JPH04310285A - Treatment of undissolved matter generated in process for manufacturing aqueous solution of slaked lime - Google Patents

Abstract

PURPOSE:To subject the undissolved matter generated when an aqueous slaked lime solution is prepared to neutralization treatment. CONSTITUTION:The undissolved matter in a dissolving tank 5 is transferred to a reaction tank 11 through an undissolved matter withdrawing pipe 19 and carbon dioxide is sent in the reaction tank 11 from a carbon dioxide cylinder 15 through a gassifier 16 and a diffusing pipe 12. Slaked lime contained in the undissolved matter is reacted with carbon dioxide in the reaction tank 11 to be precipitated as water-insoluble calcium carbonate. At this time, when pH is adjusted to 9.5-11.0 by carbon dioxide, the amount of formed calcium carbonate becomes max. A separated liquid being a supernatant is further transferred to a neutralizing tank 13 to be reacted with carbon dioxide sent from the carbon dioxide cylinder 15 through the gassifier 16 and the diffusing pipe 12. Since a small amount of calcium carbonate contained in the separated liquid is dissolved as calcium bicarbonate and pH is suppressed to about 7, treated water can be discharged. Calcium carbonate formed in the reaction tank 11 is withdrawn to be reutilized.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a method for treating undissolved substances generated during the production process of slaked lime aqueous solution, and in particular,
This invention relates to a method for treating insoluble matter generated when producing a slaked lime aqueous solution used for adjustment.

0002

BACKGROUND OF THE INVENTION It is well known that the use of slaked lime to adjust the pH of tap water has the effect of suppressing red water and is also beneficial for health. However, injecting slaked lime powder as it is not only causes dust to scatter when it is added, worsening the working environment, but it is also difficult to adjust the amount added, and insoluble matter in the slaked lime may clog the filter or water supply. This is not preferable because it increases turbidity. From this point of view,
A saturated solution of slaked lime is used to adjust the pH and alkalinity of tap water.

[0003] Conventionally, an upflow blanket type continuous slaked lime dissolving device called a saturator (dissolving tank) has been used in Europe for the above purpose. In addition, in Japan, it can be found in Japanese Patent Publication No. 62-23634, Special Publication No. 63-2890, Special Publication No. 63-2891, Special Publication No. 35574-1980, Special Publication No. 63-58762, and Special Publication No. 63-58763. Various types of dissolution tanks have been developed.

0004

[Problems to be Solved by the Invention] However, saturators do not remove impurities contained in slaked lime, such as magnesia, silicic acid, iron oxide, and other insoluble substances such as calcium carbonate produced when slaked lime reacts with carbon dioxide gas. A certain amount of undissolved material must always be removed for removal. This undissolved material contains a large amount of undissolved slaked lime, and the pH
Since the value is as high as 12 to 13, it is problematic to discharge it directly into rivers under the Water Pollution Control Act. Therefore, the undissolved substances are neutralized with acid and discharged. However, in this case, if cheap sulfuric acid is used, the reaction time will be longer than 30 minutes due to the low solubility of calcium sulfate produced in the reaction, and the pH will increase further.
Because of its poor stability, it takes a lot of effort and time to accurately adjust the pH. Neutralization with hydrochloric acid solves this problem, but since water treatment plants are also public places, there are risks associated with transporting, handling, and storing hydrochloric acid. Furthermore, there is a risk of excessive injection of acid, which causes problems such as the need for alkali injection equipment.

[0005] Furthermore, in Japanese Patent Publication No. 62-23634, the undissolved material taken out is further dissolved with water and the pH is adjusted to 1.
A slaked lime aqueous solution with a concentration of 0 or less is used together with a slaked lime saturated solution to adjust the pH of tap water. Although this method alleviates the problem of neutralization of undissolved substances, it causes water quality problems because trace amounts of impurities are mixed into tap water. Further, a large amount of water is required to dissolve undissolved substances, and the pH value of the slaked lime aqueous solution is not constant, so it is difficult to optimally control the injection amount of the slaked lime aqueous solution.

