Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a process and an
apparatus for removing sludge from polluted salt melts such as
those used, for example, for nitriding metals, preferably iron
containing metals. These salt baths are used on a large scale
and consist primarily of carbonates and cyanates of alkali
metals. Some of these baths also contain alkali metal cyanides.
These salt melts are normally operated at an operating temperature
of 560 to 600C.
During the treatment of structural members of an iron
material these baths become clogged with sludge to a varying
degree, i.e., in that adhering steel particles, as for example,
turnings or grindings, accumulate on the bottom of the treatment
vessel or remain in a finely dlvided form in a state of suspension.
Likewise the treatment of scaled or unmachined surfaces (casting
or forging skin) result in the pollution of the bath. Further-
more the slude component is also formed in that nitride
particles scale off from the charaing aids, such as baskets or
racks, after repeated treatment and remain in the bath. The
"sludge" thus formed consists chiefly of iron nitride and iron
oxide.
Other salt baths, as for example, cooling baths based
on hydroxide, become increasingly thick liquids due to the
formation of insoluble carbonates so that the melting point of
the baths increases and the cooling effect decreases. Therefore,
this kind of salt bath must be purified.
Since the sludge content of the nitride bath greatly
influences the reproducibility of the expected nitriding result
a continuous purification of the bath is very important. There-
fore, various devices for removing the sludge are used in
practice. However, the fact that the operation must be
interrupted in order to remove the sludge, i.e., the removal
of the sludge must be carried out on the non-charged bath, has
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been a common factor in all the systems used heretofore.
Furthermore in all the systems used heretofore the
removal of the sludge is carried out with perforated plates,
wire mesh screens or even with mineral wool, or glass wool which
are housed in a suitable device. In many cases said device
consists of one or several superposed basket-like structures of
perforated plate, which are adapted to the shape of the
crucible and, when required, thev are covered with a wire mesh
screen. Even simple, disc-shaped devices with an added rim of
perforated plate are used. For the xemoval of the sludge the
device is placed into the non-charged salt bath and after a
settling andquiescent phase it is withdrawn. The major portion
of the sludge particles present in the bath settle on the wire
mesh screen or on the glass wool. A cylindrical vessel having
a hinged bottom of perforated plate is also disclosed in German
Offenlegungsschrift No~ 2,645,669. Said vessel is placed with
its opened bottom into the bath, whereupon the bottom is closed
and the entire device is again withdrawn. Filtration of the bath
is thus attained. In order to attain a complete removal of the
sludge from the salt melts, the procedure must be repeated
several times in all the conventional sludge removing processes
and devices~
In all the conventional procedures the continuous
interruptions of the operation, which is required for the
purification of salt baths and can be substantial when re~uire-
ments of purity of the bath are high, is a disadvantage. In many
cases the removal of the fine suspended particles is insufficient.
Moreover in the case of sensitive structural membexs the results
of the treatment vary since the sludge content of the bath
increases between two removals of sludge and the results of the
treatment thus deteriorate. However, in the case of sensitive
structural members the way out, namely to remove the sludge
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after each charge, is very cumbersone and time-consuming.
Therefore, the present invention provides a process
and apparatus for removing sludge from polluted salt baths,
particularly nitride salt baths, which operate with as little as
possible an interruption of the operation and also assure complete
removal of the sludge. They also are suitable for any bath size
and are simple and uncomplicated to handle.
According to the present invention there is provided
a process for removing sludge from a polluted salt bath in which
during the operation of the bath a por~ion of the salt melt is
continuously purified.
Thus in accordance with the present invention during
the operation some of the salt melt is continuously purified.
This purification is carried out in that a portion of the salt
melt is continuously passed over a filter to remove the sludge
and that the purified melt is returned to the bath. The portion
of the melt which is passed over the filter is preferably such
that the total volume of the salt bath is purified once within
a period of one to five hours.
The present invention also provides an apparatus for
removing sludge from polluted salt bath including a filter
for removing the sludge and a pump immersed in the salt
adapted to continuously recirculate a portion of the salt melt
from the bath through said filter.
The apparatus according to the invention thus includes
a pump immersed into the salt bath and a filter into which
a portion of the melt is continuously fed.
The filter is disposed above and outside the bath.
Continuous removal of sludge from the bath is thus possible.
This has the advantage of a higher productivity of the bath
since it is no longer necessary to interrupt the operation.
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A further advantage lies in that the state of the
bath is continuously the same with respect to the sludge content
i.e., the minimal sludge content of the bath does not vary over
a lengthy period of time. Complete uniformity of the treatment
results is thus attained.
The present invention will be further illustrated
by way of the accompanying drawings in which:
Fig. 1 is a schematic of an apparatus for effecting
the process according to the embodiment of the present invention;
Fig. 2 is a detail of the filter system of Fig. 1
and;
Fig. 3 is a detail of an alternate filter system
of Fig. 1.
