Aggregates
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Processes: Air cooling, expanding, pelletizing

Air-Cooling Process

Molten slag can be directed to an open pit in which slag is cooled to a solid crystalline form by ambient air.  ASTM C 125 defines air-cooled blast furnace slag (ACBF) as "The material resulting from solidification of molten blast-furnace slag under atmospheric conditions.  Subsequent cooling may be accelerated by application of water to the solidified surface."

 

Solidified slag typically has a vesicular structure with many non-connected cells. When crushed, air cooled slag is very angular with few elongated pieces. The rough vesicular texture of slag results in a higher surface area as compared to smoother aggregates of equal volume; this rough vesicular surface provides excellent bonding characteristics.

 

Air-cooled blast furnace slag is an extremely versatile and durable construction material. Slag has an economic advantage over most natural aggregates due to it's lower per unit weight, thereby reducing dead load. The advantages are:

  • Lower per unit weight - reducing dead-load
  • Lower transportation and labor costs
  • Excellent aggregate bond characteristics in concrete
  • Greater yield for all construction applications
  • Improved skid resistance and stability in asphaltic pavements
  • Improved fire resistance for masonry and concrete applications

 

Most blast furnaces have two cooling pits.  One pit can be cooled and excavated while the other is being filled with molten slag.

 

The air-cooled slag is excavated by front-end loader and loaded directly onto tractor-trailers.  It is then shipped to the crushing and screening facility where it is processed into numerous value-added construction materials.

 

These processed products include granular aggregates for road base and fill, aggregates for concrete, and raw material for manufacture of mineral wool.

 

Expanding Process

The innovative, expanded slag process was developed by Lafarge to produce a lower density aggregate for value added applications such as lightweight concrete block and masonry.

 

To produce a lightweight solid, a stream of molten slag is mixed with water.  The mixture of slag and water then flows into a receptacle. The interaction between the slag and water occurs for a predetermined period of time to form an expanded solid.  This is accomplished without any additional cooling water, which would otherwise quench the material and vitrify it.  In the receptacle, a vaned rotor operates to segment the interacting mixture of slag and water and projects the segmented mixture through the air to collect in a pile.

 

True Lite Lightweight AggregateTM brand aggregate is the product of this process.  True Lite offers design advantages through:

  • Weight reduction and lower dead load
  • Increased fire resistance
  • Improved sound attenuation
  • Enhanced thermal insulation

 

These advantages apply to lightweight masonry and structural concrete applications, as well as other applications.

 

Gradations and Densities: Expanded blast furnace slag is available in a number of gradations including: Structural Coarse (1/2" x 3/16"), Coarse (3/8" clear), Fine (-3/16") and Blend (-3/8"). Densities range from 688 kg/m3 (43 lbs/ft3) to 950 kg/m3 (60 lbs/ft3), depending on gradation.

 

Pelletizing Process

The innovative slag pelletizing process was developed by Lafarge to produce a value added product benefiting the construction industry.  Pelletizing blast furnace slag makes it suitable for use as either a lightweight aggregate, such as LitexTM, or as a raw material that can be ground into slag cement, such as VitrexTM.

 

Normally fed to the pelletizer directly from the runner system of a smelting furnace, molten slag flows down a vibrating feed plate over a large spinning drum, while being quenched with water. The combination of water and sudden air-cooling - as pellets fly through the air from the force of the drum's impact - turn the slag into hard-vitrified pellets possessing an internal micro-cellular structure.

 

The treatment of the molten blast furnace slag with water results in a foaming action that leads to a unique internal cellular structure within each slag pellet.

 

Lafarge's development of the pelletizer followed many years of experience with other slag processing methods, each having limitations. In addition to environmental compatibility, the slag pelletizer offers simplicity of operation, economy of space, low capital investment, low energy and water consumption and low operating costs, while providing product versatility.

 

The raw pelletized product can be shipped directly to the customer, taken to the Lafarge docking facility for export or transferred to a crushing and screening facility where it can be further processed into a number of other products.

 

Gradations and Densities: Pelletized slag is available in a number of gradations, including: Structural Coarse (1/2" x 3/16"), Coarse (3/8" clear), Fine (-3/16") and Blend (-3/8"). Densities range from 912 kg/m3 (54 lbs/ft3) to 1105 kg/m3 (69 lbs/ft3), depending on gradation.

 

Construction materials produced through pelletizing at Lafarge include lightweight aggregates for concrete, concrete block and lightweight engineered fill, and raw material for grinding into slag cement.  Pelletized slag in masonry and structural concrete offers advantages, including fire resistance, sound absorption, thermal properties and reduced dead load.

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