GB1602218A

GB1602218A - Process for purifying an off-gas containing sulphur dioxide

Info

Publication number: GB1602218A
Application number: GB12087/78A
Authority: GB
Original assignee: Kernforschungsanlage Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 1977-03-30
Filing date: 1978-03-28
Publication date: 1981-11-11
Also published as: CH633455A5; JPS53120674A; FR2385433A1; DE2713992A1

Abstract

In a process for purifying a sulphur dioxide-containing exhaust gas by passing the exhaust gas into a sulphite-ion-forming aqueous solution, oxygen is fed to the aqueous solution. As catalyst, activated charcoal is added which was heated to red heat at a temperature above 1000 DEG C in a reducing or inert atmosphere and cooled. In this process, the process steps required for eliminating the toxic activity of sulphur dioxide are substantially reduced. The process proceeds with the use of active compounds which do not have to be regenerated or which only need to be regenerated to a very limited extent.

Description

(54) PROCESS FOR PURIFYING AN OFF-GAS CONTAINING SULPHUR DIOXIDE (71 We, KERNFORSCHUNGSANLAGE JULICH GESELLSCHAFT MIT BESCHRÄNKTER HAFTUNG, of Postfach 1913, 5170 Jfflich, Federal Republic of Germany, a Body Corporate organised according to the Laws of the Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement : The present invention relates to a process for purifying an off-gas containing sulphur dioxide by passing the off-gas into an aqueous solution which forms sulphite ions.

Off-gases containing SO, must be cleaned as extensively as possible, before release into the atmosphere, because of the toxic effect of sulphur dioxide. In mixtures with air, SO produces poisoning symptons at a concentration of as low as 0.04C;0. Hence, the removal of sulphur dioxide from offgases is of considerable importance. Num erous processes for purifying the gases have been disclosed. For example, known processes include adsorption processes and the chemical bonding of the SO. by reaction with carbonates, magnesium oxide, ammonia or citric acid, see for example 'Chemical Engineering". 1972, issue 14, pages 80 et seq. and "Chemical Engineering", 1973, issue 2. pages 59 et seq. Wash ing processes, wherein the SO is taken up by the wash solution, with formation of sulphite ions, are also known. To carry out the known processes, additional chemicals are needed for bonding the sulphur dioxide, and! our additional process steps are necessary in order to regenerate the expensive absorbents or to recover the sulphur compounds formed during the purification processes. Again, when washing sulphur dioxide out of gases, a further treatment of the wash water is necessary, to remove the sulnhite ions which are also toxic.

The invention seeks to provide a process for purifying off-gases containing sulphur dioxide, in which the process steps required to eliminate the toxic effect of sulphur dioxide are or can be substantially reduced, and which take place using active materials which require only very limited regeneration, if any.

The present invention provides a process for purifying an off-gas containing sulphur dioxide comprising passing the off-gas into water or an aqueous solution to form sulphite ions therein, supplying oxygen to the solution, and providing in the solution a catalyst for the oxidation of sulphite ions to sulphite ions with simultaneous reduction of oxygen, the said catalyst provided in the solution being activated charcoal which has been heated at a temperature above 1000"C in a reducing or inert atmosphere or in vacuo and then cooled. The activated charcoal may be coated with nickel, silver or platinum, which are metals suitable for cathodes at which oxygen can be electrochemically reduced in fuel cells.

It has been found that activated charcoal heat treated in the above manner has a surface structure which is particularly suitable for the oxidation of the sulphite, dissolved in the aqueous solution, if oxygen is sup plied. The sulphite is oxidised to sulphate at the surface of the activated charcoal, with simultaneous reduction of oxygen in an amount required for the oxidation of the sulphite. Activated charcoal which has been heated at -a temperature of at least 1100 C in a hydrogen atmosphere for at least 30 minutes is preferred. Charcoal which has been heated for 30 minutes in vacuo at not less than 1100 C and then cooled has also proved a very suitable catalyst.

Air, steam or carbon dioxide can also be employed in the production of an inert atmosphere provided that care is taken to limit the quantity of these gases so that the surface of the activated charcoal is chemically attacked to only a commensurately limited extent.

In the presence of oxygen and sulphide in the acqueous solution an electrochemical potential is set up - without external supply of current - which potential is intermediate between the redox potential of the redox system oxygen/oxygen re duction product and the redox potential of the redox system sulphite/sulphite oxidation product. Under these conditions the sulphite is oxidised to sulphate at the catalyst, with reduction of the oxygen introduced into the aqueous solution. As mentioned, the activated charcoal may be metal coated. Essentially, the particular catalyst materials should be selected in accordance with its chemical compatability with the aqueuos solution, which may be acid or alkaline, taking into account any further materials which may dissolve out of the off-gas, and taking economic aspects into account.

