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Szego Mill Technology
The Szego Mill consists of rollers with helical ridges and grooves that rotate inside a
vertical cylinder called the stator (see diagram). The rollers are free to move in the
radial direction, and grinding takes place between the ridges of the rollers and the
stator. The number, diameter, and length of the rollers varies with the mill (stator)
diameter, which is incorporated in the mill designation, for example, the SM-320 mill
has a stator with an inside diameter of 320 mm. This mill has four 110 mm diameter
rollers with a height of 400 or 600 mm.
The Szego Mill is suitable for both dry and wet grinding, and has the ability to process
thick pastes. The material to be ground is fed continuously by gravity, or pumped into
a top feed cylinder. As it moves down the cylinder, it is subjected to crushing and
shearing between the roller ridges and the stator. The grooves aid transport of pasty
material through the mill. Ground product is discharged at the bottom. Crushing force
is created mainly by the radial acceleration of the rollers, and shearing action is
induced in wet grinding by high velocity gradients between roller ridges and the stator.
Grinding forces can be controlled by the speed of rotation and the mass of the rollers.
The rotational speed of the central shaft of the roller assembly is adjustable, and
determines the roller peripheral speed. The quality of the product after one pass
through the mill is determined by the number, length and diameter of the rollers, the
feed rate, the angular velocity, and the characteristics of the material being ground.
An important feature of the Szego Mill is the ability of the roller grooves to aid the
transport of material through the mill, thus providing a means to control the residence
time, the number of times material is compressed, and, hence mill capacity and
product size distribution. This transporting action is particularly important with
materials that do not flow readily by gravity, such as pastes and sticky materials. The
ridge/groove size ratio can be changed to increase or decrease the effective pressure
acting on the particles. The common groove shapes are rectangular and tapered; the
latter will decrease the chance of particles getting stuck in the grooves, as will
The operating variables for the mill are the material feed rate, its consistency
(percentage of solids, if wet), and the rotational speed of the rotor. Typically, the
rotational speed is between 400 and 1200 rpm, depending on mill size, which
translates to roller peripheral velocities of 6 to 10 m/s. The mill is characterized by
high capacity per unit volume and modest power consumption. It is very versatile; in
wet grinding it can handle highly viscous materials such as thick pastes, i.e., high
solids concentrations, without extreme loss of efficiency. Within limits, not only particle
size but also particle shape can be controlled, for example, from granular to flaky.
Szego Mills are available in laboratory (160 mm diameter) and pilot (220 mm) sizes, as
well as in small industrial sizes (320 and 500 mm) capable of throughputs up 10
tonne/h. Compared to a ball mill, the throughput per unit mill volume is of the order of
30 times higher, and the specific power consumption is typically 30% lower. While the
Szego Mill is a compact and efficient grinder for many applications, a special niche is
wet grinding at high solids loadings. A toothpaste-like consistency appears to give
the best grinding efficiency.
Size reduction from 6 mm to below 10 microns can be achieved. Grinding can be
combined in one step with high energy mixing, leaching, beneficiation, and chemical
reaction in open, closed and locked-cycle circuits.
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