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Oleate Flotation of a Nigerian Baryte: The Relation between Flotation Recovery and Adsorption Density at Varying Oleate Concentrations, pH, and Temperatures
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
186, 225–233 (1997)
CS964501
Oleate Flotation of a Nigerian Baryte: The Relation between Flotation Recovery and Adsorption Density at Varying Oleate Concentrations, pH, and Temperatures OGBONNAYA OFOR 1
CHRISTOPHER NWOKO
AND
School of Science, Federal University of Technology, Owerri, Imo State, Nigeria Received January 23, 1996; accepted July 22, 1996
Crude Nigerian baryte was floated with sodium oleate as collector at varying oleate concentrations, pulp pH, and temperatures. Using ‘‘pure’’ baryte selected from the crude baryte, oleate adsorption densities at the baryte – water interface were determined also at varying oleate concentrations, pulp pH, and temperatures, using electrical conductivity measurements. The specific surface area of the pure baryte powder was determined using the method of adsorption of paranitrophenol in aqueous solution. The variation of oleate adsorption density and flotation recovery of baryte with oleate concentration, pulp pH, and temperature was studied. At ambient temperature ( 297C ) and pulp pH ( 8.2 ) flotation recovery of baryte increased with increasing oleate adsorption density, to a maximum recovery at the critical micelle concentration of sodium oleate. Flotation recovery increased with increasing adsorption density to about 100% recovery at maximum oleate adsorption density at pH 9.0 to 10.0 in the region of the zero point of charge ( zpc ) of the Nigerian baryte and to about 100% recovery and maximum oleate adsorption density at the temperature range 30 to 407C, also at the zpc of the baryte. It is considered that oleate chemisorbs onto the Nigerian baryte predominantly at ambient temperature and up to 407C; above 407C the chemisorbed species probably dissociate from the mineral surface while the primary mode of adsorption becomes physical processes. q 1997 Academic Press Key Words: baryte; electrical conductance; specific surface area; adsorption/desorption density; flotation recovery; oleate.
potential. Cooke et al. (2) obtained improved flotation recovery and selectivity of an iron oxide mineral from its ore at elevated temperatures, using a soap collector. Ofor (3) showed that the efficiency of separation of hematite from its high silica ore by using potassium ricinoleate in a threeingredient emulsion increased with increasing temperature; optimum separation occured within the temperature range 30 to 607C. In the present investigation, the adsorption density of sodium oleate soap on a ‘‘pure’’ Nigerian baryte surface in aqueous medium was determined at varying oleate concentrations, pulp pH, and temperatures. Desorption studies were carried out at the ambient temperature and the natural pulp pH. Electrical conductivity measurements were used to monitor oleate concentration and adsorption. The percent recovery of baryte was determined from the flotation of the crude Nigerian baryte using sodium oleate collector, at varying oleate concentrations, pulp pH values, and temperatures. The variations of oleate adsorption density and percent recovery of baryte, with oleate concentration, pulp pH, and temperature, were examined and correlated. The objective of the study is to relate mineral flotation to interfacial adsorption. EXPERIMENTAL
INTRODUCTION
Materials Several researchers have studied the adsorption of surfactants onto a mineral surface in an aqueous medium in order to bring about the selective flotation of the mineral. Shergold and Mellgren (1) showed that at a given pH, the percentage of recovery of hematite increases with increasing adsorption density of sodium dodecylsulfate, to about 100% at the surfactant concentration corresponding to zero zeta 1
To whom all correspondence should be addressed.
The baryte employed in this study occurs in the Cretaceous sandstone in the Aloshi baryte veins in the River Benue valley in the Benue State of Nigeria and was supplied by the Nigerian Mining Corporation. Thin sections and polished blocks of representative samples of the crude baryte were prepared and were studied by optical microscopy using the Vickers petrological microscope. The thin sections were studied under transmitted light; the polished surfaces were studied under reflected light (4).
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TABLE 1 Conductance and Adsorption/Desorption Density at Varying Oleate Concentrations at 297C and pH 8.2 (4) (6) (8) (9) (3) Equilibrium (5) Oleate Equilibrium Oleate (1) (2) Square-root conductance Equilibrium adsorbed (7) conductance desorbed (10) Initial oleate Initial of initial after aleate oleate per gram Adsorption after oleate per gram Desorption (11) desorption baryte 1 density 1 Degree of concentration conductance oleate adsorption concentration baryte 1 density 1 1 1003 1 102 concentration 1 102 1 1003 1003 102 1 102 1004 102 desorption 1 1002 (ms/cm) (mole/liter) (mole/g) (mmole/m2) (ms/cm) (mole/g) (mmole/m2) (10)/(7) (%) (mole/liter) (ms/cm) 1.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0 34.0
1.70 2.50 3.30 4.10 4.65 5.15 5.65 6.00 6.45 6.80 7.15 7.42 7.80 8.15 8.45 8.70 9.10 9.30
3.16 4.47 6.32 7.75 8.94 10.00 10.95 11.83 12.65 13.42 14.14 14.83 15.49 16.12 16.73 17.32 17.89 18.44
0.20 0.50 1.75 2.40 3.00 3.55 4.10 4.65 5.13 5.45 5.95 6.40 6.80 7.18 7.45 7.60 7.90 8.25
0.50 0.83 2.50 3.90 5.30 6.81 8.56 10.25 12.21 13.39 15.70 17.70 19.53 21.19 22.78 24.30 25.80 27.40
In thin section, baryte occurred as the main constituent, with quartz, goethite, and limonite as the main mineral impurities. On chemical analysis, the baryte was assayed at 85.9% BaSO4 , 8.41% SiO2 , 5.08% FeO, 0.064% Na2O, 0.042% MgO, 0.035% K2O, 0.020% CaO, 0.008% PbO, and 0.002% ZnO (analysis by the chief chemical analyst at Nigeria Petroleum Corporation R & D Division, Port Harcourt, Nigeria). The main impurities in the crude baryte are thus silica, which occurs as quartz, and iron, which occurs as goethite and limonite. A representative sample of the crude baryte was crushed to 100% minus 8 mesh (2000 mm, British Sieve Standard); fully liberated baryte particles were sorted out microscopically until a sizable quantity of pure baryte was obtained. This baryte was assayed at 99.6% BaSO4 . Adsorption tests were carried out on the pure baryte. Liberation studies were carried out on sieve portions of 100% minus 8 mesh aliquots of the crude baryte, by microscopic counting. These studies established the size-of-grind at 080 mesh (200 mm), at which particle size optimal liberation of the component minerals was obtained. Thus, to prepare the crude baryte for flotation, aliquot portions (200 g) were ground in a laboratory rod mill for 12 min at 50% solids to reduce all of the material to 080 mesh. For uniformity, the pure baryte was also reduced by dry
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0.50 1.17 1.50 2.10 2.74 3.19 3.44 3.75 3.80 4.34 4.34 4.34 4.47 4.81 5.22 5.70 6.20 6.60
1.85 4.32 5.54 7.73 10.12 11.79 12.71 13.83 13.98 15.90 15.90 15.90 16.50 17.76 19.25 21.03 22.88 24.35
0.15
—
—
—
0.90
2.25
0.83
10.74
1.40
6.25
2.31
19.60
1.55
8.01
2.95
21.30
1.80
10.56
3.90
24.50
2.00
13.30
4.92
30.90
2.15
14.80
5.47
30.80
2.25
17.00
6.27
29.80
grinding to 100% 080 mesh for adsorption tests. Flotation tests were carried out in a three-liter capacity Wemco cell, using sodium oleate as collector and methylisobutylketone (MIBK) (S.G. Å 0.8) as frother. The oleic acid and sodium hydroxide pellets were Mayer and Baker products of ‘‘Analar’’ grade, not less than 98% in purity. Solutions of Analar HCl and NaOH were used for pH adjustments. The sodium oleate solutions were prepared from oleic acid and sodium hydroxide and used immediately for the adsorption tests in order to avoid ‘‘aging’’ of the samples (5). Conductance was measured using a conductivity meter of the dipping cell type. Temperature was maintained at a constant value {0.17C using a Therm-o-Watch electronic temperature controller. All adsorption tests were carried out using distilled water; for flotation tests tap water was used. Adsorption Tests The conductance of 80 cm3 standard sodium oleate solutions was measured at different initial oleate concentrations, at the average ambient temperature of 297C, and at the natural pH of 8.2 ( Table 1, columns 1 and 2 ) . From the measurements, a standard calibration graph of conductance versus the square root of initial oleate concentration was plotted ( Fig. 1 ) .
