JP7269850B2 - Recycling method of nitric acid aqueous solution, electrolytic processing method using the same, recycling liquid of nitric acid aqueous solution, and recycling system - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for recycling an aqueous nitric acid solution, an electrolytic processing method using the method, a recycling liquid for an aqueous nitric acid solution, and a recycling system.

Various developments have been made so far on methods for treating waste liquids containing nitrate ions. As this type of technology, for example, the technology described in Patent Document 1 is known.

In Patent Document 1, by adding ferrous sulfate, hexavalent chromium is reduced to trivalent chromium with low toxicity, and by adding calcium hydroxide, trivalent chromium is precipitated as a hydroxide, and as sludge, the front A pretreatment of sedimentation to the bottom of the treatment tank is described (paragraph 0027). While the sludge deposited in the pretreatment is recovered, the liquid separated from the sludge is supplied to the nitric acid separation equipment, and nitrate ions, sulfate ions and monovalent cations remain in the liquid. (Paragraph 0028).

JP 2016-172219 A

However, as a result of investigation by the present inventors, it was found that there is room for improvement in terms of production stability in the method for recycling a waste liquid containing nitrate ions described in Patent Document 1 above.

A common pretreatment method employs the ferrite method and uses ferrous sulfate (FeSO ₄ ).
Residual sulfate ions derived from ferrous sulfate must be removed from the recycle liquid because they may alter metal workability. When the ferrite method and the alkali precipitation method are used together, a pH adjuster such as calcium hydroxide is added. A pH adjuster can generally degrade metal workability.
As a result, the number of man-hours required for removing the remaining sulfate ions, the pH adjuster, or the resulting precipitates is greatly increased, resulting in a decrease in production efficiency.

In response to this, the inventors have investigated a new method that does not use the pretreatment method described above.
That is, a method of separating water and nitrate ions from a waste liquid containing metal ions such as chromium ions using a reverse osmosis membrane to obtain (recycle) an aqueous nitric acid solution was investigated.
However, it was found that if the concentration of Cr ⁶⁺ in the effluent is too high, the reverse osmosis membrane is destroyed.
As a result of further intensive research based on such knowledge, by reducing the concentration of Cr ⁶⁺ in the waste liquid to a predetermined value or less, even if a waste liquid containing metal ions such as chromium ions is used, the reverse osmosis membrane Since destruction is suppressed, the present inventors have found that the nitric acid aqueous solution can be stably recycled from the waste liquid, leading to the completion of the present invention.

According to the invention,
A recycling method for reusing a waste liquid containing nitrate ions, water, and chromium ions, comprising:
a reduction step of reducing Cr ⁶⁺ contained in the waste liquid to Cr ³⁺ so that the concentration of Cr ⁶⁺ in the waste liquid is 5.0 ppm or less;
after the reduction step, a separation step of permeating the waste liquid containing Cr ³⁺ through a reverse osmosis membrane to obtain a permeate liquid containing nitrate ions and water.

Also according to the present invention,
An electrolytic processing method for performing electrolytic processing using a recycled nitric acid aqueous solution,
An electrolytic machining method is provided, which includes a step of electrolytically machining a metal member using the permeated liquid obtained by the above recycling method as a working fluid.

Also according to the present invention,
A recycled aqueous nitric acid solution,
nitrate ions;
water and,
a metal ion containing one or more selected from the group consisting of Al, Co, Cr, and Ni,
Provided is a recycled aqueous nitric acid solution having a Cr ⁶⁺ concentration of 5.0 ppm or less in the recycled liquid.

Also according to the present invention,
A recycling system for reusing a waste liquid containing nitrate ions, water, and chromium ions, comprising:
a reduction treatment facility for reducing Cr ⁶⁺ contained in the waste liquid to Cr ³⁺ so that the concentration of Cr ⁶⁺ in the waste liquid is 5.0 ppm or less;
a nitric acid separation facility for permeating the waste liquid containing Cr ³⁺ obtained in the reduction treatment facility through a reverse osmosis membrane to obtain a permeated liquid containing nitrate ions and water;
A recycling system is provided, comprising:

ADVANTAGE OF THE INVENTION According to this invention, the recycling method of nitric acid aqueous solution excellent in manufacturing stability, the electrolytic processing method using the same, the recycling liquid of nitric acid aqueous solution, and a recycling system are provided.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the structural outline of the recycling system which concerns on this embodiment. FIG. 2( a ) is a diagram for explaining a test for membrane performance evaluation of the reverse osmosis membrane 40 , and FIG. 2( b ) is a diagram for explaining a membrane life evaluation test for the reverse osmosis membrane 40 .

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, in all the drawings, the same constituent elements are denoted by the same reference numerals, and the description thereof will be omitted as appropriate. Also, the drawings are schematic diagrams and do not correspond to actual dimensional ratios.

