Manufacture of Wet-Process

The reactions in the digestion of apatite with sulfuric acid are complex. The calcium phosphate portion of apatite reacts with sulfuric acid as follows:



whereupon, depending upon the process conditions, the calcium sulfate precipitates either as its dihydrate or as its hemihydrate (see below). The "fluoride part" of the apatite is either removed as gaseous silicon tetrafluoride in the presence of silica and absorbed in scrubbers as aqueous hexafluorosilicic acid, or is largely removed as the sparingly soluble sodium hexafluorosilicate by adding a sodium compound. Part of the fluoride remains in the acid.

The other components of apatite (iron, aluminum, uranium) partly pass into solution as salts and are partly precipitated with the calcium sulfate. Any carbonate present produces carbon dioxide during digestion. If sedimentary noncalcined apatite is utilized, the phosphoric acid obtained is colored black by the organic impurities. In principle there are two digestion processes:
processes in which calcium sulfate dihydrate (gypsum) is formed and is separated off;
processes in which calcium sulfate hemihydrate occurs as an intermediate of final product.

Three variants of the hemihydrate process are operated industrially, one or other having more or less importance:
variant 1: calcium sulfate precipitates directly as its hemihydrate and is filtered off;
variant 2: calcium sulfate precipitates initially as its dihydrate and is subsequently recrystallized as its hemihydrate;
variant 3: calcium sulfate precipitates initially as its hemihydrate and is recrystallized as its dihydrate.

The recrystallization in variant 3 can proceed either after the calcium sulfate hemihydrate had been filtered off or in situ.

Processes yielding anhydrite (anhydrous calcium sulfate) have little industrial importance.

The crucial step in the digestion of apatite is the formation of calcium sulfate. Its properties, in particular, its ability to be filtered, are very important e.g. for throughput optimization. The incorporation of phosphate into the crystal lattice of calcium sulfate reduces the phosphate yield and can render the calcium sulfate unusable in the building industry.

The choice of process depends upon a number of factors such as:
raw material price
source of the apatite
energy costs and energy availability at the plant site
possibilities regarding the further utilization of the calcium sulfate byproduct.

The most widely used process is the dihydrate process. The process parameters of the different processes are compared in Table 1.5-6.


 

Process developers:
(1)Dihydrate: Prayon, Dorr, St. GobaidRhone Progil, Fisons, Kellog-Lopker etc.
(2)Hemih-ydrate, variant I: Fisons
(3)Hemihydrute, variant 2: Prayon, Nissan, Mitsubishi, Fisons etc.
(4)Hemihydrate, variant 3: BreyedHeurty, Dorr, Fisons

The most important requirements for modern plants operating the dihydrate process are:
(1)reaction temperature of ca. 80°C
(2)separate feed for ore and sulfuric acid
(3)vacuum evaporation cooling to ensure a constant reaction temperature (exothermic reaction!)
(4)monitoring of sulfuric acid (sulfate) concentration and if necessary recycling part of the dihydrate

These measures enable a uniform growth of the dihydrate crystals which is indispensable for efficient filtration.

The phosphoric acid obtained in the dihydrate process has a concentration of 28 to 32% (as P₂O₅). The yield, based on the phosphorus content of the apatite, is ca. 95%. Modern plants have reactors with capacities up to over 1000 t P₂O₅ per day. Fig. 1.5-1 shows a (simplified) flow sheet of a dihydrate plant.

The three industrially operated variants of the hemihydrate process produce different products:

With variant 1, in which temperatures of 90 to 100°C are used and in which the hemihydrate is directly filtered off, a relatively concentrated acid (45 to 50% P₂O₅) is obtained, but the yield of diphosphorus(V) oxide is low.

With variant 2, in which dihydrate is obtained initially at 60 to 70°C and the filter cake resulting from filtration is recrystallized to the hemihydrate by slurrying in sulfuric acid at temperatures > 90°C, a phosphoric acid with a content of 33 to 38% (as P₂O₅) is obtained and the yield of diphosphorus(Ⅴ) oxide is very high.

With variant 3, in which the hemihydrate is formed at 90 to 100°C and changes into the dihydrate upon cooling to 50 to 60°C, the diphosphorus(Ⅴ) oxide content of the calcium sulfate is less than 0.3% due to the recrystallization and the yield of diphosphorus(Ⅴ) oxide is higher than that with the dihydrate process. The Nissan process is typical of this process variant.
 
