Dross or Slag


New Member
What is the difference between dross and slag? In the welding industry slag is produced with OFC and dross is produced with plasma cutting. Both are waste material but the terms are different and I would like to know the exact reasons for the difference.
  • foxfirebrand

    Senior Member
    Southern AE greatly modified by a 1st-generation Scottish-American mother, and growing up abroad.
    Dross is finer than slag. Both are waste or byproducts of smelting and metallurgy.

    Slag is the crude material shed by high-grade ore during smelting, and it remains behind when the metal is poured off in a molten state. Tailings are cruder still, and are the rock and trash material separated from crude ore during the fining process-- including matter separated in the preliminary process, where crushing is employed and no heat is applied. Heated low-grade ore also leads tailings, but sometimes the word "slag" is applied to this stuff, especially after it is hauled out and dumped-- "slag heap" and "tailings dump" are synonymous terms, but this is strictly true only because slag is dumped with the tailings. In the non-mining-industry population, this fact has led to a blurring of the distinction.

    Dross tends to be lighter than the metal being processed, and can be part of the late stages of smelting or more advanced metallurgy, dealing with alloys. It is skimmed when possible, but sometimes has to be physically chipped and wire-brushed from the top surfaces of ingots during the cooling process.

    So slag is confused with tailings, which are the rocky matrix ore is contained in, and are lighter than ore. And it is the waste product of smelting ore, but low-grade ore yields crude metal which requires further smelting. Dross is a byproduct of metal (sometimes a useful raw product) and is therefore sometimes the result of smelting, sometimes refining.

    Two more things add to the confusion. Dross can be heavier than the metal being processed (especially in the case of aluminum), and the type that is useless is also destined for the slag heap. And the word itself is kinda "old world" or related to precious metallurgy, and in the U.S. anyway, slag can be a catchall term that includes dross.

    That was pretty easy to sort out, wasn't it?
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