A method for recycling an aqueous nitric acid solution according to the present embodiment will be outlined.

Generally, waste liquids containing nitrate ions, water, chromium ions, and the like are generated in the process of various metal surface treatments. Examples of metal surface treatment include electrolytic processing, electrolytic polishing, chemical conversion treatment, pickling, etching, passivation treatment, and the like.

Among them, electrolytic processing will be described as an example.
Sodium nitrate is mainly used for electrochemical machining. Sodium nitrate provides good machining accuracy, but Cr ⁶⁺ may remain in the machining fluid and sludge when electrolytically machining metal materials containing chromium. Also, the strong electric field could oxidize Cr ³⁺ to Cr ⁶⁺ .

On the other hand, sludge of molten metal is generated during electrolytic machining and enters between the machining electrode and the workpiece, which may reduce the machining accuracy. On the other hand, by using an aqueous nitric acid solution with an appropriate concentration, it is possible to effectively reduce the amount of metal hydroxide and the like generated and improve the processing accuracy.
When electrolytic processing is performed using an aqueous nitric acid solution, the workability fluctuates as the amount of eluted metal increases. For this reason, it has been common practice to dispose of the aqueous nitric acid solution after it has been used within a certain range.

As described above, nitrate ions, water, and chromium ions are contained in waste liquid generated in metal surface treatment such as electrolytic processing.

Therefore, in view of the above circumstances, the inventors of the present application have studied recycling of the nitric acid aqueous solution.
That is, the recycling method of this embodiment is a nitric acid recycling method for reusing a waste liquid containing nitrate ions, water, and chromium ions.

The recycling method includes a reduction ^step of reducing Cr ⁶⁺ contained in the waste liquid to Cr 3+ so that the concentration of Cr ⁶⁺ in the waste liquid is 5.0 ppm or less, and after the reduction step, the waste liquid containing Cr ³⁺ is recycled. and a separation step of permeating through a reverse osmosis membrane to obtain a permeate containing nitrate ions and water.

Based on the findings of the present inventors, a technical relationship between chromium ions and deterioration of reverse osmosis membranes has been found. That is, although the detailed mechanism is not clear, Cr ⁶⁺ affects the deterioration of the reverse osmosis membrane, but Cr ³⁺ does not affect the deterioration of the reverse osmosis membrane as compared to Cr ⁶⁺ .

Although the detailed mechanism is not clear, metal Cr contained in the processed metal dissolves as Cr ⁶⁺ during electrolytic processing. When this Cr ⁶⁺ is passed through the reverse osmosis membrane, it destroys the polyamide membrane forming part of the reverse osmosis membrane and is reduced to Cr ³⁺ . As a result, it is considered that the polyamide membrane is destroyed when a waste liquid containing chromium ions is passed through, resulting in the loss of the ability of the reverse osmosis membrane to block metal ions.

Therefore, it has been found that suppressing the generation of Cr ⁶⁺ or reducing Cr ⁶⁺ to Cr ³⁺ suppresses destruction of the polyamide membrane even when a waste liquid containing chromium ions is passed through.

Therefore, if Cr ⁶⁺ is reduced to Cr ³⁺ and the concentration of Cr ⁶⁺ in the waste liquid is appropriately controlled, even if the waste liquid containing chromium ions is used for a reverse osmosis membrane, its deterioration (reduction of membrane life and membrane characteristics) It was found that the production stability of nitric acid recycling can be improved by suppressing variations in

According to this embodiment, it is possible to realize a recycling method that stably obtains a recycled nitric acid aqueous solution.
Moreover, the recycled nitric acid aqueous solution obtained by the recycling method can be reused for metal surface treatment such as electrolytic processing.

The configuration of the recycling method of this embodiment will be described in detail below with reference to FIG.
FIG. 1 is a diagram showing a schematic configuration of a recycling system 100. As shown in FIG.

The recycling system 100 is a recycling system for reusing waste liquid containing nitrate ions, water, and chromium ions. The recycling system 100 includes a reduction treatment facility 110 that reduces Cr ⁶⁺ contained in the waste liquid to Cr ³⁺ so that the concentration of Cr ⁶⁺ in the waste liquid is 5.0 ppm or less, and the Cr obtained in the reduction treatment facility 120. a nitrate separation facility 120 for permeating a waste liquid containing ³⁺ through a reverse osmosis membrane 40 to obtain a permeate liquid containing nitrate ions and water.

A waste liquid tank 20 in FIG. 1 temporarily stores waste liquid generated in metal surface treatment such as electrolytic processing.
Based on a predetermined amount or predetermined timing, the waste liquid in the waste liquid tank 20 is supplied to the buffer tank 22 through the pipe 80a.

