An ore containing 90 wt% MgSO4(H2O and the balance insoluble minerals is fed to a dissolution tank at a rate of 60,000 lbm/h along with fresh water and a recycle stream. The tank contents are heated to 120°F, causing all of the magnesium sulfate monohydrate in the ore to dissolve, forming a solution 10°F above saturation. The resulting slurry of the insoluble minerals in MgSO4 solution is pumped to a heated ?lter, where a wet ?lter cake is separated from a solids-free ?ltrate. The ?lter cake retains 5 lbm of solution per 100 lbm of solids. The ?ltrate is sent to a crystallizer in which the temperature is reduced to 50°F, producing a slurry of MgSO4(7H2O crystals in a saturated solution that is sent to another ?lter. The product ?lter cake contains all of the crystals and entrained solution in a ratio of 5 lbm solution per 100 lbm crystals. The ?ltrate from this ?lter is returned to the dissolution tank as the recycle stream.
Solubility data: Saturated magnesium sulfate solutions at 110°F and 50°F contain 32 wt% MgSO4 and 23 wt% MgSO4, respectively.
(a) Explain why the solution is ?rst heated (in the dissolution tank) and ?ltered and then cooled (in the crystallizer) and ?ltered.
(b) Calculate the production rate of crystals and the required feed rate of fresh water to the dissolution tank. (Note: Don‘t forget to include water of hydration when you write a mass balance on water.)
(c) Calculate the ratio lbm recycle/lbm makeup water.
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