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Beneficiation of borax by reverse flotation in boron saturated brine
Journal of Colloid and Interface Science 290 (2005) 426–430 www.elsevier.com/locate/jcis
Beneficiation of borax by reverse flotation in boron saturated brine Emin Cafer Çilek ∗ , Hasan Üresin S. Demirel University, Department of Mining Engineering, Mineral Processing Division, TR 32260, Isparta, Turkey Received 24 November 2004; accepted 16 April 2005 Available online 6 June 2005
Abstract Flotation is one of the plausible methods for recovering borax fines discharged as fine waste to the tailings dam in the Kirka borax processing plant. A literature review dealing with the flotation behavior of boron minerals reveals that clay minerals in the boron ores coat boron minerals and thus deteriorate the quality of boron concentrates produced by direct flotation. The main objective of this study is therefore to recover borax fines from the tailings of the concentrator by reverse flotation. A three-level-factor experimental design was used to determine the main and interaction effects of variables selected on the metallurgical performance of reverse flotation. An analysis of variance for experimental results indicates that interaction effects of the variables for concentrate quality and recovery of B2 O3 is nonsignificant and the most important variable for grade of concentrate and recovery is the collector dosage. It is shown that a concentrate assaying 11.25% B2 O3 with 89.90% B2 O3 recovery could be produced by means of single-stage (rougher) reverse flotation. Additionally, in order to produce a sufficient-quality concentrate, a multistage reverse flotation scheme involving rougher, scavenger, and two cleaners was devised. A final concentrate containing 23.47% B2 O3 with 81.78% B2 O3 recovery was obtained from these tests. The reverse flotation method can be thus considered as an important option for the beneficiation of borax fines. 2005 Elsevier Inc. All rights reserved. Keywords: Borax; Reverse flotation; Fine particles; Slime coating; Experimental design
1. Introduction Similarly to the processing of other boron minerals of commercial importance, borax is concentrated through scrubbing followed by classification to remove undesirable clay minerals in the Kirka borax processing plant in Turkey. Since borax is a soluble boron mineral, the saturated boron brine is used as a liquid phase in the boron derivatives plant. However, not only the process flowsheet used to produce the borax concentrate in the plant but also the nature of gangue minerals within the ore tends to discharge large amounts of clay minerals together with unrecoverable borax fines in the tailings dam. Recovery of borax from the tailings dam in this plant is very important because disposal of this fine-sized material causes environmental pollution as well as economic * Corresponding author.
E-mail address: [email protected] (E. Cafer Çilek). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.04.046
loss. Additionally, recovery of borax from this waste material has great economic and operational advantage because industrial application of a suitable process can proceed easily without additional mining operations [1,2]. Considerable literature exists on the electrokinetic behavior and flotation responses of salt-type minerals such as trona, sylvite, and borax [3–12]. In these studies, collector adsorption in soluble salt flotation with alkyl amines and carboxylates is explained by the heterocoagulation of the oppositely charged collector colloids and soluble salt mineral particles. In other words, due to their high solubilities, salt-type minerals release multivalent cations that react with oppositely charged collector anions to form insoluble colloidal precipitates [3–9]. The effect of montmorillonite-type clay minerals on flotation of boron minerals has been investigated comprehensively by Celik and co-workers [6]. They have concluded that in comparison to the semisoluble boron minerals (colemanite and ulexite), borax is a soluble boron mineral and
E. Cafer Çilek, H. Üresin / Journal of Colloid and Interface Science 290 (2005) 426–430
unique in that it easily floats in its hydrated state but shows almost no flotation in its anhydrous form (Na2 B4 O7 ) with either cationic or anionic collectors at room temperature. To find the reasons for such low recoveries, they performed microflotation studies with boron minerals in the presence and the absence of clay-type minerals. One of their important conclusions is that while all boron minerals floated with both anionic and cationic surfactants in the absence of clay, even as little as 1% clay addition reduced the flotation recoveries considerably. Additionally, effect of clay coating could be minimized either at relatively high collector concentrations or under flotation conditions where clay and colemanite were made similarly charged [6]. One of the physical ways to enhance adsorption of collectors is to condition the flotation pulp by ultrasonic treatment. Celik and co-workers [12] have used in situ sonication in the flotation of colemanite. They have identified that in situ ultrasonic treatment selectively detaches clay particles from the surface of colemanite and facilitates the adsorption of anionic collector molecules. In other words, application of in situ sonication remarkably restores the floatability of colemanite [12]. As far as can be ascertained from the literature, there are limited studies on electrokinetic properties of borax in saturated solutions. The zeta potential of borax in