Aluminum Industry Primer V2
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Transcript of Aluminum Industry Primer V2
AN EDE PRIMER ON
ALUMINUM SMELTERS
An Internal Training Document
For distribution to and use by FM Global employees only
Larry J. Moore, PEPrincipal Engineer
Mining and Metallurgical RefiningStaff Engineering
JULY 2008
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Aluminum Smelter EDE Primer July 2008
Summary
FM Global has a well developed and institutionalized mine industry specialists program
called the MINERS Program which includes comprehensive engineering standards and
training for mining occupancies. This specialists program also covers as a subset
aluminum refining and smelting but not downstream secondary processes such as
rolling mills.
A fundamentals class entitled “Understanding and Quantifying the Hazards in Aluminum
Smelters” was developed and deployed in 2005 and 2006. There are currently about 40
field (EH and FH) engineers who attended one of the aluminum fundamentals classes or
are seasoned senior engineers with some aluminum smelter expertise.
An industry specific Operating Standard – OS 7-64 Aluminum Industry - has been
published for many years and this covers in detail the entire industry from refining to
consumer products. This standard also includes a comprehensive tutorial on the industry
as a Reference document.
Procedures for field engineering servicing of aluminum facilities are also detailed in the
HRG and Visit Planner.
As a direct result of a pending 2008 summer candidate campaign for Alcoa Aluminum
several operations expressed concern that adequately trained field engineering
resources were not available in all operational locations. The primary concern involves
the smelting part of the aluminum production chain but extends to the upstream refining
and downstream converting operations as well.
To address this gap in geographical resources the Principal Engineer offered to deploy a
“crash” abbreviated class on aluminum smelting for those field engineers who have no
experience in the industry and had not attended one of the fundamentals classes.
FBI operations accepted the offer and a class was deployed July 24th and 25th in the
Paris office for European field engineers.Copyright 2008 Factory Mutual Insurance Company. All rights reserved. Do not reproduce this report. It is for distribution to and use by FM Global employees only. Do not distribute beyond these groups
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Aluminum Smelter EDE Primer July 2008
A class for North and South American Operations was discussed but could not be
deployed in a suitable time frame to aid in the upcoming candidate campaign.
Instead the Principal Engineer agreed to develop basic EDE guidelines to help
inexperienced engineers know where to spend their time in complex smelting facilities.
The following EDE Primer supplements OS 7-64 but is not a replacement for studying
and understanding this comprehensive OS and its excellent Reference document.
This is also not a substitute for following the HRG and Visit Planner nor should these
guidelines overrule any Operating Requirements.
The following guidelines only cover aluminum smelting. On site power generation and
other occupancies associated with aluminum industry are not covered.
Aluminum Industry Overview:
The aluminum industry is comprised of the following general manufacturing segments:
• Mining/Concentration: A mine extracts and produces concentrated bauxite ore
• Refining : A refinery produces alumina from bauxite ore in the Bayer digestion
process
• Primary Smelting : A smelter (a/k/a reduction works) produces aluminum metal by
electrolytic reduction of alumina.
• Secondary Melting : These plants process recycled materials or remelt bar stock
from smelters for purification. These plants are heavy industrial occupancies with
principle hazards being molten metals, rolling operations with typical Equipment
and Facility hazards and large robust equipment like presses.
• Consumer products : Downstream metal working processes like rolling, wire
drawing, etc produce final products.
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Aluminum Smelter EDE Primer July 2008
The following flow diagram demonstrates the aluminum industry production chain.
Some aluminum industry facts:
• Aluminum is the third most abundant element in the Earth's crust and constitutes
7.3% by mass.
• In nature it only exists in very stable combinations with other materials
(particularly as silicates and oxides)
• Its existence was first established in 1808
Is it Aluminium or Aluminum? The word is derived from the Latin Alumen for Alum
(Potassium aluminium sulphate). In 1761 French Chemist Louis-Bernard Guyton de
Morveau proposed the term Alumine.
In 1808 the name Alumium was proposed for the metal.
