Introduction To Mineral Processing, Lecture notes of Mineralogy and Mineral Processing

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Introduction to Mineral

Processing

Lecture: Davhana M

Introduction

Mineral Processing and Extractive Metallurgy Mineral processing is a major division in the science of Extractive Metallurgy. Extractive metallurgy has been defined as the science and art of extracting metals from their ores, refining them and preparing them for use. Within extractive metallurgy, the major divisions in the order they may most commonly occur are, Mineral Processing (or Beneficiation), Hydrometallurgy, Pyrometallurgy, and Electrometallurgy. The last steps in the winning of metals are in Physical Metallurgy where the composition and treatment of metals are varied to provide desired physical and mechanical properties. In mineral processing, a number of unit operations are required to prepare and classify ores before the valuable constituents can be separated or concentrated and then forwarded on for use or further treatment. The field of mineral processing has also been given other titles such as mineral dressing, ore dressing, mineral extraction, mineral beneficiation, and mineral engineering. These terms are often used interchangeably.

The three primary types of run-of-mine materials:

• Run-of-mine consisting of useful materials. These could include granites, building sand, limestone, coal and clays. Note that materials in this category are not classified as minerals.
• Run-of-mine containing useful minerals. The minerals in this category among others include fluorite, apatite, diamonds and gemstones, vermiculite, barite, wollanstonite and chromite and are often referred to as industrial minerals. Other examples are i) barite that is used as weighing agent in oil drilling mud and ii) vermiculite, which is used for sound and thermal insulation. The unit value of this class of minerals is low but the purity is high, approaching a chemical grade. The minerals in this class are used directly for industrial applications once they are separated from a gangue content that must be low to start with. The low unit value only allows for marginal treatment costs.

• Run-of-mine containing value bearing minerals. This class of run-of-

mine is similar to the previous descriptions. However in this case, the

target mineral obtains its value from the contained metal and these

categories of deposits are referred to as metaliferrous. For example,

an ore containing the mineral chalcopyrite (CuFeS2) derives its value

from the contained copper. Chalcopyrite does not in and of itself have

any direct use as a mineral. Once chalcopyrite is concentrated

(separated from the gangue), it requires further trewatment to

extract copper via chemical (hydrometallurgical or pyrometallurcial)

methods. A list of some valuable minerals is provided in Table 1.

Processing Approach and Method In broader terms, mineral processing consists of two functions. Firstly, it involves the preparation and liberation, of the valuable minerals from waste minerals and secondly, the separation these values into two or more products, called concentrates. The term separation in this case is synonymous with concentration. These functions are carried out within the constraints of the following three rules.

• The first rule deals with the conservation of mass. The total flow of the material into the process plant equals the total flow out.
• The second rule relates to the quality or grade of the concentrate product. In practice, it is impossible to produce a concentrate consisting of only one mineral.
• The third rule is a corollary of the second. It is impractical to recover all of the valuable minerals into the concentrate.

The typical sequence of operations in the process plant.

Comminution

• The comminution process actually begins during the mining stage

through the use explosives, excavators or scrapers for softer material.

This is necessary in order to generate a material that is transportable

by haul trucks or conveyors. Comminution in the mineral processing

plant is carried out in a sequential manner using crushers and screens

followed by grinding mills and classifiers. The various types of

comminution equipment including their general application are

described in detail below.

Crushing Equipment Primary Crushers – Jaw and Gyratory

• Within the crushing circuit, a primary crusher reduces material down to a size that can be conveyed and fed to the secondary crushing circuit. The two most common primary crushers used for coarse run-of-mine material are the jaw and gyratory crushers. These primary crushers break rock through compressive forces created by a hard moving surface forcing and squeezing the rocks towards a hard stationary surface.
• A Jaw Crusher reduces large rocks by dropping them into a flat “V” shaped space created between a fixed surface and a movable surface. The compression is created by forcing the rock against the stationary plate as shown in figure 3. The opening at the bottom of the jaw plates is the crusher product size gap. The rocks remain in the jaws until it is small enough to pass through this adjustabe gap at the bottom of the jaws.
• In a gyratory crusher, a round moving crushing surface is located within a round hard shell which serves as the stationary surface (figure 3). The crushing action is created by the closing the gap between the hard crushing surface attached to the spindle and the concave liners (fixed) mounted on the main frame of the crusher. The gap is opened and closed by an eccentric drive on the bottom of the spindle that causes the central vertical spindle to gyrate.

Secondary Crushers – Cone Crusher The most common type of secondary crusher is the cone crusher. A cone crusher is very similar to the gyratory but has a much shorter spindle with a larger diameter crushing surface relative to its the vertical dimension. The eccentric motion of the inner crushing cone is similar to that of the gyratory crusher. Impact Crushers

• Impact crushers involve the use of high speed impact rather than compression to crush material. They utilize hinged or fixed heavy metal hammers or bars attached to the edges of horizontal rotating disks. The bars repeatedly strike the material to be crushed. Then the material is thrown against a rugged solid surface which further degrades the particle size. Finally, the material is forced over a discharge grate or screen by the hammers through which the finer particles drop while larger particles are swept around for another crushing cycle until they are fine enough to fall through a discharge grid. This type of crusher is normally used on soft materials such as coal or limestone due to the high wear experienced by the impact hammers, bars and inner surfaces. These crushers are normally employed for secondary or tertiary crushing.

Roll Crushers

• Rolls crushers consists of a pair of horizontal cylindrical rollers through which material is passed. The two rollers rotate in opposite directions nipping and crushing material between them. These types of crushers are used in secondary or tertiary crushing applications. They are seeing a significant increase in use due to advances in their design and the improved liberation of minerals in the crushed product.

Semi-Autogenous Mill

Rod, Ball and Pebble Mills

• The products from AG or SAG mills typically feed secondary grinding mills with particles that range in size from 5 cm down to below 100 microns (0.1 mm). The final particle size is determined by downstream processing requirements. Grinding is carried out as a wet process with water content between 50 – 70% by weight.
• Rod mills are long cylinders filled with steel rods that grind by compressive forces and abrasion. The length of the cylinder is typically 1.5 to 2.5 times longer than the diameter. As the mill turns, the rods cascade over each other in relatively parallel fashion. One of the primary advantages of a rod mill is that it prevents over- grinding of softer particles because coarser particles act as bridges and preferentially take the compressive forces. Rod mills can take particles as coarse as 5 cm. Many of the newer operations tend to install ball mills in combination with SAG mills and avoid rod mills due the cost of the media, the cost of replacing rods and general maintenance costs. Many older operations have rod mills in combination with ball mills.

Rod, Ball and Pebble Mills

Size-Separation: Screening and Classification The size distribution of the particles must be controlled for a number of reasons at various stages of a mineral processing plant:

• To enable undersized material to bypass the crushing or grinding circuit and to retain oversized particles for further size reduction,
• To provide an optimum particle size material for efficient processing in the downstream separation and concentration systems, and
• To prepare product that meets particle size specifications required for the market place. There are two distinct methods for separation of particles based on size: screening and classification. Screening: In its simplest configuration, a screen is a hard perforated surface with a matrix of fixed dimension apertures. The material is presented to the screen surface so that material finer than the apertures falls through the screen and the oversize is conveyed to the discharge end of the screen. Screening is generally difficult below 0.5 mm. Classification: Classification techniques takes advantage of the principle that particles of the same density but of different sizes settle in a fluid at different rates. Exploiting the difference in the settling rates allows for separation based on size. Classification is usually carried out at particle sizes that are considered to be too fine for sorting efficiently by screening methods.