JP2012200687A - Method of treating waste liquid containing hydrofluosilicic acid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of treating waste liquid containing hydrofluosilicic acid capable of easily removing hydrofluosilicic acid in the waste liquid containing hydrofluosilicic acid using an inexpensive material, capable of efficiently collecting it as highly-pure calcium fluoride, and capable of obtaining the treatment liquid with a low degree of fluoride ions.SOLUTION: The treated waste liquid is introduced into a reaction tank 1 as a preparation process. An inorganic metal salt is injected into and reacted with the waste liquid. Solid-liquid separation is performed in a solid-liquid separation tank 3. The preparation liquid separated in the solid-liquid separation tank 3 is introduced into a collection device 4, and sequentially passed to filling tanks 4a, 4b, 4c filled with calcium carbonate particles. Thus, calcium fluoride is generated and collected. Sludge containing a silicofluoride salt separated in the solid-liquid separation tank 3 is drained and separated. The separate liquid is transferred to the reaction tank 1.

Description

  The present invention relates to a method for recovering calcium fluoride from a waste liquid containing silicohydrofluoric acid, and more particularly to a method for recovering high-purity calcium fluoride from a waste liquid containing silicohydrofluoric acid and hydrofluoric acid. Is.

  In factories such as semiconductors, there is a step of wet-etching an oxide film of a silicon wafer with hydrofluoric acid, and waste liquid containing high concentration hydrofluoric acid is generated. Regarding the treatment of such waste liquid, various methods for recovering hydrogen fluoride have been studied from the viewpoint of reducing environmental burden and recycling. In particular, it is reacted with a calcium compound and recovered as calcium fluoride. Many methods for producing raw materials such as hydrogen acid (AHF) have been proposed. In such a recovery method, an effort is made to efficiently recover high-purity calcium fluoride. However, in the case of a waste liquid containing silicohydrofluoric acid, the hydrosilicofluoric acid reacts with the calcium compound. Therefore, it is precipitated as calcium silicofluoride, or decomposes and silicic acid is precipitated in a gel state, and the purity of recovered calcium fluoride is lowered.

  In Patent Document 1 (Japanese Patent Laid-Open No. 5-237481), as a method for recovering high-purity calcium fluoride from waste liquid containing hydrofluoric acid and silicohydrofluoric acid, the silicon concentration in the waste liquid is reduced by dilution or the like. It has been shown that high purity calcium fluoride can be recovered by reacting with a calcium compound. However, in this method, a method of diluting is shown as a means for lowering the silicon concentration in the waste liquid, but such a method is originally not suitable for the treatment of the waste liquid containing a high concentration of hydrofluoric acid. .

  That is, in recent years, a large amount of waste liquid containing high concentrations of hydrofluoric acid and hydrofluoric acid has been generated in silicon wet etching processes in solar cell manufacturing and dry etching processes using gaseous fluoride in silicon wafer manufacturing processes. is doing. With such a waste liquid, it is difficult to adjust the concentration by dilution. Further, when the concentration is lowered by dilution, the processing capacity is increased, so that the processing efficiency is lowered.

  In Patent Document 2 (Japanese Patent Laid-Open No. 2009-196858), silicohydrofluoric acid is decomposed by mixing and neutralizing a waste liquid containing silicohydrofluoric acid and hydrofluoric acid with a sodium compound (alkali). A silica slurry mainly composed of a mixture of insoluble silica and an aqueous solution of sodium fluoride is produced. Insoluble silica is separated from the silica slurry, and a calcium compound is added to the separated water, and an acid is added to adjust the pH to 2 or lower. The method of depositing and recovering calcium fluoride is shown.

However, the method of Patent Document 2 has the following problems.
1) A solution containing a strong acid such as hydrofluoric acid is used for wet etching of silicon, and wastewater containing such a strong acid is produced. To neutralize such wastewater to pH 7-10, A large amount of alkali is required.
2) To recover as calcium fluoride after neutralization, it is necessary to lower the pH of the wastewater to 2 or less, and a large amount of acid is required.
3) Since a neutralization step and an acid addition step are necessary, the steps are complicated, and there are problems in equipment costs and space.
4) Since neutralization ripening occurs, cooling equipment is required.

In Patent Document 3 (Japanese Patent Laid-Open No. 2000-72482), fluoride is added to a glass cleaning waste liquid containing fluorosilicic acid and hydrofluoric acid, and fluorosilicic acid and fluoride in the liquid are reacted to precipitate fluorosilicate. And a method for regenerating the glass cleaning solution by removing the fluorosilicate.

