IE903419A1 - Process for recovering pure hydrochloric acid from¹HCL-containing flue gases - Google Patents

Abstract

The invention relates to a process for obtaining pure, approximately 18 to 20% strength hydrochloric acid from a contaminated, 1-17% strength hydrochloric acid obtained by washing or quenching of HCl-containing flue gas with water. In this process, the contaminated hydrochloric acid is converted in a first step into approximately 18-20% strength hydrochloric acid by distilling off water and this acid is distilled off in a second step from the impurities remaining in the still bottom.

Description

The invention relates to a process for obtaining pure, approximately 18 to 20% strength hydrochloric acid from a contaminated, 1-17% strength hydrochloric acid obtained by washing or quenching of HC1-containing flue gas with water. In this process, the contaminated hydrochloric acid is converted in a first step into approximately 18-20% strength hydrochloric acid by distilling off water and this acid is distilled off in a second step from the impurities remaining in the still bottom.

PATENTS ACT, 1964 PROCESS FOR RECOVERING PURE HYDROCHLORIC ACID FROM HC1-CONTAINING FLUE GASES 3E0TI0N MW HOECHST AKTIENGESELLSCHAFT, a Joint Stock Company organized and existing under the laws of the Federal Republic of Germany, of D-6230 Frankfurt am Main 80, Federal Republic of Germany. -1IE 903419 -Μ HOECHST AKTIENGESELLSCHAFT - HOE 89/F 315 Dr.MA/St Description Process for recovering pure hydrochloric acid from HC1containing flue gases In many chlorination reactions, residues arise which must be incinerated, such as, for example, in the production of vinyl chloride or of chlorinated hydrocarbons. In these residues, the chlorine content sometimes amounts to more than 50% of the weight. Moreover, the residues frequently contain only a little or no hydrogen, so that the formation of water in the incineration is restricted to a minimum. In this case, it is possible to recover HC1 gas by means of suitable process conditions in the incineration. Such a process is described, for example, in German Offenlegungsschrift 2,262,875, where a concentrated hydrochloric acid is formed which is separated by desorption into HCl gas and azeotropic hydrchloric acid. However, the process described can be used for recovering HCl gas only if the material to be incinerated has a high chlorine content and a low hydrogen content.

Moreover, this will in general involve relatively small plants which have to reprocess only a restricted quantity of residues. The investment costs and the personnel requirement for these plants are less favorable than in larger-scale plants.

In practice, however, the incineration of special refuse is in general carried out in large-scale plants, with a throughput of varying quantities of both chlorine-containing and chlorine-free residues of different composition. As a result, the flue gas has varying contents of HCl and H20. Moreover, yet other compounds, for example SO2 and compounds of mercury, cadmium and arsenic, are frequently also present, and sometimes also the halogens chlorine, bromine and iodine. The purification of the flue gases starts in general with precipitation of the dust by means of filters, in - 2 particular by means of electrostatic precipitators. For separate scrubbing of HCl and S02 out of the flue gas, a two-stage process is conventional, such as is described, for example, by G. Klinke in Chemie-Anlagen und Verfah5 ren, 1976, No. 8, pages 84-86. In this process, the flue gas is scrubbed in the first stage with water, the hydrogen chloride being virtually completely removed, if this first stage is divided into main scrubbing and afterscrubbing (both with water). The hydrochloric acid thus formed has in general a relatively low HCl content which can amount to about 3 g of HCl/kg of hydrochloric acid up to more than 100 g/kg. The concentration of azeotropic hydrochloric acid (202 g of HCl/kg of hydrochloric acid) is, however, not reached in such residue incineration units. In the second stage of the flue gas purification unit, the S02 is then scrubbed out with caustic soda solution.

The hydrochloric acid arising in the first stage cannot be used in the form in which it is produced. On the one hand, it contains impurities which have been scrubbed out of the flue gas with the water, and, on the other hand, it is the relatively low concentration of the hydrochloric acid which militates against efficient neutralization.

