- Chemical metallurgy is the branch of metallurgy that deals with the extraction of metals from naturally occurring compounds and their refinement to levels of purity suitable for commercial use.
- Primary Metals: While iron is the most used metal, it lacks important properties such as corriosion resistance. From the beggining of the nineteeth century, copper, nickel, lead, zinc and tin and their alloys found use as substitutes for iron because they had properties cast iron and steel were lacking.
- Secondary Metals: This group includes the metals cadmium, cobalt, and mercury and the metalloids arsenic, antimony, and bismuth. They are mainly by-products of the primary metals but also form their own deposits.
- Light Metals: These are beryllium, alumnium, magnesium and titanium. They are used in pure state and in alloys, characterized by light weight and high strength hence they are valuable materials of construction.
- Refractory Metals: This group of metals in composed of the transition metals tugsten, molybdenum, noibium, tantalum, titanium, zirconium, halfnium, vanadium, rhenium, and chromium. All these metals have high melting points. They are mainly used as alloying elements in steel but also are used in the elemental form. Some resist high temperature wihtout oxidation, and some are very hard, having excellent wear and abrasion resistance.
- Scattered Metals: This group of metals and metalloids is composed of scandium, germanium, gallium, indium, thallium, hafnium, rhenium, selenium and telluriuum. They do not form minerals of their own but occur in very small amounts in the ores off other common metals. Thus, gallium occurs with aluminium in bauxite, selenium and tellurium in copper and nickel sulfide ores etc.
- Ferroalloy Metals: This groups of metals is composed of chromium, manganese, silicon, and boron. They were mainly used as alloying elements to steel in form of ferroalloys but now are also used in the elemental form.
- Minerals: Naturally occuring solid crystalline consisting of one or more metals in association with nonmetals whose composition vary withing certain limit. It has well defined physical properties.
- Ores: Aggregate of minerals from which one or more metals or minerals may be extracted economically, with profit, and on a large scale. An ore may have wide variation in composition and in physical and chemical properties. Whe the percent of metal is too low for profitable extractiong the rock ceases to be an ore.
- Gangue: Some minerals in ore that are not useful from the points of view of metal extraction are known as gangue.
- The process of separating two or more minerals, into two or more products , each of increased value is called Ore Dressing.
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Mineral processing involves the use of physical processes to manipulate ore particule size, and concentrate valuable minerals using the processes of separation based on properties of the ore such as density, chemical composition, electrostatic , magnetic or surface properties.
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Liberation of the valuable minerals from the gangue is accomplished by comminution, which involves crushing, and if necessary, grinding , to such a particle size that the product is a mixture of relatively clean particles of mineral and gangue.
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An intimate knowledge of the texture of the ore is essential for the efficient mineral processing.
- **Relativerly coarse grain size, and compact morphology of chromite **grains makes liberation from olivine gangue fairly easy
- Liberation of Chalcopyrite is fairly difficult due to "chain like distribution"
- Separate "clean" concentrates of lead and zinc will be difficult to produce in Galena and sphalerite intergrown. Contamination of concentrates with other metal is also likely.
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The grade or assay usually refers to the content of the marketable end product in the material. In metallic ores the % metal is often quoted as grade.
- For very low-grade ores (e.g gold) metal content may be expressed as ppm (equivalent to grams per tonne)
- Some metals are sold in oxide form, and hence the grade may be quoted in terms of the marketabel oxide content.
- In non-metallic operations, grade usually refers to the mineral content , e.g %CaF2 in fluorite ores. Diamond ores are usually graded in carats per 100 tonnes (1 carat is 0.2g)
- Coal is graded according to it's ash content
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The recovery in the case of the concentration of a metallic ore, is the percentage of the total metal contained in the ore that is recovered from the concentrate.
- A recovery of 90% means that 90% of the metal in the ore is recovered in the concentrate and the 10% is lost in the tailings.
- The recovery, when dealing with non-metallic ores, referes to the percentage of the total mineral contained in the ore that is recovered into the concentrate.
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The ratio of concentration is the ratio of the weight of the feed to the weight of the concentrates.
It is the measure of the efficiency of the concentration process, and closely related to the grade of the concentrate; the value of the ratio of concentration will generally increase with the grade of concentrate.
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Solid Split is the inverse of ratio of concentration.
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The enrichment ratio is the ratio of the grade of the concentrate to the grade of the feed, and again is related to the efficiency of the process.
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[!SLIDE 19]
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Thus, a 30% solids slurry by mass means 30g (or kg or tonnes) of solidsin 70g (or kg or tonnes) of water
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- A crusher is a machine designed to reduce large solid chunks of raw material into smaller chunks.
