what is phosphoric acid , Production Process , flow sheet and process description
Phosphorous
Phosphorus is the chemical
element that has the symbol P and atomic number 15. As phosphorus was the 13th
element to be discovered and can be used in explosives, poisons and nerve
agents is referred as "the Devil's element". It is essential for
life. It is a component of DNA, RNA, ATP, and also the phospholipids that form
all cell membranes. Prepared from urine it was the first element to be
discovered science ancient times.
Hennig Brand discovered phosphorous during his
experiment on urine in 1669. Robert Boyle was the first to use phosphorus to
ignite sulfur-tipped wooden splints, forerunners of our modern matches, in
1680.
Due to its high reactivity, phosphorus is never found
as a free element on Earth. Phosphorus as a mineral is present in its maximally
oxidized state, as inorganic phosphate rocks from which it can be extracted
out. Phosphorous which is primarily extracted from calcium phosphate rocks
consider as an expensive mineral but, with the increasing demand of phosphorous
products like phosphoric acid, synthetic fertilizer and phosphate salts. The
situation demands modification in extraction method as well as product
manufacture.
Elemental phosphorus exists in
two major forms
1. White phosphorus
2.
Red
phosphorus,
White phosphorus was first made
commercially, for the match industry in the 19th century, by distilling off
phosphorus vapour from precipitated phosphates, mixed with ground coal or
charcoal, which was heated in an iron pot, in retort. The precipitated
phosphates were made from ground up bones that had been degreased and treated
with strong acids. Carbon monoxide and other flammable gases produced during
the reduction process were burnt off in a flare stack. This process became
obsolete when the submerged arc furnace for phosphorus production was
introduced to reduce phosphate rock. The electric furnace method allowed
production to increase to the point where phosphorus could be used in weapons
of war.
Red phosphorous is one of the most common allotropes
of phosphorous and it is considered to be derivative of P4 molecule it exists
in an amorphous (Non crystalline) network of phosphorus atoms. It is found to
be more stable than white phosphorus (another naturally occurring phosphorus
allotropes) red phosphorous is characterised by its deep red colour and powdery
texture. Powdery texture white phosphorous undergoes a gradual transformation
to yield the red phosphorous allotrope.
Properties
Ø
Translucent like
white waxy solid
Ø
Poisonous in
nature
Ø
Less stable and
more reactive
Ø
Does not
dissolved in water
|
Period |
3 |
|
Atomic number |
15 |
|
State at 20°C |
Solid |
|
Electron Configuration |
[Ne] 3s23p2 |
|
Melting Point |
44.15°C |
|
Boiling Point |
280.5°C |
|
Specific Gravity |
1.82 |
|
Atomic mass |
30.974 |
PHOSPHORIC ACID
Phosphoric acid (H3PO4), also known as
orthophosphoric acid or phosphoric(V) acid, is a mineral (inorganic) acid.
Orthophosphoric acid molecules can combine with themselves to form a variety of
compounds which are also referred to as phosphoric acids.
Phosphoric acid is one of the most popular acids that is used in many
industries, especially in the manufacturing of fertilizers. The salts of this
acid which are known as phosphates are used mainly in agriculture and even at
home.
Phosphoric acid falls into the category of weak acids. It is also
referred to as orthophosphoric acid which helps us to easily distinguish it
from other phosphoric acids such as polyphosphoric acid. Another name for this
acid is phosphoric(V) acid. Phosphoric acid’s formula is written as H3PO4.
This acid is a non-toxic acid and in its pure form, it is a solid at room
temperature. It has a molar mass of 97.99 g/mol.
H3PO4 is one of the most important and useful
mineral acids. The acid is mostly available in the form of an aqueous
solution (almost 85%) and is odorless, colorless, and non-volatile liquid. The
solution is a sticky liquid and even though this acid is categorized as a weak
acid it can still cause irritation or burns in the skin as well as damage to
the eyes and membranes in the nose.
Phosphoric acid melts at a temperature just above room
temperature (about 20°C; 68°F), so would be expected to occur as a solid under
those conditions. As a solid, the acid is a white crystalline material with a
strong tendency to absorb moisture from the air. In fact, phosphoric acid may
also occur as a supercooled liquid at room temperature. A supercooled liquid is
one that remains in a liquid state at temperatures below its freezing point, at
which temperature it would be expected to be a solid. As a liquid, phosphoric acid is
a colorless, odorless, syrupy liquid whose character is sometimes described as
sparkling.
