JP2008043865A - Method for treating waste water containing phosphoric acid and zinc - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for treating waste water containing phosphoric acid and zinc, by which the production amount of sludge as industrial waste is reduced. <P>SOLUTION: The method for treating waste water containing phosphoric acid and zinc comprises; a first step of adding an alkaline agent 22 to the water 20 to be treated which contains phosphoric acid and zinc to convert zinc in the water 20 to be treated into zinc hydroxide; a second step of separating the zinc hydroxide from the water 24 to be treated which is obtained at the first step, as solid content 30; and a third step of making the water 32 to be treated which is obtained at the second step, pass through a seed crystal-packed phosphorus crystallization tank 18 to crystallize phosphorus. A precipitation separation treatment or a membrane separation treatment is preferably adopted at the second step. In the phosphorus crystallization tank 18 at the third step, the water 42 to be treated is made to pass through the phosphorus crystallization tank in an upward stream by using circulating water of the amount being 10-30 times as much as that of the water to be treated to form the fluidized bed 44 of a seed crystal. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a method for treating wastewater containing phosphoric acid and zinc, and in particular, wastewater containing phosphoric acid and zinc suitable for zinc phosphate bath wastewater discharged from a pre-coating treatment process for metal substrates. It relates to the processing method.

  When a metal base material is painted at a metal product manufacturing plant or the like, generally, after the surface of the base material is degreased as a pretreatment, a zinc phosphate treatment is performed to form a rust-preventive coating. In this zinc phosphate treatment, the base material is immersed in a bath filled with the zinc phosphate-containing liquid to form a zinc phosphate coating on the base material surface. Next, the base material pulled up from the bathtub is rinsed, and the zinc phosphate-containing liquid adhering to the base material surface is washed away (for example, see Patent Document 1).

  Therefore, in this type of factory, when the base material after the zinc phosphate treatment is rinsed, a large amount of washing waste water containing phosphoric acid and zinc at a high concentration is generated. Conventionally, coagulation precipitation treatment has been widely used as a method for purifying cleaning wastewater containing such phosphoric acid and zinc in high concentrations. However, in the coagulation sedimentation treatment, a large amount of sedimentation sludge containing phosphoric acid or zinc is generated, and the disposal of the treatment requires a great deal of cost.

On the other hand, a phosphorus crystallization method is known as a method for removing phosphorus from phosphorus-containing water (see, for example, Patent Document 2). Phosphorus crystallization method is a method of crystallizing phosphorus on the seed crystal surface as calcium phosphate crystals such as hydroxyapatite phosphate by adding calcium compound to phosphorus-containing water and passing it through a fixed bed or fluidized bed of seed crystal It is. According to this phosphorus crystallization method, the obtained calcium phosphate crystals can be effectively used as a fertilizer and contribute to resource recovery. Therefore, if the phosphorus crystallization method is applied to the waste water containing phosphoric acid and zinc as described above, waste water treatment that also serves as resource recovery can be realized.
JP 2003-160878 A JP 2001-334274 A

  However, in order to apply the phosphorus crystallization method to the waste water containing phosphoric acid and zinc, when the pH is adjusted to the alkali side suitable for the crystallization reaction, zinc in the water to be treated is precipitated as zinc hydroxide. It has been found that zinc hydroxide interferes with the crystallization reaction of phosphorus.

  The object of the present invention is to improve the above-mentioned problems of the prior art, reduce the amount of sludge generated, and avoid the interference of phosphorus crystallization reaction by zinc hydroxide, while the phosphorus in the treated water is removed by the phosphorus crystallization method. An object of the present invention is to provide a method for treating waste water containing phosphoric acid and zinc that can be recovered.

In order to achieve the above object, a method for treating wastewater containing phosphoric acid and zinc according to the present invention comprises adding an alkaline agent to water to be treated containing phosphoric acid and zinc, and adding zinc in the water to be treated. A first step of forming zinc hydroxide, a second step of separating the zinc hydroxide from the water to be treated that has undergone the first step, and a phosphorus crystal in which seed water has been filled with the water to be treated that has undergone the second step And a third step of crystallization treatment by passing water through the precipitation tank.
In the second step, precipitation separation treatment or membrane separation treatment is preferably employed.

  Further, in the present invention, the phosphorus crystallization tank is a fluidized bed type phosphorus crystallization tank, and the waste water is passed through the tank in an upward flow using 10 to 30 times as much circulating water as the amount of the waste water. It is characterized by forming a fluidized bed of seed crystals filled in the tank by the upward flow of water.

