ELSEVIBR Int. J. Miner. Process. 48 (1996) 183-196

Modified column flotation of mineral particles

Jorge Rubio ’

Depto. Engenharia de Minas-PPGEM, Uniuersidade Federal do Rio Grande do Sul, Au. 0. Aranha 99/513, Port0 Alegre, RS, 90035-I 70, Brazil

Abstract

This work summarizes flotation results obtained in a modified column which selectively separates drained particles from the froth zone and uses a secondary wash water system between the feed and the froth zone. Flotation results on gold, copper, lead-zinc and fluorite ores are reported. The combination of separating the froth drop-back material as a “third-product” and secondary washing improved, the concentrate grades when compared to the conventional column cell. When the modified column was used for “rougher flash” flotation or as a cleaner of copper ores; clean copper concentrates analyzing 33-40% copper were obtained (33% recovery). Flotation recovery of gold from tailings was as much as 15%, with concentrate grades higher than 160 g/t. As a cleaning stage in lead-zinc ore flotation, recoveries of both sulfides were of the order of 92-94% with grades up to 80-82%, as compared to 70% in the “conventional” column. With the fluorite ore, recoveries of the order of 94%, were achieved with high selectivity (about 96% CaF,) at high flotation rates. The performance of the modified column is better than the conventional column due to improved mass transfer conditions. Finally, data on the influence of some cell design parameters are reported and the potential practical applications of this type of cell are discussed.

Keywords: flotation; mineral particles; modified column; ore processing

1. Introduction

Column flotation is today a subject of great interest in mineral processing with steadily growing research and industrial application (Finch and Dobby, 1990; Dobby and Finch, 1991; Agar, 1991). Major advantages of columns include low capital and operating costs, better adaptability to automatic control and improved metallurgical performance. The latter is well recognized in cleaner circuits, where there is a small

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184 J. Ruhio/Int. J. Miner. Process. 48 (19961 183-196

improvement in the flotation of the very fine particles usually present (Ahmed and Jameson, 1985; Dobby and Finch, 1986; Espinoza-Gomez et al., 1988; Yoon et al., 1989).

Further, concentrate grades are higher in flotation columns compared to conventional cells, mainly because of the froth washing action which rejects many of the entrained particles (Yianatos et al., 1986, 1988; Falutsu and Dobby, 1989a,b; Finch and Dobby, 1990).

A flotation column is a reactor composed mainly of a collection zone and a froth zone. The collection zone has the objective of attaching hydrophobic particles to bubbles and the froth zone is responsible for the carrying capacity and froth enrichment (Finch and Dobby, 1990; Agar, 1991). Yet, some drained material from the froth returns to the collection zone, recirculate and remains at the pulp-froth interface exiting the column randomly.

The aim of this paper is to show results found with a modified flotation column wherein the froth zone has been “separated” from the collection zone. This artifact was firstly utilized by Falutsu and Dobby (1989a,b), to measure froth flotation recoveries and rates on a laboratory scale. Also, Rubinstein and Gerasimenko (1993), Rubinstein (1994) and Rubinstein and Badenicov (1995) have described a similar column design but no specific results have been given. Here, this modified column (three-product column or 3PC) has been modified by the author to improve mass transfer rates and collection capacity. Other studies include the characterization of the amount and quality of drop-back material for various column designs. Lastly, potential applications of the use of the modified column are discussed.

2. Experimental

2. I. Materials

2.1.1. Fluorite ore Samples of fluorite ore were obtained from Santa Catarina, Southern Brazil and the

flotation feed had particle size and fluorite distributions as shown in Table 1. Main gangue components were quartz, 34%; feldspar, 47.11% and other minor silicates. The

Table 1 Particle size distribution of the fluorite feed sample

Size (km)

-150+105 - 105+90 -90+53 -53+38 -38

Fluorite grade

in % (w/w)

29.04 16.33 15.98 15.51 23.52

in % CaF,

47.43 49.00 47.95 48.70 46.47

J. Rubio/Int. J. Miner. Process. 48 (1996) 183-196 185

reagents used were “Tall Oil” as collector (600 g/t), sodium silicate as gangue depressant (350 g/t), and sodium carbonate as a pH regulator (about 10.5).

