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Investigating sodium sulphate as a phosphate depressant in acidic media
Separation and Purification Technology 124 (2014) 163–169
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Investigating sodium sulphate as a phosphate depressant in acidic media T.F. Al-Fariss a, Y. Arafat a, F.A. Abd El-Aleem a, A.A. El-Midany a,b,⇑ a b
Phosphate and Mineral Processing Chair, Chemical Engineering Dept., King Saud University, Saudi Arabia Mining, Petroleum, and Metallurgy Dept., Faculty of Engineering, Cairo University, Egypt
a r t i c l e
i n f o
Article history: Received 22 November 2013 Received in revised form 13 January 2014 Accepted 16 January 2014 Available online 28 January 2014 Keywords: Sedimentary phosphate Calcareous phosphate Reverse flotation Sodium sulphate Depressant
a b s t r a c t Reverse flotation is a commonly used technique for separating carbonate impurities from sedimentary phosphate ores using fatty acids collectors. Although, oleic acid represents one of the famous fatty acids that have been used as a collector in phosphate flotation circuits, it is a non-selective collector. Therefore, the selection of depressing agent is the most controlling factor. In this study, sodium sulphate was used as a phosphate depressant. The role of sodium sulphate in separating phosphate from its impurities and producing an acceptable concentrate grade for phosphoric acid production (equals or more than 30% P2O5) was evaluated using augmented factorial design. The collector dose, depressant dose, solid %, flotation time and pH were chosen as main affecting variables. The results showed that the addition of sodium sulphate improves the phosphate grade and recovery especially at highly acidic pH. They showed also that the solid % and the pH represent additional key factors in achieving a good grade concentrate due to their role in controlling the amount of ionic species in the flotation pulp. A concentrate contains >32% P2O5 was obtained with a recovery ranges from 84% to 87%. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Most of the world production of phosphate rocks is used in phosphoric acid production as well as fertilizers manufacturing. The depletion of easy-to-beneficiate siliceous phosphate ores attracts more attention to solve the problems associated to the flotation of calcareous phosphate ores which require further research work [1–10]. Reverse flotation, in particular, showed promising results with different types of phosphate ores all over the world [1,2,11–13]. It depends on depressing phosphate and floating carbonate minerals by anionic collectors at acidic pH [7,10,14]. The most extensively used collectors in the reverse flotation scheme are long-chain fatty acids and their salts, especially oleic acid and sodium oleate [14]. On the other hand, several types of depressants were reported in the literature such as: sodium silicate, H2SO4, H3PO4, starch, carboxyl methyl cellulose (CMC), tannic acid, aluminium sulphate, fluosilicic acid, fluoric acid, (Na, K) Tartarat, sodium carbonate/bicarbonate, sodium tripolyphosphate, diphosphonic acid, sulforganic compound, and dipotassium hydrogen phosphate [15–18].
⇑ Corresponding author at: Mining, Petroleum, and Metallurgy Dept., Faculty of Engineering, Cairo University, Egypt E-mail addresses: [email protected], [email protected] (A.A. El-Midany). http://dx.doi.org/10.1016/j.seppur.2014.01.024 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.
Reverse flotation of calcareous phosphate encounters some difficulties due to its successful pH range is acidic. High solubility of metal cations such as Ca-ions and Mg-ions represents one of the controlling factors in accomplishment of good phosphate grade and recovery. The effectiveness of a phosphate depressant measured by its effect on the phosphate recovery as well as its interaction with the type and level of collector used [19]. Selection of suitable depressant depends on some characteristics that should be present in the used depressant such as the depressant specificity towards the mineral, that needs to be depressed, and their pH dependence. The functional groups of the depressant play an important role in its working mechanism. A dual function is needed in the depressant which is the adsorption to the needed to be depressed mineral in addition to improve its hydrophilicity. It was reported that the depressants with sulphate groups may work preferentially in depressing phosphate at acidic pH especially when the sulphuric acid and its sulphate salts were used as pH modifiers. On the other hand, the used depressant should have no or very low competition with the collector functional groups [19]. Therefore, the present work aims at studying the role of sodium sulphate (Na2SO4) as a phosphate depressant in beneficiating Al-Jalamid phosphate ore by flotation process using oleic acid as a collector. Sodium sulphate, Na2SO4, depressing action of the phosphate mineral as well as its effect on the flotation concentrate grade and recovery was studied using experimental design in
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terms of the controlling factors; namely: pH, collector dosage, solid content, depressant dosage and flotation time. The collector and depressant interaction was also investigated. 2. Experimental 2.1. Materials 2.1.1. Phosphate sample A low-grade sample of phosphate ore, Al-Jalamid area, Northern region of Saudi Arabia, was used in the current study. The representative sample was primarily crushed by jaw crusher followed by screening to get the 4.25 + 0.106 mm size fraction. The 4.75 + 4.25 mm cut was taken to an attrition mill for further grinding. After several grinding and sieving steps the 0.425 + 0.106 mm cuts were collected and mixed together thoroughly to prepare the flotation feed. 2.1.2. Chemical reagents Oleic acid of 99% purity was used as a collector (Aldrich Chemicals, Germany). Analytical grade of NaOH and H2SO4 were used as pH regulators. In addition, sodium sulphate (Na2SO4) was used as a phosphate depressant. 2.2. Methods 2.2.1. Chemical analysis Wet chemical analysis of samples was conducted using standard methods for phosphate analysis. Magnesium, calcium and iron oxides were determined by inductive couple plasma spectrometer (ICP). Phosphorous was determined by spectrophotometric method using ‘Perkin–Elmer, model Lambda 3B’ spectrophotometer. 2.2.2. Scanning Electron Microscope (SEM) The surface characteristics of the phosphate ore samples, before and after the flotation process, were identified using scanning electron microscopy (SEM-JEOL 840). 2.2.3. Flotation experiments The experimental runs were conducted in a Wemco Fagergren type flotation cell with a volume capacity of two litres. Feed sample was 0.425 + 0.106 mm size fraction. The feed was added to the flotation cell with water to get a required pulp density. The pulp density (solid %) was adjusted to be 50% at conditioning and 25% during flotation. Then the required amount of depressant (sodium sulphate) was added and conditioned for two minutes. Then the measured amount of collector was added and pH value was adjusted by adding sulphuric acid. The air was turned on after one minute of collector addition. The flotation time was taken according to the statistical design. Both the concentrate and the float were filtered, dried and weighed. Concentrate and float were analysed for P2O5 content. 2.2.4. Factorial experimental design A series of 18 experiments, following a two-level randomized 251 fractional factorial design (FFD) with two centre points (two repeated represented by experiments 17 and 18, Table 3), were designed based on the important factors that affect the reverse flotation process. This design was augmented with ‘‘star’’ design (extra 10 experiments). The studied parameters are pH, collector dosage, solid content, depressant dosage and flotation time. The other operating parameters, such as air flow rate and particles size were maintained constant. The levels of the studied parameters are specified in Table 1.
