Those chunks happen when moisture gets into the pile and melts the salt together. Really bad when you put damp salt in a tailgate hopper and it clumps up like that. One of the benefits of treating your stockpile with Magic is that it will help prevent clumping. Not eliminate it completely, because if you get snow or water into the pile it will wash the Magic off the salt and you will get clumps in that spot. But I've had a lot less clumping and crusting in my pile since I started using Magic.
After using magic for about 3 storms now,I agree with Alan,it does stay loose and free much more so than regular salt.I still empty the hopper though,it will bind up if you leave it in there long enough.
When I used smaller spreaders at a local school, we had problems with large chunks too. Not sure what size spreader you are talking about, but we had cheap walk behind broadcast spreaders. What I did, was use a circular saw to cut the bottoms out of some plastic milk crates. Then I put the flat plastic "screen" I cut, in the spreader, there was a ridge in the hopper, and the screen fit perfect. Never had a problem with clumps again. I recommend a screen of some type for all spreaders. It will help with clumps that are in bagged salt or in your bulk pile. They will <B>not</B> help if you allow salt to sit in the spreader and clump.
Plowguy, Magic is a liquid deicer that you incorporate into your stockpile. If you are useing bagged product there is also pretreated, bagged salt available. The Magic coating keeps the salt granules from "melting" together from atmospheric moisture. I won;t help if your material gets rained or snowed on though, as the Magic will wash off. In addition it will keep your salt working down to lower temperatures (near 0 deg.) and at higher temperatures you can reduce your salt application by 30% and still get the same results. Also, the corrosive nature of the salt is greatly reduced when it is mixed with Magic.
Before anyone jumps on this, I am NOT selling the stuff, only using it with good results. John Parker (NY SnowPros) is a distributor and can give you full details and costs, as well as steering you to a supplier near you.