EP0759011A1

EP0759011A1 - Process for extracting or recovering acids

Info

Publication number: EP0759011A1
Application number: EP96903826A
Authority: EP
Inventors: Josef Dipl.-Ing. Dr. Jeney
Original assignee: Sad Edv-Systemanalyse und -Systemdesign GmbH
Current assignee: Sad Edv-Systemanalyse und -Systemdesign GmbH
Priority date: 1995-03-07
Filing date: 1996-03-06
Publication date: 1997-02-26
Also published as: WO1996027554A1; KR970702819A; ATA39595A; AT412001B

Abstract

The invention concerns a process for extracting or recovering acids, in particular hydrofluoric and nitric acids from metal-containing solutions (1). The process in question is characterised by the fact that: the metal-containing solution is concentrated by evaporation (002) at temperatures at which essentially no decomposition of the nitric acid occurs; the metal-containing residue obtained by that evaporation is treated pyrohydrolytically (007), converting the metal fluorides to metal oxides (9); and acids present in the roaster gases (10) are removed from the latter. With the claimed process, hydrofluoric acid and nitric acid can be recovered in high yields and with low outlay.

Description

Process for the recovery or recovery of acids

The invention relates to a process for the recovery or recovery of acids, in particular hydrofluoric acid and nitric acid, from metal-containing solutions.

Metal-containing solutions, which contain hydrofluoric acid and nitric acid, fall in particular in the metal industry as nitric acid mixed acid pickling in the surface treatment of stainless steels. Special alloys and special metals. They contain free hydrofluoric acid, free nitric acid and iron dissolved as fluoride. Chrome and nickel.

These solutions must be renewed when a metal concentration of 60-70 g / 1 is reached. Because the disposal of the used pickling acid is problematic due to the environmental regulations and ever increasing disposal costs. Several methods have already been developed to recover hydrofluoric acid and nitric acid from the used solutions.

Known methods for the recovery of hydrofluoric acid and nitric acid are e.g. the solvent extraction. Dialysis procedure and the use of ion exchangers.

It is known from AT-PS 376.632. To recover hydrofluoric acid from pickling liquors by crystallizing the fluorides present in the liquors, bringing them into contact with steam in countercurrent at temperatures of 200 to 900 ° C. and hydrofluoric acid being condensed out of the resulting gas phase.

This known method is generally based on the principle of pyrohydrolysis. This procedure provides. that in the above case, under certain thermodynamic conditions, the metal fluorides initially in the pickling acid according to the equations

(1) 2 FeF ₃ (s) -r 3 H _: O (g) → Fe ₂ O ₃ (s) - 6 HF (g) (2) NiF ₂ (s) + H ₂ O (g> → NiO ( s) + 2 HF (g)

(3) 2 CrF ₃ is) - 3 H ₂ O (g) → Cr ₂ O ₃ (s) ^ 6 HF (g)

be converted into the respective metal oxides, free hydrofluoric acid being formed. At the high temperatures required for this process, however, there is at the same time a decomposition of the nitric acid which is also in the pickling acid according to the equation

(4) 4 HNO, (g) → 4 NO (g) - 3 O; (g) * 2 H _: O (g)

instead The decomposition of nitric acid is particularly important at temperatures above 200 ° C

In the event of a deficit of hydrofluoric acid in the mixed acid pickling, metal pickling occurs in the pickling. These are used in the pyrohydrolytic treatment according to the equation

(5) 4 Fe (MO) 3 → - Fe ₂ O ₃ (s) -r 12 NO (g) + 9 O _: (g)

also decomposed

In any case, with each pyrohydrolytic treatment, depending on the existing thermodynamic conditions, it can be expected that nitric acid introduced with the pickling agent will decompose. According to Gmehn, 2.4% at 240 ° C. at 550 ° C, however, 83.4% of the nitric acid has already decomposed

In AT-PS 395 312 it was therefore proposed, for the recovery of acids from metal-containing solutions, to release the acids from the solution by means of spraying grates and to absorb and or condense the gases formed during spraying at a temperature of 0 to 70 ° C. In particular, it is described to carry out the absorption or the condensation in two columns, the yields of hydrofluoric acid and nitric acid being improved by controlling the column temperature

During the spray rust, the pyrohvdrolysis reactions described above and also the 40 to 50% nitric acid decomposition take place. Therefore, the process according to AT-PS 395 312 provides for the recovery of the nitric acid from the decomposition products resulting from the spray rust by the absorption of water on NOτ according to the equation (7) 3 NO; (g) + H _: O (I) - ♦ 2 ENT; (aq) + NO (g)

in front.