Referring to Figs. 1 and 2, the salt melt (1) is in
a titanium crucible (2). An air compressor (3), for example, an
air lift pump with the compressed air contact (4) and the
ascending pipe (5) is dipped into the salt melt (1). The salt
flow conveyed by the air lift pump (3) passes into the filter
system (6) and leaves this system through the discharge pipe (7).
The filter system (6) comprises, for example, a double-walled
sheet-iron jacket (8 and 9). The intermediate space is insulated
with glass, slag or mineral wool (10) to avoid heat losses and
the recrystallization of bath components associated therewith.
A pot of perforated plate (11), into which a sheeth of fine-meshed
wire net is inserted (12), is disposed within said insulating
jacket.
The liquid salt is pumped into the filter sheeth, which
is open at the top. The sludge particles are retained on passing
this filter. The purified salt melt leaves the filter system
through the discharge system (7). In order to assure that in the
case of an obstructed filter, complications due to overflowing
of the pot are avoided, an overflow (13) is provided. Filter
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sheeths filled or obstructed with sludge can be very easily
exchanged without having to ~urn off the air lift pump or having
to remove it from the bath since the ascending pipe (5) can be
rotated about its longitudinal axis towards the bath.
Another feasible embodiment of the sludge removing
unit (6) is shown in Figure 3. In this case it consists of a
simple box (14), which may also be insùlated to avoid heat
losses. Sheet iron barriers (15) of varying heights are disposed
in the box (l4). The liquid salt melt conveyed by the air
lift pump (3) passes into the first chamber (16) of the sludge-
removing unit (6~. When the first chamber (1) is filled the
salt melt overflows into the further chambers until it reaches
the discharge pipe (7). Since the salt melt must slowly pass
through several chambers, it is very quiescent whereby the
sludge particles can settle on the bottom. When the chamber is
filled with sludge, the viscous sludge can be removed therefrom
simply by dumping it out.
The advantages of the process according to the invention
are explained in greater detail by means of the Examples
hereafter.
Example 1
A crucible furnace with a salt bath crucible of
titanium having a diameter of 800 mm and a depth of 1500 mm
contains 1100 kg of nitriding salt. At an operating temperature
of 580C synchronized bodies of steel C45 having a charge
weight of 400 kg are dipped in a rack of 60 kg into the bath
as the members to be treated. The time of treatment (residence
time in the bath) is one hour, the treatment being performed
as a three-shift operation.
In conventional sludge removal the sludge had to be
removed four times within the three shifts, i.e., every six
hours. Between two sludge removals the content of water-insolu-
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ble constituents in the bath varied between 0.017% (immediately
after the removal of the sludge) and 0.19% (immediately prior
to the next removal of the sludge).
The operation of removing the sludge took 30 minutes
in each case, so that within three shifts - taking into account
the time required for charging the bath and removing the members
- a maximum of 18 charges with 7200 kg could be treated.
When using the process for removing sludge according
to the invention and an apparatus according to Figure l and 2,
the content of water-insoluble constituents was constant at
0.015%. The filter sheeths could be exchanged during the
operation so that a maximum of 20 charges with a total of 8000
kg could be treated.
However, the fact that apart from the increased
productivity a distinct improvement of both the quality and the
uniformity of the treated material was noticed as a result of
the constantly low sludge content is even more important. 540
kg of the bath are passed over the sludge removing unit per hour.
Example 2
A crucible furnace with a salt bath crucible of
titanium having a diameter of 600 mm and a depth of 1300 mm
contained 550 kg of nitriding salt. The operating temperature
was 550C. The members to be treated were crankshafts of steel
42 CrMo 4. The weight of the charge was 240 kg and the weight
of the rack 30 kg. The time of treatment (residence time in the
bath) was two hours in a two-shift operation.
In this example the sludge was removed after every
two charges, requiring 25 minutes. The content of water-insoluble
constituents was 0.013% immediately after the removal of the
sludge and 0.12~ immediately prior to the next removal of the
sludge. When using an apparatus for removing sludge according
to the invention corresponding to the Figures 1 and 2, the
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content of insoluble constituents in the bath could be reduced
to a constant value of 0.014% at a throughput of 200 kg of salt
melt per hour~ While previously six charges with a total of 1440
kg could be treated within two shifts, seven charges with 1680
kg were treated when applying the continuous sludge removal.
Example 3
A tank furnace having the size of 800 x 600 x 1300 mm
was filled with an alkali hydroxide melt as the cooling bath.
The temperature of the bath was 350C.
Members according to Example 2 were dipped into this
bath during a treatment time of 10 minutes. By entraining foreign
salts and converting the hydroxide the content of insoluble
carbonate in`the bath increased, whereby the melting point
increased and the cooling effect decreased, resulting in an increase
in treatment time.
In conventional sludge removal the bath had to be
cooled after some time in two stages and the sludge was removed
by means of a screen.
With the apparatus according to the invention, the bath
could be kept free from sludge during the entire running time
while the entire bath was passed over the sludge removing apparatus
once in five hours.
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