GB1585869A - Method of producing sulphuric acid - Google Patents

Description

(54) METHOD OF PRODUCING SULPHURIC ACID (71) We, LENINGRADSKY TEKHNOLOGI- CHESKY INSTITUT IMENI LENSOVETA, of Moskovsky prospekt 26, Leningrad, and NAU CHNOISSLEDOVATELSKY INSTITUT PO UDOB RENIAM I INSEKTOFUNGITSIDAM IMENI PRo-- FESSORA YA.V.SAMOILOVA, of Leninsky prospekt, 55, Moscow, both Union of Soviet Socialist Republics, both Corporations organised and existing under the laws of Union of Soviet Socialist Republics, 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:: This invention relates to a method of producing sulphuric acid, which is widely used in the chemical, petro-chemical, machinebuilding, and other industries.

The disadvantages- of the contact method of producing sulphuric acid are the com plèxity of the contact' apparatus which is provided with- many heat-exchangers arranged between the contact shelves for ensuring a high degree of conversion, and also the comparatively low intensity of the process because of using air containing 79 per cent by volume of inert admixtures (nitrogen, argon, etc).

There is also known a method for-preparing sulphuric acid from sulphur dioxide and oxygen, in which unreacted sulphur dioxide is returned to the oxidation stage.

A gas mixture containing sulphur dioxide, oxygen, sulphur trioxide and nitrogen, heated to a temperature of 450 to 470"C in an external heat-exchanger, is delivered to a converter with stationary catalyst beds and with the- same number of gas heat-exchangers.

The conversion of sulphur dioxide to sulphur trioxide is only about 70 per cent per single pass which is due to the high temperature of the gas in the conversion process.

The temperature- in the converter is 680"C.

From the converter the gas mixture is delivered into an external heat-exchanger where it is cooled, and then passed into a condenser; From- the condenser, the gas mixturet having a temperature of 80 to 100"C, passes through three series-connected absorbers where sulphur trioxide is isolated from the gas mixture. As the gas passes through the absorbers and a spray trap, part of it is released to the atmosphere, the remainder being recycled.

Oxygen and sulphur dioxide are delivered into the system in separate streams. Oxygen is admitted to the second absorber and sulphur dioxide is delivered at a point behind the spray trap. Unreacted sulphur dioxide, together with fresh portions of sulphur dioxide and oxygen, are passed through a coke filter, where acid mist is separated, and delivered to a centrifugal pump that ensures circulation of the gas mixture in the system.

The disadvantage of this method is multiple recirculation of the gas mixture to ensure the required conversion (99.5 per -cent), which involves increased consumption of power for recycling unreacted sulphur dioxide, increases the overall size of the apparatus, and hence decreases its efficiency.

The volume of the gas mixture circulating in the system exceeds by 3S times the volume of fresh portions of gases introduced in the system. In this method it is impossible to use technical oxygen containing more than 1 per cent by volume of inert gases.

In addition to circulate gases, centrifugal machines are used that can normally operate only if the gas is thoroughly separated from the acid mist, and if the sulphur dioxide delivered from the furnace is cooled.

Furthermore, the method employs complicated multishelf contact apparatus with heat-exchangers installed between the shelves, and also a condenser, three absorbers, a spray trap, a coke filter, and a centrifugal pump.

The present invention provides a method of producing sulphuric acid, comprising catalytically oxidizing sulphur dioxide with oxygen, the said oxidation being effected in a fluidized bed of a catalyst, withdrawing heat directly from the oxidation stage by means of a heat exchanger, and converting sulphur trioxide from the gas mixture obtained from the oxidation stage into sulphuric acid as the end product, returning part of the resulting gas mixture which contains unreacted sulphur dioxide, excess oxygen and inert gas to the oxidation stage by means of an injector, a gas mixture containing sulphur dioxide being supplied to the injector from an apparatus for converting sulphur or sulphur-containing material into sulphur dioxide, the said gas mixture supplied to the injector being under a pressure higher than that in the circulation system comprising the said steps of catalytic oxidation of sulphur dioxide, conversion of sulphur trioxide into sulphuric acid and returning of unreacted sulphur dioxide, the said steps of catalytic oxidation of sulphur dioxide, conversion of sulphur trioxide into sulphuric acid and returning of unreacted sulphur dioxide being effected under a pressure of from 2 to 20 atmospheres, the excess oxygen at the entry to the said circulation system relative to the stoichiometric amount necessary for sulphur dioxide oxidation not exceeding 1.5 %.

