Method of and apparatus for the low-temperature milling of materials

Low temperature embrittlement of materials which otherwise cannot be readily comminuted is carried out by treating the materials, g.g. scraps of synthetic resin containing components which might otherwise be released into the atmosphere, with a circulated cooling gas to cause the embrittlement of the materials. The latter are then conveyed through a mill, e.g. a pin-type attrition mill, in a carrier gas and are comminuted therein. The cooling gas is passed continuously around a closed circulation path so that any released components remain trapped in the circulated gas which, in turn, is cooled to a sufficiently low temperature by a separately displaced cooling fluid passed in indirect heat exchanging relationship with the cooling gas stream along the closed path of the latter.

FIELD OF THE INVENTION 
The present invention relates to a process and an apparatus for the 
comminution of a material, e.g. plastic scraps, which is to be milled 
after embrittlement. More particularly, the invention relates to the cold 
milling of the material after it has been embrittled by chilling it in 
direct heat exchange with a cooling gas stream and is carried into the 
mill and through the mill by a carrier gas. 
BACKGROUND OF THE INVENTION 
German Pat. No. 1,778,559 (U.S. Pat. No. 3,633,830) describes a process for 
the comminution of materials which are relatively soft, stretchable, 
plastic or thermoplastic at normal temperatures and hence tend to be 
stretched upon milling at such temperatures. Such materials include 
thermoplastic foil scraps and the like. In this process the material to be 
comminuted is subjected to embrittlement by direct heat exchange with a 
cold gas stream and, after embrittlement, is conveyed through the mill by 
a carrier gas stream. 
The cooling gas stream and the conveyor gas stream can be the same gas, 
e.g. gas derived from liquified nitrogen, the latter being mixed with one 
or both of the gas streams or with the common gas stream, the excess being 
discharged from the apparatus after heat exchange with the comminuted 
material. 
Especially in the case of synthetic-resin scraps, the material to be 
comminuted and the comminuted material can contain easily volatilized 
substances, e.g. plasticizers, which cannot be safely released into the 
atmosphere even in the smallest quantities. The conventional process thus 
has significant disadvantages. For example, if release of such easily 
volatilized substances is to be avoided and a portion of the gas serving 
as the carrier or cooling gas is released into the atmosphere, it is 
necessary to provide additional devices, such as filters, adsorbers or the 
like, to recover the easily volatilized substances. In practice it is 
found that the latter substances are only partly removed by such 
techniques. 
Another disadvantage of the conventional process, especially when liquefied 
nitrogen is used as the cooling source, is that the nitrogen carrier gas 
present in the mill has a relatively high density and, because the mill 
operates at high speed, considerable energy loss is encountered as a 
result of gas friction. 
OBJECTS OF THE INVENTION 
It is the principal object of the present invention to provide a cold 
milling process whereby the aforementioned disadvantages are avoided. 
It is another object of the invention to provide a process for the 
low-temperature or cryogenic milling of substances which contain readily 
volatilizable materials not desirable for release into the atmosphere, 
which avoids such release in a low-cost and efficient manner. 
It is still another object of the invention to provide a process for the 
purposes described in which energy losses are minimized and which is of 
greater economy than the prior-art processes. 
Yet another object of the invention is to provide an apparatus for carrying 
out the improved process. 
SUMMARY OF THE INVENTION 
These objects and others which will become apparent hereinafter are 
attained, in accordance with the present invention, by circulating the 
cooling gas in a closed path as a circulated gas stream and cooling this 
gas, before it is brought into direct heat exchange with the material or 
substance to be milled, by indirect heat exchange with a separately 
displaced coolant. Advantageously the coolant has a composition different 
from that of the cooling gas. 
More particularly, the process for comminuting cold-embrittlable material 
comprises the steps of: 
(a) chilling the material to a temperature sufficiently low to embrittle it 
by passing the material in direct heat exchange with a cooling gas; 
(b) entraining the chilled material through a mill in a carrier gas in 
which the chilled material is milled and the milled product is conveyed 
out of the mill; 
(c) continuously circulating the cooling and carrier gas in a gas stream 
over a closed path; and 
(d) cooling this gas stream to a temperature of at most the temperature of 
embrittlement in step (a) by passing the gas stream at a location along 
its closed path in indirect heat exchange with a separately displaced 
coolant fluid. 