its saturated solutions was measured by Muduroglu and co-workers [11] and Celik and co-workers [8]. Their studies have shown that borax and montmorillonite-type clay minerals carry negative charges at all practical pH values. The potential-determining ions for all boron minerals are the constituent lattice ions, i.e., Na+ (for borax), Ca2+ (for colemanite and ulexite), B4 O2− 7 , and the counterion, as well as the H+ and OH− ions, which control the ratio of 2− HCO− 3 /CO3 [6,9,11]. Although both borax and clay particles apparently carry negative charges, slime coating is still inevitable; this is attributed to the interaction of positively charged sites on the clay edges and negatively charged borax surfaces. In order to design an experimental program, numerous direct flotation tests were carried out. Results of these tests are not given here, because they were not significant from the metallurgical performance point of view. However, it must be noted that even if the floatability of borax fines in this material is sufficient using one of the collector types (anionic and cationic), borax fines could not be successfully separated from the gangue minerals using direct flotation. In the light of the previous investigations and the results of the preliminary tests, the use of a reverse flotation method to recover borax fines was found to be plausible. The main objective of this present study is therefore to investigate the possibility of using reverse flotation to recover borax fines from the tailings of the Kirka borax processing plant. A statistical experimental design was used to elucidate and characterize the relationship between metallurgical performance of reverse flotation and some important variables.
427
2. Experimental 2.1. Material A representative pulp sample was taken from the hydrocyclone overflow discharged to the fine waste tailings dam. Solid content of the pulp sample was 8.5% by weight, which was determined by means of dewatering and drying at 25 ◦ C. Particle size of the solid phase in the pulp sample containing 3.4% B2 O3 was less than 0.038 mm. The main minerals within the ore are borax, montmorillonite-type clay minerals, and carbonate minerals, mainly those of dolomite and calcite [1,3–12]. B2 O3 content of the boron-saturated brine, which is the liquid phase of the pulp sample (pH 9.43), was 15.37 g B2 O3 (∼84.15 g borax/L saturated brine). In order to use it in the tests, a sufficient amount of the boronsaturated brine was prepared by using more than the required amount of borax, considering effects of temperature and pH on the solubility of borax, and then the saturated solution was separated from the insoluble particles by dewatering. 2.2. Method In the preliminary study, several direct flotation tests using different collector types were conducted to determine the floatability of borax fines prior to the designed experiments. As mentioned previously, the results of direct flotation tests were unimportant from the metallurgical performance point of view. In addition to these, a few collectorless flotation tests were performed. The most important point that must be noted from the collectorless flotation tests is that concentrate (float fraction) containing 2.56% B2 O3 with 42.96% recovery was obtained. It was determined that the float fraction consisted of clay minerals. This can only be attributed to the mechanical entrainment and the slime coating. These results clearly demonstrated that the presence of clay has a very important adverse effect on both floatability of borax and grade of the float fraction. It is evident that many cleaner stages are needed to obtain a high-quality borax concentrate. In the light of these findings, it was decided to apply a reverse flotation method for recovering borax fines, and the float fraction or the froth is therefore termed as tailings in following sections. A three level-factor experimental design was used to determine main and interaction effects of variables selected on the metallurgical performance of reverse flotation. A complete replication of this design requires 27 experiments. All flotation tests were performed using a Denver laboratory flotation cell with a 1.5-L cell volume. After 5 min of pH conditioning, the required amount of sodium silicate (Na2 SiO3 ) was added to the pulp. The collector sodium oleate (C17 H33 COONa) was added and conditioned for an additional 2 min. The frother DF-250 [CH3 (OC3 H6 )4 OH] was added to the pulp and conditioned for another 1 min and then the air was introduced. The froth thickness during the
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Table 1 Levels of variables used in the experiments Variables pH Na2 SiO3 dosage (g/t) Na oleate dosage (g/t)
Levels Low
Medium
High
8.0 500 100
9.0 750 150
10.0 1000 200
tests was kept constant by adding saturated solution into the froth zone. Several reverse flotation tests were carried out to determine levels of variables such as collector (sodium oleate) dosage and the pulp pH. In the case of reverse flotation, it is expected that the main gangue minerals must be transported into the froth by true flotation (dolomite and calcite) and mechanical entrainment (clay minerals). On the other hand, the adverse effect of these components by means of coagulation and slime coating must be prevented or minimized. Therefore, sodium silicate (ratio of SiO2 to Na2 O is 2.32) dosage was chosen as the third variable in the experimental design. Grade of concentrate (c) and recovery (R) were chosen as response variables. Levels of the variables used in the experiments are given in Table 1.
