Mill (grinding)

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An ancient Chinese tomb model of a foot-powered mill, Eastern Han Dynasty (25 - 220 AD), Freer Gallery of Art.

A grinding mill is a unit operation designed to break a solid material into smaller pieces. There are many different types of grinding mills and many types of materials processed in them. Historically mills were powered by hand (mortar and pestle), working animal, wind (windmill) or water (watermill). Today they are also powered by electricity.

The grinding of solid matters occurs under exposure of mechanical forces that trench the structure by overcoming of the interior bonding forces. After the grinding the state of the solid is changed: the grain size, the grain size disposition and the grain shape.

With following aims is grinding very important for the process engineering:

magnification of the surface area of the solid ,
manufacturing of solid with wanted grain size,
making pulping of resources possible.

1 Grinding laws
2 Grinding degree
3 Grinding machines
4 Types of grinding mills
5 See also
6 External links

[edit] Grinding laws

In spite of a great number of studies in the field of fracture schemes there is no formula known, which connects the technical grinding work with grinding results. To calculate the needed grinding work against the grain size changing three half-empirical models are used:

KICK for d > 50 mm

$W_K=c_k*(ln (d_A) - ln (d_E))\,$

BOND for 50 mm > d > 0.05 mm

$W_B= c_B*\left(\sqrt\frac{1}{d_A} - \sqrt\frac{1}{d_E}\right)\,$

RITTINGER for d < 0.05 mm

$W_R=c_R*\left(\frac{1}{d_A} - \frac{1}{d_E}\right)\,$

with W as grinding work in kJ/kg, c as grinding coefficient, d_A as grain size of the source material and d_E as grain size of the ground material.

A reliable value for the grain sizes d_A and d_E is d₈₀. This value signifies that 80% (mass) of the solid matter has a smaller grain size. The BOND's grinding coefficient for different materials can be found in various literature. To calculate the KICK's and RITTINGER's coefficients following formulas can be used

$c_K=1.151*c_B*(d_{BU})^{-0.5}\,$

$c_R=0.5*c_B*(d_{BL})^{0.5}\,$

with the limits of BOND's range: upper d_BU = 50 mm and lower d_BL = 0.05 mm.

[edit] Grinding degree

To evaluate the grinding results the grain size disposition of the source material (1) and of the ground material (2) is needed. Grinding degree is the ratio of the sizes from the grain disposition. There are several definitions for this characteristic value:

Grinding degree referring to grain size d₈₀

$Z_d=\frac{d_{80,1}}{d_{80,2}}\,$

Instead of the value of d₈₀ also d₅₀ or other grain diameter can be used.

Grinding degree referring to specific surface

$Z_S=\frac{S_{v,2}}{S_{v,1}}=\frac{S_{m,2}}{S_{m,1}}\,$

The specific surface area referring to volume S_v and the specific surface area referring to mass S_m can be found out through experiments.

Pretended grinding degree

$Z_a=\frac{d_1}{a}\,$

The discharge die gap a of the grinding machine is used for the ground solid matter in this formula.

[edit] Grinding machines

In materials processing a grinder is a machine for producing fine particle size reduction through attrition and compressive forces at the grain size level. See also crusher for mechanisms producing larger particles.

[edit] Ball mill

A typical type of fine grinder is the ball mill. A slightly inclined or horizontal rotating cylinder is partially filled with balls, usually stone or steel, which grinds material to the necessary fineness by friction and impact with the tumbling balls. The feed is at one end of the cylinder and the discharge is at the other. Ball mills are commonly used in the manufacture of Portland cement.

[edit] Rod mill

A rotating drum causes friction and attrition between steel rods and ore particles.

[edit] SAG mill

A rotating drum throws large rocks and steel balls in a cataracting motion which causes impact breakage of larger rocks and compressive grinding of finer particles. Attrition in the charge causes grinding of finer particles. SAG is an acronym for Semi Autogenous Grinding, and applies to mills that utilize steel balls in addition to large rocks for grinding.

[edit] Autogenous mill

A rotating drum throws large rocks in a cataracting motion which causes impact breakage of larger rocks and compressive grinding of finer particles. Attrition in the charge causes grinding of finer particles. Also known as ROM or "Run Of Mine" grinding.

[edit] Pebble mill

A rotating drum causes friction and attrition between rock pebbles and ore particles. May be used where product contamination by iron from steel balls must be avoided.

[edit] High pressure grinding rolls

The ore is fed between two rollers which are pushed firmly together while their rotating motion pushes the ore through a small gap between them. Extreme pressure causes the rocks to fracture into finer particles and also causes microfracturing at the grain size level.

It consists of a pair of vertical cylindrical rollers through which material is passed. The two rollers rotate in opposite directions, "nipping" and crushing material between them. A similar type of intermediate crusher is the edge runner, which consists of a circular pan with two or more heavy wheels known as mullers rotating within it; material to be crushed is shoved underneath the wheels using attached plow blades.

[edit] Buhrstone mill

Another type of fine grinder commonly used is the buhrstone mill, which is similar to old-fashioned flour mills.

[edit] Types of grinding mills

Ball mill
Colloid mill
Disk mill
Grist mill, also called Flour mill or corn mill
Hammer mill
Jet mill
Mortar and pestle
Oil mill
Wiley mill
Windmill

[edit] See also

[edit] External links

Animation of Horizontal Grinder

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Categories: Mills | Tools | Mining equipment