InJernalional Journal o/mineral Processing, 38 (1993) Elsevier Sciencc Publishers 8. Y., Amsterdam

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1 InJernalional Journal o/mineral Processing, 38 (1993) Elsevier Sciencc Publishers 8. Y., Amsterdam K. Zhong', T. V. Vasudevan and P. Somasundaran* Henry Krumb School of Mines, Columbia University, New York. NY USA (Received 15 June 1992; accepted after revision 2 November 1992) ABsrRACf A multi-pronged approach involving dissolution, adsorption, precipitation and flotation studies was used to determine the role of surface area and porosity on the floatability of apatites of different type, sedimentary or endogenic, and origin. When the samples were not aged prior to collector (p0- tassium oleate) addition, the floatability was controiled by the dissolution (of calcium) and adsorption (of oleate) behaviors which, in turn, were governed by the surface area. It appears that the surface constituted by pores had a lesser influence than the external surface, on the adsorption and the dissolution characteristics. This was suggested to be due to the slow diffusion of calcium through the pores, which results in reduced dissolution rate, as well as the non-participation of a substantial portion of the pores in the adsorption process. When the samples were aged prior to oleate addition, bulk precipitation of calcium oleate complex was found to playa crucial role. Since bulk precipitation is not an interfacial process, the effect of surface area was less with the aged samples. INTRODUCTION ;.\ '. t;,.: Flotation is the most widely used method for beneficiating endogenic and sedimentary phosphate ores. The flow sheet designed for processing these ores depends on both the type of ore, endogenic or sedimentary, and on the nature of impurities, quartz or carbonates, to be removed. In the case of endogenic phosphate ores, removal of siliceous impurity is usually accomplished in a single step involving flotation of phosphate minerals at alkaline ph using fatty acid (anionic surfactant) as the collector (Houot, 1982). Sedimentary ores, on the other hand, require an additional flotation step, involving use of amines ( cationic surfactants ), for the removal of the entrained quartz from the first stage (Houot, 1982; Lawver et al., 1982). Higher quartz entrainment encountered with sedimentary ores, compared with endogenic ores, is possibly due to the use of relatively large fatty acid dosage which produces voluminous 'Present address: Wuhan Institute of Chemical Technology, China. *Corresponding author Elsevier Science Publishers B. V. All rights reserved.

2 178 K. ZHONG ET AL froth. High fatty acid requirement of sedimentary phosphates, relative to that of endogenic phosphates, can be attributed to their low floatability. Higher floatability of the endogenic phosphates than that of the sedimentary phosphates, has been attributed to the differences in both the physical properties and the chemical composition (Sorensen, 1973; Mishra, 1979; Houot, 1982; Mishra, 1982). Sorensen (1973) found that the floatability of apatite depends on its fluorine content with the floatability increasing with increase in the fluorine content. This implies that the higher fluorine content of the endogenic phosphates (Houot, 1982) is the reason for their greater floatability than that of the sedimentary phosphates. Increase in floatability with increase in fluorine content, in turn, has been attributed to the hydrogen bond fonnation between the oxygen atoms of the oleate and the fluorine atoms of the apatite (Sorensen, 1973). However, chemisorption of oleate on to calcium sites of apatite is the more widely accepted adsorption mechanism (Du Rietz, 1975; Hanna and Somasundaran, 1976; Ananthapadmanabhan and Somasundaran, 1985). Mishra (1979, 1982) has reported that, to achieve similar flotation recoveries, ten times higher oleate concentration is required for an amorphous fluorapatite than for a well crystallized apatite. Higher floatability of crystalline apatite has been attributed to the sharp edges of the particles in this sample, which were not present on the particles of the amorphous phosphate mineral. Houot (1982), on the other hand, attributes the greater floatability of the endogenic phosphates to their smaller surface area. The effect of surface area on the floatability can be two-fold: (i) To achieve a given required surface coverage, higher surfactant levels will be required for finer particles; (ii) the rate of dissolution of calcium ions, which can deplete the dissolved oleate by precipitation, will be higher for finer particles. The contributions of these two factors have not been systematically investigated in the past. It is clear from the above discussion that a definite answer does not exist regarding the origin of the difference, either physical properties or the chemical composition, in the floatability behavior of apatites of endogenic and sedimentary nature. In the present study, the effect of both the type of phosphate ore, endogenic or sedimentary, and the surface area on the oleate floatability of apatite has been systematically investigated. A multi-pronged approach consisting of flotation, adsorption and precipitation studies was used to identify the controlling mechanisms involved in the anionic flotation of different types of apatites. EXPERIMENTAL M ateria/s Minerals: Two samples of apatite of the endogenic type and two of the sedimentary type were used. The origin of these apatites and their P20S contents

