Concentrated Nitric Acid (Via Mg(NO₂)₃ Process)

Concentrated Nitric Acid

The maximum boiling azeotrope of nitric acid and water (69 percent nitric acid) requires extractive distillation to produce concentrated nitric acid when starting with acid that is weaker than the azeotrope. Sulphuric acid has been used for this extractive distillation, but its use requires high maintenance costs. Magnesium nitrate is being used in several commercial plants in extractive distillation of nitric acid and was selected rather than other nitrate salts since it has the most favourable combination of physical properties. The weak nitric acid solution from battery limits is introduced of the Dehydrating Tower. This solution is concentrated to 98.5% NO3H with diluted magnesium nitrate which is used as dehydrating agent.

The nitric acid vapors produced in the bottom of the column and leaving the top are condensed and cooled in the Condenser. The condensed product isa 98.5% wt nitric acid solution. A portion of this liquid is returned as reflux to the top of the column by means of the Acid Reflux and Product Pumps. The remainder acid product is sent to battery limits.

The concentrated magnesium nitrate solution used as dehydrating agent is continuously fed to the top of the Dehydrating Tower. This concentrated nitrate solution removes most of the diluting water of the weak nitric acid solution feed when they mix inside the tower. As the salt solution goes down the tower, its concentration in water increases.

Part of the salt solution collected in the bottom of the tower is sent to the Salt Concentrator Heater where a certain amount of water is evaporated. The concentrated solution formed in this evaporation is separated in the Salt Evaporator from where it is recirculated, as previously said, to the middle of the tower by means of the Concentrated Salt Pump.

The remaining portion of the liquid settled in the bottom of column is recycled to the Dehydrating Tower through the Dehydrating Tower Reboiler.

The Salt Evaporator drum is provided with an U tube heater to avoid freezing during shutdowns. The attached drawing shows the critical operation zones for magnesium nitrate. The water vapors produced in the Salt Evaporator during the salt concentration operation are condensed in the Salt Concentrator Condenser and drained to Battery Limits.

The uncondensed gases leaving the dehydrating Tower Condenser pass through the Gas-Liquid Separator and, jointly with the non condensed vapors from the Salt Concentrator Condenser, are aspired by the Vacuum Pumps used to produce vacuum in the Dehydrating Tower and in the Salt Concentrator Condenser.

Salt Preparation Unit

Magnesium nitrate solution must be periodically filtered in a Salt Filter to remove residues produced during the nitric acid concentration operation.

All the salt solution hold-up should be emptied to one of the Storage Salt Tanks prior to the filtering operation.

The filtration of the magnesium nitrate solution should be done after dilution with water to 30/36% to avoid salt freezing (Diluted salt solution is pumped by means of the Diluted Salt Pump to the Precoat Mix Tank where precoat agent is added to the first salt charge. This first precoat charge is recalculated with the Filter Feed Pump through the Salt Filter).

Meanwhile, circuit and equipment should be made up with clean magnesium nitrate solution salt, by means of the Dilute Salt Pump, with product from the Salt Storage Tank that should be previously filled with clean salt solution.

Consumption figures (per ton of product /calc. as 100% HNO3/)

Raw Materials

Weak Nitric Acid                                        1005 kg
Auxiliary Magnesium Carbonat                     0.6 kg

Utilities

MP Steam                                                2375 kg
LP Steam                                                     70 kg
Cooling Water                                              95 m³
Electric Power    16.3 kWh