  According to the method for treating wastewater containing phosphoric acid and zinc according to the present invention, most of zinc contained in the water to be treated becomes zinc hydroxide in the first step, and the water produced in the first step in the second step. Zinc oxide is separated and removed from the water to be treated as a solid content. For this reason, in the third step, phosphoric acid in the water to be treated can be stably crystallized as hydroxyapatite phosphate on the surface of the seed crystal by a crystallization reaction while avoiding interference by zinc hydroxide. As a result, the treated water that has undergone the third step becomes clear water from which phosphoric acid and zinc have been removed.

  Moreover, the zinc hydroxide separated in the second step can be reused as an industrial raw material. The hydroxyapatite phosphate crystallized on the seed crystal surface in the third step can also be recovered together with the seed crystal and effectively used as a fertilizer or an industrial raw material. Therefore, sludge as industrial waste hardly occurs with respect to waste water containing phosphoric acid and zinc, and effective waste water treatment that also serves as resource recovery can be realized.

FIG. 1 is a system diagram showing a first embodiment of a processing method according to the present invention. The processing apparatus for carrying out this method is mainly composed of the reaction tank 10 in the first step, the aggregation tank 12 in the second process, the precipitation separation tank 14, the adjustment tank 16 in the third process, and the phosphorus crystallization tank 18. Composed.
In the first step, water to be treated 20 containing phosphoric acid and zinc is supplied to the reaction tank 10. For example, sodium hydroxide is added to the reaction tank 10 as the alkaline agent 22 to adjust the pH of the water to be treated 20 to 7-9. As a result, the zinc of the water 20 to be treated becomes zinc hydroxide. In addition, in this reaction tank 10, in addition to the alkaline agent 22, a chelating agent that promotes the production of zinc hydroxide may be added. However, inorganic flocculants such as polyaluminum chloride and ferric sulfate react with phosphoric acid in the water to be treated 20 to form a solid, and thus should not be added to the reaction vessel 10.

In the second step, the water to be treated 24 that has passed through the first step is supplied to the coagulation tank 12. An appropriate amount of the polymer flocculant 26 is added to the coagulation tank 12, and the solid content mainly composed of zinc hydroxide in the water to be treated 24 is flocked. The treated water 28 that has passed through the coagulation tank 12 is supplied to the subsequent precipitation separation tank 14, and the flocked solid content in the treated water 28 is precipitated and separated. Since the separated solid content 30 is mainly composed of zinc hydroxide as described above, it can be reused as an industrial raw material.
The to-be-processed water 32 which is the supernatant water of the precipitation separation tank 14 contains phosphoric acid, and is supplied to the adjustment tank 16 of the third step in a state where the pH is maintained at 7-9. Circulating water 34 circulating in the phosphorus crystallization tank 18 flows into the adjustment tank 16, and the water to be treated 32 and the circulating water 34 are merged and mixed. Further, a calcium compound 36 such as calcium chloride or calcium hydroxide is added to the adjustment tank 16. The addition amount of the calcium compound 36 is an amount necessary and sufficient for the added calcium to react with phosphorus in the water to be treated 32 to generate phosphate hydroxyapatite whose molecular formula is represented by Ca ₅ (PO ₄ ) ₃ OH. Usually, the reaction equivalent to phosphorus is 1 to 1.3 times.

  As described above, the pH of the water to be treated 32 is maintained at 7 to 9, and this pH is in a range suitable for the crystallization reaction in the subsequent phosphorus crystallization tank 18. However, the pH adjusting agent 38 may be added to the adjusting tank 16 in order to further optimally accelerate the crystallization reaction. The adjustment tank 16 is equipped with a stirrer 40, and the added calcium compound 36 and the pH adjuster 38 are uniformly mixed with the water to be treated 32.

  The treated water 42 whose calcium concentration and pH are adjusted in the adjustment tank 16 is supplied from the bottom of the phosphorus crystallization tank 18 to the phosphorus crystallization tank 18. The phosphorus crystallization tank 18 is a fluidized bed type, and is filled with a seed crystal having a particle size of about 1 mm. As the seed crystal, a phosphate ore mainly composed of calcium phosphate is preferably used. However, the seed crystal is not limited to this, and for example, bone charcoal or calcium silicate can be used.

  The treated water 42 introduced from the bottom of the phosphorus crystallization tank 18 is allowed to flow upward into the phosphorus crystallization tank 18, whereby the filled seed crystal flows and the seed crystal enters the phosphorus crystallization tank 18. Fluidized bed 44 is formed. In the process in which the water to be treated 42 passes through the fluidized bed 44 and contacts the seed crystal, hydroxyapatite phosphate crystallizes on the surface of the seed crystal, and most of the phosphorus in the water is recovered. Most of the treated water 42 that has passed through the fluidized bed 44 is collected by the water collecting means 46 disposed in the upper part of the tank, and returned to the adjustment tank 16 as circulating water 34. In addition, an amount of treated water 48 corresponding to the flow rate of the treated water 32 is discharged from the upper part of the phosphorus crystallization tank 18 to the outside of the system.

