Selective spherical agglomeration of fine salt-type mineral particles in aqueous solution

ELS E V I E R

Colloids and Surfaces A: Physicochemicaland EngineeringAspects 96 (1995) 277-285

COLLOIDS AND SURFACES

A

Selective spherical agglomeration of fine salt-type mineral particles in aqueous solution Zygmunt Sadowski Department of Inorganic Chemistry and Metallurgy, Technical University of Wroctaw, 50-370 Wroclaw, Poland Received 13 July 1994; accepted 21 October 1994

Abstract

Salt-type minerals dispersed in aqueous solution with sodium oleate were spherically agglomerated at the critical surfactant concentration. When the pH range corresponded to the pHpzc of carbonate minerals, the spherical agglomeration disappeared. The selective separation of barite from a carbonate mineral suspension (ratio 1 : 1) was realized when sodium lignin sulphonate was added using the spherical agglomeration procedure. The effect of adding sodium lignin sulphonate was analyzed using FT-IR spectroscopy and thin-layer wicking techniques. The results suggest that the addition of the modifier changes the hydrophobicity of the surface and probably prevents the adhesion of bulk precipitated surfactant salts to the carbonate mineral surfaces.

Keywords: Adsorption; Barite; Carbonate minerals; Sodium lignin sulphonate; Spherical agglomeration

1. Introduction

One of the important problems in the mineral processing of ores is the selective recovery of fine mineral particles. A method that can improve the fines recovery is the selective agglomeration of the valuable components from valueless fines which are kept dispersed [ 1,2]. Salt-type minerals such as barite, calcite, dolomite and magnesite are characterized by high solubility compared to the oxide-type minerals. The surface of the salt-type minerals is naturally hydrophilic and highly energetic in aqueous solution [-3,4]. Fatty acids and their soaps are commonly used as collectors for the salt-type mineral flotation process [-5-7]. The resemblance of the surface properties of salt-type minerals to each other results in their separation being difficult. As a consequence, this separation requires the use of modifier reagents which the adsorption collector selectivity enhances. 0927-7757/95/$09.50© 1995ElsevierScienceB.V. All rights reserved SSDI 0927-7757(94)03042-1

The modifier reagents are usually classified into two large groups: inorganic and organic compounds. The action of the modifier reagents is realized by (i) preventing collector adsorption on a mineral surface; (ii) making the mineral surface hydrophilic. One of the most widely used inorganic modifier reagents is sodium silica, which prevents the reaction of oleate species with surface sites [7]. Since polysilic acid is partly ionized in an aqueous solution, it is also used to control the charge density at the mineral-solution interface. The most commonly used organic modifying agents are polyelectrolytes such as starch, tannin, dextrin and quebracho. Somasundaran [8] found that starch increased the oleate adsorption of calcite particles; it was due to clathrate formation between starch molecules and oleate ions. This complex compound is hydrophobic in the interior and hydrophilic on the exterior. The adsorption of

278

Z. Sadowski/Colloids Surfaces A: Physicochem. Eng. Aspects 96 (1995) 277 285

the starch helix results in the hydrophobic properties of the calcite surface being destroyed. Competition between sodium oleate and quebracho for the adsorption sites on the calcite surface has been observed [9-11]. Iskra et al. [9] concluded that "calcium bridging" is responsible for the quebracho depressing action. Also, the mechanism of the interaction of tannins with calcium minerals is based on chemisorption through calcium ions [12]. In our previous work [13,14], sodium lignin sulphonate was found to be a selective dispersing agent for barite suspensions. The group of synthetic polymers that are modifying reagents include low molecular weight polyacrylates such as Cataflot P-40, manufactured by Perrefitte-Auby Co. (France) and poly(ethylene oxides). Some investigators of the behaviour of these synthetic depressor reagents have emphasized the fact that a combination of hydrophobic interactions and hydrogen bonding is responsible for the adsorption [ 15,16]. Finely, the mineral surface is covered by a strong hydrophilic protective layer. The study presented here was undertaken to obtain an insight into the surface chemical aspects of the spherical agglomeration of salt-type minerals. One goal of this work was to demonstrate the influence of both pH and modifier reagents on the selectivity of the spherical agglomeration of salttype minerals in aqueous solutions.