The waste liquid in the waste liquid tank 20 contains nitrate ions, water, and chromium ions. In addition, metal components that are soluble in the aqueous nitric acid solution and components that are insoluble therein may be included.
Metal components include metal ions such as nickel, iron, aluminum, and copper.
Examples of insoluble components include metals including tungsten and tantalum, and metal compounds (metal hydrates).
These may be used alone or in combination of two or more.

The recycling method may include a filtration step of filtering the waste liquid using a filter before the separation step.

The waste liquid from the waste liquid tank 20 of FIG. Filtration can remove insoluble components in the waste liquid that are insoluble in the aqueous nitric acid solution. For filtration, for example, bag filters, pleated filters, UF filtration and the like can be used.
The opening size of the filter can be selected as appropriate, and may be, for example, about 0.1 μm to 1 μm.
Filters used for filtration may be washed as necessary.

In the recycling method, Cr ⁶⁺ contained in the waste liquid is reduced to Cr ³⁺ so that the concentration of Cr ⁶⁺ is 5.0 ppm or less (reduction step). For the reduction step, a known reduction method may be employed, and for example, a step of contacting the waste liquid with a Cr ⁶⁺ reducing agent may be used.

As a contact method, for example, a powdery reducing agent may be added to the waste liquid, or the waste liquid may be passed through a filter medium containing the reducing agent. The addition of the reducing agent and the passing of the liquid through the reducing agent may be performed once or multiple times. Among these methods, the method of adding a reducing agent may be used from the viewpoint of production efficiency.

The reducing agent is not particularly limited as long as it is a substance having a property of reducing Cr ⁶⁺ , but preferably iron or reduced iron may be included.
Examples of the powdery reducing agent include elemental iron powders such as iron powder, reduced iron powder, Daraiko iron powder (made from scrap iron), and atomized iron powder. Among these, iron powder and reduced iron powder may be used.

Here, the reduction mechanism of Cr ⁶⁺ in the waste liquid using elemental iron powder is considered as follows.
Cr ⁶⁺ +3e ⁻ →Cr ³⁺
Fe ²⁺ →Fe ³⁺ +e ⁻
At this time, the waste liquid containing nitrate ions has a pH of about 1. Therefore, since the oxidized iron element powder dissolves in the waste liquid, it is not necessary to remove it with a filter or the like afterwards. In addition, it becomes possible to adopt a process in which coprecipitation of trivalent chromium and iron hydroxide does not occur. Manufacturing efficiency can also be improved in that filtering is not required. In addition, pH adjustment of the waste liquid becomes unnecessary.

Generally, sulfates such as ferrous sulfate and sodium bisulfite are sometimes used as Cr ⁶⁺ reducing agents.
When sulfate is used, sulfate ions remain in the waste liquid. Sulfate ions can alter metal workability. On the other hand, the use of sulfates such as sodium bisulfite requires large fluctuations in pH. As a result, the number of man-hours in the recycling process increases in terms of removing sulfate ions and adjusting the pH, resulting in a decrease in production efficiency. As a result, it is difficult to obtain a cost advantage.
In addition, when the ferrite method (using ferrous sulfate) and the alkali precipitation method are used in combination, if calcium hydroxide is used as a pH adjuster, Cr ³⁺ becomes hydroxide (Cr(OH) ₃ ) in the waste liquid. precipitate. Residual pH modifiers can generally degrade metal workability. Therefore, it becomes necessary to remove the pH adjuster and remove the precipitate, which reduces production efficiency.

On the other hand, the production efficiency can be improved by using iron element powder. Further, the recycling method of the present embodiment may be configured so as not to include the step of converting Cr ³⁺ into hydroxide and precipitating it in the waste liquid. As a result, a significant cost advantage can be secured.

The lower limit of the amount of iron element powder to be added is, for example, 2 times or more, preferably 2.5 times or more, and more preferably 3 times or more in terms of mass with respect to the mass of chromium element contained in the waste liquid. Thereby, the concentration of Cr ⁶⁺ in the waste liquid can be reduced within the desired range. On the other hand, the upper limit of the amount of iron element powder to be added is, for example, 5 times or less, preferably 4.5 times or less, more preferably 4 times or less in terms of mass with respect to the mass of chromium element contained in the waste liquid. be. As a result, generation of heat and NO ₂ can be suppressed in the waste liquid to which the iron element powder is added. Therefore, manufacturing stability can be enhanced.

The reduction treatment equipment 110 in FIG. 1 reduces the waste liquid in the buffer tank 22 .
The buffer tank 22 may include, for example, a mechanism for adding a reducing agent and a filtering medium containing the reducing agent.
The reduced waste liquid in the buffer tank 22 is supplied to the concentrated liquid tank 30 through the pipe 80b.

