GB2171688A

GB2171688A - Process and apparatus for detoxifying nitrite-containing effluents

Info

Publication number: GB2171688A
Application number: GB08604866A
Authority: GB
Inventors: Karl-Heinz Knirsch
Original assignee: Wieland Werke AG
Current assignee: Wieland Werke AG
Priority date: 1985-02-27
Filing date: 1986-02-27
Publication date: 1986-09-03
Also published as: ATA42586A; DK162763B; AT392459B; DK162763C; FR2577918B1; DE3506825C2; GB2171688B; DE3506825A1; GB8604866D0; FR2577918A1; DK83086A; DK83086D0

Abstract

The detoxification is carried out by amidosulphuric acid (NH2SO3H) in a reaction vessel in which an amidosulphuric acid starting solution with a pH value of about 5> pH >/= 2 is introduced first and the nitrite liquor is then sprayed in at several points below the liquid level, crystalline amidosulphuric acid being added simultaneously onto the surface such that a pH value between about 5> pH >/= 2 is maintained. This procedure ensures an excess of NH2SO3<-> ions at all times, so that the formation of nitrous gases is prevented.

Description

SPECIFICATION Process and apparatus for detoxifying nitrite-containing effluents The invention relates to a process for the batchwise detoxification of nitrite-containing effluents, in particular nitrite-containing liquors, in a reaction vessel.

During oxidative pickling of metals in H2SO4-HNO2 mixtures, toxic nitrous gases (NO,) are formed which are absorbed in counter-current by sodium hydroxide solution, for example in gas scrubbers. The nitrite-containing liquors thus obtained must not be allowed to pass into the effluent, but must be detoxified. This is effected by oxidation by means of hypochlorite or electrolysis to give nitrate, which results in a large increase in the salt content of the effluent, which in turn can cause eutrophication of the water courses. By contrast, detoxification of the nitrite-containing liquors by reduction of the nitrogen in the NO2- ion to elemental nitrogen by means of amidosulphuric acid NH2SO3H (hereinafter referred to as ASS) represents a process which does not pollute the environment.The reaction proceeds as follows: NO2- + NH2SO2H#N2 t + HSO4- + H2O (compare, for example, R. Weiner: "Die Abwasser der Galvanotechnik und Metallinudstrie [Electroplating and Metal Industry Effluents]"; Eugen G. Leuze Verlag, fourth edition, 1973, page 166, and Hoechst AG data sheet "Hoechst amidosulphonic acid for nitrite destruction", November 1980 edition).

In the hitherto conventional detoxification by a standing or batch process, the effluents are pumped into a reaction vessel, the detoxification chemicals are added and the whole is thoroughly mixed, the detoxified liquids then being passed over filtration devices. The filtrate is discharged into water courses or into the sewer system, and the residues are dumped on tips.

In the detoxification process for nitrite-containing liquors, the liquor is introduced first and ASS is applied to the surface of the liquor. It is alsmost inevitable that at the point where the ASS is introduced nitrous gases (NO,) are formed simultaneously along with the detoxification reaction, namely by local over-acidification, especially if the nitrite concentrations are relatively high, that is to way above about 10 g per litre of liquor. The nitrite-containing liquors usually obtained from the gas scrubber units described above, however, contain concentrations of approximately 30 to 60 g of nitrite per litre of liquor, so that the working rate on detoxification must be very low.A certain improvement was achieved, for example, by adding a relatively large quantity of sodium hydroxide solution to the nitrite liquor in order to reduce the formation of nitrous gases when the ASS is admixed. The actual detoxification reaction was then initiated by adding sulphuric acid, but the reaction then starts spontaneously at about pH = 5, and this manifests itself particularly by a rise in temperature and a volume increase due to extensive gas evolution (N2). Foaming of the solution over the edge of the reaction vessel is often unavoidable, since the reaction easily runs away. The procedure described admittedly reduces but does not exclude the formation of nitrous gases.

It is therefore the object of the invention to develop the detoxification process in such a way that the disadvantages described above are avoided.

According to the invention, the object is achieved by the following features: a) an amidosulphuric acid starting solution of a pH value of about 5 > pH 3 2 is initially introduced and b) the nitrite liquor is then sprayed in at several points below the liquid level, crystalline amidosulphuric acid being added at the same time onto the surface, so that a pH value between about 5 > pH 3 2 is is maintained.

This procedure always ensures the necessary excess of NH2SO2- ions, so that the occurrence of nitrous gases (NO,) is prevented. At the same time a high working rate is possible, and the reaction is easily controllable, since it can immediately be stopped by closing the nitrite liquor feed.

Since the detoxification takes place below the liquid level, it is also possible to apply crystalline ASS, since there is no nitrite on the surface which, due to local over-acidification, could cause the formation of nitrous gases. As compared to the known process with the addition of ASS dissolved in water, the introduction of ASS in crystalline form results at the same time in a saving of up to about 50% of the volume of the detoxification charge, since the solubility of ASS in water at room temperature is only about 180 litre.

According to a special embodiment of the invention the pH is adjusted to a value between about 4 > pH > 3.

Preferably, the nitrite liquor is introduced in the vicinity of the bottom of the reaction vessel, in particular in a finely distributed form.

It is advantageous when the amount of ASS starting solution is about 1 to 20% of the end volume of the charge and contains about 1 to 20% of the precalculated quantity of ASS. The pH value of pH 2 thus established is adjusted to the required pH value of 5 > pH 3 2 by adding alkali hydroxide solution to the starting solution.

