JPS5811273B2 - Treatment method for wet phosphoric acid factory wastewater - Google Patents

Description

Detailed Description of the Invention The present invention involves reacting a saturated gypsum solution or gypsum slurry produced in a gypsum separation process in a wet phosphoric acid production process with fluorine-containing wastewater produced in a waste gas treatment process, and recovering the resulting precipitate. The present invention also relates to a wastewater treatment method characterized by obtaining a clear supernatant liquid.

Generally, in a wet phosphoric acid production method, phosphate rock is treated with sulfuric acid to produce a phosphoric acid solution, and gypsum is produced as a by-product.

In this decomposition reaction, fluorine compounds are generated according to the following reaction formula.

Ca5F(PO4)3+5H2SO4+X-H20→5
CaSO4+XH2O+3H3PO4+HF (1
)4HF+5i02→5tF4+2H20(2)3Si
F4+2H20→2H2StFa+5i02 (3)H
2S+Fa→SiF4+2HF (4)
That is, when sulfuric acid is reacted with phosphate rock, the fluorine contained in the phosphate rock becomes hydrogen fluoride, and this hydrogen fluoride reacts with silica in the phosphate rock or added to form silicon tetrafluoride, most of which is It reacts with water to form silicofluoric acid, which is present in the phosphoric acid solution.

Therefore, most of the fluorine in the wet phosphoric acid is thought to remain in the phosphoric acid solution as silicofluoric acid, but some of it remains in the phosphoric acid solution when the phosphoric acid solution is heated, as shown in equation (4) above. ,
It decomposes into silicon tetrafluoride and hydrogen fluoride.

Fluorine gas and fluorine-containing wastewater are generated in wet phosphoric acid factories as described above, but since both exhaust gas and wastewater are subject to strict regulations for pollution prevention, when producing and concentrating 9-phosphoric acid, Significant efforts are being made to recover and remove fluoride from wastewater.

In addition, the saturated gypsum solution or gypsum slurry produced in the process of separating by-product gypsum from the phosphoric acid solution according to equation (1) above also contains suspended substances, phosphorus, fluorine, etc., which are subject to pollution prevention regulations. Therefore, wastewater treatment is carried out depending on the water quality.

Conventionally, the treatment of wastewater containing hydrofluoric acid, silicofluoric acid, sodium silicofluoride, etc., such as cleaning water generated from the waste gas treatment step T in the wet phosphoric acid production process and condensed water generated from the phosphoric acid liquid concentration process, has been carried out in large quantities. Hydrogen fluoride in wastewater can be removed by diluting it with water and discharging it into the sea, or by adding calcium salts such as slaked lime or calcium chloride to remove hydrogen fluoride from wastewater using the following method:
2HF + Ca (OH) 2 → CaF 2 + 2H 20] to fix it as calcium fluoride and remove it.

However, among these conventional treatment methods, it is extremely difficult to implement the former method now that discharge into ocean areas is shifting from conventional concentration regulations to total volume regulations.

In addition, the latter method requires a large amount of calcium salt, which results in the formation of a large amount of sludge, and since calcium salt is relatively expensive, wastewater treatment costs increase, or the fluoride produced Since calcium is fine, it has drawbacks such as difficulty in separating it from a solution.

In addition, in the washing water generated in the gypsum separation process, that is, the saturated gypsum solution or the gypsum slurry, conventionally, suspended substances such as gypsum contained in it are sedimented and removed, and then harmful substances such as phosphorus and fluorine, which are regulated for pollution prevention, are removed. In order to remove these harmful substances, more precise wastewater treatment is being carried out, but this method requires large equipment and expense, and has drawbacks such as the difficulty of bringing these harmful substances below regulated values. .

In order to overcome and eliminate these conventional drawbacks, the present inventors have conducted various studies and found that gypsum-containing wastewater and fluorine-containing wastewater in the above-mentioned wet phosphoric acid manufacturing process, which had been treated separately until now, and the wastewater at an appropriate pH and molar ratio C.
a2+/(2F-+2/3(PO43-)), the dissolved substances are immobilized to form a precipitate, and the undissolved substances contained therein are separated from the liquid, thereby reducing the efficiency of the precipitate. It was discovered that it is possible to obtain clear treated water with a low fluorine content by removing the fluorine, and based on this knowledge, the present invention was completed.

