Brazilian Journal of Chemical ISSN 0104-6632 Printed in Brazil Engineering www.scielo.br/bjce Vol. 34, No. 03, pp. 821 – 830, July – September, 2017 dx.doi.org/10.1590/0104-6632.20170343s20160040 ELECTROFLOTATION OF PRECIPITATED PHOSPHATE FROM SYNTHETIC SOLUTION S. G. da Cruz1*, M. B. de M. Monte2 and A. J. B. Dutra1 1Metallurgical and Materials Engineering Program, PEMM/COPPE, Federal University of Rio de Janeiro, P.O. Box 68.505, Rio de Janeiro-RJ, Brazil. 2Vale Technology Institute (ITV Mining), Ouro Preto, Brazil. *E-mail: [email protected]; Phone: +55 21 39388548 (Submitted: January 20, 2016; Revised: April 13, 2016; Accepted: May 10, 2016) ABSTRACT – The phosphate recovery from a 20 mg L-1 Na2HPO4 solution through precipitation with Ca(OH)2 followed by electroflotation was evaluated. X-ray diffraction and particle size measurements indicated that the precipitates were a mixture of hydroxyapatite and calcium-deficient hydroxyapatite, with size ranging from 3 to 10 mm. Electroflotation with Na-oleate as surfactant was used to recover the precipitates. The surfactant adsorption was evaluated through zeta potential measurements. The influence of Na-oleate concentration, pH and bubble size on phosphate recovery was investigated. In the absence of Na-oleate, a phosphate recovery of 50% was achieved, while in the presence of 20 mg L-1 of Na-oleate it was increased to 90%, at pH 11. The phosphate recovery also increased with Ca(OH)2 concentration increase and bubble size decrease, reaching 100% at 300 mg L-1 Ca(OH)2 and bubble size around 39 µm. Keywords: Phosphate recovery, precipitation, electroflotation. INTRODUCTION the environment has been considered an important issue for quality of life support, leading to the search for new Industrial and agricultural expansion currently technologies to recover this element (Cordell et al., 2009), demands a growing exploitation of phosphorus sources offering opportunities for its recycling, and contributing in the environment to supply human needs. This wide to phosphorus sustainability (Morse et al., 1998). At the utilization of phosphates leads to a considerable amount present time, chemical and biological processes are the most of phosphorus-containing wastes being discharged into used for phosphate removal from wastewaters. Among the water bodies, demanding previous treatment to avoid biological processes, activated sludge is considered the eutrophication. Additionally, rainwater can wash the most versatile and efficient, while among the chemical soil, removing phosphate fertilizers which can exacerbate processes, precipitation through the use of aluminum, iron still further the eutrophication issue, a phenomenon and calcium salts has been widely employed (Jenkins and characterized by algae overpopulation and the rapid Ferguson, 1971; Morse et al., 1998). Aluminum and iron growth of aquatic plants, which impair the penetration of salts present the disadvantage of generating sludge, which light in the water, thus reducing photosynthesis reactions cannot be reused or reclaimed (Urano and Tachikawa, and, consequently, the amount of dissolved oxygen in 1991). This problem does not occur with calcium salts, as the water, suffocating aquatic animals and converting the they react with phosphate to generate precipitates such as water body into an open sewer (Hosni et al., 2008). Thus, hydroxyapatite, monetite, brushite and amorphous calcium the development of phosphorus management strategies in phosphate, among others, which are compounds normally * To whom correspondence should be addressed 822 S. G. da Cruz, M. B. de M. Monte and A. J. B. Dutra exploited in phosphate rock mining for fertilizer production (Brahmi et. al, 2015a,b; Bouguerra et al, 2015a,b; and present the potential for reuse and recycling. However, Kuokkanen et al., 2015). The main difference between calcium phosphate precipitates are difficult to settle, electrocoagulation and electroflotation lies in the electrode requiring the use of flocculants (Liu and Chang, 2009). type. In electrocoagulation soluble anodes of aluminum or Thus, there is a need to employ other solid/liquid separation iron are used, while insoluble anodes, such as RuO2 coated techniques, such as flotation, which is a well established titanium, are used in electroflotation. The technique used technique for minerals separation and is also commercially in this study has the advantage of generating more and used for wastewater treatment (Hoseinian et al., 2015; smaller bubbles than those generated by electrocoagulation Matis, 1995; Rubio et al., 1996; Rubio, 1998a,b; Rubio et (Chen, 2003). Furthermore, electrocoagulation generates a al., 2002; Voronin and Dibrov, 1999). This technique is more complex sludge, with aluminum or iron hydroxides, based on the different hydrophobicity of the particles to turning its reuse and recycling more difficult. be separated. Thus, hydrophobic particles can be captured Considering the need for alternative treatments to by gas bubbles and float to the liquid surface. To be recover phosphate from wastewaters and the success effective, flotation requires the