With variant 3 with recrystallization after filtering off of the
hemihydrate, the acid obtained has a concentration as high as in variant 1 (> 42% P₂O₅). After filtration, the hemihydrate is slurried in dilute acid and recrystallized to the dihydrate. This process calls for very uniformly-shaped hemihydrate crystals and for very complicated recycling of the acid. However, it uses particularly little energy, since the acid produced does not need to be concentrated and the phosphate ore used does not have to be so finely ground as with the dihydrate process. A comparison of the classical dihydrate process with the Fisons hemihydrate process with filtration and conversion of the hemihydrate is given in Table 1.5-7.

Fig. 1.5-2 shows a flow sheet of the Fisons hemihydrate process.

Concentration of acid: The phosphoric acid produced in several of the above-mentioned processes may, depending upon the application (currently mainly fertilizer manufacture), have to be concentrated. Fertilizer production requires acids containing between 40 and 54% diphosphorus(Ⅴ) oxide. For transportation purposes further concentration to 52 to 72% diphosphorus(Ⅴ) oxide is required. The use of certain evaporation processes, e.g. submerged burner, vacuum evaporation etc., to concentrate the phosphoric acid is problematical due to the high corrosiveness of the acid, the formation of precipitates and the release of acid-containing gases (fluorine compounds and phosphoric acid mist). Furthermore, they are very energy intensive.

Purification of wet-process acid: Wet process acid is very impure. During concentration and subsequent standing, wet-process acid deposits a considerable fraction of its insoluble impurities as sediment. These "deslimed" acids are transportable. Table 1.5-8 gives the average composition of such acids.

Further concentration to acids with diphosphorus(Ⅴ) oxide contents of ca. 67 to 72% reduces the fraction of certain impurities even further e.g. the fluoride content is thereby reduced to 0.4%.

Much more extensive purification is possible by:
(1)precipitation of troublesome ions;
(2)multistage liquid-liquid countercurrent extraction of the phosphoric acid.

In precipitation purification, heavy metal ions such as copper and arsenic are precipitated as their sulfides and sulfate is precipitated as barium sulfate.

In liquid-liquid extraction three types of extraction agents are used:
(1)completely water miscible agents, such as methanol or isopropanol;
(2)partially water miscible agents, such as butanol, isoamyl alcohol, n-heptanol;
(3)water immiscible agents, such as tri-n-butyl phosphate or di-isopropyl ether.

Upon mixing the impure phosphoric acid with the organic extraction agent two phases are always formed: one consists of a solution of the purer phosphoric acid in the organic extraction agent and the other of an aqueous phosphoric acid solution together with the impurities. Pure acid can be recovered from the organic phase by stripping with water, distilling off the solvent or temperature
variation. Depending upon the process used, phosphoric acid yields between 85 and 98% are obtained.

The quality of the acids produced is, depending upon the process used, between industrial and food grades. The strongly contaminated acid remaining after production of pure phosphoric acid can in some processes still be used for fertilizer manufacture.

In 1993 the capacity for extracting purified phosphoric acid in the USA and Western Europe was together 850 ^. 10³ t/a.

Processes to extract impurities from phosphoric acid and thereby to purify the acid are currently not operated industrially. However, it is possible to extract uranium from phosphoric acid.

Manufacture of Furnace Phosphoric Acid

Furnace phosphoric acid is obtained by the combustion of white phosphorus in air and the absorption of the resulting diphosphorus(Ⅴ) oxide in water (in practice dilute phosphoric acid).

Two processes are industrially operated: the "IG" and "TVA (Tennessee Valley Authority)" processes. In the TVA-process combustion and absorption take place in separate towers, in the IG-process in a single tower. In this process the walls of the towers are protected from the hot phosphorus flame by pumped phosphoric acid. This pumped phosphoric acid removes the heat of reaction by being circulated through a heat exchanger and provides the water for phosphoric acid formation. The acid produced is extracted from the pumped phosphoric acid. Phosphoric acid mist in the tail gases (6 to 8% of residual oxygen) is removed before venting, for example by venturi scrubbers. The construction materials used have to exhibit very high corrosion resistance.

(Poly)phosphoric acids are thereby produced with diphosphorus(V) oxide contents of 54.5 and 61.5%. (In the USA, polyphosphoric acids with diphosphorus(Ⅴ) oxide contents of 76 or 84% are often produced to save on transport costs. These are diluted on site before use.) Furnace phosphoric acids are very pure. If necessary, the low arsenic content can be reduced to less than 0.1 ppm by treatment with hydrogen sulfide and filtration of the sulfide formed.