[0006] Furthermore, Special Publication No. 63-2890, Special Publication No. 63-2890,
No. 3-2891, Special Publication No. 63-35574, Special Publication No. 1983
No. 3-58762 and Japanese Patent Publication No. 63-58763 disclose that undissolved matter is taken out from the dissolving tank, but there is no description of how to dispose of the undissolved matter, so problems like the saturator mentioned above may not occur. arise.

Therefore, although a saturated aqueous solution of slaked lime has long been considered desirable for adjusting the pH of tap water, there is no effective solution to the above problem, and the use of slaked lime in tap water is limited, leading to the early development of treatment methods. was desired.

[0008]

[Means for solving the problem] In order to meet the above request,
As a result of various studies on methods for neutralizing the undissolved matter taken out from the dissolution tank, the present inventors found that if the undissolved matter was extracted from the dissolution tank for producing a slaked lime aqueous solution and the undissolved matter was reacted with carbon dioxide gas, the undissolved matter could be neutralized. Since it becomes dissolved calcium carbonate and settles, it prevents the pH from increasing after neutralization, and provides a safe and steady pH.
It was discovered that H adjustment can be performed. In particular, when neutralizing with carbon dioxide gas, the undissolved matter is removed at pH 9.5 to 11 in the first step.
After neutralizing to 0 and precipitating calcium carbonate, it is desirable to further neutralize the separated liquid with carbon dioxide gas.

[0009]

[Function] Generally, when a slaked lime solution containing suspended matter at a saturation concentration or higher is neutralized to pH 7 and left to stand, a phenomenon is observed in which the pH increases due to redissolution of the suspended matter. For example, when undissolved substances taken out from a dissolution tank are neutralized with sulfuric acid, not only the time required for neutralization cannot be determined, but also the pH
Even if acid addition is stopped at 7, an increase in pH is observed for 2 to 3 hours.

On the other hand, when carbon dioxide gas is used for such neutralization, the remaining slaked lime is easily converted to calcium carbonate once the pH is neutralized to 9.5 to 11.0 (preferably around pH 10.0). It precipitates and can be recovered together with the calcium carbonate contained in the undissolved matter. Therefore, after collecting the precipitate, if the separated liquid is neutralized again with carbon dioxide gas, the pH value can be easily adjusted to 5.8 to 8.6.
Moreover, since the slight remaining calcium carbonate becomes bicarbonate and dissolves, it can meet the standards set by the Water Pollution Control Law. Even when an excess of carbon dioxide gas is supplied, the pH does not fall below 6.3, and there is no need to inject alkali, unlike when an excess of acid such as hydrochloric acid is used. Therefore, by neutralizing the undissolved matter taken out from the dissolution tank with carbon dioxide gas, safe and steady pH adjustment can be performed. Further, by implementing the present invention, it becomes possible to continuously inject a slaked lime saturated solution into tap water. On the other hand, the recovered calcium carbonate can be directly used in the rubber, plastic, fertilizer industries, etc., and can also be used as a raw material for quicklime and slaked lime, so it can be used in a wide range of applications and does not waste resources.

[0011]

EXAMPLE Next, the processing method according to the present invention will be explained in detail with reference to FIG. In FIG. 1, reference numeral 1 denotes a slaked lime preliminary dissolution tank, in which slaked lime is dissolved in water and made into a slurry. Its concentration is generally 5-15%
is adjusted to a concentration value in the range of . The slaked lime slurry adjusted to the above concentration is supplied by a slaked lime slurry transfer pump 3 to a slaked lime slurry supply pipe 6 vertically arranged in a dissolution tank (saturator) 5. The dissolution tank 5 preferably includes a stirring tube 7 communicating with the slurry supply pipe 6, a stirrer 9, and a draft tube 10 as shown in the figure. The slaked lime slurry supplied to the dissolution tank 5 rises in the stirring tube 7 while being stirred and mixed with water that is supplied at a constant rate from the water supply pipe 18 shown in the figure, adjusted by the flow meter 4, and then rises in the draft tube 10. descend. Undissolved slaked lime and insoluble matter are
A blanket zone is formed at the bottom of the dissolution tank 5, and due to the suction force of the agitator 9, the slaked lime slurry is continuously supplied through the slaked lime slurry supply pipe 6, and the water is also continuously supplied through the supply pipe 18, and the mixture is stirred. The fluid circulates through the tube 7 and draft tube 10. In this way, by supplying water while fluidly circulating the slaked lime slurry, it is possible to suppress the outflow of slaked lime particles and to continuously obtain a slaked lime aqueous solution with a concentration close to saturation. The concentration of the slaked lime aqueous solution at this time varies depending on the temperature, but is approximately 0.1% to 0.18%. It varies depending on the supply rate of slaked lime slurry. The slaked lime aqueous solution overflows a triangular notch-shaped overflow weir provided at the top of the dissolution tank 5, is collected in a trough 8, and is guided to an injection point 17 from there.

