Studies On The Preparation Technology Of Compound Fertilizers With High Magnesium Phosphate Tailings

As the phosphate rock resources depleting dramatically,floating process was applied in order to meet the requirements of phosphate rock quality that the wet-process phosphoric acid production needed.Large numbers of phosphate tailings were produced every year which not only took up quantities of land,but also caused the ecological environment pollution.The main elements such as phosphorus,magnesium and calcium that existed in phosphate tailings were essential nutrients for plant growth.To prepare compound fertilizers by phosphate tailings could not only use these elements effectively but also obtain economic,environmental and social benefits.In this paper,the research object was the high magnesium phosphate tailings.The process characteristics of the tailings were analyzed.Then the process of decomposing the phosphate tailings by concentrated sulfuric acid and waste phosphoric acid respectively were studied based on the technological characteristics.Phosphate tailings were made into high,medium and low concentration compound fertilizers after decomposing by concentrated sulfuric acid or waste phosphoric acid or concentrated sulfuric acid and waste phosphoric acid step by step.The formulas were calculated by the quick mathematical software MathCAD.The main research contents and results were as follows:?1?The major substance in the phosphate tailings was CaMg?CO₃?₂,the content of MgO was 18.65%,CaO was 32.30%and P₂O₅ was 4.09%.The reactivity of the phosphate tailings was 99.94%.With high reactivity,the phosphate tailings were more easily to be decomposed.The resistance coefficient of the phosphate tailings was 4.30,which indicated the resistant property and the decomposing ability in the mixed acid were strong.The phosphate tailings belonged to serious foaming low-concentration phosphate rock since the foam stabilizing index was up to 2505.8mL·min.It indicated that the foaming property and foam stability of the tailings were strong.?2?The main factors in the process of acidolysis using concentrated sulfuric acid such as the mass fraction of sulfuric acid,the sulfuric acid dosage,the reaction time and the size of tailings were studied by single factor experiments and orthogonal experiments.The results indicated that the conversion rate of MgO and P₂O₅ were significantly affected by the size of the tailings.Meanwhile,variation trends of the conversion rate of MgO and P₂O₅ were similar to some extent.The optimal experiment results for the size of tailings,the mass fraction of sulfuric acid,sulfuric acid dosage and the reaction time were determined to be 0.13 mm,98%,105%,and20 min,respectively.Under this condition,the conversion rate of MgO was 93.88%,the conversion rate of P₂O₅ was 87.19%,the content of free acid was 5.89%and the free water content was 9.60%.?3?The main factors in the process of acidolysis using waste phosphoric acid such as the waste phosphoric acid dosage,the reaction time and the size of tailings were studied by single factor experiments and orthogonal experiments.The results indicated that the conversion rate of MgO was significantly affected by the size of the tailings,while,the conversion rate of P₂O₅ was significantly affected by the reaction time.Meanwhile,variation trends of the conversion rate of MgO and P₂O₅ were not exactly the same.The optimal experiment results for the size of tailings,waste phosphoric acid dosage and the reaction time were determined to be 0.13mm,105%,and 90 min,respectively.Under this condition,the conversion rate of MgO was74.58%,the effective phosphorus content was 18.96%,the conversion rate of P₂O₅was-1.98%,the content of free acid was 4.32%and the free water content was34.75%.?4?Different formulas were designed to prepare high,medium and low concentration compound fertilizers.To prepare compound fertilizers with phosphate tailings that decomposed by concentrated sulfuric acid was not feasible,since the materials were bonded into blocks and the distribution of the nutrient elements were uneven and the following drying and grinding processes were difficult.To prepare compound fertilizers with phosphate tailings that decomposed by waste phosphoric acid had many advantages,such as simple operation,mild condition and low cost.Through this method,compound fertilizers that conformed to the national standard could be prepared.However,it was obtained on the basis of the compromise of magnesium conversion rate and phosphorus conversion rate?-1.98%?,besides,the reaction time was prolonged.To prepare compound fertilizers with phosphate tailings that decomposed step by step could ensure the conversion rate of magnesium in the formulas,but unable to prevent the degeneration of the effective phosphorus in the waste phosphoric acid,resulting in approximately 1%absolute error between the actual measured value and the preset value.