One man borrowed money from me 2 years ago to go into business, as he had no job in Qatar. He paid half then he disappeared from my area and at present I do not know his whereabouts. Now can I consider the balance, money that I could not recover as my Zakah money?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The procedure you are asking about, i.e. giving up a loan considering as Zakah is not correct and it is not considered as paying the obligatory Zakah. The reason is (Allah knows best) that the Zakah should be carried out with the intention of Zakah at the time of giving it to those who are liable to receive it. The money you gave as loan to your friend was given as a loan not as Zakah. Also, in this way you are trying to protect your money by not paying Zakah.
Anyway, if you want to subtract a sum of your money only to ease him from his debt you may do it; doing so is highly appreciated in Shariah and Allah will grant you good reward for this act. Allah Says (interpretation of meaning): {And if the debtor is in a hard time (has no money), then grant him time till it is easy for him to repay, but if you remit it by way of charity, that is better for you if you did but know.}[2:280].
However, considering the loan, as Zakah is not valid. Thus, one has still to pay the amount of Zakah due on him. He can pay it to the indebted person or to any other needy Muslim from amongst the following eight categories: "As-Sadaqât (here it means Zakât) are only for the Fuqarâ' (poor), and Al-Masâkin (the poor) and those employed to collect (the funds); and for to attract the hearts of those who have been inclined (towards Islâm); and to free the captives; and for those in debt; and for Allâh's Cause (i.e. for Mujâhidûn - those fighting in the holy wars), and for the wayfarer (a traveller who is cut off from everything); a duty imposed by Allâh. And Allâh is All-Knower, All-Wise." [9:60].
Allah knows best.