Format And Type Fonts


 

 CCHHEEMMIICCAALL  EENNGGIINNEEEERRIINNGG  TTRRAANNSSAACCTTIIOONNSS  
 

VOL. 39, 2014 

A publication of 

 
The Italian Association 

of Chemical Engineering 

www.aidic.it/cet 
Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Peng Yen Liew, Jun Yow Yong  

Copyright © 2014, AIDIC Servizi S.r.l., 

ISBN 978-88-95608-30-3; ISSN 2283-9216 DOI: 10.3303/CET1439130 

 

Please cite this article as: Prisciandaro M., Lancia A., Musmarra D., di Celso G.M., 2014, Gypsum scale inhibition on 
process equipment surfaces: a review, Chemical Engineering Transactions, 39, 775-780  DOI:10.3303/CET1439130 

775 

Gypsum Scale Inhibition on Process Equipment Surfaces:  
A Review 

 

Marina Prisciandaro
a
, Amedeo Lancia

b
, Dino Musmarra

c
,  

Giuseppe Mazziotti di Celso
d
 

a
Dipartimento di Ingegneria Industriale, dell'Informazione e di Economia, Università dell’Aquila, viale Giovanni Gronchi 

18, 67100 L'Aquila, Italy 
b
Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università “Federico II” di Napoli, P.le 

V. Tecchio, 80, 80125 Napoli, Italy 
c
Dipartimento di Ingegneria Civile, Design, Edilizia e Ambiente, Seconda Università degli Studi di Napoli, Real Casa 

dell’Annunziata, Via Roma 29, 81031 Aversa (CE), Italy 
d
Univ. of Teramo, Faculty of Bioscience, Via C. Lerici, 1, 64023 Mosciano S.A. (TE), Italy. 

marina.prisciandaro@univaq.it 

The aim of the paper is to quantify and to compare the antiscalant effect of several additives on gypsum 

precipitation by measuring the retard on the induction period for nucleation. Through a well-assessed laser 

light scattering technique previously devised, the induction times are measured, and then used to estimate 

thermodynamic parameters such as the activation energy for nucleation, in a calcium sulfate 

supersaturated solution with the addition of NTMP, PBTC and citric acid. Experiments are carried out at a 

fixed additive concentration level (equal to 0.05 g/L), with the temperature varying in the range 15 – 35 °C. 

A comparison among different additives allows to define which is the most active in retarding gypsum 

scale formation. 

1. Introduction 

Gypsum scale deposition on process equipment surfaces has several disadvantages: in particular, when 

scales crystallize on heat transfer surfaces, they offer a resistance to the heat flow and they can 

accumulate in pipelines, orifices and other flow passages seriously impeding the process flow. Moreover, 

calcium sulfate scales, together with calcium carbonate scales, are the major cause of fouling in reverse 

osmosis membranes, resulting in a continuous decline in desalted water production thus reducing the 

overall efficiency and increasing operation and maintenance costs. Therefore, from an economic point of 

view, the formation of calcium sulfate mineral scales is an obstacle to the recovery of potable water from 

sea or brackish waters, as well as to the industrial utilization of many natural waters. 

Calcium sulfate, CaSO4, has several forms, i.e. calcium sulfate dihydrate (commercially known as 

gypsum), calcium sulfate anhydrous (anhydrite), calcium sulfate hemihydrate, present in two different 

structures, -hemihydrate and -hemihydrate (stucco or plaster of Paris). In natural deposits, the main 

form is the dehydrate; however, some anhydrite is also present in most areas, although to a lesser extent 

(Kirk-Othmer, 2011). Besides occurring naturally in the environment, calcium sulfate can be obtained by 

precipitation. In particular, calcium sulfate may crystallize as gypsum, calcium sulfate hemihydrate and 

anhydrite. In the recent years, there has been a revival of interest in calcium sulfate for biomedical 

applications, in which it is of great interest to synthesize uniform low-dimension crystals of calcium sulfate 

through wet chemical process (Sandhya et al., 2012). Gypsum is also obtained as a by-product of various 

chemical processes. The main sources are processes that involve gas scrubbing, i.e. burning sulfur-

containing fuels, such as coal, used in electrical power generating plants, and the chemical synthesis of 

chemicals, such as sulfuric acid, phosphoric acid, titanium dioxide, citric acid, and organic polymers.  