Throughout this specification all the pressure values given are manometric.

The aforesaid steps of catalytic oxidation of sulphur dioxide, conversion of sulphur trioxide into sulphuric acid and returning of unreacted sulphur dioxide are preferably effected under a pressure of from 5 to 12 atmospheres.

Preferably sulphur dioxide delivered to the stage of catalytic oxidation is obtained by melting sulphur, evaporating it by bubbling oxygen through the molten sulphur, and oxidizing the sulphur vapour by oxygen in a fluidized bed of an inert material or a wear resistant oxidic catalyst. Attention is drawn in this respect to our co-pending Application No. 46081/77 (Serial No. 1,560,524).

Sulphur is preferably evaporated and its vapour oxidized under a pressure of from 2 to 35 atm.

To oxidize sulphur vapour in a fluidized bed, it is preferred to use quartz sand, aluminosilicate, or silica gel as inert material, and to use vanadium oxide or iron oxide catalysts as wear resistant oxidic catalysts.

The invention will be further described, by way of example only, with reference to the accompanying drawing, which is a flowsheet of the method according to the invention of producing sulphuric acid.

As can be seen from the drawing, sulphur dioxide obtained by burning sulphur, or raw materials containing sulphur, in an apparatus 1, is delivered to a further apparatus 2 where it is oxidized with oxygen in a fluidized bed of a wear-resistant oxidic catalyst, e.g. vanadium oxide. Heat liberated by oxidation of sulphur dioxide to sulphur trioxide is withdrawn by heat-exchangers 3 located in the reaction zone.

An oxidant containing as much oxygen as possible and as little inert gases (nitrogen, argon, etc.) as possible is preferably used to oxidize sulphur dioxide to sulphur trioxide.

This intensifies the oxidation process and decreases the dimensions of the equipment and pipe-lines.

As sulphur dioxide is oxidized to sulphur trioxide, the gas mixture containing sulphur trioxide, sulphur dioxide, oxygen, and inert gases is delivered to the stage of conversion of sulphur trioxide to sulphuric acid, for example to an absorber 4, where sulphur trioxide is absorbed by sulphuric acid. Sulphuric acid as the end product is discharged from the system through a line 5.

Part of the gas mixture discharged from the absorber 4 is withdrawn from the circulation system through a line 6, and the remaining part of the gas mixture is returned to the oxidation stage.

Sulphur trioxide can be isolated from the gas mixture by cooling to the temperature at which sulphur trioxide is liquefied. Liquid sulphur trioxide is dissolved in sulphuric acid to give sulphuric acid as the end product.

The processes of sulphuric dioxide oxidation to sulphur trioxide, conversion of sulphur trioxide to sulphuric acid, and return o fthe unreacted sulphur dioxide into the system are effected under a pressure from 2 to 20 atm. The elevated pressure increases the conversion of sulphur dioxide to sulphur trioxide by 5 to 20 per cent; however, the gradient of conversion decreases with increasing pressure, and elevation of pressure above 20 atmospheres is unreasonable.

It is preferred that sulphur dioxide be prepared according to the following method.

Molten sulphur, having a temperature of 120 to 150"C, is delivered through a line 7 into a bubbling chamber 8 of the apparatus 1. Sulphur is evaporated in the bubbling chamber by the heat of the reaction of sulphur oxidation with oxygen, which latter is fed into the chamber through a line 9.

The quantity of oxygen delivered for bubbling should be calculated with respect to the heat and material balance.

The temperature of the molten sulphur is near to the boiling point of sulphur, provided the heat is not removed from the bubbling bed. Excess heat can be withdrawn by heatexchangers 10 placed in the liquid sulphur bed. In this case the temperature can be considerably lower than the boiling point of sulphur.

The gas mixture obtained by bubbling oxygen through the bed of molten sulphur contains sulphur vapour, sulphur dioxide, and inert gases. The gas mixture is passed through a grating 11 into a fluidized bed of an inert material. Oxygen is delivered into the fluidized bed through a line 12 in the quantity required for complete oxidation of sulphur vapour, or in the quantity corresponding to stoichiometric as calculated for the summary reaction of sulphur oxidation to sulphur trioxide, or in an excess of up to 1.5 per cent. Heat is withdrawn from the reaction zone by heat-exchangers 13 located in the fluidized bed.

