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Optimization of Processing Water Treatment and Its Reuse in Bauxite Flotation LIQING SUN1,2, ZILONG MA2,*, LULU LIAN1,2, QING WANG1,2, LIJUAN SUN1,2 and JIE LIU1,2 1School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China 2National Engineering Research Center of Coal Preparation and Purification, Xuzhou 221116, Jiangsu, China ABSTRACT: The optimum condition for the sedimentation process of concentrate pulp and tailing pulp in bauxite flotation was determined based on the supernatant turbidity study. The flotation tests were conducted to study the effect of recycled water turbidity and recycling time on the concentrate grade and recovery. The results demonstrate that recycled water with turbidity over 95 Nephelometric Turbidity Units (NTU) cannot achieve the ideal concentrate compared to fresh water. In addition, recycled water that was reused twice in bauxite flotation demonstrated reduced selectivity. This is mainly caused by the accumulation of chlorine ions, which may enhance the entrainment of clays in the concentrate. INTRODUCTION T diasporic bauxite mineral resources have a wide distribution in China, which always have a low Al2O3 to SiO2 (A/S) mass ratio ranging from 4 to 6 [1]. Flotation has been identified to be an effective method for separating fine and valuable minerals from other composites [2–4]. For the diasporic bauxite ore, the flotation method can improve the A/S ratio to meet the requirements for Bayer Technology. The volume of water required for processing the diasporic bauxite is at least 3 m3 per ton. In order to preserve water and reduce water discharge, the processing water must be recycled in the flotation. Solid-liquid separation after flotation has attracted extensive attention worldwide [5–8]. Sedimentation is the primary technique in the dewatering process [9]. In most cases, efficient separation of the solid and process liquid is quite challenging and complicated. As the sizes of the solid particles are often less than 10 μm, it is difficult for the fine particles to settle down or settle quickly. In addition, the clay minerals such as kaolinite, illite, and pyrophyllite are often negatively charged and mutually exclusive, making the slurry stable [10– 15]. To overcome these challenges, coagulants or floche *Author to whom correspondence should be addressed. E-mail: [email protected] culates are extensively used to promote sedimentation [6,16,17]. Polyacrylamide (PAM) is commonly used as a flocculant in water treatment, but is less efficient when used alone, due to the weak interaction between PAM and the particles or the repulsive force between the chain of PAM and the particles [18–20]. Therefore, coagulates such as aluminum sulfate and poly-aluminum chloride (PAC) are added before PAM to neutralize the charge on the particle surface [21,22]. So far, various reports have studied the process water recycling in flotation, and the results show that the flotation results were closely related to the water quality [5,23–25]. The process water often contains numerous ions, agents, and particle residue, which is considerably different from fresh water. The ions may originate from mineral dissolution and reagent addition [26–28]. The mineral dissolution also contributes a small amount of substances to the flotation water, due to the low solubility from certain materials [29]. However, these constituents can be enhanced through processing water recycling, thus affecting the flotation performance. For example, in a complex sulphide flotation plant, the accumulation of copper ions can activate the sphalerite flotation [30]. Similar to mineral dissolution, the reagent also contribute a small portion of substances, and these substances can also accumulate and affect the flotation performance [31]. In this study, we aim to optimize the processing water treatment and investigate its reuse capability in Journal of Residuals Science & Technology, Vol. 14, No. 2—April 2017 1544-8053/17/02 163-08 © 2017 DEStech Publications, Inc. doi:10.12783/issn.1544-8053/14/2/19 163 164 L. SUN, Z. MA, L. LIAN, Q. WANG, L. SUN and J. LIU bauxite flotation. The influence of the pH value and the combination reagent of coagulant-flocculant on sedimentation efficiency were investigated to define the optimum experimental conditions for coagulation and flocculation. The flotation tests were carried out to study the relationship between water turbidity and flotation performance. Moreover, we also studied the processing water that was obtained by sedimentation of pulp under the optimum conditions and reused twice in flotation. MATERIALS AND METHODS Materials The bauxite ore used in this study were obtained from Daoer in Shanxi Province. The lumps of the bauxite were first crushed by a jaw crusher, then screened and grounded to ensure that 85% of the minerals pass the 200-mesh sieve. The chemical analysis is shown in Figure 1, which indicates that the bauxite sample contains 60% aluminum oxide and 15% silicon dioxide, with an A/S ratio of 4. The concentrate pulp and tailing pulp used for sedimentation study were also obtained in positive floatation from Shanxi Daoer investment co., LTD. The sample was mixed evenly through riffle before the basic characteristic analysis. The chemical