The effect of addition of flocculating agents, viz., alum (KAl(SO4)2·12H2O) (up to 10 wt%) and polyaluminum chloride ([Al(OH)aClb]n, a &gt;1.05; n~15) (up to 6 wt%) on the solubility behavior of calcium sulfate dihydrate (gypsum, CaSO4·2H2O) in aqueous NaCl solutions has been examined at 35 °C. The solubility of CaSO4·2H2O decreases with the addition of alum while it increases with the addition of polyaluminum chloride without any significant shift in solubility maximum. Density (ρ) and speed of sound (u) have also been determined for the quaternary systems (CaSO4·2H2O+NaCl+alum/PAC+H2O) at 35 °C and used to estimate solution isentropic compressibility (κs) and ion-hydration (nh) characteristics. Empirical equations describing the s, ρ, u and κs as a function of solution composition are presented.

Bharmoria, Pankaj 

Kumar, Arvind 

Shukla, Jignesh 

Trivedi, Tushar  J

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Indian Journal of Chemistry Vol. 55A, January 2016, pp. 57-63       Effect of flocculating agents on solubility behavior of calcium sulfate dihydrate (gypsum) in aqueous sodium chloride  system and solution properties at 35 °C Jignesh Shuklaa, Tushar J Trivedib, Pankaj Bharmoriaa  & Arvind Kumara, * aSalt and Marine Chemicals Division, CSIR-Central Salt and Marine Chemicals Research Institute, GB Marg,  Bhavnagar 364 002, India Email: arvind@csmcri.org/ mailme_arvind@yahoo.com bGraduate School of EEWS (Energy Environment Water Sustainability), KAIST, 291 Daehak-ro, Yuseong Gu,  Daejeon 305-701, Republic of Korea  Received 17 September 2015; revised and accepted 01 January 2016 The effect of addition of flocculating agents, viz., alum (KAl(SO4)2·12H2O) (up to 10 wt%) and polyaluminum chloride ([Al(OH)aClb]n, a >1.05; n~15) (up to 6 wt%) on the solubility behavior of calcium sulfate dihydrate (gypsum, CaSO4·2H2O) in aqueous NaCl solutions has been examined at 35 °C. The solubility of CaSO4·2H2O decreases with the addition of alum while it increases with the addition of polyaluminum chloride without any significant shift in solubility maximum. Density (ρ) and speed of sound (u) have also been determined for the quaternary systems (CaSO4·2H2O+NaCl+alum/PAC+H2O) at  35 °C and used to estimate solution isentropic compressibility (κs) and ion-hydration (nh) characteristics. Empirical equations describing the s, ρ, u and κs as a function of solution composition are presented. Keywords: Solution chemistry, Solubility, Flocculating agents, Gypsum, Density, Speed of sound Studies on aqueous electrolyte systems are important for understanding ionic equilibrium,  ion-solvent and ion-ion interactions.1,2 Such studies are even more important for developing the processes for recovery of various inorganic  salts from the marine resources like sea and  subsoil waters and industrial effluents containing different salts. Gypsum (CaSO4·2H2O) is one of  the important marine salts which is precipitated in large quantities during the solar evaporation of natural brines prior to the production of common salt (NaCl). It finds several applications, for example in cement industry, writing chalk, soil additive, or as a food additive, etc.3,4 Because of very low solubility of CaSO4·2H2O in water/brines, the Ca2+ and SO42- ions present in feed brines precipitate out and form scale in pipes and other production equipment once its saturation limit exceeds the solubility limit.5 For prevention of scale formation it is necessary to alter the solubility of CaSO4·2H2O with organic or inorganic additives. Therefore, understanding the effect of various kinds of additives on the solubility behavior of CaSO4·2H2O in brines is of practical importance. Along with the solubility behavior, knowledge of other physical properties of solution such as density, speed of sound and estimated values of compressibility therein is also a necessary prerequisite6,7 while processing for the industrial applications.  