BENEFICIATION OF A HIGH DOLOMITIC PHOSPHATE ORE: A BENCH SCALE OPTIMIZATION STUDY. K. ZHONG1, T. V. VASUDEVAN and P. SOMASUNDARAN2

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1 Minerals ~ring, Vol. 4, No. 5/6, lip , 1991 Printed in G1'eat Britain /9\ $ ~ \99\ Perpmon Press pic BENEFICIATION OF A HIGH DOLOMITIC PHOSPHATE ORE: A BENCH SCALE OPTIMIZATION STUDY K. ZHONG1, T. V. VASUDEVAN and P. SOMASUNDARAN2 H~nry Krumb School of Mines, Columbia University, New York, NY 10027, U.S.A. Current address: Wuhan Chemical Engineering Institute, Wuhan, China P.R.C. 2 To whom correspondence should be addressed (Received 7 August 1990; revision accepted 26 September 1990) ABSTRACT A three stage flotation scheme was employed to beneficiate a low grade. high dolomitic phosphate ore from Florida. The reagent scheme used. developed on the basis of our previous fundamental studies. consisted of oleic acid added as collector. mineral oil and ethoxylated sulfonate as co-collectors and 1. hydroxy ethylidene 1.1- diphosphonic acid (Dequest 2010) as francolite depressant. The three stage scheme was designed to remove fine dolomite (minus 210 microns) in the first stage. quartz in the secolrd and coarse dolomite (plus 210 microns) in the filral stage. Using a systematic optimizatiolr procedure. which included a comprehensive factorial design. a 420xl05 microns phosphate ore assaying 9.8 wt% P20S' 1.9 wt% MgO and 58 wt% acid insolubles was beneficiated to a product analyzing 31 wl% P20S' 0.9 wt% MgO and 6.0 wt% acid insolubles at a phosphate recovery of 71 wt%. Keywords Optimization; beneficiation; phosphates; carbonates; francolite; dolomite; flotation; oleic acid; ethoxylated sulfonate; phosphonic acids INTRODUCTION Development of a suitable process to beneficiate low grade, high dolomitic phosphate ores is important for extending the rapidly depleting reserves of Florida phosphates. The currently used commercial process of double flotation [1] is not adequate for reducing the dolomitic impurity level to less than 1.0 weight percent MgO in the concentrate, as stipulated by the phosphate industry. During the past decade, studies conducted by investigators in the Florida phosphate industry [2,3,4,5] and the Tennessee Valley Authority [6] have resulted in the development of a number of processes. However., all these studies originated on a bench scale and the fundamentals involved in the separation scheme have not been established. This leads to serious limitations for optimization of these processes and also their use for beneficiating ores of different origin and composition. Recently, Moudgil and Chanchani [7,8] and Ince [9] have conducted some fundamental studies which resulted in the development of two processes for the removal of dolomite from apatite. Extension of these fundamental studies to beneficiate the natural ores on a bench scale was also reported [10]. However, a systematic optimization of the important variables was not attempted and no guidelines are yet available for processing such complex ores. In this study, beneficiation of a high dolomitic Florida phosphate Ore on a bench scale using a three stage flowsheet is discussed. The reagent scheme employed included the use of ethoxylated sulfonate as a co-collector along with the conventionally used oleic acid - 563

