History of Phosphoric Acid Technology (Evolution and Future Perspectives)

KEMWorks • Symphos 2011 • P1104

Phosphoric Acid Technology – History, Evolution & Future Perspectives

Chris Earl (KEMWorks) & Mohamed Belghiti Aloui (OCP)

Plenary Lecture presented at Symphos 2011, Marrakech May 2011

The first use of phosphate as agricultural fertilizer is lost in history. Like the origin of fire, mankind

accidentally discovered that certain soil amendments significantly improved yield of important food

crops. One of the earliest records is of the use of “bird dung” (guano) in Carthage about 200 BCE. Thus

phosphate use started in N. Africa not too far from here.

While it was known that bones, fish and guano were effective fertilizers, the active ingredient was not

identified until 1669 by the alchemist Brandt, who was trying to turn lead into gold. The British chemist

Boyle first prepared phosphoric acid in 1694. He discovered that mixing the products of burning

phosphorus with water produced a liquid with acidic properties.

Phosphate fertilizers mainly consisted of bones and guano until the middle 19th century. Treatment of

bones with sulfuric acid was patented by J. Lawes of Britain in 1842. Lawes subsequently modified his

patent to include treatment of mineral phosphates. The famed German chemist Liebig noted in 1857

that mineral phosphates could be solubilized with sulfuric acid as readily as bones and thus be made

much more effective as fertilizer. In only five years the production of Single SuperPhosphate reached

200,000 tons per annum in Great Britain.

The first Wet Process Phosphoric Acid (WPA) plants were built between World War 1 & 11 and typically

produced 25 – 50 t/d P2O5 using decantation to separate the precipitated gypsum. The Dorr Company of

Connecticut, USA was a pioneer in development of the WPA process using counter-current washing of

gypsum in sequential thickeners thus producing about 22% P2O5 acid. By the 1920’s the last thickeners

in the wash train were replaced by continuous filters supplied by The Oliver Company and product

strength rose to 25% P2O5. Eventually, the two companies combined to form the Dorr-Oliver

Corporation which was much later acquired by Jacobs Engineering.

In 1932 Dorr-Oliver built the largest phosphoric complex to date at Trail, BC, Canada for Cominco. It

consisted of three trains each consuming 150 t/d of phosphate.

Single or Double Superphosphate remained the primary phosphate fertilizer until after WW 11. Mostly

it was produced in small plants often local to sources of phosphate and sulfuric acid. Baltimore and

Charleston in the USA were centers of such production because they were ports that could receive

phosphates and sulfur and also re-ship the final products.

After WW11 TVA did some innovative research to demonstrate the practicality of granulating high

analysis fertilizers such as GTSP and DAP. The advantage of these materials was lowered transportation

costs and in the case of DAP, simultaneous provision of essential nitrogen. By the 1960’s the explosion

of the phosphoric industry took off as all the major players in the industry started construction of large

WPA plants and integrated fertilizer complexes producing DAP and GTSP.


Milestones in Wet-Process Phosphoric Acid Production

(Acknowledgements’ to Waggaman)

Period Process Acid Concentration% P2O5

Single Train PlantCapacity t/d P2O5

1870 -1915 Batch 10 - 15 5?1915- 1919 Counter-current decantation 22 - 23 101920 - 1931 Counter-current decantation plus

continuous filtration25 40

1932 – 1952 Slurry Recycle 30 - 33 1001952 – 1961 Slurry Recycle 30 – 33 2501962 – 1965 Single Reactor, Tilting Pan Filter 30 – 32 850

1968 Single Reactor, Tilting Pan Filter 30 14001998 Single Reactor, 5 Belt Filters 28 2650

One of the authors (Earl) was the process engineer responsible for the 1968 plant listed above. It was in

fact a two train plant built for Freeport Sulphur at Uncle Sam, Louisiana, USA. This plant processed

Florida phosphate which was shipped by barge across the Gulf of Mexico then up the Mississippi River.

Each reactor-filter system was nominally rated at 900 t/d but essential features such as vacuum cooling

were over-sized. This was in fact a normal practice since the Owners wished to pay the minimum license

fee. At that time these plants installed the largest Bird-Prayon tilting pan filters ever built comprising

150 m2 of pan area. The plants soon attained 1400 t/d capacity and Freeport had to add another

sulfuric acid plant. In the 1970’s the plant was converted to wet rock grinding as part of a general trend

in the USA to conserve energy.

