Molten Metal & Forming Processes
A Technical and Commercial
Comparison of Alloying Products
for the Aluminium Casthouse
Introduction
Making elemental additions during the melting
phase enhances the mechanical and physical properties of aluminium alloys. A remelt manager has several alternative methods available for introducing the
element required; pure metal, aluminium hardeners
and high concentrate tablets are all techniques commonly employed. The process of selecting the optimum alloying method is dependent on several criteria; the price of the additive, dissolution times, metal
cleanliness, the temperature required of the liquid
metal bath to disperse alloying element, addition
recovery levels, casting rates, health & safety issues,
environmental impact and any factors affecting downstream activities.
The purpose of this paper is to compare aluminium
based hardeners with compacted powder tablets.
Although, historically, hardeners are perceived to be
more expensive than tablets and this is often paramount in the purchasing decision, it is necessary to
explore all the aforementioned factors, including a
detailed economic investigation, to determine the true
total cost of producing the final value added product.
Tablets
The elemental addition is in powder form, usually at
75% - 80% concentration level and balance 20% 25% aluminium powder with or without a flux inclusion, compacted into a puck or tablet.
By David Evans, Steven Munnoch
and Steve Martin,
Avon Metals, UK
tive) technical understanding of the metal quality
implications associated with adding different elements to aluminium alloys and how this affects the
final product performance.
Method
Separate additions of Avon hardeners, flux-free
tablets and flux-containing tablets were made to pure
aluminium in order to produce a series of 3003 and
8006 melts. The effects of the additions on the level of
inclusions were monitored using the Prefil® (Pressure
Filtration) test. The flow-rate of molten metal through
a micro filter at constant temperature and pressure
was monitored and used to plot a graph of weight filtered vs. time. Inclusions in the metal, such as oxide
films, quickly build up on the filter surface during a
test, reducing the flow-rate through the filter. Therefore the slope and overall shape of the weight filtered
vs. time curve indicates the level of inclusions present
in the metal (Figure 1).
A full description of the Prefil test and quantitative
assessment
of
residues
is
available
at
www.metalgate.net.
Hardeners
Aluminium based hardeners or master
alloys are concentrated alloys of an element
pre-dissolved into a primary or secondary
(scrap) based aluminium matrix. They are
most commonly produced in ingot, waffle &
sow form.
Both methods employed will ultimately
achieve the final specification in the slab or
billet, which can then be rolled or extruded
to the final product shape.
Comparison 1 - Metal Quality
In order to compare hardeners with tablets
in terms of their effect on molten metal quality (melt inclusion content), Avon Metals
called upon the expertise of N-Tec Limited.
Recently N-Tec has developed a (quantitaAPT ALUMINIUM - Process & Product Technology
Figure 1: Introduction to Prefil Curves – ‘clean’ vs. ‘dirty’ metal
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Molten Metal & Forming Processes
It is clear from the graphs
that for both 3003 and 8006
alloys the addition of Avon
hardener keeps the cleanliness of the metal well within
the Target Production Window for melting/holding furnace systems.
Flux-containing tablets initially disperse quicker than
those without, but the flux
quickly introduces inclusions
into the melt that significantly
reduce the quality of the metal
to a substandard product.
Flux-free tablets disperse
their weight better over a
longer time period than their
flux – containing counterparts,
Figure 2: Combined Prefil Windows showing World Class Production Window for Sheet
however the absence of flux
and Foil (3003 and 8006) and Target Production Window for melting/holding furnaces
has an adverse effect on the
dissolution and recovery of
In this report Prefil flow rate curves will be presentthe element and as a result the flux free tablets fail to
ed against both the Prefil World Class Production
keep the metal quality within the Target Production
Window for Sheet and Foil (shaded dark grey) and the
Window.
Prefil Target Production Window for melting/holding
These inclusions, if not fully filtered out, will result
furnace systems (shaded light grey), Figure 2. This
in hardspots which will cause "ripping" in rolling foils
enables the reader to compare the experimental Prefil
and "deep graining" in extruded products.
results with relevant industrial windows for metal
Comparison 2 – Melt Temperatures
quality at the casting head and in the melting/holding
Tablets are made as additions when the bath is liqfurnaces.
uid and molten metal analysis can be determined.
Figures 3 & 4, respectively, illustrate the maximum
High concentrations require a higher temperature in
flow rate hardener performance against fluxed and
order for dissolution of the element in liquid alumininon fluxed tablets when producing 3003 and 8006
um. Hardeners are added to the initial cold charge
alloys with Mozal primary aluminium.
and are analytically effective as liquid state is
Results
achieved. A low dispersal temperature and first time
specification are a major gain.
