new equipment for froth characterisation in a flotation column: sampling system and on line sensor. Proc. Int. Symp. Column Flotation, Column 96, CIM Annu. Meet., August, Montreal, pp. 51-62. Mclvor, R.E., Finch, J.A., 1990. A guide to interfacing of plant grinding and flotation operations. Miner. Eng. 4 (1), 9-23. Morizot, G., Crois6, G., Houot, R., 1988. Polymetallic ore from Chessy: is it sometimes possible to obtain good metallurgical results by flotation of complex sulphie ore? Proc. XVIth IMPC, Stockholm, pp. 293-315. Morizot, G., Save, M., Conil, P., McKey, J., 1991a. Shrinkage of roughing and cleaning stage with column flotation. The Chessy case. Proc. Int. Symp. Column Flotation, Column 91, CIM Annu. Meet., Sudbury, Canada, pp. 75-76. Morizot, G., Save, M., Conil, P., Mangeot, M., 1991b. Column versus mechanical flotation: application to the Chessy (France) polymetallic project, Ottawa, CIM, Canada. Copper' 91, Cobre' 91, Int. Symp., Ottawa, pp. 277-290. Morizot, G., Watsford, R.M.S., Gallet, M., 1991c. Optimisation of the milling design of the Chessy project, metallurgy. Proc. Mining Industry Optimization Conference, June, Sydney, pp. 65-70.

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Nice, R.W., Brown, P.J., 1995. The design of a base metals separation process. Proc. XIX IMPC, San Francisco, Calif., Vol 3, pp. 137-143. Vedrine, H., Broussaud, A., Conil, P., De Matos, C.F., 1990. Mod61isation de la cin&ique de flotation d'un minerai sulfur6 polym&allique. Soci&6 de l'Industrie Min6rale, Les Technique, mars-avril, 79-87, 72, pp. 2-3. Villeneuv,~, J., Guillaneau, J.C., Durance, M.V., 1994. Flotation modeling: a wide range of solutions for solving industrial problems. Miner. Eng. 8 (4/5), 409-420. Villeneuv,e, J., Da Silva, R.V.G., Birro, E., Martins, M.A.S., Durance, M.V., Guillaneau, J.C., 1996. Advanced use of column flotation models for process optimization. Proc. Int. Symp. Column Flotation, Column 96, CIM Annu. Meet., August, Montreal, pp. 51-62.