Manufacture of Sulphuric acid and Sulphates, Chemistry tutorial


Sulphuric acid is the other heavy inorganic chemical that is produced and employed in a large scale. It has long been the chemical that is prepared in the biggest quantities on a world scale. Sulphuric acid is prepared from elemental sulphur. This element is obtained via mining sulphide- containing ores or in extremely pure form from underground deposits by the Frasch method. Though, the large-scale consumption of petroleum and natural gas has changed the condition. This is so as sulphur takes place as an impurity on most fossil fuels and should be eliminated before the fuels are processed. Therefore, such fuels have become the major source of sulphur. Sulphuric acid is a good catalyst for most of the reactions and is as well employed to produce sulphates, like ammonium sulphate, sodium sulphate and aluminum sulphate. We shall consider how sulphuric acid is produced and as well the manufacture of several sulphates.

Manufacture of Sulphuric Acid:

There are two main methods utilized in the production of sulphuric acid. These are the lead-chamber method and the contact method. The lead-chamber method is the older of the two methods. Though, all over the world, sulphuric acid is now manufactured by the contact method. Let us just talk concerning the Lead-chamber method in brief.

The lead-chamber method results an aqueous sulphuric acid having 62% to 78% sulphuric acid. The major principle behind the procedure, is that moist S02 (that is, sulphur dioxide) in presence of nitrogenous oxides (that is, active catalyst) is oxidized to sulphur trioxide (SO3) by the oxygen present in air. Sulphur trioxide is formed to react by water to produce sulphuric acid. The reaction is as illustrated below:

2SO2(g) + O2(g) (catalyst) → 2SO3(g)

SO3(g) + H2O(l) → H2SO4(l)

The Contact Process:

The preparation of sulphuric acid via contact method is mainly based on the catalytic oxidation of SO2 to SO3. The given steps are comprised in the preparation of sulphuric acid.

1) Preparation of SO2

2) Purification of SO2

3) Oxidation of SO2

4) Absorption of SO3

5) Dilution Oleum.

Let us illustrate all the steps one by one:

1) Preparation of SO2:

Sulphur dioxide is basically obtained through burning sulphur,

S(s) + O2(g) → SO2 (g)    

By roasting pyrite (that is, iron sulphide) or other metal sulphides

4FeS(g) + 7O2(g) → 2Fe2O3(s) + 4SO2(g)

Or via burning hydrogen sulphide,

H2S(g) + O2(g) → SO2(g) + H2O(g)

2) Purification of SO2:

SO2 consists of a number of impurities like dust particles, arsenous oxides, vapors and so on. Such impurities should be eliminated to avoid the catalyst poisoning. How is this accomplished? SO2 is first passed via the dust chamber where steam is spread over the gas to take away dust particles, which settle down. Fe(OH)3 is as well sprayed over to take away oxides of Arsenic. SO2 is then passed via a washing tower after cooling. The gas is sprayed through water to eliminate any other soluble impurities. The gas is now dried by passing through dying tower where concentrated H2SO4 (that is, dehydrating agent) is sprayed. H2SO4 eliminates moisture from SO2.

To make sure that the gas is free from Arsenic oxide (As2O3), a poison for the catalyst, it is passed via a test box where a strong beam of light is thrown against the gas. Whenever there is no scattering of light in the box, it is a sign that the gas is free of As2O3.

3) Oxidation of SO2 to SO3:

This occurs in the contact tower where the catalyst, vanadium (v) oxide, V2O5 is filled in various pipes. Here, SO2 reacts with air (O2) to produce SO3. Under this condition, 98% SO2 is transformed to SO3.

4) Absorption of SO3 in H2SO4:

SO3 is not directly passed in water, as a dense fog of minute particles of H2SO4 is formed. Thus, it is dissolved in concentrated H2SO4 to make pyrosulphuric acid (that is, oleum).

SO3 + H2SO4 → H2S2O7 (Oleum)

5) Dilution of Oleum:

Oleum is now diluted by water to form H2SO4 of required concentration.

H2S2O7 + H2O → 2H2SO4

Pure sulphuric acid is manufactured by this method.

Manufacture of Sulphates:

As we are familiar that sulphuric acid is employed in the production of sulphates like aluminum sulphate, ammonium sulphate, sodium sulphate and so on. The preparation of ammonium sulphate has been illustrated under 'Inorganic Fertilizers'. In this, we will consider the preparation of aluminum sulphate. 

Aluminum sulphate, Al2(SO4)3 is generally applied in the pulp and paper industry and in the purification of water. Alum is made up in a simple two- step method from aluminum trihydrate and sulphuric acid according to the given reaction:

2Al(OH)3 + 3H2SO4 + 8H2O →  Al2(SO4)3.14H2O

Step 1: Reacting the Raw Materials:

The aluminum trihydrate is transported and stored in one tonne bags. Each and every bag is carried by hoist to the feed hopper, where it is opened and put across to the reaction tank. The reaction water, trihydrate and sulphuric acid batch are stirred for a period of time with water vapor exiting via the stack. 

Step 2: Making the Final Product:  

After the reaction period, either liquid or solid alum is prepared. If liquid, the concentrated batch is run to the dilution tank filled by dilution water where it cools over time. The liquid alum is then filtered by pumped to storage.

Whenever solid is prepared, the concentrated alum is run to the casting pans where it sets, aided via fan cooling. The pans are fork lifted to storage racks and from there; they are taken to the crushing plant and dropped to a hopper. The pieces are broken further via a large kibbler followed through a small kibbler and then screened. The screened alum is rotary crushed and further screened to either return to the rotary crusher or be bagged as the ground product. Dust is constantly collected via an extraction system and is sold as product.

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