JPH04110100A - Production of pure water from seawater - Google Patents

Abstract

PURPOSE:To obtain extremely high-quality water from seawater by treating seawater with an acid such as sulfuric acid, then controlling the pH to a high value, drawing off the deposit and desalting the residual seawater. CONSTITUTION:Seawater is adjusted to low pH with a strong acid contg. sulfate ion and then to high pH with a strong alkali, the deposit generated is removed, and the seawater is distilled until its amt. is reduced to a specified value to produce distilled water. A several % dil. sulfuric acid is used as the strong acid contg. sulfate ion and used to control the seawater to low pH. Sodium hydroxide, potassium hydroxide, calcium oxide or their mixture are used as the strong alkali. The residual seawater is conventionally distilled in a well- known desalting plant or distilled under reduced pressure. The seawater is concentrated to 10-20%, the seawater concentrated to <=20% is cooled to deposit salt, and the salt and solid residue are utilized for respective purposes.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing pure water from seawater by distillation, and particularly relates to a method for producing relatively highly purified pure water from seawater.

[Prior art and problems to be solved by the invention] Methods for producing pure water from salt water such as seawater include distillation, coagulation, reverse osmosis using a semipermeable membrane, and ion exchange membrane. There is an electrodialysis method, but the coagulation method has a higher plant cost than the distillation method, and the electrodialysis method costs 5,000 yen.
It is not suitable for salt water containing more than ppm of salt, and especially for seawater containing about 35,000 ppm of salt, distillation and reverse osmosis are the main methods of desalination. In particular, the distillation method has a larger production capacity than the reverse osmosis method, and is popular because it does not require expensive equipment such as high-pressure pumps and reverse osmosis modules, and it makes effective use of the heat of steam. Various plants have been developed that employ multi-stage distillation, flash distillation, vapor compression methods, etc.

When seawater is supplied to the various plants mentioned above after simple pretreatment and desalinated by distillation, if the saltwater is concentrated beyond a certain point, chlorine gas will be mixed into the steam, and the resulting freshwater may meet the standards for some uses. may not be suitable. For this reason, the salt water that has been concentrated to some extent is discarded, making it impossible to efficiently obtain pure water and making it impossible to utilize seawater as a whole.

Another drawback is that salt precipitates called scales form on the seawater surface during the process of concentrating saltwater, which has an interfering effect on heat transport and productivity.

Methods for preventing scale include adding sulfuric acid to seawater and then using a degassing device to remove CO, ``-, etc., which cause scale, and adding alkali metal salts after degassing to precipitate and remove calcium compounds. has been proposed (Japanese Unexamined Patent Publication No. 102492/1983).

However, all of these methods are aimed at preventing scale, and require a degassing device, making the device complicated and large, and there are limits to the concentration of salt water.

It is an object of the present invention to provide a method for producing highly pure water from seawater using an extremely simple device with less scale generation. A further object of the present invention is to provide a method for producing pure water from seawater that enables effective use of both salt and water contained in seawater.

[Means for Solving the Problems] In order to achieve the above object, the present inventor has conducted intensive research on the use of seawater, and has found that either saltwater obtained in the process of separating salt from seawater or pure water obtained by distillation. It was discovered that the quality is extremely good when used in manufacturing, leading to the present invention. That is, the method for producing pure water from seawater of the present invention is as follows:
After adjusting seawater to a low pH with a strong acid containing sulfate ions, a strong alkaline agent is added to increase the pH, the precipitate that is generated at this time is removed, and the remaining seawater is distilled to a predetermined amount to produce pure water. It is something to do.

As the strong acid containing sulfate ions used to lower the pH of seawater, several percent dilute sulfuric acid can be used, or a 3 to 5 percent concentrated acid in an aqueous solution containing activated calcium phosphate can be used. An aqueous solution (hereinafter abbreviated as P-8 acid) obtained by adding sulfuric acid to remove the precipitate can also be used. This P-8 acid exhibits strong acidity with a pH of about 0.2, and unlike deleterious substances such as sulfuric acid, it does not cause any abnormality when it comes in contact with the skin, and can be used as a highly safe acid that is easy to handle. Dilute sulfuric acid or P-
The seawater can be adjusted to a low pH of 2 or less by adding several percent of the 8-acid to seawater and leaving it for 2 to 3 hours. At this time, almost no precipitates are formed, but some precipitates are removed together with the original suspended matter in the seawater by means such as filtration.

