High gradient wet magnetic separator design and experiment with chromite ores

Magnetic separators have been used in mineral processing in different fields such as ore dressing, recycling and interactive protection since the late 19th century. During magnetic separation in mineral processing, when particles with different magnetic features in an ore are exposed to a high gradient magnetic field formed in a specific area, separation process can be carried out by magnetic forces which are generated according to these particles' properties. Although particles with strong magnetic sensitivity are removed easily from mixture via strong magnetic force developed in the direction of magnetic field, particles with weak magnetic sensitivity may be left in the separation media due to insufficient resultant magnetic force intensity. Therefore, the magnetic field strength and the gradient used in separation have significant effect on high efficiency of ore dressing performed by using magnetic field. Minerals displaying ferromagnetic property can be easily separated from the beneficiation medium with a low magnetic field strength by means of having strong magnetic sensitivity. In addition to this, since most of the minerals are paramagnetic with weak magnetic sensitivity, high magnetic field strength is required for successful seperation of these minerals. Minerals having no magnetic sensitivity are called diamagnetic and the seperation process can be carried out by pretreatment of these minerals with paramagnetic liquids in order to obtain magnetic properties. Besides, in some practices performing a range of heat treatment on weak magnetic featured minerals transforms them into minerals with high magnetic sensitivity so that separation process can be executed. Dry enrichment of oversized paramagnetic ores such as hematite, chromite, and manganese with residual magnetic field of 5000 Gauss and above is successful, however it is not possible to beneficiate these ores via dry magnetic separation when fine particle sizes are in question. Hence, these ores are dressed with either electromagnetic seperators or gravity methods. Electromagnetic seperators which perform wet beneficiation for fine sizes are expensive and imported. Also, they have high operating costs because of high energy consumption. In this thesis study, development of a high gradient magnetic seperator with 10.000-12.000 Gauss magnetic field for enrichement of fine sized ores is targeted. Within this thesis scope, basic geometric dimensions of drum, types of magnets used in the drum, and suitable systems were determined, first drafts of magnetic structure are drawn, domain analysis of magnetic design using computer assisted finite element methods and the optimization of magnetic structure were done. According to this obtained data, a prototype was produced and the benefitiation experiments of chromite ores with particle sizes below 1 mm which are gathered from different regions of Turkey were conducted. Chromite ores having paramagnetic features were enriched at fine sizes with magnet designs developed within this thesis. However, development and optimization of beneficiation prosess flow scheme were not aimed in ore dressing experiments. The main point is to study enrichment of these fine sized ores by magnetic seperator. The purpose of the thesis involves introducing a new and productive device for ore dressing industry by proposing an alternative to problems arising from gravity separation methods (for example beneficiation with shaky tables of chromite ores with olivine doesn't conclude effectively) which are used for fined sized chromite ores. Hence, developing an energy-efficient and low-cost magnetic seperator aimed as an alternative to wet beneficiation of fine sized paramagnetic minerals by shaking tables for the first time in both the world and Turkey using permenant separators benefitted technologically and economically to a great extent. The greatest progress of beneficiation via magnetic separation was achieved in 1960 when Jones magnetic separator with high magnetic field strength was developed and industrial scale production started from the 1970s onwards. Again in 1970s, continous permanent magnetic separators (RE magnetic separators) were manufactured using rare earth elements such as neodymium (Nd), aluminum (Al) and cobalt (Co) for high strength magnetic field supply. Until that era those devices have been known as the strongest continous magnetic separators which had field strengths up to 22.000 Gauss. Nowadays, this type of magnetic separators are fabricated by different companies as drum or roll versions for dry separation processes. Magnetic separators with "superconductors" which were developed as laboratory scale devices in 1980s and found industrial use in early 1990s are new generation separators providing the highest magnetic field strength in modern-day. 50 000 Gauss (5 Tesla) magnetic field strength can be achieved with these separators. Magnetic field strength of devices manufactured by Eriez and Outotec companies can go up to even 150000 Gauss. Eddy-current separators and magnetic liquids are again recently developed technologies used for separation of nonferrous solids. Most of this recent development have been mainly focused on wet magnetic separators with very high field strength. Ever-increasing need for beneficiation of fine sized ores with low magnetic sensitivity and compulsory raw material supply from different sources fueled the ongoing studies in this field. In the last 20 years, a vast number of wet magnetic separators with high field strength have been developed notably. The common trait of these devices is a ferromagnetic material based filler called "matrix". This material is placed inside a mobile system which is located between the poles of electromagnets. The function of the filler is to increase magnetic field strength and rate of field strength change namely gradient. In other words, raising the attractive force on mineral particles is achieved by increasing magnetic field gradient rather than field strength. Magnetic field gradient is a parameter as important as magnetic field strength influencing enrichment process. In mineral processing, magnetic separators are classified as "high magnetic field" and "low magnetic field" according to applied magnetic field strength and "dry" or "wet" method according to type of separation medium. During enrichment of particles with diameter size above 1 mm dry methods can be performed easily, however, using the same method for particles with diameter size below 1 mm requires increase of medium powder which causes environmental problems subsequently. Besides, agglomeration of particles under the effect of electrostatic attractive forces and magnetic field plus trapped nonmagnetic particles inside these agglomerates reduces the efficiency of separation process significantly. Maintaning the magnetic field gradient inside magnetic separator as high as possible has a considerable effect on minimising agglomeration problem. Although agglomeration issues arise in magnetic separators when dry methods are performed, design enhancement studies regarding the increase of magnetic field gradient are promising in overcoming this problem.