Lecture 21 Phosphorous

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LECTURE–20: PHOSPHOROUSPHOSPHORIC ACID

CHEMICAL TECHNOLOGY (CH-206)

Department of Chemical Engineering

http://www.iitr.ac.in/Main/pages/index.html

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PHOPHOROUS (P)

In 1669, Hennig Brand discovered phosphorous. In 1680, Robert Boyle was the first to use

phosphorus to ignite sulfur-tipped wooden splints. It is used in explosives, poisons and nerve agents is

referred as "the Devil's element". It is a component of DNA, RNA, ATP, and also the

phospholipids that form all cell membranes. It is not found in its free element form on Earth, due

to its high reactivity. Phosphorus as a mineral is present in oxidized

state, as inorganic phosphate rocks. Phosphorous is primarily extracted from calcium

phosphate rocks consider as an expensive mineral..

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PHOPHOROUS (P) Elemental phosphorus exists in two major forms

White phosphorus 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. CO 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 production was further increased, electric furnace method.

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PHOSPHATE ROCK Phosphate rock is a natural mineral found as a

geological deposit in the form of sedimentary rocks containing various amounts of calcium phosphates.

India alone is estimated to have about 140 million tons of rock phosphate deposits, most of which are however, of low grade and with substantial impurities unsuitable for the production of phosphate fertilizers.

Phosphate rock is used as a phosphatic fertilizer after grinding called phosphorite or mineral phosphate, or as a primary source of phosphorus.

The ground rock (60-100mesh size) phosphate can be applied directly to the acidic soil.

For neutral or alkaline soils acid-treated rock phosphate (like superphosphate) is used. Crops like rubber, tea, coffee, apples and fruit plantations of

oranges are suitable for direct application of rock phosphate.

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PHOSPHATE ROCK Francolite (calcium carbonate-fluorapatite) of

formula [Ca5(PO4,CO3)3(F,OH)] is the most predominant mineral of phosphate.

Four kinds of phosphate rocks are recognized, with the phosphorus content varying from 2 to 21%: hard rock phosphate, soft rock phosphate, land pebble phosphate and river pebble phosphate.

Phosphate rock contains phosphorous in an apatite form which is water insoluble.

The citrate solubility can vary from 5 to 17 % of the total phosphorus, depending on the chemical nature of the rock and the size to which it is ground.

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PHOSPHATE ROCK

The efficiency of the ground rock phosphate can be increased by Mixing with soluble phosphorus and fertilizers Mixing with elemental sulfur or sulfur-producing

compounds Using phosphate solubilizing micro-organisms

More than 90% of rock phosphate is used for production of superphosphate and phosphoric acid.

Less than 8 % is used directly as soil fertilizer and about 2 % as animal and poultry feed.

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PHOSPHATE ROCK: PURIFICATION

Clays (kaolinite, illite, smectites and attapulgite), quartz and other silicates (feldspars), carbonates (calcite and dolomite), secondary phosphates (phosphates bearing iron and aluminum) and iron oxides (geothite, hematite and magnetite) are the common impurities which are associated with phosphate rock.

The ore is upgraded and the impurities are removed by Floatation method.

Calcium phosphate is obtained after removal of various impurities present in phosphate rock.

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Froth floatation is generally employed with siliceous ores when other less expensive or less complicated techniques fail to produce phosphate concentrates suitable for chemical processing.

Prior to its conditioning for floatation, the floatation feed of phosphate rocks is delimed.

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PHOSPHATE ROCK: PURIFICATION In the floatation of phosphate ores, apatite particles are

generally directly transferred to the froth fraction (direct floatation) by using anionic collectors such as fatty acids.

The anionic collectors selectively attach themselves to the phosphate particles, render them hydrophobic and lift them to the surface by the froth and air bubbles formed.

The mineral bearing froth may simply overflow the cells or paddles or may be skimmed off.

Quartz and other silicates are removed from the bottom of the floatation cells.

A second stage of floatation may be required to remove silica from the phosphate-rich float by cationic collectors (usually amines), when silica is floated and the phosphate particles settle to the underflow.

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A selective floatation of carbonates from phosphate rock is rather difficult owing to the similarity in the physicochemical properties of the carbonate and phosphate minerals.