For all above mentioned question, the understanding of gypsum nucleation mechanism is of great 

importance in different fields. Despite the fact that considerable research has been going on over the past 

decades on the formation of calcium sulfate in aqueous media there is still some uncertainty as to the  



 

 

776 

 
NTMP PBTC CA 

   

Figure 1: Structure of the three tested additives 

mechanism of formation of this salt, because of the widely variable conditions of the solutions in which the 

salt formation takes place, including temperature, pH, ionic strength and composition and the presence of 

foreign ions and/or water soluble compounds (Lioliou et al., 2006). In the present paper, experimental 

results relative to the measurement of induction time for gypsum nucleation as a function of temperature 

are presented, with the addition in solution of several additives and namely citric acid (CA), NTMP, PBTC, 

whose structures are shown in Figure 1; they are able to retard the formation of gypsum nuclei, through 

the increasing of induction times.  

1.1 Induction time measurement and nucleation mechanisms 
The study of the effects of an additive on gypsum nucleation can be carried out by evaluating the induction 

period, defined as the time elapsing between the onset of supersaturation and the formation of critical 

nuclei, or embryos.  

The various experimental techniques detect with a different resolution the first portions of the crystalline 

phase, that nucleates and grows in a supersaturated solution. As a result, under otherwise equal 

conditions, induction time may not have the same value when measured by different techniques.  

However, this time primarily depends on solution supersaturation and temperature and it is the sum of two 

components: nucleation time (tn), related to the appearance of the critical nuclei, and growth time (tg), 

connected to the growth process, leading from critical nuclei to measurable crystals. Depending on the 

relative values of these two time periods, induction time can be influenced by nucleation alone (tn>>tg, 

nucleation-controlled induction period), by both mechanisms (tntg, nucleation-and-growth-controlled 

induction period) or by growth alone (tn<<tg, growth-controlled induction period) (Söhnel and Mullin, 1988). 

While tg can be estimated by a kinetic growth expression, tn is more difficult to quantify. Nevertheless, it is 

possible to discriminate whether the appearance of the new solid phase is controlled by nucleation and/or 

by growth, on the basis of the dependence of tind on supersaturation.  

In particular, if the process which takes place is truly homogeneous nucleation, i.e. it occurs in a clear 

solution under the effect of supersaturation alone, tind is inversely proportional to the nucleation rate, 

defined as the number of nuclei formed in solution per unit time and volume.  

In this case, as shown by Mullin (2001) and Söhnel and Garside (1992), it is possible to use the 

experimental knowledge of the induction period to estimate the activation energy (Eact) for nucleation 

based on the dependence of tind on temperature.  

Moreover, tind dependence on supersaturation allows to determine the interfacial tension (sl) between 

crystals and the surrounding solution.  

In this paper, the three tested additives already compared to study their effect on gypsum interfacial 

tension (Prisciandaro et al., 2009), are now examined to evaluate their influence on activation energy. 

2. Materials and Methods 

The experimental apparatus consists of a stirred reactor with a related optical device and is schematically 

sketched in Figure 2. The reactor is a batch cylindrical crystallizer, made of glass, with a working volume of 

1.1x10
-3

 m
3
 and a diameter of 0.09 m. The crystallizer is surrounded by a water jacket for temperature 

control; stirring is provided by a two–blade polypropylene stirrer, with rotation rate ranging between 1 and 

10 s
-1

. An off–take tube, placed at half of the working height of the vessel, allows to remove samples of the 

suspension. 