2.1.2. Gold ores Sample 1: A representative sample of a reagentized rougher feed from a local gold

mill located near Copiapo-Chile, was used for the laboratory studies. The mineralogi- cal analysis showed that this gold ore consisted mainly of quartz, limonite, hematite, pyrite and chalcopyrite. Gold liberation in the minus 74 p,m fraction was about 82% and chemical analysis is shown in Table 2.

Sample 2: This gold sample corresponded to the gold flotation tailings of the roughercleaner gold plant mentioned above. Flotation of gold from this sample was carried out in a pilot scale column.

Sample 3: This was a low grade copper-gold flotation tailings located also near Copiap6’ - Chile. The chemical analysis yielded 0.40 g/t Au and 0.25% Cu.

2.1.3. Copper ore Copper sulfide ores used were from mills belonging to Codelco - Chile. Sample 1: This sample corresponded to a reagentized rougher feed, was homogenized

and quartered following conventional procedures. The sample was 78% minus 210 Frn and the copper content was between 1.1 and 1.4%. The gangue was mainly quartz, silicates, carbonates, fluorides.

Flotation test conditions were as follows: 35% solids content with lime addition to adjust pulp pH to 11; SF-l 13 (sodium isopropyl xanthate, 30-50 g/t) and a mixture of frothers (DF-250, MIBC and pine oil), 20 g/t.

Sample 2: This sample was a rougher concentrate collected before and after regrind- ing. The one after regrinding was 67.14% minus 53 Km, with a copper grade of 4.76% and 58.48% “insolubles” (gangue material insoluble in acid). The one before regrinding was 57.75% minus 53 km, with a copper grade of 4.57% and 59.95% insolubles.

Samples were homogenized in a 150 liters conditioner which fed the column at 20.8 + 0.45% solids content for the ground sample and 31.7 + 0.51% for the unground one. Reagents used and flotation pH were the same as for sample 1.

2.1.4. Lead-zinc sulfide ore The gore was from Companhia Brasileira de Cobre, Southern Brazil, and samples used

were bulk rougher concentrates usually analyzing about 49% sulfides. Amy1 xanthate was used as a bulk collector at 40 g/t; CuSO, as an activator of ZnS at 40 g/t and pine oil as a frother at 80 g/t.

Table 2 Chemical analysis of the gold ore sample

Au (g/t) Ag k/t) cu (%) Fe (%) SO, (o/o) 3.70 4.50 1.36 14.15 59.24

186 J. Rubio/ht. J. Miner. Process. 48 (19961 183-196

2.2. Methods

2.2.1. Columns Flotation columns used are shown in Figs. 1 and 2. The “conventional” laboratory

cell (Fig. 1) usually had a 2.54 cm diameter and a height of at least 229 cm. Froth zone height was usually 50 cm (varying between 30 and 75 cm); the collection zone height varied between 50 and 100 cm; and the froth zone height varied between 20 and 75 cm.

The modified column was usually of the same height unless otherwise indicated. The intermediate zone varied between 30 and 90 cm. Details of the operation of these column cells can be found elsewhere (Santander et al., 1994; Valderrama et al., 1995; Cabral, 1995). Gas hold-up was determined by the method reported by Finch and Dobby (1990) and Dobby and Finch (1986).

In the modified, 3PC column cell the zones are as follows: (1) The collection zone, between the air sparger and the feed point (see tube 3 in Fig.

2). (2) The secondary cleaning or intermediate region, between the feed and the water

inlet II (4 in Fig. 2). (3) The inflection zone located between the water inlet II and the upper part of tube 2

(3 in Fig. 2). (4) The froth drop-back collector or third product, located at the end of tube 1 (2 in

Fig. 2). (5) The cleaning or froth zone at the top of the column (1 in Fig. 2).

Fig. 1. Conventional column flotation.

J. Rubio/ ht. J. Miner. Process. 48 (1996) 183-196 187

,- Wash Water 1

Concentrate - 1

TUBE 1

2 3 Wash Water 2 -

4

Froth drop-back L (third product)

J- Feed

3 - Froth drop-back zone 4 - intermediate zone 5 - Collection zone

t Air

Fig. 2. Modified column flotation, 3PC cell.

