Method of burning waste, especially PVC waste, comprised essentially of plastic

A method of burning waste comprised essentially of plastic, especially PVC waste. The waste is burned in a revolving cylindrical furnace accompanied by the addition to the waste of at least one finely ground solid additive having at least one component with a melting temperature higher than the melting temperature of the plastic of the waste.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of burning waste comprised 
essentially of plastic, especially PVC waste. 
It is known (German Publication "Mull und Abfall", 6/84, Pages 169-175) to 
collect plastic, especially PVC plastic, separately from household refuse 
and to burn it in a combustion unit. In addition to being generated in 
household refuse, plastic also results as a residual product in industry, 
for example during the manufacture of windows and the like, during 
building renovation or the like. 
During the combustion of waste that comprises essentially plastic in a 
combustion unit that is provided with grate firing, there exists the 
danger that due to the low melting point of the plastic waste, for example 
in a temperature range of 70.degree.-200.degree. C., molten plastic falls 
through the grating into the air boxes and is thus lost to the combustion. 
It is an object of the present invention to provide a method of the 
aforementioned general type that essentially guarantees a more complete 
combustion. 
SUMMARY OF THE INVENTION 
This object is realized in that the waste comprised essentially of plastic 
(plastic waste) is burned in a revolving cylindrical furnace accompanied 
by the addition of at least one finely ground solid additive having at 
least one component with a melting temperature higher than the melting 
temperature of the plastic. 
Due to the combustion in the revolving cylindrical furnace, and due to the 
addition of the finely ground solid additive, the pyrolysis phase during 
the combustion of the plastic (melting of the plastic, softening and 
partial combustion of volatile components and coking) are influenced in 
such a way that the coke that is formed does not cake up the revolving 
cylindrical furnace and has a looser structure that burns as efficiently 
as possible. This can essentially be attributed to the fact that after the 
plastic melts it becomes disposed upon the finely ground additive, thus 
making available a considerably greater surface area for the further steps 
of the pyrolysis phase. In other words, a coke structure is formed that 
leads to a substantially more complete combustion. It is preferred that 
such an additive be used that even at the end of the combustion the 
melting temperature of the at least one component is not reached. The 
temperature at the end of the revolving cylindrical furnace can, for 
example, be in the range of from 1100.degree.-1300.degree. C. The 
terminology "melting temperature of the plastic" in the specification and 
the claims also includes a temperature window, since with a mixture of 
various plastics one cannot expect a uniform melting temperature of the 
plastic components. The window can be between 70.degree. and 200.degree. 
C., but preferably between 80.degree. and 150.degree. C. 
Due to the combustion in a revolving cylindrical furnace, an adequate 
retention time for the essentially uniform distribution of the softened or 
melted plastic upon the particles of the solid additive is ensured. 
Although the combustion of solid waste, especially specialty waste, 
industrial waste or the like, in a rotary kiln has already been proposed, 
with the general combustion of plastic waste caking and complete 
combustion problems can occur. 
In a preferred manner, as the finely ground solid additive one or more 
additives from the following group are selected: quartz sand, fine gravel, 
stone chippings, bauxite, pure clay, oxidized ore, waste glass, granite, 
residue from combustion, especially slag, lignite, bituminous coal, 
especially highly volatile bituminous coal, asphalt road surface that has 
been reduced in size, chips of old tires. 
With a portion of the aforementioned additives, in addition to the 
additives a fuel is also introduced into the process to enlarge the 
surface area in the event that the heating value of the plastic waste is 
not adequate by itself. With fuels an additional effect can be achieved in 
that the volatile constituents, by means of gasification, also contribute 
to a loosening of the coke structure and hence to an improved combustion. 
With the combustion of PVC plastic waste, one must take into consideration 
that this plastic waste has a high proportion of chlorine and in addition, 
as with other plastics, contains at least one additive. Such additives 
are, for example, stabilizers in the form of inorganic heavy metal salts, 
metallic soaps, especially of Ba, Cd, Pb, Zn, Ca and other 
metallic-organic compounds. Furthermore, PVC can also be protected against 
UV light, by having UV absorbers added thereto. To provide color to 
plastic products, inorganic and organic pigments are added. Furthermore, 
plasticizers may be required. In addition to the additives mentioned up 
till now, plastic mixtures can also contain other additives, such as 
propellants, adhesives, fillers, lubricants, antistatic agents, 
fungicides, and the like; with regard to quantity, the fillers are of 
particular significance since they can be present in the plastic, 
especially in PVC, up to 50% by weight. 
When a plastic, especially PVC, is used that contains at least one 
additive, the finely ground solid additive is such a one that together 
with the additive leads to an extract-resistant residue from combustion. 
In a particularly preferred manner, when plastic is burned that has an 
alkali and/or alkaline earth containing filler, an SiO.sub.2 and/or 
Al.sub.2 O.sub.3 containing additive is used; for this purpose, one can 
use additives from the group of: quartz sand, fine gravel, stone chippings 
on the one hand, or bauxite, pure clay on the other hand, as well as 
combustion residue that is produced during the combustion of the plastic 
waste and that has possibly been reduced in size. Constituents introduced 
with the other additives can also be bonded in a partially 
extract-resistant manner by the formation of silicates and/or aluminates. 
