What are the ways in which people are robbed of their humanity through alienation in Narrative of the Life of Frederick Douglass and “Life in the Iron Mills”?

There are many ways people are robbed of their humanity in The Narrative of the Life of Frederick Douglass. One way is, as you mentioned, through alienation. Slaves are alienated from the rest of humanity through the denial of their identity.
In Douglass's time, slaves could not properly ascertain their age. They were not allowed to have official records of their date of birth. Therefore, slaves could only guess at their age, a major source of unhappiness for Douglass throughout his life. For their part, slaves could not even ask their masters for information about any part of their background, as such inquiries were considered "improper and impertinent."
Douglass was also kept in the dark about his paternal heritage. Others mentioned that his father was white, but Douglass had no means of ascertaining this fact. Additionally, Douglass was also parted from his mother when he was still an infant. It was then the custom of slaveholders to keep female slaves estranged from their children. This is another way in which slaves were kept alienated: they were denied the opportunity to form strong familial bonds. Thus, they were robbed of their humanity. Douglass's mother died when he was seven years old. Tragically, he was never allowed to visit her during her illness, and he was even denied permission to attend her funeral.
Slaveholders also robbed their slaves of their humanity by denying them equal access to the justice system that whites depended on. Thus, slaves had no legal rights. In fact, killing a slave was not even considered a crime. Men killed slaves with impunity and boasted of doing so openly. Douglass relates how his wife's cousin was brutally murdered by the wife of Mr. Giles Hick. The young girl had been tasked to watch Mrs. Hick's baby. Due to her lack of sleep, however, she failed to hear the baby's cries one night. Finding that her slave was slow to move, Mrs. Hicks picked up an oak stick and broke the girl's nose and breastbone. The girl died not long after.
We find the same sort of inhumanity and alienation in "Life in the Iron Mills." In this story, the working poor are looked at with suspicion, distaste, and disapproval. They are treated as nothing more than disheveled specimens of humanity. Because many work in the steel mills, their bodies are often coated in grime. There is little opportunity or means of keeping clean. The workers are also paid poorly and must content themselves with living accordingly. Often, there is little to eat, and this causes malnutrition among the working immigrant population.
Since there is little opportunity for children in these communities to receive adequate schooling, many grow up with little hope of bettering their lives. Like their forefathers before them, they grow up to toil away in stifling hot steel mills. The workers are kept alienated from civilized society by their lack of education. Without education, social advancement is all but denied to them.