Calcium sulfate precipitation in the presence of water-soluble polymers

doi:10.1016/j.jcis.2006.07.054

Journal of Colloid and Interface Science

Volume 303, Issue 1, 1 November 2006, Pages 164-170

https://doi.org/10.1016/j.jcis.2006.07.054 Get rights and content

Abstract

The effect of four different polymers on the precipitation of calcium sulfate was investigated in the present work. The degree of inhibition was estimated from measurements of the calcium ion activity and from specific solution conductivity measurements in the supersaturated solutions during the course of the precipitation process. The effects of polyacrylic acid (PAA, three different polymers with average molecular weight 2000, 50,000, and 240,000, respectively) and of a co-polymer of PAA with polystyrene sulfonic acid (PSA, average molecular weight $< 20, 000$ ) were investigated with respect to their effect on the kinetics of spontaneous precipitation of calcium sulfate salts. The results of the kinetics experiments suggested that the spontaneous precipitation from supersaturated calcium sulfate solutions at 25 °C yielded exclusively calcium sulfate dihydrate (gypsum) both in the absence and in the presence of the polymeric additives. The induction times, preceding the formation of the solid increased in all cases in the presence of the polymeric additives. Polymer concentrations as low as 2.0 ppm increased induction time from practically zero to 10 min. The rates of precipitation were reduced according to the solutions content in the polymers added and precipitation was completely suppressed in the presence of 6.0 ppm of the polymers tested, depending on their molecular weight. The lower the molecular weight of PAA, the more efficient was the threshold inhibition and the stronger the reduction of the rates of spontaneous precipitation. PSA yielded the poorest inhibition efficiency in comparison with the PAA, possibly because of the relatively lower affinity of the sulfonate groups for the calcium ions of the surface of the solid forming. The kinetics results analysis assuming Langmuir-type adsorption of the polymeric molecules on the growing supercritical gypsum nuclei showed different affinity for the polymers tested in agreement with the respective inhibition efficiency, in the order: PAA1 > PAA2 > PSA > PAA3. The presence of the polymers in the supersaturated solutions resulted in modification of the precipitated gypsum crystals morphology.

Graphical abstract

The presence of polyacrylic acid in supersaturated solutions retards the spontaneous precipitation of calcium sulfate dihydrate and causes changes of the morphology of the precipitated crystals.

Introduction

The formation of tenaciously adhering calcium sulfate scale in a number of processes from water desalination to heat exchangers and processes involving heating of water is a persistent problem [1]. Although six different calcium sulfate crystal forms are known to exist [2], three different salts are usually encountered in natural formations and scale precipitates: calcium sulfate dihydrate (CaSO₄⋅2H₂O, CSD), calcium sulfate hemihydrate (CaSO₄ $\cdot 1 / 2$ H₂O, CSH) and anhydrous calcium sulfate (CaSO₄, CSA). Both the CSA and the CSH salts may undergo further dehydration via phase transformation to the anhydrous form [3]. Despite the fact that considerable research has been going on during the past decades on the formation of calcium sulfate in aqueous media there is still large uncertainty concerning the mechanism of formation of this salt because of the largely 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. A large number of the published studies agree on the fact that the formation of the calcium sulfate nuclei is initiated on solid substrates. These substrates may be either metallic surfaces of heat exchangers or crystals of the same or different substrates [4], [5], [6], [7], [8], [9].

Among the most important objectives of the mechanistic investigations has been the possibility to control the formation of the various forms of calcium sulfate. The main effort to this end has been focused in the use of water soluble inhibitors which may act either as threshold inhibitors which block the development of the supercritical nuclei [10], or as retarders of the growth of the calcium sulfate crystals [11], [12], [13], [14], [15], [16]. Several investigations have been carried out on the influence of trace amounts of polymeric scale inhibitors on both the precipitation and crystal habit modification of calcium sulfate forms [12], [17], [18], [19]. Polymers containing carboxylic groups such as carboxymethyl cellulose (CMC), polymethacrylic acid (PMA), and polyacrylic acid (PAA) were found to be particularly effective as CaSO₄⋅2H₂O growth inhibitors [12].

In the case of polymers in solution, there is general agreement that inhibition of the formation or the growth of the salt nuclei is effected by adsorption of these molecules on the active growth sites. Polymers tend to adsorb on solids from solutions due both to van der Waals [20] and/or electrostatic interactions [21], [22]. The length of the polymer chains therefore as well as the functional groups present, which through ionization regulate the electrostatic charges of the polymers, are of primary importance for the investigation of the role of the respective polymeric additives in the crystal growth of calcium sulfate.

In the present work, we have addressed the problem of the effect of PAA on the scale formation of gypsum (CSD) using polymers of markedly different molecular weight. Moreover, in order to compare the relative importance of the presence of the carboxylic groups, a co-polymer of PAA with sulfonated polystyrene was tested. The sulfonate groups are more strongly ionized in comparison to the carboxylic groups and are expected to promote stronger electrostatic interactions between the polymer and the surface of the calcium containing crystals which form in the supersaturated solutions. The effect of the water soluble polymeric additives was investigated in experiments in which calcium sulfate precipitation took place spontaneously from unstable supersaturated solutions, past the lapse of measurable induction time characteristic of the time frame needed for the formation of the supercritical nuclei and the subsequent initiation of the precipitation process. In order to accentuate the effect of the additives, the supersaturated solutions conditions selected for the test experiments in the absence of polymeric additives, yielded spontaneous precipitation with practically zero induction time.

Section snippets

Experimental

Crystal growth experiments were carried out in a 0.250-dm³ double-walled Pyrex vessel thermostated at $25.0 \pm 0.2 ° C$ by water circulation from a constant-temperature bath. Stock calcium chloride and sodium sulfate solutions were prepared from the respective crystalline solids (Merck, pro analisi). The solutions were filtered through membrane filters (0.22 μm, Millipore) and standardized by atomic absorption spectrometry (Perkin Elmer A Analyst 300) and ion chromatography (Dionex) for calcium and

Results and discussion

In all experiments of the present work, the pH of the supersaturated solutions was about 5.0 and it was not adjusted. It is established that pH over a wide range (3.0–8.0) does not affect the kinetics of spontaneous precipitation of CSD [9], [27], [28]. Three types of PAA polymers of different molecular weights and one polysulfonic acid polymer were tested as inhibitors of the calcium sulfate precipitation. The first three polymers are characterized by the same binding capacity with respect to

Acknowledgements

The authors wish to acknowledge financial support by the General Secretariat for Research and Technology, Ministry of Development, through PENED Program Contract M413/2002.

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