Phosphoric Acid Structure
When we talk about the structure of phosphoric acid,
the central phosphorus atom is bonded together with an oxygen atom through
a double bond.
It is also connected to three hydroxyl (-OH) groups through single bonds.
Phosphoric Acid
Properties
Below we will look at the chemical and physical
properties of H3PO4.
Physical Properties
- Pure
phosphoric acid is normally in the form of a white crystalline solid.
- It
has a melting point of 42.4° C.
- The
acid is colourless.
- It
is also odourless and a viscous liquid with a density of 2.030 g.cm-3.
- H3PO4 is
non-toxic and non-volatile.
Chemical Properties
- Phosphoric acid or H3PO4 can
release up to three H+ ions. Due to this property, it can react
differently in comparison to other mineral acids.
- Reaction with bases usually results in the
formation of three classes of salts.
- When the molecules of phosphoric acid are exposed
to high temperatures it forms dimers, trimers and even long polymeric
chains as seen in poly phosphoric acids and meta-phosphoric acids.
Phosphoric
Acid Uses
Phosphoric acid is one of the most popular chemical
compounds that have several uses in different industries and even in products
that we consume. Here are some popular uses of H3PO4.
. In Agriculture
One of the most common uses of phosphoric acid is in
the agriculture domain. It is widely used in the production of fertilizer and
as a flavouring agent in animal or poultry feed.
In Dentistry
It is also used in dentistry where dentists often use
the chemical compound as an etching solution and for cleaning the teeth.
Phosphoric acid is also found in mouth cleaning products. Alternatively,
phosphoric acid is found in anti-nausea medicines.
Treatment of Rust
Phosphoric acid is also used in treating rusts and
removing them from metal components. It is used in the process of the phosphate
conversion coating. This helps in corrosion resistance.
Skincare Products
Phosphoric acid mostly used in adjusting or
controlling the pH
level in skincare products. It
is used in toothpaste, soaps, and detergents as well.
In The Food and Beverage Industry
Phosphoric acid is often used as a food additive and
is mainly utilized to acidify foods and beverages. It helps in creating a
certain taste.
Beverages
Food
grade phosphoric acid is used in the beverage industry as an acidulant and
flavoring agent. The presence of 0.05 % phosphoric acid contributes to the
unique taste of cola drink. Phosphoric acid is also a less expensive additive
for beverages than citric, tartaric, acetic and lactic acids.
Cheese
Phosphoric
acid is used in the dairy industry as an acidulant to cause curdling in the
production of cottage cheese.
Egg products
Phosphoric
acid is used to minimize the viscosity and browning in the spray drying of egg
albumen.
Fats and oils
Phosphoric
acid is added to fats and oils as an emulsifier and to aid in the control of
fatty peroxides. It is also used as a degumming agent during vegetable oil
refining.
Gelatin
Phosphoric
acid is sometimes used to improve the settling rates and clarity of gelatins.
Jams and Jellies
Small
amount of Phosphoric acid can be added to jams and jellies for acidifying,
buffering and preservation.
Sugar refining
In the
refining of raw sugar, phosphoric acid is reacted with lime to form a calcium
phosphate precipitate which aids in the filtration of particulates.
Other Applications
Ø
It is used in
phosphoric acid fuel cells.
Ø
Production of
activated carbon.
Ø
Compound
semiconductor processing.
Ø
It is used in
sanitizing brewing and dairy industries.
Phosphoric
Acid Hazards
Phosphoric acid is mostly non-toxic and does not cause
harm to the skin or any part of the body in low concentration. It is dangerous
only at higher concentrations and can cause severe skin irritation or burns and
even damage to the eyes. H3PO4 can also cause
irritation in the respiratory tract if vapours are inhaled. This acid should be
stored in a metallic or coated fibreboard container (with polyethylene inner
package) and kept in a cool and well-ventilated place.
Preparation
of Phosphoric Acid
Phosphoric acid is usually prepared or manufactured
using two different processes. These include:
Ø
Thermal process.
Ø
The ‘wet’
process.
The Thermal
process
In this, phosphorus is heated or burnt at high temperature in the
presence of air. The burning results in the generation of phosphorus pentoxide
which is then condensed to form a white powder. It is then hydrated in a
separate process to obtain phosphoric acid.