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at the ambient temperature using 80 cm3 oleate solution at the optimum initial concentration of 0.018 M. The results are presented in Table 2. From the graph of adsorption density versus pH shown in Fig. 3, the optimum pH at which maximum adsorption occurred was determined to be pH 9.5. Finally, using the same volume (80 cm3 ) and initial concentration (0.018 M) of sodium oleate solution at the optimum pH of 9.5, adsorption density was measured at varying temperatures ranging from ambient to 807C (Table 3). The graph of adsorption density versus temperature (Fig. 4) gave the optimum temperature at 407C, at which maximum oleate adsorption occurred. Desorption Test Each solid residue after filtration at equilibrium adsorption was placed in 80 cm 3 deionized water, shaken vigorously for 1 day using a mechanical shaker, and filtered. The equilibrium conductance and hence the quantity of oleate desorbed per gram of baryte were determined. The results are presented in columns 8 and 9, respectively, of
FIG. 1. Standard calibration graph of conductance vs square root of initial oleate concentration.
One gram of the pure baryte powder was added to each 80 cm3 standard oleate solution; the mixture was shaken vigorously with a mechanical shaker for 1 h to attain equilibrium adsorption and then filtered until a clear filtrate was obtained. Preliminary tests established that equilibrium was essentially complete in less than 45 min. The equilibrium conductance of the clear filtrate was measured (column 4) and the equilibrium oleate concentration (column 5) was read from the standard calibration graph. The number of moles of oleate adsorbed at the aqueous baryte surface from the 80 cm3 oleate solution was then determined (column 6). The specific surface area of the pure baryte particles was determined to be 2.71 m2 per gram using the method of adsorption of paranitrophenol in aqueous solution, following the procedure of Gilles and Nakhwa (6). The equilibrium adsorption density was then calculated in micromoles of oleate adsorbed per m2 of baryte surface (column 7). The graph of adsorption density versus initial oleate concentration was plotted (Fig. 2), from which the optimum initial oleate concentration at which maximum adsorption occurred was determined to be 0.018 M. Adsorption density was measured at different pH values
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FIG. 2. Composite graph of oleate adsorption density and percent recovery of baryte vs initial oleate concentration.
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TABLE 2 Oleate Adsorption Density at Varying Mineral Pulp pH Values (at Ambient Temperature 297C)
(1) pH
(2) Conductance before pH adjustment and before oleate adsorption 1 102 (ms/cm), k1
(3) Conductance after pH adjustment and before oleate adsorption 1 102 (ms/cm), K2
(4) Conductance afer pH adjustment and after oleate adsorption 1 102 (ms/cm), K3
3.0 4.0 5.0 6.0 7.0 8.2 9.0 10.0 11.0 12.0
6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00
125.00 122.40 107.50 102.30 90.10 75.90 75.00 95.20 115.60 120.90
125.40 122.75 107.75 102.40 90.10 75.80 74.85 95.00 115.90 121.28
(5) Conductance due to pH ions 1 102 (ms/cm), K4 (Åk2 0 K1)
(6) Conductance due to oleate adsorption at equilibrium 1 102 (ms/cm), k5 (Åk3 0 k4)
119.00 116.40 101.50 96.30 84.10 69.90 69.00 89.20 109.60 114.90
6.40 6.35 6.25 6.10 6.00 5.90 5.85 5.80 6.30 6.38
Table 1. The desorption density, i.e., the amount of oleate desorbed at equilibrium from 1 m2 of the pure baryte surface, was computed. The results are presented in Table 1, column 10. The composite graph of adsorption density and desorption density against equilibrium concentration at the ambient temperature ( 297C ) and natural pH ( 8.2 ) is shown in Fig. 5. Flotation Tests The crude Nigerian baryte (200-g portions), pulverized to 100% minus 80 mesh, was floated in a 3-liter Wemco cell using sodium oleate as collector and methylisobutylketone (MIBK) as frother. Flotation was carried out first at the natural pH of 8.2 and the ambient temperature of 297C, using 80 cm3 of sodium oleate and collector solution at varying initial concentrations. The results are shown in Table 4. Figure 2 shows the graph of the percentage of recovery of baryte versus initial oleate concentration. From this graph the optimum oleate collector concentration of 0.018 mole/ liter or 2.2 kg per ton of baryte was obtained, which gave the maximum baryte recovery of 99.3% in concentrate. Using this optimum oleate dosage, flotation was repeated at varying mineral pulp pH values at ambient temperature. Table 5 is a presentation of the results. The graph of the percentage of recovery versus pH is shown in Fig. 3. This graph shows that the maximum recovery of 99.9% occured at pH 10. Finally, using the optimum collector dosage and pH value, flotation was carried out at varying mineral pulp temperatures. The results are shown in Table 6. Figure 4 shows the graph of percentage of recovery versus temperature.
