Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCEDURE FOR THE MANUFACTURE OF HIGH CONCENTRATION
MANGANESE MINITABLETS FOR ALUMINIUM BATH ALLOYING. AND THE
DEVICE FOR EXECUTING IT
DESCRIPTION
OBJECT OF THE INVENTION
The present invention refers to a procedure for the manufacture of
high concentration manganese (Mn) minitablets for aluminium (AI) bath
alloying,
the purpose of which is to produce Mn minitablets with a 90-98% concentration
of
this metal, for adding in AI smelting.
The object of the invention is to produce a minitablet product
composed of Mn and AI powder whose first component is obtained by electrolysis
and grinding, while the second component is an atomised powder produced by
means of mechanical processes, both components being them mixed and
compacted to form minitablets with a high Mn concentration.
A further object of the invention is the device for the execution of the
above-mentioned procedure, the device where the loading, dispensing,
compacting and final forming of the minitablets take place.
BACKGROUND OF THE INVENTION
The alloying of aluminium baths with manganese has changed
substantially in recent times, and from the original addition of lumps of
metal,
which gave rise to serious problems of purity and dissolving rate, there has
been
a shift towards two different concepts of alloying: on the one hand, the use
of
parent alloys, consisting of AI and Mn manganese alloys with a 10 to 25% Mn
content, and, on the other, the addition of powdered Mn by means of injecting
the
powder into the furnace. Although both methods are still employed today, their
use has declined drastically since the first compact Mn pellets were
introduced
towards the end of the seventies. These pellets, which came in the form of
tablets, minitablets or briquettes, combine concepts of the two previous
methods,
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take advantage of their strong points and reduce their drawbacks. The pellets
consist of Mn powder in a concentration usually above 75% compacted using AI
powder as the binding agent, a flux, or a mixture of both, in a concentration
of up
to 25%. These materials substantially reduced the amount of cold material
which
is added to the AI furnace in the alloying operation in comparison with parent
alloys. Furthermore, parent alloys usually contain 75 to 90% second smelting
aluminium, which could give rise to problems in the molten metal, besides
calling
for a stock 4 times higher than that of compacted powder alloying agents.
Moreover, they are easy-to-use materials that do not require the investment in
equipment and safety that is necessary for powder injection.
The great financial step that was taken on changing parent alloys
containing a maximum of 25% Mn to compact alloying agents whose Mn content
is 75% or more has generated constant pressure on the manufacturers of
compact alloying agents to obtain materials that, while being effective in the
AI
bath alloying process, also succeed in increasing the Mn concentration in the
compact alloying agent. In this respect, no materials are available on the
market
that contain a percentage above 85% Mn, due mainly to the problems of
compactibility of Mn, an abrasive and non-ductile material. In addition, it is
suspected that the material may not dissolve as quickly as the compact
materials
of lower Mn concentration due to the reduced proportion of AI and/or flux,
which
also act as disintegrators of the compact when this is put into the furnace,
as
reported by the scientific literature on the subject.
As the active alloying element of the compacts is Mn, the decreased
AI content brings a series of advantages for the founder. The amount of
material
to be added to the furnace is smaller, which means that less cold material is
added to the AI bath and that raw material stocks are reduced. Similarly,
there is
a cut in material transport costs, which will be significantly lower than
those of
75% or 80% compacts. Besides this, the price of products depends less on the
value of AI, subject to the changes in its quotation on the London Metal
Exchange, and since AI is currently more expensive than Mn, the cost of the
set
of the raw materials used in production would also be lower. Lastly, we have
to
consider that the founder/user is not interested in adding a material (AI
powder)
to his furnace that he is able to self himself and which, moreover, has a
value
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added due to atomisation, which is lost when smelting it again.
Despite these financial advantages, no compact Mn materials have
appeared on the market with a concentration of 90% or more. The attainment of
this objective raises a series of scientific challenges when it comes to flow
production of these materials. On the one hand, experience indicates that the
pressing process has to be improved in order to be able to reach these Mn
percentages. On the other, the raw materials have a series of factors that may
be
modified when it comes to achieving a better performance. In addition, it has
to
be confirmed whether it is really necessary to have compacts in the furnace
with
AI powder concentrations of more than 10% or 15% for the Mn dissolving rate to
be acceptable, or whether compacts with less than 10% AI dissolve in the
furnace
at a suitable rate.
