Mineral Processing Mineral Processing is the process where raw mined materials are refined to become useful components for industry. Image 1 is a picture of some of the many minerals that are mined and processed. Mineral Processing is involved in production of three main products: aggregates (stone and rock), minerals (gold, lead, and iron), and coal. Mineral Processing is also used in the recycling industry. There are 4 main steps in Mineral Processing: 1. Characterization, 2. Liberation, 3. Separation, 4. Disposition. Figure 1
Image from http://www.bgsd.k12.wa.us/hml/jr_cam/science/rocks/images/minerals.jpg
Characterization Characterization is the first step of mineral processing. It is when the material being processed is identified and its chemical/physical properties are noted. Characterization answers two basic questions: What am I processing? and What property can I use to help process the material?
In the Characterization step, Sampling is often performed. Sampling answers the first question; what am I processing? There are a variety of sampling techniques but each involves taking a representative sample which is analyzed to identify the components of the feed material. When the feed material is identified, important chemical/physical properties of the material can be identified. The most important property is size. Other important properties are density, magnetic susceptibility, and electrical conductivity.
Liberation Liberation is the 2nd step in the process where dissimilar elements in the feed material are freed from one another. This is often accomplished by size reduction. There are two classes of size reduction machines: Crushers and Grinders. Crushers deal with large particles usually particles greater than 1 mm diameter while grinders deal with particles smaller than 1mm diameter.
Crushers handle particles larger than 1mm diameter. Grinders handle particles smaller than 1 mm diameter. There are many types of crushers but all either crush by Compression or Impact. Particles are compressed or impacted until they break into smaller pieces. Typical crushers have a reduction ratio of 40/6. This means that if you put a 40 mm diameter particle in them they will give you a range of particles about 6 mm in diameter. Figure 2 is an image of a particular type of crusher called a Jaw Crusher. It uses compression to crush the material. Figure 2
Image from http://www.penncrusher.com/images/imagelib/content/PIC_jaw_crushers_1.jpg
As with crushers, there are a variety of grinders but they all use the same concept for grinding. They either have the feed particles smash against themselves or the feed particles smash against stronger material for size reduction. When the feed particles grind against themselves it is called an Autoginous Mill. When particles grind against a stronger material, it is called a Ball or Rod Mill; depending on the shape of the stronger material used. Image 3 is a picture of how a ball mill works. Figure 3
Image from http://www.ktf-split.hr/glossary/image/ball_mill.gif.
The mineral processing industry has created a Work Index Value to help understand the hardness of the feed material. This index assigns values to minerals based on their difficulty to grind. Therefore a high work index means the mineral is hard to grind. The coal industry also assigns values based on the coal’s hardness. Theses values are called the Hardgrove Grindability. However, in the Hardgrove Grindability Index higher values signify softer, easily ground coal.
Work Index Value-Higher Values are harder to grind. Hardgrove Grindability-Higher Values are softer and easier to grind.
Separation Separation is most important step of mineral processing. It is when the different components of the feed material are separated from each other. Separation uses chemical and physical properties to achieve this. Particles are most often separated by size, density, magnetic
susceptibility, and electrical conductivity. Aggregate mineral processing usually skips this step and continues directly to disposition. Size separation is primarily done by Screening. Figure 4 is an image of a series of different Mesh Size screens. Screening is often done in series, so multiple streams of different size ranges can be created. There are two systems of mesh sizing: Tyler and U.S. series. In both systems, larger mesh sizes correspond to smaller hole sizes.
Tyler Series- Larger Mesh Sizes mean smaller holes in the screen. U.S. Series- Larger Mesh Sizes mean smaller holes in the screen. Figure 4
Image from http://www.rtg.wa.edu.au/loanpool/belmont/sieves.jpg
Density separation is performed by Hydrocyclones and Dense Medium Separations. Dense Media Separation involves pouring the material into a specially prepared fluid that has a density between the components of the feed material. Material that floats has a lower density than the fluid while material that sinks has a density greater than the fluid. Figure 5 shows how particles react in a dense media separation. Hydrocyclones work on the same premise as dense media separators but the material and prepared fluid are spun around utilizing Centrifugal Forces. Feed material that comes out of the top of the hydrocyclone is the less dense material. Feed material that comes out the bottom of the hydrocyclone is the dense material. Figure 6 is an image of how a hydrocyclone works.