2 S64 L bmng et ai. mineral oil mixture, and a phosphonic acid depressant. These reagents were identified through fundamental studies and showed promising results in microflotation tests [II]. Ethoxylated sulfonate was added due to its high salt tolerance since non-selective precipitation of metal- surfactant complexes is reported to be an important reason for the loss in selectivity observed during the separation of salt-type minerals [12]. Another important feature of the current study is the use of a comprehensive factorial design and a systematic optimization procedure employed in each stage. Work described in this paper summarizes the results obtained from the optimization study Materials EXPERIMENTAL Phosohate ore: High dolomitic phosphate ore was supplied by the Florida Institute of Phosphate Research (FIPR). The as-received ore sample was deslimed and screened using 710 and 105 microns screens. The undersize material was discarded while the oversize fraction was crushed and ground, followed by desliming and screening using a 105 microns sieve. The oversize fraction obtained from the latter step had a high dolomite content and, therefore, was mixed with the 110xl05 microns fraction of the as-received ore. The composite material, used as the flotation feed, assayed 11.0 wt% PzOs, 2.2 wt% MgO and 54 wt% acid insolubles. A 420xlOS microns fraction, also used as flotation feed, was obtained by screening the composite 710xl05 microns feed. The 420xl05 microns fraction assayed 9.8 wt% PzOs, 1.9 wt% MgO and 58.3 wt% acid insolubles. Reagents: Collectors: A mixture of fatty acids containing 70 wt% oleic acid, was obtained from Fluka, A. G. Extra heavy white mineral oil and ethoxylated sulfonate, used as co-collectors, were purchased from Sonneborn and sons Inc., and PPG Inc., respectively. Modifier: I, hydroxyethylidine-i,i-diphosphonic acid (Dequest 20 I 0), obtained from Monsanto chemical co., was used as the phosphate depressant. Other Reagents: Nitric acid and potassium hydroxide, used as ph modifying reagents, were supplied by Fisher Scientific company. Calcium, magnesium and phosphorous standards, used for chemical analysis, were purchased from Environmental Resources Associates. Methods Aooaratus: Bench scale flotation was carried out using a Denver. Model D-l flotation machine provided with a 6.9 cm diameter impeller and a 1.5 litre cell. An agitation intensity of 1500 rpm was maintained during conditioning and flotation., f Procedure: Five hundred grams of feed were conditioned at the desired pulp density for a known time interval after which make-up water was added in order to maintain a pulp density of 36 weight percent during flotation. All experiments were carried out using tap water and flotation was continued till completion. Analysis: P20S and MgO contents of the feed and notation products were determined as follows: 5 mls of concentrated hydrochloric acid was added to I gram of solids, and heate.d to dryness. 5 mls of concentrated nitric acid and 5 mls of concentrated hydrochloric acid were then added, heated to expel the fumes, made up to a known volume ( 500 mls or I lit.), and the solution analyzed by inductively coupled plasma emission spectroscody.

3 Beneficiation of a phosphate ore S6S Design of Flowsheet EXPERIMENTAL DESIGN, RESULTS AND DISCUSSION Dolomite and Quartz are the main gangue minerals in the phosphate ore and since these two minerals exhibit vastly different flotation behavior, in terms of their response to the fatty acid collector, it is difficult to remove both the minerals in a single stage. Also, the feed analysis indicated that the major fraction (50 wt%) of the dolomite present was in the coarse size range (plus 210 microns). Since coarse dolomite is known to display poorer flotation response than the finer particles [13], an attempt was made to remove the two fractions in different stages. Therefore, a three stage beneficiation scheme was conceived in which dolomite would be removed in two stages and Quartz in a third stage. Preliminary experiments conducted led to a flowsheet and reagent scheme described in Figure 1. The flowsheet described in the figure was designed to achieve beneficiation by removing the majority of the fine dolomite during the first stage, quartz and some portion of coarse dolomite during the second and the remaining coarse dolomite during the third stage. Feed 500 a Condit/ooing (ph = 5.5) Reagent (In sequence) TIme. min. Deq EO. sulfonate 1 Oleic acid 2 Minerai oil 1 Float 1 (dolomite) Flotation Stage I Waste water Flotation Stage II (Fresh water) Conditioning (ph= 7-7.5) Reagent (In seq.) EO. sulfonate 1 Oleic acid 1 Mineral oil 1 Time,mln Acid washing (ph 4-4.5), 2 min. Sink (quartz + dolomite) C<XJditioning (ph = 5-5.5) Reagent (In seq.) TIme. min. Deq EO. sulfonate 1 Oleic acid 1 Mineral Flotation Stage III Float 2 Sink 1 (dolomite) (Francolite) Fig.l Flowsheet of three stage flotation scheme