  The amount of circulating water 34 is normally set to 10 to 30 times the amount of treated water 32. The seed fluidized bed 44 is properly formed by the upward flow of the water 42 to be treated. Moreover, since the circulating water 34 dilutes the to-be-processed water 32, even if the fluctuation | variation of the phosphoric acid concentration of the to-be-processed water 32 is large, it plays the role which mitigates the bad influence.

  In the phosphorus crystallization tank 18, if crystallization on the surface of the seed crystal is continued by long-term operation, the seed crystal is enlarged due to the crystallized phosphate hydroxyapatite and the fluidity is lowered. Therefore, the seed crystal enlarged at an appropriate timing is extracted from the phosphorus crystallization tank 18 and updated to a new seed crystal. The recovered enlarged seed crystals are used effectively as fertilizers. Alternatively, the enlarged seed crystals can be adjusted to a certain particle size by crushing or other means and reused as seed crystals.

  According to the method for treating wastewater containing phosphoric acid and zinc according to this embodiment, most of the zinc contained in the water to be treated 20 becomes zinc hydroxide in the reaction tank 10 in the first step. In the second step, the zinc hydroxide produced in the first step is flocked in the coagulation tank 12 and separated and removed as a solid content 30 in the precipitation separation tank 14. For this reason, in the phosphorus crystallization tank 18 in the third step, phosphoric acid in the water to be treated is stably crystallized as hydroxyapatite phosphate on the surface of the seed crystal by crystallization reaction while avoiding interference by zinc hydroxide. Can do. As a result, the treated water 48 that has undergone the third step becomes clarified water from which phosphoric acid and zinc have been removed. Moreover, the solid content 30 mainly composed of zinc hydroxide separated in the second step can be reused as an industrial raw material. The hydroxyapatite phosphate crystallized on the seed crystal surface in the third step can also be recovered together with the seed crystal and effectively used as a fertilizer or an industrial raw material. Therefore, sludge as industrial waste hardly occurs with respect to waste water containing phosphoric acid and zinc, and effective waste water treatment that also serves as resource recovery can be realized.

  FIG. 2 is a system diagram showing a second embodiment of the processing method according to the present invention. In FIG. 2, elements denoted by the same reference numerals as those in FIG. In the present embodiment, the second step is constituted by the membrane separation tank 50. The membrane separation tank 50 is equipped with a membrane separation means 52 constituted by a microfiltration membrane or an ultrafiltration membrane. Zinc hydroxide in the water to be treated 24 having undergone the first step is subjected to membrane separation by the membrane separation means 52. Then, the solid content 30 is separated and removed from the water to be treated 24. The phosphoric acid in the for-treatment water 24 permeates through the microfiltration membrane or the ultrafiltration membrane and is contained in the for-treatment water 32 and sent to the adjustment tank 16. According to the second embodiment, since a precipitation separation tank with a small water area load and a large required installation area is not required, the apparatus scale can be reduced.

It is a systematic diagram showing a first embodiment of a processing method according to the present invention. It is a systematic diagram which shows 2nd Embodiment of the processing method which concerns on this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Reaction tank, 12 ... Coagulation tank, 14 ... Precipitation separation tank, 16 ... Adjustment tank, 18 ... Phosphorus crystallization tank, 20 ... Water to be treated, 22 ... Alkaline agent, 26 ... ... Polymer flocculant, 30 ... Solid content, 34 ... Circulating water, 36 ... Calcium compound, 38 ... pH adjuster, 40 ... Stirrer, 44 ... Fluidized bed, 46 ... Water collecting means, 48 ... treated water, 50 ... membrane separation tank.

Claims (3)

  A first step in which an alkaline agent is added to the water to be treated containing phosphoric acid and zinc to make zinc in the water to be treated as zinc hydroxide, and the zinc hydroxide is separated from the water to be treated in the first step. Phosphoric acid and zinc, comprising: a second step, and a third step in which the water to be treated having passed through the second step is passed through a phosphorus crystallization tank filled with seed crystals for crystallization treatment Of wastewater containing water.   The method for treating wastewater containing phosphoric acid and zinc according to claim 1, wherein the second step is a precipitation separation treatment or a membrane separation treatment.   The phosphorus crystallization tank of the third step is a fluidized bed type phosphorus crystallization tank, and the treated water is flowed upward into the tank using circulating water 10 to 30 times the amount of treated water. The method for treating wastewater containing phosphoric acid and zinc according to claim 1, wherein a fluidized bed of seed crystals filled in the tank is formed by upward flow.

Images