Adding nitric acid to the potassium chloride plating solution by mistake, this company’s rescue measures are worth learning!

01

Cause of failure

The picker in the workshop took the nitric acid by mistake to the place where the potassium chloride hangs the galvanizing line to adjust the pH value. When the team process manager used hydrochloric acid to adjust the pH value, he did not look at the label carefully and mistakenly added nitric acid as hydrochloric acid to the tank.

Because it is a large tank of 8000L, there is no obvious failure after adding nitric acid, but a small amount of black shadows appear on the plated parts, which did not attract attention at that time. We operate in a three-shift system. The day shift personnel did not notice, nor did the night shift personnel notice. They still added nitric acid as hydrochloric acid to the tank again.

After adding nitric acid twice, the black shadow of the plated parts becomes more serious. The technologist thought it was caused by the high pH and added it again. After this addition, the amount of nitric acid became very large, resulting in pitch black surface and rough and loose coating.

This kind of malfunction has never happened before, and it was then discovered that nitric acid was mistakenly used as hydrochloric acid. After calculation, the amount of nitric acid added reached 2g/L.

02

Fault analysis and treatment

After finding the reason, what should I do?

One method is to change the anode to stainless steel, and the current density is 3～4A/dm.

Electrolysis treatment is carried out under the condition of bath temperature 5O℃, so that nitrate radicals decompose and escape on the anode; then replace the zinc anode, and then carry out electrolysis treatment at a current density of 1 to 2A/dm.

Another method is to use a low current density of 0.05～0.10A/dm for long-term electrolysis when the mass concentration of nitrate ions is not high.

If there is too much nitrate, only change the plating solution. According to this, the time will be very long, and the workshop production task is urgent, so this is not the best way.

Then it is to rebalance the plating solution, but the company will lose a lot. So is there a better way? It seems that you have to experiment and find a new way in the experiment.

First of all, we take the fault liquid to the laboratory for Hull cell test. On the test piece, 2/3 of the high-end area is black, but about 1/4 of the low-area area can still see a little more normal color. We have done the following treatments respectively to the plating solution.

(1) Use a beaker to take 2000 mL of the faulty liquid, heat it to boiling, and keep it for 10 minutes.

See if some nitric acid can be evaporated at high temperature. After cooling to room temperature, make a Hull cell test piece. Upon observation, there is no effect.

(2) Take another 2000mL fault solution, add 5g/L powdered N-type activated carbon at room temperature, and stir for 10min.

Make the Hull cell test piece. After observation, the blackness of the high current area becomes slightly lighter.

(3) Take another 2000mL fault solution, add H2O2 2mL/L, stir for 20min and filter.

Make the Hull cell test piece. After observation, there is no effect.

(4) Take another 2000mL of the fault solution, add H2O2mL/L at room temperature, stir for 10mini and heat to over 95℃, keep for 10rain; at 95℃, add 5g/L of powdered N-type activated carbon and stir for 10min; while Add analytically pure NaOH 4g/L (after dissolving in water) and stirring.

After a short pause, the activated carbon and flocs suspended in the beaker quickly sink, the upper plating solution is clear, and the bottom sediment is dense. Pour out the supernatant liquid and let it cool. Make a test piece for the Hull trough.

At this time, the black shadows on the test piece are almost gone.

I used this method three times in a row, all with the same result, indicating that the method is effective.

According to the above test results, the treatment process of the large tank is worked out as follows:

(1) Test the plating solution of each plating tank, and make up the plating solution with insufficient composition, then add H2O2 2mL/L (oxidized Fe2+) at room temperature, and stir for 30min;

(2) Heat the plating solution to above 95℃ and keep it for 30rain;

(3) When the temperature drops to about 90℃, add 5g/L of N-type powdered activated carbon and stir for 30min;

(4) Add analytically pure NaOH 4g/L while it is hot (NaOH is pre-dissolved in a small bucket), while stirring;

(5) Let stand for 2h;

(6) In order to shorten the processing cycle, the titanium tube in the special tank circulates cold water to force cooling;

(7) When the temperature of the plating solution is about 40℃, filter the plating solution back to the plating tank with a filter;

(8) Adjust the pH value of the returned plating solution to about 5.8 with hydrochloric acid, and add a brightener, electrolyze it at a low current density of 0.1A/din for 6h, and then try plating.

Using the above process, it takes about 18h to process 8000L of plating solution. The plating solution of 11 tanks is processed in about a week. In about 10 days, the entire workshop resumed production.

03

Conclusion

The above treatment method can achieve the expected treatment effect. The author believes that the following two points have played a key role.

(1) The plating solution is heated to reach the boiling point. In addition, the powdered activated carbon has a large specific surface area of ​​micropores, and its ability to adsorb various impurities is strong, and it can also adsorb nitrogen oxides in the plating solution.

(2) Since hydrogen peroxide can oxidize divalent iron, the last addition of NaOH makes the oxidized trivalent iron become ferric hydroxide and quickly precipitate.

The iron hydroxide floc also has an excellent adsorption effect. At the same time, the suspended activated carbon and zinc hydroxide colloid are co-precipitated, so that the plating solution can be quickly clarified. This provides great convenience for subsequent filtering of the plating solution.

The mistake of adding drugs this time is a very profound lesson. How to avoid the occurrence of similar accidents is a problem that we need to learn by analogy and improve.