A cargo of iron ore is sold on a specification, not a photograph. The spec sheet is a short table of chemical assays, and every line on it moves money. A buyer reads it to answer one question. What will this ore cost to turn into steel, and what will it be worth delivered. Learning to read the sheet is learning to see the furnace behind the numbers.
The sheet is deceptively plain. A handful of elements, each a number to two or three decimal places, printed on a single page. Yet a trader who knows the trade can read a delivered cost and a processing headache straight off that page, while a newcomer sees only chemistry. The gap between those two readings is what this guide is about. Every element has a destination in the process, and the assay is really a forecast of where each one will end up.
Fig. 1 A concentrate assay and what each line signals to a buyer
Fe percent, the headline
The first line is total iron, written as Fe percent. It is the single most quoted figure and the base of pricing. Higher iron means more metal per ton and less waste to handle, melt, and remove as slag. The global benchmark is set at 62 percent. Ore above that earns a premium, ore below it a discount. A concentrate at 69.8 percent sits well above the benchmark, which is the starting point of its value, though iron alone never tells the whole story.
SiO2 and Al2O3, the gangue
Silica, SiO2, and alumina, Al2O3, are the two main gangue oxides. They are the rock that came with the iron. Both end up in the slag, the molten waste that floats on the iron and is tapped off. More gangue means more slag, more flux to manage it, more energy to keep it fluid, and less furnace volume for iron. Silica is the larger fraction in most ores. Alumina is watched even more closely, for a specific reason covered below.
Phosphorus, and why low P avoids a whole stage
Phosphorus is a tramp element. In steel it causes cold shortness, a brittleness that shows up under stress at low temperature. Most steel grades hold a tight ceiling on phosphorus. The trouble is that phosphorus in the ore largely follows the iron into the metal. Removing it means running a dephosphorization stage, an extra step of oxidizing and slagging under controlled conditions that costs time, reagents, and yield. An ore that arrives with phosphorus already low lets a mill skip or shorten that step. A concentrate near 0.028 percent phosphorus is in that friendly range.
Alumina and the tuyeres
Alumina deserves its own line of thought. In a blast furnace, alumina raises the viscosity of the slag. A stiff, sticky slag does not drain cleanly. It hurts gas flow through the burden and it puts thermal and chemical load on the tuyeres, the nozzles that blast hot air into the base of the furnace. Operators therefore prize low alumina and a favorable silica-to-alumina ratio. Ore that keeps alumina down protects furnace stability and tuyere life, which is why the number gets scrutiny out of proportion to its size.
Sulfur and loss on ignition
Two more lines commonly appear. Sulfur, S, embrittles steel and must be controlled, though much is driven off during sintering. Loss on ignition, LOI, measures volatiles and bound water that leave when the ore is heated. High LOI means part of the shipped weight vanishes in the furnace, so buyers account for it. These are secondary to iron and phosphorus, but they are not ignored.
How penalties and premiums are applied
The spec sheet becomes a price through a schedule of adjustments. A contract sets a base price at a reference grade, then adds and subtracts. There is usually a premium per iron unit above the base and a discount below it. There are penalty scales for silica, alumina, phosphorus, and sulfur above stated thresholds, charged per tenth of a percent. A clean, high-grade concentrate collects the grade premium and avoids the impurity penalties at the same time. That is the whole logic of the sheet. Value is grade earned minus penalties owed.
Moisture, size, and physical terms
The chemistry is the core of the sheet, but a full specification also carries physical terms. Ore is quoted on a dry basis, yet it ships wet, so a moisture figure sets how much of the loaded weight is water the buyer is not paying for as iron. Particle size distribution matters too. Lump, fines, and concentrate behave differently in a furnace or a pellet plant, and a size that is out of window can attract its own handling penalty. These lines rarely dominate a negotiation, but a buyer reads them because they affect delivered tonnage and downstream handling.
Why consistency is its own value
One quality that never appears on a single assay is consistency, and it may matter as much as any line that does. A mill tunes its process to a feed. Grade and impurities that hold steady cargo to cargo let operators run closer to their limits with less reblending and fewer surprises. An ore that swings, even around a good average, forces conservative operation. When a producer can show that its concentrate holds a tight, repeatable specification, that reliability is a real part of the value, even though it lives in the record of many spec sheets rather than any one of them.
Reading it as a buyer would
Put together, the sheet tells a story in five or six numbers. Iron sets the base. Silica and alumina set the slag burden and the penalty exposure. Phosphorus decides whether a costly stage is needed. Sulfur and LOI trim the edges. A metallurgist does not admire a spec sheet. They cost it, line by line, into a delivered number. Once the sheet reads that way, the difference between two ores that both say iron ore becomes obvious.