4 S66 K. ZHoNG et ai. ~lo~~ion of ~)omite from Francolite - First stare In this stage, fine dolomite was removed by flotation using a mixture of oleic acid and mineral oil (1:1 by weight) as collector and co-collector respectively. Ethoxylated sulfonate was used as an additional co-collector, while Dequest 2010 was used as the francolite depressant. Effects of ph and conditioning pulp density were examined in addition to the effect of reagent dosage using the factorial design. Factorial Desi2n: A five-factor, three-level fractional factorial design [14] was employed in the first stage. Details of various factors and their levels are shown in Table I. Selectivity index as defined below was used as the performance criterion [15]: 51 - % P20S Recovery + % MgO Rejection where suffix I represents the first stage TABLE 1 Factors and levels for stage I Code Factor Level (1) A Oleic acid/mineral oil, g/t (50:50 by weight) B Ethoxylated sulfonate, gft c Pulp density, wto/o D ph E Dequest 2010, 9/T Variance analysis of the effect of various factors was carried out and the results obtained are summarized in Table 2. The parameter F, which represents the effect of a factor or an interaction in a normalized form, is obtained by dividing the residual mean of square of a factor or an interaction by that of the error. F-test indicates the relative significance of a factor over that of the experimental error. A comparison of the F values calculated for different factors showed that at 95% confidence level (Fo.os = 3.74) only the effect of ethoxylated sulfonate was significant (F= 10.2). This suggests that non-selective precipitation of metal-surfactant complexes on minerals is an important reason for the loss in selectivity observed during separation of salt-type minerals and that by using salttolerant reagents selectivity can be improved. Qotjmjzation - First Sta2e:. Optimization for the first stage was carried out following the path of steepest ascent [16,17]. Since it was found from the factorial experiments that only the ethoxylated sulfonate level was significant, optimization of only this factor was attempted while the other parameters were maintained at constant levels. The results of the experiments, summarized in Table 3, showed the selectivity index to increase with ethoxylated sulfonate dosage in the tested range of 50 to 150 gft. However, increase in ethoxylated sulfonate dosage above 100 glt caused excessive foaming making the operation difficult. Hence, from a practical point of view, 100 gft ethoxylated sulfonate would represent the optimum dosage.

5 Beneficiation of a phosphate ore 567 TABLE 2 Variance analysis for stage I Code Factor/ Inter. Sum of square Degrees of freedom Mean of squares F A O.A/M.O B EO. S c Pulp Density D ph E Deq Total TABLE 3 Results of optimization for stage I Oleic acid/minerai oil (1:1) = 600 9/t Dequest 2010 = 40 9/t ph = 5.5 Pulp density = 55 wt% EO Prod. Weight g/t % Assay, 0/0 P20S MgO Ins Dis., % P205 MgO Ins Float Float Float Float Float Experiments described thus far were conducted using a 710xlO5 microns feed. Since 420xlO5 microns fraction is more commonly used in the Florida phosphate plants, an attempt was made to determine the effect of feed size on the flotation performance. The results obtained showed that the optimum condition for flotation established with the 710xlO5 microns fraction held valid also for the 420xlO5 microns feed although the selectivity index value obtained with the latter was higher (Table 4). Therefore, with the practical application of the process in mind, experiments under stages II and III were carried out with the 420xlO5 microns fraction. Flotation of francolite from Quartz - Second Stage The main objective of this stage was to reject the Quartz in the sink. Flotation of francolite using oleic acid/mineral oil mixture and ethoxylated sulfonate was carried out at the natural ph (7-7.5) of the suspension. Since preliminary results showed that a portion of the coarse M F ""1I.-r

