JPS5839230B2 - Control device for phosphate chemical treatment - Google Patents

Description

[Detailed description of the invention] The present invention relates to a control device for metal phosphate chemical conversion treatment.

Conventional phosphate chemical treatment of metals generally consists of degreasing → water washing →
The process consists of washing with water → film formation → washing with water → washing with water.

At the film formation stage, chemicals are replenished according to the consumption of processing liquid components due to film formation and removal of the processing liquid, and the acid ratio, total acidity, zinc ion concentration, etc. of the processing liquid are controlled to be constant. There is.

'i! In addition, a sodium nitrite aqueous solution is usually supplied as a film formation reaction accelerator to control the nitrite ion concentration in the treatment liquid to be constant.

However, with the above management method, sodium ions that are unnecessary for film formation are added to the treatment solution, and nitrite ions in the treatment solution are oxidized and converted to nitrate ions. This will increase the nitrate ion concentration in the liquid.

By the way, in the phosphate chemical treatment line generally employed at present, as mentioned above, a part of the treatment liquid goes out to the next washing process in the form of taking out, so if you replenish it according to the consumption by taking out, Sodium ions and nitrate ions are not accumulated in the processing solution more than necessary, and the ion concentration balance of the processing solution composition is maintained.

However, if some of the treatment liquid is carried out and the amount going to the next washing process is reduced, and the composition of the replenishing chemicals does not match the conditions of the chemical conversion treatment line, some components may accumulate and increase. , the balance between consumption and supply of ions in the treatment liquid composition is disrupted.

For example, abnormal accumulation of sodium ions and nitrate ions can lead to formation defects such as yellow rust and scabbing.

In particular, it was devised to solve the problem of waste liquid treatment caused by discharging a large amount of washing water containing processing liquid generated in the washing process to the outside, and the problem of water resource protection caused by using a large amount of washing water. In the chemical conversion treatment apparatus disclosed in Japanese Unexamined Patent Publication No. 48-69728, the accumulation of unnecessary ions in the treatment liquid increases, making it necessary to renew the treatment liquid at an early stage.

To this end, a method has been proposed in which only nitrite ions are supplied to the treatment solution to solve the problems caused by using nitrite (e.g., the above-mentioned sodium salt) as a film formation reaction accelerator. -24535).

This method involves exchanging some of the nitrate ions in the treatment solution to nitrite ions using an anion exchange method (using an anion exchange resin), but it requires regeneration of the exchange resin, and There are other problems, such as complicated workability and disposal of sodium nitrate and other substances associated with the regeneration process.

An object of the present invention is to provide a control device that is capable of managing the nitrite ion concentration in a treatment liquid using a method other than the above-mentioned anion exchange method for replenishing nitrite ions.

The nitrite ion replenishment method adopted in the present invention is a method of reducing a portion of nitrate ions in the processing solution to nitrite ions by cathodic electrolytic reduction reaction disclosed in Japanese Patent Application No. 51-49023 previously filed. It is.

When nitric acid is reduced in the treatment solution (i.e. acidic phosphate film chemical conversion treatment solution) in this way, NO3+ 3 H++ 2 e #HNO2+ H20
The fact that the reduction stops at the nitrous acid stage, as shown in , is noteworthy given that many previous studies have focused on the production of hydroxyamines and ammonia through the reduction of nitric acid.

The present invention is based on a method of replenishing nitrite ions to a treatment liquid by such a cathodic electrolytic reduction reaction, and further develops this method to extract the nitrite ion concentration in the treatment liquid as an oxidation-reduction potential and perform treatment based on this. This invention relates to a control device that can manage the concentration of nitrite ions in a liquid.

That is, the control device of the present invention is characterized by: a means for measuring the nitrite ion concentration in an acidic phosphate film chemical conversion treatment solution containing nitrite ions as a film formation reaction accelerator; and a reference potential, and a means for increasing or decreasing the DC current density according to the control signal and supplying the DC current to both poles in the processing solution. The nitrate ions contained therein are reduced to nitrite ions, thereby maintaining the nitrite ion concentration at a predetermined value.

Next, the present invention will be specifically described with reference to the accompanying drawings.

Reference numeral 1 denotes a chemical conversion tank containing a treatment liquid, and the object to be treated is either immersed in this tank or sprayed with the tobacco treatment liquid to undergo phosphate conversion treatment.

