how to melt a gold dust?

Answer 1

Gold melts at 1064+ degrees Celsius. So any oven that generates similar temperature can melt gold.
But why do you want to melt gold ? If your purpose is purification, dissolving it in Aqua Regia (1:3 ratio of conc. HNO3 and conc H2SO4) or making an amalgam with mercury and then heating it will serve the purpose. It also gets dissolved in KCN. Moreover, there is also electrolysis. (But be careful, both mercury vapour and potassium cyanide are EXTREMELY hazardous).
If, on the other hand, your objective is to give it a specific shape, use a blow pipe as a goldsmith does.

Answer 2

heat