Note: Descriptions are shown in the official language in which they were submitted.
2~212~2
RAW MATERIAL MOUIJDS AND PROCESS FOR THE MANUFACTURE
OF RAW MATERIAL MOULDS
The invention relates to a process for the manufacture of raw material
moulds, particularly for the manufacture of silicone or silicone alloys in
an electro cupola furnace, wherein quartz sand, a non-caking carbon carrier,
preferably petrol coke, and a pitch containing binder are mlxed to ~orm a
starting mixture, the starting mixture is compressed to briquette blanks,
and the briquette blanks are processed to raw material mouldæ through a
solidifying heat treatment. For the manufacture of such raw material
moulds, quartz sand of a grain size range between .05 and .2 mm is generally
used, and preferably quartz sand of narrower grain slze bands vi~hin that
range. It is understood that iron or iron oxide of small particle size may
be admixed with the starting mixture of the raw material moulds for the
production of ferro silicone. Iron or iron oxide may also be added to the
heating ch~mber of the electro cupola, preferably in the form of pellets or
individual pieces. The heating chamber includes the raw material moulds and
quartz. The invention further relates to raw materlal moulds manufactured
by this process.
Raw material moulds for the above identified application must conform to
certain physical and chemical requirements. For the production of silicone
it is a chemical requirement known from British Patent No. G~ 20 84 122,
that silicone carbide is formed first in the raw material moulds, du~ing
their journey through an upper part of the electro cupola, in accordance
with the equation
SiO2 ~ 3 C , SiC ~ 2 CO
To achieve this, the total carbon content of the raw material moulds must be
appropriately selected. The carbon content is generally selected to be
higher than stoichiometrically required (British Patent No. GB 21 50 128).
- It is a physical requirement that the raw material blanks do not break apart
during their journey through the lower part of the electro cupola, and
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PAT 15848-1
especially during the reduction of silicone to silicone carbide, so that the
silicone carbide may further react according to the equation
SiO2 ~ SiC = 3 Si ~ 2 C0
It will be readily apparent to a skilled person that further reactions ta~e
place in the electro cupola. For example, at the high temperatures present
in the furnace and in accordance with the equation
SiO2 ~ C = SiO + CO
highly volatile silicone monoxide is produced and is partly lost, which
reduces the yield of the silicone production and interferes with the heat
balance o~ the process.
In a prior art process disclosed in German Published Application No.
DE 37 24 541, of which the present invention is an improvement, raw material
moulds are produced which are made of a mixture of quartz sand, a pitch
containing binder and a fine grained carbon carrier, but are free of caking
coal and are formed in a binder aided briquette forming process. The heat
treatment is carried out in a rotary kiln, a lower part of which is filled
sufficiently high with quartz sand to provide that the heat treatment is
carried out in a quart~ sand submersion bed. Thia process has proven to be
advantageous. Particularly, the raw material moulds manufactured using such
2S a process substantially withstand al] stresses durlng their journey through the electro cupola so that it is guaranteed that the above mentioned
chemical processes can take place in an especially controlled way. However,
with respect to the long term properties oE the raw material moulds before
their use in an electro cupola, raw material moulds manufactured in
accordance with the prior art process may be much improved. The mechanical
stability of such raw material moulds decreases with time during storage,
and in soma cases decreases so far as to become unacceptable. This may be
explained by air diffusing into the raw material moulds and, especially
during outside storage, water penetrating into the raw material moulds,
which may lead to a reduction of the binding forces of the raw material
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PAT 15848-1
2~12~ ~
mould bonding structure. This is not the case for raw Material moulds
designed for the sa~e application, but manufactured using a high temperature
briquette forming process as described in German Published Application No.
DE 30 09 ~08, wherein raw material moulds are compressed directly from a
starting mixture which has a temperature of 350 to ~50 C without subjecting
them ~o a solidifying heat treatment. However, it is a disadvantage of such
a high temperature briquette forming process that it is costly.
The present invention provides a process for the manufacture of raw
material moulds, especially for the production of silicone or silicone
alloys in an electro cupola furnace, which process leads to raw material
moulds which not only fulfill all physical and chemical requirements during
their journey through the electro cupola furnace, but are also characterized
by excellent long term properties and substantially retain their stability
during transpor~ and outside storage.