PO

HO

HO

C N

C

C

H H

H H

P O

OH

OH

P O

OH

OH

H

H

PO

HO

HO

C C

H2C

H2C

H2C C

C OH

OH

OH

O

O

O

C

O

HO

C

H2C

H2C

C

C OH

OH

OH

O

O



 

 

777 

The stream removed by the off–take tube is sent, by a peristaltic pump, to an analysis flow–through cell, 

and then is conveyed again to the crystallizer. Since we are interested in induction time measurements, it 

is reasonable to think that crystal nuclei are so small that they are not damaged while passing through the 

pumping device, though located after that measurement cell; this assumption was indeed confirmed by 

optical microscope analysis. The cell, made of quartz, is 7x10
-2 

m long, with a square section of 2.5x10
-5

 

m
2
 and 2.5x10

-3 
m thickness. A 10 mW He–Ne laser beam (Io=632.8 nm) is focused on the cell, orthogonal 

to its walls; the beam, whose diameter is 2 mm, is vertically polarized. On the path of the laser beam, 

placed at 45° with respect to its direction, a beam splitter is provided in order to divide the laser beam into 

two parts; one is used to illuminate the measurement cell, while the other, collected by a photodiode, 

allows to check the stability and the intensity of the laser beam (Io). The signal of the scattered light (Isca) is 

collected by two lenses of focal 120 and 50 mm, at 90° with respect to the laser beam; this signal is sent, 

through a quartz optical fiber which ends on an interferential filter, to a photomultiplier tube, connected to a 

power supply with voltage variable in the range of 0 - 1,000 V. The signal of the transmitted light (Itrans) is 

collected by a photodiode located beyond the cell, at 0° with respect to the laser beam. The two analogue 

signals of scattered and transmitted light, together with Io, are collected by a recorder device. 

Supersaturated solutions of calcium sulfate were prepared by mixing clear aqueous solutions of reagent-

grade CaCl2·2H2O and Na2SO4 (Applichem, Darmstadt, Germany). The additive solutions were prepared 

by adding PBTC (C7H11O9P as 50 %w/w solution in water, available under the commercial name of 

Bayhibit
®
 AM kindly provided by Lanxess Deutschland GmbH, Leverkusen) to bidistilled water,  

The dissolved Ca
2+

 ion concentration was measured by ethylenediaminetetraacetic acid titration using 

Murexide (Applichem, Darmstadt, Germany) as an indicator, while the SO4
2-

 ion concentration was 

measured by means of turbidity measurements carried out in a spectrophotometer (Hach model 2010). 

The phosphonate ion concentration was determined by converting it into the orthophosphate form with 

potassium persulfate and a UV lamp and analyzing the free orthophosphate by means of a 

spectrophotometer (Hach model 2010).  

Once prepared, all the solutions were filtered, by using a 0.45 m filter (Millipore, HVLP 4700) and a 

vacuum pump (Vacuubrand, MZ4C), in order to eliminate all foreign materials inevitably present in the 

solution, and then mixed directly into the reactor. The additive aqueous solution was added to the Na2SO4 

solution and then fed to the reactor. The equimolar concentration of CaCl2·2H2O and Na2SO4 in the reactor 

varied between 50 and 290 mol/m
3
, while the additive concentration was 0.05 g/L in the reactor.  

 

 

temperature 

controller

 

crystallizer

 

recycle pump

 
photodiode

photomultiplier  

 
He-Ne laser  

cell

 

beam 

splitter

lenses 

and stop  

 
 

      
          photodiode 

 

optical 

fiber

 

data 

acquisition

 

filter

 

voltage supply 

0-1000V
 

CaCl2 x 2 H2O

 

Na2SO4

feed
  

 

storage tanks

T

additive

 

 

Figure 2: Sketch of the experimental apparatus used to measure the induction time 

The additive concentration range was selected by considering the dosage range typically used as a scale 

inhibitor in industrial process equipment.  