An example of the acidic phosphate film chemical conversion treatment solution used here is an aqueous solution with a pH of about 3 and having the following composition.

Zinc ion 0.08 Sodium ion 0.30 tt Phosphate ion 1
．． 00 tt Nitrate ion 0.50
tt Nitrite ion 0.008 tt In the treatment liquid as described above, from the viewpoint of chemical conversion treatment, the nitrite ion concentration is preferably in the range of 0.004 to 0.02 weight φ.

When this nitrite ion concentration generally falls below a predetermined concentration within the above-mentioned range, it becomes subject to management by this device.

In this case, it is usually desirable that the nitrate ion concentration of the treatment liquid is 0.2 weight or more.

If the concentration is too low, the production efficiency of nitrite ions will decrease, which will be disadvantageous in terms of equipment.

Reference numeral 2 denotes an electrolytic cell, which is connected to the chemical conversion tank 1 through a circulation path by a pump 3 and a pipe 4, and which reduces part of the nitrate ions in the treatment liquid from the chemical conversion tank 1 to nitrite ions. is supplied to the chemical conversion tank 1.

As the cathode used in the electrolytic layer 2, metals with a high hydrogen overvoltage that generate little or substantially no hydrogen that adversely affects the cathode electrolytic reduction reaction, such as zinc, copper, etc.
Preferably, the material is made of lead, titanium, mercury, tin, or the like.

For the anode, use sparingly soluble or insoluble metals that are difficult to melt and consume during electrolysis, such as platinum, platinum-plated titanium, noble metal oxides (titanium or tantalum with a thin film of noble metal oxides such as ruthenium and iridium formed), and carbon. ,
Preferably, the material is made of lead dioxide, triiron tetroxide, stainless steel, or the like.

A twisted zinc electrode can be used as the anode.

The voltage applied to the electrodes is usually 3 to 20V.

Further, the current density is 0.01 to 30 A/drn'.

The relationship between this current density and the amount of nitrite ions produced by electrolysis is shown in FIG. 2.

The amount of nitrous acid produced on the vertical axis in this figure is the temperature difference of the acid in the treatment liquid at the inlet and outlet of the electrolytic cell.

That is, when the current density is zero, the amount of generation is also zero.

In the illustrated apparatus, an example is described in which the electrolytic layer 2 is provided, but a configuration may be adopted in which both the electrodes are immersed in the chemical conversion bath 1 without providing this.

5 is a means for measuring the nitrite ion concentration in the treatment liquid in the chemical conversion tank 1 as an oxidation-reduction potential, and the details are as follows.

In the chemical conversion treatment process, the object to be treated, which is an iron cylinder or steel, elutes ferrous ions into the treatment liquid.

This increase in the concentration of ferrous ions deteriorates the quality of the coating formed on the object to be treated, so that the ferrous ions are oxidized to ferric ions by nitrite ions.

Therefore, this ferrous ion is measured as a redox potential by a pair of electrodes consisting of a platinum electrode and a reference electrode.

The fact that the elution of ferrous ions is measured in response to the redox potential in this way corresponds to the consumption of nitrite ions.

Specifically, this method is disclosed in Japanese Patent Publication No. 10051/1983 entitled "Automatic Control Method for Addition of Nitrite in Acidic Phosphate Coating Bath Liquid".

Another measurement method is to continuously collect the treated solution through a separate line, add a certain amount of cerium salt solution to it, stir it, and continuously send the solution to a tube equipped with a platinum/comparison electrode to reduce the oxidation. An automatic titration device that measures the potential may be used, or an automatic titration device that periodically samples the treated solution, adds a few drops of dilute sulfuric acid, titrates it with potassium permanganate, and detects the end point of the titration using the redox potential. Good too.

Reference numeral 6 denotes means for amplifying the output signal from the means 5, which amplifies the input signal level required by the electronic proportional control fawn 7 (i.e., 4 to 20 mADC, 1 to 5 VDC, etc.).

The means 7 includes an amplifying means 9 capable of performing a comparison operation using the output signal of the means 6 and the output signal of the reference potential generation means 8 built in the means 7 as input signals, and extracting a control signal as an output, and further It has a proportional action function that allows you to change the adjustment sensitivity.