This reaction is highly exothermic, so intensive cooling of the absorption column is necessary

For reaction (7), however, it is first necessary that NO present primarily in the roasting gas according to the equations

(8) 2 NO (g) - O; (g) → 2 NO: lg) or (9) NO + H ₂ θ ₂ → NO: * H ₂ 0

is reacted with oxygen or other oxidizing agents such as hydrogen peroxide to NO ₂ .

However, this method has several disadvantages:

For example, the recovery rates achievable according to this process are unsatisfactory. It is described that a maximum recovery rate of 33% of the nitric acid originally used is possible in one column. Only through additional oxidation measures. which naturally entails increased chemical expenditure and thus higher costs, a recovery rate of approx. 40% can be achieved in the second column.

According to AT-PS 395 312, the total amount of nitric acid recoverable from the metal-containing solution is a maximum of about 80% of the amount used when additional oxidation measures are used, the rest of the unreacted nitrogen oxides must be separated from the rust gases, e.g. in DeNOx systems, and is no longer available for the recovery of nitric acid

Furthermore, this method must be cooled efficiently in order to prevent the reaction to nitric acid from being decomposed again when the temperature is reached due to the exothermic nature of the reaction. This cooling consumes large amounts of energy. The present invention has accordingly set itself the task of overcoming the described disadvantages of the prior art to provide a method in which hydrofluoric acid and nitric acid can be recovered efficiently and economically from metal-containing solutions in high recovery rates

This task is solved according to the invention in that the metal-containing solution is evaporated at temperatures at which essentially no decomposition of the nitric acid takes place, that the metal-containing residue of the evaporation is treated pyrohydrolytically, the metal fluids being converted into metal oxides, and in that the resulting rust gases are separated from the rust gases

Surprisingly, it has been found that hydrofluoric acid and nitric acid can be separated from the metal-containing solutions by the sequence of process steps hydrofluoric acid and nitric acid with jerk rates of up to 90% and above, without having to resort to additional measures for oxidation or intensive cooling

The invention is based on the knowledge of removing a large part of the nitric acid in the metal-containing solution from the solution by evaporation below the decomposition temperature of the nitric acid. The small amount of nitric acid remaining in the metal-containing residue is indeed partially decomposed in the reactor as described in the beginning, but this is The extent of the resulting nitric acid losses is so small that additional oxidation of the nitrogen oxides formed and conversion to nitric acid can be completely avoided

It is clearly evident that this procedure saves all previously necessary measures for the recovery of the nitric acid decomposed during the pvrohvdrolytische treatment, so that it is only necessary. Separating the acids from the rust gases economically using known measures. In particular, the method, which is inherently uneconomical, can be avoided by the process according to the invention, first of all by decomposing a substance to be recovered and then having to assemble it again at great expense

The evaporation of the metal-containing solution preferably takes place at temperatures below 200 ° C. and particularly advantageously in a temperature range from 80 to 120 ° C. it is ensured that no significant decomposition of the nitric acid takes place during evaporation

In an advantageous embodiment of the invention, the evaporation is carried out at reduced pressure. This enables a gentler treatment of the metal-containing solution

The preliminary treatment of the metal-containing residue is preferably carried out in a spray rust reactor

It has been shown that the process according to the invention can be carried out even more efficiently if the temperature in the burner reactor is 500 to 700.degree. C., preferably 550 to 650.degree. C. At these higher temperatures, the pyrohvdrolysis reactions take place more quickly due to the fact that If only small amounts of nitric acid get into the spray rust reactor at all, the decomposition of the nitric acid, which also occurs more strongly at these higher temperatures, is of little importance

An advantageous embodiment of the method according to the invention is characterized in that the rust gases emerging from the reactor for pyrohvdrolviischen treatment are brought into contact with the metal-containing residue of the evaporation in a flue gas scrubber

