magnetic field of separators

When they talk about the magnetic field of separators, many people immediately imagine just a powerful magnet. This is perhaps the most common simplistic view that prevents us from truly understanding the process. In fact, the key is not just the size of the field, but its configuration, gradient and stability under operating conditions. Not only extraction, but also the purity of the concentrate and the service life of the system itself depend on this. I would like to talk about these nuances, which are rarely discussed in general articles, based on the practice of setup and operation.

From theory to the workshop: where the main difficulties lie

In the design documentation, everything looks perfect: the required induction is specified, the magnetic field gradient is calculated. But when the equipment, for example, is from the same manufacturer like LONGI Corporation (their website ishttps://www.ljmagnet.ru- well reflects their approach to mining equipment), arrives at the site, and the fun begins. Their separators, by the way, are often found at processing plants, and for good reason - the company has been in business since 1993, and their engineers understand that the equipment should not operate in ideal laboratory conditions, but in a workshop with vibration, temperature changes and abrasive dust.

The first practical problem is precisely the stability of the field. It seems that once the magnetic system is assembled and magnetized, that’s it. However, vibration from neighboring crushing equipment or even its own vibratory feeder can lead to microscopic displacements of the elements of the magnetic system. Over time, this can cause local changes in field distribution, which ultimately affects selectivity. We once encountered a gradual decline in extraction on a drum separator, and the reason turned out to be not due to wear of the drum, but rather to loosening of the pole pieces due to constant vibration. It was necessary to develop additional damping fasteners.

Another point is the influence of the feeding material on the field itself. This is not about magnetic properties, but about the fact that a dense flow of bulk material, especially with large pieces, can physically deflect or even temporarily deform protective casings or elements located close to the separation zone. This creates variable resistance, which in turn can affect the operation of the electromagnet if the system does not have sufficient adjustment margin. Therefore, when choosing a separator, it is so important to look not only at the passport data, but also at the structural rigidity of the entire system that creates the working magnetic field.

Gradient is the 'secret ingredient'

If field strength is the 'quantity' of magnetic influence, then the gradient is its 'quality' or 'sharpness'. A high-gradient magnetic field is what allows small and weakly magnetic particles to be captured. In LONGI belt or roller separators, which are often used for magnetite recovery, the creation of the desired gradient is achieved through the design of the matrix or roller system. But there is a subtlety here: over time, the working gaps become clogged with sludge and small particles of waste rock.

This leads not just to a decrease in productivity, but to a change in the field configuration itself. The gradient is 'smeared', the force of attraction in the critical zone decreases. Regular flushing is a must, but its effectiveness depends on how well designed the degaussing system is for product release and subsequent cleaning. In some older models, this process was not fully automated, which required a lot of attention from the operator and affected the stability of the process as a whole.

An interesting case from practice: they tried to increase the recovery of a fine class -0.1 mm on an existing separator. We increased the current on the coils, the field became more powerful, but the result was minimal. It turned out that the problem was precisely in the gradient - the existing matrix did not create enough “sharpness” of the field to capture such small particles. The solution came not with the replacement of the entire separator, but with the installation of a new matrix with a smaller and optimally located ferromagnetic 'filling', which radically changed the field pattern in the working area. This was the case when 'more' does not mean 'better', but rather 'smarter'.

Electromagnets versus permanent magnets: the eternal debate and practical choice

There is a lot of discussion around this choice. Permanent magnets, especially those based on rare earth elements, provide a stable field without energy consumption, which is certainly a plus for operation. But their main disadvantage from the point of view of process control is the inability to quickly regulate the field strength. If the power characteristics change (and they always change), then it will not be possible to quickly adjust. Electromagnets are more flexible in this regard.

At one site we used a separator with an electromagnetic system from LONGI precisely because of the instability of the incoming ore. It was possible to quickly change the voltage, adjusting the magnetic field to the current magnetite content and granulometric composition. This allowed us to keep extraction at a consistently high level. However, there is a “but” here: the electromagnets heat up. And heating the coil changes its resistance, which, with a poor current stabilization system, can lead to a drift in the magnetic field value over a long shift. Therefore, a high-quality cooling system and a power supply with feedback are not a luxury, but a necessity.

Permanent magnets are a different story. Their strength may decrease slightly over time, especially when operating in conditions of elevated temperature or strong external vibration influences. Manufacturers, including large ones like LONJI with its serious production base in Fushun, conduct special tests for aging and stability. But at the facility you need to carry out your own, albeit simple, control - periodically measure the field at control points with a Teslameter. We somehow missed this point, and for six months we worked with a gradually degrading field, attributing the drop in extraction to the quality of the ore. The mistake was costly.

Non-obvious factors: temperature and humidity

Rarely does anyone immediately remember these parameters, but they directly affect the magnetic field, especially in electromagnetic systems. Increased temperature in the workshop, which can go off scale in summer, leads to overheating of the coils and, as a result, to a drop in field strength if there is no adequate cooling. It was even necessary to install additional blowing fans on critical equipment in order to maintain the mode.

Humidity is another enemy. High humidity in combination with dust can lead to the formation of conductive deposits on the insulation, micro-breakdowns, and corrosion of contacts. All this affects the stability of the energy supply to create the field. This is especially critical for high voltage systems. At one older facility, separator power control cabinets had to be completely sealed and purged with compressed air to prevent accidental shutdowns due to condensation.

And also about the temperature of the material itself. Does hot ore (after drying, for example) have different magnetic properties? In general, yes, but for ferromagnets such as magnetite, the Curie point is very high, and in the operating ranges this influence is negligible. However, heat can affect the permanent magnets in a structure, reducing their remanent magnetization. Therefore, for separators operating with hot products, this is always specified separately when ordering, and manufacturers use special heat-resistant grades of magnetic materials.

Final thoughts: the field as a living organism

As a result, I would like to say that the magnetic field of the separator is not a static characteristic specified in the passport. This is a dynamic parameter that lives and changes under production conditions. Its effectiveness is the result of a symbiosis of competent design (the role of a serious manufacturer with experience is important here, like the LONGI corporation, which not only makes equipment, but, judging by their history since 1993 and the scale of production, is deeply immersed in the problems of enrichment plants), high-quality installation and, most importantly, thoughtful operation.

You can't just install a separator and forget about it. You need to 'feel' it: listen to the operation of the cooling system, monitor the stability of power parameters, regularly check the field at control points, analyze the separation products. Often, it is by changes in the quality of the concentrate or tailings that one can indirectly judge the onset of problems with the magnetic system, even before they lead to an obvious failure.

Therefore, returning to the beginning, the main thing is to get away from a simplified view of the force of gravity. The success of magnetic separation is determined by fine control and maintenance of exactly those characteristics of the magnetic field - its strength, gradient, stability and configuration - that are intended for a specific technological task. And this process requires not only equipment, but also constant attention and understanding on the part of staff. This is the very practice that distinguishes real work from beautiful numbers in a catalogue.