NO753583L - - Google Patents

Description

According to the general practice, it is common for steel and steel alloys to be given a surface pickling before the metal is given the final surface treatment. The pickling usually takes place by the goods to be processed being treated in a bath, which is most often hydrochloric acid-based or sulfuric acid-based, and the purpose is usually that scale, corrosion product or also that more or less successful coatings from returned goods are to be removed before the terminated process. During use, these baths are enriched with a number of metal salts, usually in the first place. iron salts, while the content of free acid in the baths drops to such a low value that the baths are no longer sufficiently effective. It is then necessary to replace them with new and more effective pickling baths.

The disposal of the used pickling baths today represents a major problem for many companies. While it was previously practiced that the baths were discharged into the nearest recipient (watercourse, fjord, sea area), this is no longer considered acceptable practice. The used pickling baths are required to be cleaned and detoxified before any discharge, and the authorities have drawn up guidelines for the permitted residual content of waste substances.

Today, there are several alternative options for choosing a process for this detoxification of pickling baths. In certain alternatives, the ferrous pickling baths - which are then often assumed to be virtually free of dissolved metal salts other than iron - are transported to larger, central treatment facilities, which may be based on roasting, pyrolysis, ion exchange, liquid-liquid extraction techniques, or. the like.

What these process alternatives have in common is that the treatment plants will be very capital-intensive and also relatively complicated to operate. They can therefore only come into question for the relatively large companies, or for companies that are advantageously located geographically in relation to a central facility, so that the transport of the highly corrosive pickling baths is simplified.

For the other companies, the only realistic alternative seems to be the construction of precipitation facilities for conventional precipitation of metal hydroxides and deposition of solids.

This method is also burdened with significant weaknesses. It is known that by the conventional practice of precipitation, of hydroxides of iron, chromium, zinc, copper, cadmium etc. get very voluminous droppings, which have very poor drainage properties. Conventional precipitation plants therefore usually include both a dilution operation, a pH-controlled dosing of neutralizing agent (to pH 8 - 10),

a thickening operation and the use of a filter press for dewatering the solids. The end product is a filter cake of considerable volume, with a characteristic water content of 75 - 80%. The amount of filter cake from a pickling bath with, for example, 100 g/l Fe 2 + will amount to approx. 1.2 - 1.3 nos ^ per mJ ^ original stain bath.

The deposition of such a filter cake can often .by

on significant problems.

According to the present invention, these operations can be greatly simplified by simple means, - which opens up the possibility of a significantly simplified method in terms of equipment. The invention is based on the surprising discovery that by optimizing the precipitation conditions for iron and other dissolved metal salts, it is possible to obtain oxidic precipitations of the same metals, which have such physical and chemical properties that a number of advantages are achieved. This has been shown in laboratory experiments that by simultaneously ensuring that the following conditions are maintained -

1) high temperature in the solution (t > 80°C),

2) slow addition of neutralizing agent, and

3) a correspondingly slow oxidation of Fe 2 + to Fe<3+>

have succeeded in precipitating almost the entire solution's content of dissolved iron in a form that causes the solid to assume previously completely unknown properties, and which, for example, makes the use of a filter for the liquid/solid separation unnecessary. The solid consists for a significant part of Fe^O^.

If, in addition to iron, the solution also contains zinc and other metal ions in lower concentrations, which is usually the case if the pickling bath is from hot-dip galvanizing companies, it is also possible to achieve a selective precipitation of the iron as described above at a pH of < 5j0, after which in the next step a hot precipitation (t > 80°C) of zinc and other metal ions is carried out by slow addition of neutralizing agent to a final pH of 9.0 - 11.0. This method produces a precipitate of zinc, which essentially has the same favorable physical properties as previously described for the iron precipitate.

The advantages of the process are described in the following example, which nevertheless does not represent any limitation for the process:

Example 1.

