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I had read about the two important methods used for the concentration of an ore - Roasting and Calcination. I have a doubt regarding why both the methods are carried out at temperatures not above the ore's melting point.

After some research, I have been able to conclude that one of the reasons is the formation of complicated slag (i.e., difficult to remove from the ore) that tends to form if the ore is molten.

Could someone please explain what exactly happens so that a complicated slag is formed? Also, are there more reasons behind keeping the temperature below the ore's fusion temperature (any specific reasons pertaining to roasting and calcination individually)?

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The purpose of roasting a/o calcination is oxidation a/o decomposition of original ore to form metal oxides.

It is desirable to keep the intermediate product solid to obtain porous product. This allows easy access for further reduction by carbon monoxide by coke or charcoal reduction process. If oxides were in form of a solidified melt, the reduction arrangement would be problematic.

Additionally, some ores or oxides have too high melting point to be practical or easily possible to melt them.

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