The characteristics of raw materials required for the cement production differ at a wide range. The limestone deposits used today as base material, show very different properties regarding hardness, abrasiveness and stickiness. To optimize the crushing process as well as the investment and operating costs, different types of crushers are available. To make a proper selection, several important points should be considered in advance.
The very first step should always be an analysis of the quarry, and especially the characteristics of the raw material. Of prime importance is the hardness of the material to be crushed. To describe this property the compressive strength, the Los Angeles Index or the Mohs Hardness should be determined and a point load test should be carried out.
The abrasiveness of the material has to be described by its Silica and free quartz content, as well as the abrasion index. Just as important is an expression of the stickiness. For this reason the moisture, the clay content and, in very difficult applications, the handling coefficient (shear test) has to be ascertained. Based on this data an overview of the selection criteria allows a pre-choice of the required crusher (Fig.1).
Primary crushing of limestone for the cement production requires high crushing ratios to reduce the material in a single step, down to the required grain size (approx. 0 – 25/80mm) for the raw mills. It should always be the aim to have as few crushing steps as possible, in order to minimize the investment and maintenance costs and time. According to the above overview, one step crushing can be achieved by operating Hammer Crushers or Impact Crushers. These crusher types are fast running machines providing high kinetic energy, resulting in strong impact forces and high crushing ratios. The allowable maximum compressive strength of 150 Mpa is normally sufficient for most of the limestone applications.
The silica content is an important issue to be considered before using Hammer or Impact Crushers. Due to the high speed of the rotors, the crushing tools (hammers or impact bars) are vulnerable to wear.
The limit for these machines is 8-12% silica content. Before making a final decision, the abrasiveness of the raw material has to be analyzed very carefully to avoid suffering from high maintenance costs.
If the silica content is above mentioned values, a crushing system with a small speed difference between the material to be crushed and the crushing tools should be used.
As final material characteristics, the moisture content has to be taken into account. While the Impact Crusher is limited to a moisture content of max. 8%, the Double Shaft Hammer Crusher can take on a moisture content of up to 20%.
This special feature allows charging limestone and sticky materials like e.g. clay at the same time, to obtain a pre-homogenization within the crusher.
Wherever the silica or the moisture content is too high for hammer or impact crushers, a two-step crushing system is a better solution for reducing the material to the requested grain size for the raw mills.
Available for these two-step systems are Double Roll Crushers, RollSizers, Jaw Crushers and Gyratory/Cone Crushers, which are all able to operate in abrasive material. But while the crushing ratio of these crushers is approx. the same (1:5 to 1:6), their ability to crush hard and/or wet material differs.
Jaw and Gyratory/Cone crushers reduce the material by high pressure, which makes them perfect for processing hard materials with a compressive strength of up to 400 Mpa. Because of the relatively low compressive strength of limestone (< 150 Mpa), these crushers are mainly used for highly abrasive limestone. On the other hand these machines are sensitive to high moisture contents, which have to be considered very carefully.
Double Roll Crushers and RollSizers combine pressure and impact, as well as shear and tensile forces for crushing. They are limited regarding hardness of the material (max. 175 Mpa), but they are able to crush very sticky and abrasive materials.
The previously given explanations show, that for every kind of limestone there is a suitable crusher combination to produce the required grain size distribution.
But not only for limestone, also for the necessary additives (like e.g. clay, lime, puzzolan, sandstone, iron ore, schist etc.) with their different material characteristics, the proper crushing method can be realized.
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