Anthony in Epicurious
The trick to pulling off a lo-fi frozen drink like this one is in the dilution. Alcohol, as you may know, has a much lower freezing point than water. “Pure ethyl alcohol (the type of alcohol we drink) freezes at about -174 degrees Fahrenheit,” says Anthony Caporale, Director of Spirits Education at the Institute of Culinary Education. Caporale says you can achieve a granita-like level of frozen if you cut the alcohol content to about 15% with non-alcoholic mixers.
https://www.epicurious.com/expert-advice/frozen-slushy-negroni-without-any-gadgets