Thus the cooling gas is continuously recirculated along its closed 
recirculation path so that any components of the material to be comminuted 
which are volatilized in and mixed with the cooling gas remain in the 
closed path and are never released into the atmosphere. 
According to an important feature of the invention, the carrier gas stream 
is advantageously circulated along a closed path and is brought into heat 
exchange with a separately displaced coolant to reduce the temperature of 
this carrier gas stream. 
Thus the material to be milled and the material being milled are 
continuously surrounded by an atmosphere of gas fully separated from the 
ambient atmosphere and displaced along a closed circulating path 
independently from the coolant which abstracts heat from the gas streams 
in direct contact with the milled material and the material to be milled. 
Advantageously, the cooling and/or carrier gas is constituted by a gas 
which is lighter (lower in specific gravity or density) than the coolant. 
Thus the carrier gas can be relatively expensive helium or a gas with a 
similarly low density which is never lost from the circulating system 
because it is never released into the atmosphere. Because of the low 
specific gravity or density of the gas in direct contact with the milled 
material or the material to be milled, the losses of energy by gas 
friction are held to a minimum and, especially with mills operating at 
high speed, the milling efficiency is significantly increased. It is thus 
possible to operate with mills which otherwise would generate large 
amounts of gas friction, e.g. pin or attrition mills having rotary disks 
carrying the attrition pins. 
Even with jet mills it is possible to attain a high comminution efficiency 
since the jet velocity is a function of the speed of sound in the gas 
present in the mill which, in turn, increases inversely with the molecular 
weight of the gas. Thus, because of the low molecular weight of a gas such 
as He, as contrasted with N.sub.2 as used heretofore, the jet velocity can 
be increased and the milling made more efficient. 
Furthermore, because of the substantially higher thermal conductivity of 
low-density gases such as He, as contrasted with, for example, N.sub.2, 
the temperature increase resulting from milling is rapidly conducted away 
so that the degassing of substances or materials containing readily 
volatile components is markedly reduced or eliminated. 
Since the coolant does not directly contact the materials and can be 
released into the atmosphere, it can be constituted by a relatively 
inexpensive fluid serving as the energy carrier, e.g. liquefied N.sub.2 
which can be brought into indirect heat exchange with the cooling gas 
stream by a heat exchanger in which the liquefied N.sub.2 is evaporated. 
As noted, this coolant does not come into direct contact with the material 
to be milled or the milled material and hence the physical or chemical 
relationship of the coolant to the milled material is of no significance 
in the process according to the present invention. It is thus unnecessary 
to provide any aftertreating devices, such as absorbers, filters or the 
like, for any of the coolant which may be released into the atmosphere. 
According to a further feature of the invention, the coolant is passed in 
indirect heat exchange with the material to be milled in order to bring 
about a precooling of the latter. 
It has been found, quite surprisingly, that the consumption of the energy 
carrier, i.e. the low-cost coolant, and hence the energy consumption of 
the system of the present invention is lower than that of conventional 
processes. This can be attributed at least in part to the reduced friction 
losses and the fact that the system of the present invention permits more 
exact temperature control, brings about higher efficiency of heat transfer 
to the material to be milled and from the material which has been milled, 
and also allows the temperature to be maintained constant more readily in 
spite of changes in the throughput of the milling system. 
It has been found to be advantageous, with respect to the energy 
consumption and indeed to improve the energy economies of the system, to 
pass the cooling gas stream in counterflow to the material to be milled 
and subsequently in counterflow to the milled material. 
The system has been found to be especially economical and convenient when, 
in accordance with the rate of flow in the materials to be comminuted and 
the temperatures most suitable therefor, the throughput of the cooling gas 
stream and/or the carrier gas stream is adjustable, e.g. by appropriate 
valves in the path of these fluids. 
