Certain solid substances, known as hydrated compounds.

Chapter 3, Problem 3.20

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Certain solid substances, known as hydrated compounds. have well-defined molecular ratios of water to some otherspecies. which often is a salt. For example, calcium sulfate dihydrate (commonly known as gypsum, CaS04 . 2H2 0), 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.) Solid gypsum is formed in a crystallizer and leaves that unit as 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 aftlter cake. The filter cake, which is 95.0 wt% solid gypsum and the remainder CaS04 solution, is fed to a dryer in which all water (including the water of hydration in the crystals) is driven off to yield anhydrous (waterfree) CaS04 as product. A flowchart and relevant process data are given below. Solids content of slurry leaving crystallizer: 0.35 kg CaS04 . 2H20/L slurry CaS04 content of slurry liquid: 0.209 g CaS04/100 g H20 Specific gravities: CaS04' 2H20(s), 2.32; liquid solutions. 1.05 (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 CaS04 in the gypsum, and the mass of CaS04 in the liquid solution. (c) Calculate the percentage recovery of CaS04-that is, the percentage of the total CaS04 (precipitated plus dissolved) leaving the crystallizer recovered as solid anhydrous CaS04.