Magnesium hardness

Magnesium hardness = Total hardness - Calcium hardness 

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Noncarbonate or permanent calcium hardness, if present, is not affected by treatment with lime alone. If noncarbonate magnesium hardness is present in an amount greater than 70 ppm and an excess hydroxyl alkalinity of about 5 ppm is maintained, the magnesium will be reduced to about 70 ppm, but the calcium will increase in proportion to the magnesium reduction. 

Analyses of deposits from lower pressure boilers may vary widely and may comprise several major constituents from a wide spectrum of minerals, although typically there are one or two dominant materials present such as calcium and magnesium hardness salts and iron oxides. 

Total hardness is equal to the sum of the calcium hardness and the magnesium hardness. 

In a condenser leak in which magnesium hardness exceeds the bicarbonate alkalinity, the pH may be lowered sufficiently to induce significant acid attack, especially if an internal all-volatile treatment program (AVT) is employed ... 

External treatment also meant the removal of calcium and magnesium hardness by zeolite softening using either a variety of silcate-based, natural zeolites [such as analcite, (Na AljS O, 2 2H20)], or manufactured carbon zeolites. 

FWCaH — FW calcium hardness, in mg/1 (ppm) CaC03 FWMgH = FW magnesium hardness, in mg/1 CaC03 FWMA = FW methyl orange/total alkalinity, in mg/1 CaC03 RBWPA — Required BW phenolphthalein alkalinity, in mg/1 CaC03 COC = Cycles of FW concentration required in the BW... 

M = the magnesium hardness expressed as CaC03 in the FW C = the calcium hardness expressed as CaC03 in the FW S = the silica expressed as Si02 in the FW... 

Novel ion-exchange technology has recently been developed for purification of brine. Different resins have been developed for removal of sulphate impurities as well as calcium and magnesium hardness. The process is very simple and since only water is consumed to regenerate the ion-exchange resins, the operating costs are extremely low. The equipment, which is similar to that currently widely utilised for purification of waste acid, is very compact. It is expected that commercial-scale systems of both types will be installed later in 2000. 

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. 

Total hardness is the sum of calcium hardness and magnesium hardness. 

Some surface water supplies have only 10 to 50 ppm or more of total calcium and magnesium hardness they are naturally soft waters and may also be described as lean waters, due to dissolved solids from all sources being limited to perhaps only 30 to 60 ppm TDS. Cooling systems using this quality of water as makeup may employ cycles of concentration (COC) of 7 to 1 Ox or more. Control of hardness scales tends not to be an onerous task for modem polymeric scale inhibitors, but lean water formulations must allow for more aggressive conditions and a real risk of some metal wastage. 

Table 12.5 lists other standard methods for the determination of major ions carbonate and bicarbonate (alkalinity)50 51 67 68 and calcium and magnesium (hardness).50 51 70-72... 

Heavy-Duty Liquids, Heavy-duty laundry liquids have gained much importance and much interest in the literature over the last few years. Products in the industry today include both built and nonbuilt heavy-duty liquids. Built heavy-duty liquids are those which contain a builder such as tetrapotassium pyrophosphate (TKPP) or sodium citrate in order to sequester calcium and magnesium hardness ions. Nonbuilt heavy-duty liquids contain no builders and make up for that shortage by considerably increased active contents in order to maintain performance(8-14). Built heavy-duty liquids can have either of the following types of formulations ... 

For this purpose the determination of the magnesium hardness is easily effected by treating a measured volume of the carefully... 

Mg""] eq = 2[Mg""]. [Mg""] is the equivalent molar concentration in terms of the magnesium ion that also subsumes all concentrations. Considering Eqs. (10.2) and (10.3) simultaneously and expressing the molar concentrations of the calcium and magnesium hardness in terms of calcium only. 

As shown in all the previous reactions, the removal of the magnesium hardness, both carbonate and noncarbonate, can use just one chemical. This chemical is normally lime, CaO. 

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. 

Now, consider the fractional variables. For example, consider/Mgca and/o,. As in the mass variables, the first subscript of the fractional variable refers to the type of fraction of the mass. Thus, in /MgCa. the type of fraction is the fraction of Mg and, considering the fact that when Mg and Ca are used as subscripts, they refer to the noncarbonate forms of hardness,/MgCa stands for the fraction of the noncarbonate form of magnesium hardness, with Ca reminding about the relationship./ gca could just simply be written as /wg and they would be the same, but Ca is there, again, just as a reminder that it is also involved in the chemistry of the reaction. By the convention we have just discussed, /ca stands for the fraction of the noncarbonate form of calcium hardness. As a further example, to what does /cauco, refer The is the fraction of the calcium carbonate (bicarbonate) hardness. 

In addition, define/i as the overall removal of the total calcium hardness,/2 as the overall removal of the total magnesium hardness, and/as the overall removal of total hardness. Recall that V is the total volume of water treated and that the... 

Example 10.4 For the raw water of Example 10.2, the lime-soda process in the split treatment mode is used to remove the total hardness to 120 mg/L as CaC03 containing magnesium hardness of 30 mg/L as CaCOs. Calculate the chemical requirements in the first stage. 

Fraction of noncarbonate hardness of calcium removed Overall removal of the calcium hardness Overall removal of the magnesium hardness Fraction of Ca(HC03)2 removed... 

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