Advanced Powder Technol., Vol. 11, No. 2, pp.

187 – 197 (2000)

© VSP and Society of Powder Technology, Japan 2000.

Original paper

A novel cryogenic grinding system for recycling scrap tire

peels

S. B. LIANG 1,∗ and Y. C. HAO 2

1 College of Chemical Engineering, Dalian University of Technology, Dalian, 116012, PRC
2 Department of Laboratory M&E, Dalian University of Technology, Dalian, 116012, PRC

Received 14 July 1999; accepted 29 October 1999

Abstract—By using a thermal separator and a vortex mill, a cryogenic grinding system was
established in which experiments were performed to grind scrap tire peels into fine powders. The
new grinding device is more effective than mechanical mills such as the ACM pulverizer. The
cryogenic grinding results also show that rubber products of sizes less than that 50 μm are difficult
to attain. Sharp corners and smooth surfaces were visible on the ground rubber particles that have
the much better flowability. The optimal feed rate and size of scrap tire peels were investigated.
The characteristics of final rubber products and the energy consumption in the grinding process were
examined in detail. The exergy efficiency of the cryogenic grinding system is 10.9%.

Keywords: Thermal separator; vortex mill; scrap tire peels; recycling; cryogenic grinding; exergy
efficiency.

1. INTRODUCTION

Scrap rubber tires can be ground into fine powders under the cryogenic state. The
cryogenic grinding of rubber scrap in liquid nitrogen atmosphere produces cube-
like particles as small as 75 μm. Liquid nitrogen is usually used as a refrigerant for
grinding rubber materials in most size-reduction mills (usually mechanical mills)
due to its inert property, low-pressure storage, rapid heat transfer, low boiling point
at atmospheric pressure, moderately high refrigeration value and easily controlled
flow. In cryogenic grinding, the size of scrap rubber is reduced from granular to
small chips which are then fed into a loading hopper and from there by means of
a cooling conveyor into the size-reduction mill where additional liquid nitrogen is
injected as required by the automatic grinding temperature control system. One
major feature of the size-reduction mill is that 50– 80% of the power input is trans-
mitted into heat. The temperature level in the size-reduction mill chamber must

∗ Correspondence concerning this paper should be addressed to S. B. Liang.

188 S. B. Liang and H. Yunchen

be maintained below approximately −100 ◦ C. The consumption of liquid nitrogen

needed to grind 1 kg of scrap tire peels in the cryogenic grinding system is as high as
2– 3 kg. For commercial scale grinding of scrap tire peels in a cryogenic state, it is
not convenient to use such a large amount of liquid nitrogen. Consequently it makes
the grinding cost much higher and the final fine rubber products are too expensive.
According to the features of the cryogenic grinding, a new cryogenic grinding sys-
tem which mainly includes a thermal separator and a vortex mill was established.
This cryogenic grinding system not only maintains the favorable properties which
exist when utilizing liquid nitrogen, but also decreases the energy consumption of
recycling scrap tire peels.

1.1. Thermal separator [1]

In this investigation, instead of liquid nitrogen, a special device developed in the
laboratory was used for expanding compressed air through equipment designed as a
thermal separator, which belongs to the field of wave machinery, i.e. mechanization
of shock tubes. It is composed of several rotating nozzles and a large number of sta-
tionary tubes mounted radically around the periphery of rotating nozzles. Figure 1a
shows the main structure of the thermal separator. During operation, the rotating
nozzles sweep past a series of stationary shock tubes in order of precedence. When
any shock tube aligns with a rotating nozzle, for a time interval it is exposed to driver
gas at high pressure. The appearance of the high-pressure gas is equivalent to the di-
aphragm break in a normal shock tube. A shock wave propagates into the tube, and
to accelerates, heats and compresses the gas originally in the tube. After removal of
the nozzle, the tube is exposed to the lower pressure exhaust pipe. Under the action
of the rarefaction wave, the flow direction of gases in the tube is changed. Conse-
quently the gas injected into the tube will be exhaled into the exhaust pipe. The tem-
perature of the discharged gas is decreased due to a waste of energy. The shocked
gas dissipates heat energy by means of heat transfer across the tube wall. When the
driver gas in the tube is exhausted, the gas dynamic is completed. A large reduction
of refrigeration efficiency (i.e. isentropic efficiency) may occur as a result of several
types of no-ideal effects such as leakage and mixing of gases, boundary layer of the
in-tube, interface mixing, and heat transfer. At present the refrigerating efficiency
had achieved 80% in the laboratory and 75% in industrial applications. The ther-
mal separator possesses unique qualities such as lower price, easy operation, simple
maintenance, high throughput and good antierosion against drops or particles.