3 FLOAT ASI UTY OF DIFFERENT APATITES: ROLE OF SU RF ACE AREA AND POROSITY 179 TABLEt are given in Table I. The two endogenic fluorapatites, Australian and Canadian, and the sedimentary apatite from Idaho, collophane, were purchased from Ward's Scientific Establishment, Inc. The francolite was supplied by Agrico Mining Co., Horida. All four apatite samples were crushed in an agate mortar and screened using 65 and 150 mesh sieves. Oversize fractions were again crushed and screened. 65 X 150 mesh fraction was used for all the experiments. Reagents: Oleic acid specified to be of greater than 9 purity was purchased from Aldrich Chemical Co. Potassium hydroxide and nitric acid, used as ph modifiers, and potassium nitrate, used for adjusting the ionic strength, were obtained from Fisher Scientific Co. Potassium oleate solution was prepared by dissolving oleic acid in potassium hydroxide solution at ph II. Methods Dissolution tests: One gram of apatite was added to 100 ml of I X 10-3 kmol/m3 KNO3 solution in a 150 ml beaker. A calcium ion selective electrode (Orion Model 93-20) and a ph electrode were dipped into the slurry which was stirred for a specified time period using a magnetic stirrer. The ph of the slurry was maintained at 9.5 :!:O.I and the response of the calcium selective electrode was recorded at known time intervals. Flotqtion tests: A modified Hallimond tube was used for the flotation studies. One gram of apatite was added to 100 ml of I X 10-3 kmol/m3 KNO3 solution containing a known amount of potassium oleate, and stirred for 5 minutes. ph of the slurry was maintained at 9.5:!: 0.1 during the conditioning. The slurry was then transferred quickly into the Hallimond tube and the flotation was carried out for 30 seconds using nitrogen at a flow rate of 80 mil min. In tests where the effect of aging was studied, one gram of solids was stirred with 50 ml of I X 10-2 kmol/m3 KNO3 solution for 10 minutes following which 50 ml of I X 10-2 kmol/m3 KNO3 solution containing the potassium oleate was added. The procedures for conditioning and flotation were the same as discussed above. It is to be pointed out here that no measurable amount, by weight, of fines smaller than 105 microns ( 150 mesh) were gen-

4 180 K. ZHONG ET AL erated during conditioning and flotation and, therefore, the surface area measured for the feed sample was used in the calculations. Turbidity measurement: The turbidity was measured using a DRT -1 OOB turbidimeter made by HF Scientific Inc. In apatite-oleate systems, the suspension used for turbidity measurements was obtained after centrifuging the slurry at 3000 g for 5 minutes in order to remove the apatite fines. Control studies using mixtures of calcium chloride and potassium oleate solutions showed that at the centrifugal force and the time used, calcium oleate precipitates did not settle down. Determination of oleate concentration: Dissolved oleate concentration was measured using a Dohrmann DC 90 total organic carbon (TOC) analyzer. Before the analysis, apatite particles were separated from the slurry using the procedure described under the turbidity measurement section. Calcium oleate precipitates were separated by centrifuging the suspension at 15,000 g for 120 minutes using a Sorvall RC-5B refrigerated centrifuge. RESULTS AND DISCUSSION Surface areas of apatites Specific surface areas of the four apatite samples, obtained by BET method using nitrogen as the adsorbate, are given in Table 2. This table also shows the specific external surface area (range) calculated from the average particle size (mesh size) using the correlation of Harris (1959) as well as that oforr and Dallavale (1959 )*. It can be seen that the measured specific surface areas TABLE 2 Comparison of experimental and calculated surface areas. Calculated external surface area-: lOm2/gm Sample - Australian fluorapatite Canadian fluorapatite Francolite, Florida Collophane, Idaho Surface area, m2/gm experimental S.80 Pore area, % estimatedb "Calculated using the correlations of Harris (1959) and Off and Dallavale (1959). bestimated using the calculated value of external surface area and the experimentally determined BET surface area *Harris has correlated the mesh size with the surface area obtained by the air permeability method (external surface area; includes surface roughness) for coal particles, while Off and Dallavale have provided conversion factors for obtaining BET surface area (which includes pore area) from the surface area obtained by the air oerrneability method for a variety of minerals.

