EP0635586B1 - Process for regenerating hydrochloric acid from pickling installations - Google Patents

Description

The invention relates to a process for the regeneration of hydrochloric acid from pickling plants, comprising the thermal cleavage of iron chloride in the used pickling acid to iron oxide and gaseous hydrochloric acid, at least one reducing compound being admixed with the used pickling acid.

Pickling is an indispensable process step in metallurgical process engineering for the production of steel products. Hydrochloric acid and sulfuric acid, as well as other acids and acid mixtures, are particularly suitable as the pickling medium. Due to various circumstances, partly due to the achievable quality of the end products and partly due to the fact that it can be completely regenerated, pickling with hydrochloric acid or mixtures containing it has become increasingly popular over the past 30 years. The effect of the acid is to dissolve the layers of scale that form on the steel surface through previous processes such as rolling, annealing, etc. This happens after the following chemical reaction: $${\text{(1) FeO + 2 HCI → FeCl}}_{\text{2nd}}{\text{+ H}}_{\text{2nd}}\text{O}$$

The pickling process therefore consumes acid (HCl) to a point where the solution is saturated with iron chloride and can no longer be used for pickling.

Various processes for the regeneration of hydrochloric acid from pickling plants are already known, such as FR 1.399.534. Here, the formation of free chlorine or iron chloride is prevented by, for example, adding sulfur and roasting the pickling solution. Another method for the regeneration of hydrochloric acid from pickling plants is described in US Pat. No. 3,443,991. Here, hydrogen or other reducing gases are added to the furnace, which prevent the development of chlorine and form gaseous hydrochloric acid, which in one subsequent absorber is absorbed. The article "About the pickling of hot baths with hydrochloric acid ...." (Materials and Corrosion, Volume 18, Issue 8, August 1967, pages 673 to 680) basically describes the regeneration of hydrochloric pickling baths and in particular represents a spray roasting system used for this.

Für diesen Vorgang der thermischen Spaltung haben sich zwei Prozesse durchgesetzt:

• a) das Sprühröstverfahren, bei dem die verbrauchte Beizlösung mit dem Eisenchlorid in einen leeren Reaktor, der durch Brenner direkt beheizt ist, eingesprüht wird, wobei sich ein feines, staubförmiges Eisenoxid bildet,
• b) das Fließbettverfahren, bei dem die Lösung in einen Fließbettreaktor eingedüst wird, der ein Bett aus kugelförmigen Eisenoxidpartikeln enthält, die durch die Brennergase und die Fluidisierungsluft in Schwebe gehalten werden, wobei sich ein grobkörniges Eisenoxid bildet.

It has been shown that the spent pickling acid and in particular the iron chloride contained therein can be split by a thermal process, whereby iron oxide is formed on the one hand and hydrochloric acid is recovered on the other hand, which can be returned to the pickling process. This happens after the following reaction: $${\text{(2) 2 FeCl}}_{\text{2nd}}{\text{+ 2 H}}_{\text{2nd}}{\text{O + 0.5 O}}_{\text{2nd}}{\text{→ Fe}}_{\text{2nd}}{\text{O}}_{\text{3rd}}\text{+ 4 HCl}$$

Two processes have prevailed for this process of thermal fission:

• a) the spray roasting process, in which the spent pickling solution with the iron chloride is sprayed into an empty reactor which is directly heated by a burner, a fine, dusty iron oxide being formed,
• b) the fluidized bed process, in which the solution is injected into a fluidized bed reactor which contains a bed of spherical iron oxide particles which are kept in suspension by the burner gases and the fluidizing air, with a coarse-grained iron oxide being formed.

In both processes, undesirable gaseous by-products can form due to various side reactions, which are often toxic pollutants and can be removed only with difficulty or with great technical effort using conventional technology.

These pollutants include the compounds NO and NO ₂ (collectively referred to as nitrogen oxides, NO _x ), which on the one hand can form themselves from the atmospheric nitrogen through the combustion process, and on the other hand can form nitrogen compounds, such as inhibitors, from the pickling bath.

