Noncarbonate hardness

The lime or lime—soda process results in the precipitation of calcium as calcium carbonate and magnesium as magnesium hydroxide. The solubiUties of these compounds are shown in Figure 4 as functions of pH. When lime is used alone, only the carbonate hardness is reduced. The carbonate hardness is present as calcium or magnesium bicarbonate. The additional use of soda ash can reduce the noncarbonate hardness by providing additional carbonate ion. The reactions involved in the various steps of the process are Hsted below ... 

The reaction of calcium noncarbonate hardness with soda ash ... 

The soda ash provides CO 3 because available CO 3 is consumed (eqs. 3—5). The noncarbonate hardness may be represented as sulfate, although any anion except carbonate or bicarbonate could be present. 

The reaction of magnesium noncarbonate hardness with lime and with soda ash is a two step reaction since reaction 6 produces a reasonably soluble calcium salt that must react with in order to cause calcium precipitation ... 

From reactions 2—6, it can be seen that the addition of lime always serves three purposes and may serve a fourth. It removes, in order, COg, calcium carbonate hardness, and magnesium carbonate hardness (reactions 8, 9, and 10, respectively). Where magnesium noncarbonate hardness must be removed, the lime converts it to calcium noncarbonate hardness (reaction 6). Soda ash, then, removes noncarbonate hardness according to reaction 5. 

The salts that cause permanent hardness are calcium sulfate, CaS04, calcium chloride, CaCl2, magnesium sulfate, MgS04, and magnesium chloride, MgCP. These are known as nonalkaline or noncarbonate hardness salts and cannot be removed by boiling they must be removed by chemical treatment. 

Permanent hardness or noncarbonate hardness is that portion of the total hardness that cannot be removed by heating water (e.g., chloride or sulfate hardness) and is the difference between total hardness and total alkalinity. 

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. 

Conversion of the permanent, noncarbonate hardness salts such as calcium sulfate or chloride produces more insoluble calcium carbonate using soda ash. The total noncarbonate hardness is the sum of the calcium sulfate or chloride present in the original RW plus that formed from magnesium sulfate or chloride in the previous reaction. 

Noncarbonate Hardness. This is the presence of other salts of Ca and Mg. 

Total Hardness. This is the sum of the carbonate and noncarbonate hardness. 

Permanent Hardness. This is Ca and Mg residue in the water after boiling and differs from noncarbonate hardness because it also measures the carbonate remaining in solution. 

It is interesting to note that when the total alkalinity is less than the total hardness, then calcium and magnesium are present in compounds other than carbonates, bicarbonates and hydrates. In this case, the amount of hardness equivalent to total alkalinity is the carbonate hardness the remainder is the noncarbonate hardness. 

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 of a water sample is a measure of its capacity to precipitate soap. The presence of calcium and magnesium ions in water essentially contributes to its hardness. Other polyvalent ions, such as aluminum, also cause hardness. Their effect, however, is minimal, because these polyvalent ions occur in water often in complex forms and not as free ions. As a result, they cannot precipitate soap. Although calcium is not the only cation causing hardness, for the sake of convenience, hardness is expressed as mg CaC03/L. Similarly, anions other than carbonate, such as bicarbonate, also cause hardness in water. To distinguish the contributions of such anions from carbonates, hardness is sometimes termed as carbonate hardness and noncarbonate hardness. This can be determined from alkalinity. The relationship is as follows ... 

When total hardness is equal to or less than the sum of carbonate and bicarbonate alkalinity, all hardness is noncarbonate hardness only and there is no carbonate hardness. 

Two basic types of hardness are associated with the ions causing hardness carbonate and noncarbonate hardness. When the hardness ions are associated with the HCO3 ions in water, the type of hardness is called carbonate hardness, otherwise, the type of hardness is called noncarbonate hardness. An example of carbonate hardness is Ca(HC03)2, and an example of noncarbonate hardness is MgCl2. 