Several treatments have been proposed, including floatation, calcination, acid washing, magnetic separation and heavy media separation for the removal of free carbonates from the phosphates.

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PHOSPHATE ROCK: PURIFICATIONProcess Raw materials

and reagents Main products and derivatives

By- products

Acidulation Phosphate rock, Sulfuric acid, phosphoric acid, hydrochloric acid ammonia, potassium chloride

Superphosphate, phosphoric acid (wet process) triple super phosphate , mono ammonium phosphate, diammonium phosphate

Fluorine compounds vanadium, uranium (limited)

Electric-furnace reduction

Phosphate rock, siliceous flux, coke (for reduction), electrical energy, condensing water

Phosphorus, phosphorus pentoxide and halides, phosphoric acid, triple superphosphate, various Na,K,NH4,Ca salts; mono potassium phosphate

Fluorine compounds, CO, slag (for RR ballast aggregate, fillers, ferrophosphorus

Calcium metaphosphate

Phosphate rock, phosphorus, air or oxygen, fuel

Calcium metaphosphate

Fluorine compounds

Calcination or defluorination

Phosphate rock silica, water or steam, fuel

Defluorinated phosphate

Fluorine compounds

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YELLOW PHOSPHOROUS

Raw materials Basis: 1 ton Phosphorus Calcium phosphate 6804 kg Sand 2018 kg Coke 1202 kg Carbon electrode consumption 22.68 kg Electricity 13000 kWH

Reaction2Ca3(PO4)2 + 10C + 6SiO2 CaSiO3 + P4 + 10CO

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YELLOW PHOSPHOROUS

11500C15000C

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YELLOW PHOSPHOROUS

11500C and 1500 0C

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YELLOW PHOSPHOROUS Rock phosphate is crushed as fine powder. The powdered rock phosphate is mixed with sand

and coke powder in the required proportion and charged into electric furnace having electrodes at bottom.

The base of furnace is heated with carbon blocks as the temperature increases due to electrical heating.

Reaction of rock phosphate with sand starts at about 11500C resulting into calcium silicate and P2O5.

Further increasing the temperature to 15000C carbon particle reacted with P2O5 there by phosphorous and CO formed which is collected from top outlet.

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YELLOW PHOSPHOROUS

Residual calcium silicate settles down at bottom in form of slag which is taken out time to time from the outlet provided at bottom of the furnace.

Product gases which is mixture of phosphorous and CO is cooled in a water cooler thereby phosphorous solidify and CO gas is separated.

Purification of phosphorous is carried out by melting it and treating with chromic acid (mixture of K2Cr2O7 and H2SO4).

The carbon and silicon impurities are removed due to oxidation.

Pure phosphorous which is pale yellow colour is washed with water before it is casted into sticks.

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YELLOW PHOSPHOROUSRECOVERY OF THE PRODUCTS

The fumes emerging from the furnace are first freed from any mineral and other fine reagents which may have been carried up at less than 3000C so that the phosphorous is not condensed out.

The remaining gases consist of phosphorous, CO and SiF4, are sent to a bottom of tower in which water is sprayed from two different heights. The temperature should not go below 600C.

The phosphorous condenses out but does not solidify, is collected under water which reacts with the SiF4 gas, converting into metasilicic and fluorosilicic acids.

SiF4 + 3H2O H2SiO3 + 2H2SiF6

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YELLOW PHOSPHOROUSRECOVERY OF THE PRODUCTS

The fluorinated components are subsequently recovered from the solution.

CO which is completely freed from phosphorous and fluorine compounds is then cooled, dried and subsequently used as a fuel.

The slags consisting of CaSiO3, which are produced in the furnace and subsequently discharged from outlet provided at base of furnace, are good additives for cements, air-port runway construction and antiskid conglomerates.

The liquid phosphorous, after decolouration with activated carbon is filtered and solidify to yellow phosphorous.

It is stored under water.

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RED PHOSPHOROUS

Raw material Yellow phosphorous

Batch process Yellow phosphorous obtain from earlier process is

taken into iron pot provided with safety outlet and thermometer jackets.

