At present, domestic production of Glauber's salt production process mainly adopts multi effect evaporation in vacuum, the water soluble solid nitrate mine after mining as raw material. Raw material impurities, into the vacuum evaporation system, implementation of Na2SO4 crystallization, then get the finished product. The production process of the grain size of the product depends mainly on the evaporation crystallization process. In addition, impurities, in the production process of pH value, the degree of collision also affect the product particle size.
A, supersaturation
The degree of supersaturation of solution is one of the key factors affecting the degree of inorganic salt. Solute crystallization from solution out there are two processes, one is the formation of crystal nucleus, namely the crystal nucleation process; another is the growth of the crystal nucleus is called crystal growth. Both nucleation and crystal growth, all need to depend on in the process, solute precipitation is to generate a nucleus or crystal growth. The nucleation dominated process necessary for the formation of small size products, while the growth to process oriented generation of larger diameter crystal. The solubility curves of Figure 1 L1 of a substance under certain temperature for the solubility curve, L2 supersaturated concentration curve. Solution for unsaturated zone below L1, not precipitated, as a saturated zone above L2, only in this region to nucleation, crystal precipitation. For the solute from a to B, then C, and finally to the evaporation process of D, the following formula can be used to indicate the degree of supersaturation:
LnS= lnCd
LnCb= 2 Delta x M
P * R * T * r= 2 δ
Rho RT x 1R
Type: S -- the supersaturation of the solution (mol/kg);
Delta -- crystal surface tension (N/m)
M -- the material molecular weight (mol)
P -- crystal density (kg/m3
)
R -- the molar gas constant (J/mol · K)
T -- temperature (K)
R -- the crystal radius (mm)
Cd -- arbitrarily in a certain temperature point d supersaturated solution concentration of Cb -- B concentration in the saturated solution at a temperature in the above formula, for a particular solution δ, M, P, R, and T are constants, so the crystal size is only related to the degree of supersaturation of solution the supersaturation of the solution, and inversely proportional to the supersaturation of the solution, which is greater, the smaller the particle size of crystals. To the production of large granular sodium sulfate, must be in the production process as much as possible to reduce the degree of supersaturation of solution
Effect of two, calcium, magnesium ions and impurities
In sodium sulfate evaporation process, along with the precipitation of sodium sulfate, magnesium ion concentration increase, system consists of crystalline sodium sulfate to sodium sulfate double salt with 3Na2SO4 · MgSO4 of saturation line, the precipitation of sodium and magnesium sulfate crystallization, because of the small size of it, is not easy to subside in sodium sulfate solution, crystallization in sodium sulfate solution with high degree of saturation is no longer continue to grow, affecting the product particle size. In addition, according to the common ion effect, magnesium sulfate and calcium sulfate existed, the sodium sulfate solubility decreased, resulting in unit time of sodium sulfate precipitation caused by sodium sulfate crystal number increased, grain refinement. On the other hand, due to the production of anhydrous sodium sulfate is a process for the production of downstream, in the production process due to the added since the evaporation and the existence of liquor, so that the number of nucleation nitrate solution greatly exceed the standard, in the presence of sediment and suspended matter in nitrate solution, greatly increasing the number of nuclei, will inevitably lead to grain refinement.