Ionic strength effects in diamine flotation of quartz and magnetite

Minerals Engineering, Vol. 5, Nos 10-12, pp. 1287-1294, 1992 Printed in Great Britain

0892-6875192 $5.00+0.00 0 1992 Pergamon Press Ltd

IONIC STRENGTH EFFECTS IN DIAMINE FLOTATION OF QUARTZ AND MAGNETITE

J.L. SCOTT5 and R.W. SMITHf

0 Windsor Minerals Inc., Windsor, Vermont, USA t University of Nevada, Reno, Nevada

ABSTRACT The flotation of quartz and magnetite was studied as a function of pH when using N-alkyl-1, 3 diaminopropanes and n-dodecylamine as collectors. The alkyl chains of the diamines were 8, 12 and 16 carbons in length. Two collector concentrations were used and flotation studied in the absence of additives and in the presence of NaCl. The chain length effect on flotation using the diamines is comparable to that observed in flotation using monoamines. All the amines are relatively stronger collectors for quartz than for magnetite. The NaCl, in the concentration studied, 0.6 kmol/m3, functions as a depressant for the minerals. The 12 C diamine is a stronger collector for the minerals than the 12 C monoamine. Probable surface chemical phenomena giving rise to the observed experimental results are discussed. Keywords

Quartz, magnetite, diamine and monoamine flotation, ionic strength. INTRODUCTION

Diamines can be used in reverse iron ore flotation of quartz from magnetite and other iron ore minerals. They appear to be stronger collectors for silicates than monoamines of comparable chain length [l-5]. Inorganic electrolytes are sometimes present in flotation water due either to use of brackish water or to environmental requirements of recycling of discharge water. However, relatively little research work has been done to date exploring the relative collecting power of the diamines for quartz and magnetite under various conditions of ionic strength and collector chain length. In amine flotation the presence of the electrolytes can markedly affect flotation [5-g]. At least some of the effects are due to simple increases in ionic strength. Indeed, a number of studies have investigated the effect of ionic strength on the flotation when using long chain collectors, both anionic and cationic, [5-g]. It has been reported that the addition of an inorganic salt can increase or decrease flotation recoveries depending on the system [7, 10-121. It has also been shown that the surface charge on oxide minerals can be reduced by the presence of a monovalent salt [ 13- 151. This paper reports on a study of the effect of chain length and ionic strength on the diamine flotation of quartz and magnetite and the effect of ionic strength on dodecylamine flotation of the minerals. It is hoped that the better understanding of the process involved will lead to better selection of operating parameters in actual mono or diamine flotation of ores containing the two minerals. 1287

1288

J . L . SCOTT and R. W. SMITH

EXPERIMENTAL

The minerals studied were purchased from Wards Natural Science Establishment, Inc. The quartz was from Hot Springs, Arkansas and the magnetite was from Ishpeming, Michigan. The size fraction used in the microflotation studies was -212 #m, +45 #m (- 65, +325 mesh). High purity quartz crystals and magnetite samples were crushed and then ground with a mortar and pestle. The materials were hand screened to the desired size fraction and then r o - t a p p e d for 30 min. After dry screening the samples were leached in concentrated HC1 for 5 min and then allowed to stand for 10 min. The samples were next rinsed with double distilled water until the pH of the rinse water was the same as that of the water alone. The samples were oven dried at about 95 °C. The amines studied were high purity materials obtained from Armac Co., McCook, Illinois. The diamines were N-alkyl-1, 3 diaminopropanes of chain lengths 8, 12, and 16 C. The monoamine was high purity n-dodecylamine. Acetate salts of the amines were prepared using reagent grade acetic acid. The inorganic salt used for maintaining ionic strength was reagent grade NaCI. HCI and carbonate free NaOH were used for pH control. All solutions were prepared with double distilled water. The samples were prepared for Hallimond tube flotation by first agitating the mineralcollector solution suspensions for 5 min in a water bath held at 35 °C for experimentation with the 16C diamine or at 25 °C for all other experimentation. The samples were then transferred via a glass funnel to the Hallimond tube. After a total conditioning time of 7 min flotation was initiated. In the experimentation 1 gm mineral samples in 170 ml of collector solution were used. i00 ml of purified nitrogen were passed through the cell at a rate of 2 ml/sec. No stirring of the samples was employed. RESULTS Figure 1 illustrates the flotation of quartz (la) and magnetite (lb) as functions of pH when using the four different amines as collectors. In all cases collector concentration was 1 x 10 .5 k m o l / m 3. All data on quartz in this figure and in subsequent figures are from a previous publication [5]. All of the collectors are obviously stronger collectors for quartz than for magnetite. Evident from the results are the chain length effects, the relative stronger collecting power of the 12 C diamine compared to the 12 C monoamine, and the apparent double flotation peaks for the 12 C diamine quartz flotation. Note that the chain length effect is as expected for long chain collectors [3,5,16]. The effect of increasing ionic strength via addition of 0.6 kmol/m 3 NaCI is demonstrated in Figure 2 for both quartz (2a) and magnetite (2b). Note the decrease in quartz flotation recovery in all cases, especially with the shortest chain amine and the virtual absence of magnetite flotation using all the collectors at all pH values. The effect of raising collector concentration in the absence of NaC1 to 1 x 10 -4 kmol/m 3 is illustrated in Figures 3a and 3b. Evident is the general marked increase in flotation in all cases for both minerals (except for the case of quartz flotation using the 16C diamine where there was little increase in the already excellent flotation using the lesser amine concentration). Note that the flotation trough between the flotation peaks of the quartz 12 C diamine system is eliminated. When ionic strength was increased to 0.6 kmol/m 3 by NaCI addition at amine concentrations of 1 x 10 -4 k m o l / m 3 the curves of Figures 4a and 4b were obtained for quartz and magnetite, respectively. It is observed that in all cases, except for magnetite flotation using the 8C diamine, much of the depression is reversed by the increased collector concentration. In fact magnetite flotation with this one exception is better than for flotation using 1 x 105 k m o l / m 3 amine concentrations in the absence of NaCI.

