NO130723B - - Google Patents

Description

Method for preventing oxidation of

alkali sulfite and/or alkali bisulfite to

alkali sulfate.

The treatment of a SO^-containing gas with an aqueous alkali sulphite-

Unloading is an operation which nowadays is carried out very often. For example, an SO^-containing gas resulting from the combustion of sulphur, sulphur-containing minerals, sulfur-containing fuel or the like is often washed with an aqueous alkali solution to produce alkali sulphite. for recovery or removal of SO,.,. In these treatments, the SO^-containing gas which is treated usually contains oxygen which causes oxidation of alkali sulphite and/or alkali bisulphite to alkali sulphate. The alkali sulphate thus formed is considered an unwanted pollutant in the aqueous ground

of the fact that it is useless in said treatments and also creates difficulties in the recovery of SO^ from the resulting solution by heating or treatment with MgO, ZnO or the like. Removal of the contaminating alkali sulphate from the aqueous alkali sulphite solution thus becomes necessary for the extraction operation of SO2.

For example, the reaction when treating a SO^-containing gas with an aqueous sodium sulphite solution can be expressed by the following reaction equation:

The SO 2 -containing gas subjected to such treatment usually contains 0.05 to 15 volume percent SO 2 and 1 to 20 volume percent 0 2 . The oxygen reacts with the sodium sulphite produced above in the presence of a catalytic substance or light, forming Na2S01+ as shown by the following reaction equation:

The Na2S0L(_) thus formed can no longer release SO^ upon heating. In other words, Na2S0^ cannot be converted into sodium sulphite by a simple process such as heating and therefore becomes useless when treated with a SOg-containing gas.

To prevent the oxidation of alkali sulphite to alkali sulphate has

it has previously been proposed to mix different antioxidants into the aqueous alkali sulphite solution. Examples include the addition of hydroquinone, quinoline, glycine, catechol

and similar proposed in the literature (cf. "Ing.Eng.Chem",,

27,588 (1935), Belgian Patent No. 706.¥f9, Japanese Patent Document No. 20162/1970, etc.) However, the effectiveness of these antioxidants is greatly reduced when metal ions such as Fe, Cu, and Co ions are present in an aqueous solution of alkali sulphite at a

concentration higher than approx. 50 parts per million. Unfortunately, these metal ions are almost always present in SO 2 -containing gases that are usually treated, and when such gases are treated with an aqueous alkali sulfite solution, they are carried into the resulting solution. Contamination, especially with Fe ion, is inevitable on

because it can come from the treatment equipment itself and also from fuel ash.

It has now been found that when treating an SO^-containing gas with an aqueous alkali sulphite or bisulphite solution, the presence of certain chelating agents as antioxidants is quite effective in preventing the conversion to alkali sulphate even in the presence of contaminating metal ions.

The present invention thus relates to a method

to prevent oxidation of alkali sulphite and/or alkali bisulphite to alkali sulphate by treating aqueous solutions of alkali sulphite and/or alkali sulphite with gases containing SO^ and oxygen, by adding an antioxidant to

the aqueous solution, and the peculiarity of the method according to the invention is that a compound selected from the group of chelating acids consisting of ethylenediamine-N,N,N',N'-tetraacetic acid (hereinafter referred to as "EDTA") is used as an antioxidant. nitrilotriacetic acid (hereinafter referred to as "NTA"), 1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid (hereinafter referred to as "CyDTA"), N-oxyethylenediamine-N,N,N'-triacetic acid (hereinafter referred to as "HEDTA"), ethylene-glycol-bis(^.-amL some ty leter )-N,N,N' ,N' - tetraacetic acid (hereinafter referred to as "GEDTA") and ethylenediamine-N,N,N' ,N<!->tetrapropionic acid (hereinafter referred to as "EDTP), their alkali salts (e.g. sodium salt, potassium salt, ammonium salt) as well as mixtures thereof, the treatment being carried out at a temperature of from room temperature to 110°C.

When, for example, an aqueous solution of sodium sulphite (i.e.

Na^SO^ + NaHSO^) and which possibly also contains Fe-ion is brought

in countercurrent contact with air at 50°C, the effect of different antioxidants in the solution on the oxidation of sodium sulphite to Na2S(\ will be seen in comparison in the following table 1.

As will be seen from the table above, the effect of known antioxidants such as hydroquinone is noticeably reduced in the presence of Fe-ion, while EDTA-2Na qg NTA-3Na is hardly affected by Fe-ion and can maintain its effect to a significant extent.

It has never previously been known in the field that these chelating agents effectively prevent the oxidation of alkali sulphite. It is thus worth noting that they not only stop the oxidation-accelerating effect of metal ions, but also

reduces the actual oxidation rate of alkali sulphite with oxygen.

