Our sustainable, dry method for slag granulation derives value from waste and has the potential to transform the metal and cement industries.

The challenge

Water-intensive slag waste processing

Slag is a high volume by-product from the metal smelting process. Globally, hundreds of millions of tonnes of molten slags are produced each year.

Close-up of molten steal showing spiral of glowing molten metal  ©

Slag is commonly air-cooled in large pits and used for landfill or used for road-based materials after crushing and screening. In some modern integrated steelworks, molten blast furnace slag is granulated using water jets — producing a glassy product which can be used as valuable feed in cement manufacturing by replacing ordinary Portland cement and significantly reducing the greenhouse gas emissions associated with the calcinations process.

However, water granulation consumes large volumes of water and may generate acid mist causing air pollution. More importantly, the wet process does not recover the large amount of high-grade heat contained in the molten slag. On cooling from around 1500 °C to ambient temperature, one tonne of molten slag releases about 1.8 GJ of heat. This could amount up to 800 PJ of heat globally from blast furnace slags alone and if recovered and utilised, could potentially reduce emissions by up to 60 million tonne each year.

Our response

Recover heat and create valuable waste products

[Music plays and text appears: Forging our future in green steel]

[Image changes to show foundry workers working with molten metal]

[Image changes to show Mark Cooksey, Group Leader, Sustainable Process Engineering]

Mark Cooksey: Steel making is one of the world’s largest industries and it produces over 300 million tonnes of slag every year.  So slag is the waste product from iron making. 

[Image changes to show the spinning disc for the slag granulation process and then moves back to Mark Cooksey]

At CSIRO we spent more than a decade working on advanced technology to deal with that slag in a way that’s more environmentally friendly.

[Image changes to zoom in on foundry workers working with a crucible of molten metal]

The current methods are dump it on the ground which is perfectly acceptable.  It’s not hazardous but that means you waste a large amount of material that could be used for another use and you lose all the heat, a gigajoule of heat for every tonne of slag.

[Image changes to show Mark Cooksey]

The other way of dealing with slag is to granulate it using water. 

[Image changes to show granules of slag being poured through the hands]

This produces good granules that can be used for cement manufacture.

 [Image changes to show Mark Cooksey]

The trouble is you use 1,000 litres of water for every tonne of slag so that’s a large environmental cost. 

[Images flash through of the process of dry slag granulation]

What we’ve developed here is dry slag granulation.  We can granulate the slag, produce a good quality product that can be used for cement manufacture but do it in a way that doesn’t involve water. 

[Image changes to show Mark Cooksey]

If you use our products to produce cement for each tonne you use you basically save 800 kilograms of CO2 emissions.

[Image changes to show Mark Cooksey in front of the spinning disc showing the process of moving slag into the spinning disc]

[Image changes to show the inside of the spinning disc]

Our dry slag granulation technology involves pouring molten slag from above our rig down to a spinning disc.  That disc is spinning at high speed and it atomises the molten slag into small granules.

[Image changes to show molten slag being atomised and collected in the taurus]

They solidify as they are travelling through the air and are collected in a taurus that surrounds the disc.  The really smart bit of the technology is the disc. 

[Image changes to show Mark Cooksey]

That involved a lot of computer modelling and design to come up with a disc that can produce consistent granules reliably.  Other people that have worked on this technology have struggled with that part of the process.

[Image changes to show two smaller images one of the disc and the other of the molten slag being atomised and then moves back to Mark Cooksey]

Air is used to blow the granules around the taurus.  The granules are ultimately collected from various points and the hot air is extracted out of the top and that’s how we recover the heat energy. 

[Image changes to show Prof Gang Wei, Director, China Engagement for CSIRO Manufacturing and Minerals Flagships]

Prof. Gang Wei: We have developed in partnership with a Chinese company M.C.C.E.  Through this collaboration we can commercialise CSIRO leading technology to benefit both nations through reducing carbon dioxide emission and to save water and most importantly recover energy as well.  And at the end of the day it’s a global benefit for the society.

[Image changes to show Jonathan Law, Director, Mineral Resources Flagship]

 [Image changes to show the M.C.C.E. webpage]

[Image changes to show Jonathan Law, Director, Mineral Resources Flagship]

Jonathan Law: We’ll be working with a company called M.C.C.E. in China.  China produces 50% of the world’s steel and they will be developing a pilot facility at one of the Chinese steel facilities to test this technology and then to take it to market, first in China but ultimately around the world. 

[Camera zooms in on the M.C.C.E. webpage]

This collaboration could be really important for a number of reasons.

[Image changes to show steel works]

Firstly it brings Chinese production technologies together with Australian science and secondly if implemented around the world it’ll have a huge impact on water and energy use and greenhouse gas emissions.

[Image changes to show Jonathan Law, Director, Mineral Resources Flagship]

[Image changes to show street lights along a road to the city]

Just to give you an example, if this technology was deployed everywhere in the world it would equate to about 14% of Australia’s annual energy usage, about 10% of Australia’s greenhouse gas emissions.

[Image changes to show the CSIRO webpage]

 The success is based on the CSIRO flagship programme. 

 [Image changes to show Jonathan Law, Director, Mineral Resources Flagship]

This lets us think strategically about what the important technologies Australia and the world will need in the future and dry slag granulation is a great example of us thinking about a demand in the future, working with our scientific collaborators to get the technology to a commercial ready stage and now we’re ready to commercialise it.

 [Music plays and text appears: CSIRO Big ideas start here, www.csiro.au] 

Forging our future in green steel :  This smart Australian technology is set to transform global steelmaking by making a cement product from blast furnace waste, saving water, recovering energy and reducing greenhouse gas emissions.

We developed a new, integrated dry granulation and heat recovery process which promotes sustainability and full value recovery.

Molten slag is atomised under centrifugal forces exerted by a spinning disc to produce droplets which are then quenched and solidified using air to recover the heat. This method produces glassy slag for cement manufacturing and simultaneously recovers waste heat as hot air which can then be used onsite for drying, preheating or steam generation.

Compared with the water granulation process, the dry granulation process provides a much more sustainable approach, overcoming major shortcomings of the existing method through saving water, eliminating sulphur emission and recovering high-value waste heat.

The results

A sustainable process to lower water use and increase value

The dry granulation process is destined to replace the conventional water granulation process, delivering sustainable, environmentally friendly and full value recovering process with benefits of:

  • waste heat recovery
  • huge savings in water
  • reduced air pollution
  • lower capital costs.

By converting low value slow cooled blast furnace slag into a high value material for the cement/construction industry, this process turns a waste product into wealth.

We are currently negotiating with international engineering companies to commercialise the dry slag granulation technology. Discussions are now underway with steel producers in preparation for further scale up (processing 1-2 tonne/min slag) and plant trials at one of Australia's blast furnaces within 1-2 years.

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