Table 1 The operating parameters and their levels. Operating parameters
Units
Symbol
Low ()
High (+)
pH Oleic acid dosage Solids content Na2SO4 dosage Flotation time
– kg/t % kg/t min
A B C D E
4.5 1.75 10 0.0 2.0
6.5 5.5 30 15 5.0
The experimental results will be fitted to a second-order model, which enables the prediction of the output responses namely; P2O5%, and P2O5 recovery within the studied region, Table 1. The statistical software package Design-Expert, Stat-Ease, Inc., Minneapolis, USA was used for regression analysis of the experimental data and to plot the contour graphs. Analysis of variance (ANOVA) was used to estimate the statistical parameters. 3. Results and discussion 3.1. Chemical analysis and XRD Al-Jalamid ore is a calcareous phosphate ore of sedimentary origin and consists mainly of calcium fluorapatite Ca10 (PO4)6F2 with other impurities like Cl, SiO2, CaCO3, Al2O3, Fe2O3 and MgO. This ore deposit is characterised by its low P2O5 content (20–25%), high calcite (CaCO3), CaO:P2O5 weight ratio above 2.0 with low amount of silica among the other impurities [13]. Table 2 shows the chemical analysis of 0.425 + 0.106 mm size fraction used in the flotation experiments of this study. In addition, the X-ray diffraction analysis indicated that the main ore phases are fluorapatite, calcite and silica [20]. 3.2. Statistical analysis The factorial design results of phosphate reverse flotation are given in Table 3 in terms of P2O5%, and P2O5 recovery %. It is clear that the acidic pH gives a higher grade where the P2O5% reaches 35% with reasonable recovery 84% at 10%, 5.5, 3 kg/t, 7.5 kg/t and 3.5 min for solid %, pH, collector dosage, Na2SO4 dosage and flotation time, respectively, Table 3. The analysis of variance (ANOVA) indicated that the R2 is 0.8520 and 0.9537 and the standard deviation is 1.94 and 8.35 for P2O5% and P2O5 recovery %, respectively. The analysis shows that the main factor (A) is the most effective and significant factor for P2O5% while the main factors (A, B, C, D, E) and their interaction AB, AC, AD, AE are significant model terms for P2O5 recovery %, within 95% confidence interval.
Table 2 Chemical sample.
a
characteristics
of
phosphate
Constituents
% Weight (Dry basis)
P2O5 CaO Fe2O3 Al2O3 MgO Na2O K2O F Cl SiO2 L.O.I.a
22.6 50.72 0.10 0.23 0.19 0.32 <0.050 2.630 1.540 2.37 18.25
L.O.I = Loss on Ignition.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Factors
Response
A Solid %
B pH
C Collector dose (kg/t)
D Na2SO4 dose (kg/t)
E Flotation time (min)
P2O5%
P2O5 Recovery %
10 30 10 30 10 30 10 30 10 30 10 30 10 30 10 30 20 20 10 30 20 20 20 20 20 20 20 20
4.5 4.5 6.5 6.5 4.5 4.5 6.5 6.5 4.5 4.5 6.5 6.5 4.5 4.5 6.5 6.5 5.5 5.5 5.5 5.5 4.5 6.5 5.5 5.5 5.5 5.5 5.5 5.5
1 1 1 1 5 5 5 5 1 1 1 1 5 5 5 5 3 3 3 3 3 3 1 5 3 3 3 3
0 0 0 0 0 0 0 0 15 15 15 15 15 15 15 15 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 0 15 7.5 7.5
2 5 5 2 5 2 2 5 5 2 2 5 2 5 5 2 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 2 5
33.32 27.01 32.76 27.18 29.34 26.51 31.82 27.73 33.57 27.11 32.98 26.61 34.44 25.81 27.97 27.38 34.22 34 35.7 27 28.2 32.48 29.73 33.62 31.63 30.34 32.04 33.14
86.98 93.72 25.11 92.5 6 92.6 34.33 94.17 85.24 92.7 78.24 93.14 86.66 91.34 6.83 92.22 86.04 87 84.2 93.7 92.1 87.25 93.5 81.1 84.3 88.23 80.5 82.5
3.3. Effect of depressant dosage The selection of sodium sulphate, as a one of sulphate salts, was based on how effective were sulphuric acid and other sulphate salts such as aluminium sulphate as depressants in the sense that the sulphate ions play an important role in depressing phosphate [21]. It offers a reverse concentration gradient of sulphate ions to reduce the solubility of calcite and the release of calcium ions (Ca2+ and CaOH+) due to gypsum (calcium sulphate) formation as a result of the instability of calcite surface, at acidic or nearly acidic pH. Fig. 1a and b shows the contour plot of the effect of sodium sulphate and collector dosages on phosphate grade and recovery at pH 4.5 and 6.0. At zero depressant level, an increase in the phosphate grade was observed where the grade increased from 22.4 to 27.5 and 29% P2O5 at pH 4.5 and 6, respectively with a recovery reaching 100%, Fig. 1. It is clear that the higher the collector dose, the lower the grade and recovery. The maximum grade (29% P2O5) and recovery (100%) were observed at low collector (1 kg/t) and depressant dosages (3 kg/t) at pH 6. While at pH 4.5, a relatively higher dosage of the depressant (7.5 kg/t) is needed to achieve the maximum grade (27.5% P2O5) with the same recovery (100%). The higher depressant dosages, above which the maximum grades were achieved for both pH values, lead to reducing the grade. According to the observed trends in Fig. 1, it is suggested that there are three ways that the depressant may affect the chemistry of the separation pulp. First, the solubility of calcite (CaCO3) is much higher than phosphate especially in acidic media, though Ca++ ion concentration around calcite particles is higher and CaSO4 forms quicker on the surface of calcite particles. The presence of sodium sulphate in the pulp results in CaSO42H2O precipitation on calcite surface and enhances calcite floatability. The excess of sodium sulphate leads to the precipitation of CaSO42H2O onto apatite which increases the apatite floatability [22,23]. At pH 4.5, the high
reactivity between the calcite surface and sulphuric acid as well as formation of tiny CO2 bubbles increases the instability of the calcite surface and reduces the chance of attachment of collector to calcite surface. At the same time, the phosphate surface, especially in presence of high depressant dosages, tends to form a gypsum layer too – similar to calcite surface – which reduces the collector selectivity. The same phenomena are present at pH 6 but at lower rates which increase the chance to float the calcite, reduce the gypsum formation at phosphate surface, formation and adsorption of molecular dicalcium phosphate (CaHPO4) on the phosphate particles and consequently increase the grade of the concentrate to 29% at pH 6 rather than 27.5% at pH 4.5 [23,24]. Second, the sulphate ions, as potential determining ions, increase the calcite/gypsum surface negativity, change the surface properties and widen the separation gap between calcite and phosphate. However, this effect depends on the kinetics of gypsum formation on both calcite and phosphate. Third, it helps in prevention of calcium ions release to the solution by forming a stable gypsum layer due to presence of opposing concentration gradient of sulphate ions. At higher depressant dosages, the higher recovery is noticed, especially at pH 4.5. Such higher recovery values can be attributed to increase the negativity of the pulp which leads to the instability of the collector, as anionic collector, and collector – particle repulsion. In addition, increasing the ionic strength adversely affects the bubbles stability at froth layer. 3.4. Effect of collector dosage The collector dosage effect on the reverse flotation of phosphate in presence of sodium sulphate as a depressant is shown in Fig. 1. It is obvious that increasing the collector dosage inversely affects the P2O5 concentrate grade and recovery and the sensitivity of grade to be changed under different collector dosages was compensated with depressant additions.
