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Critical issues in Material Selection for Phosphoric Acid Plants

Dr MP Sukumaran Nair FIE

The Fertilizers And Chemicals Travancore (FACT) Ltd

Cochin, India

(Abstract)

Techno-economic considerations are vital in selecting the material of construction for

phosphoric acid plant equipment. This is mainly due to the vulnerability of commonly

available materials to withstand the combined effects of corrosion and abrasive erosion

accelerated by operating temperature and concentration of the medium. Even though pure

phosphoric acid is less corrosive than other stronger mineral acids the impurities in the rock

phosphate raw material that goes into solution upon acidiculation of the rock

phosphates makes phosphoric acid highly corrosive. Rock phosphate is a mined product and

its composition varies from mine to mine. Moreover good quality rock phosphate deposits are

getting depleted and industry is increasingly exploring the use of lower grades of rock

phosphate deposits. The increased presence of impurities in these varieties of rock adds on to

the corrosive nature of the slurry. Extremes of operating conditions such as reaction

temperature and velocity of movement of slurry particles also aggravate corrosion. Thus

selection of material for construction of different equipment in the phosphoric acid plants and

its reliability and economics of operation would depend on these factors. Over the years in its

search for corrosion resistant and cost effective materials, the industry has witnessed a

transition from austenitic to super austenitic to Duplex varieties of stainless steels and

development of several proprietary alloys. Surface hardening of stainless steels by

carburization, development of high chromium white cast irons, improved designs of equipment

facilitated by techniques such as computational fluid dynamics (CFD), life cycle analysis and

costing influence the material selector of the day in making his judgment prudent. This paper

is an attempt to critically review the above issue of material selection in the light of problems

encountered in actual operating environment of the plant.

Nature of corrosion in Phosphoric Acid Plants

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Material selectors are worried not about the corrosive nature of pure phosphoric acid vis-à-vis

stronger acids like sulphuric and hydrochloric acids, but on the nature of impurities in the raw

material rock phosphate that goes into solution upon acidiculation and render phosphoric acid

highly corrosive. Rock phosphate is a mined product and its composition varies from mine to

mine. Moreover good quality rock phosphate deposits are getting depleted and industry is

increasingly exploring the use of lower grades of rock phosphate deposits. Commonly

available Phosphate Rock mineral deposits are

Tri-calcium Phosphate Ca3(PO4)2 (46 % P2O5)

Fluorapatite Ca10(PO4}6 F2 (42 % P2O5)

Carbonate apatite Ca10(PO4)6 CO3 (41 % P2O5) and

Hydroxyapatite Ca10(PO4)6 (OH)2 (42 % P2O5)

Around 60 grades of phosphate rock are available from the deposits in 24 countries world over.

Typical analysis of high grade and low grade Rock phosphates are given below:

Table 1: Analysis of Rock Phosphates

Constituent High Low ( wt % dry basis )

CaO : 53.2 44.25 P2O5 : 37.35 26.7 Fe2O3 : 1.71 0.15 Al2O3 : 2.3 0.1 MgO : 1.65 traces SiO2 : 16 0.65 F : 3.88 0.9 Cl : 0.105 0.004 CO2 : 6.5 0.9 Na2O : 2.2 0.05 K2O : 0.07 0.006 Organics : 2.4 0.18

It can be seen that any source of rock phosphate essentially contains impurities like iron and

aluminium, sodium and potassium, magnesium, fluorine, silica and chlorides etc. Besides other

impurities such as Ba, Mn, Ni, Cu, Zn, As, Sr, V, Cr, Pb, Hg, Co, Cd, U, Th, Y, Ti, Se may

also be present in very small quantities. From the corrosion point, the presence and

concentration of Fluorine and chlorine make selection of material of construction (MoC) of

equipment difficult. Other impurities affect scaling tendency and excise interference in

operation of the phosphoric and end product processes.

The corrosive nature of the slurry in phosphoric acid production is also dependant on factors

like the temperature of slurry (usually around 80 oC) and velocity of movement of slurry

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particles (peripheral speed of impeller, agitator etc.) also. The reaction slurry contains around

40 per cent solids by weight and the abrasive nature of the gypsum crystals further aggravate

the corrosion on materials of construction of the equipment. Thus selection of material for

construction of different equipment in the phosphoric acid plants would depend on these

considerations.

