Certain solid substances, known as hydrated compounds, have well-defined molecul

Question

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, CuSO_4 middot 2H_2 O), has 2 moles of water per mole of calcium sulfate; alternatively, ii 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 (CaCO_3) 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 CaS04 solution. The slurry flows from the crystallizer to a filter in which the particles are collected as a filter cake. The tiller cake, which is 95.0 wt% solid gypsum and the remainder CaSO_4 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) CaSO_4 as product. A flowchart and relevant process data are given below. Solids content of slurry leaving crystallizer: 0.35 kg CaSO_4 2H_2 O/L slurry CaSO_4 content of slurry liquid: 0.209g CaSO_4/100g H_2 O Specific gravities: CaSO_4 middot 2H_2 O(s), 2.32; liquid solutions, 1.05

Explanation / Answer

Crystallizer: When the aqueous solution of CaSO4 is feed into the crystallizer, the latter initiates the formation of solid gypsum particles. Essentially, it is responsible for the conversion of the aqueous solution into a slurry containing gypsum particles suspended in CaSO4 solution

Filter: This unit is reponsible for filtering out or separating the gypsum particles (filter cake) from the aqueous solution (filterate)

Dryer: Gypsum, which is CaSO4.2H2O has 2 moles of water. The dryer essentially sucks out this water of hydration and leaves behind anhydrous CaSO4.

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