What exactly do you mean by permanent hardness of water?
Kindly refer to the following link:
https://www.meritnation.com/ask-answer/question/what-is-permanent-hardness-of-water/acids-bases-and-salts/5697575
https://www.meritnation.com/ask-answer/question/what-is-permanent-hardness-of-water/acids-bases-and-salts/5697575