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  Manufacture of Nitric acid, Chemistry tutorial

Introduction:

Nitric acid as well termed as aqua fortis and spirit of inter, is a highly corrosive mineral acid. This is the most significant and helpful oxyacid of nitrogen. Nitric acid is of great commercial significance in the manufacture of different fertilizers, explosives, fibres, dyestuffs and plastics. A growing and very significant utilization of nitric acid is the substitution of sulphuric acid in the acidulation of phosphate rock. Pure nitric acid is generally colorless however the impure form tends to obtain a yellow cast because of decomposition into oxides of nitrogen and water. Nitric acid is entirely miscible with water. This solution consists of a boiling temperature of around 120.5oC at 1atm. Commercially available nitric acid is an azeotrope having water at a concentration of 68% HNO3, that is the ordinary concentrated nitric acid.

Manufacture of Nitric Acid:

Nitric acid is prepared on an industrial scale via three methods. These are:

1) From Chile saltpetre or nitrate

2) Brikland and Eyde process or the arc process

3) From oxidation of ammonia or the Ostwald's process

We shall now illustrate each of these methods and what it comprises. 

Manufacture of nitric acid from Chile Saltpetre:

This has been the process of preparation of nitric acid from early days. Nitric acid has been made up from sodium nitrate extracted from the Chile saltepetre (this comprises 35 to 60% NaNO3, a few KNO3 and NaCl) or nitre (that is, potassium nitrate). The chilesaltpetre is lexiviated by water and then crystallized to obtain crystals containing around 95% NaNO3 and some KNO3.    

How in practicality is nitric acid made up from NaNO3? Well, a mixture of around equivalent weight of sodium nitrate and 93% sulphuric acid is heated in a cast iron retort fitted by an outlet pipe close to the bottom to take out the solution of NaHSO4. The reactants are heated to around 2000C by the hot gases increasing from the furnace that is heated by coal fire. The vapors of nitric acid are cooled and condensed in water cooled silica pipes and the acid is collected in the stone ware receiver.

The uncondensed vapors are scrubbed by water in an absorption tower packed by stone ware balls and via which cold water is made to circulate. The dilute nitric acid collected in the receiver is re-circulated till it becomes concentrated. The residue (that is, sodium bisulphate) in the retort is eliminated via the outlet pipe at the bottom of the retort. The reaction which occurs in this method is as illustrated:

NaNO3 + H2SO4 → NaHSO4 + HNO3

The Arc method for manufacturing nitric acid:

In the arc method, carbon-dioxide and moisture are first eliminated from air and the dry air is then passed via a chamber having an electric arc stuck between two copper electrodes connected by an A.C dynamo. Cold water is circulated constantly via these electrodes. A powerful electromagnet put at right angles to the copper electrodes spreads the arc in form of a disc. The chamber is as well given with inside suction pumps by means of which air is drawn in via the holes of the refractory fire work and then forced rapidly across the flame. At the temperature of the flame that is around 2000oC, the oxygen and nitrogen in the air join to form nitric oxide. The exit gases leaving the chamber at around 1000oC are allowed to pass via the fire tube boilers in which steam is generated. The gases are then allowed to pass via oxidation chambers. In the oxidation chambers, the nitric oxide is further oxidized to nitrogen peroxide. The exit gases from the oxidation towers are then passed via a sequence of absorption towers filled by broken quartz via which cold water or dilute nitric acid is constantly sprayed from the top. The gases enter the first absorption tower at the base and leave at the top. The third tower is fed by cold water and the dilute nitric acid collected at the base is re-circulated to the top of the proceeding tower. In this manner, around 50% HNO3 is obtained at the base of the first tower. 

In the last absorption tower, the remaining gases leaving it contain traces of oxides of nitrogen. Such gases are allowed to pass via two wooden towers in which a dilute solution of sodium carbonate is sprayed. The solution at the base of the sodium carbonate tower is evaporated and the crystals of sodium nitrate collected. 

The reaction is as shown:

N2 + O2 → 2NO

2NO + O2 → 2NO2

4NO2 + 2H2O + O2 → 4HNO3

The Ostwald's Process:

This is the modern process of manufacturing nitric acid and around 90% of nitric acid is now made up by this method. The method entails four basic steps. These are:

  • Oxidation of NH3 to NO
  • Oxidation of NO to NO2
  • Absorption of NO2 in water
  • Concentration of HNO3

The essential raw materials for the preparation of nitric acid via Ostwald's process are anhydrous ammonia (that is, obtained from synthetic Haber method), filtered air, water and platinum rhodium gauze as catalyst.

How is nitric acid acquired by this method? Let us in brief illustrate the process.

A mixture of compressed dry air and anhydrous ammonia in the ratio of around 89:11 parts is fed to the catalyst chamber. The mixture readily passes via a platinum rhodium gauze cylinder that is suspended vertically and closed at the bottom by a silica lid. This cylinder acts as a catalyst, which is electrically heated initially to 700oC to start the reaction. The heat of reaction keeps this temperature and ammonia is rapidly oxidized to the nitric oxide. 

The product gases from the chamber, encompassing 10 to 12% nitric oxide are passed via heat recovery units, a quench unit for rapid cooling and then to the oxidizer absorber system. Whenever cooled to around 200oC, the nitric oxide undergoes further oxidation via air to nitrogen dioxide. The oxidation occurs in the oxidation towers that are vertical iron cylinders lined by acid proof stone. 

The NO2 is further cooled to around 50oC and absorbed in absorption towers filled by broken quartz over which cold water is constantly sprayed. The product obtained from the absorption towers includes 57 to 60% HNO3 solution that can then be concentrated.

Uses of Nitric Acid:

Nitric acid is mainly employed in the manufacture of:

Fertilizer (example: ammonium nitrate)

  • Explosive (Example: nitroglycerine, cellulose polynitrate, and so on)
  • Fibres
  • Plastics
  • Dye stuffs

This is as well employed in the purification of gold and silver.

Nitric acid serves up as an oxidizing agent and is as well employed as a laboratory reagent.

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