The present study concentrates on the flow production and
performance in the AI furnace of alloying minitablets (cylindrical in shape)
containing Mn in a concentration of more than 90%, AI being the remaining
material. Although it would be desirable to have this concentration available
in
standard sized tablets as well, the need to apply high pressures to the
material
means that the study is complicated if the size of the compact diameter is
larger
than 40 mrn. On the other hand, fluxes were initially rejected in this study
insofar
as they are materials whose binding action is considerably inferior to that of
AI
powder.
With regard to the raw materials to be used, Mn is the first limitation of
the study. The chemical requirements of Al baths involve the use of Mn of a
high
chemical purity, usually above the 99.7% level, which can only be assured if
the
Mn is produced by electrolysis. At present electrolytic Mn is only produced in
the
Republic of South Africa and the People's Republic of China, which reduces the
possibilities of finding materials with different specifications. Mn, which is
usually
in flake form, has to be converted into powder by grinding. The material
normally
used in the compacting of Mn minitablets has a grain size of less than 450
microns. Mn powder is highly abrasive, a property that is enhanced if the
amount
of fines (powder below 100 microns) increases, and which has a direct effect
on
the pressing quality and the average life of the materials (punches and liner)
of
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the press in which the material is compacted.
The situation is very different with regard to the AI powder involved.
There is a great variety of AI powders on the market that may be used in
continuous industrial processes and with different applications. In the case
of
compacting Mn half-tablets, it is normal to use AI fractions above 100 microns
and below 1000 microns insofar as grain size is concerned. These fractions are
the ones that AI producers generally regard as a by-product in their
production
processes, inasmuch as the fine fractions of AI (below 100 microns) are the
ones that have valuable applications in aeronautics and pyrotechnics on
account
of the explosive property of AI. This fact means that again the production of
a
material of specific characteristics for the compacting of Mn tablets is
skewed or
subject to production conditions independent of the application that this
study
sets out to examine.
In general, the AI used in the production of Mn minitablets is a gas-
atomised powder, although materials may be used that are obtained by
mechanised atomisation procedures, annealed materials or micronised swarf. As
a rule, atomised powders are the most suited to the requirements of the main
functions of AI.
In the production of the Mn minitablets, AI acts as a binding agent,
whereas electrolytic Mn, being highly abrasive and non-ductile, is a material
that
does not compact on its own. Potential improvements in the process apparently
lie in the application of higher pressures to the material so as to enable
these
materials to be compacted. Apart from using higher performance hydraulic units
and applying greater force to the pressing punches, another possibility is to
reduce the diameter of the minitablets, as the smaller the area of application
is,
the greater the actual pressure. This represents a problem at industrial
level, as
smaller diameter minitablets give rise to lower productivity (minitablets
weigh
less). To overcome this problem, it is necessary to work with several punches
at
the same time, and the pressing process has to be effective for all the
minitablets
made in a cycle. This means that all the liners have to be filled properly
with the
material to be compacted, that this must be mixed properly and not be
different in
each of the liners in which it is received, and that the material must flow
smoothly
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to these liners. In this respect, it is extremely important to stop the
mixture of Mn
and AI powders from becoming separated at any time in the process (a problem
that could arise easily since the two materials have widely differing
densities) and,
furthermore, that the equipment should be suitably sized so as to apply the
5 pressure needed for compacting.
DESCRIPTION OF THE INVENTION
The procedure that is advocated offers a solution to the problems and
difficulties mentioned in the previous section, for which purpose it is
specified
that, starting from the two components used, which have to be mixed, namely Mn
and AI, Mn minitablets with a concentration of more than 90% should be
compacted by using Mn produced by electrolysis and ground from flakes of Mn of
a chemical purity of 99.7% or more, which is subjected to a screening process
with a sieve with a mesh of less than 450 microns; the special feature of the
Mn
grinding process is that it is controlled so that the content of fine Mn
powder, with
a size of less than 100 microns, should not be more than 15%, as above this
proportion the compacting of Mn minitablets cannot be assured with over 90%
Mn in their composition.
The procedure also includes the fact that the most suitable AI for
successfully compacting Mn minitablets is atomised powder, which is produced
by mechanical processes, with controlled size distribution, its nominal grain
size
intervals being between 100 and 800 microns, with over 80% of the powder in
the
350-720 micron range.