Another pollutant is chlorine, which is formed in the form of elemental chlorine (Cl ₂ ) in the above processes by oxidation of HCl according to the so-called Deacon equilibrium: $${\text{(3) 2 HCl + 0.5 O}}_{\text{2nd}}{\text{→ Cl}}_{\text{2nd}}{\text{+ H}}_{\text{2nd}}\text{O}$$

The equilibrium constants of this homogeneous gas reaction are well known and are, for example, at Temperature (K) log Kp 500 0.9 600 - 0.7 700 - 1.9 800 - 2.8

It can be seen from this that the equilibrium at low temperatures is predominantly on the right side of the equation, but on the other hand the reaction kinetics at these temperatures is too slow to cause substantial chlorine formation. At temperatures around 700 K, which corresponds approximately to the temperature of the exhaust gas from a spray roasting reactor, the chlorine concentration can be calculated as follows:

P _Cl2 = Kp * (P _HCl ) ² * (P _O2 ) ^0.5 / P _H2O

With an HCl content of, for example, 5% and an O ₂ content of 3.5% and an H ₂ O content of 45% - this corresponds to a typical composition of a reactor off-gas - a Cl ₂ content of approx. 35 ppm or 110 mg / m ³ . These amounts of chlorine can vary depending on the conditions, the excess of oxygen being a particular influencing factor, since it often has to be kept high in order to achieve a certain oxide quality or for fluidization.

The chlorine once formed can only be removed from the exhaust gases with difficulty, but this is unavoidable since, for example, the TA air only allows a content of 5 mg / m ³ . For example, laundry is part of the chlorine reduction State of the art with sodium thiosulfate: $${\text{(4) Well}}_{\text{2nd}}{\text{S}}_{\text{2nd}}{\text{O}}_{\text{3rd}}{\text{+ 4 cl}}_{\text{2nd}}{\text{+ 5 h}}_{\text{2nd}}{\text{O → 2 NaHSO}}_{\text{4th}}\text{+ 8 HCl}$$

However, this type of chlorine removal requires expensive gas scrubbers and a corresponding consumption of chemicals. Waste water to be disposed of also arises.

The object of the present invention is to provide a method of the type mentioned in the introduction, in which the occurrence of the pollutants mentioned can be avoided in a simple and inexpensive manner in the recovery of hydrochloric acid from used pickling solutions.

To this end, it is provided according to the invention that at least one compound is added to the spent pickling acid which contains nitrogen with an oxidation number which has a reducing effect on NO, NO ₂ and Cl ₂ , such as, for example, ammonium compounds, ammonia, urea or amides.

$${\text{(5a)\hspace{1em}\hspace{1em}\hspace{1em}3 NO}}_{\text{2}}{\text{+ 4 NH}}_{\text{3}}{\text{\hspace{1em}\hspace{1em}\hspace{1em}→3,5 N}}_{\text{2}}{\text{+ 6 H}}_{\text{2}}\text{O}$$

$${\text{(6)\hspace{1em}\hspace{1em}\hspace{1em}3 Cl}}_{\text{2}}{\text{+ 2 NH}}_{\text{3}}{\text{\hspace{1em}\hspace{1em}\hspace{1em}→6 HCl + N}}_{\text{2}}$$

The pollutants NO _x and chlorine are oxidizing in relation to the added substances, so that they react in the following way, for example: $${\text{(5) 3 NO + 2 NH}}_{\text{3rd}}{\text{→ 2.5 N}}_{\text{2nd}}{\text{+ 3 H}}_{\text{2nd}}\text{O}$$

$${\text{(5a) 3 NO}}_{\text{2nd}}{\text{+ 4 NH}}_{\text{3rd}}{\text{→ 3.5 N}}_{\text{2nd}}{\text{+ 6 h}}_{\text{2nd}}\text{O}$$

$${\text{(6) 3 cl}}_{\text{2nd}}{\text{+ 2 NH}}_{\text{3rd}}{\text{→ 6 HCl + N}}_{\text{2nd}}$$

In reaction (5), part of the reactor and the iron oxide in it take on the function of a catalytic converter.

According to a further feature of the invention, it is provided that the spent pickling acid, together with at least one compound which contains nitrogen with an oxidation number which has a reducing effect on NO, NO ₂ and Cl ₂ , is fed into a venturi scrubber and then in one Reactor, which can be a spray roasting reactor or a fluidized bed reactor, is thermally split. On the one hand, this allows simple recovery of the hydrochloric acid and, at the same time, the production of very pure oxide which, due to its structure, is extremely suitable for further use.

