The authors investigated effects of coexisting ions on the deposition of calcium carbonate from the brine procuced by ion exchange membrane method.
1. Under the air-tight condition, there was no deposition of calcium carbonate even though, the brine was given a large amount of such salts as NaC1, MgC1₂ and CaC1₂ and was held for 20 hours at room temperature. However, the deposition of calcium carbonate was observed when the alkalinity of the brine attained 5 meq/l by addition of sodium bicarbonate or when pH was increased to 7.6 with sodium hydroxide.
2. In case the brine was agitated under the open-air condition, the concetration of carbonate ion increased in line with the release of CO₂ gas, resulting in the deposition of calcium carbonate.
3. Magnesium and phosphate both indicated a good inhibiting effect on the deposition of calcium carbonate. The investigation which was made on their effects on crystallization of calcium carbonate by X-ray analysis revealed that phosphate ion converted the crystalline form of calcium carbonate from to calcite and calcite monohydrate, but magnesium ion showed no effect on crystallization of calcium carbonate. That magnesium ion produces an inhibiting effect on the deposition of calcium carbonate seemed to be attributable to the fact that the solubility of calcium carbonate increased with a rise in the concentration of magnesium ion, thereby marking the growth of nuclear difficult.

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In order to evaluate the merits of the electrodialytic concentration of the solution discharged from the multistage flash (MSF) process, the author conducted an electrodialysis of the solution that had been prepared to have about the same concentration as the discharged solution having two-fold concentration of saline water. The solution was electrodialyzed at high current density for 4000 hours in a large-scale apparatus having an effective membrane area of 72 dm². The results were as follows:
1. The operation was stable, and no water splitting was observed.
2. There was observed an increase in the concentration of the concentrate and in the output of NaCl resulting from the employment of the high current density. However, no remarkble rise was recognized in the consumption of electric energy.
3. When the permselectivitcyo efficienfto r bivalentc ationso f cation membrane (T_Na^Ca and T_Na^Mg) showed an increase, some agent was added to the feed solution to inhibit the increase in the permselectivity coefficient. As compared with the effect of the agent observed in the electrodialysis of saline water, the effect of the agent in the electrodialysis of the high cocentration solution discharged from the MSF process proved to be less. However, it was considered possible to raise the effect to some extent by increasing the amount of the added agent.
4. There were much less ionic products of calcium sulphate and calcium carbonate of the concentration solubility product or the limit of scale formation. Therefore, there was little risk of formation of calcium sulphate and calcium carbonate. As a matter of fact, there was no trouble of scale formation during the electrodialysis.
5. Results obtained from an electrodialysis of the solution discharged from a MSF plant which was made with a smaller apparatus, were nearly equal to those obtained from the electrodialytic concentration of the artificially prepared solution.
6. When the electrodialytic concetration of the discharged solution from the MSF process or hightemperature and high-concentration solution was carried out at high current density, it wa considered necessary to pay careful attentions to the pre-treatment of feed solution, the splitting of membrane, and the breakage of anode.

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In order to prevent deposition of calcium carbonate in concentrating sea water by ion-exchange membrane method, the authors studied effects by of addition of acid to feed sea-water or circulated brine. In this study, electrodialytical tests were carried out within the desalting range of 17-75% by adding hydrochloric acid so the feed sea-water or the circulated brine to adjust the pH value of the brine to be 4-5. Thus, the effects addition of acid on concentration characteristics and prevention of calcium-carbonate deposition were examined.
The results of these experiments revealed that there was little effect on concentration characteristics, but no deposition of calcium-carbonate scale was observed when the pH of brine was kept under 5.
Under ordinary electrdialytical condition, the cost of added acid was estimated at 10 yen per ton of sodium chloride when sulfuric acid was added to the circulated brine.

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A study was made on the designing of an apparatus for the MSF process in connection with the national research and development project on desalination.
A simple and similar model was employed for this study, and on the basis of this model, such parameters as the number of evaporating stages, the length of each stage, the velocity of brine flowing in tubes, and the ratio of brine concentration were optimized in full consideration of the production cost of water. A sensitivity analysis made in this study indicated quantitatively that major parameters to the production cost of water were the cost of heat-transferring tubes and the cost of steam, and that the most sensitive parameter to the performance ratio was the number of evaporating stages.
In addition, a more detailed model was used for this study, and a computer program was prepared to again stage-by-stage variables by marking a heat transferring area constant for each stage. A sensitivity analysis was also made on the number of stages, fouling factors, the temperature of exhausted brine, the temperature of sea water, and concentration ratios. As the result, the temperature of sea water was found the most sensitive.

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