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Size Reduction by Crushing Methods

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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/315487098 SIZE REDUCTION BY CRUSHING METHODS Technical Report · March 2017 DOI: 10.13140/RG.2.2.28195.45606 CITATION READS 1 12,303 1 author: A. Balasubramanian University of Mysore 361 PUBLICATIONS 424 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Geochemical Modelling of Groundwater for Prevention of Incrustation in the Water Supply Systems of Salem, Tamil Nadu, India View project GEOCHEMICAL MODELLING OF GROUNDWATER FOR PREVENTION OF INCRUSTATION IN THE WATER SUPPLY SYSTEMS OF SALEM DISTRICT, TAMIL NADU, INDIA View project All content following this page was uploaded by A. Balasubramanian on 22 March 2017. The user has requested enhancement of the downloaded file. SIZE REDUCTION BY CRUSHING METHODS By Prof. A. Balasubramanian Centre for Advanced Studies in Earth Science, University of Mysore, Mysore Objectives: In the field of extractive metallurgy, mineral dressing is the process of separating commercially valuable minerals from their ores. There are two major categories of processing. One is comminution and the other one is separation. The process "Comminution" involves size reduction and size-wise classification called as screening or separation. The objective of this lesson is to provide the details of size reduction methods involved in mineral processing using crushing equipments. Crushing is the first step of mineral processing where the ore/rocks from the mine site is fed into the mechanical equipment in order to reduce the size of masses for subsequent usage by liberating the valuable mineral from the gangue. 1.0 Introduction: Mineral processing can involve four general types of unit operations. They are: 1) comminution – the process of particle size reduction; 2) sizing – separation of particle sizes by screening or classification; 3) concentration by taking advantage of physical and surface chemical properties; and 4) dewatering – solid/liquid separation. Size Reduction is mainly done through crushers and mills. Crushing and grinding are the two primary comminution processes. Crushing is normally carried out on the "run-of-mine" ore. The grinding process which is normally carried out after crushing, may be conducted on dry or slurried material. 2.0 The size reduction process: Minerals being crystals have a tendency to break into endless numbers of sizes and shapes every time they are introduced to energy. The difficulty in size reduction lays in the art of limiting the number of over and under sizes produced during the reduction. If this is not controlled, the mineral will follow its natural crystal behaviour, normally ending up in over-representation of fines. 3.0 Comminution Comminution is particle size reduction of materials. Comminution may be carried out on either dry materials or slurries. Crushing and grinding are the two primary comminution processes. Crushing is normally carried out on "run-of-mine" ore, while grinding (normally carried out after crushing) may be conducted on dry or slurried material. In comminution, the size reduction of particles is done by three types of forces: compression, impact and attrition. 3.1 Compression and impact forces are extensively used in crushing operations while attrition is the dominant force in grinding. The primarily used equipment in crushing are-jaw crushers, gyratory crushers and cone crushers whereas rod mills and ball mills, closed circuited with a classifier unit, are generally employed for grinding purposes in a mineral processing plant. Crushing is a dry process whereas grinding is generally performed wet and hence is more energy intensive. Crushing is done using mechanical crushers. There are several types of crushers available in processing. 1 All crushers have a limited reduction ratio meaning that size reduction will take place in stages. 3.2 Operation stages: The operating stages in minerals processing have remained the same for several years. In this, the hard, abrasive and inhomogeneous mineral crystals are o be treated in special ways in order to extract maximum value out of each size fraction. The following diagram shows the stages in operation of size reduction from crushing to upgrading. 3.3 Crushing of rock and minerals: Crushing is the largest process operation in minerals processing. The goal of this process is to produce rock or (more seldom) mineral fractions to be used as raw material for other industrial production. The quality parameters are normally strength, size and shape. There are two kinds of equipments used for crushing works. one is by using crushers and other one is by using impactors. This diagram illustrates the stages of size reduction from 1000mm to 4 mm. 2 Size reduction of ores is normally done in order to liberate the value minerals from the host rock. This means that we must reach the liberation size, normally in the interval 100 – 10 micron, see value curve 1 above. 