JPH02277594A - Treatment of cement-containing water - Google Patents

Abstract

PURPOSE:To remove the hexavalent chromium in the cement-contg. water without adjusting the pH of the water by this simple, economical and easily controllable method by treating the water with a water-soluble barium compd. or a water-soluble strontium compd. CONSTITUTION:Strongly alkaline cement-contg. water contg. hexavalent chromium and the soln. of a water-soluble barium compd. and/or a watersoluble strontium compd. are introduced into a mixing reaction vessel 7 without adjusting the pH, agitated, and subjected to a reaction. The hexavalent chromium in the form of chromate ion is converted to barium chromate or strontium chromate as the crystal of a difficultly soluble compd. in the vessel 7, and the water to be supplied to the reaction vessel also contains various solid materials on the jobsite. Accordingly, the water is introduced into a settling tank 3 along with the solid materials, and allowed to stand to remove the solid materials. The removal of hexavalent chromium is completed at this stage. By this method, the water is effectively treated even on the highly alkaline side without adjusting the pH.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for treating cement-containing water.

[Conventional technology]

Hexavalent chromium has a strong oxidizing power, and if it leaks into the environment, it is f!
Various laws and regulations are in place to prevent pollution of 7 environments as hazardous substances. It's under control.

Cement contains a small amount of hexavalent chromium, which is mixed in during the manufacturing process. When water is mixed with cement and it hardens (concrete), hexavalent chromium is fixed inside the cement and cannot be leached out, but if it comes into contact with water before hardening, it dissolves in the water and moves with the water. . Therefore, it may be necessary to remove hexavalent chromium from the water if it is to be discharged into a public place.

The conventional method for removing it is to take advantage of the fact that hexavalent chromium exhibits strong oxidizing power on the acidic side of pH 3 or less and is highly reactive.
Add acid to cement fart water containing hexavalent chromium to reduce pi to 7 AMIL, then add a solution containing ferrous ions to reduce hexavalent chromium to trivalent chromium ions, and further reduce the trivalent chromium ions. Taking advantage of the fact that valent chromium ions become hydroxide precipitates at around pl (7), alkali is added to the solution containing trivalent chromium ions, the pH is neutralized to 6 to 8, and the reaction is aged. , trivalent chromium is precipitated and removed as chromium hydroxide.

[Problem that the invention seeks to solve]

Cement-containing water generally exhibits strong alkalinity due to the high alkalinity of cement. Therefore, in the conventional method, the pH
A large amount of acid is required for the acidification in the third step, and a large amount of alkali is also used to neutralize the pH 3 liquid to a pH of 6 to 8 in order to discharge it to the public and to generate chromium hydroxide. In need of.

Furthermore, the redox reaction of ferrous ions to hexavalent chromium ions is slow, requires a long reaction time, and requires a large southern tank, and the precipitate of chromium hydroxide is glue-like. Therefore, the sedimentation rate is low, requiring a long time and a large sediment removal tank to remove the sediment.

As described above, the conventional hexavalent chromium removal method involves a complicated process, which is economically burdensome, and there are many cases where the hexavalent chromium is discharged untreated.

The object of the present invention is to provide a simple, economically advantageous and easily managed method for removing hexavalent chromium contained therein from cement-containing water without adjusting the pH. The aim is to contribute to pollution prevention.

(Method for solving point 1!1111) The present invention provides a method for adding a solution containing a water-soluble barium compound/or a water-soluble strontium compound to highly alkaline cement-containing water containing hexavalent chromium without any adjustment. This is a method in which either one of them is added and hexavalent chromium in the form of chromate ions is converted into a hardly soluble compound called barium chromate or strontium chromate.

Hexavalent chromium generally exists in the form of CrO4''° (chromate ion) and Cr,07''' (chromate ion), and between both ions there is 2Cr04'-+H''=
Cr2072-+H20 (11 flats relationship exists.

Equation (1) shows that the higher the pH of the wastewater containing hexavalent chromium, and the lower the concentration of hexavalent chromium (the more diluted it is), the more stable chromate ions exist according to Le Chatelier's law. It shows.

