Hematite: the main iron ore in Brazilian mine
This stockpile of iron ore pellets will be used in
steel production.
Iron ores are
rocks and
minerals from which
metallic iron can be economically extracted. The
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 (
Fe
3O
4),
hematite (
Fe
2O
3),
goethite (
FeO(OH)),
limonite (
FeO(OH).n(H2O)) or
siderite (
FeCO3).
Ores carrying very high quantities of hematite or magnetite (greater than ~60% iron) are known as "natural ore" or "
direct shipping ore", meaning they can be fed directly into iron-making
blast furnaces. Most reserves of such ore have now been depleted. 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.
[1] Indeed, it has been argued that iron ore is "more integral to the
global economy than any other commodity, except perhaps oil".
[2]
Sources
Metallic iron is virtually unknown on the surface of the Earth except as iron-nickel alloys from
meteorites and very rare forms of deep mantle
xenoliths. Although iron is the fourth most abundant element in the
Earth's crust, comprising about 5%, the vast majority is bound in
silicate or more rarely
carbonate minerals. The
thermodynamic barriers to separating
pure iron
from these minerals are formidable and energy intensive, therefore all
sources of iron used by human industry exploit comparatively rarer iron
oxide minerals, primarily
hematite.
Prior to the industrial revolution, most iron was obtained from widely available
goethite or
bog ore, for example during the
American Revolution and the
Napoleonic wars. Prehistoric societies used
laterite as a
source of iron ore. Historically, much of the iron ore utilized by
industrialized
societies has been mined from predominantly hematite deposits with
grades in excess of 70% Fe. These deposits are commonly referred to as
"direct shipping ores" or "natural ores". Increasing iron ore demand,
coupled with the depletion of high-grade hematite ores in the United
States, after
World War II led to development of lower-grade iron ore sources, principally the utilization of
magnetite and
taconite.
Iron ore mining methods vary by the type of ore being mined. There
are four main types of iron ore deposits worked currently, depending on
the mineralogy and geology of the ore deposits. These are magnetite,
titanomagnetite, massive hematite and
pisolitic ironstone deposits.
Banded iron formations
Processed taconite pellets with reddish surface oxidation as used in the steelmaking industry, with a
US Quarter (diameter: 24 mm (0.96 in)) shown for scale
Banded iron formations (BIFs) are
sedimentary rocks containing more than 15% iron composed predominantly of thinly bedded iron minerals and
silica (as
quartz). Banded iron formations occur exclusively in
Precambrian rocks, and are commonly weakly to intensely
metamorphosed. Banded iron formations may contain iron in
carbonates (
siderite or
ankerite) or
silicates (
minnesotaite,
greenalite, or
grunerite), but in those mined as iron ores,
oxides (
magnetite or
hematite) are the principal iron mineral.
[3] Banded iron formations are known as
taconite within North America.
The mining involves moving tremendous amounts of ore and waste. The waste comes in two forms, non-ore bedrock in the mine (
overburden or interburden locally known as mullock), and unwanted minerals which are an intrinsic part of the ore rock itself (
gangue). The mullock is mined and piled in
waste dumps, and the gangue is separated during the
beneficiation process and is removed as
tailings.
Taconite tailings are mostly the mineral quartz, which is chemically
inert. This material is stored in large, regulated water settling ponds.
Magnetite ores
The key economic parameters for magnetite ore being economic are the
crystallinity of the magnetite, the grade of the iron within the banded
iron formation host rock, and the contaminant elements which exist
within the magnetite concentrate. The size and strip ratio of most
magnetite resources is irrelevant as a banded iron formation can be
hundreds of meters thick, extend hundreds of kilometers along
strike, and can easily come to more than three billion or more tonnes of contained ore.
The typical grade of iron at which a magnetite-bearing banded iron
formation becomes economic is roughly 25% iron, which can generally
yield a 33% to 40% recovery of magnetite by weight, to produce a
concentrate grading in excess of 64% iron by weight. The typical
magnetite iron ore concentrate has less than 0.1%
phosphorus, 3–7%
silica and less than 3%
aluminium.