This grain size distribution is coarse enough to enable the material to
be compacted and fine enough not to retard the dissolving rate, through having
reduced the number of AI grains with the increased Mn concentration in the
minitablet.
The invention also refers to the device for executing the foregoing
procedure, consisting of a hopper for the reception of an Mn and AI mix with
the
afore-mentioned characteristics, there being a central product diffuser in
this
hopper which forces the product to flow through the sides of the hopper to
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prevent the mix directly reaching the feeder of a second hopper which
discharges into the respective pressing or compacting chamber, where pressing
punches will come into action.
The device has appropriate means that enable maximum, minimum
and safety levels to be kept under control in the compacting chamber so that
it
remains at a level of filling all the time such that none of the punches may
try and
make an off-load compacting stroke.
As one of its main innovative features, besides the afore-mentioned
central diffuser, the device includes a honeycomb dispensing valve interposed
between the feed hopper and the compacting chamber, which is provided with a
series of dies that are mounted on a support integral with the actual feed
hopper,
so that the support-hopper assembly is able to run along guides, in either
direction, under the action of a pneumatic device, on which guides there is in
turn
a moving punch support mounted, also driven by a pneumatic ram, so that the
support-hopper movement is independent of the moving punch movement,
although such movements must be synchronised in order to fill, press, compact
and eject the formed minitablet.
Besides the aforesaid central diffuser and the location and use of the
honeycomb dispensing valve, as an innovative feature, the device also includes
three electrical control means to monitor the maximum, minimum and safety
levels, corresponding to compacting chamber filling.
DESCRIPTION OF THE DRAWINGS
To supplement the description being given and in order to assist a
better understanding of the features of the invention, in accordance with a
preferred example of a practical embodiment of same, as an integral part of
this
description a set of drawings is adjoined, wherein, for purely illustrative
and non-
restrictive purposes, the following is represented:
Figure 1.- It shows the graph corresponding to the standard grain size
distribution of the Mn used in the invention procedure. The y axis contains
grain
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size intervals in millimetres, and the x-axis the percentage by volume of each
fraction. Grain size was measured by laser diffraction with dry method sample
insertion.
Figure 2.- It shows a representation referring to the micrograph of the
AI powder in granules used in the invention procedure.
Figure 3.- It shows the graph referring to the standard grain size
distribution of the AI used in the invention procedure. The y axis contains
the
grain size intervals in millimetres, and the x-axis the percentage by weight
of each
fraction. Grain size was measured by a sieve tower.
Figure 4.- It shows a diagrammatic, partially sectional, side elevational
view of the device for the execution of the invention procedure.
Figure 5.- It shows an elevational view, front and sectional in this case,
of the same device as in the previous figure.
PREFERRED EMBODIMENT OF THE INVENTION
The invention procedure, designed to produce Mn minitablets by
compacting, with a concentration of more than 90% of this metal, is based on
using electrolytic Mn ground from flakes of a chemical purity of 99.7% or
more.
The product is then screened with a sieve with a mesh of less than 450
microns,
since it has been found that materials containing significant fractions of a
larger
grain size give rise to much lower dissolving rates in the aluminium furnace.
The
grinding process is controlled so that the content of Mn fine powders (below
100
microns) is more than 15%, as above this percentage it has been found that the
compacting of minitablets cannot be assured with more than 90% Mn in its
composition. Figure 1 shows the graph referring to the standard grain size
distribution of the Mn used.
The tests made indicate that the AI powder most suited for compacting
Mn tablets with a concentration of more than 90% is powder atomised by
mechanical procedures, the special performance of this AL powder being due to
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its spongy granule structure that permits suitable fluidity on the metal
surfaces of
hoppers but which maintains sufficient air holes in the grains, so that the
material
is endowed with greater compressibility. Figure 2 shows the micrograph of the
AI
powder in grains, according to a microscope enlargement of this type of
powder.
The foregoing AI powder also has a controlled grain size distribution,
its nominal grain size intervals being between 100 and 800 microns, with over
80% powder between 350 and 720 microns. This grain size distribution is coarse
enough to enable the material to be compacted and fine enough not to retard
the
dissolving rate, through having reduced the number of AI grains (which trigger
the
dissolving reaction on the minitablet Mn in the furnace) with the increased Mn
concentration in the minitablet. Figure 3 shows fihe graph referring to the
standard
AI grain size distribution in the grains used.
