Actually they did send a lot of them back to africa. to Liberia.

Liberia is one of only two modern countries in Sub-Saharan Africa without roots in the European Scramble for Africa. Beginning in 1820, the region was colonized by freed American slaves with the help of the American Colonization Society, a private organization that believed ex-slaves would have greater freedom and equality in Africa. Slaves freed from slave ships were also sent there instead of being repatriated to their countries of origin. In 1847, these colonists founded the Republic of Liberia, establishing a government modeled on that of the United States and naming the capital city Monrovia after James Monroe, the fifth president of the United States and a prominent supporter of the colonization. The colonists, known as Americo-Liberians, monopolized the political and economic sectors of the country despite comprising only a small percentage of the largely indigenous population.

[link to en.wikipedia.org]

But then the local populace killed just about all the slave desendents at one time or another.

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