1.2. Vortex mill [2]

The cold air from the refrigerator, as low as −100 ◦ C, is first sent to the cooling
chamber (a fluidized bed) where the scrap tire peels become brittle. Then the cold
air stream from the refrigerator at a temperature of −73 ◦ C at 0.35 MPa (abs.) is
sent to the vortex mill to grind the brittle scrap tire peels, which are delivered
simultaneously from the cooling chamber into the vortex mill. Figure 1b shows
 A novel system for recycling scrap tire peel 189

(a) (b)
Figure 1. (a) Thermal separator and (b) vortex mill.

the gas and particles flow pattern in the vortex mill. Having been cooled below
the glass transition temperature of the rubber constituents, the scrap tire peels enter
the chamber of the vortex mill by the suction action created by the high-speed air,
which also enters the chamber through a nozzle on the wall of the vortex mill along
the tangential direction. At the outlet of the nozzle, the Mach number of the airflow
approaches unity. Thus an intensively turbulent swiveling flow field forms in the
chamber of the vortex mill. Under this condition, the particles of the scrap tire peels
move towards the chamber wall of the vortex mill quickly and at the same time the
impact among the particles of the scrap tire peels is little or weak. The grinding of
the particles does not occur in this area. The intensive impact of the high-speed air
on the particles of the scrap tire peels usually occurs in the vicinity of the nozzle
outlet, where the air velocity is reduced to accelerate that of the particles through
the transfer of kinetic energy. At the same time the particles impact violently and
then the grinding of the particles appears and continues along the near-wall area in
the chamber of the vortex mill until ground fine powders of scrap tire peels separate
from the bigger ones through the slots in the chamber wall of the vortex mill and
instantaneously enter the cyclone. Here the cyclone not only serves to separate fine
particles from the air but may also be used to recover the cold energy of ground fine
rubber powders by recycling the air in the loop of the cyclone and pre-cooler.
The significant advantage of the cryogenic grinding process is that by applying a
vortex mill the heat evolution during the grinding process is less and can be easily
removed in comparison with mechanical grinders. Another advantage is that the
cold energy stored in the ground fine rubber powders can be reclaimed to pre-cool
the scrap tire peels entering in the pre-cooler. Compared with jet mills or fluid
energy mills [3, 4], the vortex mill needs relative low gas pressures.

2. CRYOGENIC GRINDING PROCESS

A schematic diagram of the cryogenic grinding process is illustrated in Fig. 2.

There are three advantages for choosing the vortex mill as a grinding apparatus:
(1) The scrap tire peels can be ground into fine powders.
190 S. B. Liang and H. Yunchen

Figure 2. Cryogenic grinding process. 1, Air compressor; 2, water cooler; 3, dryer; 4, blower(1);
5, heat exchanger; 6, thermal separator; 7, vortex mill; 8, cryogenic cooling chamber; 9, cyclone;
10, blower(2); 11, cryogenic precooler.

Table 1.
The physical properties of air and rubber

λg Cg ρg μg λp Cp ρp
(W/mK) (kJ/ kg K) (kg/ m)3 (kg/ ms) (W/ mK) (kJ/ kg K) (kg/ m)3
0.02 1.005 1.293 17.09 0.163 2.093 1200
(−40 ∼ −100 ◦ C) (273/Tg ) (384/(Tg (−20 ∼ −90 ◦ C) (−20 ∼ −90 ◦ C)
+111))
(Tg /273)1.5

λ, heat conductivity; C, heat capacity; ρ, density; μ, viscosity; g, air; p, particles.

(2) In contrast to the conventional grinding apparatus (i.e. mechanical grinders),

such as two-roll grooved rubber mills, hammer mills and cracker mills, the
vortex mill works steadily and safely. No occasional maintenance is necessary
since no rotating parts are involved.
(3) Only compressed air is used as the power source of grinding and no extra power
supply is required. The gas pressure needed by the vortex mill is lower than that
needed by jet mills or fluid energy mills.
In addition, the compressed air entering at 0.35 MPa(abs.) would certainly bring
a cooling effect to over-compensate for the thermal energy generated there by the
friction of the colliding rubber particles and maintain the grinding condition at a
rather constant temperature.
The physical properties of air and rubber used in the cryogenic grinding system
are given in Table 1.
The cryogenic refrigerating power Q (kJ/ kg) supplied by the thermal separator
for the cryogenic cooling and grinding of scrap tire peels in the cryogenic grinding
process is given by:
Q = i1 − i2 + Q (kJ/ kg), (1)
 A novel system for recycling scrap tire peel 191