3. Results and discussions The gangue minerals, which can be classified into two groups from the flotation point of view, must be transported into the froth in reverse flotation of borax. Results of the collectorless flotation have shown that montmorillonite-type clay minerals can be transported sufficiently into the froth by mechanical entrainment. However, carbonate minerals can be transported into the froth under flotation conditions where these minerals are rendered hydrophobic by use of a carboxylate (e.g., sodium oleate) in an alkaline medium. Results of reverse flotation tests randomly conducted under the conditions described in the previous section are illustrated in Fig. 1. Parallelism of the lines in Fig. 1 indicates that the interaction effects of the variables are negligible for all responses. In other words, the pH–sodium oleate dosage and sodium oleate dosage–sodium silicate dosage variable interactions appear to have no significant effect on the grade of concentrate and recovery. The main and two-order interaction effects of variables on the responses at 95% significant levels are summarized in Table 2. One interesting point to be noted from Fig. 1, however, is that a concentrate of maximum grade with reasonable recovery was obtained from the flotation test conducted at pH 8, which was contrary to what is observed for the flotation behavior of the carbonate minerals. It is well known that the zeta potential of calcite is positive in the pH range 7–11 with a maximum occurring at pH 9.5 and the oleate anions are chemisorbed on the surface of calcite. The maximum adsorption of sodium oleate occurs in the pH range 10–11 [15–18]. For these reasons, this result is unexpected and it may be attributed to inherent difficulties
Fig. 1. Effects of the variables considered on the grade of concentrate (dashed lines) and recovery.
such as high ionic strengths, high-viscosity brines, and the presence of clay minerals. Celik and co-workers [6] present an excellent discussion of the electrokinetic and flotation behavior of boron minerals in the presence and the absence of clay and multivalent ions. As mentioned above, the negligible interaction effect for each response was determined from the experimental results illustrated in Fig. 1. The same conclusions were obtained from an analysis of variance for each response (c and R) given in Table 2. One interesting point to be noted from the analysis of variance is that Na oleate dosage is the most significant variable affecting both grade of concentrate and recovery. In addition to this, values of F statistics indicate that the sodium silicate dosage is relatively more important than
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429
Table 2 ANOVA for grade of concentrate (c, B2 O3 , %) and recovery (R, B2 O3 , %) Source of variance
Degrees of freedom
F calculated [theoretical]
Mean squares R
The pulp pH (1) Na2 SiO3 dose (2) Na oleate dose (3) (1) × (2) (1) × (3) (2) × (3) Error Total
2 2 2 4 4 4 8 26
c
127.78 33.93 809.75 0.44 1.365 0.123 2.77
R 46.11[4.46]*
0.422 0.584 2.465 0.0008 0.0193 0.001 0.020
12.24[4.46]* 292.22[4.46]* 0.16[3.84] 0.49[3.84] 0.044[3.84]
c 21.1[4.46]* 29.078[4.46]* 122.69[4.46]* 0.04[3.84] 0.965[3.84] 0.05[3.84]
* Significant at 95%.
Fig. 3. Flowsheet of multistage reverse flotation tests. Table 3 Results of multistage reverse flotation tests
Fig. 2. Recovery and grade of tincal concentrate as a function of flotation time.
the pulp pH for grade of concentrate. This result shows that decrease in coagulation and/or the slime coating of the clay minerals in the pulp is more important than decrease in mechanical entrainment in producing a high-grade concentrate. This is consistent with what is generally expected in flotation practice. Furthermore, in the light of these evaluations, an additional test was conducted at pH 8, high dosage of sodium oleate, 350 g/t, and a low to medium level dosage of sodium silicate, 650 g/t. A concentrate containing 11.41% B2 O3 with 90.55% recovery was obtained. It must be noted that all flotation tests were conducted as a single stage (or rougher flotation) and flotation time was 2 min in these experiments. However, flotation time is a very critical variable of the process. In order to obtain the desired final grade of concentrate, the rougher flotation is completed at optimum flotation time and the tailing (the froth product in this case) is fed into the scavenger bank. Therefore, optimum flotation time must carefully be determined from flotation kinetic tests [13,14]. For these reasons, a flotation kinetic test was performed to determine the optimum flotation time in this step of the experimental program and results are illustrated in Fig. 2. As can be followed from Fig. 2, increase in grade of the rougher concentrate is negligible above the flotation time of 5 min.