As in the case of calcium hardness, magnesium can also be present in the form of carbonate and noncarbonate hardness. The K,p of Mg(OH)2 is a low value of 9(10 ). Thus, the hardness is removed in the form of Mg(OH)2. To remove the carbonate hardness of magnesium, a source of the OIT ion is therefore added to precipitate the Mg(OH)2 as shown in the following softening chemical reaction ... 

In natnral waters, the form of noncarbonate hardness normally encountered is the one associated with the sulfate anion although, occasionally, large quantities of the chloride and nitrate anions may also be found. The softening reactions for the removal of the noncarbonate hardness of magnesinm associated with the possible anions are as follows ... 

Magnesium, whether in the form of the carbonate or noncarbonate hardness, is always removed in the form of the hydroxide. Thus, to remove the total magnesium hardness, more lime is added to satisfy the overall stoichiometric requirements for both the carbonates and noncarbonates. (Later, we will also discuss the requirement of adding more lime to raise the pH.) The pertinent softening reactions for the removal of the noncarbonate hardness of magnesium follow. 

As noted before, the calcium ion is removed in the form of CaC03. This is the reason for the use of the second chemical known as soda ash for the removal of the noncarbonate hardness of calcium. Thus, another set of chemical reactions involving soda ash and calcium would have to written. The pertinent softening reactions are as follows ... 

Some of the calcium hardness of these reactions would be coming from the by-product noncarbonate hardness of calcium that results if lime were added to remove the noncarbonate hardness of magnesium. 

It is worth repeating that soda ash is used for two purposes only to remove the original calcium noncarbonate hardness in the raw water and to remove the by-product calcium noncarbonate hardness that results from the precipitation of the noncarbonate... 

Because the raw water is exposed to the atmosphere, there will always be some CO2 dissolved in it. As will be shown later, carbon dioxide also consumes hme. We wiU, however, ignore this requirement for the moment, and discuss it in the latter part of this chapter. Ignoring carbon dioxide, the amount of hme needed comes from the requirement to remove the carbonate hardness of calcium and the requirements to remove both the carbonate and the noncarbonate hardness of magnesium. We will first calculate the amount of hme required for the ranoval of the carbonate hardness of calcium. 

The removal of the noncarbonate hardness of magnesium using lime as the precipitant produces a corresponding amount of noncarbonate hardness of calcium as a by-product. This by-product requires the use of soda ash for its removal. Thus, the amount of lime needed for the removal of this magnesium noncarbonate hardness will be discussed in conjunction with the determination of the stoichiometric soda ash required to remove the by-product calcium noncarbonate hardness that results. 

The amount of soda ash needed comes from the requirement to remove the noncarbonate hardness of calcium. In addition, if the noncarbonate hardness of magnesium was precipitated using lime, additional soda ash will also be required to remove the... 

To calculate the soda ash requirement for the calcium noncarbonate hardness by-product, let M gca represent the mass of the magnesium species that precipitates and results in the production of the additional calcium hardness cation, where the Ca, again, is written as a reminder. Refer to Eqs. (10.17), (10.18), and (10.19) to see how the calcium hardness is produced from the precipitation of noncarbonate magnesium. The number of equivalents of the calcium hardness produced is equal to the number of equivalents of the noncarbonate hardness of magnesium precipitated [which is equal to MMgCa/(Mg/2), where Mg/2 is the equivalent mass of Mg as obtained from Eqs. (10.17), (10.18), and (10.19)]. Because this is calcium hardness, we aheady have the method of determining the amount of soda ash needed to remove it as shown in Equation (10.25). Letting this amount be is... 

The solids produced in water softening plants, if not put to use, pose a disposal problem. Conceptually, because of their basic nature, they can be used in absorption towers that use alkaline solutions to scrub acidic gas effluents. These solids, as far as the lime-soda process is concerned, come from the solids produced in the removal of (1) the carbonate hardness and (2) the noncarbonate hardness. 

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