Editing Iron ore (section)

====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.