Stages
Products
Grade, B2 O3 (%)
Recovery, B2 O3 (%)
Rougher
CR TR CS TS CC1 TC1 CC2 TC2
11.01 0.27 0.94 0.11 17.26 3.63 24.37 7.42
95.46 5.54 3.71 1.83 81.43 14.03 67.07 14.36
Scavenger Cleaner-1 Cleaner-2
C: concentrate; T : tailing.
However, above this time, the borax particles instead of the gangue minerals are transported into the froth product. Therefore, the optimum flotation time for the rougher flotation was determined to be 5 min. In order to produce a high-grade borax concentrate, the rougher concentrate was cleaned twice and the rougher tailing was also scavenged as shown in Fig. 3. In the conditioning period of cleaner stages and scavenger stage, same reagent dosages, which were employed in the single stage tests, were added into the feed of the scavenger and cleaner stages (i.e., concentrate or tailing of the rougher stage). It is evident from the multistage tests results given in Table 3 that a final concentrate containing 24.37% B2 O3 with 81.78% B2 O3 recovery could be produced. Traditionally, the grade of concentrate is inversely proportional to recovery in the normal flotation operation. In the case of the reverse flotation, however, the grade of concentrate is directly proportional to recovery, as shown in Fig. 4.
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of concentrate with reasonable recovery was obtained from the reverse flotation test conducted at pH 8. This result is not consistent with what is observed for the flotation behavior of the carbonate minerals, it may be attributed to the presence of excessive amount of ions and clay in the pulp. Furthermore, it was observed that multistep cleaning of the rougher concentrate could produce a final concentrate containing 24.37% B2 O3 with reasonable recovery. Based on the results of this study, the reverse flotation method can be considered an important option for the beneficiation of the borax fines.
Acknowledgments Fig. 4. Correlation between grade of concentrate and recovery.
Because the middlings products (i.e., tailings of the cleaner stages and concentrate of the scavenger stage) did not recirculate in the multistage reverse flotation tests, the recovery according to initial feed was low (67.07%). However, these products are normally recirculated in continuous flotation systems such as the locked cycle flotation tests, the pilot scale tests, or a plant scale operation. Therefore, it can be safely assumed from Fig. 4 that a higher quality concentrate with a higher recovery could be produced from one of these continuous systems.
4. Conclusions Flotation is one of the plausible methods for recovering borax fines from the hydrocyclone overflow discharged to the tailings dam in the Kirka borax processing plant. The previous studies, published by several authors cited above, have shown that a common problem encountered in all types of boron minerals is the presence of significant amounts of clay-type minerals, which adversely affect flotation recoveries due to slime coatings. The main objective of this study is therefore to recover borax fines from the tailing of the Kirka borax processing plant by reverse flotation method. In the reverse flotation case, recovery of borax fines can be achieved by transferring of dolomite and calcite into the froth by true flotation, whereas clay minerals are transported by mechanical entrainment. As far as can be evaluated from the experimental results and their statistical treatments, amount of collector (sodium oleate) and amount of dispersant (Na2 SiO3 ) have important role for the production of a high quality borax concentrate. Interestingly, a concentrate containing the maximum grade
The authors are grateful to Eti Boron Co. for providing access to the Kirka borax processing plant and valuable assistance in the sampling campaign and the anonymous referees for their helpful comments on an earlier version of this paper.
