Dear Mahmood Hashim generally I would suggest to use either a heating mantle (see attached picture) or, even better, an oil bath. If you use a round-bottom flask to hold your n-hexane, then a hot plate is unsuitable anyway. The boiling point of n-hexane is 69 °C, so I think you need around 100 °C to keep it refluxing during the Soxhlet extraction process.