Purification of oil

1. A method for purification of contaminated oil, comprising the steps of:

providing contaminated oil and a separation aid in a tank ( 3 );

waiting for allowing a sludge phase comprising the separation aid together with impurities from the oil to settle in a bottom part ( 4 ) of the tank ( 3 ); and

reusing the sludge phase for purification of new contaminated oil by mixing the sludge phase with the new contaminated oil.

2. A method according to claim 1 , further comprising the steps of:

transferring clean oil out from the tank when the sludge phase has settled in the bottom part ( 4 ) of the tank ( 3 );

adding new contaminated oil to the tank;

mixing the newly added contaminated oil with the sludge phase previously settled in the bottom part of the tank; and

waiting for allowing a new sludge phase comprising the old sludge phase and new impurities from the new oil to settle in the bottom part of the tank.

3. A method according to claim 2 , further comprising the steps of:

removing a part of the sludge settled in the bottom part ( 4 ) of the tank ( 3 ) before new contaminated oil is added to the tank; and

compensating the amount of separation aid lost by the removing of a part of the sludge by adding to the tank an amount of fresh separation aid corresponding to the amount of removed separation aid.

4. A method according to claim 1 , further comprising the step of detecting the presence of an oil phase or a sludge phase in at least one position within the tank and according to the detection remove a certain amount of sludge before new contaminated oil is added to the tank.

5. A system for purification of contaminated oil, said system comprising a tank ( 3 ), wherein said tank comprises:

at least one inlet ( 5 ) for providing contaminated oil and separation aid to the tank;

at least one first outlet ( 7 ) for taking out purified oil from the tank after separation;

at least one second outlet ( 9 a , 9 b ) for taking out sludge comprising the separation aid together with the impurities from the oil after separation; and

a mixer ( 13 ) arranged in the tank for mixing newly added contaminated oil with old sludge;

a sludge removing device ( 21 ) connected to the at least one second outlet ( 9 ) and arranged for removing sludge from the tank ( 3 );

a separation aid adding device ( 23 ) connected to the at least one inlet ( 5 ) and arranged for adding a separation aid to the tank ( 3 );

at least one content detection sensor ( 25 ) arranged for detecting the presence of an oil phase or a sludge phase in at least one position within the tank; and

a control device ( 27 ) connected to said at least one content detection sensor ( 25 ) and to said sludge removing device ( 21 ) for controlling the sludge removing device to remove a certain amount of sludge but not all the sludge from the tank ( 3 ) in dependence of the detection,

said control device ( 27 ) further being connected to the separation aid adding device ( 23 ) and being arranged to control the separation aid adding device to add new separation aid in an amount corresponding to the amount of separation aid being removed from the tank in the removed sludge.

6. A system according to claim 5 , comprising at least two second outlets ( 9 a , 9 b ) provided at different levels in a bottom part ( 4 ) of the tank ( 3 ).