In the past when good quality rock deposits were plenty, the level of impurities in rock

phosphates was comparatively low and were within acceptable limits. With the depletion of

major pioneering mines, use of low-grade phosphates with higher content of impurities has

become necessary due to economic and practical considerations. The current practice is to

blend of different grades of rock phosphate to keep the impurity content within acceptable

limits, go for high quality material of construction or adopt life cycle costing as the basis of

material selection for critical equipment.

Impurities in Rock Phosphates

Chlorine

Chlorine in the rock exists as mineral chlorides and also come from water used in the

beneficiation and other process. Chlorides upon acidiculation form hydrogen chloride which is

highly corrosive and spoil the passive film on the metal surface. Chloride attack is severe in

shallow pits on the surface of equipment parts leading to pitting corrosion. P.Baker has

established that the exponential effect of chlorine corrosion with temperature.

Fluorine

Fluorine is present in all phosphate rocks to the tune of 10 to 14 per cent of P2O5. When

acidulated, it forms hydrofluoric acid. If there is sufficient quantity of reactive silica in the

rock, this may end up in production of hydroflurosilicic acid (H2SiF6) thus lowering the

corrosive effect. To a lesser extent aluminium also help to fix fluorine as aluminium silico

fluoride. Acceptable corrosion rates are achieved with a F/SiO2 ratio of 1.8. Other wise, the

excess hydrogen fluoride (HF) will remain as free acid and cause excessive corrosion.

Presence of Ca, Na and Mg though lead to scale formation do not contribute to corrosion.

Ferric (Fe3+) ion promotes the formation of a passivation film inhibiting corrosion.

Sulphuric acid

The slurry in reaction tanks contains around 20–40 mg /litre (mgl) sulphuric acid. The

optimum level for each rock is different based on rock characteristics. It is reported that

increasing sulfuric acid concentration in the attack slurry from 20 -40 mgl will enhance

corrosion by a factor of 4 to 10.

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Reaction conditions

Velocity of Slurry flow

Due to presence of abrasive silica, erosion corrosion occur parts of equipment such as

impellors and pipelines. Abrasion corrosion depends on the peripheral speed of equipment

parts. Increase in peripheral speed will result in enhanced erosion corrosion rate exponentially

by a factor of 2.4.

Temperature.

Increase in slurry temperature severely accelerates corrosion. The rate corrosion rate is

doubled upon increase of temperature from 70oC to 85oC.

Combination effect of corrosion factors

In the normal operating environment of the plant, corrosive impurities, temperature of slurry

and its velocity synergistically impact and the effect of corrosion is much higher. Calcined

rocks show more corrosive tendency than uncalcined rocks due to its reductive nature. Use of

mineral and organic oils for defoaming of slurry cause damage to rubber linings.

Complexing Ratio

It is well known that silica and aluminium readily form complexes with fluorine and thus

fixation of fluorine and hydrogen fluoride will reduce its corrosive effect. The tendency to form

such complexes is defined by a molar ratio as

Complexing Ratio = ( % SiO2/60 + % Al2O3/102) / ( % F/114)

If the rock contains an excess of silica and alumina with respect to fluorine, the ratio tends to

be less than 1 and thus the corrosivity will be lower. Impurities such as Fe, Mg, Ca, and Na

which under the reaction conditions of the attack tank form soluble or insoluble fluoro

aluminates or silicates that settle on the metallic surface as scales also reduce the impact of

corrosion.

Development of metallurgy for Phosphoric acid application

In the 1960s, stainless steels of 300 series grade were widely used in Phosphoric acid plant

for slurry and acid services. AISI 316L and 317L were employed for years as material for

pumps, piping and filters in phosphoric acid plants. Over the years, the developments in the

metallurgy of alloy steels and improved heat treatment processes gave birth to special grade

alloy steels with better corrosion and erosion resistance. These special grade alloy steels

contains elements like chromium nickel and molybdenum in various proportions and also trace

elements like tantalum, titanium niobium, copper etc in small quantities. Thus high nickel

chromium alloys of the types N08020 (Carpenter 20-Cb), N10276 (Hastelloy C-276), N08825

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(Incolly 825), N06985 (Hastelloy G-3), N 06030 (Hastelloy G-30) etc became well accepted as

the material of construction for critical equipment in phosphoric acid plants.

