ImI P rocesses

The drive for the IMI processes came initially from the need to find a use for copro-duced hydrochloric acid [48]. During the 1950s, Israeli scientists developed and pat-ented the Aman process [49] to produce magnesium oxide from the chloride salts of the Dead Sea. The coproduct from this process was hydrochloric acid that was known to be as effective as sulfuric and nitric acid in dissolving phosphate rock.

Unlike the sulfuric acid process where the liquid phosphoric acid is separable from the solid calcium sulfate, in the hydrochloric acid process, as shown in the follow-ing equation, both phosphoric acid and the coproduced calcium chloride remain in solution:

Ca PO F10( 4)6 2+20HCl→10CaCl2+6H PO3 4+2HF (2.34) The breakthrough came when Baniel et al. developed the solvent extraction process that allowed the separation of calcium chloride from phosphoric acid [50]. Also in the 1950s, geologists undertook surveys for phosphate rock formations in Israel to allow the superphosphate plant at Haifa [51] to run with alternative rock sources.

Several sources were discovered of which the first to be mined, in 1952, was the Oron deposit, located 25 miles southwest of the Dead Sea. Subsequently, the Zin and

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Arad deposits were mined. Clearly, the economics of a relatively pure phosphoric acid, made from a local rock with a free acid, looked very attractive. In 1966, Arad Chemical Industries was formed and built a purified acid plant, based on the IMI process and designed to produce 165,000 tpa P₂O₅. The process is highly corrosive and required the use of plastics (PVC and polyethylene) for materials of construction that were still in a relatively early stage of development. In turn, these materials lim-ited maximum process temperatures to 60°C and affected the morphology of some equipment, for example, the cylindrical shape of the IMI settlers. A block diagram of the Arad process is shown in Figure 2.44.

Given most wet process acid is derived from sulfuric acid, IMI went on to develop other processes: firstly, one where a stream arising from hydrochloric acid dissolution of rock is combined with wet acid and then processed as per the standard route, and secondly, the closed route where the calcium chloride is recycled around the plant, a process that could run after start-up without hydrochloric acid [25]. Subsequently, these processes have developed in terms of both the solvent used and the extraction equipment.

Acid storage

Purification

Clarification

Clarification

Clarification Stripping

Extraction Water

Waste

HCI

Washing Evaporation

Distilled water

Solvent recovery HCI gas

Exhaust gas treatment Phosphate rock

Acid filtration

Product acid 95% H3PO4

FIGURE 2.44 Arad plant block diagram.

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In 1966, IMI announced it had given an exclusive license to the Litwin Corporation of Wichita, Kansas covering the United States and Canada. The license included the process package, engineering, and design, for IMI’s cleaning process to purify wet process phosphoric acid. In the announcement, IMI gave some details of a 2 tpd pilot plant, including a flow sheet, and analysis of the clean and residual acids resulting from the solvent extraction of a 53.5% P₂O₅ Florida wet acid. The results were not particularly impressive, the plant being only a single-stage extraction and strip with no scrubbing; nevertheless, the principle was demonstrated, and it was clear that a range of solvents were screened for the process. One distinguishing feature of this process is the use of different solubilities at different temperatures. Figure 2.45 shows the IPE–H₃PO₄–H₂O ternary diagrams at 5°C and 55°C and can be compared to Figure 2.19 that shows the same system at 30°C.

Extraction is carried out at 5°C as it is more effective at that temperature. As the temperature is raised, the three-phase zone moves toward the plait point, and as a result, the extract separates into a solvent-rich (low phosphoric acid content) layer and a heavier more phosphoric acid–rich extract. When the acid is stripped from this solution at the higher temperature, it is at a higher concentration than if it were stripped at 5°C. It is also the case that the volume of extract to be handled at the higher temperature is significantly lower than if the temperature had not been raised;

therefore, smaller equipment may be used.

At this time, IMI was working with A&W, and with input from A&W, Litwin Frenkel prepared a scheme for a potential plant at Whitehaven. The selected

H₃PO₄

5°C

55°C

H₂O

80

60

40

20

20

40

60

80

80 60 40 20 IPE

FIGURE 2.45 IPE–H3PO4–H2O at 5°C and 55°C ternary diagram.

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solvent was IPE, with extraction taking place at low temperature in a single mixer with two parallel settlers. The loaded solvent was then scrubbed in three mixer–

settlers and then stripped in a single mixer–settler at 50°C. Both the raffinate from the extraction settlers and the product acid from the stripping settler were transferred to solvent stripping columns to boil off the IPE that was recycled back to the stripping section. Finally, the product acid was concentrated in a conven-tional vacuum concentrator.

