Pumping Sodium Hypochlorite
Sodium hypochlorite is produced mainly through electrolysis of sodium hydroxide and chlorine.
Due to the safety aspects of transporting and handling chlorine in its elemental state, the production of sodium hypochlorite is produced onsite where possible.
As a compound, sodium hypochlorite is unstable and will breakdown into sodium chloride and sodium chlorate. As a concentrated solution it has a shelf life of approximately 28-90 days depending on the conditions of storage. In such items as household cleaners where sodium hypochlorite is a constituent ingredient, sodium hydroxide is added to slow the rate of degradation.
Sodium hypochlorite is highly corrosive and aggressive to human tissue and many metals.
Sodium hypochlorite is a widely used chemical, principally for its oxidizing nature, anti-microbial properties and bleaching power on materials such as textiles.
- Sodium hypochlorite is commonly used in effluent and water treatment. Thanks to its anti-microbial properties it can be dosed in to sewage and/or clean water. The solution is added to raw sewage when it enters a waste water plant. At clean water sites the solution is added to ‘chlorinate’ the water and provide it with a working agent to maintain its potable nature until it reaches the tap.
- Sodium hypochlorite is also added to water in water systems as a biocide and into swimming pools to disinfect the water.
- The bleaching properties of sodium hypochlorite also make an invaluable process fluid for the textile and paper industry.
- Effluent can be treated for odour control and the neutralizing of industrial effluents, particularly those from metals processing.
- In medical treatment, the chemical is used in extremely dilute concentrations for skin conditions such as eczema and in root canal dental procedures for necrotic tissue.
Sodium hypochlorite requires great consideration with regards to the selection of materials. This is due to the strong oxidizing nature of the chemical. It is recommended that most metals be avoided in any pump specification. Metals such as aluminium, cast iron and stainless steel will suffer corrosion though higher-end alloys such as titanium, Hastelloy or duplex may be acceptable depending on the concentration. Elastomer materials such as Acetal, Buna, nylon and polyurethane should also be avoided.
It is recommended to use a non-metallic build from materials such as polypropylene or PTFE with wet-side materials such as PTFE, ceramic, hypalon, EPDM, ETFE, PVC and viton.
At wastewater treatment sites the chemical may be dosed using a peristaltic hose or tube pump or a metering pump, utilizing diaphragms to deliver a quantity of chemical determined by a probe sensor such as turbidity or pH.
The positive displacement nature of these pump are used as they can provide the necessary priming suction for extraction from a tank/IBC at low flow rates and be easily programmed for a consistent delivery.
The peristaltic pump can handle gaseous fluids due to the nature of the working principle, which is particularly accommodating to hypochlorite solution as it typically ‘gasses off’ causing vapour lock in other pumps.
System and pumps are often designed with venting at the discharge and highest point in the system to prevent vapour lock. Pump heads may have accessory attachments such as a manual or automated bleed valve to release the trapped gas.
Scaling can also occur in pipework, particularly in hard water areas and in some cases may block the line. Regular flushing is recommended.
Where larger quantities or sodium hypochlorite are being transferred it is recommended using a centrifugal mag drive pump for the containment of the fluid. Depending on the flow rate and site requirements the solution can be transferred using an all-non metallic build such as a complete polypropylene body, though it is recommended avoiding carbon bearing assemblies and lesser quality magnets as these will suffer chemical corrosion.
Where the hypochlorite solution is more aggressive and chemical compatibility is paramount, a cast-iron construction with thick ETFE/PTFE lining can be the optimum solution – providing the strength of a metallic build with the security of the inert material lining. Magnet/bearing materials such as inconel, samarium cobalt and silicon carbide are recommended. Although a build of this quality may be occur a greater initial purchase price, these pumps are known to have been installed for decades where lesser materials although compatible, will eventually become more brittle, though this may still allow a pump life of several years.
Where the liquid is moved with pulsation such as with an AODD pump or with more volatility the hypochlorite may be more prone to ‘gassing off’. For this it is recommended the stroke length be set to 100% and for bleed valves to be installed with the pump and system to prevent vapour lock.