During the last forty years the wet process phosphoric acid industry has evolved under the influence of

several major factors. Among the most important are:

Changes in phosphate quality

Greatly improved instrumentation and control

Improved materials of construction

Improved equipment & processes

Phosphate Quality

In the early 1960’s it was not unusual for a process contractor to guarantee production of 32% P2O5 filter

acid at 96% overall efficiency. Indeed this might still be possible today using high quality BuCraa

phosphate from southern Morocco. However, as the phosphate industry grew rapidly between 1960

and 1980, the huge consumption of phosphate meant that domestic plants were forced to consume

significantly lower grade concentrates. The higher grades were dedicated to the export trade or GTSP

production so that whereas in 1960 31.1% P2O5 was considered minimum feed grade, by 1985 some USA

plants were feeding phosphate with only 28% P2O5. This had two effects: first, the lower grade often

meant that the feed contained a lot more deleterious contaminants such as iron, aluminum,

magnesium, etc; second, since many plants were now feeding a wet phosphate slurry, which was limited


to about 68% solids because of viscosity constraints, the phosphate slurry contained a lot more water

which diminished the available filter wash water. This combination of process conditions meant that

many plants were limited to maximum filter product strength of 28% P2O5 and in some cases as low as

26%. Fortunately, during the same period Sulfuric Acid Plant technology was improving and byproduct

steam recovery was significantly increased which provided the means to adequately concentrate the

weak acid to a quality suitable for DAP or Merchant Acid Grade.

Improved Instrumentation and Control

Prior to 1950 the means to control addition of phosphate, sulfuric and recycle acids to the reactor were

very crude. Unreliable scales and dipper wheels for acid flow were the “state-of-the-art”. Even as late

as 1968 when the Freeport Sulphur plant was built using Prayon technology, the recycle acid was split

between the various reactor cells using a large “splitter box”. This quaint device was a large rectangular

box that received all the flow of recycle acid from the filter and then allowed it to be sent to three

different cells by means of adjusting large conical plugs in the outlet pipes at the bottom. Orifice meters

could not be used because scale formation rapidly decreased their accuracy.

In this “modern plant” finely ground dry phosphate was fed using a Hardy Scale which employed a batch

bucket system that filled and discharged three times per minute using a classic beam balance system.

Later conversion to wet phosphate slurry meant that magnetic flowmeters (EMF) and density

instruments could reliably and accurately control this most basic of plant inputs. EMF flowmeters are

now routinely used for concentrated sulfuric acid and recycle acids.

If you walked into the control room of a 1960’s era phosphoric acid plant you would see a huge wall of

instrument displays mostly relying on pneumatic conduits to relay the measurement and control signals

back and forth between the field and panel instruments. Each control instrument had tuners for

derivative or proportional band analog control. Often temperatures and pressures were recorded on

charts by pens with different colored inks. The charts and ink supplies required frequent changes.

Many operators and control technicians were employed to monitor, adjust or maintain the controls.

Today we have grown used to powerful digital computer systems that not only provide precise control

from a single monitor but also can record gigabytes of data and plant history for review and trouble-

shooting.

Materials of Construction

In the early days of wet process phosphoric acid plant designers relied on heavily on lead and rubber

lined steel. Although stainless steel alloys were manufactured in the 1920’s they were little used in

industry until after WW 11. 1960’s era plants in the USA used mainly 316L alloy or for a more corrosive

phosphate, such as N. Carolina, 317L. Alloy 20 was popular for pumps and valves, just as it is today. In

the 70’s and 80’s it was realized that adding Molybdenum to the alloy conferred extra resistance to

Fluorine attack. Thus 904L, a super-alloy high in Molybdenum, became almost the standard for filter

pans and central valves plus the preferred material for agitator shaft and blades.


Again in the 1960’s most piping was rubber-lined (RL) steel. As reliable glass-reinforced polyester pipe

became widely available it replaced RL pipe except for very large sections such as for overhead vapor

and slurry circulation pipes. In recent years even this last application has given way to the use of 904L

ducts up to one meter diameter. The reason being that vacuum cooler outlet pipes are prone to scaling

and their cleaning often resulted in rubber damage which greatly extended the downtime.

