Formation and Dissolution of High- Temperature Forms of Calcium Sulfate Scales: The Influence of Inhibitors
Authors: Nancollas G.H. , Gill J.S. 
Published: December 2007

Abstract
The formation of calcium sulfate (CaSO4) scales is a persistent problem. At the high ambient temperatures often encountered in the field, not only do the mineral phases exhibit reduced solubility, but hydrate phase changes may take place. Therefore, the overall scaling reaction may reflect not only the growth of a thermodynamically stable high-temperature phase but also the concomitant dissolution of other metastable hydrates that may be favored kinetically. In the work described here, the growth and dissolution of the -hemihydrate and anhydrite phases of CaSO4 have been investigated at temperatures from 90 to 150 deg. C. The growth and dissolution rates of anhydrite follow a kinetic equation second-order in super- and under-saturation, respectively. Both processes appear to be surface controlled. In contrast, the dissolution of hemihydrate follows a rate equation first-order in undersaturation, and the phase transformations are influenced markedly by temperature changes. Typical organic polyphosphonate and polyacrylate scale inhibitors markedly reduced the formation rates of both hemihydrate and anhydrite at concentrations as low as 10-7M.

Introduction
The formation of calcium sulfate (CaSO4) scales in geothermal applications, evaporative desalination plants, cooling towers, and petroleum engineering is plants, cooling towers, and petroleum engineering is a persistent problem. In many cases, formation of the hard crystalline deposits limits the efficiency of a particular process or plant design. Much work has particular process or plant design. Much work has been done in an attempt to characterize the formation products of the scale minerals below which solutions will remain metastable and above which catastrophic formation of solids takes place. Glater and Schwartz made solubility measurements of CaSO4 hemihydrate and anhydrite and proposed a method of scaling threshold computation as a function of temperature and salinity. A heated metal wire was used by Adachi and Tanimoto to induce scale formation in CaSO4 solutions, and the effective supersolubility of CaSO4 hemihydrate was calculated from the wall temperature of the wire and the concentration of solution. Such approaches are based on the use of thermodynamic solubility products for calculating the concentrations of products for calculating the concentrations of sparingly soluble scale minerals, but since the scale process usually takes place on a solid substrate, it is process usually takes place on a solid substrate, it is much more important to take into account kinetic factors when attempting to understand and control scaling. There now is considerable evidence that crystal growth takes place through the formation of metastable intermediates that initially are favored kinetically but that rapidly transform to the thermodynamically stable phases typically observed in the field after equilibrium has been reached.In the case of CaSO4 scale formation, the problem is made worse by a decreasing solubility with increasing temperature and by the salt's ability to crystallize from aqueous solution in at least three forms: the dihydrate, the alpha- and beta-hemihydrates, and the anhydrite. The stability of these phases and the kinetics of transformation from one to another depend on such factors as the temperature and ionic strength of the solutions. On heated surfaces, it is quite likely that more than one hydrate phase will form. Following initial formation of the low-temperature dihydrate, the reduced heat-transfer coefficient accompanying scale buildup will lead to an increase in temperature and the formation of hemihydrate and anhydrite phases in the underlying scale deposits. Therefore, the growth of one phase upon another may be important in describing the overall scaling process.
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