Minerals Engineering 69 (2014) 97–101

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Minerals Engineering

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A modification in the flotation process of a calcareous–siliceousphosphorite that might improve the process economics

http://dx.doi.org/10.1016/j.mineng.2014.07.0170892-6875/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Address: P.O. Box: 87, Helwan, Cairo, Egypt. Tel.: +20 225010642, +20 2 25010643; fax: +20 2 25010639.

E-mail address: Khaled_yassin@yahoo.com (K.E. Yassin).

Tawfik R. Boulos, Ahmed Yehia, Suzan S. Ibrahim, Khaled E. Yassin ⇑Central Metallurgical Research & Development Institute, Helwan, Cairo, Egypt

a r t i c l e i n f o

Article history:Received 23 March 2014Accepted 22 July 2014

Keywords:PhosphateCalciteFlotationOleic acid

a b s t r a c t

The amenability of a low-grade Egyptian phosphorite to flotation for separation of both calcareous andsiliceous gangue minerals by just pH control was investigated. The ore, assaying 19.39% P2O5, 16.1%L.O.I. and 12.41% A.I. is mainly composed of francolite and hydroxy apatite minerals consolidated intothree different phosphatic varieties according to texture and origin, i.e. coarse phospho-chem, sharp-edged phospho-clast and fine cementing phospho-mud. This was endorsed by microscopic investigationof thin sections. X-ray diffraction analysis of the ore sample showed that the main gangue minerals arecalcite and quartz with minor dolomite and some gypsum.

Anionic flotation of calcite, under pH4.5, was successfully conducted on the �0.25 + 0.074 mm phos-pho-chem fraction without any use of phosphate depressants. This was followed by direct flotation ofphosphate after raising the pH to 9. Mechanical cleaning of the phospho-concentrate was carried out,without any addition of the collector to get rid of the entrained silica. About 3 kg/t of oleic acid wasrequired for the whole process which was added step-wise 0.5 kg/t each except for the first step whichwas 1.0 kg/t to activate the flotation pulp. Phospho-concentrate assaying 30.54% P2O5, 8.7% L.O.I. and5.76% A.I. with a P2O5 recovery of 64.34% was finally obtained without the use of expensive depressants,e.g. phosphoric acid or sodium silicate.

A trial to explain the results in view of others’ findings and in terms of the ore mineralogical character-istics was shown.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction implemented, on a commercial scale perhaps before the end of

The flotation of siliceous sedimentary phosphate is becoming acommon-place practice after the Florida ‘‘Crago’’ technology pavedthe road to fatty acids reverse or direct flotation circuits. The pro-cess has been become a conventional technology worldwide and anumber of important references that summarize the history andthe associated chemical reagents combination base are reported(Giesekke, 1985; Moudgil and Sumasundaran, 1986; Sis andChander, 2003; Lawver et al., 1980; Lawendy and McClellan, 1993).

On the other hand, the processing of calcareous siliceoussedimentary phosphate still represents a continuous industrialworldwide challenge (Blazy and Samama, 2001; MohammadKhaniet al., 2011; Al-Fariss et al., 2013; Abouzied et al., 2009; Houoteet al., 1985; Sis and Chander, 2000; Zheng and Smith, 2000;Abdel-Khalek, 2000; Amankonah and Sumasundaran, 1985;Ananthapadmanabhan and Sumasundaran, 1984; Anzia andHanna, 1987; El Shall et al., 1996). Although the process had been

the last century, yet there is a lot of discrepancy about the best strat-egy to achieve selectivity. That was attributed to the similarities inthe surface properties of both the phosphate and the carbonateminerals (Abdel-Khalek, 2000; Amankonah and Sumasundaran,1985; Ananthapadmanabhan and Sumasundaran, 1984). Houoteet al. showed that all their zero points of charge (ZPC) occur in theacid pH range of less than pH 5.5 (Houote et al., 1985). All the con-stituents of the carbonated sedimentary phosphate ores react in anearly identical manner as regards the collectors, particularly thefatty acids and their salts (Abdel-Khalek, 2000; Amankonah andSumasundaran, 1985). It was postulated that the surface propertiesof the carbonaceous phosphate are affected not only by the phos-phate’s own solution chemistry but also by the dissolved speciesfrom other salt-type minerals in the system (Amankonah andSumasundaran, 1985; Ananthapadmanabhan and Sumasundaran,1984). These dissolved species can have a marked effect on theirinterfacial properties. On the other hand, the solution chemistryof fatty acid collectors is another important parameter in the flota-tion process where Ca2+, found in solution, activate quartz impuri-ties and thus degrades the quality of the phospho-concentrate

98 T.R. Boulos et al. / Minerals Engineering 69 (2014) 97–101

(Amankonah and Sumasundaran, 1985; Ananthapadmanabhan andSumasundaran, 1984).