References [1] T. Batar, B. Kahraman, E. Cirit, M.S. Celik, Int. J. Miner. Process. 54 (1998) 99–110. [2] A. Gul, G. Bulut, H.M. Tarkan, Y. Kaytaz, Southern Hemisphere Meeting on Minerals Technology, Rio de Janeiro, Brazil, 2001 pp. 189–193. [3] J.D. Miller, S. Veeramasuneni, M.R. Yalamanchili, Int. J. Miner. Process. 51 (1997) 111–123. [4] M.S. Celik, M. Hancer, J.D. Miller, J. Colloid Interface Sci. 235 (2001) 150–161. [5] O. Ozcan, J.D. Miller, Miner. Eng. 15 (2002) 577–584. [6] M.S. Celik, M. Hancer, J.D. Miller, J. Colloid Interface Sci. 256 (2002) 121–131. [7] Y. Akin, M. Hancer, M.S. Celik, in: SME Annual Meeting, Salt Lake City, UT, 2000, pp. 1–6. [8] M.S. Celik, E. Yasar, H. El-Shall, J. Colloid Interface Sci. 203 (1998) 254–259. [9] M. Hançer, Doygun Çözeltilerde Boraksın Flotasyon Kimyasının Incelenmesi, M.Sc. Thesis, ITU Graduate School of Natural and Applied Sciences, Istanbul, 1994, in Turkish. [10] Y. Akın, M.S. Celik, in: Proceedings of Industrial Raw Materials Symposium, 1995, pp. 135–142, in Turkish. [11] M. Muduroglu, M.S. Celik, M. Hancer, J.D. Miller, in: Minerals Processing on the Verge of the 21st Century, Balkema, 2000, pp. 231– 236. [12] M.S. Celik, I. Elma, M. Hancer, J.D. Miller, in: Mineral and Coal Processing, Balkema, 1999, pp. 153–157. [13] G.E. Agar, J. Chia, L. Requis-C, Miner. Eng. 11 (1998) 347–360. [14] B.A. Wills, Mineral Processing Technology, sixth ed., Butterworth– Heinemann, Oxford, 1997, pp. 284–316. [15] C.A. Young, J.D. Miller, Int. J. Miner. Process. 58 (2000) 331–350. [16] Y. Liu, Q. Liu, Miner. Eng. 17 (2004) 855–863. [17] Y. Liu, Q. Liu, Miner. Eng. 17 (2004) 865–878. [18] Y. Cebeci, I. Sonmez, J. Colloid Interface Sci. 273 (2004) 300–305.
Beneficiation of borax by reverse flotation in boron saturated brine Emin Cafer Çilek ∗ , Hasan Üresin S. Demirel University, Department of Mining Engineering, Mineral Processing Division, TR 32260, Isparta, Turkey Received 24 November 2004; accepted 16 April 2005 Available online 6 June 2005
Abstract Flotation is one of the plausible methods for recovering borax fines discharged as fine waste to the tailings dam in the Kirka borax processing plant. A literature review dealing with the flotation behavior of boron minerals reveals that clay minerals in the boron ores coat boron minerals and thus deteriorate the quality of boron concentrates produced by direct flotation. The main objective of this study is therefore to recover borax fines from the tailings of the concentrator by reverse flotation. A three-level-factor experimental design was used to determine the main and interaction effects of variables selected on the metallurgical performance of reverse flotation. An analysis of variance for experimental results indicates that interaction effects of the variables for concentrate quality and recovery of B2 O3 is nonsignificant and the most important variable for grade of concentrate and recovery is the collector dosage. It is shown that a concentrate assaying 11.25% B2 O3 with 89.90% B2 O3 recovery could be produced by means of single-stage (rougher) reverse flotation. Additionally, in order to produce a sufficient-quality concentrate, a multistage reverse flotation scheme involving rougher, scavenger, and two cleaners was devised. A final concentrate containing 23.47% B2 O3 with 81.78% B2 O3 recovery was obtained from these tests. The reverse flotation method can be thus considered as an important option for the beneficiation of borax fines. 2005 Elsevier Inc. All rights reserved. Keywords: Borax; Reverse flotation; Fine particles; Slime coating; Experimental design
1. Introduction Similarly to the processing of other boron minerals of commercial importance, borax is concentrated through scrubbing followed by classification to remove undesirable clay minerals in the Kirka borax processing plant in Turkey. Since borax is a soluble boron mineral, the saturated boron brine is used as a liquid phase in the boron derivatives plant. However, not only the process flowsheet used to produce the borax concentrate in the plant but also the nature of gangue minerals within the ore tends to discharge large amounts of clay minerals together with unrecoverable borax fines in the tailings dam. Recovery of borax from the tailings dam in this plant is very important because disposal of this fine-sized material causes environmental pollution as well as economic * Corresponding author.