An illustrative embodiment of process according to the invention is described in the following Example: Example Activated charcoal powder having a particle size of up to 0.1 mm is heated for one hour in vacuo at a temperature of 1,100 C, and cooled. This pretreated activated charcoal was suspended in an aqueuous solution which was brought to a pH value of 11 with sodium hydroxide.

The concentration of the activated charcoal in the aqueous solution was 10% by weight.

Off-gas containing sulphur dioxide was blown, simultaeonusly with air, through the aqueous solution. In doing so, the sulphur dioxide constituent of the offgases first went into the aqueous solution, with formation of sulphate. The sulphite was oxidised at the active charcoal catalyst to sulphate, and at the same time the oxygen introduced with the blown-in air was reduced at the surface of the active charcoal in an amount required for the oxidation of the sulphite. Purified gas leaves the aqueous solution.

Apart from activated charcoal which was heated in vacuo at 1,100"C, activated charcoal heated for one hour in a hydrogen atmosphere at 1,100"C was also used.

Using 10% by weight of the activated charcoal powder in the aqueous solution, comparable results were achieved.

The activated charcoal may be coated with nickel, silver or platinum, which also catalyse the oxidation of sulphite in aqeuous solution, to give sulphate, if oxygen is supplied simultaneously. These are metals suitable for the production of cathodes at which oxygen is reduced electrochemically in fuel cells. Large active metal catalyst surfaces per unit volume are provided by coating onto activated charcoal.

WHAT WE CLAIM IS: 1. A process for purifying an off-gas containing sulphur dioxide, comprising passing the off-gas into water or an aqeuous solution to form sulphite ions therein, supplying oxygen to the solution, and providing in the solution a catalyst for the oxidation of sulphite ions to sulphate ions with simultaneously reduction of oxygen, the said catalyst provided in the solution comprising activated charcoal which has been heated at a temperature above 1000"C in a reducing or inert atmosphere or in vacuo and then cooled.

2. A process according to claim 1 wherein there is employed as catalyst activated charcoal which has been heated for at least 30 minutes at not less than 1l00 C in a hydrogen atmosphere.

3. A process according to claim 1 wherein there is employed as catalyst activated charcoal which has been heated for at least 30 minutes at not less than 1100 C in vacuo.

4. A process according to claim 1 wherein the activated charcoal is coated with nickel, silver or platinum.

5. A process for purifying an off-gas containing sulphur dioxide substantially as herein described with reference to the Example.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. duction product and the redox potential of the redox system sulphite/sulphite oxidation product. Under these conditions the sulphite is oxidised to sulphate at the catalyst, with reduction of the oxygen introduced into the aqueous solution. As mentioned, the activated charcoal may be metal coated. Essentially, the particular catalyst materials should be selected in accordance with its chemical compatability with the aqueuos solution, which may be acid or alkaline, taking into account any further materials which may dissolve out of the off-gas, and taking economic aspects into account. An illustrative embodiment of process according to the invention is described in the following Example: Example Activated charcoal powder having a particle size of up to 0.1 mm is heated for one hour in vacuo at a temperature of 1,100 C, and cooled. This pretreated activated charcoal was suspended in an aqueuous solution which was brought to a pH value of 11 with sodium hydroxide. The concentration of the activated charcoal in the aqueous solution was 10% by weight. Off-gas containing sulphur dioxide was blown, simultaeonusly with air, through the aqueous solution. In doing so, the sulphur dioxide constituent of the offgases first went into the aqueous solution, with formation of sulphate. The sulphite was oxidised at the active charcoal catalyst to sulphate, and at the same time the oxygen introduced with the blown-in air was reduced at the surface of the active charcoal in an amount required for the oxidation of the sulphite. Purified gas leaves the aqueous solution. Apart from activated charcoal which was heated in vacuo at 1,100"C, activated charcoal heated for one hour in a hydrogen atmosphere at 1,100"C was also used. Using 10% by weight of the activated charcoal powder in the aqueous solution, comparable results were achieved. The activated charcoal may be coated with nickel, silver or platinum, which also catalyse the oxidation of sulphite in aqeuous solution, to give sulphate, if oxygen is supplied simultaneously. These are metals suitable for the production of cathodes at which oxygen is reduced electrochemically in fuel cells. Large active metal catalyst surfaces per unit volume are provided by coating onto activated charcoal. WHAT WE CLAIM IS:

1. A process for purifying an off-gas containing sulphur dioxide, comprising passing the off-gas into water or an aqeuous solution to form sulphite ions therein, supplying oxygen to the solution, and providing in the solution a catalyst for the oxidation of sulphite ions to sulphate ions with simultaneously reduction of oxygen, the said catalyst provided in the solution comprising activated charcoal which has been heated at a temperature above 1000"C in a reducing or inert atmosphere or in vacuo and then cooled.