In 1971, a large purified acid plant was built in Coatzacoalcos, Mexico. This plant is still in operation and is the largest example of the IMI process outside Israel. The plant is rated at 200,000 tpa P₂O₅ and has been the subject of considerable develop-ment since the mid-1990s when A&W assumed full ownership.

The IMI Coatzacoalcos process is shown in Figure 2.46. The production site lies on the Gulf of Mexico and has direct access to ship, rail, and road transport. Sulfur and phosphate rock (for many years Moroccan Khourigba K-10 but more recently from other sources) are imported. Sulfur is burnt to sulfuric acid on a Monsanto unit and wet acid made on a twin-stream Prayon dihydrate plant. Steam from the sulfuric acid plant is used to concentrate the wet acid. The PWA plant receives 52% P₂O₅ wet acid into two 180 m³ day tanks. The wet acid, at about 50°C, is pumped to the first IMI mixer–settlers that are in parallel at 30 m³/h. The mixers have a volume of 4 m³ and the settlers, 66 m³; both are made from polypropylene-lined glass-reinforced plastic. These mixers also receive an aqueous stream from the scrubbing train and a partially loaded cold solvent from the second extraction mixer–settler. As the acid and solvent mix, the extraction reaction raises the liquid temperature to 35°C–40°C.

The lower aqueous phase flows by gravity to an intermediate storage and is then pumped to the second mixer–settler. In the second mixer, the aqueous stream con-tacts a chilled solvent stream. More acid is extracted into the solvent at 5°C–10°C.

The aqueous stream from this settler is raffinate and is pumped away to make GTSP.

The warm, loaded solvent passes through a heater into a separation vessel where a clean solvent phase is recovered and pumped through the solvent chiller. The acid-loaded phase is pumped forward to the scrubbing train.

The scrubbing train comprises six mixer–settlers, 3 and 27 m³, respectively. The aqueous layer from the first settler is pumped back to join the wet acid feed to the first mixers. A proportion of the aqueous feed to the second scrubbing mixer is split off and sold as a relatively high-impurity pure acid. Sodium hydroxide is added to the aqueous feed to the fourth mixer to aid scrubbing, and some water is added to the sixth mixer to cause a partial release of acid to carry out the scrubbing.

The scrubbed solvent is held in intermediate storage prior to flowing forward to the two stripping mixer–settlers. Water is added to both releasing two purified acid grades.

Solvent is removed from the raffinate and product acids by steam stripping against cooling water to condense water and refrigerant to condense out the solvent.

Product acids are directed to thermosyphon vacuum concentrators and onto deflu-orination via a series of economizers in a steam stripping column. Food grade acid is then pumped to the dearsenication section and reacted with sodium sulfide solution, mixed with filter aid, filtered batchwise on filter presses, and then air blown to ensure that residual hydrogen sulfide is removed from the acid.

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The final steps are decolorization utilizing carbon-filled columns, final concen-tration, and passage through a guard filter prior to final product storage.

The plant continues to undergo development. Under A&W ownership, two major additions were made to the plant, the dearsenication unit and the decolorization unit, as well as many smaller incremental improvements to increase consistency, out-put, and product quality. Development continued under Rhodia and now Innophos and also covered safety issues. The original plant was built to the standards of its time, the late 1960s and early 1970s, just before the accidents that led to the much needed development of hazard and operability studies, safer designs, and so on. The solvent, IPE, is flammable; the process requires that the solvent is chilled, which requires a refrigeration plant, which was originally a propane unit. Furthermore,

Cyclone

FIGURE 2.46 IMI Coatzacoalcos plant block diagram.

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the mixer–settlers are made from flammable plastics. Clearly, the fire potential is high. This is mitigated by extensive nitrogen blanketing and the use of hydraulic drives on the mixers, thus reducing the number of electric drives. Despite these mea-sures, the control room was built in the center of the plant, in order to be close to operations, but also potentially in the middle of a plant fire. This position was recti-fied as part of a safety improvement project.

The plant at Coatzacoalcos is now second only to the PCS plant at Aurora in scale;

it continues to develop and has shown that solvent extraction with IPE is capable of producing food grade acid in large volumes.

In 1993, Haifa Chemicals built a new plant at Mishor Rotem with a capacity of 32,000 tons P₂O₅ food grade acid and 25,000 tons food grade phosphate salts (equivalent to 12,000 tons P₂O₅). The process is based on the original IMI HCl route and is able to incorporate additional merchant grade acid made via the sulfuric acid route. The plant uses AmOH as solvent and Bateman pulse columns.