This ensures that, on the one hand, the rust gases are cooled from the metal-containing residue to such an extent that the decomposition reactions of the nitric acid are stopped and, on the other hand, the warm content of the rust gases can be used for water evaporation and the residual dust of the rust gases is washed out by the diluted metal-containing residue

The rust gases are preferably passed through at least one absorption column to separate the acids present in the rust gases

The separation of the acids from the rust gases by washing out in absorption columns is a known technique, whereby in comparison to the known methods of the prior art Technology, however, no additional measures have to be taken to regenerate the decomposed nitric acid

The temperature in the absorption column is preferably kept above 70.degree. C., which means that essentially no cooling is carried out. This is not necessary because no measures are taken according to the invention to recover from the decomposition products which are likewise present in the rust gases in small quantities To gain nitric acid

In a further advantageous embodiment of the process according to the invention, part of the acidic solution emerging from the absorption column is circulated over the column

This means that the washing liquid accumulates more and more with absorbed acids and thus higher concentrations of hydrofluoric and nitric acids can be achieved

In a further advantageous manner, the metal-containing residue of evaporation with absorption liquid is dissolved or diluted from at least one of the absorption columns used

Particularly suitable as a liquid for dissolving or diluting the metal-containing residue is washing liquid from a second or third absorption column, in which no significant acid concentrations occur anymore. In this way, even slightly acidic wastewater can be recycled in a cycle

The method according to the invention is explained in more detail below using an exemplary embodiment and with reference to the accompanying drawing, the subject matter of the invention being in no way restricted by this exemplary embodiment

The drawing shows a system for carrying out a preferred embodiment of the inventive method in a schematic manner

A metal-containing solution (1) is obtained. Eg spent pickling acid, from the pickling plant or, if necessary, an intermediate storage tank (001) into an evaporator (002) in the evaporator (002) the metal-containing solution is evaporated at a temperature below the decomposition temperature of the nitric acid, the free gaseous acids present in the vapors being separated off in a condenser (003) and in liquid form as recondensed mixed acid (2) in a mixed acid storage tank (004 ) are transferred. The evaporator (002) is preferably a single-stage or multi-stage vacuum evaporator, in which the evaporation is carried out under reduced pressure, for example under pressure reduction to 0.5 bar, in one-stage systems.

The evaporation residue (3) can be dissolved or diluted in fresh water or preferably with the outlet (5) of an absorption column (012). The main part of the evaporation residue contains metal fluorides as well as small amounts of metal nitrates and residues of non-evaporated nitric acid.

This diluted evaporation residue (6) is then conveyed into a reactor for pyrohvdrolviischen treatment, preferably a spray rust reactor (007). The diluted evaporation residue (6) preferably passes through a flue gas scrubber (006) in which it comes into contact with roasting gases (8) from the spray roasting reactor (007) and is partially evaporated. A part of the evaporation residue (6) can advantageously be circulated through the flue gas scrubber (006).

In the spray roasting reactor (007), in a particular reactor atmosphere (approx. 50 vol.% H;> approx. 5 vol.% O ₂ ), the temperature conditions are set which enable the reactions (1) to (3) described at the outset to take place .

In particular, a temperature of 500 to 700 ° C., preferably 550 to 650 ° C., is set in the burner level.

The diluted evaporation residue (11), which is at most branched off from the flue gas scrubbing circuit or drawn off after a single pass, is sprayed below the reactor head into the spray roasting reactor (007), with reactions (1) to (3) and at the same time the decomposition of the nitric acid still present in the evaporation residue according to the equations (4) and optionally (5) also given at the beginning. From the Pvrohvdrolvse reactor (007), the resulting metal oxides (9) are continuously drawn off at the lower end ) in the flue gas scrubber (006). where they are also cooled by the diluted evaporation residue (6) from the container (005). At this point the decomposition reactions of the nitric acid are stopped due to the reversal of the equilibrium position of the reaction

In the flue gas scrubber (006), residual dust from the rust gases is also taken up by the diluted evaporation residue (6) and thus returned to the reactor (007)

