properties of ferrite

When they talk about the properties of ferrite, many people immediately look into textbooks - coercive force, remanent induction, Curie point. This, of course, is the basis, but in real work with magnetic cores, especially in mining equipment, theory often diverges from practice. The most common mistake is to assume that high Br and HcB values ​​automatically give better efficiency. In fact, if the ferrite for the separator is overloaded in terms of coercivity, you can get instability in alternating fields and overheating, which in quarry conditions results in downtime. We had such cases in our workshop when they tried to take the “most powerful” one. material without taking into account the actual working cycle.

From theory to workshop: how the properties of ferrite manifest themselves in iron

Take, for example, magnetic systems for enrichment separators. The key here is not the maximum numbers, but the stability of properties in the temperature range from -20 to +80 ° C, which are common in Siberia or the Urals. Ferrite brand Y30H, which we have been using for a long time, seems to be suitable according to the passport. But in the cold, its coercive force could “float”, and the separator began to lose its fine class. Together with the technologists, we had to review not only the brand, but also the geometry of the cores - sometimes it’s easier to increase the mass but maintain stability than to chase record parameters.

Another point is mechanical strength. Ferrite is brittle, everyone knows that. But in vibrating feeders or crushers fromLONGI Corporationshock loads are constant. It happened that a batch of magnetic plates arrived with ideal magnetic properties, but after a month of operation, a network of cracks appeared in the jaw crusher. Magneticproperties of ferriteat the same time, they did not formally change, but due to microcracks, the capture efficiency dropped by 15–20%. It was necessary to introduce additional control for impact strength for critical components, although this point is not included in GOST standards for magnetic ceramics.

Therefore, now when choosing a material, we first look at the operating conditions in the equipment passport, and then at the numbers from the certificate. We often order test batches from trusted suppliers and run them on a stand that simulates real cycles - with vibration, temperature changes and long-term operation under load. This is the only way to see how a specific batch of ferrite will behave.

Granulometry and compaction: what they don’t write about in reference books

The magnetic properties of ferrite are greatly influenced by what happens before sintering. Uniformity of granules, pressing pressure, orientation of the field during molding - little things that decide everything. I remember about ten years ago we worked with a factory that supplied us with blanks for pole pieces. The magnetic parameters varied within 10% from batch to batch, although there was only one raw material. It turned out that the problem was in the old mold and that the operators did not monitor the moisture content of the powder before pressing. Humidity changed the fluidity, the compaction density turned out to be uneven, and after sintering this resulted in a scatter in the coercive force.

Now at its production inLONGI CorporationWe strictly control these stages. Especially for critical products such as magnetic drums for wet separation separators. There, even a slight heterogeneity can lead to the formation of “dead” ones. zones on the drum and loss of concentrate. Technologists with experience can visually determine the quality of the pressing by the fracture of the workpiece - if the structure is non-uniform, it means that the magnetic properties will be “striped”.

This leads to another practical conclusion: it is not always worth taking ferrite with passport properties “above average”. Sometimes it is more reliable and cheaper to take material with slightly lower but stable performance, which gives a predictable result in the finished product. For mass production of equipment, as at our site in Fushun, where up to 4,000 units of equipment are assembled per year, this is a matter of both reputation and economics.

Temperature factor and losses in real cycles

The Curie point is good, but before it, ferrite can greatly change its behavior. In the converter technology used in our electromagnetic feeders, eddy current losses and hysteresis are important. And they strongly depend not only on the composition of the ferrite, but also on the frequency of magnetization reversal. There was a project where we switched from a frequency of 50 Hz to 400 Hz to increase performance. The standard Y33 ferrite did not work here - it got so hot that we had to install additional cooling.

I had to delve into the topic and test materials with cobalt and nickel additives, which stabilize properties in a wide frequency range. It is more expensive, but for a specific order for high frequency separators it was the only solution. By the way, on the websitehttps://www.ljmagnet.ruYou won’t find this in the product section - such nuances usually remain in internal test reports.

In field conditions, at processing plants, temperature is the main enemy. In summer, indoor temperatures around the separator can exceed 50°C. If the ferrite is selected without a margin for the temperature coefficient of induction, then by the evening the separation efficiency may drop noticeably. We learned from our mistakes - now for hot climates, in deliveries, for example, to Kazakhstan, we immediately add ferrite with low TCI and additionally calculate the ventilation system of the magnetic system.

Interaction with other materials of construction

The properties of ferrite are not only about the material itself, but also about how it works in an assembly. Contact with the metal body, heating from neighboring components, even the paint used to cover the magnetic plate - all this has an effect. Previously, no one paid attention to the fact that a thick layer of powder paint applied to protect against abrasive can act as a heat insulator and impair heat dissipation from the core. As a result, the ferrite worked at a temperature 10–15 degrees higher than the calculated one, and its service life was reduced.

Another example is fasteners. If the magnetic element is rigidly fixed in a steel groove, and the entire structure is subject to vibration, microcracks may appear over time. We switched to a mount with damping pads made of special rubber that is non-magnetic and can withstand temperature. This simple solution eliminated a lot of problems with breakdowns in crushing equipment.

Therefore, when developing new equipment inLONGI CorporationMagnetic engineers work in conjunction with designers and assembly technologists from the very beginning. You cannot first design a magnetic system and then “fit in” it. her into the car. It is necessary to immediately take into account thermal deformations, vibration loads and ease of maintenance. Only then will the properties of ferrite laid down in the laboratory be fully realized in work.

Quality control: distrust and double-check

You should never blindly trust certificates for magnetic ceramics, even from trusted suppliers. We have a simple but demonstrative stand in our laboratory: a reference magnetic assembly from a separator that has worked stably for years. We compare any new batch of ferrite elements with it according to indirect but practical parameters - the tearing force of a standard steel plate at different temperatures, the demagnetization time after removing the field.

There were cases when, according to the passport, two batches of ferrite from one plant were identical, but in fact one produced 8% less magnetic flux in the assembly. The reason turned out to be that in the second batch the cooling mode after sintering was slightly changed, which affected the microstructure. The material passed the basic parameters (Br, Hcb), but not the behavior in the unit.

That is why in an enterprise that employs more than 1,200 people, most of whom are engineers and technologists, its own strong laboratory is so important. You can buy the most modern raw materials, but if you do not know how to properly test and apply them, taking into account all the described subtleties, then the equipment will not work as intended. The properties of ferrite are not a set of numbers, but knowledge of how these numbers behave in iron, under the roar of crushers and in the dust of a quarry. That's what's truly valuable.