The application of phosphate depressants, particularly phospho-ric acid, while floating carbonates in acidic media (pH below 5.5)has been successfully conducted by so many workers to achievegood selectivity (Zheng and Smith, 2000; Abdel-Khalek, 2000;Amankonah and Sumasundaran, 1985; Ananthapadmanabhan andSumasundaran, 1984; Anzia and Hanna, 1987; El Shall et al.,1996; Boulos et al., 2000). It was predicted, from thermodynamicconsiderations, that selective flotation of carbonates from phos-phates in acidic media can be enhanced by minimizing the freeCa2+ in solution and by increasing HPO4

2� in the system (Abdel-Khalek, 2000). That is what happens by addition of phosphoric acidor any chelating agent. But very high doses of phosphoric acid arerequired, sometimes reaching 8–10 kg/t (Lawver et al., 1980;Boulos et al., 2000; Henchiri, 1993; Kiukkola, 1980) which will cer-tainly add to the production cost of the process.

However, not all calcareous siliceous sedimentary phosphoritesare the same to achieve the same amount of Ca2+ or HPO4

2� in pres-ence of phosphoric acid. The dolomite in the Egyptian Abu Tarturphosphate (Aluisse-Sofremines, 1978) e.g. became ferruginatedand combines with the pyrite, forming another mineral termedankerite. Therefore, taking the geological history and the mineral-ogical composition into consideration, every ore should be takenjust as a case study, i.e. not to generalize.

In this respect, the following article is another different casestudy of a calcareous siliceous phosphorite which will be subjectedto flotation without the use of any chemical depressants lookingfor some savings in the production costs of the process.

2. Experimental

The sample under investigation was an East Mediterranean sed-imentary phosphate rock of Egypt. After coning and quarteringmethods, a batch of it was finely ground to less than 0.074 mmfor complete characterization methods. Complete chemical analy-sis of the original sample was conducted using an Axios, SequentialWD-2005 X-ray fluorescence unit. A BRUKER X-ray diffractometer(Germany) of type AXS D8 ADVANCE with Cu target (k = 1.540 Åand n = 1) at 40 kV potential and 40 mA current was used for char-acterization and mineral phase identification of the sample. Thepowdered samples was analyzed using XRD with scanning speedof 2�/min. Types of the phases in the samples were identified usingthe X-ray powder data file, published by the American Standard forTesting Material (ASTM).

Wet size distribution analysis SDA of the original sample wasalso carried out and accompanied by chemical analysis of the dif-ferent size factions.

Oleic acid obtained from Riedel-de Haen (Germany) was used asthe flotation collector after mixing with kerosene at 1:1 ratio byvolume. High-grade sodium carbonate and hydrochloric acid wereused as pH regulators.

The ore sample was crushed using gyratory crusher in a closedcircuit with 0.6 mm screen. The �0.6 mm product was wet groundin a rod mill (1:1 solid/liquid ratio, 6 rods and 10 min. grindingtime) to prepare �0.25 mm + 0.074 mm size fraction. This fractionwas used as a feed for the flotation experiments.

The ‘‘Denver D12’’ flotation cell employed in this investigationwas a simulation of the commercial sub-aeration types. Flotationexperiments were conducted under the following pre-determinedoptimum conditions:

– Pulp temperature 23 �C.– Soft water usage (tap water).– Collector added as 1:1 kerosene/oleic acid solution.

– HCl and Na2CO3 as pH regulators.

