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Thread: How to divide molten Metal from Slag (the Blast Furnace)

  1. #1 How to divide molten Metal from Slag (the Blast Furnace) 
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    Check out this Animation of a Blast furnance

    The animation fails to convey what I believe is the most vital thing: how the molten iron is separated from the slag.

    Two things come to mind - melting point, and buoyancy. Does iron have a lower melting point than the slag? Also, is the slag always displaced to the surface by the molten iron?


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    The molten iron is denser than the waste slag, so it collects at the bottom. Most modern blast furnaces are designed so that when a given amount of ore is put in, the slag line will always be at the right level for the bleed openings, so it can be skimmed or drawn off. Depending on the material, the slag is sometimes put back into the furnace again to make sure all the valuable metals are removed.


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    WYSIWYG Moderator marnixR's Avatar
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    iron has a higher melting point than slag + as far as i'm aware the difference in density is sufficient to prevent any mixing

    the process is relatively free of turbulence (especially when compared with e.g. what an oxygen lance does in a BOS vessel) and the time scales are relatively extended so that (1) you don't get much physical entrapment and (2) any slag that finds itself entrapped in iron has time to float out
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    I'm a geologist but have little experience with industrial ores. The principal issue, however, is the composition of the iron ore itself before it ever goes into a furnace. Is it magnetite? hematite? etc. Depending on the ore used, is it procesed before going into the furnace... or is the furnace and temperatures configurated differently for each type of ore? The slag will be different for various ores. Off the top of my head that animation might be for a higher grade ore like hematite.
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    Your probably right. Most blast furnaces are pretty specific to the type of ore being put into them. You usually find the blast furnaces located in the same regions as the ore, so that the properties can always be relied on.

    Whatever the actual chemical process, the slag more or less "floats" above the metal when the furnace temperature reaches a point at which the slag and metal separate. It's kinda like taking cream off a vat of milk.
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    Quote Originally Posted by Wolf
    The molten iron is denser than the waste slag, so it collects at the bottom. Most modern blast furnaces are designed so that when a given amount of ore is put in, the slag line will always be at the right level for the bleed openings
    This interests me very much, because it's a mystery to me how they determine how much metal is within an ore. Surely the amounts of metal in an ore vary? Therefore, the slag line must vary depending on the quality of the ore. High quality ore's, I imagine would have a higher slag line. Low quality ore's, would have a lower slag line.

    Do you have any advice or leads to help me become more clear on this?

    Depending on the material, the slag is sometimes put back into the furnace again to make sure all the valuable metals are removed.
    I imagine that the slag line would be relatively high if one were to glean the slag.
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    Quote Originally Posted by marnixR
    iron has a higher melting point than slag + as far as i'm aware the difference in density is sufficient to prevent any mixing
    Ok, that's clear.

    the process is relatively free of turbulence
    Not sure what you mean. What does it mean for the process to be hindered by turbulence?

    the process is relatively free of turbulence (especially when compared with what an oxygen lance does in a BOS vessel)
    what is an oxygen lance? What is a BOS vessel?

    and the time scales are relatively extended so that (1) you don't get much physical entrapment
    The times scales are extended in contrast to what exactly?
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    Quote Originally Posted by Jellyologist
    I'm a geologist but have little experience with industrial ores. The principal issue, however, is the composition of the iron ore itself before it ever goes into a furnace. Is it magnetite? hematite? etc.
    What methods do we use to determine if an ore is magnetite, hematite, etc?

    is [the ore] procesed before going into the furnace?
    What does it mean to process an ore?

    ... or is the furnace and temperatures configured differently for each type of ore?
    Well, what are the different factors we need to consider?

    Melting point is an obvious one.

    Are all ore's containing metals treated to undergo oxidation/reduction?

    Are some metals less dense than the slag?

    The slag will be different for various ores. Off the top of my head that animation might be for a higher grade ore like hematite.
    What's the key differences between higher grade ore and lower grade ore? Maybe in your answer, you can refer to the periodic table...
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    Quote Originally Posted by Wolf
    Your probably right. Most blast furnaces are pretty specific to the type of ore being put into them.
    I wonder how many different types of blast furnaces there are.
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    Quote Originally Posted by remit
    Quote Originally Posted by Jellyologist
    I'm a geologist but have little experience with industrial ores. The principal issue, however, is the composition of the iron ore itself before it ever goes into a furnace. Is it magnetite? hematite? etc.
    What methods do we use to determine if an ore is magnetite, hematite, etc?

