Understanding Iron Ores and Mining of Iron Ore

Understanding Iron Ores and Mining of Iron Ore

Iron (Fe) is an abundant and a widely distributed element in the in the crust of the earth, constituting on an  average ranging from 2 % to 3 % in sedimentary rocks to 8.5 % in basalt and gabbro. Its supply is essentially limitless in almost all regions of the world.

However, most of this iron is not in a form which can be used in current iron making practices. Hence only that part of the total iron in the crust of the earth which is available to the steel industry both economically and spatially, may correctly be termed iron ore. However, what constitutes iron ore varies widely from place to place and time to time. There are many factors which determine whether iron bearing mineral can be classified as an iron ore, but basically it is a question of economics. Keeping this concept in mind, a logical definition of iron ore for commercial purposes is ‘iron bearing material that can be economically used at a particular place and time under then current cost and market price conditions.’

Because iron is present in many areas, it is of relatively of low value and thus a deposit must have a high percentage of Fe to be considered ore grade. With the advent of improved methods of beneficiation, concentration and agglomeration, the variety of iron bearing materials that can now be used has been broadened and many low grade material types which were once considered uneconomic, are now being considered as iron ore. Typically, a deposit must contain at least 25 % Fe to be considered economically recoverable.

Over 300 minerals contain iron but five minerals are the primary sources of iron ore. They are (i) magnetite (Fe3O4), (ii) hematite (Fe2O3), (iii) goethite (Fe2O3.H2O),  (iv) siderite (FeCO3), and (v) pyrite (FeS2). The first three are of major importance because of their occurrence in large economically minable deposits. However most of the iron ore mined around the world falls under the first two categories.

Hematite has a chemical composition of Fe2O3 corresponding to 69.94 % iron and 30.06 % oxygen. It has a colour from steel gray to dull red or bright red, can be either earthy, compact or crystalline, and has a specific gravity of 5.26. Common varieties are termed crystalline, specular, martite (pseudomorphic after magnetite), maghemite (magnetic ferric oxide), earthy, ocherous, and compact. Hematite is one of the most important iron ores. It has a wide occurrence in many types of rocks and is of varying origins. It occurs associated with vein deposits, igneous, metamorphic, and sedimentary rocks, and as a product of the weathering of magnetite. Some low grade deposits of disseminated crystalline hematite have been successfully treated by both gravity and flotation techniques to produce high quality concentrates.

Magnetite has a chemical composition of Fe3O4, corresponding to 72.36 % iron and 27.64 % oxygen. It has a colour of dark gray to black, and a specific gravity, 5.16 to 5.18. It is strongly magnetic, sometimes possessing polarity so it will act as a natural magnet. Magnetite occurs in igneous, metamorphic, and sedimentary rocks. The magnetic property of magnetite is important, for it permits exploration by magnetic methods and makes possible the magnetic separation of magnetite from gangue materials to produce a high quality concentrate. It has become increasingly important as a source of iron because of the continued improvements in magnetic concentration techniques and in the expanded use of the high grade products.

Mining of iron ores

Mining (extraction), beneficiation, and processing of iron ore produces iron and steel. Mining is defined as removing ore material from a deposit and encompasses all activities prior to beneficiation. High grade iron ores (iron content greater than 62 %) are simply crushed, screened and shipped directly to the iron making unit (e.g. blast furnace). Low grade iron ores containing lower iron content need beneficiation activities other than crushing, screening and washing for increasing their iron content. To be competitive, iron ore mining need to be done on a very large scale. There are two mining methods which are commonly employed. They are (i) surface mining, (ii) underground or shaft mining.

The decision to employ underground or surface mining techniques is dependent on the proximity of the ore body to the surface.  Majority of Iron ore mined around the world,  is exclusively by surface mining techniques. However, there are also some operating underground iron mines in the world. There are two basic surface mining methods designed to extract iron ore from surface deposits. These are open pit and open cut methods. Nearly all the large iron ore mines in the world, with the exception of a few, are being worked by open pit methods.

The process of mining iron ore requires massive resources. These resources  include heavy industrial mining equipment and a skilled labour force. The equipment used includes bulldozers, scrapers, haul trucks (heavy haulers), dumpers, front end loaders, hydraulic excavators, hydraulic and electric shovels ( stripping shovels, loading shovels), draglines, bucket wheel excavators, dredges, water tankers, blast hole drills (diamond bit rotary drills), and heavy duty conveyors. Besides crushers and screens with associated equipment are needed for crushing and screening of ore to merchantable product.

