The Prayon process for the purifica

The Prayon process for the purification of wet acid by solvent extraction is the most
prolific of the PWA processes. Plants have been built in Puurs (1976) and Engis
(1983) in Belgium, Korea, Indonesia, Brazil (originally in 1987 and subsequently
uprated), and Morocco (1998), the latter two being the largest with capacities around
100,000 tpa P2O5
. Prayon was in the race with the other producers in the 1970s to
secure patents for their own process [52]. The distinguishing features of the Prayon
technology are as follows: the use of an 85%–95% IPE, 5%–15% TBP solvent mix-ture; the use of a proprietary stacked mixer–settler column for the solvent extraction
section; and the use of barium carbonate for desulfation.
At a high level, the Prayon process is the same as most other PWA processes and
comprises a pretreatment step to condition the wet acid, a solvent extraction step,
and a posttreatment step to bring the product up to food grade. The block diagram of
the Prayon process is shown in Figure 2.47and assumes a 54% P2O5
wet acid feed
requiring full pretreatment.
The first stage of pretreatment is desulfation. Ground phosphate rock and/or
barium carbonate and activated silica are added to the feed acid in the desulfation
reactor. The calcium in the ground rock or the barium reacts to form a solid sulfate
(barium being the less soluble of the two), and silica is added to mop up excess
fluoride. The exact amounts of these additives depend upon the impurity levels
in the feed acid; however, the goal is to reduce sulfate to around 0.3% and fluo-ride levels to less than 0.1%. The acid is pumped from the desulfation reactor to
the concentration section where the acid is brought up to 62% P
2O5
in a standard
pumped circulation vacuum concentrator. Vapor from the concentrator body enters
a separator; the gas phase, including both HF and SiF4
, passes to a fluorine recovery
system where the gases are condensed, forming fluosilicic acid that is then exported
for sale in the water fluoridation market. The amount of silica added is controlled
to ensure the formation of fluosilicic acid. The acid from the concentrator is held
in intermediate storage then pumped to filtration on filter presses. The filter cake
is pumped back to the wet acid plant and the filtrate goes forward to settling, aided
147 Purification of Phosphoric Acid
by a flocculating agent. Following settling, the acid is heated to 150°C with steam
in a graphite tube heat exchanger, mixed with 5 kg/ton P
2O5
sodium chlorate, and
allowed to oxidize in the PTFE-lined oxidation reactor. Oxidation destroys organic
compounds that both hinder solvent extraction and impart color to the acid; it also
alters the oxidation state of some elements and may make the acid more corrosive to
some equipment. The hot, oxidized acid passes out of the reactor and is flash cooled
and is transferred to the dearsenication reactor that is also fed with sodium sulfide.
The sulfide reacts with arsenic that is now at +5 oxidation state and therefore does
not precipitate. Unlike other dearsenication/sulfiding processes, the plant in Brazil
does not have either carbon addition or filtration; the acid is pumped forward to the
solvent extraction section. In Morocco, sulfiding is carried out as part of the pro-cessing of wet acid prior to feeding the PWA plant.
Water
Cooling Sulding
Refrigeration
unit
Cooling and
storage
Oxidation
Settling
Scrub
Extract
Cooling
and ageing
Vacuum
concentration
Ground rock
and silica
Fluoride
recovery
Desulfation
Pressure
lters
Condensate
Steam
Strip
Steam
Storage
Condensate
Solvent
cooling
Steam
Deuorination Decolorization
NaClO3
HF and SiF4
Feedacid
130,000 tpa P2O5
54% P2O5acid
Raˆnate
35,000 tpa P2O5
35% P2O5acid
Industrial acid
10,000 tpa P2O5
35% P2O5acid
Foodgrade acid
95,000 tpa P2O5
62% P2O5acid
Filter cake to
wet acid plant
H2
SiF6
for sale
Na2S
Vacuum
concentration
Carbon
treatment
NaClO3or H2O2
FIGURE 2.47 Prayon plant block diagram.
148 Phosphoric Acid: Purification, Uses, Technology, and Economics
The pretreated acid is cooled against brine to 10°C–15°C and fed to the extrac-tion column where it contacts the 85% IPE/15% TBP solvent. The acid/solvent feed
ratio is 5 by volume. The Prayon extraction column for overall plant capacities in
the range 25–50,000 tpa P
2O5
is 2.4 m diameter by 8 m tall and has five mixer–set-tler stages. The temperature is maintained in the column by internal cooling coils
to counteract the exothermic nature of this solvent extraction and because the acid
solubility in the solvent is better at lower temperatures. The proportion of P
2O5
extracted into the solvent varies with feed acid quality but is in the range 70%–80%.
Raffinate leaves the base of the column with up to 1% dissolved solvent at 35% P2O5
concentration and returns to the wet acid plant. The plant in Brazil was uprated by
adding a Kühni column and extracting further P2O5
from this stream with sulfuric
acid following the same principles as the A&W UFEX process and the R–P total
exhaustion process.
Following extraction, the loaded solvent is pumped to the washing (scrubbing)
column, the same diameter as the extraction column but with only four mixer–settler
stages. The loaded solvent is washed with water, transferring some acid and impuri-ties to the aqueous phase. Unlike other processes, this wash acid then leaves the plant
and is available for technical grade applications. The water addition rate is controlled
and set based on the impurity profile of the purified acid.
The washed acid is pumped forward to the stripping column that is contacted
with warm water to release the acid into the aqueous phase. The stripping column is
1.5 m diameter and 5 m tall, comprises three stacked mixer–settlers, and operates at
50°C. Purified acid leaves the column at 45% P2O5
and is directed to steam stripping
to remove solvent. The solvent from the stripping column flows to the solvent stor-age. As it is pumped back around the solvent circuit, it passes through a coalescer to
allow removal of water.
The purified acid is held in intermediate storage before going forward for carbon
treatment in a series of four 1.8 m diameter activated carbon columns to ensure any
trace solvent and organic species are removed. The activated carbon is regenerated
with sodium hydroxide.
The acid is then concentrated in a multiple effect concentrator up to 62%–63%
P2O5
. At this stage, the acid contains approximately 300 ppm F. Acid intended for
food grade is then sent for defluorination. The original Prayon design was for batch
defluorination; however, in Brazil and Morocco, this is done on a continuous basis
with steam in a PTFE-lined column.
The final stage of treatment is decolorization that is carried out either with NaClO3
at 170°C or with hydrogen peroxide.
If the sulfate level is too high, another posttreatment step is included where bar-ium hydroxide is added to precipitate barium sulfate.
The Prayon process has proved to be flexible. The plants built in Indonesia, Korea,
and Brazil were very similar to the first full-scale plant in Puurs, Belgium (which was
initially 40,000 tpa P
2O5, compared to 25,000 tpa P
2O5
for the other three). Brazil in
particular has undergone many developments, including the conversion from batch
to continuous defluorination, extraction from the raffinate stream, and the use of
barium in desulfation. Furthermore, the Brazilian plant has demonstrated a flexibil-ity to deal with quite significant changes in feed acid impurity levels. The plant was
149 Purification of Phosphoric Acid
initially commissioned with Cajati acid, a relatively pure wet acid from an igneous
rock but has also managed with Goiásfertil acid, also from an igneous rock but with
high iron levels. The plant also utilizes acid from Moroccan Yousouffia and Ben
Guerir sedimentary rock with different levels of organics and heavy metals. A fur-ther strength of the Brazilian operation is the integrated pilot plant that allows the technical team to evaluate and plan the introduction of different acids