Zinc sulfate monohydrate

Physical Properties

Zinc sulfate monohydrate is a white, free-flowing powder that is hygroscopic and readily soluble in water. It is slightly soluble in alcohol and insoluble in acetone. This compound serves as a key raw material for the manufacture of zinc salts and zinc barium white.

The anhydrous sulfate is a colorless rhombohedral crystalline solid; refractive index 1.658; density 3.54 g/cm3; decomposes at 600°C; soluble in water, methanol, and glycerol. 
The heptahydrate, ZnSO4.7H2O, is a colorless crystalline solid; metallic taste; rhombohedral crystals; effloresces; refractive index 1.457; density 1.957 g/cm3 at 25°C; melts at 100°C; loses all its water molecules at 280°C; decomposes above 500°C; very soluble in water, 96.5 g/100mL at 20°C; soluble in glycerol, 40 g/100 mL; insoluble in alcohol. 
The hexahydrate, ZnSO4.6H2O constitutes colorless monoclinic or tetragonal crystals; density 2.072 g/cm3 at 15°C; loses five water molecules at 70°C; soluble in water.

Occurrence and Uses

Zinc sulfate occurs in nature as the mineral, zinkosite. The heptahydrate, ZnSO4•7H2O is the mineral, goslarite. The salt is used as a mordant in calicoprinting, in making rayon, in preserving wood, in animal feeds, in electroplating, and in preparing many zinc compounds.

Production

Zinc sulfate is produced as an intermediate in recovering zinc from mineral zinc blende, ZnS (see Zinc, Recovery). The mineral is roasted at about 1,000°C to form zinc oxide and sulfur dioxide which, on prolonged heating in excess air, converts to zinc sulfate:
2ZnS + 3O2 → 2ZnO + 2SO2
2ZnO + 2SO2 + O2 → 2ZnSO4
In the zinc recovery process, roasted products are leached with sulfuric acid, whereupon zinc oxide is converted to sulfate.
ZnO + H2SO4 → ZnSO4 + H2O
Also, zinc sulfate can be prepared by reacting metallic zinc with dilute sulfuric acid followed by evaporation and crystallization:
Zn + H2SO4 → ZnSO4 + H2

Chemical Properties

White, free-flowing powder. Soluble in water; insoluble in alcohol.

Electrical Conductivity

Uses

Zinc sulfate monohydrate is used as a zinc supplement in animal feeds, fertilizers, and agricultural sprays. It is a specific compound fertilizer for increasing rapeseed yield.
Zinc sulfate monohydrate may be used in the following applications:

Purification Methods

Crystallise it from aqueous EtOH or dilute H2SO4 below 39o when it forms the heptahydrate, and between 39o and 70o it forms the hexahydrate, and above 70o the monohydrate is stable. The anhydrous salt is obtained from the hydrates by heating at 280o or lower temperatures in a current of dry air. It decomposes to ZnO and SO2 at 767o. The solubility of the heptahydrate in H2O is 5.88% at 0o, 61.92% at 30o, 66.61% at 35o and 70.05% at 39o.

References

[1] S. LEE. Fabrication and Characterizations of Hot-Melt Extruded Nanocomposites Based on Zinc Sulfate Monohydrate and Soluplus[J]. Applied Sciences-Basel, 2017, 7 1: 902. DOI:10.3390/APP7090902.