Its simple. Graphite anode is used because when carbon react with oxygen librated at anode it forms CO2 gas which is a exothermic reaction and maintain the temperature of process and not allow molten aluminum to solidify. Because the density of aluminum in solid form is less dense than cyrolite and hence it can't be able to extract from the bottom.

It is simple. Graphite anode is used because when carbon react with oxygen liberated at anode it forms $\ce{CO2}$ gas which is a exothermic reaction and maintain the temperature of process and not allow molten aluminum to solidify. Because the density of aluminum in solid form is less dense than cryolite and hence it can't be able to extract from the bottom.