Diamine flotation of quartz and magnetite

100r / 90 p ] 80

5 conc. = 1 x 10 - ~ ~ With no other f additions ~ ~

1289

_~g-'~ /~\ [//// \ \

60

i '° 50

40

30 ,'7

20 10 0

1

2

Figure I=

3

4

5

6

7 pH

8

9

10

11

12

13

14

r e c o v e r y of q u a r t z as a f u n c t i o n of pH.

Flotation

100 Conc. -- 1 x 10--" With no other additions

90 A

80 70

o ¢J er c o

IJ.

60

50 40 30,

20 10i 0

I 1

Figure l b

2

3

4

5

6

7 8 9 pH F l o t a t i o n r e c o v e r y of m a g n e t i t e

10

11

I 12

I 13

I 14

as a f u n c t i o n of pH.

Fig.1 Flotation recovery of quartz (la) and magnetite (lb) as a function of pH; collector concentration: l x l 0 "5 kmol/m3; no NaCI addition; • dodecylamine, O 8C diamine, zx 12C diamine, [] 16C diamine; Quartz data from reference [5]. DISCUSSION A number of observations can be made directly from consideration of Figures 1 - 4. These are: 1.

All the amines are stronger collectors for quartz than for magnetite;

2.

Considering the 8, 12, and 16 C diamines the longer the chain length the better the flotation at a set amine concentration;

,

4.

The 12 C diamine is a stronger collector than the 12 carbon monoamine; The addition of 0.6 kmol/m 3 NaCl strongly depresses the flotation of quartz when 1 x 10 "s k m o l / m 3 amine is present;

1290

J. L, SCOTT and R. W. SMITH

100

Conc. = 1 x 1 0 - 5 M 90 80

ro ~

It

so

" --

L.

0u m

50

.2

4o

--

30

o u.

20 10

1

2

3

4

s

6-

7

8-~

;0

.-12

13

14

pH

Figure ~

Flotation recovery of quartz as a function of p H .

100

Conc. = 1 x 1 0 - 5 M With 0.6 M NaCI

90 80

7o =, o u • n-

60

~_

40

_~

3o

50

i, 2O 10

0

J

~

1

2

~v 3

4

5

6

7 pH

8

9

10

11

12

I

I

13

14

Figure 2b Flotation recovery of magnetite as a function of p H .

Fig.2 Flotation recovery of quartz (2a) and magnetite (2b) as a function of pH; collector concentration: l xl0 "5 kmol/m3; NaC1 concentration: 0.6 kmol/m3; • dodecylamine, O 8C diamine, zx 12C diamine, [] 16C diamine; Quartz data from reference [5].

.

This concentration of NaC1 at the same amine concentration almost completely depresses the flotation of magnetite regardless of amine chain length;

.

Increasing the amine concentration to 1 x 10 .4 kmol/m 3 when 0.6 kmol/m 3 NaCI is present generally reduces the depression of both minerals, except for the case of magnetite flotation using the 8C diamine where there is little flotation regardless of amine concentration.

Most of the noted phenomena can be explained based on a study of the pk a values, amine solubilities, collector valences, chain length effects and some reasonable assumptions about the electrostatic charge on magnetite and quartz as a function of pH. The quartz system was thoroughly studied in a past publication [5] and much of the reasoning for this system can be extended to the magnetite system although overall much less detailed data is available on the aqueous surface chemistry of magnetite.

