Which chemical is commonly used with lime softening?

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In lime softening, you not only precipitate hardness minerals but also need help pulling together the tiny particles for clear separation. A coagulant like ferric sulfate is added because, at the alkaline pH produced by lime, ferric ions form ferric hydroxide flocs that are large, dense, and capable of capturing fine colloids and the precipitated solids. These flocs settle out more readily, improving clarification and sludge handling, which is why ferric sulfate is commonly used with lime softening. Other options don’t serve as the primary coagulant in this process: calcium chloride is a salt, sodium hydroxide is mainly a pH adjuster, and alum is another coagulant but ferric sulfate generally provides stronger, faster coagulation under lime-softened conditions.