Mining of Iron Ores
Mining of Iron Ores
Iron ore is a type of mineral and rock from which metallic iron is extracted economically. This ore is normally rich in iron oxides and vary in color from dark grey, bright yellow and deep purple to rusty red. The iron itself is usually found in the form of magnetite (Fe3O4), hematite (Fe2O3), goethite (FeO(OH)), limonite (FeO(OH).n(H2O)) or siderite (FeCO3).
The process of mining consists of discovery of an iron ore deposit through extraction of iron ore and finally to returning the land to its natural state. It consists of several distinct steps. The first is discovery of the iron ore deposit which is carried out through prospecting or exploration to find and then define the extent, location and value of the ore body. This leads to a mathematical resource estimation of the size and grade of the deposit. Exploration and evaluation consist of identification and quantification of ore bodies by using a range of geological, geophysical and metallurgical techniques. In its simplest forms exploration involves drilling in remote areas to sample areas. The data from exploration activities is logged, mapped, analyzed and interpreted often by using models.
After the ore body has been evaluated, a detailed plan for mining is developed. This detailed plan identifies which ore bodies are to be mined and in what sequence in order to deliver the required iron ore product at an appropriate cost. The process of mine planning is an important step before the start of mine development and it continues on day to day basis once the mine becomes operational.
To gain access to the iron ore deposit within an area, it is often necessary to mine through or remove waste material (also known as overburden) which is not of an interest. The total movement of ore and waste constitutes the mining process. Often more waste than ore is mined during the life of a mine, depending on the nature and location of the ore deposit. Removal and placement of overburden is a major cost in the mining operation.
Mining techniques usually consist of two types based on excavation methods. These are surface mining and underground mining (sub surface mining). Iron ore is almost exclusively mined by surface operations. The most predominant surface mining methods used for mining of iron ores are open pit mining methods and open cut mining methods. However a few underground iron ore mines are also in operation around the globe. The decision for employing surface mining or underground mining is dependent on the proximity of the ore body to the surface. Open pit and open cut mining are the least expensive techniques for the ore extraction. Overburden and stripping ratios are important in determining whether a deposit is to be mined. The stripping ratio describes the unit of overburden that must be removed for each unit of crude ore mined. A typical open pit iron ore mine is shown in Fig 1
Fig 1 A typical open pit iron ore mine
On the basis of mining methods, iron ore mining can be broadly divided into two categories namely manual mining and mechanized mining. Large iron ore mines are mechanized mines while manual mining methods are employed in the small mines.
Manual mining method is normally limited to float ores. Mining of reef ore is also being done manually on a small scale. The float ore area is dug up manually with picks, crow bars, and spades, and then the material is manually screened to separate + 10 mm float ore which is then stacked up. The waste is thrown back into the pits. As regards to the reef ores, holes of 0.6 m deep and 30-40 mm diameters are drilled with hand held jackhammers operated with portable compressors. These holes are with a spacing of about 0.6 m and each hole is charged with 150-200 grams of gun powder or gelatin cartridges. The blasted broken ore is manually screened, stacked for loading in dumpers for dispatch.
Most of the large iron ore mines are mechanized mines. In these mining is done to extract iron ore from surface deposits. In these mines all the operations are mechanized and mining is invariably done through systematic formation of benches by drilling and blasting.
The recovery of material is done from an open pit in the ground. Overburden consisting of surface vegetation, soil and rock material is removed (stripped) to reach buried ore deposits. Overburden is continually removed during the life of the mine as the high wall is cut back to permit deepening of the pit. In the process of mining ore benches are developed for the purpose of drilling, blasting and hauling of the ore to the crushing plant. The height of the benches is dependent on several factors such as output requirement, shape, size and depth of occurrence of ore body, geological disturbance suffered by the ore body, hardness and compactness of the ore body, type and the size of deployed for the mining operations etc. The length of the face is dependent on various factors such as contours of deposit, output required, variation in grade, blending requirements and capacity of loading machinery etc. The width of the bench is governed to a large extent by the size of the largest machinery deployed.
Drilling in the benches of open pit mining is done for production of iron ore with mechanized drills specific for each mining method. The main objective of drilling operations is to create a hole of suitable diameter, depth and direction in rock for explosives to be placed for blasting activities. The drilling of the holes is done normally in a particular pattern which depends on the bench height, the diameter of the hole, the drilling machinery deployed, nature of rock and the types of explosives used. The blast holes are usually vertical but can be inclined for obtaining better blasting results.
The objective of blasting is to espose the ore body for extraction as well as to break up the ore. The main requirement for an explosive to be used in mine is the ability to achieve complete conmbustion without an external oxygen supply. Explosive materials used during blasting during earlier days are mainly nitoglycerine, carbonaceous material and an oxidizing agent. Sometimes emulsion or gel explosive cartridges are used as explosive material. However these days, the most common explosives used are mixture of ammonium nitrate and fuel oil (ANFO). The explosives are detonated by a high explosive blasting cap and/or primer. Now a days nonel is used as detonator. Nonel is shock tube detonator designed to initiate explosions. Instead of electric wires, a hollow plastic tube delivers the firing impulse to the detonator, making it immune to most of the hazards associated with stray electrical current. It consists of a small diameter, three-layer plastic tube coated on the innermost wall with a reactive explosive compound, which, when ignited, propagates a low energy signal, similar to a dust explosion. The reaction travels at approximately 2,000 m/sec along the length of the tubing with minimal disturbance outside of the tube. The design of nonel detonators incorporates patented technology, including the cushion disk (CD) and delay ignition buffer (DIB) to provide reliability and accuracy in all blasting applications
Areas for open pit mining are selected using the mining plan. Identified areas are then tagged. The ore is mined from large open pits by progressive extraction along steps or benches. The benches provide access to progressively deeper ore, as upper level is removed. After the soil and overlying rock are cleared, the ore is drilled and blasted. The portion of the ore body to be removed is first drilled by using rigs in an appropriate pattern. The drilled holes are then loaded with explosive mixtures, charged and blasted. The resulting blast breaks the material to a size required for digging.
Following blasting, the broken ore is loaded for transport by shovels, excavators or front end loaders onto large dump trucks for its transportation to the crushing and washing plant. The transportation is facilitated by maintaining mine haul roads. Overland belt conveyors are used to transport the ore material where there are long distances between the pits and crushing plants and between the crushing plants and loading sites. The wide holes in the ground created by drilling, blasting and ore removal are referred as ‘open pits’.
Crushing and washing of ores
In crushing and washing plants the processing of the ore is carried out. Processing of the ore ranges from simple crushing and screening and separating various size fractions of the ore to the processes that beneficiate or upgrade the quality of the iron ore products. This is done by physical processes, which remove impurities by difference in particle density or size gravity or size separation. Processing may be wet or dry. Further ore handling, washing and screening operations are mechanized in the crushing and washing plants.
The processed ore is stockpiled and blended to meet product quality requirements before reclaimed and loaded either in railway wagons or dumpers for dispatch to the customers.
Waste materials generated as a result of open pit mining include overburden, waste rock, and mine water containing suspended solids and dissolved materials. Other waste materials may include small quantities of oil and grease spilled during extraction. Mine water contains dissolved or suspended constituents similar to those found in the ore body itself. These may include traces of aluminum, antimony, arsenic, beryllium, cadmium, chromium, copper manganese, nickel, selenium, silver, sulphur, titanium and zinc etc.