高リン含有廃棄物(塗装スラッジ)からのリン酸カルシウムの合成および特性評価

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  • Synthesis of chemical phosphate fertilizer by recovery process from industrial waste of the paint sludge
  • コウリン ガンユウ ハイキブツ(トソウ スラッジ)カラ ノ リンサン カルシウム ノ ゴウセイ オヨビ トクセイ ヒョウカ

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The aim of our study is to develop a practical method to recover phosphorus from industrial wastes in order to obtain phosphorus compounds for fertilizer use. Among the various phosphorus-rich wastes, we used the paint sludge which is generated during the Parkerizing process, because the sludge contained large amount of phosphorus. The sludge was free from toxic heavy metals such as cadmium or mercury, but contained oil and polymer substances. We chose an alkali extraction method for phosphorus recovery from the sludge. Before phosphorus extraction, the sludge was heated at 200℃ for 3 hours and at 850℃ for 6 hours in an electrical furnace in order to remove oil and polymer substances. Then, phosphorus was extracted by potassium hydroxide solution (1〜12 mol L^<-1>) under controlled temperature (30〜70℃). We needed avoid contamination of some metal elements, such as iron or zinc. Iron was not dissolved in extracted phosphorus solution. Zinc was removed from the solution containing extracted phosphorus by lowering the pH of the solution to about 4.0 with 6 mol L^<-1> nitric acid or by employing electrodeposition. After that, calcium salt was added to the solution, and the extracted phosphorus was precipitated at different pH (pH 3〜14). Results showed that an optimal condition of phosphorus extraction from the sludge was incubation with 3 mol L^<-1>potassium hydroxide at 70℃. Removal efficiency of zinc by electrodeposition was better than that by neutralization with nitric acid. In addition, as an optimal condition of phosphorus precipitation, we chose an addition of calcium solution which dissolved calcium carbonate in nitric acid and precipitation at alkali condition (pH 12). The recovered phosphorus was precipitated as calcium phosphate containing few and little impurities. Thus, we propose that this method is applicable for recycling phosphorus in the paint sludge and producing phosphorus fertilizer.

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