Editing Iron ore (section)

==Smelting==
{{Main|blast furnace|bloomery}}
Iron ores consist of [[oxygen]] and iron atoms bonded together into molecules. To convert it to metallic iron, it must be [[smelting|smelted]] or sent through a [[direct reduction]] process to remove the oxygen. Oxygen-iron bonds are strong, and to remove the iron from the oxygen, a stronger elemental bond must be presented to attach to the oxygen. Carbon is used because the strength of a [[carbon-oxygen bond]] is greater than that of the iron-oxygen bond at high temperatures. Thus, the iron ore must be powdered and mixed with [[coke (fuel)|coke]], to be burnt in the smelting process.

[[Carbon monoxide]] is the primary ingredient of chemically stripping oxygen from iron. Thus, the iron and carbon smelting must be kept in an oxygen-deficient (reducing) state to promote the burning of carbon to produce {{Chem|C||O}} and not {{Chem|C||O|2}}.

* Air blast and charcoal (coke): 2 C + O<sub>2</sub> → 2 CO
* Carbon monoxide (CO) is the principal reduction agent.
** Stage One: 3 Fe<sub>2</sub>O<sub>3</sub> + CO → 2 Fe<sub>3</sub>O<sub>4</sub> + CO<sub>2</sub>
** Stage Two: Fe<sub>3</sub>O<sub>4</sub> + CO → 3 FeO + CO<sub>2</sub>
** Stage Three: FeO + CO → Fe + CO<sub>2</sub>
* Limestone calcining: CaCO<sub>3</sub> → CaO + CO<sub>2</sub>
* Lime acting as flux: CaO + SiO<sub>2</sub> → [[Calcium silicate|CaSiO<sub>3</sub>]]

===Trace elements===
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 determines how the slag behaves and the operational characteristics of the iron produced. Ideally, iron ore contains only iron and oxygen. In reality, this is rarely the case. Typically, iron ore contains a host of elements which are often unwanted in modern steel.

====Silicon====
Silica ({{Chem|Si||O|2}}) is almost always present in iron ore. Most of it is slagged off during the smelting process. At temperatures above {{convert|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. It is not uncommon to find up to 1.5% Si in European cast iron from the 16th to 18th centuries.

The major effect of silicon is to promote the formation of grey iron. Grey iron is less brittle and easier to finish than white iron. It is preferred for casting purposes for this reason.  British metallurgist [[Thomas Turner (metallurgist)|Thomas Turner]] reported that silicon also reduces shrinkage and the formation of blowholes, lowering the number of bad castings. However, too much silicon present in the iron leads to increased brittleness and moderate hardness.{{sfn|Turner|1900|p=287}}

====Phosphorus====
[[Phosphorus]] (P) has four major effects on iron: increased hardness and strength, lower [[Liquidus and solidus|solidus]], increased fluidity, and cold shortness. Depending on the use intended for the iron, these effects are either good or bad. Bog ore often has a high phosphorus content.{{sfn|Gordon|1996|p=57}}

The strength and hardness of iron increases with the concentration of phosphorus. 0.05% phosphorus in wrought iron makes it as hard as medium-carbon steel. High-phosphorus iron can also be hardened by cold hammering. The hardening effect is true for any concentration of phosphorus. The more phosphorus, the harder the iron becomes and the more it can be hardened by hammering. Modern steel makers can increase hardness by as much as 30%, without sacrificing shock resistance by maintaining phosphorus levels between 0.07 and 0.12%. It also increases the depth of hardening due to quenching, but at the same time also decreases the solubility of carbon in iron at high temperatures. This would decrease its usefulness in making blister steel ([[Cementation process|cementation]]), where the speed and amount of carbon absorption is the overriding consideration.

The addition of phosphorus has a downside. At concentrations higher than 0.2%, iron becomes increasingly cold short, or brittle at low temperatures. Cold short is especially important for bar iron. Although bar iron is usually worked hot, its uses{{example needed|date=April 2018}} often require it to be tough, bendable, and resistant to shock at room temperature. A nail that shatters when hit with a hammer or a carriage wheel that breaks when it hit a rock would not sell well.{{citation needed|date=April 2018}} High enough concentrations of phosphorus render any iron unusable.{{sfn|Rostoker|Bronson|1990|p=22}} The effects of cold shortness are magnified by temperature. Thus, a piece of iron that is perfectly serviceable in summer might become extremely brittle in winter. There is some evidence that during the [[Middle Ages]] the very wealthy may have had a high-phosphorus sword for summer and a low-phosphorus sword for winter.{{sfn|Rostoker|Bronson|1990|p=22}}