Sometimes steam is also added to the burner where a
condensed form of polyphosphoric acids is produced. The products are then
directly passed into a hydration tower where the gaseous phosphorus oxide is absorbed
and phosphoric acid is obtained. Nonetheless, a purer product is obtained in
the first process.
Out of number of processes available among them latest
three production processes are very common:
Ø
Using
phosphate rock and blast furnace
Ø
Using
phosphate rock and electric furnace
Ø
Oxidation
and hydration of phosphorous.
Using
phosphate rock and blast furnace
The blast furnace process was widely used in the first
three decades of 20th century. Resulting phosphoric acid can be used in
manufacturing of insecticide, pesticides, detergents etc. but not for
fertilizers
Reactions
Ø
Ø
Ø
|
Steam |
|
To phosphorous plant Phosphporous vapour |
|
Steam boiler |
|
Water |
|
Binder Coke |
|
Sand |
|
Phosphate rock |
|
Water |
|
Air |
|
Blast Furnace |
|
Hot blast stove |
|
Phosphoric acid |
|
Slag |
|
Ferro phosphorous |
|
Figure: Manufacturing of Phosphoric acid
using blast furnace |
|
Compressed air |
|
Briquetic press pressure 5000psi |
|
Dust collector |
Phosphate rock is pulverized and mixed with coke powder and binder is compressed to 5000 psi resulting into the briquettes. Briquettes are dried and charged along with sand and additional coke powder from top of the blast furnace.
The preheated air (1000 – 11000C) is charged
from bottom of the blast furnace via tuyere. A tuyere is cooled copper conical
pipe numbering 12 in small furnace and up to 42 in large furnace through which
hot air is blown in to the furnace. Preheated air leads to burning of
briquettes giving temperature rise up to 13700C.
The coke acts as reducing
agent as well as fuels. About 760kg of coke is consumed in reduction of
phosphate rock to phosphorous and remaining generates heat by combustion with air. Reaction is
completed in the furnace itself producing P2O5 and calcium silicates as slag.
The product gases also
contain carbon monoxide and nitrogen along with dust particles. For
purification, it is passed through cyclone separator and phosphorous condenser.
Thus, P2O5
and elemental
phosphorous are separated out.
Hot P2O5 gases are cooled in the
heat exchanger. Therefore, superheated steam is produced and a part of
gas is taken into regenerative blast furnace.
As a result, the entire
phosphorous and phosphorous pentoxide is cooled and purified before taken into
hydrating towers. Purification of phosphoric acid includes removal of arsenic
by hydrogen sulfide treatment followed by filtration
Using
phosphate rock and electric
The phosphate rock is
reduced to elemental phosphorous by the action of coke and heat in the presence
of sand in electric arc furnace subsequent oxidation of phosphorous gives
phosphorous pentoxide which on hydration gives the product phosphoric acid.
|
Water |
|
CO2 |
|
Sand |
|
Coke breeze |
|
Sand filter |
|
Water |
|
Phosphate rock |
|
Size |
|
Ferro |
|
phosphorous |
|
Electric Furnace 2400 oF |
|
|
|
H2S / H2O |
|
Phosphoric acid 85 % Air Figure: Manufacturing of Phosphoric acid
using Electric furnace |
|
Water |
|
Sintering& Sizing |
|
Purifier |
Phosphate rock after proper grinding
and primary purification is taken into sintering oven where it is nodulized and
granulized so that fast oxidation of the separated phosphorous takes place.
Temperature of 10950C is maintained in electric furnace so that maximum amount of
elemental phosphorous extracted out and oxidation takes place. Since fluoride
of phosphorous and calcium are the common impurity which reacts with sand
giving fluorosilicates as the slag.
The gases from the furnace,
phosphorous and carbon monoxide are removed by the suction process and the oxidation product P2O5 is taken into hydration
column which gives P2O5 to H3PO4 at about 850C. Purification of phosphoric acid is carried
out by H2S
to remove Arsenic, H2SO4 to remove calcium salts and Silica to remove
fluorides. All the byproducts are removed before concentrating the acid and
filtering it as final product.