Table 2: Alloys for Phosphoric Acid plants : Chemical composition % wt

Grade PREN C Si Mn P S Cr Ni Mo Others

316 L 26 0.020 max

0.50 max

1.7 max

0.015 max

0.010 max 17.5 14.5 2.5

317 L 31 0.030 max

1 max

2 max

0.045 max

0.030 max

18-20

11-15 3-4

N08020 (Carpenter 20Cb-3) 28

0.060 max

1 max

2 max

0.035 max

0.035 max

19-21

32.5-35 2-3

Cu 3-4 Cb+Ta 1 max W 3-4.5

N 10276 (Hastelloy C-276) 69 0.01 max

0.08 max

1 max

0.03 max

0.015 max 16 57 16

V 0.35 Co 2.5 W 4

N 08825 (Incoloy 825) 31 0.05 max

0.5 max

1 max

0.03 max

0.030 max

19.5-

23.5 38-46

2.5-3.5

Al 0.2 Ti 0.6-1.2 Co 2 Cu 1.5-3

N 06985 (Hastelloy G-3) 45

0.015 max

1 max

1 max

0.04 max

0.030 max

21-23.5 48 6-8

Cb + Ta 0.5 max W 1.5 max Cu 1.5-2.5 Co 5 max

N 06030 (Hastelloy G-30) 46 0.03 max

0.8 max

1.5 max

0.04 max

0.02 max

28-31.5 43 4-6

Cb + Ta 0.3- 1.5 W 1.5 -4 Cu 1-2.4 Co 5 max

To withstand the service environment of the corrosive fluid nature, erosion and temperature

tailor made alloys were developed under strictly controlled conditions and later their

mechanical properties like strength, weldability, workability etc were improved by special

treatment process. The higher the content of elements like chromium and nickel in the steel,

greater the corrosion resistance and service life and cost of the material.

During the 1970’s Prayon Technologies, Belgium built several plants all over the world with

HV series of material for most equipment. The supplier of this proprietary material alloys HV-

9 and HV-90A cast super-austenitic alloys produced by the Belgian casting shop Usines E.

Henricot, a foundry which was closed. The compositions of alloys HV-9 , HV-9A, HV-93and

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HV-90A though of similar types, were changed slightly but significantly in the past 30 years

to adjust to increasingly more difficult corrosion conditions.

Table 3: Chemical composition of HV grades % wt

Grade PREN C Si Mn P S Cr Ni Mo other

HV 9 35 0.04 max

1 max

2 max

0.030 max

0.030 max

20-23

24-26 4 Nb

HV 9A 36 0.04 max

1 max

2 max

0.030 max

0.030 max

20-23

24-26 4.5

Cu 1-2, Nb

HV 93 37 0.03 max

1 max

2 max

0.030 max

0.030 max

19-22

23-27

4.8-5.5 Nb

HV 90 A 37

0.03 max

1 max

2 max

0.030 max

0.030 max

19-22 30

4.8-5.5 Nb

Indeed these super-austenitic grades are difficult to produce particularly as thick cast parts. As

these parts are used in very aggressive media (phosphoric acid slurries containing high enough

contents in H2SO4, HCl, HF, H2SiF6 , Chlorides etc) the metallurgical condition must

withstand the corrosive impact. Heat treatments must be carefully controlled, i.e. full solution

annealing at 1050°C-1150°C for long enough time depending on the thickness, and then rapid

quenching in order to prevent re-precipitation of intergranular intermetallic phases and

chromium carbides. Any delivery must be carefully checked by metallographic examinations

and well suited corrosion tests.

Pitting resistance index

Pitting resistance index -- a means of comparing corrosion resistance of stainless steel—

provide a useful guideline for material selection among different alloys for corrosive in

environments. Pitting resistance equivalent number (PREN) or

Pitting resistance index = % Cr + 3.3 X % Mo + l6 X % N

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New Generation Alloys

UNS N.08904

The super austenitic alloy 904 L is fully austenitic steel with low carbon content. It has

superior corrosion resistance against pitting, crevice corrosion and stress corrosion cracking

for the full range of concentrations and temperature up to 110 oC , than ordinary acid resistant

stainless steels and classical stainless steel brands used for phosphoric acid plants It has a good

weldability and formability and high mechanical strength. Due to a low carbon content chances

for hot cracking and intergrannular corrosion after weld are minimal. It is well suited for

attack tanks, agitators, heat exchangers , filters , centrifuges and pumps.

