Iron ores are rocks and minerals from which metallic iron can be economically extracted. The Iron ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, 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). Iron Ores carrying very high quantities of hematite or magnetite (greater than ~60% iron) are known as "natural ore" or "direct shipping ore", meaning that those can be fed directly into iron-making blast furnaces. Iron ore is the raw material used to make pig iron, which is one of the main raw materials to make steel. 98% of the mined iron ore is used to make steel. Iron Ore is the fourth most abundant element in the Earth's crust.
MILL SCALE
Mill scale, often shortened to just scale, is the flaky surface of hot rolled steel, iron oxides consisting of Iron(+II,III) oxide, hematite, and magnetite. Mill scale is formed on the outer surfaces of plates, sheets or profiles when they are being produced by rolling red hot iron or steel billets in rolling mills. Mill scale is composed of iron oxides mostly ferric and is bluish black in color. It is usually less than 1 mm (0.039 in) thick and initially adheres to the steel surface and protects it from atmospheric corrosion provided no break occurs in this coating. Because it is electro-chemically cathodic to steel, any break in the mill scale coating will cause accelerated corrosion of steel exposed at the break. Mill scale is thus a boon for a while until its coating breaks due to handling of the steel product or due to any other mechanical cause. Mill scale is a nuisance when the steel is to be processed. Any paint applied over it is wasted since it will come off with the scale as moisture laden air get under it. Thus mill scale can be removed from steel surfaces by flame cleaning, pickling, or abrasive blasting, which are all tedious operations that waste energy. This is why shipbuilders used to leave steel delivered freshly rolled from mills out in the open to allow it to 'weather' till most of the scale fell off due to atmospheric action. Nowadays most steels mills can supply their produce with mill scale removed and steel coated with shop primers over which welding can be done safely.
Iron ore pellets are spheres of typically 8–18 mm (0.31–0.71 in) to be used as raw material for blast furnaces. They typically contain 67%-72% Fe and various additional material adjusting the chemical composition and the metallurgic properties of the pellets. Typically limestone, dolostone and olivine is added and Bentonite is used as binder.
The process of pelletizing combines mixing of the raw material, forming the pellet and a thermal treatment baking the soft raw pellet to hard spheres. The raw material is rolled into a ball then fired in a kiln to sinter the particles into a hard sphere. The configuration of iron ore pellets as packed spheres in the blast furnace allows air to flow between the pellets, decreasing the resistance to the air that flows up through the layers of material. The configuration of iron ore powder in a blast furnace is more tightly-packed and restricts the air flow. This is the reason that iron ore is preferred in the form of pellets rather than in the form of finer particles.
Pig iron is the intermediate product of smelting iron ore with a high-carbon fuel such as coke, usually with limestone as a flux. Charcoal and anthracite have also been used as fuel. Pig iron has a very high carbon content, typically 3.5–4.5%, which makes it very brittle and not useful directly as a material except for limited applications.
Pig iron is typically poured directly out of the bottom of the blast furnace through a trough into a ladle car for transfer to the steel mill in mostly liquid form; in this state, the pig iron is referred to as hot metal. The hot metal is then charged into a steelmaking vessel to produce steel, typically with an electric arc furnace or basic oxygen furnace, by burning off the excess carbon in a controlled fashion and adjusting the alloy composition. Earlier processes for this included the finery forge, the puddling furnace, the Bessemer process, and the open hearth furnace. Modern steel mills and direct-reduction iron plants transfer the molten iron to a ladle for immediate use in the steel making furnaces or cast it into pigs on a pig-casting machine for reuse or resale. Modern pig casting machines produce stick pigs, which break into smaller 4–10 kg pieces at discharge.
Effects of Iron Ore Elements on Steel Making
The inclusion of even small amounts of some elements can have profound effects on the behavioral characteristics of a batch of iron or the operation of a smelter. These effects can be both good and bad, some catastrophically bad. Some chemicals are deliberately added such as flux which makes a blast furnace more efficient. Others are added because they make the iron more fluid, harder, or give it some other desirable quality. The choice of ore, fuel, and flux determine how the slag behaves and the operational characteristics of the iron produced. Typically, iron ore contains a host of elements the effect of which in Steel making are described below.