Currently magnetite iron ore is mined in
Minnesota and
Michigan in the
U.S., Eastern
Canada and North
Sweden. Magnetite bearing banded iron formation is currently mined extensively in
Brazil, which exports significant quantities to
Asia, and there is a nascent and large magnetite iron ore industry in
Australia.
Direct shipping (hematite) ores
Direct shipping iron ore (DSO) deposits (typically composed of
hematite) are currently exploited on all continents except
Antarctica, with the largest intensity in
South America,
Australia and Asia. Most large hematite iron ore deposits are sourced
from altered banded iron formations and rarely igneous accumulations.
DSO deposits are typically rarer than the magnetite-bearing BIF or
other rocks which form its main source or protolith rock, but are
considerably cheaper to mine and process as they require less
beneficiation due to the higher iron content. However, DSO ores can
contain significantly higher concentrations of penalty elements,
typically being higher in phosphorus, water content (especially
pisolite sedimentary accumulations) and aluminum (
clays within pisolites). Export grade DSO ores are generally in the 62–64% Fe range
[citation needed].
Magmatic magnetite ore deposits
Occasionally
granite and
ultrapotassic igneous rocks
segregate magnetite crystals and form masses of magnetite suitable for
economic concentration. A few iron ore deposits, notably in
Chile, are formed from
volcanic flows containing significant accumulations of magnetite
phenocrysts. Chilean magnetite iron ore deposits within the
Atacama Desert have also formed
alluvial accumulations of magnetite in streams leading from these volcanic formations.
Some magnetite
skarn and
hydrothermal deposits have been worked in the past as high-grade iron ore deposits requiring little
beneficiation. There are several granite-associated deposits of this nature in
Malaysia and
Indonesia.
Other sources of magnetite iron ore include metamorphic accumulations of massive
magnetite ore such as at
Savage River,
Tasmania, formed by shearing of
ophiolite ultramafics.
Another, minor, source of iron ores are magmatic accumulations in
layered intrusions which contain a typically
titanium-bearing magnetite often with
vanadium.
These ores form a niche market, with specialty smelters used to recover
the iron, titanium and vanadium. These ores are beneficiated
essentially similar to banded iron formation ores, but usually are more
easily upgraded via
crushing and
screening. The typical titanomagnetite concentrate grades 57% Fe, 12% Ti and 0.5%
V
2O
5.
[citation needed]
Beneficiation
Lower-grade sources of iron ore generally require
beneficiation, using techniques like crushing,
milling,
gravity or heavy media separation, screening, and silica
froth flotation to improve the concentration of the ore and remove impurities. The results, high quality fine ore powders, are known as
fines.
Magnetite
Magnetite is
magnetic, and hence easily separated from the
gangue minerals and capable of producing a high-grade concentrate with very low levels of impurities.
The grain size of the magnetite and its degree of commingling with the silica
groundmass
determine the grind size to which the rock must be comminuted to enable
efficient magnetic separation to provide a high purity magnetite
concentrate. This determines the energy inputs required to run a milling
operation.
Mining of banded iron formations involves coarse crushing and screening, followed by rough crushing and fine grinding to
comminute
the ore to the point where the crystallized magnetite and quartz are
fine enough that the quartz is left behind when the resultant powder is
passed under a magnetic separator.
Generally most magnetite banded iron formation deposits must be
ground to between 32 and 45 micrometers in order to produce a low-silica
magnetite concentrate. Magnetite concentrate grades are generally in
excess of 70% iron by weight and usually are low phosphorus, low
aluminium, low titanium and low silica and demand a premium price.
Hematite
Due to the high density of
hematite relative to associated
silicate gangue, hematite beneficiation usually involves a combination of beneficiation techniques.
One method relies on passing the finely crushed
ore over a bath of solution containing
bentonite or other agent which increases the
density of the solution. When the density of the
solution is properly calibrated, the hematite will sink and the
silicate mineral fragments will float and can be removed.
Production and consumption
Iron is the world's most commonly used metal - steel, of which iron
ore is the key ingredient, representing almost 95% of all metal used per
year.
