Objection 1. It would seem that that fire will not engulf the wicked. For a gloss on Malachi 3:3, "He shall purify the sons of Levi," says that "it is a fire consuming the wicked and refining the good"; and a gloss on 1 Corinthians 3:13, "Fire shall try every man's work," says: "We read that there will be a twofold fire, one that will cleanse the elect and will precede the judgment, another that will torture the wicked." Now the latter is the fire of hell that shall engulf the wicked, while the former is the fire of the final conflagration. Therefore the fire of the final conflagration will not be that which will engulf the wicked.
Objection 2. Further, that fire will obey God in the cleansing of the world: therefore it should receive its reward like the other elements, especially since fire is the most noble of the elements. Therefore it would seem that it ought not to be cast into hell for the punishment of the damned.
Objection 3. Further, the fire that will engulf the wicked will be the fire of hell: and this fire was prepared from the beginning of the world for the damned; hence it is written (Matthew 25:41): "Depart... you cursed... into everlasting fire which was prepared for the devil," etc., and (Isaiah 30:33): "Tophet is prepared from yesterday, prepared by the king," etc., where a gloss observes: "From yesterday, i.e. from the beginning--Tophet, i.e. the valley of hell." But this fire of the final conflagration was not prepared from the beginning, but will result from the meeting together of the fires of the world. Therefore that fire is not the fire of hell which will engulf the wicked.
On the contrary, are the words of Psalm 96:3, where it is said of this fire that it "shall burn His enemies round about."
Further, it is written (Daniel 7:10): "A swift stream of fire issued forth from before Him"; and a gloss adds, "to drag sinners into hell." Now the passage quoted refers to that fire of which we are now speaking, as appears from a gloss which observes on the same words: "In order to punish the wicked and cleanse the good." Therefore the fire of the final conflagration will be plunged into hell together with the wicked
I answer that, The entire cleansing of the world and the renewal for the purpose of cleansing will be directed to the renewal of man: and consequently the cleansing and renewal of the world must needs correspond with the cleansing and renewal of mankind. Now mankind will be cleansed in one way by the separation of the wicked from the good: wherefore it is said (Luke 3:17): "Whose fan is in His hand, and He will purge His poor, and will gather the wheat," i.e. the elect, "into His barn, but the chaff," i.e. the wicked, "He will burn with unquenchable fire." Hence it will be thus with the cleansing of the world, so that all that is ugly and vile will be cast with the wicked into hell, and all that is beautiful and noble will be taken up above for the glory of the elect: and so too will it be with the fire of that conflagration, as Basil says in Psalm 28:7, "The voice of the Lord divideth the flame of fire," because whatever fire contains of burning heat and gross matter will go down into hell for the punishment of the wicked, and whatever is subtle and lightsome will remain above for the glory of the elect.
Reply to Objection 1. The fire that will cleanse the elect before the judgment will be the same as the fire that will burn the world, although some say the contrary. For it is fitting that man, being a part of the world, be cleansed with the same fire as the world. They are, however, described as two fires, that will cleanse the good, and torture the wicked, both in reference to their respective offices, and somewhat in reference to their substance: since the substance of the cleansing fire will not all be cast into hell, as stated above.
Reply to Objection 2. This fire will be rewarded because whatever it contains of gross matter will be separated from it, and cast into hell.
Reply to Objection 3. The punishment of the wicked, even as the glory of the elect, will be greater after the judgment than before. Wherefore, just as charity will be added to the higher creature in order to increase the glory of the elect, so too whatever is vile in creatures will be thrust down into hell in order to add to the misery of the damned. Consequently it is not unbecoming that another fire be added to the fire of the damned that was prepared from the beginning of the world.
Source: Summa Theologica (New Advent)