On the other hand, the undissolved slaked lime and calcium carbonate forming the blanket zone at the bottom of the dissolution tank 5 are removed by an undissolved matter drawing pipe 19 provided at the bottom of the dissolution tank 5.
is introduced into the reaction tank 11 by. It is desirable that the undissolved matter be drawn out at regular intervals, preferably by setting a time using a timer. The reaction tank 11 is connected from a carbon dioxide gas cylinder 15 to a vaporizer 16 and a gas supply pipe or aeration pipe 12.
Carbon dioxide gas is supplied via. The undissolved slaked lime introduced into the reaction tank 11 reacts with the carbon dioxide gas dispersed in the liquid to turn into calcium carbonate and precipitate. The pH at this time was 9.
By adjusting the pH to 5 to 11.0, preferably 10.0, the amount of calcium carbonate produced is maximized. Since calcium carbonate and undissolved substances generated in the reaction tank 11 are separated from the solution and precipitated, the supernatant separated liquid is further led to the neutralization tank 13. Carbon dioxide gas is similarly supplied to the neutralization tank 13 from a carbon dioxide gas cylinder 15 via a gas supply pipe or an aeration pipe 14 . The separated liquid in the neutralization tank 13 is neutralized to pH around 7 by carbon dioxide gas dispersed in the liquid. The supernatant liquid in the reaction tank 11 contains a small amount of calcium carbonate, but when it reacts with carbon dioxide gas in the neutralization tank 13, it becomes bicarbonate and is completely dissolved. Furthermore, even if excess carbon dioxide gas is supplied to the neutralization tank 13, the pH of the effluent water 21 will not be lower than approximately 6.3, and an alkaline agent must be added to adjust the pH value of the effluent water. do not need. Reaction tank 11
Calcium carbonate and a small amount of undissolved impurities produced in the above are extracted by a drawing tube 20 and can be reused as auxiliary materials such as rubber, fertilizers, or raw materials for producing quicklime.

[0013]

[Effects of the Invention] As described above, by neutralizing the undissolved matter extracted from the dissolution tank with carbon dioxide gas, it is possible to adjust the pH more safely and steadily than when using an acid. Further, no alkali injection equipment is required, and the product taken out after neutralization has a wider range of uses and higher utility value than when treated with acid. Furthermore, if the present invention is used in an apparatus for producing a slaked lime aqueous solution for adjusting the pH of tap water, it is possible to obtain a slaked lime aqueous solution with few impurities, and in addition, insoluble matter can be continuously treated, which is different from conventional examples. Thus, the slaked lime aqueous solution can be continuously injected into tap water without installing multiple devices.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the apparatus used to implement the invention. 5 Dissolution tank 11 Reaction tank 12 Aeration pipe 13 Neutralization tank 14 Aeration pipe 15 Carbon dioxide cylinder 16 Vaporizer 19 Undissolved matter withdrawal pipe 21 Discharge water

Claims (2)

[Claims] Claim 1: Treatment of undissolved matter generated in the process of producing a slaked lime aqueous solution, characterized by drawing out undissolved matter from a dissolution tank for producing the slaked lime aqueous solution, and neutralizing the drawn out undissolved matter with carbon dioxide gas. Method. 2. In the neutralization with carbon dioxide gas, in the first step, undissolved substances are neutralized with carbon dioxide gas to a pH of 9.5 to 11.0, calcium carbonate is produced and precipitated, and then the separated liquid is Claim 1 characterized in that the is neutralized with carbon dioxide gas.
A method for treating undissolved substances generated during the manufacturing process of the slaked lime aqueous solution described above.