2.3. Flotation of fluorite

2.3.1. Conventional column Rate constants were determined according to the method proposed by Contini et al.

(1988). Thus, pulps with 5 and 13% by weight and collector concentrations of 600 and 1000 g/t were respectively used. Other selected experimental data were as follows:

- air surface rate, v(a) = 0.66 * 0.15 cm/s, (0.2 * 0.02 l/min>; _ tailings removal rate, u(t) = 0.99 f 0.02 cm/s, (0.3 * 0.02 l/min>; - feed rate, u(f) = 0.82 + 0.05 cm/s, (0.25 5 0.02 l/min>; and - bias surface rate, u(b) = 0.16 + 0.03 cm/s, (0.05 + 0.01 l/min>. After 20 min of flotation (equilibration period), samples were taken within 5 min

followed by 10 min intervals between each subsequent aliquot. Triplicate samples were then weighed, filtered, dried at 60°C sized by wet screening and the material of each size fraction was analyzed for CaF$.

2.3.2. klodi$ed column Concentrate, tailings, and drop-back products were collected simultaneously and

samples were treated and analyzed as described above. The operating conditions were as follows: feed flow, 225-230 ml/min; gas flow,

188 .I. Rubio/Int. .I. Miner. Process. 48 (19%) 183-196

200 ml/mm; wash water II, 100 ml/min; bias surface rate, 50-60 ml/mm; drop-back rate 100 ml/mm.

2.4. Copper ore

The 3PC cell used had a collection zone height of 100 cm, an intermediate zone of 90 cm and a froth zone height which varied between 20 and 75 cm. Operating parameters were feed flowrate, 1.02 cm/s; air flowrate, 0.941 cm/s; wash water I rate, 0.325 cm/s; wash water II rate, 0.207 cm/s and drop-back rate, 0.328 cm/s.

2.5. Gold ore

Flotation was conducted at laboratory (rougher sample) and pilot scale for the tailings sample using an “in situ” column, 6 m tall and 5.08 cm diameter. Other operating parameters were similar to those employed with the copper ore.

2.6. Lead-zinc ore

Columns used were similar to those used in the flotation of fluorite, copper or gold; and for the 3PC column, several different design parameters were studied (Cabral, 1995).

3. Results and discussion

3.1. Fluorite ore

Tables 3-5 present comparative results of fluorite flotation obtained with the conventional and the 3PC modified column of various designs.

Results obtained show certain advantages of the 3PC cell compared with the conventional column. For example, the separation of fluorite was obtained at higher flotation rates with cleaner concentrates. Also, gas hold-up values were higher in this column because of the lower solids content (Rodriguez-Lopez, 1991; Rubio and Rodriguez-Lopez, 1992).

Results presented by size fraction show that the rate constants are higher for the modified column, the values decreasing for the finest fraction. According to Ahmed and

Table 3 Comparative results of column flotation of fluorite fines. Data are the averages of at least 3 separate experiments. Results expressed in terms of CaF,

Recovery, % Grade, %

Conventional column, gas hold-up = 10.99 94.9 92.9 3PC cell, gas hold-up = 14.76 93.9 96.0

J. Rubio/Int. J. Miner. Process. 48 (1996) 183-196 189

Table 4 Column flotation of fluorite: comparative rate constants between columns as a function of particle size

Size, )*rn k, min-’

Conventional column 3PC cell

- 150+9c1 1.18 2.12 -90+53 0.95 1.57 -53f44 0.84 1.48 -44+38 1.03 1.42 -38+25 0.82 1.14 -25+1 0.67 0.3 1

Jameson (1985), the top particle size limit is higher in columns because does not affect the stability of rising bubble-particle aggregates units. Therefore, the size effect can be explained as an increase in the collision efficiency and adhesion for increasingly coarser particles up to a point where the weight factor prevents levitation.

