Why Add Rock to a Furnace Full of Rock? Something for the Waste to Join

Ore is a mixture of a metal compound and a great deal of rock, and heating it enough to release the metal does not separate the two, since the unwanted material has an extremely high melting point and stays as a solid mass through which the metal cannot drain. Adding a flux changes that, because the flux and the waste minerals combine into compounds that melt at a far lower temperature than either does alone. The resulting liquid floats on the denser molten metal as a separate layer, which can be drawn off, leaving the metal clean. The furnace therefore produces two liquids rather than a metal embedded in solid rock.

The choice depends on what the waste minerals are:

• •Limestone, by far the most used, which decomposes in the furnace to give lime
• •Silica or sand, used where the waste is already rich in lime
• •Fluorspar, which thins the molten layer and helps it flow
• •Borax, used in small-scale and jewellery work
• •Charcoal or coke, which is fuel and reducing agent rather than flux
• •Recycled slag from previous runs, which is common practice

The material drawn off is slag, and it is produced in enormous quantities, historically as a nuisance and now as a product. Blast furnaces generate roughly a quarter to a third of a tonne of it for every tonne of iron. Cooled slowly it forms a hard crystalline rock used as aggregate for roads and concrete. Cooled rapidly by quenching in water it forms a glassy granulate that, when ground, reacts with lime and water much like cement, and is used to replace a substantial fraction of the cement in concrete, which reduces emissions considerably since cement manufacture is among the largest industrial sources. Ancient slag heaps are also archaeological evidence, and analysing them reveals what was being smelted, at what temperature and with what success.

Working out that an addition was needed took a long time and the archaeological record shows it happening. The earliest copper smelting used ores that happened to contain their own fluxing minerals, so no addition was required and none was understood, and the technique worked only with particular ores from particular places. Deliberate addition appears later and spread unevenly, with some regions producing consistently better results than others for centuries without the knowledge transferring. Bloomery ironmaking, which was the standard method in Europe for two thousand years, never reached temperatures high enough to melt the metal at all, producing a spongy mass that had to be hammered repeatedly to squeeze out the slag trapped within it, which is why wrought iron carries visible slag stringers.

Beyond forming a drainable liquid, the added material performs several further jobs at once. It absorbs impurities from the metal itself, with sulphur and phosphorus being the critical ones in ironmaking since both ruin the finished product, and adjusting the chemistry of the molten layer controls how much of each is removed. It covers the metal surface and excludes air, preventing the metal from oxidising back into a compound. It dissolves oxide films that would otherwise prevent droplets of metal from merging into a single mass. In soldering and welding, a flux performs only that last function, cleaning the surfaces so the metals actually join, which is why the same word covers two rather different operations.

Waste minerals in ore stay solid at smelting temperatures and trap the metal, so a flux is added to combine with them into compounds that melt far lower and float off as a separate layer. Limestone is the usual choice. The resulting slag is produced by the tonne and is used as aggregate and as a cement substitute, and the flux also removes sulphur and phosphorus and excludes air from the metal.