Technology
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Liquid fertilizers have long storage life Allied Chemical method prevents precipitation of sediments in liquid ammonium polyphosphate fertilizers Allied Chemical has disclosed details of its technology for making highanalysis liquid fertilizers that have a long storage life. Patents of the technology will be issued soon. The fertilizers, of grades such as 10-34-0 and 11-37-0 (N, P 2 0 5 , and K 2 0 concentrations), sold under the tradename Poly-N, are made in a unit at the company's Geismar, La., plant. The long storage life of Allied's liquid ammonium polyphosphate fertilizers results from a method to prevent precipitation of sediment. The method was developed by Dr. W. P. Moore, who now is director of research for the company's agricultural division. Much of the sediment or sludge comes from precipitation of a magnesium ammonium pyrophosphate, which forms during storage if the magnesium oxide concentration in the liquid fertilizer is high. The magnesium oxide comes from phosphate rock. Sediment or sludge in liquid fertilizers can plug application equipment or cause changes in composition (altering the "guaranteed analysis" of the product). Prevention or removal of sludge formerly has been expensive and has delayed acceptance of liquid fertilizers. Storage. If the magnesium oxide concentration in a liquid ammonium polyphosphate is about 0.4%, the fertilizer has a storage life of only about a month before precipitation clouds the solution. If the magnesium oxide concentration is 0.2%, the storage life is a year; if 0.1%, nearly two years. In the Allied process, magnesium is removed by forming an insoluble magnesium aluminum fluophosphate in the phosphoric acid after the acid is made, Dr. Moore tells C&EN. The insoluble material is separated before the phosphoric acid is converted to superphos-
Moore: precipitate impurities
phoric acid (P2O5 concentrations of more than 70%) for ammonium polyphosphate fertilizer. Allied has also patented other methods to remove impurities from wet-process phosphoric acid that include other alkaline earth ions that form precipitates. These methods include organic solvent extraction and ion exchange. However, cost and final performance of these methods are not competitive with inorganic precipitation, Dr. Moore says. Removed. After most of the magnesium is removed from the wet-process phosphoric acid, the acid is converted to superphosphoric acid. This conversion involves submerged combustion of the acid with the products resulting from burning natural gas with air. The heat supplied causes o-phosphoric acid (H 3 P0 4 ) to polymerize to polyphosphoric acid—a mixture of dimer, trimers, small amounts of higher polymers, along with unreacted o-phosphoric acid. Water produced in the reaction is also driven off, helping to increase the degree of polymerization. The superphosphoric acid is then ammoniated to make ammonium polyphosphate, filtered, and sent to storage. Allied has also developed a direct process to make liquid ammonium polyphosphate fertilizers in semicom-
mercial quantities. In this process, strong phosphoric acid, sometimes containing relatively small quantities of polyphosphate, is reacted with preheated anhydrous ammonia. During the reaction, additional polyphosphate forms. Advantages. Liquid materials' share of the fertilizer market continues to increase largely because of better crop yields per acre, but also because of convenience in handling and in subsequent processing to make various grades of fertilizer. These advantages of liquid fertilizers mean improved profitability for crop growers. At least 65% of the total phosphate in Allied's Poly-N is in the form of polyphosphate. Because forms of phosphoric acid other than ortho are more soluble, this high polyphosphate content speeds plant growth, Allied says. The ammonium polyphosphates also dissolve or chelate any added micronutrients and supply appreciable quantities of iron in chelated form to the soil, all necessary for good crop yields. Allied makes superphosphoric acid in this unit at its Geismar, La., plant