5 FLOAT ABIUTY OF DIFFERENT APATITES: ROLE OF SURFACE AREA AND POROSITY 181 of the two sedimentary apatites, collophane and francolite, and the endogenic Canadian fluorapatite are much higher than the calculated external surface area. This suggests that the surface of these three apatite samples, unlike that of the endogenic Australian fluorapatite, is constituted predominantly of pores. Floatabilities of apatites Aoatabilities of the four apatite samples are compared in Fig. 1. Aotation curves shown in this figure are nonnalized based on surface area; it provides a comparison of the amounts of oleate required to float a unit surface area of apatite. The flotation curves obtained for the two sedimentary phosphates, francolite and collophane, and the endogenic Canadian fluorapatite overlap indicating that the floatability is determined only by surface area for these three samples. However, a much higher oleate concentration is necessary to float a unit surface area of the endogenic Australian fluorapatite. At least three different phenomena are involved in the anionic flotation of phosphate minerals, namely the dissolution of calcium ions from the surface, adsorption of oleate on the surface and bulk precipitation of calcium-oleate complex. Of these, dissolution and adsorption are interfacial phenomena and, therefore, the dissolution rate and the total adsorption depend on the particle surface area. Relative contributions of these three factors would determine INITIAL OLEATE CONCENTRAnON X 10$ kmol./m3 z SPECIFIC SURFACE AREA m II Fig. I. Comparison of floatabilities of different apatites. Aotation curves nonnalized based on surface area.

6 182 K.ZHONGETAL the collector dosage required to obtain a given flotation recovery. It is suggested that in the case of the two sedimentary apatites and the endogenic Canadian fluorapatite, dissolution (rate) and adsorption control the flotation process, as the floatability depends only on the surface area. Dissolution characteristics Dissolution behavior of the four apatite samples is compared in Fig. 2 which shows the concentration of dissolved calcium per unit surface area of the mineral as a function of time. It can be seen that the dissolution curves of the two sedimentary phosphate minerals, francolite and collophane, overlap indicating the sole dependence of the dissolution rate on the surface area. However, such a dependence is not seen for the case of the two endogenic phosphate minerals: the specific rate of dissolution of the Australian fluorapatite is much higher than that of the Canadian fluorapatite. Also, the dissolution rate of the Canadian fluorapatite is closer to that of the sedimentary phosphates. In the case of the sedimentary phosphates and the Canadian fluorapatite, pore area contributes predominantly to the overall surface area and it is possible that the diffusion through the pores control the dissolution rates. This can account for the much lower specific rates of dissolution of the sedimentary phosphates and the Canadian fluorapatite compared to that of the Australian fluorapatite. Comparison of figures I and 2 shows a good correlation between the flotation and the dissolution characteristics exhibited by the four apatite samples. --a ',I.1 01,e: r.. c... M N +. Co)..' I. AGING TIME. min. Fig. 2. Comparison of the dissolution behavior of different apatites. Dissolution curves nor malized based on surface area.

7 A.oA T ABIUTY OF DIFFERENT APATITES: ROLE OF SURFACE AREA AND POROSrrY 183 The dissolution rates of the two sedimentary phosphates and the Canadian fluorapatite are much lower than that of the Australian fluorapatite, whereas their floatability is much higher than that of the Australian fluorapatite. This suggests that the oleate is depleted by the dissolved calcium and that the calcium dissolution rate should control the flotation process under the experimental conditions employed. Adsorption characteristics Flotation and dissolution behavior of the four apatite samples, discussed in the previous sections, suggested the possibility of bulk precipitation of oleate with calcium. Therefore, to accurately determine the real adsorption density using the solution depletion method it is necessary to determine first the amount of oleate lost by precipitation. Precipitation studies To determine the level of oleate precipitation by turbidity measurements, calibration using mixtures of standard solutions of calcium chloride and potassium oleate was carried out first. The initial concentration of potassium oleate was maintained at 1.2 X 10-4 kmoljm3, while the calcium chloride concentration was varied in the range of5x 10-8to 7.4x 10-5 kmoljm3. After mixing the calcium chloride and potassium oleate solutions, the turbidity and the residual calcium ion concentration (using calcium selective electrode) were measured. A plot of turbidity vs. residual calcium concentration is shown ;:I 8? INITlAL CALCIUM CONCENTRATION. kmol./m s Fig. 3. Plot of turbidity vs. residual calcium concentration.