Silica (SiO2) is almost always present in iron ore & is one of the principal deoxidizers used in steelmaking. Most of it is slagged off during the smelting process. At temperatures above 1300 °C some will be reduced and form an alloy with the iron. The hotter the furnace, the more silicon will be present in the iron. The major effect of silicon is to promote the formation of gray iron. Gray iron is less brittle and easier to finish than white iron. It is preferred for casting purposes for this reason. In low-carbon steels, silicon is generally detrimental to surface quality. Phosphorous increases strength and hardness and decreases ductility and notch impact toughness of steel. The adverse effects on ductility and toughness are greater in quenched and tempered higher-carbon steels. Phosphorous levels are normally controlled to low levels. Higher phosphorus is specified in low-carbon free-machining steels to improve machinability. Phosphorous is a deleterious contaminant because it makes steel brittle, even at concentrations of as little as 0.6%. Phosphorus cannot be easily removed by fluxing or smelting, and so iron ores must generally be low in phosphorus to begin with.
Sulfur decreases ductility and notch impact toughness especially in the transverse direction. Weldability decreases with increasing sulfur content. Sulfur is found primarily in the form of sulfide inclusions. Sulfur levels are normally controlled to low levels. The only exception is free-machining steels, where sulfur is added to improve machinability. Sulfur can be removed from ores by roasting and washing. Roasting oxidizes sulfur to form sulfur dioxide which either escapes into the atmosphere or can be washed out. In warm climates it is possible to leave pyritic ore out in the rain. The combined action of rain, bacteria, and heat oxidize the sulfides to sulfates, which are water soluble.
Aluminum is widely used as a deoxidizer. Aluminum can control austenite grain growth in reheated steels and is therefore added to control grain size. Aluminum is the most effective alloy in controlling grain growth prior to quenching. Titanium, zirconium, and vanadium are also valuable grain growth inhibitors, but there carbides are difficult to dissolve into solution in austenite. However, as per few experts, it does increase the viscosity of the slag. This will have a number of adverse effects on furnace operation. The thicker slag will slow the descent of the charge, prolonging the process. High aluminum will also make it more difficult to tap off the liquid slag.
Copper in significant amounts is detrimental to hot-working steels. Copper negatively affects forge welding, but does not seriously affect arc or oxyacetylene welding. Copper can be detrimental to surface quality. Copper is beneficial to atmospheric corrosion resistance when present in amounts exceeding 0.20%. Weathering steels are sold having greater than 0.20% Copper.
Lead is virtually insoluble in liquid or solid steel. However, lead is sometimes added to carbon and alloy steels by means of mechanical dispersion during pouring to improve the machinability.
Nickel is a ferrite strengthener. Nickel does not form carbides in steel. It remains in solution in ferrite, strengthening and toughening the ferrite phase. Nickel increases the hardenability and impact strength of steels.
Titanium is used to retard grain growth and thus improve toughness. Titanium is also used to achieve improvements in inclusion characteristics. Titanium causes sulfide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending.
Vanadium increases the yield strength and the tensile strength of carbon steel. The addition of small amounts of Vanadium can significantly increase the strength of steels. Vanadium is one of the primary contributors to precipitation strengthening in micro alloyed steels. When thermo mechanical processing is properly controlled the ferrite grain size is refined and there is a corresponding increase in toughness. The impact transition temperature also increases when vanadium is added.
BASICS OF SOME IMPORTANT ELEMENT ANALYSIS OF IRON ORE
1. Analysis of Iron content as Fe%
Laboratory Sample of Iron Ore is dissolved by boiling in 1 : 1 Hydrochloric Solution. To the boiling solution Stannous Chloride solution is added to reduce Iron with slight excess. The unused stannous chloride is destroyed by Mercuric Chloride & the reduced Iron is titrated with standard Dichromate solution using Barium Diphenylamine Sulphonate.
2. Analysis Ferrous Iron as FeO%
Ferrous Iron ore is determined by dissolving Laboratory Sample of Iron Ore diluted Hydrochloric acid in the atmosphere of Carbon Di Oxide & filtrating with Standard Potassium Dichromate solution.
3. Analysis of Silica in Iron Ore as SiO2%
Laboratory Sample of Iron Ore is dissolved in Hydrochloric acid & baked for dehydration of Silica at 110 deg C. It is again dissolved with Hydrochloric acid & diluted with water, boiled, filtered & the residue is ignited and Silica is determined by Hydrofluorisation. The residue is fused with Sodium Carbonate & extracted with Hydrochloric Acid, added to the main filtrate & it is measured for further analysis.
