A step in the direction of a circular economy: waste from marble quarries as aggregates for iron smelting

Die Eisenerzpellets werden benötigt, um die Verhüttung im Hochofen möglichst effektiv zu machen.

From Brazil comes a process to use fragments of marble extraction instead of limestone

Limestone is an important material for the production of iron and steel. It is extracted in quarries – nowadays the question arises as to whether another calcite rock could be used in its place. This calcite rock should be produced in other industrial processes anyway, and would not have to be extracted.

One will not immediately think of marble, because the material has a special reputation and also a special price, but its quarries produce large quantities of fragments, which usually go to the landfill.

After all, the recovery of the marble extracted from the quarry to ornamental stone is on average only 20%.

Marble waste, then, as a substitute for limestone in iron smelting, is the idea – but where, close to each other, is there an iron industry and a natural stone industry with marble extraction?

The answer to the question now comes from Brazil, namely from the state of Espírito Santo in the south of the country. In the area around the town of Anchieta, the multinational Samarco Mineração processes local iron ore into pig iron and steel, also using limestone from the surrounding area.

However, in the area of about 60 km from the smelters, there are also quarries where natural stone companies extract marble. The amount of marble residue by far does not correspond to the dimensions in which lime is needed for smelting – but the initial question was only whether limestone can in principle be replaced by another calcite rock.

It is a step in the direction of a circular economy, where residual materials from one process become new materials for another process.

Ein Teil des Forschungsteams von Samarco.

The old idea moved forward when 5 years ago representatives of the Sindirochas Regional Natural Stone Association visited the smelters and met with the iron cooks there. Samarco then set up the research project. Tales Machado, then president of Sindirochas, now president of the Brazilian-wide stone association Centrorochas, was there at the time.

After many material analyses, calculations, and visits to marble quarries, Samarco has now developed a process for the innovative use of marble waste and applied for a patent.

Since the approval in October 2022, 21,000 tons of marble waste have been used as aggregates in iron production.

In detail, the process involves the production of iron pellets. The ore must be in this form so that it can be optimally melted and liquefied in the blast furnace.

The pellets are produced in a process before the melting in the blast furnace – marble waste can now be used for this instead of the previous limestone.

Currently, 30% of the pellets are already produced with marble aggregate.

Afterward, in the blast furnace itself or in the subsequent step of converting the pig iron into steel, the marble waste can also take over the role of the limestone or quicklime: it triggers chemical processes with which the temperature required for prevention is lowered.

It is also important because it draws impurities from the ore and transfers them to the slag. Such impurities include sulfur and phosphorus, even small amounts of which make the iron unfit for use.

Samarco has had to overcome numerous challenges to ensure that marble can also play the desired role in the furnaces. When asked, the company gives us a few details: “The calcitic marble must have a CaO content between 49% to 55%, as well as an MgO and SiO2 content between 0% to 5%.“

Samarco Mineração (Portuguese)

Centrorochas (Portuguese)

Photos: Samarco

Stone powder for ceramics or for bricks

For some time now, attempts have been underway in Brazil and elsewhere to use stone powder from the sawing of natural stone raw blocks for the production of ceramics or bricks. The aim is to replace the clay that has been needed up to now. The topic is also becoming more and more interesting economically.

Research from Brazil (Portuguese 1, 2)

(11.08.2023, USA: 08.11.2023)