The high heat-transfer coefficient of the fluidized bed material makes it possible to maintain the temperature of the bed at from 600 to 700"C.

A high temperature wear-resistant catalyst can be used as the material of the fluidized bed in the apparatus 1 instead of inert material. This will ensure partial oxidation of sulphur dioxide to sulphur trioxide.

The processes of sulphur evaporation and oxidation of its vapour are preferably effected under a pressure of 2-35 atmospheres.

Carrying out the stage of sulphur burning under a pressure higher than that at the stage of sulphur dioxide oxidation makes it possible to utilize the energy of gas for circulation of the gas mixture in the system.

The gas mixture is circulated in the system by injectors 14.

In the method described, sulphur dioxide is oxidized in a fluidized bed of a catalyst with removal of heat liberated in the reaction directly from the catalyst bed, as a result of which gases containing up to 60 per cent by volume of sulphur dioxide and up to 40 per cent by volume of oxygen are processed.

Carrying out the process of sulphur dioxide oxidation under a pressure of from 2 to 20 atm. makes it possible to attain a conversion of 90 to 95 per cent per pass.

This decreases the volume of gas circulating in the system by 3 to 4 times as compared with the previously known circulating systems used in the manufacture of sulphuric acid, and this makes it possible to use technical oxygen containing up to 5 per cent by volume of inert admixtures. Moreover, the overall dimensions of the apparatus can be markedly reduced.

Using an injector in the method eliminates the need for cleaning the gas discharged from the absorbers from sulphuric acid mist; in addition the gas delivered from the furnace does not require cooling.

The method described for the manufacture of sulphuric acid has no stages of cleaning or purifying gases, and this significantly simplifies the process.

Utilizing a fluidized bed in the processes of sulphur vapour combustion, and sulphur dioxide oxidation, and also using immersed heat-exchangers, simplifies the equipment and makes it possible to do without gas heat-exchangers.

Since the great part of the heat is utilized in the process, vapour, characterized by high energy, can be produced.

The invention will be further described with reference to the following illustrative Examples.

Example I A gas mixture containing 65.5 per cent by volume (64,328 kg/hr) of sulphur dioxide, 34.0 per cent by volume (16,309 kg/hr) of oxygen, and 0.5 per cent by volume (213 kg/hr) of inert gases was delivered under a pressure of 15 atm and at a temperature of 650"C into an injector, where the gas mixture was mixed with a circulating gas.

At the exit from the injector, the gas composition was as follows: 0.04 per cent by volume (78 kg/hr) of sulphur trioxide, 49.2 per cent by volume (71,269 kg/hr) of sulphur dioxide, 37.94 per cent by volume (268,249 kg/hr) of oxygen, and 12.82 per cent by volume (79,327 kg/hr) of nitrogen. The temperature of the gas mixture was 350 to 550"C and the pressure was 10 atmospheres.

The mixture was passed into a contact apparatus, 5.5 m high and 3.2 in diameter.

In the contact apparatus sulphur dioxide was oxidized to sulphur trioxide in a fluidized bed of wear-resistant material, which was a vanadium oxide catalyst. The temperature of oxidation was 450--550"C, and the conversion was 90 per cent. Excess heat was withdrawn by heat exchangers located in the catalyst bed.

The gas mixture discharged from the converter had the following composition: sulphur trioxide 57.92 per cent by volume (80,256 kg/hr), sulphur dioxide 6.27 per cent by volume (7,127 kg/hr), oxygen 19.46 per cent by volume (10,789 kg/hr), and inert gases 16.35 per cent by volume (7,932 kg/hr).

The gas mixture was delivered into a bubbling or foaming absorber, 5 metres high and 3 metres in diameter, where sulphur trioxide was absorbed in sulphuric acid at a temperature of from 70 to 250"C.

After absorption, the gas mixture had the following composition: sulphur trioxide 0.14 per cent by volume (80 kg/hr), sulphur dioxide 14.89 per cent by volume (7,127 kg/hr), oxygen 46.17 per cent by volume (10,789 kg/hr) and inert gases 38.8 per cent by volume (7,932 kg/hr).

