Temporary hardness removal

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

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. 

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. 

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 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. 

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) ... 

The fundamental reason for using lime-soda softening processes is to reduce the temporary hardness (carbonate hardness) content of the raw water in order to minimize risks of carbonate scaling in the user s cooling systems. Often some of the permanent hardness (noncarbonate hardness) is also removed, as is some silica. The principal temporary hardness salt is calcium bicarbonate, formed by dissolution of limestone (calcium carbonate) by water containing dissolved carbon dioxide. 

Hardness in water can be divided into two types - temporary and permanent. Temporary hardness is caused by the presence of dissolved calcium or magnesium hydrogencarbonates. Temporary hardness is so called because it is easily removed by boiling. 

Temporary hardness is easily removed from water by boiling. When heated, the calcium hydrogencarbonate decomposes, producing insoluble calcium carbonate. 

Removal of hardness Temporary hardness is removed by boiling. Both temporary and permanent hardness are removed by ... 

Temporary hardness can be removed by boiling. When temporary hard water is boiled, calcium carbonate or magnesium carbonate is formed. This settles down as a precipitate. 

Temporary hardness. Probably because of the ease with which, these impurities may be removed, the acid carbonates of calcium and magnesium (and in some cases ferrous iron) are said to constitute temporary hardness in water. This type of hardness may be removed by boiling the water, whereupon the soluble acid carbonates are converted to the insoluble normal carbonates, which may be removed by filtration ... 

Lime-soda process. Combination of the use of slaked lime for the removal of temporary hardness and sodium carbonate for the removal of permanent hardness constitutes the lime-soda process for softening natural waters. This method is commonly used in municipal water-treatment plants and is a cheap and yet fairly effective process. If sufficient time can be allowed, the insoluble carbonates may be permitted to settle out in settling basins, or they may be more rapidly removed by means of filters. Frequently, iron or aluminum salts are added, and these hydrolyze to form gelatinous precipitates of ferric or aluminum hydroxides. As these precipitates slowly settle, they carry with them the insoluble normal carbonates, as well as any other suspended matter such as sand, clay, or organic matter which is sometimes slow in settling otherwise. 

Another satisfactory process for the determination of total hardness, based on a somewhat similar principle, is due to Blacher.4 The water is first titrated with decmormal hydrochloric acid until it is neutral to methyl orange, as in the method described above for temporary hardness. After the removal of the carbon dioxide by a current of air, the methyl orange is bleached by the addition of a drop of bromine water a little phenolphthalein and a few drops of alcoholic potassium hydroxide are added, the liquid is just decolorised with dccinormal hydrochloric acid and is then titrated with an alcoholic decmormal solution of potassium palmitate until a decided red colour is produced. The quantity of the potassium palmitate solution required is proportional to the total hardness. 

Alkalies are also powerful germicides. The addition of hme to water in accordance with Clark s process for removing temporary hardness effects the removal of the bulk of the contained bacteria.1 This is largely due to the mechanical effect of sedimentation which causes the bacteria to be carried down with the lime, only a portion of them being killed. After a time they rise again into the water. By addition of excess of hme, the germicidal action is increased, but even with 0-2 per cent, of hme (calculated as CaO) or 20 parts per 10,000, the sterilisation is not usually complete,5 whilst at this concentration the water is too strongly alkaline for most purposes. 

Bicarbonate hardness is also referred to as temporary hardness, because it may be removed by heating the water. This reverses the reactions, which put the calcium and magnesium into solution (Eqs. 5.10 and 5.11). 

They also deposit insoluble calcium or magnesium carbonate onto heated surfaces to cause scale buildup, so that heating for removal is not of practical use. Interestingly, addition of calcium in the form of slaked lime (Ca(OH)2) can efficiently remove much of the temporary hardness of natural water supplies (Eqs. 5.12-5.14). 

The second equivalent of calcium hydroxide is necessary to remove magnesium temporary hardness because the magnesium carbonate product obtained using one equivalent of calcium hydroxide is still somewhat soluble. 

The sodium carbonate formed from the removal of temporary hardness is available to precipitate any calcium or magnesium salts present as permanent hardness. Thus, the usual water treatment for removal of hardness involves adding a mixture of quicklime (CaO) and sodium carbonate in accordance with the amount of permanent hardness present. Sometimes a coagulant such as alum (Al2(S04)3 I4H2O) or ferrous sulfate will be added to help coagulate and settle any finely divided hardness in suspension [18]. 

Temporary hardness is removed by boiling, but this is impractical in daily use. The carbon dioxide can, however, be extracted from the bicarbonate by the action of an alkali, calcium hydroxide being the one which is commonly used. The reaction is as follows ... 

In order to calculate the exact quantity required to remove temporary hardness, the free carbon dioxide must be known and allowed for. 

Permanent hardness may be determined in the following manner. A quantity of the water measuring 100 ml is boiled gently for 45 minutes, cooled, and made up to its original volume with cold carbon-dioxide-free water. After mixing well, the precipitated temporary hardness is removed by filtration, and the permanent hardness determined in the filtrate by titration with soap solution. The temporary hardness will be the difference between the total and the permanent hardnesses. 

This hardness, which can be removed by heating, is called temporary or carbonate hardness. Temporary hardness is derived from contact with carbonate (limestone and dolomite). Hardness which cannot be removed by boiling is called permanent or non-carbonate hardness and it is due to anions, such as chloride, nitrate, sulfate and silicate. This hardness does not contribute to scale formation. Contact with gypsum would result in permanent hardness. Calcium hardness is that due to Ca only, while magnesium hardness is due to Mg only. Magnesium hardness can be calculated from a determination of total and calcium hardness ... 

Temporary hard water may be made soft by removing the excess of carbonic acid, which preserves the acid carbonate from decomposition, by boiling the water, as stated above, or by neutralizing the acid with a base, such as ammonium hydroxide or calcium hydroxide. Both kinds of hard water may be softened by the addition of a reagent which will form an insoluble compound by reaction with the salt which is causing the hardness. Borax is often used for this purpose. 

The role of lime in the process is two-fold. Firstly, it creates the required high pH which attacks the pathogens and, secondly, it removes pathogens by physical absorption on the floes produced by the lime as a result of removal of temporary hardness and precipitation of dissolved metals. 

The hardness may be classified as temporary hardness which is removed on boiling, or permanent hardness which is due to dissolved salts. 

This type of hard water is called temporary hard water, because it can be removed by boiling. Permanently hard water contains calcium and/or magnesium salts which are not decomposed when the water is boiled. Calcium hydrogencarbonate solution decomposes on boiling to calcium carbonate (scale), which can block pipes or fur kettles in hard water districts ... 

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