Physical phenomena such as crystallization,  phase stability and solubility of CaSO4·2H2O in  water and aqueous solutions containing electrolytes have been widely reviewed.4,8 The solubility  behavior of CaSO4·2H2O in brines has been particularly extensively studied in the past.8-15 We  are continuing our research program on measurements and interpretation of aqueous CaSO4·2H2O  solubility and solution properties in presence of electrolytes and non-electrolytes. Ternary systems (CaSO4·2H2O+NaCl+H2O and CaSO4·2H2O+CaCl2+ H2O),16-19 quaternary systems (CaSO4·2H2O+NaCl+ CaCl2+H2O, CaSO4·2H2O+NaCl+MgCl2+H2O and (CaSO4·2H2O+NaCl+H2O+MNO3, where M = Na,  K, NH4)20-22 and (CaSO4·2H2O+NaCl+H2O+organic solvents)23-24 have been investigated in our laboratory in the past few years.  In recent studies on solar salt production from natural brines it has been observed that Ca2+ and SO42- ion impurities are dramatically reduced on addition of flocculating agents in saturated  brines such as alum or PAC.25,26 Therefore, in continuation of our research program on solubility studies of gypsum in brines, herein we intend to study the effect of alum and polyaluminium chloride on the solubility behavior of CaSO4·2H2O in aqueous NaCl solutions. We also report the data on solution properties such as density and speed of sound for these systems. The primary data have been used to determine solution isentropic compressibility of the solutions.  INDIAN J CHEM, SEC A, JANUARY 2016   58Experimental  Flocculating agent, polyaluminum chloride (PAC powder form, LR grade, CAS No. 1327-41-9)  was purchased from National Chemical, assay:  Al2O3 content: 28–30 % (measured in our lab 26.52%). Alum (non-ferric) commercial grade and other inorganic salts such as CaSO4·2H2O, NaCl, (>99.5 mol%) obtained from SD Fine Chemicals (Mumbai) were used without further purification after drying in an oven at 70 °C. Solutions were prepared by weight, using an analytical balance with a precision of ±0.0001 g (Denver Instrument APX-200) in Millipore grade water. Stock solutions were prepared by adding oven dried NaCl to the solutions containing fixed amounts of PAC or alum. A range  of solutions with different NaCl and fixed PAC or alum concentrations saturated with CaSO4·2H2O  were then made by diluting stock solutions with aqueous PAC or alum solutions and adding excess CaSO4·2H2O. The resulting solutions were stirred in a thermostatically controlled water bath. After the solutions were stirred with an electrical paddle for about 24 h, liquid samples were withdrawn periodically and analyzed for different ions as described elsewhere.16 In brief, Ca2+ and Cl– concentrations were determined volumetrically using standard EDTA and AgNO3 solutions respectively with an error of <0.2%, and SO42- concentration was determined gravimetrically using barium chloride precipitation as per the standard analytic procedure with an estimated error of <0.1%.  The density (ρ) of the solutions was measured with an Anton Paar (model DMA 4500) vibrating-tube densimeter with a resolution of 5×10−2 kg m−3 at  35 °C. The densimeter was calibrated with doubly distilled and degassed water, with dry air at atmospheric pressure, and also against the densities of aqueous NaCl solutions.27 Sample temperature  was controlled by a built-in peltier device to within ±0.03 K and the accuracy of the measurements was 0.01%. Reproducibility of the results was confirmed by performing at least three measurements for each sample. Speed of sound (u) in the salt solutions was measured at 51600 Hz using a concentration analyzer (model 87, SCM Laboratory Sonic Composition Monitor) based on the sing-around technique28 at  35 °C with a single transducer cell, immersed in a water bath with temperature controlled to ±0.01 °C. The analyzer was calibrated by measurements of speeds of sound in water as a reference28, with an estimated error <±0.1 m·s-1. Reproducibility of the results was ensured by performing at least three experiments for each concentration.   