Temporary hardness

The hydrogencarbonate ion, produced in nature by this reaction, is one of the main causes of temporary hardness in water. Carbon dioxide is fairly soluble in water, 1 cm dissolving 1.7 cm of the gas at stp. The variation of solubility with pressure does not obey Henry s law, since the reaction... 

Magnesium and calcium hydrogencarbonates are known in solution and are responsible for temporary hardness in water. 

For temporary hardness due to magnesium carbonate, more lime is required, since the magnesium precipitates as the hydroxide (less soluble than the carbonate) ... 

The dealkalization process removes the temporary hardness in water. This uses an acid resin bed for regeneration—in this case sulfuric acid (H2SO4). 

Many analyses quote total hardness. Some give temporary hardness (or carbonate hardness) and permanent hardness (or non-carbonate hardness), usually in consistent units so that the values can be added together to give the total hardness. The total hardness is actually the quantity of calcium (Ca) - - magnesium (Mg) in the water. If the total is not given directly, the values given for these two constituents must be added, after conversion to mg/1 as CaC03 if necessary. 

Another reaction of bicarbonate is that in boiling water it combines with any hardness present to produce scale, while releasing CO2 into the steam. This hardness is called the temporary hardness. Its concentration therefore depends on the lessor amounts of either hardness or bicarbonate. Most waters contain more hardness than bicarbonate, so that the temporary hardness is usually equal to the bicarbonate content. The temporary hardness quoted in analyses is often the only information available on the bicarbonate content of the water. 

After the temporary hardness has been removed, any calcium and magnesium which remains is still capable of forming a scum with soap, and can also react to form boiler scale. This is called the permanent hardness. 

Most natural waters contain more hardness than bicarbonate. Only a few sources in the UK, usually from wells in sandstone strata, contain more alkalinity than hardness. In most cases the temporary hardness greatly exceeds the permanent hardness. This is especially tme of the hard alkaline waters, which come from chalk and limestone measures. 

This three-stage process is used for waters of high alkalinity and hardness. It actually removes most of the temporary hardness and so reduces the TDS of the water. However, in the process it increases the proportion of silica in the remainder. Any residual temporary hardness and the permanent hardness are softened in a conventional softener. 

The effect is best illustrated by a numerical example (Table 31.4). Let us take the case of hard and alkaline deep well water such as that found to the north of London, whose main characteristics are shown in the first column of Table 31.4. The second column shows its quality after de-alkalization has removed nine-tenths of the temporary hardness and converted it into CO2 gas. This is removed from the water by stripping it with air in a packed degassing column, and the product then softened in the third stage to yield the product shown in the third column. 

Carbon dioxide, from the decomposition in the boiler of temporary hardness salts present in some waters, causes corrosion of steel steam pipework and cast iron valves and traps. Corrosion inhibitors may be used, but the choice of inhibitor must take into account the other materials in the system. Neutralizing amines such as morpholine or cyclohexylamine are commonly used. 

Calculate the temporary hardness of the water by subtracting the permanent hardness from the total hardness. 

Temporary hardness or carbonate hardness is removed by heating water to precipitate the carbonate salt (e.g., bicarbonate hardness salts) and is equal to or less than the total alkalinity. 

Where hardness removal is required, the simplest pretreatment method for smaller, lower pressure boiler plants (below 200-300 psig) is to use a cation-exchange softener. This removes the calcium and magnesium at source and converts the bulk of temporary hardness salts into sodium bicarbonate (NaHC03), which decomposes to form sodium carbonate (soda ash) but does not scale under normal boiler operating conditions. 

The principal temporary hardness salt in raw water is calcium bicarbonate, formed by dissolution of limestone (calcium carbonate) by... 

The lime-soda softening process reduces the temporary hardness (icarbonate hardness) content of the RW, and often some of the permanent hardness (noncarbonate hardness) and some silica is also removed. 

Partial addition of lime converts the temporary hardness salt (calcium bicarbonate) to carbonate. Fifteen to 20 ppm of the calcium carbonate remains in solution, while the remainder precipitates as an insoluble salt. Further amounts of lime convert other temporary hardness bicarbonate salts (Mg, Na, etc.) to soluble carbonates ... 

Primarily the sum of Ca and Mg salts in water, although it may include other metal salts such as Al, Mn, Sr, and Zn. Temporary hardness (carbonate hardness) is that portion of the total hardness that can combine with C03 or HC03. The balance is non-carbonate or permanent hardness and is caused by Ca or Mg nitrates/sul-fates/chlorides, etc. Permanent hardness is equivalent to the excess of hardness over alkalinity. 

Temporary hard water contains divalent cations, such as Ca2+, Mg2+, and Fe2+ and the bicarbonate anion, HC03 . Heating produces, H20(1), C02 (g), and a carbonate precipitate. Permanent hard water does not form a precipitate upon heating the ions it contains, such as S042 are thermally stable. 

Temporary hard water is softened by the addition of an alkaline (basic) material. All the substances listed form alkaline solutions except NH4C1. NH4+ hydrolyzes to form an acidic solution, and thus, cannot be used to soften temporary hard water. 

Temporary hard water contains HCCV. Quicklime is CaO ... 

Temper heat treatment of metals Temporary Hardness see Carbonate hardness... 

Temporary hardness, caused by hydrogen carbonate (or bicarbonate) ions, can be removed through boiling. For example, calcium hydrogen carbonate, often present in temporary hard water, is boiled in a teapot to remove its hardness. In this, process, a scale, composed of insoluble calcium carbonate, forms on the inside of the kettle. ... 

Boiling does not remove magnesium salts, nor CaCOs at its own solubility level, nor noncarbonate calcium (i.e., Ca2+ that is counterbalanced by Cl- or another anion of which the Ca2+ salt is freely soluble). The hardness that remains after boiling is called permanent hardness. Permanent hardness plus temporary hardness is called the total hardness. 

Boiling. Boiling will remove temporary hardness, but the resulting precipitation of CaC03 may be precisely what one is trying to avoid. 

Lime-soda softening. Lime-soda softening involves removal of the temporary hardness by adding the calculated amount of hydrated lime (Section 11.1) ... 

So, we need to add Ca(OH)2 equivalent to the temporary hardness plus the magnesium hardness (which is just the total hardness, if noncarbonate Ca2+ is absent), and Na2C03 equivalent to the permanent (i.e., total minus temporary) hardness. Clearly, if lime-soda softening is to be effective, accurate analyses for Ca2+, Mg2+, and temporary hardness are needed, and the lime must be accurately weighed out accordingly. 

Temporary hardness, due to Ca(HC03)2, is lost by heating. Permanent hardness is derived from other salts, such as CaSO , and is not affected... 

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