As briefl y mentioned above, the conversion yield of the
magnesiothermic reduction under mild vacuum was markedly
improved compared to that under the conventional atmospheric
pressure (Ar atmosphere) process. In the fi nal product, the
summed mass fraction of Mg 2 Si and SiO 2 was less than 1.5%.
After the acid (HCl) washing step to completely remove Mg 2 Si,
which accompanies a minor collection loss estimated at ≈3–4%,
the Si extraction yield (the fi nal mass of the collected Si divided
by the calculated mass of Si in the original sand particles) was
about 95% (fi nal collection yield). In contrast, the sample prepared
by the conventional atmospheric-pressure reduction process
contained a signifi cant fraction of MgSi 2 and unreacted
SiO 2 (Supporting Information Figure S6); similar results were
also reported in previous studies. [ 5,14 ] The fi nal extraction yield
of the atmospheric-pressure process for the same sand particles
is estimated to be only 40–50%, showing the strong advantage
of the mild-vacuum process. Figure 5 b presents that, after
the HCl washing process, the low-pressure process produced
pure Si structures, while the conventional reduction resulted in
a mixture of Si and unreacted SiO 2 . Due to a high conversion
yield, our low-pressure reduction process does not require a
dangerous HF acid washing process to remove unreacted SiO 2 ,
thereby signifi cantly simplifying the overall process. Moreover,
the mild vacuum reduction technique has an additional important
advantage of considerably shorter processing time than
previous studies. [ 14,15,20,24 ] Our study suggests a useful processing
strategy to improve the cost-effectiveness of the magnesiothermic
reduction process as well as signifi cant reduction of
material costs.