 

 

778 

 
The supersaturation ratio S was calculated considering the liquid–solid equilibrium between Ca

2+
 and 

SO4
2-

 ions and solid CaSO4·2H2O, as described by the following equation: 

2 2
4 2 4 2Ca SO 2H O CaSO 2H O

 
     (1) 

so that it is: 

2 2
24

2
 


H OCa SO

ps

a a a  
S

K
 (2) 

where aJ represents the activity of the J species (J= Ca
2+

, SO4
2-

 and water) expressed as the product of 

the molality (mJ) and the activity coefficient (J), and Kps is the solubility product of gypsum. The value of 

Kps was calculated as a function of temperature by means of the following relationship (Marshall and 

Slusher, 1966): 

 psln K 390.96 152.62logT 12545.62 T 0.08T     (3) 

The concentration values of the J species were calculated by solving a numerical model based on the 

equilibria that take place in the aqueous solution, and namely the acid dissolution equilibria of each 

additive, if present. The activity coefficients in the supersaturated solution were calculated by using a 

modified Deybe-Hückel equation, as reported in a previous paper (Prisciandaro et al., 2006). All the 

experiments were carried out for a fixed value of additive concentration (0.05 g/L) while the 

supersaturation ratio changed in the range 2÷5. The temperature was varied in the range 15÷35 °C. The 

pH range varied in the interval 3.1÷4.5. The induction period was evaluated by measuring the intensity of 

scattered and transmitted light signals as a function of time. These signals were processed to evaluate tind 

by adopting two parallel procedures, one graphical and the other one numerical. These procedures, 

described in detail elsewhere (Lancia et al., 1999), gave quite similar (±10 %) results. 

3. Results and discussion 

Experimental results can be grouped in two series: the first includes tests conducted by setting the 

temperature at the value T=25 °C, the additive concentration at cadditive = 0.05 g/L with changing the 

supersaturation S in the range 2÷5; the second series comprises experiments carried out at a fixed 

supersaturation and additive concentration, and namely S = 4.32 and cadditive = 0.05 g/L, while varying the 

temperature in the range 15÷35 °C (precisely, T = 15, 25, 35 °C). Figure 3 reports all the tests belonging to 

the first experimental series, showing the dependence of induction period on supersaturation at T = 25 °C. 

Figure 3 shows that the induction period for gypsum nucleation continuously decreases with increasing 

supersaturation. Mostly, this figure prove that NTMP is the additive that mostly increases the induction 

period, thus retarding the nucleation, followed by citric acid and then by PBTC. This last, at least in the 

tested experimental conditions, does not augment a lot the induction period values with respect to the 

absence of any additive. 

Experimental data belonging to the second series were used to estimate the activation energy for gypsum 

nucleation according to an Arrhenius relationship: 

exp
 

  
 

act
ind

E
t A

RT
 (4) 

where A is a constant, R the gas constant and Eact the activation energy.  

In particular Eq.(4) was reported as a continuous line in Figure 4, and from the slopes of these straight 

lines the values of activation energies have been estimated. The procedure has been repeated for different 

supersaturation levels, and the averaged values reported in Table 1 have been obtained. The analysis of 

the Table reveals that if compared with the baseline (i.e. the Eact value obtained in the absence of any 

additive, Eact=30 kJ/mol, (Lancia et al., 1999)), the activation energies obtained in the presence of CA and 

NTMP from (Prisciandaro et al., 2005) and later (Prisciandro et al., 2009) are quite similar, while the value 

obtained in the presence of PBTC (Prisciandaro et al., 2012) is higher; this circumstance is quite 

unexpected, since PBTC is the less active in retarding gypsum nucleation among the testes additives. This 

has been explained with a change in nucleation mechanism, which passes from the nucleation controlled 

one to a growth controlled one, characterized by a higher activation energy (Prisciandaro et al., 2012). 



 

 

779 

 

Figure 3: Induction times for gypsum nucleation as a function of S. T=25°C, cadditive=0.05 g/L.  

----------: no additive; □: citric acid; : PBTC; ×: NTMP 

 

Figure 4: Induction period as a function of the inverse of temperature for cadditive=0.05 g/L, S=4.32.  

