Chemistry - What does it mean to 'fuse' ores and why do we use alkali metals in the process?

Solution 1:

"To fuse" is another word for "to melt" (e.g. "heat of fusion"). Specifically, if you say you want to fuse two materials, you melt them in the hope that they will mix.

In this case, you melt the carbonate, and hope that the chromite will dissolve in it. Because e.g. $\ce{Cr2O3}$ has a melting point of $\pu{2435 °C}$, chromite ($\ce{Fe(II)Cr2O4}$) typically around $\pu{2140 °C}$. That is out of reach with a common Bunsen burner, and even if you did reach it, it would still be the same insoluble mess after cooling down.

When your chromite is dissolved in alkali carbonate, the mixture easily dissolves in $\ce{HCl}$, and you can properly run your wet analytics.

Soldering for example is a very similar process: Your soldering iron cannot nearly melt copper, but the molten tin/lead/etc. fuses (partly dissolves/melts) with the copper surface, and you get a continous metallic connection.

Solution 2:

As a comment of Karl's message, I would like to add that chromite $\ce{FeCr_2O_4}$ is the most important Chromium mineral. When mixed with $8$ times its weight of sodium carbonate, and heated to high temperature, $\ce{Na_2CO_3}$ melts at $850°$C and reacts with chromite and air according to

$\ce{8 Na_2CO_3 + 4 FeCr_2O_4 + 7 O_2 -> 8 Na_2CrO_4 + 2 Fe_2O_3 + 8 CO_2 }$.

In this equation, both $\ce{Fe}$ and $\ce{Cr}$ atoms are oxidized. And this produces a mixture of soluble sodium chromate and insoluble iron oxide, which is separated from the chromate by dissolution into water.