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P2O5%
P2O5 Recovery %
15.00
15.00
26.33
26.63
D: Depressant dose, kg/t
D: Depressant dose, kg/t
(a)
11.25
26.93 27.49
7.50
27.24
3.75
26.63
11.25 100.0
7.50 97.4
94.5
3.75
91.7
26.33
88.8
0.00
0.00 1
2
3
4
5
1
C: Collector dose, kg/t
P2O5%
(b)
15.00
2
3
4
5
C: Collector dose, kg/t
P2O5 Recovery % 15.00
26.1
26.6
88.8 94.5
91.7
11.25
D: Depressant dose, kg/t
D: Depressant dose, kg/t
27.2
27.8
28.4
7.50 28.8
3.75 29.0
0.00
11.25 97.4
100.0
7.50
3.75
0.00 1
2
3
4
5
1
2
3
4
5
C: Collector dose, kg/t
C: Collector dose, kg/t
Fig. 1. Effect of depressant and collector dosages on phosphate grade and recovery at pH (a) 4.5 and (b) 6.0 [30% solids, flotation time 5 min].
Fig. 1 indicates that the depressant addition is more effective at pH 4.5 than at pH 6. In addition, the collector dosage (1.5–2 kg/t) is needed to improve the grade to 27.5% P2O5 while at pH 6 the minimum dosage of oleic acid is needed not only to initiate the flotation but also to increase the phosphate grade by 5–7% P2O5. It is worth to mention that a higher grade (29% P2O5) was reached at pH 6 even without adding depressant. The main difference between pH 4.5 and pH 6 is the amount of free Ca-ions. The Concentration of Ca-ions, of course, is higher at pH 4; therefore, the essential function of sodium sulphate is to precipitate multivalent cations (mainly Ca) from the pulp and allowing the calcite-collector selective interaction. It was reported that above 30–50 mg/L calcium as a threshold levels negatively affects the anionic flotation performance [25,26]. Moreover, the depression of phosphate in acidic media is possibly due to the adsorption (or formation) of aqueous CaHPO4 on its surface, thereby preventing surfactant adsorption. Free Ca2+ in solution can increase the formation of aqueous CaHPO4. Therefore, the presence of free Ca2+ ions is one of the most controlling factors using fatty acids in the reverse flotation of phosphate [22–27]. However, the excess of Ca ions and its dissolution at high rates not only deteriorate the formation of CaHPO4 or CaHPOþ 4 but also results in consumption of collector by forming Ca-oleate as well as formation of gypsum, especially in presence of sulphate ions, on both calcite and phosphate leading to separation non-selectivity. Therefore, adding the sodium sulphate, in addition to adjusting pH by sulphuric acid, was intended to minimise/remove the free Ca2+ from the solution by forming Ca-sulphate and enhance the formation of HPO2 4 at phosphate surface [22,23].
3.5. Effect of pH The pH of the pulp is a key factor in phosphate reverse flotation in acidic media. It controls the stability/instability of calcite surface due to the release of calcium ions (Ca2+ and CaOH+) as well as formation of CaHPO4 or CaHPO4+ on phosphate surface. Fig. 2 shows the results of the pH and depressant dosage effects on the P2O5 grade and recovery in the concentrate at two collector dosages, i.e. 1.0 and 5.0 kg/t. Similar trends for grade and recovery are observed with variation of the collector dosage. It was noticed that the maximum grade (P29% P2O5) is attained at pH 5.5–6 similar to previous studies that found the best pH range for phosphate reverse flotation is pH 5.5–6 [22,23]. It is also interesting to notice that such maximum grade was achieved at almost the same depressant dosages whatever the collector dosage is. The slightly higher grades at low collector dosage can be attributed to the higher tendency of the collector to be adsorbed on the calcite surface rather than phosphate which causes a better selectivity. The instability of both surfaces of calcite and phosphate at pH 4.5 and producing a variety of multi-valent species negatively affect the collector adsorption. The reduction of these ionic species by raising the pH to 6 enhances the concentrate grade and recovery. In addition, in the case of formation of gypsum layer, the sulphate ions play an important role in determination of the gypsum coated particle surface charge where such ions can act as potential determining ions and reduce the zeta potential of calcite from 9 to 2.5 when it covered by gypsum layer. It is known that the zeta potential of the CaSO4 is negative between pH 2.5 and 10.5, thus, the
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15.00
27.2 27.2
D: Depressant dose, kg/t
P2O5 Recovery %
P2O5% 15.00
94.5
28.1
97.4 28.8
D: Depressant dose, kg/t
(a)
11.25 29.5
7.50
29.8
3.75
11.25 100.0
7.50
100.0
3.75 97.4
27.2
94.5
0.00
0.00 4.5
5
5.5
6
6.5
4.5
5
5.5
B: pH
P2O5%
(b)
6.5
P2O5 Recovery %
15.00
15.00
26.6 26.6
88.8
27.2
91.7
28.1
D: Depressant dose, kg/t
D: Depressant dose, kg/t
6
B: pH
11.25 28.8 27.2
7.50
29.2
3.75
11.25
93.4
7.50
94.5
3.75 91.7
26.6
88.8
0.00
0.00 4.5
5
5.5
6
6.5
4.5
5
B: pH
5.5
6
6.5
B: pH
Fig. 2. Effect of depressant and pH on phosphate grade and recovery at 30% solids, flotation time 5 min, and collector dosages (a)1.0 kg/t and (b) 5.0 kg/t.
excess amounts of Ca-ions are likely to play as an activator between the anionic collector and negative surface of gypsum in addition to formation of sodium and/or calcium oleate. The interaction between gypsum and sodium/calcium oleate forms a mixture that precipitates on the phosphate and calcite (more on calcite) as indicated in Fig. 3. In addition, Fig. 3 shows a clear difference in the particle surface for bare phosphate particles, Fig. 3a, in comparison to either float and concentrate particles, Fig. 3b and c, respectively. This difference is related to the formation of a complex of Ca-sulphate and Na/Ca-oleate at the floated particles, which mainly calcite particles, while the concentrate particle, which mainly phosphate particles, are covered by collector only.
