A minyan is required to be able to recite Kaddish. There is one exception though – if the Chazzan begins his repetition of the Shemone Esray with a minyan, and then someone leaves making the minyan no longer present, he may still recite the Half Kaddish after Tachanun and the Full Kaddish after U’va L’tzion. This is because these two Kaddishim are considered as contiguous with Shemone Esray. These are the only Kaddishim that are allowed to be recited without a minyan (and only if the Shemone Esray was begun with a minyan). The Mourner’s Kaddish after Aleinu and/or Shir Shel Yom is NOT allowed to be recited without a minyan.