Study On Removal Of Calcium And Magnesium In Rare Earth Wastewater By Chemical Precipitation

Ammonium sulface wastewater is the main source of water pollution in the rare earth production process. It contains large amounts of Ca2+, Mg2+ and ammonia. So far, the treatment case of rare earth wastewater is less at home and abroad, waste water discharge on the surrounding environment caused serious pollution. Because we use the electrodialysis- evaporation method to treat the waste water in the actual process, we need to remove the calcium and magnesium in the wastewater to reduce the calcium,magnesium on the effect of evaporation or membrane fouling treatment process. Aiming at the difficulty of rare earth wastewater treatment, this subject adopted the method of one step precipitation and stepwise precipitation to treat ammonium sulface wastewater. By comparing the two solutions of calcium and magnesium removal as well as economic benefits, the final choice is stepwise precipitation.One step precipitation, namely adding oxalic acid or ammonium bicarbonate to the wastewater,producing precipitation of calcium and magnesium ions, mainly studied pH value and dosage of precipitation agent how to affect the removal efficiency of the calcium and magnesium. We also compared the treatment effects of two precipitating agents. The following conclusions can be obtained:(1) Experiments were carried out using oxalic acid as precipitation agent, pH=9.0, n(NH4HCO3) :n(Ca2++Mg2+)=2:1, the removal ratio of Ca2+ reached 99.1%, at this time, the removal ratio of Mg2+ reached36.8%. The sediments were calcium oxalate hydrate, magnesium oxalate hydrate and ammonium sulfate.(2) Experiments were carried out using ammonium bicarbonate as precipitation agent, pH=9.0,n(NH4HCO3) : n(Ca2++Mg2+)=2:1, the removal ratio of Ca2+ reached 72.2%, at this time, the removal ratio of Mg2+ reached 50.1%. The sediments were dolomite(CaMg(CO3)2) and magnesium sulfate hydrate(MgSO4·7H2O) and can not be recycled.Stepwise precipitation, namely adding oxalic acid and phosphate successively to the wastewater,producing calcium oxalate and magnesium ammonium phosphate precipitation respectively, mainly studied pH value, dosage of precipitation agent, aging time and seed amount how to affect the removal efficiency of the calcium and magnesium. The results showed that, the optimum condition of Ca C2O4 precipitate was seed(calcium oxalate) amount = 1.5 per thousand, pH=7.5, n(H2C2O4) : n(Ca2+)=1.2:1, aging time was 2hours, the removal ratio of Ca2+ reached 92.2%, at this time, the concentration of Mg2+ was almostunchanged; the optimum condition of MAP precipitate was pH=7.5, n(NH4H3PO4) : n(Mg2+) =0.9:1, aging time was 2 hours, the removal ratio of Ca2+ reached 81.8%, and the removal ratio of Mg2+ reached 98.1%;the sediments were Ca C2O4 and MgMH4PO4·6H2O by X-ray diffraction. In the production of oxalic acid regeneration experiment, the results showed that, the calcium oxalate conversion rate can reach 74 percent within the acceptable range that oxalic acid regeneration process is feasible.