E ﬀ ect of Surfactant on Water Content of Phosphogypsum

: Phosphogypsum is a kind of solid waste produced in wet process of producing phosphoric acid, which a ﬀ ects the ﬁltration rate and water content of phosphogypsum. The e ﬀ ects of single surfactant sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), stearic acid (SR) and polydimethylsiloxane (PDMS) and coupled surfactants on the water content of phosphogypsum were investigated. The results show that, during the leaching process, surfactant strengthened the interfacial interactions between molecules through hydrophobic and hydrophilic orientation on solid–liquid interface, reduced the surface tension and contact angle to improve the ﬁltration rate and reduced the soluble phosphorus loss, thereby improving the leaching rate of phosphate rock and reducing the water content of phosphogypsum. Among them, the water content of phosphogypsum was better controlled by stearic acid and sodium dodecylbenzene sulfonate than the other surface surfactants. Compared with the blank group, the stearic acid and sodium dodecylbenzene sulfonate increased the ﬁltration rate of phosphogypsum by 24.34%, the moisture content decreased by 3%, and the phosphate leaching rate increased by 4.36%.


Introduction
Key processes during the wet process of producing phosphoric acid are the leaching behavior and crystallization process of phosphogypsum. CaSO 4 ·2H 2 O is the main content of phosphogypsum; its crystal structure and particle size can directly affect the filtration rate and the loss rateof phosphate. Many materials can improve the crystal structure during the production process of CaSO 4 ·2H 2 O, e.g., activated carbon, activated silicone, aluminum sulfate, and some inorganic compounds [1][2][3][4] as well assome polymers and surfactants, phosphoric acid, and carboxylate.
Rashad [5][6][7][8][9][10][11] simulated the production process of phosphate acid with sulfate and calcium hydrophosphateand showed that the existence of hex-adecyl trim-ethyl ammonium bromide, 1,2-benzodiazepines, Al 3+ and Mg 2+ can improve the crystallization of phosphogypsum, while citric acid has the opposite effect. Zhang [12] found that increased crystallization temperature can enlarge particle size and improve the crystal structure in optimal crystallization time. Zhang [13] simulated the production process with calcium chloride and sulfate, finding that the crystal structure of phosphogypsumcan change from acicular crystal to prismatic crystal, bowknot shape crystal, and spherical crystal, thusenlarging the particle size. Most research is focused on improving crystal structure of CaSO 4 ·2H 2 O with pure chemicals, while ignoring the effect of impurities such as Mg 2+ , Al 3+ , Fe 3+ , and SiO 2 during the process [14][15][16]. During the wet process of producing phosphoric acid, the effect of the impurities in the phosphate rock cannot be ignored. However, few studies focused analyze the effect of surfactants on the growth behavior of phosphogypsum crystal and water content of phosphogypsum. This study investigated the effects of single surfactant SDS, SDBS, SR and PDMS on the water content of phosphogypsum. At the same time, the effect of SDBS+SDS, SDBS+SR and SDBS+PDMS on the water content of phosphogypsum was also investigated. The regulation mechanism of single surfactants and two-element compound surfactants on water content of phosphogypsum was analyzed from the following aspects: the interfacial tension of solution, the contact angle of system solid-liquid interface, the morphology of CaSO4·xH2O, and the fractal growth behavior of phosphogypsum.

Materials
The SDS, SDBS, SR and PDMS used were of analytical grade, purchased from Kelong Co., Ltd, Chengdu, China, and used as received without purification.The phosphate rock was collected from Sinochem Fuling Chongqing Chemical Industry Co., Ltd., Chongqing, China. The chemical composition of the phosphate rock is listed in Table 1; the phase composition measure by X-ray diffraction (XRD-6000, Shimadzu, Japan) is detailed in Figure 1a. The phosphate was mainly composted of Ca5(PO4)3F, SiO2, CaMg(CO3)2. The phosphogypsum was produced by acid leaching of phosphate rock; the composition and phase are detailed in Table 2 and Figure 1b. The results shown in Figure 1b indicate that the phosphogypsum existed as CaSO4·2H2O, SiO2, and FeS2.

Experimental Procedure
All experiments were performed in an agitation reactor (self-made) with liquid-to-solid of 2.5:1 g/mL. A predetermined amount of phosphate rock and water was added to the reactor to produce

Experimental Procedure
All experiments were performed in an agitation reactor (self-made) with liquid-to-solid of 2.5:1 g/mL. A predetermined amount of phosphate rock and water was added to the reactor to produce homogeneous slurry under constant stirring. The slurry was heated to a predetermined temperature. Next, the H 2 SO 4 was added to the reactor, and then the surfactant. After the required reaction time, the leachate was separated from the residue by vacuum filtration. The residue was dried in an oven and grounded to fine particles and analyzed by Scanning Electron Microscopy (SEM; S4800; HITACHI, Japan). The concentration of phosphate was determined by quinoline phosphomolybdate gravimetric method.

