Flotation of silicated gangue iron ores: Mechanism and effect of starch

Minerals Engineering, Vol. 11, No. I, pp. 71-76, 1998

Pergamon 0892---6875(97)00D9-8

© 1997 Published by Elsevier Science Ltd All rights reserved. Printed in Great Britain 0892-6875/98 $19.00+0.00

TECHNICAL NOTE FLOTATION OF SILICATED GANGUE IRON ORES: MECHANISM AND EFFECT OF STARCH

S. MONTES-SOTOMAYOR °, R. HOUOT* and M. KONGOLO* § Universidad de Santiago de Chile, Facultad de Quimica y Biologia, Santiago, Chile t" Laboratoire Environnement et Mineralurgie, 54500-VandIuvre, France. E-mail: houot @ensg.u-nancy.fr (Received 7 July 1997; accepted 20 October 1997)

ABSTRACT Hematite floatability is always lower than that of quartz under the same physico-chemical conditions. Indeed, up to pH> 9, the presence of molecular amine in solution is detrimental to hematite flotation. Starch aztsorbs specifically on both minerals (more on quartz than on hematite). However, starch adsorbed on quartz will desorb in alkaline medium in the presence of alkylammonium salt according to the collector concentration and pH (competing adsorption). This is not the case for hematite for which the starch-mineral affinity is stronger than that of the quartz. © 1997 Published by Elsevier Science Ltd. All rights reserved. Keywotnts Flotation depressants, flotation collectors, iron ores

INTRODUCTION Historically, direct flotation methods with anionic reagents were the first to be tested and applied on iron oxides. Reverse flotation of silica and silicates was then studied, the ferriferous concentrate being collected at the bottom of the cell after depression of iron minerals by reagents such as starch or dextrine (1--4). The purpose of fltis work is to examine the origin and the nature of collector-mineral or starch-mineral interactions. MATERIALS AND METHODS A high purity qum'tz was obtained from "Mercedita" deposit in Ovalle (Chile) (SiO2= 99.8% with 0.04% Tests were carried out with 100-200 lain and < 100 lava mineral fractions.

AI203 and F e 2 0 3 ) .

Hematite comes fi:om "Santa Fe" mine (Chile). Hematite content is about 90%, with goethite 2%, magnetite Presented at Minerals Engineering '97, Santiago, Chile, July-August 1997

71

72

s. Montes-Sotomayoret

al.

1%, quartz 4%, chlorite 0.45%, TiO 2 0.18%, feldspars 0.1%, apatite 0.01% as main impurities. BET analysis revealed that the hematite surfaces exhibit some microporosity (5.6< r< 12.4/~). The same size fractions were selected. Primary alkylamines with minimal purity of 99% were obtained from Fluka, and were transformed to chlorhydrate of alkylammonium by the Ralston method [5]. The starch is a corn starch, Camelia variety, with an amylose content of 28.5%, an average molecular mass in weight Mp = 15.2 106, and a molecular mass in number Mn = 7.8 106. Flotation tests were carded out with a modified Hallimond flotation cell using a porous bottom (pores of 10-20 ~n). Streaming potential was used to determine the Zeta potential of the 100-200 pm mineral fractions [6]. Adsorption isotherms were obtained by using the rests method.

RESULTS AND DISCUSSION

Flotation tests (Tables 1-2). Under equivalent physico--chemical conditions, quartz floats better than hematite. With a 35% iron content mix, iron oxide flotation is observed to be independent of the presence of quartz. In contrast, for an iron content of 55%, quartz acts as a carder mineral for hematite. Hematite floatability in a mix with quartz is minimal at pH 10 for all collector concentrations (molecular amine is present). TABLE 1

Collector concentr. x 10-5 M/I 3 5 7 10 16 30 50 70 100

Quartz recovery as a function of size distribution, depressant adding and collector concentration (pH=lO)

100-200 larn without starch 100 100 100 100


100 97 97 90

69 54 46 36

without starch 54 66 72 80

Quartz <1001am with 20ppm starch 65 66 76 74 70 60 55 42 28

Recovery % Mixture Mixture 4/3 4/3 without with starch starch 20 14

Mixture 6/1 without starch

Mixture 6/1 with starch 14

33 48 56 49 48 37

54 60 61 62

41 46 52 57

41 59 57 61

63

Streaming potential Figure 1 shows the variation of zeta potential as a function of pH for quartz in the presence of varying concentrations of alkylammonium chloride. The zeta potential curve of quartz is drastically modified with alkylammonium chloride. In all cases, the floatability is maximal (at constant pH) for concentrations that are greater than or equal to that corresponding to the change of sign point (PCR). For this PCR, the concentration in molecular amine resulting from the hydrolysis of amine salt is always lower than the corresponding amine solubility. Hematite recovery reaches a maximum above the mineral IEP at pH= 6. Therefore, the floatability is due to either the electrostatic interaction between the alkylammonium ion and the negatively charged hematite,

Flotation of silicated gangue iron ores

73

or to a specific adsorption of the collector on neutral sites or already modified by amine adsorption. The final result is a positively charged surface. Specific adsorption of starch on hematite is evidenced by the displacement of the 1EP of the mineral towards a more acidic value (pH=5). TABLE

Hematite recovery as a function of size distribution, depressant adding and collector concentration (pH=10). Mixture 4/3 = iron content 35 %; Mixture 6/1 = iron content 55%

