Introduction
Banded Iron Formation (“BIF – pronounced Biff”) is the most important source of iron ore today. It occurs on all continents and iron content can be monumental. Here in Australia, the Hamersley Range in the West is thought to have initially contained 100 trillion tonnes of iron. This is probably less than 5% of the world’s total iron in BIF. Of course a trillion is not a large number in today’s inflated world. But even so, at current production rates that would be enough to last the world for a few hundred thousand years or so.

So the world is not short of iron, but what about iron ore. “Ore” is a mineral or an aggregate of minerals from which a valuable constituent, especially a metal, can be profitably mined or extracted. The primary iron in BIF occurs as mostly magnetite and lesser hematite1 at typical grades of 20-35% iron. Most of this iron is not economically exploitable. It is not ore.

While it sometimes seems today that enthusiastic promoters can call almost any BIF, or anything related to BIF, an ore, this is not always justified. Below we briefly explain some of the things to look for, but first…

Some background
BIF’s are enigmatic chemical precipitates characterized by the presence of alternating layers of iron-rich and amorphous silica-rich layers at all scales. The boundary between layers is sharp, even at microscopic scale. Their origin and manner of formation continues to spark quarrelsome debate within the geological fraternity.

At the risk of adding more fuel to the debate (and of creating try-hard metaphors), the classic theory is that the layers were formed in the sea. Bacterial photosynthesis released oxygen which then combined with dissolved iron to precipitate iron oxides. The banding cycle is thought to relate to variations in available oxygen. The variation is possibly seasonal, but probably not.

The vast majority of BIF’s range in age from 3.3 billion years old to around 2.1 billion years old. About 90% of all know BIF’s were deposited around 2.2 to 2.5 billion years ago. While iron-bearing units were deposited later than 2.1 billion years ago, they are not considered true BIF’s.

Now for something different but related. The Great Oxidation Event, also known as the Oxygen Crisis, began around 2.4 billion years ago. This event was the point at which dissolved iron was unable to consume all the oxygen produced by photosynthesis. Oxygen thus began to accumulate in the oceans and ultimately the atmosphere. For the environmentally minded, the event lead to the greatest extinction in history – that of the Earth’s anaerobic residents. Further, it is thought to have reacted with atmospheric methane leading to catastrophic climate change. Some villain that oxygen.

Anyway, enough of the history lesson, on to the ore. I only consider BIF-related ore below. Other types of iron ore, infrastructure, country risk, logistics, transport, markets and so on will be left for another time. Nevertheless it should be noted that the development of iron ore deposits is usually constrained by factors other than the ore itself.

Bedded Iron Ore
The primary magnetite/hematite in BIF’s can be enriched in situ through natural processes to iron grades of 60% or more. These processes oxidize the magnetite and remove the silica, resulting in deposits which are primarily comprised of hematite along with lesser goethite. They are often of sufficient quality to be classed as Direct Shipping Ore (“DSO”), which is to say they need no or negligible beneficiation, often just crushing and screening, before shipping to the customer.

One of the largest hematite deposits in the world (and the biggest mine, 5 by 0.5 kilometres) is Mount Whaleback, operated by BHP at Newman in Western Australia. It had an original resource of 1.7 billion tonnes of high quality iron ore at a grade of 64% iron. In Brazil, the Carajás mining district contains multiple hematite deposits with an initial total resource of more than 17 billion tonnes with grades of over 64% iron.

The most important factors in determining the economic value of bedded iron ore are the physical and chemical characteristics. The ore must be able to be crushed without producing an undue amount of fines, which impede circulation in a blast furnace. Fines can be easily dealt with but at a cost. Generally, hematite ore needs a minimum grade of 55%, and realistically greater than 62%, mainly because the lower the grade the higher the contaminants. However, lower grade bedded iron ore can often readily be beneficiated to DSO at low cost.