Table 4: UNS N 08904 Chemical composition

Grade Element C Si Mn P S Cr Ni Mo Cu

UNS.N 08904 %

0.020 max

1.00 max

2.0 max

0.030 max

0.020 max

19-21

24-26

4.0-5.0

1.0-2.0

Mechanical Properties

Tensile Strength 600 N/mm2 Elongation 45 % min 0.2% Yield Point 240 N/mm2 Hardness 155 HB PREN 35

These following proprietary steel grades are covered by the general equivalences

UNS N.08904, Werkstoff nr. 1.4539, AFNOR Z.2.NCDU.25.20 or DIN

X.1.NiCrMoCu.25.20.5.

Trade Names

URANUS.B-6 from Creusot-Loire (France)

2-RK-65 from Sandvik (Sweden),

904-L from Udelholm (Sweden)

Jessop 700 (USA).

UNS N.08028

This is a multipurpose austenitic stainless extra low carbon alloy for service in highly

corrosive conditions. It is characterized by very high corrosion resistance in strong acids, very

good resistance to stress and intergrannular corrosion in various environments, resistance to

pitting and crevice corrosion and also posses good weldability. It is used for pumps, piping,

heat exchangers, evaporators and agitators. A proprietary alloy of this family viz; Sanicro 28

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is in service in the phosphoric acid industry for the last three decades with excellent corrosion

protection at temperatures in the 80-85 deg C rang and liquid velocities of 3.5 m/sec.

Table 5: UNS N 08028 Chemical composition

Grade Element C Si Mn P S Cr Ni Mo Cu

UNS.N 08028 %

0.020 max

0.6 max

2.0 max

0.025 max

0.015 max 27 31 3. 5 1. 0

Mechanical Properties

Tensile Strength 550-750 N/mm2 Elongation 40 % min 0.2% Yield Point 220 N/mm2 Hardness 90 HB

PREN 39 Other general equivalences are

ASTM: N08028, EN: 1.4563, EN Name: X 1 NiCrMoCu 31-27-4, W Nr.: 1.4563

DIN: X 1 NiCrMoCuN 31 27 4, SS: 2584 , AFNOR: Z1NCDU31-27-03

Trade Names

Sanicro 28 (Sandvik, Sweden)

UR B28 (Creusot-Loire, France)

UNS S.31254

Another austenitic high Molybdenum grade alloy stainless steel which is coming up in

phosphoric acid service is of the 904hMo type and is designated as UNS 31254 .

Table 6: UNS S 31254 Chemical composition

Grade Composition C Mn Si P S Cr Mo Ni Cu N

UNS

S31254 Nominal %

0.020

max

1.00

max

0.80

max

0.030

max

0.010

max 20 18 6.1 0.7 0.20

Mechanical Properties

Tensile Strength 650+ N/mm2 Elongation 35 % min 0.2% Yield Point 300 N/mm2 Hardness 96 HB

PREN 83

Other general equivalences are

Werkstoff Nr. 1.4529, DIN X.1.NiCrMoCuN.25.20.6 (German)

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UNS S.31254 (US)

Z1NCDU20.18.06AZ, AFNOR (French)

Trade Names

254SMO Sandvik (Sweden)

19.25.HMo (USA)

UR B 25 (Creusot-Loire , France)

654SMO Avesta

Duplex Steels

UNS S 31803

Duplex grade stainless steel grades containing 20-27 % Cr and 4-8 % Ni and 2.5 -3.5 % Mo

are capable of withstanding high mechanical stress under severe corrosive condition as in

phosphoric acid plants. Compared to austenitic grades Duplex type of steel have twice the

yield strength and a high resistance against general corrosion, stress cracking and erosion. As

these alloys contain lesser nickel than in austenitic high grade steels , they are less costly .

Though the weldability is good, after heat treatment it has a strong tendency for embrittlement

of the heat affected zones. This restrict its application in the processing industry. The erosion

and corrosion resistance of this material to the hot slurry and cold phosphoric acid service

essentially depends on the post treatment actions carried out during the manufacture of the

same.To a certain extent the disadvantages caused by the heat treatment operation can be

contained by carefully controlling the chemical composition. It has an extensive application in

the phosphoric acid industry for pumps and agitators, slurry piping and vessels.

Table 7: UNS S 31803 Chemical composition

Grade Element C Si Mn P S Cr Ni Mo N

UNS.S 31803 %

0.030 max 1.0max

2.0 max

0.030 max

0.02 max

21-23

4.5-6.5

2.5-3.5

0.08-0.20

Mechanical Properties

Tensile Strength 680-900 N/mm2 Elongation 25 % min 0.2% Yield Point 450 N/mm2 Hardness 290 HB

PREN 34

Other general equivalences are

American designation UNS S31803, European designation X2CrNiMoN 22-5-3

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Werkstoff nr 1.4462, J.93370 (USA).