Strong alkaline agents precipitate alkaline earth metals such as calcium and magnesium, whose solubility decreases at high pI (lower pI), and salts such as sulfates of other metals.

Such strong alkaline agents include sodium hydroxide,
Potassium hydroxide, calcium oxide, or a mixture thereof is used. The amount of strong alkaline agent is sufficient as long as it is at least the amount that can achieve the above purpose, and usually sodium hydroxide (solid) is 3% based on seawater, and an aqueous solution of calcium oxide plus sodium hydroxide is used. Add about 5% and leave for 10 hours or more. As a result, the seawater becomes basic with a pH of 13 or more, and a precipitate is formed.

This precipitate contains various metal elements and is useful in itself, and is used for other purposes after solid-liquid separation. The remaining seawater contains chlorine, calcium,
Magnesium etc. have significantly decreased compared to raw seawater, and p
When distilling salt water with H13 or higher, high-quality pure water can be obtained without evaporation of chlorine even when concentrated to about 10%.

Table 1 (Unit: mg/U) Is it possible to distill this remaining seawater by a conventional method in a known desalination plant? 47492, Japanese Patent Application No. 1-40092), it is preferable to carry out this step, thereby making it possible to recover and utilize the entire seawater resource. Distillation may be performed under reduced pressure. Additionally, seawater can be concentrated by 10 to 20%. In addition, the seawater after concentrating to less than 20% can be cooled to precipitate salts, or the salts and solids left behind can be used for their respective purposes, and their uses have already been described. The present applicant has disclosed this method (Japanese Patent Application Laid-open No. 47492/1982, Japanese Patent Application No. 40092/1999).

[Example] 1. Preparation of P-3 acid 50 g of powder mainly composed of calcium phosphate made from calcined animal bones was dissolved in fresh water 19 to obtain an aqueous solution (1) with a pH of 13 or higher. 5% concentrated sulfuric acid was added to the aqueous solution (1) to produce P-8 acid with a pH of 0.2.

2. Treatment of seawater Add the above p-s acid 1ON to seawater 5009,
After standing for a period of time, impurities were removed by filtration. As a result, the pH of the seawater became 1.6. Next, 3% NaOH was added to the seawater 5009 whose pH had been lowered, and the mixture was left for 10 hours.

At this time, the seawater after removing the generated precipitate had a pH of 13.

The remaining 10 U of seawater was distilled to a concentration of 19 to obtain 8.7 U of water. The results of the water quality test are shown in Table 2. In addition,
Table 2 also shows, as a comparative example, the results obtained when seawater was distilled to 40% of its original volume.

Table 2 As is clear from Table 2, the water obtained by the method of the present invention has a content of chlorine ions and evaporation residue that meet the water quality standard values based on the Water Supply Law (200 μg/l, 500 mg/l, and 500 mg/l, respectively).
D) The water was of good quality and was much lower than that in D). In particular, pure water with less evaporation residue was obtained compared to when seawater was directly distilled.

The water obtained in this way can be used as drinking water, washing water, boiler water, etc. In particular, it can be used as calcium ion water (Patent No. 1507714) by dissolving activated calcium phosphate, which is a product of high-temperature calcined bones. I can do something.

[Effect of the invention]

Claims (1)

[Claims] 1. After adjusting seawater to a low pH with a strong acid containing sulfate ions, adding a strong alkaline agent to increase the pH, removing the precipitate generated at that time, and distilling the remaining seawater. A method for producing pure water from seawater, characterized by producing pure water. 2. The method for producing pure water from seawater according to item 1, characterized in that the strong acid used is a strong acid obtained by adding sulfuric acid to an aqueous solution in which activated calcium phosphate is dissolved and removing a precipitate.