ion exchange
Technologies - Ion Exchange
Ion exchangers are insoluble granular substances; their molecular structures include acid or base radicals capable of permutating, without any visible change to their physical appearance and without any change in solubilisation, the positive or negative ions that are «fixed» to these radicals, with ions of the same sign found in solution in their surrounding liquid. This permutation termed ion exchange allows us to alter the ionic composition of the liquid undergoing treatment, without modifying the total number of charges that exist in this liquid prior to the exchange.
Originally, ion exchangers consisted of earth pigments (zeolites) and then synthetic mineral compounds (silico-aluminates) followed by organic compounds, the latter now almost exclusively being used under the name of resins. These materials are supplied either as granules or, in most cases, as beads.
Gel type resins have a homogenous structure.
Macroporous type backbone resin porosity is obtained by adding a foaming solvent. The resulting macropores create a break in the crystalline structure that becomes opaque to light. Macroporous resins are heavily reticulated and have a better organic matter adsorption and desorption capacity.
Related Information
> Basics of Ion Exchange
Cation Exchangers
Cation exchangers can be divided into two groups :
Highly Acid Exchangers
One of the characteristics of these exchangers is the presence of sulphonic radicals HSO3– that have an acidity similar to that of sulphuric acid. At present, these are sulphonated polystyrenes obtained from :
The products created by this preparation are virtually single function products. Their physical and chemical properties vary according to the percentage of divinylbenzene to styrene, called the reticulation or crossing rate and which will usually vary from 6 to 16%.
For conventional fixed bed applications, the cationic exchange reticulation rate is approximately 8%.
In the case of high velocity (continuous or intermittent), short cycle treatments or the treatment of water containing traces of oxidants, resins with higher reticulation levels are used for both gel and macroporous structures.
Low Acid Excahngers
These exchangers are polyacrylic resins characterised by the presence of carboxylic radicals HCO2, that can be likened to certain organic acids such as formic acid or acetic acid. They differ from high acid exchangers on two counts :
they only fix cations Ca2+, Mg2+, Na+,Fe2+ ; Mn2+ … bonded to bicarbonates but cannot exchange cations that are in equilibrium with strong anions (SO42-, Cℓ–, NO3–);
they regenerate easily (reaction of a strong acid on weak acid salts); the reaction is total, with regeneration levels that are close to the stoichiometry.
Anion Exchangers
We have:
Their behaviour in the presence of acids will be different:
low alkalinity ion exchangers will not fix very weak acids such as carbonic, boric acid, or silica unlike the high alkalinity exchangers that fix them totally;
high alkalinity exchangers alone are capable of reacting with strong base salts (salt cut-off) according to reaction type
whereas high alkalinity exchangers are virtually insensitive to this phenomenon;
Low Alkalinity Anion Exchangers
These products are usually tertiary amines. Primary amines are rarely used and have very low alkalinity.
Their backbone usually has a polystyrenic or macroporous structure, or a polyacrylic structure Polyacrylic resins have a greater capacity and retain carbonic acid but are difficult to rinse.
High Alkalinity Anion Exchangers
These products are quaternary amines. They usually have a polystyrenic or acrylic backbone with a gel or macroporous structure.
In the case of the polystyrenic variants, we have the higher alkalinity type 1 with trimethylammonium groups and type 2 with dimethylethanolammonium groups that usually have a slightly lower alkalinity level.
Type 1: high alkalinity, strong affinity for silica and carbon dioxide, low capacity and poor regeneration performance.
Type 2: lower alkalinity, lower affinity for silica and carbon dioxide, less chemically stable, higher capacity, better regeneration performance.
Polyacrylics have an alkalinity that is halfway between types 1 and 2. They are good at eluting organic matter but do not withstand temperatures of over 35°C too well.
There are also dual function polyacrylic resins that have both high and low functions in the same bead. These resins have a high exchange capacity but can only be used on water that does not contain much silica. Colloidal silica is not trapped by ion exchange resins.
Deionised Water
Deionisers that produce high-quality deionised water using ion exchange resins.
Deionisation (or demineralisation) removes total dissolved solids from water using ion exchange. Our technologies control the electric charge of ions in water by attracting non-water ions and replacing them with water ions, removing the solids and leaving pure water.
What Is Deionised Water?
Deionised water is deeply demineralised, ultrapure water with the resistivity close to 18 megohm-cm. In order to obtain the high quality pure deionised water, a multi-stage water purification process can be used. Deionisation (or demineralisation) uses a multi-stage water purification process which removes total solids from water using ion exchange. Our technologies control the electric charge of ions in water by attracting non-water ions and replacing them with water ions, removing the solids and leaving pure water.
After pre-cleaning, the water is supplied to the reverse osmosis membrane, and then the water is filtered through a special deionisation medium, which removes the rest of the ions in the water.
Deionised Water Systems Explained
A deionisation system can produce a lower conductivity effluent compared to the RO permeate
A deionisation system reduces the water losses (part of the treated deionised water will be stored to periodically regenerate the resin but the volume of waste coming from the regeneration of the resin is significantly lower than the concentrate)
Reverse Osmosis is a high pressure process compared to ion exchange, so a deionisation system is characterised by lower energy consumption compared to an RO system during operation
With a deionisation unit you can use ion-selective resin to remove specific compounds, whereas an RO unit cannot be a selective process
Continuous Electrodeionisation (CEDI)
EDI system combining ion exchange resins and ion-selective membranes with a current for high-purity water production.
What Is Continuous Electrodeionisation (CEDI)?
Electrodeionisation (EDI) is a high-efficiency water demineralisation process. Systems use electricity, resins and ion exchange membranes to separate dissolved impurities (ions) from water. Continuous electrodeionisation (CEDI) means that the electric current continuously regenerates the required resin mass.
CEDI Water Treatment Systems
CEDI systems avoid issues with exhausted resin beds requiring regeneration or replacement. CEDI systems don't require any water treatment chemicals and they provide a consistent flow of high-quality water, suitable for many applications. Reverse osmosis (RO) water treatment systems are typically used prior to CEDI to ensure that the CEDI system is not overloaded with high levels of salts.
CEDI Applications And Benefits
CEDI process water systems are used in a wide range of applications and industries:
Food and beverage
Pharmaceutical
Chemical processing
The key benefits of CEDI in these applications include:
Chemical-free water treatment
Continuous water flow with consistent, known water quality
Zero resin bed regeneration downtime
Fast system installation and startup
Standardised systems mean short lead times
Flexible system design with high flow rates