The time at which colonization of freed slaves was really en vogue was in the 1820s and 30s with the American Colonization Society. So, there were few people who seriously thought about it as a viable option by 1860, and the sheer numbers of slaves had grown to over 4 million by that time.
The idea that all blacks, slave or not, could be sent back to Africa was always a pipe dream. In the days of Monroe’s American Colonization Society and compensated emancipation, yes, tens of thousands of former slaves were sent back to form the colony of Liberia. While this was an interesting experiment, the effort was doomed to failure.
First, since all slaves had to have been imported by 1808 at least, and were much more likely the descendants of slaves who had been imported even earlier than that, we had no way of knowing which area or nation of Africa they originated from. Even if we did, that doesn’t mean we controlled the land, which had already been colonized in part by the European powers, or was under the control of African civilizations that weren’t looking for immigrants.
Both before and during the war, the North did not have control over the territory with the bulk of the slave population in them. Logistically and financially, how could we have expected to liberate and expatriate 4 million slaves anywhere? Especially when our best efforts up to that point had only colonized 25 – 30,000 to that point.
Add to this the fact that slaves spoke the Africanized dialect of English, and had no knowledge of African languages, and would only be able to communicate with the few we had sent to Liberia.
Next, consider what it would take, either in a war setting or during peacetime, to acquire control over 4 million slaves? Even if we could compensate the owners for them, what would that accomplish? It would not obviate the economic necessity of slavery in the Deep South, or abolish the institution as a whole, so all we would have done is giving millions of dollars to slave owners who would then replace their slaveholdings piecemeal.
Politically, even with a Republican-led war Congress, Lincoln would not have won approval for the large expenditures of time, money and resources to achieve full colonization. The failure of the Constitutional Convention to successfully and permanently address the slavery issue, coupled with Manifest Destiny and American expansion in the 1800’s made the Civil War the only means of practical abolition.