(a)
3.6. Effect of solid % Fig. 4 shows the results of the solid % and depressant dosages on the P2O5 grade and recovery in the concentrate at 3 kg/t oleic acid and pH 5.5. It shows the effect of depressant on grade and recovery at different solid %. It is clear that the grade exceeds 33% P2O5 at 10% solids which is not the case at higher solid % (20% or 30%). However, the recovery at 10% solids is the lowest. In other words, the recovery increases with increasing the solid %. The higher grade at lower solid % and the higher recovery at higher solid % are a result of relatively lower concentration of dissolved ionic species that interact with either phosphate surface or collector molecules and affect the separation grade and recovery. The reduction of calcium
(b)
(c)
Fig. 3. SEM pictures for the shape of (a) phosphate ore, (b) float, and (c) concentrate at 3 kg/t Na2SO4.
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P2O5%
P2O5 Recovery %
15.00
15.00
D: Depressant dose, kg/t
D: Depressant dose, kg/t
28.6
11.25
34.5
30.5 33.5
7.50
32.5
29.5
31.5
3.75
11.25
60.0
7.50
3.75
90.0
80.0
50.0
70.0
98.2
40.0
30.0
0.00
0.00 10
15
20
25
30
10
15
A: Solid %
20
25
30
A: Solid %
Fig. 4. Depressant and solid % effects on phosphate grade and recovery at 3 kg/t collector, pH 5.5 and 3 min flotation time.
P2O5% 15.00
P2O5 Recovery % 15.00
29.7
D: Depressant dose, kg/t
D: Depressant dose, kg/t
30.4
11.25
31.6
31.3 30.9
7.50
30.9
3.75
92.8
11.25 99.2
7.50
87.2
3.75
96.3 92.8
30.4
82.5
0.00
0.00 2.00
2.75
3.50
4.25
5.00
2.00
E: Flotation time, min
2.75
3.50
4.25
5.00
E: Flotation time, min
Fig. 5. Depressant and flotation time effects on phosphate grade and recovery at 3 kg/t collector, pH 5.5% and 25% solids.
ions and the ionic valence in the solution may lead from one side to the lower oleic acids consumption by its adsorption on calcium carbonate or ca-sulphate and/or formation of Ca-oleic complexes [28].
3.7. Effect of flotation time Fig. 5 shows the results of the flotation time and depressant dosages on the P2O5 grade and recovery in the concentrate at 3 kg/t oleic acid, 25% solids and pH 5.5. It shows that the flotation time slightly affects the P2O5% in the concentrate. Increasing the flotation time increases the phosphate recovery. A concentrate containing 31% P2O5 and 99% P2O5 recovery was achieved at 3 min flotation time.
4. Conclusions Sodium sulphate was tested as a phosphate depressant in the phosphate reverse flotation using oleic acid as a collector. A low grade phosphate ore from Al-Jalamid deposit was used in this study. The main role of sodium sulphate is to depress the phosphate by utilising Ca-ions on the phosphate surface to increase its hydrophilicity. It also helps in interaction with calcium ions
and precipitates them as calcium sulphate as a stable layer on calcite surface or in the solution. The depressing effect of sodium sulphate as a function of collector dosage, pH, flotation time and solid percent was studied using augmented factorial design. It revealed the importance of pH and solid % as main players in controlling the release of Ca-ions in the solution as well as their interaction with calcite, phosphate or even with the collector. It emphasises that the formation of the gypsum on calcite surface as well as the present of CaHPO4 is the main reasons behind the phosphate hydrophilicity and depression and consequently increase the separation selectivity. A concentrate with >32% P2O5 and a recovery exceeding 84% can be achieved. Acknowledgement The authors would like to express their appreciation and thanks to Phosphate and Mineral Processing Chair, Chemical Engineering Dept., King Saud University for supporting this research work. References [1] T.F. Al-Fariss, H.O. Ozbelge, S.S.E.H. Elnashaie, S.M. AbdulRazik, F.A. Abdel Aleem, N. Gassem, Preliminary investigation for the production of wet-process phosphoric acid from Saudi phosphate ores, Fert. Res. 28 (1991) 201–212.
T.F. Al-Fariss et al. / Separation and Purification Technology 124 (2014) 163–169 [2] T.F. Al-Fariss, K.A. El-Nagdy, F.A. AbdEl-Aleem, A.A. El-Midany, Column vs. mechanical flotation for calcareous phosphate fines upgrading, Particul. Sci. Technol. 31 (2013) 488–493. [3] A.M. Elmahdy, S.E. El-Mofty, M.A. Abdel-Khalek, N.A. Abdel-Khalek, A.A. ElMidany, Bacterially induced phosphate–dolomite separation using amphoteric collector, Sep. Purif. Technol. 102 (2013) 94–102. [4] A.M. Elmahdy, S.E. El-Mofty, N.A. Abdel-Khalek, A.A. El-Midany, Do Pseudomonas Aeruginosa bacteria affect the selectivity of dolomite/apatite separation?, Tenside Surfact Det. 48 (2011) 439–444. [5] A.A. El-Midany, H. El-Shall, S. Svoronos, Modeling of PVA-coated dolomite floatability in acidic media, Powder Technol. 209 (2011) 25–28. [6] A.A. El-Midany, H. El-Shall, S. Svoronos, Bubbles growth and their stability in reactive flotation process, Chem. Eng. Process. – Process Intensification 48 (2009) 1534–1538. [7] A.M. Elmahdy, A.A. El-Midany, N.A. Abdel-Khalek, S.E. El-Mofty, Effect of oleate/bacteria interactions on dolomite separation from phosphate ore, Tenside Surfact. Det. 46 (2009) 340–345. [8] A. El-Midany, H. El-Shall, R. Stana, Mechanisms involved in reactive flotation of dolomite, Miner. Metall. Process., SME 26 (2009) 94–100. [9] N.A. Abdel-Khalek, A.M. Elmahdy, A.A. El-Midany, S. Farrah, Optimization of bio-flotation of carbonaceous impurities from phosphate ore, Miner. Process. Extractive Metall., Trans. Inst. Min. Metall. C UK 117 (2008) 38–42. [10] A.M. Elmahdy, N.A. Abdel-Khalek, A.A. El-Midany, Statistical significance of some operating parameters on dolomite removal from phosphate using amphoteric collector, Miner. Metall Process, SME 24 (2007) 51–56. [11] H. El-Shall, Evaluation of Dolomite Separation Techniques’’, FIPR Report 02094-108, Florida Institute of Phosphate Research, Bartow, Florida, October 1994. [12] H. El-Shall, P. Zhang, R. Snow, Comparative analysis of dolomite/francolite flotation techniques, Miner. Metall. Process. 7 (1996) 135–140. [13] T.F. Al-Fariss, S.S.E.H. El-Nashaie, F.A Abd El Aleem, H.O. Ozebelge, Pilot Plant Studies on the Production of Phosphoric Acid from Saudi Phosphate Ores, Final Report on the Project No. AR-9-38 Submitted to KACST, 1993. [14] H. Sis, S. Chander, Reagents used in the flotation of phosphate ores: a critical review, Miner. Eng. 16 (2003) 577–585. [15] F. Guo, J. Li, Separation strategies for Jordanian phosphate rock with siliceous and calcareous gangues, Int. J. Miner. Process 97 (2010) 74–78.