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(7) Equilibrium concentration 1 1003 (mole/g)
(8) Oleate adsorbed per gram baryte 1 1003 (mole/g)
(9) Adsorption density 1102 (mmole/m2)
15.90 15.47 15.05 14.38 14.00 13.39 13.16 13.28 15.33 15.61
2.10 2.53 2.95 3.62 4.00 4.61 4.84 4.72 2.67 2.39
7.74 9.34 10.89 13.35 14.76 15.90 17.85 17.86 9.86 8.81
From Fig. 4 the maximum flotation recovery of 99.9% was obtained at 30 to 407C. For all flotation tests, pulp density was fixed at 10% solids; MIBK dosage was constant at 4 kg per ton of crude baryte; the pulp was conditioned for 10 min at an impeller agitation speed of 1520 rpm; and flotation was performed for 5 min. For each flotation test, 80 cm3 oleate solution was used. Each concentrate was dried at 1107C, weighed, and analyzed for its BaSO4 content. The percentage of recovery of baryte in each concentrate was calculated. RESULTS
Adsorption density increased rapidly with the increase in equilibrium oleate concentration up to the concentration value of 13.39 1 10 03 M at the adsorption density value of 15.9 1 10 2 mmole/m 2 (Fig. 5). Beyond this oleate concentration value, adsorption density remained constant as oleate concentration increased. This value of 0.013 mole/liter is thus the critical micelle concentration (CMC) of sodium oleate, and 15.9 1 10 2 mmole/m 2 is the quantity of oleate required to attain complete monolayer coverage of the Nigerian baryte surface at ambient temperature and pH. Figure 2 shows that flotation recovery of baryte increased with increasing adsorption density of sodium oleate to a maximum at the initial collector concentration of 0.018 mole/liter. Figure 3 shows that at ambient temperature (297C average), as pH increased adsorption density increased rapidly to a maximum value of 17.85 1 10 2 mmole/m 2 at pH 9.5, representing 26.7% oleate extraction per gram of baryte, and
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rapidly to a maximum recovery of 99.9% at pH 10.0, in the region of the zpc, and decreased with further increase in pH. Figure 3 establishes that flotation recovery of baryte increased with increasing adsorption density of sodium oleate to about 100% recovery at maximum oleate adsorption density, at pH 9.5–10.0, corresponding to zero-point-of-charge (zpc) of the baryte. Figure 4 shows that at the optimum adsorption pH of 9.5, adsorption density increased with increase in temperature to a maximum value of 20.75 1 10 2 mmole/m 2 at 407C, representing 31.2% oleate extraction per gram of the pure baryte. Beyond this temperature level, adsorption density decreased even more slowly with further increase in temperature. The rise from 26.7% oleate extraction at 297C to 31.2% oleate extraction at 407C at the same pH emphasizes the significance of increased temperature in oleate adsorption onto the aqueous baryte surface. Like adsorption density, percent recovery increased with increase in temperature, to a maximum recovery of 99.9% at 30 to 407C. Beyond 407C, percent recovery decreased as temperature increased (Fig. 4). Figure 4 establishes that flotation recovery of baryte increased with increasing adsorption density of sodium oleate to about 100% recovery and maximum adsorption density at the temperature range of 30 to 407C, at the zpc of baryte. ANALYSIS OF RESULTS
FIG. 3. Composite graph of oleate adsorption density and percent recovery of baryte vs pulp pH.
then decreased fairly slowly with further increase in pH. This pH (9.5) of maximum oleate adsorption is thus the zero point of charge (zpc) of the Nigerian baryte. Also, from Fig. 3, the percent recovery of baryte increased equally
Free Energy of Adsorption The adsorption density versus pH isotherm (Fig. 3) shows that oleate adsorption continued appreciably several pH units beyond the zpc (pH 9.5) of the pure Nigerian baryte. This
TABLE 3 Oleate Adsorption Density at Varying Pulp Temperatures (at Optimum pH 9.5)
(1) Temperature (7C)
(2) Conductance before pH adjustment to pH 9.5 optimum 1 102 (ms/cm), K1
(3) Conductance after pH adjustment and before oleate adsorption 1 102 (ms/cm), K2
(4) Conductance after pH adjustment and after oleate adsorption 1 102 (ms/cm), K3
29 35 40 45 50 55 60 80
6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00
7.50 7.50 7.50 7.50 7.50 7.50 7.50 7.50
6.80 6.70 6.65 7.05 7.20 7.40 7.60 7.75
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(5) Conductance due to pH ions 1 102 (ms/cm), k4 (Åk2 0 K1)
(6) Equilibrium conductance due to unadsorbed oleate in solution 1 102 (ms/cm), k5 (Åk3 0 k4)
(7) Oleate adsorbed per gram baryte 1 1004 (mole/g)
(8) Adsorption density 1 102 (mmole/m2)
1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50
5.30 5.20 5.15 5.55 5.70 5.90 6.10 6.25
47.75 53.44 56.23 38.98 32.98 23.75 17.44 7.73
17.85 19.72 20.75 14.39 12.17 9.76 6.43 2.85
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The sign and magnitude of the DG 0ads values show that oleate adsorption at the aqueous baryte surface is feasible at both acid and alkali pH. The feasibility increases with increasing pH, to a maximum in the region of the zpc of the Nigerian baryte, and decreases at higher pH values. The similarity in the variation pattern of the percentage of recovery and of adsorption density at varying pH and temperatures shows that mineral flotation is an interfacial phenomenon; it proceeds through the adsorption of the collector ions onto the aqueous mineral surface. The oleate flotation of the Nigerian baryte at ambient temperature (297C) is feasible at both acid and alkali pH and reaches a maximum at the zpc of the baryte. Beyond this zpc region, DGads as well as flotation recovery of baryte decrease. The observed variation of DGads with pH indicates that some definite change must be occuring in the nature of the adsorption process. Effect of Temperature Houwing and Salomon (8) have stated that while physical adsorption decreases with an increase in temperature, chemisorption increases as temperature increases. Chemisorption would thus be the dominant mode of adsorption up to 407C.
FIG. 4. Composite graph of oleate adsorption density and percent recovery vs pulp temperature.
is evidence of chemisorption (7). Adsorption at the ambient temperature (297C) is thus specific, in the Stern plane, involving chemical bond formation between the oleate ion and the baryte surface. The Stern–Graham equation is applied to assess the magnitude of the standard free energy of adsorption ( DG 0ads ), G Å zrc exp
( DGads ) , RT
where G is the adsorption density in mole/cm2 , z is the valency of the adsorbed ion, r is the effective radius of the adsorbed ion (5.2 1 10 08 cm for the carboxylate ion), c is the equilibrium concentration in mole/cm3 , R is the gas constant, and T is the absolute temperature. Applied to conditions involving oleate adsorption at varying pH values at the ambient temperature of 297C (Table 2), the DG 0ads values presented in Table 7 were obtained. Table 7 shows that DG 0ads is negative at the pH range studied; it increases in magnitude as pH increases, to a maximum at pH 9.0 to 10.0, and falls rapidly at higher pH values.
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FIG. 5. Oleate adsorption and desorption isotherms at 297C (ambient) (pH 8.2).
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TABLE 4 Flotation Recovery of Baryte at Varying Oleate Collector Concentrations at Ambient Temperature (297C) and Natural Pulp pH (8.2)
Serial number
Collector (sodium oleate) concentration in moles per liter (M)
Weight of concentrate (g)
Weight of concentrate (%)
Assay of concentrate for BaSO4 (%)
Weight of feed (g)
Assay of feed for BaSO4 (%)
Recovery of BaSO4 (%)
1 2 3 4 5 6 7 8 9
0.001 0.002 0.006 0.010 0.014 0.018 0.022 0.026 0.030
140.07 159.75 166.83 169.41 171.80 175.30 165.00 152.17 145.05
70.04 79.88 83.42 84.71 85.90 87.65 82.50 76.09 72.53
90.50 92.90 94.10 95.40 96.30 97.30 95.90 93.20 90.00
200 200 200 200 200 200 200 200 200
85.90 85.90 85.90 85.90 85.90 85.90 85.90 85.90 85.90
73.79 86.38 91.38 94.07 96.30 99.28 92.10 82.55 75.99
Beyond 407C the chemisorbed species probably dissociate from the surface while the principal mode of adsorption becomes physical processes. This would account for the fall in adsorption density and consequently in flotation recovery at temperatures above 407C. Increase in temperature from 30 to 407C had practically no effect on flotation recovery, which was about 100% at that temperature range in the region of the zpc of the Nigerian baryte. This is at variance with the adsorption test results, which showed that oleate extraction increased from 26.7% at 297C to 31.2% at 407C, signifying that the increase in temperature from 30 to 407C had a definite effect on adsorption density. The influence of hydrometal ions inherent in the pulp, besides pure baryte and aqueous ions, could have brought about the variance. Nature of Adsorption The adsorption isotherm of sodium oleate soap on the Nigerian baryte at 297C is given in Fig. 5. The initial slope
is convex to the concentration axis; it defines an increase in adsorption with increase in the external solution concentration, up to 15.9 1 10 2 mmole/m 2 adsorption density. The upper part of the curve is a plateau which indicates surface saturation in which all possible sites in the original baryte surface are filled (complete monolayer coverage). The isotherm is thus an L2 curve according to the classification of Gilles et al. (9). This group of isotherms is generally referred to as the Langmuir-type of isotherm. Since the oleate ions are adsorbed on baryte as single monofunctional units, there appears to be marked localization of the forces of attraction over the carboxylate group leading to very strong interaction (chemisorption) with baryte at this part of the ion. Figure 5 further presents the desorption isotherm of the oleate soap on the Nigerian baryte, also at 297C (Table 1, column 10 vs column 5). The isotherm, and in particular the degree of desorption presented in column 11 of Table 1, show that a small amount of the adsorbed oleate desorbed.
