Pumping Sulphuric Acid

The most widely used chemical in industry, sulphuric acid is utilized in almost every market sector in the world.

Sulphuric is an aggressive acid, available in several concentrations from chemical manufacturers, or where a large quantity is used, the acid may be produced on site. Depending on the concentration, sulphuric has an SG ranging from 1–2. The chemical can be either clear or cloudy and is very stable and non-flammable.

• Production of hydrochloric, nitric and phosphoric acids
• Phosphate fertilizer, where sulphuric is converted into phosphoric acid
• Petroleum production where impurities are removed by the acid
• Paper industries where sulphuric is used as part of chlorine dioxide generation and pH correction
• Sugar bleaching
• Wastewater treatment
• Synthetic textiles production such as cellulose fibres
• Pickling of metals
• Catalyst agent for crude oil and petroleum production
• Agrichemicals for potato crop spraying
• Production of Rayon as a reagent with the pulped wood product

The most significant consideration is the safety of the surrounding workforce by preventing exposure of the fluid directly or by the fluid vaporising. 

When the pump leaks, sulphuric acid may also react with exposed metal, releasing toxic fumes. Sulphuric acid is extremely poisonous and will attack the respiratory system, the eyes, nervous system, teeth and the circulatory system.

Although sulphuric is non-flammable in its natural state, the formation of hydrogen gas can be extremely dangerous. The hydrogen gas can form where the corrosion of metallic parts, water ingress (such as valves, boilers, absorption systems etc) and sulphuric react to form the gas. The gas eventually accumulates in ‘pockets’ or stagnant areas of the system and creates a potential risk of explosion.

When reacting with metals, the acid can form sulphur dioxide, which is extremely flammable and poses a significant fire risk.

Depending on the concentration of the acid, the formulation can be extremely corrosive to metals with stainless steel being especially vulnerable.

In recirculation applications such as in the pickling of metals, pickling acid must be collected and re-used. The collected fluid may have abrasive metal particles and contaminants which may be reactive and emit small quantities of toxic fumes and/or vaporise as part of the process.

The pump must have compatible materials. The selected materials largely depend on the concentration and the temperature of the acid. Where the acid is dilute, stainless steel can be quickly corroded such as in the manufacture of batteries. 

Commonly, non-metallic materials such as PP, PE, PTFE, EPDM or FPM are used. The limitation of the non-metallic materials is usually higher temperatures and higher concentrations. 

Where the acid is much more concentrated and a higher temperature, high-grade alloys may be used which have been formulated for chemical resistance and to handle a rise in temperature. 

The aggressiveness of the acid can rise exponentially with the temperature, so the exact concentration and operating system conditions must be known when specifying a pump. Typical materials may be types of titanium, duplex or Hastelloy. 

Where the acid is very high (above 200oc) in temperature and concentration, specialist materials such as cast iron/silicon alloys can be used.

The containment of the acid is vital. The most secure pump type for this is a mag drive centrifugal pump as there are no mechanical seals which wear and will eventually fail or require regular inspection and replacement. 

The mag drive pump uses a magnet assembly to provide the drive to the impeller with o-rings to seal the pump casing. Providing the o-rings and wet-side materials are compatible the unit is 100% leak-free. The magnet and bearing assemblies are also made of high grade materials, such as silicon carbide, samarium cobalt and inconel. 

Magnet assembly pump technology is now preferable to shaft-sealed models, largely due to containment. 

Double shaft-sealed pumps will inevitably fail and allow the fluid into the chamber, which should trigger an alarm sensor, notifying the engineering team. Should the pump catastrophically fail and expose the sulphuric acid to the environment, the surrounding area and workforce will be at risk. Even with the advance of modern seal types, the integrity of the fluid is inherently vulnerable.

Traditionally, the cost of a mag drive pump has always been higher than the shaft-sealed equivalent, however with the cost of ownership including more maintenance inspections, servicing and spares as well as the potential clean-up costs and accident report administration, the mag drive models now make much greater sense, economically.

Sulphuric acid is often handled by air-operated double diaphragm pumps (AODD) such as transfer from an IBC. AODD pumps offer a great deal of flexibility with materials for the casing and wet-side parts. Accessories are available should a diaphragm rupture and cause a leak.

Where a dosing process is required, a peristaltic pump can provide an accurate and reliable flow. 

With a range of hose and flange materials available such as EPDM and PVDF, the pump specification can be tailored to the type of acid and deliver a consistent flow rate. In the event of a hose rupture the fluid will mix with the lubricant. It is an important consideration to select a compatible lubricant such as a silicone type.

For mixing and blending sulphuric solutions the fluids can be mixed to a homogeneous state by using inline mixers. A ‘static mixer’ is a chain of helical mixing elements which fit inside a pipeline, providing turbulent and cutting forces to mix the fluid.

The fluid may also be mixed or blended using a tank and agitator where turbulent force is added by a rotating impeller. Both of these mixing types require the appropriate materials selection, usually a non-metallic dipped coating.