Diamineflotationof quartzandmagnetite

1291

Conc.= 1 x 10-4M With no other additions 100 90 A

80

60 r,- 50

i '° 40 "~ 30 n20I

I0 I

I

I

Figulre ~ ~otaiion;eco6verioHf~ua~ als°a~nc:i~n13fpH4. 100

Conc. = 1 x 10-4M

,/(~'~

~

90 80 ro

o>

6o

g:

so

g

40

~" 20 10 1 Figure

2

3

4

5

6

7 pH

8

9

10

11

12

13

14

81) Flotation recovery of magnetite as a function of pH.

Fig.3 Flotation recovery of quartz (3a) and magnetite (3b) as a function of pH; collector concentration: lxl0 -4 kmol/m3; no NaCI addition;, dodecylamine, O 8C diamine, zx 12C diamine, [] 16C diamine; Quartz data from reference [5]. Considering, firstly, the pk a values and solubilities of dodecylamine and the diamines, the ~k a value for dodecylamine is 10.63, the solubility of the molecular species is about 2 x 10kmol/m 3 at 22-25 °C and the Krafft temperature is about 26 °C [17]. For the 12 C diaminopropane the Pkal and Pka2 values are at about 6.8 and 9.3, respectively [2,5]. The solubility of the molecular species was determined to be about 3.6 x 10 -7 kmol/m 5 at 2225 °C. By analogy with data for primary monoamines the pk a values of the diamine propanes should change very little with chain lengths from 8 to 16 C. Thus, the pk a values should be about the same for the 8, 12 and 16 C diaminopropanes. The solubility of the molecular species should decrease about 1/3 for each CH 2 group added to the chain. Thus, ~he solubilities of the 8 and 16 C diamine molecular species should be very roughly 4 x 10" and 4 x 10 -9 kmol/m 3, respectively. The isoelectric point (iep) of quartz lies at about pH 2 and that of magnetite at pH 5-6.5 [18,19]. Thus, quartz is more negatively charged than magnetite and all the amines should function as stronger collectors for it than for magnetite. The effect should be particularly

1292

J . L . SCOTT and R. W. SMITH

marked at pH values from slightly more basic than the iep of magnetite (a little above pH 6.5) down to pH values a little more basic than the iep of quartz (a little above pH 2). Past work on quartz [5] demonstrated the importance of the more acidic pK a values of the diamines as compared to dodecylamine (and the other monoamines) and of the doubly charged amine species in promoting stronger flotation of quartz when using the 12 C diamine as compared to the 12 C monoamine. Since the magnetite (iep ~ 5 - 6.5) is less negatively charged than quartz and since PKaq ~ 6.8 for the diamines used, the maximum concentration of the 2+ species will be at pH values where the magnetite has a low negative charge, is uncharged or is positively charged. Thus, strong interaction between the mineral and this diamine species is unlikely.

loot/

9OFWith0"6MNaC'-Conc. = 1 x 1 0 - 4 M

II

I

/

["~'--,J~

"

--

80

6O

~, so ~

40

~

3o

N 2O 10

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

pH Flotation recovery of quartz as a function of pH.

Figure i s 100

Conc. = 1 x 1 0 - 4 M With 0.6 M NaCI

90 80~ "-" ¢p

=, 0 (.l II

~

70

60 50

3o 2O

10 0

I

I

I0

I

I~

I

,-"t

I

I

2

3

4

5-

6

~7

8

~

'

9

10

11- 12

I

I

13

14

pH

Figure 4b Flotation recovery of magnetite as a function of pH.

Fig.4 Flotation recovery of quartz (4a) and magnetite (4b) as a function of pH; collector concentration: 1x 10 -4 kmol/m3; NaCI concentration: 0.6 kmol/m3; • dodecylamine, O 8C diamine, zx 12C diamine, [] 16C diamine; Quartz data from reference [5].

Diamine flotation of quartz and magnetite

1293

Additionally, in earlier work [4,5] the possibility of surface precipitation of amine species on quartz was considered and deduced to be an important mechanism in quartz flotation. Castro, et al. [20] and Laskowski [21] have also shown the importance of surface precipitation in amine flotation. Again, since there will be a more positive charge on hematite it, thus, can be assumed that a greater bulk amine concentration will be required before such precipitation occurs. Regardless of mechanisms involved it can readily be seen from the various figures that an increase in ionic strength tends to decrease flotation for both minerals. The decrease in flotation can in some cases lead to less selectivity. However, by adjusting amine concentration and using amines of different chain length it appears that conditions can always be found that will give good selectivity. As a word of caution it should be noted, however, that the effect of the presence of both minerals together on selective flotation was not investigated. Thus, the possibility of a small solubility of magnetite with subsequent adsorption of aqueous Fe(II) or Fe(IIl) species and its probable depressing effect on quartz was not considered. Should this phenomenon take place a depressant(s) would be required for selective flotation. ACKNOWLEDGEMENT The financial support of this work by the National Science Foundation Grant ENG 7910010 is gratefully acknowledged.