[2]
It is used primarily in structural engineering applications and in
maritime purposes, automobiles, and general industrial applications
(machinery).
Iron-rich rocks are common worldwide, but ore-grade commercial
mining
operations are dominated by the countries listed in the table aside.
The major constraint to economics for iron ore deposits is not
necessarily the grade or size of the deposits, because it is not
particularly hard to geologically prove enough tonnage of the rocks
exist. The main constraint is the position of the iron ore relative to
market, the cost of rail infrastructure to get it to market and the
energy cost required to do so.
Mining iron ore is a high volume low margin business, as the value of iron is significantly lower than base metals.
[5]
It is highly capital intensive, and requires significant investment in
infrastructure such as rail in order to transport the ore from the mine
to a freight ship.
[5] For these reasons, iron ore production is concentrated in the hands of a few major players.
World production averages two billion metric tons of raw ore
annually. The world's largest producer of iron ore is the Brazilian
mining corporation
Vale, followed by Anglo-Australian companies
BHP Billiton and
Rio Tinto Group. A further Australian supplier,
Fortescue Metals Group Ltd has helped bring Australia's production to second in the world.
The seaborne trade in iron ore, that is, iron ore to be shipped to other countries, was 849m tonnes in 2004.
[5] Australia and Brazil dominate the seaborne trade, with 72% of the market.
[5] BHP, Rio and Vale control 66% of this market between them.
[5]
In
Australia iron ore is won from three main sources: pisolite "
channel iron deposit" ore derived by mechanical erosion of primary banded-iron formations and accumulated in alluvial channels such as at
Pannawonica, Western Australia; and the dominant metasomatically-altered
banded iron formation related ores such as at
Newman, the
Chichester Range, the
Hamersley Range and
Koolyanobbing,
Western Australia. Other types of ore are coming to the fore recently, such as oxidised ferruginous hardcaps, for instance
laterite iron ore deposits near
Lake Argyle in Western Australia.
The total recoverable reserves of iron ore in
India are about 9,602 million tones of
hematite and 3,408 million tones of
magnetite[citation needed].
Chhattisgarh,
Madhya Pradesh,
Karnataka,
Jharkhand,
Odisha,
Goa,
Maharashtra,
Andhra Pradesh,
Kerala,
Rajasthan and
Tamil Nadu are the principal Indian producers of iron ore. World consumption of iron ore grows 10% per annum
[citation needed] on average with the main consumers being China, Japan, Korea, the United States and the European Union.
China is currently the largest consumer of iron ore, which translates
to be the world's largest steel producing country. It is also the
largest importer, buying 52% of the seaborne trade in iron ore in 2004.
[5]
China is followed by Japan and Korea, which consume a significant
amount of raw iron ore and metallurgical coal. In 2006, China produced
588 million tons of iron ore, with an annual growth of 38%.
Iron ore market
Over the last 40 years, iron ore prices have been decided in closed-door negotiations between the small handful of miners and
steelmakers which dominate both spot and contract markets. Traditionally, the first deal reached between these two groups sets a
benchmark to be followed by the rest of the industry.
[2]
This benchmark system has however in recent years begun to break
down, with participants along both demand and supply chains calling for a
shift to short term pricing. Given that most other
commodities
already have a mature market-based pricing system, it is natural for
iron ore to follow suit. To answer increasing market demands for more
transparent pricing, a number of financial exchanges and/or clearing
houses around the world have offered iron ore swaps clearing. The CME
group, SGX (Singapore Exchange), London Clearing House (LCH.Clearnet),
NOS Group and ICEX (Indian Commodities Exchange) all offer cleared swaps
based on The Steel Index's (TSI) iron ore transaction data. The CME
also offers a Platts based swap, in addition to their TSI swap clearing.
The ICE (Intercontinental Exchange) offers a Platts based swap clearing
service also. The swaps market has grown quickly, with liquidity
clustering around TSI's pricing.