6 568 K. ZHoNG et ai. dolomite was also rejected along with Quartz, the selectivity criteria used included dolomite removal efficiency also. TABLE 4 Optimization results for 420 x 105 microns feed EO.S. g/t Prod. Weight % Assay, % P20S MgO Ins Dis., % P 205 MgO Ins 51, Sink Total Sink Total Sink Total : Thus, SII = [2 x % PzOs recovery + % (quartz + dolomite) rejection - 200] / 2 The type of separation attempted in this stage is widely practised in the Florida phosphate industry and is relatively simple. Therefore, the factorial design employed was limited to two factors (oleic acid/mineral oil, A; ethoxylated sulfonate, B) and two levels(40 and 60 g/t of A; 20 and 40 g/t of B). Results from the factorial tests showed that effects of oleic acid/mineral oil and ethoxylated sulfonate, and their interactions, were significant. Optimization studies were conducted following the path of steepest ascent. Optimum reagent dosages found were 40 g/t ethoxylated sulfonate and 118 g/t oleic acid/mineral oil mixture. The maximum selectivity achieved was 49.5 which corresponds to a phosphate recovery of 89 wt% and, dolomite and Quartz removal efficiencies of 26 and 95 wt%, respectively. Coarse Dolomite Flotation - Third Sta26 In this stage. oleic acid - mineral oil mixture and ethoxylated sulfonate were used as collectors for dolomite and Dequest 2010 as a francolite depressant. as in~e first stage. Since the feed to this stage was obtained after the original feed was processed through two stages the amount and. therefore. the pulp density used in the third stage was relatively lower (25 wt%). Also. since the effects of ph and pulp density were not found to be significant in the first. stage. these. factors were maintained at fixed levels. serectivity index, used was the same as 10 the f 1.rst stage; A three-factor, two-level factorial design was used and various factors and levels employed are summarized in Table 5. Variance analysis, carried out as described in:~~age I, showed that at 95% confidence level (FO.O5-10) both ethoxylated sulfonate (F- ~) and Dequest 2010 (F - 199) dosages were significant variables (Table 6). The import.ce of Dequest 2010 dosage can be attributed to the enhanced hydrophobicity of francolite ijstage III feed, caused by oleate treatment in stage II. "? (2)

7 Beneficiation of a phosphate ore TABLE 5 Factors and levels for stale III S69 Cod. Factor Level, gft.1 +1 A Dequest B EO.S c O.A./M.O TABLE 6 Variance analysis for stage III Factor/ Inter. Sum of squares Degrees of freedom Mean of squares F A Deq B EO.S c O.A.jM.O AxB Error Total 1379 '7 Optimization, carried out following the path of steepest ascent, showed that the optimum reagent scheme was oleic acid/mineral oil at 182 g/t, ethoxylated sulfonate at 57 g/t and Dequest 2010 at 57 g/t. Selectivity index obtained under the optimum condition was 43.8 which corresponds to a P20S recovery of 92.4 wt% and a dolomite rejection of 51.4 wt%. Overall Flowsheet Beneficiation of 420xlOS microns flotation feed using a three stage beneficiation scheme, under optimized conditions, produced a phosphate concentrate containing 28 wt% P20S and 1.3 wt% MgO. Marginally lower P20S level in the phosphate concentrate was easily increased to the level desired by the phosphate industry (> 30 wt P20S) by employing cleaning operations subsequent to stages II and III which removed the entrapped Quartz and the dolomite gangue, respectively. The concentrate thus obtained using the three stage beneficiation scheme, which includes cleaning steps subsequent to stages II and III, contained 31 wt% P20S, 0.9 wt% MgO and 6.0 wt% acid insolubles. The phosphate recovery was 71 wt%.