2 liters of pickling bath from a hot-smelling company with

following analysis:

Fe<2+>=119 g/l, Zn<2+>= 68 g/l, HC1 = 18 g/l - milk of lime was added to pH approx. 1.0, heated to 90°C in a reaction vessel with a stirrer and finely dispersed air added as oxidizing agent. The pH was then adjusted to 3 3.5 and maintained by repeated additions of the milk-of-lime suspension. After 18 hours the reaction was terminated, the sludge settled to the bottom and the clear solution decanted from the solid.

The solution from the previous step was again heated to approx. 90°C, further milk of lime was added over 4-6 hours, until the pH was brought up to 9»0 - 10.0. The suspension was then set to sedimentation and decantation, - and the solid was allowed to drain by draining off the filtrate. Analyses: Applied pickling bath: Fe 2 +..= 119 g/l Zn<2+>= 68 g/l HC1 = 18 g/l Iron precipitation

After decantation: H20 = 20$ Analysis on unwashed

dry matter: Fe = kj>%

Zn = 3.0$

Ca = h, 0%

Cl = 8.3%

Liter weight for sludge: 2.4 kg/l S sludge volume: 12% of original pickling bath volume.

Analysis of washed dry matter:

Fe = $60.0

Zn = 0.02$

Approx = 0.2%

Cl_= 0.8%

Analysis of filtrate from iron precipitation:

Zn = 68.0 g/l

Fe = 0.3"

PH = 3.5

Analysis of zinc precipitation:

Zn = 26.00$?

Fe = 0.50$ Ca = 12.2%

Cl = 21.5%

Analysis of filtrate from zinc precipitation:

Zn = 3.0 mg/l

Fe = 1.0 mg/l

p-H = 10.0

Depending on the conditions in the process in question, one chooses, according to the previous description, either to isolate the iron precipitation from the secondary precipitation of zinc - and any other metal compounds, - or one chooses to carry out a combined precipitation.

In that. first alternative, one has the opportunity to precipitate any valuable components in the "solution in such a form that they can be disposed of for a separate treatment, - for example, with a view to processing during recycling.

In the second alternative, the prerequisites for a simple technical solution to the drainage/disposal problems are achieved.

The process described above is suitable both for pickling baths based on hydrochloric acid and for baths based on sulfuric acid.

Claims (7)

1. Process for the treatment of ferrous pickling baths from surface treatment companies, characterized in that the solution is heated to a temperature of between 60°C and the boiling point of the solution, the pH is regulated to 2-5, preferably 3 4, by slowly adding a suitable neutralizing agent to maintain the pH of the solution within the given limits, while at the same time ensuring that it takes place with a suitable oxidizing agent a slow oxidation of the solution's content of divalent iron ions. 2. Process according to claim 1, characterized in that the iron-containing pickling bath is obtained from pickling of steel in connection with hot-dip galvanizing. 3. Process according to claims 1 and 2, characterized in that, after complete precipitation of the solution's iron content, the solution's pH is slowly raised to 9 - 10.0, at a temperature between 60°C and the solution's boiling point, whereby also the solution's zinc and other content metal ions are precipitated in a sludge that has very favorable physical properties. 4. L Process according to claims 1-3, characterized in that the oxidizing agent used is one of the following oxidizing agents: air, oxygen, chlorine, chlorate, hypochlorite, persulfate, peroxydisulfate, permanganate, manganese dioxide, hydrogen peroxide. 5. Process according to claims 1-4, characterized in that the oxidation takes place at such a rate that between 2 and 20 g/l - preferably approx. 3 - 6 g/l iron is brought to oxidize and precipitate per hour. 6. Process according to claims 1-3, characterized in that the addition of neutralizing agent for the secondary precipitation of zinc and other dissolved metal ions takes place over a time period of at least 4-6 hours, preferably 10-20 hours. ' 7. Process according to claims 1-6, characterized in that one of the following neutralizing agents is used as neutralizing agent: CaO, Ca(OH)2, NaOH, CaCO^, Na^O^, NH^OH.