where i1 is the specific enthalpy of the inlet air of the compressor; i2 is the specific
enthalpy of the exit air of the water cooler, and Q is the dissipated heat energy
due to heat transfer across the tube wall of the thermal separator and is determined
mainly by the pressure ratio of the inlet pressure to the exit pressure of the thermal
separator and the isentropic efficiency of the thermal separator. The average surface
temperature of the tube wall of the thermal separator can reach levels as high as
150 ◦ C. It is obvious that the isentropic efficiency of the thermal separator is less
than the turbo expander because of their quite different principles of operation.
The isentropic efficiency η of the thermal separator used in the cryogenic grinding
process is as high as 74%. It is given by:
1 − T2 /T1
η= , (2)
1 − (p1 /p2 )−(k−1)/ k
where T1 and T2 represent, respectively, the temperature of the inlet and outlet
air of the thermal separator, p1 and p2 represent, respectively, the pressure of the
inlet and outlet air of the thermal separator, and k = Cp /Cv , where C is the heat
capacity.

3. EXPERIMENTAL RESULTS AND ANALYSIS

In the cryogenic grinding system as shown in Fig. 2, compressed air at a flow rate
of 720 N m3 /h was supplied by two air compressors of 2V-6/ 8 in parallel. The
maximum pressure of supply air was 0.9 MPa(abs.). The grinding chamber of
the vortex mill was 300 mm i.d. The bulk of the product was collected in the
cyclone receiver and a small fraction, less than 4 wt%, was carried over to the
cryogenic cooling chamber by the bulk exhausted air from the chamber of vortex
mill.

3.1. Effect of the type of mills on the cryogenic grinding of scrap tire peels
The comminution results of scrap tire peels in the cryogenic state by using a vortex
mill and a ACM pulverizer, which is one kind of mechanical grinder, are compared.
The feed rate of the ACM pulverizer was 80– 120 kg/ h and the total power needed
by the ACM pulverizer was 12.5 kW. While the ACM pulverizer worked, the cold
air from the thermal separator at a temperature of −80 ◦ C at atmospheric pressure
is sent to the grinding zone of the ACM pulverizer. When the vortex mill of the
grinding chamber of 300 mm i.d. is utilized, the compressed air from the thermal
separator at a temperature of −73 ◦ C at 0.35 MPa(abs.) is sent to it. In both cases,
the average feed size of scrap tire peels is about 5– 6 mm and prior to entering the
mill chambers the scrap tire peels must be maintained below the glass transition
temperature of rubber constituents in the cryogenic cooling chamber. Figure 3
shows product particle size distribution of scrap tire peels ground by both the ACM
pulverizer and the vortex mill. These are their optimum ground particle sizes. It
192 S. B. Liang and H. Yunchen

Figure 3. Product size distribution of the ACM pulverizer and the vortex mill.

is noted that the ground product of the ACM pulverizer in which grinding power
consumed was mostly transferred into heat absorbed by the ground material, and
was coarser than that of the vortex mill and agglomerated easily on account of the
inefficiency due to the application of excessive impact energy. The vortex mill
occupies a special place among grinding equipment, since grinding in the vortex
mill (e.g. jet mill or fluid energy mill) largely eliminates the accumulation of thermal
energy in the ground particles. Cooling due to the very great gas to solid ratio and to
the expansion of the gas more than compensates for the heat energy generated by the
collision of the particles and by the friction between them. The cooling effect also
permits the treatment of thermally labile materials, e.g. rubber and plastic. Sharp
corners and smooth surfaces were visible on the ground rubber particles which have
much better flowability. This supports the assumption that high-velocity collision
during fine grinding under the cryogenic state alters the crystal structure, i.e. the
energy content of the material. Thus, in the use of the ground rubber product, its
more active state must be taken into consideration, i.e. mechanochemical activation
must form a consciously applied part of the preparatory operations.