Higher values of rate constants obtained in the modified column flotation improve fluorite recovery without much affecting the concentrate selectivity (grades of the order of 96%C!aF, in one stage). Fluorite losses in tailings are very low and occur basically in the ultra-fines where drainage is particularly efficient in columns (Rubio and Rodriguez- Lopez, 1992; Valderrama et al., 1995)

The third product was composed of middlings, of drained CaF* fines and of entrained fine gangue (Fig. 3).

3.2. Copper ore

3.2. I. Rougher flash jlotation Flotation of copper sulfides in the 3PC showed (Table 6) that it is possible to obtain

very clean concentrates (about 40% copper and 2% “insolubles”) with a recovery of 32.5%. The enrichment ratio was of the order of 33 and the drained material analyzed 7% copper and 51% insolubles with recovery of approximately 12.3%.

High enrichment ratios obtained characterize the 3PC as a very selective column, collecting mainly those particles having high collection probabilities. Low mass recover-

Table 5 Modified 3PC cell flotation of fluorite: Influence of design parameters on separation performance and gas hold-up

Test Sk, cm recovery, % CaF*, YC gas hold-up - Collection Zone Intermediate Zone Froth Zone

1 154 90 35 88.50 93.95 13.20 2 100 30 70 92.38 92.53 13.95 3 100 90 50 90.87 96.30 14.21 4 50 90 50 93.93 96.04 14.76

190 J. Rho/ ht. J. Miner. Procrss. 48 (19%) 183-196

c

I Concentrate

Third product (drop-back)

7 O-25 25-38 38-44 44-53 53-88 88-l 49

Particle size range, pm

Fig. 3. Column flotation of fluorite fines in the three products, 3PC cell.

ies are due to the fact that the feed material was too coarse (25% more than 150 pm), where some particle sedimentation was observed (Santander et al., 1994).

These results suggest the use of the 3PC as a “flash flotation” unit recovering about 32% of the copper content, thus producing marketable concentrates. The third product material is composed of coarse and middlings particles and because of the grade, could be reground and join the flotation circuit for rougher concentrates.

Table 7 shows flotation results of copper sulfides using the 3PC cell before and after regrinding of the concentrates.

These results show that independently of grinding, concentrate grades were greater than 33% copper with 2% insolubles, with enrichment ratios near 7. Moreover, with the unground material; drained material yielded final copper grades of the order of 28-29% copper at 41-44% recovery (Santander et al., 1994).

These values show on the one hand that no matter what the feed is, concentrates in

Table 6 Average flotation values obtained for the copper sulfides in the 3PC cell

Product

Concentrate Drained material Tailings Feed

Grade, % Copper recovery, % Enrichment ratio (Cu)

cu Insolubles

40.42 2.02 32.55 33.40 1.31 51.10 12.30 6.09 0.44 67.60 55.15 1.41 65.50

J. Rubio/Int. .I. Miner. Process. 48 (1996) 183-196 191

Table I Flotation of copper rougher concentrates in the 3PC cell. Tests 1 and 2, ground concentrate (2 rounds); Tests 3 and 4 without grinding. C = concentrate, D = drained material, T = tailings, F = feed

Tests 1 2 3 4

Copper grade, % C D T F

Insolubles, % C D T F

33.40 34.70 34.50 33.60 21.20 19.40 27.80 29.50

0.85 0.64 0.97 1.43 4.16 4.16 4.60 4.54

3.29 2.02 2.04 2.00 22.60 23.30 10.60 7.36 66.60 66.60 66.80 69.10 58.48 58.48 62.40 57.50

Copper recoueiy, % C 38.53 31.21 36.05 29.37 D 51.95 48.68 41.37 44.35

Enrichment ratios C D

7.01 7.28 7.50 7.40 4.45 4.07 6.04 6.49

the 3PC column will always produce rich concentrates and depending on the mineral system. composite concentrates; i.e. concentrate plus drained material will also yield high grades. In the case of unground sample, the composite had a final copper grade > 31% Cu at 77% copper recovery. Thus, floating rougher concentrates in this column would decrease cell volume requirement in the circuit, and would reduce considerably the amount of mass to grinding.

Results not reported here, revealed that the conventional column operating in similar conditions did not produce such an enrichment, but a higher copper recovery.

