Certain solid substances, known as hydrated compounds, have well-defined molecular ratios of water to some other species. For example, calcium sulfate dihydrate (commonly known as gypsum, (CaSO₄•2H₂O), has 2 moles of water per mole of calcium sulfate; alternatively, it may be said that 1 mole of gypsum consists of 1 mole of calcium sulfate and 2 moles of water. The water in such substances is called water of hydration. 

In order to eliminate the discharge of sulfuric acid into the environment, a process has been developed in which the acid is reacted with aragonite (CaCO₃) to produce calcium sulfate. The calcium sulfate then comes out of solution in a crystallizer to form a slurry (a suspension of solid particles in a liquid) of solid gypsum particles suspended in an aqueous CaSO₄ solution. The slurry flows from the crystallizer to a filter in which the particles are collected as a filter cake. The filter cake, which is 95.0 wt% solid gypsum and the remainder CaSO₄ solution, is fed to a dryer in which all water (including the water of hydration in the crystals) is driven off to yield anhydrous (water-free) CaSO₄ as product. A flowchart and relevant process data are given below. 

(a) Briefly explain in your own words the functions of the three units (crystallizer, filter, and dryer). 

(b) Take a basis of one liter of solution leaving the crystallizer and calculate the mass (kg) and volume (L) of solid gypsum, the mass of CaSO₄ in the gypsum, and the mass of CaSO₄ in the liquid solution. 

(c) Calculate the percentage recovery of CaSO₄—that is, the percentage of the total CaSO₄ (precipitated plus dissolved) leaving the crystallizer recovered as solid anhydrous CaSO₄. 

(d) List five potential negative consequences of discharging H₂SO₄ into the river passing the plant.

Solids content of slurry leaving crystallizer: 0.35 kg CaSO4-2H2O/L slurry CaSO4 content of slurry liquid: 0.209g CASO,/100 g H,O Specific gravities: CASO4-2H2O(s), 2.32; liquid solutions, 1.05 Water vapor CaSO,(aq) Slurry Filter cake CRYSTALLIZER FILTER DRYER Filtrate CaSO,(s)