Conversion of yellow phosphorous to red phosphorous is exothermic reaction and strict maintenance of temperature in range 2400C to 2500C is required as conversion do not take place below 2400C and above 2500C accident chances are there.

After complete conversion product is washed with NaOH solution as yellow phosphorous is soluble but red phosphorous is not soluble in NaOH

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RED PHOSPHOROUS

Continuous process In the process liquid white phosphorous is

maintained at boiling point for 5-6 hrs to achieve 35 to 50% conversion.

The product is taken into screw conveyer in which unreacted phosphorous vaporizes which is recrystallize and recycled.

The red phosphorous is of high purity and therefore doesn't require further purification.

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RED PHOSPHOROUS

Block diagram of manufacturing process

Process diagram of manufacturing process

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PHOSPHOROUS: PROPERTIES Molecular formula : P Molecular weight : 30 gm/mole Appearance : White, red and black

solid Odour : Irritating odour Boiling point : 280.5 0C Melting point : 44.2 0C Density : 2.2 – 2.34 gm/mL Solubility : Insoluble in water and

soluble in carbon disulfide

White phosphorus is a highly reactive, waxy, white-yellow, transparent solid with acrid fumes. It emits a weak green glow (luminescence) in the presence of oxygen. White phosphorus ignites spontaneously in air.

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PHOSPHOROUS: APPLICATIONS Red phosphorus is used in fireworks, smoke bombs and pesticides. Black phosphorus no significant commercial uses due to least reactivity. White phosphorus and zinc phosphate are mainly used as a poison for

rats. Used in making incendiary bombs, tracer bullets and for producing smoke

screen Used in fertilizers. Used in the manufacture of PCl3, PCl5, P2O5 and phosphorus bronze. Organophosphorus compounds used in detergents, pesticides and nerve

agents, and matches. Phosphorus is one of the most essential mineral in the body and is ranked

second to calcium. About 80% of all phosphorus is present in human body in the form of calcium

phosphate in the teeth and bones. It also participates in several vital functions of the body, such as energy

metabolism, synthesis of DNA and the absorption and utilization of calcium. Phosphorus plays a role in facilitating optimal digestion. It helps in the normal functioning of the kidneys and ensures proper discharge of

wastes. Adequate levels in body is essential to maintain normal brain functions Phosphorus helps maintain a good hormonal equilibrium.

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PHOSPHORIC ACID

Phosphoric acid (H3PO4), also known as orthophosphoric acid, it 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 used for specialty application, such as anticorrosive and food industry.

The continuous process of phosphoric acid production uses liquid white phosphorous at the boiling condition for 5 to 6 hrs so that about 35–50 % white phosphorous is converted to red phosphorous.

The hot red phosphorous is taken in a screw conveyer which along with inert gases gives a solid pure red phosphorous as product.

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PHOSPHORIC ACID: MANUFACTURING PROCESSES

The modern manufacturing methods of phosphoric acid are following: Blast furnace process Electric furnace process Oxidation and hydration of phosphorous Wet process or strong sulfuric acid leaching

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PHOSPHORIC ACID: MANUFACTURING PROCESSESBLAST FURNACE PROCESS

Raw materials Basis: 1 ton phosphoric acid (100%) Phosphate rock 2290kg Sand (Silica) 6800kg Coke 3175kg (reducing agent as

well as fuel) Briquette binder 227kg Air 450000 ft3

Reactions Ca3(PO4)2 + 3SiO2 + 5C 2P + 5CO + 3CaSiO3

2P + 5CO + 5O2 P2O5 + 5CO2

P2O5 + 3H2O 2H3PO4 (85-90% yield)

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Block diagram for manufacturing of Phosphoric acid using blast furnace

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Process diagram for manufacturing of Phosphoric acid using blast furnace

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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” small

furnace and up to 42” 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.

About 760kg of coke is consumed in reduction of phosphate rock to phosphorous and remaining generates heat by combustion with air.

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PHOSPHORIC ACID: MANUFACTURING PROCESSES BLAST FURNACE PROCESS

Reaction is completed in the furnace itself producing P2O5 and calcium silicates as slag.

