Foundry Filler Sands:
Types, Uses and Advantages
Filler sands, also known as ladle sands or nozzle sand is a granular refractory material used to avoid contact between molten steel and slide-gate system.
The casting process in steelmaking involves passing the steel ladle from tapping to teeming. This is continued until the semi-finished product is formed. One major factor that can hamper this process is the 'non-free opening' of the slide-gate system, blocking the flow of steel from the teeming ladle to the tundish. It is at this point that filler sands come in. They are used at the inner area of the nozzle and the well block to prevent contact between the poured molten steel and the slide-gate system. It is used in the secondary steel refining process.
Of all types of filler sands, Chromite-based sands are the most used. For the most part, the most important properties of a filler sand are particle-size distribution, low thermal expansion, flexibility, refractoriness, composition, and packing. These properties have a significant impact on the formation of the sintered crust and its properties when the filler sand makes contact with liquid steel.
Perhaps, the most important property of filler sands is its ability to form sintered crusts of about the right thickness when it is brought in contact with molten metal.
The ideal filler sand must have a good balance between surface melting and refractoriness as well as produce a splintered surface layer. Even so, the splintered layer must not be too thick.
Operators in foundry industries often experience difficulties in removing the sand due to sintering. When the sand used gets in the way of the nozzle, oxygen burning becomes necessary. Oxygen burning, however, worsens the quality of the steel produced and in some cases, affects the whole process.
This ladle-free opening rate is what causes the most nightmare to metallurgists and operators in foundry industries. It is also for this reason that filler sands are used and preferred over other means of keeping the ladle open like using an oxygen lance.
When the filler/ladle/nozzle sand is composed mainly of chrome ore, it is referred to as chromite-based sand. Chromite-based sand is composed primarily of chromite sand and some additives. Asides its advantages of good fluidity, high melting point, and high density; it is never over-sintered.
When the chromite sand present in the ladle nozzle filling sand is over 60%, it forms a continuous sintered layer when used. This is because the chromite sand is in continuous distribution.
When used under very high temperatures, the FeO constituent of the chromite sand dissolves and forms a secondary spinel. This secondary spinel, in turn, changes the volume of the sintered layer and causes cracking when the slide gate is opened.
After the opening of the slide gate, the unsintered filling sand located at the lower part of the nozzle will flow out very quickly. Consequently, forming cracks on the sintered layer. The sintered layer is then broken under the static pressure of molten steel. With that, automatic opening is achieved. This chromite based ladle nozzle filling sand is adopted by most large steel plants.
In general, ladle filler sand is composed mainly of refractory raw material in silica sand and chromite sand. Putting things in perspective, filler sand is composed of, , , , with a bulk density of about 1.6-2.2g/cc.
How Filler Sands Are Used
Although there are several ways of placing the sand into the ladle, two of these methods are the most common. In the first process, the filler sand is divided into several plastic bags. The sands are then thrown into the centre of the tapping hole. Then, the filler sand then gradually deteriorates down.
Otherwise, you can position a tube of metal over the tapping hole. After that, pour in the sand such that it covers the sliding gate system.
Another method of placement used by some foundry industries is placing a sand patron when the ladle is to be cleaned. The patron used is made of sheet metal and will most often than not melt when tapping the hole.
It is extremely important that the sintered layer formed is thin yet not too thin. A higher pressure will be required to get the ladle to open spontaneously if the sintered layer is too thick. If it comes out too thin instead, the sand can permeate, clogging the nozzle in the process.
When the filler sand is heated using a high-temperature ladle, its components – chromite and silica – will react to a liquid formation. This liquid form is then transformed into a high strength layer as the sintering process continues.
The period between when the filler sand is in the ladle and when the molten metal is poured into the ladle is referred to as pre-sintering time.
Ideally, the temperature and holding time should be kept low so as to increase the effectivity of the production and reduce environmental impact.
During the refining process that takes place in the ladle, the already sintered layer is worn down gradually by erosion. The wearing rate of the sintered layer is essential for the free opening rate of the ladle, as without it, the sintered layer would become too thick.
The casting process is used for casting metals which cannot be cast with a continuous casing. This is due to the significant differences in composition on solidifying. What this means is that on cooling, the whole material doesn’t solidify at once, resulting in different microstructures and compositions.
Consequently, it is preferable to ingot cast the metals. This is done by uphill casting where the molten metal is discharged into a permanent mold either from the top or bottom.
However, just before then, the liquid metal in the bottle of the ladle is allowed to flow out with the filler sand in order to get cleaner steel.
The molds are then left to be filled with the molten metal, then left to cool before they are taken for milling.
Advantages and Disadvantages of Different Forms of Filler Sands
While the most common forms of filler or ladle sands are chromite-based or silica sands, some filler sands are zircon-based. However, they have some disadvantages in their application relative to chromite-based and silica-based sands.
Zircon-based ladle/filler sands have the disadvantage of adhering to the well and nozzle refractories. Over time, this leads to a deterioration of the ladle, which in some cases might necessitate the replacement of these mechanical parts. In addition, its supply and pricing are constantly fluctuating.
The silica-based filler sand is a lower cost option and has a few advantages that it the more preferred in some foundry countries. Fine carbon, which is one of its main components, is used to limit permeation of the molten steel into the sand bed. It is also used to reduce some of the corrosive effects of the molten steel on the well and nozzle assembly.
Foundry chromite sand or chrome sand is one of the best filler sands available. It has good heated volume stability as well as high thermal conductivity. This is evident when it comes in contact with the molten metal.
In addition, it has excellent resistance to basic slag, ferric oxide, and other chemical reactions. It has the characteristics of the solid phase sintering and does a good job of preventing the infiltration of molten metal. It is suitable for the production of all forms of stainless steel, alloy steel, and carbon steel. When used, sand defects such as inclusions, among others, are reduced.
Filler Sands Vs. It's Alternatives
For the most part, using filler sands has a host of advantages compared to oxygen lance, which could be used as an artificial means of ensuring “free opening.”
Asides providing a high free opening rate, the use of ladle/filler sands also increases productivity. Oxygen lances have been found to decrease or lower the quality of steel produced in many foundry industries due to steel oxidation. This is, however, in sharp contrast to what is obtainable when filler sands are used.
Filler sands help increase the quality of steel as well as overall productivity. It also helps increase the efficiency of the production process since the number of steps is reduced.
Another advantage of using filler sands is that thermal loss during the steelmaking process is reduced and even the steel re-oxidation.
This re-oxidation in steel is the cause of many defects in steel making. The melt will re-oxidize when the steel bath is exposed to oxygen from an external source following deoxidation.
Usually, the dissolved oxygen content in the steel is low after deoxidation. Re-oxidation occurs when this melt is exposed to oxygen gas or the surrounding air. This extra oxygen added to the deoxidated melt (re-oxidation) increases the amount of secondary inclusion during solidification.
Filler sands are without question a necessity in steel production. They reduce if not eliminate sand defects in steel production. Furthermore, chromium-based filer sands are one of the most preferred and widely used kinds of filer sand in the foundry industry, owing to its relatively better chemical and thermal properties.