In the recycling method, after the reduction step, the waste liquid containing Cr ³⁺ is permeated through the reverse osmosis membrane 40 to obtain a permeated liquid containing nitrate ions and water (separation step).

Nitrate separation facility 120 of FIG. 1 comprises concentrate tank 30 , reverse osmosis membrane 40 , and permeate tank 60 .

An example of the separation process using the reverse osmosis membrane 40 is as follows.
Waste liquid is supplied from the concentrated liquid tank 30 to the reverse osmosis membrane 40 via the pipe 80c.
The reverse osmosis membrane 40 treats waste liquid containing nitrate ions, water, chromium ions, and the like. Using a pump 82 (high pressure pump), for example, under a pressure condition of about 0.5 MPa to 4.0 MPa, more preferably about 2.5 MPa to 3.0 MPa, the waste liquid may be pressure-fed to the reverse osmosis membrane 40. good.
In the reverse osmosis membrane 40, the waste liquid is separated into a permeate containing nitrate ions and water, and a concentrate containing chromium ions and other metal ions.
The permeated liquid is supplied to the permeated liquid tank 60 via the pipe 80d.

As described above, a permeated liquid containing nitrate ions and water, that is, an aqueous nitric acid solution can be obtained by the reverse osmosis membrane 40 .

The upper limit of the concentration of Cr ⁶⁺ in the waste liquid supplied to the concentrated liquid tank 30 is 5.0 ppm or less, preferably 1.0 ppm or less, more preferably 0.3 ppm or less, and even more preferably 0.01 ppm or less. As a result, destruction of the reverse osmosis membrane 40 can be suppressed, so that manufacturing stability can be enhanced. On the other hand, the lower limit of the concentration of Cr ⁶⁺ in the waste liquid is not particularly limited, but may be 0 ppm (below the detection limit).

In the present embodiment, the concentration of Cr ⁶⁺ in the waste liquid can be controlled by appropriately selecting the type and amount of each component contained in the waste liquid, the reduction treatment of the waste liquid, and the like. Among these factors, the use of a reducing agent such as elemental iron powder and the appropriate adjustment of the amount of elemental iron powder added are factors for making the concentration of Cr ⁶⁺ in the waste liquid within a desired numerical range. mentioned.

The recycling method includes a Cr concentration ^measurement step of measuring the concentration of Cr ⁶⁺ in the waste liquid after the reduction step and before the separation step. It may include a first notification step of performing. As a result, it is possible to suppress the waste liquid containing the Cr ⁶⁺ concentration higher than the reference value from being supplied to the reverse osmosis membrane 40 . When the notification is detected, part or all of the recycling system 100 may be stopped.

The reduction treatment facility 110 of FIG. 1 may have a sensor 50 . The sensor 50 measures the concentration of Cr ⁶⁺ in the waste liquid supplied from the buffer tank 22 to the concentrate tank 30 via the pipe 80b.

As the sensor 50, an oxidation-reduction potential (ORP) meter, a Cr concentration sensor for measuring the concentration of hexavalent chromium by absorptiometry, or the like is used. These may be used alone or in combination of two or more. By using more than one, stable concentration control becomes possible.

In the recycling method, the liquid temperature of the waste liquid may be controlled so that the liquid temperature is 20° C. or higher and 35° C. or lower during the separation step. Manufacturing stability can be improved by controlling the temperature of the waste liquid contacting the reverse osmosis membrane 40 within an appropriate range.
Specifically, the lower limit of the liquid temperature of the waste liquid supplied to the reverse osmosis membrane 40 is, for example, 20° C. or higher, preferably 25° C. or higher, and more preferably 28° C. or higher. As a result, it is possible to suppress a decrease in the amount of permeate separated by the reverse osmosis membrane 40 . This enables stable recycling. On the other hand, the upper limit of the liquid temperature of the waste liquid is, for example, 35° C. or lower, preferably 33° C. or lower, and more preferably 30° C. or lower. As a result, deterioration of the metal blocking ability of the reverse osmosis membrane 40 can be suppressed. Therefore, an increase in the replacement frequency of the reverse osmosis membrane 40 can be suppressed.
Although the detailed mechanism is not clear, it is thought that the aperture of the reverse osmosis membrane 40 fluctuates when the liquid temperature is too high.

In FIG. 1, the liquid temperature of the waste liquid supplied to the reverse osmosis membrane 40 may be adjusted by an arbitrarily placed temperature adjusting mechanism such as a heater. As an example, the buffer tank 22 may be equipped with a heater. This makes it possible to stably control the liquid temperature of the waste liquid in the concentrated liquid tank 30 .

As the reverse osmosis membrane 40 in FIG. 1, a known one can be used.
The pore size (opening) of the reverse osmosis membrane 40 can be appropriately selected according to impurities such as metal ions, but may be about 0.1 nm.

