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Comparison of dry and wet grinding process in an electromagnetic mill is presented in this paper. The research was conducted in a batch copper ore grinding. Batch mode allows for precise parametrization and constant repetitive conditions of the experiments. The following key aspects were tested: processing time, feed size, size of the grinding media, mass of the material and graining media, and density of the pulp. The particles size distribution of the product samples was analyzed in the laboratory after each experiment. The paper discusses the experimental results as well as the concept of dry and wet grinding and classification circuits for the electromagnetic mill. The main points of the discussion are the size reduction effectiveness and power consumption of the entire system. V
Comminution is one of the most energy-consuming technological processes, which aims to bring the material to the desired particle size [1,2,3]. The analysis of comminution consists of establishing the relations between the particle size distribution of products, the physical properties of the material, the comminution energy consumption and the parameters of the machines [4].
Comminution of raw material is carried out for two main reasons [1,5]: obtaining a final product, according to the requirements (main operation in the case of aggregates) and, mineral liberation, that is, bringing the material to such a state that it liberates the injection of the useful ingredient from the gangue (preparatory operation to a beneficiation process). Comminution of raw materials is widely used in many industries starting from mineral processing to the chemical, construction, food, cosmetic and pharmaceutical industries.
Generally, the comminution can be carried out mechanically and chemically. Mechanical comminution is the result of the operation of external forces or special forces on the material, while chemical comminution leads to the removal of part of the particles by dissolving, digesting or forming a different substance. Mechanical comminution is performed for dividing individual particles of material into smaller parts by smashing, breaking, attrition, pealing, cutting, crushing and other actions. Figure 1 shows three models of particle size reduction in mechanical treatment which follow from the type of machine used for a specific comminution operation. The choice of the machine is usually determined by the feed particles size and the requirements for the product obtained as a result of grinding. These issues are closely related to the efficiency of comminution machines [6,7]. Conventional grinding of mineral raw materials is carried out by crashing of the material using grinding media which refers to the model (b) in Figure 1. There is no shape control of the obtained particles, which often results in low technological value of the product.
This research particularly concerns ultra-fine comminution, which is a high energy consuming process. Conventional devices, such as tumbling ball mill, limit power consumption which follows form physical constraints such as, e.g., speed or grinding media size restrictions. In turn, the quality of the product decreases. For example, at low speed, large grinding media in a tumbling mill generate mainly impact and abrasive stresses, which, for micron- or sub-micron-size particles, do not work well. Thus, mineral processing society and device manufacturers constantly strive to develop less energy-intensive, more efficient solutions, with possibility to precisely define of the product properties such as shape and particle size [8]. This particularly concerns the case of the hard-to-enrich non-ferrous ores where one of the most important directions for modern technologies are new solutions designed for ore grinding to obtain grain sizes in the range of micrometers and development of conditions for effective beneficiation within such grain size [9].
A novel solution, the electromagnetic mill (EMM) can face the above problems. Small ferromagnetic particles which serve as the grinding media in EMM are put into motion in rotating electromagnetic field and devote EMM to ultra-fine comminution. Mainly model (a) and partially model (b) presented in Figure 1 characterize EMM performance what influence the shape of the product particles. The most exciting property of EMM is very short processing time and relatively easy optimization of EMM energy consumption.
EMM allows the use of both of the two principal methods of grinding: dry and wet. This paper presents a research concerning comparison of these two technologies used in EMM installation.
The essential advantages of wet grinding over dry one are as follows :
lower energy consumption per unit of mass,
better efficiency of the mill,
no dust,
lower noise.
Disadvantages compared to dry grinding:
the wear on the media and liners is usually considerably greater,
less material in the range of the very small particle size is produced,
drying the comminuted material usually needs more expensive dust collecting equipment than raw material drying,
shut down and switching on procedures are more complex in the wet case therefore the wet process is less convenient for day or single shift operations.
Unique characteristics of raw material emphasize the property of wet or dry technologies. For example, if sulphide minerals are processed, wet grinding is preferred because of the following reasons:
downstream (wet) processing requirements,
higher energy efficiencies associated with wet grinding,
requirement for a low moisture content feed (nominally less than 2% moisture) for dry milling is difficult to fulfill,
fine sulphides tend to oxidize in the air,
dry grinding often produces strong agglomerates and incrustation build-ups (depending on the fineness of the grind) which are difficult to subsequently disperse,
the surface properties of dry ground minerals are different than the same minerals wet ground.
The analysis presented above is preliminary and focuses mostly on the product particles size. Further tests and particles analysis methods (laboratory flotation, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis, X-ray fluorescence (XRF) spectroscopy, Brunauer–Emmett–Teller (BET) surface area or 3D particles analysis among others) for copper ore and other materials will be applied in the future to study the EMM product properties. Effectiveness comparison with other grinding solutions (e.g., ball mill, vibratory mill) will be performed as well. During flotation tests, the same material samples will be processed in EMM and in a laboratory ball mill for specific periods of time (to obtain the same product size). Flotation comparison tests aim to determine if the specific method of size reduction in EMM can improve copper recovery in the flotation process.
Wet technology is more effective than dry if the energy consumption factor is concerned. This follows mainly from energy consumption comparison of the transport system in dry technology (pneumatic transport) and in wet technology (hydraulic pumping). In typical operating conditions, the D100 EMM dry case needs 3 kW of power while the wet case needs 1.6 kW (for material transport system operation, apart from constant power for the mill). There are few other aspects simplifying the wet installation when compared with the dry one: maintenance and service of the pneumatic transporting system is more expensive and inconvenient due to dust and necessity of filter cleaning or replacing. On the other hand, the wet product often needs drying which causes additional energy-costs.
An important factor in wet and dry technologies comparison are control issues which are significantly simpler in the wet case. Control of the pump output is performed using inverters which perfectly set the required frequency of the pump drive. Thus, flow may be accurately maintained and in turn a mean grinding time can be appropriately set.
The methodology of batch grinding in EMM has proven to be effective for the purpose of comparing different process parameters. It allows us to contrast the results without influencing external disturbances. Moreover, it allows for the continuous process parametrization and estimation of energetic efficiency. Several issues were noticed, however, mainly for short processing times. The results obtained for larger graining media or higher densities should be studied further, primarily with respect to continuous process in grinding and classification circuits. The methodology for compete circuit parametrization based on the batch experimental results is the natural goal.
Wet grinding technology in EMM has been showed to be more effective than the dry technology. When the energy required for the pneumatic and hydraulic transport system is compared, that difference is even clearer. Research on dry grinding is, however, necessary, because some material can only be grinded in dry technology. Comparison of dry grinding results between EMM and other standard solutions showed certain advantages of such a processing method. Further research on product parameters is, however, necessary. Estimation of the processing time required for a specific size of the product grains and sharp edges is one of the challenging problems. Product grains reactivity, i.e., floatability of copper ore based on EMM grinding process parameters is another important issue. Influence of the processed material moisture on the grinding process in EMM will be studied further as well, also for the moisture control purposes. The EMM can be viewed as very competitive in relation to other milling solutions and at the same time it delivers a wide range of research problems.
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