DE1129144B - Process for the regeneration of alkali solutions containing arsenite which have been used to remove carbon dioxide from gas mixtures - Google Patents

Description

The invention relates to a method for the regeneration of alkali solutions containing arsenite, which served to remove carbon dioxide from gas mixtures, by excreting one Reduction of the absorption capacity causing arsenate or the compounds of the pentavalent Arsenic from the alkali solutions.

It is known to reduce the arsenate content in a carbon dioxide scrubbing solution in such a way that the arsenate is precipitated by crystallization, and magnesium salts can also be added, which leads to the formation of the sparingly soluble magnesium arsenate. A method for reducing arsenic acid or arsenic compounds in an acidic environment is also known. ¹ p

On the other hand, the invention is based on the object of reducing the arsenate to arsenite under certain conditions Carry out conditions and thereby the disadvantages of the known regeneration processes to avoid.

The object is achieved in that the arsenate-containing solution is withdrawn from the circuit and the arsenate in the alkaline medium of the solution itself in the presence of reducing agents causing the reduction of the arsenate to arsenite by heating to 150 ° C, preferably exceeding 250 to 280 ^{Temperatures below 0} C are reduced to arsenite.

The inventive method is the ineffective or inhibitory arsenate for the purpose of Carbon dioxide absorption converted into active arsenite.

Formaldehyde, acetaldehyde, Methyl alcohol, ethyl alcohol, formic acid, oxalic acid, tartaric acid, hydrazine and its salts, Hydrosulfites, arsenic, antimony, bismuth, sulfides, hydrocyanic acid, Cyanide or iron cyanide can be used.

Furthermore, it is useful that the heating provided according to the invention in a closed System is made while maintaining the liquid phase.

A modification of the process according to the invention is that the regeneration is fed Solution is enriched with carbon dioxide.

It is also possible to add the reducing agent containing arsenate and adding the reducing agent Solution first evaporated to dryness and the salt-like residue in the presence of non-oxidizing Gases heated to a temperature exceeding 150 ° C.

containing alkali solutions,

which were used to remove carbon dioxide from gas mixtures

Applicant:

»Vetrocoke« S.p.A., Porto Marghera, Venice (Italy)

Representative: Dipl.-Ing. H. Bohr, Dipl.-Ing. S. Staeger,

Munich 5, Müllerstr. 31,

and Dr.-Ing. H. Fincke, Berlin-Lichterfelde West,

Patent attorneys

Claimed priority:

Italy from March 7th and April 26th 1958

(No. 585 464, and No. 6477)

Giuseppe Giammarco,

Porto Marghera, Venice (Italy),

has been named as the inventor

When the improvement according to the invention is used, those agents are preferably selected from these agents which exert their reducing effect without leaving a foreign compound in the solution which would accumulate and require a subsequent cleaning process. From this point of view, it is particularly preferred to use free organic acids such as formic acid and oxalic acid, etc., which exert their reducing effect by _{decomposing in water and CO 2} , e.g. B. according to the following implementations:

As ₂ O ₅ + 2 HCOOH = As ₂ O ₃ = 2 CO ₂ + 2 H ₂ O or
As ₂ O ₅ + 2 (COOH) ₂ = As ₂ O ₃ + 4 CO ₂ + 2 H ₂ O

According to one possible application of the improved method according to the invention, for alkaline solution containing the arsenate to be reduced of one of the abovementioned reducing agents

