The class D extinguisher contains a powder know as METAL-X. When it is used it comes out in a slow soft stream so as not to send the burning metal flying all over the place. You wave it back and forth to cover the metal and it puts out the fire by melting and coating the chips. Actually it isolates the burning metal from the rest of it before it gets going. Mag has to burn its self out. While we carry it on our trucks for use on aircraft, in the shops they just use straight talc as METAL-X is a very corrosive substance and can do a number on your machinery. The maint. guys said that you have to practicaly dis-assembly the machine to get it all out of the nooks and crannies after you use it, or you end up with rust where your can not see it until it is too late. The talc basicly does the same thing as the METAL-X by isolating the burning metal but it does not melt and stick to the Mag. It is cheaper and you can tell right away, before starting the job if there is any moisture in it as it clumps. Sand that feels dry may have enough water trapped in it to cause small steam explosions or generate hydrogen gas and have it flare or burp, throwing the metal around. Mag burns bright white and it is like looking at a welding arc trying to get it covered so be careful.
Mag chips need to be kept to a minimum on and around the machines cutting it and the tools need to be kept sharp at keep the heat down. We make the shops use heavy steel bins or 55 gallon drums with secure fitting lids to hold the chips and they must be emptied every night or when full, just like a rag can. No major fires yet, so the system must be working.
About 20 years ago a local shop that did work for Boeing, Lockheed, GE and Thiokol had a major mag fire. As they were not carefull with the chips they lost the shop in about 40 minutes. It was around a 15,000 Square foot building too. As with any shop good housekeeping helps to minimize problems when they do occur, and they always do when you least expect it.
If you cut any of the combustable metals or grind a lot of Aluminum take care with the chips and the grinding swarf and you should not have any problems.