In Prob. 23.2 it is found that the sludge retains solution varying with the concentration as shown in Table 23.4. If a 12 percent solution of the NaOH is to be produced, how many stages must be used to recover 97 percent of the NaOH?

Prob. 23.2

A five-stage countercurrent extraction battery is used to extract the sludge from the reaction Na₂CO₃ + CaO + H₂O → CaCO₃ + 2NaOH

The CaCO₃ carries with it 1.5 times its weight of solution in flowing from one unit to another. It is desired to recover 99 percent of the NaOH. The products from the reaction enter the first unit with no excess reactants but with 0.6 kg of H₂0 per kilo gram of CaCO_3.

(a) How much wash water must be used per kilogram of calcium carbonate?

(b) What is the concentration of the solution leaving each unit, assuming that CaCO₃ is completely insoluble?

Table 23.4:

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TABLE 23.4 NaOH, wt% Kg solution/kg CaCO, 1.50 5 1.75 10 2.20 15 2.70 20 3.60