As the filtration membrane, for example, a polyamide membrane such as an aromatic polyamide membrane, a cellulose acetate membrane, a polysulfone membrane, or the like can be used. Among these, an aromatic polyamide film is suitable. Cellulose acetate membranes and polysulfone membranes may not sufficiently block metal ions and permeate them.

The recycling method may include a pretreatment step of washing the reverse osmosis membrane 40 and immersing it in an aqueous nitric acid solution containing no metal ions before the separation step. As a pretreatment, SBS (sodium bisulfite) or the like, which is a membrane preservation solution for the reverse osmosis membrane 40, is washed with ion-exchanged water or the like, and then treated with a nitric acid solution containing no metal ions and having a concentration of about 17% for 1 to 24 hours. You may perform the process which carries out the degree circulation. As a result, even if the novel reverse osmosis membrane 40 is used, the permeated liquid can be obtained relatively stably.

The recycling method may include a step of washing the reverse osmosis membrane 40 with an alkaline solution after the separation step. That is, by washing the reverse osmosis membrane 40 after use, it is possible to stably obtain the permeated liquid.
As a cleaning method, for example, sodium hydroxide having a pH of about 11 is passed through the reverse osmosis membrane 40 to remove components adhering to or clogging the membrane. After an appropriate period of use, washing may be carried out for several minutes to a day or so, depending on the degree of contamination.

The reverse osmosis membrane 40 may be used in a cross-flow system or a dead-end system. From the viewpoint of production efficiency, the cross-flow method is preferred. In the cross-flow method, the waste liquid continues to flow along the surface of the filtration membrane at a certain flow rate. This separates the filtrate and the concentrate.

In the separation process, the reverse osmosis membrane 40 is used to obtain a concentrated liquid that has not passed through the reverse osmosis membrane 40 in addition to the permeated liquid, and the obtained concentrated liquid is returned to the reverse osmosis membrane 40. and permeating.

In FIG. 1, the concentrated liquid separated by the reverse osmosis membrane 40 is collected in the concentrated liquid tank 30 through the pipe 80e. The concentrated liquid supplied from the pipe 80e is mixed with the waste liquid supplied from the pipe 80b in the concentrated liquid tank 30, and is passed through the reverse osmosis membrane 40 again. Thereby, the recovery efficiency of nitric acid in the waste liquid can be enhanced.

Part of the concentrated liquid repeatedly concentrated by the reverse osmosis membrane 40 may be discarded in the waste tank 32 .

The permeated liquid separated by the reverse osmosis membrane 40 is collected in the permeated liquid tank 60 via the pipe 80d.

After the separation step, the recycling method includes a metal ion concentration measuring step of measuring the concentration of metal ions in the permeated liquid. A notification step may be included.

Nitrate separation facility 120 of FIG. 1 may include sensor 52 . The sensor 52 measures the concentration of metal ions in the permeated liquid supplied from the reverse osmosis membrane 40 to the permeated liquid tank 60 via the pipe 80d.

As the sensor 52, a metal concentration sensor or the like that measures the metal concentration by absorptiometry is used.
A nickel concentration sensor may be used as an example of a metal concentration sensor. By controlling the metal concentration such as the nickel concentration, it becomes possible to control the concentration of metal ions in the permeated liquid and to detect when the reverse osmosis membrane 40 should be replaced.

If a metal ion concentration equal to or higher than a set value is detected, part or all of the recycling system 100 may be stopped. The reverse osmosis membrane 40 is cleaned or replaced as necessary.

The permeated liquid recovered in the permeated liquid tank 60 becomes a nitric acid aqueous solution containing nitrate ions and water.
The aqueous nitric acid solution may not contain metal ions such as chromium ions, but may contain a small amount of one or more metal ions. The total concentration of metal ions in the permeated liquid is sufficiently lower than the total concentration of metal ions in the waste liquid in the waste liquid tank 20 .

Examples of the metal ions in the aqueous nitric acid solution recovered as the permeated liquid include metal ion species contained in the waste liquid tank 20, such as chromium ions (Cr ³⁺ ), nickel ions, aluminum ions, cobalt ions, and the like. . These may be used alone or in combination of two or more.

The upper limit of the total concentration of metal ions in the aqueous nitric acid solution as the permeate is, for example, 10 mg/l or less, preferably 5 mg/l or less, more preferably 3 mg/l or less. Among these, the upper limit of the concentration of at least one metal ion, for example Cr ³⁺ , in the aqueous nitric acid solution is, for example, 3 mg/l or less, preferably 1 mg/l or less, more preferably 0.5 mg/l or less. be. On the other hand, the lower limit of the total metal ion concentration and the lower limit of the Cr ³⁺ ion concentration may be, for example, 0 mg/l or more, or 0.01 mg/l or more.