2. Materials and methods

2.2. Surfactant and modifier reagents The anionic surfactant used in these studies was pure sodium oleate (SOL) provided by the Sigma Chemical Company. Cataflot P-40 is an acrylic polymer of low molecular weight which has been specially developed by the Perrfitte-Auby Company (France) for depressing and dispersing calcite and dolomite in the ore flotation. Another modifier agent used in these studies was sodium lignin sulphonate (SLS), purchased from the Crown Zellerbach Chemical Co. Low molecular weight poly(ethylene oxide) (PEO) was obtained from the Aldrich Chemical Company, Inc.. The other reagents were purchased from POCH in Gliwice (Poland). All reagents were used without further purification.

2.3. Spherical agglomeration The agglomeration experiments took place in a Teflon vessel which contained 100 ml of the mineral suspension. In each experiment, 2.0 g of mineral powder were suspended in a 100 ml solution. A small amount of n-heptane (0.5 0.6 ml) was added under various mixing conditions. Generally, the agglomeration time was 50 s and the mixing speed was 10000rev min -1. The agglomerates formed in this procedure were separated by screening (120 mesh). The recovery was determined by measuring the per cent of material remaining on the screen by weight.

2.1. Minerals

2.4. Adsorption measurements

Highly pure samples of the minerals barite, calcite, dolomite and magnesite were supplied by the WARD'S Natural Science Establishment Inc. These samples were dry-ground in an agate mortar and dry-screened to obtain - 4 0 ~tm fractions. Measurements of the specific surface area of these samples by the BET nitrogen adsorption method gave the values 2.8m 2 g-1 1.4m 2 1.1 m 2 g 1 and 1.5 m 2 g-1 for calcite, dolomite, magnesite and barite, respectively. These samples were utilized in both the adsorption and the spherical agglomeration tests.

The amount of oleate adsorbed on the mineral surface was determined from the difference between the initial and final concentrations. A defined quantity of dry mineral sample (1 g) was dispersed in 100 ml of an aqueous solution of sodium oleate. The time of equilibrium for the sodium oleate adsorption was 8 h. After equilibration, the dispersed solid was separated with a centrifuge at 200g for 5 min. The concentration of sodium oleate was analyzed using a two-phase titration technique [-17]. In the case of the modifier agent application, the mineral powder was first conditioned in SLS

279

Z. Sadowski/Colloids Surfaces A." Physicochem. Eng. Aspects 96 (1995) 277-285

solution before the addition of sodium oleate solution.

100

2.5. Infrared spectroscopy study The spectra of the mineral samples were obtained using a Perkin-Elmer Model 1600 infrared spectrophotometer equipped with a multiple internal reflection (MIR) accessory. This technique was found to be suitable for the detection of surface complexes on the mineral surface under the experimental conditions used. All the spectra were recorded between 4000 and 700 cm -~. A germanium internal reflecting crystal was used. It was suitable for the analysis of high refractive index samples with a high water content.

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2.6. Thin-layer wicking technique The thin-layer wicking experiments were carried out using glass plates (microscopic slides) on which the mineral powder was deposited. The preparation of a thin layer of powder was described in detail by Chibowski and co-worker [-18,19]. The same mineral powder was used for both the adsorption and agglomeration tests. The plates were equilibrated for 24 h with n-heptane vapour. The slide was placed in a horizontal position in the specially designed chamber. The test liquid was transported from its vessel to the powder layer by means of a flannel wick. The surface of the liquid tested was at the same level as the slide surface. The speed of liquid penetration was determined by measuring the distance travelled by the liquid as a function of time.

T

510-3

concentration,

Sodium oIeafe

kmo[/m3

Fig. 1. Spherical agglomeration of salt-type minerals as a functionof sodiumoleateconcentration. 3 x 10 .3 mol 1-1 for magnesite, barite, dolomite and calcite respectively. The pH significantly affects the solubility of carbonate minerals. For this reason, the effect of pH on the spherical agglomeration of carbonate minerals has been investigated. Fig. 2-4 present the results for calcite, and dolomite magnesite suspensions at the CSCs. These data demonstrate that all investigated carbonate suspensions have a

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3. Experimental results and discussion

~

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Fig. 1 presents several curves demonstrating the effect of the sodium oleate concentration on the recovery of spherical agglomeration when the salttype mineral suspensions were used. It should be noted that all mineral suspensions show a characteristic behaviour. A sharp increase in the recovery rate takes place at the critical surfactant concentration (CSC), which is characteristic for each mineral suspension. These CSCs were approximately equal to 3 x 10 -4, 5 x 10 -4, 6 x 10 -4 and