Trade Names

URANUS-45.N, ( from Creusot-Loire France)

SAF 2205 (Sandvik, Sweden)-

CD4MCu Duriron Co

Lewmet 15 ( Chas A. Lewis & Co, USA)

Industry operators admit that proprietary material procured from different suppliers have

given prolonged service in some cases and at the same time failed very fast on other occasions.

Instances of failure of this grade of material have been reported in the case of agitator blades

and pumps. It is noticed that the material is very vulnerable to corrosion due to increased levels

of chloride in the service medium. The sourcing of good quality material for a specific service

in itself a big challenge for the material selector and the user. ASTM test procedures may be

adopted as acceptance standards for chemical composition, mechanical properties, corrosion

tests and post repair checks.

UNS S32550

S32550 are a group of super duplex stainless steel with 25per cent Cr and excellent resistance

to pitting and erosion . The molybdenum and nitrogen additions have been optimized in order

to obtain the best corrosion resistance properties even for heavy plates. High nitrogen content

improves the structural stability particularly in heat affected zones. Its corrosion resistance is

much better than N08904 and roughly equivalent to 6 Mo super austenitic alloys. Copper

addition increase the corrosion resistance properties, particularly in sulphuric acid media.

It is a cost efficient grade for offshore, marine, phosphoric acid, sulphuric acid applications in

presence of chlorides.

The duplex microstructure provides high mechanical properties and thus the alloy is best

suited for abrasion corrosions for pumps, agitators, rakes etc.

Table 8: UNS S 32550 Chemical composition

Grade Element C Si Mn P S Cr Ni Mo Cu N

UNS.S 32550 %

0.030 max 0.80max

1.5 max

0.035 max

0.02 max

24-26

5.5-8 3-5 0.5-3

0.2-0.35

Mechanical Properties

Tensile Strength 770 N/mm2

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Elongation 25 % min 0.2% Yield Point 550 N/mm2 Hardness 310 HB

PREN 52

Other general equivalences are

EURONORM 1. 4507 - X2 Cr Ni Mo Cu N 25.6.3 AFNOR Z3 CNDU 25.07 AZ ASTM UNS S32550/S32520 Trade Names

URANUS-52.N+ ( Creusot-Loire France)

Ferralium 255 (Langley Alloys, UK)

White Cast Irons

ASTM Specification A532 covers the composition and hardness of abrasion-resistant white

irons.. The high alloy white cast irons, which contain the alloying elements above 4 percent,

with their predominant carbides in the microstructures, provide the high hardness that is

necessary to withstand erosion without degradation. The supporting matrix structure is

adjusted by alloy content and heat treatment to develop the most cost effective balance

between resistance to abrasive wear and the toughness required. The high alloy white irons are

readily cast to the shapes required for the handling of abrasive medium such as slurries.

White irons offer considerable versatility in properties for a variety of abrasion-resistant

applications. The composition of the white iron used to produce a given casting can be selected

and heat treatments specified to develop the carbide distribution and the matrix microstructures

that will give maximum service life and cost effectiveness.

The alloyed irons all contain chromium to prevent the formation of graphite and to insure the

stability of the carbides in the microstructure. Many of the alloy white irons also contain

nickel, molybdenum, copper or combinations of these metals to prevent or to minimize the

formation of pearlite in the microstructure.

Alloyed martensitic white irons develop Brinell hardnesses in the range 500-700. For many

abrasion-resistant applications, the more expensive alloyed white cast irons with martensitic

matrix structures provide the most economical service.

The high alloy white irons are classified into three groups as:

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1. Nickel Chromium white irons, a low chromium group (1-4% chromium and 3-5%

nickel, with one modification which contains 7-11% chromium)

2. Chromium Molybdenum irons, a high chromium group (14-28% chromium, 1-3%

molybdenum, often alloyed further with additions of nickel or copper.

3. High chromium white irons, (25-28 % chromium with molybdenum and/or nickel up to

1.5%)

The above categories of white irons though possess outstanding wear resistance their acid

corrosion resistance are poor. Hence new varieties have been developed with very high

chromium content up to 50 per cent and containing carbon up to 3 percent. Several pump

manufacturers have developed proprietary versions of high chromium white irons containing

34-50 per cent of chromium, 2.5 per cent of carbon together with Ni, Mo, Mn, Si, Cu and other

micro alloying elements for the phosphoric acid service.

