| When minerals are placed in a magnetic field, there are three reactions which may occur. First, they are attracted to the magnetic field. Second, they are repulsed by the magnetic field. And third, no noticeable reaction to the magnetic field occurs.
Particles that are attracted to the magnetic field are called magnetic. But, there are two classifications of magnetic particles, strongly magnetic particles, such as iron and magnetite, and weakly magnetic particles, such as rutile, ilmenite, and chromite. Strong magnetic particles may be easily separated with a separator having a low intensity magnetic field of 400 - 600 gauss. Paramagnetic particles (weakly magnetic) require a higher intensity magnetic field to separate them, generally ranging from 6,000 to 20,000 gauss.
Particles that are repulsed by a magnetic field are called diamagnetic. A line of separators called Eddy Current Separators, takes advantage of the diamagnetic particles, separating them from other material. One of the largest uses currently is in the recycling industry, where wire and metals made from copper and aluminum are separated from plastics. When product, such as aluminum, passes over the eddy current , the spinning magnets inside the shell generate an eddy current in the aluminum thus creating a magnetic field around the piece of aluminum. The polarity of the magnetic field of the aluminum is the same as the rotating magnets, causing the aluminum to be repelled away from the separator. Product such as plastic, glass, or other process materials simply fall off the end of the separator. An eddy current is defined as the currents caused by voltages induced by changing flux, and tend to oppose the change of the flux.
Non magnetic particles, such as gold, quartz, and pyrite, are not amenable to magnetic separation, but some magnetic material may be removed from the feed. For instance, in a few situations, plants using gravity concentration for recovering gold, used magnetic separators to remove the high concentration of magnetite that was recovered with the gold, prior to further processing.
Magnetic separation is generally a low cost method of recovery, unless high intensity separators are required. The electro-magnetic high intensity separators that produce 20,000 gauss, tend to be expensive. However, the rare earth magnetic separators are relatively inexpensive and can produce magnetic fields around 6,000 gauss. So, when looking for a process to recover valuable minerals, magnetic separation should not be overlooked, if some of the material is magnetic or para-magnetic.
And as a comment on choosing magnetic separators, when I was working on a project involving separating some magnetite, rutile from a spiral concentrator concentrate, a new engineer just out of school, came up to me and said that he had a way to change the process and both eliminate a stage of separation, while saving money. I asked him to explain this idea and he began telling me that he would simply eliminate the first stage of low intensity separators and send everything directly to the high intensity magnetic separators. This would eliminate the need for one complete circuit, and would only "slightly" overload the high intensity separators. He further explained that by slightly increasing the size of the high intensity magnetic separators, this problem would be solved. Then he stood back, waiting for my approval.
"Well" I began, "you have definitely been giving this some thought. But you do not have two critical pieces of information, that would make your plan unacceptable. First, by sending highly magnetic material to a high intensity magnetic separator, it would rapidly fill the magnetic surface and blind off the separator working area for any weakly magnetic material, so you would reduce the efficiency of the separator by 50% to 70% or more. The magnetic separator salesman would like you, though because he could retire after you purchased enough of the very expensive high intensity magnetic separators to make your idea work. Which brings me to the second point, low intensity drum separators cost a fraction of what a high intensity magnetic separator would cost. These 20,000 gauss separators cost several hundred thousand dollars each, while this drum separator costs around $20,000, each. Almost a factor of 10. So, while it is a interesting concept, it would cost more to recover the titanium from the rutile than one could ever sell it for, which makes it not feasible. But don't let that discourage you from looking at alternate methods, because that is how discoveries are made."