On the other hand, cement-containing water has sufficient conditions for the stability of this chromate ion.

It is highly alkaline and has a hexavalent chromium concentration of 1
It is very low, about 0 ppm or less, and is in a diluted state. Therefore, hexavalent chromium in water containing cement exists as chromate ions.

When trying to remove hexavalent chromium from cement-containing water in this way, since the liquid is alkaline, the bonding reaction with these metal ions does not require any pH adjustment, making it extremely difficult to remove hexavalent chromium. This is the most important point of the present invention.

The present inventors have conducted various studies on techniques that satisfy these conditions, and as a result, they have come up with the present invention.

That is, the present invention is a method for treating cement-containing water, which is characterized in that the cement-containing water is treated with a water-soluble barium compound or a water-soluble strontium compound.

Next, the present invention will be explained in more detail, but it is not limited to the following invention.

First, the water-soluble barium compound used in the present invention will be explained.

The reaction between barium ions and chromate ions is expressed as follows, and the reaction rate is extremely high.

Ba'+ CrO4''=BaCrOa+2) When this reaction is occurring in water, Ba'+ and CrO4'-
The following relationship holds true between the concentrations of (solubility M).

(Ba") ・(Cr 04"-)=1. sxI
Q-” (31 []; Concentration ■09/Rabbit (edited by the Japan Society for Analytical Chemistry; from the Analytical Chemistry Data Book) The water pollution control law wastewater standard for hexavalent chromium is 0.5 mg/Q, and the concentration of hexavalent chromium in water containing cement is X mg of valent chromium
/g, the amount of barium ions required to reduce the content to 0.5 mg/g or less can be determined from equation (2) and +31 as follows.

From formula (2), 1 is included in hexavalent chromium;
The amount of barium ions that react with g/4 is 2.64・Xm
It is g. (3) From formula 1, 0.5 mg of hexavalent chromium
The amount of barium ions that must be added in order to
By adding mf of 1mg or more to 11z, it is possible to reduce hexavalent chromium to 0.5mg/L or less9.
The second term is almost negligible, so it is proportional to the content 2,64
It will almost be determined by Xmg/g.

-a Since X is less than 10 mg/4 U, the amount of barium ions required for removal is 26
．． A dose of 4 mg or less is sufficient.

Barium ions are added as an aqueous solution, but barium chloride (Ba CL
t) is suitable. However, other barium sources may be used as long as they ionize barium in water, are economically viable, and pose no pollution problems.

26.4mg/4 barium ion is 39.9mg/9
of barium chloride (Ba CIJtl), and the amount of solution containing this amount of barium chloride is added.

In addition, for water-soluble strontium compounds, reaction (4) is established by replacing Ba''* in formula (2) with Sr'''', and the solubility product is expressed by formula (5).

Sr"+CrO4'-=Sr Cr 04
(4) (Sr”l −(Cr 04!-1=3.
6X10-% (5) As in the case of barium, the amounts of strontium ion and strontium chloride required to lower the concentration of 10 mg/4 chromium to 0.5 mg/R or less are found to be 17 mg each per 1 solution. , 31 mg, and the corresponding amount of strontium chloride (m) is added.

On the other hand, it is also possible to add barium chloride and strontium chloride in solid form, but it takes time for them to dissolve and diffuse in the liquid, and the reaction does not proceed during that time, requiring a large volume as a reaction tank. The advantage of high reaction rate in (2) and (4) cannot be utilized, and since the amount added is so small that it is difficult to measure and add it industrially with high precision, these solution conditions It is preferable to add it with

There is no major limit to the concentration of the solution, but the higher concentration is determined by the saturation solubility of each salt, approximately 163 g/inclusive for barium chloride and strontium chloride, 157
g/Aderu.

Next, water-soluble barium compounds used in the present invention include water-liquefied barium, perchlorated barium, barium oxide, barium bromide, barium nitrate, barium tetracyanoplatinate tetrahydrate, and water-soluble strontium compounds. Examples include strontium nitrate, strontium acetate, strontium perchlorate, and strontium bromide.