In the USA many large plant operators adopted a pragmatic approach and started using HDPE pipe for

most if not all applications in the reactor/filtration system. First, it was cheap, and totally immune to

corrosion. Second, many operators also had mining and beneficiation plants that used large quantities

of HDPE pipe, so they had good experience with the fusion welding and installation of this material. Not

being restrained by warranty and litigation considerations the plant operators often installed HDPE in

conditions outside the manufacturer’s recommendation. Most HDPE pipe is limited to use below 60 0C

and has no design strength at that temperature. Some USA plants routinely installed HDPE for

conditions up to 820C by using very heavy wall thickness and providing continuous support. HDPE

became almost the standard material for the filter down-legs. Its smooth surface and flexibility

extended the time between wash cycles. One of the most extreme applications of HDPE has been in the

fabrication of the reactor mixing tees where concentrated sulfuric acid and recycle acid are mixed

beneath the 820C slurry. Originally installed at an IMC plant in Florida in the 1990’s this simple and

inexpensive mixing tee has become widespread and shown a very long life compared to the use of

expensive super-alloys.

Improved Equipment & Processes

Phosphate rock preparation for reaction was generally considered part of the phosphoric acid plant and

most contractors supplied grinding mills close to the reactor system. When plants were smaller than

100 t/d P2O5 often ring/roller mills such as the Raymond Mill were used. Larger plants generally used

ball mills and from the 60’s on they became almost standard practice. Although wet grinding of

phosphate had been employed earlier in a few plants it started to be widely used in the USA in the early

1970’s in response to the first major energy crisis. Although the addition of water with the phosphate

meant potentially lower filter recovery, the process was rapidly adopted for a several important

reasons:

Most USA producers used locally produced phosphate, thus wet transport was not a big cost

issue

Major savings in energy and operating cost resulted from shutting down the rock dryers

Dust scrubbers and bag-houses were not required and nuisance dust was eliminated

Storage and metering of phosphate became easier

When dry phosphate was fed to the reactor it was common practice, at least in the Prayon process, to

dilute the strong sulfuric acid to about 65% with fresh water. This generated tremendous heat so the

diluted acid was cooled in a graphite tube heat exchanger prior to reactor addition. These “dilution-

coolers” were high maintenance equipment since any impurities in the fresh water e.g. silica would

precipitate and plug the tubes. The fragile tubes often broke when being cleaned. Once the plant was


converted to wet slurry phosphate it was no longer possible to dilute the sulfuric acid with water

because of water balance issues. To many operators the elimination of the dilution-cooler was another

advantage of the conversion to wet grinding.

The idea of pre-diluting and cooling the strong sulfuric acid was to prevent it from immediately

dehydrating the gypsum crystals present in the reactor and converting them to poorly formed hemi-

hydrate. In fact temporary bypass of the dilution-cooler often caused plant capacity to decrease by 50%

due to poor filterability. Since fresh water could not be used for dilution once wet slurry phosphate was

employed, the solution was to substitute the recycle acid as the diluent. This was accomplished in a

mixing tee installed on the reactor roof.

A disadvantage of wet grinding which eliminated the dilution-cooler was that considerable extra cooling

load was placed on the cooling system which was most often consisted of reactor slurry circulated

through a vacuum chamber i.e. Flash Cooling. Increased heat load meant larger temperature changes

and increased scaling in the vessel and circulation piping. In 1960’s and 1970’s Prayon Process plants

the Flash Cooler was elevated some distance above the reactor and was mostly empty except for a

curtain or crude spray of slurry. This required medium head pumps and circulation was designed for a

50C temperature drop. Often plants were run at 150% capacity and after conversion to wet rock

grinding they experienced 10-120C which meant that the Flash Cooler system required very frequent

cleaning.

The Prayon Process eventually arrived at a good solution when it proposed installing the Flash Cooler at

a low level so that it operated with a flooded pool of slurry. This allowed the use of low-head axial flow

pumps which could generate very large flows and thus minimize temperature drop and scaling. Modern

cooling systems can operate for a very long time without requiring to be cleaned. This increased

circulation also benefitted the reaction system and provided better efficiencies through lowered co-

crystallized losses and improved filterability.

Phosphoric acid filtration has improved significantly in the last forty years. Larger filters are available

and they operate at higher speed. In 1965 a filtration rate of 5 t/m2 P2O5 was the highest used for

design, nowadays it is common to consider 10 t/m2 or more for certain phosphates. The filter operation

was also very complex system requiring air/gas separators for all the filtrates and four compartment

agitated seal tanks for managing filtrate recycle and product discharge. A modern plant today

incorporates all the gas-liquid separation in the central valve which then can pipe the filtrates directly to

self-regulating pumps. Apart from simplifying the operation this type of filtration system design also

eliminated ten pieces of equipment which provided a saving of investment and maintenance costs.