  However, the method of Patent Document 3 is a method in which fluorosilicic acid is removed from the waste liquid and recovered as a glass cleaning liquid containing hydrofluoric acid, and recovery as calcium fluoride is not shown. In Patent Document 3, in order to increase the hydrofluoric acid purity of the recovered liquid, fluoride is added to generate hydrofluoric acid. Therefore, it is necessary to add an expensive fluoride with high purity. Moreover, since hydrofluoric acid concentration becomes high by the production | generation of hydrofluoric acid, in order to produce | generate calcium fluoride from a collection | recovery liquid, it is necessary to dilute and there is much waste.

Japanese Patent Application Laid-Open No. 5-237482 JP 2009-196858 JP 2000-72482 A

  In order to solve the conventional problems as described above, the object of the present invention is to easily remove hydrofluoric acid from a hydrofluoric acid-containing waste liquid using an inexpensive material, and to obtain high-purity calcium fluoride. As well as a method for treating a hydrofluoric acid-containing waste liquid capable of efficiently collecting a treatment liquid having a low fluoride ion concentration.

The present invention is the following method for treating a hydrofluoric acid-containing waste liquid.
(1) a pretreatment step in which an inorganic metal salt is added to a waste liquid containing silicohydrofluoric acid and hydrofluoric acid to form a hardly soluble silicofluoride salt and separated;
And a recovery step of recovering the pretreatment liquid as calcium fluoride by reacting the pretreatment liquid with a calcium compound.
(2) The method according to (1) above, wherein the recovery step is a step of allowing the pretreatment liquid to flow through the calcium carbonate packed bed to cause a reaction, and recovering it as calcium fluoride.
(3) The method according to the above (1) or (2), wherein the inorganic metal salt is Na, K, Cs, Rb, Ba salt.
(4) The method according to any one of (1) to (3) above, wherein the calcium compound is calcium chloride, oxide, hydroxide, carbonate or bicarbonate.

In the present invention, the hydrosilicic acid-containing waste liquid to be treated is a waste liquid containing hydrofluoric acid (H ₂ SiF ₆ ) and hydrofluoric acid (HF) as constituent components. The hydrofluoric acid-containing waste liquid may contain other fluorides and other components. Examples of such a hydrofluoric acid-containing waste liquid include a waste liquid discharged in a process of wet etching Si with an HF aqueous solution such as a solar battery, but may be a waste liquid discharged from another process. The concentration of silicohydrofluoric acid in the waste liquid is 0.01 to 10.0 w / v%, particularly 0.013 to 6.0 w / v%, and the concentration of hydrofluoric acid is 0.1 to 5.0 w / v%. Particularly, 0.5 to 3.0 w / v% of waste liquid is suitable for the treatment of the present invention. In the present invention, “w / v%” means “g / 100 mL”.

In the present invention, when treating the waste liquid containing silicohydrofluoric acid and hydrofluoric acid, a pretreatment step for producing and separating a hardly soluble silicofluoride salt, and producing and collecting calcium fluoride are obtained. High-purity calcium fluoride can be efficiently recovered by combination processing with the recovery step.

  The pretreatment step is a step of adding an inorganic metal salt to a waste liquid to be treated containing silicohydrofluoric acid and hydrofluoric acid to produce a hardly soluble silicofluoride salt and separating it. The inorganic metal salt to be added may be a metal salt that reacts with silicofluoric acid to form a hardly soluble salt, but a monovalent or divalent metal salt is particularly preferable. Examples of such inorganic metal salts include Na, K, Cs, Rb, and Ba salts. Na salts and Ba salts that do not dissolve precipitates even in an acid solution are particularly preferable. The Ca salt is removed because the salt precipitated by addition is easily dissolved in the acid solution. The anion serving as a metal counter ion is not limited, but an anion that does not interfere with precipitation in the pretreatment step and the recovery step is preferable, and examples thereof include chloride, hydroxide, nitrate, carbonate, bicarbonate and the like.

  The inorganic metal salt is added by adding a powder or an aqueous solution of the inorganic metal salt to the waste liquid to be treated. In the case of carbonate or bicarbonate, it can be added in powder form. In this case, the precipitate is peeled off due to the generation of carbon dioxide gas, so that the reaction proceeds quickly without reducing the concentration, which is preferable. In the case of other salts, it is usually added in the form of an aqueous solution, but the concentration is not particularly limited, and is usually about 1 to 50% by weight, preferably about 5 to 25% by weight.