Table 9: High Chromium White Iron : Chemical composition

Grade Element C Si Mn P S Cr High chromium cast iron %

2.5-2.9 0.6-0.8

1-1.2

0.04 max

0.04 max

21-23

Mechanical Properties

Tensile Strength 410-690 N/mm2

Hardness 430 HB

Trade Names

Gastite T90G of GIW industries, USA

349 XL EnviroTech, USA

Table 10 : Materials for Phosphoric acid plant equipment

Equipment

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Commonly used Material of Construction

Attack tank reactor, Digestion

vessel

CS or RCC with Rubber lining and Carbon brick lining at

bottom and sides. CS / RCC with rubber lining at top.

UNS No N08904 , N08825, N08020, S 32520

Agitator for hot slurry /

filtrate service

Impeller & Shaft UNS No N 06985, N 06030, S 31254, S 32520

Agitator for cold acid service Impeller & Shaft UNS No N08904

Filter

Trays UNS No N08904

Cloth – Polypropylene

Scrubber fan

Impeller – SS 316 L

Casing – cast steel rubber lined (CSRL)

Flash Cooler Pump

Impeller – UNS No N8028 , S 31254

Casing - UNS No N8028

Slurry Circulator

Impeller & casing – UNS No N8028

Filter feed pump

Impeller and Casing

UNS No N08904, S.31254, N08028

UNS S 31803 , S 32520

Scrubber tower CSRL with PVC coupling / PTFE spray nozzles.

Exhaust fan

Impeller SS 316 L

Casing CSRL

Wash acid pumps

Impeller UNS S 31803

Casing UNS No N08020

Mixing head UNS No N10276

Evaporator

UNS No N08028, S 31254, N 06985, N 06030

Scrubber Circulation Pump

(Weak Fluorosilicic Acid )

Impeller and Casing UNS No N08020

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Sludge Pump

Impeller and Casing UNS No N08904, N 06030

Cold Phosphoric Acid Pump Impeller and Casing UNS No N08904, S 31803

Vacuum Pump

Impeller Bronze 90%, Cu 10%

Casing Cast Iron

Life Cycle Costing

Traditionally the selection of a material for a given application has been on the basis of the

lower purchase price. We may either go for cheaper low grade alloy steels with lesser service

life and frequent replacements or costlier higher grade alloy steel with increased service life

and lower frequency of replacements. It is now recognized that the cheapest purchase price

may not be the most economic choice if we consider additional costs due to installation,

regular maintenance and periodic replacement are also considered. In the case of critical plant

equipment production loss on account of unscheduled down-time of the equipment are also to

be seen. In many industries this lost time cost far outweighs all other costs, and must certainly

be included in estimates of life cycle cost. Life Cycle Costing (LCC) in material selection

quantifies the total cost—operation, maintenance, replacement etc-- over the entire life of the

equipment with the aim of selecting the most economic alternative. Thus the choice of the

material for a specific service as for phosphoric acid ultimately is decided by the operation and

maintenance philosophy adopted for running the plant.

Often the material is selected for the worst corrosion scenario and with due consideration to

economics. However safety, environmental consequences and unexpected loss of production

due to plant outages remain prime considerations. This approach in material selection always

help to improve the bottom line of operation.

References

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1. Phosphates and Phosphoric Acid, Raw Materials, Technology, and Economics of the Wet Process, Becker P., Marcel Dekker, Inc., New York, 1989.

2. Phosphoric Acid, Slack A.V., Ed., Marcel Dekker, Inc., New York, 1968.

3. Solving Corrosion Problems in Wet Process Phosphoric Acid Plants Mr. C.M. Schillmoller & Mr. P.K. Bhattacharjee, Fertiliser Industry Annual review , 1994.

4. Corrosion and wear resistant alloys in phosphoric acid service, Phosphorous and Potassium, No.208, British Sulphur Publishing, UK, 1997.

5. Corrosion Manual, Part 1, Wet Phosphoric Acid, IFDC, Muscle Shoals, USA, 1981. 6. Iron Casting Handbook, American Foundrymen’s Society, Des Plaines, Illinois, USA. 7. Websites: www.sandvik.com, www.nidi.org, www.hayensintnl.com, www.giwindustries.com, www.cartech.com,