Advantageously, the exhaust gas originating from the thermal decomposition is subjected to a wash, preferably with rinsing water from a rinsing system connected downstream of a pickling system, whereby the pollutant values in the exhaust gas can be reduced further.

That according to a further feature in a method as described in the previous paragraph at least one compound which contains nitrogen with an oxidation number which has a reducing effect on NO, NO ₂ and Cl ₂ has been added to the flushing water before the exhaust gas is scrubbed by the thermal cracking additional advantage that all acidic compounds and residues of chlorine are chemically removed by the nitrogenous compound during washing.

In the following description, two preferred exemplary embodiments of the method according to the invention will be explained in more detail with reference to the accompanying drawings.

1 and 2 show schematically exemplary plants for carrying out the method according to the invention using a spray roasting reactor.

The spent pickling acid is introduced into a venturi scrubber 2 via a line 1. The gases coming from the reactor 3, for example a spray roasting reactor, are fed into the venturi scrubber 2 via a line 4. The aqueous solution from the venturi scrubber 2 is passed through the pump 6 via a line 5 to the spray device 7 of the reactor 3, to which gas and air for combustion and oxidation are fed via a line 8. The oxide formed by the spray roasting process is via a line 9, preferably withdrawn from the reactor 3 by means of a rotary valve, not shown.

After the venturi scrubber 2, the exhaust gas from the reactor 3 is subsequently fed to a first column 10 and via line 23 to a second column 11 for further purification. Both columns 10, 11 are preferably charged with water, to which chemicals can optionally be added to support the cleaning action, via lines 13 and the residual liquid is removed via lines 25.

After the second column 11 there is also a scrubber 12, to which the exhaust gas is fed via line 23 'and fresh water via a line 13 and the waste water is removed via a line 25. The cleaned exhaust gas is then fed to a chimney 15 by a fan 14 and released into the atmosphere.

The compound or the mixture of compounds containing nitrogen with a low oxidation number is fed into the supply line 1 for the used pickling acid to the venturi scrubber 2 via a line 16. It applies to the amount of nitrogen-containing compound (s) supplied that it must be admixed at least in a stoichiometric ratio to the pollutants contained, the lower the pollutant content in the exhaust gas, the greater the excess of the nitrogen-containing compound or the mixture thereof is. Depending on the initial values, at least five times, but preferably at least ten times, the amount is added instead of the stoichiometrically necessary amount for chlorine. For nitrogen oxides, the minimum addition amounts are twice, preferably three times, the stoichiometrically necessary amounts.

2 is constructed similarly to that of FIG. 1, but no second column is provided. To reference the source for the To produce used pickling acid, a rinsing system 18 and the previous pickling system 19 are shown. The rinsing system 18 is supplied with fresh water via a line 13, while its wastewater - with the residual content of pickling acid contained therein - is fed to the scrubber 12 via the line 20.

In the line 20 leading to the scrubber 12, the nitrogen-containing compound or the mixture of such compounds is mixed with the water from the rinsing system 18 via a further line 17. This has the effect that elemental chlorine still present in the scrubber 12 forms a chemical compound with the nitrogen-containing compound, for example forms ammonium chloride with ammonia, and is thus removed from the exhaust gas. The cleaned exhaust gas is finally released into the atmosphere via a chimney 15.

The solution emerging from the first column 10 and containing the hydrochloric acid formed during the thermal cleavage is fed via line 21 to the pickling plant 19.

If it is still necessary to further reduce pollutants, the pickling acid used in this process variant can also be mixed directly via line 16 with at least one compound containing nitrogen with a low oxidation number before the pickling acid enters the venturi scrubber 2.

Embodiment A: Trial 1:

In a test plant similar to that in FIG. 1, used pickling acid from a steelmaking company was treated. The acid contained 119.4 g / l Fe ²⁺ , 6.8 g / l Fe ³⁺ and a total of 224 g / l HCl.

The feed rate of the venturi washer was 16 l / h and that of the spray roasting reactor was 10 l / h. The temperature in the burner level is 645 ° C and in the upper area of the reactor 389 ° C. The amount of gas was 2.8 m ³ / h, the amount of air 28 m ³ / h and the O ₂ content 5% (based on dry volume).

After the first column, a Cl ₂ content of 24.6 mg / m ³ exhaust gas was measured without the addition of nitrogen-containing compounds.

