Process engineering and chemistry

Concentration of ore

Unwanted rocks, sand and grit from the mineral ore are called gangue or matrix. These have to be removed so that the mineral ore is concentrated with higher percentage of metal. Ores are mined from deep within the earth’s crust in the form of rocks. The minerals are embedded in these rocks. The rocks are first crushed into smaller pieces by crushers. Then they are ground to powder by ball mill and other processes so that powdered ore is obtained. Depending on the type of ore, hydraulic washing, froth floatation process, magnetic separation and chemical separation techniques are applied for concentrating an ore.

i) Hydraulic washing : As the name suggests, hydraulic washing process is done by washing the ores with streams of water. If an ore is heavier or denser than the gangue, then the gangue particles are washed way with the stream. The heavier or denser ore particles remain behind and can be collected. Hydraulic washing is done for ores that have tin or lead, as they are found to be heavier than the gangue.

ii) Froth-floating process : This process is used for sulphide ores. Oils can wet sulphides. Oil floats on water. Sulphide ores are first ground to powder and water is added. Then pine oil is added and the emulsion is agitated by passing compressed air. Oil and froth float on the surface along with the sulphide ore. The gangue particles being insoluble in oil remain at the bottom of the water tank. The froth is removed and allowed to settle down. This is called the froth-floating process. This process is used for sulphide ores of Cu, Pb and Zn.

iii) Magnetic separation : This method of concentration can be applied when the gangue and the ore particles have different magnetic properties. For example if the ore particles are magnetic in nature and if the gangue particles are non-magnetic, then a strong magnet can be brought and the magnetic ore particles can be sucked out from the powdered ore. The figure below shows how magnetic separation is done. The powdered ore is poured over a conveyer belt. One of the rollers of the belt is made out of magnet. The magnetic roller makes the magnetic ore particles stick on the belt and these are moved at a distance before they are collected. The gangue particles, being non-magnetic in nature, do not get attracted to the roller and fall in a heap below the roller itself. Iron ores like magnetite, chromite, and manganese ore like pyrolusite are concentrated by this process.

A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally iron.

In a blast furnace, fuel, ore, and flux (limestone) are continuously supplied through the top of the furnace, while air (sometimes with oxygen enrichment) is blown into the lower section of the furnace, so that the chemical reactions take place throughout the furnace as the material moves downward. The end products are usually molten metal and slag phases tapped from the bottom, and flue gasesexiting from the top of the furnace. The downward flow of the ore and flux in contact with an upflow of hot, carbon monoxide-rich combustion gases is a countercurrent exchange process.

Blast furnaces are to be contrasted with air furnaces (such as reverberatory furnaces), which were naturally aspirated, usually by the convection of hot gases in a chimney flue. According to this broad definition, bloomeries for iron, blowing houses for tin, and smelt mills for lead would be classified as blast furnaces. However, the term has usually been limited to those used for smelting iron ore to produce pig iron, an intermediate material used in the production of commercial iron and steel.

Modern process

Blast furnace placed in an installation
1. Iron ore + limestone sinter
2. Coke
3. Elevator
4. Feedstock inlet
5. Layer of coke
6. Layer of sinter pellets of ore and limestone
7. Hot blast (around 1200 °C)
8. Removal of slag
9. Tapping of molten pig iron
10. Slag pot
11. Torpedo car for pig iron
12. Dust cyclone for separation of solid particles
13. Cowper stoves for hot blast
14. Smoke outlet (can be redirected to carbon capture & storage (CCS) tank)
15: Feed air for Cowper stoves (air pre-heaters)
16. Powdered coal
17. Coke oven
18. Coke
19. Blast furnace gas downcomer

Blast furnace diagram
1. Hot blast from Cowper stoves
2. Melting zone (bosh)
3. Reduction zone of ferrous oxide (barrel)
4. Reduction zone of ferric oxide (stack)
5. Pre-heating zone (throat)
6. Feed of ore, limestone, and coke
7. Exhaust gases
8. Column of ore, coke and limestone
9. Removal of slag
10. Tapping of molten pig iron
11. Collection of waste gases

Modern furnaces are equipped with an array of supporting facilities to increase efficiency, such as ore storage yards where barges are unloaded. The raw materials are transferred to the stockhouse complex by ore bridges, or rail hoppers and ore transfer cars. Rail-mounted scale cars or computer controlled weight hoppers weigh out the various raw materials to yield the desired hot metal and slag chemistry. The raw materials are brought to the top of the blast furnace via a skip car powered by winches or conveyor belts.

