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Pumping Nitric Acid

Nitric acid is a highly corrosive and toxic mineral acid that is colourless or yellow-tinged depending on decomposition of oxides of nitrogen and water parts. Nitric acid is widely used as an agent for nitration and oxidization. 

Commonly available between 52- 68%, commercial grade nitric acid is predominantly used in the production of ammonium nitrate, a key compound in fertilizer. Higher concentrations are available such as 86%-95%, known as fuming acid due to the vapours seen when exposed to the air.

Nitric acid is highly reactive with most metals and some non-metallic materials. S.G. is approximately 1.42-1.5.

Applications

  • Used in compounds for explosives and munitions
  • Processed with ammonia for the production of ammonium nitrate for fertilizer
  • As an oxidizer in liquid rocket fuel
  • Treatment of wood for ageing/aesthetics
  • Nylon/Polyurethane
  • Pickling of stainless steel
  • CIP solutions
  • Processing of spent nuclear fuel

Pumping considerations

The two major considerations for handling nitric acid are compatible materials and the containment from the working environment and workforce.

Nitric acid reacts with most metals and selected materials such as stainless steel and aluminium can only be recommended for very dilute concentrations (5-10%). Natural rubber and Buna are also severely affected.

For commercial grade concentrations it is recommended to specify non-metallic wet materials including polypropylene, Kynar, Teflon and ETFE. 

Where higher pressures are in use, a metal construction with non-metallic wet-side coating is an excellent choice. 

Metals such as stainless steel (specific alloy grades), Hastelloy and titanium may be used however this is subject to a degree of corrosion per annum which will increase tolerances, particularly for pumps such as rotary gear. 

Nitric acid is highly reactive on contact with skin, metallic surfaces and may fume in contact with oxygen. These fumes are highly toxic and can affect skin, eyes, teeth, the respiratory system and more. It is therefore essential to have 100% containment of the acid. 

In many premises where nitric acid is handled, any leak may result in evacuation causing immense costs and downtime.

Pump specification

A non-metallic mag drive type pump is recommended for any type of large flow rate. This is due to the excellent containment properties, chiefly as there are no mechanical seals in contact with the fluid. The pumps are 100% leak-free as an o-ring sealing system contains the fluid. 

Mag drive pumps are available in materials that are highly resistant to chemicals with non-metallic ETFE and PTFE lining/cast iron construction widely used. Magnet/bearing assemblies include inconel, ceramic and specialist composites to allow for harsh fluids. 

Where higher temperatures are in use, more advanced alloys can be specified.

For smaller or container transfer amounts, an air-operated double diaphragm (AODD) pump. For very dilute concentrations a standard non-metallic and even metallic type can be used, however for commercial grade and above, it recommended an all non-metallic build should be used, specifically PTFE, PVDF and Viton. 

The ‘solid-block’ type AODD pumps are ideal as there are no metallic parts in contact with the fluid or air. Where the nitric acid is being handled in close proximity to the workforce many premises place the pump inside a fuming cabinet should the pump fail and a leak occur. Many of these AODD pump models are compact builds and operate using only compressed air, making them excellent for ATEX rated zones.