analysis (Figure 1) indicates that the minerals in the concentrate and tailing pulp are 65%, 50% aluminum oxide and 11%, 23% silicon dioxide, with the A/S ratio of 5.9 and 2.1, respectively. The particle sizes of bauxite ore, concentrate pulp, and tailing pulp were analyzed by MasterSizer 2000 (Malvern Instruments). As shown in Figure 2, the results indicate that bauxite ore, concentrate pulp, and tailing pulp have a median diameter (d50) of 8 μm, 6 μm, and 13 μm, respectively. The PAM and PAC used as flocculant and coagulant were purchased from Zhengzhou in Henan province. Sodium oleate, synthesized by oleate and NaOH in the laboratory, was used as collector in the flotation tests. Na2CO3 (AR) and HCl (AR) were used in the tests as pH modifiers. Sodium hexametaphosphate (AR) was used as dispersant. Distilled water was used in all of the experiments except the ones that require recycled water. Figure 1. The chemical components of: (a) bauxite ore, (b) concentrate pulp and (c) tailing pulp. Optimization of Processing Water Treatment and Its Reuse in Bauxite Flotation 165 Ion-chromatographic Measurements The ion content in the samples was determined in an ICS-1100 (Thermo Fisher, America). The suspension was first centrifuged to form clear water samples, and then passed through a filter membrane to ensure no particles left in the sample. RESULTS AND DISCUSSION Optimization of Processing Water Treatment in Bauxite Flotation Influence of pH Value on Pulp Sedimentation Figure 2. Cumulative size distribution of bauxite ore, concentrate pulp and tailing pulp. Bench-scale Flotation Tests At pre-test stage, the concentrate pulp was more difficult to settle than the tailing pulp, as the same amount of PAM was used as flocculant. The suitable dosage was developed for concentrate pulp and tailing pulp respectively. The pH value is an important factor for PAM adsorption in minerals [32]. In order to get the qualified recycling water, the influence of pH on the sedimentation of concentrate pulp and tailing pulp were studied under the suitable dosage of PAM. The results are provided in Figure 3. The sedimentation performance was evaluated according to the supernatant turbidity. It can be seen that the concentrate pulp shows the best sedimentation performance in neutral pH solution. The tailing pulp exhibited the best sedimentation in the pH range of 6 to 7. When the pH value is higher than 7, the supernatant turbidity increases with pH, which indicates reduced sedimentation performance. The flotation tests were carried out in a 750 ml flotation cell using one roughing-one scavenging-one cleaning flowsheet. The pulp was made by adding 320 g minerals into the flotation cell, followed by adding deionized water to the cell volume and subsequent mixing procedure. In the roughing process, the pH value of the pulp was adjusted to 10 using sodium carbonate, and then 60 g/t sodium hexametaphosphate was added as dispersant. Conditioning time for adjusting the pH value and the dispersant were 6 and 5 minutes, respectively. The collector was added two times, and the dosage was 400 g/t and 300 g/t, with the conditioning time of 4 and 3minutes, respectively. Cleaning process was carried out without reagents addition, while 300 g/t collector was added in the scavenging process. During the tests, the impeller speed was 1600 r/min, and the temperature was 25°C ± 2°C. Figure 3. Influence of pH on sedimentation of concentrate pulp and tailing pulp (Cconcentrate = 80 g/t, Ctailing = 40 g/t). Methods Settlement Tests The settlement tests were carried out in a 500 ml cylinder with a stopper. The concentration of the slurry used in the tests was 5% (g/ml). The PAC was added first to the slurry, followed by turning the cylinder upside down three times to mix uniformly. Next, PAM was added followed by the same mixing procedure. The supernatant height was collected during the tests. After settlement, the supernatant was collected, and the turbidity was measured with a 2100 N turbidimeter (HACH, America). 166 L. SUN, Z. MA, L. LIAN, Q. WANG, L. SUN and J. LIU Effect of Combination Agent on Sedimentation With the addition of PAC into the wastewater, the time for floc formation can be reduced, and the characteristics of the flocs were enhanced [33]. Therefore, PAC was used coordinated with PAM in the study, and the effect of combination agent on the sedimentation of concentrate pulp and tailing pulp were studied. The sedimentation performance is evaluated according to the settling rate and the supernatant turbidity (See Figure 4). As PAC is used alone, the settling rate is the minimum one for both concentrate pulp and tailing pulp, compared to the case when PAC is used coordinated with PAM. Meanwhile, the supernatant turbidity decreases over the concentration of PAC. When PAC is used coordinated with fixed dosage of PAM, the settling rate almost decreases with the increasing of PAC concentration. For the tailing pulp, the supernatant tur- bidity decreases with PAC concentration. For the concentrate pulp, as PAC is used alone, the supernatant turbidity decreases with PAC concentration. However, there is no obvious regularity when the PAC is used with PAM. Generally, particles tend to coagulate and aggregate through the electrostatic patch effect, charge neutralization, bridge