3.4 Feed material: All operations in size reduction, both crushing and grinding are of course determined by the feed characteristics of the minerals (rock/ore) moving into the circuit. The key parameters we need are the “crushability or grindability”, also called work index and the “wear profile”, called abrasion index. 4.0 Crushing of rock and minerals: Crushing is the largest process operation in minerals processing. The goal is to produce rock or (more seldom) mineral fractions to be used as rock fill or ballast material for concrete and asphalt production. Quality parameters are normally strength, size and shape. The kinds of materials processed are Limestone, Granite, Gabbro, Basalt, River Stone, Coal Gangue, Quartz, Diabase, Iron Ore, Copper Ore, Zinc Ore and Manganese Ore. 4.1 The art of crushing: Crushing means different things for different operations and the production goals are not always equal. In Mineral dressing, these two approaches are adopted: 1. Crushing of rock and gravel 2. Crushing of ore and minerals. There are three stages in crushing as stage 1, 2 and 3. In each stage the reduction in size ranges as referred to as R1,R2 and R3. This diagram illustrates the stages with equipments and reduction ratios. From 1000mm to 100 micron levels the mass is crushed. 3 4.2 Size control: Neither crushers nor grinding mills are very precise when it comes to the correct sizing of the end products. The reason is to find partly in the variation of the mineral crystals compounds (hard-soft, abrasive – non abrasive), partly in the design and performance of the equipment. Size control is the tool for improvement of the size fractions in the process stages and in the final products. For the coarser part of the process, screens are used (in practise above 1-2 mm). In the finer part we have to use classification with spiral classifiers. 5.0 Types of Crushers: A crusher is a machine designed to reduce large rocks into smaller rocks, gravel, or rock dust. Crushers may be used to reduce the size, or change the form, of waste materials so they can be more easily disposed of or recycled, or to reduce the size of a solid mix of raw materials (as in rock ore), so that pieces of different composition can be differentiated. Crushing is the process of transferring a force amplified by mechanical advantage through a material made of molecules that bond together more strongly, and resist deformation more, than those in the material being crushed do. Crushing devices hold material between two parallel or tangent solid surfaces, and apply sufficient force to bring the surfaces together to generate enough energy within the material being crushed so that its molecules separate from (fracturing), or change alignment in relation to (deformation), each other. 5.1 Jaw crusher A jaw crusher is generally used as a primary crusher in a crushing circuit. Product is fed into the top of the jaw crusher by an vibrating grizzly feeder. The eccentric rotating drive shaft causes the movable jaw to oscillate crushing the aggregate against a fixed jaw. Jaw crushers are run on belt drives driven by an electric motor or diesel engine. Jaw crushers are used extensively throughout the aggregate and mineral processing industry. This diagram illustrates the structure of a jaw crusher. Operating principle: Jaw crushers operate according to the principle of pressure crushing. The crushing material is crushed in the wedge-shaped pit between the fixed crusher jaw and the crusher jaw articulated on an eccentric shaft. The material is crushed by the elliptic course of movement and transported downwards. This occurs until the material is smaller than the set crushing size. The feed cavity of the Jaw Crusher consist of moveable jaw and fixed jaw, which imitate the movement of the animal’s two jaws to complete the materials’ crushing . Jaw Crushers have found their extensive application for the mid crushing of the various ores and large-size materials block in the field of mining, smelting, building material, highway, railway, water conservancy and chemical industry, etc 4 5.2 Gyratory crusher : A gyratory crusher is similar in basic concept to a jaw crusher, consisting of a concave surface and a conical head; both surfaces are typically lined with manganese steel surfaces. The inner cone has a slight circular movement, but does not rotate; the movement is generated by an eccentric arrangement. As with the jaw crusher, material travels downward between the two surfaces being progressively crushed until it is small enough to fall out through the gap between the two surfaces. The gyratory crushers are robust crushers with modern features. They are designed to give optimal capacity in primary crushing, increasing the total capacity in the mining crushing process. These crushers have a large feed opening and a grooved mantle, making them suitable for crushing large boulders.
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