Chapter 3, Problem 3.26P

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, CaSO₄2H₂O), 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. (More information about hydrated salts is given in Chapter 6.)

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 front tlte crystallizer to a filter in which the particles arc 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 arc given below.

Solids content of slurry leaving crystallizer 0.35 kg CaSO₄-2H₂O/L slurry CaSO₄ content of slurry liquid: 0.209g CaSO₄/100g H₂O Specific gravities: CaSO₄-2H₂O(s), 2.32; liquid solutions, 1.05

1. Briefly explain in your own words the functions of the three units (crystallizer, filter, and dryer).
2. Take a basis of one liter of solution leaving the crystallizer and calculate tlte mass (kg) and volume (L) of solid gypsum, the mass of CaSO₄ in the gypsum, and the mass of CaSO₄ in the liquid solution.
3. 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.
4. List five potential negative consequences of discharging H₂SO₄ into the river passing the plant.