To economize on ASS, it is advisable to add ASS only up to that quantity which was previously calculated for the total quantity of nitrite liquor to be detoxified and then to maintain the required pH range of about 5 > pH 5 2, in particular 4 > pH > 3, by the addition of dilute mineral acid, in particular sulphuric acid, below the liquid level. The mineral acid likewise is preferably introduced in the vicinity of the bottom of the reaction vessel.

The invention at the same time relates to a reaction vessel for carrying out the process according to the invention. The reaction vessel is characterized by a region which conically tapers towards the bottom and in which a distributor pipe, provided with perforations, for the nitrite liquor is provided.

The cone angle a of the conical region is preferably between 200 and 600.

It is also advisable to provide additionally a distributor pipe, provided with perforations, for the mineral acid in the conically tapering region.

The invention is explained in more detail by reference to the illustrative embodiment which follows.

The drawing shows a reaction vessel 1 in longitudinal section, which has a region 3 which tapers coni cally towards the bottom 2. The cone angle a is about 200 - 600. In this region 3, a distributor pipe 4, which has perforations 5, is provided for the nitrite liquor. In parallel thereto, a distributor pipe 6 with perforations 7 is provided for the dilute mineral acid.

A device 8 is provided for continuous measurement of the pH value. The liquid level at two different heights is indicated by 9' at the start of the detoxification and by 9 at the end of the detoxification.

Numerical example of the detoxification of a charge of 5,000 litres of NO2 --liquor (with about 43 g of NO2-/Iitre of liquor).

500 kg of ASS are precalculated for the detoxification of this quantity of liquor.

In the reaction vessel 1, an ASS solution with about 25 kg of ASS in 250 litres of water (= 5%) is first introduced in such a way that at least the distributor pipe 4 is covered by the solution and the pH sensor 8 is immersed in the liquid. The pH value is then adjusted with NaOH to about 5 > pH 3 2. The nitrite solution is sprayed into the solution through the perforations 5 of the distributor pipe 4 below the liquid level rising from 9' to 9. With simultaneous addition of ASS, the pH range is adjusted with ASS to about 5 > pH 3 2 until the remaining 475 kg of ASS have been consumed. The ASS is sprinkled as crystals onto the surface. A pH range of about 5 > pH 3 2 is then adjusted by the addition of dilute H2SO4 which is introduced through the perforations 7 of the distributor pipe 6. This is continued until the entire 5,000 litres of nitrite liquor have been detoxified.

No formation of nitrous gases is observed during the execution of these process steps.

After the conventional neutralization by addition of NaOH up to about pH = 7, the mixture is filtered through a chamber filter press. The filtrate passes via a final pH control into the external water course or into the sewer system.

For example, about 6,000 litres of nitrite liquor/ hour are detoxified by the process described in a reaction vessel 1 of approximately 7,000 litres capacity.

Claims

1. Process for the batchwise detoxification of nitrite-containing effluents in a reaction vessel in which the nitrite reacts with amidosulphuric acid in the acidic pH range with the formation of elemental nitrogen and hydrogen sulphate, comprising the following features: (a) an amidosulphuric acid starting solution of a pH value of about 5 > pH 2 is initially introduced, and (b) the nitrite liquor is then sprayed in at several points below the liquid level, crystalline amidosulphuric acid being added at the same time onto the surface, so that a pH value between about 5 > pH 3 2 is maintained.

2. Process according to claim 1, wherein a pH value between about 4 > pH > 3 is maintained.

3. Process according to claim 1 or 2, wherein effluent is a nitrite liquor which is introduced in the vicinity of the bottom of the reaction vessel.

4. Process according to any preceding claim, wherein the nitrite containing effluent is introduced in fine distribution.

5. Process according to any preceding claim, wherein the amidosulphuric acid starting solution amounts to about 1 to 20% of the final volume of the charge and contains about 1 to 20% of the previously calculated quantity of amidosulphuric acid.

6. Process according to any preceding claim, wherein the pH value of the amidosulphuric acid starting solution is adjusted by adding alkali hydroxide solution.

7. Process according to any preceding claim, wherein the amidosulphuric acid is added only up to a quantity which was precalculated for the total nitrite containing effluent quantity to be detoxified and that the required pH range of about 5 > pH 3 2, is maintained by addition of dilute mineral acid below the liquid level.

8. Process according to claim 7, wherein the amidosulphuric acid is added only up to a quantity which was precalculated for the total nitrite containing effluent quantity to be detoxified and that the required pH range of about 4 > pH > 3 is maintained by addition of dilute mineral acid below the required level.

9. Process according to claim 7 or 8, wherein sulphuric acid is used as the mineral acid.

10. Process according to any of claims 7 to 9, wherein the mineral acid is introduced in the vicinity of the bottom of the reaction vessel.

11. Reaction vessel for carrying out the process according to claims 1 to 10, wherein said vessel has a region which tapers conically towards the bottom thereof and in which a distributor pipe, provided with perforations, for the nitrite liquor is provided.

12. Reaction vessel according to claim 11, wherein the cone angle a of the conical region is between about 20 and 60 .

13. Reaction vessel according to claim 11 or 12, wherein additionally a distributor pipe, provided with perforations, for the mineral acid is provided in the conically tapering region.

14. Process substantially as herein described with reference to and as shown in the accompanying drawings.

15. Reaction vessel substantially as herein described with reference to and as shown in the accompanying drawings.