- Lime Softening Chemistry
- Lime Softening Plant
- Lime Softening Water Treatment
- Lime Softening Water Treatment
- Lime Softening Chemistry
- Lime Softening Reactions For Kids
Lime softening, also known as Clark's process,[1] is a type of water treatment used for water softening which uses the addition of limewater (calcium hydroxide) to remove hardness (calcium and magnesium) ions by precipitation. The process is also effective at removing a variety of microorganisms and dissolved organic matter by flocculation.[2]
History[edit]
- In a softening-reactor a bed of sand and pellets is floating in the water. By adding chemicals like caustic-soda or lime a reaction is triggered that will force the calcium out of the water and on.
- Therefore, lime softening consists of three separate sections: reaction (which converts hardness to a low solubility salts), precipitation (which happens after oversaturation of water with low soluble salts), and the sedimentation of generated particles. There are three main types of lime softening systems; Cold Lime.
Lime Softening Chemistry
A water softening experiment was conducted in replicate to observe the changes in parameters such as total hardness, calcium hardness, magnesium hardness, alkalinity and pH with varying dosages of lime. A lime dosage range of 30-180% of the stoichiometric amount was chosen for the experiments. Lime-Soda Water Softening Process: Carbon dioxide and carbonate hardness (calcium and Magnesium bicarbonate) are complexed by lime. In this process Calcium and Magnesium ions are precipitated by the addition of lime (Ca(OH)2) and soda ash (Na2CO3). Lime-Soda Ash Water Softening Reactions 7 Early Water Softening Studies 1? The Water Softening Process 19 Excess lime treatment 25 Split treatment 28 Split treatment with sludge recycle 31 Use of water treatment systems 35 Limitations on Hardness Removal 35 Magnesium removal 35 Calcium removal 37 Ionic strength of solution 43. Soda lime water softening process Carbon dioxide and carbonate hardness (calcium and Magnesium bicarbonate) are complexed by lime. In this process Calcium and Magnesium ions are precipitated by the addition of lime (Ca(OH)2) and soda ash (Na2CO3).
Lime softening was first used in 1841 to treat Thames River water. The process expanded in use as the bactericidal effect of the process was discovered. Lime softening greatly expanded in use during the early 1900s as industrial water use expanded. Lime softening provides soft water that can, in some cases, be used more effectively for heat transfer and various other industrial uses.
Chemistry[edit]
As lime in the form of limewater is added to raw water, the pH is raised and the equilibrium of carbonate species in the water is shifted. Dissolved carbon dioxide (CO2) is changed into bicarbonate (HCO−
3) and then carbonate (CO2-
3). This action causes calcium carbonate to precipitate due to exceeding the solubility product. Additionally, magnesium can be precipitated as magnesium hydroxide in a double displacement reaction.[3]
3) and then carbonate (CO2-
3). This action causes calcium carbonate to precipitate due to exceeding the solubility product. Additionally, magnesium can be precipitated as magnesium hydroxide in a double displacement reaction.[3]
In the process both the calcium (and to an extent magnesium) in the raw water as well as the calcium added with the lime are precipitated. This is in contrast to ion exchange softening where sodium is exchanged for calcium and magnesium ions. In lime softening, there is a substantial reduction in total dissolved solids (TDS) whereas in ion exchange softening (sometimes referred to as zeolite softening), there is no significant change in the level of TDS.
Lime softening can also be used to remove iron, manganese, radium and arsenic from water.
Future uses[edit]
Lime softening is now often combined with newer membrane processes to reduce waste streams. Lime softening can be applied to the concentrate (or reject stream) of membrane processes, thereby providing a stream of substantially reduced hardness (and thus TDS), that may be used in the finished stream. Also, in cases with very hard source water (often the case in Midwestern USA ethanol production plants), lime softening can be used to pre-treat the membrane feed water.
Waste products[edit]
Lime softening produces large volumes of a mixture of calcium carbonate and magnesium hydroxide in a very finely divided white precipitate which may also contain some organic matter flocculated out of the raw water. Processing or disposal of this material may be a cost to the process.
References[edit]
- ^Mellor, J W, Intermediate Inorganic Chemistry, Longmans, Green & Co, London, 1941, p. 202
- ^USBR - Lime Softening fact sheetArchived 2011-06-13 at the Wayback Machine
- ^'Lime Softening'. Retrieved 4 November 2011.
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