4. Analysis of Alumina in Iron Ore as Al2O3%
To the Solution of the Iron Ore Laboratory Sample Sodium Phosphate is added & the acidity adjusted. Sodium Thiosulphate when added in excess reduces Iron to the Ferrous State & precipitates Aluminum as Phosphate. Alumina is calculated from the weight of Aluminum Phosphate.
5. Analysis of Phosphorous in Iron Ore
The laboratory sample of Iron Ore is dissolved in Hydrochloric Acid & evaporated to dryness nearly. Add Nitric Acid (10 cc) two three times to drive out Hydrochloric acid & dehydrate Silica. Add Water to dissolve the sample and filter the solution & collect the solution in a conical flask. Then precipitate the Phosphorous with Aluminum Molybdate after making properly acidic concentration & appropriate temperature. Filter the precipitate which after making acid free is dissolved in known excess of standard alkali. The unreacted alkali is back titrated with standard Nitric Acid.
6. Analysis of Sulphur in Iron Ore
Oxidation of Sulphur in Iron Ore is done by Bromine & Concentrated Nitric Acid. After that Acid Mixture (Hydrochloric Acid + Nitric Acid) is added for decomposition at lower heat. Evaporate the solution to dryness & cool. Make the solution with Hydrochloric Acid. The Iron is removed by Methyl Iso Butyl Ketone. After that Sulphur in the solution is precipitated with Barium Chloride as Barium Sulphate from which Sulphur content is analysed.
Analysis of Mn, Ti, CaO, MgO, Na2O, K2O & Trace Analysis of Arsenic, Nickel, Copper, Lead, Zinc, Vanadium etc. in Iron Ore is carried out as per the requirement.
MSK’s Facilities for Iron Ore Analysis in the World:
TML & FMP
Transportable Moisture Limit (TML) of Iron Ore Bulk cargo by Flow Table Test Method When solid bulk cargoes, such as Iron ores contain high moisture, they get subjected to cyclic forces due to rise in pore water flow pressure, it results in steep rise in loss of strength of material to be held in definite shape and there is loss of stability of the cargo causing the material to flow like liquid. This is called liquefaction of solid bulk cargo.
Due to natural oscillation of the Marine Vessel in sea, the cargo experiences further drift with often separation of water from the cargo as well. Due to cargo shifting there is possibility of Vessel capsizing in the sea. This shifting occurs when the moisture content in the cargo reaches certain limit characteristic for that cargo.
To prevent the risk of liquefaction, International maritime Organization (IMO) has specified tests to be carried out as per BC Code to determine upper bound acceptable of moisture content of cargo, which is defined by the Flow Moisture Point (FMP). The Transportable Moisture Limit (TML) is derived from the FMP.
BC Code has recommended three different tests as below:
1. The Flow Table Test
2. Penetration Test
3. Proctor / Fagerberg Test
The Flow Table test is widely adopted for carrying out the test of FMP & TML.
MSK has developed the Test equipment and Test facility at its various Labs in India & abroad.
Principle (Flow table Test):
Adjustment of moisture content of sample by mixing with water so that plastic deformation occurs during dropping of flow table; the sample is considered to be at its flow moisture point.
TML = 90 % of Flow Moisture Point
Reference to the Standard on which the procedure is based
BC Code: Appendix – 2
Equipment Used:
1. Standard flow table & frame (ASTM Designation [ C 230 – 68 ] – Sec. 3 )
2. Flow table mounting ( ASTM Designation [ C 230 – 68 ] – Sec. 3 )
3. Mould ( ASTM Designation [ C 230 – 68 ] – Sec. 3 )
4. Tamper – The required tampering pressure may be achieved by using calibrated, spring – loaded tampers or some other suitable design of tamper that allows a controlled pressure to be applied via a 30 mm diameter tamper head.
5. Scales and weights (ASTM Designation [ C 109 – 73 ] – Sec. 3 ) and suitable sample containers.
6. Glass graduated measuring cylinder and burette having capacities of 100 – 200 ml and 10 ml respectively.
7. A hemispherical mixing bowl approximately 30 cm diameter, rubber gloves and drying dishes or pans.
8. A drying oven with controlled temperature up to approximately 1100C and this oven should be without air circulation.
Sampling:
To ensure that TML result is representative, increments of material shall be taken either:
a) From stockpile
b) During loading or discharging a vessel.
Test Procedure:
FMP is derived from the observations by prescribed method and TML is calculated as
TML = 0.90 x FMP
Point to be noted here that TML / FMP is a function of Granularity of Cargo, it’s not applicable for Lumpy cargo.