US2087230A - Method for regenerating phosphoric pickling bath - Google Patents

Description

Patented July 20, 1937 PATENT OFFICE METHOD FOR REGENERATING- PHOS- PHQRIC PICKLING BATH Jean Adrien Marie Bigeon, Colombes, France, assignor to La Compagnie Nationale de Matieres Colorantes et Manufactures de Produits Chimiques du Nord Reunies, Etablisscments Kuhlmann, Paris, France, a corporatio of France No Drawing.

3 Claims.

My invention has for its object the regeneration of the pickling baths constituted by phosphoric acid, generally .used for ferrous metals.

It is a well known fact that commercial phosphoric acid, used at the concentration of 10 to 12% of- P205 and at the temperature of about 80 0., constitutes for all ferrous metals such as cast iron, soft steel, shaped and rolled steel and generally speaking cast iron and steel of any description and obtained in any manner whatever, a pickling agent the action of which is much more regular than any other mineral acid such as hydrochloric or sulphuric acid.

This fact can be explained in several manners: either the phosphoric acid, by reason of its natural viscosity, does not enter as deeply as hydrochloric or sulphuric acids into the pores and intercrystalline interstices of the metals; or else.

the viscosity of the inert combinations of iron and phosphoric acid formed from the beginning of the pickling in the microscopic recesses of the metal opposes the subsequent penetration of free acid into these recesses or again'the combinations of iron and phosphoric acid, not saturated in iron, become bound throughtheir free acid valences to the metal wall, according to the theory of adsorption and form a protecting coat opposing any subsequent attack.

.Whatever'may be the mechanism of the pickling action of phosphoric acid on iron, the surfaces treated by this acid are perfectly smooth whereas the pickling by means of hydrochloric or sulphuric acids produces rough and dull surfaces.

This pickling by means of phosphoric acid in a first bath is generally followed by an inimersion in a second bath of acid iron phosphate which deposits on the metal a film forming a yielding and resistant support for the paints or varnish to be applied subsequently. This process ensures a longer life for the painting of iron'worksthan any other process of pickling, sanding included. On the other hand it is a known fact that such pickling leads to a very high consumption 01 phosphoric acid.

The phosphoric pickling bath, very active a the beginning of .its action, becomes soon sluggish, while still containing in the free state more than half of the Pros employed' A very small amount of iron in dissolution is suflicient for paralyzing the pickling action of the phosphoric acid, probably by reason of the rapid .and considerableincrease of the viscosity of the bath which increase of viscosity is due to Application March 5, 1936, Serial In France July 16, 1935 the presence of a compound of phosphoric acid and divalent iron.

The regeneration of the bath by means of successive addition of fresh phosphoric acid is delusive because it soon becomes incapable of pickling, for a specific weight of 18 to 20 Baum, when the contents of free P205 are below 8%.

It is then necessary to get prematurely rid of the pickling bath. Part (about A) of this first bath may be used after dilution to 23% of free P205 for forming the second bath i. e. the bath of acid phosphate of iron. The remainder of the first bath is thrown away. When the pickling is effected in this manner the efiiciency of the P205 employed is underneath 50%.

Now I have found that it is possible to regenerate the pickling bath which has become too sluggish by freeing the phosphoric acid combined with divalent iron, by means of an addition of an acids produces iron sulphate or ferrous chloride,-

non-ionized in the bath, which frees the phosphoric acid heldin combination by the iron.

-It is found that the. quality of the bath after the addition of sulphuric or hydrochloric acid is strictly the same as before said addition. The steel sheets pickled in a bath of phosphoric acid regenerated by sulphuric or hydrochloric acid have the same smooth and brilliant appearance as those which have been pickled in a bath of phosphoric acid containing no'addition of another mineral acid.

Certain easy precautions should be kept in mind when performingthis regeneration.

In the first place it is indispensable to. never leave any free sulphuric or hydrochloric acid in the regenerated phosphoric bath. To this end the amount of sulphuric or hydrochloric acid added at any moment into the bath must always be proportional to the amount of iron existing at the moment considered in the bath. Theoretically for transforming into ferrous sulphate or .chloride the totality of the iron existing in the phosphoric bath to be regenerated, there should be added about 100 parts in weight of sulphuric acid at 66 Baum or 220 parts in weight of hydrochloric acid at 20-22" B. for'56 of iron. Practically the maximum is 100 parts of sulphuric acid at 60 Baum or 200 parts of hydrochloric acid at 20-2 2 Baum for 50 of iron.

There is, in fact, no interest in setting in liberty, at each addition of sulphuric or hydrochloric acid, all the phosphoric acid combined with the iron; what is important is only the maintenance of a constant pickling power.

The addition of sulphuric acid is efiected under the form of sulphuric acid at 60 Baum because such an addition into a bath at 85 C. presents no danger whereas acid at 66 Baum would cause projections of liquid.

Before proceeding-With an addition of sulphuric or hydrochloric acid, I measure the iron contents by means of potassium permanganate. This measuring method is the best for the purpose as it has been noticed that almost all the iron dissolved in the bath is in the ferrous state. However when the measuring is performed in a bath which has been submitted to a beginning of regeneration through hydrochloric acid, it is nec-' essary to observe the usual known precautions required by the presence of hydrochloric acid.