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[16] A.l. Abramov, G. Onal, S. Atak, M.S. Celik, Mechanism of reverse flotation calcareous phosphate ores, in: H. El-Shall, B.M. Moudgil, R. Wiegel (Eds.), Beneficiation of Phosphate: Theory and Practice, SME, 1993, pp. 281–288. [17] J.S. Hanna, I. Anazia, Fatty acid separation of siliceous carbonate phosphates, Int. Metall. Process. 10 (1990) 84–89. [18] I.Z. Zafar, M.M. Anwar, D.W. Pritchard, Innovations in beneficiation technology for low grade phosphate rocks, Nutr. Cycl. Agroecosys. 46 (1996) 135–151. [19] P. Zhang, R. Snow, A.E.C. Peres, H. El-Shall, A. El-Midany, Depressants in nonsulfide mineral flotation, in: M.C. Furestenau, G. Jameson, R. Yoon (Eds.), Proceeding of Froth Flotation: A Century of Innovation Symposium, Society for Mining, Metallurgy, and Exploration, Inc. (SME), 2007. [20] A.A. El-Midany, F.A. Abd El-Aleem, T.F. Al-Fariss, Why relatively coarse calcareous phosphate particles are better in a static-bed calciner?, Powder Technol 237 (2013) 180–185. [21] M. Mohammadkhani, M. Noaparast, S.Z. Shafaei, A. Amini, E. Amini, H. Abdollahi, Double reverse flotation of a very low grade sedimentary phosphate rock, rich in carbonate and silicate, Int. J. Miner. Process. 100 (2011) 157–165. [22] N.A. Abdel-Khalek, Evaluation of flotation strategies for sedimentary phosphate with siliceous and carbonates gangues, Miner. Eng. 13 (2000) 789–793. [23] D.A. Elgillani, A.Z.M. Abouzeid, Flotation of carbonates from phosphate ores in acidic media, Int. J. Miner. Process. 38 (1993) 235–256. [24] M.A. Abdel-Zaher, Physical and thermal treatment of phosphate ores—an overview, Int. J. Miner. Process. 85 (2008) 59–84. [25] G.A. Gruber, B.M. Moudgil, P. Somasundaran, Understanding the Basics of Anionic Conditioning in Phosphate Flotation, Report of Investigation, Florida Institute of Phosphate Research (Bartow, FL), 1995, Publication number: 02090-121. [26] R.C. Guimaraes, A.C. Araujo, Reagents in igneous phosphate ores flotation, Miner. Eng. 18 (2005) 199–204. [27] P. Sumasundaran, L. Zhang, Role of surface chemistry of phosphate in its beneficiation, in: P. Zhang, H. El-Shall, R. Wiegel (Eds.), Beneficiation of Phosphates: Advances in Research and Practice, SME Co., USA, 1999, pp. 141– 1154. [28] B. Nanthakumar, D. Grimm, M. Pawlik, Anionic flotation of high-iron phosphate ores—Control of process water chemistry and depression of iron minerals by starch and guar gum, Int. J. Miner. Process. 92 (2009) 49–57.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Investigating sodium sulphate as a phosphate depressant in acidic media T.F. Al-Fariss a, Y. Arafat a, F.A. Abd El-Aleem a, A.A. El-Midany a,b,⇑ a b
Phosphate and Mineral Processing Chair, Chemical Engineering Dept., King Saud University, Saudi Arabia Mining, Petroleum, and Metallurgy Dept., Faculty of Engineering, Cairo University, Egypt
a r t i c l e
i n f o
Article history: Received 22 November 2013 Received in revised form 13 January 2014 Accepted 16 January 2014 Available online 28 January 2014 Keywords: Sedimentary phosphate Calcareous phosphate Reverse flotation Sodium sulphate Depressant
a b s t r a c t Reverse flotation is a commonly used technique for separating carbonate impurities from sedimentary phosphate ores using fatty acids collectors. Although, oleic acid represents one of the famous fatty acids that have been used as a collector in phosphate flotation circuits, it is a non-selective collector. Therefore, the selection of depressing agent is the most controlling factor. In this study, sodium sulphate was used as a phosphate depressant. The role of sodium sulphate in separating phosphate from its impurities and producing an acceptable concentrate grade for phosphoric acid production (equals or more than 30% P2O5) was evaluated using augmented factorial design. The collector dose, depressant dose, solid %, flotation time and pH were chosen as main affecting variables. The results showed that the addition of sodium sulphate improves the phosphate grade and recovery especially at highly acidic pH. They showed also that the solid % and the pH represent additional key factors in achieving a good grade concentrate due to their role in controlling the amount of ionic species in the flotation pulp. A concentrate contains >32% P2O5 was obtained with a recovery ranges from 84% to 87%. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Most of the world production of phosphate rocks is used in phosphoric acid production as well as fertilizers manufacturing. The depletion of easy-to-beneficiate siliceous phosphate ores attracts more attention to solve the problems associated to the flotation of calcareous phosphate ores which require further research work [1–10]. Reverse flotation, in particular, showed promising results with different types of phosphate ores all over the world [1,2,11–13]. It depends on depressing phosphate and floating carbonate minerals by anionic collectors at acidic pH [7,10,14]. The most extensively used collectors in the reverse flotation scheme are long-chain fatty acids and their salts, especially oleic acid and sodium oleate [14]. On the other hand, several types of depressants were reported in the literature such as: sodium silicate, H2SO4, H3PO4, starch, carboxyl methyl cellulose (CMC), tannic acid, aluminium sulphate, fluosilicic acid, fluoric acid, (Na, K) Tartarat, sodium carbonate/bicarbonate, sodium tripolyphosphate, diphosphonic acid, sulforganic compound, and dipotassium hydrogen phosphate [15–18].
⇑ Corresponding author at: Mining, Petroleum, and Metallurgy Dept., Faculty of Engineering, Cairo University, Egypt E-mail addresses: [email protected], [email protected] (A.A. El-Midany). http://dx.doi.org/10.1016/j.seppur.2014.01.024 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.