Planning and development

The process of mining of iron ore from discovery of an iron ore deposit body through extraction of iron ores and finally to returning the land to its natural state 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 to estimate the size and grade of the deposit.

Mining iron ore begins at ground level. Ore is identified by diamond drilling core samples on a grid several meters into the earth. iron ore rock comprises a substantial percent of iron and the rest is impurities such as alumina and silica. These samples are analyzed and categorized so that mining engineers can accurately develop a mine plan.

The development of the mine includes mine planning to evaluate economically recoverable portion of the deposit, the metallurgical characteristics of the ores, ore recoverability, engineering concerns, crushing, and infrastructure requirements. The proportion of a deposit that is economically recoverable is dependent on the enrichment factor of the ore in the area.

To gain access to the mineral deposit within an area it is often necessary to mine through or  remove waste material which is not of immediate 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 body. Waste removal and placement is a major cost to the mining operation.

The general occurrence, size and shape of an iron ore deposit is determined during the exploration phase. Knowledge of the deposit is determined in more detail through development work. It is often necessary during the development of a mine to determine, in considerable detail, the position and nature of geological structures which affect ore distribution and availability.

After sufficient detailed information is obtained, various combinations of operating plans are studied using maps and sections prepared for this purpose. These show the size and shape of the ore body, ore compositions and laboratory test results. From these graphic representations, the quantities of ores and waste materials are determined by the application of volume weight factors. Computers are commonly used in the preparation of tonnage estimates and in the preparation of detailed mining plans. Through the use of these systems, comparative evaluations of various mining methods and plans are made to determine the most favorable plan for each particular deposit and to schedule the mining of the deposit.

Accordingly, it is necessary to plan the mine production so as to generate a steady stream of iron ore whose composition is consistently close to the target grades. This need drives the process of developing an exploration prospect into a viable mining project. Because the composition of the ore deposit can only be coarsely sampled prior to mining, and because the economic and financial conditions tend to be quite volatile, any mine plan is at best tentative, subject to revision in the light of changing knowledge about the ore deposit as it is progressively exposed during mining operations.

The mine development and planning does not cease when production begins, because of the need to respond to changing conditions as a result of the unfolding of knowledge about the ore body, generated as more drill holes provide more assays distributed over the ore body’s volume as it is mined.

The planning, development and operation of an open pit mine are usually based on a rectangular block model. This block model comprises a set of rectangular blocks, with dimensions corresponding to the smallest mineable unit, says 50 meters square horizontally by 10 meters vertically. For each block, estimates are made of the grade (iron, plus each of the contaminants such as alumina, silica and phosphorus).

The block model is an evolving and adaptive information system. It is initially based upon the interpolation of data from samples taken during exploratory drilling. During the development and operation of the mine the block model is continuously revised by in-fill drilling, data from blast holes drilled for placing explosives, and from the assays of mined ore as it is crushed and analyzed. At any stage of operation, mining selection and sequencing decisions are to be based upon the imperfect block model information currently available, so as to produce ore for shipment that matches target grade within specified tolerances.

Open pit mining

Surface mining methods are designed to extract ore from surface deposits. The ore deposit is exposed by stripping away a layer of earth, sometimes only a few meters thick. This material which need to be stripped is known as ‘overburden’. Overburden may consist of unconsolidated material, rock, clay, gravel, and lean ore material. The depth to which open pit mining is carried depends upon the grade of the ore, the nature of the overburden and the stripping ratio. The stripping ratio is the amount of overburden and waste that has to be handled for each unit of ore mined.  It is described as the unit of the overburden that must be removed for each unit of crude ore mined. Stripping ratios increase with the quality of the ore being mined and cost factors related to beneficiation and transportation.

The economic stripping ratio varies widely from mine to mine, depending upon a number of factors. In the case of direct shipping ores, it may be as high as 6:1 or 7:1. In the case of low grade ore , a stripping ratio of less than 1.5:1 is often considered as an economic limit.

For mining of the iron ore, it is essential to uncover the mine area by first stripping of the overburden. The overburden is loaded by large hydraulic shovels into production trucks, which haul it to contour dumps. These dumps are environmentally designed to match the surrounding area.

In open pit mining, removal of overburden may continue through a large part of the life of a mine as the pit walls are cut back to permit deepening of the mine to recover ore in the bottom. Unconsolidated materials are excavated by power shovels, draglines or power scrapers, depending on local conditions. Other materials are generally excavated with power shovels.

