Slags are by- or co-products of the iron and steel making process. BF (blast furnace) slag is generated in iron production and EAF (Electric Arc Furnace) slag is generated by steel production. The same is with Basic Oxygen Furnace (BOF) slag. Processing of EAF using traditional methods involves crushing and grinding. However, the new technology of atomising molten slag is an innovative process that produces high-specification products with multiple uses.
Slag Atomising Technology (SAT) is invented, patented and commercialised by Ecomaister Co., Ltd of Korea. Many plants are successfully in operation and many are under construction, 5-6 projects under negotiation. SAT is a strategic approach to overcome the environmental problems of electric arc furnace slag. A modern technology and operating system, the unique JIT technological management operation of hazardous materials. JIT means 'Just In Time' operation, since the molten waste slag is immediately converted to environmental-friendly PS balls as soon as it is dumped into the SAT system.
Introduction
EAF slag is a waste material with a huge production volume in melt shops (15-20% of melt capacity). It contains metal residuals and can not be treated easily and with cost effective methods. Slag atomising technology (SAT) is a new system adopted (invented) to atomise molten slag out of electric arc furnaces in a most effective way, and is most economical compared to traditional methods of aging and crushing. The products of SAT process are PS balls, spherical shaped with different diameters.
Slag Atomising Technology (SAT)
Slag Atomising Technology is a process that converts molten slag (1300-1350ºC) into small spherical balls with a diameter range of 0.1 to 4.5mm. The process consists of a high speed air blowing system with catalyst and water exposed to the stream of falling molten slag out of a tundish into the yard. The high speed air flow with assistance of water and catalyst employed for this purpose, through a fast heat exchange, converts the slag stream into spherical balls with glassy finishing. The PS balls structure is typically a stable spinel structure that induces superior physical characteristics to the balls. PS balls are classified by size through a sieving machine.
The main part of molten slag is processed through the atomising system, usually accounts for 75-80% of the molten slag. The remaining part of the slag comprises the heavy materials including the useful metal that is collected on the bottom of the carrying pot. Our experience in Korean SAT plants represents an average of 3% of metal content out of total slag weight. About 20% of slag is dumped into the slag cooling pit and crushed with mechanical crushing systems after cooling. The crushed slag is passed through magnetic separators to separate the metal. The metals are collected as recycled metal and the remaining slag is an iron free material suitable for cement plant admixture with maximum size of 4.5mm.
Comparison of conventional slag processing (CSP) versus new SAT process
A) Slag status of conventional method (CSP): In general slag that is processed after cooling by water in normal temperature. This slag processed by CSP contains free CaO about 0.1 % to 20%. Over 1% of free CaO is the primary factor of expansion of slag itself. Free CaO is also a factor of pollution, and caused the restrict ion of its applications.
Free CaO + H2O reacts to Ca(OH)2
It means that free CaO will be changed to Ca(OH)2 when it reacts with H2O. After all Ca(OH)2 is weathered continuously, so the slag's usage should be restricted for above reason.
For the civil work applications, the slag produced by CSP should be weathered in a slag yard for one to six months or aged by steam which will add cost.
B) Slag statues in new SAT method: As explained, free CaO and Fe2O3 are scattered into solid slag assuming the form of the molecule itself if slag is processed by CSP, eventually free CaO and Fe2O3 is blown off in the air after weathering. To apply this new technology, let the molten slag cool immediately by the high-speed air and water, the various unstable elements assume the forms of CaO-Fe2O3, SiO2-Fe2O3, Mg-Fe2O3.
There shall be no free CaO in the products, and the surface will be glassy with spinel structure. The spinel structure is the combined form of CaO-Fe2O3, CaO-SiO2 and the like.
Graph 1 shows the analysis result of XRD of slag processed by SAT and shows the formation of dicalcium ferrite {2CaO- Fe2O3} and larnite (β- 2CaO-SiO2}.Besides, this new technology enables the slag to take on a granulated-round shaped form which comes into good use in concrete mixtures. The chemical ingredients of PS balls are nearly the same as the slag of the conventional method but the free CaO content is below 0.15% while the content of the conventional method is over 1%. For the physical characteristics, the specific gravity of PS balls proved 3.56 as the content involves 2CaO-Fe2O3, Fe2O3 and Fe2O4, and the absorption rate of water indicates 0.42% while the natural sand is about 1%. That was the result of SAT, a new approach to process molten slag, which blocked the PS balls from the air-hole. This technology enables PS Ball diversity in various applications.