The reaction of silicofluoric acid and the inorganic metal salt in the pretreatment step is presumed to be according to the following formula [I].
H ₂ SiF ₆ + MxA → MxSiF ₆ ↓ + H ₂ A ... [I]
(In the formula [I], M represents a cation atom such as Na or Ba, x represents the number of atoms, and is omitted when the number of atoms is 1. A represents an anion atom such as OH or Cl. In the formula, it is indicated as bivalent.)

As shown in the above formula [I], the inorganic metal salt reacts with silicohydrofluoric acid as much as it is added, and immediately precipitates as a hardly soluble silicofluoride salt. An amount corresponding to the equivalent may be added. The amount added may not be accurate. Generally, when the reaction equivalent is 1, it can be about 0.8 to 1.5 times, preferably about 0.9 to 1.2 times. If an excessive amount of inorganic metal salt is added, hydrofluoric acid contained in the waste liquid is neutralized, and a hardly soluble salt such as NaF or BaF ₂ may be precipitated. Is preferably added at pH 4 or less. Since the precipitated silicofluoride salt is easily separable, it can be easily separated and removed with a small amount of entrained water by general solid-liquid separation such as sedimentation separation.

  The recovery step is a step of reacting the pretreatment liquid separated in the pretreatment step with a calcium compound to recover fluoride ions such as hydrofluoric acid contained in the pretreatment liquid as calcium fluoride. Examples of the calcium compound to be reacted here include calcium chloride, oxide, hydroxide, carbonate or bicarbonate. The method of reacting the calcium compound is not limited, and powder or an aqueous solution can be added to the waste liquid to be treated for reaction, and the precipitated calcium fluoride can be recovered by solid-liquid separation. A method is preferred in which the solution is passed through to react and recovered as calcium fluoride. Depending on the wastewater, the hydrofluoric acid concentration may be diluted and transferred to a calcium fluoride recovery facility so that the concentration of hydrofluoric acid is optimal for the calcium fluoride recovery method. In consideration, it is preferable to dilute after the silicofluoride salt is precipitated and removed with an inorganic metal salt.

A method of collecting and reacting a pretreatment liquid through a calcium carbonate packed bed, which is a preferable method of the recovery process, and recovering it as calcium fluoride is disclosed in, for example, Japanese Patent No. 26005629. The pretreatment liquid is passed through the series to react with the layer, and the liquid that has exited the final stage packed bed is taken out as the treatment liquid. Then, calcium fluoride is recovered from the first-stage packed bed where the reaction is completed, and calcium carbonate-filled layer is formed by newly filling calcium carbonate, and this is operated in a merry-go-round manner as a new final stage. is there.

The reaction of hydrofluoric acid and calcium compound in the recovery step is presumed to be according to the following formula [II].
2HF + CaAx → CaF ₂ ↓ + H ₂ Ax... [II]
(In the formula [II], Ax represents an anion atom such as OH, Cl, CO _3, etc., but in the formula, it is a divalent display.)

  When the pretreatment liquid is passed through the calcium carbonate packed bed for reaction and recovered as calcium fluoride, fluoride ions such as hydrofluoric acid in the pretreatment liquid are contained in the calcium carbonate particles forming the packed bed. Therefore, all fluoride ions such as hydrofluoric acid in the pretreatment liquid are reacted and recovered without controlling the amount to be reacted.

  In addition, the calcium carbonate forming the packed bed reacts sequentially with the fluoride ions penetrating into the calcium carbonate particles, so the entire calcium carbonate particles become calcium fluoride. Complete. The entire packing of the first-stage packed bed after completion of the reaction can be recovered as it is as calcium fluoride. Since the recovered calcium fluoride has a high purity, it is used as a raw material for producing anhydrous hydrofluoric acid for dry etching and the like.

  The silicofluoride salt separated in the pretreatment step can be recovered and used as it is, or can be disposed of as waste. However, the silicofluoride salt can be converted into silica and fluoride salt by reacting with alkali in the neutralization decomposition step. By sending the decomposed and separated fluoride salt to the pretreatment step or the recovery step and reacting it as fluoride ions, the recovery efficiency of calcium fluoride can be further increased. The separated fluoride salt may be sent to the recovery step since the silicic fluoride has been removed, but if it contains silica or silicic fluoride that cannot be removed, it is preferably sent to the pretreatment step for further reaction.