Surface conditioning of 250 g samples of the deslimed phosphateore was carried out in the flotation cell for 5 min at an impeller speedof 2000 rpm and pH 4.5 (using dilute HCl acid). After pH adjustmentat 4.5, conditioning with the collector was conducted for a further5 min at a pulp density of 50% solids. The pulp was diluted to 30% sol-ids and flotation was commenced after decreasing the motor speedto 1500 rpm and opening the tap for aeration. Step addition of thecollector in 0.5 kg/t doses was employed to float calcite. After calciteseparation, the pulp pH was raised to pH 9 (using sodium carbonate)and oleic acid was then added stepwise to float the phospho-concen-trate, leaving the silica depressed in the flotation cell. Mechanicalcleaning of the concentrate, i.e. flotation without addition of collec-tor, was then done to clear it from the entrained silica. No phosphoricacid was added to depress the phosphate in the first cycle and sim-ilarly no sodium silicate was used to depress silica in the second one.Products of the phosphoconcentrates and tailings were dried andanalyzed for P2O5and SiO2.

3. Assessment of results

3.1. Sample characterization

Fig. 1 illustrates the XRD analysis of the phosphate sample. Evi-dently the rock consists of two main phosphate minerals, francoliteand hydroxyapatite. Calcite and quartz are the main gangue miner-als, with minor dolomite and gypsum.

Table 1 depicts the XRF of the original sample from which it isclear that it is of low grade, with 19.59% P2O5, containing consider-able amount of calcite, 48.4% CaO and 17.5 Los of Ignition (L.O.I.)and appreciable content of silica, 10.46%.

Table 2 shows the wet chemical/size analysis of the originalphosphate sample. Results show that the P2O5% was low in coarsesize >22.40 mm and then shows moderate increase in gradebetween �22.4 + 0.83 mm, reaching 20.07% P2O5 with weightrecovery reaching 81.26%. Appreciable enrichment in P2O5% wasnoticed in the size �0.83 + 0.106 mm reaching in average 23.47%,with weight recovery reaching 8.06%. Again P2O5% showed adecrease below 0.106 mm and then remarkable deterioration toabout 11.39% below 0.074 mm.

Sink–float tests, using pure bromoform with density 2.89 g/cm3,showed that more than 65% by wt. of the �0.25 + 0.074 mm prod-uct was of high phosphate content that reached more than 33.43%P2O5, with low contents of calcite and silica reaching 6.95% and0.77%, respectively, Table 3.

Mineralogically, the phosphate mineral was shown in differentshapes and types in addition to calcareous shells as the majoraccompanying phase, Fig. 2. This phosphate type was mostly isotro-pic cryptocrystalline carbonate fluorapatite (francolite) present asgrains or as mud called cellophane. Most of the phosphate mineralwas detected as fish bones and shell fragments, peloids and intra-clasts. Fewer amounts as ooids set in a calcareous cemented matrix.However, phosphate types could be classified according to textureand origin into three main categories, i.e. phospho-mud (phosphatefilling cavities), phosphochem, and phosphoclasts, Fig. 2.

(P) Phosphatic pellets in rounded and elliptical shapes (brownin color in ordinary light (A&C) and isotropic between crossednicols.

(L) Phosphatic lithoclastic in irregular shape (brown to whitishbrown in color in ordinary light (A&C) and isotropic betweencrossed nicols (B&D).

(OF) Phosphatic bone fragments which appear white in ordinarylight (A&C) and show gray first order interference color betweencrossed nicols (B&D).

Fig. 1. XRD of original phosphate sample.

Table 1XRF analysis of original sample.

Constituent wt.%

CaO 48.40P2O5 19.59SiO2 10.46MgO 0.66Al2O3 1.04Fe2O3 1.50S 0.85L.O.I. 17.50

Table 2Wet size/chemical analysis of the original phosphate sample.

Fraction (mm) wt.% P2O5% A.I.a% L.O.I.b%

+22.4 2.03 16.84 2.15 24.9516.00 8.92 20.30 16.14 14.3511.20 21.39 19.10 15.03 16.008.00 16.62 20.04 8.33 17.556.68 7.97 22.02 9.36 15.364.00 15.79 19.88 9.25 16.453.33 3.61 20.93 10.48 15.392.36 3.91 20.21 10.36 15.321.17 2.12 21.01 11.35 14.150.83 0.93 21.34 11.81 13.160.59 0.92 23.69 9.85 12.720.42 0.89 23.66 9.32 12.230.29 1.19 23.61 9.28 11.980.21 0.77 24.99 8.16 12.540.106 4.29 23.07 10.75 12.970.088 0.51 19.53 16.87 14.070.074 0.32 18.56 18.47 14.390.045 7.82 11.39 30.99 15.64Org. 100.00 19.59 12.84 15.89

a Acid insoluble.b Loss on ignition.

