[timrice]

Author here

Surface contact is one reason you want an ice/water slurry instead of just ice, but the real reason is that ice melting consume a lot more energy than just ice being warmed up to it's melting point.

The ice will quickly come up to it's melting (equilibrium!) point, without cooling the ice cream mixture very much. Remember, we're trying to freeze the ice cream (not just cool it down), which is proportionally just as thermodynamically expensive as melting ice. Bringing the ice up to it's melting point alone won't suck enough heat out of the ice cream mixture to freeze it.

[hammock]

This right here is the explanation that clicks for me. It’s not enough to say “salt makes the ice colder than 32”. Which might cause one to wrongly assume you do it so the ice cream freezes “faster.”

What you say here is the reason WHY that is needed in the first place, and you say it very clearly

[nkurz]

Remember, we're trying to freeze the ice cream (not just cool it down), which is proportionally just as thermodynamically expensive as melting ice.

Is it just proportional, or is it actually pretty close to 1:1? That is, how accurate is the view that if you want to freeze 1L (or kg) of ice cream you need to melt 1L (or kg) of ice? Although I guess ice cream is not just frozen water, so perhaps that forms a fixed proportion. Alternatively stated, how much ice do you need to start with to freeze a given quantify of ice cream?

[Dylan16807]

According to some random post on google, ice cream needs about 2/3 as much energy per gram to melt or freeze.

Of course you also have to worry about losses, extra margin, and the heat of stirring.

[iamkroot]

You're right, I should have said equivalently :D