Super

The manufacture of superphosphate from apatite and sulfuric acid proceeds according to the idealized overall equation:



This reaction takes place in two steps: anhydrite and phosphoric acid being formed in the fast first step. This phosphoric acid then reacts slowly - over a period of weeks- ("curing") with further apatite producing mono-calcium phosphate hydrate. Part (ca. 10 to 40%) of the fluoride contained in the apatite is expelled in the form of gaseous silicon tetrafluoride, the rest remains in the superphosphate. Any ato-apatite and calcium carbonate in the fluoro-apatite react forming carbon dioxide.

The manufacture of superphosphate proceeds in five stages:
(1)grinding of the apatite
(1)reaction with sulfuric acid
(1)solidification and crushing of the primary reaction product
(1)"curing" - completion of the reaction
(1)comminution and possible granulation of the end product

The grinding of the apatite (≥33.5% P₂O₅), if necessary after prior crushing, yields a material with a particle size of, for example, 90% <150 µm. The reaction with the ca. 70% sulfuric acid is currently mainly carried out continuously in mixing units with stirrers or in a stainless steel conical stirrer-less mixing funnel developed by TVA. On average 60 kg sulfuric acid is necessary per 100 kg of apatite.

The initially liquid digestion mixture solidifies within 5 to 20 min and then has to be crushed. A large number of apparatuses have been developed for this primary size reduction e.g. chamber systems in which the material solidifies and is then mechanically removed. In modern continuous plants the reaction mixture is, for example, placed on long rubber conveyor belts upon which it solidifies. During solidification gaseous fluoro-compounds escape and have to be removed and absorbed.

To complete the after-reaction, the product has to "cure" for several weeks. Then it is crushed and supplied. Since it is dusty and has a tendency to cake, it is often granulated. This can take place both before and after the curing.

In the Federal Republic of Germany superphosphate is marketed with 18% citrate-soluble P₂O₅, whereas in the USA the P₂O₅ content is 20 to 21 %.

Triple Superphosphate

The manufacture of Triple Superphosphate proceeds according to the equation:



Its manufacture is very similar to that for superphosphate. Apatite with a P₂O₅ content ≥31 % is ground to e.g. 70% < 74 µm and wet-process acid with 52 to 54% P₂O₅ are used as starting materials. The molar ratio of CaO to P₂O₅ in the final product should be between 0.92 and 0.95 and the P₂O₅ content should be ca. 47%. The solidification of the reaction mixture occurs faster than in the manufacture of superphosphate.

In the USA, triple superphosphate is often used as a granulate, which can be produced using various methods. In the Dorr-Oliver process, for example, a still liquid digestion mixture is deposited on circulating triple phosphate granules, which are then sieved and dried. The ratio of separated to recirculated granules is 1:12. In general, however, triple superphosphate is first produced and then granulated with water and steam.

Ammonium Phosphates

The following ammonium phosphates are used as fertilizers either separately or as mixtures:
(1)monoammonium phosphate NH₄H₂PO₄ (MAP)
(2)diammonium phosphate (NH₄)₂HPO₄ (DAP)
(3)ammoniumpolyphosphates [NH₄PO₃]_n (APP)

Tri-ammonium phosphate is not a commercial product because of its high ammonia vapor pressure. Mono- and diammonium phosphate are used as solid fertilizers, whereas ammonium polyphosphate is mainly utilized in solution as a liquid fertilizer, since unlike the orthophosphates, it is very soluble and is more difficult to granulate than the orthophosphates. As a result of its complexing properties, it also keeps impurities (iron, aluminum, magnesium etc.) in solution. Ammonium phosphate fertilizers are relatively impure (purity ca. 85%), due to their being prepared with nonpurified wet-process acid. Commercial monoammonium phosphate contains 11 to 13% N and 48 to 53% P₂O₅ (theoretically 12.2% N, 61.7% P₂O₅). Commercial diammonium phosphate contains 16 to 18% N and 46 to 48%  P₂O₅ (theoretically 21.2% N, 53.7%  P₂O₅).

Solid fertilizers: In the manufacture of solid ammonium phosphate fertilizers there are two main problems:
(1)manufacture of storable, noncaking products
(2)least possible energy consumption during manufacture

To achieve these aims the manufacturing steps: neutralization of phosphoric acid with ammonia in an exothermic reaction and production of solid materials (e.g. by granulation or prilling) are linked with one another. Several of the many processes for the manufacture of ammonium phosphates will be discussed below.

In the TVA granulation process, slurries of ammonium phosphates, with a deficit or excess of ammonia with respect to mono-ammonium phosphate, are produced by the reaction of ammonia with phosphoric acid. These slurries are granulated by adding the deficient quantities of acid or ammonia necessary for a stoichiometric product and recycling the fine fraction. The granulate is then dried in, for example, rotary dryers, a process which requires considerable energy.

