Editing Siderite (section)

== Carbonate iron ore ==<!-- 'Spathic iron ore' redirects here -->
Although [[carbonate]] iron ores, such as siderite, have been economically important for steel production, they are far from ideal as an ore. 

Their hydrothermal mineralisation tends to form them as small [[ore lens]]es, often following steeply [[dip (geology)|dip]]ping [[bedding plane]]s.{{efn-lr|Some siderite, along with [[goethite]], also forms in [[bog iron]] deposits,{{sfn|Sedimentary Geology|page=304}} but these are small and economically minor.}} This keeps them from being amenable to [[opencast mining|opencast working]], and increases the cost of working them by mining with horizontal [[stoping|stope]]s.{{sfnp|Jones|2011|page=34–35,37}} As the individual ore bodies are small, it may also be necessary to duplicate or relocate the pit head machinery, [[winding engine]], and pumping engine, between these bodies as each is worked out. This makes mining the ore an expensive proposition compared to typical [[ironstone]] or [[haematite]] opencasts.{{efn-lr|Both ironstones and [[banded iron formation]]s are sedimentary formations, thus the economically viable deposits may be considerable thicker and more extensive.<ref name="SG, 302" />}}<!-- is a serial comma needed after "considerable" to make sense regarding three items, or should "considerable" be changed to "considerably"? -->

The recovered ore also has drawbacks. The carbonate ore is more difficult to [[Smelting|smelt]] than a haematite or other oxide ore. Driving off the carbonate as carbon dioxide requires more energy and so the ore 'kills' the [[blast furnace]] if added directly. Instead the ore must be given a preliminary roasting step. Developments of specific techniques to deal with these ores began in the early nineteenth century, largely with the work of [[Sir Thomas Lethbridge, 2nd Baronet|Sir Thomas Lethbridge]] in [[Somerset]].<ref name="Jones, 17" /> His 'Iron Mill' of 1838 used a three-chambered concentric roasting furnace, before passing the ore to a separate reducing furnace for smelting. Details of this mill were the invention of Charles Sanderson, a steel maker of Sheffield, who held the patent for it.<ref>{{Cite patent
  |country=GB
  |number=7828
  |gdate=October 1838
  |title=Smelting Iron Ores
  |inventor=Charles Sanderson
}}</ref>

These differences between spathic ore and haematite have led to the failure of a number of mining concerns, notably the [[Brendon Hills Iron Ore Company]].{{sfnp|Jones|2011|page=99}}

Spathic iron ores are rich in manganese and have negligible phosphorus. This led to their one major benefit, connected with the [[Bessemer process|Bessemer steel-making process]]. Although the first demonstrations by Bessemer in 1856 were successful, initial attempts by others to replicate his method infamously failed to produce good steel.{{sfnp|Jones|2011|page=16}} Work by the metallurgist [[Robert Forester Mushet]] showed that the reason for the discrepancy was the nature of the Swedish ores that Bessemer had innocently used; they were very low in phosphorus. Using a typical European high-phosphorus ore in Bessemer's converter gave a poor quality steel. To produce high quality steel from a high-phosphorus ore, Mushet realised that he could operate the Bessemer converter for longer, burning off all the steel's impurities including the unwanted phosphorus, but also the carbon (which is an essential ingredient in steel), and then re-adding carbon, along with manganese, in the form of a previously obscure ferromanganese ore with no phosphorus, [[spiegeleisen]].{{sfnp|Jones|2011|page=16}} This created a sudden demand for spiegeleisen. Although it was not available in sufficient quantity as a mineral, steelworks such as that at [[Ebbw Vale Steelworks|Ebbw Vale]] in South Wales soon learned to make it from the spathic siderite ores.{{sfnp|Jones|2011|page=158}} For a few decades, spathic ores were therefore in demand and this encouraged their mining. In time though, the original 'acidic' liner of the Bessemer converter, made from siliceous sandstone or [[ganister]], was replaced by a 'basic' liner in the newer [[Sidney Gilchrist Thomas|Gilchrist Thomas process]]. This removed the phosphorus impurities as [[slag]] produced by chemical reaction with the liner, and no longer required spiegeleisen. From the 1880s, demand for the ores fell once again and many of their mines, including those of the [[Brendon Hills]], closed soon after.