[6]
By April 2011, over US$5.5 billion worth of iron ore swaps have been
cleared basis TSI prices. By August 2012, in excess of one million
tonnes of swaps trading per day was taking place regularly, basis TSI.
A relatively new development has also been the introduction of iron
ore options, in addition to swaps. The CME group has been the venue most
utilised for clearing of options written against TSI, with open
interest at over 12,000 lots in August 2012.
Singapore Mercantile Exchange (SMX) has launched the world first global iron ore futures contract, based on the
Metal Bulletin
Iron Ore Index (MBIOI) which utilizes daily price data from a broad
spectrum of industry participants and independent Chinese steel
consultancy and data provider Shanghai Steelhome's widespread contact
base of steel producers and iron ore traders across China.
[7] The futures contract has seen monthly volumes over 1.5 million tonnes after eight months of trading.
[8]
This move follows a switch to index-based quarterly pricing by the world's three largest iron ore miners -
Vale,
Rio Tinto and
BHP Billiton - in early 2010, breaking a 40-year tradition of benchmark annual pricing.
[9]
Available iron ore resources
Available world iron ore resources
The Iron ore reserves at present seem quite vast, but some are
starting to suggest that the math of continual exponential increase in
consumption can even make this resource seem quite finite. For instance,
Lester Brown of the
Worldwatch Institute has suggested iron ore could run out within 64 years based on an extremely conservative extrapolation of 2% growth per year.
[10]
Available Australian iron ore resources
Geoscience Australia calculates that the country's "economic
demonstrated resources" of iron currently amount to 24 gigatonnes, or 24
billion tonnes.
[citation needed]
The current production rate from the Pilbara region of Western
Australia is approximately 430 million tonnes a year and rising. Experts
Dr Gavin Mudd (Monash University) and Jonathon Law (CSIRO) expect it to
be gone within 30 to 50 years (Mudd) and 56 years (Law).
[11]
These estimates require on-going review to take into account shifting
demand for lower grade iron ore and improving mining and recovery
techniques (allowing deeper mining below the groundwater table).
Future Pilbara production capacity
In 2011, leading Pilbara based iron ore miners - Rio Tinto, BHP
Billiton and Foscues Metal Group - all announced significant capital
investment in the development of existing and new mines and associated
infrastructure (rail and port). Collectively this would amount to the
production of 1,000 million tonnes per year (Mt/y) by 2020. Practically
that would require a doubling of production capacity from a current
production level of 470 Mt/y to 1,000 Mt/y (an increase of 530 Mt/y).
These figures are based on the current production rates of Rio 220 Mt/y,
BHP 180 Mt/y, FMG 55 Mt/y and Other 15 Mt/y increasing to Rio 353 Mt/y,
BHP 356 Mt/y, FMG 155 Mt/y and Other 140 Mt/y (the latter 140 Mt/y is
based on planned production from recent industry entrants Hancock, Atlas
and Brockman through Port Hedland and API and others through the
proposed Port of Anketell).
A production rate of 1,000 Mt/y would require a significant increase
in production from existing mines and the opening of a significant
number of new mines. Further, a significant increase in the capacity of
rail and port infrastructure would also be required. For example, Rio
would be required to expand its port operations at Dampier and Cape
Lambert by 140 Mt/y (from 220 Mt/y to 360 Mt/y). BHP would be required
to expand its Port Hedland port operations by 180 Mt/y (from 180 Mt/y to
360 Mt/y). FMG would be required to expand its port operations at Port
Hedland by 100 Mt/y (from 55 Mt/y to 155 Mt/y). That's an increase of
420 Mt/y in port capacity by the three majors Rio, BHP and FMG and about
at least 110 Mt/y from the non-major producers. Based on the
rule-of-thumb of 50 Mt/y per car dumper, reclaimer and ship-loader the
new production would require approximately 10 new car dumpers,
reclaimers and ship-loaders.
New rail capacity would also be required. Based on the rule-of-thumb
of 100 Mt/y per rail line, increasing production by approximately 500
Mt/y would require 5 new single rail lines. One scenario is an extra
rail line for all the majors: BHP (from double to triple track), Rio
(double to triple track), FMG (single to double track) and at least two
new lines. New lines have been proposed by Hancock to service the Roy
Hill mine
[citation needed] and QR National to service non-major producers.