By setting the upper limit of the total concentration of metal ions in the nitric acid aqueous solution to the above upper limit or less, no additional process is required when using the recycled nitric acid aqueous solution as a chemical solution for metal surface treatment such as electrolytic processing. Become. That is, it is not necessary to analyze and adjust the concentration of metal ions by ICP or the like, or to measure and adjust the concentration of nitric acid by neutralization titration or the like. This eliminates the need for expensive analytical equipment and simplifies the recycling process, resulting in cost advantages.

Here, an experiment conducted by the inventor will be described with reference to FIG.
FIG. 2( a ) is a diagram for explaining a test for membrane performance evaluation of the reverse osmosis membrane 40 , and FIG. 2( b ) is a diagram for explaining a membrane life evaluation test for the reverse osmosis membrane 40 .

Tests for membrane performance evaluation showed the following results.
First, a waste liquid (aqueous solution containing a plurality of kinds of metal ions and nitrate ions) before filtration was put into the concentrated liquid tank 30 shown in FIG. 2(a).
The metal ion concentrations of the waste liquid before filtration were chromium ions: 58.00 mg/l, nickel ions: 205.00 mg/l, aluminum ions: 13.90 mg/l, and cobalt ions: 58.80 mg/l. . Metal ion concentrations were measured using an ICP emission spectrometer. In addition, all chromium ions were Cr ³⁺ when measured by absorptiometric method to measure hexavalent chromium.
Subsequently, using the cross-flow method, the waste liquid before filtration was pressure-fed from the concentrated liquid tank 30 to the reverse osmosis membrane 40 (aromatic polyamide membrane, opening size: 0.1 nm) through the pipe 80c. The permeated liquid was recovered in the permeated liquid tank 60 through the pipe 80d, and the concentrated liquid was recovered in the concentrated liquid tank 30 through the pipe 80e. The collected concentrate was supplied to the reverse osmosis membrane 40 again. This evaluation test was conducted for 50 hours (1000 L processing).
After that, the metal ion concentrations of the concentrated liquid in the concentrated liquid tank 30 after 50 hours were chromium ions: 157.00 mg/l, nickel ions: 598.00 mg/l, aluminum ions: 27.30 mg/l, cobalt Ions: 125.00 mg/l.
On the other hand, the metal ion concentrations of the permeated liquid in the permeated liquid tank 60 after 50 hours were chromium ions (Cr ³⁺ ): 0.27 mg/l, nickel ions: 1.55 mg/l, and aluminum ions: 0.96 mg. /l, cobalt ions: 0.27 mg/l.
From the above membrane performance evaluation test, it was found that an aqueous nitric acid solution from which metal ions were sufficiently removed was obtained as the permeated liquid.

Tests for membrane life evaluation showed the following results.
First, aqueous solution samples 1 to 3 containing chromium ions and nitrate ions were put into the concentrated liquid tank 30 of FIG. 2(b) as waste liquid before filtration. The Cr ⁶⁺ concentrations in aqueous samples 1-3 were 0 ppm, 0.4 ppm, and 6.0 ppm, respectively. The Cr ⁶⁺ concentration was adjusted by adding iron powder and adjusting the amount of iron powder added. Cr ⁶⁺ concentration was measured using diphenylcarbazide absorptiometry.
Subsequently, using a cross-flow method, the aqueous solution sample in the concentrated liquid tank 30 is pressure-fed to the reverse osmosis membrane 40 (aromatic polyamide membrane, opening size: 0.1 nm) through the pipe 80c, and the concentrated liquid is was recovered in the concentrated liquid tank 30 through the pipe 80e and the permeated liquid through the pipe 80f. The collected concentrate and permeate were supplied to the reverse osmosis membrane 40 again. This evaluation test was conducted for three months.
In the membrane life evaluation test, the case where the membrane could be used without problems for 3 months was evaluated as ⊚, the case where the membrane could be used for 1 week to 1 month was evaluated as ◯, and the case where the reverse osmosis membrane 40 was destroyed immediately after the start of the test was evaluated as ×. .
As a result, Aqueous solution sample 1 of Example 1 was evaluated as ⊚, Aqueous solution sample 2 of Example 2 was evaluated as O, and Aqueous solution sample 3 of Comparative example 1 was evaluated as ×.
From the above, for an aqueous solution sample (waste liquid) containing nitrate ions, water, and chromium ions, as in Examples 1 and 2, Cr ⁶⁺ was reduced to Cr ³⁺ so that the concentration of Cr ⁶⁺ was 5.0 ppm or less. It was found that compared with Comparative Example 1, an aqueous nitric acid solution can be stably obtained by permeating the aqueous solution sample through the reverse osmosis membrane.