Reverse flotation of calcareous phosphate encounters some difficulties due to its successful pH range is acidic. High solubility of metal cations such as Ca-ions and Mg-ions represents one of the controlling factors in accomplishment of good phosphate grade and recovery. The effectiveness of a phosphate depressant measured by its effect on the phosphate recovery as well as its interaction with the type and level of collector used [19]. Selection of suitable depressant depends on some characteristics that should be present in the used depressant such as the depressant specificity towards the mineral, that needs to be depressed, and their pH dependence. The functional groups of the depressant play an important role in its working mechanism. A dual function is needed in the depressant which is the adsorption to the needed to be depressed mineral in addition to improve its hydrophilicity. It was reported that the depressants with sulphate groups may work preferentially in depressing phosphate at acidic pH especially when the sulphuric acid and its sulphate salts were used as pH modifiers. On the other hand, the used depressant should have no or very low competition with the collector functional groups [19]. Therefore, the present work aims at studying the role of sodium sulphate (Na2SO4) as a phosphate depressant in beneficiating Al-Jalamid phosphate ore by flotation process using oleic acid as a collector. Sodium sulphate, Na2SO4, depressing action of the phosphate mineral as well as its effect on the flotation concentrate grade and recovery was studied using experimental design in
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terms of the controlling factors; namely: pH, collector dosage, solid content, depressant dosage and flotation time. The collector and depressant interaction was also investigated. 2. Experimental 2.1. Materials 2.1.1. Phosphate sample A low-grade sample of phosphate ore, Al-Jalamid area, Northern region of Saudi Arabia, was used in the current study. The representative sample was primarily crushed by jaw crusher followed by screening to get the 4.25 + 0.106 mm size fraction. The 4.75 + 4.25 mm cut was taken to an attrition mill for further grinding. After several grinding and sieving steps the 0.425 + 0.106 mm cuts were collected and mixed together thoroughly to prepare the flotation feed. 2.1.2. Chemical reagents Oleic acid of 99% purity was used as a collector (Aldrich Chemicals, Germany). Analytical grade of NaOH and H2SO4 were used as pH regulators. In addition, sodium sulphate (Na2SO4) was used as a phosphate depressant. 2.2. Methods 2.2.1. Chemical analysis Wet chemical analysis of samples was conducted using standard methods for phosphate analysis. Magnesium, calcium and iron oxides were determined by inductive couple plasma spectrometer (ICP). Phosphorous was determined by spectrophotometric method using ‘Perkin–Elmer, model Lambda 3B’ spectrophotometer. 2.2.2. Scanning Electron Microscope (SEM) The surface characteristics of the phosphate ore samples, before and after the flotation process, were identified using scanning electron microscopy (SEM-JEOL 840). 2.2.3. Flotation experiments The experimental runs were conducted in a Wemco Fagergren type flotation cell with a volume capacity of two litres. Feed sample was 0.425 + 0.106 mm size fraction. The feed was added to the flotation cell with water to get a required pulp density. The pulp density (solid %) was adjusted to be 50% at conditioning and 25% during flotation. Then the required amount of depressant (sodium sulphate) was added and conditioned for two minutes. Then the measured amount of collector was added and pH value was adjusted by adding sulphuric acid. The air was turned on after one minute of collector addition. The flotation time was taken according to the statistical design. Both the concentrate and the float were filtered, dried and weighed. Concentrate and float were analysed for P2O5 content. 2.2.4. Factorial experimental design A series of 18 experiments, following a two-level randomized 251 fractional factorial design (FFD) with two centre points (two repeated represented by experiments 17 and 18, Table 3), were designed based on the important factors that affect the reverse flotation process. This design was augmented with ‘‘star’’ design (extra 10 experiments). The studied parameters are pH, collector dosage, solid content, depressant dosage and flotation time. The other operating parameters, such as air flow rate and particles size were maintained constant. The levels of the studied parameters are specified in Table 1.
Table 1 The operating parameters and their levels. Operating parameters
Units
Symbol
Low ()
High (+)
pH Oleic acid dosage Solids content Na2SO4 dosage Flotation time
– kg/t % kg/t min
A B C D E
4.5 1.75 10 0.0 2.0
6.5 5.5 30 15 5.0
The experimental results will be fitted to a second-order model, which enables the prediction of the output responses namely; P2O5%, and P2O5 recovery within the studied region, Table 1. The statistical software package Design-Expert, Stat-Ease, Inc., Minneapolis, USA was used for regression analysis of the experimental data and to plot the contour graphs. Analysis of variance (ANOVA) was used to estimate the statistical parameters. 3. Results and discussion 3.1. Chemical analysis and XRD Al-Jalamid ore is a calcareous phosphate ore of sedimentary origin and consists mainly of calcium fluorapatite Ca10 (PO4)6F2 with other impurities like Cl, SiO2, CaCO3, Al2O3, Fe2O3 and MgO. This ore deposit is characterised by its low P2O5 content (20–25%), high calcite (CaCO3), CaO:P2O5 weight ratio above 2.0 with low amount of silica among the other impurities [13]. Table 2 shows the chemical analysis of 0.425 + 0.106 mm size fraction used in the flotation experiments of this study. In addition, the X-ray diffraction analysis indicated that the main ore phases are fluorapatite, calcite and silica [20]. 3.2. Statistical analysis The factorial design results of phosphate reverse flotation are given in Table 3 in terms of P2O5%, and P2O5 recovery %. It is clear that the acidic pH gives a higher grade where the P2O5% reaches 35% with reasonable recovery 84% at 10%, 5.5, 3 kg/t, 7.5 kg/t and 3.5 min for solid %, pH, collector dosage, Na2SO4 dosage and flotation time, respectively, Table 3. The analysis of variance (ANOVA) indicated that the R2 is 0.8520 and 0.9537 and the standard deviation is 1.94 and 8.35 for P2O5% and P2O5 recovery %, respectively. The analysis shows that the main factor (A) is the most effective and significant factor for P2O5% while the main factors (A, B, C, D, E) and their interaction AB, AC, AD, AE are significant model terms for P2O5 recovery %, within 95% confidence interval.
Table 2 Chemical sample.