The amount of gas withdrawn from the circulating system was 681 kg/hr, and the remaining gas was returned to the system by the injector owing to rarefaction produced by the gas mixture jet issued from the injector under a pressure of 15 atm. The heat withdrawn from the stage of sulphur evaporation, sulphur vapour oxidation, and sulphur dioxide oxidation was utilized to generate steam at a temperature of 4400C and a pressure of 40 atm.

The above-described process yielded 2364 tons of sulphuric acid per day, the conversion of sulphur dioxide in the system being 99.7 per cent. The recirculation factor was 0.3, and the degree of absorption was 99.9 per cent.

Example 2 Liquid sulphur having a temperature of 120--150"C was delivered into a bubbling chamber of a molten sulphur combustion furnace at a rate of 30,234 kg/hr. A mixture of oxygen (4,560 kg/hr) and nitrogen (20 kg/hr) was delivered into the same chamber under a pressure of 25 atm. The furnace was a steel apparatus, lined on the inside.

The diameter of the furnace was 3 metres and the height thereof was 10 metres. As oxygen was bubbled through the molten sulphur bed, having a temperature of 650"C, sulphur dioxide was formed and sulphur was evaporated by the heat liberated in this reaction. Excess heat was withdrawn by heat-exchangers located in the molten sulphur bed.

The gas mixture containing 81.2 per cent by volume (9120 kg/hr) of sulphur- > vapour, 18.38 per cent by volume (25,674 kg/hr) of sulphur dioxide, and 0.42 per cent by volume (20 kg/hr) of inert gases was passed through a grating into a fluidized bed of silica gel (particle size 1.5+0.5 mm). A mixture of oxygen (41,698 kg/hr) and nitrogen (183 kg/hr) was delivered into the same bed.

Sulphur vapour was completely oxidized: to sulphur dioxide in the fluidized bed of silica gel at a temperature of 650 to 660"C.

Excess heat liberated in the reaction of sulphur vapour oxidation of sulphur dioxide was withdrawn by heat-exchangers installed in the silica gel bed.

The gas mixture emerging from the furnace consisted of sulphur dioxide (64.95 per cent by volume, or 60,468 kg/hr), oxygen (34.55 per cent by volume, or 16,024 kg/hr), and inert gases (0.5 per cent by volume, or 203 kg/hr).

The gas mixture, having a temperature of 650--700"C and a pressure of 15 atm., was delivered into the injector where it was mixed with the circulating gas, having. the following composition: sulphur trioxide . 0.59 per cent by volume (735 kg/hr); sulphur dioxide ... 6.68 per cent by volume (6,539 kg/hr); oxygen ... ... 74.45 per cent by volume (36,828 kg/hr); inert gases 18.36 per cent by volume (7,997kg/br).- The composition of the gas after mixing in the injector was as follows: sulphur trioxide ... 0.32 per cent by volume (735 kg/hr); sulphur dioxide ... 34.83 per cent by volume (67,000 kg/hr); oxygen ... ... ... 55.14 per cent by volume (52,852 kg/hr); inert gases ... 9.72 per cent by volume (8,200 kg/hr); The temperature of the mixed gas was 470--550"C, and the pressure thereof was- 15 atmospheres. The gas mixture was' deliveried into a contact appparatus- having a diameter of 2.5 metres and a height of 5 metres.

In the contact apparatus, sulphur dioxide was oxidized to sulphur trioxide in a fluidized bed of a wear-resistant vanadium oxide catalyst, at a temperature of 480 to 550 C.

The conversion was 90 per cent. Excess heat was withdrawn by heat-exchangers installed in the catalyst bed.

The gas mixture emerging from the contact apparatus consisted of 37.51 per cent by volume (76,120 kg/hr) of sulphur trioxide, sulphur trioxide ...

sulphur dioxide ...

oxygen ...

inert gases 1337.5 kg/hr of this gas mixture was withdrawn from the system, while the remaining gas was returned into the injector by the action of vacuum produced in the injector by the issuing jet of the gas fed from the furnace under a pressure of 25 atmospheres.

Heat withdrawn from the stage of sulphur evaporation, sulphur vapour oxidation, and sulphur dioxide oxidation was utilized for the generation of steam at a temperature of 440"C and a pressure of 40 atm.

The above-described process yielded 2,200 tons of the acid per day, the conversion of sulphur dioxide to sulphur trioxide being 99.8 per cent. The recirculating factor was 0.68 and the degree of absorption was 99.9 per cent.