203 579/254

are attacked in a corresponding stoichiometric amount in light chrome steels as well as others in Excess in relation to the experiments to be reduced to arsenite, such as porcelain and quartz, Amount of arsenate added, the resulting are unusable. On the basis of the attempts was Solution - preferably after heating - one found that this disadvantage can be remedied, pressurized autoclave at temperature 5 if precious metals are used; of these is of over 150 ° C, as defined above, preferably with success silver in the form of a thin-walled interior between 250 and 280 ° C, the lining of the autoclave has been used, is supplied because in the last-mentioned tempe- What the choice of favorable temperatures at the raturspanne the implementation in the technical field application of the method according to the invention Success expires. ..-- "". io, it should be noted that the conversion time with In operation it was found that the reducing increase in temperature decreases and that a Effect of the above agents also on the formation of elevated temperature, e.g. B. over 250 ° C, despite the aufvon elemental arsenic can lead to corrosion which, however, leads to the use of precious metals prolonged treatment in the autoclave gradually caused by metals, and the increased operating pressure in the according to an implementation of the following type: 15 Autoclave - as mentioned above - preferable

':. ■ is because it accelerates the reduction process

3 As ₂ O ₅ , -f 4 As = 5 As ₂ O ₃ ^ _n J _-

is reabsorbed by the solution because the arsenic According to another method carried out itself - as mentioned above - to the reducing, the reduction of the arsenate to arsenite goes with the help Belonged to fabrics; the reducing effect of the reducing agents mentioned above The agent used ultimately always leads to the same thing when one is dry, i. H. with solid phase that quantitative result. It can therefore reach temperatures in excess of 150 ° C. In It can be assumed that the basic reduction, in this case, is the arsenate to be reduced Implementation in reality consists of two substances which contain one another and which are followed by a reducing substance Reactions exists, with the first 25 enriched solution earning to the dry state Effect of the reducing agent to form evaporates, then the salt-like residue at space of Arsenic metal leads and at the second this pressure to a temperature exceeding 150 ° C in the latter - in turn, reducing the same with the presence of an inert, non-oxidizing gas, implements the arsenate. such as nitrogen or carbon dioxide, heated;

This second reaction is obviously 3 ° after this treatment, the salt-like residue

by stirring, which facilitates contact, dissolved and reused,

favorable; It was thus established that the stirring of the. It is to be noted that in general the

Solution significantly promotes the reduction, so that for dry reduction as with the heating in the autoclave

this takes about 3 to 4 hours. a small amount of arsenic metal is formed, which

In addition, it was found that the reduction in 35 in this case was also due to local over-

Arsenate to arsenite is slightly larger at high temperature according to the heating.

Invention is then facilitated when the solution In this process, the reduction occurs in the

has a higher CO ₂ content or if the corrosion characteristic in the liquid phase does not show up,

the same alkali present at least in part so that ordinary steel can be used and

In the form of bicarbonate. It is therefore recommended that a stainless nickel-chromium steel is well suited; the

absent, the arsenate to be reduced ent- Dry reduction has the disadvantage that only

holding solution a corresponding amount of coal can be worked discontinuously while

to add dioxide or the solution to be heated to, on the other hand, the reduction in the liquid phase

that point of the * cycle for CO ₂ absorption can be carried out,

to infer from which it is most important to 45 As regards the choice of favorable temperatures,

contains carbonate, d. H. at the end of the absorption stage. so are the higher temperatures in this case too,

Finally, the reaction with concentrated solutions, for example in the above-mentioned range between trated solutions, is more favorable, which also has the advantage of 250 and 280 ° C, which is preferable, which, however, brings the temperature that with the contents of the decomposition autoclave, which is harmful in this case, remains the same a larger amount of arsenate can be reduced 50 reaction of the arsenite in arsenic metal and arsenates. For this purpose, the begins to be reduced, should not exceed. Arsenate, also beforehand, from the CO ₂ absorbing solutions. . In the presence of ammonia or precipitates as magnesium - ammonium arsenate, for example, can be obtained from a device for washing CO ₂ under then advantageously the reduction process pressure by means of a pressure enriched with arsenate. Sodium arsenite solution is continuously a 15 l / h

In the technical application of the quantity of solution corresponding to the invention with the following improved process, the result was obtained: special difficulties with regard to the building material with _{methyl orange titratable} Na ₂ O .. 85.2 g / l

for the autoclave in which the reduction is taking place. _{Ah A§2} Q ₃ expressed arsenate 82 g / l