[12][13]
A 1,000 Mt/y production rate needs to be further considered by
proponents and government. Areas of further consideration include new
port space at Anketell to service the West Pilbara mines, growth at Port
Hedland (BHP has announced the development of an outer harbour at Port
Hedland), rail rationalisation and the regulatory approval requirements
for opening and maintaining a ground disturbance footprint that supports
1,000 Mt/y of production including, amongst other things, native title,
aboriginal heritage and environmental protection outcomes.
Smelting
Iron ores consist of
oxygen and iron atoms bonded together into molecules. To convert it to metallic iron it must be
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, to be burnt in the smelting process.
However, it is not entirely as simple as that.
Carbon monoxide
is the primary ingredient of chemically stripping oxygen from iron.
Thus, the iron and carbon smelting must be kept at an oxygen deficient
(reducing) state to promote burning of carbon to produce
CO not
CO
2.
- Air blast and charcoal (coke): 2 C + O2 → 2 CO.
- Carbon monoxide (CO) is the principal reduction agent.
- Stage One: 3 Fe2O3 + CO → 2 Fe3O4 + CO2
- Stage Two: Fe3O4 + CO → 3 FeO + CO2
- Stage Three: FeO + CO → Fe + CO2
- Limestone calcining: CaCO3 → CaO + CO2
- Lime acting as flux: CaO + SiO2 → CaSiO3
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 determine 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 (
SiO
2) is almost always present in iron ore. 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. 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.
Turner (1900, pp. 192–197) reported that silicon also reduces shrinkage and the formation of blowholes, lowering the number of bad castings.
Phosphorus
Phosphorus
(P) has four major effects on iron: increased hardness and strength,
lower solidus temperature, 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 (
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),
where the speed and amount of carbon absorption is the overriding
consideration.
The addition of phosphorus has a down side. 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 often require it to be tough,
bendable, and resistant to shock at room temperature. A nail that
shattered when hit with a hammer or a carriage wheel that broke when it
hit a rock would not sell well. High enough concentrations of phosphorus
render any iron unusable (
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 (
Rostoker & Bronson 1990, p. 22).
Careful control of phosphorus can be of great benefit in casting
operations. Phosphorus depresses the liquidus temperature, allowing the
iron to remain molten for longer and increases fluidity. The addition of
1% can double the distance molten iron will flow (
Rostoker & Bronson 1990, p. 22). The maximum effect, about 500 °C, is achieved at a concentration of 10.2% (
Rostocker & Bronson 1990,
p. 194). For foundry work Turner 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 (
Turner 1900, pp. 202–204).
There are two remedies for high phosphorus iron. The oldest, and
easiest, 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, the
owner of Marlboro Iron Works did not appear to know about it in 1772.
Given Zane's reputation for keeping abreast of the latest developments,
the technique was probably unknown to the ironmasters of Virginia and
Pennsylvania.
Phosphorus
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. The
iron pillar of India which does not rust is protected by a phosphoric composition.
Phosphoric acid is used as a rust converter because phosphoric iron is less susceptible to oxidation.
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
aluminum 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 very hard to reduce. As a result aluminium contamination
of the iron is not a problem. However, it does increase the viscosity
of the slag (
Kato & Minowa 1969, p. 37 and
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; don't use ore or a lime source with a high aluminium
content. Increasing the ratio of lime flux will decrease the viscosity (
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 (
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 century 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 has to 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 (
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 (
Rostoker, Bronson & Dvorak 1984, p. 760). According to
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,
ironmasters looked for another ore. When mineral coal was first used in European blast furnaces in 1709 (or perhaps earlier), it was
coked. Only with the introduction of
hot blast from 1829 was raw coal used.
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 (
Turner 1900, pp. 77). However, historically (at least), iron sulfide (
iron pyrite FeS
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 (
Rostoker & Bronson 1990, p. 21).
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