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Bauxite Mining
Alumina Refining
Aluminum Smelting
Refined Metal
RollingMills
Extrusion ProcessesOther
TransformationProcesses
End Use Recycling
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Aluminum Smelter EDE Primer July 2008
Aluminium was adopted by the International Union of Pure and Applied Chemists in
order to conform with the "ium" ending of most elements. By the mid-1800s both
spellings were in common use. Charles Dickens commented that both names were too
difficult for the masses to pronounce.
The first US company was called the Pittsburgh Reduction Aluminum Company and the
metal gradually began to be known only as Aluminum in the US. In 1907 this company
became the Aluminum Company of America (ALCOA).
In 1925 the American Chemical Society decided to adopt the name Aluminum in their
official publications. Most of the world has kept the “i” in Aluminium but it is interesting
to note that the name for the metal's oxide, Alumina has been universally accepted over
its more convoluted alternatives, Alumine and Aluminia.
Both Aluminium and Aluminum thus have an equal claim to etymological and historical
justification.
Exposure Driven Engineering (EDE) Guidelines
The following gives a brief overview of the principal EDE FH and EH hazards found in
electrolytic aluminum reduction smelters.
Protection guidance for various identified exposures is detailed in OS 7-64, Aluminum
Industry, and other operating standards and is not repeated herein except generally.
A smelter consists of distinct and common processes and infrastructure. While there are
variances this is typical of modern smelters using Pre-Baked Anode (PBA) technology.
Older Soderberg technology is not described in this paper as it is now more uncommon
and many of the same hazards and exposures are the same with PBA. Soderberg is well
covered in OS 7-64.
Production of primary aluminum requires the following systems and infrastructure
• delivery, storage, and handling of raw materials
• production of anodes
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Aluminum Smelter EDE Primer July 2008
• production and handling of molten metal
o electrolytic reduction process
o casting
• processing of waste emissions
• electrical supply and distribution
• other utilities and administration support
Figure 1: Modern aluminum smelter with raw material delivery systems and carbon plant
at bottom, potlines at top center, and electrical distribution at top right
Delivery, storage and raw material handling systems
Primary bulk raw materials consist of
• Alumina (a purified form of aluminum oxide) produced in an upstream alumina
refinery and the primary feedstock used for production of aluminum metal.
• Liquid or solid organic pitch (resin binder)
• Petroleum coke (carbon)
• Cryolite, solid sodium aluminum fluoride additive used as a conductive flux and
molten solvent in potline cells
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Aluminum Smelter EDE Primer July 2008
These are often delivered to the site by marine vessel (common for alumina if the
refinery is remote and if there is a nearby port to the smelter) or by surface transport
such as trucks or rail. Cross country rubber belt conveyor systems generally are used to
transport these materials from point of entry to storage silos of bins at the plant site. Hot
liquid pitch is generally delivered by rail or truck and piped into heated tanks.
Primary EDE hazards for raw material delivery and distribution systems are:
• Windstorm exposure at exposed port facilities with large rail mounted cranes and
ship unloaders. Long narrow levees servicing ship ports with conveyor and piping
infrastructure are highly susceptible to hurricane or cyclone wind and sea surge
damage. Wind induced toppling or severe movement of rail mounted cranes and
ship unloaders can occur.
• Fires in semi-mobile equipment like ship unloaders and their on-board electrical
and lubrication systems
• Fires in combustible docks and buildings on docks
• Fires in liquid pitch systems and storage tanks
o Sprinklers
o Interlocks
o Confinement
o Drainage
o Protection Reference: OS 7-99, Thermal Oil Systems; 7-32 Flammable
Liquid Operations.
