After Study On The Equilibrium Of The Sodium Carbonate In The Wet Production Process

Sodium percarbonate can be produced by a variety of process routes, normally employing a reaction between hydrogen peroxide and a source of sodium carbonate. In some variations, often called collectively "wet processes" the sodium percarbonate crystallises out of solution. During some reactions, sodium chloride or sodium sulfate or ethanol is added as the salting-out agent in order to decrease the composition of sodium carbonate present in the mother solution as a further consequence enables greater production to be obtained from a reactor of a given size. The use of a salting-out agent is a common practice in wet processes, in view of the significant advantages. It is well known that the production of sodium percarbonate is based on the phase diagram of the quaternary systems Na₂CO₃-NaCl-H₂O₂-H₂O, Na₂CO₃-Na₂SO₄-H₂O₂-H₂O andNa₂CO₃-C₂H₅OH-H₂O₂-H₂O. However no studies have been reported on the phase diagram of these quaternary systems so far. It is important to investigate the phase diagram for designing and optimizing the procedure of sodium percarbonate. In this paper, solid-liquid equilibrium for the quaternary systems Na₂CO₃-NaCl- H₂O₂-H₂O, Na₂CO₃-Na₂SO₄-H₂O₂-H₂O and Na₂CO₃-C₂H₅OH-H₂O₂-H₂O were studied at a temperature of 293.1 K using the moist solid method,the solid phases were confirmed by Schreinmaker's method. Phase diagrams were constructed in the oblique projection according to the J·neck method. Besides, the phase diagrams were calculated and optimum mass ratio of materials were determined.1. The phase diagram of the quaternary system Na₂CO₃-NaCl-H₂O₂-H₂O at 293.1 K inclues four crystallization fields, which correspond to Na₂CO₃·10H₂O, NaCl, Na₂CO₃·1.5H₂O₂·H₂O and Na₂CO₃·2H₂O₂·H₂O. Five crystallization lines of the salt: the crystallization line of the salt Na₂CO₃·10H₂O and NaCl; the crystallization line of the salt Na₂CO₃·10H₂O and NaCl; the crystallization line of the salt NaCl and Na₂CO₃·1.5H₂O₂·H₂O; the crystallization line of the salt Na₂CO₃·2H₂O₂·H₂O and NaCl and the crystallization line of the salt Na₂CO₃·1.5H₂O₂·H₂O and Na₂CO₃·2H₂O₂·H₂O. Two isothermal invariant points: co-saturated point of Na₂CO₃·10H₂O, NaCl and Na₂CO₃·1.5H₂O₂·H₂O, co-saturated point of NaCl, Na₂CO₃·1.5H₂O₂·H₂O and Na₂CO₃·2H₂O₂·H₂O.The solubility of sodium percarbonate decreases with increase in the concentration of sodium chloride. Based on 1 kg hydrogen peroxide with a concentration of 30% (mass fraction), the optimal theoretical dosage of Na₂CO₃ is from 0.62 kg to 0.64 kg, and NaCl, 0.2 kg. The optimal mass ratio of m(Na₂CO₃) to m(NaCl) is 3.2～3.0: 1.2. The phase diagram of the quaternary system Na₂CO₃-Na₂SO₄-H₂O₂-H₂O at 293.1 K is divided into four crystallization fields, which correspond to Na₂CO₃·10H₂O,Na₂SO₄·10H₂O, Na₂CO₃·1.5H₂O₂·H₂O and Na₂SO₄·0.5H₂O₂·H₂O. Six crystallization lines of the salt: coexistence of the salts Na₂CO₃·10H₂O and Na₂CO₃·1.5H₂O₂·H₂O; the coexistence of the salts Na₂CO₃·10H₂O and Na₂SO₄·10H₂O; the coexistence of the salts Na₂SO₄·10H₂O and Na₂CO₃·1.5H₂O₂·H₂O; the coexistence of the salts Na₂SO₄·0.5H₂O₂·H₂O and Na₂CO₃·1.5H₂O₂·H₂O; the coexistence of the salts Na₂SO₄·10H₂O and Na₂SO₄·0.5H₂O₂·H₂O; the coexistence of the salts Na₂CO₃·1.5H₂O₂·H₂O and Na₂CO₃·2H₂O₂·H₂O. Three isothermal invariants points: the co-saturated point of Na₂CO₃·10H₂O,Na₂SO₄·10H₂O and Na₂CO₃·1.5H₂O₂·H₂O; the co-saturated point of Na₂SO₄·10H₂O, Na₂CO₃·1.5H₂O₂·H₂O and Na₂SO₄·0.5H₂O₂·H₂O; the co-saturated point of Na₂CO₃·1.5H₂O₂·H₂O, Na₂SO₄·0.5H₂O₂·H₂O and Na₂CO₃·2H₂O₂·H₂O.The solubility of sodium percarbonate decreases with increase in the concentration of sodium sulfate. Based on 1 kg hydrogen peroxide with a concentration of 30% (mass fraction), the optimal theoretical dosage of Na₂CO₃ is from 0.55 kg to 0.63 kg, and Na₂SO₄, 0.17 kg. The optimal mass ratio of m(Na₂CO₃) to m(Na₂SO₄) is 3.2～3.7:1.3. The phase diagram of the quaternary system Na₂CO₃-C₂H₅OH-H₂O₂-H₂O at 293.1 K is divided into three crystallization fields, which correspond to Na₂CO₃·10H₂O, Na₂CO₃·1.5H₂O₂·H₂O and Na₂CO₃·2H₂O₂·H₂O. Two crystallization lines of the salt: the crystallization line of Na₂CO₃·10H₂O and Na₂CO₃·1.5H₂O₂·H₂O, the crystallization line of Na₂CO₃·1.5H₂O₂·H₂O and Na₂CO₃·2H₂O₂·H₂O. One isothermal invariants points: the co-saturated point of Na₂CO₃·10H₂O, Na₂CO₃·1.5H₂O₂·H₂O and Na₂CO₃·2H₂O₂·H₂O. The solubility of sodium percarbonate decreases with increase in the concentration of ethanol. Based on 1 kg hydrogen peroxide with a concentration of 30% (mass fraction), the optimal theoretical dosage of Na₂CO₃ is from 0.63 kg to 0.65 kg, and C₂H₅OH, 0.99～1.00 kg. The optimal mass ratio of m (Na₂CO₃): m (C₂H₅OH) is 2.52～5.19:1.