The Caso4 <sub> 4 </ Sub>-of Ca (oh) <sub> 2 </ Sub>-h <sub> 2 </ Sub> O System Thermodynamic Equilibrium And Sulfate Removal Process Research

Large amounts of acidic sulfate wastewater containing heavy metals are discharged every year from the sulfate system in a number of industries such as nonferrous smelting,electroplating,mining and alkali making.It will be harmful to human health and ecological balance of water if that wastewater is discharged without any treatment.Presently the lime-milk neutralization process is a conventional technique.But in the practice,it was found out that the pH value of the neutralization-hydrolysis solution could not be adjusted to the expected value when the concentration of SO₄^2- was high,which brought the adverse effect to the purification and reuse of industrial wastewater.In order to find out the variation law of equilibrium concentration of sulfate ion in neutralization-hydrolysis solution and the effect of sulfate ion concentration on the pH adjustment of neutralization-hydrolysis solution,then to provide basis for the properly utilizing of Ca(OH)₂ in the treatment of heavy-metal wasrewater,the variation law of equilibrium concentration of sulfate ion in CaSO₄-Ca(OH)₂-H₂O system which based on the thermodynamics was discussed in this research,the Pitzer theory for multi-component system was applied to calculate the equilibrium solubility of CaSO₄-Ca(OH)₂-H₂O system and the optical method was adopted to determine the equilibrium solubility of this system.Besides, preliminary technological study on the removal of sulfate ion from industrial wastewater by double salt method was carried out in this research.The pc-pH diagram of the CaSO₄-Ca(OH)₂-H₂O system at 25℃were constructed according to the theory of thermodynamic equilibrium. The interaction characteristics between the solubility of CaSO₄ and Ca(OH)₂ can be found out from the diagram.CaSO₄,Ca(OH)₂ and solution coexist when the pH value of solution is about 13.2.CaSO₄ with the minimum solubility of 0.411 g/L is in equilibrium with solution when the pH value is lower than 13.2,and Ca(OH)₂ with the minimum solubility of 2.749 g/L is in equilibrium with solution at the pH value over 13.2.The variation law of equilibrium concentration of sulfate ion in CaSO₄-Ca(OH)₂-H₂O system was studied in detail by using simulated water sample.The results show that the SO₄^2- equilibrium concentration is nearly constant with the alue of 0.01235 mol/L when pH value is 3.0～12.0,and there is little influence of pH value on the SO₄^2- equilibrium concentration;and the SO₄^2- equilibrium concentration increases significantly when pH value is higher then 12.0.The effect of Na⁺ on the equilibrium concentration of sulfate ion in CaSO₄-Ca(OH)₂-H₂O system is studied,the results show that the SO₄^2- equilibrium concentration is nearly constant with the value of 0.033 mol/L when pH value is 3.0～12.0 and the SO₄^2- equilibrium concentration increases significantly when pH value is higher then 12.0. The variation law of equilibrium concentration of sulfate ion in CaSO₄-Ca(OH)₂-H₂O system isn't affected by Na⁺,but the equilibrium concentrations of SO₄^2- increase with the addition of Na⁺,and the increase of SO₄^2- has a linear relationship with the increase of Na⁺,this illuminates that the SO₄^2- equilibrium concentrations increase with the existence of Na⁺ in this system.Pitzer theory was adopted to calculate the relationship between pH value and SO₄^2- equilibrium concentrations,and the theoretical calculation is in agreement with experimental data.The phase diagram of CaSO₄-Ca(OH)₂-H₂O system at 25℃which bases on the thermodynamics was drawn,and the physical definition of each area was studied,the effect of sulfate ion concentration on the adjustment of pH values in the neutralization-hydrolysis solution were analyzed in detail.The liquid area ABCD represents the area of unsaturated solution,the concentrations of both CaSO₄ and Ca(OH)₂ are in instauration,and the scope of this area is restricted by the solubility curves BC and CD,the pH value of neutralization-hydrolysis solution could be adjusted arbitrarily as the concentration of sulfate ion is in this area,and the maximum pH value is decided by the curve CD;The area EDCF is a two-phase area which is composed of solid Ca(OH)₂ and solution containing CaSO₄ and Ca(OH)₂,and solid Ca(OH)₂ is in equilibrium with solution in this area,and the adjustment of pH value of neutralization-hydrolysis solution is decided by the line CD as the concentration of sulfate ion is in this area;The area BCG is a two-phase area which is composed of solid CaSO₄·2H₂O and solution containing CaSO₄ and Ca(OH)₂,and solid CaSO₄·2H₂O is in equilibrium with solution in this area,and the adjustment of pH value of neutralization-hydrolysis solution is decided by the line BC as the concentration of sulfate ion is in this area;The area FCGH is the three-phase area which is composed of co-saturated solution,solid Ca(OH)₂ and solid CaSO₄·2H₂O,and the co-saturated solution is in equilibrium with both solid Ca(OH)₂ and solid CaSO₄·2H₂O,the pH value of neutralization-hydrolysis solution could not be adjusted as the concentration of sulfate ion is in this area,and the pH value is constant and decided by the composition of co-saturated solution for point C.This research puts forward a technology to remove the sulfate ion from wastewater by forming a double salt.The effects of aluminum salt dosage,solution pH value,reaction time,reaction temperature and initial concentration of SO₄^2- on the removal of SO₄^2- were investigated.The optimum conditions for the removal of sulfate are as follows:aluminum salt dosage is 0.7 g/100mL,the pH value of solution is 12.0,reaction time is 80 min,the initial concentration of sulfate ion is higher than 1300 mg/L, and the reaction temperature has little effect on the reaction.Three factors three levels orthogonal experiments were designed for the several important influencing factors,and range analysis and variance analysis were carried out for the experiment results.The results indicate that the pH value is the most important factor for the removal of sulfate ion,the influencing sequence of each factor is solution pH value＞aluminum salt dosage＞reaction time.