Experimental Study on Backfilling Mine Goafs with Chemical Waste Phosphogypsum

Table 1

Main impurity content distribution in PG [4].

Impurity types	Solubility	Main form of impurities
Phosphate compounds	Dissolvable	H3PO4, (H2PO4)^−, and (HPO4)^2−
	Eutectic	CaHPO4·2H2O
	Indissolvable	Phosphate complex (with Fe, Al, alkali metal, etc.)
Fluoride	Dissolvable	(SiF6)^2− and F^−
	Indissolvable	CaSiF6, CaF2, and Na3AlF6
Organics	Indissolvable	Defoaming agent and scale inhibitor
Heavy metal	—	Cr, Cu, Zn, Cd, etc.
Other impurities	Dissolvable	K^+ and Na^+
	Indissolvable	SiO2, Fe, Sr, Mg oxide, and clathrate