Abstract
Calcium sulfate is one of the most common inorganic salts with a high scaling potential. The solubility of calcium sulfate was modeled with the Pitzer equation at a temperature range from 273.15 to 473.15 K from published solubility data, which was critically evaluated. Only two Pitzer parameters, β(1) and β(2), with simple temperature dependency are required to model the solubility with excellent extrapolating capabilities up to 548.15 K. The stable temperature range for gypsum is 273.15–315.95 K, whereas above 315.95 K the stable phase is anhydrite. Hemihydrate is in the metastable phase in the whole temperature range, and the obtained metastable invariant temperature from gypsum to hemihydrate is 374.55 K. The obtained enthalpy and entropy changes at 298.15 K for the solubility reactions are in good agreement with literature values yielding solubility products of 2.40 × 10–05, 3.22 × 10–05, and 8.75 × 10–05 for gypsum, anhydrite, and hemihydrate, respectively. The obtained Pitzer model for the CaSO4–H2O system is capable of predicting the independent activity and osmotic coefficient data with experimental accuracy. The mean absolute average error of activity coefficient data at 298.15 K is less than 2.2%. Our model predicts the osmotic coefficient on the ice curve within 1.5% maximum error.
Highlights
Scaling or precipitation fouling, mainly forming a solid layer on equipment surfaces or piping networks, is a persistent problem encountered in many industrial processes, causing production losses, standstills, downtime and process efficiency decrease due to the reduction of equipment volume and material flow, increased heat transfer resistance, corrosion, and wearing out of construction materials.[1]
The stability regions of CaSO4 hydrates depend on solution conditions, and they are influenced by temperature and composition of the aqueous solution
Instead of comparing the calculated and measured molality, the difference in Gibbs energy was selected to fit the parameters of the Pitzer model
Summary
Mainly forming a solid layer on equipment surfaces or piping networks, is a persistent problem encountered in many industrial processes, causing production losses, standstills, downtime and process efficiency decrease due to the reduction of equipment volume and material flow, increased heat transfer resistance, corrosion, and wearing out of construction materials.[1]. The demand for process water circulation in hydrometallurgical processes will build up more and more complex and concentrated aqueous solutions, increasing the possibility of scaling. The need of thermodynamic understanding of a multicomponent aqueous solution is required, since laboratory analyses only, are not enough to comprehend the scaling potential and its variations with temperature and concentration. Calcium sulfate forms stable hydrates with 0, 1/2, and 2 molecules of crystalline water, with the chemical names of anhydrite (AH: CaSO4), hemihydrate (HH: CaSO4·0.5H2O), and dihydrate, i.e., gypsum (DH: CaSO4· 2H2O). Understanding the phase equilibria of CaSO4 as a function of temperature and other electrolytes is of great theoretical significance and practical importance, making it possible to estimate its scaling potential and facilitate the synthesis of calcium sulfate materials in industrial processes
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