Peel and devein the prawns, discarding the heads and shells. Place the prawns in a bowl, add the lime juice, salt and pepper, and mix well. Set aside for 10 minutes. Then rinse and pat dry with paper towels. Dust the prawns lightly with flour. Heat the oil in a wok and deep-fry the prawns for 1 to 2 minutes until golden. Remove from the oil and set aside to drain.