8 S70 IC.. ZH>NO el ai. CONCLUSIONS A reagent scheme which included the use of ethoxylated sulfonate as a co-collector, to the conventionally used oleic acid/mineral oil mixture, and a phosphonic acid francolite depressant (DeQuest 2010) was employed to beneficiate a high dolomitic Florida phosphate ore. Using a three stage beneficiation scheme, which included two cleaning steps, a flotation feed containing 9.8 wt% P2.5.' 1.9 wt% MgO and 58 wt% insolubles was concentrated to a product assaying 31 wt% P20S, 0.9 wt% MgO and 6 wt% insolubles at a phosphate recovery of 71 wt%. Comprehensive factorial design followed by optimization was carried out for all three stages. In the first stage, where the majority of the fine dolomite particles were removed, only the effect of ethoxylated sulfonate was found to be significant. However, in the third stage, employed for the removal of coarse dolomite, effects of both ethoxylated sulfonate and Dequest 2010 were found to be significant. The effectiveness of ethoxylated sulfonate was attributed to its salt-tolerance property due to which non-selective precipitation of metal-surfactant complexes is minimized. The importance of Dequest 2010 in stage III was suggested to be due to the enhanced hydrophobicity of francolite in the feed to that stage. It was shown that by using an appropriate combination of reagents with conditions and dosage optimized systematically, a low grade complex phosphate ore can be beneficiated to a high grade concentrate at a reasonably good phosphate recovery. ACKNOWLEDGEMENTS The authors wish to acknowledge the Florida Institute of Phosphate Research (FIPR Project # ) and the National Science Foundation (NSF Project # MSM ) for their financial support of this work. I. 2. REFERENCES Crago, A. Process of Concentrating Phosphate Minerals U. S. Patent No (1940). Lawver, J.E., Murowchick, B.L. & Snow, R.E. Beneficiation of South Florida High Carbonate Phosphates 15MA-. Technical Economic Con!erel,ces, Orlando, Florida, Preprints TA/78/1 (1978). 3. Snow. R.E Beneficiation of Phosphate Ores U.S Patenl No (1979). 4 Dufour, P., Pelletier, B., Predali, J.J. & Ranchin, G. Beneficiation of South Florida Phosphate Rock with High Carbonate Content Proceedings, Second InternaJional Symposium on Phosphorous Compounds, Boston, p. 247 (1980). s. 6. Lawver, J.E. Wiegel, R.L. Snow, R.E. & Hwang, C.L. Phosphate Reserves Enhancement by Beneficiation Mining Congress Journal, 68, p. 27 (1982). Lehr, I.R. & Hsieh, S.S. Beneficiation of High Carbonate Phosphate Ores U. S. Patent 4'.287,053 (1981). 7. Moudgil, B.M. & Chanchani, R. Flotation of Apatite and Dolomite using Sodium Oleate as Collector Minerals and Metallurgical Processing, 2(1), p.13 (1985). 8. Moudgil, B.M. &; Chanchani, R. Selective Flotation of Dolomite from Francolite using Two-stage Conditioning Minerals and Metallurgical Processing, 2(1), p.19 (1985).

9 Beneficiation of a phosphate ore Ince, D.E. Effect of Sodium Chloride on the Selective Flotation of Dolomite from Apatite Ph.D Thesis. University 01 Florida (1987). 10. Moudgil, B. M., Ince, D., Vasudevan, T. V. & Sober, D. Bench-Scale Optimization of the Two-Stage Conditioning Process for Apatite-Dolomite Separation Minerals and Metallurgical Processing, 7(1), p. 53 (1990). 11 Somasundaran, P. Beneficiation of Dolomitic Phosphates, Final Report (Jan. 1989), Florida Institute of Phosphate Research, Bartow, Florida. 12. Ananthapadmanabhan, K. P. & Somasundaran, P. Role of Dissolved Mineral Species in Calcite-Apatite Flotation Minerals and Metallurgical Processing, 1(2), p. 36 (1984). 13, Moudgil, B.M. Separation of Dolomite from South Florida Phosphate Rock - Phase II, Final Report (July 1987), Florida Institute of Phosphate Research, Bartow, Florida. 14. Anon. Normal Statistical Methods (in chinese), p. 68, Institute of Mathematics of the Academy of Sciences of China (1975). 15 Sresty, G. C. & Somasundaran, P. Selective Flocculation of Synthetic Mineral Mixtures Using Modified Polymers Int. J. Min. Process., 6, p. 303 (1980). 16. Cochran, W.G. & Cox, G.M. Experimental Designs 2nd Edn, p. 357, John Wiley & Sons, New York (1957). 7. Somasundaran, P. & Prickett, G. O. Optimization of Flotation Operation using Statistical Methods Trans AIME, 244, p.369 (1969).

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