That is, the present invention aims to economically treat the wastewater generated in the wet phosphoric acid production process, and to make the process into a closed system by circulating and reusing the purified treated wastewater. , gypsum-containing wastewater discharged from the filtration process and fluorine-containing wastewater discharged from the exhaust gas treatment process were treated as Ca2+/(2F-+2/3(PO4
3-)) molar ratio of 1.1 to 1.5 and pH of 8 to 1
A method for treating wet phosphoric acid factory wastewater, which is characterized in that the reaction is carried out in the range of 1, the generated precipitate is separated and removed, and the P solution is recycled as washing water in the system for gypsum or separator P cloth, etc. can be achieved effectively.

Gypsum saturated solution such as P cloth washing water of gypsum separator generated from the filtration process in the wet phosphoric acid manufacturing process, or gypsum slurry liquid, cleaning liquid of fluorine-containing exhaust gas from the phosphoric acid manufacturing process, and phosphoric acid concentration process. When reacting with fluoride-containing wastewater such as steam condensed water at a pH of 8.0 to 11.0 and a molar ratio of 1.1 to 1.5 as described above,
Insoluble apatite and calcium fluoride are efficiently produced by calcium in gypsum-containing wastewater and fluorine and phosphoric acid in fluorine-containing wastewater.

In a preferred embodiment, this generated precipitate is separated and recovered, and the resulting clear treated water is reused as plaster or separated iF cloth washing water in the filtration process, thereby making the wastewater treatment a closed system. do.

In this case, in the method of the present invention, if phosphoric acid is contained in the fluorine-containing wastewater, the phosphoric acid is fixed as an insoluble substance and separated and recovered according to the following formula.

(5x+y)Ca2++(x+2y)F-+3xPO4
3-→xca5(PO4)3F+VCaF2 In addition, both wastewaters cause a quantitative imbalance, resulting in the above molar ratio Ca2+/(2F-+2/3(PO43-))=1.
1 to 1.5, a large amount of fluorine ions are present in the p-liquid because fluoroapatite and calcium fluoride are insufficiently produced.

For example, if there is a calcium deficiency, supplement with slaked lime, and if there is an excess of calcium or a deficiency of fluorine, supplement with phosphoric acid.

The method of the present invention does not require a large amount of expense and equipment, such as the repeated immobilization of fluorine with calcium salts, etc., in order to reduce the fluorine content in wastewater to below the legal regulation value, unlike conventional methods. No special process is required.

In other words, by using gypsum-containing wastewater generated from the filtration process to treat fluorine-containing wastewater, the wastewater treatment in the wet phosphoric acid process becomes a closed system, and there is no need to discharge wastewater outside the process, so fluorine in the treated water can be reduced. Precise processing is not required to reduce the concentration of regulated substances such as below the legally regulated values.

Therefore, there is no need for complicated processes that require a large amount of expense and large equipment as in the past.

Furthermore, because it is a closed system, most of the fluorine, phosphoric acid, calcium, etc. in the wastewater are fixed as insoluble substances and are recovered and reused, while the treated liquid is reused as washing water for the gypsum separator, etc. Since it is not discharged outside the process,
The method of the present invention is extremely advantageous as a wet phosphoric acid process wastewater treatment method because it is expected to have a significant effect not only on pollution prevention but also on improving the yield of various raw materials.

In the present invention, the above-mentioned reactions of both wastewaters may be carried out in a batchwise or continuous manner, although either of them may not be sufficient, but it is preferable to adopt a continuous manner from various points of view.

In this case, to correct the imbalance of calcium and fluorine in both wastewaters and adjust the molar ratio, use lime milk if there is a calcium deficiency, and use phosphoric acid solution if there is an excess of calcium or a fluorine deficiency. The pH can also be adjusted using alkalis such as sodium hydroxide and soda carbonate, all of which are easily available.

The present invention will be explained below with reference to Examples.

Example 1 In the wet phosphoric acid manufacturing process, 744 ppm of calcium ions (Ca2+), 60 ppm of phosphate ions (PO43-), and sulfate ions (SQ) were generated from the phosphoric acid filtration process.
, 2-) Wastewater 101 with pH 3.0 containing 1784 ppm
and fluorine ions generated from the fluorine-containing gas treatment process (
F-) 8000ppm and silica (Sin2) 2105p
After stirring and mixing 960 ml of wastewater containing PM,
(OH)2100g/l slaked lime as Ca(OH)2, 6.587g, and 48% strength soda as NaCH
By adding 16.28 g as
9.5, stirred for 20 minutes and reacted to produce insoluble apatite and calcium fluoride,
This was sent door to door to obtain 25.7 g of solid matter.

On the other hand, calcium ion (Ca2+) 47pp as mud liquid
m, 10.4 A of filtrate containing 53 ppm of fluorine ions (F-) and 0.24 ppm of phosphate ions (PO,3-)
The slurry was recycled and used as washing water for phosphogypsum.

