sodium persulfate microwave heating cause crystallized particles I use Sodium persulfate to etch a copper board to produce a PCB. I mix 1 dl sodium persulfate with 1 liter hot water. When the water get cold, I heat it up by microwave. After the microwave, I see such crystallized particles at the bottom of the jar. Please note that there were not any solid particles in the acid before the microwave. I wonder what are these particles? is it sodium persulfate? if it is so, why it is crystallized when it is heated up?
Solubility of water increases as the temperature of the water increases, right? Then why it causes solid particles in the car?

Btw, if I put these crystallized particles into the same solvent, they don't dissolve, however, if I put them into a new hot water cup, they dissolve.
If it is sodium persulfate and if they are crystallized because of the microwave, can't I reuse them again?
If it is copper particles, isn't it supposed to be in such color?
It looks to me like copper sulphate ($\ce{CuSO4·nH2O}$). If I'm right then you can't use it for etching. For this you need oxidant (persulphate). Try to solve this solid in water. You should get light blue solution. Stick into it a nail (i.e some metal iron source). If it cover itself with a brownish powder like in wiki-link you gave, then it's a copper(+2) salt.