Studies of the effects of electrolytes on the rates of precipitation of calcium sulphate dihydrate using an ion‐selective electrode

Abstract

A calcium ion selective electrode has been used to follow the acceleration of the rate of precipitation of calcium sulphate dihydrate by inorganic electrolytes at 15, 25 and 35°C. At zero and low added electrolyte concentrations, the rate of precipitation is strictly proportional to the square of the supersaturation, has an activation energy 75 kJ mol-' in agreement with previous studies and is indicative of a surface diffusion controlled process. However, with electrolytes containing sulphate ions in excess of 0.05 mol dm-3 marked departures from a square law are observed; the rate is best represented as the sum of a square and first order term in supersaturation, the first order term becoming dominant at concentration of c. 0.1 mol dnr3. Under these conditions the rate has an activation energy (29 kJ mol-l) which is close to that for the volume diffusion of ions in aqueous solutions and indicates that the surface process has been accelerated to such an extent that the diffusion of ions from solution to the crystal face has become the slowest and, hence, the rate controlling step. It appears that the electrolytes accelerate the surface process by increasing the rates of dehydration of the calcium and sulphate ions at the interface.

Calcium sulfate precipitation pathways in natural and engineered environments

“…He et al, 1994 37.0 Induction times of precipitation between 25-90 °C Lancia et al, 1999 62.8 Growth of seed crystals between 30-90 ºC Smith and Sweet, 1971 66.9±6.3 Growth of seed crystals between 25-55 ºC (Rapid growing faces only) Lui and Nancollas, 1973 58.6 Growth of seed crystals between 60-105 ºC. Nancollas et al, 1973 75±6.3 Growth of seed crystals between 10-30 ºC Mile et al, 1982 60.0 Growth of seed crystals between 25-60 ºC Witkamp et al, 1990 61 Growth of seed crystals between 10-40 °C He et al, 1994 64.…”

“…Smith and Sweett 1971;Van Rosmalen et al 1981;Christoffersen et al 1982) or the calcium concentration of the solution (e.g. Liu and Nancollas 1970;, 1975Packter 1974;Brandse et al 1977;Barcelona and Atwood 1978;Christoffersen et al 1979;Mile et al, 1982;He et al, 1994), or by observing growth in situ using microscopic techniques (e.g. Liu and Nancollas 1973;Tadros and Mayes 1979).…”

“…Gypsum crystal growth occurring from seeded or unseeded solutions at relatively low saturation and ionic strength is best described by a reaction of second order, i.e. n = 2 (see, among others, Liu and Nancollas, 1970;Smith and Sweett 1971;Liu and Nancollas, 1973;Brandse et al 1977;Christoffersen et al, 1982;Mile et al, 1982;Witkamp et al, 1990;He et al, 1994;De Meer et al, 2000;Reznik et al, 2009). Only Barcelona and Atwood (1978), who performed growth experiments using natural seawater, found a third-order rate dependence on supersaturation, whereas Packter (1974) reported growth to be a fourth-order process.…”

Inorganic Scaling in Membrane Desalination: Models, Mechanisms, and Characterization Methods

“…The concentration of specific scale-forming ions (e.g., calcium) can be measured using an ion selective electrode (ISE). ISE converts the activity of a specific ion dissolved in a solution into an electrical potential on the basis of either ion exchange or ion transport mechanisms. , Using the ISE, direct and quantitative evaluation of the scale formation is available. For example, the amount of calcium precipitation can be determined by comparing the concentrations of Ca 2+ in a solution before and after scale formation …”