The non-equilibrium behavior of salt forming was experimentally determined for the system of MgSO4-Na2SO4-H2O during evaporation. The experiments of evaporation were carried out at a constant boiling temperature of 100 ℃ by a heating agent of fixed temperature of 140 ℃. The used evaporation intensity was about 140～160 g/(h•L). The compositions of liquid and solid phases were analyzed at the occurring point of the primary nucleation and the following points of further evaporation. Based on the results, the concept of primary and extended salt forming region was given. The experimental results show that: (1) Both 3Na2SO4•MgSO4 and Na2SO4•MgSO4•2.5H2O belong to the same-composition complex-salt in an equilibrium state, but change to different-composition complex-salt in a non-equilibrium state. (2) When evaporating solution which was co-saturated for Na2SO4 and 3Na2SO4•MgSO4 in equilibrium diagram, the Na2SO4 was only crystallized out. The Na2SO4 can be only crystallized up to 60.86% of the total Na2SO4 in the original solution. The same behavior was found for the co-saturated solution of 3Na2SO4•MgSO4 and Na2SO4•MgSO4•2.5H2O. (3) The primary salt-forming region is largely different from the equilibrium region. Based on the Jänecke index, the width of the primary forming region of Na2SO4 was extended from 21.02 to 32.76, the width of the primary forming region of Na2SO4•MgSO4•2.5H2O was reduced from 41.40 to 25.71. (4) Under the condition of having crystal seeds, the width of salt-forming region is larger than that of the primary salt-forming region in different degrees for different salts, for example the region width of Na2SO4 and 3Na2SO4•MgSO4 increased by 7.72 and 8.81 respectively. The overlay region of the primary and extended salt-forming region was found in the non-equilibrium state. The kinds of salt forming depend on those of crystal seeds.

Key words: phase diagram, boiling evaporation, non-equilibrium state, MgSO4-Na2SO4-H2O