Results and Discussion
The surfactant was an amphiphilic structure containing hydrophobic group and water-based group; it was energized at surface or interface of the solution and changed the physicochemical property (surface tension and contact angle) to display effect of wetting or solubility ( Figure 2). homogeneous slurry under constant stirring. The slurry was heated to a predetermined temperature. Next, the H2SO4 was added to the reactor, and then the surfactant. After the required reaction time, the leachate was separated from the residue by vacuum filtration. The residue was dried in an oven and grounded to fine particles and analyzed by Scanning Electron Microscopy (SEM; S4800; HITACHI, Japan). The concentration of phosphate was determined by quinoline phosphomolybdate gravimetric method.

Results and Discussion
The surfactant was an amphiphilic structure containing hydrophobic group and water-based group; it was energized at surface or interface of the solution and changed the physicochemical property (surface tension and contact angle) to display effect of wetting or solubility ( Figure 2).

Effect of Surfactant
The results shown in Table 3 Table 4. The addition of SDBS could decrease the surface tension, which was more effective than SDS, SR and PDMS. However, adding more species was more effective than single species, and the performance of SDBS+SR was better than SDBS+SDS or SDBS+PDMS.
The addition of surfactant could decrease the interface energy and block the hydrogen bond to rearrange the water molecule, and then increased the bond-angle of water [17] (Figure 3),thus decreasing the surface tension and enlarged the contact area of leaching solvent and phosphate rock, which favored the mass transfer rate and improved the leaching efficiency.

Effect of Surfactant
The results shown in Table 3 Table 4. The addition of SDBS could decrease the surface tension, which was more effective than SDS, SR and PDMS. However, adding more species was more effective than single species, and the performance of SDBS+SR was better than SDBS+SDS or SDBS+PDMS. The addition of surfactant could decrease the interface energy and block the hydrogen bond to rearrange the water molecule, and then increased the bond-angle of water [17] (Figure 3), thus decreasing the surface tension and enlarged the contact area of leaching solvent and phosphate rock, which favored the mass transfer rate and improved the leaching efficiency.

Effect of Surfactant on Morphology of Phosphogypsum
The SEM of phosphogypsumis shown in Figure 4. The results indicate that morphology of the phosphogypsum was displayed as tabular and the particle size was about 20-25 μm without surfactant. The phosphogypsum grew to 25-30 μm with the addition of SDBS, while the phosphogypsum changed to virgate with the addition of SR and PDMS, and the particle size grew to 30-35 μm. The particle size changed little with the addition of two species of surfactant, remaining 20-30 μm.
The addition of SDBS, SDS, or PDMS could increase the interfacial energy and hinder the formation of crystal nucleus, which avoided the formation of fine grain [18]. In addition, the addition of SDBS, SDS, orPDMS decreased the supersaturating of the phosphate rock, and avoided the deposition of fine grain on the surface of rock. The diffusion velocity of Ca 2 ＋ and SO4 2− was increased due to the change of surface tension, which was beneficial for the growth of phosphogypsum.

Effect of Surfactant on Morphology of Phosphogypsum
The SEM of phosphogypsumis shown in Figure 4. The results indicate that morphology of the phosphogypsum was displayed as tabular and the particle size was about 20-25 µm without surfactant. The phosphogypsum grew to 25-30 µm with the addition of SDBS, while the phosphogypsum changed to virgate with the addition of SR and PDMS, and the particle size grew to 30-35 µm. The particle size changed little with the addition of two species of surfactant, remaining 20-30 µm.  The fractal dimension of the phosphogypsum was calculated according to Equation (1) and the results are displayed in Table 5. The correlation of contact angle with fractal dimension was calculated (see Equation (2)) and the R 2 was 0.9986 shown in Figure 5. Low fractal dimension indicated low contact area, which was not beneficial for adsorption of water at low fractal The addition of SDBS, SDS, or PDMS could increase the interfacial energy and hinder the formation of crystal nucleus, which avoided the formation of fine grain [18]. In addition, the addition of SDBS, SDS, or PDMS decreased the supersaturating of the phosphate rock, and avoided the deposition of fine grain on the surface of rock. The diffusion velocity of Ca 2+ and SO 4 2− was increased due to the change of surface tension, which was beneficial for the growth of phosphogypsum. The fractal dimension of the phosphogypsum was calculated according to Equation (1) and the results are displayed in Table 5. The correlation of contact angle with fractal dimension was calculated (see Equation (2)) and the R 2 was 0.9986 shown in Figure 5. Low fractal dimension indicated low contact area, which was not beneficial for adsorption of water at low fractal dimension. However, it was good for separation of the leachate and phosphogypsum, as well as increasing the filtrate rate and decreasing water content in the phosphogypsum. dimension. However, it was good for separation of the leachate and phosphogypsum, as well as increasing the filtrate rate and decreasing water content in the phosphogypsum.

