Research Of Ammonia Recycling In Rare Earth Separation Process

Treating ammonia-nitrogen wastewater produced in the process of rare earth(RE)extraction has always been a problem in the RE industry. Based on a previous study on the ammonia-free leaching for ionic RE mine, we designed a process route for RE separation that enhances the efficiency for wastewater treatment. This process route consists of 3 steps, including internal circulation of ammonia-nitrogen enrichment,alkali-hydrolyzation to make ammonia and acidification with sulfuric acid to make hydrochloric acid. Moreover, we perform systematic studies on each individual step within the new route.Our experimental data shows: [1] under normal temperature, the solubility of ammonium chloride in RE solution decreased with the increasing of RE ion level and RE atomic numbers; [2] when the concentration of ammonium chloride was saturated,the solution viscosity increased with the increasing of RE atomic numbers; [3] the concentration of ammonium chloride had no impact on the oil-water p Hase fractionation. Even in saturated ammonium chloride, we did not encounter any problems that were frequently found in the extraction process, such as emulsification and unsuccessful p Hase separation.Extraction experiments show that the extracting capacity and separation coefficient in saturated ammonium chloride is very close to those of the conventional process. The similarities include: [1] the extraction capacity of light RE is smaller than heavy RE; [2] light RE separation coefficient increased with the decreasing of the solution acidity, and heavy RE behaves oppositely compared with light RE; [3] the saturated ammonium chloride in the stripping acid did not affect the stripping process.The secondary extraction rate of light RE was above 98%, and, for heavy RE, it was above 80%.Results from alkaline hydrolysis experiment show that magnesium hydroxide is a suitable decomposition for ammonium chloride, and the alkaline hydrolysis reaction can only happen above boiling temperature(~103°C). The most optimal reaction condition is at 130°C with 15% excessive magnesium hydroxide. In addition, certain degree of vacuum treatment accelerated the reaction progress.Sulfuric acid acidification experiments show that modulation with the temperature, vacuity and sulfuric acid concentration can accelerate the acidification process. The optimized conditions are as follows: 15% excessive sulfuric acid,temperature at 130°C and vacuum degree at 0.05 MPa. The XRD analysis identified the acidification product as magnesium sulfate monohydrate, not magnesium hydrogen sulfate. However, the magnesium sulfate monohydrate obtained in this study contained more than 1% chloride, which exceeded the standard requirement. To solve this issue, we utilized the ethanol to absorb the water, so that the product can be recrystallized and re-purified. The re-purification conditions are as follows: under room temperature around 25°C, with the p H value above 1.5, initial concentration of magnesium sulfate being 1.5mol/L and volume ratio between ethanol and magnesiumsulfate being 1:1, the precipitation rate of magnesium sulfate is more than 85% with less than 1% chloride contamination.