Example 3 The procedure was the same as described in Example 1, except that the pressure of the gas mixture was 8 atm. The pressure in the circulating system was 2 atm. The conversion of sulphur dioxide in the contact apparatus was 85 per cent. The diameter of the contact apparatus, the capacity of which was the same as that of the apparatus utilized in Example 1, was 7 metres and the height thereof was 12 m.

The diameter of the absorber, the capacity of which was the same as that of the absorber utilized in Example 1, was 3.8 metres and the height thereof was 6 metres.

WHAT WE CLAIM IS:- 1. A method of producing sulphuric acid, comprising catalytically oxidizing sulphur dioxide with oxygen, the said oxidation being effected in a fluidized bed of a catalyst, withdrawing heat directly from the oxidation stage by means of a heat exchanger, and converting sulphur trioxide from the gas mixture obtained from the oxidation stage into sulphuric acid as the end product, returning part of the resulting gas mixture which contains unreacted sulphur dioxide, excess oxygen and inert gas 4.14 per cent by volume (6,700 kg/hr) of sulphur dioxide, 46.81 per cent by volume (37,775 kg/hr) of oxygen, and 11.54 per cent by volume (8,201 kg/hr) of inert gases.

The mixture was delivered into a foaming absorber, having a diameter of 2.5 metres and a height of 4 metres. Sulphur trioxide was absorbed in the foaming absorber in sulphuric acid at a temperature of from 70 to 250"C.

The-composition of the gas discharged from the absorber was as follows: 0.51 per cent by volume (761 kg/hr); 6.68 per cent by volume (6,700 kg/hr); 74.45 per cent by volume (37,775 kg/hr); 18.36 per cent by volume (8,201 kg/hr).

to the oxidation stage by means of an injector, a gas mixture containing sulphur dioxide being supplied to the injector from an apparatus for converting sulphur or sulphur-containing material into sulphur dioxide, the said gas mixture supplied to the injector being under a pressure higher than that in the circulation system comprising the said steps of catalytic oxidation of sulphur dioxide, conversion of sulphur trioxide into sulphuric acid and returning of unreacted sulphur dioxide, the said steps of catalytic oxidation of sulphur dioxide, conversion of sulphur trioxide into sulphuric acid and returning of unreacted sulphur dioxide being effected under a pressure of from 2 to 20 atmospheres, the excess oxygen at the entry to the said circulation system relative to the stoichiometric amount necessary for sulphur dioxide oxidation not exceeding 1.5%.

2. A method as claimed in Claim 1, in which the said steps of catalytic oxidation of sulphur dioxide, conversion of sulphur trioxide into sulphuric acid and returning of unreacted sulphur dioxide are effected under a pressure of from 5 to 12 atmospheres.

3. A method as claimed in Claim 1 and 2, in which sulphur dioxide delivered to the stage of catalytic oxidation is obtained by melting sulphur, evaporating it by bubbling oxygen through the molten sulphur, and oxidizing sulphur vapour by oxygen in a fluidized bed of an inert material or a wearresistant oxidic catalyst.

4. A method as claimed in Claim 4, in which sulphur is evaporated and its vapour oxidized under a pressure of from 2 to 35 atm.

5. A method as claimed in Claim 3 or 4, in which quartz sand, aluminosilicate or slica gel is used as the said inert material.

6. A method as claimed in claim 3 or 4, in which a vanadium oxide or iron oxide catalyst is used as the said wear-resistant oxidic catalyst.

7. A method of producing sulphuric acid

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

Claims (9)

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

   diameter of 2.5 metres and a height of

   5 metres.

   In the contact apparatus, sulphur dioxide was oxidized to sulphur trioxide in a fluidized bed of a wear-resistant vanadium oxide catalyst, at a temperature of 480 to 550 C.

   The conversion was 90 per cent. Excess heat was withdrawn by heat-exchangers installed in the catalyst bed.

   The gas mixture emerging from the contact apparatus consisted of 37.51 per cent by volume (76,120 kg/hr) of sulphur trioxide, sulphur trioxide ...

   sulphur dioxide ...

   oxygen ...

   inert gases 1337.5 kg/hr of this gas mixture was withdrawn from the system, while the remaining gas was returned into the injector by the action of vacuum produced in the injector by the issuing jet of the gas fed from the furnace under a pressure of 25 atmospheres.

