JPS6265787A - Treatment of waste water of concrete - Google Patents

Abstract

PURPOSE:To easily and thoroughly remove hexad Cr by adding and mixing to and with ferrous sulfate to alkaline waste water of concrete in which hexad Cr and slaked lime are dissolved to settle and separate the hexad Cr and neutralizing the separated liquid. CONSTITUTION:The ferrous sulfate is added and mixed to and with the alkaline waste water of concrete in which the hexad Cr and slaked lime are dissolved. The hexad Cr is settled as insoluble Cr hydroxide and is subjected to a solid- liquid sepn. The separated water is neutralized. More specifically, the treating operation is made simple without the need for making many stages of pH adjustments as in the conventional practice; in addition, chemicals are less used and the treatment cost is considerably reduced. The removal of salts by the use of carbon dioxide in the above-mentioned neutralization treatment is possible. This method has, therefore, high applicability as a treatment of waste water with the lower cost burden in a small-scale factory. The method is satisfactorily applicable as well in the case in which a low salt concn. is required as effluent.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a concrete wastewater treatment method for removing hexavalent chromium from wastewater generated by cleaning equipment, etc. in a concrete handling department such as a concrete manufacturing factory.

(Prior art) Concrete wastewater generally contains slaked lime (Ca (OH) 2 ) eluted from concrete in an almost saturated state, so it has a high alkalinity exceeding Pl + 12, and chromium accompanying concrete raw materials is removed during concrete production. Hexavalent chromium (Cr), which is easily soluble when oxidized during kiln roasting,
”). Therefore, in order to discharge this wastewater, the discharge standards (total Cr2PPn↓, Cr”0.5PPm
↓, pH 5.8 to 8.6>, it is necessary to remove hexavalent chromium and neutralize the liquid.

Conventionally, the typical method used to treat such concrete wastewater is to first add hydrochloric acid (11cj) to the wastewater to make it acidic beyond the neutralization point and below pH 3, and then add sodium sulfite (NaH3O3) or Ferrous sulfate (FeSO4
・Add and mix a single element such as 7H20) to reduce hexavalent chromium to trivalent chromium (Cr3÷), then add thorium hydroxide (NaOH) or slaked lime to adjust the pH to 9-IO. Trivalent chromium is converted into insoluble chromium hydroxide (Cr
(Oll) 3) is precipitated, this is separated into solid and liquid, removed as sludge, and the separated liquid is further neutralized to adjust the pH.
This is a method of discharging the water as neutral with a pH of 5.8 to 8.6.

(Problems to be Solved by the Invention) However, in the above-mentioned conventional treatment method, wastewater with a pH of 12 or higher is once reduced to a pH of 3 or lower, and then reduced again to P)
Return the pH to 9-10 and further increase the pH to 5.8-8.6 after solid-liquid separation.
Since the multi-step pH adjustment is performed, the processing and drying process becomes extremely complicated, and the processing cost becomes extremely high due to the large amount of chemicals used for pH adjustment. In addition, in the above treatment method, F
Since acid is used, calcium components and P in Genko water are
The sodium or calcium component used for H adjustment is contained in the final effluent as calcium chloride (CaCe2) or 1-lium chloride (NaCρ), and it is impossible to remove this salt, so a low salt concentration is required. There is a drawback that the treatment method cannot be used when

This invention was made to solve the above-mentioned conventional problems, and it is possible to completely remove hexavalent chromium very easily and at low cost without performing multi-step pH adjustment, and it is also possible to remove salt. The purpose is to provide a method for treating concrete wastewater.

(Means for Solving the Problems) In order to achieve the above object, the inventors have
As a result of repeated research, it has been found that in the treatment of concrete wastewater, the lower the pH, the greater the effect of the reducing agent, and for this reason, it is common sense to treat wastewater to a level of P13 or less, as mentioned above. However, we have discovered that even if ferrous sulfate is added directly to the wastewater, hexavalent chromium can be completely removed as water-soluble chromium hydroxide.

That is, the method for treating concrete wastewater according to the present invention involves adding and mixing ferrous sulfate to alkaline concrete wastewater containing dissolved hexavalent chromium and slaked lime, precipitating hexavalent chromium as insoluble chromium hydroxide, and performing solid-liquid separation. It is characterized by neutralizing the separated liquid.

(Structure and operation of the invention) As mentioned above, the concrete wastewater to be treated is almost saturated with slaked lime, and generally the P-alkalinity is Ca.
It exhibits alkalinity with a pH of 12 or higher at 2000 PPm or more in terms of C0+, and the chromium attached to the raw material contains a maximum of about 5 PPm as total chromium, most of which is oxidized during kiln roasting during concrete production, making it easily soluble. Dissolved as hexavalent chromium.

When ferrous sulfate is added to the above wastewater, Ca (Oll) 2 +Cr6+ FeSO4・71120 Ca (Off) 2 +Ca5O< +Cr (OH
) 3↓+Fe (011) As shown in 3↓, hexavalent chromium is reduced to trivalent chromium and precipitates as insoluble chromium hydroxide, and the iron component also precipitates as iron hydroxide. Therefore, by solid-liquid separation, the chromium component is removed as sludge, and the slaked lime (': a
(0:l) 2 ) and calcium sulfate (CaSO4) are dissolved, but the amount of calcium sulfate is a trace amount corresponding to the ferrous sulfate used.