Careful control of phosphorus can be of great benefit in casting operations. Phosphorus depresses the liquidus, allowing the iron to remain molten for longer and increasing fluidity. The addition of 1% can double the distance molten iron will flow.{{sfn|Rostoker|Bronson|1990|p=22}} The maximum effect, about {{convert|500|C|abbr=on}}, is achieved at a concentration of 10.2%.{{sfn|Rostoker|Bronson|1990|p=194}} For foundry work Turner{{sfn|Turner|1900|}} felt the ideal iron had 0.2–0.55% phosphorus. The resulting iron filled molds with fewer voids and also shrank less. In the 19th century some producers of decorative cast iron used iron with up to 5% phosphorus. The extreme fluidity allowed them to make very complex and delicate castings, but they could not be weight-bearing, as they had no strength.{{sfn|Turner|1900|pp=202–204}}

There are two remedies{{according to whom|date=April 2018}} for high-phosphorus iron. The oldest, easiest, and cheapest, is avoidance. If the iron that the ore produced was cold short, one would search for a new source of iron ore. The second method involves oxidizing the phosphorus during the fining process by adding iron oxide. This technique is usually associated with puddling in the 19th century, and may not have been understood earlier. For instance, Isaac Zane, owner of Marlboro Iron Works, did not appear to know about it in 1772. Given Zane's reputation{{according to whom|date=April 2018}} for keeping abreast of the latest developments, the technique was probably unknown to the ironmasters of [[Virginia]] and [[Pennsylvania]].

[[Phosphorus]] is generally considered to be a deleterious contaminant because it makes steel brittle, even at concentrations of as little as 0.6%. When the [[Gilchrist–Thomas process]] allowed the removal of bulk amounts of the element from cast iron in the 1870s, it was a major development because most of the iron ores mined in continental Europe at the time were phosphorous. However, removing all the contaminant by fluxing or smelting is complicated, and so desirable iron ores must generally be low in phosphorus to begin with.

====Aluminium====
Small amounts of [[aluminium]] (Al) are present in many ores including iron ore, sand, and some limestones. The former can be removed by washing the ore prior to smelting. Until the introduction of brick-lined furnaces, the amount of aluminium contamination was small enough that it did not have an effect on either the iron or slag. However, when brick began to be used for hearths and the interior of blast furnaces, the amount of aluminium contamination increased dramatically. This was due to the erosion of the furnace lining by the liquid slag.

Aluminium is difficult to reduce. As a result, aluminium contamination of the iron is not a problem. However, it does increase the viscosity of the slag.{{sfn|Kato|Minowa|1969|p=37}}{{sfn|Rosenqvist|1983|p=311}} 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 aluminium will also make it more difficult to tap off the liquid slag. At the extreme, this could lead to a frozen furnace.

There are a number of solutions to a high-aluminium slag. The first is avoidance; do not use ore or a lime source with a high aluminium content. Increasing the ratio of lime flux will decrease the viscosity.{{sfn|Rosenqvist|1983|p=311}}

====Sulfur====
[[Sulfur]] (S) is a frequent contaminant in coal. It is also present in small quantities in many ores, but can be removed by [[calcining]]. Sulfur dissolves readily in both liquid and solid iron at the temperatures present in iron smelting. The effects of even small amounts of sulfur are immediate and serious. They were one of the first worked out by iron makers. Sulfur causes iron to be red or hot short.{{sfn|Gordon|1996|p=7}}

Hot short iron is brittle when hot. This was a serious problem as most iron used during the 17th and 18th centuries was bar or wrought iron. Wrought iron is shaped by repeated blows with a hammer while hot. A piece of hot short iron will crack if worked with a hammer. When a piece of hot iron or steel cracks, the exposed surface immediately oxidizes. This layer of oxide prevents the mending of the crack by welding. Large cracks cause the iron or steel to break up. Smaller cracks can cause the object to fail during use. The degree of hot shortness is in direct proportion to the amount of sulfur present. Today, iron with over 0.03% sulfur is avoided.

Hot short iron can be worked, but it must be worked at low temperatures. Working at lower temperatures requires more physical effort from the smith or forgeman. The metal must be struck more often and harder to achieve the same result. A mildly sulfur-contaminated bar can be worked, but it requires a great deal more time and effort.

In cast iron, sulfur promotes the formation of white iron. As little as 0.5% can counteract the effects of slow cooling and a high silicon content.{{sfn|Rostoker|Bronson|1990|p=21}} White cast iron is more brittle, but also harder. It is generally avoided, because it is difficult to work, except in China where high-sulfur cast iron, some as high as 0.57%, made with coal and coke, was used to make bells and chimes.{{sfn|Rostoker|Bronson|Dvorak|1984|p=760}} According to {{Harvard citation text|Turner|1900|pp=200}}, good foundry iron should have less than 0.15% sulfur. In the rest of the world, a high-sulfur cast iron can be used for making castings, but will make poor wrought iron.

