DROSS FORMATION ,TYPES

By | April 14, 2016

Dross is a byproduct that is formed due to the galvanization process by the reaction of molten zinc and loose particles of iron created in the kettle. It is a byproduct or a waste material that can also be recycled in order to recover more metal from the same.here we will discuss dross formation.

What is dross?

Dross is basically defined as a byproduct, which is formed due to the process of galvanization. It is the byproduct formed by the reaction of molten zinc and loose particles of iron in the galvanizing kettle. This is known to be a metallurgical reaction that is formed between steel and iron with the help of molten zinc in the kettle. Dross is actually a byproduct of metal smelting. Metal smelting is the process where metal is extracted from rocks called ore. Dross is also formed due to other hot liquids as well. For e.g. if you are making some chicken soup the fat that accumulates and floats on the surface while simmering the soup is also called dross.

Though Dross is known to be a byproduct or a waste material created due to oxidization procedures, the definition of dross is not just limited to the same. It is basically a waste matter that forms due to an amalgamation of different metals or also while galvanizing and that needs to be removed in a variety of ways.

It is a collection of solid impurities that float on a molten metal. It forms on the surface of metals that have low-melting-points. The prominent examples are zinc, tin, wrought iron, lead, aluminum or alloys by oxidation.

Dross is different from slag. First of all, unlike slag, dross is a solid. It is not a waste material like slag.

DROSS

                         Fig 1.Dross

Why is dross formed?(dross formation)

Though Dross is termed to be waste materials, the question arises that why is this waste material actually formed? Dross is a mass of solid impurities that is found to be floating on a molten metal as well as dispersed metal as in wrought iron. It usually forms on the surface of metals that has a low meltingpoint. These comprise of metals like lead, aluminum, zinc and alloys. Dross can be of solid as well as liquid forms. They cannot be named entirely as waste materials because Aluminum dross can always be recycled and can also be used as secondary steel making deoxidation slag. Dross can be very easily formed as well as removed

How is dross formed?

Metal dross can be formed very easily and it is unavoidable because when metal and alloys are melted, they form dross. The main objective of dross formation is basically to minimize losses from both ends- from the economic viewpoint as well as from the gaining from the optimum quality of cast iron and wrought products. Aluminum losses are mainly due to the formation of dross during melting and casting. It has been seen that aluminum alloys contain magnesium that can trigger the process of oxidation. This is the way of is forming dross .It has been found that dross is usually formed in humid atmospheres where the amorphous oxide that covers magnesium and aluminum-magnesium at 750°C provides an effective barrier that protects the molten metal. This magnesium in the absence of water forms different kinds of oxide that actually is the onset of dross formation. The exudation of molten material through the surface oxide is known to be the main cause of dross formation.

How can dross be removed?

Researchers have tried out various methods to reduce or remove dross formation. Manufacturers have also tried to introduce various designs to squeeze or crop dross that has not really been very successful due to the environmental conditions as well as high temperatures. Nitrogen is used to reduce the formation of dross but it is not really applicable in many areas as the operating costs are really high and extravagant. There are chemical methods for removal of dross as well such as powders that can also help in removal of dross as well after it is formed.

Dross reducing powders are available since a long time. They are very effective when it comes to removing dross. These powders are known to be inorganic chemicals that produce a chemical effect on the surface. It is an oxidizing process that helps in separating the metals from the dross. Many of these powders comprise of zinc chloride or ammonium tetrafluoroborate which is known to be hazardous to the health as well as toxic and give out a smoke that is bad for health. Powders are also added to the solder pot after the formation of dross and are applied during the maintenance. The solder bath temperature is removed below 450°F and a layer of powder is applied to cover the dross and manually chop it into the dross. The remaining dross is removed along with the powder mess. Some powder may remain in the pot that may cause further oxidation to produce more dross, which again can be removed.

The different Types of dross

There are different kinds of dross, which are formed due to oxidation of different metals among which aluminum is the most prevalent one. Aluminum dross is a mixture of metallic and non-metallic Aluminum oxide. Dross is formed on the surface of molten aluminum smelter. It is recycled to recover Aluminum.

Types of dross

Dross can be broadly categorized into two types.

Floating Dross: The free particles of dross floating throughout the kettle are called Floating Dross. It consists of long inter-metallic spikes interwoven in clumps. It looks like a hollow structure.

Bottom Dross: As the name suggests, we can understand that dross particles that settle to the bottom of the galvanizing kettle are called Bottom Dross.

Storage of Dross

The dross can be stored in a covered shed or a bunker. There is a quality inspection team that keeps the record of the dross by visually inspecting the quality of the dross. Dross is also broken and pressed into lumps. The loose material is passed through the enrichment process.

Recycling of aluminum dross

Aluminum dross is a byproduct of aluminum smelting process and can be mechanically recycled to separate the residual aluminum metal from the aluminum oxide. Dross is recycled with the help of thermo-mechanical extractions. This process does not require any toxic chemicals that are used in the process of extracting aluminum. The dross is first crushed properly and then separated into aluminum metal-rich particles as well as aluminum oxide rich particles, which are based on the density. These metals are then melted to remove the dross.

 

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