TABLE 5 Flotation Recovery of Baryte at Varying pH Values (Initial Collector Concentration 0.018 M; Temperature of Pulp 297C (Ambient)
Serial number
pH
Weight of concentrate (g)
1 2 3 4 5 6 7 8 9
3 5 6 7 8 9 10 11 12
0 150.20 158.70 148.60 168.05 172.44 173.75 168.70 160.22
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Weight of concentrate (%)
BaSO4 assay of concentrate (%)
Weight of feed (g)
BaSO4 assay of feed (%)
Recovery of BaSO4 (%)
0 75.10 79.35 74.30 84.03 86.22 86.88 84.35 80.11
0 89.00 92.10 94.00 95.30 98.00 98.80 97.90 95.60
200 200 200 200 200 200 200 200 200
85.90 85.90 85.90 85.90 85.90 85.90 85.90 85.90 85.90
0 77.81 85.17 81.31 93.22 98.37 99.86 96.13 89.16
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TABLE 6 Flotation Recovery of Baryte at Varying Temperatures (Initial Collector Concentration 0.018 M; pH 10)
Serial number
Collector concentration (M)
1 2 3 4 5 6
0.018 0.018 0.018 0.018 0.018 0.018
Temperature (7C)
Weight of concentrate (g)
Weight of concentrate (%)
Assay of concentrate for BasO4 (%)
Weight of feed (g)
Assay of feed for BaSO4 (%)
Recovery of BaSO4 (%)
29 40 50 60 70 80
173.75 175.60 166.50 156.65 145.91 130.40
86.88 87.80 83.25 78.33 72.96 65.20
98.80 97.70 96.70 96.00 95.40 94.90
200 200 200 200 200 200
85.90 85.90 85.90 85.90 85.90 85.90
99.86 99.92 93.72 87.53 81.02 72.03
This further confirms that at the natural pH of 8.2 at ambient temperature (297C) oleate is chemisorbed onto the aqueous baryte surface. However, that desorption occured at all indicates that the physical adsorption process contributed, even if in a small measure, to the overall adsorption process. The graphical summary of equilibrium concentrations of oleate species in solution at various pH presented by Jung et al. (10) shows that within the pH range 3–12 the active 0 oleate species in solution comprise 01 0 , 01 20 2 , H01 2 , and H01. Jung et al. hold the view that H01 molecules adsorb specifically, in the Stern layer, without bond formation with baryte. These researchers consider that the ionic species would most probably chemisorb at the baryte surface through bond formation; barium cleates are probably formed. The graphical summary under reference further shows that the concentration of the active oleate species in solution reach the maximum at about pH 8.0 and begin to decrease at pH 9–10. The latter pH is the region of the zpc of baryte. This would account for the decrease in adsorption density and percentage of recovery beyond pH 9.5 to 10.0. TABLE 7 DG0ads at Various Baryte Pulp pH Values
(1) pH
(2) Equilibrium oleate concentration 1 1003 (mole/liter)
(3) Adsorption density 1 102 (mmol/ m2 )
(4) DG0ads (Kcal/mole)
3.0 4.0 5.0 6.0 7.0 8.2 9.0 10.0 11.0 12.0
15.90 15.47 15.05 14.38 14.00 13.39 13.16 13.28 15.33 15.61
7.74 9.34 10.89 13.35 14.76 15.90 17.85 17.86 9.86 8.81
06.90 07.04 07.14 07.30 07.37 07.44 07.53 07.52 07.07 07.00
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A further reason for the decrease would be that the preponderance of OH 0 ions above pH 10 would lead to OH 0 ions competing with oleate ions for adsorption at the mineral surface. CONCLUSION
Mineral flotation is an interfacial phenomenon involving the adsorption of collector ions onto the aqueous mineral surface. Oleate flotation of Nigerian baryte is thermodynamically feasible at both acid and alkali pH. Oleate adsorption density and flotation recovery of Nigerian baryte increased with increasing pH up to the zero-point-of-charge (zpc) of the baryte (pH 9.5). Beyond pH 10, the percentage of recovery and adsorption density decreased. Flotation recovery of baryte increased with increasing oleate adsorption density to a maximum at the initial oleate concentration of 0.018 mole/liter at the ambient temperature of 297C and natural pH of 8.2, at the critical micelle concentration of 0.013 mole/liter for sodium oleate. Flotation recovery increased with increasing adsorption density to about 100% recovery at maximum oleate adsorption density, at pH 9.0 to 10.0 in the region of the zpc of the Nigerian baryte, and to about 100% recovery and maximum oleate adsorption density at the temperature range 30 to 407C, also at the zpc of the baryte. Oleate chemisorbs onto the aqueous baryte surface at ambient temperature and up to 407C, with a minor contribution of physical adsorption. Beyond 407C the chemisorbed species probably dissociates from the baryte surface, while the principal mode of adsorption becomes physical processes. Among the stable oleate species identified in solution, Hol is considered to adsorb specifically in the Stern layer without 0 bond formation; the ionic species 01 0 , 01 20 2 , and HO1 2 would chemisorb most probably through bond formation; barium oleates are probably formed at the mineral surface. Factors that would account for the decrease in adsorption
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density and percent recovery beyond pH 9.5 to 10.0 would include: (i) the concentration of the oleate species in solution beginning to decrease in the pH region 9–10; (ii) the preponderance of OH 0 ions above pH 10 would lead to OH 0 ions competing with oleate ions for adsorption at the mineral surface. REFERENCES 1. Shergold, H. L., and Mellgren, O., Trans. Inst. Miner. Metall. C 78, 121–132 (1969).
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2. Cooke, S. R. B., Iwasaki, I., and Choi, A. S., Trans. AIME 217, 76– 83 (1960). 3. Ofor, O., J. Miner. Metall. Process., August 1 (1992). 4. David, J., ‘‘Ore Microscopy and Ore Petrography,’’ 297–303. Wiley, New York, 1982. 5. Han, K. N., Healy, T. W., and Fuerstenau, D. W., J. Colloid Interface Sci. 44, 407–414 (1973). 6. Gilles, C. H., and Nakhwa, S. N., J. Appl. Chem. 12, 266–273 (1962). 7. Fuerstenau, D. W., in ‘‘Chemistry of Biosurfaces’’ (I. M. L. Hair, Ed.), Vol. 1, p. 143. Dekker, New York, 1971. 8. Houwing, R., and Salomon, G., Eds., ‘‘Adhesion and Adhesives,’’ 2nd ed., Vol. 1. Elsevier, Amsterdam/New York, 1965. 9. Gilles, C. H., MacEwan, T. H., Nakhwa, S. N., and Smith, D., J. Chem. Soc., 3973–3993 (1960). 10. Jung, R. F., James, R. O., and Healy, T. W. J. Colloid Interface Sci. 118(2), 463–472 ( 1987).