Results and discussion Experimental results of CaSO4·2H2O solubility in aqueous-NaCl solutions containing fixed molality of alum or PAC and the solution densities and speed of sounds at 35 °C are given in Table 1. The dependence of CaSO4·2H2O solubility in NaCl+H2O+alum or PAC systems and other solution properties as a function of NaCl concentration has been correlated by means of a polynomial type equation, 2 31 2 3( ) ( NaCl) ( NaCl) ( NaCl)F Q Ao A m A m A m= + + +  … (1) where Q represents a general measured property (solubility, density, speed of sound) or derived function (isentropic compressibility) and m is the molality (mol kg-1) of NaCl in the solution. The values of the parameters Ai were evaluated by the method of least squares using non-linear method with all points weighted equally. The parameters Ai and standard deviations σ are given in Table 2. The solubility pattern of CaSO4·2H2O in pure brines and brines containing different concentration of alum or PAC is compared in Figs 1 and 2 respectively. The solubility of CaSO4·2H2O in brine solutions decreases significantly with increase in alum concentration, whereas in the case of PAC the solubility increases with increase in concentration. The decrease in solubility of CaSO4·2H2O with the addition of alum may be due to the common ion effect of (SO42–). Dissolution of CaSO4·2H2O in water follows the equilibrium:  )1(OH2)aq(SO)aq(Ca)s(OH2.CaSO224224++⇔−+  … (2) Excess amount of SO4-2 ions will shift the dissolution equilibrium to the left, reducing the concentration of Ca2+, and thus effectively reducing the solubility of the solid. The solubility maximum of CaSO4·2H2O was not altered by the addition of alum and an amount of 10 wt% alum could reduce the solubility higher than 80% with almost linear pattern. In the case of PAC added CaSO4·2H2O solutions in brine, interionic forces play a role in enhancing the CaSO4·2H2O solubility wherein all the ions in solution, besides possessing tightly-held waters of hydration, tend to attract oppositely-charged ions ("counter-ions") around them. These ion-counterion bodies appear to be almost electrically neutral, which keeps  them  from  interacting  with  each  other (as to  NOTES   59  Table 1—Solubility of CaSO4·2H2O in NaCl+alum/ PAC+H2O, solution density (ρ) and speed of sound (u) at 35 °C NaCl (mol·kg-1) CaSO4 (mol kg-1) ρ (kg m-3) u (m s-1)  NaCl (mol kg-1) CaSO4 (mol kg-1) ρ (kg m-3) u (m s-1) 1% Alum  3% Alum 0.0508 0.0223 1009.1 1545.2  0.1009 0.0125 1019.8 1553.8 0.7568 0.0255 1036.0 1578.2  0.7568 0.0165 1041.9 1585.7 1.1604 0.0280 1047.9 1596.8  1.0595 0.0181 1053.3 1598.6 1.6650 0.0305 1063.3 1622.1  1.6144 0.0201 1074.4 1624.8 2.1694 0.0321 1083.3 1643.4  1.9181 0.0215 1082.3 1640.6 2.4721 0.0321 1094.7 1658.8  2.3207 0.0220 1097.1 1659.1 2.9407 0.0318 1110.1 1680.9  2.8757 0.0218 1117.7 1684.3 3.4814 0.0310 1129.5 1707.2  3.2793 0.0204 1133.7 1706.5 4.0361 0.0293 1146.5 1734. 2  3.8343 0.0180 1150.7 1730.6 4.4397 0.0275 1162.6 1752.9  3.9856 0.0175 1155.4 1737.9 4.8938 0.0261 1177.7 1775.8  4.6919 0.0145 1182.5 1771.8 6% Alum  10% Alum 0.0503 0.0115 1040.9 1568. 5  0.0727 0.0058 1062.8 1593.2 0.5549 0.0138 1056.5 1589.1  0.5838 0.0060 1075.9 1618.4 1.1099 0.0141 1078.4 1620.5  1.0216 0.0065 1091.5 1637.9 1.4631 0.0143 1089.1 1637. 9  1.3621 0.0072 1105.2 1653.6 1.9677 0.0143 1105.4 1659.4  1.8481 0.0080 1121.0 1675.7 2.3712 0.0142 1118.1 1680.4  2.1886 0.0085 1131.2 1688.1 2.7746 0.0141 1133.9 1701. 7  2.5770 0.0085 1143.9 1707.7 3.2792 0.0135 1149.3 1724.4  3.0151 0.0082 1157.7 1729.7 3.7332 0.0127 1165.2 1748.3  3.4583 0.0077 1172.3 1745. 