Study of the CaSO4 deposits in the presence of scale inhibitors

The water state in the CaSO4 solutions where concentrations ranged from 0.68–7.08 g/l was studied by the differential-scanning calorimetry. In the low-temperature range, 248.8 K, a peak was detected at the beginning of a CaSO4 concentration of 1.7 g/l. Its appearance could be attributed to an initial moment of the crystallization, that is, a formation of the CaSO4 nuclei. The resulting increase in the CaSO4 concentration (up to 4.7 g/l) promotes a growth of the low-temperature peak. The effect of the inhibitors on the water state in the CaSO4-containing solutions was examined to clarify the role played by these potential scalants. Solutes of different chemical natures were used as inhibitors. As the thermograms demonstrated, an addition of the inhibitors into the system tested did not influence the formation of the CaSO4 nuclei but it precluded the resulting growth of crystals. An increase in concentration of the inhibitors results in a decrease of lowtemperature peak. This testifies to the slowing down of the formation process of the CaSO4 nuclei. Better results were established by the presence of polyacrylamide and carboxymethyl cellulose while those with the presence of PE-1 polyelectrolyte were somewhat worse. The use of polyacrylamide as an inhibitor has been shown to increase the retentivity of the OPAM-KN membrane up to 0.90.

An appearance of the CaSO4 nuclei at surplus of SO42– ion was established to occur at a CaSO4 concentration higher than 0.68 g/l, which is a much lower saturation concentration of CaSO4 (2.04 g/l). Table 1 Variation in the water state in the OPAM-KN membrane vs. time in the CaCl2+Na2SO4 system at the concentration of calcium and sulfate ions is 1.3 and 3.4 g/l, respectively, containing also PAA at its concentration 5 ppm Initial membrane in Without inhibitor distilled water τ=1h τ=4h Moisture content, % Free water, g/g Bound water, g/g Retention coefficient Volume flux, µm/s 0.85 0.16 — 7.8 0.82 0.08 0.85 5.8 0.87 0.20 0.48 7.7 Membrane +PAA In the presence of PAA at a pressure of 0.6 τ = 1 h τ=4h MPa 0.84 0.90 0.75 0.20 0.15 0.16 — 0.88 0.90 5.9 4.2 4.1 At lower concentrations of the inhibitor under study, a growth of crystals appear in the CaSO4 solutions. The chemical nature of an inhibitor plays an essential role in the CaSO4 scaling prevention processes on the membranes. When using the membranes characterized by a small fixed-surface charge (pressure-driven membrane processes), it is expedient to apply the polyelectrolytes having the negative charge (PAA and CMC). As for electrodialysis where the membranes having a fixed-surface charge are used, the anion-exchange polyelectrolytes are better (PE-1). An addition of the PAA inhibitor into the desalination process accomplished by the OPAM-KN membrane results in an increase in its retentivity from 0.34 to 0.90.