The cleaned rust gases (15) pass from the flue gas scrubber (006) into a first absorption column Kl (01 1), the acids in the rust gases (15) being absorbed with water. In order to achieve the highest possible concentrations of hydrofluoric acid and nitric acid, part ( 16) the mixed acid regenerated in this way is circulated over the absorption column Kl (01 1). The rest (12) of the regenerated mixed acid from the column Kl (011) is pumped to the mixed acid storage tank (004)

Advantageously, the outflow from column K 1 (01 1) is cooled to just above 70 ° C. by means of cooling means (016)

The second absorption column K2 (012). which is also fed with water, serves primarily as an exhaust gas scrubber.A part of the washing liquid (5) created here can be used to dissolve or dilute the metal-containing evaporation residue. Part of the washing liquid can also be circulated in the column K2 (012)

The waste gases (13) still present after the second wash column (012) are in an alkaline scrubber W 1 (013). preferably freed from the last traces of acid using NaOH or other alkaline reagents and passed to a DeNOx plant (014) for the catalytic reduction of NOx with ammonia or urea The exhaust gases (14) freed from acids and nitrogen oxides in this way are discharged into the atmosphere by an exhaust gas fan (015). The exhaust gas fan (015) naturally also effects the entire gas transport through the plant, starting with the extraction of the roasting gases (10) from the reactor (007) up to the pushing out of the cleaned exhaust gases (14) through the chimney (014).

Example:

500 l of a pickling agent (1) with a composition according to the following table, a temperature of 30 ° C. and a density of 1.23 kg / l were placed in a test installation constructed analogously to the drawing.

Concentration (g l) This corresponds to concentration (g 1)

FeF ₃ (aq) 88.9 total Fe 44

HF (aq) 33.7 Total HF 81

HNO ₃ (aq) 350 total HNO ₃ 350

H ₂ O 757.4 total H ₂ O 757.4

In this context, the sum of Fe is the sum of the metals Fe. Ni and Cr understood, the majority of Fe being formed.

This pickling agent (1) was evaporated in batches in a vacuum evaporator at a pressure between 200 and 500 mbar and at temperatures between 80 and 90 ° C. This gave a metal-containing concentrate (3) with the following composition:

Evaporation residue stream (3)

% By weight kg / h

H ₂ O (l) 20.6 41.6

FeF ₃ (s) 44.0 88.9

HF (aq) 0.0 0.0

HHNNOO ₃₃ ((aaqq)) 2266..22 52.9

FeOO _? ) ₃ (s) 9.2 16.9

Temperature: 100 ° C

kg / h% of (l) total Fe 44.0 100.0

Total HF 43.1 53.2

Total ENT, 61.5 17.6 The mixed acid evaporating in the evaporator (002) was recondensed in the condenser (003) to a condensate according to the table below and transferred to the mixed acid tank (004). A total of 440 1 condensed mixed acid (2).

Condensed mixed acid (2)

≠ kg / h

H ₂ O (l) 81 1.7 719.5

FeF ₃ (aq) 0.0 0.0

HF (aq; 42.8 37.9 H HNNOO ₃₃ ((aaqq)) 3 32255..55 288.5

Temperature: 30 ° C kg / h% of (l)

Total Fe 0.0 0.0

Total HF 37.9 46.5

SSummummee HHNNOO ₃₃ 228888..55 81.5

The evaporation residue, i.e. H. the metal-containing concentrate (3). was rinsed from the evaporator (002) with water into the stirred tank (005) and dissolved there with further addition of water while stirring, resulting in 380 1 low-acid, metal-containing solution (6) with the following composition:

Diluted evaporation residue (6) g / i kg / h

H ₂ O (l) 930.7 682.8

FeF ₃ (aq) 108.7 81.0

HF (aq) 0.0 0.0

HNO3 (aq) 65.1 48.2

Fe (NO ₃ ) 3 22.8 16.9

Temperature: 30 ° C Density: 1.13 kg / 1

kg / h% of (l)

Total Fe 44.0 100.0

Total HF 43.1 54.3

Total ENT ₃ 61.5 18.0 The low-acid pickling agent (6) was then pumped from the container (005) into the flue gas scrubber (006), with a partial stream being circulated through the scrubber (006) and another partial stream (1 1) in a spray rust reactor (007) subjecting the pyrohydrolysis to subordinate has been