There are a number of remedies for sulfur contamination. The first, and the one most used in historic and prehistoric operations, is avoidance. Coal was not used in Europe (unlike China) as a fuel for smelting because it contains sulfur and therefore causes hot short iron. If an ore resulted in hot short metal, [[ironmaster]]s looked for another ore. When mineral coal was first used in European blast furnaces in 1709 (or perhaps earlier), it was [[coke (fuel)|coked]]. Only with the introduction of [[hot blast]] from 1829 was raw coal used.

====Ore roasting====
Sulfur can be removed from ores by [[Roasting (metallurgy)|roasting]] and washing. Roasting oxidizes sulfur to form [[sulfur dioxide]] (SO<sub>2</sub>), which either escapes into the atmosphere or can be washed out. In warm climates, it is possible to leave [[Pyrite|pyritic]] ore out in the rain. The combined action of rain, [[bacteria]], and heat [[Oxidation|oxidize]] the sulfides to [[sulfuric acid]] and [[sulfate]]s, which are water-soluble and leached out.{{sfn|Turner|1900|pp=77}} However, historically (at least), iron sulfide (iron [[pyrite]] {{Chem|Fe||S|2}}), though a common iron mineral, has not been used as an ore for the production of iron metal. Natural weathering was also used in Sweden. The same process, at geological speed, results in the [[gossan]] [[limonite]] ores.

The importance attached to low-sulfur iron is demonstrated by the consistently higher prices paid for the iron of Sweden, Russia, and Spain from the 16th to 18th centuries. Today sulfur is no longer a problem. The modern remedy is the addition of [[manganese]], but the operator must know how much sulfur is in the iron because at least five times as much manganese must be added to neutralize it. Some historic irons display manganese levels, but most are well below the level needed to neutralize sulfur.{{sfn|Rostoker|Bronson|1990|p=21}}

Sulfide inclusion as [[manganese sulfide]] (MnS) can also be the cause of severe [[pitting corrosion]] problems in low-grade [[stainless steel]] such as [[SAE 304 stainless steel|AISI 304 steel]].<ref name="StewartWilliams1992">{{cite journal |last1=Stewart |first1=J. |last2=Williams |first2=D.E. |title=The initiation of pitting corrosion on austenitic stainless steel: on the role and importance of sulphide inclusions |journal=Corrosion Science |volume=33 |issue=3 |year=1992 |pages=457–474 |issn=0010-938X |doi=10.1016/0010-938X(92)90074-D|bibcode=1992Corro..33..457S }}</ref><ref name="WilliamsKilburn2010">{{cite journal |last1=Williams |first1=David E. |last2=Kilburn |first2=Matt R. |last3=Cliff |first3=John |last4=Waterhouse |first4=Geoffrey I.N. |title=Composition changes around sulphide inclusions in stainless steels, and implications for the initiation of pitting corrosion |journal=Corrosion Science |volume=52 |issue=11 |year=2010 |pages=3702–3716 |issn=0010-938X |doi=10.1016/j.corsci.2010.07.021|bibcode=2010Corro..52.3702W }}</ref> Under oxidizing conditions and in the presence of moisture, when [[sulfide]] oxidizes, it produces [[thiosulfate]] anions as intermediate species, and because the thiosulfate anion has a higher equivalent electromobility than the [[chloride]] anion due to its double negative electrical charge, it promotes pit growth.<ref name="NewmanIsaacs1982">{{cite journal |last1=Newman |first1=R. C. |last2=Isaacs |first2=H. S. |last3=Alman |first3=B. |title=Effects of sulfur compounds on the pitting behavior of type 304 stainless steel in near-neutral chloride solutions |journal=Corrosion |volume=38 |issue=5 |year=1982 |pages=261–265 |issn=0010-9312 |doi=10.5006/1.3577348}}</ref> Indeed, the positive electrical charges born by Fe<sup>2+</sup> cations released in solution by Fe [[oxidation]] on the [[Anode|anodic]] zone inside the pit must be quickly compensated / neutralized by negative charges brought by the [[Electrokinetic phenomena|electrokinetic]] migration of anions in the capillary pit. Some of the [[Electrochemistry|electrochemical]] processes occurring in a capillary pit are the same as those encountered in [[capillary electrophoresis]]. The higher the anion electrokinetic migration rate, the higher the rate of pitting corrosion. [[Electrokinetic phenomena|Electrokinetic transport]] of ions inside the pit can be the rate-limiting step in the pit growth rate.