The device for executing the procedure is represented in figures 4 and
5, comprising a hopper (1 ) for reception and storage of the mix, which is fed
in
through the respective filler neck (2), a mix which, as stated, is composed of
Mn
and AI. The mix has to be homogeneous and, on being received in the hopper
(1 ), it falls on a centrally positioned diffuser (3), a diffuser (3) that has
a conical
layout and is supported on legs (4), so that this diffuser forces the product
to flow
through the sides of the hopper (1 ) and never directly onto the feeder hopper
(5)
provided at the outlet of the hopper (1 ), and from which hopper (5) the
product
moves onto the compacting hopper (6). The diffuser (3) prevents the effects of
product separation and assures continuous fluidity at the same level of
product in
the hopper (1 ). The compacting hopper (6) is a vertical continuation of the
feeder
hopper (5), so that the former defines a chamber which maintains a product
level
and in which the compacting is done by means of both fixed punches (7) and
moving punches (8).
The compacting hopper (6) is provided with a series of dies (9), of
varying number depending on the size of the device, and the product or Mn and
AI powder reaches these dies (9) by way of a honeycomb valve (10) interposed
between the feeder hopper (5) and the compacting hopper (6), so that a metered
amount of product passes through this valve and is loaded onto each one of the
dies (9), as the honeycomb valve (10) forms a sort of drum-sector that is
loaded
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with a given quantity of product so that, when this valve turns through an
angle,
the corresponding sector load discharges on the compacting hopper (6) and the
product reaches the respective die (9). The dies are arrayed on a support (11
)
which is integral with the actual compacting hopper (6), and that support-
hopper
assembly is mounted on guides (12), along which it may move in either
direction
under the action of a pneumatic device, on which guides (12) there is in turn
a
moving punch (8) support (13) mounted, also driven by a pneumatic ram or
device. The support-hopper movement is independent of the moving punch
movement, although such movements must be synchronised in order to fill,
press, compact and eject the formed minitablet.
The fixed punches (7) are arranged co-axially facing the moving
punches (8), the latter being installed on a static support (14).
In this way, when the support (11 ) with the compacting hopper (6)
moves forward, the die (9) is filled and then trips the moving punch (8),
which
advances and compacts the material located between it and the fixed punch (7).
The moving punch (8) then moves back and the support-hopper assembly slides
slightly forward so that the fixed punch (7) ejects the minitablet, whereupon
the
cycle starts over again.
It is essential for this device to maintain a minimum product column
level in the compacting chamber (6), so that none of the punches attempts to
compact an empty die, which would result in the breakage of the punches and
column or chamber. This level is maintained by the use of three electrical
controls
and the afore-mentioned honeycomb valve (10), controls which correspond to
references A, B and S, and which indicate the maximum level, minimum level
and safety level of the product in the compacting chamber (6), all of this in
such a
way that the safety level S causes the device to shut off if the product drops
below this level because there will be a risk of emptying the chamber, whereas
level B is the product level that permits a reproducible column weight to be
maintained capable of assuring suitable fluidity and consistent reproducible
filling
at all the punches. When the product has reached that level, the honeycomb
valve (10) opens and dispenses more product from the hopper. This honeycomb
valve (10) closes when the product reaches the maximum level A.
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To obtain proper compacting of the half-tablet with an Mn
concentration of 90% or more, it is necessary to work with punches capable of
applying a pressure of 7500 Kglcm2 of punch. In a practical example a check
was
made on the mechanical strength of the product obtained with 90% and 95% Mn,
in the conditions explained, a mechanical strength check that was carried out
by
means of a drop test consisting of dropping a number of minitablets onto a
cement floor from a height of 1 m, recording the number of impacts required to
cause breakage and for the loss of 2% weight of the minitablet.
10 Minis Mn 90% Minis Mn 95%
Number of tests 5 5
Drops to 2% weight loss 31 1.30.6
Drops to breakage 3.70.6 2.30.3
Dissolving tests of these Mn minitablets with concentrations of 90% or
more were conducted in AI baths, using for this purpose a rotary gas-fired
semi-
industrial furnace with a capacity of 400 kg AI. The experiments were
performed
in accordance with regular standard processes for the addition of minitablets,
bath slag removal, stirring and sample collection. The samples were analysed
by
spark spectrophotometry.
