Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BRIEF SUMMAR~ OE THE INV NTION
~ his invention relates generally to methods for pyro-
lytic separation of polymeric organic compounds from objects or
substances that are sub~tantially unaffected by the applied heat.
Such methods are useful ~n the polymer related industries to
remove a variety of plastics from such objects as dies, blow
molding heads, breaker plates, screen packs and filters, spinner-
ettes, extruder screws, pumps, nozzles and other such tooling.
The parts so cleaned may then be reused The process is
also useful for reclaiming metals by separation from such polymers,
and for reclai~ling certain constituents of the polymers.
More specificall~ the invention relates to improvements
in the fusing of such pol~mers as are ~lowable at temperatures
below those at which appreciable degradation is initiated, and
improvements in the removal of plastics having an ash o~ high
carbon content after pyrolysis has removed the volatile consti -
~tuents.
There are numerous methods for removal of deposits of
plastlc from metal parts. Except for the methods employing solvents,
ultrasonic cleaning or a combin~tion of these, the methods
generally involve heating to a temperature usually in the range
from 800 degrees to 900 degrees F within which the plastic degrades.
During degradation volatile constituents which are generally
combus~ihle are evolved, leaving a residue that is frequently
composed largely of carbon. If oxygen is present oxidization
occurs, leaving an i.norganic dust or powder residuum.
Prior methods of cleaning include, for example, covering
the parts with molten salt; or heating the parts in an oven in
an atmosphere of air~ inert gas or steam; or immersing the parts
in a hed of heated aluminum oxide; or heating the parts with a
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7~
1 blow torch or upon a hot plate. Each of these methods has one or
more disadvantages such as the need for periodic replacement of
salt or other medium of immersion, atmospheric cont~mination by
smoke or vapor, lengthy t:lme periods for completion of the cleani.ng,
annealing or distortion of parts from which the plastic is to be
removed, danger to operators from salt eruption, spillage or other
hazards, residues such as salt or aluminum oxide left on parts
requir.ing further cleani.ng operations, and limitations either to
speciic xesins, ~o limited amounts of resin, or. to the cleaning
of parts that do not have complicated shapes.
Many of the foregoing disadvantages have been overcome
by a particular process of vacu~n pyrolysis described in an article
by Don Biklen, entitled "Now: Remove Plastic Deposits From
Extruder Parts the Modern Way", published in the SPE Journal for
July, 1973, Volume 29, page 25. According to this method, plastic
- coated parts are placed in a chamber fitted with radiant heating
~lements and with a vacuum pump leadi:ng to a waste line. Under
vacuum the parts are heated. When the temperature reaches 800 to
900 degrees F the plastic decomposes, certain constituents vaporize
~9 and the vapors are pulled off throug:h a condensate trap where -
- most of the solids condense upon contact with water. Vapors and
water leaving the trap enter the vacuum pump. The water and vapor
move through the pump to a separator from which the water passes
to a drain ox recirculates to the vacuum pump and the vapor is
e~hausted to atmosphere through a vent stack either directly or
through a gas-fired afterburner. ..
A refinement of this process also ha~ been in use ~y
the assignee of the present application, involving a second vacuum
chamber or collection receptacle installed beneath the first
vacuum chamber and in space communication therewith through a
--2~
1 connecting pipe. The second chamber is maintained at a substan~'
tially lower temperature than the first chamber. Upon heating,
certain plastics in the first chamber become flowable at higher
temperatures and drop to a collection,tray leading through -the
connecting pipe to the second chamber, where the temperature is
low enough to cause resolidification~ The second chamber has a
collection pan and door for removal of the collected plastic.
This refinement permits the retrieval of a substantial portion
sf ~he total plastic material without its pyrolysis.