Next, the present invention will be explained by examples.

Example: Since the amount of wastewater generated at the site is often irregular at different times, it is advantageous to receive it in a storage tank 42 and then pour it out in a fixed amount from that tank for treatment, as the entire device can be compact, but the amount of wastewater It is not necessarily necessary if the amount is small.

The flowmeter 1 is installed because the water-soluble barium compound must be added in proportion to the flow rate.

Next, the cement-containing water is mixed with, for example, a barium chloride solution and introduced into a reaction tank 7 where it is stirred in order to react with a water-soluble barium compound. The metering pump 2 is proportional to the product of the flow rate of 1 and the concentration of hexavalent chromium, and the barium chloride mw! is added so that the optimum amount of barium ions for the reaction is added. This is to control the flow rate of Although barium chromate crystals are formed in this tank, the water fed to the reactor may also contain various other solids generated on site. Therefore, in order to separate them, the water is led to the skin acid separation tank 3 together with the solids and allowed to stand still.

Since the purpose of this tank is solid-liquid separation, it is possible to use various filtration devices and other separation devices in addition to the shell separation tank to achieve that purpose.

At this stage, the removal of hexavalent chromium is completed.

Clear water from which solids and barium chromate have been removed still has a pH of about 9 to 13, so neutralization with acid is required before it can be discharged, and the water is led to neutralization tank 9 where the pu
After it has been neutralized to 6 to 8, it should be released into public places. The filter acid used for this purpose is used to prevent the excess pyram ions or strontium ions from flowing into the reaction tank as much as possible when adding excess pyram ions or strontium ions to the reaction tank to ensure the reaction is complete. It is preferable to neutralize with sulfuric acid in order to form a completely harmless sulfuric acid compound that easily binds to sulfuric acid ions.

This neutralization (mulberry production) may be performed before the solid matter group operation.
If this happens, the chromic acid in barium chromate will change to dichromic acid according to equation (2) and dissolve, so pH control must be strict.

Since cement particles are also included in the solids, it must be taken into consideration that the amount of consumed acid increases due to the reaction between the added acid and these particles. Furthermore, a method may also be used in which a primary precipitation tank is provided in advance of the mixing reaction tank 7, and a water-soluble barium compound is added to the supernatant water.

Although the above is an example of a process for continuously treating water containing cement, it is also possible to treat it in batches when the amount of water is small.

In that case, it would be possible to combine the mixing, reaction tank 7, and sedimentation separation tank 8 into the same tank, and use it as the sedimentation separation tank after the reaction.

In addition, in the case of continuous treatment, since the reaction rate of equations (2) and (4) is very high, a line mixer can be used instead of the mixing and reaction tank 7.

The following Figure 2 shows an example of hexavalent chromium removal according to the present invention. The figure shows 1 cement water containing 1 omg/4 of hexavalent chromium with various pH values! 1, add 27 mQ# of barium chloride solution with a barium ion concentration of 1 g/e, and add barium ion 2
7 mg), after neutralization to pH 8, surface load 2
Solid matter was sedimented and separated under conditions of m'/hr, and the hexavalent chromium concentration of the supernatant water was measured and graphed.

This shows that if ρI() is maintained above 5 breaths, the concentration of 61 chromium in the supernatant water is well below the regulation f110.5mg/@.

〔effect〕

The present invention has the advantage that by treating cement-containing water with a water-soluble barium compound or a highly water-soluble strontium compound, it is possible to effectively treat cement-containing water even on the highly alkaline side without having to adjust the pH of the cement-containing water to 1m. .

[Brief explanation of drawings]

Figure 1 is a diagram showing the treatment flow for carrying out the method of the present invention, and Figure 2 shows the pH of the liquid after treatment of cement-containing water.
It is a diagram showing H and hexavalent chromium concentration.

Claims (1)

[Claims] A method for treating cement-containing water, comprising treating the cement-containing water with a water-soluble barium compound or a water-soluble strontium compound.