- Crushers are commonly classified by the degree to which they fragment the starting material, with:
- coarse crushers not reducing it by much
- intermediate crushers fragmenting it much more significantly
- grinders reducing it to a fine powder
- The two best known types of coarse crushers are:
- jaw crusher
- gyratory crusher
- The crushers are normally fed with rocks, up to about 1 meter is size, while the grinders are usually fed with rocks crushed down to a maximum size of about 50mm.
- The mechanisms of size reduction during crushing and grinding are different. The chief difference being that in crushing operations, the size reductions is more by compression and impact and less by attrition while in grinding, the forces of attrition are much greater.
- In an open circuit operation, feed material is only run through a crusher once. Whereas in a closed circuit operation, the cooarser fraction is collected and re-crushed in the same unit until it's of a size that will pass through the product screen.
- Jaw crusher consists of a set of vertical jaws, one jaw being fixed and the other being moved back and forth relative to it. The jaws are further apart at the top than at the bottom, forming a tapered chute so that the material is crushed progressively smaller and smaller as it travels downward until it is small enough to escape from the bottom opening. The movement of the jaw can be quite small, since complete crushing is not performed in one stroke.
- Gyratory crusher (or cone crusher) is similar in basic concept to a jaw crusher, consisting of inner and outer vertical crushing cones; the outer if oriented with it's wide end upward and the inner cone is inverted relative to the outer with it's apex upward. The inner cone has a slight circular movement, but does not rotate.
- Impact crusher: Hammer mills involve the use of impact rather than pressue to crush material. They utilize heavy metal bars attached to the edge of horizontal rotating disks by hinges, which repeatedly striker the material to be crushed. The material is contained withing a cage, with opening on the bottom of the desired size to allow pulverized material to escape. This type of crusher is usually used with soft material such as coal.
- Roller crusher: Intermediate crusher consists of a pair of horizontal cylinderical roller through which material is passed. The two rollers rotate in opposite directions, "nipping" and crushing material between them.
- A similar type of intermediate crusher is the edge runner, which consists of a circular pan with two or more heavy wheel known as mullers rotating within it; material to be crushed is shoved underneath the wheels using attached plow blades.
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Grinders are for fine particle size reduction through attrition and compressive forces at the grain size level.
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Ball Milling: A slightly inclined or horizontal rotating cyclinder is partially filled with balls, usually of stone or steel, which grinds material to the necessary fineness by friction and impact with the tumbling balls. The feed is at one end of the cylinder and discharge is at the other.
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Rod milling: A rotating drum causes friction and attrition between steel rods and ore particles.
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SAG milling: A rotating drum throws large rocks and steel balls in a cataracting motion which causes impact breakage of larger rocks and compressive grinding of finer particles. Attrition in the charge causes grinding off finer particles.
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Autogenous milling: Same as SAG mills but no steel balls are used.
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Autogenous milling means a process in which the size of the constituent pieces of a supply of rock is reduces in a tumbling mill purely by the interaction of the pieces, or by the interaction of the pieces with the mill shell, no other gridning mediuum being employed.
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Pebble milling: A rotating drum causes friction and attrition between rock pebbles and ore particles. May be used where product comtamination by iron from steel balls must be avoided.
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High Pressue Grinding Rolls: The ore is fed between two rollers which are pushed firmly together whille their rotating motion pushed the ore through a small gap between them. Extreme pressure causes the rocks to fracture into finer particles and also causes microfacturing at the grain size level.
It consists of a pair of horizontal cylindrical rollers through which material is passed. The two rollers rotate in opposite directions. A similar type of intermediate crusher is the edge runner, which consists of a circular pan with two or more heavy wheels know as mullers rotating within it; material to be crusshedis shoved underneath the wheels using attached plow blades.
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Buhrstone mill: Another type of fine grinder; similar to old-fashioned flour mills.
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The two main features of interest for designing a plant for size reduction are:
- The power required for size reduction - is the sum of the work required to crush or grding the rock as well as rotate the mill.
- The choice of crushers and grinders
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The oldest theory, Von Rittinger (1867), stated that the energy consumed in size reduction is proportional to the area of new surface produced.
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The second oldest theory, Kick (1885), stated that the energy required is proportional to the reduction in volume of the particles.
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Often referred to as the “third theory”, Bond (1952) stated that the energy input is proportional to the new crack tip length produced in particle breakage.
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The rule of thumb to modify the critical speed to suit dry and wet conditions of milling is:
- For dry grinding, multiply by a factor of 0.65
- For wet grinding, mutliply by a factor of 0.70