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m

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z

i

8

9

10

11

12

pH Fig. 2. Effect of pH on the sodium oleate adsorption and

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Z. Sadowski/Colloids Surfaces A. Physicochem. Eng. Aspects 96 (1995) 277 285

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minimum recovery at a pH value which corresponds to the pHpzc of these minerals (calcite, pH 8.5, dolomite, pH 9.5; and magnesite, pH 9.5). It also appears that the low recovery range correlates well with the low adsorption density of sodium oleate on the mineral surface. The behaviour of the barite suspension was different (Fig. 5). The recovery rate of spherical agglomeration was equal to 100% when the pH equals or is greater than 6.0 and a small dependence of pH on the adsorption density of sodium oleate on the barite surface was observed. The results obtained with simple mineral suspen-

Fig. 5. Effect of the sodium oleate adsorption and spherical agglomeration of barite suspensions.

sions suggest that the two-mineral mixture (baritecarbonate) can be easily separated over a specific pH range, using sodium oleate as the surfactant and the spherical agglomeration procedure. Unfortunately, selectivity was not achieved for the 1 : 1 mixed mineral suspensions. The loss in selectivity was mostly due to heteroaggregation of both mineral particles. According to the model of the spherical agglomeration of salt-type minerals [20], low selectivity and heteroaggregation is the result of heterocoagulation in the first step of the spherical agglomeration process. To avoid heterocoagulation, different types of modifier reagents were employed in the spherical agglomeration experiments. As shown in Table 1, the addition of sodium lignin sulphonate to the tested suspensions protected only the spherical agglomeration of carbonate mineral suspensions. The addition of 1000 g-I of Cataflot P-40 caused all the mineral suspensions not to agglomerate. On the other hand, the addition of PEO (poly(ethylene oxide)) did not stop the spherical agglomeration. The effect of the sodium lignin sulphonate addition on the selectivity of the spherical agglomeration of mineral suspensions is presented in Table 2. The spherical agglomeration experiments were conducted at the CSC and in the vicinity of pH 10. As can be seen, both the yield and the recovery of the one-step separation process are satisfactory. The mechanism of the action of sodium lignin

281

Z Sadowski/Colloids Surfaces A: Physicochem. Eng. Aspects 96 (1995) 277 285 Table 1 Effect of modifying reagents on the spherical agglomeration of salt-type mineral suspensions Mineral

Calcite Dolomite Magnesite Barite

Concentration of SOL (mol 1 1)

Concentration of modifying reagent (1000 g t - 1t SLS

Cataflot P-40

PEO

3x 6x 4x 5x

Coagulation Coagulation Coagulation S. agglomeration

Coagulation Coagulation Coagulation Coagulation

S. S. S. S.

10 3 10 4 l0 4 10-4

agglomeration agglomeration agglomeration agglomeration

Table 2 Separation of barite from carbonate minerals (1 : 1 mixture) in the presence of sodium lignin sulphonate Mineral

Calcite Barite Dolomite Barite Magnesite Barite

Concentration of SOL ( m o l l 1)

Concentration of SLS (g t 1)

7 x 10 4

1000

6.5 x 10 -4

750

6 x l0 4

1000

Wt.%

Assay carbonate (per cent)

Recovery (per cent)

37.42 a 62.58 b 40.23 a 59.77 b 44.27 a 55.73 b

8.13 71.26 6.21 77.73 4.65 84.59

6.30 93.62 5.10 94.90 4. l 8 95.81

a Concentrate. b Tailing.

sulphonate, which was used as an enhancer of the separation selectivity, has been investigated using the FT-IR technique (Figs. 6A and 6B). The adsorbed form of sodium oleate on the salttype mineral surfaces can be analyzed by use of the band positions in the IR spectra. Some workers [21-24] who studied the mechanism of oleate interactions on calcite and fluorite have shown that the carboxylic group has characteristic frequencies in the range 1400-1800cm -1. The carbonyl stretching frequencies for unsaturated fatty acids possessing double bonds generally occur in the range 1690-1720 cm-1. T h e - C = C - s t r e t c h i n g vibrations give rise to a band between 1600 and 1680 cm-1. The asymmetric stretching vibration of the carboxylate groups corresponds to the peak at 1558.7 cm 1. The presence of calcium oleate and the oleate anions in the form of a bilayer suggests a band at 1575.8cm -1. The small peak at 1554-1555 c m - 1 is attributed to the monocoordinated surface calcium oleate and the peak at 1539.0cm i corresponds to the calcium oleate

