Please, let's not have any other mechanics like this. I assume that asking to make sifting automatic is futile. Therefore I want to suggest another option.
Seeing how fun smelting wrought iron can be if you have some company I think we could apply something similar to sifting.
A sluice box. From wikipedia http://en.wikipedia.org/wiki/Gold_mining:
Using a sluice box to extract gold from placer deposits has long been a very common practice in prospecting and small-scale mining. A sluice box is essentially a man made channel with riffles set in the bottom. The riffles are designed to create dead zones in the current to allow gold to drop out of suspension. The box is placed in the stream to channel water flow. Gold-bearing material is placed at the top of the box. The material is carried by the current through the volt where gold and other dense material settles out behind the riffles. Less dense material flows out of the box as tailings.
Sounds easy - build a sluice box, put it in a river and keep filling with rubble. However, I'm pretty sure devs have already considered it and decided against it. Probably because it would be too easy. So it needs to be spiced up a bit.
You don't put a sluice box in the river - you put it on the ground. And have to pour water on top of it. The faster / more water you pour the faster it processes the rubble - the box would have a meter for amount of water which would decrease quickly. Pouring water should not be instant - it should take 2-5 seconds. Now, the trick is - one person pouring water would achieve efficiency slightly lower than manually panning. But have 2 people pouring it and efficiency would increase 3-4 times. Have a third person dropping rubble and you get a nice factory.
Sluice box could also come with a slightly lower chance to find gems. And maybe a chance to find rocks / salt / granite, so that they keep filling output window and have to be cleaned often - with the full output window you just waste rubble and get nothing.