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Hardness

The document discusses the calculation of water hardness and its units of measurement. It defines hardness as the concentration of calcium carbonate (CaCO3) in water. The units used to measure hardness include parts per million (ppm), milligrams per liter (mg/L), degrees Clarke (°Cl), and degrees French (°Fr). The specification of water hardness for different industrial uses such as boilers, paper, and textiles is outlined. Scale and sludge formation in boilers from hard water is explained, including the disadvantages of each and methods for prevention and removal.

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Hardness of any substance in equivalents of CaCO3 can be calculated as Hardness= [Mass of hardness producing substance in mg/L] X [Eqv. wt of CaCO3] ________________________________________________________ [Eqv. Wt of hardness producing substance] [Mass of sub. in mg/L]x[Mol. Wt of CaCO3] ___________________________________ Mol. Wt of Subs. Or= 1
For e.g. If a sample of hard water contains 10 mg/L of MgSO4, then its CaCO3 equivalent is given by Mass of MgSO4 (mg/L)X mol. Wt of CaCO3 __________________________________ Mol. wt of MgSO4 = 10x100/120 = 1000/120 = 8.33 mg/L (ppm) CaCO3 equivalents of MgSO4 (Hardness) 2 =
DIFFERENT UNITS OF HARDNESS 1. PPM: May be defined as the no. of parts (by wt) of CaCO3 equivalent hardness +nt per million parts (by wt) of water 2. mg/L: It is the no of mg of CaCO3 equivalent hardness present per liter of water 1mg/L=1mg of CaCO3 eq. hardness per liter of water As mass of 1 L water = 1 Kg = 1000g = 1000x1000mg = 106 mg; So, 1mg/L = 1mg of CaCO3 eq. hardness per 106 mg of water=1ppm 1mg/L = 1ppm 3 One litre of water has a mass of almost exactly one kilogram when measured at its maximal density, which occurs at about 4 °C.
3. Degree Clarke (°Cl): defined as the no. of parts of CaCO3 equivalent hardness present per 70,000 parts of water. It is denoted by °Cl 1°Cl= 1 part of CaCO3 equivalent hardness present per 70,000 parts of water 4. Degree french (°Fr): defined as the no. of parts of CaCO3 equivalent hardness in 105 parts of water. Denoted by °Fr. 1°Fr= 1 part of CaCO3 equivalent hardness in 105 parts of water 4
Relationship between different units of hardness 1ppm=1part per 106 parts of water, 1°Fr = 1part per 105 parts of water °Cl = 1part per 70,000 parts of water; So, 106 ppm = 105 °Fr = 70,000 °Cl Or 1ppm = 0.1 °Fr = 0.07 °Cl 5
6 SPECIFICATION OF WATER FOR DIFFERENT USE • Boilers • Paper Industry • Textile Industry • Sugar Industry • Beverage Industry • Laundries • Cooling Systems • Pharmaceutical Industries • Cooking
Boilers: • Water used in boilers should be free from hardness because hard water causes sludge and scale formation which prevent efficient heat transfer • Dissolved solids may cause caustic embrittlement (material of boiler becomes brittle due to accumulation of caustic substances) 7
8 Paper Industry: • Should be free from silica (produces cracks in paper) and hardness (increases ash content of paper) • Fe & Mn can affect brightness & color of the paper • Water should be free from alkalinity as alkaline water consume more alum and increase cost of production.
9 Textile Industry: • Should be free from hardness as hard water precipitates basic dyes and decreases the solubility of acidic dyes • Presence of Fe, Mn salts causes uneven dyeing on fabrics • Should be free from turbidity, colour and organic matter
Sugar Industry: • Should be free from sulphates, carbonates and nitrates otherwise crystallization and refining of sugar becomes difficult and sugar obtained is deliquescent (absrob moisture from air) Laundries: • Hard water increases consumption of soaps Cooking: • Hard water increases fuel consumption and cooking time 10
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SCALE & SLUDGE FORMATION 14 In Boilers Water is continuously converted into steam, resulting increase in conc. of dissolved salts due to evap. of water. When the conc. reaches to the saturation point, the dissolved salts start precipitating out.
Sludge: If the pptes are soft, loose and slimly, these are called sludges. Scales: If the pptes are hard and adhering on the inner walls of boiler, these are called scales. 15
SLUDGE Soft, loose and non-sticky precipitate formed within the boiler  It can be easily removed by a wire brush  It is formed at the colder portion of a boiler due to substances which have greater solubility in hot water than in cold water (e.g. MgSO4 , MgCl2, CaCl2) 16
17 Disadvantages of sludge 1. Sludge is a poor conductor of heat, leads to increase in fuel consumption 2. Decreases efficiency of boiler 3. Leads to blocking of pipes (as sludge settles in the regions of poor water circulation) Prevention of sludge formation  by using soft water  by removing a portion of water having conc. salts and replacing it by fresh water (blow down operation)
SCALE Hard deposits firmly adhering to the inner walls of boiler (very difficult to remove) Cause of formation: 1. At high temp, the bicarbonates +nt in water are converted into insoluble carbonates (scales) Ca(HCO3)2 → CaCO3 ↓ + H2O + CO2 scale 2. Magnesium salts get hydrolysed at high temperature formimg scales MgCl2 + 2H2O → Mg(OH)2 ↓ + 2 HCl Scale 3. Solubility of CaSO4 decreases with temperature, so CaSO4 is ppted as scale at high temperature Some Scale Constituents: CaSO4, CaCO3, Mg(OH)2, Fe2O3, SiO2, Na2SO4 18
Disadvantages of scales: 1. Scales are poor conductors of heat, so leads to the wastage of fuel 2. Distortion of boiler material Prevention: Internal treatment of boiler feed water Removal of Scales: • By wire brush or by blow down operation • Sudden cooling • By chemical dissolution- CaCO3 by 5-10% HCl, CaSO4 by EDTA 19