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 ore is removed. After the soil and overlying rock are cleared, the ore is drilled and blasted. The object of blasting is to expose the ore body for extraction or to create approach (horizontal passages) that can be used to access the ore body. Blasting is also used to break up ore.

Drilling and blasting is done to break consolidated materials into sizes capable of being handled by mining equipment as well as crushing and screening facilities. It is also sometimes done to loosen ore banks ahead of power shovels to increase the efficiency of loading.

The portion of the ore body to be removed is first drilled in a specific pattern. The drilling is conducted with large mechanized drilling rigs.  The chief 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. Typically the drilled blast holes are 400 mm in diameter and around 10-12 meter deep. Around 400 holes are drilled in a blast pattern.

Before the blast the drilled holes are filled with explosive mixtures. The main requirement for an explosive to be used in mine blasting is the ability to achieve complete combustion without an external oxygen supply. In the past, explosives used in blasting were comprised of nitroglycerine, carbonaceous material, and an oxidizing agent. Today, the most common explosives used are mixtures of ammonium nitrate fertilizer and fuel oil (called ANFO). The explosive is detonated by a high explosive blasting cap and/or primer. In some cases, emulsion or gel explosive cartridges are used.

Once prepared, the mine site is cleared of workers and equipment, and the blast is detonated. Each of the holes is detonated just a millisecond apart, resulting in a pile of crude iron ore that is broken apart to a minus 2 m x 2 m size. The wide holes in the ground created by drilling, blasting, and ore removal are referred to as ‘open pits’.

Following blasting, the fractured ore is known as run of mine (ROM) ore. ROM ore is loaded by huge electrical shovels, hydraulic excavators, or front-end loaders onto large capacity dump trucks, which haul it to crushing and screening station.

Crushing and screening

Iron ore of merchantable grade must be properly sized prior to charging to the blast furnace. Present blast furnace technology commonly requires crushing and screening of direct charge lump ore finer than 10 mm and coarser than 30 mm. The specific size selected is based on the characteristics of the ore and is specified so as to maintain high stack permeability and also allow sufficient time for reduction of coarser material. Consequently, crushing and screening are an integral part of ore producing facilities.

Many mines employ two to three stages of crushing. Some mines have the primary crusher located in the mine, using conveyors to transport the crushed ore to the secondary and tertiary crushers or directly to the mills. The crushing stages will reduce the iron ore from several feet in diameter at the primary stage to six inches down to one-half or three-eighths of an inch as a final product. The crusher product is fed to the milling operation for further size reduction

Crushing commonly involves a primary jaw crusher with secondary crushers operating in closed circuit with vibrating screens. Equipment selection is determined largely by the friability of the ore. Most of the screening operations on high grade ores are dry except when the fines fraction can be effectively upgraded by de-sliming.

The minus 10 mm fines produced by crushing and screening are most commonly agglomerated by sintering, or sometimes ground for pelletizing.

The mining program at individual mines is developed to produce a uniform product. Although there are multiple handling steps involved in most loading and shipping systems they do not often provide enough mixing to meet quality assurance standards now required by the steel plants, especially if both size consistency and chemistry standards are specified. Sophisticated combined blending and load out facilities are now almost universally provided in the iron ore mines.

Stacking and reclaiming systems are used quite often. Stacking results in layering of the iron ores. Each successive layer represents an iron ore that may differ in size consistency or chemical composition from adjacent layers. The elongated pile is built up to a height limited by the stacking capability of the stacking machine. The ore may then be reclaimed for use by bucket wheel excavators, frontend loaders, or a scraper cross conveyor. Removal of ore from the face of the pile results in a stream of material that is a uniform blend of ore from all the layers. The blended ore is then dispatched to the customers.

The various steps in mining are shown in Fig 1.

Steps in open pit mining of iron ore

Fig 1 Steps in open pit mining of iron ore

Environmental issues

Materials generated as a result of open-pit mining include overburden, waste rock, and mine water. Other wastes may include small quantities of oil and grease spilled during extraction. Mine water usually contain 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, sulfur, titanium, and zinc.

Water causes a variety of problems in iron ore mining operations. Except in rare instances, such as in hilltop mining or mining under desert conditions, water must be collected in sumps, wells or underground workings and pumped out of the mine. Such drainage water is often utilized directly to make up for water losses in concentration operations.