3.2. Performance of a vortex mill with different grinding parameters

3.2.1. Effect of feed rate. Figure 4 shows the product size distribution obtained
when the scrap tire peels cooled below their glass transition temperature are fed in
at a constant size of 5– 6 mm but at different feed rates. In Fig. 4 we only compare
the different percents of 60, 80, 150 and 200 meshes of ground rubber products. The
compressed air from the thermal separator at a temperature of −73 ◦ C at 0.35 MPa
(abs.) is sent to the vortex mill of which the grinding chamber was 300 mm i.d. It
must be noted that the vortex mill has an optimum feed rate of scrap tire peels in
order to obtain the finest grind. The product size distribution also shows that the
fine product sizes below 50 μm (or 300 mesh) are very rare, although by applying
the vortex mill in which the compressed air comes from the compressors directly at
 A novel system for recycling scrap tire peel 193

Figure 4. Effect of feed rate.

the ambient temperature at the identical pressure, material such as limestone and
talcite, etc., can be ground into fine powders with average sizes below 20 μm.
This shows that not only scrap tire peels are extremely difficult to grind into fine
powders but also the cryogenic comminution, especially by using the vortex mill, is
characterized by the unique features of all its process. Comminution in the vortex
mill, where the particles in the gas jet achieve a velocity of more than 100 m /s in
a very short time irrespective of the decrease of the gas velocity of sound at low
temperature (about −100 ◦ C), is primarily the result of high-velocity collisions. It is
possible to measure experimentally the large quantities of energy which are mainly
converted into heat. Their magnitude is about 10– 105 times that of the surface free
energy γ [5]. A mathematical estimation can be made which shows that the energy
density in the vicinity of the crack at high fracture propagation speeds is so large
that the corresponding mathematically computed temperature — in reality there is
probably no thermodynamic equilibrium — is often above the melting point. As
a result, thermal activation at the top of the crack also contributes to splitting the
material, quite apart from the tensile stress. Of course this is a consequence of
the mechanical stress. The total energy which is adsorbed at the crack tip in the
form of surface free energy, plastic deformation involving heat generation and other
forms of energy must be taken from the stress field. Schonert [6] found that roughly
1 μm is the limit below which a particle exposed to the effect of some force does not
break but undergoes compression and consequent deformation, and loses its rigidity.
This phenomenon is called ‘microplasticity’ [5, 6]. Microplasticity involves the
possibility of energy uptake. During the cryogenic grinding of scrap tire peels, the
limit may increase sharply since they are much more thermally sensitive than other
materials [7, 8]. That is to say that in the cryogenic grinding process, much finer
rubber powders (e.g. below 50 μm) are impossible to attain.
194 S. B. Liang and H. Yunchen

Figure 5. Effect of feed particle sizes.

3.2.2. Effect of feed size. Figure 5 shows the product size distribution of two
different feed sizes of scrap tire peels which were ground by the vortex mill at
identical grinding parameters to those mentioned above. It is obvious that the scrap
tire peels at constant feed size of 8– 18 mesh nearly did not grind. This observation
agrees qualitatively with the theory of Griffith. The general starting point of his
arguments is that smaller particles have fewer defects, i.e. fewer effective flaws.
The stress necessary to initiate a fracture must therefore be greater. This theory is
also valid for grinding scrap tire peels in the cryogenic state. Griffith’s condition
is G  2γ (here G is the specific crack extension energy). It corresponds to
the ideal brittle fracture which, however, does not occur according to the present
state of knowledge, especially for cryogenic grinding. Actually the specific fracture
energy β is a determinant, but the exact value of β of scrap tire peels is not clear
below the glass transition temperature.

3.2.3. Effect of the temperature and pressure of the grinding compressed air.
When the inlet temperature of the compressed air is near or equal to the glass
transition point, fine rubber powders are obtained in the cryogenic grinding process.
As the inlet temperature increases, the performance of the vortex mill decreases.
When the temperature of the compressed air from the thermal separator is 20 ◦ C
higher than the glass transition point of the rubber constituents, the sample scrap
tire peels could not be ground in the vortex mill. The experimental data are in good
agreement with the corresponding calculations by a numerical method of transient
heat transfer [9].
The effect of the pressure of the grinding air was examined by using two
different size vortex mills with grinding chambers diameters of 200 and 300 mm,
respectively. Figure 6 shows the size distribution of product particles for the two
different size vortex mills. As the pressure of the grinding air was raised from 0.35
to 0.45 MPa(abs.), the product at the same feed rate of scrap tire peels became finer.
 A novel system for recycling scrap tire peel 195

Figure 6. Effect of compressed air pressure.