Here, the amount of ferrous sulfate to be used is the theoretically required amount corresponding to hexavalent chromium plus the theoretically necessary amount corresponding to dissolved oxygen in wastewater, or an amount slightly lower than this is sufficient. Yes, it is possible to almost completely remove hexavalent chromium by precipitation as chromium hydroxide, even if it is not used in excess. In other words, it is generally known that Fe2+ is rapidly oxidized by dissolved oxygen to Fea+ under highly alkaline conditions, and this is considered to be a competitive reaction that favors Fe2+ as a reducing agent, and the dissolved oxygen of Fe2'' is Although a corresponding amount is added in excess, it has been found that the reduction reaction of Cr''-=CrJ+ proceeds somewhat preferentially, as is clear from the results of Examples described later.

The theoretical required amount of ferrous sulfate above is: C"6÷+3Fe"Cr"+3Fe-(
1) 'AO2+2Fe''+5H20- 2Fe (Oll) 3 + 4H-(II) Above (1
) From formula (II), 3.22PPm (FeSO4-
15.97 PPm as 71120), iOxygen I
7 as Fe2+ for PPm. OPrm (F
eSO47) 120 as 34.69 PP+w)
It is.

Incidentally, during the solid-liquid separation described above, a sedimentation aid or a flocculant may be used to facilitate separation of the precipitate.

As the separated liquid from which sludge has been removed as described above is highly alkaline, it is neutralized to a discharge standard pH of 5.8 to 8.
Neutralize to conform to 6.

Although various means can be employed for this neutralization treatment, the most suitable method is a neutralization method by blowing carbon dioxide gas. That is, when carbon dioxide gas is blown into the separated liquid, slaked lime precipitates as calcium carbonate as follows: CaCO3↓+H20+CaSO4. By separating this precipitate by filtration or the like, it becomes possible to remove salt from the effluent water. However, excess carbon dioxide gas produces calcium bicarbonate (Ca (HCO3) 2), which is remelted, so the neutralization point at which the total amount of dissolved solids is the minimum is PH.
It is around 9. In addition, even in the above-mentioned precipitation separation of calcium carbonate, a flocculant or a flocculation aid may be used to facilitate separation.

(Example) A treatment test using simulated wastewater will be shown below as an example of the present invention.

Dissolve slaked lime (Ca (Oll) 2 ) in tap water until it reaches a saturated state, then add sodium dichromate (Na 2 ).
A simulated wastewater was prepared by dissolving Cr207) as CrB+ to give 5 PPm.

This simulated wastewater:! 00va 1 to capacity 50 (1m It
of FeSO in a beaker and stirred with a jar tester.
A 2% by weight aqueous solution of 4.71120 was added in an amount shown in Table 1 below, stirring was continued for 5 minutes, a precipitation aid was added to complete precipitation, and solid-liquid separation was performed. When the separated liquid was analyzed, the residual chromium and removal rate were as shown in Table 1.

The theoretically required amount of FeSO4.7*20 is 5 (Cr") x 15.20, assuming dissolved oxygen to be 10PPa+.
97+lO(02)×34.69 425 PPm.

Table 1 In addition, when the simulated wastewater was reduced and solid-liquid separated in the same manner as above except that rapid stirring was performed using a magnetic stirrer instead of the relatively slow stirring using the jar tester, residual chromium and solid-liquid separation in the separated liquid were confirmed. The removal rate was as shown in Table 2. In addition, the theoretically required amount of FeSO47H20 is 5, since the amount of dissolved oxygen measured was 8.70 PPm.
(Cr”) X 15.97+8.7(02) X 3
4.69-381 PPIm.

Table 2 From the results in Tables 1 and 2 above, it can be seen that the amount of hexavalent chromium contained in concrete wastewater is the theoretically necessary amount corresponding to ferrous sulfate, hexavalent chromium, and dissolved oxygen even in the high pH range of raw wastewater. It can be seen that by directly adding an amount slightly less than this, it is almost completely removed.

Next, put liquid 1a after the solid-liquid separation into a beaker, blow carbon dioxide gas into it while stirring, take samples at each PH in Table 3, and add PAC (polyaluminum chloride) as a coagulant to Af2Q.
When 10 PPn+ and auxiliary agent 2 PPm were added and filtered, the filtrate was analyzed, and the values shown in Table 3 below were obtained.

P-alkali, m-alkali, TH,, CI2- in the table
1SiO2(7) Values are in PPm.

From Table 3-F, neutralization treatment with carbon dioxide gas is advantageous in that the total amount of dissolved solids is minimized at a pH of around 9 (as a filtrate, IJ 8.3).

(Effects Unique to the Invention) According to the treatment method of the present invention, hexavalent chromium can be almost completely removed by simply adding and mixing ferrous sulfate to concrete wastewater, followed by solid-liquid separation and neutralization. As a result, there is no need to perform multi-stage PHLm adjustment as in the past, making the processing operation extremely simple, reducing the amount of chemicals used and greatly reducing processing costs. Salt is possible. Therefore, the method of the present invention is highly applicable as a low-cost wastewater treatment method even in small-scale factories, and is fully applicable even when a low salt concentration is required in the effluent water.

Claims (1)

[Claims] The method is characterized by adding ferrous sulfate to alkaline concrete wastewater containing dissolved hexavalent chromium and slaked lime, precipitating hexavalent chromium as insoluble chromium hydroxide, separating solid and liquid, and neutralizing the separated liquid. How to treat concrete wastewater.