In the TVA tubular reactor process (e.g. pipecross reactors), anhydrous ammonia alone, or diluted with an equal quantity of water, is reacted with phosphoric acid. The reaction product is directly added to recycled particles in an adjacent granulation unit. At this point, further ammonia or acid can be added to the granulating material. The process is designed to utilize most of the heat of neutralization in the drying of the granules.

Non-granular (powdery) mono-ammonium phosphate is obtained in the Swift process by reacting liquid ammonia with phosphoric acid containing 50%  P₂O₅ in an impeller-stirred reactor. The reaction products (finely divided monoammonium phosphate and steam, temperature ca. 126°C) are fed in at the top of a tower. The steam is driven out by a counter-current of air from below, the solid ammonium phosphate sinking to the bottom. The wet-process acid used
in this process does not have to be deslimed beforehand.

The manufacture of ammonium phosphates can be combined with the production of mixed fertilizers e.g. ammonium phosphate with ammonium sulfate (partial substitution of sulfuric acid with phosphoric acid) or ammonium nitrate. The ammonium phosphates can also be converted into mixed fertilizers during granulation by
adding potassium salts, urea etc.

Liquid urnrnoniurn phosphate fertilizers: Ammonium polyphosphates can be manufactured from phosphoric acids containing either high or low concentrations of poly-phosphoric acid, or from orthophosphoric acid solutions.

Initially, polyphosphoric acid was used which was produced by the combustion of white phosphorus, solutions with 11% N and 37% P₂O₅ being obtained. These "furnace acids" are currently too expensive for fertilizers, due to increased energy costs, and the polyphosphoric acids now used are manufactured from wet-process acid.

If polyphosphoric acids with a high content of P₂O₅ in the polymer form are used (40 to 50% of total P₂O₅ as polyphosphoric acid), the reaction with ammonia has to be carried out with cooling to avoid hydrolysis to orthophosphates. The energy intensive nature of poly-phosphate production has resulted in the current use of acids with about 20 to 30% of  P₂O₅ in the polymer form. In a tube reactor developed by TVA, the acid is reacted with gaseous ammonia in an exothermic reaction at 230 to 240 °C, whereupon a considerable proportion of the orthophosphate is condensed to polyphosphates with the elimination of water. The resulting melt is dissolved in an aqueous solution of the end product and the required quantities of water and, if necessary, ammonia is added at the same time. Solutions with 11% N and 37%  P₂O₅ are generally produced, with 60 to 68% polyphosphate.

In 1985 in the USA, there were more than 135 TVA tube reactor plants operating, each producing 25 t of product per hour. A problem with these reactors is scaling of aluminum and magnesium phosphates on the reactor walls, which results in enforced down time for scale removal.

In a process developed by the company Swift, orthophosphoric acid is used as the starting material. Preheated acid is reacted in a special reactor with gaseous ammonia at high temperatures. At a temperature of 300°C the proportion of polyphosphates in the end product is 60%.

In addition to these soluble fertilizers, suspensions of ammonium phosphate, which may contain other fertilizer substances, are very important, particularly in the USA.

Nitrophosphates

In addition to the digestion of apatite with sulfuric acid to phosphoric acid or superphosphate, the digestion of apatite with nitric acid is also of major industrial importance, the digestion with hydrochloric acid on the other hand having no industrial importance. The exothermic reaction with nitric acid proceeds, in principle, as follows:



50 to 60% nitric acid is used up to 20% in excess. The fluoride largely remains in the reaction mixture. Part of the nitric acid is reduced to nitrogen oxides, which have to be removed from the tail gases.

Unlike the digestion with sulfuric acid, calcium is present in a soluble form. Since calcium nitrate is very hygroscopic, it is mostly either converted or separated (and then further utiliLed). [In a process developed by Lonza AG the calcium nitrate remains in the mixture. The digestion mixture is dehydrated up to the point that only 2 to 3 moles of water are present per mole calcium nitrate. Then the not yet solidified mass is granulated and coated with, for example, basic slag (Thomas meal) or calcium cyanamide etc.]

The conversion can occur:
(1)either by adding ammonia and carbon dioxide to the reaction mixture, whereupon ammonium nitrate and (only citrate soluble) calcium hydrogen phosphate (dicalcium phosphate) are produced in addition to calcium carbonate (Carbonitric process):



(2)or by adding sulfate as sulfuric acid, ammonium sulfate or potassium sulfate, whereupon the calcium is converted in an analogous reaction to calcium sulfate. The free phosphoric acid forms calcium hydrogen phosphate upon neutralization (Sulfonitric process).

An example of calcium separation is the Odda process, in which, depending on the temperature reached, a small part or a large part of the calcium nitrate crystallizes out as its tetrahydrate. The calcium-depleted mother liquor is then neutralized with ammonia. The calcium nitrate separated can, for example, be converted into calcium carbonate and ammonium nitrate by reacting with ammonia and carbon dioxide and then after filtering off the calcium carbonate, worked up to ammonium nitrate:



The calcium carbonate is then generally mixed with a 97% ammonium nitrate melt to lime ammonium nitrate melt to lime ammonium nitrate ("calnitro").