   Heat withdrawn from the stage of sulphur evaporation, sulphur vapour oxidation, and sulphur dioxide oxidation was utilized for the generation of steam at a temperature of 440"C and a pressure of 40 atm.

   The above-described process yielded 2,200 tons of the acid per day, the conversion of sulphur dioxide to sulphur trioxide being 99.8 per cent. The recirculating factor was 0.68 and the degree of absorption was 99.9 per cent.

   Example 3 The procedure was the same as described in Example 1, except that the pressure of the gas mixture was 8 atm. The pressure in the circulating system was 2 atm. The conversion of sulphur dioxide in the contact apparatus was 85 per cent. The diameter of the contact apparatus, the capacity of which was the same as that of the apparatus utilized in Example 1, was 7 metres and the height thereof was 12 m.

   The diameter of the absorber, the capacity of which was the same as that of the absorber utilized in Example 1, was 3.8 metres and the height thereof was 6 metres.

   WHAT WE CLAIM IS:- 1. A method of producing sulphuric acid, comprising catalytically oxidizing sulphur dioxide with oxygen, the said oxidation being effected in a fluidized bed of a catalyst, withdrawing heat directly from the oxidation stage by means of a heat exchanger, and converting sulphur trioxide from the gas mixture obtained from the oxidation stage into sulphuric acid as the end product, returning part of the resulting gas mixture which contains unreacted sulphur dioxide, excess oxygen and inert gas 4.14 per cent by volume (6,700 kg/hr) of sulphur dioxide, 46.81 per cent by volume (37,775 kg/hr) of oxygen, and 11.54 per cent by volume (8,201 kg/hr) of inert gases.

   The mixture was delivered into a foaming absorber, having a diameter of 2.5 metres and a height of 4 metres. Sulphur trioxide was absorbed in the foaming absorber in sulphuric acid at a temperature of from 70 to 250"C.

   The-composition of the gas discharged from the absorber was as follows: 0.51 per cent by volume (761 kg/hr); 6.68 per cent by volume (6,700 kg/hr); 74.45 per cent by volume (37,775 kg/hr); 18.36 per cent by volume (8,201 kg/hr).

   to the oxidation stage by means of an injector, a gas mixture containing sulphur dioxide being supplied to the injector from an apparatus for converting sulphur or sulphur-containing material into sulphur dioxide, the said gas mixture supplied to the injector being under a pressure higher than that in the circulation system comprising the said steps of catalytic oxidation of sulphur dioxide, conversion of sulphur trioxide into sulphuric acid and returning of unreacted sulphur dioxide, the said steps of catalytic oxidation of sulphur dioxide, conversion of sulphur trioxide into sulphuric acid and returning of unreacted sulphur dioxide being effected under a pressure of from 2 to 20 atmospheres, the excess oxygen at the entry to the said circulation system relative to the stoichiometric amount necessary for sulphur dioxide oxidation not exceeding 1.5%.
2. 2. A method as claimed in Claim 1, in which the said steps of catalytic oxidation of sulphur dioxide, conversion of sulphur trioxide into sulphuric acid and returning of unreacted sulphur dioxide are effected under a pressure of from 5 to 12 atmospheres.
3. 3. A method as claimed in Claim 1 and 2, in which sulphur dioxide delivered to the stage of catalytic oxidation is obtained by melting sulphur, evaporating it by bubbling oxygen through the molten sulphur, and oxidizing sulphur vapour by oxygen in a fluidized bed of an inert material or a wearresistant oxidic catalyst.
4. 4. A method as claimed in Claim 4, in which sulphur is evaporated and its vapour oxidized under a pressure of from 2 to 35 atm.
5. 5. A method as claimed in Claim 3 or 4, in which quartz sand, aluminosilicate or slica gel is used as the said inert material.
6. 6. A method as claimed in claim 3 or 4, in which a vanadium oxide or iron oxide catalyst is used as the said wear-resistant oxidic catalyst.
7. 7. A method of producing sulphuric acid

   substantially as herein described with reference to the accompanying drawing.
8. 8. A method of producing sulphuric acid substantially as herein described in any of the foregoing Examples.
9. 9. Sulphuric acid produced by the method as claimed in any of claims 1 to 8.