Removal of Calcium and Other Metals

Another problem caused by hard water is the formation of mineral deposits. For example, when water containing calcium and bicarbonate ions is heated, insoluble calcium carbonate is formed  

This product coats the surfeces of hot water systems, clogging pipes and reducing heating efficiency. Dissolved salts such as calcium and magnesium bicarbonates and sulfates can be especially damaging in boiler feedwater. Clearly, the removal of water hardness is essential for many uses of water. 

Several processes are used for softening water. On a large scale, such as in community watersoftening operations, the lime-soda process is used. This process involves the treatment of water with lime, Ca(OH)2, and soda ash, Na2C03. Calcium is precipitated as CaCOj and magnesium as Mg(OH)2. When the calcium is present primarily as bicarbonate hardness, it can be removed by the addition of Ca(OH 2 alone  

Some large-scale lime-soda softening plants make use of the precipitated calcium carbonate product as a source of additional lime. The calcium carbonate is first heated to at least 825°C to produce quicklime, CaO  

The quicklime is then slaked with water to produce calcium hydroxide  

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