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CS964501
Oleate Flotation of a Nigerian Baryte: The Relation between Flotation Recovery and Adsorption Density at Varying Oleate Concentrations, pH, and Temperatures OGBONNAYA OFOR 1
CHRISTOPHER NWOKO
AND
School of Science, Federal University of Technology, Owerri, Imo State, Nigeria Received January 23, 1996; accepted July 22, 1996
Crude Nigerian baryte was floated with sodium oleate as collector at varying oleate concentrations, pulp pH, and temperatures. Using ‘‘pure’’ baryte selected from the crude baryte, oleate adsorption densities at the baryte – water interface were determined also at varying oleate concentrations, pulp pH, and temperatures, using electrical conductivity measurements. The specific surface area of the pure baryte powder was determined using the method of adsorption of paranitrophenol in aqueous solution. The variation of oleate adsorption density and flotation recovery of baryte with oleate concentration, pulp pH, and temperature was studied. At ambient temperature ( 297C ) and pulp pH ( 8.2 ) flotation recovery of baryte increased with increasing oleate adsorption density, to a maximum recovery at the critical micelle concentration of sodium oleate. Flotation recovery increased with increasing adsorption density to about 100% recovery at maximum oleate adsorption density at pH 9.0 to 10.0 in the region of the zero point of charge ( zpc ) of the Nigerian baryte and to about 100% recovery and maximum oleate adsorption density at the temperature range 30 to 407C, also at the zpc of the baryte. It is considered that oleate chemisorbs onto the Nigerian baryte predominantly at ambient temperature and up to 407C; above 407C the chemisorbed species probably dissociate from the mineral surface while the primary mode of adsorption becomes physical processes. q 1997 Academic Press Key Words: baryte; electrical conductance; specific surface area; adsorption/desorption density; flotation recovery; oleate.
potential. Cooke et al. (2) obtained improved flotation recovery and selectivity of an iron oxide mineral from its ore at elevated temperatures, using a soap collector. Ofor (3) showed that the efficiency of separation of hematite from its high silica ore by using potassium ricinoleate in a threeingredient emulsion increased with increasing temperature; optimum separation occured within the temperature range 30 to 607C. In the present investigation, the adsorption density of sodium oleate soap on a ‘‘pure’’ Nigerian baryte surface in aqueous medium was determined at varying oleate concentrations, pulp pH, and temperatures. Desorption studies were carried out at the ambient temperature and the natural pulp pH. Electrical conductivity measurements were used to monitor oleate concentration and adsorption. The percent recovery of baryte was determined from the flotation of the crude Nigerian baryte using sodium oleate collector, at varying oleate concentrations, pulp pH values, and temperatures. The variations of oleate adsorption density and percent recovery of baryte, with oleate concentration, pulp pH, and temperature, were examined and correlated. The objective of the study is to relate mineral flotation to interfacial adsorption. EXPERIMENTAL
INTRODUCTION
Materials Several researchers have studied the adsorption of surfactants onto a mineral surface in an aqueous medium in order to bring about the selective flotation of the mineral. Shergold and Mellgren (1) showed that at a given pH, the percentage of recovery of hematite increases with increasing adsorption density of sodium dodecylsulfate, to about 100% at the surfactant concentration corresponding to zero zeta 1
To whom all correspondence should be addressed.
The baryte employed in this study occurs in the Cretaceous sandstone in the Aloshi baryte veins in the River Benue valley in the Benue State of Nigeria and was supplied by the Nigerian Mining Corporation. Thin sections and polished blocks of representative samples of the crude baryte were prepared and were studied by optical microscopy using the Vickers petrological microscope. The thin sections were studied under transmitted light; the polished surfaces were studied under reflected light (4).
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TABLE 1 Conductance and Adsorption/Desorption Density at Varying Oleate Concentrations at 297C and pH 8.2 (4) (6) (8) (9) (3) Equilibrium (5) Oleate Equilibrium Oleate (1) (2) Square-root conductance Equilibrium adsorbed (7) conductance desorbed (10) Initial oleate Initial of initial after aleate oleate per gram Adsorption after oleate per gram Desorption (11) desorption baryte 1 density 1 Degree of concentration conductance oleate adsorption concentration baryte 1 density 1 1 1003 1 102 concentration 1 102 1 1003 1003 102 1 102 1004 102 desorption 1 1002 (ms/cm) (mole/liter) (mole/g) (mmole/m2) (ms/cm) (mole/g) (mmole/m2) (10)/(7) (%) (mole/liter) (ms/cm) 1.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0 34.0
1.70 2.50 3.30 4.10 4.65 5.15 5.65 6.00 6.45 6.80 7.15 7.42 7.80 8.15 8.45 8.70 9.10 9.30
3.16 4.47 6.32 7.75 8.94 10.00 10.95 11.83 12.65 13.42 14.14 14.83 15.49 16.12 16.73 17.32 17.89 18.44
0.20 0.50 1.75 2.40 3.00 3.55 4.10 4.65 5.13 5.45 5.95 6.40 6.80 7.18 7.45 7.60 7.90 8.25
0.50 0.83 2.50 3.90 5.30 6.81 8.56 10.25 12.21 13.39 15.70 17.70 19.53 21.19 22.78 24.30 25.80 27.40
In thin section, baryte occurred as the main constituent, with quartz, goethite, and limonite as the main mineral impurities. On chemical analysis, the baryte was assayed at 85.9% BaSO4 , 8.41% SiO2 , 5.08% FeO, 0.064% Na2O, 0.042% MgO, 0.035% K2O, 0.020% CaO, 0.008% PbO, and 0.002% ZnO (analysis by the chief chemical analyst at Nigeria Petroleum Corporation R & D Division, Port Harcourt, Nigeria). The main impurities in the crude baryte are thus silica, which occurs as quartz, and iron, which occurs as goethite and limonite. A representative sample of the crude baryte was crushed to 100% minus 8 mesh (2000 mm, British Sieve Standard); fully liberated baryte particles were sorted out microscopically until a sizable quantity of pure baryte was obtained. This baryte was assayed at 99.6% BaSO4 . Adsorption tests were carried out on the pure baryte. Liberation studies were carried out on sieve portions of 100% minus 8 mesh aliquots of the crude baryte, by microscopic counting. These studies established the size-of-grind at 080 mesh (200 mm), at which particle size optimal liberation of the component minerals was obtained. Thus, to prepare the crude baryte for flotation, aliquot portions (200 g) were ground in a laboratory rod mill for 12 min at 50% solids to reduce all of the material to 080 mesh. For uniformity, the pure baryte was also reduced by dry
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0.50 1.17 1.50 2.10 2.74 3.19 3.44 3.75 3.80 4.34 4.34 4.34 4.47 4.81 5.22 5.70 6.20 6.60
1.85 4.32 5.54 7.73 10.12 11.79 12.71 13.83 13.98 15.90 15.90 15.90 16.50 17.76 19.25 21.03 22.88 24.35
0.15
—
—
—
0.90
2.25
0.83
10.74
1.40
6.25
2.31
19.60
1.55
8.01
2.95
21.30
1.80
10.56
3.90
24.50
2.00
13.30
4.92
30.90
2.15
14.80
5.47
30.80
2.25
17.00
6.27
29.80
grinding to 100% 080 mesh for adsorption tests. Flotation tests were carried out in a three-liter capacity Wemco cell, using sodium oleate as collector and methylisobutylketone (MIBK) (S.G. Å 0.8) as frother. The oleic acid and sodium hydroxide pellets were Mayer and Baker products of ‘‘Analar’’ grade, not less than 98% in purity. Solutions of Analar HCl and NaOH were used for pH adjustments. The sodium oleate solutions were prepared from oleic acid and sodium hydroxide and used immediately for the adsorption tests in order to avoid ‘‘aging’’ of the samples (5). Conductance was measured using a conductivity meter of the dipping cell type. Temperature was maintained at a constant value {0.17C using a Therm-o-Watch electronic temperature controller. All adsorption tests were carried out using distilled water; for flotation tests tap water was used. Adsorption Tests The conductance of 80 cm3 standard sodium oleate solutions was measured at different initial oleate concentrations, at the average ambient temperature of 297C, and at the natural pH of 8.2 ( Table 1, columns 1 and 2 ) . From the measurements, a standard calibration graph of conductance versus the square root of initial oleate concentration was plotted ( Fig. 1 ) .
