Abstract
In his previous paper, the author reported that sodium hexameta-phosphate was an effective inhibiter on the thermal decomposition of carbonates in the brine produced by the ion exchange membrane method.
In this paper, the author made a study on prevention of the scale deposition of calcium sulfate hemihydrate and sodium penta-calcium sulfate (Na2SO4·5CaSO4··3H20) during salt manufacture by the ion exchange membrane method. For scale prevention were used various additives such as ortho- (SOP), pyro- (SPP), tripoly- (STP), hexameta- (SHP), and ultra- (SUP) phosphates. These phosphates were added to the decarbonated brine together with the suspension of calcium sulfate dihydrate. The brine vessel was immersed in a shaking bath at 95°C.
The results of this study were as follows:
1. When the inhibition effect was viewed from the P205 contents of condensed sodium phosphates mentioned above, the SHP and SUP having larger contents than the others indicated a higher inhibition effect. The inhibition value of the SHP was 56 (CaSO4 ppm/added phosphate ppm) after one hour.
2. The results of X-ray diffractometry, DTA, TGA, and chemical analysis of the deposited crystals showed that when the SHP was added more than 50 ppm, CaSO4·2H20 was transformed into II·CaSO4 and that when the other condensed sodium phosphates of 50 ppm were added, CaSO4·2H20 was transformed into CaSO4·1/2 H2O or Na2SO4·5CaSO4·3H20.
3. When one hour passed after the addition, the concentration of the SHP and the SUP showed a decrease of less than 20% against the initial concentration as the result of their decomposition to insoluble phosphate. The other condensed sodium phosphates, on the other hand, showed a decrease of 75-95%.
4. The above results made it obvious that the SHP had a large inhibition effect and transformationeffect from CaSO4·2H20 to II·CaSO4, and that the SHP was the most effective of all the condensed sodium phosphates.