    ...
    Not too difficult. Just crush a few representative samples and look at the crystal structures under a microscope. Determine levels of silica, other metals and so on. A few simple tests to confirm. Today most ore (maybe all in large operations?) will be magnetite.

    There's always differences in ores from place to place and this would result in differences in the pig iron and eventual steel. I'd assume iron is refined down to acceptable standards and sold as such. Other metals such as precious and non-ferrous (alluminum) would hve more exact standards.
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    WYSIWYG Moderator marnixR's Avatar
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    ok, basic lesson on blast furnaces :

    1. you charge alternative layers of coke, ferrous charge and slag-forming components from the top; the ferrous charge these days is mostly sinter (=fines melted beforehand, cooled and broken to give a better size distribution than lump ore) and pellets (fines made into a sludge and then turrned into little marbles of ferrous material); lump ore these days is only a minor component because the the gas permeability of the blast furnace is very important and is better controlled through using pellets and sinter
    2. the iron content of the ores received is well-known and comes with the bill of lading from the ship; the iron content of sinter and pellets can be controlled during their production
    3. oxygen enriched air is being blown through tuyeres into the blast furnace just above the level of the hearth; this rises through the stack and reacts in counterflow with the ferrous charge and the coking coal
    4. the solid charge gradually softens and partially melts as it moves down; usually the coke remains the last solid component to support the rest of the stack; slag and hot metal sinks down to the sump or hearth where they separate into 2 layers, a heavier iron layer at the bottom and a lighter slag layer at the bottom
    5. at regular intervals 1 of 2 tapholes gets opened, which initially draws pure iron from the furnace; as the liquid level drops, a mixture of slag and hot metal is drawn for the rest of the tap duration
    6. the tapped material goes through a refractory running system which is designed to separate off slag as the lightest component and divert it into a slag pit, whilst the hot metal ends up being poured into a torpedo car - this is a refractory-lined cigar shaped container that sits on a bogey pulled by a loco
    7. once full the torpedo car is pulled to the BOS (Basic Oxygen Steel) steelmaking plant where the hot metal is charged together with about 20% steel scrap - in this process an lance introduces a stream of oxygen into the iron charge and burns off the excess carbon to turn liquid iron into steel

    hope this post describes the process somewhat better than my first post which, in hindsight, was rather cryptic - i had assumed a greater familiarity with iron & steelmaking processes than i should have
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    First, I have a minor question I'm hoping one of you can help me out with, it's a very simple question, largely related to semantics:

    What do we call the mass of rock that is quarried for ore?


    Quote Originally Posted by Jellyologist
    Quote Originally Posted by remit
    Quote Originally Posted by Jellyologist
    I'm a geologist but have little experience with industrial ores. The principal issue, however, is the composition of the iron ore itself before it ever goes into a furnace. Is it magnetite? hematite? etc.
    What methods do we use to determine if an ore is magnetite, hematite, etc?

    ...
    Not too difficult. Just crush a few representative samples and look at the crystal structures under a microscope.
    I'm guessing it would be important to test a sample of rock every few feet along the area being quarried. The content of a sample may be radically different from another sample that was quarried only a few feet away. Right? I imagine that any serious large scale operation might involve hundreds of samples within a square kilometer, just to get a sense of the regularity from sample to sample. The goal is to assess a large bulk of rock that is being quarried for ore, and my intuition tells me that many samples every few feet, and perhaps at varying depths, need to be taken, in order to get an idea of how widespread the regularity is in a given area.

    I'm sure there are scientific terms to express the sort of thing i'm getting at, and I wish I knew them.

    Determine levels of silica, other metals and so on. A few simple tests to confirm.
    Does anyone here have an opinion about a career in assaying ore's? Sounds interesting enough to me. One gets to use a microscope, conduct lab experiments to test for a sample's properties, who knows, maybe such a job might involve actually going out on the field.

    Today most ore (maybe all in large operations?) will be magnetite.
    How come? Is it simply more prevalent?
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  14. #13  
    WYSIWYG Moderator marnixR's Avatar
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    Quote Originally Posted by remit
    How come? Is it simply more prevalent?
    highest iron content
    "Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
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  15. #14  
    WYSIWYG Moderator marnixR's Avatar
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    btw, this one may be of interest, a blast furnace blowing its top
    fortunately you can always close its bell top, and i assume that's how this flame was extinguished
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