• Fires in coke (carbon) handling systems
• Possible dust explosion hazard with coke and solid pitch handling systems (Note:
petroleum coke has a low Kst, has been determined in tests to be hard to ignite
and thus represents a very low explosion hazard. Solid pitch fines are similar to
coke fines. Explosion hazards can generally be controlled by good housekeeping
rather than special protection due to very low overall risk)
o Reference: OS 7-76, Combustible Dusts
• Fires in rubber belt conveyors and bucket elevators
o Sprinklers
o Shutdown interlocks
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Aluminum Smelter EDE Primer July 2008
o Reference: OS 7-11, Rubber Belt Conveyor Systems
• Mechanical damage to or toppling of large ship unloaders and cranes due to
stress cracking, corrosion, fatigue or overloading
• Implosion of suction unloading systems
• Breakdown of large motors and suction blowers
• Electrical breakdown or fires in transformers or switchgear
• Impact by marine vessel
• Port blockage
Anode production (PBA processes):
The pre-baked anode smelting process requires anodes and cathodes for the electrolytic
process. The cathode is a carbon liner added when the cell (pot) is newly built and
lasting the life of the pot. Anodes are large carbon blocks “glued” together under
pressure using a pitch resin binder with copper metal rods for conducting electricity from
bus bars. Anodes are consumable products and the carbon that is consumed is the
reducing agent for the reduction process per the following equation.
2 Al2O3 (solid) + 3 C (solid) à 4 Al (liquid) + CO2 (gas)
To put the anode demand in perspective
• 1 ton of aluminum requires 1100 lbs (500 kg) of carbon anode
• The life of one anode is about 30-40 days
• Daily consumption of anodes is from 600 to 1100 t/day
Because of the large continuous demand for anodes a PBA smelter usually requires an
on-site production facility to produce rodded anodes. Anodes can rarely be produced
economically elsewhere and delivered to the plant although there are plants that
specialize only in anode production.
This facility –called the Carbon or Paste Plant - consists of the following primary
processes:
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Aluminum Smelter EDE Primer July 2008
• Mixing of the resin-coke (carbon) blend and forming of a “green” anode block
under pressure and temperature. This is done in a multi-story building
featuring hot liquid pitch pumping and piping systems, mixers and blenders,
thermal oil dryers and hydraulic presses. Hot liquid pitch, thermal oil,
hydraulic oils and carbon combine to produce a high combustible loading with
flammable liquids.
• Adding copper metal conductive rods to the green anode block. Typically
done in a continuous process by drilling openings in the block, and inserting
and “gluing” the rods in place with molten iron. This process can produce fine
dusts of coke and dry pitch that can be combustible.
• Curing (baking and drying) of the green anode block to form the final product
which is sent to the potline. The green anodes are delivered to in-ground
baking furnaces, which consist of a series of refractory brick lined pits with
hollow, surrounding interconnected flue walls. Anodes are packed into the
pits with a blanket of coke covering the anodes and filling the space between
the anode blocks and the walls of the pits.
• The pits are heated with natural gas for a period of several days. The flue
system of the furnace is arranged so that hot gas from the pits being fired is
drawn through the next few sections of pits to preheat the next batch of
anodes before they are fired. Air for combustion of the gas travels through the
flues of previously fired sections, cooling these anodes while reheating the
air. The anodes are fired to approximately 2100o F (1150°C), and the cycle of
placing green anodes, preheating, firing, cooling, and removal is approx. two
weeks.
• Large waste emission systems associated with anode forming and baking
ovens consisting of off-gas collection ducts (often called ring mains),
scrubbers, precipitators etc.