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at the ambient temperature using 80 cm3 oleate solution at the optimum initial concentration of 0.018 M. The results are presented in Table 2. From the graph of adsorption density versus pH shown in Fig. 3, the optimum pH at which maximum adsorption occurred was determined to be pH 9.5. Finally, using the same volume (80 cm3 ) and initial concentration (0.018 M) of sodium oleate solution at the optimum pH of 9.5, adsorption density was measured at varying temperatures ranging from ambient to 807C (Table 3). The graph of adsorption density versus temperature (Fig. 4) gave the optimum temperature at 407C, at which maximum oleate adsorption occurred. Desorption Test Each solid residue after filtration at equilibrium adsorption was placed in 80 cm 3 deionized water, shaken vigorously for 1 day using a mechanical shaker, and filtered. The equilibrium conductance and hence the quantity of oleate desorbed per gram of baryte were determined. The results are presented in columns 8 and 9, respectively, of
FIG. 1. Standard calibration graph of conductance vs square root of initial oleate concentration.
One gram of the pure baryte powder was added to each 80 cm3 standard oleate solution; the mixture was shaken vigorously with a mechanical shaker for 1 h to attain equilibrium adsorption and then filtered until a clear filtrate was obtained. Preliminary tests established that equilibrium was essentially complete in less than 45 min. The equilibrium conductance of the clear filtrate was measured (column 4) and the equilibrium oleate concentration (column 5) was read from the standard calibration graph. The number of moles of oleate adsorbed at the aqueous baryte surface from the 80 cm3 oleate solution was then determined (column 6). The specific surface area of the pure baryte particles was determined to be 2.71 m2 per gram using the method of adsorption of paranitrophenol in aqueous solution, following the procedure of Gilles and Nakhwa (6). The equilibrium adsorption density was then calculated in micromoles of oleate adsorbed per m2 of baryte surface (column 7). The graph of adsorption density versus initial oleate concentration was plotted (Fig. 2), from which the optimum initial oleate concentration at which maximum adsorption occurred was determined to be 0.018 M. Adsorption density was measured at different pH values
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FIG. 2. Composite graph of oleate adsorption density and percent recovery of baryte vs initial oleate concentration.
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TABLE 2 Oleate Adsorption Density at Varying Mineral Pulp pH Values (at Ambient Temperature 297C)
(1) pH
(2) Conductance before pH adjustment and before oleate adsorption 1 102 (ms/cm), k1
(3) Conductance after pH adjustment and before oleate adsorption 1 102 (ms/cm), K2
(4) Conductance afer pH adjustment and after oleate adsorption 1 102 (ms/cm), K3
3.0 4.0 5.0 6.0 7.0 8.2 9.0 10.0 11.0 12.0
6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00
125.00 122.40 107.50 102.30 90.10 75.90 75.00 95.20 115.60 120.90
125.40 122.75 107.75 102.40 90.10 75.80 74.85 95.00 115.90 121.28
(5) Conductance due to pH ions 1 102 (ms/cm), K4 (Åk2 0 K1)
(6) Conductance due to oleate adsorption at equilibrium 1 102 (ms/cm), k5 (Åk3 0 k4)
119.00 116.40 101.50 96.30 84.10 69.90 69.00 89.20 109.60 114.90
6.40 6.35 6.25 6.10 6.00 5.90 5.85 5.80 6.30 6.38
Table 1. The desorption density, i.e., the amount of oleate desorbed at equilibrium from 1 m2 of the pure baryte surface, was computed. The results are presented in Table 1, column 10. The composite graph of adsorption density and desorption density against equilibrium concentration at the ambient temperature ( 297C ) and natural pH ( 8.2 ) is shown in Fig. 5. Flotation Tests The crude Nigerian baryte (200-g portions), pulverized to 100% minus 80 mesh, was floated in a 3-liter Wemco cell using sodium oleate as collector and methylisobutylketone (MIBK) as frother. Flotation was carried out first at the natural pH of 8.2 and the ambient temperature of 297C, using 80 cm3 of sodium oleate and collector solution at varying initial concentrations. The results are shown in Table 4. Figure 2 shows the graph of the percentage of recovery of baryte versus initial oleate concentration. From this graph the optimum oleate collector concentration of 0.018 mole/ liter or 2.2 kg per ton of baryte was obtained, which gave the maximum baryte recovery of 99.3% in concentrate. Using this optimum oleate dosage, flotation was repeated at varying mineral pulp pH values at ambient temperature. Table 5 is a presentation of the results. The graph of the percentage of recovery versus pH is shown in Fig. 3. This graph shows that the maximum recovery of 99.9% occured at pH 10. Finally, using the optimum collector dosage and pH value, flotation was carried out at varying mineral pulp temperatures. The results are shown in Table 6. Figure 4 shows the graph of percentage of recovery versus temperature.
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(7) Equilibrium concentration 1 1003 (mole/g)
(8) Oleate adsorbed per gram baryte 1 1003 (mole/g)
(9) Adsorption density 1102 (mmole/m2)
15.90 15.47 15.05 14.38 14.00 13.39 13.16 13.28 15.33 15.61
2.10 2.53 2.95 3.62 4.00 4.61 4.84 4.72 2.67 2.39
7.74 9.34 10.89 13.35 14.76 15.90 17.85 17.86 9.86 8.81
From Fig. 4 the maximum flotation recovery of 99.9% was obtained at 30 to 407C. For all flotation tests, pulp density was fixed at 10% solids; MIBK dosage was constant at 4 kg per ton of crude baryte; the pulp was conditioned for 10 min at an impeller agitation speed of 1520 rpm; and flotation was performed for 5 min. For each flotation test, 80 cm3 oleate solution was used. Each concentrate was dried at 1107C, weighed, and analyzed for its BaSO4 content. The percentage of recovery of baryte in each concentrate was calculated. RESULTS
Adsorption density increased rapidly with the increase in equilibrium oleate concentration up to the concentration value of 13.39 1 10 03 M at the adsorption density value of 15.9 1 10 2 mmole/m 2 (Fig. 5). Beyond this oleate concentration value, adsorption density remained constant as oleate concentration increased. This value of 0.013 mole/liter is thus the critical micelle concentration (CMC) of sodium oleate, and 15.9 1 10 2 mmole/m 2 is the quantity of oleate required to attain complete monolayer coverage of the Nigerian baryte surface at ambient temperature and pH. Figure 2 shows that flotation recovery of baryte increased with increasing adsorption density of sodium oleate to a maximum at the initial collector concentration of 0.018 mole/liter. Figure 3 shows that at ambient temperature (297C average), as pH increased adsorption density increased rapidly to a maximum value of 17.85 1 10 2 mmole/m 2 at pH 9.5, representing 26.7% oleate extraction per gram of baryte, and
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rapidly to a maximum recovery of 99.9% at pH 10.0, in the region of the zpc, and decreased with further increase in pH. Figure 3 establishes that flotation recovery of baryte increased with increasing adsorption density of sodium oleate to about 100% recovery at maximum oleate adsorption density, at pH 9.5–10.0, corresponding to zero-point-of-charge (zpc) of the baryte. Figure 4 shows that at the optimum adsorption pH of 9.5, adsorption density increased with increase in temperature to a maximum value of 20.75 1 10 2 mmole/m 2 at 407C, representing 31.2% oleate extraction per gram of the pure baryte. Beyond this temperature level, adsorption density decreased even more slowly with further increase in temperature. The rise from 26.7% oleate extraction at 297C to 31.2% oleate extraction at 407C at the same pH emphasizes the significance of increased temperature in oleate adsorption onto the aqueous baryte surface. Like adsorption density, percent recovery increased with increase in temperature, to a maximum recovery of 99.9% at 30 to 407C. Beyond 407C, percent recovery decreased as temperature increased (Fig. 4). Figure 4 establishes that flotation recovery of baryte increased with increasing adsorption density of sodium oleate to about 100% recovery and maximum adsorption density at the temperature range of 30 to 407C, at the zpc of baryte. ANALYSIS OF RESULTS
FIG. 3. Composite graph of oleate adsorption density and percent recovery of baryte vs pulp pH.