In the experiments, the results of which are shown in Table 1, it was used

air with an oxygen content of 21% by volume. As the rate of oxidation in the treatment of an alkali sulphite solution is generally found to decrease with the decrease in the partial pressure of oxygen and as an off-gas or an outlet gas for the production of sulfuric acid usually contains 2 to 12 volume percent oxygen, the formation of an alkali sulphate in the treatment of such a gas according to the invention becomes so small that one can disregard it.

Sodium sulphite, potassium sulphite, ammonium sulphite etc. are used as alkali sulphite. In an aqueous solution, these may be present together with the corresponding alkali hydrogen sulphite (e.g. sodium hydrogen sulphite, potassium hydrogen sulphite, ammonium hydrogen sulphite). The concentration of alkali sulphite (and the corresponding alkali hydrogen sulphite) varies depending on the species and can be from 20 to 35 percent by weight in the case of sodium sulphite (i.e.

Na2S0^ + NaHSO^), from 30 to 65 percent by weight in the case of potassium sulphite (i.e. K^SO^ + KHSO^) and from 20 to 50

percent by weight in the case of ammonium sulphite (i.e. (M1+)2S0^ + NH^HSO^).

The concentration of the antioxidant in the aqueous alkali sulphite solution varies depending on the metal ion concentration in the absorption solution and will usually be from 50 to 2,000 parts per litre. million, based on the weight of the aqueous sulphite or bisulphite solution.

The treatment of a SO2-containing gas with the aqueous alkali sulphite solution containing the antioxidant according to the invention is carried out as mentioned at a temperature of from room temperature to 110°C.

As illustrated above, the present invention is effective when

it is necessary to prevent oxidation of alkali sulphite by using the aforementioned chelating agents. Thus, the operation for removing alkali sulphate, which is required in normal processes, can be omitted.

Practical and preferred embodiments of the present invention are shown in the following examples in which the stated percentages are percentages by weight.

Example 1.

To an aqueous solution essentially consisting of Ah-, 6%

Na2S03, 16.9$ NaHSO^, 0.2% NagSO^ and 68.3$ water and containing

100 parts per million (ppm) Fe<++>1,300 ppm EDTA-2Na was added

and the resulting solution was countercurrently contacted with air

at 50°0- The formation per hour of NagSO^ was measured at 0.052%/hour as

corresponded to 30$ of what would have been the case of using the same aqueous solution as above, but without EDTA-2Na.

Example 2.

To an aqueous solution consisting essentially of 7.2%

K2S0^, 5V$KHS0^ and 38.8$ water and containing 200 ppm Fe<++>, 250 ppm EDTA-2Na was added and the resulting solution was brought into contact with air as in example 1. The formation per hour of K^SO^ was measured at 0.02$/hour which corresponded to 35"$ of what would have been in the case of using the same aqueous solution as above, but without EDTA-2Na.

Example 3.

To an aqueous solution as in example 1 containing 200 ppm Fe<++ >bee, 500 ppm NTA-3Na was added and the resulting solution was brought into contact with air as in example 1. The formation per hour of NagSO^ was measured at 0.086$/hour which corresponded to 50$ of what would have been the case using the same aqueous solution as above, but without NTA-3Na.

Example h.

To an aqueous solution as in Example 1 containing 100 ppm Fe<++>, 350 ppm of an antioxidant as indicated in Table 2 was added, and the resulting solution was brought into contact with air as in Example 1.

The oxidation ratio is shown in Table 2.

Claims (2)

1. Method for preventing oxidation of alkali sulphite and/or alkali bisulphite to alkali sulphate by treating aqueous solutions of alkali sulphite and/or alkali bisulphite with gases containing SO^ and oxygen, by adding an antioxidant to the aqueous solution, characterized in that a compound selected from the group of chelating acids consisting of ethylenediamine-N,N,N',N'-tetraacetic acid, nitrilotriacetic acid, 1,2-diaminocyclohexane-N,N,N',N' is used as antioxidant -tetraacetic acid, N-oxyethylethylenediamine-N,N,N'-triacetic acid, ethylene-glycol-bis (B-aminoethyl ether)-N,N,N*,N'-tetraacetic acid and ethylene-diamine-N,N,N', N'-tetrapropionic acid, their alkali salts and mixtures thereof, the treatment being carried out at a temperature of from room temperature to 110°C. 2. Procedure as stated in claim 1, characterized in that the antioxidant is used in a concentration of from 50 to 2,000 parts per million, based on the weight of the aqueous sulphite or bisulphite solution.