• Automated or manual systems for delivering pre-baked anodes to the potline
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Aluminum Smelter EDE Primer July 2008
Figures 2 and 3: Enclosed and open multi-story carbon plants
Primary EDE hazards for Carbon (Paste) Plants are:
• Fires in thermal oil systems in multi-story buildings
o Sprinklers
o Interlocks
o Confinement
o Drainage
o Protection Reference: OS 7-99, Thermal Oil Systems; 7-32 Flammable
Liquid Operations; 7-14, Protection of Chemical Process Structures
• Fires in hydraulic oil systems with high pressure spray potential
o Sprinklers
o Interlocks
o Protection Reference: OS 7-98, Hydraulic Fluid Oil Systems
• Fires in hot liquid pitch storage, pumping and piping systems with pool potential
o Sprinklers
o Interlocks
o Confinement
o Drainage
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Aluminum Smelter EDE Primer July 2008
o Protection Reference: 7-32 Flammable Liquid Operations
• Fires in conveyor systems (both rubber belt and bucket)
o Sprinklers
o Shutdown interlocks
o Protection Reference: OS 7-11, Rubber Belt Conveyor Systems
• Fires in pitch or bake oven fumes collection (waste emission) systems
(combustible deposits inside ducts, precipitators, scrubbers etc)
o Housekeeping
o Internal fire protection
• Fires in electrical cables, transformers and switch rooms
o Products of combustion (POC) detection at minimum
o Additional fire protection as needed
• Low dust explosion hazard with coke and solid pitch handling systems (See
Material Handling above)
• Mechanical damage to gears and motors on ball mills, extruders, and presses
o Maintenance
o Vibration analysis
o Spares
o NDE
• Electrical breakdown of transformers and switchgear
o Maintenance, inspection and testing
o Spares
o Clean, cool and dry environments
Production and handling of aluminum metal
At a primary smelter this generally consists of
• Electrolytic reduction process
• Casting of ingots, billets or other shapes to customer specification
Electrolytic reduction process
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Aluminum Smelter EDE Primer July 2008
The Hall-Heroult process is the method by which alumina (Al2O3) is separated into its
component parts of aluminum metal and oxygen gas by electrolytic reduction. It is a
continuous process with alumina being dissolved in cryolite bath material (sodium
aluminum fluoride) in electrolytic cells called pots and with oxidation of the carbon
anodes. The bath is kept in its molten state by the resistance to the passage of a large
electric current. Pot temperatures are typically around 1688-1769° F (920°- 980°C). The
aluminum is separated by electrolysis and regularly removed by siphoning for
subsequent casting. The pots are connected electrically in series to form a ‘potline.’
A potline is a group of 100 to 300 electrolytic cells or "pots" that are connected in
electrical series. An aluminum smelter consists of one or more potlines
In each pot, direct current passes from carbon anodes, through the cryolite bath
containing alumina in solution, to the carbon cathode cell lining and then to the anodes
of the next pot and so on (see Figures 4 and 5). Steel bars embedded in the cathode
carry the current out of the pot while the pots themselves are connected through an
aluminum bus-bar system. The pot consists of a steel shell in which the carbon cathode
lining is housed. This lining holds the molten cryolite and alumina in solution and the
molten aluminum created in the process. An electrically insulated superstructure
mounted above the shell stores alumina automatically delivered via a sealed system and
holds the carbon anodes, suspending them in the pot.
The electrolyte, which fills the space between the anodes in the pot, consists of molten
cryolite containing dissolved alumina. A solid crust forms at the surface of the electrolyte.
The crust is broken periodically and alumina is stirred into the electrolyte to maintain the
alumina concentration.
Approximately 13 -16 kilowatt-hours of direct current electrical energy, 2.2 lbs (1/2 kg) of
carbon, and 4.4 lbs (2 kg) of aluminum oxide are consumed per kg of aluminum
produced.
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Aluminum Smelter EDE Primer July 2008
Figures 4 and 5: Cross section views of pot
Figures 5 and 6: Photo of a modern PBA pot and series of pots in a potline building.
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Aluminum Smelter EDE Primer July 2008
As the electrolytic reaction proceeds, aluminum, which is slightly denser than the pot
bath material, is continuously deposited in a metal pool on the bottom of the pot while
oxygen reacts with the carbon material of the anodes to form oxides of carbon, primarily
large amounts of carbon dioxide. As the anodes are consumed during the process, they
must be continuously lowered to maintain a constant distance between the anode and
the surface of the metal, which electrically is part of the cathode. The anodes are
replaced on a regular schedule.
The vigorous evolution of carbon dioxide at the anode helps mix the added alumina into
the electrolyte but carries off with it any other volatile materials and even some fine
solids. If any carbon monoxide does form it usually burns to carbon dioxide when it
contacts air at the surface of the crust. Compounds of fluoride formed in side reactions
are the other main volatile product.