For example, Automatic Control Equipment Handbook (published by Ohmsha) No. 99~
It has the adjustment function disclosed on page 102, and the control signal level is 4-20 mADct or 1-5 VDC.

The means 8 built into the means 7 supplies the oxidation-reduction potential of the processing solution to form a uniform and good film, and is composed of a DC potentiometer circuit or a precision variable resistor for setting a reference voltage.

The output signal is set to a predetermined nitrite ion concentration.

If the concentration setting value is unknown, a processing liquid may be separately prepared and the means 5 may be immersed in the processing liquid and supplied.

The means 10 into which the control signal of the means 7 is introduced is a means for supplying direct current to the electrolytic cell 2 in order to generate nitrite ions necessary for the treatment liquid by electrolysis.

The current density of the supplied direct current is automatically increased or decreased by the current adjustment section 11 according to the control signal of the means 7, and is supplied to the electrolytic cell 2 within the above-mentioned range.

The operation of this means 11 can be explained, for example, in the booklet 43-6 "About parts using thyristors" published by Nippon Battery ■ (30
H) Those that perform the functions disclosed in the TAI version can be adopted.

A rectifier 12 is constructed of a group of semiconductor rectifying elements for receiving alternating current power having a commercial frequency (504 to 60 hertz), changing the voltage to the above-mentioned required voltage, and further obtaining direct current. Built-in.

1 When the object to be treated is immersed in the treatment liquid in the chemical conversion tank 1 or the treatment liquid is sprayed onto the object and recovered, and the nitrite ion concentration in the treatment liquid decreases, the means 5 detects this and The measurement signal is amplified by means 6 and transmitted to means 7.

At that time, when the means 7 is pressed, either a positive or negative deviation signal is generated between the reference potential signal and the output signal of the means 7 is also increased or decreased, that is, a control signal proportional to the measurement signal is generated. can get.

To explain the relationship between the control signal amount and current density,
As the control signal increases, the current density also increases.

That is, when the control signal level reaches 20 mADc,
In the means 10, the maximum current density (specifically, 8 A/dm2) in terms of nitrite ion production current efficiency is supplied to the electrolytic cell 2 to promote an increase in the amount of nitrite ion produced, and the chemical conversion tank 1
The concentration at is returned to a predetermined value.

Next, when the nitrite ion concentration in the chemical conversion tank 1 reaches a predetermined value, the control signal decreases, and the current density also decreases.

Therefore, the amount of nitrite ions produced to be applied to the electrolytic tank 2 is also reduced, and the system operates to prevent excessive supply to the chemical conversion tank 1.

On the other hand, when the chemical conversion treatment is stopped and the consumption of nitrite ions in the treatment liquid decreases, and the concentration thereof reaches a predetermined value or more, the control signal correspondingly decreases and the current density of the means 10 also becomes minimum, The amount of nitrite ions produced is also small.

By repeating such adjustment operations, the nitrite ion concentration in the treatment liquid is maintained approximately constant.

As is clear from the above explanation, according to the present invention, a means for measuring the nitrite ion concentration in the chemical conversion tank as a redox potential is provided, so that a difference occurs from the redox potential value corresponding to a predetermined concentration. By doing so, the current density supplied to the electrolytic cell can be set, and the nitrite ion concentration in the treatment liquid can be automatically maintained at a predetermined concentration (o, oos±0.0008 weight cup).

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of the apparatus of the present invention, and FIG. 2 is a graph showing the relationship between the amount of nitrite ion produced and current density, where 1 is a chemical conversion tank, 2 is an electrolytic tank, and 5 is a redox potential. 7 is a measuring means, 7 is an electronic proportional control means, 8 is a reference potential supply means, and 10 is a direct current adjustment supply means.

Claims (1)

[Claims] 1. Means for measuring the nitrite ion concentration in an acidic phosphate film chemical conversion treatment solution containing nitrite ions as a film formation reaction accelerator, as a redox potential, means for proportionally controlling a deviation signal between the measured potential and a reference potential; and a means for increasing or decreasing the DC current density according to a control signal and supplying the DC current to both poles in the processing liquid, reducing nitrate ions in the processing liquid to nitrite ions in proportion to the current density, thereby A control device for phosphate chemical treatment, characterized in that the nitrite ion concentration can be maintained at a predetermined value.