Evaluation of flotation strategies for sedimentary phosphates with siliceous and carbonates gangues

Minerals Engineering, Voi. 13, No. 7, pp. 789-793, 2000

Pergamon 0892--6875(00)00064-9

© 2000 Published by Elsevier Science Ltd All fights reserved 0892-6875/00/$ - see front matter

TECHNICAL NOTE EVALUATION OF FLOTATION STRATEGIES FOR SEDIMENTARY PHOSPHATES WITH SILICEOUS AND CARBONATES GANGUES

N.A. A B D E L - K H A L E K Central Metallurgical Research and Development Institute, P.O. Box 87 Helwan, Cairo, Egypt E-mail: [email protected] or [email protected] (Received 2 December 1999; accepted 2 May 2000) ABSTRACT Different flotation strategies to separate both calcite and silica from a sedimentary phosphate ore are studied in this paper. The results show that a single stage for flotation of phosphate or calcite is not enough to obtain high grade concentrates. Among three different processes for carbonate flotation, the results reveal better selectivity when phosphoric acid is used as depressant for phosphate. A circuit consists of a flotation stage for carbonate and another for either phosphate or silica presents the best strategy for upgrading such ore. Separation of calcite in the first stage seems to be essential where the selectivity margin tends to operate in favor of carbonate rather than phosphate mineral. Concentrates with P20s above 30 % are obtained while using such flotation strategy from a feed ground to -0.15 ram. © 2000 Published by Elsevier Science Ltd. All rights reserved

Keywords Froth flotation; flotation depressants; flotation collector; liberation; fine particle processing

INTRODUCTION Separation of carbonates from phosphates is extremely complex due to their similar physicochemical properties. So, the attention ofXesearchers has been directed to improve the selectivity of their separation. Reverse flotation of carbonate gangue with depression of the phosphate is one of the more promising techniques that have been tried by several investigators (El-Shall et al., 1996; Elgillani and Abouzeid, 1993; Anazia and Hanna, 1987). They have shown that selective flotation of dolomite from phosphate can be achieved under slightly acidic conditions, provided an apatite depressant is added to the system (Moudgil and Stanasundaran, 1986). Consequently, when these flotation processes were adopted, the search had focused not on carbonate, but on possible phosphate depressant systems (Anazia and Hanna, 1987; Smani et al., 1975; AI-Fariss et al., 1992). Meanwhile, direct flotation of phosphate from silica at Sebaiya west beneficiation plant, Egypt, did not give satisfactory results due to the presence of calcite impurities. This paper evaluates different flotation strategies to sepmate both calcite and silica from such Egyptian sedimentary phosphate ore.

789

790

N.A. Abdel-Khalek

MATERIALS AND METHODS Materials

A phosphate sample from Sebayia west, Egypt, was used in this study. The sample was ground, unless otherwise mentioned, to -0.25 mm. The size fraction of +0.045 mm was used as a flotation feed. The sample contained - 20.9 % P205, 25.8 % acid insoluble (A.I) and 13.6 % loss on ignition (L.O.I). XRD and mineralogical analysis showed that phosphate was present as francolite while the main gangue minerals, in decreasing order, were quartz, calcite and montmorillonite. Flotation tests were performed using oleic acid (Westvaco, USA) as a collector for either phosphate or calcite. Custamine 6622-62C (cationic fatty amine acetate) by Westvaco, USA, was used as a cationic collector for silica. NH4OH solution (BDH product, UK) was used as a pH modifier in the alkaline region. Either H3PO4 or H2SO4 (BDH product, UK) was used as a modifier. Sodium silicate was used as a silica depressant. Methods

The flotation tests were conducted using Denver D12 flotation machine. In phosphate flotation, a sample was conditioned with pre-determined amounts of sodium silicate (0.25 kg/t) and oleic acid-fuel oil mixture (2.0 kg/t). The pH was maintained constant at 9.5. Three flotation processes were tested to separate calcite from the phosphate. Calcite was floated with 2.0 kg/t oleic acid using H3PO4 or H2SO4 (pH 5.5) as a phosphate depressant. Addition of 0.3 kg/t aluminum sulfate and 0.6 kg/t Na-K tartrate as a depressant, at pH 5.5 or 7.8, was also tested. Cationic flotation of silica was performed using 0.5-kg/t amine at neutral pH. The different alternatives for flotation of both calcite and phosphate were also investigated. In these experiments the samples were subjected to de-oiling using H2SO4 and rinsing with water twice before the second stage of flotation.

RESULTS Flotation of carbonate

Table 1 shows the results of flotation of carbonate by different processes. The results illustrate that application of these processes can reduce carbonates where LOI is decreased from 13.6 %, in the feed, to about 6.9-8.9% in concentrates. The results also show that phosphoric acid gives better grade and recovery than the other techniques. However, the general low grade of concentrates confirms that application of a single stage for flotation of carbonate or phosphate (as in Sebayia West flotation plant) is not enough and application ofa flowsheet to treat both calcite and silica is essential to produce high-grade concentrates. TABLE 1 Results of single-stage flotation for carbonate

Case

1

2 3 4

Type of Depressant Aluminum sulfate and sodium potassium tartrate (at pH 5.5) Aluminum sulfate and sodium potassium tartrate (at pH 7.8) Phosphoric acid at pH 5.5 Sulfuric acid at pH 5.5 Plant data for single phosphate flotation (rougher + 2 cleaning stages)

P205 % 22.5

g.l.