Ideally iron ore contains only iron and oxygen. This is never the case. The most significant contaminants are silica, alumina and phosphorus. There are many other elements that are deleterious to the value of iron ore. Arsenic, barium, chlorine, cobalt, copper, nickel, lead, antimony, strontium, zinc and zirconium are some.

Phosphorus is the bad boy. There are 8-10 billion tonnes of high phos (more than 0.10% by weight) iron ore in Western Australia that no one wants. Phosphorus today is impossible to remove from bedded iron ore. Avoid.

Silica and alumina should be below around 5% and 2% respectively. Anything much higher would generally not be considered ore. Both can be removed by beneficiation and /or fluxing in the blast furnace, but this can be a significant expense.

Finally, a desirable loss on ignition (“LOI”) is 7-10%. LOI is defined as the amount of water that vaporises at a temperature of 1000oC.

 

Detrital Iron Ore
The fragments from erosion of bedded deposits can be deposited in economic concentrations where natural traps exist. These are referred to as detrital iron deposits. The quality of the ore depends upon the quality of the deposit from which they are derived. Because of the way they form, detrital deposits can often have a very high proportion of high quality lump ore.

Today a number of companies classify low grade, highly contaminated iron-rich material as detrital iron ore on the basis that it can be beneficiated to produce an economic product. The risk here to the investor is twofold. Can it be upgraded and, if so, at what cost. Take care.

Channel Iron Ore
Economic deposits of iron ore can form through the deposition of tiny eroded grains of hematite in river channels. These are called channel iron deposits. They appear to be unique to Western Australia.

The grains are deposited by erosion and the iron in ground water accretes around the grains to form pea-sized grains known as pisolites. The pisolites are in turn cemented by further deposition of iron. Most channel iron ore mined in Western Australia is DSO. Because of the nature of the iron (hematite with a goethite matrix) these deposits tend to be lower in iron content which is offset by a higher LOI.

The same comments regarding low grade detrital iron apply to low grade channel iron deposits.

Magnetite Iron Ore
Beneficiated2 magnetite from BIF forms a substantial part of the world’s iron ore production. It is a well understood and generally straightforward process.

The most important factors that determine whether BIF can be economically beneficiated are iron content, crystal size and type, and contaminating elements. Deposit size and mining parameters are not usually relevant given the immense size of BIF formations.

Typically, the BIF would need to be at least 25% iron and the magnetite and silica able to be separated by grinding to no more than 30 to 45 microns. The concentrate should grade in excess of 63% iron and have low phosphorous, aluminium, silica, and titanium.

The process required to produce magnetite ore from BIF is very expensive to build and operate when compared with DSO ore. Once mined the ore must be ground very fine before the magnetite and silica can be magnetically separated. The magnetite slurry must then be dried and filtered. It is then typically rolled into balls or pellets and roasted.

Most BIF’s can be readily beneficiated; capital and operating costs are the determining factor regarding commercialisation.

Final Word
Bedded is best. If it is not direct shipping ore then make certain it is amenable to cost effective beneficiation to DSO standard before investing.

Beneficiating magnetite from BIF is well understood and works just fine; but it is expensive. Take care that you understand where your investment sits in the costs curve as the iron ore price has significant risk on the downside in the years ahead.

A good detrital or channel is a thing of beauty; DSO and negligible mining/beneficiation costs. But. Take extreme care with low grade contaminated “detrital”.

 

 

—oo0oo—

1 Iron can exist in a wide range of oxidation states with the most common being the +2 (ferrous) and +3 (ferric) states. The most common form of iron in BIF is magnetite (Fe3O4) which contains iron in both the +2 and +3 states and comprises 72.36% iron when pure. Hematite (Fe2O3) contains iron in the +3 state and comprises 69.94% iron when pure. The hydrated oxides of iron, goethite and limonite, are also mined but are much less common.

2 Beneficiation is the process of separating ore (in this case magnetite) from waste or gangue (in this case mostly silica) by any of a variety of processes.

19 April 2012 Commodities