It was found that at 150 ⁰ C over- _{As Ag Q from} | _{depressed arsenite 68} J ) _x

rising temperatures and especially with

Temperatures of about 250 ° C a corrosion 65 The solution is continuously at 0.45 l / h

effect occurs, the offset at the same time by the alkali and 80 ° / ₀ formic acid solution and then

arsenic is caused, which is why not only

homely steel, but also the rust-free nickel-clad autoclave of about 501 contents.

out, in which the solution is ^{heated to 265 0} G and reaches a pressure of 45 atü. When leaving the autoclave, the solution has the following composition:

With methyl orange, titrable Na ₂ O .. 103.2 g / l _{Arsenate expressed as As 2} O ₃ .... 24.5 g / l _{Arsenite expressed as As 2} O ₃ 125.5 g / l

Example 2

A solution containing 193 g / l of K ₂ O, 121 g / l of _{arsenite expressed as As 2} O ₃ with 45 g / l of _{arsenate expressed as As 2} O ₃ becomes 21 g / l of formic acid, used for CO _{2 absorption and titrable with methyl orange} added, whereupon the solution is fed to an autoclave at a temperature of 235 ° C. and a pressure of approximately 30 kg / cm ^2. After the treatment, it can be seen that the _{arsenite content expressed as As 2} O ₃ has increased to 153 g / l, while that of the _{arsenate expressed as As 2} O _{3 has} decreased to 13 g / l.

Example 3

With a similar solution as in Example 2 using a stoichiometrically corresponding one Amount of oxalic acid, d. H. 41 g / l, similar results are obtained.

Example 4

With a solution containing 195 g / l K ₂ O titrable with methyl orange, 126 g / l _{arsenite expressed as As 2} O ₃ and 46 g / l _{arsenate expressed as As 2} O ₃ , to which 14 g / l tartaric acid are added, the decrease the expressed as as ₂ O ₃ Arsenatgehalt after heating to 28O ⁰ C at 41 g / l.

Example 5

In a solution according to Example 4, 7 g / l formaldehyde is added and the was heated to 25O ⁰ C, it was found that the expressed as As ₂ O ₃ arsenate is lowered to 31 g / l and expressed as As ₂ O ₃ Arsenite increases to 141 g / l.

Example 6

When the solution according to Example 4 is heated to 260 ^° C. in the presence of 5 g / l methyl alcohol, the _{arsenate expressed as As 2} O ₃ drops to 35 g / l.

Example 7

When using the solution according to Example 4 and heating to 200 ^° C. in the presence of 32 g / l potassium hydrosulfite, the _{arsenate expressed as As 2} O ₃ drops to 34 g / l.

Example 8

When 7.5 g / l of hydrazine are added and the solution according to Example 4 is heated to 260 ° C., the _{arsenate expressed as As 2} O ₃ drops to 16 g / l.

Example 9 · '

Using the same solution as in Example 4 and 14 g / l of arsenic metal by heating to 23O ⁰ C the expressed as As ₂ O ₃ arsenate to 23 g / l decreases.

Example 10 ^6s

_{A K 2} O used for CO ₂ absorption and 193 g / l titratable with methyl orange, 121 g / l as As ₂ O ₃

55 arsenite expressed and 45 g / l solution containing arsenate expressed _{as As 2} O ₃

29 g / l of potassium cyanide are added, whereupon the solution is fed to the autoclave ^{at a temperature of approximately 240 ° C.} After the treatment, it can be seen that the _{arsenite content expressed as As 2} O _{3 has} increased to 145 g / l, while that of the _{arsenate expressed as As 2} O _{3 has} decreased to 21 g / l.