a
characteristics
of
phosphate
Constituents
% Weight (Dry basis)
P2O5 CaO Fe2O3 Al2O3 MgO Na2O K2O F Cl SiO2 L.O.I.a
22.6 50.72 0.10 0.23 0.19 0.32 <0.050 2.630 1.540 2.37 18.25
L.O.I = Loss on Ignition.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Factors
Response
A Solid %
B pH
C Collector dose (kg/t)
D Na2SO4 dose (kg/t)
E Flotation time (min)
P2O5%
P2O5 Recovery %
10 30 10 30 10 30 10 30 10 30 10 30 10 30 10 30 20 20 10 30 20 20 20 20 20 20 20 20
4.5 4.5 6.5 6.5 4.5 4.5 6.5 6.5 4.5 4.5 6.5 6.5 4.5 4.5 6.5 6.5 5.5 5.5 5.5 5.5 4.5 6.5 5.5 5.5 5.5 5.5 5.5 5.5
1 1 1 1 5 5 5 5 1 1 1 1 5 5 5 5 3 3 3 3 3 3 1 5 3 3 3 3
0 0 0 0 0 0 0 0 15 15 15 15 15 15 15 15 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 0 15 7.5 7.5
2 5 5 2 5 2 2 5 5 2 2 5 2 5 5 2 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 2 5
33.32 27.01 32.76 27.18 29.34 26.51 31.82 27.73 33.57 27.11 32.98 26.61 34.44 25.81 27.97 27.38 34.22 34 35.7 27 28.2 32.48 29.73 33.62 31.63 30.34 32.04 33.14
86.98 93.72 25.11 92.5 6 92.6 34.33 94.17 85.24 92.7 78.24 93.14 86.66 91.34 6.83 92.22 86.04 87 84.2 93.7 92.1 87.25 93.5 81.1 84.3 88.23 80.5 82.5
3.3. Effect of depressant dosage The selection of sodium sulphate, as a one of sulphate salts, was based on how effective were sulphuric acid and other sulphate salts such as aluminium sulphate as depressants in the sense that the sulphate ions play an important role in depressing phosphate [21]. It offers a reverse concentration gradient of sulphate ions to reduce the solubility of calcite and the release of calcium ions (Ca2+ and CaOH+) due to gypsum (calcium sulphate) formation as a result of the instability of calcite surface, at acidic or nearly acidic pH. Fig. 1a and b shows the contour plot of the effect of sodium sulphate and collector dosages on phosphate grade and recovery at pH 4.5 and 6.0. At zero depressant level, an increase in the phosphate grade was observed where the grade increased from 22.4 to 27.5 and 29% P2O5 at pH 4.5 and 6, respectively with a recovery reaching 100%, Fig. 1. It is clear that the higher the collector dose, the lower the grade and recovery. The maximum grade (29% P2O5) and recovery (100%) were observed at low collector (1 kg/t) and depressant dosages (3 kg/t) at pH 6. While at pH 4.5, a relatively higher dosage of the depressant (7.5 kg/t) is needed to achieve the maximum grade (27.5% P2O5) with the same recovery (100%). The higher depressant dosages, above which the maximum grades were achieved for both pH values, lead to reducing the grade. According to the observed trends in Fig. 1, it is suggested that there are three ways that the depressant may affect the chemistry of the separation pulp. First, the solubility of calcite (CaCO3) is much higher than phosphate especially in acidic media, though Ca++ ion concentration around calcite particles is higher and CaSO4 forms quicker on the surface of calcite particles. The presence of sodium sulphate in the pulp results in CaSO42H2O precipitation on calcite surface and enhances calcite floatability. The excess of sodium sulphate leads to the precipitation of CaSO42H2O onto apatite which increases the apatite floatability [22,23]. At pH 4.5, the high
reactivity between the calcite surface and sulphuric acid as well as formation of tiny CO2 bubbles increases the instability of the calcite surface and reduces the chance of attachment of collector to calcite surface. At the same time, the phosphate surface, especially in presence of high depressant dosages, tends to form a gypsum layer too – similar to calcite surface – which reduces the collector selectivity. The same phenomena are present at pH 6 but at lower rates which increase the chance to float the calcite, reduce the gypsum formation at phosphate surface, formation and adsorption of molecular dicalcium phosphate (CaHPO4) on the phosphate particles and consequently increase the grade of the concentrate to 29% at pH 6 rather than 27.5% at pH 4.5 [23,24]. Second, the sulphate ions, as potential determining ions, increase the calcite/gypsum surface negativity, change the surface properties and widen the separation gap between calcite and phosphate. However, this effect depends on the kinetics of gypsum formation on both calcite and phosphate. Third, it helps in prevention of calcium ions release to the solution by forming a stable gypsum layer due to presence of opposing concentration gradient of sulphate ions. At higher depressant dosages, the higher recovery is noticed, especially at pH 4.5. Such higher recovery values can be attributed to increase the negativity of the pulp which leads to the instability of the collector, as anionic collector, and collector – particle repulsion. In addition, increasing the ionic strength adversely affects the bubbles stability at froth layer. 3.4. Effect of collector dosage The collector dosage effect on the reverse flotation of phosphate in presence of sodium sulphate as a depressant is shown in Fig. 1. It is obvious that increasing the collector dosage inversely affects the P2O5 concentrate grade and recovery and the sensitivity of grade to be changed under different collector dosages was compensated with depressant additions.
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P2O5%
P2O5 Recovery %
15.00
15.00
26.33
26.63
D: Depressant dose, kg/t
D: Depressant dose, kg/t
(a)
11.25
26.93 27.49
7.50
27.24
3.75
26.63
11.25 100.0
7.50 97.4
94.5
3.75
91.7
26.33
88.8
0.00
0.00 1
2
3
4
5
1
C: Collector dose, kg/t
P2O5%
(b)
15.00
2
3
4
5
C: Collector dose, kg/t
P2O5 Recovery % 15.00
26.1
26.6
88.8 94.5
91.7
11.25
D: Depressant dose, kg/t
D: Depressant dose, kg/t
27.2
27.8
28.4
7.50 28.8
3.75 29.0
0.00
11.25 97.4
100.0
7.50
3.75
0.00 1
2
3
4
5
1
2
3
4
5
C: Collector dose, kg/t
C: Collector dose, kg/t
Fig. 1. Effect of depressant and collector dosages on phosphate grade and recovery at pH (a) 4.5 and (b) 6.0 [30% solids, flotation time 5 min].
Fig. 1 indicates that the depressant addition is more effective at pH 4.5 than at pH 6. In addition, the collector dosage (1.5–2 kg/t) is needed to improve the grade to 27.5% P2O5 while at pH 6 the minimum dosage of oleic acid is needed not only to initiate the flotation but also to increase the phosphate grade by 5–7% P2O5. It is worth to mention that a higher grade (29% P2O5) was reached at pH 6 even without adding depressant. The main difference between pH 4.5 and pH 6 is the amount of free Ca-ions. The Concentration of Ca-ions, of course, is higher at pH 4; therefore, the essential function of sodium sulphate is to precipitate multivalent cations (mainly Ca) from the pulp and allowing the calcite-collector selective interaction. It was reported that above 30–50 mg/L calcium as a threshold levels negatively affects the anionic flotation performance [25,26]. Moreover, the depression of phosphate in acidic media is possibly due to the adsorption (or formation) of aqueous CaHPO4 on its surface, thereby preventing surfactant adsorption. Free Ca2+ in solution can increase the formation of aqueous CaHPO4. Therefore, the presence of free Ca2+ ions is one of the most controlling factors using fatty acids in the reverse flotation of phosphate [22–27]. However, the excess of Ca ions and its dissolution at high rates not only deteriorate the formation of CaHPO4 or CaHPOþ 4 but also results in consumption of collector by forming Ca-oleate as well as formation of gypsum, especially in presence of sulphate ions, on both calcite and phosphate leading to separation non-selectivity. Therefore, adding the sodium sulphate, in addition to adjusting pH by sulphuric acid, was intended to minimise/remove the free Ca2+ from the solution by forming Ca-sulphate and enhance the formation of HPO2 4 at phosphate surface [22,23].