use of surfactants to render of flotation for apatite separation from other minerals, the particle surface more hydrophobic. The efficiency of the objective of this paper was to assess the viability of this process depends on the probability of particle–bubble recovering phosphate from a dilute synthetic solution, collision and attachment. For small particles there is a need simulating a polishing stage of a municipal sewage for small bubbles such as those produced by dissolved air treatment station by precipitation with calcium hydroxide flotation or by electrolysis, which produce even smaller followed by electroflotation. bubbles (Venkatachalam et al., 1992; Burns et al., 1997). Electroflotation is a process where particulate matter and/ MATERIALS AND METHODS or polluting substances are floated through their adhesion on tiny hydrogen and oxygen (and other gases, depending Calcium phosphate precipitate was obtained from a on the electrolyte) bubbles generated electrolytically as synthetic solution with 20 mg L-1 of Na2HPO4 and 50 a result of water decomposition. The application of this mg L-1 of Ca(OH)2. The precipitating agent Ca(OH)2 technique to ore fines recovery has been the object of concentration range was previously determined through investigation since 1946 (Miettinen et al., 2010). Bhaskar a Jar test equipment. The precipitate was analyzed by Raju and Khangaonkar (1984) studied the electroflotation X-ray diffraction (Shimadzu model XRD-6000), with of chalcopyrite ultrafines (~4 μm) in a modified Hallimond CuKα radiation. The zeta potential was determined with tube with platinum anodes and copper cathodes. The a Zetasizer Nano Series (Nano-ZS). The particle size authors observed that electrolytically generated bubbles of the precipitate and the average bubble diameter were rendered better floatability (~81.3%) compared to the determined with a Malvern Instruments Mastersizer floatability with an oxygen cylinder in a conventional 2000SM capable of analyzing particles with diameters Hallimond tube (~39.1%). Ketkar et al. (1991) studied the between 0.1–2000 µm. electroflotation of fine and ultrafine quartz particles (<20 The average bubble diameter was determined with mm) in a modified Hallimond tube, with a stainless steel the help of a specially designed acrylic cylindrical cell cathode and platinum as anode, and reported a recovery with a volume of 1 liter, to fit in a Malvern Instruments of 94% of the particles. Sarkar et al. (2010) compared the Mastersizer 2000SM device. The cathode was a stainless flotation of silica ultrafines (~13 μm) in a Denver flotation steel mesh and the anode an RuO2 coated titanium grid. cell using both air and hydrogen as the sparged gases with A constant electrical current was applied to the electrodes an electroflotation cell, and reported that much higher using a DC power supply. The influence of pH and current recoveries (~95%) were achieved when the electroflotation density on the bubble diameter in a 0.1 mol L-1 NaCl cell was used. solution was evaluated. The feasibility of electroflotation applied to water and Electroflotation tests were carried out in a 1-liter industrial wastewaters treatment has been demonstrated by several authors (Aoudj et al., 2015; Casqueira et. al., 2006; volume acrylic cell, with an AISI 316 stainless steel Nahui et al., 2008; Vu et al., 2014); however, there is little cathode and RuO2 coated titanium as anode, as shown in information in the literature about the use of electroflotation Figure 1. A cathode–anode potential of 3.5 V, producing for phosphate recovery from aqueous solutions. Vlyssides a cathodic current density of 6.25 mA cm-2, was used et al. (2002) studied the electrolytic treatment of domestic in all the electroflotation experiments. Sodium oleate wastewater with an electrolytic cell with a Ti/Pt anode and (CH3(CH2)7CH=CH(CH2)7COO−Na+) was used as an AISI 304 stainless steel cathode. The authors reported surfactant for the phosphate precipitate. that the phosphate concentration decreased by 98% of its The influence of Na-oleate concentration, pH, calcium initial value (17.5 mg L-1) in 60 minutes. hydroxide concentration and bubble size on the phosphate The electrocoagulation process has already reported recovery and on the zeta potential of the precipitate was by several authors as an alternative wastewater treatment evaluated. The solution used in the electroflotation tests Brazilian Journal of Chemical Engineering Electroflotation of precipitated phosphate from synthetic solution 823 was 20 mg L-1 of Na2HPO4, 50 mg L-1 of Ca(OH)2 and Precipitation of amorphous calcium phosphate 0.1 mol L-1 NaCl as electrolyte with 0.1 mol L-1 HCl or NaOH as pH regulators. All reagents used were obtained 9Ca2+ + 6PO43- → Ca9( PO4)6 .nH2O (1) from VETEC and were analytical grade. For each experiment, a phosphate solution with Hydrolysis of PO43- a definite concentration of Ca(OH)2 was fed into the electroflotation cell. After 20 minutes the phosphate PO43- + H2O → HPO42- + OH- (2) concentration of the residual solution was determined by the photometric method (HI 83099, HANNA Instruments).