The product gases also contain CO and N2 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. Superheated steam is produced and a part of gas is taken

into regenerative blast furnace. As a result the entire phosphorous and P2O5 is cooled and

purified before taken into hydrating towers. Purification of phosphoric acid includes removal of arsenic

by hydrogen sulfide treatment followed by filtration.

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Engineering Aspects: Blast furnace is made of high temperature resistant

refractories brick. Blast furnace have accessories of hot blast stove for

supply of compressed preheated air having temperature 1000 – 11000C, briquette press for preparation of briquettes of Phosphate rock and coke, dust collector (cyclone separator) for removal of dust particles from product stream.

Also, two outlets for removal of slag and ferrophosphorous are provided at the bottom of the furnace.

The top of the blast furnace is closed as it operates at high top pressure.

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There are two different systems are used for charging of briquettes, coke and silica.

One is having double bell system which is often equipped with movable throat armour and other is bell less top.

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PHOSPHORIC ACID: MANUFACTURING PROCESSES BLAST FURNACE PROCESS There are two construction techniques to support the

blast furnace. One utilized lintel or support ring at the bottom of the shaft

upon which the higher level of the furnace rests. The other is free standing construction requiring and

independent support for the blast furnace top and gas system.

The required expansion both thermal as well as pressure for the installation is below for the lintel i.e. in bosh/belly area in lintel type furnace, while compensator for expansion in the free standing furnace is at the top.

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PHOSPHORIC ACID: MANUFACTURING PROCESSESELECTRIC FURNACE PROCESS

Raw materials Basis: 1 ton phosphoric acid (100%) Phosphate rock 2225kg Sand (silica) 680kg Coke breeze 400kg Caron electrode 8kg Air 100000 ft3 Electricity 4070KWH

ReactionsCa3(PO4)2 + 3SiO2 + 5C 2P + 5CO + 3CaSiO3 ΔH = 364.8 kcals

2P + 5CO + 5O2 P2O5 + 5CO2

P2O5 + 3H2O 2H3PO4 87-92% yield ΔH = 44.9 kcals

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PHOSPHORIC ACID: MANUFACTURING PROCESSES ELECTRIC FURNACE PROCESS

Block diagram for manufacturing of Phosphoric acid using electric furnace

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Process diagram for manufacturing of Phosphoric acid using electric furnace

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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 flourosilicates as the slag.

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PHOSPHORIC ACID: MANUFACTURING PROCESSESPHOSPHATE ROCK AND ELECTRIC FURNACE

The gases from the furnace, phosphorous and CO 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.

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PHOSPHORIC ACID: MANUFACTURING PROCESSESPHOSPHATE ROCK AND ELECTRIC FURNACE

Advantages of electric arc furnace over other methods It can handle low-grade phosphate rock,

provided the major impurity is silica. Presence of iron and aluminum oxides are not

objectionable as in the wet process. Siliceous rocks containing 24% phosphorus as

P2O5 are acceptable. The by-product CO, is used as a fuel for

calcination.

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PHOSPHORIC ACID: MANUFACTURING PROCESSESOXIDATION AND HYDRATION OF PHOSPHOROUS

Raw materials Basis: 1 ton phosphoric acid (100%) Phosphorus 300kg Air 46000 ft3 Steam Water

Reactions 2P + 2½O2 P2O5

P2O5 + 3H2O 2H3PO4 (94 – 97% yield)

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Block diagram for manufacturing of Phosphoric acid

19800C

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Process diagram for manufacturing of Phosphoric acid

19800C

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The molten phosphorous is sprayed into combustion chamber along with preheated air and superheated steam.

Combustion of phosphorous increases the temperature up to 19800C.

Furnace is made of acid proof structural bricks, graphite, carbon and stainless steel.

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.

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PHOSPHORIC ACID: MANUFACTURING PROCESSESWET PROCESS OR FROM STRONG SULFURIC ACID

Raw materials Basis: 1 ton phosphoric acid (100%) Phosphate rock 1635kg Sulfuric acid 1360kg

Reaction

Ca10F2(PO4)6 + 10H2SO4 + 20H2O 10CaSO4.2H2O + 2HF + 6H3PO4There are two processes i.e. dihydrate and hemihydrates (CaSO4.2H2O

and CaSO4.1/2H2O) are used for production of phosphoric acid.

