FAQIs coal magnetic? Why can you use magnets to clean coal?
A few facts about magnetic fields. Everything is magnetic to some degree. The range of levels of magnetism is very wide, of the order of millions to one. Magnetic fields are generated by motion of electric charges. A current in a wire generates a magnetic field surrounding the wire. The magnetic field lines around an element of electric current are more or less circles surrounding the current. Rotation of an electrically charged body, such as an electron, also makes a magnetic field emanating from and surrounding the body. The earth is magnetized. No doubt you have seen a drawing of the field lines emerging from the magnetic North Pole and looping back to the magnetic South Pole. This is a dipole field, the most elemental of all magnetic fields. Magnetic field lines are unusual. Unlike the lines of the electric field which begin and end on electric charges, the lines of the magnetic field close on themselves. The lines of the earth’s magnetic field emerge at the North Pole and reenter through the South Pole and pass through the earth to connect with the lines emerging from the North Pole.
There are three types of magnetic materials, diamagnetic, paramagnetic, and those exhibiting collective magnetism such as ferromagnetism. Paramagnetism and diamagnetism are weak forms of magnetism. Generally speaking, the MagMill separates weakly paramagnetic minerals from weakly diamagnetic coal.
The essence of magnetic separation technology is in knowing the relative degrees of magnetism of materials and in knowing how to generate magnetic forces. There are tables of values of magnetic properties called magnetic moment or magnetic susceptibility. Values of atomic susceptibilities are shown in the following figure showing atomic susceptibilities of the elements at room temperature (taken from the American Institute of Physics Handbook, Third Edition, page 5-224, ed. D. Gray, ©The McGraw-Hill Companies, Inc., 1972; reprinted with permission from McGraw-Hill). Some values of the magnetic susceptibility of minerals naturally found in coal are given in the table below.
Paramagnetic: Pulled into magnetic field
Diamagnetic: Pushed out of magnetic field
Coals and most of the minerals they contain are feebly magnetic and are unresponsive to conventional wet or dry magnetic separators. Their magnetic susceptibilities are hundreds of thousands of times less than that of pure iron. Even though the minerals are feebly magnetic, MagMill LLC's patented dry separators can separate them from coal if they are liberated from the carbon matrix. The carbonaceous components of coal are diamagnetic (pushed out of a magnetic field). The feebly magnetic minerals are paramagnetic (pulled into a magnetic field).
How does the MagMill remove trace metals from coal?
Mercury, arsenic, selenium and other trace metals can form solid solutions with sulfide minerals. Magnetic separation of paramagnetic sulfides removes surface bound trace metals such as mercury and arsenic. This can be a significant portion of these metals in the coal. The graph shows the relationship between mercury and iron pyrites for coal samples withdrawn from operating pulverizers at power plants in Ohio, Pennsylvania and West Virginia.
Does the MagMill work for all coals?
Over seventy coals with ash values ranging from 5 to 40% and sulfur values ranging from 0.5 to 5% have been tested for their response to magnetic separation. The coals represented all ranks except anthracite and were from all major US coals fields. By and large all coals exhibited consistent and good reductions in ash. Sulfur reductions were better for mill concentrated samples extracted from pulverizers than for raw coals.
How big is the MagMill? Will it fit into my plant?
The MagMill consists of the plant's existing pulverizer(s), a first stage belt separator, our electromagnet, chiller and dc power supply. The first installations will most likely be one separator servicing one pulverizer, but later will be one separator servicing a bank of pulverizers. The Bechtel Corporation prepared an engineering evaluation for Allegheny Energy's Fort Martin plant (a relatively new plant with plenty of room) and for Ameren/CIPS's Meredosia plant (an older facility with little extra space) and concluded that a MagMill retrofit could be installed in both plants. Projected footprints of the units are shown below. As a rule of thumb, the capacity of the separator will be one-third to one-half the capacity of the pulverizer. The power supply and chiller can be located remote from the magnet.
What will it cost?
The installed cost of a MagMill retrofit is a function of its size. There is an economy of scale for larger machines, meaning that the cost per unit size decreases as the size of the unit increases.The costs will include a capital charge and operating costs, including labor, maintenance, electricity, cooling water, and waste disposal.
How will I save?
Pre-combustion removal of ash and abrasive minerals such pyritic sulfur and its associated trace metals will reduce maintenance costs throughout the plant, reduce slagging and fouling, and will reduce the burden of post-combustion clean-up. Costs for SO2 and mercury allowances will be lowered.
Beta prototype testing showed an increase in output can be achieved at lowered power draw. This is important, especially for pulverizer limited plants, since a significant portion of the output of a pulverizer that is down can be made up by increasing the output of another. It is important to note that although pulverizer output can be increased, overall emissions are lowered.
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