8 4.0361 0.0119 1174.9 1763.6  3.6790 0.0075 1179.1 1754.2 4.5908 0.0114 1195.7 1786.0  4.2796 0.0070 1199.6 1782.9 1% PAC  3% PAC 0.0958 0.0406 1002.7 1534.1  0.3392 0.0623 1011.2 1550.3 0.6162 0.0495 1023.5 1558.5  0.7753 0.0668 1028.4 1582.2 1.0660 0.0540 1041.6 1586.1  1.2114 0.0698 1045.3 1600.5 1.5506 0.0565 1060.6 1611.3  1.6475 0.0708 1062.3 1635.4 2.1805 0.0595 1077.9 1641.6  2.0835 0.0708 1078.6 1653.4 2.7135 0.0599 1098.5 1667.6  2.5681 0.0708 1096.1 1687.8 3.2434 0.0599 1117.7 1694.2  3.0042 0.0703 1113.5 1713.2 3.7310 0.0594 1135.8 1718.1  3.4887 0.0679 1131.0 1720.6 4.0702 0.0579 1148.2 1739.1  3.9248 0.0653 1144.9 1756.8 4.4578 0.0562 1163.1 1760.6  4.3125 0.0624 1162.0 1783.4 5.0878 0.0527 1186.8 1793.4  4.6032 0.0608 1172.2 1802.7 6% PAC 0.0171 0.0651 1019.8 1539.5 2.7926 0.0853 1137.6 1682.6 0.5681 0.0716 1042.4 1563.2 2.9432 0.0847 1143.4 1704.3 1.0130 0.0772 1062.9 1586.2 3.5592 0.0794 1170.4 1726.5 1.5469 0.0816 1085.5 1612.2 4.0640 0.0721 1192.1 1747.7 1.8207 0.0833 1096.4 1638.3 4.3806 0.0655 1204.9 1761.8 2.3819 0.0856 1118.9 1662.9     INDIAN J CHEM, SEC A, JANUARY 2016   60 Table 2—Parameters Ai and standard deviations σ of Eq. (1) for the system CaSO4·2H2O+NaCl+alum/ PAC+H2O at 35 °C Additive A0 A1 A2 A3 σ Solubility (mol kg-1) 1% Alum 0.0221 0.0179 0.00571 –0.00311 1.87×10-4 3% Alum 0.0118 0.00715 -0.0017 -8.64×10-4 2.32×10-4 6% Alum 0.0112 0.0064 -0.0056 0.00259 5.87×10-5 10% Alum 0.0059 -0.0014 0.0029 -9.75×10-4 8.44×10-5 1% PAC 0.0397 0.01693 -0.0041 2.54×10-4 6.41×10-4 3% PAC 0.0587 0.01293 -0.0034 2.22×10-4 4.64×10-4 6% PAC 0.0647 0.01299 -0.0004 -5.6×10-4 2.54×10-4 Density, ρ (kg m-3) 1% Alum 1007.8 0034.7 - - 1.24 3% Alum 1015.8 0035.3 - - 1.16 6% Alum 1039.0 0033.8 - - 1.09 10% Alum 1059.2 0032.7 - - 1.19 1% PAC 1000.9 0036.3 - - 1.7 3% PAC 0999.0 0037.5 - - 0.98 6% PAC 1019.0 0042.4 - - 0.67 Speed of sound, u (m s-1) 1% Alum 1541.90 47.55 - - 0.92 3% Alum 1549.01 47.46 - - 0.91 6% Alum 1565.17 48.70 - - 1.83 10% Alum 1591.93 44.64 - - 1.58 1% PAC 1528.81 51.62 - - 2.08 3% PAC 1547.00 58.56 - - 2.64 6% PAC 1525.08 51.87 - - 3.05 Isentropic compressibility, 1012×κS (Pa-1) 1% Alum 409.03 -39.27 1.95 - 1.00 3% Alum 388.02 -37.85 2.11 - 0.63 6% Alum 368.85 -31.05 1.47 - 0.54 10% Alum 404.34 -38.96 1.95 - 1.44 1% PAC 426.37 -42.04 1.95 - 1.61 3% PAC 425.12 -47.01 2.64 - 2.75 6% PAC 416.36 -45.64 2.61 - 2.01  form a precipitate). No alteration in solubility maximum of CaSO4·2H2O in solutions with added alum may be due to less perturbation of water by the ions furnished by it. In PAC added solutions, the solubility maximum of CaSO4·2H2O shifted slightly towards the higher salinities and the solutions formed a gel like structure at PAC concentration >6%.  Density (ρ) and speed of sound (u) values for the system CaSO4·2H2O+NaCl+H2O+alum/PAC at 35 °C as a function of NaCl concentration are reported in Table 1 and are plotted in Figs 3 and 4, respectively. Both, ρ and u were found to increase linearly with an increase in NaCl concentration in the solution and also depended upon the type of flocculating agent in the solution. The ρ and u of solutions were higher for the solutions containing either alum or PAC. Solutions containing 6% PAC became highly dense at higher brine concentrations. Similar behavior was also observed for u values wherein it was quite high in solutions containing 6% PAC. The density and speed of sound largely depends upon how ions affect the structure of neighboring water molecules in the solution. Normally the small ions cause a high degree of electrostatic ordering of neighboring waters.  For larger ions, electrostatic effects are smaller,  hence water structure is dominated by water-water hydrogen bonding in the first shell around the ion. The  high density   in  6%  PAC  containing  solutions   NOTES   61     Fig. 2—Comparison of solubility of CaSO4·2H2O at 35 °C in aqueous NaCl-solutions containing coagulant polyaluminum chloride (PAC) at varying concentrations. [Lines are polynomial fit to the experimental data].   