As electrolysis progresses, the aluminum oxide content of the bath is decreased and is
intermittently replenished by feed additions from the pot's alumina storage to maintain
the dissolved oxide content at about 2 to 5 percent. If the alumina concentration falls to
about 1.5 to 2 percent, the phenomenon of "anode effect" may occur. During anode
effect, the bath fails to wet the carbon anode, and a gas film forms under and about the
anode. This film causes a high electrical resistance and the normal pot voltage, about 4
to 5 volts, increases 10 to 15 times the normal level. Correction is obtained by computer
controlled or manual procedures resulting in increased alumina content of the bath.
The most critical issues associated with potline operation are
• Power supply and all electrical equipment must be designed to provide maximum
reliability and continuity of operations.
• Raw material handling, pneumatic systems, instrumentation, computer and
process controls must be arranged, protected and maintained in good working
order.
• Centralized exhaust and effluent treatment systems must be protected and
maintained.
• Back up systems or alternative operating capabilities must be in place for all
critical support systems.
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Aluminum Smelter EDE Primer July 2008
• Loss of critical support systems for more than a few hours can cause pots or
potlines to freeze.
A Potline process consists of the following systems and infrastructure:
• Electrical power
o Electrolysis power to pots and potlines is delivered from rectiformers by
metal (usually aluminum) bus bars. Control systems power is usually by
electrical cable. Both systems are typically in a space or basement below
the pots.
• Compressed air
o Used for control air for regulating equipment and for pneumatic power for
raw materials (alumina, Cryolite, etc.) delivery systems
• Raw materials conveying system
o Alumina and Cryolite are delivered to storage bins by elevated rubber belt
conveyor systems and are fed continuously into pots by pot tending
machines
o Anodes are delivered by mobile pot tending machines and inserted into
pots as they are consumed
• Fumes and effluent exhaust and handling system
o Off-gases consisting primarily of CO2 and fines containing alumina and
Cryolite fluorides are captured in waste emission ducts and sent to
scrubbers and precipitators for recovery of valuable feed stock and air
cleaning
• Service cranes and mobile equipment
o Replenishing feed hoppers
o Changing PB anodes (or feeding Soderberg pots with anode paste)
o Tapping metal
Primary EDE hazards for potline processes are:
• Loss of a critical utility can cause potline freeze
o A potline freeze is caused when there is an interruption to the pot or
potline for more than “several “(4 to 8) hours. This can result in the metal
and electrolyte in the pot freezing (solidifying) and this cannot generally
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Aluminum Smelter EDE Primer July 2008
be re-melted by electrical means due to poor conductivity. The frozen
electrolyte often needs to be physically removed. This can result in all or
many of the pots needing to be torn apart and rebuilt, a long and costly
process. (Refer to OS 7-64, Aluminum Industry - Reference Document -
for a more detailed description. Also refer to OS 7-64 – Section 5.0 Loss
Expectancy Guidelines - for a procedure to quantify potline freeze.)
o A potline freeze might be caused by many different sub- events including:
• Uncontrolled fire at anode paste plant
• Fire/explosion anode baking ovens/fume extraction system
• Loss of main electrical switchgear/substation
• Loss of main auxiliary power switchgear
• Loss of more than one power group/transformer
• Fire exposure to DC bus bars
• Molten metal pot tap out damaging DC bus bar
• Loss of potline gas treatment center
• Electrical utility service interruption
• Loss of compressed air service
• Loss of environmental waste effluent treatment (fume handling)
• Loss of shipping facility (unloader, conveyers)
• Loss of casting center (molten metal explosion)
• Loss of critical production equipment (ball mill, induction furnace,
paste mixer)
• Loss of main DCS control system
• Loss of cooling water
• Events that can cause a loss of electrical power:
o Area-aide power outage such as weather event destroying power lines
o Failure of on-premises generation
o Switchgear or transformer failure
o Failure involving component within the DC bus bar network
o Sudden tap out from a pot damaging bus bars or control cables
o Transformer fire damaging bus bars
o Poor coordination of electrical system resulting in failure
Methods to reduce risk of electrical failure
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Aluminum Smelter EDE Primer July 2008
o Redundant incoming power supply lines
o Proper layout of power lines, transformers, bus bars
o Testing, inspection and maintenance of electrical equipment
o Conduct thermography of welds, joints, risers.
