L9_Lime_soda_softening_2009

L9_Lime_soda_softening_2009 - Lime-Soda Softening EVEG 3110...

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Lime-Soda Softening EVEG 3110 Version dated 2/10/03
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Source of Hardness Soil Rock Formations Under low pH, basic materials (e.g. limestone) can be dissolved Rain Topsoil Zone of intense bacterial action CO 2 in large amounts Some action on basic compounds Subsoil Zone of lesser bacterial action CO 2 in small amounts Continued action on basic compounds Limestone Zone of intense chemical action CaCO 3 + H 2 CO 3 Ca(HCO 3 ) 2 MgCO3 + H 2 CO 3 Mg(HCO 3 ) 2 Davis and Cornwell, 2 nd edition
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Terminology Mg ++ = Magnesium ion Ca ++ = Calcium ion Hardness=Mg ++ +Ca ++ expressed in terms of CaCO 3 or equivalence (for all practical purposes) • Ca(OH) 2 = Lime or hydrated lime CaO=Quick Lime Na 2 CO 3 = Soda Ash
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Terminology CaO+H 2 O Ca(OH) 2 +heat Quick lime is an alternate source for hydrated lime
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Benefield, Judkins, Weand, 1982c Hardness Distribution
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Davis and Cornwell (1991), 2 nd edition
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Softening Objectives To reduce total hardness to a range of 50 to 100 mg-CaCO 3 /l To assure the resulting water is non corrosive and potable
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Remember the Carbonate Alkalinity System CaCO 3 <===>Ca ++ + CO 3 -- Limestone carbonate CO3 -- + H + <====> HCO 3 - carbonate bicarbonate HCO 3 - + H + <====>H 2 CO 3 bicarbonate carbonic acid H 2 CO 3 <===> H 2 0 + CO 2 carbonic acid carbon dioxide Increasing pH
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CO 2 (aq) + H 2 0  H 2 CO 3 HCO 3 - + H + C0 3 = + 2H + Increasing pH Increasing OH - concentration pH 7-8 CO 2 (gas)
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Hydroxyl addition OH - H 2 0 + CO 2 + H 2 0  H 2 CO 3 HCO 3 - + H + C0 3 = + 2H + Hydroxyl addition drives the reactions to the right as the H + ions are removed pH=10-11
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Hydroxyl addition OH - H 2 0 + C0 3 = CO 2 + H 2 0  H 2 CO 3 HCO 3 - + H + C0 3 = + 2H + Conversion of carbon dioxide and bicarbonate to carbonate occurs At the same time pH=10-11
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This note was uploaded on 05/20/2010 for the course EVEG 3110 taught by Professor Malone during the Spring '10 term at LSU.

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L9_Lime_soda_softening_2009 - Lime-Soda Softening EVEG 3110...

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