Table 3Sink/float separation test using pure bromoform of �0.25 + 0.074 mm flotation feed.

Size fraction (mm) Product wt.% P2O5% A.I.% L.O.I.%

�0.25 + 0.074 Float 33.93 9.16 15.96 25.92Sink 66.07 33.43 0.77 6.95

T.R. Boulos et al. / Minerals Engineering 69 (2014) 97–101 99

(F) Calcareous shells.(C) Carbonate cement by which phosphate components

cemented.

3.2. Flotation of the phospho-chem and phospho-clasts size fraction

Table 4 depicts the results of the flotation tests of the relativelycoarse size cut, �0.25 + 0.074 mm, at various sequences and modeof oleic acid additions.

The following conclusions could be reported:

1. It was easy to float calcite at pH 4.5, without chemicaldepression of phosphate. Actually, the depression of thislatter mineral at this pH was so strong, sometimes, toclog the performance of the cell impeller.

2. The mode of addition of the collector plays a detrimentalrole on the flotation sequence, e.g. by increasing the col-lector consumption from 0.5 kg/t (No. 1) to 2 kg/t (No. 9),

Fig. 2. Petrography pictures of phosphate original sample.

Table 4Effect of sequence and manner of oleic acid addition on flotation.

No. Oleic acid (kg/t) Product wt.% P2O5 L.O.I A.I.

% % Rec. % % Dist. % % Dist.

1 0.5 Calcite 11.38 15.82 7.77 21.99 16.08 10.52 17.872 0.5 Phosphate 76.34 25.84 85.17 14.76 72.42 2.01 22.903 – Silica 4.34 8.01 1.50 7.28 2.03 61.76 40.014 – Middling 7.94 15.52 5.32 18.52 9.45 16.71 19.80

Total 100 23.14 99.76 15.56 100 6.70 100

5 1.75 Calcite 10.23 10.93 4.89 26.27 17.24 11.97 18.066 – Phosphate 64.70 23.69 67.05 16.41 68.10 4.24 40.467 0.25 Phosphate 22.90 27.84 27.89 9.65 14.17 8.24 28.008 – Silica 2.17 3.38 0.17 4.64 0.35 78.46 13.54

Total 100 27.86 100 15.59 100 6.78 100

9 2 Calcite 7.87 7.81 2.67 31.34 15.10 8.81 10.5510 – Phosphate 38.98 22.13 37.52 18.84 44.97 3.72 22.0711 0.25 Phosphate 51.08 26.59 59.08 12.13 37.94 6.21 48.2812 – Silica 2.08 8.03 0.73 7.63 1.95 60.38 19.12

Total 100 22.99 100 16.33 100 6.57 100

13 1 Calcite 17.42 11.33 8.47 29.47 34.11 4.27 10.9114 0.5 Calcite 9.74 8.21 3.43 32.44 20.99 4.80 6.8615 0.5 Calcite 6.15 12.93 3.41 27.31 11.16 7.35 6.6316 0.5 Phosphate 16.38 29.56 20.78 10.85 11.81 4.46 10.7117 0.5 Phosphate 47.96 30.87 63.54 7.96 25.37 6.20 43.6018 – Silica 2.35 9.16 0.92 5.13 0.80 66.08 22.77

Total 100.0 23.43 100.0 15.05 100.0 6.91 100.0

Original 100 23.30 14.82 6.82

100 T.R. Boulos et al. / Minerals Engineering 69 (2014) 97–101

no corresponding increase occurred on the amount offloated calcite. But when this dose of collector was addedstepwise 0.5 kg/t each, it ended up with the flotation of 3cuts of calcite assaying 29.47%, 32.44% and 27.31% L.O.I.at a total% weight of 33.31% (No. 15). The first dose ofthe collector, however, should be high enough (1.0 kg/t)for pulp activation.

