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]
Excess Lime Softening Water
Excess Lime: Lime addition for softening: For waters with high calcium, high magnesium, and carbonate hardness; CaO = carbonic acid + + + concentration total alkalinity magnesium hardness excess lime dose process may be one or two stages Soda ash addition for softening: Na 2CO 3 = none Carbon dioxide for pH adjustment after softening: CO. A water softener uses specially-formulated salt pellets that slowly dissolve in the hard water and counteract the mineral deposit’s ability to adhere to surfaces. With this method, your household water will be “softer” and less likely to produce lime deposits. Sketch a drawing of a typical lime softening facility, identifying processes and points of chemical addition in the appropriate order for each of the following: 1. Straight lime 2. Split-lime 3. Lime-soda ash split 4. Lime-soda ash 5. Solids contract unit Differentiate between gravimetric and volumetric dry feeders.
History[edit]
Cite advantages and disadvantages of using lime softening for iron and manganese removal. Differentiate between the terms 'feeding' and 'mixing' and their relation to coagulation. Differentiate between split lime treatment and excess lime treatment. Describe the solids contact process. Lime Softening Study Guide - January 1994 Edition. The two-stage, excess lime softening process provides the most complete softening. It is capable of removing calcium and magnesium carbonate and noncarbonate hardness, down to the solubility limits of about 30 to 40 mg/L of calcium hardness and about 10 mg/L of magnesium hardness. Excess lime and the absence of alkalinity caused by carbonates. The plateau between 60% and 90% of lime is due to the presence of extra alkalinity. Any excess lime added in this region converts bicarbonates to carbonates and those carbonates combine with the free calcium ions to form precipitates.
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]
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. How to edit mov files in adobe premiere pro.
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. Macklemore white privilege ii lyrics.
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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