Effect of Surfactant on Filtrate Rate of Phosphogypsum
The addition of surfactant not only decreased the surface tension and contact angle, and changed the wetting characteristics of the phosphate rock, but also obtained large size of phosphogypsum. The results shown in Table 6 display the effect of surfactant on the filtrate rate of phosphogypsum. The filtrate rate was 905.80 kg·m -2 ·h -1 without surfactant. The addition of surfactant could affect the filtrate rate significantly. The rate increased 13% (with SDS), 20% (with SDBS), 16.7% (with PDMS), 20.86% (with SDS+SDBS), 24.34% (with SR+SDBS) and 17.41% (PDMS+SDBS), respectively.
The addition of SDBS, SDS, PDMS, SDBS+SDS, SDBS+SR, or SDBS+PDMS could increase the interfacial energy and hinder the formation of crystal nucleus, avoiding the formation of fine grain [19].The addition of surfactant also increased the growth rate of crystal and the filtrate rate.

Effect of Surfactant on Filtrate Rate of Phosphogypsum
The addition of surfactant not only decreased the surface tension and contact angle, and changed the wetting characteristics of the phosphate rock, but also obtained large size of phosphogypsum. The results shown in Table 6 display the effect of surfactant on the filtrate rate of phosphogypsum. The filtrate rate was 905.80 kg·m −2 ·h −1 without surfactant. The addition of surfactant could affect the filtrate rate significantly. The rate increased 13% (with SDS), 20% (with SDBS), 16.7% (with PDMS), 20.86% (with SDS+SDBS), 24.34% (with SR+SDBS) and 17.41% (PDMS+SDBS), respectively. The addition of SDBS, SDS, PDMS, SDBS+SDS, SDBS+SR, or SDBS+PDMS could increase the interfacial energy and hinder the formation of crystal nucleus, avoiding the formation of fine grain [19]. The addition of surfactant also increased the growth rate of crystal and the filtrate rate.

Effect of Single Surfactant
The phosphogypsum was dried in an oven at 45 • C for desorption of free-water; the loss of phosphogypsum mass is shown in Figure 6. The water content was calculated according to Equation (3) by weighing method. The mass of phosphogypsum in the oven was not changed, which indicated that the free-water was extirpated.

Effect of Single Surfactant
The phosphogypsum was dried in an oven at 45 °C for desorption of free-water; the loss of phosphogypsum mass is shown in Figure 6. The water content was calculated according to Equation (3) by weighing method. The mass of phosphogypsum in the oven was not changed, which indicated that the free-water was extirpated. The results shown in Figure 6 indicate that the water-loss rate was faster than blank group with addition of SDS, SDBS, PDMS and SR. Table 7displays the effect of single surfactant on water content of phosphogypsum. The water content of phosphogypsum was 20.3%, and decreased to 18.0%, 19.4% and 18.9%, respectively, with addition of SDBS, SDS and PDMS, while increased to 23.1% with addition of SR. SDS and SDBS are anionic surfactants containing hydrophilic group and oil-based, while the silicon methyl group in PDMS is strongly hydrophobic. Tables 3 and 4show that the addition of SDS, SDBS or PDMS could decrease the surface tension of phosphogypsum and contact angle and was beneficial for separation. In addition, the size of phosphogypsum grew up with the addition of surfactant, which could increase the filtrate rate and decrease the water content.  The results shown in Figure 6 indicate that the water-loss rate was faster than blank group with addition of SDS, SDBS, PDMS and SR. Table 7 displays the effect of single surfactant on water content of phosphogypsum. The water content of phosphogypsum was 20.3%, and decreased to 18.0%, 19.4% and 18.9%, respectively, with addition of SDBS, SDS and PDMS, while increased to 23.1% with addition of SR. SDS and SDBS are anionic surfactants containing hydrophilic group and oil-based, while the silicon methyl group in PDMS is strongly hydrophobic. Tables 3 and 4 show that the addition of SDS, SDBS or PDMS could decrease the surface tension of phosphogypsum and contact angle and was beneficial for separation. In addition, the size of phosphogypsum grew up with the addition of surfactant, which could increase the filtrate rate and decrease the water content.
Appl. Sci. 2018 The effect of coupled surfactant on the water content of phosphogypsum was investigated with SDBS+SDS (1:1), SDBS+SR (1:1), and SDBS+PDMS (1:1). The loss of phosphogypsum mass is shown in Figure 7. The results shown in Figure 8 indicate that the water-loss rate was faster than blank group with addition of SDBS+SDS (1:1), SDBS+SR (1:1) or SDBS+PDMS (1:1). Table 8 displayed the effect surfactant on water content of phosphogypsum. The water content was decreased to 18.0%, 17.3% and 18.6%, respectively, with addition of SDBS+SDS (1:1), SDBS+SR (1:1), and SDBS+PDMS (1:1). The synergistic effect was not obvious, as SDS and SDBS had the same hydrophobic group. SR reacted with SDBS to form complex compound (Figure 8), which was easy to overcome the low hard water of SDBS. The coupling of SDMS and SR showed good synergistic effect.  The results shown in Figure 8 indicate that the water-loss rate was faster than blank group with addition of SDBS+SDS (1:1), SDBS+SR (1:1) or SDBS+PDMS (1:1). Table 8 displayed the effect surfactant on water content of phosphogypsum. The water content was decreased to 18.0%, 17.3% and 18.6%, respectively, with addition of SDBS+SDS (1:1), SDBS+SR (1:1), and SDBS+PDMS (1:1). The synergistic effect was not obvious, as SDS and SDBS had the same hydrophobic group. SR reacted with SDBS to form complex compound (Figure 8), which was easy to overcome the low hard water of SDBS. The coupling of SDMS and SR showed good synergistic effect.