Example 11

A solution used for CO ₂ absorption and containing 195 g / l K ₂ O titrable with methyl orange, 129 g / l _{arsenite expressed as As 2} O ₃ and 43 g / l _{arsenate expressed as As 2} O ₃ becomes 44 g / l potassium ferrocyanide , K ₄ [Fe (CN) ₆ ], whereupon the solution is fed to the autoclave at a temperature of 200 ° C. and a pressure of approximately 16 kg / cm ^2. After the treatment, it can be seen that the _{arsenite content, expressed as As 2} O ₃ , has risen to 156 g / l, while that of the _{arsenate expressed as As 2} O _{3 has} decreased to 16 g / l; the precipitate of the iron compounds formed is removed from the solution by decanting.

Example 12

As an example of the beneficial influence of a higher CO ₂ content in the solution, the following figures are given: With a 93 g / l Na ₂ O titratable with methyl orange, 56 g / l _{arsenite expressed as As 2} O ₃ and 54 g / l as as ₂ O ₃ expressed arsenate containing, in an autoclave in the presence of 25 g / l of formic acid heated to 26O ⁰ C solution of the arsenate reduced to 16 g / l, when in the starting solution, the CO ₂ -Na ₂ O molar ratio 1.15 to 10 g / l with a CO ₂ -Na ₂ O molar ratio = 1.35 and to only 2 g / l with a CO ₂ -Na ₂ O molar ratio = 1.75.

Example 13

As an example of a reduction in a concentrated solution, a 71 g / l Na ₂ O titratable with methyl orange, 119 g / l _{arsenite expressed as As 2} O ₃ and 150 g / l _{arsenate expressed as As 2} O ₃ with 70 g / l formic acid added and the ^{solution heated to 270 °} C. found that the _{arsenate expressed as As 2} O ₃ drops to 1.5 g / l.

Example 14

As an example of a dry reduction, a _{solution containing 191 g / l with K 2} O methyl orange, 126 g / l _{arsenite expressed as As 2} O ₃ and 49 g / l _{arsenate expressed as As 2} O ₃ , mixed with 23 g / l formic acid is used evaporated by boiling at atmospheric pressure until completely dry; the salt-like residue is heated to 220 ° C. _{at atmospheric pressure in a CO 2 atmosphere and during}

Held at this temperature for 30 minutes; those redissolved in the same initial volume of the solution Salts show that the arsenate content has dropped to 12 g / l; there will be the formation of 1.9 g / l arsenic metal established.

Claims (1)

PATENT CLAIMS: 1. Process for the regeneration of arsenite-containing alkali solutions for removal of carbon dioxide from gas mixtures, by excretion of the arsenate causing a reduction in the absorption capacity or the compounds of pentavalent arsenic from the alkali solutions, characterized in that the arsenate-containing solution is withdrawn from the circuit and the arsenate in the alkaline medium of the solution itself in the presence of the reduction of the arsenate to arsenite reduced causing reducing agents in excess by heating at 15O ⁰ C, preferably lying 250-280 ⁰ C temperatures to arsenite. 2. The method according to claim 1, characterized in that that as reducing agents formaldehyde, acetaldehyde, methyl alcohol, ethyl alcohol, formic acid, oxalic acid, tartaric acid, hydrazine and its salts, hydrosulfites, arsenic, antimony, bismuth, sulfides, hydrocyanic acid, cyanides or iron cyanides can be used. 3. The method according to claims 1 and 2, characterized in that the heating is carried out in a closed system while maintaining the liquid phase. 4. Process according to claims 1 to 3, characterized in that the regeneration supplied solution is enriched with carbon dioxide. 5. The method according to claims 1 and 2, characterized in that solution containing the arsenate and mixed with the reducing agent initially evaporated to dryness and heating the salt-like residue in the presence of non-oxidizing gases to 15O ⁰ C for rising temperature. 8th. Considered publications:
German Auslegeschrift No. 1 000 356;
Gmelin, Handbuch der inorganic Chemie, Edition, Vol. As, 1952, pp. 351 to 358. © 209 579/254 5.62