By their nature tablets are
prone to sinking and cause
non-dispersion which results
in further quantities being
required to achieve target elemental addition – this can
result in the melt running out
of specification due to the
lower part of the melt absorbing the extra amount of element. Many producers avoid
this problem by raising the
temperature of the melt to
ensure rapid dissolution of the
element. Pure aluminium
melts at 660°C but in order to
ensure homogenous dispersion of the element in the carrier tablet and avoid stratification in chemical composition,
Figure 3: Alloy 3003 Prefil curve
2
APT ALUMINIUM - Process & Product Technology
Molten Metal & Forming Processes
tures and longer melting times
required to effectively disperse tablets have a detrimental effect on the exposed furnace linings. Replacement
and maintenance regimes are
significantly increased.
Proprietary filters and
degassers to clean the liquid
metal of inclusions are costly
consumables and slow down
the metal flow rate.
Figure 4: Alloy 8006 Prefil curve
the melt must be raised to at least 750°C.
Comparison 3 – Volume throughput
Consider that the hardener is added at the start of
the melt and melts with the primary aluminium.
Tablets are added at the end of the primary melting
phase - therefore increasing the total time of the melting cycle. This is especially critical when the melting
and alloying are performed in the same furnace as this
is often the plant bottleneck and has a knock-on effect
on plant throughput. Increased stirring and drossing
off procedures are also required to disperse the element properly.
After the furnace has been drossed off the cycle time
of the next melt is often increased by retaining a liquid heel or bath. This can hide pyramids of undissolved concentrate, which can often reappear in the
next rush test, or remain undetected because it
breaches the upper spectrum of the equipment setting.
The use of hardeners increases the volume of
castable product. Whereas tablets will increase standing time, reducing volume available and requiring
more fuel to maintain the liquid state, with the obvious tax and environmental implications. The longer
the time metal is in the melting furnace the more dross
forms, reducing yield.
Due to the high yields, hardeners can be calculated
to become part of the initial cold charge i.e. melted
with the primary aluminium at low temperature. This
significantly reduces the amount of energy required
and shortens the length of the melt time giving
increased volume throughput - more tonnes produced
in a given time span equals more revenue, giving an
edge in an ever more competitive world market.
Comparison 4 – Plant life
The flux inclusions as well as the higher temperaAPT ALUMINIUM - Process & Product Technology
Comparison 4 - Environmental Issues
Environmental
costs
increase, more dross is formed
(requiring associated disposal)
and the lower the yield of
contained metal the more
oxide formed. Pure metal and
clean elements in hardeners do not add oxides,
whereas powdered products have a substantially
increased exposed surface area promoting natural
oxide formation. Also it has been shown that for manganese tablets there is an exothermic reaction
between the manganese and aluminium components
of the tablet, observed as a glowing of the tablet on
the surface of the melt.
Tablets both with and without flux add hydrogen
gas to the melt requiring further costly degassing.
Microscopic analysis of final product has shown maximum performance is achieved utilising hardeners.
Tablets take longer and require a higher melting
temperature than master alloys in order for effective
dissolution to occur. Due to new Climate Change
Levy legislation penalising energy usage in order to
reduce global warming, every energy unit conserved
in the melting process will help to reduce this tariff.
Raising aluminium to excessive temperatures in order
to disperse high concentrate or pure metal additions
will ultimately become uneconomic and environmentally unfriendly.
The volatile chemical reaction required and encouraged, to disperse tablets also means that fume formation is increased and therefore filter bag life is
reduced.
Comparison 5 - Health & Safety
Tablet additions have an advantage in terms of storage volumes, however, the cardboard packaging is
often damaged, leaving product strewn over the casthouse floor. Hardeners, by their physical form are
stored and used as a raw material, being added to the
melt at the outset adding little or no extra handling
time.
When using tablets care needs to be taken to minimise the risk of a powder explosion and that the fur3
Molten Metal & Forming Processes
nace men do not inhale or ingest the fine particles.
Flux containing tablets emit fumes when charged to
the furnace.
The risk to safety around the facility becomes higher the longer the charge door is open and low emissions become more of a problem.
Comparison 6 - Cost of elemental addition
Pricing basis: - Prime €1595, AlMn20 €1813,
Mn80% €2030, £/€ Conversion Rate = 1.45
(Note: prices and exchange rates may vary at time
of printing).