It is noted that the optimal pressure of the grinding air needed by the vortex mill in
the cryogenic grinding system will be determined by the final cost of ground fine
particle products of scrap tire peels.

3.3. Exergenic analysis of the cryogenic grinding process

According to the whole grinding parameters of the cryogenic grinding process
which worked in the optimum state, the exergy losses generated in equipment and
subprocesses are calculated.
The exergy of circulating air at each point is given by:
e = h − h0 − T0 (s − s0 ), (3)
where h, s and e, respectively, represent the enthalpy, entropy and exergy of air at a
certain temperature (T ) and pressure (p); h0 , s0 and T0 , respectively, represent the
enthalpy, entropy and exergy of air at the environmental atmosphere state.
The exergy of heat δQ is given by:
 
T0
de = 1 − δQ. (4)
T
The physical exergy of ground material is given by:
 
T
e = Cp (T − T0 ) − T0 ln , (5)
T0
where Cp is the heat capacity of the ground material, T is the temperature of the
ground material, e is the physical exergy of the ground material and T0 is the
temperature of the environment.
The results are given in Table 2. The exergy efficiency of the cryogenic grinding
process is 10.9%. The significant losses of exergy took place in the compressors and
the vortex mill. About 1500 kWh will be consumed for grinding 1000 kg of scrap
196 S. B. Liang and H. Yunchen

Table 2.
The exergy losses (kW) in the cryogenic grinding system

Input of Exergy Exergy loss Exergy loss Exergy loss Exergy loss Exergy
exergy available in the com- in the heat in the ther- in the vor- loss in the
pressor exchanger mal separa- tex mill cooling
tor chamber
90 9.87 28.70 6.64 13.74 27.36 2.69

tire peels at the experimental level. This shows that the cryogenic grinding of scrap
tire peels by means of the thermal separator and the vortex mill is more economical
than that by means of liquid nitrogen.

4. CONCLUSION
Scrap tire peels can be ground into fine powders in the cryogenic grinding process
consisting of a thermal separator and a vortex mill. The major feature of the
cryogenic grinding system is that the energy consumption of recycling scrap tire
peels is considerably lower than that of using liquid nitrogen. Compared with
mechanical grinders such as a ACM pulverizer, the vortex mill can treat thermally
labile materials and eliminates the accumulative heat produced in the ground
particles. The data obtained in this work show that rubber products sizes less than
about 50 μm are difficult to attain. This corresponds with the theory of fracture
mechanics. There is an optimum feed rate of scrap tire peels in the cryogenic
grinding process. The experimental results also show that Griffith’s theory on
particle flaws is valid for the cryogenic grinding of the scrap tire peels. As the
pressure of the grinding air was raised, the product at the same feed rate of scrap
tire peels became finer. The losses of exergy in the process largely took place in the
compressors and the vortex mill.

REFERENCES
1. Y. Hongru, The novel approach for generation of cryogenic test flow in a transonic wind tunnel,
Exp. Measurem. Fluid Mechan. 11 (1), 1– 5 (1997).
2. D. I. Eskin and N. A. Doronolev, The research of jet vortex mill, in: Proc. Int. Conf. Fine Grinding,
Classification and Agglomeration Science and Technology, Penn State University, pp. 41– 44
(1997).
3. M. Menyhart and L. Miskiewicz, Comminution and structural changes in a jet mill, Powder
Technol. 15, 261– 266 (1976).
4. B. Dobson and E. Rothwell, Particle size reduction in a fluid energy mill, Powder Technol. 3,
213– 217 (1969/70).
5. H. Rumpf, Physical aspects of comminution and new formulation of a law of comminution,
Powder Technol. 7, 145– 159 (1973).
6. K. Schonert and K. Steier, Die Grenze Der Zerkleinerrung bei Kleinen Korngroßen, Chem. Ingr.-
Tech. 43 (13), 773– 777 (1971).
 A novel system for recycling scrap tire peel 197

7. T. Oshima and Y. L. Zhang, The effect of the types of mill on the flowability of ground powders,
Advanced Powder Technol. 6, 35– 45 (1995).
8. P. Rajalingam, J. Sharpe and W. E. Baker, Ground rubber tire/ thermoplastic composites: Effect
of different ground rubber tires, Rubber Chem. Technol. 66, 664– 667 (1993).
9. S. B. Liang, Y. Q. Fang and X. Y. Zheng, Transient heat transfer of scrap tire peel and its fine
powder in the cryogenic grinding process, Chem. Engng. Commun. 163, 133– 144 (1998).