Lecture 26. Other Phosphate Fertilizers Part 2

Transcription

1 Lecture 26 Other Phosphate Fertilizers Part 2 Rhenania Phosphate Rhenania Phosphate is another thermally produced phosphate fertilizer. It is made by calcining a mixture of phosphate rock, sodium carbonate, and silica in a rotary kiln at C [15]. Enough sodium carbonate is used to form the compound CaNaPO 4 and enough silica to form Ca 2 SiO 4 with the excess calcium. Typical charge proportions are one part of sodium carbonate to three parts of phosphate rock and enough silica to raise the SiO 2 content of the product to about 10%. The product contains 28% - 30% P 2 O 5, which is nearly all soluble in neutral or alkaline ammonium citrate solution even though much of the flurine remains in the product. It is applied to the soil in pulverized form or granulated in small granules with potash salts. Some grades are produced containing magnesium or boron, which are added during granulation as kieserite or borax, respectively. A somewhat similar product, Roechling phosphate, uses a soda slag that is a byproduct from the steel industry. Also, the naturally occuring minerals, trona (sodium sesquicarbonate) or natron (sodium carbonate ), may be used. Experimants have shown that a similar product can be made by sintering potassium carbonate with phosphate rock and silica to produce a product grade of The phosphate compound in this product is presumed to be CaKPO 4. The overall reaction in producing Rhenania phosphate is assumed to be: Ca 10 F 2 (PO 4 ) 6 +4Na 2 CO 3 +2SiO 2 6CaNaPO 4 +2Ca 2 SiO 4 +2NaF +4CO 2 Any grade of phosphate rock can be used, but since the grade of the product is determined by the grade of the rock, a high grade is preferred. Dicalcium Phosphate Dicalcium Phosphate is a common constituent of nitrophosphate fertilizers and of compound fertilizers formed by ammoniation of superphosphates. The process of production consists of dissolving phosphate rock in hydrochloric acid and then precipitating dicalcium

2 phosphate by stepwise addition of limestone and slaked lime. The product is recovered by filtration and washing, and the remaining solution of calcium chloride may be used or discarded. Magnesium Phosphates Monomagnesium, dimagnesium, and trimagnesium phosphates are known to be effective fertilizers, but there is no known commercial production of these materials for fertilizer use. No doubt small percentages of these compounds are formed in processing phosphate rock containing magnesium. Introduction Urea Superphosphate (USP) Urea and sulfuric acid form the following complexes: CO(NH 2 ) 2 H 2 SO 4 and 2CO(NH 2 ) 2 H 2 SO 4 There are two compounds that correspond respectively to 3.6 moles of urea to 1 mole of H 2 SO 4 and 1.8 mole of urea to 1 mole of acid. While the melting point of urea is C, both have a melting point of about 10 0 C. The preparation of the mixtures of urea, sulfuric acid, and water at the mole ratios, 3:6:1 and 1:8:1, is exothermic in both cases. Heat release with the first ratio is lower than with the second one and allows the preparation of the mixture under stable and reliable conditions at an equilibrium temperature of 60 0 C C, which is ideal to acidulate phosphate rock. In the manufacture of USP, the reaction of acidulation may be written as follows. Ca 3 (PO 4 ) 2 + 2H 2 SO 4 + (8a+2f) CO(NH 2 ) 2 + (e + 2bx) H 2 O 2a [CaSO 4 4CO(NH 2 ) 2 ] + 2b(CaSO 4 xh 2 O) + eca(h 2 PO 4 ) 2 H 2 O + f[ca(h 2 PO 4 ) 2 2CO(NH 2 )] With a+b = 1, and e+f = 1. It will be noticed that urea is associated with calcium sulfate rather than water of hydration. But the sulfuric acid to rock ratio has not changed. Identification of the Reaction Products X-ray analysis of the product showed that: There is no more free urea.

3 There is a substantial amount of tetra urea calcium sulfate. P 2 O 5 as monocalcium phosphate may be linked to two ureas. Properties of USP Properties % Weight Total nitrogen 20.9 Urea nitrogen 19.3 Ammoniacal nitrogen 1.6 Total P 2 O Citrate-soluble P 2 O Water-soluble P 2 O SO H 2 O 1.0 Storage Properties USP can be used as produced, i.e., in powrered form, or as a granular material. In the later case, its physical properties are quite similar to those of urea-based NP and NPK grades. The product stores well. Its critical relative humidity is 65% - 70% at 20 0 C; consequently, it is suitable for bulk storage. Agronomic Interest

4 To determine whether this new fertilizer with urea in the molecular structure has agronomic properties typical traditional fertilizers, agronomic tests were done with maize and rice. USP was tested in comparision with the same quantities of nitrogen and phosphate supplied by DAP and urea. In both cases the same input of potassium was supplied by potassium chloride (KCl) containing 60% K 2 O. Tests were carried out in five different combination of clay, silt, sand, and loam soils. An increase in yeild was found at the opotimum nitrogen input. Thus, for a yield of 11,500 kg/ha of maize, the fertilization rate can be reduced by 40 kg/ha of nitrogen when using USP. USP of the grade with 60% K 2 O potassium chloride was compared with a mixture comprising ammonium sulfate, TSP, and potassium chloride. Only one type of soil was used for this comparison; a loam soil composed of clay, sand and silt. With the same fertilization rate of 140 kg/ha nitrogen, the yield was increased by 10%. The tests also showed that splitting the application of nitrogen, which was advisable in traditional fertilization, was no longer necessary with USP. The tests have shown that USP affects maize and rice fertilization very positively. At constant yield the consumption of nitrogen can be cut by at least 10%. Advantages of the Process Zero liquid effluents, near zero flurine emission. Because the flurine in the phosphate rock is entirely recovered in the USP, a single-stage scrubbing unit satisfies the most stringent standards while SSP requires three or four stages. Moreover, the scrubbing liquor is recycled into the preparation of the urea-sulfuric acid mixture. The phosphoric acid route is avoided. This is one of the rare processes that allow for the production of a urea-based compound fertilizer without using phosphoric acid, thus avoiding its costs and nuisances. Granulation plants can be retrofitted to operate the USP process. The USP process cost effective. USP is cost effective because it uses the lowest cost raw materials based on concentrated sulfuric acid, it produces a drier product that rquires no additional drying when used in powdered form or saves 40% of the drying energy when it is granulated. The production technology is simple and requires limited capital cost. It can be produced in SSP or TSP plants after an easy and cheap revamping.

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