Trial 2:

Conditions were set as in Example 1. 2.0 g / l ammonium chloride (NH ₄ Cl) were added to the pickling acid which was fed to the venturi scrubber. The chlorine measurement showed no detectable chlorine.

Trial 3:

With the same pickling acid as in experiments 1 and 2, the venturi was charged with 16 l / h and the reactor with 8.2 l / h. The temperature at the burner level was now 554 ° C and at the top of the reactor 390 ° C. The amount of gas was 2.1 m ³ / h, the amount of air 28 m ³ / h and the O ₂ content was now increased to 12% (based on dry volume). After the first column, a Cl ₂ content of 107 mg / m ³ exhaust gas was measured without the addition of nitrogen-containing compounds. It was thus shown that the increase in the oxygen content led to a considerable increase in the chlorine concentration.

Experiment 4:

In a further experiment with the same conditions as in experiment 3, 5.7 g / l ammonium chloride was added to the pickling acid added to the venturi. The chlorine measurement after the first column now showed a significant reduction in the chlorine content compared to experiment 3 to 37 mg / m ³ , ie a reduction of 65%.

Embodiment B: Trial 1:

In an industrial spray roasting plant, an experiment was carried out with the pickling acid from embodiment A, in which the temperature in the burner level was 600 ° C. and at the top in the reactor 415 ° C. The feed amount of the reactor was 4500 l / h, the gas consumption 480 m ³ / h, the air consumption 5570 m ³ / h and the amount of exhaust gas (at 85 ° C) 12500 m ³ / h.

The NO _x content in the chimney was measured and the averaging gave a value of 180 ppm consisting of 150 ppm NO and 30 ppm NO ₂ . This corresponds to a total of 2.5 kg NO / h.

Trial 2:

20% ammonium chloride solution was added to the pickling acid in accordance with the table below, the amount being increased gradually. The equivalent amounts of NH ₄ Cl were calculated according to equation (5) for NO. NH ₃ / NO NO content (ppm) 1: 1 160 2: 1 100 3: 1 80 6: 1 50 10: 1 25th

Note: At the temperatures in the reactor, NH ₄ Cl forms ammonia (NH ₃ ): $${\text{(7) NH}}_{\text{4th}}{\text{Cl → NH}}_{\text{3rd}}\text{+ HCl}$$

It was therefore possible to achieve a considerable reduction in the NO _x content in the exhaust gas, depending on the respective NH ₃ excess.

Claims (10)

1. Process for regeneration of hydrochloric acid from pickling lines comprising thermal decomposition of iron chloride in the spent pickling acid into iron oxide and gaseous hydrochloric acid, where at least one reducing compound is added to the spent pickling acid, characterized by the fact that the compound contains nitrogen at an oxidation number having a reducing effect on NO, NO₂ and Cl₂.
2. Process according to claim 1, characterized by the fact that ammonium compounds, ammonia, urea or amides are used as the reducing compound.
3. Process according to claim 1 or 2, characterized by the fact that said spent pickling acid is fed to a Venturi washer together with at least one compound containing nitrogen at an oxidation number having a reducing effect on NO, NO₂ and Cl₂ and is then thermally decomposed in a reactor.
4. Process according to claim 3, characterized by the fact that said thermal decomposition takes place in a spray roasting reactor.
5. Process according to claim 3, characterized by the fact that said thermal decomposition is carried out in a fluidized bed reactor.
6. Process according to one of the claims 1 to 5, characterized by the fact that the off-gas from said thermal decomposition is subjected to washing.
7. Process according to claim 6, characterized by the fact that said washing is carried out using the rinse water from a rinsing plant downstream of the pickling plant.
8. Process according to claim 6 or 7, characterized by the fact that at least one compound containing nitrogen at an oxidation number having a reducing effect on NO, NO₂ and Cl₂ is added to the rinsing water before washing the off-gas from said thermal decomposition.
9. Process according to one of the preceding claims, characterized by the fact that at least one compound containing nitrogen at an oxidation number having a reducing effect on NO, NO₂ and Cl_2, is added in at least fivefold, preferably ten-fold the stoichiometric quantity in relation to the chlorine content and/or in at least double, preferably triple the stoichiometric quantity in relation to nitrogen oxides.
10. Process according to claim 9, characterized by the fact that said compound is added in at least ten-fold the stoichiometric quantity in relation to the chlorine content and/or in at least triple the stoichiometric quantity in relation to nitrogen oxides.

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