For carrying out the process of the present invention, it has been found to 
be most advantageous to provide an apparatus which comprises a cooling 
tower, a comminuting device (e.g. a jet or attrition mill) connected to 
the tower, a collecting vessel or bin for the comminuted material, and 
gas-tight material gates between the aforementioned components. A 
compressor is provided to circulate the cooling gas stream and/or the 
carrier gas stream, and a counterflow heat exchanger is provided along the 
circulating path for traversal by the circulated gases and by the 
aforementioned separately displaced coolant. 
For precooling of the material to be comminuted in the cooling tower, the 
latter can be provided with a heat exchanger, e.g. a coil, which is 
traversed by the coolant connected via a duct with the counterflow heat 
exchanger which, in turn, can be connected to a source of the coolant, 
e.g. a source of liquid N.sub.2. 
For control of the temperatue of the material to be comminuted and the 
throughflow of the cooling gas stream and/or the carrier gas stream, the 
counterflow heat exchanger is provided with a set of cooling-gas conduits 
having adjustable valves connecting the counterflow heat exchanger with 
the inlet of the comminuting device, at least one further duct being 
connected to the outlet of the comminuting device for carrying off the gas 
therefrom. 
For the reheating to ambient temperature of the material which has been 
comminuted and recovering the cold which is stored in the comminuted 
product as enthalpy of cooling, the counterflow heat exchanger is 
connected with the cooling column via a duct. A further duct connecting 
the cooling column with the collecting vessel so that a portion of the 
circulated gas stream at least is passed in direct heat exchange with the 
milled product in the latter, and a duct connects the heat exchanger with 
this collecting vessel.

SPECIFIC DESCRIPTION 
The cold-milling apparatus of the present invention comprises a cooling 
tower 1 closed on opposite ends by gas-tight material-passing gates 4, 5, 
in which the mterial to be milled is cooled. A pin-type attrition mill 2 
is provided below the tower and is connected by another gate 6 with a 
collecting vessel or bin 3 for the milled product. This container 3 is 
closed from the atmosphere by still another gate 7. 
To cool the material to be milled, a duct 20, leading from a counterflow 
heat exchanger 9, carries the cooling gas into the base of the tower 1. 
The cooling gas is preferably He. 
The cooling gas after being warmed in direct heat exchange with the 
material in the tower 1, is carried off at the top of the tower 1 via a 
duct 21 opening into the base of the bin 3. The gas is further warmed by 
abstracting cold from the milled product in this container or bin 3 and is 
carried from the top of the latter via line 22 to a compressor 8. The 
compressed gas is delivered via line 19 to the other side of the 
counterflow heat exchanger 9. The compressor 8, line 19, the counterflow 
heat exchanger 9, line 20, the tower 1, line 21, the bin 3 and line 22 
constitute a closed circulation path for the cooling gas stream. 
At several locations along the counterflow heat exchanger 9, communicating 
with the cooling gas space thereof, are lines 12, 13 and 14 provided with 
respective controllable valves 15, 16 and 17 for diverting a portion of 
the cooling gas into an inlet for the attrition mill to serve as the 
carrier gas. A separating vessel 23 is provided between the mill 2 and the 
bin 3 from which gas (carrier gas) is withdrawn via line 18 and passed 
through a filter 24 which removes dust of the milled product. The carrier 
gas rejoins the cooling gas stream at the inlet to the compressor 8. Thus 
a closed path for the carrier gas stream is established via line 18, 
filter 24, compressor 8, line 19, lines 12, 13 or 14, mill 2 and the 
separating chamber 23. 
In the counterflow heat exchanger 9, the cooling gas stream and the carrier 
gas stream are cooled via a separately displaced coolant fluid delivered 
via line 25 from a suitable source, e.g. an air rectification 
installation, tank, liquefaction device or the like. The cooling fluid is 
liquid N.sub.2. Since the coolant does not fully lose its cooling power 
within the counterflow heat exchanger 9, it is passed in a gaseous stage 
through a heat exchanger 10, e.g. a coil, within the tower 1 to precool 
the material therein. The nitrogen is thereafter released into the 
atmosphere at 10a through a valve 10b.