In order to provide a constant speed of pickling, which is an indispensable condition for regular working and leads to less expense in labor and heating steam, it is preferable to not add all the sulphuric or hydrochloric acid at once when the phosphoric bath is nearly exhausted, but to add it in several times, for instance in four times, when the 'iron contents of the bath reach 13, 26, 39 and 52 kgs. per cubic meter.

I have noticed that after the fourth regeneration, the viscosity of the bath is much smaller than if I had simply reinforced the bath through additions of phosphoric acid. For this reason the bath may be used down to contents of about 4% of free P205.

In practice the regeneration through sulphuric or hydrochloric acid doubles the efiiciency of use of the phosphoric acid employed as'well as the duration of the pickling bath.

As the solubility of the ferrous sulphate and iron chloride in the phosphoric bath is comparatively small, it is preferable not to add in all more than 100 kgs. of sulphuric acid at 60 B. or 200 kgs. of hydrochloric acid at 20-22 B. for 240 kgs. of phosphoric acid at 57 B. (50% of P205) employed. If this precaution were not observed I would risk producing in the phosphoric bath a crystallization of iron sulphate or chloride. As a. bath at 10-11% of P205 contains about 240 kgs. of phosphoric acid at 50% of P205 per cubic meter, it is easy to bear in mind that the amount of sulphuric acid at 60 B. to add into it should notrise beyond 100 kgs. per cubic meter of bath or that the amount of hydrochloric acid at 20-22 B. should not rise beyond 200 kgs. per cubic meter of bath.

It should be noted that the phosphoric pickling bath regenerated by means of, sulphuric or hydrochloric acid should never serve for preparing the second bath or bath of iron phosphate.

For preparing this second bath I use solely an exhausted, but not regenerated phosphoric bath so as to introduce into the second or phosphate bath only these traces of iron sulphate or chlo-v ride introduced by the iron sheets coming out of the phosphoric bath which traces the most sensitive reactions are incapable of showing and the effect of which on the second or phosphate-bath can be but negligible.

Example 1 (sulphuric acid) Using a first or phosphoric pickling bath of 10 cubic meters, containing originally 11% of P205 and kept at 0., the duration of pickling of a sheet iron which was at the beginnin 2 minutes had risen to 30 minutes after 8 days of working.

The amount of divalent iron measured by means of potassium permanganate is then 14 kgs. per cubic meter of bath; the acidimetric measure of P205 after elimination of the iron salts through the classical acetone process gives out contents of 7%. I

If the second bath or iron phosphate bath requires being renewed at this moment (this bath serves for the'duration of several pickling baths) I throw away this second bath and replace it by 2,500 litres of the first or phosphoric bath and 7,500 litres of water. These figures correspond to a second bath of same volume as the first or phosphoric bath which is not compulsory.

On the other hand the phosphoric first bath is returned substantially to its original volume by an addition of about 600 kgs. of phosphoric acid containing 50% of P205 (corresponding to the 240 kgs. contained in each cubic meter of the phosphoric bath removed for reconstituting the second) and of the required volume of water.

I add then 250 kgs. of sulphuric acid at 60 B. which raise the contents of free P205 to 9.5% and reduce the duration of pickling of a sheet iron to 15 minutes.

The pickling being then continued at the same rate analysis shows successively on the 10th, 12th and 14th days contents of iron of 26, 38 and 51 kgs. per cubic meter. On each of these days I add 250 kgs. of sulphuric acid at 60 B, i. e. in all 1000 kgs. After the fourth regeneration, the work may be continued up to the 15th day.

Example 2 (hydrochloric acid) A pickling bath of 10' cubic meters containing originally 10.5% of P205 and'held at 50 C. is found to be too sluggish after an eight. days use. The measuring of the divalent iron by the permanganate method gives out contents of 12 kgs. per cubic meter with 7.5% of free P205.

If the second or .phosphatation bath requires same rate and I find on the 10th, 12th and 14th days, 25, 37 and 49 kgs. of ferrous iron per cubic meter. I regenerate each time through an addition of 500 kgs. of hydrochloric acid at 20-22 B. and I continue the pickling up to the 15thday.

What I claim is:

1. In the method for pickling ferrous metals with phosphoric acid baths containing at least 10% P205 the step which consists in adding to the bath when the bath has lost part of its activity by combination of the phosphoric acid with iron, sulphuric acid in amounts not above that corresponding to saturation of the iron present in the bath.

2. In the method for pickling ferrous metals with phosphoric acid baths containing at least 10% P205, the step which consists in adding to the bath, when the bath has lost part of its activity by combination of the phosphoric acid with iron, parts in weight at the most of sulphuric acid at 60.Baum for 50 parts of iron present in iron, sulphuric acid in amounts not above that corresponding to saturation of iron present in the bath and not exceeding 100 kgs. of sulphuric-acid at 60 Baum for each 240 kgs. of phosphoric acid at 50% of P205 contained in the bath.

JEAN ADRIEN MARIE BIGEON.