Small black holes evaporate fairly quickly from Hawking radiation. The rate increases dramatically the smaller the mass. Since the only time primordial black ho

Small black holes evaporate fairly quickly from Hawking radiation. The rate increases dramatically the smaller the mass. Since the only time primordial black holes would have formed was in the Big Bang there is a lower limit based on the black hole surviving 13.8 billion years. iirc the number is on the order of 10^12 kilograms.