iron ferrite

When they say “iron ferrite,” many immediately imagine the same dark brown or black powder that goes into magnetic circuits. But if you dig deeper, in production, especially in mining and processing equipment, everything is not so simple. A common mistake is to assume that the main thing is the magnetic permeability from the material data sheet. In fact, under conditions of vibration, shock loads and abrasive dust, the stability of properties from batch to batch and the mechanical strength of the core itself after pressing and sintering are much more important.

From theory to workshop: where the nuances lie

Take, for example, the production of separators based oniron ferrite. The technology seems to be well-established: batch preparation, pressing, high-temperature sintering. But here’s the point: even minor temperature fluctuations in the furnace, say by 15-20 degrees in the sintering zone, can lead to magnetic losses in the finished product increasing by unacceptable percentages. And this is not revealed immediately, but already at the stage of assembly and testing of the finished separator. I had to deal with when a batch of cores, supposedly made according to the same regulations, gave a spread in induction of up to 10%. They looked for the reason everywhere - in the powder, in the mold. It turned out that the problem was uneven wear of the muffle furnace heaters, which was not diagnosed in time.

Another practical point is the purity of raw materials. In theory,iron ferrite- these are Fe2O3 and oxides of other metals. But in practice, the powder may contain foreign inclusions, for example, traces of oil from equipment or microparticles of abrasive. When pressed, they become centers of microcracks. Later, when the separator operates under load, these cracks can grow due to vibration, which will lead to splitting of the core. More than once I have seen such “fatigue” ones. destruction on old machines after several years of operation. Therefore, now in production, for example, at the same enterprise of the LONGI Corporation, they pay great attention to the preparation and control of raw materials at the entrance. Not just a passport from the supplier, but your own, additional analysis.

By the way, about LONJI. On their websitehttps://www.ljmagnet.ruyou can find information that the company has been operating since 1993 and specializes in mining and processing equipment. When dealing with such a scale - a total area of ​​140,000 m2 and production of about 4,000 units of equipment per year - issues of stability and repeatability of material parameters become especially acute. It's one thing to make a prototype of a magnetic system oniron ferrite, and it is quite another to provide the same characteristics for thousands of separators. Here, it is impossible to do without a deep study of the technology for preparing and processing magnetic powder.

Sintering and geometry problems

Sintering is a completely different story. It would seem that the process is standardized. But the shape of the product makes its own adjustments. Thin-walled rings or complex profiles are sintered differently than solid cylinders. The problem of shrinkage and warping arises. This is especially critical for high-gradient separators, where the gap between the poles must be maintained with high accuracy. Uneven shrinkage of the core nullifies all calculations of the magnetic field.

We once tried to work with powderiron ferritefrom a new supplier. According to the passport, everything was perfect, even better than the old one. But when sintering complex-profile workpieces for rotors, an increased percentage of defects was obtained due to warping. They began to figure it out. It turned out that the new powder had a slightly different granulometry - a narrower fractional composition. This led to a change in the kinetics of compaction during sintering. We returned to the old, trusted supplier, but with stricter control over granulometry for each batch. Sometimes simplicity and predictability are more important than “ideal”? on the characteristics paper.

Another practical observation: after sintering, mechanical processing is often required - grinding the seating surfaces. It is important here not to overheat the part. Overheating locally changes the microstructure in the surface layer, a so-called “defective layer” appears, which may have reduced magnetic permeability and increased losses. This will then affect the heating of the core during operation. Therefore, grinding modes - speed, feed, cooling - are also part of the technological chain of working withiron ferrite, which is not always written about in textbooks.

Integration into the final product

A high-quality ferrite core itself is only half the battle. How it will work in a magnetic system depends on a lot of factors. For example, on the method of fastening. Rigid fixation with epoxy compounds in an aluminum case - seemingly reliable. But epoxy and ferrite have different coefficients of thermal expansion. With cyclical temperature changes (and they are inevitable in the workshop), mechanical stresses can arise, which over time will lead to a crack. I have seen such cases on equipment operating under conditions of sudden daily temperature changes.

Therefore, in some structures they began to switch to elastic methods of fixation or leave damping gaps. But here another question arises: vibration. A loose core can “play”, which is unacceptable. It turns out that we need to look for a balance. In modern developments, for example, for processing equipment, combined fastening systems are often used. This is precisely the area where the experience of practicing engineers, who have seen how equipment behaves in real conditions for years, is invaluable. The LONGI company, with its long history and staff of more than 1,200 people, of which over 60% are specialists with higher education, has certainly accumulated a huge database of such practical solutions that cannot be found in open sources.

A separate discussion concerns protective coatings. Nakediron ferritequite fragile and sensitive to moisture. The mining industry has an aggressive environment. Therefore, cores are often coated with a thin layer of epoxy or special varnish. But there are pitfalls here too. A coating that is too thick can create an unwanted air gap in the magnetic circuit. Too thin - it won't protect. It is necessary to accurately dose and control the thickness. This, again, is a question of technology and production culture.

Looking to the future and alternatives

Nowadays there is a lot of talk about rare earth magnets and their advantages. Yes, they have a higher energy product. But when it comes to large industrial systems that require large volumes of magnetic material, cost becomes a determining factor.Iron ferritewas and still remains a “workhorse” for many types of magnetic separators, especially for the extraction of large and highly magnetic fractions. Its manufacturability, sophisticated processes and relatively low price are huge advantages.

The main directions of development, in my opinion, lie not in the search for a replacement for the material, but in improving its “environment”. This includes more accurate computer modeling of magnetic fields taking into account the real properties of a specific batch of ferrite, and the development of new composite structures where ferrite elements are optimally combined with other materials, and the introduction of systems for monitoring the state of the magnetic system directly during equipment operation.

It is likely that over time, new brands of powders with more stable characteristics or improved compressibility will appear. But the fundamental principles will remain. The main thing is to understand thatiron ferrite- not an abstract material from a reference book, but a living product, whose final properties are born at the intersection of chemistry, metallurgy, mechanics and engineering experience. Experience that allows companies like LONGI to create reliable equipment that works in the most difficult conditions for years. It is this practical experience, and not bare numbers from the specifications, that ultimately determines whether the separator will effectively extract the valuable fraction from the pulp or become a headache for the processing plant.