2

Collector concentr. x

10-5 M/I

100-200 ~'n without starch

<1001~n

10

11 13 16 23 24 22

3 5 7 10 16 30 50 70 100

Hematite Recovery % <1001ma Mixture Mixture 4/3 with 4/3 10ppm without with starch starch starch 2 11 4 11 4 11 6 13 13 23 14 14 26 21 28 21 19 32 19 22 21 22

without starch 6 9 8

36 46 50

100 80

~

Mixture 6/1 without starch

15 14 16 18

Mixture 6/1 with starch

17 17 20 28

26 50

o lo-4.M/L. N 3¢-4.M/L. u 5o-4.M/L. ÷ H20

60

U ~ '10 20 0

-20 -40 -60 -80 -100

" 0

' 2

"

' 4

"

' 6

"

' 8

"

' 10

pH

12

Fig.1 Quartz streaming potential variation as a function of pH and collector (C12) concentration. Furthermore, the interaction between adsorbed starch and alkylammonium ions is revealed by the displacement of the IEP (Figure 2) towards more alkaline pH values (9< pH <10). Starch adsorption on minerals in the presence of alkylamine Adsorption per gram of mineral is higher for hematite than for quartz. Both curves tend to exhibit adsorption maxima. The values obtained experimentally are lower than the values corresponding to a monolayer, which indicates that starch molecules that do not form loops are formed on the surface at

74

S, Montes-Sotomayor et al.

equilibrium. Figure 3 presents the adsorption of alkylamine chloride on both minerals. The amounts of adsorbed starch are also reported in Figure 4. In the case of hematite, the amount of adsorbed starch remains constant along the isotherm. In the case of quartz, starch adsorption seems to compete with surfactant adsorption as the amount of starch adsorbed decreases (from 0.0208 to 0.0116 mg/g ) along the isotherm.

30 25 20 15 10

-

•~

u *

lOppm +le-4MB am. lOppm + 5e-4M/l am.

+

10 ppm without amine

-5 -10 -15 -20 -25 -30

,

2 Fig.2

t

,

4

I

6

~

I

8

m

I

10

pH

12

Hematite streaming potential variation as a function of pH and dodecylamine concentration (10ppm starch adding).

ii ooo 8

Z

0

10

20

30

40

50 Ce, ppm

Fig.3 Starch adsorption isotherms on quartz and hematite (pH=9.5) It can then be inferred that: on hematite, the adsorbed quantity of collector reaches a maximum of 3 lanol/m 2 for a collector equilibrium concentration of 2"10-4Mol/1, which is much less than a monolayer. on quartz, starch and collector molecules are competing for the surface, with increasing collector adsorption as its concentration increases. In basic medium, the quartz tloatability is thus enhanced.

Flotation of silicated gangue iron ores

75

CONCLUSIONS At pH=10 and when mixed with quartz, hematite floatability decreases strongly for all collector concentrations, compared with recoveries observed at lower pH. In alkaline medium, adsorbed starch maintains hematite depression.

]c: 0.0116 mg/g

9-

[]•quartzIh(~matite

"~'~/mg/g

7" 04

E

5;-

d

4-

E 0

0.0208mg/g

/

I

0.32

3--

2-

1Ci],

10 "8

g,

, ,,~.1

.....10' -7

,

, , ~,.d

........

,

~

10 -6

, ,~,,,,I

,

, , ,~,,,I,

. . . . . . ' -5

10

10

.'

.' .' '"".....

-4

10

-3

Ce, M/I

Fig.4 Alkylamine adsorption isotherms on quartz and hematite (20ppm starch and pH=7) These results are in agreement with starch specific adsorption, highlighted by the decrease of hematite IEP in the presence of starch. This displacement of hematite IEP reveals the strength of mineral-starch interaction. In the presence of collector, the residual charge becomes zero or positive (from pH 4 to I0). Similar surface charge results are observed for quartz. However two important differences should be pointed out for hematite: adsorption isotherms show that starch cannot be desorbed with increasing concentration in alkylamrnonium ions, the structure of the adsorbed layer must be completely different because for very low starch concentrvXions (as low as 5 ppm), floatability is absent or poor between pH 6 and 10.

REFERENCES °

2. 3. 4. 5.

• Houot, R., Beneficiation of iron ore by flotation - - Review of industrial and potential applications. Int. J. o f Miner. Process., 1983, 10: 183-204. Polgaire, J.L., Rtactifs amints appliquts ~tla flottation inverse de minerals de fer de type itabirite. Th~se Doct-lng, 1976, Nancy, 211p. Pinto, C.L.L, de Araujo, A.C. & Peres, A.E.C., The effect of starch, amylose and amylopectin on the depression of oxi-minerals. Minerals Eng., 1992, 5,nos 3-5 March-May, 469--478. Montes-S.otomayor, S., La flottation inverse des minerais de fer ~ gangue silicatte. Mtcanisme et effet de l'amidon de mais. Th~se de Doctorat ~s Sciences, 1993, INPL, Nancy: 214p. Ralston, A.W., Hoffman, E.J., Hoerr, C.W. & Selby, W.M., Studies on high molecular weight aliphatie amines and their salts. J. Amer. Chem. Soc., 1941, 63,No4: 1598-1601.

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S. Montes-Sotomayoret al.

Cases, J.M., Les ph6nom6nes physictr--chimiques ~ l'interface. Applications au proc6d6 de flottation. Th~se d'Etat, Nancy, Mdmoires des Sciences de la Terre no13, 1968, 120p.

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