Advantages of SAT
Basically, it is possible to ignore any environmental dispute related to the storage or disposal of PS balls. PS balls can be used for various applications (water treatment, construction material, abrasive, desulphuriser, etc):
- Improvement of work-conditions by reducing dust, noise and wastewater.
- Saving space in slag yards.
- The traditional method of crushing and aging is an obstacle of increasing production capacity, SAT is the unique and best solution.
- Enhancing the company's reputations as an environmental-friendly manufacturer and protecting against a civil appeal.
- Improvement of metal-collecting efficiency by adopting SAT.
What is a PS ball?
PS ball is an abbreviation for Precious Slag ball, a newly born useful material generated from waste material. It has a glassy surface with spinel structure. PS Balls are spherical in shape and 0.1 to 4.5mm in diameter, a non-expansionary material with no free lime.
The PS balls have many applications, thanks to their physical and chemical specifications. Most importantly, PS balls are environmentally-benign materials produced by a sophisticated technology, and free of any pollution issues.
Characteristics of PS balls
Spinel structure, is the main characteristic of this material, a stable physical and chemical structure that removes the main factor of pollution. Free CaO and FeO are scattered into the solid slag assuming the form of the molecule itself. If the slag is processed by the conventional method, eventually free CaO and FeO are blown off in the air after weathering. But SAT as a new technology lets the molten slag cool immediately by the high-speed air, catalyst and water, the various unstable elements assume the forms of CaO-Fe2O3, SiO2-Fe2O3, Mg-Fe2O3. There shall be no free CaO in the products and the surface will be glassy with spinel structure.
PS Ball has a superior characteristic compared with sand on compressive strength, hardness and anti-weathering.
Chemical & Physical properties of PS ball
Typical properties are based on the products of Korean EAF & BOF slag, see tables below.
| Classification | Component Ration (%) | Remarks | |||||
| CaO | Fe2O3 | SiO2 | Al2O3 | MgO | Etc. | ||
| Pohang Branch | 40.4 | 28.2 | 11.7 | 3 | 6.5 | 10.2 | Converter slag |
| Dangjin Branch | 17.4 | 44.2 | 20 | 7.6 | 6.2 | 4.6 | Electric arc furnace slag |
| Physical properties |
Hardness (V/C) | Diameter | Specific gravity | Water permeability | Comprehensive strength |
| 739.8 | 0.1-4.5mm | 2.3kg/l | 5.3x102cm/sec | 323kgf/cm2 |
Applications of PS balls
As a newly born material, PS balls have superior physical and chemical characteristics that create a great diversity of applications.
1) Shingle roofs
2) Manholes
3) Abrasive blasting material
4) Road pavements
5) Weight materials
6) Noise barriers and radiation shielding
7) Cement admixture
8) Floor material
9) Weak ground improvement
10) PC piles
11) Water and waste water treatment
12) Filter materials
13) Non-sliding floors
14) Bricks and prefabricated concrete items
15) Anti-wearing tiles
Conclusion
Slag Atomising Technology (SAT) is a new technology to manage the slags produced by EAFs, to produce valuable products out of waste materials. All products are environmentally-friendly materials.
Compared to conventional methods of EAF slag processing, SAT is more versatile, most economical and has few environmental constraints. All products from this process are useful in one way or another, the most important is the capacity to convert waste material into reusable materials. PS balls as end products are very uniform (uniformity coefficient 1.22 compared to sand with 1.64) with a much higher compression strength (161% increased CS rather than sand), good hardness of 740 Vickers (62 Rockwell).
It is a patented technology (acquired patents from US, Japan and Korea), with many honourable awards and certificates from different administrations and Environmental authorities. SAT is already in operation in Korea with a total capacity of 800,000mt/year in different steel mills. 300,000mt/year under construction started operation during second half of 2006, many plants are operating or are under construction and negotiation in Malaysia, South Africa, China, Taiwan and Spain.
Slag Atomising Technology is a multi-function, multi-application system, a great managerial approach to solve environmental and technical problems inherent in waste material.