However, in some inst~nces difficulty has been exper-
ienced with this refinement because of the failure of the fus~ed
plastic to flow from the heated vacuum chamber quickly enough to
prevent its deyxadation as the temperature rises~ This difficulty
has been causecl primaril~ because of the viscous nature of the
molten plastic which impedes its flow, in many instances blocking
the conne,-ting pipe~ Attempts have been made to reduce this
probl~m by surroundnng the connecting pipe with a heater, but
this expedient is only partially successful.
This inv~ntion compr~ses improvements upon the process
XO described by Biklen and, its later refinements, by facilitating the
removal of fused pol~mer ~rom the heated vacuum chamber so that
it may be collected efflciently and resolidified with minimal or
negligible degradation. Thereby, the remaining polymer that is
subjected to pyrolys,is comprises only a fraction of the total
p,olym~r to be separated from the contents of the heated chamber.
Improvements are al50 made to the process of pyrolysis following
the removal of the volatile constituents, when the remaining ash
is of high carbon contentO By the latter improvements, the resid-
uum, frequently comprising substantially only inorganic pigments
or fillers, is a dust or powder that is easil~ cleaned from the
parts.
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1 BRIEF DESCRIPTION OF THE DRA~ING
The drawing is a partially schematic representation of
apparatus adapted to carry out the process of this invention in
its preferred form and variations.
~ETAILED DESCRIPTION
.
Referring to the drawing, the reference number 12 repre-
sents a chamber unit comprising a three~part vacuum enclosure. The
-
parts comprise a first chamber 14, a second chamber or collection
receptacle 16, and a connecting section 18 which is preferably but
not necessarily a vertical pipe of substantial diameter as
hereinafter further described. The first chamber is preferably
lined with stainless steel, insulated and fitted with one or more
electrically energized radiant heating elements 20, a heating
control the.rrnocouple (not shown~, a xack 22, a door 24 or
insertion or withdrawal of the rack, and a collection tray 26. The
tray has a central opening or discharge point 28 located on the
axls of the section 18 so that molten polymer flowing from the
tray falls by ~ree fall completely out of contact with the.walls
of the section 18 onto a mass 30 of congealed or resolidified
polymer in the chamber 160
The section 18 is of such dimensions and orientation as
to i.solate the second chamber 16 thermally from the first chamber
14 so that the temperature o the chamber 16 i5 substantially
below that of the chamber 14. Th~s permits solidification to occur
immediately aS the polymer reaches the chamber 16. The chamber 16
has a door 32 or removal of the resolidified polymer after the
process is completed.
Preferably, the chamber 14 is arranged with means (not
shown) for loading polymer-laden pieces 34 on the rack 22
externally of the c~amher 14 and for inserting the loaded rack
~4--
1 into the chamber. An air valve 33 is also provided for a purpose
hereinafter described.
A pipe 36 leads -to a condensate trap or water mist
scrubber 38 havin~ a nozzle 40 connected to a source of water and
producing a water spra~ 41. The water spra~ impinges on the vapors
passing to the trap 38 from the chamber 14.
A pipe 42 is connected from the trap 38 to a vacuum pump
44 of the water ring type. An arrow 46 represents a supply of water
to the pump from a recirculation tank 48 described below.
A pipe 50 is connected from the pump 44 to a separator
5~. The separatox has a water trap that separates the water carrying
trapped pol~ymer particles ~rom the remaining vapor t the water
passing through a pipe 54 to the recirculation tank 48 and the vapor
passing through a pipe 56 to a gas-fired afterburner 58, the
exhaust of which leads to a stack 60. In many cases the afterburner
can be omitled as the effluent is within the atmosphere safety
limits of t~pical communitiesO
A source o~ makeup water is connected through a valve
~2 to-~he recirculation tank 48. The tank 48 is of conventiolnal
constxuction and serves to filter out and retain the trapped
polymer partic~es in the waterl discharglng clean water to the
pump 44 as indicatid by the arrow 46. Suitable means are provided
for periodicall~ cleaning out the tank 4B. The valve 62 is opened
to provide only the makeup water required to replace a correspond-
ing amount of water discharged throuyh a P-trap 64 to a drain. The
rate of discharge is adjusted to maintain the temperature of the
recirculated water at a value below B5 degrees F. Maintenance oE
the water temperature below the stated value has been found to
improve the efficienc~ o~ the pump 44.