then decreased fairly slowly with further increase in pH. This pH (9.5) of maximum oleate adsorption is thus the zero point of charge (zpc) of the Nigerian baryte. Also, from Fig. 3, the percent recovery of baryte increased equally
Free Energy of Adsorption The adsorption density versus pH isotherm (Fig. 3) shows that oleate adsorption continued appreciably several pH units beyond the zpc (pH 9.5) of the pure Nigerian baryte. This
TABLE 3 Oleate Adsorption Density at Varying Pulp Temperatures (at Optimum pH 9.5)
(1) Temperature (7C)
(2) Conductance before pH adjustment to pH 9.5 optimum 1 102 (ms/cm), K1
(3) Conductance after pH adjustment and before oleate adsorption 1 102 (ms/cm), K2
(4) Conductance after pH adjustment and after oleate adsorption 1 102 (ms/cm), K3
29 35 40 45 50 55 60 80
6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00
7.50 7.50 7.50 7.50 7.50 7.50 7.50 7.50
6.80 6.70 6.65 7.05 7.20 7.40 7.60 7.75
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(5) Conductance due to pH ions 1 102 (ms/cm), k4 (Åk2 0 K1)
(6) Equilibrium conductance due to unadsorbed oleate in solution 1 102 (ms/cm), k5 (Åk3 0 k4)
(7) Oleate adsorbed per gram baryte 1 1004 (mole/g)
(8) Adsorption density 1 102 (mmole/m2)
1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50
5.30 5.20 5.15 5.55 5.70 5.90 6.10 6.25
47.75 53.44 56.23 38.98 32.98 23.75 17.44 7.73
17.85 19.72 20.75 14.39 12.17 9.76 6.43 2.85
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The sign and magnitude of the DG 0ads values show that oleate adsorption at the aqueous baryte surface is feasible at both acid and alkali pH. The feasibility increases with increasing pH, to a maximum in the region of the zpc of the Nigerian baryte, and decreases at higher pH values. The similarity in the variation pattern of the percentage of recovery and of adsorption density at varying pH and temperatures shows that mineral flotation is an interfacial phenomenon; it proceeds through the adsorption of the collector ions onto the aqueous mineral surface. The oleate flotation of the Nigerian baryte at ambient temperature (297C) is feasible at both acid and alkali pH and reaches a maximum at the zpc of the baryte. Beyond this zpc region, DGads as well as flotation recovery of baryte decrease. The observed variation of DGads with pH indicates that some definite change must be occuring in the nature of the adsorption process. Effect of Temperature Houwing and Salomon (8) have stated that while physical adsorption decreases with an increase in temperature, chemisorption increases as temperature increases. Chemisorption would thus be the dominant mode of adsorption up to 407C.
FIG. 4. Composite graph of oleate adsorption density and percent recovery vs pulp temperature.
is evidence of chemisorption (7). Adsorption at the ambient temperature (297C) is thus specific, in the Stern plane, involving chemical bond formation between the oleate ion and the baryte surface. The Stern–Graham equation is applied to assess the magnitude of the standard free energy of adsorption ( DG 0ads ), G Å zrc exp
( DGads ) , RT
where G is the adsorption density in mole/cm2 , z is the valency of the adsorbed ion, r is the effective radius of the adsorbed ion (5.2 1 10 08 cm for the carboxylate ion), c is the equilibrium concentration in mole/cm3 , R is the gas constant, and T is the absolute temperature. Applied to conditions involving oleate adsorption at varying pH values at the ambient temperature of 297C (Table 2), the DG 0ads values presented in Table 7 were obtained. Table 7 shows that DG 0ads is negative at the pH range studied; it increases in magnitude as pH increases, to a maximum at pH 9.0 to 10.0, and falls rapidly at higher pH values.
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FIG. 5. Oleate adsorption and desorption isotherms at 297C (ambient) (pH 8.2).
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TABLE 4 Flotation Recovery of Baryte at Varying Oleate Collector Concentrations at Ambient Temperature (297C) and Natural Pulp pH (8.2)
Serial number
Collector (sodium oleate) concentration in moles per liter (M)
Weight of concentrate (g)
Weight of concentrate (%)
Assay of concentrate for BaSO4 (%)
Weight of feed (g)
Assay of feed for BaSO4 (%)
Recovery of BaSO4 (%)
1 2 3 4 5 6 7 8 9
0.001 0.002 0.006 0.010 0.014 0.018 0.022 0.026 0.030
140.07 159.75 166.83 169.41 171.80 175.30 165.00 152.17 145.05
70.04 79.88 83.42 84.71 85.90 87.65 82.50 76.09 72.53
90.50 92.90 94.10 95.40 96.30 97.30 95.90 93.20 90.00
200 200 200 200 200 200 200 200 200
85.90 85.90 85.90 85.90 85.90 85.90 85.90 85.90 85.90
73.79 86.38 91.38 94.07 96.30 99.28 92.10 82.55 75.99
Beyond 407C the chemisorbed species probably dissociate from the surface while the principal mode of adsorption becomes physical processes. This would account for the fall in adsorption density and consequently in flotation recovery at temperatures above 407C. Increase in temperature from 30 to 407C had practically no effect on flotation recovery, which was about 100% at that temperature range in the region of the zpc of the Nigerian baryte. This is at variance with the adsorption test results, which showed that oleate extraction increased from 26.7% at 297C to 31.2% at 407C, signifying that the increase in temperature from 30 to 407C had a definite effect on adsorption density. The influence of hydrometal ions inherent in the pulp, besides pure baryte and aqueous ions, could have brought about the variance. Nature of Adsorption The adsorption isotherm of sodium oleate soap on the Nigerian baryte at 297C is given in Fig. 5. The initial slope
is convex to the concentration axis; it defines an increase in adsorption with increase in the external solution concentration, up to 15.9 1 10 2 mmole/m 2 adsorption density. The upper part of the curve is a plateau which indicates surface saturation in which all possible sites in the original baryte surface are filled (complete monolayer coverage). The isotherm is thus an L2 curve according to the classification of Gilles et al. (9). This group of isotherms is generally referred to as the Langmuir-type of isotherm. Since the oleate ions are adsorbed on baryte as single monofunctional units, there appears to be marked localization of the forces of attraction over the carboxylate group leading to very strong interaction (chemisorption) with baryte at this part of the ion. Figure 5 further presents the desorption isotherm of the oleate soap on the Nigerian baryte, also at 297C (Table 1, column 10 vs column 5). The isotherm, and in particular the degree of desorption presented in column 11 of Table 1, show that a small amount of the adsorbed oleate desorbed.