This is achieved in a process in accordance with the invention, wherein a
pitch co~taining binder, which is made of a binder mixture of pitch and
caking coal and has a temperature between 100 and 200 C, is mixed with
quartz sand and a non-caking carbon carrier at a mixing temperature within
the same temperature range as the binder, the resulting starting mixture is
compressed to raw material blanks starting at the mixing temperature, and
the blanks are subjected to a solidi~ying heat treatment which has an end
temperature of over 450 C for production of the raw material moulds. In a
process in accordance with the invention, the briquette forming process
which is used for the production o~ the moulds is a binder aided briquette
forming process such as the process generally used in the manufacture of
mineral coal briquettes. Thereiore, the technology of known binder aided
briquette forming processes may be employed despite the high quartz sand
content of the moulds in the present invention. Generally, commonly known
briquette presses and pressures of 1 to 2 t/cm2 may be employed.
It is an unexpected result that, when the above mentioned binder aided
briquette forming process is used and the above mentioned parameters are
employed, the resulting raw material moulds comply with all the
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PAT 158~8-1
2~212
above-mentioned requirements even with respect to their long term
properties. This is especially apparent, when a binder mixture ls used,
which is a pitch/coal alloy. A pitch/coal alloy in accordance with the
invention is given, if the pitch and the ca~lng coal ~re, so to speak,
dissolved in each other so that an integration of the two components into a
new binder alloy has taken place. This may be readily achieved, especially
when coal of sufficiently fine grained is used. In raw material moulds
manufactured by a process in accordance with the in~ention, the pitch/coal
alloy is s~rprisingly unaffected by air and humidity di~fusing into the raw
material moulds. Several variants of the raw material mould manu~acturi~g
process in accordance with the present invention are within the scope of the
invention, Especially good results are achieved if the non-caking carbon
carrier, the quartz sand and the binder mixture have the same temperature
during their admixture and if the admixture is carried out at a preferred
temperature of about 160 C. In a preferred embodiment, the starting mixture
contains 20 to 40% per weight of quartz sand, the binder Mixture and, for
the remainder, petrol coke and activators. A number of activators adapted
to be used in the present process are readily apparent to a person skilled
in the art.
In contrast to the above described prior art process taught in German
Published Applicatlon No. DE 37 24 541, wherein the solidifying heat
treatment must be carried out in a rotary kiln, in accordance with the
present invention, the heat treatment may also be effected in a different
manner, for example, on a moving grate which passes through an appropriate
oven or an appropriate heat chamber and is loaded with a single layer or
multiple layers of raw material moulds. However, in a preferred embodiment
of the process in accordance with the invention, the blanks are subjected to
the solidifying heat treatment in a rotary kiln. A very careful heat
treatment of the blanks and a high final rigidity of the moulds may be
achieved if raw material blanks, which have a higher specific weight than a
shooting weight of the quartz sand, are produced by selecting an appropriate
mixing ratio of the components in the starting mixture and an appropriate
- degree of compression of the starting mixture during the forming of the raw
materlal ~.oulds, and are fed to a heated rotary kiln for the solidifying
heat treatment, which kiln is filled with quartz sand to such a height that
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PAT 15848-1
. 2
the heat treatment of the blanks i~ e~f~cted ln a quartz sand submersion
bed. The quartz sand preferably has a t~mperatur~ of about 500 to 530 C, at
least towards the exit end of the rotary kiln. In order to prevent the
blanks from contacting each other in the kiln and to prevent their abrasion,
the kiln is preferably filled with a volume of quartz sand which is larger
than twice the volume of the voids within a loose filling of ~he kiln with
the blanks. For the remainder oE the process in accordance with the
invention, recourse to the technology disclosed in German Published
Application No. DE 37 24 541 may be had. Accordingly, the raw material
blanks may be formed in a pellet forming apparatus, which yields
substantially spherical pellets, or in a briquette forming press. The use
of the latter is preferred, since the specific weight of the briquette
blanks may be controlled through an adjustment of the briquetting force of
the press. The raw material blanks are fed into the kiln through an entry
end thereof together with a volume of quartz sand required for retaining the
volume of the submersion bed substantially constant. Both the produced
moulds and the heated quartz sand leave the kiln at an exit and thereof.