connection, and sweep flocculation in series or synchronously [34]. In reference 35, the diameter of the particles studied was 3 μm, and the coagulation region can be divided into two parts, negative zone (part I) and positive zone (part II) according to the zeta potential (See Figure 5 ). The main mechanism was ascertained to be the charge-neutralization in part I and sweep flocculation in part II. Thus, the coagulation behavior of the concentrate pulp in this work agrees well with the reference, since the particle diameter is similar. Figure 4. Effect of combination agent on supernatant turbidity and settling rate of concentrate pulp and tailing pulp. Optimization of Processing Water Treatment and Its Reuse in Bauxite Flotation 167 Figure 5. Zeta potential and residual turbidity as a function of coagulant dosage: (a) zeta potential and (b) residual turbidity ([Al]T equals to the total aluminum concentration added in) [35]. Reuse of Processing Water in Bauxite Flotation Processing Water Reused Twice in Bauxite Flotation Processing Water with Different Turbidity Reused in Bauxite Flotation The pulp was pre-treated as mentioned above, and the supernatant with 100 NTU was used with recycling water, which was referred to as the first cycle. Subsequently, the pulp of concentrate, middling, and tailing were treated by sedimentation process. All of the pulps were settled around pH 7, and then PAC along with PAM were added into the concentrate pulp and tailing pulp as described above, while the middling pulp were treated as tailing pulp. The supernatant was then mixed and used for flotation, which was referred to as the second cycle. The flowsheet of recycling water used in flotation was shown in Figure 6. After reusing the recycling water twice, the flotation results are shown in Table 2. As the recycling water is reused in the first cycle, the concentrate with A/S ratio of 6.4 is measured, and the recovery of Al2O3 reached 72%, which was higher than using fresh water. However, when the recycling water was settled for the sec- In order to determine if the processing water treated by the sedimentation process was suitable for the flotation test, the water was reused in flotation, and the results are listed in Table 1.As can be seen from the table, when the turbidity of the water is 1908 NTU and 95 NTU, the final A/S ratio is less than 6, which cannot meet the demand of the bauxite production. Comparing the results from sample No.1 to No.4, we can conclude that the A/S ratio of the concentrate increases with the decreasing of water turbidity. This is mainly because that the high turbidity is induced by ultrafine particles such as kaolinite and illite, which are hard to settle. When the processing water with high turbidity is reused in flotation, the ultrafine particles can be floated to concentrate through coating and foam entrainment, which lowers the concentrate grade. Table 1. Flotation Results of Bauxite using Recycle Water with Different Turbidity. Number No.1 No.2 No.3 No.4 Concentrate (%) Turbidity of Recycle Water (NTU) Recovery (%) Al2O3 (%) SiO2 (%) A/S 1908 95 57 0 61 67 72 49 61 62 62 66 13 12 10 10 4.7 5.2 6.2 6.6 Figure 6. Flowsheet of recycling water used in flotation. 168 L. SUN, Z. MA, L. LIAN, Q. WANG, L. SUN and J. LIU Figure 7. Concentration of Cl– in recycled water. ond time and reused in flotation, the recovery of Al2O3 increases to 86%, and the A/S ratio of the concentrate decreases to 4.9, which indicates reduced selectivity in bauxite flotation. In bauxite, diaspore is the main mineral, and kaolinite is the typical clay mineral. During the sedimentation process, the PAC coagulates the ultrafine particles through compressing the double electrode layer on the clay surface. It is generally recognized that effective particle aggregation is caused by the positively charged aluminum of PAC [37,38], so the chloride ions accumulate in the process water. The concentration of the chloride ions in the recycle water is shown in Figure 7. Inorganic electrolytes such as NaCl are favorable to form smaller stable bubbles [36], and the smaller bubbles can enhance the flotation performance through increasing particle-bubble collision probability [37]. Thus, increasing ionic strength can result in increased gangue entrainment [38,39]. CONCLUSIONS The sedimentation of bauxite concentrate pulp and tailing pulp were investigated. The results show that neutral pH is beneficial to the sedimentation of concentrate pulp and tailing pulp. Optimization of PACPAM dosage can achieve supernatant with turbidity about 100 NTU. Based on the sedimentation results, the flotation tests were carried out to study the effect of water tuebidity and recycle time on the A/S ratio and recovery of the concentrate. Results show that the A/S ratio increases with the decreasing of water turbidity. When the process water treated based on the optimum sedimentation condition is reused in bauxite flotation for two cycles, the A/S ratio are 6.3 and 4.9, respectively. By measuring the ion content of the water, chloride ions are found to accumulate in the processing water, which may increase gangue entrainment in bauxite flotation. ACKNOLEDGEMENTS Table 2. Flotation Results of Bauxite that Reused Recycling Water Twice. 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