% 22.9

L.O.I. % 8.85

% P205 Recovery 92.9

21.7

22.1

8.11

93.2

23.2 23.0 22.924.1

21.9 21.4 11.012.8

6.87 7.43

93.2 83.0

Evaluation of flotation strategies for sedimentary phophates with siliceous and carbonates gangues

791

Design of two circuits for flotation of carbonate and phosphate A circuit consists of a stage for phosphate flotation and a second for calcite, or vice versa, has been studied using the different techniques mentioned before. Table 2 illustrates these results. These results indicate that the best design is to float carbonate in the fn'st stage (case 7 or 8) followed by phosphate in the second stage. In these cases, better grade is obtained in comparison to the other designs. The results in Table 2 also reveal that the grade obtained while using H3PO4 (case 7 or 9) is always better than its respectiw,• option with H2SO4 (case 8 or 10). TABLE 2 Design of two flotation circuits for carbonates and phosphate Case

10

Design of flotation circuits Case 1 + flotation of phosphate Case 2 + flotation of phosphate Case, 3 + flotation of phosphate Case 4 + flotation of phosphate Rew,~rse to case 7 (Flotation of phosphate followed by ease 3) Rew~rse to case 8 (Flotation of phosphate followed by ease 4)

P205 % 25.2 25.2 27.9 26.1 25.02

A.I.

% 19.5 20.7 17.2 19.1 19.2

L.O.I. % 8.4 8.1 6.87 6.8 7.8

% P205 Recovery 86.7 86.1 80.45 77.9 84.3

24.2

20.5

7.9

76.5

Design of two circuits for flotation of carbonate and silica A design of two circuits for flotation for both carbonate and silica impurities is also studied, the results of which are shown in Table 3. It is known that amine flotation is very sensitive to the presence of slimes and in turn lower grade and recovery are obtained in case of floating silica gangue in the first stage before flotation of carbunates.-Anionic flotation of carbonate in the first stage, followed by a second stage for amine flotation of silica, gives the best grade and recovery. The results in Table 3 also show that P205 can be successively increased to 30.7 % with decreasing the feed size to -0.15 mm. TABLE 3 Design of two flotation circuits for carbonates and silica impurities Case

11 12 13 14 15 16

Design of flotation circuits Case 3 + amine flotation of silica Case 4 + amine flotation of silica Reverse to case 3 (Amine flotation of silica + Case 3) Reverse to case 4 (Amine flotation of silica + Case 4) Case 11 (Flotation feed 0.20 x 0.045 mm) Case 11 (Flotation feed 0.15 x 0:045 mm)

P205 % 27.2 26.1 26.3

A.I.

L.O.I. %

% 17.0 18.6 18.6

6.87 7.43 7.9

% P205 Recovery 85.9 80.3 80.4

24.4

21.6

6.7

72.8

29.11 30.7

14.8 10.2

6.43 6.2

78.4 72.6

DISCUSSION Extensive studie.,~have been conducted to achieve effective separation of dolomite from Florida phosphate (El-Shall et al., 1996). The poor selectivity encountered with flotation of carbonaceous phosphates has been attributed to the similarities in the surface properties of the constituent minerals. The surface properties of phosphate are affected not only by phosphate's own solution chemistry but also by the dissolved species from other salt-t313e minerals in the system (Sumasundaran and Zhang, 1999). These dissolved species can have a marked effect on their interfacial properties. So, the apatite surface can be converted to calcite and vice versa throug~ surface reactions or bulk precipitation of the more stable phase (Ananthapadmanabhan