Furthermore, the silicate and/or aluminate formation offers the advantage 
that the desired recovery of the chlorine fraction can be quantitatively 
optimized since during the combustion there is no extensive formation of 
water soluble calcium chloride. 
It is expedient to reduce the size of the plastic waste prior to the 
combustion. It also appears to be expedient to mix the plastic waste and 
the solid, finely ground additive together prior to introduction into the 
revolving cylindrical furnace. The mixture can be stabilized by adding a 
wetting liquid. A calcium chloride solution can be used as a wetting 
agent, especially a calcium chloride solution that results during the 
cleaning of the flue gas accompanied by simultaneous recovery of hydrogen 
chloride as washings. This discharge can also have a slurry-like 
consistency. The recovery of hydrogen chloride from the flue gas during 
the combustion of PVC is part of the state of the art and need not be 
described in greater detail here. 
It is also possible to use as a mixing aid either the washings from the 
extraction treatment of the combustion residue alone or in admixture with 
the washings from the flue gas treatment. 
The inventive method also offers a simple possibility of removing finely 
ground solid materials that are contaminated with organic compounds, 
especially chlorine containing hydrocarbons, in that pursuant to the 
teaching of the present invention they are introduced into the revolving 
cylindrical furnace as finely ground additives. This addresses in 
particular contaminated ground that is formed from contaminated sands 
and/or gravel.

DESCRIPTION OF PREFERRED EMBODIMENTS 
PVC-waste 1 is continuously or intermittently supplied to a mixer 2. This 
mixer can have various configurations, for example it can be a paddle 
mixer, a mixing screw, a plowshare mixer, or the like. In the mixer 2 the 
PVC waste is mixed with recirculated residue from combustion 3, quartz 
sand 4, and lignite 5 accompanied by the addition of aqueous calcium 
chloride slurry 6. It can also be possible to just add water 7 in place of 
the slurry. It is to be understood that the drawing illustrates only one 
exemplary embodiment; in view of the foregoing explanations in the 
introduction to the specification, the supply of one of the finally ground 
solid additives 3, 4 or 5 may be sufficient. However, with regard to the 
calcium chloride slurry that originates from the flue gas scrubbing or 
purification, it is expedient that it be returned in its entirety to the 
combustion since the calcium chloride itself has no appreciable 
possibility for use. 
The mixture is conveyed via a feed mechanism 8 to a revolving tubular or 
cylindrical kiln or furnace 9, which could also be provided with a 
secondary combustion chamber. 
As already mentioned above, of the additives to the PVC, the quantity of 
the chalk (CaCO.sub.3) is of particular significance. In the revolving 
cylindrical furnace, by using SiO.sub.2 additives, the following reactions 
are brought about: 
EQU CaCO.sub.3 +SiO.sub.2 =CaSiO.sub.3 +CO.sub.2 
EQU CaO+SiO.sub.2 =CaSiO.sub.3. 
By means of these reactions, an extract-resistant Ca compound is 
synthesized, essentially avoiding that to any significant extent a 
formation of calcium chloride occurs. For the residue of calcium chloride 
formation, the following equation applies: 
EQU CaCl.sub.2 +SiO.sub.2 +H.sub.2 O=CaSiO.sub.3 +HCl. 
The calcium carried along by the PVC is converted into a nearly insoluble 
calcium silicate. Tests have shown that the reaction of the SiO.sub.2 With 
the calcium chloride at temperatures above 1000.degree. C. occurs 
practically entirely from the left toward the right. The residue from 
combustion 3, the sand 4, and the inert components of the lignite 5 lead 
to a looser or less compact structure of the coke that is formed during 
the combustion, as a result of which an essentially complete combustion is 
achieved. 
The residue from combustion 10a that is withdrawn from the revolving 
cylindrical furnace 9 is subjected in a residue from combustion treatment 
unit 11 to a known washing and possibly to a heavy metal extraction. A 
portion 3 of the residue from combustion is, as described previously, 
conveyed to the mixer 2 after being reduced in size or pulverized in the 
unit 12. 
The flue gas 10b that is withdrawn from the revolving cylindrical furnace 
is conveyed to a flue gas treatment unit 13, from which scrubbed or 
purified flue gas 14 and hydrogen chloride 15 as well as a CaCl.sub.2 
containing flue gas treatment washings 16 are withdrawn. These washings 
can either be conveyed alone to the mixer 2 or together with the washings 
17 of the residue from combustion treatment unit after precipitation of 
heavy metals. At 18, the portion of the extract-resistant residue from 
combustion that is not returned to the mixer is withdrawn. If no return of 
residue from combustion is intended, the entire quantity of residue from 
combustion is withdrawn at 18. 
Especially when PVC waste is burned, the inventive method offers the 
advantage that the coke structure is loosened up in a manner that 
facilitates the combustion, that at least a significant portion of the 
additives present in the PVC are converted into extract-resistant residue 
from combustion, and that a relatively high proportion of the hydrogen 
chloride produced during the combustion can be recovered. It is to be 
understood that the heat that is released during the combustion can be 
utilized by heat exchange with the flue gas, especially for the generation 
of steam. 
The plastic waste does not necessarily consist of a full 100% plastic, 
since impurities, such as hardware components in the case of plastic 
windows, can be present and would be introduced into the revolving 
cylindrical furnace. 
The present invention is, of course, in no way restricted to the specific 
disclosure of the specification and drawing, but also encompasses any 
modifications within the scope of the appended claims.