REFERENCES

.

Bleier, A., Goddard, E. D. and Kulkarni, R. D., The Structural Effects of Amine Collectors on the Flotation of Quartz, in: Flotation, A. 34. Gaudin Memorial Volume, ed. Fuerstenau, M. C., 117-147 (1976).

.

Smith, R. W., Cationic and Amphoteric Collectors, in Reagents in Mineral Technology, eds. Somasundaran, P. and Moudgil, B., Marcel Dekker, 219-256 (1988).

.

Smith, R. W., Structure - Function Relationships of Long Chain Collectors, in Challenges in Mineral Processing, eds. Sastry, K. V. S. and Fuerstenau, M. C., SME/AIME, 87-116 (1989).

.

Smith, R . W . and Scott, J . L . , Mechanisms of Dodecylamine Flotation of Quartz, Mineral Process. Extract. Metall. Rev., 7, 81-94 (1990).

.

Scott, J. L. and Smith, R. W., Diamine Flotation of Quartz, Minerals Engineering, 4, 141-150 (1991).

.

Onoda, G. Y. and Fuerstenau, D. W., Amine Flotation of Quartz in the Presence of Inorganic Electrolytes, in VII International Mineral Processing Congress, ed. Arbiter, N., Gordon and Breach, Publ., 301-306 (1964).

.

Rajala, J. A. and Smith, R. W., Ionic Strength, Collector Chain Length and Temperature Interactions in Alkyl Sulfate Flotation of Hematite, Trans. S M E / A [ M E , 274, 1947-1953 (1984)

.

Wei, T. L. and Smith, R. W., Anionic Activator Function in Cationic Flotation of Hematite, in Reagents in the Mineral Industry, eds. Jones, M. J. and Oblatt, R., Inst. Min. Metall., London, 41-45 (1984).

.

Wei, T. L. and Smith, R. W., Anionic Depressant Function in Anionic Flotation of Hematite, Intern. Journ. Mineral Process., 21, 93-103 (1987).

1294

J. L. SCOTT and R. W. SMITH

10.

Fuerstenau, D. W. and Modi, H. J., Flotation of Corundum. An Electrochemical Interpretation, Trans. AIME, 217, 381-387 (1960).

11.

Somasundaran, P., Interfacial Chemistry of Particulate Flotation, in Advances in Interfacial Phenomena of Particulate/Solution/Gas Systems; Applications to Flotation Research, AIChE Symposium Series 150, 71, 1-15 (1975).

12.

Kulkarni, R. D. and Somasundaran, P., Effects of Reagentizing, Temperature and Ionic Strength and Their Interactions in Hematite Flotation, Trans. AIME, 262, 120125 (1977).

13.

Gaudin, A. M. and Fuerstenau, D. W., Quartz Flotation with Anionic Collectors, Trans. AIME, 202, 66-72.(1955).

14.

Gaudin, A. M. and Fuerstenau, D, W., Quartz Flotation with Cationic Collectors, Trans. AIME, 202, 958-962 (1955),

15.

Fuerstenau, D. W. and Modi, H. J., Steaming Potentials of Corundum in Aqueous Organic Electrolyte Solutions, Journ. Electrochem. Soc. Journ., 106,336-341 (1959).

16.

Fuerstenau, D. W., Healy, T. W. and Somasundaran, P., The Role of the Hydrocarbon Chain of Alkyl Collectors in Flotation, Trans. SME/AIME, 229, 321-325 (1964).

17.

Dervichian, D. G., La Cristallisation et la Solubilite des Agents de Surface, in Vortrage in Originalfassung, II[ Internationaler Kongress fur Grenzflachenaktiv Stoffe, 1, Koln, 182-188 (1961).

18.

Ney, P., Zeta Potential und Flotierbarkeit yon Mineralen, Springer Verlag, 126-128 (1973).

19.

Parks, G. A., The Isoelectric Points of Solid Oxides, Hydroxides and Aqueous Hydroxo Complex Systems, Chem. Rev., 65, 177-198 (1965).

20.

Castro, S. H., Vurdela, R. M. and Laskowski, J. S., The Surface Association and Precipitation of Surfactant Species in Alkaline Dodecylamine Hydrochloride Solutions, Colloids Surf., 21, 87-100 (1986).

21.

Laskowski, J. S., The Colloid Chemistry and Flotation Properties of Primary Aliphatic Amines, in Challenges in Mineral Processing, eds. Sastry, K. V. S. and Fuerstenau, M. C., SME/AIME, 15-34 (1989).