Effect of Surfactant on Leaching Efficiency
The effect of addition of surfactant on the leaching efficiency of phosphate was investigated and the results are shown in Table 9. The leaching efficiency was increased by about 3.32% (SDBS and SDS), 1.35% (PDMS), 3.49% (SDBS+SDS), 4.36% (SDBS+SR) and 2.49% (SDBS+PDMS), respectively. The addition of surfactant decreased the surface tension and contact angle, favoring the mass transfer during the leaching process, which improved the leaching efficiency. The addition of surfactant could increase the interfacial energy and hinder the formation of crystal nucleus, which avoided the formation of fine grain and improved the decomposition rate of phosphate ore.

Conclusions
The effects of single surfactants SDS, SDBS, SR and PDMS and coupled surfactants SDBS+SDS, SDBS+SR and SDBS+PDMS on the water content of phosphogypsum were investigated. The results show that, during the leaching process, surfactant strengthened the interfacial interactions between molecules through hydrophobic and hydrophilic orientation on solid-liquid interface, reduced the surface tension and contact angle to improve the filtration rate and reduced the soluble phosphorus loss, thereby improving the leaching rate of phosphate rock and reducing the water content of phosphogypsum. Among them, the water content of phosphogypsum was better controlled by SR + SDBS than by other surface surfactants. Compared with the blank group, SR + SDBS increased the filtration rate of phosphogypsum by 24.34%, decreased the moisture content by 3%, and increased the phosphate leaching rate by 4.36%. In addition, the fractal dimension of phosphogypsum crystal increased with the increase of contact angle of solid-liquid interface, and the mathematical relationship between fractal dimension and contact angle was well fitted.

Effect of Surfactant on Leaching Efficiency
The effect of addition of surfactant on the leaching efficiency of phosphate was investigated and the results are shown in Table 9. The leaching efficiency was increased by about 3.32% (SDBS and SDS), 1.35% (PDMS), 3.49% (SDBS+SDS), 4.36% (SDBS+SR) and 2.49% (SDBS+PDMS), respectively. The addition of surfactant decreased the surface tension and contact angle, favoring the mass transfer during the leaching process, which improved the leaching efficiency. The addition of surfactant could increase the interfacial energy and hinder the formation of crystal nucleus, which avoided the formation of fine grain and improved the decomposition rate of phosphate ore.

Conclusions
The effects of single surfactants SDS, SDBS, SR and PDMS and coupled surfactants SDBS+SDS, SDBS+SR and SDBS+PDMS on the water content of phosphogypsum were investigated. The results show that, during the leaching process, surfactant strengthened the interfacial interactions between molecules through hydrophobic and hydrophilic orientation on solid-liquid interface, reduced the surface tension and contact angle to improve the filtration rate and reduced the soluble phosphorus loss, thereby improving the leaching rate of phosphate rock and reducing the water content of phosphogypsum. Among them, the water content of phosphogypsum was better controlled by SR + SDBS than by other surface surfactants. Compared with the blank group, SR + SDBS increased the filtration rate of phosphogypsum by 24.34%, decreased the moisture content by 3%, and increased the phosphate leaching rate by 4.36%. In addition, the fractal dimension of phosphogypsum crystal increased with the increase of contact angle of solid-liquid interface, and the mathematical relationship between fractal dimension and contact angle was well fitted.