3. It was also possible to float phosphoconcentrates at pH 9assaying 29.26% and 30.87% P2O5 with just 2 incrementsof collector, 0.5 kg/t each. The silica contaminating suchconcentrates ranged between 4.46% and 6.2% which isacceptable in terms of the high degree of liberation ofthese phospho-chem and phospho-clast, i.e. there wasnot any need to use a silica depressant (Nos. 16 and 17).

4. High-grade silica tailing, assaying 66.1% A.I. and just 9.16%P2O5 was left in the flotation cell (No. 18). This was theentrained silica which was liberated from the phosphaticcalcareous shell, Fig. 2.

5. A combined phospho-concentrate (Nos. 16 and 17).assaying 30.54% P2O5, 8.7% L.O.I. and 5.74% A.I. wasobtained within this circuit at an overall P2O5 recoveryof 84.34%.

Most workers, in this field, are sure of phosphate depression inacidic medium, below pH5. Nevertheless, all of them recom-mended the application of phosphoric acid as a depressant duringcalcite flotation, (Zheng and Smith, 2000; Abdel-Khalek, 2000;Amankonah and Sumasundaran, 1985; Ananthapadmanabhan

T.R. Boulos et al. / Minerals Engineering 69 (2014) 97–101 101

and Sumasundaran, 1984; Anzia and Hanna, 1987; El Shall et al.,1996; Boulos et al., 2000). In the meantime, sodium silicate wasalso employed for silica depression in the same circuit (Dho andIwasaki, 1990). Irrespective of the suggested explanation aboutthe role of phosphoric acid in this respect, the process was toostrong in the current sample to block the flotation cell impellerwithout any depressants.

FIPR suggested that the ionization of HCl acid in low pH mightenhance the formation of an electronegative film on the phosphateparticles by the chloride ions leading to the hindrance of oleic acidadsorption on the phosphate particles and hence their depression(Institute of Phosphate Research (FIPR), 1988). Selective flotationof phosphate from calcite was predicted to occur only under acidicconditions when the calcite is expected to dissolve, (Biswas, 1967).In this article, flotation was conducted in the pH range of 4.5. Theaction of acids is probably the preferential dissolution of calcitesurface and the generation of carbon dioxide bubbles, that nucle-ates on its surface and remains there depending on the porosityand hydrophobicity of calcite. The presence of CO2 enhanced theselectivity of calcite flotation with respect to apatite even withthe use of low collector dosage due to the suppression of the elec-trostatic barrier for the anionic collector adsorption, (Predali, 1969;Sampat Kumar et al., 2003).

Besides, at low pH, oleic acid is essentially present as neutralmolecules (Albuquerque et al., 2012). Consequently, these neutralmolecules are physically bind to the calcite surface, which isslightly positively charged, through hydrogen bonding withH2CO3 species improving the selective flotation of calcite.

In addition, the higher solubility of calcite than apatite,(Somasundaran et al., 1985), is due to calcite exhibiting greatersurface reactivity and availability of calcium at its surface for thechemisorptions reaction of oleate species to occur.

However, it does make sense that the mineralogical characteris-tics of a sample make a difference in flotation processes. Simply, thebehavior of a free phosphate particle might differ from that of a bin-ary or tertiary particle. The extremely high degree of liberation of thedifferent minerals of the current sample, reaching about 83%, endedup with the accumulation of the carbonaceous gangue minerals inthe +22.4 mm size cut, the high-grade phosphate (reaching 33–34% P2O5 in sink/float test) in the intermediate �0.25 + 0.074 mmand the free silica in the �0.074 phospho-mud fraction. Under suchconditions the depression of phosphate particles will be, in additionto the influence of the electronegative film on their surfaces, due tothe selective behavior of the free phosphate particles.

4. Conclusion

Anionic flotation of calcite from a calcareous siliceous sedimen-tary phosphorite was successfully conducted at pH 4.5 after natu-ral depression of phosphate without any addition of phosphoricacid. Raising the pH to 9, flotation of phosphate proceeds leavingsilica in the bottom of the flotation cell without using silica depres-sant. Correlation of what happened in this case study with others’finding and with the mineralogical composition of the sample wascarried out.

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