8 186 K. ZHONG ET AL TABLE 4 Dissolved calcium concentration and excess oleate requirement due to aging. Solids concentration = wt%; agingtime= 10 minutes; conditioning time=5 minutes; conditioningph=9.5:t0.1 Sample Ca2+ kmol/m3x 10' Excess oleate kmol/m) x 105 Ratio oleate/ca2+ Australian fluorapatite Canadian Iluorapatite Francolite, Florida Collophane,ldaho S 0- X B. C -'... ;:: z ' Florida. Idaho ph = ' INITIAl OLEATE CONCENTRATION x 10 kmol.jm' 2 SURFACE AREA m / 9 Fig. 5. Comparison offloatabilities of sedimentary apatites. Rotation curves normalized based on surface area. phane, are shown in Fig. 5. It can be seen that the amounts of oleate required to float a unit surface area of the sample is different for the two samples. It is to be noted that the initial oleate concentration and the adsorption density required to obtain a given flotation yield depended only on the surface area and not the origin, when the two sedimentary phosphates were not aged prior to the oleate addition. Absence of correlation between the floatability and the surface area for the aged samples suggests the bulk calcium oleate precipitation (which does not depend on the surface area) to playa crucial role in the flotation process, under the aging conditions.,

9 A.OATABILITY OF DIFFERENT APATITES: ROLE OF SURFACE AREA AND POROSITY 187 CONCLUSIONS (a) By using a multi-pronged approach involving dissolution, adsorption, precipitation and flotation tests the role of surface area on the floatability of apatites was elucidated. (b) The surface of the two sedimentary phosphates, francolite and collophane, and the Canadian fluorapatite, was found to be constituted predominantly by pores. (c) In the case of un aged samples, floatability behavior of different apatites was found to be controlled by their dissolution and adsorption characteristics. ( d) Dissolution and adsorption characteristics were found to be dependent on both the external surface area and the pore area; the influence of pore area, however, appeared to be less important. ( e) Lower rates of dissolution of the two sedimentary phosphates and the endogenic Canadian fluorapatite, than that of the endogenic Australian fluorapatite, were attributed to the slower diffusion through the pores; the result also suggested that a substantial fraction of the pores does not participate in the adsorption process. (f) When the samples were aged, bulk precipitation of calcium oleate was found to playa governing role. (g) The floatability, dissolution and adsorption behaviors of the endogenic Canadian fluorapatite were found to be closer to those of the sedimentary phosphates, than to those of the endogenic Australian fluorapatite suggesting that the surface area, rather than the type of the mineral (endogenic or sedimentary), plays a governing role. ACKNOWLEDGEMENT The authors would like to acknowledge the financial support of the Florida Institute of Phosphate Research (FIPR) and the National Science Foundation (NSF). REFERENCES Ananthapadmanabhan, K.P. and Somasundaran, P., Surface precipitation of inorganics and surfactants and its role in adsorption and flotation. Colloids Surf., 13: Du Rietz, C., Chemisorption of collectors in flotation. In: XI Int. Miner. Process. Congr., pp Hanna, H.S. and Somasundaran, P., Flotation of salt-type minerals. In: M.C. Fuerstenau (Editor), Flotation. A.M. Gaudin Memorial Volume, AI ME Publications, New York, pp Harris, C.C., Dewatering Fine Coal. Ph.D. Thesis, University of Leeds, Leeds. Houot, R., Beneficiation of phosphatic ores through flotation: review of industrial applications and potential developments. Int. J. Miner. Process., 9:

10 188 K. ZHONG ET AL Lawver, J.E., Wiegel, R.L., Snow, R.E. and Hwang, C.L., Phosphate reserves enhancement by beneficiation. Min. Congr. J., 68: 27. Mishra, S.K., The electrokinetic properties and flotation behavior of apatite and calcite in the presence of dodecylamine chloride. Int. J. Miner. Process., 6: Mishra, S.K., Electrokinetic properties and flotation behavior of apatite and calcite in the presence of sodium oleate and sodium metasilicate.lnl J. Miner. Process., 9: Off, Jr, C. and Dallavalle, J.M., Fine Particle Measurement. McMillan, New York, p Sorensen, E., On the adsorption of some anionic collectors on fluoride minerals, J. Colloid Interface Sci., 45 (3): ;.