Figure 5
Image from http://www.coleparmer.com/techinfo/images/centrifugation-4.gif
Figure 6
Image from http://flotrend.com/images/hydrocyclone2.jpg
Hydroclones are used extensively with feed particles smaller than 100 micrometer diameter. Dense media separation is used with larger particles. Hydrocyclones are used with small particles because small particles need the centrifugal force to speed separation.
Dense Media Separation is used for particles 100 micrometers diameter and greater. Hydrocyclones are for particles 100 micrometers diameter and smaller. Magnetic susceptibility separation is done by exposing the feed material to a magnet. Feed material that is attracted to the magnet is magnetic and falls into one bin. Feed material that is unaffected by the magnetic is non-magnetic and falls into another bin. Magnetic materials include iron and magnetite. Figure 7 shows how the process of magnetic susceptibility separation.
Figure 7
Image from http://www.steinert.com.au/img/photos/magdrum.jpg
Electrical conductivity separation is achieved by using a High Tension Separator. A high tension separator charges the feed particles. Feed Particles that retain an Electrical Charge fall into one bin while those that do not fall into another bin. Depending on which bin the particle falls into it is classified as a Conductors or Resistor. Conductors retain an electric charge. Examples include iron, copper, silver and aluminum. Resistors do not hold an electric charge. Examples include soil, sand, plastic, and lead. Figure 8 shows how a high tension separator works. Figure 8
Image from http://www.freepatentsonline.com/6797908-0-large.jpg
Disposition Disposition is the final step of mineral processing. It involves the removal of waste in an environmental acceptable manner and the sale of the product.
Glossary Autoginous mill- a type of grinder where particles smash against each other causing a size reduction in the particles. Ball mill- a type of grinder where particles smash against a harder sphere-shaped material causing a size reduction in the particles. Centrifugal force- the apparent force associated with rotation that pushes objects away from the center of rotation Characterization- the first step of mineral processing where the material being processed is identified and its chemical/physical properties are noted. Compression- the force associated with a particle being “squeezed.” Conductor- a material that retains an electrical charge. Crusher- a name given to a group of machines in mineral processing that reduce the size of 1 mm diameter and larger feed particles. Dense Media Separation- a process where materials are separated by density using a special created fluid. Disposition is the final step of mineral processing. It involves the removal of waste in an environmental acceptable manner and the sale of the product. Electrical Charge- when a material holds a surplus of negatively charged electrons. Grinders- a name given to a group of machines in mineral processing that reduce the size of 1 mm and larger feed particles. Hardgrove Grindability- a system of measuring the hardness of coal where higher values signify softer material. High Tension Separator- a machine that uses electricity to separate particles by electrical conductivity. Hydrocyclone- a machine that separates based on density. It does this by utilizing the centrifugal force associated with rotation. Impact- a force caused by a particle “smashing” into a hard surface. Particles experience this force while they rapidly decelerate by hitting a hard immovable surface.
Liberation- the 2nd step of mineral processing where dissimilar elements in the feed material are freed from one another. Mineral Processing- the process where raw mined materials are refined to become useful components for industry. Mesh size- a value corresponding to the size of the holes in screens. Resistor- a material that does not hold an electrical charge. Rod mill- a type of grinder where particles smash against a harder cylinder-shaped material causing a size reduction. Sampling- a test where a small portion of a material is taken. The small portion is then analyzed. Screening – a process where material is poured threw a wire mesh. Feed particles that are retained on the screen are larger than the holes in the screen. Feed particles that pass through the screen are smaller than the holes in the screen. Separation- the 3rd step of mineral processing when the different components of the feed material are separated from each other. Tyler series- a group of meshes that have varying hole sizes. Larger mesh sizes correspond to smaller openings in the mesh. U.S. series- a group of meshes that have varying hole sizes. Larger mesh sizes correspond to smaller openings in the mesh. Work Index Value- a system of measuring material hardness where higher values signify harder material
Source M.S. Klima, F.F. Aplan, and P.T. Luckie. Elements of Mineral Processing: Lecture Notes. The Pennsylvania State University. ProCopy CourseWorks. 2010.