The object was then to produce a hydrochloric acid which is as pure and technically utilizable and azeotropic or almost azeotropic, from the dilute and contaminated hydrochloric acid such as is formed in the first stage of flue gas scrubbing. The flue gases can originate from the incineration of diverse materials. HCl-containing flue gases are produced, for example, in domestic refuse incinerators. The quantity of HCl which must be scrubbed out of the flue gas bears a relation to the chlorine content of the refuse. At about 0.7% of chlorine in the waste, about 5 kg of HCl/tonne of refuse result. In the incineration of special refuse of different composition, for example residues from various production processes in a large chemical works, the flue gas contains fluctuating - 3 quantities of HC1 which, per tonne of waste, can amount to more than ten times the value to be expected in domestic refuse incineration. On average, the chlorine content of the wastes, which have to be processed in such a residue incineration unit, is frequently about 5%. For such cases with fluctuating, but overall relatively high HC1 content, a process is required which allows reliable recovery of the valuable hydrochloric acid even under more difficult conditions. In general, the scrubbing waters from the first stage of flue gas scrubbing have HC1 contents of approximately 1-15% by weight, preferably 3-10% by weight. The impurities present in the scrubbing waters are all those formed in the gaseous state during the combustion and dissolve in the scrubbing waters which are acidic (due to HC1), and also residual fractions of filter dust, unless this has been deposited on the electrostatic precipitators. The same also applies to aerosol residues which dissolve in the acidic scrubbing water. For example the following can be present in such an acidic scrubbing waters compounds of arsenic and heavy metals such as zinc, cadmium and lead, and especially mercury chlorides or also metallic mercury, and additionally bromine and iodine compounds as well as sulfates or sulfites.

Surprisingly, very pure azeotropic or almost azeotropic hydrochloric acid can be recovered from such contaminated scrubbing waters.

The invention relates to a process for recovering pure, about 18 to 20% hydrochloric acid from a contaminated 1-17% hydrochloric acid obtained by water-scrubbing or -quenching of HC1-containing flue gas, which comprises converting the contaminated hydrochloric acid by distilling off water in a first stage to about 18-20% hydrochloric acid and distilling off the latter in a second stage from the impurities which remain in the bottom product. In the first stage, the scrubbing water from flue gas scrubbing is evaporated to such an extent that - 4 the HC1 concentration in the evaporated solution corresponds to an azeotropic or almost azeotropic hydrochloric acid, i.e. is about 18-20% by weight. Preferably, such a quantity of water is evaporated off that an azeotropic hydrochloric acid results. In the presence of major quantities of arsenic or bromides or iodides in the scrubbing water, it is frequently advisable to add an oxidizing agent to the scrubbing water, namely before or during the evaporation. The quantity of oxidizing agent which may be added depends on the content of arsenic compounds, bromides and iodides in the scrubbing water, an excess doing no harm. Examples of oxidizing agents which can be used are chlorine (introduction of chlorine gas), hypochlorites or hydrogen peroxide. On evapora15 tion, bromine and iodine pass into the vapor phase and hence ultimately into the condensate from which they can be recovered if desired. The oxidizing agent which may be added also has the function of converting any arsenic compounds present from the trivalent to the pentavalent stage and thus of preventing AsC13 from being distilled off. In the incineration of residues which contain little arsenic, however, the addition of oxidizing agents can be omitted. The major part of the bromine and iodine then do not pass over with the water vapor and remain in the distillation bottoms in the subsequent second stage.

In the second stage, the hydrochloric acid obtained in the first stage is distilled over, it being sufficient if a bottom product having a volume of only 1-5%, and frequently even only 1-2%, of the initial volume of scrubbing water remains. The distillate thus obtained has an HC1 concentration of about 18-20% by weight and is very pure. Surprisingly, impurities are still present only in such a low concentration that the acid meets in this respect the requirements of the German Pharmacopea, 9th edition.

The bottom product from the distillation, which contains virtually all the impurities (apart from bromine and - 5 iodine if an oxidizing agent is added) and especially the heavy metals, can be reprocessed. A particular advantage of the process according to the invention is that only a very small volume of bottom product is formed. As mentioned above, this frequently amounts to less than 2% of the initial volume of scrubbing water, for example about 1.5% if the scrubbing water to be processed has an HCl content of 5%. The bottom product from the distillation can be adjusted to pH 9 to 10 with lime or caustic soda solution in order to effect a precipitation of hydroxides; the heavy metal hydroxides can be separated off by filtration. A precipitation of the heavy metals by l,3,5-triazine-2,4,6-trithiol (TMT) can also be appropriate, if in fact a part of the heavy metals should be bound as complexes. It is, however, also possible to evaporate the neutralized bottom product which indeed has only a relatively small volume. In both cases, the solid residues are taken either to a landfill or to metal recovery.