3~ If desired, skimming devices and fine scxeens or other
~80~dZ
1 filters may ~e employed in the s~stem, ~re~erabl~ withln the closed
loop between the pwmp 44 and the tank 48.
~ s an altexnative, the system can ~lso be operated by
discharging the separator 52 directl~ to the drain, thus eliminat-
lng the recirculation tank 48, in sltuatlons where the illustrated
arran~ement is not re~uired.
The apparatus described aboYe is adapted for an~ of
sev~ral selected modes of operation which are next described. The
selected mode is dependent upon the type and ~uantity o~ plastic
1~ materlal to be subjected to pyrolysis~ The types of plastics gen- .
erall~ used dl~er in two important respects, namely, the property
of becoming ~usible at a temperature below that at which appreciable
degradation Qccurs, and the presence of an ash o high carbon
content following the evolution o~ substantiall~ all of the vola-
tile constituent~. E'or example~ vinyl and rubber compounds.are not
fusible to an extent rendering them flowable at temperatures below
tha-t at which these compounds begin to degrade. In contrast, most
of the other compounds axe flowable at temperatures below those at
which the~ begin to degrade, for example pol~styxenes, pol~olefins,
2~ polycarbonates9 nylons, polyesters and polypropylenes, to mention
common examples. Some pol~mers including pol~oleins and polysty-
renes do not h~ve an ash o~ high carbon content a~ter the e~olu~ion
o~ volatila components J while otller polymers such as polyesters,
nylons, vin~ls, pol~carbonates and rubber compounds do have such an
ash.
~ n use, the above-described apparatus may be controlled
manuall~ but is pre~erabl~ controlled in an automatic c~cle by a
timer lnot shown) adjusted to accommodate the characteristics o$
the particular pol~.mers and the ~uantities thereof to be removed
3~ ~rom the parts 34. The c~cle is ~enerall~ o~ 6b to 90 minutes
duration, but in some cases it can be lon~er.
l The following description is generali~ed to include the
various type~ of plastics commonly used. In the case of particular
plastics the dura-tion of each step, the level of vacuum maintained
and the temperatures of the respective chambers are appropriakely
adjusted for maxim~un efficiency, cleanliness o effluents and
rapidity of pol~mer removal.
Polymer-laden or coated parts 34 are first placed in the
chamber 14 on the rack 22, and the doors 24 and 32 are sealed. The
air valve 33 is closed. The pump 44 is then turned on to evacuate
the system comprising the entire enclosure 12~the pipe 36 and the
trap 38. Typically, the vacuum is maintained at 25 to 27 inches
of mercury~ At the same t~me the radiant heating element 20 or a
plurality of such heaters are energized to cause the temperature
within the chamber 14 to increase.
~ t a given temperature, certain of the above-stated
polymer types fuse and flow from the parts 34, dropping to the
inclined collection tray 26 and draining thererom through the
opening 28. This material then drops through the section 18 to the
substantia]ly cooler chamber 16 where it resolidifies in the mass
30. The chc~bers 14 and 16 and the connecting section 18 are so
arranged that the chamber 16 preferably does not exceed a maximum
tempera-ture of 150 degrees F during the cycle, whereas the
temperature of the chamber 14 rises substantially higher as here-
inafter described. It will be noted that since the tray 26 is
situated within the heated chamber 14, the fused polymer that falls
to the tray remains at substantially the same temperature until it
passes through and falls from the opening 28. A~ter passing from
the opening the polymer does not contact any wal] surface of the
connecting section 18 and falls freely by gravity through this
section ~o the cooler chamber 16.
The result is rapid elimination of the fused polymer that
drops from the parts 34 from the chamber 14, thereby avoiding the
clogging of pipes or passages and the possibility that khis polymer
will be retained within the chamber 14 long enough to begin to
degrade as the temperature of that chamber continues to increase.