As described above, the recycled nitric acid aqueous solution composed of the permeated liquid can be suitably used as a chemical liquid (working liquid) used for various metal surface treatments.

For example, the recycled nitric acid aqueous solution can be reused as a working fluid in electrolytic machining.

The electrolytic machining method using a recycled nitric acid aqueous solution includes a step of electrolytically machining a metal member using the permeated liquid (nitric acid aqueous solution) obtained by the above recycling method as a machining fluid.

The nitric acid concentration of the recycled nitric acid aqueous solution may be adjusted to a desired concentration based on the measurement results of the nitric acid concentration using specific gravity, neutralization titration, or the like. The desired nitric acid concentration can be adjusted as appropriate for metal surface processing, but when used in electrochemical machining fluids, the nitric acid concentration may be about 17%. As a result, it is possible to suppress the amount of metal hydroxide and the like generated, and to improve the processing accuracy.

In FIG. 1, the permeated liquid (recycled nitric acid aqueous solution) collected in the permeated liquid tank 60 is supplied to the buffer tank 62 via the pipe 80g.
The recycle liquid in the buffer tank 62 is supplied to the mixing tank 70 via the pipe 80h, the valve 72 and the pipe 80k.
In the mixing tank 70, the recycle liquid is adjusted to the desired concentration. For example, water such as city water may be supplied to the mixing tank 70 from the water supply 74 via the pipe 80j. Alternatively, nitric acid may be supplied from the new nitric acid solution tank 76 to the mixing tank 70 through the pipe 80j. The nitric acid in the new nitric acid tank 76 may be a commercially available product or a new solution before use.

The recycled liquid mixed with water, nitric acid, and other necessary components in the mixing tank 70 is supplied to the processing machine tank 12 through the pipe 80l. The concentration of recycle liquid in mixing tank 70 may be measured by sensor 54 . Depending on the concentration value of the sensor 54, water or nitric acid may be added to achieve the desired concentration.

The recycled liquid supplied into the processing machine tank 12 is supplied to the electrolytic processing machine 10 via the pipe 80m as a processing liquid for electrolytic processing.

The recycled nitric acid aqueous solution may contain nitrate ions, water, and metal ions including one or more selected from the group consisting of Al, Co, Cr, and Ni. The concentration of Cr ⁶⁺ in the recycled liquid is 5.0 ppm or less. As the concentration of Cr ⁶⁺ in the recycled liquid, the above-mentioned concentration of Cr ⁶⁺ in the waste liquid supplied to the concentrated liquid tank 30 can be used.

Further, the concentration of at least one of the metal ions in the recycled nitric acid aqueous solution may be, for example, 3.0 mg/l or less.

Processing stability can be improved by using a recycled liquid containing a small amount of metal ions as the working liquid, as compared with the case where the processing liquid contains nitrate ions and water but does not contain metal ions.
Although the detailed mechanism is not clear, when only nitrate ions and water are used, metal ions are generated after the start of electrochemical machining, and as a result, the workability fluctuates from the initial stage, resulting in variations in workability.

Further, the electrochemical machining method includes a step of obtaining a working fluid after electrochemical machining used in electrochemical machining, a step of mixing a permeated fluid (recycled fluid) with the post-electrolytic machining fluid to obtain a mixed fluid, and using the mixed fluid, A step of subjecting the metal member to electrochemical machining may be included.

In the electrolytic processing machine 10 of FIG. 1, the used working fluid used for electrolytic processing (post-electrolytic processing processing fluid) may be supplied to the processing machine tank 12 via the pipe 80n. At this time, in the processing machine tank 12, the recycled liquid supplied from the mixing tank 70 and the post-electrolytic processing processing liquid supplied from the electrolytic processing machine 10 are mixed. Then, the mixed liquid may be supplied again to the electrolytic processing machine 10 through the pipe 80m and used as the working liquid.
The mixing ratio of the recycled liquid and the post-electrolytic processing liquid in the mixed liquid can be adjusted as appropriate. . As a result, an increase in the concentration of metal ions in the working fluid can be suppressed, so that working stability can be maintained.

The machining fluid that has been used repeatedly in this manner is supplied as a waste fluid from the processing machine tank 12 to the waste fluid tank 20 through the pipe 80o when the amount of dissolved metal ions or metals falls within a predetermined range.
This waste liquid can be reused by the recycling system 100 again.

Although the embodiments of the present invention have been described above, these are examples of the present invention, and various configurations other than those described above can be adopted. Moreover, the present invention is not limited to the above-described embodiments, and includes modifications, improvements, etc. within the scope of achieving the object of the present invention.