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Process diagram for manufacturing of Phosphoric acid

4 to 8hrs 75-800C.

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The grinded phosphate rock is reacted with dilute phosphoric acid to produce melt, which in a reactor as mixed with concentrated sulfuric acid for 4 to 8hrs in the temperature range of 75-800C.

Excess air is required to control the temperature. Resulting gases includes HF and P2O5 which in the

absorption tower is separated and finally treated to give fluorosilicates and dilute phosphoric acid.

The main product in the liquid form which is phosphoric acid and calcium sulfate is filtered and washed.

Thus, gypsum and phosphoric acid are separated and after minor purification the phosphoric acid is concentrated into the evaporator.

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Throughout the plant, corrosion resistant materials of construction must be used.

The most common ones are structural carbon or nickel alloy for evaporator heat exchangers; rubber or carbon-brick for reactor linings; polyester-fiber glass in pipes, ducts, and small vessels.

Yield of phosphoric acid based on phosphorus content of raw material is 95%

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Engineering Aspects Selection of phosphate rock Grinding of rock materials Handling and storage of

phosphate rock Amount of sulfuric acid Filtration of gypsum Purification Sludge disposal

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PHOSPHORIC ACID: PROPERTIES Molecular formula : H3PO4 Molecular weight : 97.994gm/mole Appearance : White solid or colourless

viscous liquid above 420C Boiling point : 1580C (decompose) Melting point : 42.350C (anhydrous)

29.320C (hemihydrate) Density : 1.885gm/mL (liquid)

1.685gm/mL (85% solution) 2.030gm/mL (crystal at 250C)

Solubility : Soluble in water Viscosity : 147cP (100%)

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PHOSPHORIC ACID: APPLICATIONS Used for preparation of hydrogen halides Used as a "rust converter", by direct application to rusted iron, steel tools,

or surfaces. It converts reddish-brown iron(III) oxide, Fe2O3 (rust) to black ferric phosphate, FePO4

Food-grade phosphoric acid is used to acidify foods and beverages such as various colas.

Used in dentistry and orthodontics as an etching solution, to clean and roughen the surfaces of teeth where dental appliances or fillings will be placed.

As an ingredient in over-the-counter anti-nausea medications that also contain high levels of sugar (glucose and fructose).

Used in many teeth whiteners to eliminate plaque. Used as an external standard for NMR and HPLC As a chemical oxidizing agent for activated carbon production As the electrolyte in phosphoric acid fuel cells and is used with distilled

water (2–3 drops per gallon) as an electrolyte in oxyhydrogen (HHO) generators.

Also, used as an electrolyte in copper electro polishing for burr removal and circuit board planarization.

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PHOSPHORIC ACID: APPLICATIONS As a flux by hobbyists (such as model railroaders) as an aid to

soldering. As common wet etching agent in compound semiconductor

processing, Hot phosphoric acid is used in micro fabrication to etch silicon

nitride (Si3N4). It is highly selective in etching Si3N4 instead of SiO2, silicon dioxide.

As a cleaner by construction trades to remove mineral deposits, cementitious smears, and hard water stains.

As a chelant in some household cleaners aimed at similar cleaning tasks.

Used in hydroponics pH solutions to lower the pH of nutrient solutions.

As a pH adjuster in cosmetics and skin-care products. As a dispersing agent in detergents and leather treatment. As an additive to stabilize acidic aqueous solutions within

specified pH range

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ACKNOWLEDGEMENT

Slides are developed from the following references: Austin G. T., "Shreve’s Chemical Process Industries",

Fifth edition, Tata McGraw Hill, NY. Kent J.A., "Riegel's Handbook of Industrial

Chemistry,” CBS Publishers. Gopala Rao M. & Marshall Sittig, "Dryden’s Outlines

of Chemical Technology for the 21st Century", Affiliated East –West Press, New Delhi.

Mall I. D., "Petrochemical Process Technology", Macmillan India Ltd., New Delhi.

http://nptel.ac.in/courses/103106108/24

Documents

Lecture 21 Phosphorous