It is presumed that the reaction in which the silicofluoride Na salt is reacted with alkali (NaOH) to be decomposed into silica and fluoride salt in the neutralization decomposition step is based on the following formula [III].
Na ₂ SiF ₆ + 4NaOH → 6NaF + SiO ₂ ↓ + 2H ₂ O ... [III]

  As described above, according to the present invention, after the inorganic metal salt is added to the waste liquid containing silicohydrofluoric acid and hydrofluoric acid in the pretreatment step to form a hardly soluble silicofluoride salt, the pretreatment is performed. Since the liquid was reacted with the calcium compound and recovered as calcium fluoride, the hydrofluoric acid contained in the silicohydrofluoric acid-containing waste liquid was easily removed by using an inexpensive material, and high purity calcium fluoride. As a result, it is possible to obtain a treatment liquid having a low fluoride ion concentration and the like.

It is a flowchart which shows the processing method of the silicofluoric acid containing waste liquid of embodiment.

  In FIG. 1, 1 is a reaction tank, 2 is an inorganic metal salt storage tank, 3 is a solid-liquid separation tank, 4 is a recovery device, and 5 is a post-treatment device. The reaction tank 1, the inorganic metal salt storage tank 2, and the solid-liquid separation tank 3 constitute a pretreatment device. The recovery device 4 passes a series of liquids upward through a plurality of filling tanks 4a, 4b, and 4c filled with calcium carbonate particles, and when the first stage reaction is completed, the packed bed is replaced and the final stage is reached. It is configured.

  The waste liquid to be treated containing silicohydrofluoric acid and hydrofluoric acid is introduced into the reaction tank 1 from the line L1 as a pretreatment process, and the inorganic metal salt is injected into the reaction tank 1 from the inorganic metal salt storage tank 2 through the line L2. The reaction solution is introduced into the solid-liquid separation tank 3 from the line L3 and solid-liquid separated. In the reaction tank 1, a silicofluoride salt is generated according to the formula [I], and this is subjected to solid-liquid separation in the solid-liquid separation tank 3.

  Fluoride ions such as hydrofluoric acid contained are reacted with a calcium compound to produce calcium fluoride according to the formula [II], which is recovered. As shown in Japanese Patent No. 26005629, the recovery device 4 is composed of a plurality of filling tanks 4a, 4b, and 4c filled with calcium carbonate particles, so that the upward direction is gradually increased from the first filling tank 4a. The liquid is passed through the series in a flow, and the reaction is caused to proceed sequentially in each of the filling tanks 4a, 4b, and 4c.

  When the reaction in the first stage filling tank 4a is completed, the first stage filling tank 4a is disconnected, and the pretreatment liquid is sequentially passed through the next stage filling tanks 4b and 4c to continue the processing. The calcium fluoride produced in the separated first-stage filling tank 4a is taken out from the line L5 and collected, and is newly filled with calcium carbonate to form a calcium carbonate filled layer. Then, the filling tank 4a in which the packed bed is replaced is moved to the subsequent stage of the filling tank 4c, and this is connected as a new final stage to operate in a merry-go-round manner. The effluent from the filling tank 4c of the recovery device 4 is sent from the line L6 to the post-treatment device 5, where post-treatment such as coagulation treatment is performed, and the treatment liquid is discharged out of the system from the line L7.

  The sludge containing silicic fluoride salt separated in the solid-liquid separation tank 3 is taken out from the line L8 and disposed of such as dehydration. The dehydrated filtrate can be returned to the reaction tank 1. Sludge containing silicic fluoride salt can also be treated by introducing it into a neutralization decomposition tank (not shown) from line L8.

  The reaction between silicofluoric acid and the inorganic metal salt has a very high reaction rate, and forms a silicofluoride salt within 15 minutes after the reaction. This silicofluoride salt has a large specific gravity and tends to settle. For this reason, a method of directly adding an inorganic metal salt to the solid-liquid separation tank 3 without providing the reaction tank 1 or a line injection of the inorganic metal salt directly into the line L3 in front of the solid-liquid separation tank 3 without providing the reaction tank 1 The method to do is also good.

  In the above treatment, it is not necessary to use a high-purity and expensive fluoride, and an inexpensive material such as a normal inorganic metal salt can be used. The hydrosilicofluoric acid in the waste liquid can be easily removed as a silicofluoride salt, whereby it can be efficiently recovered as high-purity calcium fluoride and a treatment liquid having a low fluoride ion concentration can be obtained.

  Examples of the present invention and comparative examples will be described below. In each case, the percentage purity is w%.