E-mail address: [email protected] (E. Cafer Çilek). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.04.046
loss. Additionally, recovery of borax from this waste material has great economic and operational advantage because industrial application of a suitable process can proceed easily without additional mining operations [1,2]. Considerable literature exists on the electrokinetic behavior and flotation responses of salt-type minerals such as trona, sylvite, and borax [3–12]. In these studies, collector adsorption in soluble salt flotation with alkyl amines and carboxylates is explained by the heterocoagulation of the oppositely charged collector colloids and soluble salt mineral particles. In other words, due to their high solubilities, salt-type minerals release multivalent cations that react with oppositely charged collector anions to form insoluble colloidal precipitates [3–9]. The effect of montmorillonite-type clay minerals on flotation of boron minerals has been investigated comprehensively by Celik and co-workers [6]. They have concluded that in comparison to the semisoluble boron minerals (colemanite and ulexite), borax is a soluble boron mineral and
E. Cafer Çilek, H. Üresin / Journal of Colloid and Interface Science 290 (2005) 426–430
unique in that it easily floats in its hydrated state but shows almost no flotation in its anhydrous form (Na2 B4 O7 ) with either cationic or anionic collectors at room temperature. To find the reasons for such low recoveries, they performed microflotation studies with boron minerals in the presence and the absence of clay-type minerals. One of their important conclusions is that while all boron minerals floated with both anionic and cationic surfactants in the absence of clay, even as little as 1% clay addition reduced the flotation recoveries considerably. Additionally, effect of clay coating could be minimized either at relatively high collector concentrations or under flotation conditions where clay and colemanite were made similarly charged [6]. One of the physical ways to enhance adsorption of collectors is to condition the flotation pulp by ultrasonic treatment. Celik and co-workers [12] have used in situ sonication in the flotation of colemanite. They have identified that in situ ultrasonic treatment selectively detaches clay particles from the surface of colemanite and facilitates the adsorption of anionic collector molecules. In other words, application of in situ sonication remarkably restores the floatability of colemanite [12]. As far as can be ascertained from the literature, there are limited studies on electrokinetic properties of borax in saturated solutions. The zeta potential of borax in its saturated solutions was measured by Muduroglu and co-workers [11] and Celik and co-workers [8]. Their studies have shown that borax and montmorillonite-type clay minerals carry negative charges at all practical pH values. The potential-determining ions for all boron minerals are the constituent lattice ions, i.e., Na+ (for borax), Ca2+ (for colemanite and ulexite), B4 O2− 7 , and the counterion, as well as the H+ and OH− ions, which control the ratio of 2− HCO− 3 /CO3 [6,9,11]. Although both borax and clay particles apparently carry negative charges, slime coating is still inevitable; this is attributed to the interaction of positively charged sites on the clay edges and negatively charged borax surfaces. In order to design an experimental program, numerous direct flotation tests were carried out. Results of these tests are not given here, because they were not significant from the metallurgical performance point of view. However, it must be noted that even if the floatability of borax fines in this material is sufficient using one of the collector types (anionic and cationic), borax fines could not be successfully separated from the gangue minerals using direct flotation. In the light of the previous investigations and the results of the preliminary tests, the use of a reverse flotation method to recover borax fines was found to be plausible. The main objective of this present study is therefore to investigate the possibility of using reverse flotation to recover borax fines from the tailings of the Kirka borax processing plant. A statistical experimental design was used to elucidate and characterize the relationship between metallurgical performance of reverse flotation and some important variables.
427
2. Experimental 2.1. Material A representative pulp sample was taken from the hydrocyclone overflow discharged to the fine waste tailings dam. Solid content of the pulp sample was 8.5% by weight, which was determined by means of dewatering and drying at 25 ◦ C. Particle size of the solid phase in the pulp sample containing 3.4% B2 O3 was less than 0.038 mm. The main minerals within the ore are borax, montmorillonite-type clay minerals, and carbonate minerals, mainly those of dolomite and calcite [1,3–12]. B2 O3 content of the boron-saturated brine, which is the liquid phase of the pulp sample (pH 9.43), was 15.37 g B2 O3 (∼84.15 g borax/L saturated brine). In order to use it in the tests, a sufficient amount of the boronsaturated brine was prepared by using more than the required amount of borax, considering effects of temperature and pH on the solubility of borax, and then the saturated solution was separated from the insoluble particles by dewatering. 