TABLE 5 Flotation Recovery of Baryte at Varying pH Values (Initial Collector Concentration 0.018 M; Temperature of Pulp 297C (Ambient)
Serial number
pH
Weight of concentrate (g)
1 2 3 4 5 6 7 8 9
3 5 6 7 8 9 10 11 12
0 150.20 158.70 148.60 168.05 172.44 173.75 168.70 160.22
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Weight of concentrate (%)
BaSO4 assay of concentrate (%)
Weight of feed (g)
BaSO4 assay of feed (%)
Recovery of BaSO4 (%)
0 75.10 79.35 74.30 84.03 86.22 86.88 84.35 80.11
0 89.00 92.10 94.00 95.30 98.00 98.80 97.90 95.60
200 200 200 200 200 200 200 200 200
85.90 85.90 85.90 85.90 85.90 85.90 85.90 85.90 85.90
0 77.81 85.17 81.31 93.22 98.37 99.86 96.13 89.16
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TABLE 6 Flotation Recovery of Baryte at Varying Temperatures (Initial Collector Concentration 0.018 M; pH 10)
Serial number
Collector concentration (M)
1 2 3 4 5 6
0.018 0.018 0.018 0.018 0.018 0.018
Temperature (7C)
Weight of concentrate (g)
Weight of concentrate (%)
Assay of concentrate for BasO4 (%)
Weight of feed (g)
Assay of feed for BaSO4 (%)
Recovery of BaSO4 (%)
29 40 50 60 70 80
173.75 175.60 166.50 156.65 145.91 130.40
86.88 87.80 83.25 78.33 72.96 65.20
98.80 97.70 96.70 96.00 95.40 94.90
200 200 200 200 200 200
85.90 85.90 85.90 85.90 85.90 85.90
99.86 99.92 93.72 87.53 81.02 72.03
This further confirms that at the natural pH of 8.2 at ambient temperature (297C) oleate is chemisorbed onto the aqueous baryte surface. However, that desorption occured at all indicates that the physical adsorption process contributed, even if in a small measure, to the overall adsorption process. The graphical summary of equilibrium concentrations of oleate species in solution at various pH presented by Jung et al. (10) shows that within the pH range 3–12 the active 0 oleate species in solution comprise 01 0 , 01 20 2 , H01 2 , and H01. Jung et al. hold the view that H01 molecules adsorb specifically, in the Stern layer, without bond formation with baryte. These researchers consider that the ionic species would most probably chemisorb at the baryte surface through bond formation; barium cleates are probably formed. The graphical summary under reference further shows that the concentration of the active oleate species in solution reach the maximum at about pH 8.0 and begin to decrease at pH 9–10. The latter pH is the region of the zpc of baryte. This would account for the decrease in adsorption density and percentage of recovery beyond pH 9.5 to 10.0. TABLE 7 DG0ads at Various Baryte Pulp pH Values
(1) pH
(2) Equilibrium oleate concentration 1 1003 (mole/liter)
(3) Adsorption density 1 102 (mmol/ m2 )
(4) DG0ads (Kcal/mole)
3.0 4.0 5.0 6.0 7.0 8.2 9.0 10.0 11.0 12.0
15.90 15.47 15.05 14.38 14.00 13.39 13.16 13.28 15.33 15.61
7.74 9.34 10.89 13.35 14.76 15.90 17.85 17.86 9.86 8.81
06.90 07.04 07.14 07.30 07.37 07.44 07.53 07.52 07.07 07.00
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A further reason for the decrease would be that the preponderance of OH 0 ions above pH 10 would lead to OH 0 ions competing with oleate ions for adsorption at the mineral surface. CONCLUSION
Mineral flotation is an interfacial phenomenon involving the adsorption of collector ions onto the aqueous mineral surface. Oleate flotation of Nigerian baryte is thermodynamically feasible at both acid and alkali pH. Oleate adsorption density and flotation recovery of Nigerian baryte increased with increasing pH up to the zero-point-of-charge (zpc) of the baryte (pH 9.5). Beyond pH 10, the percentage of recovery and adsorption density decreased. Flotation recovery of baryte increased with increasing oleate adsorption density to a maximum at the initial oleate concentration of 0.018 mole/liter at the ambient temperature of 297C and natural pH of 8.2, at the critical micelle concentration of 0.013 mole/liter for sodium oleate. Flotation recovery increased with increasing adsorption density to about 100% recovery at maximum oleate adsorption density, at pH 9.0 to 10.0 in the region of the zpc of the Nigerian baryte, and to about 100% recovery and maximum oleate adsorption density at the temperature range 30 to 407C, also at the zpc of the baryte. Oleate chemisorbs onto the aqueous baryte surface at ambient temperature and up to 407C, with a minor contribution of physical adsorption. Beyond 407C the chemisorbed species probably dissociates from the baryte surface, while the principal mode of adsorption becomes physical processes. Among the stable oleate species identified in solution, Hol is considered to adsorb specifically in the Stern layer without 0 bond formation; the ionic species 01 0 , 01 20 2 , and HO1 2 would chemisorb most probably through bond formation; barium oleates are probably formed at the mineral surface. Factors that would account for the decrease in adsorption
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OLEATE FLOTATION OF A NIGERIAN BARYTE
density and percent recovery beyond pH 9.5 to 10.0 would include: (i) the concentration of the oleate species in solution beginning to decrease in the pH region 9–10; (ii) the preponderance of OH 0 ions above pH 10 would lead to OH 0 ions competing with oleate ions for adsorption at the mineral surface. REFERENCES 1. Shergold, H. L., and Mellgren, O., Trans. Inst. Miner. Metall. C 78, 121–132 (1969).
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2. Cooke, S. R. B., Iwasaki, I., and Choi, A. S., Trans. AIME 217, 76– 83 (1960). 3. Ofor, O., J. Miner. Metall. Process., August 1 (1992). 4. David, J., ‘‘Ore Microscopy and Ore Petrography,’’ 297–303. Wiley, New York, 1982. 5. Han, K. N., Healy, T. W., and Fuerstenau, D. W., J. Colloid Interface Sci. 44, 407–414 (1973). 6. Gilles, C. H., and Nakhwa, S. N., J. Appl. Chem. 12, 266–273 (1962). 7. Fuerstenau, D. W., in ‘‘Chemistry of Biosurfaces’’ (I. M. L. Hair, Ed.), Vol. 1, p. 143. Dekker, New York, 1971. 8. Houwing, R., and Salomon, G., Eds., ‘‘Adhesion and Adhesives,’’ 2nd ed., Vol. 1. Elsevier, Amsterdam/New York, 1965. 9. Gilles, C. H., MacEwan, T. H., Nakhwa, S. N., and Smith, D., J. Chem. Soc., 3973–3993 (1960). 10. Jung, R. F., James, R. O., and Healy, T. W. J. Colloid Interface Sci. 118(2), 463–472 ( 1987).
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