ferrite

When people talk about ferrites, many people immediately imagine small cores in electronics. But in our business - in mining equipment - it’s a completely different story. Here, ferrite is not just a material, but often a key element in magnetic separation systems. Many times I have encountered the fact that engineers, accustomed to high-frequency applications, underestimate the requirements for stability and mechanical strength of ferrite elements in harsh quarry conditions. Constant vibration, temperature changes from -40 to +50, abrasive dust - all this creates an environment where the laboratory parameters of the material fade into the background, and its “survivability” comes to the fore.

What do we really expect from ferrite in a separator?

If we discard theory, in practice everything depends on several simple but strict requirements. Magnetic induction should remain stable, without noticeable decline, after thousands of hours of operation under load. This is not a situation where you can simply take a ferrite with the maximum initial induction from a catalog. Often the temperature coefficient is more important. I remember that on one of the first projects we installed a separator with seemingly excellent ferrite blocks. In the summer, during the peak shift, the temperature in the work area jumped, and the extraction efficiency dropped by 15-20%. It turned out that the material had too high a negative induction TCR. We had to revise the entire design of the magnetic system, adding compensation elements.

The second point is resistance to demagnetizing fields. During operation, especially when a piece of rock is jammed or during a power failure, pulsed fields of reverse direction may occur. If the coercive force of the ferrite is insufficient, an irreversible loss of magnetic properties in local areas can occur. This is not always immediately visible, but gradually leads to the appearance of “dead zones”. on the separator drum and, as a result, to product losses. We check this not only on the stand, but also after every 500-1000 engine hours on a real object, measuring the field with a contact Teslameter at control points.

And, of course, mechanical strength. Ferrites are brittle materials. Standard slabs or segments can crack from a strong point impact during installation or from overtightening the fastening bolts. We have long switched to using ferrite elements in special shock-absorbing polyurethane cassettes. This is not only protection, but also compensation for thermal expansion - steel housing and ferrite have different coefficients, without a damping layer stresses appear over time.

Experiences and mistakes: a case with a dry separation system

I would like to give an example of one project that taught me a lot. We are talking about a dry magnetic separation system for the enrichment of iron ore concentrate. The goal was to remove waste rock at an early stage to reduce the load on crushers and mills. The concept involved the use of a powerful drum separator with a multi-pole system based on ferrite magnets.

Initially, a standard configuration with Y30 strontium ferrite blocks was chosen. On paper everything matched. But during the tests it turned out that the fine ore dust, which inevitably forms in the process, is magnetized and creates a kind of “lint”. on the surface of the drum. Over time, this layer became denser, increasing the gap between the ferrite and the processed material, which critically reduced the field gradient and separation efficiency. We had to stop the line for cleaning almost every shift.

The solution was found not in replacing the ferrite, but in changing the design. Instead of a smooth drum, they made a ribbed one, with longitudinal grooves, where less dust would clog. And the ferrite blocks themselves were enclosed in sealed casings with forced blowing of low-pressure air. This partly solved the problem. But the main conclusion was this: when working with dry materials, the calculation of the magnetic system cannot be carried out only on the pure fraction; it is necessary to model the behavior of the dust component and its effect on the magnetic field.

By the way, in this project we collaborated with engineers from the LONGI corporation. They were just offering their vision of the drum design, based on their experience in the production of mining equipment. Their website, https://www.ljmagnet.ru, can be viewed to understand the scale - the company has been operating since 1993 and produces thousands of units of equipment per year. Their practical approach, when the theory is tested immediately in the workshop or at the test site, often helps to avoid such “dust problems”. problems at the design stage.

Issues of supply and quality control of ferrite elements

One of the unobvious difficulties is ensuring the homogeneity of the ferrite batch. Let's say you order 200 standard slabs measuring 100x50x20 mm. The parameters in everyone’s passport are normal. But when you start assembling a magnetic system from them, where the plates are placed close together, a scatter in induction of 5-7% may appear. For a high-gradient separator this is already critical - instead of a uniform field, strong and weak stripes are obtained.

We have developed our own acceptance procedure. Firstly, selective measurement is not 10%, but almost 30% of the batch. Secondly, we measure not only the induction in the center of the stove, but also in the corners. It happens that due to the peculiarities of pressing or sintering, the edges have different properties from the center. If the scatter is large, such slabs are sent to less critical units or returned to the supplier. Expensive? Yes. But it’s cheaper than redoing the magnetic system of a finished separator at the customer’s site.

Another nuance is geometry. It would seem that a slab is a slab. But if the flatness tolerance is not maintained, microgaps appear between the elements when assembling the package. In the air. And he demagnetizes. As a result, the calculated adhesion force is not achieved. Therefore, now in the technical specifications we always specify not only magnetic parameters, but also strict tolerances on geometry, especially for ferrite elements that will work in prefabricated systems with minimal gaps.

Alternatives and hybrid systems: is there life without ferrite?

Of course, in recent years there has been a lot of talk about rare earth magnets based on NdFeB. Their energy product is incomparably higher. But when it comes to large-scale systems that process hundreds of tons of material per hour, price becomes a determining factor. Replacing a ton of ferrite with a neodymium magnet increases the cost of the unit by several times, not by percentages. Therefore, a complete abandonment of ferrites in industrial magnetic separation is not expected in the near future.

The path of hybrid systems seems more promising. For example, the main field is created by powerful electromagnets or rare earth elements, and ferrite concentrators or pole pieces are used to form a specific gradient or create additional capture zones. Ferrite here works not as a source of the field, but as its conductor and shaper. This allows you to use its key advantage - stability and predictability of behavior over a wide temperature range.

LONGI Corporation, the same manufacturer with an area of ​​140,000 m2 and a team of over 1,200 people, has in its portfolio solutions where ferrite elements are integrated into complex magnetic systems of fine separators. Their approach, when more than 60% of employees have higher education, is felt precisely in such complex developments where it is necessary to combine materials science, electromagnetism and mechanical engineering. This is not just iron assembly, this is engineering work where understanding the capabilities and limitations of ferrite is a basic skill.

Looking to the future: what could change?

If we talk about trends, then, in my opinion, the main development will not be in the direction of a revolutionary new material, but in the direction of optimizing the use of existing ferrites. Firstly, it is an improvement in sintering technology to increase the uniformity and mechanical strength of large-sized products. A slab measuring half a meter or more that is not afraid of shock loads is already a serious advantage.

Secondly, coverage and protection will become increasingly important. Development of durable polymer or composite coatings that do not peel off ferrite during thermal cycling and protect it from abrasive wear and the corrosive atmosphere in workshops. This directly affects the service life of all equipment.

And finally, digitalization. Introduction of sensors to monitor the temperature of ferrite elements in real time and adjust the operating modes of the separator. While this may seem like overkill at the moment, when it comes to continuous processes with high downtime costs, proactive diagnostics can save huge amounts of money. Ferrite, as the heart of the magnetic system, will require an increasingly attentive and intelligent attitude towards itself. Not as a consumable, but as a key asset in the value chain.