Hardness salts

Filtration does not remove dissolved soHds, but may be used together with a softening process, which does reduce the concentration of dissolved sohds. For example, anthracite filtration is used to remove residual precipitated hardness salts remaining after precipitation softening. 

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. 

The base exchange process removes both the temporary and permanent hardness salts from the water by allowing the water to flow through resin beads containing sodium zeolite, Na2Z. 

When the permanent hardness salt CaS04 passes through the bed, calcium zeolite (CaZ) and sodium sulfate (Na2S04 ) are formed, which are then flushed away. 

Basically, the hardness salts of calcium and magnesium ions are ex changed for sodium ions in the dealkization process the carbonate and bicarbonate salts, which cause high levels of alkalinity, are replaced with chloride ions. Reverse osmosis can also be used to produce demineralized water. 

Water, softening A chemical process used to change the chemical composition of water by the removal of hardness salts. 

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. 

Mineral constituents, including hardness salts, sodium salts (chloride, sulphate, nitrate, bicarbonate, etc.), salts of heavy metals, and silica. 

Hardness salts The hardness figures for natural supplies are very varied but most natural supplies in the U.K. fall into well-defined groups. The most important of these are ... 

Upland surface water. This is low in hardness salts having run over impervious rocks but will often be high in organic matter, i.e. fulvic and humic acids. 

Treatment of waters for boiler feed For most boilers, then, raw water from any of the above sources will require treatment followed by appropriate conditioning. In simple, low pressure plants, precipitation of the hardness salts, e.g. by lime or lime/soda, will be adequate. 

As has been noted, scaling due to inadequate water treatment may, in the short term, lead to efficiency losses but, eventually, could result in overheating and possibly to associated corrosion. Awareness of this has led to an increasing use of appropriate water treatment with increasing boiler pressure. Scaling due to hardness salts is therefore less common now than formerly, but deposits caused by other means may occur with similarly undesirable consequences. 

With high-pressure boilers operating above about 60 bar, the complete removal of hardness salts from the feed-water by one means or another is... 

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. 

The potential for hardness salts to represent a dominant part of any deposit usually is reduced with an increase in boiler pressure. This is because higher pressure boilers generally have additional and progressively more sophisticated pre-boiler treatment processes installed,... 

Given that the primary source of waterside deposition is almost always hardness salts, good operational practice dictates that (HW heating boilers aside) all boiler plants, wherever located and no matter how limited their output or pressure rating, require softened MU water as a minimum form of pretreatment. 

The need to prevent deposition on boiler surfaces (resulting principally from hardness salt scaling by calcium carbonate and other alkaline earth metals)... 

Although any given source of water typically has a wide range of dissolved minerals present, and each of these has a potential for causing difficulties to a greater or lesser extent, it is the alkaline earth salts (.hardness salts) that are always present to some degree and generally are the most troublesome in a boiler. This section discusses these salts, their presence in natural makeup (MU) water sources, and their contribution to hardness scales and deposition in boiler plants. 

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. 

Even where these organic polymer materials are employed, their ability to disperse sludge and transport hardness salts may be constrained by limited BD and other factors. Some sludge still develops. The fact remains that boilers simply are not designed to be settling and clarifying vessels. 

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. 

Chelants have the ability to take waterborne metal ions such as Ca and Mg (but also Fe and Cu) and produce soluble coordinate bond complexes. Therefore, they can be employed for the control of FW hardness salts and other related problems. 

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

Under hot BW conditions this reaction is reversible, leading to a serious risk of carbonate scale depositing on heat transfer surfaces. Consequently, many large water utilities and industries around the world continue to use the old established, but effective lime (calcium hydroxide) and soda ash (sodium carbonate) processes to soften water by precipitating out insoluble hardness salts. 

The steps involved in converting soluble hardness salts in the RW to insoluble precipitants that can be subsequently removed by filtration or clarification, are ... 

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 ... 

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. 

Cation exchange of hardness salts, using potassium chloride (KC1) as a regenerant, rather than sodium chloride. KC1 is considerably more expensive than ordinary salt (NaCl), but it is promoted as being medically and environmentally superior. 

Both IX units remove hardness salts, and by splitting the RW stream based on incoming water analysis and treated water needs, any level of alkalinity can be produced. As in other dealkalization processes, the treated water TDS is reduced proportional to the alkalinity removed. 

Scale and deposit control. This is primarily control over the deposition of hardness salts and other scale-forming minerals. 

These condensed tannins and their derivatives, all of high molecular weight, function as anionic polyelectrolyte sludge conditioners, tending to sequester hardness salts and hinder their precipitation as crystalline scales. In addition, when precipitation does occur, the condensed tannins coagulate the particles, resulting in a mobile sludge that can be easily blown down. 

Where direct addition to the boiler is possible, use a combined phos-phate-chelant program because solubilization of hardness salts will occur this limits the potential for adherence to metal surfaces. 

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