The mercury, which is present as a chloro complex can also be removed from the not yet neutralized bottom product by means of a strongly basic ion exchanger. This exchanger, which cannot be regenerated, is taken to a landfill after loading with mercury. The bottom product which is then free of mercury can also be taken to a landfill.

Example 1 In a unit for the incineration of residues from various production plants in a chemical works, flue gas was formed which, after filtration, was scrubbed with water in a first scrubbing stage. The acidic scrubbing water (see Table 1 for the composition) which then had to be reprocessed contained about 5% by weight of HCl. 7 kg of this scrubbing water were put into a heatable 10 1 round35 bottomed flask, to which a column (35 x 3.5 cm) packed with Raschig rings was fitted. The content of the flask was heated to 100-108°C, water distilling off. The distillation rate was 750 g/h. After 5,061 g of a distillate virtually free of hydrochloric acid had been distilled off, 1,933 g of crude hydrochloric acid remained in the flask. In a second distillation, the crude hydrochloric acid was distilled from a 2 1 flask through the packed column already described (280 g/h). 1,838 g of pure hydrochloric acid passed over, and 95 g of bottom product remained (see Table 1 for the composition).

Table 1: Component Scrubbing water from Distillate Distillate je from 2nd stage Residue (bottom product) from 2nd stage flue gas scrubbing from 1st staj HCl 5.3% by weight 0 19. IX by weight 16.6% by weight Inpurities (mg/kg): Fluorine 9 2.5 16 120 Bromine 500 100 2 32000 20 Iodine < 10 0 < 100 Sulfate (S0A) 590 < 1 not determined Arsenic 8 < 0.005 16 300 Cadmium 0.03 < 0.005 < 10 Chromium 0.05 < 0.005 30 25 Copper 0.7 < 0.005 30 Iron 14.6 0.04 900 Mercury 0.02 0.003 0.003 0.87 Nickel 1.6 < 0.005 60 Lead 0.8 < 0.01 45 30 Zinc 24 0.03 1400 Example 2 2 kg of the scrubbing water, described in Example 1, from flue gas scrubbing were put into a heatable 4 1 roundbottomed flask, to which a column (35 x 3.5 cm) packed with Raschig rings was fitted. The content of the flask was heated to 100-108°C. After the boiling point had been reached, sodium hypochlorite solution (with 20 g of active chlorine/1) was fed in under control during the entire distillation time. In the course of 200 minutes, 1,446 g of a first fraction were distilled off which contained virtually all the bromine. The consumption of sodium hypochlorite (calculated as active chlorine) was 740 mg. 580 g of crude hydrochloric acid remained in the flask. In a second distillation, the crude hydrochloric acid was distilled from all flask through the packed column already described (240 g/h) . 546 g of pure hydrochloric acid passed over, and 34 g of botom product remained. The compositions of the distillates and of the bottom product are shown in Table 2.

Table 2 Component Scrubbing water from flue gas scrubbing Distillate from 1st sta Distillate ige from 2nd stage Bottom product from 2nd stage 20 HC1 5.3% by weight 18.3% by weight 15% by weight Impurities (mg/kg): Fluorine 9 2.4 17 120 Bromine 500 590 < 2 < 200 Iodine < 10 0 0 < 100 25 Arsenic 8 < 0.005 1.6 Cadmium 0.03 < 0.005 Chromium 0.5 < 0.005 Copper 0.7 < 0.005 Iron 14.6 0.04 30 Mercury 0.02 0.003 0.003 0.9 Nickel 1.6 < 0.005 Lead 0.8 < 0.01 Zinc 24 0.03 HOE 89/F 315

Claims (5)

1. PATENT CLAIMS

1. A process for recovering pure, about 18 to 20% hydrochloric acid from a contaminated 1-17% hydrochloric acid obtained by water-scrubbing or -quenching of HCl-containing flue gas, which comprises converting the contaminated hydrochloric acid by distilling off water in a first stage to about 18-20% hydrochloric acid and distilling off the latter in a second stage from the impurities which remain in the bottom product.

2. The process as claimed in claim 1, wherein an oxidizing agent is added before or during the first stage.

3. The process as claimed in claim 2, wherein the oxidizing agent is chlorine or hypochlorite.

4. a process as claimed in claim 1, substantially as hereinbefore described with particular reference to the accompanying Examples.

5. Pure hydrochloric acid whenever recovered by a process claimed in a preceding claim. Dated this the 21st day of September, 199