FIELD OF THE INVENTION [0001] The present invention relates to a method and apparatus for concentrating high silica iron ores, tailings from iron ore processing plants, or an iron-containing waste stream, and more particularly to apparatus and methods for production of metallized iron-containing feed material for a blast furnace that has been conditioned to have reduced levels of gangue components. BACKGROUND OF THE INVENTION [0002] Many iron ore deposits in the world (magnetite, hematite or mixed ore) are characterized as having medium to high grade iron content (25-40% Fe Total), but contain elevated gangue components especially silica content. Those deposits that are characterized as having high silica with relative large iron oxide grain structure can be easily beneficiated by using a combination of standard comminution techniques (primary, secondary and tertiary grinding) to achieve successful iron particle liberation followed with concentrating by applying either low or medium intensity magnetic separation. Silica flotation may also be applied to the concentrate product so as to effect maximum silica rejection or removal and result in an iron ore concentrate having greater than 60% FeTotal with silica content less than 3%. In general, the coarser the iron oxide grain size, the easier and more efficient the liberation and concentrating steps become. However, finely disseminated iron ore; i.e., iron oxide grains that are tightly bound to silica, and having a grain size ranging from 2-15 microns, require substantial grinding and energy input to effect liberation (if it is even possible). High intensity grinding increases the processing cost considerably. And both magnetic separation and silica floatation may not work because the ore particle size may be too fine (for instance 100% passing 500 Mesh). As a result, these more challenging iron ore resources are usually overlooked and relegated to being unusable. SUMMARY OF THE INVENTION [0003] It has been discovered that finely disseminated iron ore can be concentrated effectively by employing a reducing roast followed by grinding and density, preferably magnetic, separation. The procedure is as follows: Initially, the iron ore is ground to relatively coarse particle size (100% minus 1/4" to 100% minus 1mm), and then subsequently thermally processed using a relatively low temperature magnetizing roast in a furnace (rotary kiln, rotary hearth or other furnace that may either be direct or indirectly fired) at (700 ~ 1,200° C) by mixing the ore with a low rank thermal coal or other cheap carbon-containing reductant. The reducing reactions in the roasting step promote conversion of the higher iron oxides to either lower iron oxide species, or metallic iron, actually causes a change in the iron crystal structure. This in turn imparts internal stresses at the iron oxide or metallic iron-silica grain boundary interfaces and generates in some ores "self liberation" where the iron species are fractured at the grain boundary interfaces. In other instances, it has been demonstrated in the laboratory that the internal stresses within the roasted ore create a condition where, if the roasted product is then be subjected to a light grind, a concentrate is produced of high grade (>55% FeTotal) with very high unit iron recoveries (>60%). [0004] The present invention is particularly useful for concentrating low grade iron ores or iron ore tailings. [0005] The apparatus consists of a comminution means or grinder 12 for reducing the particle size of the ore 10; a mixer 14 for mixing the sized ore with a reductant 16; a roasting furnace 20 for receiving and roasting the high silica iron ore; a cooler 22 (if required) to reduce the temperature of the roasted ore, a second grinder 24 and a separator, which can be a magnetic separator 26, or a wind classifier 28 and magnetic separator combined, so as to be a 'dry' coupled unit, or another type of density separator may be used in their place. OBJECTS OF THE INVENTION [0006] The principal object of the present invention is to provide an improved method of concentrating and recovering iron values from high silica iron ores. [0007] Another object of the invention is to provide an improved method of concentrating and recovering iron values from tailings from iron ore processing plants, or an iron-containing waste stream. [0008] A further object of this invention is to provide a method of increasing the iron grade of the feed by effecting pre-reduction of the iron oxide species by using a solid or gaseous reductant. [0009] Another object of the invention is to provide apparatus for concentrating and recovering iron values from high silica iron ores. BRIEF DESCRIPTION OF THE DRAWING

[0010] The foregoing and other objects will become more readily apparent by referring to the following detailed description and the appended drawing in which: [0011] The single figure is a schematic flowsheet of the invented process. DETAILED DESCRIPTION [0012] Referring now to the drawing, high silica iron ore 10 to be processed is ground in grinder 12 to reduce the particle size to at least 100% minus 1/4 inch, and preferably to a particle size of 100% minus 1.0 mm. The ground particles are mixed in mixer 14 with a low rank thermal coal or other cheap carbon-containing reductant 16 such as dried or pyrolized or carbonized grass or other organic material. The carbon-containing material 16 is selected from the group comprising: coal, coal wash plant fines, petcoke, petcoke fines, grass, cellulose, and organic matter such as waste sludge. Iron ore tailings 18 may be introduced to the mixer 14, as desired, and mixed with the ground particles and reductant to form a mixture. [0013] The mixture is then introduced into a roasting furnace 20, preferably a rotary kiln or rotary hearth type furnace in which the particulates are reduced at a temperature of about 700 - 1200°C (preferably about 900 - 1100°C) for a period of about 15 minutes to one hour. The heated particulates are then cooled to a temperature at or below the iron Curie point ( ~770°C) in cooler 22, and then introduced to a density separator, such as magnetic separator 26 which separates and removes the gangue and reductant ash 30 which are mostly silica and silicates, leaving the concentrated iron ore 32, which has a high percentage of iron. Alternatively, separation of the dense iron particles from the less dense gangue / ash particles can be effected by means of wind classification 28 or other density separation method. Magnetic separation of the wind classified intermediate product can then follow as a subsequent step. If desired the particulate from the cooler can be further ground, before being classified. Alternatively, the cooled particulate from cooler 22 or from grinder 24, or both, can be introduced directly to the magnetic separator 26. Magnetic separator 26 can be either a wet or dry separator system. It has been found that a wet magnetic separator is more efficient than a dry separator when handling fine particles, and allows the capture of fine dust. Even 'slimes' can be handled by a wet magnetic separator. SUMMARY OF THE ACHIEVEMENT OF THE OBJECTS OF THE INVENTION [0014] From the foregoing, it is readily apparent that I have invented an improved method and apparatus for concentrating and recovering iron values from finely disseminated iron ore having high silica content, by employing a reducing roast followed by grinding and magnetic separation. [0015] It is to be understood that the foregoing description and specific embodiments are merely illustrative of the best mode of the invention and the principles thereof, and that various modifications and additions may be made to the apparatus by those skilled in the art, without departing from the spirit and scope of this invention.