Oxidation and Hydration of
Reactions
Ø
Ø
|
Water |
|
COMBUSTION CHAMBER |
|
Phosphorous Air |
|
Steam |
|
Vent |
|
85% Phosphoric acid Figure:ManufacturingofPhosphoricacidbyOxidationandHydration |
The
gases from furnace which mainly contains P2O5,
steam, N2 and
small quantity of oxygen is taken into a hydration column where counter
current mixing of dilute
phosphoric acid and the gases gives the product concentrated H3PO4 of 75% to 85%
concentration. Remaining acid is trapped into packed column or electrostatic
precipitator.
The ‘wet’
process.
The wet process is
according to the acids (sulfuric acid, nitric acid or hydrochloric acid) used
to decompose the phosphate rock.
Ø Rock Phosphate with concentrated Sulphuric
Acid
Ø Rock Phosphate with dilute Nitric Acid
Ø Rock Phosphate with Hydrochloric Acid
Rock
Phosphate with concentrated Sulphuric Acid
The process using sulfuric acid is the most
common among all particularly for producing fertilizer grade phosphoric acid.
This acid, also called as
green acid. Depending upon the hydrate forms of calcium sulfate produced during
the wet process, it is classified as anhydrate, hemi hydrate and dehydrate. The
hydrate form is controlled mainly by temperature and acid concentration.
wet process, phosphoric acid is produced from a
naturally occurring crystal rock known as fluorapatite which contains the
phosphate mineral. This compound is reacted with concentrated sulphuric
acid and water. When the
reaction takes place it results in the formation of phosphoric acid and calcium
sulphate (gypsum) as well as some insoluble impurities. The extra chemical
compounds and impurities are removed by the process of filtration and
evaporation. The acid is then concentrated to ca 56-70% P2O5 (super
phosphoric acid) using vacuum distillation. The reaction can be represented as:
Ø
Ca5(PO4)3Cl
+ 5H2SO4 + 10H2O → 3H3PO4 +
5CaSO4·2H2O + HCl
Ø
The product from the ‘wet process’ acid is impure but
can be used, without further purification, for fertilizer manufacture.
Rock Phosphate with dilute Nitric Acid
The process involves
acidifying phosphate rock with dilute nitric acid to produce a mixture
of phosphoric acid and calcium
nitrate.
Ø Ca5(PO4)3OH
+ 10 HNO3 → 3 H3PO4 + 5 Ca(NO3)2 +
H2O
The mixture is cooled to
below 0 °C, where the calcium nitrate crystallizes and can be separated
from the phosphoric acid.
Ø H3PO4 +
3 Ca(NO3)2 + 12 H2O → 2 H3PO4 +
3 Ca(NO3)2·4H2O
The resulting calcium
nitrate produces nitrogen fertilizer. The filtrate is composed mainly of
phosphoric acid with some nitric acid and traces of calcium nitrate, and this
is neutralized with ammonia to produce a
compound fertilizer.
Ø Ca(NO3)2 + 4 H3PO4 +
8 NH3 → CaHPO4 + 2 NH4NO3 +
3(NH4)2HPO4
If potassium chloride or potassium sulphate is added, the result will be NPK fertilizer. The process was an
innovation for requiring neither the expensive sulphuric acid nor
producing gypsum waste.
The calcium nitrate mentioned before, can as said be
worked up as calcium nitrate fertilizer but often it is converted into ammonium nitrate and calcium carbonate using carbon dioxide and
ammonia.
Ø Ca(NO3)2 + 2 NH3 +
CO2 + H2O → 2 NH4NO3 +
CaCO3
Both products can be worked up together as straight
nitrogen fertilizer.
Rock Phosphate with Hydrochloric Acid
Different
processes have quite varied operational and availability of raw material resources,
electricity. in order to prefer rock phosphates with hydrochloric acid is the
biggest advantage over all other production processes for our organization.it
is also best suited to consume hydrochloric acid which is availabled by caustic
soda plant .as HCl is the byproduct of caustic soda plant. as we do not have
power plant we r taking electricity from others. so I think these are major
reason to prefer this process in our plant Grasim Industries Limited, Karwar.
The hydrochloric acid
system introduced by Israeli Mining Industries Ltd., the hydrochloric acid
system introduced by Dow Chemical Company
The Israeli process
utilizes hydrochloric acid as the leach to solubilize the apatite with the
production of Patented Feb. 10, 1970.
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