o Perform milli-volt drop tests of welds, joints, risers and DC
isolators.
o Conduct thermography of DC isolators.
o Protection of electrical equipment
o Availability of emergency equipment
o Conduct thermography of welds, joints, risers.
o Perform milli-volt drop tests of welds, joints, risers and DC isolators.
o Conduct thermography of DC isolators.
o Equipment and procedures to handle failure of
welds and joints (similar to handling a pot tap out that washes away a
section of busbars).
o Visually inspect welds for cracking.
o Protection References: UTH - Service Interruption
(P0229); UTH – Electrical Hazards; OS 7-64 Aluminum Industry;
Electrical operating standards
• Events that can cause a loss of
compressed air
Loss will shut down plant causing possible potline freeze
Power outage
Fire
Mechanical breakdown
• Environmental waste effluent handling systems (fume system)
o Power outage
o Fire in combustible components (e.g. bag house)
o Protection References: OS 7-64, Aluminum Industry Section
2.1.3.3; OS 7-73 Dust Collection Systems; OS 7-78 Industrial Exhaust
Systems
• Molten Material – See casting operations below
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Aluminum Smelter EDE Primer July 2008
Figure 7: Closely spaced regulating transformers exposing main AC bus bars
Casting Operations
Molten aluminum is tapped from pots and transferred (usually by mobile carriers with
crucibles) to holding furnaces in a cast house. Furnaces are generally oil or gas fired but
may be electric.
From furnaces molten aluminum is cast into shapes such as ingots, billets, sows, or
slabs. Casting stations often use water cooling baths.
Aluminum metal is highly reactive in both solid and liquid (molten) forms. Solid particles
can cause dust explosions. Solid particles however are usually not found in or near
casting operations unless there is a powder making operation or cutting of metal in the
area.
Liquid aluminum in aerosol (mist droplet) form in air can cause a violent mist explosion.
Aerosols can be formed in casting operations when chemical reactions occur or when a
strong disturbing force such as a lightning strike or a precursor explosion occurs. This
phenomenon is rare but has occurred in industrial settings with molten metal mists.
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Aluminum Smelter EDE Primer July 2008
Molten aluminum is reactive in the presence of water and some other materials such as
hydrated lime, iron and copper oxides, and fertilizers. The reaction with water is partially
a steam expansion event compounded with formation of hydrogen gas by the rapid
decomposition of water. Reaction with oxides (i.e., iron rust) is called a thermite reaction.
The presence of hydrated lime (i.e., concrete) can cause a rapid exothermic reaction
under certain ideal conditions.
Since casting operations include water cooled casters and sometimes metal lined or
concrete pits these explosion potentials can exist in modern smelters.
Fire hazards are also present from potential molten metal spills contacting combustibles.
Primary EDE hazards for casting operations are:
• Fuel explosion in furnace
Proper fuel burner combustion safeguards
• Molten metal-water explosion
Eliminate water from furnace charge
Proper design and maintenance of water cooling on casters
Good operator training
Organic coatings on metal and concrete surfaces in pits
• Chemical reaction between aluminum and lime, metal catalysts or
oxidizers
Proper understanding of hazards
Elimination of potential catalysts
Proper grounding and bonding against lightening
• Molten metal spill
Can cause fires in other combustibles
Can damage control systems
Proper location of cables, control an utility systems
Dikes and curbs
Pits
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Aluminum Smelter EDE Primer July 2008
• Fire hazards in hydraulic (furnace lifting jacks), lube oil and fuel oil
systems
Proper location
Shielding against molten metal spill
AS protection if not near molten metal
Interlocks
• Fire hazards in grouped cables or switchgear
Proper location
Shielding against molten metal spill
AS protection if not near molten metal
• Mechanical and electrical breakdown
Maintenance of equipment
• Primary protection Reference: OS 7-64 Aluminum Industry
Administrative and support facilities
The plant will have warehousing, offices, maintenance shops, buildings housing
electrical and other utilities. These buildings will typically be non-combustible in modern
smelters. Fire protection might be needed for combustible occupancy.
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