Slag and Slagging: Key Differences and Valuable Applications in the Metallurgical Industry

2025-06-23

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As molten metal glows brightly in the hot metallurgical furnace, two seemingly similar by-products 

are created - slag and float. They are often confused, but are very different in the metallurgical 

process, each plays a unique role in the resource cycle chain.


Nature and birth: two products of high-temperature smelting


Slag (Slag):


Source: mainly associated with the smelting of metal ores (such as blast furnace ironmaking, steelmaking, 

copper smelting) or fuel combustion (such as coal-fired boilers) and produce.


Formation: in the high temperature smelting process, the ore in the useless vein minerals (such as silica, 

alumina, calcium oxide, magnesium oxide, etc.), the fuel in the ash as well as deliberately added to the flux 

(such as limestone, fluorite) complex physicochemical reactions, and ultimately the formation of floating in 

the metal melting pool above the molten silicate or silica-aluminate mixtures.


Purpose: Its central role is **“inclusion and separation ”**. The slag effectively collects and dissolves harmful 

impurities (sulfur, phosphorus, etc.) in the raw material, protects the target metal from oxidation, and facilitates 

the aggregation and settling of the metal droplets to physically separate the metal from the slag.


Dross:


Source: mainly produced in the refining, melting and holding process of molten metal, especially in the treatment 

of non-ferrous metals (such as aluminum, zinc, lead, tin and its alloys) is most common.


Formation: When molten metal is exposed to the air, its surface is very easy to react with oxygen to produce a layer 

of solid or semi-solid material composed mainly of metal oxides. In addition, inclusions uplifted from the molten 

metal (e.g., fine oxide particles, melt residues, furnace lining erosion, etc.) are also enriched.


Composition: The main body is the target metal oxides (such as aluminum slag is mainly Al₂O₃, zinc slag is mainly ZnO), 

mixed with a considerable number of unoxidized metal particles (wrapped in oxides), flux components and impurities.


Resource utilization: the key path to turn “waste” into treasure


Recycling of slag:


The cornerstone of the building materials field: this is the most important destination for slag. Water quenching and 

granulation of blast furnace slag is the production of high-quality slag silicate cement active material; steelmaking slag 

after stabilization can be used as a good road-building aggregate or engineering backfill; part of the slag can be used 

to produce slag cotton (thermal insulation materials), microcrystalline glass or as a soil conditioner. The key lies in the 

scale treatment and stability control of performance.


Environmental protection and cost win-win: a large number of use of slag to replace natural sand and gravel aggregate, 

significantly reducing the consumption of natural resources mining and stockpiling slag place required for land 

occupation, reducing the environmental load, but also for the metallurgical enterprises to bring certain economic benefits.


Scum value mining:


Metal recovery is the core: the core of the value of slag is contained in the unoxidized metal particles (such as aluminum 

beads in aluminum slag). Through the crushing, screening, smelting (usually need to add salt melt cover to prevent 

oxidation) and other processes, you can efficiently recover the valuable metals, which is the primary goal of the slag 

treatment and economic drivers. The level of recovery rate directly affects the economic efficiency.


Exploration of Oxide Components: Oxide-rich residues (e.g., aluminum ash/secondary aluminum ash) after recovering 

metals are also seeking ways to be utilized, such as as raw materials for refractory materials, raw materials for water 

purification agents (polymerized aluminum chloride), ceramic additives, or admixtures for construction materials, 

etc., but these applications are usually small in scale or have high technical requirements.


Challenges and the future: towards a more efficient cycle


Challenges of slag utilization: The fluctuation of slag composition from batch to batch, the potential leaching risk of 

certain elements (e.g., phosphorus, chromium, heavy metals), and the economics of high value-added utilization

 technologies are still directions that need continuous attention and improvement. Standardized and refined 

classification and utilization is the trend.


Challenges of slag treatment: Improving metal recovery (especially fine-grained metals), reducing energy and flux 

consumption in the recovery process, developing more environmentally friendly and efficient metal recovery 

technologies (e.g., salt-free treatment), and high value-added utilization of oxide-rich residues are the focus. Source

 reduction of floating slag (e.g. improved melting processes, protective measures to reduce oxidation) is equally

 important.


Driven by both policy and technology: Increasingly stringent environmental regulations and the pressure of scarce 

resources are driving the metallurgical industry towards more efficient and environmentally friendly resource

 utilization of slag and slag floats. Innovative treatment technologies, intelligent sorting and recycling equipment

 and the construction of a circular economy industry chain will be the future direction of development.


Conclusion


Slag and slag, the metallurgical industry's “twin”, although both born in the high-temperature furnace, but has 

a very different gene and mission. Slag as a collection of silicates, its value lies in the scale of conversion into 

building materials; slag as a mixture of metals and oxides, the core of which lies in the efficient recovery of 

valuable metal resources. A deep understanding of their differences is not only necessary for the optimization

 of metallurgical processes, but also a key step in promoting efficient recycling of resources and realizing green

 and low-carbon development. In the wave of circular economy, every properly utilized slag and float slag 

signifies our deeper respect and more responsible attitude towards the earth's resources.