what propaganda technique is used in Animal Farm through this quote "all animals were equal but some are more equal than others”?
This phrase uses more than one propaganda technique but it most prominently engages in doublespeak, a term invented by Orwell in 1984 to indicate saying the opposite of what is meant. Doublespeak actually inverts meaning, such as, in 1984, calling war "peace" or the ministry devoted to torture the ministry of love (minilove). In Animal Farm, a slogan devoted to radical equality: "all animals are equal," is turned into its opposite: all animals are equal, except some are not; in other words, all animals are not equal.
Other propaganda techniques make this doublespeak possible. Using the language of the common man is another form of propaganda, such as when politicians say "God bless America" whether they mean it or not or when politicians are photographed eating grits in a diner when they are wealthy people who normally would never go near a diner. In Animal Farm's case, the equality slogan speaks the language of the common animal, so it is easy to confuse the issue by attaching doublespeak to it. Finally, the doublespeak is reinforced by conditioning: the animals have learned to feel good about the equality slogan, so anything attached to it, even it turns the meaning of the original statement upside down, is likely to evoke a "feel good" sensation.