----------: no additive; □: citric acid; : PBTC; ×: NTMP 

 



 

 

780 

 
Table 1: Estimated values for Eact at cadditive=0.05 g/L (averaged on a supersaturation range 2<S<4) 

Additive Eact [kJ/mol] 

Baseline 30 

NTMP 26 

CA 29 

PBTC 50 

4. Conclusions 

In this paper, the effect of different additives (CA, NTMP, PBTC) on gypsum nucleation kinetics is studied 

at T=15, 25 and 35 °C and at additive concentration equal to 0.05 g/L. Experimental results have been 

used to compare the inhibiting effect of different additives on gypsum nucleation and to estimate the 

activation energy for the nucleation process at various additive concentrations. The results confirm that 

NTMP is the most active of the tested additives; moreover, the comparison among the activation energies 

estimated with different additives in solution, has shown that when PBTC is present, the mechanism 

controlling nucleation kinetic changes, passing from a nucleation controlled induction period to a growth 

controlled induction period.  

References 

Kirk-Othmer Encyclopedia of Chemical Technology, 2011, Lancia, A., D. Musmarra, Prisciandaro, M. 

“Calcium sulfate” 1-22. Fifth Edition 2011, John Wiley & Sons, New York, USA. 

Lancia A., Musmarra D., Prisciandaro M., 1999, Measurement of the induction period for calcium sulfate 

dihydrate precipitation. AIChE J., 45, 390-397. 

Lioliou M.G., Paraskeva C.A., Koutsoukos P.G., Payatakes A.C., 2006, Calcium sulfate precipitation in the 

presence of water-soluble polymers. J. Coll. Int. Sci., 303, 164–170. 

Marshall W.L., Slusher R., 1966, Thermodynamics of calcium sulfate dihydrate in aqueous sodium chloride 

solutions 0-110°. J. Phys. Chem., 70, 4015-4027.  

Mullin, J.W., 2001, Crystallization, 4th edition, Butterworth-Heinemann Ltd, Oxford. UK. 

Prisciandaro M., Olivieri E., Lancia A., Musmarra D., 2009, Retardant effect of different additives on 

gypsum nucleation, Chemical Engineering Transactions Series, 17, 669-674. 

Prisciandaro M., Lancia A., Musmarra D., 2003, The retarding effect of citric acid on calcium sulfate 

nucleation kinetics. Ind. Eng. Chem. Res., 42, 6647-6652.  

Prisciandaro M., Olivieri E., Lancia A., Musmarra D., 2006, Gypsum precipitation from an aqueous solution 

in the presence of nitrilotrimethylenephosphonic acid. Ind. Eng. Chem. Res., 45, 2070-2076. 

Prisciandaro M., Olivieri E., Lancia A., Musmarra D., 2009, Gypsum Scale Control by 

Nitrilotrimethylenephosphonic Acid. Ind. Eng. Chem. Res., 48, 10877–10883. 

Prisciandaro M., Olivieri E., Lancia A., Musmarra D., 2012, PBTC as an Antiscalant for Gypsum 

Precipitation: Interfacial Tension and Activation Energy Estimation, Ind & Eng.Chem.Res., 51, 12844-

12851. 

Prisciandaro M., Santucci A., Lancia A., Musmarra D., 2005, Role of Citric Acid in Delaying Gypsum 

Precipitation, Canadian Journal of Chemical Engineering, 83, 586-592. 

Sandhya S., Sureshbabu S., Varma H.K., Komath M., 2012, Nucleation kinetics of the formation of low 

dimensional calcium sulfate dihydrate crystals in isopropyl alcohol medium, Cryst. Res. Technol., 47, 

780–792. 

Söhnel O., J. Garside, 1992, Precipitation, Butterworth-Heinemann Ltd, Oxford, UK 

Söhnel O., Mullin J.W., 1988, Interpretation of crystallization induction periods J. Coll. Int. Sci., 123, 43-50.