Selective calcium softening

One can stop at this point (selective calcium softening) or add enough further lime to precipitate the Mg2+ as Mg(OH)2. In contrast to the calcium analogs, MgC03 is fairly soluble but Mg(OH)2 is not (see Exercises 14.1 to 14.3) ... 

Tin tends to follow arsenic and antimony and is removed from lead bullion in softener slags or caustic slag from the Harris process. It is the most soluble of the three metals in caustic slags and can be selectively leached with water and precipitated from the resulting solution by the addition of lime to form calcium stannate containing around 40 per cent Sn. 

These affinity relationships are reversed in concentrated solutions. This is what makes regeneration of exhausted resins possible. An exhausted cation resin used for softening is predominantly in the calcium and magnesium form, both divalent ions. The resin is restored to its regenerated condition, the sodium form, by the introduction of 10 percent sodium chloride. This sodium chloride solution is concentrated enough (10,000 ppm) to reverse the selectivity. The driving force of the monovalent sodium ion then converts the resin back to the sodium form. 

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