2.2. Method In the preliminary study, several direct flotation tests using different collector types were conducted to determine the floatability of borax fines prior to the designed experiments. As mentioned previously, the results of direct flotation tests were unimportant from the metallurgical performance point of view. In addition to these, a few collectorless flotation tests were performed. The most important point that must be noted from the collectorless flotation tests is that concentrate (float fraction) containing 2.56% B2 O3 with 42.96% recovery was obtained. It was determined that the float fraction consisted of clay minerals. This can only be attributed to the mechanical entrainment and the slime coating. These results clearly demonstrated that the presence of clay has a very important adverse effect on both floatability of borax and grade of the float fraction. It is evident that many cleaner stages are needed to obtain a high-quality borax concentrate. In the light of these findings, it was decided to apply a reverse flotation method for recovering borax fines, and the float fraction or the froth is therefore termed as tailings in following sections. A three level-factor experimental design was used to determine main and interaction effects of variables selected on the metallurgical performance of reverse flotation. A complete replication of this design requires 27 experiments. All flotation tests were performed using a Denver laboratory flotation cell with a 1.5-L cell volume. After 5 min of pH conditioning, the required amount of sodium silicate (Na2 SiO3 ) was added to the pulp. The collector sodium oleate (C17 H33 COONa) was added and conditioned for an additional 2 min. The frother DF-250 [CH3 (OC3 H6 )4 OH] was added to the pulp and conditioned for another 1 min and then the air was introduced. The froth thickness during the
428
E. Cafer Çilek, H. Üresin / Journal of Colloid and Interface Science 290 (2005) 426–430
Table 1 Levels of variables used in the experiments Variables pH Na2 SiO3 dosage (g/t) Na oleate dosage (g/t)
Levels Low
Medium
High
8.0 500 100
9.0 750 150
10.0 1000 200
tests was kept constant by adding saturated solution into the froth zone. Several reverse flotation tests were carried out to determine levels of variables such as collector (sodium oleate) dosage and the pulp pH. In the case of reverse flotation, it is expected that the main gangue minerals must be transported into the froth by true flotation (dolomite and calcite) and mechanical entrainment (clay minerals). On the other hand, the adverse effect of these components by means of coagulation and slime coating must be prevented or minimized. Therefore, sodium silicate (ratio of SiO2 to Na2 O is 2.32) dosage was chosen as the third variable in the experimental design. Grade of concentrate (c) and recovery (R) were chosen as response variables. Levels of the variables used in the experiments are given in Table 1.
3. Results and discussions The gangue minerals, which can be classified into two groups from the flotation point of view, must be transported into the froth in reverse flotation of borax. Results of the collectorless flotation have shown that montmorillonite-type clay minerals can be transported sufficiently into the froth by mechanical entrainment. However, carbonate minerals can be transported into the froth under flotation conditions where these minerals are rendered hydrophobic by use of a carboxylate (e.g., sodium oleate) in an alkaline medium. Results of reverse flotation tests randomly conducted under the conditions described in the previous section are illustrated in Fig. 1. Parallelism of the lines in Fig. 1 indicates that the interaction effects of the variables are negligible for all responses. In other words, the pH–sodium oleate dosage and sodium oleate dosage–sodium silicate dosage variable interactions appear to have no significant effect on the grade of concentrate and recovery. The main and two-order interaction effects of variables on the responses at 95% significant levels are summarized in Table 2. One interesting point to be noted from Fig. 1, however, is that a concentrate of maximum grade with reasonable recovery was obtained from the flotation test conducted at pH 8, which was contrary to what is observed for the flotation behavior of the carbonate minerals. It is well known that the zeta potential of calcite is positive in the pH range 7–11 with a maximum occurring at pH 9.5 and the oleate anions are chemisorbed on the surface of calcite. The maximum adsorption of sodium oleate occurs in the pH range 10–11 [15–18]. For these reasons, this result is unexpected and it may be attributed to inherent difficulties
Fig. 1. Effects of the variables considered on the grade of concentrate (dashed lines) and recovery.
such as high ionic strengths, high-viscosity brines, and the presence of clay minerals. Celik and co-workers [6] present an excellent discussion of the electrokinetic and flotation behavior of boron minerals in the presence and the absence of clay and multivalent ions. As mentioned above, the negligible interaction effect for each response was determined from the experimental results illustrated in Fig. 1. The same conclusions were obtained from an analysis of variance for each response (c and R) given in Table 2. One interesting point to be noted from the analysis of variance is that Na oleate dosage is the most significant variable affecting both grade of concentrate and recovery. In addition to this, values of F statistics indicate that the sodium silicate dosage is relatively more important than
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Table 2 ANOVA for grade of concentrate (c, B2 O3 , %) and recovery (R, B2 O3 , %) Source of variance
Degrees of freedom
F calculated [theoretical]
Mean squares R
The pulp pH (1) Na2 SiO3 dose (2) Na oleate dose (3) (1) × (2) (1) × (3) (2) × (3) Error Total
2 2 2 4 4 4 8 26
c
127.78 33.93 809.75 0.44 1.365 0.123 2.77
R 46.11[4.46]*
0.422 0.584 2.465 0.0008 0.0193 0.001 0.020
12.24[4.46]* 292.22[4.46]* 0.16[3.84] 0.49[3.84] 0.044[3.84]
c 21.1[4.46]* 29.078[4.46]* 122.69[4.46]* 0.04[3.84] 0.965[3.84] 0.05[3.84]
* Significant at 95%.