There are different ways in which the raw materials are charged into the blast furnace. Some blast furnaces use a "double bell" system where two "bells" are used to control the entry of raw material into the blast furnace. The purpose of the two bells is to minimize the loss of hot gases in the blast furnace. First, the raw materials are emptied into the upper or small bell which then opens to empty the charge into the large bell. The small bell then closes, to seal the blast furnace, while the large bell rotates to provide specific distribution of materials before dispensing the charge into the blast furnace. A more recent design is to use a "bell-less" system. These systems use multiple hoppers to contain each raw material, which is then discharged into the blast furnace through valves. These valves are more accurate at controlling how much of each constituent is added, as compared to the skip or conveyor system, thereby increasing the efficiency of the furnace. Some of these bell-less systems also implement a discharge chute in the throat of the furnace(as with the Paul Wurth top) in order to precisely control where the charge is placed.

The iron making blast furnace itself is built in the form of a tall structure, lined with refractory brick, and profiled to allow for expansion of the charged materials as they heat up in the furnace during their descent, and subsequent reduction in size as melting starts to occur. Coke, limestone flux, and iron ore (iron oxide) are charged into the top of the furnace in a precise filling order which helps control gas flow and the chemical reactions inside the furnace. Four "uptakes" allow the hot, dirty gas high in carbon monoxide content to exit the furnace throat, while "bleeder valves" protect the top of the furnace from sudden gas pressure surges. The coarse particles in the exhaust gas settle in the "dust catcher" and are dumped into a railroad car or truck for disposal, while the gas itself flows through aventuri scrubber

The "casthouse" at the bottom half of the furnace contains the bustle pipe, water cooled copper tuyeres and the equipment for casting the liquid iron and slag. Once a "taphole" is drilled through the refractory clay plug, liquid iron and slag flow down a trough through a "skimmer" opening, separating the iron and slag. Modern, larger blast furnaces may have as many as four tapholes and two casthouses. Once the pig iron and slag has been tapped, the taphole is again plugged with refractory clay.

The tuyeres are used to implement a hot blast, which is used to increase the efficiency of the blast furnace. The hot blast is directed into the furnace through water-cooled copper nozzles called tuyeres near the base. The hot blast temperature can be from 900 °C to 1300 °C (1600 °F to 2300 °F) depending on the stove design and condition. The temperatures they deal with may be 2000 °C to 2300 °C (3600 °F to 4200 °F). Oil, tar, natural gas, powdered coal and oxygen can also be injected into the furnace at tuyere level to combine with the coke to release additional energy and increase the percentage of reducing gases present which is necessary to increase productivity.

Process engineering and chemistry

Blast furnaces of Třinec Iron and Steel Works, Czech Republic

Blast furnaces differ from bloomeries and reverberatory furnaces in that in latter, flue gas is in intimate contact with the iron, allowing carbon dioxide to dissolve in the iron, which lowers the melting point and changes the iron into pig iron. The intimate contact of flue gas with the iron causes contamination with sulfur if it is present in the fuel. Historically, to prevent contamination from sulfur, the best quality iron was produced with charcoal.

The blast furnaces operates as a countercurrent exchange process whereas a bloomery does not. Another difference is that bloomeries operate as a batch process while blast furnaces operate continuously for long periods because they are difficult to start up and shut down. See: Continuous production

The main chemical reaction producing the molten iron is:

Fe₂O₃ + 3CO → 2Fe + 3CO₂

This reaction might be divided into multiple steps, with the first being that preheated blast air blown into the furnace reacts with the carbon in the form of coke to produce carbon monoxide and heat:

2 C(s) + O₂(g) → 2 CO(g)

The hot carbon monoxide is the reducing agent for the iron ore and reacts with the iron oxide to produce molten iron and carbon dioxide. Depending on the temperature in the different parts of the furnace (warmest at the bottom) the iron is reduced in several steps. At the top, where the temperature usually is in the range between 200 °C and 700 °C, the iron oxide is partially reduced to iron(II,III) oxide, Fe₃O₄.

3 Fe₂O₃(s) + CO(g) → 2 Fe₃O₄(s) + CO₂(g)

At temperatures around 850 °C, further down in the furnace, the iron(II,III) is reduced further to iron(II) oxide:

Fe₃O₄(s) + CO(g) → 3 FeO(s) + CO₂(g)^[]

Hot carbon dioxide, unreacted carbon monoxide, and nitrogen from the air pass up through the furnace as fresh feed material travels down into the reaction zone. As the material travels downward, the counter-current gases both preheat the feed charge and decompose the limestone to calcium oxide and carbon dioxide:

CaCO₃(s) → CaO(s) + CO₂(g)

As the iron(II) oxide moves down to the area with higher temperatures, ranging up to 1200 °C degrees, it is reduced further to iron metal:

FeO(s) + CO(g) → Fe(s) + CO₂(g)

The carbon dioxide formed in this process is re-reduced to carbon monoxide by the coke:

C(s) + CO₂(g) → 2 CO(g)

The temperature-dependent equilibrium controlling the gas atmosphere in the furnace is called the Boudouard reaction:

2CO CO₂ + C

The decomposition of limestone in the middle zones of the furnace proceeds according to the following reaction:

CaCO₃ → CaO + CO₂

The calcium oxide formed by decomposition reacts with various acidic impurities in the iron (notably silica), to form a fayalitic slag which is essentially calcium silicate, CaSiO₃:

SiO₂ + CaO → CaSiO₃

The "pig iron" produced by the blast furnace has a relatively high carbon content of around 4–5%, making it very brittle, and of limited immediate commercial use. Some pig iron is used to make cast iron. The majority of pig iron produced by blast furnaces undergoes further processing to reduce the carbon content and produce various grades of steel used for construction materials, automobiles, ships and machinery.

