What is Ion Exchange of Water Softening?
Ion exchange is a process of water softening, often called base exchange water softening, is a chemical process in which scale-forming mineral salts (Ca²⁺ and Mg²⁺) found in water are replaced with alternative mineral salts that do not contribute to scale formation.
The base exchange or ion exchange process of water softening is one of the most common methods of water treatment in textile industry. Its function is to remove scale-forming calcium and magnesium ions from hard water. In many cases soluble iron (ferrous) can also be removed with softeners. A standard water softener has four major components: a resin tank, resin, a brine tank and a valve or controller.
Ion exchange has long been used as an excellent method of water treatment. This method is now widely applied for waste water treatment also. It offers a great potential for material recovery and water conservation. This is useful for the recovery of process chemicals and for the removal of objectionable substances. It is capable of removing salts and organics from waste water. Several special type of ion exchange resins have been developed in recent times. Such specially prepared resins to retain high molecular weight compounds and macroporous anion resins that holds organics make this method an attractive one. A specific advantage of using this process to textile waste water treatment is its applicability to highly alkaline waste streams. If the effluent is sodiumhydroxide, anion exchanger alonemaybe used.
Ion exchange is an effective, versatile means of conditioning boiler feed water. The term “ion exchange” describes the process: as water flows through a bed of ion exchange material, undesirable ions are removed and replaced with less objectionable ones.
Though expensive, ion exchange method is considered for waste water treatment owing to its capability of removing hardness and organics and yielding a water of reusable quality. However this method needs extensive pretreatment to protect the resins from fouling and damage. Another notable disadvantage is that the total volume of effluent increases as regenerate wastes are also added to the waste water stream.
For example, in softening processes, calcium and magnesium ions (hardness) are exchanged for sodium ions. In dealkalization, the ions contributing to alkalinity (carbonate, bicarbonate, etc.) are removed and replaced with chloride ions. Other dealkalization processes utilizing weak acid cation resin or strong acid cation resin in a split stream process, exchange cations with hydrogen. This forms carbonic acid which can be removed in a decarbonator tower. Demineralization is simply replacing all cations with hydrogen ions (H+) and all anions with hydroxide ions (OH–). Ion exchange materials are like storage batteries; they must be recharged (regenerated) periodically to restore their exchange capacity. With proper design and operation, ion exchange processes are capable of removing selected ions almost completely (in some cases to a fraction of a part per million).
Cation Exchangers:
There has been constant improvement in ion exchange materials since the first use of natural and synthetic inorganic products. Sulfonated coal, styrene-base resins, phenolic resins and acrylic resins are some that have been developed. Exchange capacities were greatly increased with the development of the styrene-base exchangers. These resins are manufactured in spherical, stress and strain-free form to resist physical degradation. They are stable at temperatures as high as 300°F and are applicable over a wide pH range. More dense resins, those having greater degrees of copolymer crosslinking, were specially developed for heavy duty industrial applications. These products are more resistant to degradation by oxidizing agents such as chlorine, and withstand physical stresses that fracture lighter duty materials. Weakly acidic cation exchange resins contain carboxylic and phenolic groups. They remove alkalinity by exchanging their hydrogen ions for the cations associated with the bicarbonate ion (calcium, magnesium, and sodium bicarbonates). Being weakly acidic, they will not affect the cations associated with the anions of strong acids (chlorides or sulfates). Because of almost 100% utilization of the regenerant acid, chemical operating costs will be at a minimum, and there will be little excess acid to produce objectionable waste effluents.
Anion Exchangers:
Anion exchange materials are classified as either weak base or strong base depending on the type of exchange group. Weak base resins act as acid adsorbers, efficiently removing strong acids such as sulfuric and hydrochloric. However, they will not remove carbon dioxide or silica. They are used in systems where strong acids predominate, where silica reduction is not required, and where carbon dioxide is removed in degasifiers. Preceding strong base units in demineralizing processes, weak base resins give more economical removal of sulfates and chlorides. These are two general classes of strong base anion exchangers, Types I and II, denoting differences in chemical nature. Both remove silica and carbon dioxide as well as other anions. Type I is more effective in removing silica, and is used when the combined silica and carbon dioxide content of the water contacting the exchanger is more than 25% of the total anions. When there is contamination of the water with organic matter, a more porous form of Type I resin is recommended. The Type II anion material is used in treating waters where the combined carbon dioxide and silica content is less than 25% of the total anions. This is often the case when carbon dioxide is taken out in a degasifier ahead of the anion exchanger unit.
Process Flow of Ion Exchange Process:
Ion-exchange processes fall into several categories: softening (including removal of iron and manganese), dealkalization, and demineralization. Examples of these processes are listed below:
Advantages and Disadvantages of Ion Exchange Process:
Water softening is a simple, well-documented ion exchange process. It solves a very common form of water contamination: hardness. Regeneration with sodium chloride is simple, inexpensive and can be automatic, with no strong chemicals required.
Benefits of Ion exchange water softening:
- It is a relatively simple and effective way to remove calcium and magnesium ions from hard water.
- It can be used to soften water for both residential and commercial purposes.
- It is a relatively affordable option for water softening.
The disadvantages of ion exchange water softening become apparent when high-quality water is required. Softening merely exchanges the hardness ions in the water supply for normally less-troublesome sodium ions. Since the treated water contains sodium instead of calcium or magnesium, it is still unsuitable for many uses.
Conclusions:
Ion exchange is primarily used for hardness removal in water treatment by industries to minimize scaling. In wastewater treatment, it can be used for the removal of toxic metals or recovery of precious metals. The use of ion-exchange processes affords numerous efficient and effective means of conditioning feedwater. The proper selection of the specific ion exchange process depends on water quality needs, operating convenience, and economic considerations. For effective results, the system must be carefully selected, designed, operated and maintained. Because the decision is complex, an experienced ion exchange engineer should be consulted to assist in selection and design.
References:
- Chemical Technology in the Pre-Treatment Processes of Textiles by S. R. Karmakar
- Dyeing and Chemical Technology of Textile Fibres by E. Trotman
- Chemistry of the Textile Industry By C. Carr
- Theory and Practice of Water and Wastewater Treatment, Second Edition By Ronald Droste and Ronald Gehr
You may also like:
- Demineralization Process of Water Softening
- Hardness of Water in Textile Industry: Types, Reasons and Consequences
- Determination of Hardness of Water in Textile Industry
- Water Hardness Test Method
- Water Management in Textile Industry – An Overview
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
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