3.3. Gold ore

Rougher sample (sample 1). Table 8 shows comparative results of gold column flotation at lab scale, varying the feed flowrate.

Table 8 Separati’on parameters obtained in conventional and in the 3PC cell as a function of the feed flowrate

Feed flowrate, cm/s Conventional column 3PC cell

Au grade, g/t Au recovery, % Au grade, g/t Au recovery, %

1.1 72.8 50.8 117.6 44.7 1.9 88.0 63.8 168.0 52.5

192 J. Ruhin/Int. .I. Miner. Process. 48 (lY%/ 183-196

Table 9 Flotation of gold in tailings (sample 2). Effect of air flowrate on separation parameters in the 3PC cell at pilot scale

Air tlowrate, cm/a Au grade, g/t

1.6 160.4 1.7 136.2 2.2 120.7 2.8 91.2

Au recovery, %

15.5 14.4 13.5 9.9

Tailings sample (sample 2). Table 9 show flotation results of gold present in tailings at pilot scale with the 3PC column.

Results showed a decrease in the flotation efficiency when increasing the air flowrate. Thus, for flowrates higher than 1.6 cm/s, a higher degree of turbulence was observed in the collection cell that led to a rupture of the floating bubble-particle aggregates.

These results show also the advantages of using the 3PC modified column as an enriching cell even in the treatment of tailings. Enrichment ratios of the order of 50 can be obtained. Moreover, gold grades produced in this column are higher than those obtained in the actual plant, where the tailing samples were taken. This plant uses conventional flotation (rougher-cleaner stages), and gold concentrate grades are usually in the range of 70-90 g/t gold (Valderrama et al., 1995).

The effect of flowrate is similar to that obtained before and once again, the 3PC concentrates result richer than those obtained in the conventional cell. As shown in Fig. 4, with the 3PC cell is possible to recover a small fraction of the gold contained in this low grade sample but this is not possible in the conventional cell.

14 /

12

3PC IO

c

8

.; 6

1

4

2

t 3

isA

cc

0 / ! I I

0 4 8 12 16 20 24 28

Gold recovery, %

Fig. 4. Column flotation of gold-bearing tailings. Grade recovery curves. CC = Conventional cell.

J. Rubio / ht. J. Miner. Process. 48 (1996) 183-196 193

Table 10 Flotation of low grade copper-gold tailings (sample 3). Separation parameters obtained in conventional and in the 3PC cell as a function of the air flowrate. Feed flowrate: 0.95 cm/s; wash water I: 0.29 cm/s; wash water II, 0.20 cm/s

Air flowral:e, cm/s Conventional column 3PC cell

Au grade, g/t Au recovery, % Au grade, g/t Au recovery, %

0.91 3.2 23.2 6.8 14.9 0.99 3.4 25.4 3.9 19.3 1.23 1.6 3.5 9.2 12.4 1.56 1.9 12.2 13.0 11.6

70

65

60

t 3PC

93 95 97 99

PblZn recovery, %

Fig. 5. Bulk flotation of lead-zinc ore in conventional (CC) and modified 3PC column: Grade-recovery curves.

3.4. Lead-zinc ore

Tabk 10 shows comparative results between the two columns cleaning lead-zinc rougher concentrates at pilot scale.

Table 10 and Fig. 5 show that high flotation recoveries of Pb/Zn sulfides can be obtained with the two columns. However, the 3PC yields richer concentrates, about 10% higher, at slightly lower recoveries. The effect of the specific air flowrate did not much influence the separation parameters (Cabral, 1995; Valderrama et al., 1995).

4. Final remarks

The modified 3PC column appears to be an efficient device for the separation of mineral particles possessing high flotation rate constants. Concentrates are always very high grade and compared to the conventional column they are a bit higher. The quality

194 J. Rubio/ Int. _I. Miner. Process. 48 (19961 183-196

of the drained material will depend on ore system and can be either tailed or recirculated with or without classification or regrind.

Should the ore system contain a high proportion of low grade middlings or gangue slimes, grades will be low and this “third” product may be discarded.