Fig. 3. Flowsheet of multistage reverse flotation tests. Table 3 Results of multistage reverse flotation tests
Fig. 2. Recovery and grade of tincal concentrate as a function of flotation time.
the pulp pH for grade of concentrate. This result shows that decrease in coagulation and/or the slime coating of the clay minerals in the pulp is more important than decrease in mechanical entrainment in producing a high-grade concentrate. This is consistent with what is generally expected in flotation practice. Furthermore, in the light of these evaluations, an additional test was conducted at pH 8, high dosage of sodium oleate, 350 g/t, and a low to medium level dosage of sodium silicate, 650 g/t. A concentrate containing 11.41% B2 O3 with 90.55% recovery was obtained. It must be noted that all flotation tests were conducted as a single stage (or rougher flotation) and flotation time was 2 min in these experiments. However, flotation time is a very critical variable of the process. In order to obtain the desired final grade of concentrate, the rougher flotation is completed at optimum flotation time and the tailing (the froth product in this case) is fed into the scavenger bank. Therefore, optimum flotation time must carefully be determined from flotation kinetic tests [13,14]. For these reasons, a flotation kinetic test was performed to determine the optimum flotation time in this step of the experimental program and results are illustrated in Fig. 2. As can be followed from Fig. 2, increase in grade of the rougher concentrate is negligible above the flotation time of 5 min.
Stages
Products
Grade, B2 O3 (%)
Recovery, B2 O3 (%)
Rougher
CR TR CS TS CC1 TC1 CC2 TC2
11.01 0.27 0.94 0.11 17.26 3.63 24.37 7.42
95.46 5.54 3.71 1.83 81.43 14.03 67.07 14.36
Scavenger Cleaner-1 Cleaner-2
C: concentrate; T : tailing.
However, above this time, the borax particles instead of the gangue minerals are transported into the froth product. Therefore, the optimum flotation time for the rougher flotation was determined to be 5 min. In order to produce a high-grade borax concentrate, the rougher concentrate was cleaned twice and the rougher tailing was also scavenged as shown in Fig. 3. In the conditioning period of cleaner stages and scavenger stage, same reagent dosages, which were employed in the single stage tests, were added into the feed of the scavenger and cleaner stages (i.e., concentrate or tailing of the rougher stage). It is evident from the multistage tests results given in Table 3 that a final concentrate containing 24.37% B2 O3 with 81.78% B2 O3 recovery could be produced. Traditionally, the grade of concentrate is inversely proportional to recovery in the normal flotation operation. In the case of the reverse flotation, however, the grade of concentrate is directly proportional to recovery, as shown in Fig. 4.
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of concentrate with reasonable recovery was obtained from the reverse flotation test conducted at pH 8. This result is not consistent with what is observed for the flotation behavior of the carbonate minerals, it may be attributed to the presence of excessive amount of ions and clay in the pulp. Furthermore, it was observed that multistep cleaning of the rougher concentrate could produce a final concentrate containing 24.37% B2 O3 with reasonable recovery. Based on the results of this study, the reverse flotation method can be considered an important option for the beneficiation of the borax fines.
Acknowledgments Fig. 4. Correlation between grade of concentrate and recovery.
Because the middlings products (i.e., tailings of the cleaner stages and concentrate of the scavenger stage) did not recirculate in the multistage reverse flotation tests, the recovery according to initial feed was low (67.07%). However, these products are normally recirculated in continuous flotation systems such as the locked cycle flotation tests, the pilot scale tests, or a plant scale operation. Therefore, it can be safely assumed from Fig. 4 that a higher quality concentrate with a higher recovery could be produced from one of these continuous systems.
4. Conclusions Flotation is one of the plausible methods for recovering borax fines from the hydrocyclone overflow discharged to the tailings dam in the Kirka borax processing plant. The previous studies, published by several authors cited above, have shown that a common problem encountered in all types of boron minerals is the presence of significant amounts of clay-type minerals, which adversely affect flotation recoveries due to slime coatings. The main objective of this study is therefore to recover borax fines from the tailing of the Kirka borax processing plant by reverse flotation method. In the reverse flotation case, recovery of borax fines can be achieved by transferring of dolomite and calcite into the froth by true flotation, whereas clay minerals are transported by mechanical entrainment. As far as can be evaluated from the experimental results and their statistical treatments, amount of collector (sodium oleate) and amount of dispersant (Na2 SiO3 ) have important role for the production of a high quality borax concentrate. Interestingly, a concentrate containing the maximum grade
The authors are grateful to Eti Boron Co. for providing access to the Kirka borax processing plant and valuable assistance in the sampling campaign and the anonymous referees for their helpful comments on an earlier version of this paper.
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