In a 90 tonne remelt furnace, in order to make an
addition of 1% manganese we should consider the
two methods available:
The apparent financial difference between tablets
and hardeners is slight; however the increased
throughput level of actual metal units generates significant cost savings both in time and energy.
Therefore it can be established that every tonne of
aluminium hardener utilised performs as full value
added finished product.
Conclusions
The choice of best alloying practice in the aluminium casthouse is an intricate process that must consid-
1) Tablets
80% Manganese and 20% Aluminium Powder/Flux
Recovery level variable
Melt size = 90000 kgs
Required Manganese level = 1% therefore 900 kgs Manganese required in tablet form at 80% concentrate
(900 ÷ 80% = 1125 kgs of tablets).
Therefore charge cost assuming metal at
Aluminium
Tablets
89,100 tonnes x €1,595 (£1,100)
1,125 kgs x €2,030 (£1,400)
90 tonnes
=
=
=
Therefore cost per tonne in-spec liquid metal
€142,115 (£98,010)
€2,284 (£1,575)
€144,399 (£99,585)
€1,604 (£1,107)
This assumes 100% recovery from the tablet, which is unrealistic. In clinical tests actual performance is
60% Manganese content and zero recovery from the powdered aluminium content.
Therefore, one may assume that a further 375 kgs of costly addition may be required to reach the target
level, increasing the cost of the finished product accordingly, thus: Aluminium
Tablets
89,100 tonnes x €1,595 (£1,100)
1,500 kgs x €2,030 (£1,400)
90 tonnes (after melting)
=
=
=
Therefore cost per tonne in-spec liquid metal
€142,115 (£98,010)
€3,045(£2,100)
€145,160 (£100,110)
€1,613 (£1,112)
2) Hardeners
By comparison, disregarding the incorrect yield performances of tablets, we see the following comparison:
Furnace 90 000 kilos x 1% = 900 kilos Manganese. With AlMn20 addition we need 4,500kgs of hardener.
Aluminium
AlMn20
85.5 tonnes x 1,595 (£1,100)
4.5 tonnes x €1813 (£1,250)
90 tonnes
Therefore cost per tonne in-spec liquid metal
4
=
=
=
€136,373 (£94,050)
€8,159 (£5,625)
€144,532 (£99,675)
€1,606 (£1,108)
APT ALUMINIUM - Process & Product Technology
Molten Metal & Forming Processes
er a range of factors in order to determine the most
suitable method for introducing elemental additions.
For marginal cost comparison, aluminium-based
hardeners have a massive overall beneficial effect and
are sustainable, effective products that give full
enhanced value per tonne- the cost of every tonne of
aluminium hardener utilised is transferred at full value
into the finished product. They readily go into solution
at lower liquid aluminium temperatures, minimising
the formation of dross and solubility of hydrogen.
These lower furnace temperatures also mean reduced
energy consumption and longer furnace life. Many
plants throughout Europe and North America have
conducted trials and concluded that hardeners have
not only increased productivity but have also shown
considerable cost savings in real terms and a more
controlled environmental impact.
Hardeners can be charged cold at the start of the
melt and have the whole melt cycle to disperse;
tablets are added at the end of the melt, leading to
increased cycle times and lower production.
The use of flux in tablets improves the initial dissolution rate by removing the oxide layer on the metallic powder particle; however the inclusion of flux
introduces inclusions into the alloy and reduces metal
cleanliness. Tablets remain a viable option to fine trim
melts, if required, prior to casting.
Although more voluminous, aluminium-based
hardeners are a more viable method of introducing an
APT ALUMINIUM - Process & Product Technology
elemental addition than tablets in both technical and
commercial terms providing:
• Cleaner finished product
• Higher product throughput
• Lower furnace temperature
• More environmentally/operative/plant friendly
• More cost effective
Biographies:
Authors: David Evans has over 25 years experience
in the development and marketing of aluminium
hardeners to major companies throughout Europe and
is Managing Director of Avon Metals Ltd.
Steven Munnoch has 7 years experience in the Aluminium Industry and is Commercial Director of Avon
Metals Ltd.
Steve Martin is a graduate metallurgist with over 10
years experience in the Aluminium Industry and is
Sales Manager of Avon Metals Ltd.
Contributors: M.F. Lovis of N-Tec Ltd. is a Materials
Engineering Masters graduate with post graduate work
at Oxford University.
Dr. A. Cushway of N-Tec Ltd is A Fellow of the Institute of Cast Metal Engineers and Chartered Engineer,
with many years experience in aluminium production.
R. Hovland is with Hydro in Norway. APT
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