bilayer. As can be seen in Fig. 6A, the addition of sodium lignin sulphonate causes a decrease in the peak at 1558.7 cm -1. This suggests that the adhesion of bulk precipitated calcium oleate to the mineral surface decreases. As is known, the precipitation and adhesion of surfactant salts is a condition for spherical agglomeration to occur [20]. As shown by Hu et al. [25], the intensity of the = C H 2 stretching band can be used to study the adsorption reaction on the fluorite surface. The bands related to the hydrocarbon chain occur in the range 2800-3040cm-1. The asymmetric stretching vibration of the CH 2 group has a band at 2920.5 cm -1. The asymmetric stretching vibration of the - C H 2 - group occurs at 2954 c m - 1. The addition of sodium lignin sulphonate had no effect on the peak positions (Fig. 6B). This can mean that this reagent has no effect on the chemisorption of sodium oleate on the calcite surface. The spectra of barite in the presence of both sodium oleate and sodium lignin sulphonate are shown in Fig. 7. Plitt and Kim [26] used the band appearing at 1560 cm -1 for the quantitative deter-

Z. Sadowski/Colloids Surfaces A: Physicochem. Eng. Aspects 96 (1995) 277-285

282

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283

Z. Sadowski/Colloids Surfaces A." Physicochem. Eng. Aspects 96 (1995) 277-285 100.00" ST

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Fig. 7. FT-IR spectra of barite in the presence of sodium lignin sulphonate and sodium oleate. mination of sodium oleate on the barite surface. Peck [27] reported the chemisorption of sodium oleate on barite; it was associated with the appearance of the antisymmetric carbonyl absorption band at 1543 c m - 1. Direct information about the amount of oleate adsorbed could be obtained from the height of these two peaks. The results of Fig. 7 indicated that the higher chemisorption of sodium oleate on the barite surface occurs in the presence of sodium lignin sulphonate. Information about the free energy of the mineral surface which has been contacted with both SOL and SLS would be helpful for a better understanding of the effect of the modifier reagent during the spherical agglomeration process. The results of the thin-layer wicking experiments are presented as a function of x 2 = f ( t ) in which x is a penetration distance and t is the penetration time, as shown in Fig. 8. As can be seen, the penetration rates for nheptane show a straight-line relationship. This means that Washburn's equation holds for these systems. The results obtained when the plates were

previously in contact with the n-heptane vapour are presented as open symbols in Fig. 8. The filled symbols correspond to the bare plates (without being prewetted). Based on the results obtained for precontacted plates, the effective radius (R) of the capillary systems can be calculated: 0.9615x 10-Scm, 1.9878 x 10 -5 cm and 2.1187 × 10 -5 cm for calcite, calcite + SOL and calcite + SLS + SOL respectively. For this calculation we noted that the surface tension of n-heptane is 20.3 m N m -1 (at 20°C) and the viscosity is 0.409 cP at 20°C [-28]. Next, using the data obtained from the thinlayer wicking experiments on the bare surfaces (filled symbols) the surface tension components ysew have been calculated. The modified Washburn's equation has been submitted for the calculation Rt

x 2 = ~q W a - W c WA -- Wc = 2x/Y~w Ys tw -- 27L

Z. Sadowski/Colloids Surfaces A: Physicochem. Eng. Aspects 96 (1995) 277-285

284 1200-

1000

800

t.[s] 600

~00

Co[cite

Colcite

Colcife

3 10-3S01 3 10"3 SOL kmo[/m 3 2000g/f SLS

200 0 l

,

i

10

20

D •

30

0 •

L,O

x2,[crn2]

Fig. 8. Penetration time of n-heptane into a thin layer of A calcite, [] calcite+sodium oleate and O calcite+sodium oleate+sodium lignin sulphonate versus distance squared. Filled symbols refer to the bare plates (without being prewetted).