The heating element 20 con-tinues to be energized and to
increase the temperature within the chamber 14 until the polymer
remaining on the parts 34 begins to degrade by Pvolution of its
volatile components. This degradation is not accompanied by burning
because of the lack o~ oxygen within the chamber. Ordinarily, the
temperatures required to cause such vaporization are in the range
between 800 degrees and 900 degrees E, although the apparatus is
preferably d~signed for opexating temperatures within the chamber
14 up to about 1,000 degrees F.
The vapors evolved from the parts 34 during de~radation
pass ~hrough the pipe 36 to the trap 38 where they are projected
into the path of the water spray 41~ Here, most of the solids
entrained in the vapors ar~ trapped in the waterO The water laden
wlth solids, combined with the remaining vapor, passes through the
pump 44 to the separator 52. From the separator~ substantiàlly all
of the vapors pass thrcugh the pipe 56 to the afterburner 58,
where any combustible components ma~ be burned. In practice, the
afterburner 58 is usually not required because the vapors in the
pipe 56 are sufficiently clean for exhausting directly to the
a~mosphere.
The water containing solids passes through the pipe 54
to khe separator 48r which filters out substantially all o~ the
solids and recirculates water to the pump 44.
The rate of flow of water through the trap 64 substan-
tially equals that through the nozæle 40 and ma]ceup valve 62, and
~8~
is controlled by the valve 62 to maintain the tempera-ture of the
recirculating water below approximately 85 degrees F.
After an interval of time, substantially all of the
vapori~ab:Le constituents of the polymer, which are frequen-tly
combustible, are removed from the chamber 14 leaving an ash upon
the parts 34. In certain cases this ash has a high carbon content.
For example, the ash of polyesters is approximately 86 percent
carbonO While maintaining the chamber 14 at a temperature in the
~ange of 800 degrees to 900 degrees F and.continuing to operate
the pump 44, the vacuum is gradually reduced to a lower value or
zero by introduction of air through the valve 33 to react with the
carbon, thereby producing carbon dioxide and carbon monoxide. To
facilitate this reaction the air introduced through the valve 33
may be preheated, and means may be provided to distribute it
uniformly ovex the parts 34. These gasses are withdrawn through
the pipe 36 to the trap 38 The remai:ning residue on the parts 34
after this combustion is completed is generally in the form of
dust or powder and mos-tly comprises inorganic pigments and fillers.
~t this stage the heater 20 and the pump 44 are de-en~rgi7ed, and
the door 24 may b~ opened to remove the parts 34~
It wîll be understood that the abo~e-described step of
introducing air through the valve 33 ma~ be omitted in certain
instances, for example whexe the polymer is a polyvinyl chloride
compound and it is desired to avoid the release of chlorine gas
by combustlon.
~he final cleanup of the parts 34 may be accomplished by
any of the known technique5 such as air-jet cleaning, wiping or
bead dusting at low pressure with glass, woodflour, lime or soda
particles~
~lthough the apparatus described above employs a three- :
_g_
4~2
1 part vacuum enclosure 12 including a vertical connecting pipe
section 18, other forms of construction can be used to carry out
the process. For example, a single enclosure may be provided with
a heat baffle dividing the internal space into two reyions, one
above the other. The upper region may contain the rack, the inclined
tray for molten polymer and the radiant heating means. The heat
baffle may have an aperture through which the molten polymer falls
by free fall to the lower, cooler region of the enclosure. The
heat baffle ma~ be provided with reflective surfaces to reflect
the radiant heat upwardly toward the articles on the rack.
The process described above has been employed success-
fully to remove pol~mers ~rom parts having complicated shapes. It
has also accomplished the removal of polymers from parts wikhout
distorting them or materially changing their surface finish.
This process has been further demonstrated to be safe
- for workers, for waste water disposal and for control of
atmospheric pollution.
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