Pumping Ferric Chloride
Ferric chloride is a widely utilised chemical in industry and municipal water treatment works. Available in granular format or in solution, ferric chloride is a strong acid which is poisonous, corrosive but not flammable.
Approximately 2 million tonnes are produced every year of which the majority goes to the water and waste water treatment sector and the electronics industry. Ferric is produced commonly as a by-product of metals finishing or produced on an industrial scale typically at 38-42% conc.
- More than 80 % of ferric chloride is used in the waste water industry where it provides multiple benefits. Chiefly, it is used as a flocculant where it is highly effective in removing suspended particles. Ferric also has other qualities including deodorizing, sterilizing, removal of phosphorus and at dewatering sludge, prior to disposal.
- In the production of printed circuit boards, ferric is used as an etching chemical for copper material. The ‘Edinburgh etch’ mix with citric acid provides an etching bath liquor with a faster bite for brass and copper elements which also minimises sediment build-up in the fine grooves of circuit boards.
- When reacted with ferrocyanide, ferric is used in pigment production for blue tones such as ‘prussian blue’.
- In PVC production, ferric is used as a catalyst for making ethylene dichloride.
- Ferric chloride is an active ingredient in household antiseptics.
- In bio gas production, ferric chloride can increase quality by minimizing hydrogen sulfide.
The main aspects for specifying a pump for ferric chloride are the chemical compatibility of wet-side materials and the containment of the fluid.
For transfer applications such as the delivery of ferric chloride from a large road tanker or storage vessel a mag drive centrifugal pump or air diaphragm pump are often specified.
Ferric chloride solution at 38-42% is strongly acidic with a pH of less than 1 so it is recommended that a centrifugal mag drive pump with a non-metallic lining be used as stainless steel and cast iron is susceptible to being oxidized by ferric. An ETFE, PP or PTFE lining provides a chemically compatible layer as well as the structural strength of the cast iron construction. Hastelloy C-276 or titanium may be used; however the cost is substantially increased.
As the pump is sealed using an o-ring assembly with no sealing component in contact with a moving part, there is no risk of leakage through abrasive wear, such as seen on shaft-sealed pumps. Where the pump is emptying a vessel or road tanker, a self-priming pump is often specified or a flow meter is installed. The S.G. of ferric can be as high as 1.5, therefore a trimmed impeller, increased motor power or pump size may be necessary.
It is important to note that if a mag drive pump is used in a tank transfer system, run-dry protection is used to maintain flooded suction as without the pump media providing lubrication, the bearing assembly will quickly degrade catastrophically.
A limitation with many mag drive centrifugal pumps is the inability to handle even small particles. Where ferric chloride contains (non-magnetic) particles ‘thrust balancing’ mag drive technology allows particles of up to 3mm to be handled. This benefits the process by allowing total containment with a modest solids handling capability.
Air diaphragm pumps
An air diaphragm pump (AODD) is often an alternative to a mag drive pumps, most commonly where the pump may run dry. An air diaphragm pump carries the risk of a diaphragm rupture, which will cause the air chamber to flood, potentially causing a release to atmosphere. This can be prevented by regular inspection/replacement of the diaphragms or leak detection, shutting down the pump in the event of a breach of the diaphragm. Although the AODD carries this risk, it is uncommon and providing the pump is periodically inspected, uninterrupted operation is normal.
Although an AODD has this inherent risk. it also carries the significant benefits of being able to run-dry, self-prime and handle particulates which would affect many other pump types including mag drive centrifugal models. This is particularly notable in the circulation of ferric in the finishing of circuit boards. As a pump, one AODD pump can handle a range of fluids with varying weights and characteristics, making it very versatile where there is production of multiple products.
AODD pumps are available in a wide range of metallic and non-metallic materials for chemical compatibility. AODD pumps with PP or PTFE construction are rated as excellent with most rubber, plastics and fluoropolymers suitable.
Ferric is often dosed for numerous water and wastewater treatment processes such as sludge dewatering or sterilizing. The ferric is often delivered by a peristaltic pump which provides an accurate and consistent delivery of the chemical. The peristaltic pump provides a flow directly proportional to the speed of the pump and can therefore be linked to an inverter to provide proportional dosing determined by an analytical probe reading for pH or turbidity.
The pump is especially robust as thanks to the peristaltic principle the pump it can run dry, self-prime and can easily handle higher S.G. products containing strongly abrasive particles. With an EPDM hose for chemical compatibility, a peristaltic pump makes a very attractive solution for dosing ferric chloride.
A peristaltic pump can not only dose the ferric chloride in to the effluent stream/tank, it can also handle the flocked material with a gentle pumping action to transfer it for further processing with minimal disturbance.
Peristaltic pumps can be fitted with PP flange inserts of complete PP construction to prevent corrosion. In the event of a burst hose, the pump can be fitted with a leak detection sensor to trigger an alarm mechanism. The fluid will be contained in the pump casing which can then be manually drained via a port, preventing exposure to employees and surrounding equipment.