Colloids and Surfaces A: Physicochemical and Engineering Aspects
A new method for the study of calcium carbonate growth on steel surfaces
Introduction
The growth of calcium carbonate (CaCO3) on various surfaces has been thoroughly examined in many studies but the detailed mechanisms that govern the crystal growth and shape are still unknown. Temperature and pH are known to be parameters of particular importance for CaCO3 crystallization. The solubility of the salt decreases with temperature, hence crystallization frequently occurs on warm surfaces, e.g. heat exchangers. Crystallization of CaCO3 is sometimes a desirable process, for example in biomineralization [1] but more often CaCO3 crystallization is an unwanted phenomenon. Uncontrolled growth of calcium carbonate on surfaces is usually referred to as scaling. It is a well known problem in areas as diverse as pulp and paper, oil production and water cooling processes [2]. Against this background there is a considerable interest to find compounds that efficiently prevent the formation of CaCO3 deposits on surfaces, so called scale inhibitors. Most studies related to scale inhibition have dealt with the effect of polyelectrolyte addition to calcium carbonate solutions though some later studies [3] have focused on poly(ethylene glycol) (PEG) compounds with various substituents attached. The latter polymers affect crystal shape and growth in an interesting way, and it seems possible to alter the crystal forms by the choice of end groups of the PEG chain [4]. The common method of inducing crystallization in a laboratory experiment is to mix aqueous solutions of two soluble salts such as Na2CO3 and CaCl2. Crystallization by this procedure is a fast process, giving large amounts of CaCO3 crystals that rapidly sink to the bottom of the beaker. However, it is not really a suitable technique for mimicing scaling in real systems, since scaling in large scale operations usually proceeds very slowly compared with such an experiment. In pulp mills for example, where the calcium content is usually high, the calcium carbonate layer formed on metal surfaces might increase by 0.5 mm per month [5]. In this article we describe a method to induce and monitor scaling which resembles the practical situation, for instance in a heat exchanger.
As mentioned above, a common method to alter the shapes of CaCO3 crystals is to add small amounts of a polyelectrolyte to the solution. The polyelectrolyte is negatively charged, and it is believed that the polymer attaches to the growth steps of the crystal. Adsorption of the polymer prevents the crystal from growing in an organized fashion and an amorphous structure is often formed.
Poly(acrylic acid), anionic polyacrylamide and various derivatives of these are frequently used as additives in industrial processes to prevent scaling on surfaces. It has also been shown that crystal growth can be reduced by addition of substances such as gelatin, carboxymethyl cellulose and keratin [6] though nowadays the use of these additives is probably rare. As mentioned earlier, it is also possible to use PEG compounds to achieve similar antiscaling effects as with the polyelectrolytes.
In this paper, we have extended the PEG polymer concept to surface active copolymers of PEG and poly(propylene glycol) (PPG). A diphosphate moiety is introduced at the hydroxyl terminal end of the PEG block. Diphosphates are known to bind strongly to calcium ions. The rationale behind the use of a surface active polymer as a scale inhibitor is to enhance the tendency of the polymer to interact at solid surfaces.
Section snippets
Experimental section
The starting block copolymers were provided by Akzo Nobel Surface Chemistry, Sweden. The more hydrophobic polymer had a poly(ethylene glycol) (PEG) block of molecular weight (MW) 320 g mol−1 and a poly propylene glycol (PPG) block of MW 1800 g mol−1. The more hydrophilic polymer had a PEG MW of 1800 g mol−1 and a PPG MW of 1800 g mol−1. The block copolymers were terminated in the PPG end with a methyl group in order to prevent reactions from occuring at this end. To a round bottomed, three
The polymeric scale inhibitors
The synthesis is outlined in Fig. 4. Addition of epichlorohydrin to the hydroxyl-terminal end of the block copolymer was made in the presence of strong caustic using a procedure described before [7]. An oxirane-terminated block copolymer was obtained in almost quantitative yield. It is unclear if the addition reaction proceeds as a direct substitution of the chloride by the alkoxide or if the alkoxide ion first ring-opens the epoxide to give a chlorohydrine which subsequently looses HCl to
Concluding remarks
The surface active polymeric scale inhibitors synthesized and evaluated in this work have not been optimized. The anti-scaling effect obtained, although substantial (see Table 1), is not remarkable. However, we believe that the concept of using a surface active polymer with a ligand for Ca2+ attached at the end is an interesting one. Due to the combination of surface activity and complexing ability such components should have a very strong affinity for CaCO3 covered surfaces and thus be
Acknowledgements
We would like to thank the following persons: Jonas Ekroth at BIM Kemi AB whose knowledge in industrial scaling was invaluable and Elina Sandberg at Akzo Nobel Surface Chemistry for valuable advice on the synthesis of the diphosphate polymer. We also thank KIF for financial support (to PK).
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