Jaw Crushers for efficient primary size reduction

Jaw crusher fields of application

A jaw crusher is always at the very front of the sample preparation chain, precrushing all hard and brittle materials. The RETSCH jaw crusher range is primarily used in laboratories and pilot plants under rough conditions but are also suitable for on-line quality control of raw materials.

The main fields of application for a jaw crusher include building materials, mineralogy and metallurgy, ceramics and glass, materials science and environmental analysis. They crush medium-hard, hard, brittle and tough materials such as ores, slag, oxide ceramics, hard coal or cement clinker.

RETSCH jaw crushers are powerful forced-feed crushers, available in 7 sizes. They are used for sample crushing on a laboratory scale, but, depending on the model, can also be integrated into existing process lines for continuous size reduction in online operation.

Jaw crusher function principle

In forced-feed jaw crushers the feed material passes through the no-rebound hopper and enters the crushing chamber. Size reduction takes place in the wedgeshaped area between the fixed crushing arm and one moved by an eccentric drive shaft. The elliptical motion crushes the sample which then falls under gravity.

As soon as the sample is smaller than the discharge gap width, it falls into a removable collector within the jaw crusher. The continuous gap width setting with scale ensures optimum size reduction in accordance with the set gap width. 

_{Example function principle BB 100}

Technical influences on the crushing performance

The crushing performance of a jaw crusher depends on the operational sloped angle of the jaws (1) and the ir shape, the speed and the movement behavior of the four-bar linkage. During one revolution of the four-bar linkage, the breaking jaw moves in vertical and horizontal direction. In the process, the gap width constantly moves between a minimum and a maximum (2). The nominal gap width is set at the minimum.

The interaction of a very small sloped angle of the jaws, a small change in gap width to the set gap width and high speed result in an extremely good crushing performance. This is the case, for example, with table-top units such as the BB 50 jaw crusher.

Low speed and a large sloped angle of the jaws, on the other hand, result in rather coarse grind sizes even with a medium change in gap width to the set gap width. This combination is mainly encountered in floor models that can accommodate large sample pieces, such as the BB 300 jaw crusher.

The crushing ratio of a jaw crusher results from the maximum achievable final fineness in relation to the maximum feed size. For Retsch jaw crushers, this lies between 26 and 220. A high value reflects the capacity of a jaw crusher to accept large sample pieces and provide powerful crushing performance, resulting in high final fineness.

Breaking jaw materials

Mechanical size reduction of solids inevitably results in wear of the grinding tools, the so-called abrasion. This means that during grinding, for example with grinding tools made of steel, a certain amount of steel components as well as heavy metals, chromium, etc., can be introduced into the sample. Generally, abrasion is in the ppm or ppb range.

Nevertheless, the grinding process should be carried out as contamination-free as possible. For example, in the case of subsequent analysis for heavy metals, it is advisable to select breaking jaws made of a material that has as few or no heavy metals as possible. Abrasion resistance also plays a role, which varies depending on the material. 

Breaking jaws for RETSCH's jaw crusher range are available in the following materials:

• manganese steel
• stainless steel
• stainless steel 316L
• NiHard4
• tungsten carbide
• zirconium oxide

^{The shape of the crushing jaws is determined by curvature and profiling.}

Steel and cast iron

Steels are ferrous materials whose carbon content is generally less than 2%. Chemically, steel is an alloy of iron and iron carbide. To influence the chemical and mechanical properties of steels, other metals are added (e.g. chromium and manganese).

In contrast to steel, cast iron is hard and brittle due to a carbon content of over 2%. Cast iron is not forged, but cast into the appropriate shape. 

• manganese steel
  The manganese content is between 12% - 14%, the carbon content between 1% - 1.2%. Manganese steel reaches hardness values of over 600 HV (approx. 55 HRC).
   
• stainless steel
  Corrosion-resistant steel with an extremely thin, invisible oxide protective layer which forms at chromium contents >12%. Corrosion resistance increases with the chromium content of the steel.
   
• stainless steel 316L
  Stainless steel with a combination of high chromium content of 17-19% and very low carbon content <0.03%. High corrosion resistance even in chlorinated media and high acid resistance.
   
• steel for heavy-metal free grinding 1.1750 | 1.0038
  These steels are free of chromium and nickel and can be used for sample preparation to heavy metal analysis, provided that possible iron abrasion does not interfere. They have a hardness of up to 62 HRC and are not corrosion-resistant.
   
• cast iron NiHard4
  High alloy cast iron with very high resistance to wear and impact. The hardness is 550 - 700 HBW due to a high carbon content of 2.6 - 32%.

ceramics

Ceramics are a variety of inorganic, non-metallic materials that are formed with the addition of water, dried at room temperature and then hardened by a firing process (sintering) at high temperatures, thus acquiring their characteristic properties.

• tungsten carbide
  Tungsten carbide is one of the hard metals. A cobalt content of 6 - 10% increases the toughness of the material and minimizes abrasion. Tungsten carbide is characterized by its very high hardness and wear resistance.
   
• zirconium oxide
  The main raw material for the production of zirconium oxide ceramics (ZrO₂) is the mineral zirconium (ZrSiO₄). ZrO₂ is obtained from it by melting with coke and lime. Zirconium oxide is very stable against thermal, chemical and mechanical influences and therefore very suitable for grinding tools.

Jaw Crushers - FAQ