The obtained solid matter was 1.1 g of apatite, CaF214
．． 69.5i02209, CaSO4・2H20+Ca
(OH)28. It contained Og.

Example 2 744 ppm of calcium ions (Ca2+) generated from the gypsum filtration step in the wet phosphoric acid production process.

Wastewater 1 containing 1786 ppm of sulfate ions (8042-)
Phosphoric acid (P
Ca(OH)21
00g/l of slaked lime as Ca(OH)2 is 137.6
Continuously add 4g/hr and 48% of sodium hydroxide as NaOH to give 16289/hr of Ca2+/C2F
+2/3(PO43-)) molar ratio of 1.3 and pH
was reacted for 20 minutes while adjusting the temperature to 8.5, and the solid content of insoluble apatite and calcium fluoride was 360 g/hr.
was recovered, calcium ion (Ca2+) 40ppm,
Fluorine ion (F-) 50ppm and phosphate ion (
Po, 3-) 0.3 ppm of tear fluid 1041/hr was obtained.

The slurry was recycled and used as washing water for phosphogypsum.

The solid material obtained above was 95 g of apatite, CaF21
40g, 5i0220g, CaSO4・2H20+Ca
It contained 2105g of (OH).

Example 3 960 ml of waste water containing fluorine and silica with the same composition as in Example 1 was mixed with waste water iox having the same composition as in Example 1.
9.087 g of 2100 g/l slaked lime as Ca(OH)2 and 16.28 g of 48% strength soda as NaOH were added, Ca2+/[2F-+2/
3(Po43-)] and the pH was adjusted to 10.5 with stirring for about 30 minutes to form a solid substance of insoluble apatite and calcium fluoride 29.3. g and this by furnace and calcium ion (Ca2+) 70pp
A solution 10.21 having m, 50 ppm of fluorine ions (P-) and 0.3 ppm or less of phosphate ions (PO43-) was obtained.

The obtained slurry was used for cleaning phosphate gypsum door cloths.

The solid material obtained above contained 1.1 g of apatite, CaF
214.6g, SiO, 20g, CaSO4・2H20
It contained 211.6 g of +Ca(OH).

Comparative Example While stirring and mixing wastewater having the same composition as in Example 1 at the same ratio, slaked lime with a concentration of Ca(OH) of 21009/l was mixed with Ca(OH).
) 2L and 1. B, and 48-potent soda with NaOH
16.28g was added as Ca2+/(2F-12
/3(PO43-)] molar ratio 1.0 and pH 9.0
After that, the mixture was stirred for 20 minutes and reacted to produce insoluble apatite and calcium fluoride, which were separated in a furnace to yield 23.1 g of solid matter and calcium ions (Ca2+)2.
ppm, P solution with 266 ppm of fluorine ions (F-) and 0.5 ppm of phosphate ions (PO4''-), 10.4
I got an A.

The filtrate contains a lot of fluorine ions and is unsuitable as washing water, and further requires post-treatment by adding calcium or the like.

The above solids are 1.1 g of apatite, 210 g of CaF, 8
1022g, CaSO4・2H20moCa(OH)21
It contained 0g.

Example 4 Calcium ions (Ca2
+) 800ppm, phosphate ion (PO4”-) 65p
101 gypsum wastewater containing 1920 ppm of sulfate ions (8042) and 6000 ppm of fluorine ions (F-)
pm, and wastewater 1.287 containing 1580 ppm of silica (Si02) while stirring and mixing.
7.205g of 0g/l slaked lime as Ca(OH)2
and 16.28 as NaOH of force soda of 48
g, Ca2+/[2F-12/3(po43
The molar ratio of D-)) was adjusted to 1.4 and the pH was 9.7, and the reaction mixture was stirred for 20 minutes to obtain a solid substance 28.5.9 mainly consisting of insoluble apatite and calcium fluoride.

On the other hand, 45pp of calcium ion (Ca2+) is used as F solution.
m, filtrate 10.71 with 53 ppm of fluorine ions (F-) and 0.3 ppm or less of phosphate ions (PO4"3-)
I got it.

This filtrate can be sufficiently recycled as washing water for phosphate gypsum.

Claims (1)

[Claims] 1. In treating factory wastewater generated in the wet phosphoric acid manufacturing process, gypsum-containing wastewater discharged from the filtration process and fluorine-containing wastewater discharged from the exhaust gas treatment process are treated with Ca2
+/[2F-12/3(PO43-)] molar ratio is 1.
1 to 1.5 and a pH of 8 to 11, the resulting precipitate is separated and removed, and the filtrate is recycled as washing water in the system.