From the 1980’s onward large belt filters were employed at some new installations. They had an

advantage of running at higher speed so that a m2 of belt filter area had more capacity than a m2 of a

pan or table filter. However, there was a practical size limitation to about 80 m2 so multiple filters were

required for most plants (Oswal) in India has five). Some vendors would claim filters of 100 m2 or larger

but this meant they had to have a spliced rubber belt which proved troublesome in some cases. The

attraction of belt filters was lowered installation cost and perhaps less scaling since the filter cloth can


be washed from both sides. It seemed that operators with good experience of tilting pan or table filters

were reluctant to use belt filters since it was often more difficult to keep the whole filter area clean.

The feature of using multiple filters for larger plants did show an unexpected but obvious benefit. The

reactor system does not need to shut down for the regular frequent filter wash cycle. The capacity

merely has to be reduced to match the available filter capacity. At PMP in Jorf Lasfar with two large

Profile filters the reactor could still run at 70% capacity with one filter on wash. Shutting a reactor for 8

to 12 hours every 5 – 7 days meant the system cooled off and an extra layer of fluosilicates was created

on the floor and walls. At annual inspection of single filter plants 400 mm of scale was often observed,

and upon inspection one could see that it was formed of layers like tree rings each about 5-8 mm thick.

Continuous operation of the reactor system provided a dramatic reduction of scale formation.

The Maroc Phosphore 3 & 4 plants at OCP’s Jorf Lasfar complex are an excellent example of the

evolution of phosphoric acid technology. Originally constructed in 1983 as a line of 8 x 600 t/d P2O5 it

was the largest WPA complex in the world at that time and used the Rhone-Poulenc (RP) air-cooled

reactor systems and one large table filter per reactor. Air cooling has limitations in larger plants so OCP

later modified a number of the trains to incorporate a high-level flash cooler which increased those units

capacity about 50%. Most recently OCP has announced the intention to transport the feed phosphate

187 km by pipeline from Khourigba in order to gain very large savings in drying and transport costs. The

feed will be ground to pipeline specification and pumped at 55 - 60% solids to a storage terminal at Jorf.

Needless to say, this pipeline slurry is incompatible with the existing MP 3 & 4 operations, so a major

modernization has been planned. Firstly the phosphate slurry will be thickened to minimize water

balance problem discussed above. Even so, table filters consume a large amount of fresh water to clean

the filter cloth after gypsum discharge. This water contains valuable P2O5 and must be re-used in the

process. With dry phosphate, as at the existing plants, this water can all be consumed while maintaining

reasonable product acid strength. With pipeline slurry the water balance either causes a steep drop in

plant efficiency or forces the production of uneconomically weak product acid. The intention therefore

is to modernize the plant by combining two existing R-P reactors into one plant and adding a Prayon

style rectangular reactor that will include a very large low-level flash cooler system. The RP pan filters

are at “end-of-life” and will each be replaced with a much larger Profile tilting pan filter. Thus the eight

plants will become four each with a modern reaction system and the latest multiple filter technology.

It would be remiss to present a history of wet process phosphoric acid without discussing technologies

that result in a calcium sulfate product other than dihydrate. There were various attempts at anhydrite

processes but they were never commercially successful. Hemihydrate processes have a long history in

the industry. The original motivation in Asia was to produce a valuable calcium sulfate byproduct.

Japanese companies were pioneers of this technology.

Mostly it was limited to small plants and higher temperatures, which increased corrosion, led to high

maintenance and a reputation of unreliability. There are a few hemihydrate plants in N. America mostly

built for site specific reasons. At Belledune, Canada a Prayon plant was successfully converted to Fison’s

hemi technology mainly because the sulfuric acid supply came from a smelter operation and there was


little byproduct steam to concentrate the filter acid. In N. Florida Occidental (now PCS) built a large

complex based on their own proprietary hemihydrate technology mainly because the prime product was

to be Superphosphoric Acid (70% P2O5) for export to the USSR, thus steam supply was at a premium.

More recently a large plant in Western Australia was based on hemihydrate technology and the large

complex being built in Saudi Arabia will also use hemihydrate technology. It is interesting that one of

the best applications of hemihydrate technology have been in the processing of igneous phosphates.

With dihydrate processing of igneous phosphates, although high quality, often they often had lower

performance in terms of filtration capacity. It seems that hemihydrate technology works well with

igneous phosphates such as produced in Russia and Brazil. Unfortunately, the hemihydrate process is

mostly incompatible with lower grade phosphate especially as pipeline slurries.

There is a well known African proverb that says “The future belongs to the people who prepare for it

today”. We do not know the future of the phosphoric acid industry but by understanding the

fundamental technology and the history of its development we can be well prepared for its future

evolution.

History of Phosphoric Acid Technology (Evolution and Future Perspectives)

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