Although the efficiency of blast furnaces is constantly evolving, the chemical process inside the blast furnace remains the same. According to the American Iron and Steel Institute: "Blast furnaces will survive into the next millennium because the larger, efficient furnaces can produce hot metal at costs competitive with other iron making technologies." One of the biggest drawbacks of the blast furnaces is the inevitable carbon dioxide production as iron is reduced from iron oxides by carbon and there is no economical substitute – steelmaking is one of the unavoidable industrial contributors of the CO₂ emissions in the world (see greenhouse gases).

The challenge set by the greenhouse gas emissions of the blast furnace is being addressed in an on-going European Program called ULCOS (Ultra Low CO₂ Steelmaking). Several new process routes have been proposed and investigated in depth to cut specific emissions (CO₂ per ton of steel) by at least 50%. Some rely on the capture and further storage (CCS) of CO₂, while others choose decarbonizing iron and steel production, by turning to hydrogen, electricity and biomass. In the nearer term, a technology that incorporates CCS into the blast furnace process itself and is called the Top-Gas Recycling Blast Furnace is under development, with a scale-up to a commercial size blast furnace under way. The technology should be fully demonstrated by the end of the 2010s, in line with the timeline set, for example, by the EU to cut emissions significantly. Broad deployment could take place from 2020 on.

Crude steel production
The hot metal contains spurious tramp elements such as carbon, silicon, sulphur and phosphorus. These constituents are removed in BOF steel plant converters.

Illustration of an oxygen top-blowing converter

From hot metal comes crude steel. The impurities are oxidised in converters by top-blowing oxygen through a water-cooled lance. Certain quantities of scrap, accounting for as much as 25% of the total charge, are added as cooling agents, since the oxidation process generates a strong amount of heat.

Scrap charging in a converter shop

A converter holds up to 400 t crude steel. Added along with hot metal and scrap are lime, for slag forming purposes, and alloying agents. The blowing process takes some 20 minutes. Also practised nowadays besides pure top-blowing with oxygen is combined blowing, in which inert stirring gases or oxygen is additionally injected through the converter bottom.

2.2 Steel scrap - electric arc furnace process route
Increasing importance is being attached to scrap recycling for reasons of optimum raw materials utilisation and environmental protection. Steel offers everything needed in this respect, making it a particularly eco-friendly material. Used as the melting unit these days is the electric arc furnace, whose arc makes it possible to transform electrical energy into melting heat with very good efficiency and a high energy density.
The electrical current cannot simply be taken from the public supply system. Using a transformer, it is necessary to turn a current of high voltage into a current of lower voltage (600 to 1000 v) and high amperage (55 to 78 kA). The most important parameter for the performance of an arc furnace is the specific apparent power of the transformer in relation to one tonne of charge material, in which respect values of up to 1000 kVA/t are achieved. Graphite electrodes conduct the electrical current and create the arc to the metallic charge.

A.C. electric arc furnace with eccentric bottom tap hole

The main structural elements of an arc furnace are the furnace shell with eccentric bottom tap hole system and working door, the removable roof with graphite electrodes, and the tilting mechanism. The furnace shell has a refractory lining. Arc furnace tap weights nowadays range as high as 200 t, the annual output of such furnaces being around 1.5 mill. t.
To charge the furnace, the roof is lifted and swung aside. The scrap is conveyed in large buckets over the furnace and then charged into the furnace. The roof is moved back into place and the electrodes are lowered, igniting an arc on the cold scrap. During the melt-down process, temperatures in the arc reach as high as 3500 °C, and in the steel bath as high as 1800 °C. The high temperatures also enable the dissolution of difficult-to-melt scrap alloy constituents. Additional injection of oxygen or of other fuel-gas mixtures accelerates the melt-down process. Once the required chemical composition and temperature of the steel have been attained, the furnace is emptied into a ladle by tilting.
An arc furnace can produce any steel grade, completely regardless of the charge (scrap, DRI , hot metal, as well as any combinations). Today, crude steel is produced not only in A.C. electric arc furnaces, which operate with three graphite electrodes, but also in direct-current arc furnaces fitted with only one electrode.

Direct-current electric arc furnace

This unit, besides offering more favourable conditions for the melt-down of scrap, consumes somewhat less electrical energy and electrode and refractory material.

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