Demagnetising coils are commonly used in the minerals processing industry to demagnetise magnetised slurry. The media used in the Dense Medium Separation (DMS) process is magnetic to allow for simple recovery and regeneration processes. In the past traditional demagnetising coils have been plagued by inefficiencies. Longi-Multotec now offers a solution that addresses these issues and provides continuous, high frequency demagnetisation, which will assist in preventing medium losses and may also improve cyclone efficiency.
“It is well known that recovery of the media by means of magnetic separators induces a residual magnetisation, which causes flocculation of the magnetised particles and increases the viscosity of the medium. It is possible to reduce or even eliminate these effects by demagnetising the magnetic over-dense product stream,” says Stuart Callum, Process Engineer, at Multotec’s Solid Liquid and Magnetic Separation business line.
Callum goes on to explain : “The process of demagnetisation requires an alteration of an applied magnetic field from north to south, while monotonically decreasing the field intensity steps. This elimination of the magnetic hysteresis will bring about scrambling of the magnetic domains striving towards a zero net resultant charge.”
DMS efficiency may be improved by lowering and then controlling medium viscosity as well as medium stability and density control. These improvements are clearer in cyclones, where gravitational forces are higher, compared to DMS drums.
“Over the years, demagnetising coils have been developed to improve the frequency of alternating magnetic fields as well as to decrease the distance over which the field is dissipated. These products were aimed at allowing more energy efficient and compact designs. Ironically however, this drive has led to inefficiencies in process performance,” says Willem Slabbert, Applications and Process Manager, at Multotec’s Solid Liquid and Magnetic Separation business line.
The first generation of demagnetising coils – the line frequency demagnetiser – had constant magnetic fields associated with specific sized coils which were energised with constant AC electrical energy supply. Decreasing coils sizes were installed in the direction of fluid flow, around a fluid carrying pipe, to achieve decreasing magnetic field exposure of the fluid.
“A disadvantage of this technology included high power consumption due to the need to charge numerous coils simultaneously and continuously. The line frequency demagnetiser was also not able to provide a consistently decreasing magnetic field, but rather resulted in incremental decreases. This result is highly ineffective in terms of demagnetisation,” Slabbert adds.
“Probably the biggest advance in demagnetising coil technology was the introduction of pulsing electrical energy supply to the demagnetising coil. This allowed the constant AC electrical energy supply to the panel components to be stored in the electrical circuit up to the point of discharge. Thereafter, the energy would be transferred into the conductive wires of the demagnetising coil and as the energy from the storage capacitor was reduced, the magnetic field strength would decrease consistently over time until the capacitor energy was low. The recharge and onset of the next pulsing cycle would then begin,” Slabbert explains.
This technology achieved far better results in demagnetisation, even though it still operated at
50 Hz. The single coil configuration and improved energy control also allowed capital and operational cost savings. The biggest disadvantage of this system, however, is that in a given time frame, not all the elements of the slurry are equally exposed to the magnetic field.
The consonance wave demagnetising coil was developed to address the issues experienced with the line frequency demagnetiser. A secondary coil, with a secondary control circuit, is now employed to discharge during the time that the primary circuit capacitor is charging.
“The apparent advantage of the consonance wave demagnetising coil is that even while a capacitor is charging, there is no time during which no demagnetisation occurs. However, different coil strengths and different pulsation starting times make consistency of the pulse difficult, resulting once again in issues with demagnetisation and efficiency,” Slabbert comments.
“In summary, historic generations of demagnetising coils are flawed through not complying with consistently decreasing field strength. The results of these flaws are significant inefficiencies that will impact detrimentally on the efficacy of demagnetisation,” he points out.
The constant wave, high frequency demagnetising coil has been designed to overcome the inefficiencies of previous generations of coils by generating a continuous wave of a consistently decreasing magnetic field across distance in the direction of fluid flow, at high frequencies of up to 200 Hz. This allows continuous demagnetisation of slurry through all consecutive slurry elements, thereby ensuring continuous effective demagnetisation, with ideal exposure of the entire slurry flow to the demagnetising properties.
“Even though the actual magnetic coil remains the same, new advances in electrical component technology allow improved control over the electrical energy supply to the coil. The performance advancement of the new generation Longi-Multotec demagnetising coil will allow more efficient demagnetisation, which will facilitate better viscosity management and in turn will improve cyclone performance in terms of separation efficiency and quality,” Slabbert concludes.
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