This reactor was heated with 7 m ^' h of natural gas and 85 m7 h of air, a temperature of about 550 ° C. being established in the burner level. The partial stream (1 1) of the pickling agent (6) was fed into the reactor head in an amount of 75 1 Ti injected

The roasting gases (10) were sucked out of the reactor (007) by the negative pressure of the fan (15) and passed through the rust gas line (008) first into the dust separator (009) and then into the flue gas scrubber (006). The temperature of the rust gas

(10) was approx. 300 ° C immediately after leaving the reactor. The gas (15) enriched with water vapor in the flue gas scrubber (006) then had the following composition -

Roasting gas after flue gas ashing (15)

NmVh kg / h

H _: O (g) l 145.7 920.6 F FeeFF-- ,, ((ss)) 0.0

HF (g) 48.3 43.1

NO 12.4 16.6

O: 43.5 62.1

CO ₂ 65.6 128.8

Temperature 85 ° C

kg / h% of (1)

Total HF 43.1 53.2

Total ENT; 61.5 17.6

This enriched rust gas (15) reaches the first absorption column (01 1), where the absorption of the HNO ₃ . HF and part of the NO: component in the NO according to the equation 3 NO ^ - H - »2 HNO * NO

he follows.

Since all processes taking place in the column (01 1) are highly exothermic, the column circuit (16) was cooled to approx. 75 ° C. in order to keep the vapor pressures of HF and HNO ₃ as low as possible. This also reduced the fresh water supply to almost 0.

Finally, 57.6 kg'h or. 54 1 h of a regenerated mixed acid (12) at 75 ° C. with a density of 1.07 g l and the following composition:

Regenerated mixed acid (12) g 1 kg / h

H ₂ O (l) 931.4 50.0

FeF ₃ (aq) 0.0 0.0

HF (aqj 76.1 4.1

HNO ₃ (aq) 62.5 3.4

Based on the sum of the values of material flow (2) and (12), there is a recovery rate of 91.1% for ENT ₃ and a recovery rate of 97.4% for HF. This is made possible on the basis of the method according to the invention because 81.5% of the nitric acid is already separated off by evaporation and therefore only 18.5% of HNO ₃ are still available for decomposition reactions.

In contrast, in processes known from the prior art, the entire amount of HNO _{3 is available} for decomposition reactions. A much larger amount of HNO _{3 is} decomposed. The highest achievable recovery rate when recovering from a metal-containing solution is approximately 80% according to the prior art.

Claims

claims

1 procedure for the recovery or recovery of acids, in particular hydrofluoric acid and nitric acid from metal-containing solutions, characterized in that the metal-containing solution is evaporated at temperatures at which essentially no decomposition of the nitric acid takes place, that the metal-containing residue of the evaporation is treated with pvrohvdrolviisch, a conversion of the metal fluids into metal oxides takes place, and that acids present in the rust gases formed are separated from the rust gases

2 The method according to claim 1, characterized in that the temperature when evaporating the metal-containing solution is less than 200 ° C. is preferably 80 to 120 ° C.

3 The method according to claim 1 or 2, characterized in that the evaporation is carried out at reduced pressure

4 The method according to any one of the preceding claims, characterized in that the pyrohydrolytic treatment of the metal-containing residue is carried out in a spray rust reactor

5 The method according to claim 4, characterized in that the temperature in the burner level of the spray rust reactor 500 to 700 ° C. is preferably 550 to 650 ° C

6 The method according to any one of the preceding claims, characterized in that the rust gases emerging from the reactor for pyrohydrolytic treatment are brought into contact with the metal-containing residue of the evaporation in a flue gas scrubber

7. The method according to any one of the preceding claims, characterized in that the rust gases for removing the acids located in the rust gases are passed through at least one absorption column

8. The method according to claim 7, characterized in that the temperature within the absorption column is more than 70 ° C.

9. The method according to claim 7 or 8, characterized in that part of the acidic solution emerging from the absorption column is circulated over the column.

10. The method according to any one of claims 7. 8 and 9. characterized in that the metal-containing residue of evaporation is dissolved or diluted with absorption liquid from at least one of the absorption columns used.