The durativn of the heat treatment of the blanks and thus, the length of
time the blanks remain in the kiln, is selected to provide a sufficient
solidification of the blanks. The raw material moulds are preferably
removed from the rotary kiln together with the least amount of quartz sand
possible. However, the quartz sand may also exit the kiln at substantially
the same speed as the raw material moulds in which case the quartz sand is
preferably partially recycled into the kiln in such a way that the sand
particles are recycled on average three times. Since only part of the
quartz sand filling of the kiln is recycled in such a case, the amount of
newly added, cool quartz sand may be adjusted such that the temperature oE
the quartz submersion bed is about 20Q C at the entry end of the kiln, which
is advantageous for a gentle heat treatment of the blanks. It is an
advantage of the heat treatment in a rotary kiln, that volatile components
which evaporate from the blanks during the heat treatment may be combus~ed
in the kiln above the quartz sand bed. The resulting combustion heat may be
used to retain the kiln at tha required temperature and to make the heat
treatment substantially independent of additional heat sources. Thus, the
one or more burners which are generally provided at the entry end of a
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PAT 15848-1
~)2~ 2~
rotary kiln may be required for the start of the heat treatm~nt and for
special situations only.
To optimize the process in accordance with the invention, it is preferred
to employ a binder mixture which, with resp~ct to the s~arting mixture from
whieh the moulds are pressed, contains at least 7~ per weight of pitch in
the form of crude oll pitch or coal pitch, especially electrode pitch, and
at least 12~ per weight of flne grained caking coal. The blndçr mixture
preferably contains 7 to 12~ per weight of pit~h and 12 to 14~ per weight of
fine grained caking coal. Preferably, quartz sand with a 8rain size of .05
to .2 mm and petrol coke with a grain size of less than 2 mm with at least
60~ below .5 mm, is employed.
The raw material moulds produced by the process in accordance with the
invention are characterized by their long term properties and their behavior
in the electro cupola furnace. Without departing from the s~ope of the
invention, fine, particulate minerals may be admixed with the starting
mixture. As a result, an activation of .he raw material moulds may be
achieved which is effective at temperatures above 1500 C. The raw material
moulds manufactured by a process in accordance with the invention may also
be employed as silicone or carbon carriers during the manufacture of c~st
iron in a cupola furnace.
The invention will be further described below by way of example only and
with reference to a preferred embodiment of the invention. A graph, which
is shown in Figure 1, forms part of the example. Figure 1 shows the
pressure resistance of the raw material moulds plotted over time.
1) In order to manufacture raw material moulds for the production of
silicone or silicone alloys in an electro cupola furnace, in a flrst
example, 38~ per weight of quartz sand with a grain si e of .08 to .25 mm
and 50~ per weight of petrol coke with a grain si~e of 2 mm were thoroughly
admixed with 12~ per weight of electrode pitch at a temperature of 160 C.
The softening point of the electrode pitch as defined by Kaines was 90 C
at completion of the mixing, and the resulting starting mixture had a
temperature of 145 C.
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PAT 15848-1
~21212
Substantially spherical blanks of about 2 cm diameter were produced from
this starting mixture after cooling to 105 C using a pressure of 1.5 t/cm2,
The blanks were subjected to a solidifying heat treatment in a rotary kiln
as described above. l'he so manufactured raw material moulds exhibit, after
outside storage, long term pressure resistance properties as illustrated by
curve a in the graph of Figure 1. In the acceptable pressure resistance
region, before the decrease of curve a, the moulds comply with all chemical
and physical requirements in the electro cupola fur~ace.
2) In a second example, one part per weight o the electrode pitch
referred to in example 1 and 1.8 parts per weight of caking coal with a
grain size of up to 80~ between .l and .2 mm, are melted to a pitchlcoal
alloy at a temperature of about 160 C. The first example has been repeated
with 18~ of thls pitch/coal alloy replacing the electrode pitch of the first
example. For the remaining steps of example 2, the steps of the first
example have been repeated up to the finish of the raw material moulds.
These show, a~ter outside storage, long term pressure resistance properties
as illustrated by curve b in the graph of Figure 1.
3) In a third example, the steps of example 2 have been repeated up to
the production of the blanks. However, using the same treatment time and
temperature in the heat treatment, the blanks have been hardened on a moving
grate. Curve c in the graph o~ Figure 1 shows the pressure resistance of
the resulting raw material moulds.
A comparison of curves a, b and c s~rikingly shows the improvements in the
characteristics and properties of the raw material moulds, which have been
achieved through using a process in accordance with the present inventlon,
PAT 15848-1