The average value of 7~w of the calcite surface was 44.95mN m -1 and this was close to the literature data [28]. The values for the surface treated by SOL and both SLS and SOL were 40.94mN m -1 and 40.79 mN m -1 respectively. The calculated surface free energy components of the investigated mineral powders may lead us to conclude that the addition of sodium lignin sulphonate does not affect the hydrophobicity of the calcite surface in the presence of sodium oleate. This conclusion is in agreement with the FT-IR analysis.

sions (barite-carbonate) is possible when sodium lignin sulphonate was used as a modifier reagent. This observation is in agreement with the model of the spherical agglomeration of salt-type minerals that has been proposed [20]. According to this model, the addition of surfactant to the suspension during the first step causes the hydrophobic aggregation (coagulation) of the mineral particles. The heterocoagulation process, which has been observed for the mixed mineral suspension, was a reason for the loss of selectivity. Sodium lignin sulphonate added before the surfactant addition caused a beneficial selectivity to be obtained (Table 2). The presence of sodium lignin sulphonate in the suspension did not affect the sodium oleate adsorption. However, a difference existed with the physically adsorbed calcium oleate. This could be an explanation for the observed disappearance of the spherical agglomeration of calcite and other carbonate minerals. The surface free energy of calcite particles does not change when sodium lignin sulphonate is added to the calcite suspension. The calcite surface continues to be hydrophobic. The basic conditions for the realization of spherical agglomeration require both the bulk precipitation and adhesion of sparingly soluble surfactant salts. The low concentration of dissolved ions at the pHpzc is the reason why spherical agglomeration cannot be realized. The presence of sodium lignin sulphonate probably caused a reduction in the surfactant salt adhesion to the surfaces of the carbonate mineral particles. For a better explanation of this problem, experiments involving surfactant salt adhesion will be continued.

Acknowledgement The author wishes to acknowledge the help of Mrs. W. Jagiello with the FT-IR experiments.

Appendix: List of symbols 4. Conclusion The experimental data showed that the selective spherical agglomeration of binary mineral suspen-

R t W A

effective radius time adhesion energy

Z. Sadowski/Colloids Surfaces A: Physicochem. Eng. Aspects 96 (1995) 27~285

Wc x

cohesion energy penetration distance

Greek letters 7 Lw 7Lw )'L r/

surface free energy component of liquid surface free energy component of solid surface tension of liquid liquid viscosity

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285

[-10] C. Gutierrez, Trans. AIME, 266, (1979), pp. 1918 1924. [11] F. de Castro, M. deHoces, Int. J. Miner. Process., 37, (1993), pp. 283-298. [12] J.H. Schulze, H.S. Hanna and U. Bilsing, Freiberg. Forschungsh. A, 476, (1970), pp. 33-46. [13] Z. Sadowski, R.W. Smith, J. Dispersion Sci. Technol., 10, (1989), pp. 715-738. [14] Z. Sadowski, Miner. Eng. 5, (1992), pp. 421 428. [15] R.J. Pugh, Int. J. Miner. Process., 25, (1989), pp. 101-130. [16] W.A. Eigeles, T.V. Lippa, Izv. Vyssh. Uchebn. Zaveda., Tswetn. Meta., 4, (1974), pp. 7 15, [17] E.W. Foler, T.W. Steele, Rep. no. 443, S. Af. Trans. Inst. Min. Metall. 1984. [18] E. Chibowski, J. Dispersion Sci. Technol., 6 (1992) pp. 1069-1090. [19] E. Chibowski, L. Holysz, Langmuir, 8 (1992), pp. 710-716. [20] Z. Sadowski, J. Dispersion Sci. Technol., in press. [21] V.M. Lovell, L.A. Goold, N.P. Finkelstein, Int. J. Miner. Process., 1 (1974), pp. 183-198. [22] E.G. Giesekke, Int. J. Miner. Process., 11, (1983), pp. 19-56. [23] K. Hanumantha Rao, J.M. Cases, P. De Donato, K.S.E. Forssberg, J. Colloid Interface Sci., 145, (1991), pp. 314 329. [24] K. Hanumantha Rao, J.M. Cases, K.S.E. Forssberg, J. Colloid Interface Sci., 145, (1991), pp. 330 348. [25] J.S. Hu, M. Misra, J.D. Miller, In. J. Miner. Process., 18, (1986), pp. 73 84. [26] L.R. Plitt, M.K. Kim, Trans. AIME, 256, (1974), pp. 188-193. [27] A.S. Peck, Rep. Invest. U.S. Bureau of Mines, No. 6202, 1963 1 16. [28] L. Holysz, E. Chibowski, J. Colloid Interface Sci., 164, ( 19941 245-251.