[Examples 1 and 2 and Comparative Example 1]
[Test Example 1] Silica fluoride salt removal test 1 L of a solution containing 2.0 w / v% H ₂ SiF ₆ , 10 w / v% HF and 3.0 w / v% HNO ₃ is mixed with the following inorganic metal salt: Was added. The added amount of the inorganic metal salt is a molar ratio with respect to H ₂ SiF ₆ , and Example 1 is an aqueous solution in which NaCl is 0.5 to 3.0, and Example 2 is BaCO ₃ in a ratio of 0.2 to 1.5. Added. After the addition, the mixture was stirred for 1 hour with a stirrer, and the Si concentration in the supernatant was measured with an ICP emission spectrometer. Table 1 shows the relationship between the molar ratios of inorganic metal salts added in Examples 1 and 2 and the Si concentration (the effect of dilution is corrected). As shown in Table 1, it was confirmed that the supernatant Si concentration decreased as the amount of inorganic metal salt added increased. In the molar ratio to H ₂ SiF ₆ , NaCl of Example 1 was 3.0, BaCO ₃ of Example 2 was 1.5, and the supernatant Si concentration was <100 mg / L.

As Comparative Example 1, 48 wt% NaOH was added until pH 7 and the supernatant Si concentration was measured with an ICP emission spectrometer. Table 2 shows the relationship between the Si concentration of the supernatant of Comparative Example 1 and Examples 1 and 2 and the amount of inorganic metal salt used (the effect of dilution is corrected) together with the Si concentration of the blank (no addition). As shown in Table 2, since Comparative Example 1 needs to neutralize not only H ₂ SiF ₆ but also HF and HNO ₃ , the amount of inorganic metal salt (NaOH) used may be larger than Examples 1 and 2. I understand.

[Test Example 2]; Reaction Rate Test Under the conditions of adding BaCO _{3 in} Example 2 in Table 1 (molar ratio 0.5), the relationship between the reaction time after the addition and the supernatant Si concentration was determined. The results are shown in Table 3. From Table 3, it was confirmed that the reaction between the inorganic metal salt and H ₂ SiF ₆ was almost completed within 15 minutes.

[Example 3, Comparative Example 2]:
[Test Example 3]: Calcium fluoride recovery test In contrast to the solution containing 5000 mg / L (34.7 mmol / L) of H ₂ SiF _{6 and} 10000 mg / L (500 mmol / L) of HF, BaCO ₃ ( ₂ L of molecular weight 197) was added as a powder so that the molar ratio with respect to H ₂ SiF ₆ was 1.5, and 50 L of the obtained supernatant solution was charged with 500 g of CaCO ₃ particles having a particle size of 0.3 mm and a purity of 99% or more. Water was passed through the series in an upward flow sequentially through three packed columns consisting of columns. In Comparative Example 2, the same test was performed without adding BaCO ₃ .

In the water flow of Example 3 and Comparative Example 2, the Si concentration (mg / L), HF concentration (mg / L) and pH at the inlet of the first stage packed tower and the outlet of the third stage packed tower were measured. Moreover, after passing water, the packing was taken out from the first stage packed tower, and after drying, the purity (%) of CaF ₂ was measured. These results are shown in Table 4.

  The present invention relates to a method for recovering calcium fluoride from a waste liquid containing silicohydrofluoric acid, and further to removing silicofluoride salt from the waste liquid containing silicohydrofluoric acid and hydrofluoric acid to obtain high-purity calcium fluoride. It can be used for the method of recovering.

  1: reaction tank, 2: inorganic metal salt storage tank, 3: solid-liquid separation tank, 4: recovery apparatus, 4a, 4b, 4c: filling tank, 5: post-treatment apparatus.

Claims (4)

A pretreatment step of adding an inorganic metal salt to a waste liquid containing silicohydrofluoric acid and hydrofluoric acid to form a hardly soluble silicofluoride salt and separating it;
And a recovery step of recovering the pretreatment liquid as calcium fluoride by reacting the pretreatment liquid with a calcium compound.   The method according to claim 1, wherein the recovery step is a step of allowing the pretreatment liquid to flow through the calcium carbonate packed bed for reaction to recover as calcium fluoride.   The method according to claim 1 or 2, wherein the inorganic metal salt is Na, K, Cs, Rb, Ba salt.   4. The method according to claim 1, wherein the calcium compound is calcium chloride, oxide, hydroxide, carbonate or bicarbonate.

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