3.5. Effect of pH The pH of the pulp is a key factor in phosphate reverse flotation in acidic media. It controls the stability/instability of calcite surface due to the release of calcium ions (Ca2+ and CaOH+) as well as formation of CaHPO4 or CaHPO4+ on phosphate surface. Fig. 2 shows the results of the pH and depressant dosage effects on the P2O5 grade and recovery in the concentrate at two collector dosages, i.e. 1.0 and 5.0 kg/t. Similar trends for grade and recovery are observed with variation of the collector dosage. It was noticed that the maximum grade (P29% P2O5) is attained at pH 5.5–6 similar to previous studies that found the best pH range for phosphate reverse flotation is pH 5.5–6 [22,23]. It is also interesting to notice that such maximum grade was achieved at almost the same depressant dosages whatever the collector dosage is. The slightly higher grades at low collector dosage can be attributed to the higher tendency of the collector to be adsorbed on the calcite surface rather than phosphate which causes a better selectivity. The instability of both surfaces of calcite and phosphate at pH 4.5 and producing a variety of multi-valent species negatively affect the collector adsorption. The reduction of these ionic species by raising the pH to 6 enhances the concentrate grade and recovery. In addition, in the case of formation of gypsum layer, the sulphate ions play an important role in determination of the gypsum coated particle surface charge where such ions can act as potential determining ions and reduce the zeta potential of calcite from 9 to 2.5 when it covered by gypsum layer. It is known that the zeta potential of the CaSO4 is negative between pH 2.5 and 10.5, thus, the
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15.00
27.2 27.2
D: Depressant dose, kg/t
P2O5 Recovery %
P2O5% 15.00
94.5
28.1
97.4 28.8
D: Depressant dose, kg/t
(a)
11.25 29.5
7.50
29.8
3.75
11.25 100.0
7.50
100.0
3.75 97.4
27.2
94.5
0.00
0.00 4.5
5
5.5
6
6.5
4.5
5
5.5
B: pH
P2O5%
(b)
6.5
P2O5 Recovery %
15.00
15.00
26.6 26.6
88.8
27.2
91.7
28.1
D: Depressant dose, kg/t
D: Depressant dose, kg/t
6
B: pH
11.25 28.8 27.2
7.50
29.2
3.75
11.25
93.4
7.50
94.5
3.75 91.7
26.6
88.8
0.00
0.00 4.5
5
5.5
6
6.5
4.5
5
B: pH
5.5
6
6.5
B: pH
Fig. 2. Effect of depressant and pH on phosphate grade and recovery at 30% solids, flotation time 5 min, and collector dosages (a)1.0 kg/t and (b) 5.0 kg/t.
excess amounts of Ca-ions are likely to play as an activator between the anionic collector and negative surface of gypsum in addition to formation of sodium and/or calcium oleate. The interaction between gypsum and sodium/calcium oleate forms a mixture that precipitates on the phosphate and calcite (more on calcite) as indicated in Fig. 3. In addition, Fig. 3 shows a clear difference in the particle surface for bare phosphate particles, Fig. 3a, in comparison to either float and concentrate particles, Fig. 3b and c, respectively. This difference is related to the formation of a complex of Ca-sulphate and Na/Ca-oleate at the floated particles, which mainly calcite particles, while the concentrate particle, which mainly phosphate particles, are covered by collector only.
(a)
3.6. Effect of solid % Fig. 4 shows the results of the solid % and depressant dosages on the P2O5 grade and recovery in the concentrate at 3 kg/t oleic acid and pH 5.5. It shows the effect of depressant on grade and recovery at different solid %. It is clear that the grade exceeds 33% P2O5 at 10% solids which is not the case at higher solid % (20% or 30%). However, the recovery at 10% solids is the lowest. In other words, the recovery increases with increasing the solid %. The higher grade at lower solid % and the higher recovery at higher solid % are a result of relatively lower concentration of dissolved ionic species that interact with either phosphate surface or collector molecules and affect the separation grade and recovery. The reduction of calcium
(b)
(c)
Fig. 3. SEM pictures for the shape of (a) phosphate ore, (b) float, and (c) concentrate at 3 kg/t Na2SO4.
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P2O5%
P2O5 Recovery %
15.00
15.00
D: Depressant dose, kg/t
D: Depressant dose, kg/t
28.6
11.25
34.5
30.5 33.5
7.50
32.5
29.5
31.5
3.75
11.25
60.0
7.50
3.75
90.0
80.0
50.0
70.0
98.2
40.0
30.0
0.00
0.00 10
15
20
25
30
10
15
A: Solid %
20
25
30
A: Solid %
Fig. 4. Depressant and solid % effects on phosphate grade and recovery at 3 kg/t collector, pH 5.5 and 3 min flotation time.
P2O5% 15.00
P2O5 Recovery % 15.00
29.7
D: Depressant dose, kg/t
D: Depressant dose, kg/t
30.4
11.25
31.6
31.3 30.9
7.50
30.9
3.75
92.8
11.25 99.2
7.50
87.2
3.75
96.3 92.8
30.4
82.5
0.00
0.00 2.00
2.75
3.50
4.25
5.00
2.00
E: Flotation time, min
2.75
3.50
4.25
5.00
E: Flotation time, min
Fig. 5. Depressant and flotation time effects on phosphate grade and recovery at 3 kg/t collector, pH 5.5% and 25% solids.
ions and the ionic valence in the solution may lead from one side to the lower oleic acids consumption by its adsorption on calcium carbonate or ca-sulphate and/or formation of Ca-oleic complexes [28].
3.7. Effect of flotation time Fig. 5 shows the results of the flotation time and depressant dosages on the P2O5 grade and recovery in the concentrate at 3 kg/t oleic acid, 25% solids and pH 5.5. It shows that the flotation time slightly affects the P2O5% in the concentrate. Increasing the flotation time increases the phosphate recovery. A concentrate containing 31% P2O5 and 99% P2O5 recovery was achieved at 3 min flotation time.
4. Conclusions Sodium sulphate was tested as a phosphate depressant in the phosphate reverse flotation using oleic acid as a collector. A low grade phosphate ore from Al-Jalamid deposit was used in this study. The main role of sodium sulphate is to depress the phosphate by utilising Ca-ions on the phosphate surface to increase its hydrophilicity. It also helps in interaction with calcium ions
and precipitates them as calcium sulphate as a stable layer on calcite surface or in the solution. The depressing effect of sodium sulphate as a function of collector dosage, pH, flotation time and solid percent was studied using augmented factorial design. It revealed the importance of pH and solid % as main players in controlling the release of Ca-ions in the solution as well as their interaction with calcite, phosphate or even with the collector. It emphasises that the formation of the gypsum on calcite surface as well as the present of CaHPO4 is the main reasons behind the phosphate hydrophilicity and depression and consequently increase the separation selectivity. A concentrate with >32% P2O5 and a recovery exceeding 84% can be achieved. Acknowledgement The authors would like to express their appreciation and thanks to Phosphate and Mineral Processing Chair, Chemical Engineering Dept., King Saud University for supporting this research work. References [1] T.F. Al-Fariss, H.O. Ozbelge, S.S.E.H. Elnashaie, S.M. AbdulRazik, F.A. Abdel Aleem, N. Gassem, Preliminary investigation for the production of wet-process phosphoric acid from Saudi phosphate ores, Fert. Res. 28 (1991) 201–212.
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