Fig. 1—Comparison of solubility behavior of CaSO4·2H2O at 35 °C in aqueous NaCl-solutions containing coagulant alum at varying concentrations.[Lines are polynomial fit to the experimental data].   Fig. 3—Density of aqueous-NaCl solutions saturated with CaSO4•2H2O and containing varying amounts of (a) alum, and (b) PAC at 35 °C. [Lines are linear fit to the experimental data].    Fig. 4—Speed of sound of aqueous-NaCl solutions saturated with CaSO4•2H2O and containing varying amounts of (a) alum, and (b) PAC at 35 °C. [Lines are linear fit to the experimental data]. INDIAN J CHEM, SEC A, JANUARY 2016   62 may be due to breaking of normal water structure  and formation of dense ion solvated networked structure. The composition dependence of ρ and u  has been fitted to Eq. 1. The parameters Ai  and standard deviations σ are given in Table 2. Assuming negligible ultrasonic absorption under the experimental conditions, we derived the solution isentropic compressibility κS from the u and ρ values by using the Newton–Laplace equation, 21/S uκ ρ=   … (3) Figure 5 show a comparison of κS of the CaSO4·2H2O+NaCl+H2O+alum/PAC system. The composition dependence of κS has been fitted to Eq. 1, and the parameters and standard deviations are provided in Table 2. Addition of alum/PAC in water decreased the solution compressibility in general.  The κS decreased with the increase of alum/PAC concentration. When compared at the same concentration of alum or PAC in the solution,  the solutions having alum were found less compressible. The compressibility of aqueous electrolyte solutions comprises configurational as well as vibrational effects with the former effect playing a dominant role in dilute solutions.29 At higher concentrations the isentropic compressibility is largely determined by the compression of the hydration shell of the ions. Therefore, it can be assumed that alum perturbs the bulk water structure and weakens its hydrogen bonding to a larger extent as compared with PAC, thereby making the solutions more rigid.  Acknowledgement Council of Scientific & Industrial Research,  New Delhi, India, is gratefully acknowledged for supporting the research under Chemical Science Cluster (CSC0203) project of Twelfth Five Year Plan.  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Fig. 5—Isentropic compressibility of aqueous-NaCl solutions saturated with CaSO4·2H2O and containing varying amounts of (a) alum, and (b) PAC at 35 °C. [Lines are polynomial fit to the experimental data]. NOTES   6320 Kumar A, Sanghavi R & Mohandas V P, J Chem Eng Data, 52 (2007) 902. 21 Kumar A, Shukla J, Dangar Y & Mohandas V P, J Chem Eng Data, 55 (2010) 1675.  22 Trivedi T J, Shukla J & Kumar A, J Chem Eng Data, 59 (2014) 832. 23 Kaur M, Rao K S, Singh T, Mohandas V P & Kumar A,  J Chem Eng Data, 55 (2010) 4704. 24 Trivedi T J, Pandya P & Kumar A, J Chem Eng Data 58 (2013) 773. 25 Kumar A, Mukhopadhyay I, Ghosh P K, Mohandas V P, Shukla J & Sanghavi R J, US Pat, (2011) US 8, 021, 442, B2.  26 Mukhopadhyay I, Ghosh P K, Mohandas V P, Hamidani A U, Susarala, V R K S & Sanghavi R J, US Pat, (2012) US8282690 B2 and EP1928569 B1.  27 Surdo A L, Alzola E M & Millero F J, J Chem Thermodyn, 14 (1982) 649.  28 Del Grosso V A & Mader C W, J Acoust Soc Am, 52 (1972) 1442. 29 Sile W M, Donfor A R & Litovitz T A, J Chem Phys 44 (1966) 3712.                                           INDIAN J CHEM, SEC A, JANUARY 2016   64  

Effect of flocculating agents on solubility behavior of calcium sulfate dihydrate (gypsum) in aqueous sodium chloride system and solution properties at 35 °C 

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Effect of flocculating agents on solubility behavior of calcium sulfate dihydrate (gypsum) in aqueous sodium chloride system and solution properties at 35 °C

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