Results obtained with coarse feed (copper ore) revealed that with the 3PC cell the entrainment of gangue particles into concentrate is substantially reduced and, as a result, drained material becomes a high grade product. Then, this column may also operate “open” with the two product, concentrate and drop-back, together yielding final copper concentrates.

The 3PC cell behaves as a sharp “classification” column, whose performance depends basically on the particles characteristics (hydrophobicity and size). Thus, medium size hydrophobic particles having high flotation rates are transferred rapidly to the concentrate resisting froth drainage. Middlings, or particles less hydrophobic col- lected by bubbles-depending on attachment forces and size-may suffer drainage by the wash water and may be collected as froth drop-back material together with entrained or entrapped gangue slimes, if present (Rodriguez-Lopez, 1991; Rubio and Rodriguez- Lopez, 1992; Santander et al., 1994; Valderrama et al., 199.5 ).

Column efficiency (accuracy) regarding particles being reported to any of the three products will depend on particle class concentration (hydrophobic, middlings and gangue) and on column imperfection. The latter depends on operating and column design parameters, such as the dimensions of the various zones - factors which must be controlled and optimized.

The high performance obtained with this modified column can be explained as follows:

(1) Part of the froth drop-back product in conventional columns remains at the pulp-froth interface; a fraction returns to the froth; another transfers to the tailings; and a part of the fraction in the froth is drained back again. Thus, some material keeps recirculating. This material is usually composed of low grade middlings and/or entrained (and entrapped) gangue slimes, and because it has to leave the column somewhere; it may either contaminate the concentrate or is transferred to the tailings causing losses (should the amount of valuable in middlings be high).

In conventional columns high enrichment is limited by the recirculating drop-back material but recoveries are usually high.

In the three-product flotation column, the upper part of the collection zone is “free” from froth washed byproducts and their recirculation. Therefore, this zone remains “constant” in terms of physical properties, solids content and grade. The column operates with constant pulp density, viscosity, pressure on the sparger and “hold-up” values. Bubbles also will be stable and constant in number too (Rodriguez-Lopez, 1991; Rubio and Rodriguez-Lopez, 1992; Cabral, 1995).

(2) In the 3PC modified column the “third” product can be retreated separately as follows: i. Desliming the entrained (or entrapped) gangue ultrafines to classify high grade middlings. ii. Reprocessing the middlings by regrinding or recirculating it depending on grade and degree of liberation. iii. Discard it when grade is too low.

(3) The secondary wash water reduces the degree of entrainment and entrapment of gangue slimes and is responsible for the bias water in the column. For this reason the

J. Rubio/Int. J. Miner. Process. 48 (1996) 183-196 195

so-called intermediate zone and its length is important in the 3PC column. This region is rather diluted compared to the collection zone allowing bubble-hydrophobic particles to float faster.

Because of the high accuracy for the separation of particles, this 3PC cell may be used in various forms. Thus, independently of the feed quality, enrichment will depend on the amount of liberated hydrophobic particles in the system.

5. Conchsions

The results reported here lead to the following conclusions: (I) The 3PC modified column presents certain advantages compared to the conven-

tional ce’ll in terms of enrichment ratios and flotation rates. Selective separation of the froth drop-back material avoids particle recirculation, which contaminate concentrates in conventional columns. Hence, pulp density, viscosity, hold-up and solids content in the 3PC column remains “constant” permitting a clearer cut separation.

(2) Benefits attained with the use of the 3PC cell are in the flotation of rougher feeds or concentrates to decrease mass to grinding and cell volume requirements. In tailings treatmem the cell always yields high grade concentrates. As a “rougher-flash” flotation unit, clean copper concentrates (40.42%), were obtained at 32% recoveries. As a flotation cleaner final copper grades were 32% Cu without the need of a regrinding process. With lead-zinc ores, concentrate grade was 13% greater than those obtained in the conventional column. Corresponding values for fluorite and gold were 4% and 50% respectively. Recoveries of gold particles in tailings were 15% with gold grades of 160 g/t (80% greater than in conventional cleaner circuits).

Acknowledgements

The author thanks all his students responsible for the experiments and to the Institutions that made this work possible. Special thanks to Ross Smith from the University of Nevada, Reno, NV, for discussion and English revision.

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