10 Electrolytic processing machine 12 Processing machine tank 20 Waste liquid tank 22 Buffer tank 30 Concentrated liquid tank 32 Waste tank 40 Reverse osmosis membrane 50 Sensor 52 Sensor 54 Sensor 60 Permeated liquid tank 62 Buffer tank 70 Mixing tank 72 Valve 74 Water supply 76 New nitric acid liquid tank 80a, 80b, 80c, 80d, 80e, 80f, 80g, 80h, 80i, 80j, 80k, 80l, 80m, 80n, 80o Piping 82 Pump 100 Recycling system 110 Reduction treatment facility 120 Nitrate separation facility

Claims (17)

A recycling method for reusing a waste liquid containing nitrate ions, water, and chromium ions, comprising:
a reduction step of reducing Cr ⁶⁺ contained in the waste liquid to Cr ³⁺ so that the concentration of Cr ⁶⁺ in the waste liquid is 5.0 ppm or less;
after the reduction step, a separation step of permeating the waste liquid containing Cr ³⁺ through a reverse osmosis membrane to obtain a permeated liquid containing nitrate ions and water. The recycling method according to claim 1,
The recycling method, wherein the reducing step comprises contacting with a Cr ⁶⁺ reducing agent. The recycling method according to claim 2,
The recycling method, wherein the reducing agent contains elemental iron powder. The recycling method according to claim 3,
The recycling method, wherein the amount of the iron element powder added is two to five times the mass of the chromium element contained in the waste liquid in terms of mass. The recycling method according to any one of claims 1 to 4,
The recycling method, wherein the liquid temperature in the waste liquid is set to 20° C. or higher and 35° C. or lower during the separation step. The recycling method according to any one of claims 1 to 5,
After the reduction step and before the separation step, a Cr concentration measurement step of measuring the concentration of Cr ⁶⁺ in the waste liquid,
A recycling method, comprising a first reporting step of reporting when the concentration of Cr ⁶⁺ is equal to or higher than a predetermined value in the Cr concentration measuring step. The recycling method according to any one of claims 1 to 6,
A recycling method, comprising a step of pretreating the reverse osmosis membrane by washing it and immersing it in an aqueous nitric acid solution containing no metal ions before the separation step. The recycling method according to any one of claims 1 to 7,
A recycling method including a step of washing the reverse osmosis membrane with an alkaline solution after the separation step. The recycling method according to any one of claims 1 to 8,
In the separation step,
a step of using the reverse osmosis membrane to obtain, in addition to the permeated liquid, a concentrated liquid that has not passed through the reverse osmosis membrane;
and a step of permeating the obtained concentrate through the reverse osmosis membrane.
Recycling method. The recycling method according to any one of claims 1 to 9,
A recycling method, comprising a filtration step of filtering the waste liquid using a filter before the separation step. The recycling method according to any one of claims 1 to 10,
After the separation step, the permeated liquid includes a metal ion concentration measurement step of measuring the concentration of metal ions,
A recycling method, comprising a second notification step of notifying when the concentration of metal ions is equal to or higher than a predetermined value in the metal ion concentration measurement step. The recycling method according to any one of claims 1 to 11,
A recycling method that does not include a step of converting Cr ³⁺ into a hydroxide and precipitating it in the waste liquid. An electrolytic processing method for performing electrolytic processing using a recycled nitric acid aqueous solution,
An electrolytic machining method, comprising a step of electrolytically machining a metal member using a permeated liquid obtained by the recycling method according to any one of claims 1 to 12 as a working liquid. The electrolytic processing method according to claim 13,
a step of obtaining the post-electrolytic machining fluid used in the electrolytic machining;
a step of mixing the permeated liquid with the working liquid after electrochemical machining to obtain a mixed liquid;
An electrolytic machining method, comprising the step of electrolytically machining a metal member using the mixed solution. A recycled aqueous nitric acid solution,
nitrate ions;
water and,
a metal ion containing one or more selected from the group consisting of Al, Co, Cr, and Ni,
A recycled solution of an aqueous nitric acid solution, wherein the concentration of Cr ⁶⁺ in the recycled solution is 5.0 ppm or less. The recycled nitric acid aqueous solution according to claim 15,
A recycled nitric acid aqueous solution, wherein the concentration of at least one of the metal ions is 3.0 mg/l or less. A recycling system for reusing a waste liquid containing nitrate ions, water, and chromium ions, comprising:
a reduction treatment facility for reducing Cr ⁶⁺ contained in the waste liquid to Cr ³⁺ so that the concentration of Cr ⁶⁺ in the waste liquid is 5.0 ppm or less;
a nitric acid separation facility for permeating the waste liquid containing Cr ³⁺ obtained in the reduction treatment facility through a reverse osmosis membrane to obtain a permeated liquid containing nitrate ions and water;
with a recycling system.

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