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N. A, AbdeI-Khalek

and Sumasundaran, 1984; Amankonah and Sumasundaran, 1985). The equilibrium governing the conversion of apatite to calcite can be written as: Cal0(PO4)6(OH)2(s) + 10 CO32- = 10 CaCO3(s) + 6 PO42- + 2OHOn the other hand, the solution chemistry of'fatty acids is another important element in the anionic flotation of phosphate where Ca2+, present in solution, affects the grade of concentrates by activating quartz through formation of calcium-bearing precipitates at high pH (Dho and Iwasald, 1990). The results in Table 2 (case 9 or 10) and Table 3 (case 13 or 14) may confirm this finding where low grade concentrates are produced when either phosphate or silica is floated in the first circuit before carbonate flotation. It is expected that, under these conditions, the concentration of Ca2+will be high leading to such poor selectivity. The depression of phosphate in acid media is possibly due to the adsorption (or formation) of aqueous CaHPO4 on its surface, preventing surfactaut ions from approaching the surface of the phosphate particles. Free Ca2+in solution can affect the formation of aqueous CaHPO4 (Sumasundaran and Zhang, 1999). From thermodynamic considerations it can be predicted that selective flotation of carbonates from phosphates in acidic media can be enhanced by minimizing free Ca2+in solutmn and by increasing HPO42- in the system. This can be done by: ( 1) decreas"mg free Ca2+concentration in the system to low values by adding suitable chemical reagents such as sulfuric acid or chelating agents such as oxalic acid, and (2) adding soluble salts to enhance the depression of phosphate minerals (Elgillani and Abouzeid, 1993). The results in Table 1 show that the applied processes for carbonate flotation are effective in reducing the LOL There is, however, a marginal selectivity for phosphoric acid (6.8 % LOI) in comparison to other (7.4-8.9 % LOI) methods. It is expected that both sulfuric and phosphoric acids can minimize the free Ca 2+ in solution but the concentration of HPO42- in the system might be higher in case of phosphoric acid. This may explain such preferential selectivity with phosphoric acid. Meanwhile, the results in Tables 2 and 3 reveal that the best flotation strategy is to float calcite in the first circuit while depressing phosphate with phosphoric acid. This may indicate that the selectivity margin tends to operate in favor of carbonates rather than the phosphate mineral. Als0, the results indicate that application of phosphoric acid as a phosphate depressant gives better selectivity in comparison to the other flotation designs. This could be attributed to the reaction of carbonates with the acid that results in continuous dissolution or removal of surface contaminants and ensures the availability of clean surface suitable for fatty acid adsorption. This may also reduce the precipitation of ionic species, which leads to mutual transformation of apatite in to calcite or vise versa. In addition, during such dissolution process, CO2 micro-bubbles are generated on the carbonate particle surfaces leading to enhanced oleic acid adsorption. On the other hand, the reaction of phosphoric acid with the apatite particles may produce high concentrations of orthSphosphate ionic species in the surface water layers surrounding the mineral particles. The phosphate-rich water layer is reported to be strongly hydrogen bonded to apatite and therefore depresses its flotation (Bertolucci et al., 1968).

CONCLUSIONS Among three different processes for carbonate flotation, better selectivity is noticed while using phosphoric acid as a depressant for phosphate. A flotation circuit of two stages (one for carbonate and another for either phosphate or silica) presents the best strategy for upgrading calcitic siliceous phosphates. The design of these flotation circuits is very important in determining the selectivity of the overall process. Separation of the associated carbonates (calcite) in the first flotation circuit seems to be essential where the selectivity margin tends to operate in favor of carbonate rather than phosphate mineral. The beneficial role of fine grinding of flotation feed to -0.15 mm is illusWated where concentrates with P205 above 30 % are obtained using such flotation strategy.

Evaluation of flotation strategies for sedimentary phophates with siliceous and carbonates gangues

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REFERENCES

AI-Fariss, T.S.; Ozbelge, H.O.; El-Shall, H., On the phosphate rock beneficiation for the production of phosphoric acid in Saudi Arabia, d. King Saud Univ. Eng. Sci., 1992, 4(1), 13-32. Amankonah, J.O., and Sumasundaran, P., Effects of dissolved mineral species on the electrokinetic behavior of calcite and apatite, Colloids and Surfaces, 1985, 15(3--4), 335-353. Ananthapadmanabhan, K.P., and Sumasundar~, P., The role of dissolved mineral species in calcite-apatite flotation, Minerals and Metallurgical Processing, 1984, 1(1), 36-42. Anazia, I.J. and Hanna, J., New flotation approach for carbonate phosphate separation, Minerals and Metallurgical Processing, 1987, November, 4(4), 196-202. Bertolucci, M., Jantzef, F. and Chmberlain, D.L., Chemistry of organic-inorganic interfaces, In Interaction of Liquids at Solid Substrates, ed. R.F. Gould. American Chemical Society, Advances in Chemistry Series, Washington, D.C., 1968, pp. 124-132. Dho, H. and Iwasaki, I., Role of sodium silicate in phosphate flotation, Minerals and Metallurgical Processing, 1!990, 7(4), 215-221. Elgillani, D.A. and Abouzeid, A.Z.M., Flotation of carbonates from phosphate ores in acidic media, International Journal of Mineral Processing, 1993, 38(3 -4), 235 -256. El-Shall, H., Zhang, P., and Snow, R., Comparative analysis of dolomite-francolite flotation techniques. Minerals and Metallurgical Processing, 1996, 13(3), 135-140. Moudgil, B.M., and Sumasundaran, P., Advances in phosphate beneficiation, In Advances in Mineral Processing,/~xbiter Symposium, SME, 1986, pp. 426-441. Smani, S.; Cases, J.M.; and Blazy, P., Beneficiation of sedimentary phosphate ore, Tram SME/AIME, 1975, 258(2), 176-182. Sumasundaran, P. and Zhang, L., Role of surface chemistry of phosphate in its beneficiation, In Beneficiation of Phosphates: Advances in Research and Practice, ed. P. Zhang, H. El-Shall and R. Wiegel. SME, CO, USA, 1999, pp. 141-154.

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